Oct. 2020, NPO法人 モバイル・コミュニケーション・ファンド, Docomo Mobile Science Award　　　　　　　　　　　　　　　
Kunimasa Saitoh

Jul. 2017, IEEE Photonics Society Distinguished Lecturers Award　　　　　　　　　　　　　　　
SAITOH Kunimasa

Feb. 2015, Japan Society for the Promotion of Science (JSPS), JSPS Prize　　　　　　　　　　　　　　　
SAITOH Kunimasa

Nov. 2011, 東北大学電気通信研究所, 第1回 RIEC Award　　　　　　　　　　　　　　　
Japan

Apr. 2009, 船井情報科学奨励賞　　　　　　　　　　　　　　　
Japan

Apr. 2008, 文部科学大臣表彰若手科学賞　　　　　　　　　　　　　　　
Japan

Nov. 2005, エリクソン・ヤング・サイエンティスト・アワード2005　　　　　　　　　　　　　　　
Japan

2005, The Ericsson Young Scientist Award 2005　　　　　　　　　　　　　　　

Jul. 2004, Best Paper Award in 9th OptoElectronics and Communications Conference (OECC2004)　　　　　　　　　　　　　　　

2004, Best Paper Award in 9th OptoElectronics and Communications Conference (OECC2004)　　　　　　　　　　　　　　　

Feb. 2003, 平成15年度丹羽保次郎記念論文賞　　　　　　　　　　　　　　　
Japan

2003, Niwa Yasujiro Memorial Paper Award　　　　　　　　　　　　　　　

Mar. 2002, 平成13年度電子情報通信学会学術奨励賞　　　　　　　　　　　　　　　
Japan

2002, Young Scientist Award from IEICE　　　　　　　　　　　　　　　

Mar. 2001, 平成12年度電気学会優秀論文発表賞　　　　　　　　　　　　　　　
Japan

Dec. 1999, 平成11年度電気関係学会北海道支部連合大会若手講演者表彰　　　　　　　　　　　　　　　
Japan

May 1999, 平成10年度電子情報通信学会論文賞　　　　　　　　　　　　　　　
Japan

1999, Excellent Paper Award from IEICE　　　　　　　　　　　　　　　

Design of a mode-dependent-loss controller using a long-period fiber grating with coating material for a 2LP-mode transmission system
Takumi Kimura, Takanori Sato, Masaki Wada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Journal of the Optical Society of America B: Optical Physics, 41, 9, 1960, 1967, 01 Sep. 2024
Scientific journal, We designed a mode equalizer to handle mode-dependent loss (MDL) in mode division multiplexing systems. The proposed equalizer is based on a step-index long-period fiber grating (LPFG) and designed to attenuate the LP01 core mode while providing high transmission of the LP11 core mode. For a practical fiber design, we assumed a coating material with a higher refractive index than the cladding. Consequently, numerous leaky cladding modes should be analyzed. This study focuses on identifying the optimal core parameters to efficiently equalize MDL.

Random Polarization-Mode Coupling Effects in Heterogeneous Multi-Core Fibers With Different Cladding Diameters
Gustavo Ocampo, Takanori Sato, Mayu Iizuka, Kunimasa Saitoh
Journal of Lightwave Technology, 42, 16, 5674, 5680, Institute of Electrical and Electronics Engineers (IEEE), 15 Aug. 2024
Scientific journal

A silicon mode multiplexer based on the three-stage cascaded wavelength insensitive coupler for MDM-WDM transmission system
Kodai Nakamura, Takanori Sato, Kunimasa Saitoh
IEICE Electronics Express, 21, 7, 20240065, 20240065, Institute of Electronics, Information and Communications Engineers (IEICE), 10 Apr. 2024
Scientific journal

High tolerant design of athermal Si wavelength filter using Mach–Zehnder interferometer
Takanori Sato, Kengo Maeda, Yusuke Sawada, Takuya Mitarai, Takuo Hiratani, Takuya Okimoto, Tsutomu Ishikawa, Naoki Fujiwara, Hideki Yagi, Kunimasa Saitoh
Japanese Journal of Applied Physics, 63, 4, 04SP55, 04SP55, IOP Publishing, 01 Apr. 2024
Scientific journal, Abstract

A design method for the athermal Mach–Zehnder wavelength filter, which is available in the Si-on-insulator platform, is proposed. We can obtain much higher fabrication-tolerant characteristics by introducing a condition, minimizing the dependence of the waveguide width on the athermal condition, to the wavelength filter design. Our design has the arm waveguides divided into four sections, and each section is appropriately designed to exploit athermal characteristics. We also reveal the relationship between the arm length (corresponding to the footprint) and the waveguide width (relating to the propagation loss) from the formularized equations for the wavelength filter design. One example of the Mach–Zehnder interferometer filter designed by the proposed method has lower spectral shift characteristics less than 1 pm K⁻¹ entire C band even if the waveguide width errors occur, compared with the conventional design.

Variable Mode-Dependent-Loss equalizer based on Silica-PLC for Three-Mode transmission
Takayoshi Mori, Takeshi Fujisawa, Junji Sakamoto, Yoko Yamashita, Taiji Sakamoto, Ryota Imada, Ryoto Ima, Takanori Sato, Kei Watanabe, Ryoichi Kasahara, Toshikazu Hashimoto, Kunimasa Saitoh, Kazuhide Nakajima
Optical Fiber Technology, 81, Dec. 2023
Scientific journal, We present a low-loss and compact silica-based planar lightwave circuit (PLC) mode-dependent loss (MDL) equalizer for three-mode transmission with a variable range of 2.5 dB. The proposed device utilizes a Mach-Zehnder interferometer (MZI) structure to selectively attenuate the LP01 mode while maintaining low insertion loss for the LP11a,b modes. We demonstrate that the device effectively reduces differential modal gain (DMG) in a three-mode transmission system. Additionally, we successfully equalize the pump-power dependence of the DMG in a two-LP mode erbium-doped fiber amplifier (EDFA) to less than 0.5 dB over the entire C-band. We also show that the variable range has a tradeoff relationship with the wavelength bandwidth, where too large a variable range results in non-negligible wavelength dependencies. Consequently, fixed equalizers are advantageous when large MDL compensation is required, while variable equalizers are effective for fine-tuning MDL of less than 1.2 dB.

Broadband design of silica-PLC mode-dependent-loss equalizer for 2LP-mode transmission systems
Takanori Sato, Masaki Wada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optics Express, 31, 24, 39965, 39980, 20 Nov. 2023
Scientific journal, A new configuration of mode-dependent-loss (MDL) equalizer for two linearly-polarized mode transmission systems using the silica planar lightwave circuit platform is proposed. This device acts as an LP01-mode attenuator (precisely, LP01/LP21 mode converter) to adjust the MDL keeping a high transmission of the LP11 modes. Almost all components constructing the device are based on the adiabatic mode conversion, which brings broadband operation. Especially, a newly proposed E12/E22 mode converter plays a key role in broadband MDL equalization. It is numerically revealed that the flattened spectra with designated transmission can be obtained for the wavelength from 1200 nm to 1650 nm.

Investigation of double-cladding heterogeneous step-index 2LP-mode multicore fiber with a two-ring layout
Zheyu Zhao, Takanori Sato, Takeshi Fujisawa, Taro Iwaya, Yuto Sagae, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Journal of the Optical Society of America B, 40, 10, 2511, 2518, 01 Oct. 2023
Scientific journal, This paper investigates the design of C-band double-cladding 2LP mode multicore fiber (2LP-MCF) with 125-µm cladding diameter by numerical simulation, in which a two-ring layout is used to improve crosstalk and critical bend radius. The numerical results show that the crosstalk can be less than −60 and −35 dB/km for 2LP mode eight-core and 10-core fibers, respectively, and it is also shown that the crosstalk can be suppressed compared with that in heterogeneous 2LP-MCFs without a double-cladding layer.

Bayesian design of mosaic-based mode multiplexers for various wavelength bands
Takeshi Fujisawa, Takuya Mitarai, Takuya Okimoto, Naoya Kono, Naoki Fujiwara, Takanori Sato, Hideki Yagi, Kunimasa Saitoh
Optics Express, 31, 16, 26842, 26842, Optica Publishing Group, 27 Jul. 2023
Scientific journal, Ultracompact mode multiplexers based on mosaic structure for various wavelength bands designed by Bayesian technique are investigated. C-, O-, and C + O band, TE₀-TE₁ 2-mode multiplexers can be designed with the same footprint, by only changing the mosaic-pattern, showing the great flexibility of mosaic-based devices. Bayesian direct binary search method is used for the design, and it is demonstrated that the Bayesian technique is superior to conventional design method in terms of the best-structure search for the same number of iterations. The designed devices are fabricated for Si-waveguide platform, and the proof-of-concept results are obtained. These results indicate that the mosaic-based devices are promising candidates for future compact optical transceivers.

Design of silica-PLC LP11 mode rotator based on adiabatic mode conversion
Takanori Sato, Takeshi Fujisawa, Masaki Wada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optics Express, 31, 15, 23910, 23922, 17 Jul. 2023
Scientific journal, A silica-based LP11 mode rotator, which is one of the basic and indispensable optical components for space division multiplexing, with multiple tapered trenches is proposed. Compared with the conventional interference-based LP11 mode rotator with a simple L-shape waveguide, the proposed LP11 mode rotator has many advantages in a mode conversion efficiency, an insertion loss, and a fabrication tolerance because the operation principle is based on the adiabatic mode conversion. By using an approach of the shortcut to adiabaticity, the proposed device is effectively miniaturized rather than the standard tapered structures. Among the LP11 mode rotators in the silica-based mode multi/demultiplexers, the proposed type will be a considerably promising candidate.

Mode converter using bent and twisted coupled-multicore fibers for group delay spread reduction
Takanori Sato, Kazushi Homma, Takeshi Fujisawa, Yuto Sagae, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optical Fiber Technology, 77, May 2023
Scientific journal, A mode converter using bent and twisted coupled-multicore fiber for the reduction of the group delay spread is proposed. The bent and twisted fiber, which is one of the periodic structures, can work as a long-period grating mode converter by properly setting the twist rate. To investigate the mode conversion characteristics, a simple and effective eigenmode analysis for three-dimensional periodic structures is proposed. We found that to convert the specified modes, the core layout is also important as well as the twist rate. For example, the mode conversion behaviors of 4CFs with square lattice and triangular lattice layouts are greatly different although each 4CF enables the specified mode conversion. It is revealed that, by switching the higher and lower order modes, the group delay can be equalized, resulting in the suppression of the group delay spread. It leads to reducing the complexity of the multiple-input multiple-output receiver in the mode division multiplexing system.

Investigation of heterogeneous step-index 2LP-mode multi-core fibers based on a two-ring layout with standard cladding diameter
Zheyu Zhao, Yizhou Wang, Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
OPTICS CONTINUUM, 2, 5, 1137, 1147, Optica Publishing Group, May 2023
English, Scientific journal, The numerical results of the step-index 2LP-mode 6-core and 8-core multi-core fibers (MCFs) with 125-mu m cladding diameter and two-ring layout for C-band, show that the crosstalk less than -51 and -35 dB/km can be obtained respectively. It is revealed that the two-ring layout is adapted for improving the crosstalk and critical bending radius, also effective for few-mode MCFs as well as single-mode MCFs.(c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

6-mode and 10-mode photonic lantern mode multi/demultiplexers based on silica planar lightwave circuit
Han Wang, Takeshi Fujisawa, Takanori Sato, Takayoshi Mori, Taiji Sakamoto, Yoko Yamashita, Ryota Imada, Kazuhide Nakajima, Kunimasa Saitoh
OPTICS COMMUNICATIONS, 529, ELSEVIER, Feb. 2023
English, Scientific journal, We propose a 6-mode and 10-mode photonic lantern mode multiplexer based on two-and three-layer silica planar lightwave circuitry, respectively. The 6-mode multiplexer has transmission greater than -0.19 dB at a wavelength of 1550 nm for the E11, E21, E12, E22, E31, and E13 modes. The 10-mode multiplexer is designed to only excite selected modes (E11, E21, E12, E22, E31, E13, E32, E23, E41, and E14 modes), used for fiber transmission. Each mode has transmission greater than -1.15 dB, at a wavelength of 1550 nm.

Design of Three-Mode Dual-step Index Pure Silica Core Fiber with Low Differential Mode Delay and Low Loss Characteristics
Taro Iwaya, Yuto Sagae, Takayoshi Mori, Taiji Sakamoto, Takashi Matsui, Takanori Sato, Kunimasa Saitoh, Kazuhide Nakajima
Proceedings of SPIE - The International Society for Optical Engineering, 12429, 2023
International conference proceedings, A 3-mode fiber with pure silica core and dual-step index profile is numerically investigated for realizing low DMD and low loss. We found a structure that minimizes DMD while maintaining the low loss property of the pure silica core. We also show that lower loss and larger MFD properties are restricted by the requirement for the micro-bending sensitivity.

Fabrication-tolerant, 2-mode, 4λ multiplexer based on Si waveguides for beyond Tbit/s optical Ethernet
Takeshi Fujisawa, Kunimasa Saitoh
2023 Optical Fiber Communications Conference and Exhibition, OFC 2023 - Proceedings, 2023
International conference proceedings, A fabrication-tolerant, 2-mode, 4λ multiplexer is proposed for beyond Tbit/s optical Ethernet system. Various techniques for strengthening fabrication tolerance are introduced, and a proof-of-concept device is fabricated for Si-photonics platform.

Relationship between Polarization Mode Dispersion and Crosstalk in Heterogeneous Multi-core Fibers with Different Cladding Diameters
Gustavo Ocampo, Takanori Sato, Takeshi Fujisawa, Mayu Nakagawa, Kunimasa Saitoh
2023 Optical Fiber Communications Conference and Exhibition, OFC 2023 - Proceedings, 2023
International conference proceedings, We experimentally investigate the relationship between crosstalk and polarization mode dispersion (PMD) in two heterogeneous multi-core fibers with different cladding diameters. We observed that increasing the cladding diameter improves the crosstalk, but deteriorates the PMD.

Design of Double-Cladding Heterogeneous 2LP-Mode 6-Core Fiber with Two-Ring Layout
Zheyu Zhao, Takanori Sato, Takeshi Fujisawa, Taro Iawaya, Yuto Sagae, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
2023 Opto-Electronics and Communications Conference, OECC 2023, 2023
International conference proceedings, Double-cladding heterogeneous 2LP-mode 6-core fibers (2LP-6CFs) with a two-ring layout are investigated. By optimizing core parameters in 125-μm cladding diameter, crosstalk in 2LP-6CF can be suppressed compared with that in heterogeneous 2LP-6CFs without double-cladding layer.

Proposal of an ultra-compact mode multiplexer using air-hole type photonic crystal waveguides
Han Wang, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
2023 Opto-Electronics and Communications Conference, OECC 2023, 2023
International conference proceedings, We propose an ultra-compact TE0-TE1 mode-division multiplexer based on air-hole photonic crystal waveguides. The longitudinal sizes are only 2.05 μm, which is 1/5 compared with that based on conventional Si-wire waveguides.

Proposal of 5-mode multiplexer using pillar type photonic crystal waveguides
Han Wang, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
28th Microoptics Conference, MOC 2023, 2023
International conference proceedings, We propose a 5-mode multiplexer based on 2D pillar square-lattice photonic crystal waveguides with Si pillar and air background. We then evaluated the performance of the device. The device can selectively excite 5 target modes (TE0, TE1, TE2, TE3, and TE4 modes) by choosing the input ports.

S+C+L Band Tapered Asymmetric Directional Coupler for Broadband Polarization Splitter-Rotator
Shin'Ei Ochiai, Takeshi Fujisawa, Kodai Nakamura, Takanori Sato, Makoto Okano, Kunimasa Saitoh
28th Microoptics Conference, MOC 2023, 2023
International conference proceedings, A broadband tapered asymmetric directional coupler based on Si waveguide for TE1 mode multiplexing was designed. It was fabricated for polarization splitter-rotator for future broadband digital coherent optical transmission system. Fabricated device exhibits broadband mode multiplexing operation with the insertion loss less than 1.9 dB over S+C+L band.

Wavelength insensitive three-mode directional coupler designed by wavefront matching method for broadband mode-dependent-loss equalizer
Ryoto Ima, Takeshi Fujisawa, Masaki Wada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takanori Sato, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
28th Microoptics Conference, MOC 2023, 2023
International conference proceedings, A wavelength insensitive three-mode directional coupler for mode-dependent-loss (MDL) equalizer is proposed. By using powerful optimization method, the waveguide geometry is automatically obtained, and designed device exhibits almost wavelength independent operation. The proposed device is useful for constructing broadband MDL equalizer used in multiple-input-multipleoutput signal processing in mode-division-multiplexing transmission.

High Tolerant Design of Peltier-Free Si Wavelength Filter Using Mach-Zehnder Interferometer for DWDM Application
Kengo Maeda, Takanori Sato, Takeshi Fujisawa, Takuya Mitarai, Takuo Hiratani, Takuya Okimoto, Tsutomu Ishikawa, Naoya Kono, Naoki Fujiwara, Hideki Yagi, Kunimasa Saitoh
28th Microoptics Conference, MOC 2023, 2023
International conference proceedings, High-Tolerant design of a Si wavelength filter using a Mach-Zehnder interferometer is proposed for Peltier-free and low power dissipation Si-photonics devices. Numerical results show that the proposed filter has a temperature dependence low enough for use in dense wavelength division multiplexing systems. It is less than 3 pm/K in the entire C-band, and the device has superior characteristics compared with one designed by the conventional method.

Design of PLC E31-E13 and E-LP tapered mode converters using fast quasiadiabatic dynamics
Han Wang, Takeshi Fujisawa, Takanori Sato, Masaki Wada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
IEICE Electronics Express, 20, 23, 2023
Scientific journal, The previous PLC E31 and E13 mode converters employ linear tapered structures to fulfill the adiabatic coupling condition and achieve a high mode conversion efficiency, which requires long taper lengths. In this paper, we propose two types of PLC tapered structures that can be applied in different scenarios using fast quasiadiabatic dynamics. These structures enable mode conversions of E31-E13 modes, as well as E31-LP02 and E13LP21b modes, respectively. The E31-E13 mode converter can be reduced from the previous three-stage linear taper of 12000 µm to approximately 4000 µm in length. Similarly, the E31-LP02, E13-LP21b mode converter can be reduced from the previous two-stage linear taper of over 6000 µm to approximately 4000 µm in length.

Weakly Coupled Homogeneous 3-Mode 4-Core Fiber With Standard Cladding Diameter
Yuto Sagae, Takashi Matsui, Takayoshi Mori, Taiji Sakamoto, Taro Iwaya, Takanori Sato, Kunimasa Saitoh, Kazuhide Nakajima
Journal of Lightwave Technology, 41, 12, 3950, 3956, 2023
Scientific journal, We propose a design concept of a 3-mode 4-core fiber (3M-4CF) with a common depressed (CD) layer and numerically investigated the tradeoff between the optical confinement loss and inter-core crosstalk (XT) for it and three other types of weakly coupled homogeneous 3M-4CFs, i.e., with step index (SI) cores, trench-assisted (TA) cores, and W-shaped (W) cores, with the standard 125-μm cladding diameter. We reveal that the CD-type 3M-4CF is efficient in suppressing optical confinement loss while maintaining a feasible inter-core XT. We confirm the feasibility of the proposed design concept on the basis of evaluating the optical characteristics for the fabricated CD-type 3M-4CF. We further optimize the design of the CD layer to achieve lower loss. We also show a tradeoff between the micro-bending loss sensitivity and the inter-core XT in our proposed CD structure.

Proposal of mosaic-based 2 × 2 3-dB couplers for prospective on-chip communication in mid-infrared wavelengths around 2.1  µm
Taichi Muratsubaki, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Journal of the Optical Society of America B, 39, 9, 2522, 2522, Optica Publishing Group, 01 Sep. 2022
Scientific journal, We propose two types of mosaic-based 3-dB couplers for wavelengths around 2.1 µm. For type I, the proposed device provides excess loss of dB and imbalance of over the wavelength range from 2.06 to 2.14 µm. Compared with the standard multimode-interference-based 3-dB coupler, the imbalance is improved by . For type II, coupling length in the multimode-interference region is about three times shorter than the one based on multimode-interference theory. It provides excess loss of and imbalance of over the wavelength range from 2.06 to 2.14 µm. Both proposed devices show high fabrication tolerance to hole diameter variation. Especially for type II, the fabrication-tolerant direct binary search was effective to obtain such a structure. These mosaic-based 3-dB couplers will play an important role in highly integrated photonic circuits.

Arbitrary higher-order optical spatial state generation by using spontaneously broken degeneracy modes in helically twisted ring-core hole assisted fibers
Takeshi Fujisawa, Kunimasa Saitoh
Optics Express, 30, 14, 24565, 24565, Optica Publishing Group, 04 Jul. 2022
Scientific journal, A helically twisted ring-core hole assisted fiber (HAF) is proposed for an arbitrary higher-order spatial state generation. In usual circular core fibers, HE and EH modes are degenerate and have vector field distributions. We will show the degeneracy of these modes having specific azimuthal mode order is lifted by periodically placing air-holes around the core with specific symmetry while preserving their vector field distributions. It is called spontaneously broken degeneracy (SBD) in this paper. Azimuthal order of the SBD modes can be changed with the arrangement of surrounding air-holes. By using this SBD modes, it is shown that arbitrary higher-order spatial state generation, including orbital angular momentum (OAM) state, is possible based on a geometric phase existing in twisted fibers. Furthermore, by using periodically inverted twisting, efficient OAM state generation is possible by accumulating the geometric phase. The topological charge can be changed by changing the arrangement of surrounding air-holes. Since the mechanism does not depend on a phase matching, such as a long-period grating, but on a topological effect, the wavelength dependence is very small, leading to novel and efficient mechanism for the manipulation of the spatial state of the light.

Comparison of Scalar and Vector WFM Method Through the Design of High-Δ Waveguide Lenses
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
IEEE Photonics Technology Letters, 34, 10, 533, 536, Institute of Electrical and Electronics Engineers (IEEE), 15 May 2022
Scientific journal, We demonstrate that the high refractive index difference (Δ) waveguide design using a wavefront matching (WFM) method requires a full-vector propagation analysis. The investigation was implemented through the comparison of some pairs of high- Δ waveguide lenses designed by a scalar or full-vector WFM method. The results show that a full-vector WFM method is superior to a scalar WFM method for the design of waveguide lenses with the Δ of 20% or higher. Finally, we experimentally demonstrate that the necessity of a full-vector propagation analysis for the design of Si waveguide lenses using a WFM method. Calculation and measurement results agree well and the insertion loss of a pair of Si waveguide lenses designed by a full-vector WFM method is 0.78 dB lower than that designed by a scalar WFM method at the wavelength of 1550 nm.

Hybrid algorithm based on the grey wolf optimizer and direct binary search for the efficient design of a mosaic-based device
Kodai Nakamura, Takeshi Fujisawa, Kunimasa Saitoh
Journal of the Optical Society of America B, 39, 5, 1329, 1329, Optica Publishing Group, 01 May 2022
Scientific journal, We propose a novel hybrid algorithm based on the grey wolf optimizer (GWO) and direct binary search (DBS) for the design of mosaic-based devices. The DBS algorithm leads to the local optimum structure, and the design results are changed for each trial. Thus, we need to implement the DBS design hundreds of times to find a high-performance mosaic-based device, resulting in huge computer resources and calculation time. To resolve the problem and search for a better solution, the GWO is combined with the DBS. The average and best performances of the designed devices are better by 0.5 and 1.6 points when compared with those designed by the DBS.

A Novel Algorithm of Wavefront-Matching Method for Stable and Efficient Design of Si Waveguides
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
IEEE Photonics Technology Letters, 34, 7, 359, 362, Institute of Electrical and Electronics Engineers (IEEE), 01 Apr. 2022
Scientific journal, We propose a novel algorithm of a wavefront-matching (WFM) method for the design of silicon waveguides. A conventional WFM method may not provide a stable design of silicon waveguides because of its high-index contrast. In the proposed method, a proper constraint is imposed on a conventional method depending on the device performance. Effectiveness of the proposed method is demonstrated through the design of TE0-TE1 mode converter (MC) in detail. The conversion efficiency of the TE0-TE1 MC is -0.52 dB at the wavelength of 1550 nm. As another example, TE0-TE2 MC was designed by the proposed method to additionally demonstrate its effectiveness.

Direct-binary-search algorithm for fabrication-tolerant photonic-crystal-like subwavelength structures and its application to a four-mode waveguide crossing in 2 μm waveband
Taichi Muratsubaki, Takeshi Fujisawa, Yusuke Sawada, Takanori Sato, Kunimasa Saitoh
Japanese Journal of Applied Physics, 61, 4, 042003, 042003, IOP Publishing, 01 Apr. 2022
Scientific journal, Abstract

A novel direct-binary-search algorithm for the design of fabrication-tolerant photonic-crystal-like subwavelength structures is proposed. In the proposed method, structures based on holes with different diameters are considered to enhance the device performance under diameter variation. The usefulness of the proposed method is demonstrated through the design of a fabrication-tolerant four-mode waveguide crossing in the 2 μm waveband. With the proposed method, the transmission of TE3 mode is increased by 4.9% (for diameter of 130 nm) and 9.5% (for diameter of 150 nm) on average of 15 samples, compared with the conventional direct-binary-search method. The designed device provides insertion losses of <0.87 dB and crosstalks of <−24 dB in spite of diameter variations of ±10 nm from the ideal diameter.

Multi-Core Fiber Technology for SDM: Coupling Mechanisms and Design
Kunimasa Saitoh
Journal of Lightwave Technology, 40, 5, 1527, 1543, Institute of Electrical and Electronics Engineers ({IEEE}), 01 Mar. 2022
Scientific journal, Coupling mechanisms and design issues in multi-core fibers (MCFs) for space division multiplexing are described in this paper. For uncoupled MCFs, a design procedure that considers random structural perturbations and effects of correlation length and birefringence beat length on inter-core crosstalk are presented based on coupled mode theory. Further, for coupled MCFs, a numerical estimation method of modal dispersion is described, and the method to enhance random mode mixing is presented.

Analytical expression for mode-coupling coefficient between non-identical step-index cores and its application to multi-core fiber design within 125-μm cladding diameter
Yizhou Wang, Takeshi Fujisawa, Kunimasa Saitoh
Optics Communications, 506, 127552, 127552, Elsevier BV, Mar. 2022
Scientific journal, Analytical expression is derived for the mode-coupling coefficient between non-identical step-index (SI) cores, which enables a quick estimate of the crosstalk in the heterogeneous SI multi-core fibers (Hetero-SI-MCFs). The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Using the derived analytical expression, the feasibility of Hetero-SI-MCF design within standard 125-μm cladding diameter is discussed. The designed MCFs have potential applications in space division multiplexing (SDM) systems.

Compact Power Splitters with Mosaic-based Structure Designed by Bayesian Direct-binary-search Method
Takuya Mitarai, Takeshi Fujisawa, Takuya Okimoto, Naoya Kono, Naoki Fujiwara, Yusuke Sawada, Taichi Muratsubaki, Takanori Sato, Kunimasa Saitoh, Hideki Yagi
OECC/PSC 2022 - 27th OptoElectronics and Communications Conference/International Conference on Photonics in Switching and Computing 2022, 2022
International conference proceedings, We report the first experimental demonstration of compact and low-loss 1×2 power splitters designed by Bayesian direct-binary-search method with arbitrary splitting ratio, which are comprised of photonic crystal-like mosaic structures on Si-photonic circuits.

Analysis of Crosstalk Dependence on Cladding Diameter in Heterogeneous Multi-core Fibers by Considering Polarization-mode Coupling Effects
Gustavo Ocampo, Takanori Sato, Takeshi Fujisawa, Yoshimichi Amma, Kunimasa Saitoh
OECC/PSC 2022 - 27th OptoElectronics and Communications Conference/International Conference on Photonics in Switching and Computing 2022, 2022
International conference proceedings, We considered polarization-mode coupling to investigate crosstalk dependence on cladding diameter in heterogeneous multi-core fibers. Results showed different birefringence beat length and correlation length values on two fibers with different cladding diameters.

Silica-PLC based mode-dependent-loss equalizer for two LP mode transmission
Takeshi Fujisawa, Takayoshi Mori, Junji Sakamoto, Yoko Yamashita, Taiji Sakamoto, Ryota Imada, Ryoto Ima, Takanori Sato, Kei Watanabe, Ryoichi Kasahara, Toshikazu Hashimoto, Kazuhide Nakajima, Kunimasa Saitoh
Optics InfoBase Conference Papers, 2022
International conference proceedings, A mode-dependent-loss (MDL) equalizer based on silica-PLC is proposed for three-mode transmission and experimentally demonstrated for the first time. By selectively attenuating LP01-mode, 1.5-dB MDL equalization is demonstrated for the setup with large LP11-mode loss.

Optical Fibers for Space-Division Multiplexing
Masaharu Ohashi, Shoichiro Matsuo, Tetsuya Hayashi, Katsunori Imamura, Yasuo Kokubun, Masanori Koshiba, Takayoshi Mori, Kazuhide Nakajima, Masataka Nakazawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Sasaki
Springer Series in Optical Sciences, 236, 39, 170, 2022
In book, This chapter describes the design, the transmission characteristics, and the measurements technology of multi-core fibers (MCFs), few-mode fibers (FMFs), and few-mode multi-core fibers (FM-MCFs). Moreover, the cabling technology and future perspectives of innovative optical fiber cable technologies are presented.

A 125-μm Cladding Diameter Uncoupled 3-mode 4-core Fibre with the Highest Core Multiplicity Factor
Yuto Sagae, Takashi Matsui, Taiji Sakamoto, Taro Iwaya, Takayoshi Mori, Takanori Sato, Kunimasa Saitoh, Kazuhide Nakajima
2022 European Conference on Optical Communication, ECOC 2022, 2022
International conference proceedings, The highest relative core multiplicity factor of beyond 12 is achieved in a 125-μm cladding diameter uncoupled multi-core fibre by using a common depressed layer design. Feasible inter-core crosstalk below -40 dB/km and effective area over 80 μm2 in C-L band are successfully obtained simultaneously.

Cascaded Wavefront-Matching-Method Designed Mode-Exchangers for MDL Management in MDM Transmission
Yuichi Asama, Takeshi Fujisawa, Takanori Sato, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Yoko Yamashita, Kazuhide Nakajima, Kunimasa Saitoh
Optics InfoBase Conference Papers, 2022
International conference proceedings, 6-mode-exchangers composed of side- and top-grating-like waveguides designed with an optimization algorithm is proposed for mode-division-multiplexing system. By cascading these devices, an efficient reduction of mode-dependent-loss is possible with only one-time mode exchanging operation.

Modal amplitude and phase estimation of multimode near field patterns based on artificial neural network with the help of grey-wolf-optimizer
Naoto Sugawara, Takeshi Fujisawa, Kodai Nakamura, Yusuke Sawada, Takayoshi Mori, Taiji Sakamoto, Ryota Imada, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optical Fiber Technology, 67, 102720, 102720, Elsevier BV, Dec. 2021
Scientific journal, A simple and efficient method for estimating modal amplitude and phase of multimode near field patterns (NFPs) based on artificial-neural-network (ANN) with the help of the optimization method is proposed. The inferred amplitude and phase of measured NFPs based on ANN are refined by using a grey-wolf optimizer (GWO). By using the proposed method, the image correlation between reproduced and measured NFPs is improved without re-training of ANN, which is the most time-consuming part of ANN-based numerical modal decomposition technique. Numerical examples of three and six mode cases are presented for the estimation using simple ANN. For six-mode case, the correlation is greatly improved by using the optimizer. Finally, the estimation of the measured NFPs from three-mode exchanger and six-mode mode conversion grating is implemented, and 5% improvement in the correlation value is observed for six-mode case. The proposed method offers alternative way to improve the correlation without using elaborated ANN.

A Novel Core Allocation in Heterogeneous Step-Index Multi-Core Fibers With Standard Cladding Diameter
Yizhou Wang, Takeshi Fujisawa, Yuto Sagae, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Journal of Lightwave Technology, 39, 22, 7231, 7237, Institute of Electrical and Electronics Engineers ({IEEE}), 15 Nov. 2021
Scientific journal, We numerically reveal the feasibility of designing multi-core fibers (MCFs) with the simple step-index (SI) profile within the standard 125-μm cladding diameter. We show that the standard cladding diameter enables us to incorporate five identical cores for C-band operation, and the number of cores can be further increased with the heterogeneous core arrangement. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk (XT) and critical bending radius (Rpk) are improved comparing to that of Hetero-MCFs designed by the conventional method. These new types of step-index MCFs (SI-MCFs) with standard cladding diameter have potential application in the space-division multiplexing (SDM) systems.

Arbitrary polarization and orbital angular momentum generation based on spontaneously broken degeneracy in helically twisted ring-core photonic crystal fibers
Takeshi Fujisawa, Kunimasa Saitoh
Optics Express, 29, 20, 31689, 31689, The Optical Society, 27 Sep. 2021
Scientific journal, Using orbital angular momentum (OAM) as a spatial information channel attracts a lot of attention due to its infinite multiplexing capability. The research on OAM carrying fibers is intensively studied and ring-core fibers are promising candidates for them. At the same time, generating OAM modes in those fibers are also important topic. Here, the evolutions of polarization states including OAM of light in helically twisted ring-core photonic crystal fibers (PCFs) are investigated for generating OAM states. The degeneracy of some of the mode sets is spontaneously broken, and the birefringence cause a geometric phase (GP) in the twisted PCFs while preserving the vectorial nature of the modes. It is demonstrated that an arbitrary polarization and OAM state can be generated by using the GP in uniformly twisted and twisted PCFs with periodical inversion. In the ring-core PCF presented in this paper, the degeneracy of HE31 and EH31 mode sets are broken, and OAM light with the topological charge of 2 and 4 can be generated. The wavelength dependence is very small compared with that of OAM generators based on long-period gratings (LPGs). Furthermore, by properly setting the period of the inversion, OAM light with different topological charges can be simultaneously generated with the same fiber structure. These results indicate that the proposed approach is effective for the generation of OAM modes with the conventional fiber modes.

Broadband silicon four-mode multi/demultiplexer designed by a wavefront matching method
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Optics Express, 29, 17, 27322, 27322, The Optical Society, 16 Aug. 2021
Scientific journal, We propose a broadband silicon four-mode multi/demultiplexer which consists of three asymmetric directional couplers, and the asymmetric directional couplers were designed by a wavefront matching method to operate as mode multi/demultiplexers for TE1, TE2, and TE3 modes, respectively. Simulated results show that the −0.5-dB bandwidths of normalized transmission of the couplers designed by a wavefront matching method are 112, 114, and 134 nm, respectively. Those for the conventional couplers are 80, 72, 65 nm, respectively. The superiority of asymmetric directional couplers designed by a wavefront matching method are experimentally demonstrated. In the additional investigation, ultrabroadband tapered asymmetric directional couplers are theoretically demonstrated.

Experimental Demonstration of Broadband Silicon 4-Mode (de)Multiplexer Designed by Wavefront-Matching Method
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
2021 Optical Fiber Communications Conference and Exhibition, OFC 2021 - Proceedings, Jun. 2021
International conference proceedings, A broadband silicon-based 4-mode multiplexer designed using a wavefront-matching method is experimentally demonstrated. Measured results strongly correlate with simulated results, and very flat transmission spectra are observed for the proposed device.

Six-mode scrambler based on cascaded side-wall grating waveguides
Takeshi Fujisawa, Taiji Sakamoto, Masashi Miyata, Takashi Matsui, Toshikazu Hashimoto, Ryoichi Kasahara, Takayoshi Mori, Ryota Imada, Kazuhide Nakajima, Kunimasa Saitoh
Japanese Journal of Applied Physics, 60, 6, 062002, 062002, IOP Publishing, 01 Jun. 2021
Scientific journal, A six-mode scrambler based on cascaded side-wall long-period-grating waveguides on a silica planar lightwave circuit (PLC) platform is proposed for mode-division-multiplexing (MDM) transmission. The device can be fabricated with only one-step etching since it does not use a surface grating. A mode-dependent loss (MDL) of the system is estimated and the MDL can be reduced with a multiple scrambling operation. Two gratings proposed here and a fiber-pigtailed module containing these gratings are fabricated and the expected proof-of-concept mode scrambling operation is confirmed. It is found that the connection between few-mode fiber (FMF) and PLC chip seems to be the dominant loss of the module. Considering these losses, the measured results are in good agreement with the calculated results.

Ultrasmall O-band CWDM 4? MUX based on Mosaic Structure for Dense Optical Interconnects
Kodai Nakamura, Takeshi Fujisawa, Yusuke Sawada, Takanori Sato, Kunimasa Saitoh
2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings, May 2021
International conference proceedings, An ultrasmall 4? wavelength multiplexer based on cascaded mosaic structure is proposed for dense optical interconnects for the first time. Direct-binary-search design shows the size can be drastically reduced, and is 1/20 of conventional one.

High-tolerance CWDM4 wavelength multiplexer based on 2×2/2×1 MZ filters with polarization multiplexing
Kodai Nakamura, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
IEICE Electronics Express, 18, 8, 20210110, 20210110, Institute of Electronics, Information and Communications Engineers (IEICE), 25 Apr. 2021
Scientific journal, A novel Si four-wavelength multiplexer (MUX) for CWDM system composed of two types of Mach-Zehnder (MZ) MUX, and a polarization splitter rotator (PSR) based on a tapered asymmetric directional coupler (ADC) and TE1-TM0 mode converter (MC) is proposed and experimentally demonstrated. To make the ADC fabrication-tolerant and broadband, a new design method is proposed, in which waveguide width dependence is taken into account. Although due to the waveguide width change during the fabrication process, the loss of the fabricated ADC is relatively large, a proof-of-concept four wavelength multiplexing is experimentally demonstrated.

Bayesian direct-binary-search algorithm for the efficient design of mosaic-based power splitters
Takeshi Fujisawa, Kunimasa Saitoh
OSA Continuum, 4, 4, 1258, 1258, The Optical Society, 15 Apr. 2021
Scientific journal, An efficient design method of mosaic-based power splitters based on Bayesian optimization technique is proposed. First, learning characteristics of Gaussian process (GP), which is one of the Bayesian inference techniques, is investigated to show its high regression performance. The transmission characteristics of 1×2 mosaic-based power splitters can be learned with an error of only 0.5%, which is comparable or better than simple ANN. Next, it is demonstrated that an efficient design of 1×2 mosaic-based power splitter with various splitting ratios is possible by using Bayesian optimization based on GP for selecting the next pixel. In the conventional direct-binary-search (DBS) design of mosaic-structure, the next pixel is chosen randomly. On the other hand, in the proposed method it is chosen based on the statistical information obtained by Bayesian inference. By accumulating the information of the transmission characteristics of the device obtained by electromagnetic (EM) simulation as training data, 70% reduction of the number of EM simulation compared with conventional DBS design is demonstrated. Furthermore, by using Bayesian optimization technique, it is shown that the device structure with better characteristics is obtained, compared with those obtained by conventional DBS design for the same number of EM simulation. There results indicate that the proposed method is useful for the design of mosaic-based devices.

Tunable broadband mode power dividers based on a wavelength-insensitive coupler using the thermo-optic effect for flexible modal power adjustment in a mode-division multiplexing network
Kodai Nakamura, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Journal of the Optical Society of America B, 38, 3, 946, 946, The Optical Society, 01 Mar. 2021
Scientific journal, We demonstrate tunable broadband two-mode power dividers based on a wavelength-insensitive coupler for a mode-division multiplexing system. Ultra-small TE0-TE1 and TE0-TE2 mode dividers based on Si-wire waveguides are designed based on the coupled-mode theory. Tunability is achieved by using the thermo-optic effect. In both dividers, arbitrary branching ratios can be realized over a wide wavelength range. We experimentally demonstrated for the first time, to the best of our knowledge, a tunable TE0-TE1 mode divider based on Si-wire waveguides. From the experimental results, an arbitrary branching ratio can be achieved by adjusting the injection current to a TiN heater placed on top of one of the delay lines.

Fabrication-tolerant four-mode waveguide crossing based on PhC-like subwavelength structures at 2 µm
Taichi Muratsubaki, Takeshi Fujisawa, Yusuke Sawada, Takanori Sato, Kunimasa Saitoh
Optics InfoBase Conference Papers, 2021
International conference proceedings, A four-mode waveguide crossing based on photonic-crystal-like subwavelength structures is proposed at the 2 µm waveband, together with a novel direct binary search algorithm for obtaining fabrication-tolerant structures.

Switchable Mode Converter for Four-Mode MDM System Assisted by Passive Mode Controlling Device Designed by Wavefront Matching Method
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
26th Microoptics Conference, MOC 2021, 2021
International conference proceedings, We propose a switchable mode converter which converts TE0 mode into any one of the first four TE modes. The device includes TE0-TE1 mode converter designed by wavefront matching method. Mode conversion efficiencies are larger than -0.39 dB and modal crosstalks are lower than -23 dB at 1550 nm.

Design of Resonant-Characteristics-Monitorable Si Wavelength Filter Using Face-To-Face Loop Mirrors For Heterogeneous Integrated Tunable Lasers
Takanori Sato, Takeshi Fujisawa, Takuya Mitarai, Takuo Hiratani, Takuya Okimoto, Tsutomu Ishikawa, Naoya Kono, Naoki Fujiwara, Hideki Yagi, Kunimasa Saitoh
IEEE International Conference on Group IV Photonics GFP, 2021-December, 2021
International conference proceedings, A Si wavelength filter using face-to-face loop mirrors is newly proposed. It has two resonant modes that can be excited from different access ports. By using it as an external resonator in III-V/Si hybrid lasers, the resonant characteristics in each filter can be directly monitored.

Low-Loss and Small 2 × 4λ Multiplexers Based on 2 × 2 and 2 × 1 Mach–Zehnder Interferometers With On-Chip Polarization Multiplexing for 400GbE
Takeshi Fujisawa, Junya Takano, Yusuke Sawada, Kunimasa Saitoh
Journal of Lightwave Technology, 39, 1, 193, 200, Institute of Electrical and Electronics Engineers (IEEE), 01 Jan. 2021
Scientific journal, Two silicon, 4λ O-band multiplexers (MUXs) for 400GbE using two Mach-Zehnder interferometers (MZIs) having 90-degree phase shift in their output with the help of on-chip polarization multiplexing are demonstrated. By using 2 × 2 and 2 × 1 MZIs for first 3200-GHz filters, the relative position of their spectra has 90-degree phase difference, leading to significant simplification of the tuning of peak wavelength position of the two filters. The polarization MUX is done by using an on-chip polarization-splitter-rotator (PSR) based on an asymmetric directional coupler (ADC) and a TE1-TM0 mode converter (MC) using tapered rib waveguide, designed for O-band application. Two 4λ MUXs for 1.28 and 1.3 μm bands are designed, aiming to the specified wavelength range in 400GbE. Tolerance analysis shows the proposed MUX is robust to the change of waveguide width occurring in the fabrication process, compared with those of conventional multi-stage 4λ MUXs. The averaged insertion loss of eight wavelength is 1.86 dB for the fabricated device and the demonstration of 8λ (2×4λ) MUX for 400GbE based on Si-waveguide is for the first time.

Broadband and compact silicon mode converter designed using a wavefront matching method
Yusuke Sawada, Takeshi Fujisawa, Kunimasa Saitoh
Optics Express, 28, 25, 38196, 38196, The Optical Society, 07 Dec. 2020
Scientific journal, A broadband and compact TE0-TE1 mode converter for a mode division multiplexing system designed using a wavefront matching method is realized. We present the first experimental demonstration of a silicon waveguide device designed by a wavefront matching method. In order to achieve broadband operation of the silicon mode converter, seven wavelengths are considered in its optimization process. The designed silicon mode converter is fabricated via a standard complementary metal-oxide-semiconductor technology, which enables low-cost mass production. Measurements performed using the fabricated mode converter correlate strongly with the calculated results.

Few-mode Multi-core Fibres: Weakly-coupling and Randomly-coupling
Kunimasa Saitoh
2020 European Conference on Optical Communications, ECOC 2020, Dec. 2020
International conference proceedings, Few-mode multi-core fibre (FM-MCF) design suitable for SDM transmission is described. For weakly-coupled FM-MCF, we explain how to determine fibre parameters and investigate the maximum spatial density in a limited cladding diameter. For randomly-coupled FM-MCF, we discuss how to enhance random mode-mixing between different mode-groups.

Modal Amplitude and Phase Estimation of NFP of Six-Mode FMF Based on Artificial Neural Network with the Help of Grey-Wolf-Optimizer
N. Sugawara, T. Fujisawa, K. Nakamura, Y. Sawada, T. Sakamoto, T. Matsui, K. Nakajima, K. Saitoh
2020 European Conference on Optical Communications, ECOC 2020, Dec. 2020
International conference proceedings, A simple and efficient method for estimating modal amplitude and phase of few-mode-fiber based on artificial-neural-network with the help of grey-wolf-optimizer is proposed. Experimentally obtained six-mode near-field-patterns of recently proposed six-mode mixer are analyzed and successfully reproduced for the first time.

Ultrasmall two-mode dividers based on mosaic structure designed by direct-binary-search algorithm aided by artificial neural network
Takeshi Fujisawa, Kunimasa Saitoh
25th Opto-Electronics and Communications Conference, OECC 2020, 04 Oct. 2020
International conference proceedings, Ultrasmall mosaic-based two-mode dividers are proposed for mode-division-multiplexing system. The device with arbitrary branching ratio can be designed efficiently with direct-binary-search algorithm with the help of artificial neural network.

Mode Amplitude and Phase Estimation using Multimode NFP by ANN
N. Sugawara, T. Fujisawa, K. Nakamura, Y. Sawada, T. Sakamoto, T. Matsui, K. Nakajima, K. Saitoh
25th Opto-Electronics and Communications Conference, OECC 2020, 04 Oct. 2020
International conference proceedings, A simple and efficient method for estimating modal amplitude and phase of few-mode-fiber based on artificial-neural-network with the help of grey-wolf-optimizer is proposed. Experimentally obtained three-mode near-field-patterns of proposed three-mode exchanger are analyzed and successfully reproduced.

Control of Group Delay Spread in Randomly-Coupled Multicore Fibers
Kunimasa Saitoh, Takeshi Fujisawa, Takanori Sato
25th Opto-Electronics and Communications Conference, OECC 2020, 04 Oct. 2020
International conference proceedings, The impact of core-to-core distance, bending condition, and number of cores on the group delay spread (GDS) in randomly-coupled multicore fibers (MCFs) is investigated for designing low GDS coupled MCFs. It is shown that the control of group index difference between different modes along the propagation direction in bent and twisting MCFs has strong impact on the GDS reduction.

Step index 8-core fiber with 125-m cladding diameter for O-band use
Yizhou Wang, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
25th Opto-Electronics and Communications Conference, OECC 2020, 04 Oct. 2020
International conference proceedings, Two types of step index 8-core fiber with standard cladding diameter containing outer trench layer are proposed. Based on the calculation results, the proposed fibers are suitable for O-band use.

Spatial Density and Splicing Characteristic Optimized Few-Mode Multi-Core Fiber
Taiji Sakamoto, Kunimasa Saitoh, Shota Saitoh, Yoshiteru Abe, Katsuhiro Takenaga, Azusa Urushibara, Masaki Wada, Takashi Matsui, Kazuhiko Aikawa, Kazuhide Nakajima
Journal of Lightwave Technology, 38, 16, 4490, 4496, Institute of Electrical and Electronics Engineers ({IEEE}), 15 Aug. 2020
Scientific journal, Few-mode fiber design suitable for long-haul dense space division multiplexing (SDM) transmission is proposed, where the spatial density and the splicing characteristics of the fiber is optimized. We numerically investigate the impact of the number of modes per core in a multi-core structure on the total number of spatial channels possible and the splice loss property. It is shown that increased number of mode per core enables us to improve the spatial density while maintaining feasible splice-induced differential modal attenuation. We then successfully fabricate a 10-mode 12-core fiber with a cladding diameter of 217 μm, that achieves a relative core multiplicity factor (RCMF) of 101.7 and maximum number of spatial channels among the possible few-mode multi-core fiber designs with a cladding diameter of less than 250 μm. Experiments confirm that its splice-induced differential modal attenuation characteristics were better than those of previously reported 114-spatial-channel fiber.

Geometric-phase-induced arbitrary polarization and orbital angular momentum generation in helically twisted birefringent photonic crystal fiber
Takeshi Fujisawa, Kunimasa Saitoh
Photonics Research, 8, 8, 1278, 1278, The Optical Society, 01 Aug. 2020
Scientific journal, The evolutions of polarization and orbital angular momentum (OAM) states of light in helically twisted birefringent photonic crystal fibers (TB-PCFs) are analyzed. It is shown that a circular polarization (CP) component (S3 of a Stokes parameter) is periodically excited when usual linearly polarized (LP) modes of PCF are launched. The excitation originates from a geometric phase in TB-PCFs. The S3 excitation is larger for larger linear birefringence for a fixed twisting rate. If the linear birefringence is large enough, a CP filtering behavior can be seen in addition to the S3 excitation. From the analytical consideration of the sign of the geometric phase, the TB-PCF with periodical inversion of twisting is proposed to generate arbitrary polarization state on the Poincaré sphere. Next, an OAM state generation in multimode TB-PCFs is shown for higher-order LP mode input. By observing a far-field interference pattern from TB-PCF mixed with LP01 mode, a vortex associated with the OAM state can be seen. Similar to the single-mode case, by using periodical twisting inversion, efficient OAM generation is possible. These results indicate that by simply launching fiber's LP mode into TB-PCF, arbitrary polarization and OAM states can be generated, leading to a novel mechanism for the manipulation of the spatial state of light.

High Spatial Density 6-Mode 7-Core Fiber Amplifier for L-Band Operation
Yongmin Jung, Masaki Wada, Kohki Shibahara, Saurabh Jain, Ian A. Davidson, Pranabesh Barua, John R. Hayes, Taiji Sakamoto, Takayuki Mizuno, Yutaka Miyamoto, Yusuke Sasaki, Kunimasa Saitoh, Kazuhide Nakajima, David J. Richardson
Journal of Lightwave Technology, 38, 11, 2938, 2943, 01 Jun. 2020
Scientific journal, We present a high spatial density space division multiplexed fiber amplifier (i.e., 6-mode 7-core multicore fiber amplifier) supporting 42 spatial channels. A high average modal gain of 23.1 dB is obtained in the L-band with an average differential modal gain of 3.2 dB and a minimum output power of 17.5 dBm per core per mode. The total power consumption of the amplifier is improved by a power reduction factor of 0.36 and a fully fiberized optical amplifier is realized in a cladding-pumped configuration. The assembled amplifier is then tested in a few-mode multicore fiber transmission link and simultaneously amplified 42 spatial channel fiber transmission is demonstrated with the highest spatial density and energy efficiency to date.

Experimental Demonstration of Broadband Silicon Mode Converter Designed by Wavefront-Matching Method
Yusuke Sawada, Takeshi Fujisawa, Kunimasa Saitoh
Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS, 2020-May, May 2020
International conference proceedings, A broadband Si-based TE0-TE1 mode converter is successfully designed by a wavefornt-matching method for the first time and experimentally demonstrated. The effect of the broadband design appearing in the optimum waveguide geometry is observed.

Design of small mode-dependent-loss scrambling-type mode (de)multiplexer based on PLC.
Motoki Shirata, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optics express, 28, 7, 9653, 9665, 30 Mar. 2020, [International Magazine]
English, Scientific journal, A general design rule of scrambling-type mode (de)multiplexers (mode scramblers) based on silica planar lightwave circuit (PLC) with small mode-dependent-loss (MDL) is presented for a mode-division multiplexing (MDM) system. First, we consider four- and eight-mode scramblers and demonstrate that if the number of modes is 2N, it is possible to construct small-MDL mode scramblers by using Y-branch waveguides and mode rotators. Next, a 6-mode scrambler, which can be used for four linearly polarized (LP) mode transmission in MDM system, is considered, and the MDL is large if Y-branch waveguides are cascaded simply, originating from the radiation loss of unwanted modes at the Y-branch. We propose a 2 + 4-type mode scrambler by combining 2- and 4-mode scramblers and demonstrate that it is possible to design a small MDL 6-mode scrambler.

A Tunable Mode Divider Based on Wavelength Insensitive Coupler using Thermo-Optic Effect for Gain-Equalization in MDM Network
Kodai Nakamura, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
2020 Optical Fiber Communications Conference and Exhibition, OFC 2020 - Proceedings, Part F174-OFC 2020, Mar. 2020
International conference proceedings, A tunable TE0-TE1 mode divider based on wavelength-insensitive-coupler is experimentally demonstrated for the first time. Arbitrary branching ratios can be realized by using thermo-optic heaters. The proposed device is useful for gain-equalization in MDM networks.

Low-Loss Silicon 2 × 4λ Multiplexers Composed of On-Chip Polarization-Splitter-Rotator and 2 × 2 and 2 × 1 Mach-Zehnder Filters for 400GbE
Junya Takano, Takeshi Fujisawa, Yusuke Sawada, Kunimasa Saitoh
2020 Optical Fiber Communications Conference and Exhibition, OFC 2020 - Proceedings, Part F174-OFC 2020, Mar. 2020
International conference proceedings, 2×4λ Si-photonics multiplexers for 400GbE composed of Mach-Zehnder filters and a polarization-splitter-rotator are proposed and experimentally demonstrated for the first time. Relative spectral position of two filters is locked by using 2×2 and 2×1 configurations.

Wavefront-Matching-Method-Designed Six-Mode-Exchanger Based on Grating-Like Waveguide on Silica-PLC Platform
Takeshi Fujisawa, Taiji Sakamoto, Masashi Miyata, Takashi Matsui, Toshikazu Hashimoto, Ryoichi Kasahara, Kazuhide Nakajima, Kunimasa Saitoh
2020 Optical Fiber Communications Conference and Exhibition, OFC 2020 - Proceedings, Part F174-OFC 2020, Mar. 2020
International conference proceedings, A first six-mode exchanger based on one sidewall grating-like waveguide is successfully designed with the help of strong optimization algorithm. Fabricated device compensates for mode-dependent-loss caused by fiber-waveguide junctions, showing the proof-of-concept operation.

Broadband design of silicon photonics four-mode (de)multiplexer by wavefront matching method
Yusuke Sawada, Takeshi Fujisawa, Kunimasa Saitoh
Optics InfoBase Conference Papers, Part F184-IPRSN 2020, 2020
International conference proceedings, A broadband silicon-based four-mode multiplexer is designed by a wavefront matching method. The crosstalk is suppressed for all modes in wide bandwidth and the transmission spectra becomes very flat compared with those of conventional design.

A novel Si four-wavelength multiplexer for 100/400GbE using higher-order mode composed of (a)symmetric directional couplers and TE1-TM0 mode converter.
Takeshi Fujisawa, Junya Takano, Yusuke Sawada, Taiji Sakamoto, Takashi Matsui, Kazuhide Nakajima, Kunimasa Saitoh
Optics express, 27, 25, 36286, 36296, 09 Dec. 2019, [International Magazine]
English, Scientific journal, A novel Si four-wavelength multiplexer (MUX) for 100/400GbE composed of (a)symmetric directional couplers and a rib-waveguide TE1-TM0 mode converter is proposed and experimentally demonstrated. Two-lane signals are multiplexed as TE0 modes via symmetric directional coupler (DC) and other two-lane signals are multiplexed as TE1 modes via asymmetric directional couplers (ADCs). Finally, TE1 modes are converted to TM0 modes and the device acts as a 4-lane MUX. The proposed device is fabrication-tolerant due to the removal of the 1600-GHz filter used in conventional two-stage Mach-Zehnder MUXs, and the insertion loss is smaller than those of previously reported Si-based MUXs.

Nonreciprocity Enhancement of Graphene-on-Si Waveguide using One-Dimensional Photonic Crystal
Kodai Nakamura, Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
MOC 2019 - 24th Microoptics Conference, 192, 193, Institute of Electrical and Electronics Engineers Inc., 01 Nov. 2019
English, International conference proceedings, The effect of using one-dimensional photonic crystal waveguide on the nonreciprocity of graphene-on-silicon waveguides is theoretically investigated. It is shown that the position of air-hole strongly affects the loss of forward and backward propagation modes.

Inter-core crosstalk mitigation in multicore fibers by optical CDMA spreading technique with referenced channels　　　　　　　　　　　　　　　
Yizhou Wang, Kunimasa Saitoh
Asia Communications and Photonics Conference, ACP, 2019-, OSA - The Optical Society, 01 Nov. 2019
English, International conference proceedings, In this paper, we employ optical code division multiple access spreading technique to treat the inter-core crosstalk in multicore fiber transmission. By choosing referenced channels, our results show about 3.4 dB improvement of inter-core crosstalk comparing to conventional 6-core fiber.

The Effect of Core Offset on the Mode Converting Characteristics in Twisted Single Mode Fibers
Sota Nakano, Takeshi Fujisawa, Kunimasa Saitoh
Journal of Lightwave Technology, 37, 21, 5479, 5485, Institute of Electrical and Electronics Engineers (IEEE), 01 Nov. 2019
Scientific journal

Fabrication of Ultrasmall Silicon Waveguide Lenses Designed by Wavefront-Matching Method
Takeshi Fujisawa, Kunimasa Saitoh, Yusuke Sawada
IEEE International Conference on Group IV Photonics GFP, 2019-, IEEE Computer Society, 01 Aug. 2019
English, International conference proceedings, We design and fabricate ultrasmall silicon waveguide lenses designed by a wavefront-matching (WFM) method by a standard CMOS process. It is experimentally demonstrated that a low-loss several micron free-space light propagation is possible by using the waveguide lens designed by the WFM method.

MA4.1 epitaxial design of GeSn quantum wells for optoelectronic applications (Invited)
Takeshi Fujisawa, Masakazu Arai, Kunimasa Saitoh
IEEE Photonics Society Summer Topical Meeting Series 2019, SUM 2019, Institute of Electrical and Electronics Engineers Inc., 01 Jul. 2019
English, International conference proceedings, Epitaxial designs of GeSn/SiGeSn quantum wells for optoelectronic applications are theoretically demonstrated based on many-body theory. Layer structures for high-Temperature operation lasers and electroabsorption modulators are discussed.

Cladding Diameter Dependence of Inter-Core Crosstalk in Heterogeneous Multicore Fibers
Kenta Nishimura, Takanori Sato, Takeshi Fujisawa, Yoshimichi Amma, Katsuhiro Takenaga, Kazuhiko Aikawa, Kunimasa Saitoh
OECC/PSC 2019 - 24th OptoElectronics and Communications Conference/International Conference Photonics in Switching and Computing 2019, Jul. 2019, [Peer-reviewed]
International conference proceedings, © 2019 The Institute of Electronics, Information and Communication Engineers (IEICE). From random twisting point of view, relationship between inter-core crosstalk and cladding diameter in heterogeneous multicore fibers is investigated. Calculated cladding diameter dependence is in good agreement with measured result.

Ultra-Robust Design of Mode (De)Multiplexer Based on Asymmetrical Directional Coupler Using Wire and One-Side Rib Waveguides
Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
OECC/PSC 2019 - 24th OptoElectronics and Communications Conference/International Conference Photonics in Switching and Computing 2019, Jul. 2019, [Peer-reviewed]
International conference proceedings, © 2019 The Institute of Electronics, Information and Communication Engineers (IEICE). An ultra-robust Si mode (de)multiplexer is newly proposed. The device is composed of Si wire and one-side rib waveguides. The numerical results based on the coupled mode theory show the effectiveness of our proposed design.

1.2 Pb/s throughput transmission using a 160 μm cladding, 4-core, 3-mode fiber
Ruben S. Luis, Georg Rademacher, Benjamin J. Puttnam, Tobias A. Eriksson, Hideaki Furukawa, Andrew Ross-Adams, Simon Gross, Michael Withford, Nicolas Riesen, Yusuke Sasaki, Kunimasa Saitoh, Kazuhiko Aikawa, Yoshinari Awaji, Naoya Wada
Journal of Lightwave Technology, 37, 8, 1798, 1804, Institute of Electrical and Electronics Engineers Inc., 15 Apr. 2019
English, Scientific journal, This paper extends a recent demonstration of 1.2 Pb/s transmission using a few-mode multi-core fiber with a cladding diameter of 160 μm. The fiber was a 3.37 km, 4-core, 3-mode few-mode multi-core fiber with spatial multiplexing and demultiplexing performed with especially designed mode-selective couplers. We transmitted 368 wavelength division multiplexed spatial super channels across the C and L bands. We selected polarization-division-multiplexed-256-quadrature amplitude modulation format to maximize the generalized mutual information of the transmitted signals, which were decoded using punctured soft-decision forward error correction with a code rate granularity of 0.01. The maximum crosstalk of the system was-31 dB between LP11 modes of adjacent cores, leading to a Q-factor penalty of approximately 0.5 dB. We estimated the system mode-dependent loss values ranging from 3 dB to 5 dB across C and L bands with negligible wavelength dependence.

Low-Loss and Fabrication-Tolerant Si Four-Wavelength Multiplexer using Higher-Order Mode for 100/400GbE
Junya Takano, Takeshi Fujisawa, Takanori Sato, Yusuke Sawada, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima, Kunimasa Saitoh
2019 Optical Fiber Communications Conference and Exhibition, OFC 2019 - Proceedings, Apr. 2019, [Peer-reviewed]
International conference proceedings, © 2019 OSA. A Si four-wavelength multiplexer for 100/400GbE composed of (a)symmetric directional couplers and a rib-waveguide mode-converter is experimentally demonstrated. Proposed device is fabrication-tolerant due to the removal of 1600-GHz filter used in conventional two-stage Mach-Zehnder multiplexers.

First Experimental Demonstration of Wavefront-Matching-Method-Designed Silicon Mode Converters
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
2019 Optical Fiber Communications Conference and Exhibition, OFC 2019 - Proceedings, Apr. 2019, [Peer-reviewed]
International conference proceedings, © 2019 OSA. Ultrasmall TE0-TE1 and TE1-TE2 mode converters based on Si-wire waveguides designed by wavefront matching (WFM) method are demonstrated both theoretically and experimentally. This is the first demonstration of WFM-designed Si waveguide devices.

Microscopic gain analysis of modulation-doped GeSn/SiGeSn quantum wells: epitaxial design toward high-temperature lasing
T. Fujisawa, M. Arai, K. Saitoh
Optics Express, 27, 3, 2457, 2464, The Optical Society, 04 Feb. 2019
Japanese, Scientific journal, © 2019 Optical Society of America. Threshold carrier densities of GeSn quantum well (QW) lasers and the physical reason of low-temperature lasing of current GeSn laser are investigated through the comparison of threshold carrier densities of conventional III-V QW lasers. Electrons distributed over L-band is the main cause of decreased gain for GeSn QWs. To increase the gain (and improve the laser characteristics), a modulation-doped GeSn QW is proposed and the material gain is analyzed based on many-body theory for both qualitative and quantitative simulation. Significant gain increase can be expected for n-type modulation doping QWs. The doping condition for elevated temperature lasing is discussed and it was found that material gain curve similar to III-V QW is obtained for GeSn QW with n-type modulation doping of 6 × 10 18 cm −3 . It was also found that unlike III-V QW lasers, n-type modulation doping is more effective for high-speed operation in terms of differential gain than p-type modulation doping.

A proposal of Mach–Zehnder mode/wavelength multi/demultiplexer based on Si/silica hybrid PLC platform
Misa Kudo, Shun Ohta, Eri Taguchi, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima, Kunimasa Saitoh
Optics Communications, 433, 168, 172, Elsevier BV, Feb. 2019
Scientific journal

All-Optical Diode Suppressing Broadband Backward Transmission Using Single-and Four-Port Photonic Crystal Cavities
Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
IEEE Photonics Journal, 11, 1, 4900214, 14, Institute of Electrical and Electronics Engineers ({IEEE}), Feb. 2019, [Peer-reviewed]
English, Scientific journal, © 2009-2012 IEEE. We present an all-optical diode suppressing broadband backward transmission using single-port and four-port photonic crystal (PC) cavities and its operation principle. We find that, by combining a four-port PC cavity and two power-dependent phase shifters that are composed by a 3-dB power divider and two single-port PC cavities, the backward transmission in the nonlinear-resonator-based all-optical diode becomes theoretically 0% for any input power and any input wavelength. To confirm the validity of our approach, the numerical analysis based on the nonlinear beam propagation method as well as the coupled mode theory is performed. The results prove that the forward transmission of-4 dB and large nonreciprocal transmission ratio of 35 dB are achievable.

CMOS-compatible Si-wire polarization beam splitter based on wavelengthinsensitive coupler
Yusuke Sawada, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
IEICE Electronics Express, 16, 8, 2019, [Peer-reviewed]
Scientific journal, © 2019 The Institute of Electronics, Information and Communication Engineers. An ultrabroadband silicon polarization beam splitter (PBS) based on a wavelength-insensitive coupler is fabricated by a standard complementary metal oxide semiconductor technology. The PBS has three identical directional couplers and two identical delay lines with a point symmetric layout. We experimentally demonstrate the broadband operation of the PBS. The crosstalk is less than -12 dB for both TE and TM modes over the wavelength range from 1532nm to 1649 nm. Additionally, we theoretically discuss a tunability of the PBS for the first time.

Optimum index profile of few-mode coupled multicore fibers for reducing the group delay spread
Kazuki Yoshida, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Optics Express, 27, 11, 16281, 16295, The Optical Society, 2019, [Peer-reviewed]
Scientific journal, © 2019 Optical Society of America. We numerically investigate the group delay spread (GDS) characteristics in fewmode coupled multicore fibers (FM-CMCFs) in weak random-bending conditions by using a coupled-wave theory. We qualitatively show the modal coupling between modes in the intraand the inter-mode group generated by bending. The results indicate that the modes of FMCMCFs are less likely to couple with each other in weak random-bending conditions in contradiction to single-mode coupled multicore fibers. To resolve this problem, we demonstrate an optimum index profile and core pitch, where the GDS can be reduced even if the different mode groups do not couple strongly.

Suppression of Group-Delay Spread in Coupled Two-LP-Mode Four-Core Fiber
Takanori Sato, Kazuki Yoshida, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima, Kunimasa Saitoh
31st Annual Conference of the IEEE Photonics Society, IPC 2018, 06 Nov. 2018, [Peer-reviewed]
International conference proceedings, © 2018 IEEE. We newly develop the group-delay spread (GDS) analysis for a tight-bent coupled few-mode multicore fiber (FM-MCF) using pseudo guided modes and reveal that the GDS of coupled FM-MCF can be suppressed for the first time.

GeSn/SiGeSn multiple-quantum-well electroabsorption modulator with taper coupler for mid-infrared Ge-on-Si platform
Minami Akie, Takeshi Fujisawa, Takanori Sato, Masakazu Arai, Kunimasa Saitoh
IEEE Journal of Selected Topics in Quantum Electronics, 24, 6, Nov. 2018, [Peer-reviewed]
Scientific journal, © 2018 IEEE. We propose a taper coupler electroabsorption modulator (EAM) composed of GeSn/SiGeSn multiple-quantum-well (MQW) on Ge-on-Si platform for mid-infrared (2 μm) integrated optical active devices. The epitaxial design is performed by calculating the absorption spectra of GeSn/SiGeSn MQW using many-body theory to investigate the extinction characteristics of GeSn MQW waveguides. Two types of taper couplers are considered for connecting Ge-rib waveguide and GeSn-MQW-highmesa waveguide efficiently. One is an adiabatic taper coupler (ATC) type EAM and it is useful for thin Ge-buffer structure in terms of the extinction ratio. Another is a resonant taper coupler (RTC) type EAM and it is superior to ATC type EAM for thick Ge-buffer. It is confirmed that RTC type EAM can obtain the high extinction characteristics with low-loss and shorter device length (6.87 dB, -3.97 dB, and 215 μm) compared with conventional ATC type EAM for thick Ge-buffer (5.67 dB, -4.9 dB, and 340 μm).

GeSn/SiGeSn Multiple-Quantum-Well Electroabsorption Modulator With Taper Coupler for Mid-Infrared Ge-on-Si Platform
Akie Minami, Fujisawa Takeshi, Sato Takanori, Arai Masakazu, Saitoh Kunimasa
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 24, 6, 1, 8, Institute of Electrical and Electronics Engineers ({IEEE}), Nov. 2018, [Peer-reviewed]
Scientific journal

Off-axis core transmission characteristics of helically twisted photonic crystal fibers
Fujisawa Takeshi, Saitoh Kunimasa
OPTICS LETTERS, 43, 20, 4935, 4938, The Optical Society, 15 Oct. 2018, [Peer-reviewed]
Scientific journal

Non-circular multi-core fibers for super-dense SDM
Md Nooruzzaman, Kunimasa Saitoh, Yusuke Sasaki, Katsuhiro Takenaga, Kazuhiko Aikawa, Toshio Morioka
IEICE Electronics Express, 15, 19, Institute of Electronics Information Communication Engineers, 10 Oct. 2018
English, Scientific journal, Non-circular multi-core fibers (NC-MCFs) to accommodate more than 100 cores per fiber are proposed for super-dense space-division multiplexing. The proposed MCFs have the potential for easier splicing/ connection and their applicability to highly-dense ribbon MCFs for ultra-high capacity optical transport systems.

Erratum: Full-vector finite-element beam propagation method for helicoidal waveguides and its application to twisted photonic crystal fibers (Journal of Lightwave Technology (2017) 35:14 (2894-2901) DOI: 10.1109/JLT.2017.2703889)
Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Journal of Lightwave Technology, 36, 18, 4211, 4212, Institute of Electrical and Electronics Engineers ({IEEE}), 15 Sep. 2018, [Peer-reviewed]
Scientific journal, © 2018 IEEE. We reported the development of novel beam propagation method (BPM) for helicoidal waveguides and its application to twisted photonic crystal fibers (PCFs) in [1]. Although the formulation presented in [1] is correct, we found a careless mistake in our numerical code. In this erratum, we would like to describe a corrected point in the code and demonstrate corrected results. As shown here, corrected results are very much closer to the experimental results [2]. The corrections do not affect the claim of the original paper, namely, the development of novel BPM and the BPM is more useful for analyzing twisted PCF than a guided mode analysis. II. CORRECTED POINTS Following equations are finite element matrices [Mtz ] and [Mz t ] (the equation numbers are the same as [1]). (Equation presented) In our numerical code, minus 1 is multiplied to these matrices by mistake. Here, we correct this point, and the results obtained by the corrected code are presented. Again, mathematical formulation presented in [1] is correct. III. CORRECTED RESULTS Here, the corrected major numerical results are presented. The condition of the calculation is completely the same as in [1]. For easy comparison with [1], the same Figure numbers are given in this erratum. Figure 3(a) and (b) showtransmission spectra of twisted PCF fora = 0.004, 0.006, 0.008 and 0.0108 rad/μm and α = 0.0136 rad/μm. Dots in Figs. 3(a) and (b) indicate reported measured spectra [2] for α = 0.0108 and 0.0136 rad/μm, respectively. In [1], if the value of a for the (Figure Presented) Fig. 3. Transmission spectra of twisted PCF for (a) α = 0.004, 0.006, 0.008, 0.0108 rad/μm and (b) α = 0.0136 rad/μm. Dots are reported measured data in [2]. calculation is the same with the experiment, the loss is overestimated and we had to fit the value. In [1], α = 0.008 and 0.01 rad/μm were used to compare the calculated results with the measured results for α = 0.0108 and 0.0136 rad/μm. In the corrected results, however, calculated results with α = 0.0108 and 0.0136 rad/μm are in very good agreement with the measured results. Therefore, we do not have to fit the value of a in the corrected code, which is a very reasonable result. The positions of the dips are very close to the experiment with slight difference (calculated dips are on shorter wavelength side) and the background loss at off resonance wavelength can be taken into account (the background loss calculated by the guided mode analysis is almost zero [2]). (Figure Presented) Fig. 4. Polarization power ratio as a function of z for (a) λ = 0.66 μm and (b) λ = 0.8 μm. (Figure Presented) Fig. 7. Transmission spectra of twisted PCF for different values of d. Figure 4(a) and (b) show the polarization power ratio as a function of z for λ = 0.66 and 0.8 μm. The twisting rate is α = 0.0136 rad/μm. Compared with Fig. 4 of [1], we changed the value of a from 0.01 to 0.0136 rad/μm, since we do not have to fit the value any more as shown in Fig. 3. Compared with [1], in the results obtained by the corrected code, the polarization rotation occurrs in the helicoidal system (xyz coordinate). This seems to be reasonable if we consider that the modeling space (where the numerical discretization is made) is related to the xyz coordinate system [3]. (Figure Presented) Fig. 9. Transmission spectra of twisted PCF for (a) α = 0.0108 rad/μm and (b) α = 0.0136 rad/μm. Dots are reported measured data in [2]. Figure 7 shows the transmission spectra of twisted PCF for different values of d. The twisting rate is α = 0.0136 rad/μm (which is changed from α = 0.01 rad/μm in [1]). Qualitatively, similar results are obtained compared with [1] (smaller loss for larger air-hole size). Figure 9 (a) and (b) show the transmission spectra of twisted PCF for α = 0.0108 and 0.0136 rad/μm, respectively, calculated by using the finite-element mesh shown in Fig. 8 of [1]. The positions of the calculated dips are shifted to longerwavelength side and are in excellent agreement with the measured results. From these results, the corrected BPM gives more reasonable and accurate results. At the same time, the claims and conclusion in [1] (mathematical formulation of helicoidal BPM and the claim that the BPMis useful for the analysis of twisted PCF comparedwith the guided mode analysis) are not affected by the corrected results.

Iterative unreplicated parallel interference canceler for MDL-tolerant dense SDM (12-Core × 3-Mode) transmission over 3000 km
K. Shibahara, T. Mizuno, D. Lee, Y. Miyamoto, H. Ono, K. Nakajima, Y. Amma, K. Takenaga, K. Saitoh
44th European Conference on Optical Communication (ECOC2018), 37, 6, Tu3F.6, 1569, Institute of Electrical and Electronics Engineers ({IEEE}), Sep. 2018, [Peer-reviewed]
English, International conference proceedings

Development of the Wavefront Matching Method Based on the 3-D Finite-Element Method and Its Application to Si-wire Mode Converters
Yusuke Sawada, Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
Journal of Lightwave Technology, 36, 17, 3652, 3659, Institute of Electrical and Electronics Engineers ({IEEE}), 01 Sep. 2018, [Peer-reviewed]
Scientific journal, © 1983-2012 IEEE. A fully three-dimensional wavefront matching method (WFM) based on a finite-element scheme for optical waveguide devices including waveguide discontinuities is newly developed. In contrast to the conventional WFM method based on the beam propagation method, the developed method, in principle, can be applied to optimize any geometry including reflection and structural discontinuities. The procedure for renewing refractive index distribution is described in detail. The application for Si-wire mode converters ( TE0-TE1, TE1-TE2, and TE0- TE2) are demonstrated and ultrasmall structure with low loss is obtained. We designed them to confirm the validity of the developed method. A low-loss and compact waveguide structure was obtained by the developed method automatically.

Beam propagation analysis of optical activity and circular dichroism in helically twisted photonic crystal fiber
Sota Nakano, Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Japanese Journal of Applied Physics, 57, 8, Aug. 2018, [Peer-reviewed]
International conference proceedings, © 2018 The Japan Society of Applied Physics. Transmission characteristics of a helically twisted photonic crystal fiber (PCF) are theoretically investigated by a recently developed beam propagation method for a helicoidal system. Experimentally reported features of the twisted PCF, such as periodical transmission dips, optical activity discontinuity, and circular dichroism, are successfully reproduced in a simulation. We reveal unique properties related to circular dichroism and optical activity in the twisted PCF. Through Stokes parameter evaluation, the polarization state of the input light in the twisted PCF is investigated in detail and the transition from linearly polarized that to circularly polarized light is demonstrated. Furthermore, the optical activity discontinuity is confirmed near the transmission dip.

A review on PLC-based two-mode multi/demultiplexer designed by wavefront matching method
E. Taguchi, T. Fujisawa, Y. Yamashita, S. Makino, N. Hanzawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, F. Yamamoto, K. Saitoh
IEICE Transactions on Electronics, E101-C, 7, 518, 526, Institute of Electronics, Information and Communications Engineers (IEICE), Jul. 2018, [Peer-reviewed], [Invited]
Scientific journal

Novel All-Optical Diode Based on Single-Port and Four-Port Photonic Crystal Cavities
Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
23rd Opto-Electronics and Communications Conference, OECC 2018, Jul. 2018, [Peer-reviewed]
International conference proceedings, © 2018 IEEE. All-optical diode using single-port and four-port photonic crystal cavities is proposed. The backward transmission less than -30 dB is achievable for any input power, while the forward transmission reaches 40 % without magneto-optical materials.

A Broadband Silicon Two-Mode Multiplexer Designed by Wavefront Matching Method
Y. Sawada, T. Sato, T. Fujisawa, K. Saitoh
23rd Opto-Electronics and Communications Conference, OECC 2018, E101C, 7, 518, 526, Jul. 2018, [Peer-reviewed]
International conference proceedings, © 2018 IEEE. A broadband Si-based two-mode (de)multiplexer is proposed. A wavefront matching method is used to optimize the waveguide geometry for broadband operation. Designed device has about 2.4 times bandwidth compared with a conventional device.

Si-based Mach-Zehnder wavelength/mode multi/demultiplexer for a WDM/MDM transmission system
S. Ohta, T. Fujisawa, S. Makino, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
Optics Express, 26, 12, 15211, 15220, OSA - The Optical Society, 11 Jun. 2018, [Peer-reviewed]
English, Scientific journal, We propose and experimentally demonstrate a low-loss and low-crosstalk Mach-Zehnder mode/wavelength multi/demultiplexer for WDM/MDM transmission based on a Si-photonics platform. A broadband 3-dB mode divider, which is also newly devised here, makes it possible to compose a Mach-Zehnder filter for “mode” and “wavelength” simultaneously. Transmission characteristics of fabricated 3-dB mode dividers are in excellent agreement with theoretical results. Mach-Zehnder filters using the 3-dB mode divider with a free spectral range (FSR) of 20 and 1 nm are also fabricated and the modal crosstalk is less than −24 dB in the 40-nm wavelength range for the MZ filter with an FSR of 20 nm. The tuning of the peak wavelength position by the TiN heater is also demonstrated.

Scrambling-Type Three-Mode PLC Multiplexer Based on Cascaded Y-Branch Waveguide with Integrated Mode Rotator
Takeshi Fujisawa, Yoko Yamashita, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima, Kunimasa Saitoh
Journal of Lightwave Technology, 36, 10, 1985, 1992, Institute of Electrical and Electronics Engineers Inc., 15 May 2018, [Peer-reviewed]
English, Scientific journal, A novel scrambling-type mode multiplexer (MUX) is proposed for future mode-division multiplexing with large number of modes. The proposed MUX does not aim at high modal extinction ratio as in a conventional-mode MUX, rather, all the modes are moderately mixed assuming that an MIMO-based receiver is used. The proposed configuration is superior to the conventional-mode MUX based on asymmetric directional couplers in terms of the fabrication tolerance of the waveguide width and scalability to a large number of modes. The wavelength dependence is also small since no coherent coupling devices are used, except for mode rotators. A three-mode multiplexer design based on a cascaded Y-branch waveguide and an integrated mode rotator on a silica planar lightwave circuit shows that low-loss and small wavelength dependence multiplexing is possible and a proof-of-concept device is fabricated.

Mach-Zehnder Mode/Wavelength (De)Multiplexer for WDM/MDM Optical Transmission
S. Ohta, T. Fujisawa, S. Makino, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
European Conference on Optical Communication, ECOC, 2017-, 1, 3, Institute of Electrical and Electronics Engineers Inc., 24 Apr. 2018, [Peer-reviewed]
English, International conference proceedings, We propose and experimentally demonstrate a low-loss and low-crosstalk Mach-Zehnder mode/wavelength multi/demultiplexer for WDM/MDM transmission. A broadband 3dB mode divider, which is also newly devised here, makes it possible to compose Mach-Zehnder filter for 'mode' and 'wavelength' simultaneously.

DMD-unmanaged long-haul SDM transmission over 2500-km 12-core × 3-mode MC-FMF and 6300-km 3-mode FMF employing intermodal interference cancelling technique
K. Shibahara, T. Mizuno, D. Lee, Y. Miyamoto, H. Ono, K. Nakajima, S. Saitoh, K. Takenaga, K. Saitoh
Optical Fiber Communication Conference (OFC2018), 37, 1, Th4C.6, 147, OSA, Mar. 2018, [Peer-reviewed]
English, International conference proceedings

Six-Mode Seven-Core Fiber for Repeated Dense Space-Division Multiplexing Transmission
Taiji Sakamoto, Kunimasa Saitoh, Shota Saitoh, Kohki Shibahara, Masaki Wada, Yoshiteru Abe, Azusa Urushibara, Katsuhiro Takenaga, Takayuki Mizuno, Takashi Matsui, Kazuhiko Aikawa, Yutaka Miyamoto, Kazuhide Nakajima
Journal of Lightwave Technology, 36, 5, 1226, 1232, Institute of Electrical and Electronics Engineers Inc., 01 Mar. 2018, [Peer-reviewed]
English, Scientific journal, We propose the optimum design of high spatial density few-mode multicore fiber (MCF) for repeated space-division multiplexed (SDM) transmission. Numerical analyses reveal that a hexagonally arranged six-mode seven-core structure is potentially better than the other possibility, the six-mode multicore structure, in terms of realizing high spatial density and low mode-dependent loss (MDL) characteristics at the splice point
its benefits will enable us to construct long-haul dense SDM transmission links. We fabricate a 171 μm cladding six-mode seven-core fiber and successfully demonstrate low loss, differential mode delay (DMD), crosstalk, and splice-induced MDL characteristics while maintaining the relative core multiplicity factor of more than 50. Finally, we demonstrate repeated optical 12 × 12 MIMO transmission of more than 100 km using our six-mode MCF and erbium doped fiber amplifier (EDFA).

Proposal of Si Four-Wavelength Multiplexer Using Higher-Order Mode for 100GbE
Junya Takano, Takanori Sato, Yusuke Sawada, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima, Kunimasa Saitoh
23RD OPTO-ELECTRONICS AND COMMUNICATIONS CONFERENCE (OECC2018), IEEE, 2018
English, International conference proceedings, We propose a Si four-wavelength multiplexer for 100GbE using (a) symmetric directional couplers and TE1-TM0 mode converter. The proposed device does not require 1600-GHz filter as in the conventional configuration, resulting in a large fabrication tolerance.

Design of a high-forward-transmission all-optical diode based on cascaded side-coupled photonic crystal cavities
Takanori Sato, Takeshi Fujisawa, Kunimasa Saitoh
Journal of the Optical Society of America B: Optical Physics, 34, 12, 2493, 2500, The Optical Society, 01 Dec. 2017, [Peer-reviewed]
Scientific journal, © 2017 Optical Society of America. A high-forward-transmission all-optical diode based on cascaded photonic crystal cavities is proposed. To obtain a high forward transmission and a large nonreciprocity, we generate a box-like spectrum by cascading two side-coupled photonic crystal cavities. We find that by appropriately adjusting the distance between two cavities, a high contrast of the transmission and a flat spectral lineshape with almost 100% transmission can be obtained. These characteristics are preferable for achieving a high forward transmission and a large nonreciprocity. Numerical results show that the designed all-optical diode performs perfect transmission with a large nonreciprocal transmission ratio of >50 dB.

A Mach-Zehnder mode multi/demultiplexer based on Si/silica hybrid PLC platform for WDM/MDM transmission
M. Kudo, S. Ohta, E. Taguchi, T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
30th Annual Conference of the IEEE Photonics Society, IPC 2017, 2017-, 709, 710, Institute of Electrical and Electronics Engineers Inc., 20 Nov. 2017, [Peer-reviewed]
English, International conference proceedings, A low-loss and low-crosstalk Mach-Zehnder mode/wavelength multi/demultiplexer based on silica-PLC platform is proposed for WDM/MDM transmission. The Mach-Zehnder filter for "mode" and "wavelength" can be constructed by newly designed broadband 3dB-modedivider. Furthermore, using Si/silica hybrid platform, footprint of the chip can be drastically reduced.

Coiling size dependence of group delay spread in coupled multicore fibers without intentional twisting
Kunimasa Saitoh, Takeshi Fujisawa, Takanori Sato
Journal of Lightwave Technology, 35, 20, 4559, 4566, 15 Oct. 2017, [Peer-reviewed]
Scientific journal, © 2017 IEEE. Coiling size dependence of group delay spread (GDS) in coupled multicore fibers (MCFs) without intentional twisting is investigated by using a coupled-wave theory. It is shown that the GDS does not simply decrease with increasing bending radius and the optimum coiling size to decrease GDS depends on a coreto- core distance of MCFs. It is also found that a coupled MCF with a trench-assisted profile is suitable for realizing lower GDS with higher spatial density compared with a coupled MCF with a step-index profile.

Crosstalk Analysis of Heterogeneous Multicore Fibers Using Coupled-Mode Theory
Takeshi Fujisawa, Yoshimichi Amma, Yusuke Sasaki, Shoichiro Matsuo, Kazuhiko Aikawa, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 9, 5, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2017, [Peer-reviewed]
English, Scientific journal, Intercore crosstalk of heterogeneous multicore fiber is investigated based on coupled-mode theory. Random twisting model is used for estimating the crosstalk. The crosstalk of two kinds of fibers: triangular lattice 30-core fiber with four kinds of cores and square lattice 32-core fiber with two kinds of cores is investigated both theoretically and experimentally. Unlike previous study, measured crosstalk for all the combinations of cores for both fibers is in good agreement with calculated values with single correlation length, showing the validity of the theoretical model used here.

Full-vector finite-element beam propagation method for helicoidal waveguides and its application to twisted photonic crystal fibers
Takeshi Fujisawa, Takanori Sato, Kunimasa Saitoh
Journal of Lightwave Technology, 35, 14, 2894, 2901, 15 Jul. 2017, [Peer-reviewed]
Scientific journal, © 1983-2012 IEEE. A full-vector beam propagation method based on a finite-element scheme for a helicoidal system is developed. The permittivity and permeability tensors of a straight waveguide are replaced with equivalent ones for a helicoidal system, obtained by transformation optics. A cylindrical, perfectly matched layer is implemented for the absorbing boundary condition. To treat wide-angle beam propagation, a second-order differentiation term with respect to the propagation direction is directly discretized without using a conventional Padé approximation. The transmission spectra of twisted photonic crystal fibers are thoroughly investigated, and it is found that the diameters of the air holes greatly affect the spectra. The calculated results are in good agreement with the recently reported measured results, showing the validity and usefulness of the method developed here.

Wavefront Matching Method Based on Full-Vector Finite-Element Beam Propagation Method for Polarization Control Devices
Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
JOURNAL OF LIGHTWAVE TECHNOLOGY, 35, 14, 2840, 2845, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2017, [Peer-reviewed]
English, Scientific journal, We developed a wavefront matching (WFM) method based on the full-vector finite-element beam propagation method (BPM). The WFM method is one of the optimization algorithms based on BPM, and it can deterministically design optimum wave-guide structures from desired characteristics. The developed WFM method can be applied to optical waveguides with high refractive-index contrast and polarization control devices, which require full-vector analysis. In this paper, the principle and design procedure of the full-vector WFM method are described. Its validity and effectiveness are demonstrated by designing polarization control devices, such as a polarization converter based on a vertical asymmetric taper waveguide and a polarization splitter-rotator based on taper and Y-branch waveguides.

PLC-Based LP11 Mode Rotator With Curved Trench Structure Devised From Wavefront Matching Method
Yoko Yamashita, Shuntaro Makino, Takeshi Fujisawa, Nobutomo Hanzawa, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Fumihiko Yamamoto, Kazuhide Nakajima, Kunimasa Saitoh
IEEE PHOTONICS TECHNOLOGY LETTERS, 29, 13, 1063, 1066, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2017, [Peer-reviewed]
English, Scientific journal, A compact and low-loss PLC-based mode rotator is proposed. The mode rotator with the curved trench structure is designed based on the wavefront matching method, which is an optimization algorithm based on the beam propagation method. The proposed mode rotator is 45% in size (1 mm) and has lower loss (1/5) compared with the conventional structure that has a straight trench. Furthermore, the proposed mode rotator can achieve low-mode crosstalk.

Design and Fabrication of Broadband PLC-Based Two-Mode Multi/Demultiplexer Using a Wavefront Matching Method
Yoko Yamashita, Takeshi Fujisawa, Shuntaro Makino, Nobutomo Hanzasa, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Fumihiko Yamamoto, Kazuhide Nakajima, Kunimasa Saitoh
JOURNAL OF LIGHTWAVE TECHNOLOGY, 35, 11, 2252, 2258, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 2017, [Peer-reviewed]
English, Scientific journal, A broadband and low-loss two-mode (LP01/LP11a) multi/demultiplexer (MUX/DEMUX) based on a planar lightwave circuit (PLC) is designed by a wavefront matching method, which is an optimization algorithm of PLC based on the beam propagation method. The calculated transmission loss of the optimized structure is lower than -0.3 dB in the wavelength range from 1400 to 1700 nm. Numerical results show that the proposed structure has large fabrication tolerance compared to the conventional structure. The designed structure is fabricated and broadband characteristics are well-reproduced, showing the usefulness of the proposed MUX/DEMUX.

Group delay spread analysis of coupled-multicore fibers: A comparison between weak and tight bending conditions
Takeshi Fujisawa, Kunimasa Saitoh
OPTICS COMMUNICATIONS, 393, 232, 237, ELSEVIER SCIENCE BV, Jun. 2017, [Peer-reviewed]
English, Scientific journal, Group delay spread of coupled three-core fiber is investigated based on coupled-wave theory. The differences between supermode and discrete core mode models are thoroughly investigated to reveal applicability of both models for specific fiber bending condition. A macrobending with random twisting is taken into account for random modal mixing in the fiber. It is found that for weakly bent condition, both supermode and discrete core mode models are applicable. On the other hand, for strongly bent condition, the discrete core mode model should be used to account for increased differential modal group delay for the fiber without twisting and short correlation length, which were experimentally observed recently. Results presented in this paper indicate the discrete core mode model is superior to the supermode model for the analysis of coupled-multicore fibers for various bent condition. Also, for estimating GDS of coupled-multicore fiber, it is critically important to take into account the fiber bending condition.

Low-loss, compact, and fabrication-tolerant Si-wire 90° waveguide bend using clothoid and normal curves for large scale photonic integrated circuits
Takeshi Fujisawa, Shuntaro Makino, Takanori Sato, Kunimasa Saitoh
Optics Express, 25, 8, 9150, 9159, Apr. 2017, [Peer-reviewed]
Scientific journal, © 2017 Optical Society of America. Ultimately low-loss 90° waveguide bend composed of clothoid and normal curves is proposed for dense optical interconnect photonic integrated circuits. By using clothoid curves at the input and output of 90° waveguide bend, straight and bent waveguides are smoothly connected without increasing the footprint. We found that there is an optimum ratio of clothoid curves in the bend and the bending loss can be significantly reduced compared with normal bend. 90% reduction of the bending loss for the bending radius of 4 μm is experimentally demonstrated with excellent agreement between theory and experiment. The performance is compared with the waveguide bend with offset, and the proposed bend is superior to the waveguide bend with offset in terms of fabrication tolerance.

Optimal design of 4LP-mode multicore fibers for high spatial multiplicity
Yuki Tobita, Takeshi Fujisawa, Taiji Sakamoto, Takashi Matsui, Shota Saitoh, Katsuhiro Takenaga, Kazuhiko Aikawa, Shinichi Aozasa, Kazuhide Nakajima, Kunimasa Saitoh
OPTICS EXPRESS, 25, 5, 5697, 5709, OPTICAL SOC AMER, Mar. 2017, [Peer-reviewed]
English, Scientific journal, High spatial multiplicity fiber designs are presented for homogeneous and heterogeneous 4LP-mode multicore fibers (MCFs) that support six spatial modes per core. The high-spatial-density 4LP-mode MCF design methodology is explained in detail. The influence of the number of cores on the cladding diameter (D-cl) and relative core multiplicity factor (RCMF) is investigated. The optimal core designs and MCF layouts with square and triangular lattices maintain glass fiber reliability (maximum D-cl = 250 mu m). For homogeneous 4LP-mode MCFs, a 19-core triangular-lattice fiber gives the highest RCMF of 61.7. For heterogeneous 4LP-mode MCFs, an RCMF of 65.4 is obtained for a 21-core square-lattice fiber. (C) 2017 Optical Society of America

Design and optimization of 32-core rod/trench assisted square-lattice structured single-mode multi-core fiber
Xueqin Xie, Jiajing Tu, Xian Zhou, Keping Long, Kunimasa Saitoh
OPTICS EXPRESS, 25, 5, 5119, 5132, OPTICAL SOC AMER, Mar. 2017, [Peer-reviewed]
English, Scientific journal, We propose and design a kind of heterogeneous rod-assisted and trench-assisted multi-core fiber (Hetero-RA-TA-MCF) with 32 cores arranged in square-lattice structure (SLS), and then we introduce the design method for Hetero-RA-TA-MCF. Simulation results show that the Hetero-RA-TA-32-Core-Fiber achieves average effective area (A(eff)) of about 74 mu m(2), low crosstalk (XT) of about -31 dB/100km, threshold value of bending radius (R-pk) of 7.0 cm, relative core multiplicity factor (RCMF) of 8.74, and cable cutoff wavelength (lambda(cc)) of less than 1.53 mu m. (C) 2017 Optical Society of America

Few-mode multicore fibers for long-haul transmission line
Yusuke Sasaki, Katsuhiro Takenaga, Shoichiro Matsuo, Kazuhiko Aikawa, Kunimasa Saitoh
OPTICAL FIBER TECHNOLOGY, 35, 19, 27, ELSEVIER SCIENCE INC, Feb. 2017, [Peer-reviewed]
English, Scientific journal, Few-mode multicore fibers (FM-MCFs) that enable dense space-division multiplexing (DSDM) have the potential to drastically improve the fiber capacity. In designing the FM-MCFs, several issues that originate from multicore fibers and few-mode fibers must be considered. In this paper, these design issues such as inter-core crosstalk (IC-XT) and dispersion mode delay (DMD) are discussed. A three-mode 12-core fiber with low-DMD low-IC-XT achieves long-haul DSDM transmission over 500 km. The design concept, fiber design, and characteristics of the fabricated three-mode 12-core fiber are also described. (C) 2016 The Authors. Published by Elsevier Inc.

A Metal-Assisted Silicon Slot Waveguide for Highly Sensitive Gas Detection
Yuhei Ishizaka, Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
IEEE PHOTONICS JOURNAL, 9, 1, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2017, [Peer-reviewed]
English, Scientific journal, A metal-assisted silicon slot waveguide for highly sensitive gas detection is proposed. To show the superiority of the proposed structure, the optical confinement factor in the gas region and the sensitivity are theoretically investigated using the 2-D vector finite element method. Numerical results show that the proposed structure can achieve a strong light confinement in the gas region compared with the conventional slot waveguide. The maximal optical confinement factor of 85% is obtained in the proposed structure, which is approximately 2.5 times larger than the conventional slot waveguide. Additionally, the results show that the proposed structure has a large sensitivity compared with the conventional slot waveguide. Finally, a ring resonator based on the proposed structure is analyzed to evaluate the Q factor and to increase the validity of results obtained by 2-D simulation.

Low-Loss and Low-DMD 6-Mode 19-Core Fiber With Cladding Diameter of Less Than 250 mu m
Taiji Sakamoto, Takashi Matsui, Kunimasa Saitoh, Shota Saitoh, Katsuhiro Takenaga, Takayuki Mizuno, Yoshiteru Abe, Kohki Shibahara, Yuki Tobita, Shoichiro Matsuo, Kazuhiko Aikawa, Shinichi Aozasa, Kazuhide Nakajima, Yutaka Miyamoto
JOURNAL OF LIGHTWAVE TECHNOLOGY, 35, 3, 443, 449, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2017, [Peer-reviewed]
English, Scientific journal, We investigate a few-mode multicore fiber structure suitable for realizing a high spatial density while maintaining a feasible cladding diameter. First, we discuss the impact of cladding diameter on mechanical reliability, and set a target cladding diameter of less than 250 mu m to maintain a comparable reliability to conventional fiber. We then optimize the 6-mode core profile to obtain a low differential mode delay (DMD) characteristic and reveal that a hexagonally arranged 19-core structure can realize more than 100 spatial channels with a cladding diameter of less than 250 mu m. Finally, we fabricate 6-mode 19-core fiber with a 246 mu m cladding diameter and realize a relative core multiplicity factor of more than 60 with a low loss and a DMD of less than 0.24 dB/km and 0.33 ns/km, respectively.

Long-Haul Dense Space-Division Multiplexed Transmission Over Low-Crosstalk Heterogeneous 32-Core Transmission Line Using a Partial Recirculating Loop System
Takayuki Mizuno, Kohki Shibahara, Feihong Ye, Yusuke Sasaki, Yoshimichi Amma, Katsuhiro Takenaga, Yongmin Jung, Klaus Pulverer, Hirotaka Ono, Yoshiteru Abe, Makoto Yamada, Kunimasa Saitoh, Shoichiro Matsuo, Kazuhiko Aikawa, Marc Bohn, David J. Richardson, Yutaka Miyamoto, Toshio Morioka
JOURNAL OF LIGHTWAVE TECHNOLOGY, 35, 3, 488, 498, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2017, [Peer-reviewed]
English, Scientific journal, In this paper, we present long-haul 32-core dense space-division-multiplexed (DSDM) unidirectional transmission over a single-mode multicore transmission line. We developed a low-crosstalk heterogeneous 32-core fiber with a square lattice arrangement, and a novel partial recirculating loop system. The span crosstalk of the 51.4-km 32-core transmission line was less than -34.5 dB. This allowed the transmission of polarization-division- multiplexed 16 quadrature amplitude modulation (PDM-16QAM) signals through all 32 cores over a long distance exceeding 1000 km. We demonstrate 32-core DSDM 20 wavelength-division-multiplexed PDM-16QAM transmission over 1644.8 km with a high aggregate spectral efficiency of 201.46 b/s/Hz Additionally, we examine the effect of crosstalk on the transmission performance of each core, and show that the Q-penalty has strong correlation with intercore crosstalk.

PLC-based mode multi/demultiplexers for mode division multiplexing
Kunimasa Saitoh, Nobutomo Hanzawa, Taiji Sakamoto, Takeshi Fujisawa, Yoko Yamashita, Takashi Matsui, Kyozo Tsujikawa, Kazuhide Nakajima
OPTICAL FIBER TECHNOLOGY, 35, 80, 92, ELSEVIER SCIENCE INC, Feb. 2017, [Peer-reviewed]
English, Scientific journal, Recently developed PLC-based mode multi/demultiplexers (MUX/DEMUXs) for mode division multiplexing (MDM) transmission are reviewed. We firstly show the operation principle and basic characteristics of PLC-based MUX/DEMUXs with an asymmetric directional coupler (ADC). We then demonstrate the 3-mode (2LP-mode) multiplexing of the LP01, LP11a, and LP11b modes by using fabricated PLC-based mode MUX/DEMUX on one chip. In order to excite LP11b mode in the same plane, a PLC-based LP11 mode rotator is introduced. Finally, we show the PLC-based 6-mode (4LP-mode) MUX/DEMUX with a uniform height by using ADCs, LP11 mode rotators, and tapered waveguides. It is shown that the LP21a mode can be excited from the LP11b mode by using ADC, and the two nearly degenerated LP21b and LP02 modes can be (de) multiplexed separately by using tapered mode converter from E-13 (E-31) mode to LP21b (LP02) mode. (C) 2016 Elsevier Inc. All rights reserved.

Ultrasmall silicon mode converters designed by wavefront matching method developed for waveguide discontinuity problem
Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
2016 IEEE Photonics Conference, IPC 2016, 439, 440, Institute of Electrical and Electronics Engineers Inc., 23 Jan. 2017, [Peer-reviewed]
English, International conference proceedings, The wavefront matching method for the waveguide discontinuity problem is newly developed in this study. The developed method can be applied to optimize the waveguides including reflection. The application for silicon mode converters are demonstrated and ultrasmall and low-loss structure is obtained.

Design and analysis of weakly- and strongly-coupled multicore fibers
Kunimasa Saitoh, Takeshi Fujisawa, Takanori Sato
Optics InfoBase Conference Papers, Part F53-Networks 2017, 2017, [Peer-reviewed]
International conference proceedings, © OSA 2017. Design and analysis of MCFs are described. For weakly-coupled MCFs, coupled-power theory is used to estimate crosstalk and SDM density, whereas, for strongly-coupled MCFs, coupled-wave theory is used to evaluate GDS originating from modal dispersion.

A photonic-plasmonic mode converter using mode-coupling-based polarization rotation for metal-inserted silicon platform
Yuhei Ishizaka, Masaru Nagai, Takeshi Fujisawa, Kunimasa Saitoh
IEICE ELECTRONICS EXPRESS, 14, 2, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Jan. 2017, [Peer-reviewed]
English, Scientific journal, We present a photonic-plasmonic mode converter using modecoupling- based polarization rotation for connecting a silicon wire and hybrid plasmonic waveguides on a metal-inserted silicon platform. By using the polarization rotator based on defect-introduced silicon wire waveguide, the proposed mode converter can convert the photonic TE mode into the hybrid plasmonic TM mode. At the beginning, to identify a cross-sectional structure for polarization conversion, we investigate the hybrid parameters that determine the degree of mode rotation using the two-dimensional vector finite element method. Next, using the three-dimensional vector finite element method, we investigate the conversion characteristics of the whole device including input and output ports. Numerical results show that the extinction ratio of 46 dB and the insertion loss of 0.36 dB are achieved. The tolerance of the defect is also numerically evaluated. Our proposed mode converter can reduce insertion losses compared to conventional mode converters based on a taper-introduced butt joint structure.

Design and optimization of 3-mode x 12-core dual-ring structured few-mode multi-core fiber
Jiajing Tu, Keping Long, Kunimasa Saitoh
OPTICS COMMUNICATIONS, 381, 30, 36, ELSEVIER SCIENCE BV, Dec. 2016, [Peer-reviewed]
English, Scientific journal, We adopt dual-ring structure (DRS) for the core arrangement of 3-mode (LP01, LP11a and LP11b) x 12-core few-mode multi-core fiber (FM-MCF) and then introduce the design method for this DRS-FM-MCF. After investigating the characteristics such as differential mode delay (DMD), inter-core crosstalk (XT), threshold value of bending radius (Roc), relative core multiplicity factor (RCMF) and cable cutoff wavelength (lambda(cc)), we present an optimized scheme for this DRS-FM-MCF. For the optimized DRS-FM-MCF, IDMDI is <= 100 ps/km over C+L band, the maximum XT at wavelength (lambda) of 1625 nm achieves -33 dB/ 100 km, maximum R-pk is 11.03 cm, RCMF (LP01, LP11a and LP11b) reaches 25.49 and maximum lambda(cc), is <= 1530 nm. Compared with one-ring structure (ORS), DRS has much more space to enlarge core pitch (Lambda) so that lower XT can be achieved. Furthermore, DRS has less confinement degree on mode than square lattice structure (SLS) if A and cladding diameter (D-cl) are set at similar values. It means that it is easier for DRS to make sure lambda(cc) would not be larger than the lower limit of bands. In this paper, DRS is proved as a suitable core arrangement for 3-mode x 12-core FM-MCF. (C) 2016 Elsevier B.V. All rights reserved.

Design of Homogeneous Trench-Assisted Multi-Core Fibers Based on Analytical Model
Feihong Ye, Jiajing Tu, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo, Hidehiko Takara, Toshio Morioka
JOURNAL OF LIGHTWAVE TECHNOLOGY, 34, 18, 4406, 4416, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2016, [Peer-reviewed]
English, Scientific journal, We present a design method of homogeneous trench-assisted multicore fibers (TA-MCFs) based on an analytical model utilizing an analytical expression for the mode coupling coefficient between two adjacent cores. The analytical model can also be used for crosstalk (XT) properties analysis, such as XT reduction amount versus trench width, trench depth, and other fiber structural parameters as compared with normal step-index MCFs. Furthermore, the model can be used to search for core positions for further XT reduction in nonclose-packed structures. For instance, we show that a dual-ring structure is the quasi-optimum core layout starting from an one-ring structured 12-core fiber. Based on the analytical model, a square-lattice structured 24-core fiber and a 32-core fiber are designed both for propagation-direction interleaving (PDI) and non-PDI transmission schemes. The proposed model provides a powerful tool for designing high-count homogeneous TA-MCFs.

32-core Dense SDM unidirectional transmission of PDM-16QAM signals over 1600 km using crosstalk-managed single-mode heterogeneous multicore transmission line
T. Mizuno, K. Shibahara, H. Ono, Y. Abe, Y. Miyamoto, F. Ye, T. Morioka, Y. Sasaki, Y. Amma, K. Takenaga, S. Matsuo, K. Aikawa, K. Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, M. Yamada
2016 Optical Fiber Communications Conference and Exhibition, OFC 2016, 09 Aug. 2016
International conference proceedings, We demonstrate 32-core dense space-division multiplexed (DSDM) unidirectional transmission of PDM-16QAM 20-WDM signals over 1644.8 km employing a low-crosstalk single-mode heterogeneous 32-core fiber in a partial recirculating-loop system.

Investigation on core selection approach for heterogeneous trench-assisted multi-core fiber
Jiajing Tu, Keping Long, Kunimasa Saitoh
Optics InfoBase Conference Papers, OSA - The Optical Society, 11 Jul. 2016
English, International conference proceedings, We introduce an efficient core selection approach for heterogeneous trench-assisted multi-core fiber. We firstly decide the cores at outer layer and secondly obtain a core selection region for central core with the outer cores fixed.

Theoretical Investigation of Six-Mode Multi/Demultiplexer Based on Fused-Type Multicore Fiber Coupler
Shoko Nishimoto, Takeshi Fujisawa, Yusuke Sasaki, Hitoshi Uemura, Kunimasa Saitoh
IEEE PHOTONICS JOURNAL, 8, 3, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 2016, [Peer-reviewed]
English, Scientific journal, A six-mode (LP01/LP11ab/LP21ab/LP02) multi/demultiplexer based on a fusedtype multicore fiber coupler is proposed, and the coupling characteristics are investigated. The multi/demultiplex operation is achieved by a phase matching between the desired mode of center core and the LP01 mode of the outer core in the elongated region of the fused-type multicore fiber coupler. From the numerical results based on the beam propagation method, it is found that the coupling efficiencies between the LP01 and LP11/LP21/LP02 modes are more than -0.1 dB, and the crosstalk is lower than -20 dB at the wavelength of 1550 nm.

Group delay spread analysis of strongly coupled 3-core fibers: an effect of bending and twisting
Takeshi Fujisawa, Kunimasa Saitoh
OPTICS EXPRESS, 24, 9, 9583, 9591, OPTICAL SOC AMER, May 2016, [Peer-reviewed]
English, Scientific journal, The effect of bending and twisting on the group delay spread (GDS) of strongly coupled 3-core fibers is investigated. For the random perturbation inducing modal coupling in the fiber, two physical mechanisms, microbending or macrobending with random twist, are considered. Calculated results show that both mechanisms lead to the same effect, namely, reduced GDS under strong coupling regime. Furthermore, a novel fiber structure having an air-hole at the center is proposed for reducing the GDS. By placing the air-hole, the effective index difference between fundamental and the higher order modes is reduced, leading to strong modal mixing in the fiber, and hence, small GDS. Calculated GDS of the fiber with air-hole is almost 1/5 compared with that of the fiber without air-hole. (C) 2016 Optical Society of America

An Efficient Core Selection Method for Heterogeneous Trench-Assisted Multi-Core Fiber
Jiajing Tu, Keping Long, Kunimasa Saitoh
IEEE PHOTONICS TECHNOLOGY LETTERS, 28, 7, 810, 813, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2016, [Peer-reviewed]
English, Scientific journal, We introduce an efficient core selection method for heterogeneous trench-assisted multi-core fiber. In order to make the design more flexible, the relative position and the width of one trench are usually assumed to have a proportional relation to core radius. However, it is hard to choose non-identical cores simultaneously, since cores are selected under the requirement of cable cutoff wavelength of higher order mode that is influenced by the surrounding trenches, whose relative position and width are also decided by core radii. Therefore, in this method, we first decide the cores at the outer layer and second obtain a core selection region for cores at the inner layer with the outer cores fixed. Both outer and inner cores are designed to achieve the target effective area and crosstalk.

High-Spatial-Multiplicity Multicore Fibers for Future Dense Space-Division-Multiplexing Systems
Shoichiro Matsuo, Katsuhiro Takenaga, Yusuke Sasaki, Yoshimichi Amma, Shota Saito, Kunimasa Saitoh, Takashi Matsui, Kazuhide Nakajima, Takayuki Mizuno, Hidehiko Takara, Yutaka Miyamoto, Toshio Morioka
JOURNAL OF LIGHTWAVE TECHNOLOGY, 34, 6, 1464, 1475, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2016, [Peer-reviewed]
English, Scientific journal, Multicore fibers and few-mode fibers have potential application in realizing dense-space-division multiplexing systems. However, there are some tradeoff requirements for designing the fibers. In this paper, the tradeoff requirements such as spatial channel count, crosstalk, differential mode delay, and cladding diameter are discussed. Further, the design concept and transmission characteristics of high-core-count single-mode multicore fibers are discussed. A heterogeneous multicore fiber with 30 cores and quasi-single-mode multi-core fibers with 31 cores are developed.

A crosstalk analysis of heterogeneous 30-core fibre　　　　　　　　　　　　　　　
T. Fujisawa, Y. Amma, S. Matsuo, K. Aikawa, K. Saitoh, M. Koshiba
European Conference on Optical Communication, ECOC, 1037, 1039, Institute of Electrical and Electronics Engineers Inc., 2016
English, International conference proceedings, Intercore crosstalk of heterogeneously arranged 30-core fibre having four kinds of cores is investigated. Measured crosstalk for all the combinations of cores are in good agreement with calculated values by coupled-mode theory with single correlation length, assuming longitudinal random twist.

Space-Time Coding-Assisted Transmission for Mitigation of MDL Impact on Mode-Division Multiplexed Signals
K. Shibahara, T. Mizuno, H. Takara, H. Kawakami, D. Lee, Y. Miyamoto, S. Matsuo, K. Saitoh, M. Yamada
2016 OPTICAL FIBER COMMUNICATIONS CONFERENCE AND EXHIBITION (OFC), IEEE, 2016, [Peer-reviewed]
English, International conference proceedings, A method is described for applying space-time coding implemented by Hadamard transform to SDM transmission. Experiments demonstrated that the method substantially improves mode dependent loss tolerance and enables transmission reach to be enhanced by 20%.

Multicore Fiber Technology
Kunimasa Saitoh, Shoichiro Matsuo
JOURNAL OF LIGHTWAVE TECHNOLOGY, 34, 1, 55, 66, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jan. 2016, [Peer-reviewed]
English, Scientific journal, Multicore fibers (MCFs) are expected as a good candidate for overcoming the capacity limit of a current optical communication system. This paper describes the recent progress on the MCFs for space-division multiplexing to be utilized in future large capacity long-distance transmission systems. Tradeoff issue between low crosstalk and high core density in MCFs is presented and prospect of large-space multiplicity of MCFs is discussed.

Design of a reflection-suppressed all-optical diode based on asymmetric L-shaped nonlinear photonic crystal cavity
Takanori Sato, Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
Journal of the Optical Society of America B: Optical Physics, 33, 1, 54, 61, 01 Jan. 2016, [Peer-reviewed]
Scientific journal, © 2015 Optical Society of America. A simple design method for suppressing the reflection of the all-optical diode based on the L-shaped photonic crystal (PC) cavity is proposed. Analyzing the linear resonant characteristics of the PC cavity and using the nonlinear coupled-mode theory, the strategy for obtaining reflection-suppressed structure is illustrated. Based on the design rule, a "reflection-suppressed all-optical diode" is presented, and numerical solutions indicate that the designed structure has a small reflection as well as an extremely large nonreciprocal transmission ratio.

Dense SDM (12-Core x 3-Mode) Transmission Over 527 km With 33.2-ns Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization
Kohki Shibahara, Doohwan Lee, Takayuki Kobayashi, Takayuki Mizuno, Hidehiko Takara, Akihide Sano, Hiroto Kawakami, Yutaka Miyamoto, Hirotaka Ono, Manabu Oguma, Yoshiteru Abe, Takashi Matsui, Ryohei Fukumoto, Yoshimichi Amma, Tsukasa Hosokawa, Shoichiro Matsuo, Kunimasa Saitoh, Makoto Yamada, Toshio Morioka
JOURNAL OF LIGHTWAVE TECHNOLOGY, 34, 1, 196, 204, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jan. 2016, [Peer-reviewed]
English, Scientific journal, We propose long-haul space-division-multiplexing (SDM) transmission systems employing parallel multiple-input multiple-output (MIMO) frequency-domain equalization (FDE) and transmission fiberwith lowdifferentialmode delay (DMD). We first discuss the advantages of parallel MIMO FDE technique in long-haul SDMtransmission systems in terms of the computational complexity, and then, compare the complexity required for parallel MIMO FDE as well as the conventional time-domain equalization techniques. Proposed parallel MIMO FDE that employs low baud rate multicarrier signal transmission with a receiver-side FDE enables us to compensate for 33.2-ns DMD with considerably low-computational complexity. Next, we describe in detail the newly developed fiber and devices we used in the conducted experiments. A graded-index (GI) multicore few-mode fiber (MC-FMF) suppressed the accumulation of DMD as well as intercore crosstalk. Mode dependent loss/gain effect was also mitigated by employing both a ring-core FM erbium-doped fiber amplifier and a free-space optics type gain equalizer. By combining these advanced techniques together, we finally demonstrate 12-core x 3 mode dense SDM transmission over 527-km GI MC-FMF without optical DMD management.

Wavelength-Dependence of Inter-Core Crosstalk in Homogeneous Multi-Core Fibers
Feihong Ye, Jiajing Tu, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo, Hidehiko Takara, Toshio Morioka
IEEE PHOTONICS TECHNOLOGY LETTERS, 28, 1, 27, 30, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jan. 2016, [Peer-reviewed]
English, Scientific journal, The wavelength dependence of inter-core crosstalk in homogeneous multi-core fibers (MCFs) is investigated, and the corresponding analytical expressions are derived. The derived analytical expressions can be used to determine the crosstalk at any wavelength necessary for designing future MCF wavelength-division multiplexing (MCF-WDM) networks and transmission systems.

Graded-index few-mode multi-core fiber with dual-ring structure
Jiajing Tu, Keping Long, Kunimasa Saitoh
Asia Communications and Photonics Conference, ACPC 2015, Optical Society of America (OSA), 11 Dec. 2015
English, International conference proceedings, We proposed a two-mode supporting multi-core fiber with dual-ring structure. The maximum XT11-11 achieves -33 dB/100km, maximum Rpk is 11.03 cm, and RCMF reaches 15.28. © 2015 OSA.

Enhancement of Optical Nonlinearity in Coupled Resonator Optical Waveguide Based on Slotted 1-D Photonic Crystal Cavity
Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
IEEE PHOTONICS JOURNAL, 7, 6, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 2015, [Peer-reviewed]
English, Scientific journal, A coupled resonator optical waveguide (CROW) based on a slotted 1-D photonic crystal (1-D PC) cavity is proposed, and its enhancement of the optical nonlinearity due to the slow-light effect of the CROW and the small effective area of the slotted 1-D PC cavities is demonstrated. Setting the appropriate slot configuration in terms of the tradeoff between the nonlinear parameter and the leakage loss, a large nonlinear parameter exceeding 10(4) m(-1)W(-1) can be obtained.

Measurement of inter-core crosstalk in few-mode multicore fibre
S. Saitoh, Y. Sasaki, K. Takenaga, M. Ohashi, K. Saitoh, K. Nakajima, S. Matsuo
European Conference on Optical Communication, ECOC, 2015-, Institute of Electrical and Electronics Engineers Inc., 30 Nov. 2015, [Peer-reviewed]
English, International conference proceedings, The inter-core crosstalk of a fabricated few-mode multicore fibre is theoretically and experimentally investigated. By changing the direction of the LP11 mode, the crosstalk between the LP11 mode and the LP01 mode decreased by more than 10 dB.

Quantum-Confined Stark Effect Analysis of GeSn/SiGeSn Quantum Wells for Mid-Infrared Si-Based Electroabsorption Devices Based on Many-Body Theory
Takeshi Fujisawa, Kunimasa Saitoh
IEEE JOURNAL OF QUANTUM ELECTRONICS, 51, 11, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2015, [Peer-reviewed]
English, Scientific journal, Quantum-confined Stark effect (QCSE) of group IV Ge(Sn)/SiGe(Sn) quantum wells (QWs) on Si substrate is analyzed by microscopic many-body theory for mid-infrared (mid-IR) Si-based electroabsorption devices. To show the validity of the theory, QCSE of Ge/SiGe QW is investigated and very good agreement between theory and reported measured results is obtained. Next, the QCSE of GeSn/SiGeSn QWs is analyzed and the QW design for electroabsorption modulators to obtain large extinction ratio in mid-IR region is presented. It is shown that compressive and tensile strained well and barrier layers is preferable to obtain large extinction ratio due to its large conduction band offset.

Three-dimensional finite-element time-domain beam propagation method and its application to 1-D photonic crystal-coupled resonator optical waveguide
Shuntaro Makino, Takanori Sato, Yuhei Ishizaka, Takeshi Fujisawa, Kunimasa Saitoh
Journal of Lightwave Technology, 33, 18, 3836, 3842, 15 Sep. 2015, [Peer-reviewed]
Scientific journal, © 1983-2012 IEEE. A 3-D finite-element time-domain beam propagation method is described for the first time, to the best of our knowledge, for the analysis of transmission characteristics of any kind of photonic structures. In order to avoid nonphysical reflections from computational window edges, the perfectly matched layer boundary condition is introduced. Furthermore, the present method can treat wideband optical pulses owing to the use of Padé approximation. The validity of this method is verified by numerical simulations of a photonic crystal cavity and a microring resonator. In addition, in order to demonstrate the usefulness of the developed method, a taper waveguide for a highly efficient connection between a straight waveguide and a 1-D photonic crystal-coupled resonator optical waveguide is designed.

Heterogeneous trench-assisted few-mode multicore fiber with graded-index profile and square-lattice layout for low differential mode delay
Jiajing Tu, Kunimasa Saitoh, Yoshimichi Amma, Katsuhiro Takenaga, Shoichiro Matsuo
OPTICS EXPRESS, 23, 14, 17783, 17792, OPTICAL SOC AMER, Jul. 2015, [Peer-reviewed]
English, Scientific journal, We propose a kind of heterogeneous trench-assisted graded-index few-mode multi-core fiber with square-lattice layout. For each core in the fiber, effective area (A(eff)) of LP01 mode and LP11 mode can achieve about 110 mu m(2) and 220 mu m(2). Absolute value of differential mode delay (vertical bar DMD vertical bar) is smaller than 100 ps/km over C + L bands, which can decrease the complexity of digital signal processing at the receiver end. Considering the upper limit of cladding diameter (D-cl) and cable cutoff wavelength of LP21 mode in the cores located at the inner layer, we set core pitch (Lambda) as 43 mu m. In this case, D-cl is about 220.4 mu m, inter-core crosstalk (XT) is lower than -40 dB/500km and the relative core multiplicity factor (RCMF) reaches 15.93. (C) 2015 Optical Society of America

High-density multicore fiber with heterogeneous core arrangement
Y. Amma, Y. Sasaki, K. Takenaga, S. Matsuo, J. Tu, K. Saitoh, M. Koshiba, T. Morioka, Y. Miyamoto
Conference on Optical Fiber Communication, Technical Digest Series, 2015-, Institute of Electrical and Electronics Engineers Inc., 10 Jun. 2015
English, International conference proceedings, A 30-core fiber with heterogeneous cores that achieved large spatial multiplicity and low crosstalk of less than -40 dB at 100 km was demonstrated. The correlation lengths were estimated to be more than 1 m.

A rigorous definition of nonlinear parameter γ and effective area Aeff for photonic crystal optical waveguides
Takanori Sato, Shuntaro Makino, Yuhei Ishizaka, Takeshi Fujisawa, Kunimasa Saitoh
Journal of the Optical Society of America B: Optical Physics, 32, 6, 1245, 1251, Jun. 2015, [Peer-reviewed]
Scientific journal, © 2015 Optical Society of America. A rigorous definition of nonlinear parameter ? for high-index-contrast (HIC) periodic optical waveguides, such as photonic crystal (PC) optical waveguides, is proposed. The definition is fully vectorial, and approximations assumed in previous works are unnecessary. The ? values calculated by the proposed definition are compared with those obtained by previous definitions, the rigorous nonlinear mode solver, and measured values for weakly guiding optical fibers, HIC Si-nanowire waveguides, and PC waveguides. It is demonstrated that the ? values calculated by proposed definition agree well with those of the rigorous nonlinear mode solver and experiments for all three waveguides, showing the validity of the definition.

Material Gain Analysis of GeSn/SiGeSn Quantum Wells for Mid-Infrared Si-Based Light Sources Based on Many-Body Theory
Takeshi Fujisawa, Kunimasa Saitoh
IEEE JOURNAL OF QUANTUM ELECTRONICS, 51, 5, 1, 8, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2015, [Peer-reviewed]
English, Scientific journal, Material gain of GeSn/SiGeSn quantum wells, which can be grown on Si substrate using a buffer layer, is analyzed based on microscopic many-body theory (MBT) for mid-infrared light sources based on Si photonics. MBT can consider a gain spectrum broadening associated with scattering phenomena, such as Coulomb scattering, based on quantum field theory, and does not need any artificial fitting parameters, such as a relaxation time, used in conventional analysis. Not only Gamma-but also carrier distributions in L-points are considered for the gain analysis. Using MBT, the quantum well structures maximizing the material gain and the differential gain at the threshold are investigated in terms of the well thickness, the strain, and the energy difference of quantum states between Gamma- and L-points.

Large-Mode-Area All-Solid Photonic Bandgap Fibers for the Mitigation of Optical Nonlinearities
Liang Dong, Fanting Kong, Guancheng Gu, Thomas Wade Hawkins, Maxwell Jones, Joshua Parsons, Monica T. Kalichevsky-Dong, Kunimasa Saitoh, Benjamin Pulford, Iyad Dajani
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 22, 2, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2015, [Peer-reviewed]
English, Scientific journal, There is still significant need for power scaling of fiber lasers. Large-mode-area fibers are a key for the mitigation of optical nonlinearities. In recent years, mode instability has shown itself to be an additional significant limiting factor for single-mode power scaling in the regime of a few hundred watts to kilowatts. It is better appreciated now that further power scaling requires significant high-order-mode suppression in addition to a large effective mode area in a fiber. In recent years, we have shown that all-solid photonic bandgap fibers are a superior approach due to their unsurpassed higher-order-mode suppression in large-mode-area designs, making them well suited for applications at high average powers. We will review of some of the recent progress, challenges, and prospects of all-solid photonic bandgap fibers in this invited paper.

Few-Mode Multicore Fiber With 36 Spatial Modes (Three Modes (LP01, LP11a, LP11b) x 12 Cores)
Yusuke Sasaki, Yoshimichi Amma, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 33, 5, 964, 970, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2015, [Peer-reviewed]
English, Scientific journal, The design and characteristics of a novel three-mode 12-core fiber are demonstrated. This fiber has low intercore crosstalk (IC-XT) and low differential mode group delay (DMD). In order to produce such a fiber, three new techniques are introduced to few-mode multicore fiber. A heterogeneous core arrangement with two types of cores, is used to minimize IC-XT; a multistep index, which can control DMD without sacrificing inter-mode crosstalk, is adopted as a core profile to reduce DMD; and a square lattice structure allows for the inclusion of 12 cores within a cladding diameter of 230 mu m. Optimum design of the fiber using these techniques is determined from calculations. Finally, detailed characteristics of a three-mode 12-core fiber based on the design are reported. A fabricated 40-km fiber is confirmed to have a DMD of less than vertical bar 530 vertical bar ps/km over the C + L band and an estimated worst-case IC-XT of less than -55 dB/100 km at 1550 nm. This low IC-XT allows for the transmission of 32 QAM signals.

PLC-Based Four-Mode Multi/Demultiplexer With LP11 Mode Rotator on One Chip
Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Takui Uematsu, Fumihiko Yamamoto
JOURNAL OF LIGHTWAVE TECHNOLOGY, 33, 6, 1161, 1165, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2015, [Peer-reviewed]
English, Scientific journal, We propose a PLC-based mode multi/demultiplexer (MUX/DEMUX) for mode-division multiplexing. We show numerically that the PLC-based mode MUX/DEMUX with a uniform height on one chip can multi/demultiplex four modes (LP01, LP11 a, LP11 b, and LP21 a) by using our proposed LP11 mode rotator and a parallel waveguide. We successfully multi/demultiplex the four modes and achieve a relatively low-wavelength dependence over the C-band by using our fabricated four-mode MUX/DEMUX.

Polarizing ytterbium-doped all-solid photonic bandgap fiber with similar to 1150 mu m(2) effective mode area
Fanting Kong, Guancheng Gu, Thomas W. Hawkins, Joshua Parsons, Maxwell Jones, Christopher Dunn, Monica T. Kalichevsky-Dong, Benjamin Pulford, Iyad Dajani, Kunimasa Saitoh, Stephen P. Palese, Eric Cheung, Liang Dong
OPTICS EXPRESS, 23, 4, 4307, 4312, OPTICAL SOC AMER, Feb. 2015, [Peer-reviewed]
English, Scientific journal, We demonstrate an Yb-doped polarizing all-solid photonic bandgap fiber for single-polarization and single-mode operation with an effective mode area of similar to 1150 mu m(2), a record for all-solid photonic bandgap fibers. The differential polarization mode loss is measured to be >5dB/m over the entire transmission band with a 160nm bandwidth and >15dB/m on the short wavelength edge of the band. A 2.6m long fiber was tested in a laser configuration producing a linearly polarized laser output with a PER value of 21dB without any polarizer, the highest for any fiber lasers based on polarizing fibers. (C) 2015 Optical Society of America

Comparison of homogeneous and heterogeneous 2LP-mode multicore fibers for high spatial multiplicity
Yuki Tobita, Takeshi Fujisawa, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh
Proceedings of Frontiers in Optics 2015, FIO 2015, OSA - The Optical Society, 2015
English, International conference proceedings, We compare homogeneous and heterogeneous 2LP-mode multicore fibers (MCFs) and find that the heterogeneous MCF can improve a tradeoff relationship between low inter-core crosstalk and short cable cutoff wavelength, resulting in high spatial multiplicity.

Impulse response analysis of strongly-coupled three-core fibers
Takeshi Fujisawa, Kunimasa Saitoh
Proceedings of Frontiers in Optics 2015, FIO 2015, OSA - The Optical Society, 2015
English, International conference proceedings, Impulse response of strongly-coupled 3-core fibers is analyzed from weak to strong coupling regime based on multimode generalized nonlinear Schrödinger equation. Measured and calculated impulse responses are in good agreement, showing the validity of discussion.

A high-performance polarization splitter-rotator designed by wavefront matching method
Shuntaro Makino, Takeshi Fujisawa, Kunimasa Saitoh
Proceedings of Frontiers in Optics 2015, FIO 2015, OSA - The Optical Society, 2015
English, International conference proceedings, A high-performance polarization splitter-rotator based on a mode-conversion taper and a mode-sorting asymmetric Y-branch waveguides is designed by wavefront matching method. A high transmission in a wide wavelength range is demonstrated.

Mode Dependent Loss Equaliser and Impact of MDL on PDM-16QAM Few-mode Fibre Transmission
T. Mizuno, H. Takara, K. Shibahara, Y. Miyamoto, M. Oguma, H. Ono, Y. Abe, T. Matsui, S. Matsuo, K. Saitoh, Y. Kimura
ECOC 2015 41ST EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION, IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, We experimentally evaluate the relationship between mode dependent loss (MDL) and Q penalty for few-mode fibre transmission. We employ a low-MDL recirculating loop and free-space-optics type MDL equaliser and transmit 3-mode signals with PDM-16QAM modulation.

A Principal Mode Analysis of Strongly-Coupled 3-Core Fibres
T. Fujisawa, K. Saitoh
ECOC 2015 41ST EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION, IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, Group delay spread of strongly-coupled 3-core fibres is investigated through the principal mode analysis. Calculated and measured results are in good agreement and group delay spread can be further reduced by adding an air-hole to the centre of the fibre.

A new and simple method for crosstalk estimation in homogeneous trench-assisted multi-core fibers
Feihong Ye, Jiajing Tu, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo, Toshio Morioka
Asia Communications and Photonics Conference, ACPC 2014, Optical Society of America (OSA), 14 Nov. 2014
English, International conference proceedings, A new and simple method for inter-core crosstalk estimation in homogeneous trench-assisted multi-core fibers is presented. The crosstalk calculated by this method agrees well with experimental measurement data for two kinds of fabricated 12-core fibers.

Three-dimensional finite-element mode-solver for nonlinear periodic optical waveguides and its application to photonic crystal waveguides
Takanori Sato, Shuntaro Makino, Yuhei Ishizaka, Takeshi Fujisawa, Kunimasa Saitoh
Journal of Lightwave Technology, 32, 21, 3409, 3417, 01 Nov. 2014, [Peer-reviewed]
Scientific journal, © 1983-2012 IEEE. A 3-D finite-element mode-solver, based on a self-consistent algorithm, is newly developed for nonlinear periodic optical waveguides. The developed method makes it possible to calculate various nonlinear characteristics based on optical Kerr effect rigorously, including the nonlinear phase shift, the nonlinear refractive index distribution, and γ-value. Its validity is confirmed by analyzing a silicon-nanowire waveguide and a slot waveguide. To show the usefulness of the method, the nonlinear characteristics of photonic crystal line-defect waveguides and 1-D photonic crystal coupled resonator optical waveguides are investigated. Numerical results show that in some cases, γ-value calculated by the newly developed method and approximate linear analysis differs two-orders of magnitude. For instance, γ-value calculated by developed method is 510 000 m-1W-1for a 1-D photonic crystal coupled resonator optical waveguide, whereas γ-value calculated by conventional method is only 5000 m-1W-1. This difference indicates that, to estimate the nonlinear characteristics of 3-D waveguides precisely, the developed nonlinear guided mode solver is required.

Simple analytical expression for crosstalk estimation in homogeneous trench-assisted multi-core fibers
Feihong Ye, Jiajing Tu, Kunimasa Saitoh, Toshio Morioka
OPTICS EXPRESS, 22, 19, 23007, 23018, OPTICAL SOC AMER, Sep. 2014, [Peer-reviewed]
English, Scientific journal, An analytical expression for the mode coupling coefficient in homogeneous trench-assisted multi-core fibers is derived, which has a simple relationship with the one in normal step-index structures. The amount of inter-core crosstalk reduction (in dB) with trench-assisted structures compared to the one with normal step-index structures can then be written by a simple expression. Comparison with numerical simulations confirms that the obtained analytical expression has very good accuracy for crosstalk estimation. The crosstalk properties in trench-assisted multi-core fibers, such as crosstalk dependence on core pitch and wavelength-dependent crosstalk, can be obtained by this simple analytical expression. (C) 2014 Optical Society of America

PLC-based LP11 mode rotator for mode-division multiplexing transmission
Kunimasa Saitoh, Takui Uematsu, Nobutomo Hanzawa, Yuhei Ishizaka, Kohei Masumoto, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Fumihiko Yamamoto
OPTICS EXPRESS, 22, 16, OPTICAL SOC AMER, Aug. 2014, [Peer-reviewed]
English, Scientific journal, A PLC-based LP11 mode rotator is proposed. The proposed mode rotator is composed of a waveguide with a trench that provides asymmetry of the waveguide. Numerical simulations show that converting LP11a (LP11b) mode to LP11b (LP11a) mode can be achieved with high conversion efficiency (more than 90%) and little polarization dependence over a wide wavelength range from 1450 nm to 1650 nm. In addition, we fabricate the proposed LP11 mode rotator using silica-based PLC. It is confirmed that the fabricated mode rotator can convert LP11a mode to LP11b mode over a wide wavelength range. (C) 2014 Optical Society of America

Grating inscription to few-mode multi-core optical fiber
Hiroki Ishihara, Hitoshi Uemura, Yusuke Sasaki, Koji Omichi, Takeshi Fujisawa, Kunimasa Saitoh
Optics InfoBase Conference Papers, OSA - The Optical Society, 21 Jul. 2014, [Peer-reviewed]
English, International conference proceedings, Preliminary fiber Bragg grating inscription to 2LP-mode 4-core fiber is demonstrated to confirm feasibility for optical fiber sensing application. Clear Bragg reflection spectra for LP01 and LP11 are obtained individually using all fiber-optic few-mode multi-core input/output module.

Heterogeneous 32-core fiber with square-lattice layout for high-density transmissions
Xueqin Xie, Jiajing Tu, Keping Long, Kunimasa Saitoh
Optics InfoBase Conference Papers, OSA - The Optical Society, 21 Jul. 2014, [Peer-reviewed]
English, International conference proceedings, We propose a 32-core fiber with rod/trench structures and achieve average Aeff of about 75 μm2, low XT of less than -27 dB/100km, and cable cutoffwavelength of less than 1.53 μm.

Yb-doped all-solid photonic bandgap fiber lasers
Guancheng Gu, Fanting Kong, Thomas Hawkins, Joshua Parsons, Maxwell Jones, Christopher Dunn, Monica T. Kalichevsky-Dong, Kunimasa Saitoh, Liang Dong
Fiber-Based Technologies and Applications, FBTA 2014, Optical Society of America (OSA), 18 Jun. 2014
English, International conference proceedings, A ytterbium-doped all-solid photonic bandgap fiber laser with an approximately 50μm core diameter has been demonstrated with an efficiency of ~70% and good beam quality (M2<
1.2). © OSA 2014.

Ytterbium-doped large-mode-area all-solid photonic bandgap fiber lasers
Guancheng Gu, Fanting Kong, Thomas Hawkins, Joshua Parsons, Maxwell Jones, Christopher Dunn, Monica T. Kalichevsky-Dong, Kunimasa Saitoh, Liang Dong
OPTICS EXPRESS, 22, 11, 13962, 13968, OPTICAL SOC AMER, Jun. 2014, [Peer-reviewed]
English, Scientific journal, Single-mode operation in a large-mode-area fiber laser is highly desired for power scaling. We have, for the first time, demonstrated a 50 mu m-core-diameter Yb-doped all-solid photonic bandgap fiber laser with a mode area over 4 times that of the previous demonstration. 75W output power has been generated with a diffraction-limited beam and an efficiency of 70% relative to the launched pump power. We have also experimentally confirmed that a robust single-mode regime exists near the high frequency edge of the bandgap. These fibers only guide light within the bandgap over a narrow spectral range, which is essential for lasing far from the gain peak and suppression of stimulated Raman scattering. This work demonstrates the strong potential for mode area scaling of in single-mode all-solid photonic bandgap fibers. (C)2014 Optical Society of America

Design of Three-Spatial-Mode Ring-Core Fiber
Motoki Kasahara, Kunimasa Saitoh, Taiji Sakamoto, Nobutomo Hanzawa, Takashi Matsui, Kyozo Tsujikawa, Fumihiko Yamamoto
JOURNAL OF LIGHTWAVE TECHNOLOGY, 32, 7, 1337, 1343, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2014, [Peer-reviewed]
English, Scientific journal, We investigate the fiber parameters which meet the design conditions for effectively three-spatial-mode transmission and low bending loss in a ring-core fiber for mode-division multiplexing transmission. We also clarify the effective area and the difference of the effective index while satisfying the design conditions.

Design Optimization of Large-Mode-Area All-Solid Photonic Bandgap Fibers for High-Power Laser Applications
Shota Saitoh, Kunimasa Saitoh, Masahiro Kashiwagi, Shoichiro Matsuo, Liang Dong
JOURNAL OF LIGHTWAVE TECHNOLOGY, 32, 3, 440, 449, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2014, [Peer-reviewed]
English, Scientific journal, We optimized the structural parameters of large-mode-area all-solid photonic bandgap fibers for high-power laser applications with numerical simulations. We obtained an effective mode area of greater than 1000 mu m(2) in bending condition while maintaining single-mode operation and realizing compact packaging for both 7- and 19-cell core fibers. We also found out that a core diameter of larger than 115 mu m could be achieved for the 19-cell core fibers.

Ultra-Broadband Silicon-Wire Polarization Beam Combiner/Splitter Based on a Wavelength Insensitive Coupler With a Point-Symmetrical Configuration
Takui Uematsu, Tetsuya Kitayama, Yuhei Ishizaka, Kunimasa Saitoh
IEEE PHOTONICS JOURNAL, 6, 1, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2014, [Peer-reviewed]
English, Scientific journal, An ultrabroadband silicon wire polarization beam combiner/splitter (PBCS) based on a wavelength-insensitive coupler is proposed. The proposed PBCS consists of three identical directional couplers and two identical delay lines. We design the PBCS using the 3-D finite element method. Numerical simulations show that the proposed PBCS can achieve the transmittance of more than 90% over a wide wavelength range from 1450 to 1650 nm for both TE and TM polarized modes.

Heterogeneous trench-assisted few-mode multicore fiber with low differential mode delay
Jiajing Tu, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo
OPTICS EXPRESS, 22, 4, 4329, 4341, OPTICAL SOC AMER, Feb. 2014, [Peer-reviewed]
English, Scientific journal, We propose a kind of heterogeneous multi-core fiber (Hetero-MCF) with trench-assisted multi-step index few-mode core (TA-MSI-FMC) deployed inside. After analyzing the impact of each parameter on differential mode delay (DMD), we design a couple of TA-MSI-FMCs with A(eff) of 110 mu m(2) for LP01 mode. DMD of each TA-MSI-FMC is smaller than vertical bar 170 vertical bar ps/km over C + L band and the total DMD can approach almost 0 ps/km over C + L band if we adopt DMD managed transmission line technique by using only one kind of Hetero-TA-FM-MCF. For such Hetero-TA-FM-MCF, crosstalk is about -30 dB/100km at wavelength of 1550 nm as bending radius becomes larger than 15 cm, core number can reach 12, a relative core multiplicity factor (RCMF) is 15.7, and the RCMF can even reach 26.1 if we treat LP11 mode as two special modes thanks to the multiple-input-multiple-output technology. (C) 2014 Optical Society of America

Yb-doped photonic bandgap fiber lasers with record core diameter
Guancheng Gu, Fanting Kong, Thomas Hawkins, Joshua Parsons, Maxwell Jones, Christopher Dunn, Monica T. Kalichevsky-Dong, Kunimasa Saitoh, Liang Dong
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2014
English, International conference proceedings, We have demonstrated ytterbium-doped fiber lasers in 50μm-core photonic bandgap fibers with robust single mode output, a record core diameter for active photonic bandgap fibers, with slope efficiency exceeding 70%. © 2014 OSA.

Few-Mode Multicore Fiber for Highly Dense Space-Division Multiplexing
Y. Sasaki, Y. Amma, K. Takenaga, N. Guan, S. Matsuo, K. Saitoh, M. Koshiba
2014 IEEE PHOTONICS SOCIETY SUMMER TOPICAL MEETING SERIES, 164, 165, IEEE, 2014, [Peer-reviewed]
English, International conference proceedings, Few-mode multicore fiber (FM-MCF) has the potential to improve the transmission capacity of a fiber drastically. Challenges in designing FM-MCF and characteristics of our fabricated FM-MCF for highly dense space-division multiplexing are reviewed.

PLC-based mode multi/demultiplexer for MDM transmission
N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, M. Koshiba, F. Yamamoto
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS III, 9009, SPIE-INT SOC OPTICAL ENGINEERING, 2014, [Peer-reviewed]
English, International conference proceedings, We propose a PLC-based multi/demultiplexer (MUX/DEMUX) with a mode conversion function for mode division multiplexing (MDM) transmission applications. The PLC-based mode MUX/DEMUX can realize a low insertion loss and a wide working wavelength bandwidth. We designed and demonstrated a two-mode ( LP01 and LP11 modes) and a three-mode ( LP01, LP11, and LP21 modes) MUX/DEMUX for use in the C-band.

12-core x 3-mode Dense Space Division Multiplexed Transmission over 40 km Employing Multi-carrier Signals with Parallel MIMO Equalization
T. Mizuno, T. Kobayashi, H. Takara, A. Sano, H. Kawakami, T. Nakagawa, Y. Miyamoto, Y. Abe, T. Goh, M. Oguma, T. Sakamoto, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, T. Morioka
2014 OPTICAL FIBER COMMUNICATIONS CONFERENCE AND EXHIBITION (OFC), IEEE, 2014, [Peer-reviewed]
English, International conference proceedings, We demonstrate dense SDM transmission of 20-WDM multi-carrier PDM-32QAM signals over a 40-km 12-core x 3-mode fiber with 247.9-b/s/Hz spectral efficiency. Parallel MIMO equalization enables 21-ns DMD compensation with 61 TDE taps per subcarrier.

Fused Taper Type Fan-in/Fan-out Device for 12 Core Multi-Core Fiber
H. Uemura, K. Omichi, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba
2014 OPTOELECTRONICS AND COMMUNICATIONS CONFERENCE AND AUSTRALIAN CONFERENCE ON OPTICAL FIBRE TECHNOLOGY (OECC/ACOFT 2014), 49, +, IEEE, 2014, [Peer-reviewed]
English, International conference proceedings, Fan-in/fan-out device for splicing to 12 core multi-core fiber is fabricated. Maximum loss of 4.7 dB at C-band and the worst crosstalk of -45 dB are confirmed in fan-in and fan-out with 20 m-long MCF.

Design of Few-Mode Fibers for Mode-Division Multiplexing Transmission
Motoki Kasahara, Kunimasa Saitoh, Taiji Sakamoto, Nobutomo Hanzawa, Takashi Matsui, Kyozo Tsujikawa, Fumihiko Yamamoto, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 5, 6, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 2013, [Peer-reviewed]
English, Scientific journal, We investigate the fiber parameters that meet the design conditions for low bending loss and an effectively few-mode operation on few-mode fibers with a step-index profile and a W-shaped refractive index profile for mode-division multiplexing transmission. We also clarify the limits of effective area enlargement and the difference of the effective index while satisfying the design conditions.

Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Koshiba Masanori, Yamamoto Fumihiko
Optics express, 21, 22, 25752, 25760, Optical Society of America, 04 Nov. 2013
English, We proposed a PLC-based mode multi/demultiplexer (MUX/DEMUX) with an asymmetric parallel waveguide for mode division multiplexed (MDM) transmission. The mode MUX/DEMUX including a mode conversion function with an asymmetric parallel waveguide can be realized by matching the effective indices of the LP01 and LP11 modes of two waveguides. We report the design of a mode MUX/DEMUX that can support C-band WDM-MDM transmission. The fabricated mode MUX/DEMUX realized a low insertion loss of less than 1.3 dB and high a mode extinction ratio that exceeded 15 dB. We used the fabricated mode MUX/DEMUX to achieve a successful 2 mode x 4 wavelength x 10 Gbps transmission over a 9 km two-mode fiber with a penalty of less than 1 dB. ©2013 Optical Society of America

Transmission-Efficient Structures of Bent and Crossing Silicon Slot Waveguides
Yuhei Ishizaka, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 5, 5, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2013, [Peer-reviewed]
English, Scientific journal, We present transmission-efficient bent and crossing silicon slot waveguides for compact silicon slot waveguide-based circuits. To begin with, using the 3-D vector finite-element method, we investigate the transmission characteristics of a bent silicon slot waveguide with an asymmetric slot that is connected to straight waveguides. Numerical results show that the transmission efficiency can be improved by optimizing a taper structure that is introduced between the bend of an asymmetric slot waveguide and its straight input/output waveguides. Next, we investigate the transmission characteristics of a 3-D crossing silicon slot waveguide that has a vertical coupler. Numerical results show that our crossing structure can achieve a high transmission efficiency compared with conventional structures.

Proposal of Coupled Ring Resonator Based on One-Dimensional Photonic Crystal Nanocavity
Shuntaro Makino, Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 31, 15, 2565, 2569, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Aug. 2013, [Peer-reviewed]
English, Scientific journal, We propose the coupled ring resonator based on one-dimensional photonic crystal (1-D PC) nanocavity. The proposed structure is composed by introducing 1-D PC nanocavity into the CROW based on ring resonator, and we expect that it can achieve small group velocity. We investigate the transmission characteristics of the proposed structure by using 2-D scalar finite element method (FEM) for periodic waveguide analysis. We obtained the group velocity v(g) = 0.033c with c being light velocity in vacuum because of combining 1-D PC nanocavity and CROW based on ring resonator. Moreover, we consider reduction of footprint of the proposed structure for photonic integration.

Structural dependence of nonlinear characteristics in slot waveguides composed of photonic crystal nanobeam cavities
Shuntaro Makino, Yuhei Ishizaka, Kunimasa Saitoh, Masanori Koshiba
Pacific Rim Conference on Lasers and Electro-Optics (CLEO Racific Rim), Jul. 2013, [Peer-reviewed]
English, International conference proceedings, We investigate structural dependence of nonlinear characteristics in slot waveguides composed of photonic crystal nanobeam cavities. Numerical results show that our proposed structure can realize a high effective nonlinear coefficient. © 2013 IEEE.

Mode-Division Multiplexing Transmission System With DMD-Independent Low Complexity MIMO Processing
Taiji Sakamoto, Takayoshi Mori, Takashi Yamamoto, Nobutomo Hanzawa, Shigeru Tomita, Fumihiko Yamamoto, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 31, 13, 2192, 2199, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2013, [Peer-reviewed]
English, Scientific journal, We propose a mode-division multiplexing (MDM) transmission system with differential mode delay (DMD) independent low computational complexity MIMO processing. We adjusted the delay time of the delay units in the equalizer in accordance with the DMD value of the fiber, and the functional imperfection of the mode MUX/DEMUX can be compensated for with low computational complexity by using this technique. Finally, we investigated the crosstalk characteristics of the PLC-based MUX/DEMUX and achieved an experimental WDM-MDM transmission over the C-L band with low MIMO processing complexity even when using a few-mode fiber with a high DMD.

Longitudinal Power Decay of a Weakly-Coupled Multi-Core Fiber
Itaru Ishida, Hiroki Hamaguchi, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 25, 13, 1270, 1273, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2013, [Peer-reviewed]
English, Scientific journal, Longitudinal power decay because of the inter-core crosstalk of a weakly-coupled multi-core fiber is numerically simulated by solving coupled power equation and measured using bi-directional optical time domain reflectometer technique. The power decay caused by the inter-core crosstalk is observed on the measured optical time domain reflectometer data. The simulated power decay shows good agreement with the measured power decay. The power decay of the outer core excitation is gentler than that of the center core excitation. The center core and outer cores of multi-core fibers with a hexagonal close-packed structure show the different crosstalk characteristics until the power of all cores is saturated.

Design of highly-nonlinear horizontal slot waveguide with low and flat dispersion
Masa-aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
OPTICS COMMUNICATIONS, 298, 180, 184, ELSEVIER SCIENCE BV, Jul. 2013, [Peer-reviewed]
English, Scientific journal, We present an optimum design of highly-nonlinear horizontal slot waveguides consisting of silicon and silicon nanocrystais with flat dispersion characteristics. Results from numerical simulations show that the nonlinear coefficient of 6600 W-1 m(-1) at 1550-nm wavelength and the dispersion values between -150 and 150 ps/nm/km in the S, C, and L bands can be achieved. In addition, when we limit the operating bandwidth to C-band, the most tolerant slot height is 20 nm for the 10 nm fabrication errors of waveguide width and high-index region height. (C) 2013 Elsevier B.V. All rights reserved.

409-Tb/s+409-Tb/s crosstalk suppressed bidirectional MCF transmission over 450 km using propagation-direction interleaving
Akihide Sano, Hidehiko Takara, Takayuki Kobayashi, Hiroto Kawakami, Hiroki Kishikawa, Tadao Nakagawa, Yutaka Miyamoto, Yoshiteru Abe, Hirotaka Ono, Kota Shikama, Munehiko Nagatani, Takayoshi Mori, Yusuke Sasaki, Itaru Ishida, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba, Makoto Yamada, Hiroji Masuda, Toshio Morioka
OPTICS EXPRESS, 21, 14, 16777, 16783, OPTICAL SOC AMER, Jul. 2013, [Peer-reviewed]
English, Scientific journal, We demonstrate bidirectional transmission over 450 km of newly-developed dual-ring structured 12-core fiber with large effective area and low crosstalk. Inter-core crosstalk is suppressed by employing propagation-direction interleaving, and 409-Tb/s capacities are achieved for both directions. (C) 2013 Optical Society of America

Transmission Systems Using Multicore Fibers
Yoshinari Awaji, Kunimasa Saitoh, Shoichiro Matsuo
Optical Fiber Telecommunications VIB: Systems and Networks: Sixth Edition, 617, 651, Elsevier Inc., 13 May 2013, [Peer-reviewed]
English, In book, Multi-core fiber (MCF) transmission technologies have been widely studied as the simplest form of space-division multiplexing (SDM). Many types of MCFs exist,but the most common is "Uncoupled MCF" where each individual core is assumed to be an independent optical path. The key issue in these systems is how to suppress the inter-core crosstalk and the coupling/de-coupling mechanism. Currently, many MCF varieties, coupling methods, splicing techniques, and transmission schemes have been proposed and demonstrated and despite many of the component technologies still being in the development stage, MCF systems already present the capability for huge transmission capacity. In this chapter, these component technologies and early experimental trials of MCF transmission are reviewed. At first, we provide an overview of medium- to long-haul MCF transmission and theories. Secondly, coupling technologies between MCF-SMF and MCF-MCF are reviewed. Finally, several experimental demonstrations, including transmission exceeding 100. Tb/s and over 1000. km, are described.

Slow-Light-Enhanced Nonlinear Characteristics in Slot Waveguides Composed of Photonic Crystal Nanobeam Cavities
Shuntaro Makino, Yuhei Ishizaka, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 5, 2, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2013, [Peer-reviewed]
English, Scientific journal, We investigate slow-light-enhanced nonlinear characteristics in slot waveguides composed of photonic crystal (PC) nanobeam cavities. To evaluate the effective nonlinear coefficient, numerical simulations are performed by using 3-D vector finite-element method for periodic waveguide analysis. Results show that the maximum nonlinear coefficient of 11 600 m(-1) W-1 is obtained by optimizing waveguide width and height. This high optical nonlinearity is realized owing to strong light confinement at slotted geometry and small group velocity in PC nanobeam cavities. Also, we show that there exists tradeoff between the effective nonlinear coefficient and the bandwidth in the case of changing one-period length of the proposed structure.

TE/TM-Pass Polarizer Based on Lithium Niobate on Insulator Ridge Waveguide
Emi Saitoh, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 5, 2, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2013, [Peer-reviewed]
English, Scientific journal, We propose a TE/TM-pass polarizer based on a shallow X-cut lithium niobate-on-insulator (LNOI) ridge waveguide. In shallow X-cut LNOI ridge waveguides, the leaky mode is interchanged between TE and TM polarization waves depending on the ridge waveguide height. The phenomenon does not occur in silicon-on-insulator ridge waveguides. We use this special loss mechanism for TE/TM-pass polarizers. We evaluate structural and wavelength dependencies of polarizers by a vectorial finite-element method. The extinction ratios of 108 dB/mm and 27 dB/mm are obtained for TE-pass and TM-pass polarizers.

Physical interpretation of intercore crosstalk in multicore fiber: Effects of macrobend, structure fluctuation, and microbend
Tetsuya Hayashi, Takashi Sasaki, Eisuke Sasaoka, Kunimasa Saitoh, Masanori Koshiba
Optics Express, 21, 5, 5401, 5412, Optical Society of American (OSA), 11 Mar. 2013, [Peer-reviewed]
English, Scientific journal, We have derived an intuitively interpretable expression of the average power-coupling coefficient for estimating the inter-core crosstalk of the multicore fiber. Based on the derived expression, we discuss how the structure fluctuation and macrobend can affect the crosstalk, and organize previously reported methods for crosstalk suppression. We also discuss how the microbending can affect the crosstalk in homogeneous and heterogeneous MCFs, based on the derived expression and previously reported measurement results. © 2013 Optical Society of America.

Structural dependence of nonlinear characteristics in coupled resonator optical waveguides based on slotted nanobeam cavities
Shuntaro Makino, Yuhei Ishizaka, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2013
English, International conference proceedings, We propose coupled resonator optical waveguides based on slotted nanobeam cavities. We show that our proposed structure can achieve a high effective nonlinear coefficient because of small group velocity and light confinement in slot region. © OSA 2013.

Optimized design method for heterogeneous trench-assisted multi-core fiber　　　　　　　　　　　　　　　
Jiajing Tu, Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
2013 18th OptoElectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching, OECC/PS 2013, 2013
English, International conference proceedings, By designing the cores, trench layers and core number, we propose a relative optimized design scheme for heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF), which is a bend-insensitive MCF with high density of cores and ultra-low crosstalk. © 2013 IEICE.

Fused taper type fan-in/fan-out device for multicore EDF　　　　　　　　　　　　　　　
Hitoshi Uemura, Katsushiro Takenaga, Teijiro Ori, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
2013 18th OptoElectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching, OECC/PS 2013, 2013
English, International conference proceedings, Fused taper type fan-in/fan-out device with small MFD is developed for multicore EDF. Almost similar MFD and high accuracy core arrangement as multicore EDF are achieved by using high Δ single-core fiber in elongation process. © 2013 IEICE.

Investigation of crosstalk dependencies on bending radius of heterogeneous multicore fiber　　　　　　　　　　　　　　　
Y. Sasaki, Y. Amma, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba
2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2013, 2013
English, International conference proceedings, Crosstalk characteristic of heterogeneous MCF with homogeneous A eff is investigated. It is experimentally confirmed that radius around which the crosstalk peaks (Rpk) of smaller than 50 mm is realizable by Δneff of larger than 0.0010. © 2013 OSA.

2 x 344 Tb/s propagation-direction interleaved transmission over 1500-km MCF enhanced by multicarrier full electric-field digital back-propagation
T. Kobayashi, H. Takara, A. Sano, T. Mizuno, H. Kawakami, Y. Miyamoto, K. Hiraga, Y. Abe, H. Ono, M. Wada, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Yamada, H. Masuda, T. Morioka
IET Conference Publications, 2013, 622 CP, 1254, 1256, 2013
International conference proceedings, We demonstrate 12-core fiber bidirectional long-haul transmission with sub-petabit-class capacity (2 x 344 Tb/s). Inter-core crosstalk management and multicarrier nonlinear compensation enabled the longest distance of 1500 km in SDM transmission with unidirectional capacity over 300 Tb/s.

Trench-assisted low-crosstalk few-mode multicore fiber
Yusuke Sasaki, Yoshimichi Amma, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
IET Conference Publications, 2013, 622, 13, 15, 2013, [Peer-reviewed]
English, International conference proceedings, Trench-assisted few-mode multicore fiber with high-multiplicity and low-crosstalk characteristics was designed and fabricated. The core supported two-LP-mode transmission over C+L band. The fabricated fiber realized the worst case crosstalk of smaller than -30 dB/100km at 1625 nm with cladding diameter of 195.4 μm.

Crosstalk suppressed hole-assisted 6-core fiber with cladding diameter of 125 μm
Taiji Sakamoto, Kunimasa Saitoh, Nobutomo Hanzawa, Kyozo Tsujikawa, Lin Ma, Masanori Koshiba, Fumihiko Yamamoto
IET Conference Publications, 2013, 622, 7, 9, 2013, [Peer-reviewed]
English, International conference proceedings, We report 125 μm-cladding multi-core fiber for high-density space-division multiplexing transmission. We show numerically that a hole-assisted structure can reduce both crosstalk and leakage loss. We achieved 125 μm-cladding 6-core fiber experimentally with a low crosstalk of less than -30 dB/100km and a high normalized core multiplicity of 6.0.

Crosstalk increase in tightly bent multi-core fiber due to power coupling mediated by cladding modes
Tetsuya Hayashi, Toshiki Taru, Takashi Sasaki, Kunimasa Saitoh, Masanori Koshiba
IET Conference Publications, 2013, 622, 10, 12, 2013, [Peer-reviewed]
English, International conference proceedings, We developed simplified coupled-power equations including cladding modes, and revealed that crosstalk in tightly bent multi-core fibers can be increased from the existing predictions. The crosstalk increase was proportional to the square of the bend loss coefficient, and validated by experimental results, for the first time.

Three-mode PLC-type multi/demultiplexer for mode-division multiplexing transmission
Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Kyozo Tsujikawa, Takui Uematsu, Masanori Koshiba, Fumihiko Yamamoto
IET Conference Publications, 2013, 622, 183, 185, 2013, [Peer-reviewed]
English, International conference proceedings, We demonstrated a three-mode multi/demultiplexer (MUX/DEMUX) using parallel waveguides with a uniform height for mode division multiplexing transmission. The mode conversion of the LP01 mode to the LP11 and LP21 modes in the C-band was achieved using a fabricated MUX/DEMUX.

Multicore fiber with one-ring structure
S. Matsuo, Y. Sasaki, I. Ishida, K. Takenaga, K. Saitoh, M. Koshiba
Proceedings of SPIE - The International Society for Optical Engineering, 8647, 2013, [Peer-reviewed]
English, International conference proceedings, The characteristics of a multicore fiber with one-ring structure are reviewed. The one-ring structure, which has no center core, can overcome issues on the hexagonal close-pack structure that is the most popular multicore structure. The onering structure has flexibility in the number of cores and is unrelated to the core pitch limitation due to cutoff wavelength lengthening thank to no center core structure. The one-ring structure is effective to suppress the worst case crosstalk that is crosstalk assuming all cores carry equal signal power. In the case of hexagonal close-pack structure, the worst case crosstalk of an inner core is 7.8 dB larger than that between two cores. The different worst crosstalk is observed depending on the number of nearest neighbor cores. The one-ring structure can limit the degradation to 3.0 dB for all cores. Fabricated 12-core fiber with the one-ring structure based on the simulation realized effective core area of 80 m2 and very low crosstalk less than -40 dB after 100-km propagation. © 2012 SPIE.

Recent Progress on Multi-Core Fiber and Few-Mode Fiber
S. Matsuo, Y. Sasaki, I. Ishida, K. Takenaga, K. Saitoh, M. Koshiba
2013 OPTICAL FIBER COMMUNICATION CONFERENCE AND EXPOSITION AND THE NATIONAL FIBER OPTIC ENGINEERS CONFERENCE (OFC/NFOEC), IEEE, 2013, [Peer-reviewed]
English, International conference proceedings, This paper reviews research activities concerning multi-core fiber, few-mode fiber and few-mode multi-core fiber. The characteristic of the 12-core fiber that was used for the first 1-Pb/s/fiber transmission experiments is presented.

Multicore fibers for large capacity transmission
Kunimasa Saitoh, Shoichiro Matsuo
NANOPHOTONICS, 2, 5-6, 441, 454, WALTER DE GRUYTER GMBH, 2013, [Peer-reviewed]
English, We experience Internet traffic growth of 100 times every 10 years. However, the capacity of existing standard single-mode fiber is approaching its fundamental limit regardless of significant realization of transmission technologies which allow for high spectral efficiencies. Space division multiplexing (SDM) based on multicore fibers (MCFs) has emerged as a solution to the problem of saturation of the capacity of optical transmission systems. This article presents the recent progress on the MCFs for future large capacity long-distance transmission systems. In MCFs, there is a tradeoff relationship between low crosstalk and high multiplicity, therefore the maximum number of cores and the core arrangement have to be carefully determined based on the required crosstalk level and core size. The state-of-the-art of fabricated MCFs and the transmission experiments using MCFs are reviewed. The current maximum capacity-distance product in MCF transmission is 368.2 (184.1+184.1) Pb/s/fiber km with the relative spatial efficiency of 4.7 compared with a standard single-mode fiber. In order to increase the spatial efficiency as well as the capacity-distance product further in MCFs, the possibility of heterogeneous MCFs and few-mode MCFs is also presented.

Optimized design method for bend-insensitive heterogeneous trench-assisted multi-core fiber with ultra-low crosstalk and high core density
Jiajing Tu, Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
Journal of Lightwave Technology, 31, 15, 2590, 2598, 2013, [Peer-reviewed]
English, Scientific journal, We describe and explain the design methods for the cores, trench layers and core number in multi-core fibers (MCFs) in detail. According to such method, we propose relative optimized design schemes for heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF), inside which cores are arranged in one-ring structure. This Hetero-TA-MCF is a kind of bend-insensitive MCF with high density of cores and ultra-low crosstalk. © 1983-2012 IEEE.

12-core fiber with one ring structure for extremely large capacity transmission
Shoichiro Matsuo, Yusuke Sasaki, Tsuyoshi Akamatsu, Itaru Ishida, Katsuhiro Takenaga, Kazuhiro Okuyama, Kunimasa Saitoh, Masanori Kosihba
OPTICS EXPRESS, 20, 27, 28398, 28408, OPTICAL SOC AMER, Dec. 2012, [Peer-reviewed]
English, Scientific journal, The feature of a multicore fiber with one-ring structure is theoretically analyzed and experimentally demonstrated. The one-ring structure overcomes the issues of the hexagonal close-pack structure. The possibility of 10-core fiber with A(eff) of 110 mu m(2) and 12-core fiber with A(eff) of 80 mu m(2) is theoretically presented. The fabricated 12-core fibers based on the simulation results realized A(eff) of 80 mu m(2) and crosstalk less than -40 dB at 1550 nm after 100-km propagation. The MCF with the number of core larger than seven and the small crosstalk was demonstrated for the first time. (C) 2012 Optical Society of America

Large-effective-area uncoupled few-mode multi-core fiber
Yusuke Sasaki, Katsuhiro Takenaga, Ning Guan, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 20, 26, B77, B84, OPTICAL SOC AMER, Dec. 2012, [Peer-reviewed]
English, Scientific journal, Characteristics of few-mode multi-core fiber (FM-MCF) were numerically analyzed and experimentally confirmed. The cores of FM-MCF were designed to support transmission of LP01 and LP11 modes from the point of bending loss of LP11 and LP21 modes. Inter-core crosstalk between LP11 mode was calculated to determine core pitch of fibers. It was confirmed that the fabricated fibers was two-mode transmission over C-band and L-band with the effective area of LP01 mode of about 110 mu m(2) at 1550 nm. The crosstalk of the fibers was estimated to be smaller than -30 dB at 1550 nm after 100-km propagation. The crosstalk dependence on wavelength was also measured and matched well with the simulated results. (c) 2012 Optical Society of America

Mode area scaling with all-solid photonic bandgap fibers
Fanting Kong, Kunimasa Saitoh, Devon Mcclane, Thomas Hawkins, Paul Foy, Guancheng Gu, Liang Dong
OPTICS EXPRESS, 20, 24, 26363, 26372, OPTICAL SOC AMER, Nov. 2012, [Peer-reviewed]
English, Scientific journal, There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of similar to 920 mu m(2) can be made with excellent higher order mode suppression. (C) 2012 Optical Society of America

Three-Dimensional Finite-Element Solutions for Crossing Slot-Waveguides With Finite Core-Height
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 30, 21, 3394, 3400, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2012, [Peer-reviewed]
English, Scientific journal, A three-dimensional vectorial finite element method (3-D-VFEM) for waveguide discontinuity problems is newly formulated and implemented. Through the use of the 3-D-VFEM, transmission characteristics of two types of crossing slot-waveguides with finite core-height are investigated for the first time. One is a direct crossing and the other is a crossing filling up the slots locally. Results show that three-dimensional analysis considering the finite core-height is indispensable.

Crosstalk and Core Density in Uncoupled Multicore Fibers
Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
IEEE PHOTONICS TECHNOLOGY LETTERS, 24, 21, 1898, 1901, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2012, [Peer-reviewed]
English, Scientific journal, Reduction of intercore crosstalk in uncoupled multicore fibers (MCFs) is the most critical issue for long-haul space division multiplexing transmission. The required coupling coefficient, allowable outer cladding diameter, and assumed bending radii range are important fiber design parameters for maximizing a relative core density while maintaining low-crosstalk level in an MCF. The relationships between crosstalk and core density in uncoupled MCFs with a step-index profile and trench-assisted structure are investigated in detail, and the maximum achievable relative core multiplicity factors are clarified in a limited cladding diameter.

Large Effective-Area Few-Mode Multicore Fiber
Katsuhiro Takenaga, Yusuke Sasaki, Ning Guan, Shoichiro Matsuo, Motoki Kasahara, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 24, 21, 1941, 1944, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2012, [Peer-reviewed]
English, Scientific journal, A design concept of few-mode multicore fiber is presented. Cores with a step-index profile are designed to support two LP modes over the C band and the L band with a large effective area larger than 110 mu m(2). The pitch of cores is determined regarding the inter-core crosstalk related to the LP11 mode. The two LP modes multicore fibers with four cores and seven cores show a relative core multiplicity factor (RMCF) of 7.3 and 9.2, respectively. The RCMF of the proposed two LP modes multicore fiber is larger than that of previously reported single-mode multicore fibers.

Analytical Expression of Average Power-Coupling Coefficients for Estimating Intercore Crosstalk in Multicore Fibers
Masanori Koshiba, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo
IEEE PHOTONICS JOURNAL, 4, 5, 1987, 1995, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2012, [Peer-reviewed]
English, Scientific journal, In order to realize fast and accurate estimation of intercore crosstalk in bent multicore fibers (MCFs), an analytical expression of the average power-coupling coefficient (PCC) based on an exponential autocorrelation function is, for the first time, derived, resulting in no need for heavy numerical computations. It is revealed that, when the bending radius is large and the correlation length is large, the average PCC is inversely proportional to the correlation length and to the square of the propagation constant difference Delta beta(mn) between core m and core n, and when the bending radius is small and the correlation length is large, the average PCC is proportional to the bending radius and is independent of the correlation length. When the correlation length is small, on the other hand, the average PCC is proportional to the correlation length and is independent of the bending radius. For homogeneous MCFs (Delta beta(mn) - 0) with small bending radius, the average PCC coincides with the mean crosstalk increase per unit length derived from the coupled-mode theory of Hayashi et al. that is proportional to the bending radius. Average crosstalk values calculated by using the analytical expression derived here are in excellent agreement with those of numerical solutions of coupled-power equations, irrespective of the values of bending radius and correlation length.

Design and Fabrication of Large-Mode Area Air-Clad Leakage Channel Fiber With Superior Bending Characteristics
Mrinmay Pal, Kunimasa Saitoh, M. C. Paul, Debashri Ghosh, S. K. Bhadra
IEEE PHOTONICS TECHNOLOGY LETTERS, 24, 18, 1650, 1652, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2012, [Peer-reviewed]
English, Scientific journal, We investigate the bending characteristics of leakage channel fibers (LCFs) to achieve large-mode area and effectively single-mode operation with a practically allowable bending radius of 5 cm for compact Yb-doped fiber laser applications. We design and fabricate a two-ring air-clad LCF whose bending characteristics have been studied both by numerical analysis through the full-vectorial finite element method and experimentally. The LCF has bend-loss of 0.09 dB/m and effective area of 350 mu m(2) for a 5-cm bending radius. The single-mode nature of the bent LCF at 1064 nm is presented.

Design of a Compact Two-Mode Multi/Demultiplexer Consisting of Multimode Interference Waveguides and a Wavelength-Insensitive Phase Shifter for Mode-Division Multiplexing Transmission
Takui Uematsu, Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 30, 15, 2421, 2426, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Aug. 2012, [Peer-reviewed]
English, Scientific journal, A compact two-mode (de)multiplexer (TM-MUX) based on Si nanowire for mode-division multiplexing is designed. The TM-MUX is composed of two multimode interference (MMI) waveguides and a butterfly-shape tapered phase shifter between two MMI waveguides. Numerical simulations show that the designed device is compact compared with conventional-mode (de) multiplexer and operates as a TM-MUX in the whole C-band. In addition, we show that the designed TM-MUX has relatively good fabrication tolerance.

Effectively single-mode all-solid photonic bandgap fiber with large effective area and low bending loss for compact high-power all-fiber lasers
Masahiro Kashiwagi, Kunimasa Saitoh, Katsuhiro Takenaga, Shoji Tanigawa, Shoichiro Matsuo, Munehisa Fujimaki
OPTICS EXPRESS, 20, 14, 15061, 15070, OPTICAL SOC AMER, Jul. 2012, [Peer-reviewed]
English, Scientific journal, An effectively single-mode all-solid photonic bandgap fiber with large effective area and low bending loss for compact high-power all-fiber lasers is fully investigated. The pitch dependencies of effective area, bending loss, and effectively single-mode operation are discussed numerically and experimentally. The calculation results indicate that an effectively single-mode all-solid photonic bandgap fiber with an effective area of more than 500 mu m(2) and a bending loss of less than 0.1 dB/m at a bending radius of 10 cm can be realized by choosing optimum fiber parameters. In a fabricated effectively single-mode all-solid photonic bandgap fiber with 48.0 mu m core, the effective area of 712 mu m(2), the effectively single-mode operation, and the bending loss of less than 0.1 dB/m at a bending radius of 10 cm are achieved simultaneously at 1064 nm. (c) 2012 Optical Society of America

Design and analysis of large-effective-area heterogeneous trench-assisted multi-core fiber
Jiajing Tu, Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
OPTICS EXPRESS, 20, 14, 15157, 15170, OPTICAL SOC AMER, Jul. 2012, [Peer-reviewed]
English, Scientific journal, Based on the overlap integral of electromagnetic fields in neighboring cores, a calculating method is proposed for obtaining the coupling coefficient between two adjacent trench-assisted non-identical cores. And a kind of heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) with 12 cores is proposed to achieve large effective area (A(eff)) and high density of cores. As bending radius becomes larger than 50 mm, the crosstalk value at 1550-nm wavelength of the Hetero-TA-MCF is about -42 dB after 100-km propagation and the Aeff of this Hetero-TA-MCF can reach 100 mu m(2). (C)2012 Optical Society of America

Compact Polarization Rotator Based on Surface Plasmon Polariton With Low Insertion Loss
Masa-aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 4, 3, 707, 714, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 2012, [Peer-reviewed]
English, Scientific journal, An ultracompact mode-evolution-based polarization rotator is proposed. The polarization rotator consists of silicon wire waveguide covered by silica buffer layer and linearly tapered thin aluminum. Numerical simulations using 3-D finite element method demonstrate that it is possible to obtain an 11-mu m-long polarization rotator with about -20-dB extinction ratio and less than 4.7-dB loss in C-band for both polarization conversions from TM-like mode to TE-like mode and from TE-like mode to TM-like mode.

Structural Dependence of Group Velocity and Leakage Loss in 1-D Photonic Crystal Coupled Resonator Optical Waveguide With Modulated Mode-Gap
Yuki Kawaguchi, Shuntaro Makino, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS JOURNAL, 4, 2, 300, 309, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2012, [Peer-reviewed]
English, Scientific journal, One-dimensional photonic crystal coupled resonator optical waveguide (1-D PC-CROW) based on mode-gap confinement has a possibility to reduce leakage losses compared with conventional 1-D PC-CROW constructed by placing defects periodically. However, transmission characteristics of 1-D PC-CROW based on mode-gap confinement have not been investigated so far. In this paper, we evaluate structural dependence of group velocity and leakage losses of 1-D PC-CROW by using the 3-D vector finite element method for periodic waveguide analysis. We show that such 1-D PC-CROW can realize small group velocity and low leakage losses simultaneously by investigating dispersion relationships of 1-D PC-CROW.

Low bending loss and effectively single-mode all-solid photonic bandgap fiber with an effective area of 650 mu m(2)
Masahiro Kashiwagi, Kunimasa Saitoh, Katsuhiro Takenaga, Shoji Tanigawa, Shoichiro Matsuo, Munehisa Fujimaki
OPTICS LETTERS, 37, 8, 1292, 1294, OPTICAL SOC AMER, Apr. 2012, [Peer-reviewed]
English, Scientific journal, A large-mode-area all-solid photonic bandgap fiber with a seven-cell core and five high-index rod rings is investigated. Numerical simulations show that an effective area of more than 500 mu m(2), a bending loss of less than 0.1 dB/m at a bending radius of 10 cm and effectively single-mode operation can be achieved simultaneously. A core diameter of 44.8 mu m and an effective area of 650 mu m(2) at 1064 nm are achieved in a fabricated fiber. Bending loss at 1064 nm is 0.09 dB/m at a bending radius of 7 cm. Effectively single-mode operation is also realized at a bending radius of 10 cm. (C) 2012 Optical Society of America

Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission
Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Shigeru Tomita, Masanori Koshiba
2012 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2012, 2012
International conference proceedings, An asymmetric parallel waveguide realizes low-loss mode conversion and MUX/DEMUX simultaneously over a wide wavelength range. MDM and WDM transmission is achieved with a low insertion loss of <1.6dB for LP01 and LP11 modes. © 2012 OSA.

Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission
Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Shigeru Tomita, Masanori Koshiba
Optics InfoBase Conference Papers, 2012
International conference proceedings, An asymmetric parallel waveguide realizes low-loss mode conversion and MUX/DEMUX simultaneously over a wide wavelength range. MDM and WDM transmission is achieved with a low insertion loss of <1.6dB for LP01 and LP11 modes. © 2012 OSA.

Highly bendable and effectively single-mode all-solid photonic bandgap fiber with large effective area
Masahiro Kashiwagi, Kunimasa Saitoh, Katsuhiro Takenaga, Shoji Tanigawa, Shoichiro Matsuo, Munehisa Fujimaki
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2012
English, International conference proceedings, Highly bendable large-mode-area all-solid photonic bandgap fibers for high-power fiber lasers are reported. A 712-μm2 effective area, effectively single-mode operation, and bending loss of less than 0.1 dB/m at a 10-cm bending radius are achieved. © 2012 OSA.

Design of ultra-small mode-evolution type polarization rotator based on surface plasmon polariton
Masa-Aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
Integrated Photonics Research, Silicon and Nanophotonics, IPRSN 2012, Optical Society of America (OSA), 2012
English, International conference proceedings, We propose an ultra-small polarization rotator based on a surface plasmon polariton phenomenon. Numerical simulations show that a 5-μm-long polarization rotator with extinction ratio better than -15 dB on the entire C-band is achievable. © 2012 Optical Society of America.

Large-core single-mode solid photonic bandgap fibers
Liang Dong, Kunimasa Saitoh, Fanting Kong, Paul Foy, Thomas Hawkins, Devon Mcclane
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2012
English, International conference proceedings, Mode-area scaling of single-mode fibers is critical to power scaling of fiber lasers. Significantly different guidance principle of solid photonic bandgap fibers provides new design opportunities. Recent progress in this area will be reported. © 2012 OSA.

Design and analysis of heterogeneous trench-assisted multi-core fiber under bending condition　　　　　　　　　　　　　　　
Jiajing Tu, Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
Optics InfoBase Conference Papers, 2012
English, International conference proceedings, Heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) is proposed to achieve larger effective area. The crosstalk value at 1550-nm of Hetero-TA-MCF can be lower than dB after 100-km propagation. © OSA 2012.

Design of doubly coupled resonator optical waveguides
Shuntaro Makino, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Access Networks and In-house Communications, ANIC 2012, JTu5A.9, Optical Society of America (OSA), 2012
English, International conference proceedings, We propose doubly-coupled resonator optical waveguides (D-CROWs). D-CROWs are composed of cascaded ring resonators based on 1-D photonic crystals. We show that D-CROWs realize small group velocity compared with conventional 1-D photonic crystals based CROWs. © 2012 Optical Society of America.

Large-effective-area uncoupled 10-core fiber with two-pitch layout
Y. Sasaki, K. Takenaga, Y. Arakawa, S. Tanigawa, S. Matsuo, K. Saitoh, M. Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2012
English, International conference proceedings, Characteristics of a 10-core fiber with two-pitch layout were investigated. A core pitch was optimized by simulation. The fabricated fiber realized Aeff larger than 115 μm2 and lowcrosstalk characteristics with about 200-μm cladding diameter. © 2012 OSA.

Characterisation of MC fibers: New techniques and challenges
K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba
2012 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2012, IEEE Computer Society, 2012
English, International conference proceedings, The various types of MCFs with very low crosstalk, large Aeff and low micro-bending loss were discussed. A 10-core MCF with two-pitch layout and a moderate cladding diameter was proposed. © 2012 OSA.

Large-Effective-Area Uncoupled Few-Mode Multi-Core Fiber
Yusuke Sasaki, Katsuhiro Takenaga, Ning Guan, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
2012 38TH EUROPEAN CONFERENCE AND EXHIBITION ON OPTICAL COMMUNICATIONS (ECOC), 20, 26, B77, B84, IEEE, 2012, [Peer-reviewed]
English, International conference proceedings, Characteristics of few-mode multi-core fiber (FM-MCF) were numerically analyzed and experimentally confirmed. Fabricated fibers supported two-mode transmission over C-band and L-band with the effective area of about 110 mu m(2) at 1550 nm. The crosstalks of the fibers were estimated to be smaller than -30 dB at 1550 nm after 100-km propagation.

1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) crosstalk-managed transmission with 91.4-b/s/Hz aggregate spectral efficiency
H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A. Matsuura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K. Tsujikawa, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba, T. Morioka
European Conference and Exhibition on Optical Communication, ECEOC 2012, 2012, [Peer-reviewed]
English, International conference proceedings, We demonstrate 1.01-Pb/s transmission over 52 km with the highest aggregate spectral efficiency of 91.4 b/s/Hz by using low-crosstalk one-ring-structured 12-core fiber. Our multi-core fiber and compact fan-in/fan-out devices are designed to support high-order modulation formats up to 32- QAM in SDM transmission. © 2012 OSA.

Practically deployable and effectively single-mode all-solid photonic bandgap fibre with an effective area of 1028 μm2
M. Kashiwagi, K. Saitoh, T. Ichige, S. Tanigawa, S. Matsuo, M. Fujimaki
European Conference and Exhibition on Optical Communication, ECEOC 2012, 2012, [Peer-reviewed]
English, International conference proceedings, Effective area limit for a practically deployable and effectively single-mode all-solid photonic bandgap fibre is investigated. 1028-μm 2 effective area and 0.09 dB/m bending loss at 20-cm bending radius are achieved in a fabricated fibre. © 2012 OSA.

All-solid photonic bandgap fibers for high power lasers
Liang Dong, Kunimasa Saitoh, Fanting Kong, Paul Foy, Thomas Hawkins, Devon McClane, Guancheng Gu
Proceedings of SPIE - The International Society for Optical Engineering, 8547, 2012, [Peer-reviewed]
English, International conference proceedings, There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly modedependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ∼800ïm2 can be made with excellent higher order mode suppression. © 2012 SPIE.

Homogeneous and heterogeneous multi-core fibers
Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
2012 IEEE Photonics Society Summer Topical Meeting Series, PSST 2012, 210, 211, 2012, [Peer-reviewed]
English, International conference proceedings, Reduction of inter-core crosstalk in uncoupled multi-core fibers (MCFs) is the most critical issue for long-haul space division multiplexing transmission. In homogeneous MCFs, the crosstalk is degraded as increasing a bending radius. On the other hand, in heterogeneous MCFs, the crosstalk can be drastically reduced when a bending radius is larger than a specific value. Required coupling coefficient and propagation constant difference between neighboring cores as well as allowable outer cladding diameter and assumed bending radii range are important fiber design parameters for maximizing a relative core density in a MCF. © 2012 IEEE.

Transmission characteristics of crossing slot-waveguides with finite core-height
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 887, 888, 2012, [Peer-reviewed]
English, International conference proceedings, We investigate transmission characteristics of two types of crossing slot-waveguides using three-dimensional finite element method, a direct crossing and a crossing with filled slots. Results show that three-dimensional analysis considering the finite core-height is indispensable. © 2012 IEEE.

A proposal of doubly coupled resonator optical waveguides
Shuntaro Makino, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 635, 636, 2012, [Peer-reviewed]
English, International conference proceedings, We propose doubly-coupled resonator optical waveguides (D-CROWs). D-CROWs are composed by introducing 1-D photonic crystals (PC) structure into the ring resonator based CROWs. We show D-CROWs achieve small group velocity compared with conventional 1-D PC-CROWs. © 2012 IEEE.

Enlargement of effective area of effectively single-mode all-solid photonic bandgap fiber with low bending loss
T. Ichige, K. Saitoh, M. Kashiwagi, K. Takenaga, S. Tanigawa, S. Matsuo, M. Fujimaki
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 789, 790, 2012, [Peer-reviewed]
English, International conference proceedings, Effectively single-mode all-solid photonic bandgap fiber with an effective area of 830 μm 2 and a bending loss of less than 0.1 dB/m at a bending radius of 15-cm has been demonstrated. © 2012 IEEE.

Compact polarization rotator based on surface plasmon porariton with low insertion loss
Masa-Aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 247, 248, 2012, [Peer-reviewed]
English, International conference proceedings, We propose an ultra-small polarization rotator based on a surface plasmon polariton phenomenon. Numerical simulations show that a 10-μm-long polarization rotator with extinction ratio about -20 dB on the entire C-band is achievable. © 2012 IEEE.

Large-effective-area heterogeneous trench-assisted twelve-core fiber under bending condition
Jiajing Tu, Kunimasa Saitoh, Masanori Koshiba, Katsuhiro Takenaga, Shoichiro Matsuo
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 477, 478, 2012, [Peer-reviewed]
English, International conference proceedings, Heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) is proposed to achieve large effective area and high density of cores. The crosstalk value at 1550-nm wavelength of Hetero-TA-MCF can be lower than -30 dB after 100-km propagation. © 2012 IEEE.

A large effective area multi-core fiber with an optimized cladding thickness
Katsuhiro Takenaga, Yoko Arakawa, Yusuke Sasaki, Shoji Tanigawa, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 19, 26, 543, 550, OPTICAL SOC AMER, Dec. 2011, [Peer-reviewed]
English, Scientific journal, The cladding thickness of trench-assisted multi-core fibers was theoretically and experimentally investigated in terms of excess losses of outer cores. No significant micro-bending loss increase was observed on multi-core fibers with the cladding thickness of about 30 mu m. The tolerance for the micro-bending loss of a multi-core fiber is larger than that of the single core fiber. However, the cladding thickness will be limited by the occurrence of the excess loss on outer cores. The reduction of cladding thickness is probably limited around 40 mu m in terms of the excess loss. The multi-core fiber with an effective area of 110 mu m(2) at 1.55 mu m and 181-mu m cladding diameter was realized without any excess loss. (C)2011 Optical Society of America

Multi- core fiber design and analysis: coupled-mode theory and coupled-power theory
Masanori Koshiba, Kunimasa Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo
OPTICS EXPRESS, 19, 26, 102, 111, OPTICAL SOC AMER, Dec. 2011, [Peer-reviewed]
English, Scientific journal, Coupled-mode and coupled-power theories are described for multi-core fiber design and analysis. First, in order to satisfy the law of power conservation, mode-coupling coefficients are redefined and then, closed-form power-coupling coefficients are derived based on exponential, Gaussian, and triangular autocorrelation functions. Using the coupled-mode and coupled-power theories, impacts of random phase-offsets and correlation lengths on crosstalk in multi-core fibers are investigated for the first time. The simulation results are in good agreement with the measurement results. Furthermore, from the simulation results obtained by both theories, it is confirmed that the reciprocity is satisfied in multi-core fibers. (C) 2011 Optical Society of America

Large-effective-area ten-core fiber with cladding diameter of about 200 mu m
Shoichiro Matsuo, Katsuihiro Takenaga, Yoko Arakawa, Yusuke Sasaki, Shoji Taniagwa, Kunimasa Saitoh, Masanori Koshiba
OPTICS LETTERS, 36, 23, 4626, 4628, OPTICAL SOC AMER, Dec. 2011, [Peer-reviewed]
English, Scientific journal, A multicore fiber with two-pitch layout is proposed to overcome the trade-off between core number and a cladding diameter of a standard hexagonal layout with a single-core pitch. A fabricated ten-core fiber simultaneously realizes effective area of about 120 mu m(2) at 1550nm, small crosstalk, and cladding diameter of 204 mu m. The crosstalk between the center core and outer cores is about 30 dB smaller than that between outer cores. The small crosstalk of the center core would help to keep the transmission quality of the center core at the same level as that of the outer cores. (C) 2011 Optical Society of America

Design of Optical XOR, XNOR, NAND, and OR Logic Gates Based on Multi-Mode Interference Waveguides for Binary-Phase-Shift-Keyed Signal
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 29, 18, 2836, 2846, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2011, [Peer-reviewed]
English, Scientific journal, We present a novel design method and potential application for optical XOR, XNOR, NAND, and OR logic gates for binary-phase-shift-keyed signal processing devices. The devices are composed of multi-mode interference waveguides and convert the phase information of the input signal to amplitude at the output. We apply the finite element method for numerical simulations, and the evaluated least ON to OFF logic-level contrast ratios for the XOR, XNOR, NAND, and OR logic gates are 21.5 dB, 21.5 dB, 22.3 dB, and 22.3 dB, respectively. The proposed logic gates are extremely promising signal processing devices for binary-phase-shift-keyed signals in packet switching systems.

Crosstalk behavior of multi-core fiber with structural parameter drift in longitudinal direction
Shoichiro Matsuo, Katsuhiro Takenaga, Yoko Arakawa, Yusuke Sasaki, Shoji Tanigawa, Kunimasa Saitoh, Masanori Koshiba
IEICE ELECTRONICS EXPRESS, 8, 17, 1419, 1424, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Sep. 2011, [Peer-reviewed]
English, Scientific journal, An impact of longitudinal structural parameter drift on crosstalk behavior of multi-core fibers is investigated with a newly proposed semi-analytical method which can easily simulate crosstalk under the various kinds of parameter drifts. The simulation results indicate that the structural parameter drift has an important role in moderating bending diameter dependence of crosstalk of multi-core fibers.

Limitation on Effective Area of Bent Large-Mode-Area Leakage Channel Fibers
Kunimasa Saitoh, Shailendra Varshney, Kaori Sasaki, Lorenzo Rosa, Mrinmay Pal, Mukul Chandra Paul, Debashri Ghosh, Shyamal Kumar Bhadra, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 29, 17, 2609, 2615, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2011, [Peer-reviewed]
English, Scientific journal, We investigate the bending characteristics of leakage channel fibers (LCFs) to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for compact Yb-doped fiber applications. Through numerical simulations, carried by the full-vectorial finite-element method, we present the limitations on the effective area of LCFs under bent condition and compare their limits with that of conventional step-index LMA fibers. Due to a better controllability of the low numerical aperture and a large value of the differential bending loss (similar to 20 dB/m) between the fundamental and higher order modes in LCFs, the LMA of similar to 500 mu m(2) (core diameter of similar to 36 mu m) at 1064 nm can be achieved when the optimized LCF is bent into a 10 cm bending radius.

Recent Progress on Photonic Crystal Fibers
SAITOH Kunimasa, MATSUO Shoichiro
IEEJ Transactions on Sensors and Micromachines, 131, 9, 605, 607, The Institute of Electrical Engineers of Japan, 01 Sep. 2011
Japanese, This article has no abstract.

Design Principle for Realizing Low Bending Losses in All-Solid Photonic Bandgap Fibers
Tadashi Murao, Koyuru Nagao, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 29, 16, 2428, 2435, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Aug. 2011, [Peer-reviewed]
English, Scientific journal, In this paper, the structural dependence of factor which mainly affects a bending loss property is theoretically investigated in all-solid photonic bandgap fibers (PBGFs). A design principle for realizing low bending losses is successfully figured out for the first-order photonic bandgap (PBG). In particular, one of the origins which causes the variation of bending loss property for each structural parameter is identified. In addition, we show that exploitation of a large pitch relative to a rod diameter, aiming to realize a large-mode area (LMA) structure, leads to a significant degradation of the bending loss property. Moreover, it is demonstrated that a V-value which is proposed for all-solid PBGFs is also reduced significantly for the LMA condition. The origin of the degradation is attributed to the newly-excited Bloch state which determines the second-order PBG edge.

A design method of lithium niobate on insulator ridge waveguides without leakage loss
Emi Saitoh, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 19, 17, 15833, 15842, OPTICAL SOC AMER, Aug. 2011, [Peer-reviewed]
English, Scientific journal, We evaluate structural dependency of leakage losses in lithium niobate on insulator ridge waveguides. Generally, shallow ridge waveguides based on isotropic materials have inherent leakage loss for TM-like mode. On the other hand, lithium niobate is anisotropic material, thus the optical properties of lithium niobate based ridge waveguides are different from those of isotopic material based ridge waveguides. In this paper, we investigate leakage losses of lithium niobate on insulator ridge waveguides. We show that the shallow ridge waveguide structure without leakage loss can be realized by choosing the waveguide parameters adequately. (C) 2011 Optical Society of America

エクサへの挑戦:超大容量光ファイバー通信技術の最新動向 超大容量伝送実現に向けたモード多重伝送技術
半澤信智, 齊藤晋聖, 坂本泰志, 松井隆, 冨田茂, 小柴正則
O plus E, 382, 944, 948, Aug. 2011, [Invited]
Japanese

Strong infrared radiation through passive dispersive wave generation and its control
S. Roy, D. Ghosh, S.K. Bhadra, K. Saitoh, M. Koshiba
Applied Optics, 50, 20, 3475, 3481, 10 Jul. 2011, [Peer-reviewed]
English, Scientific journal

Design of ultra compact all-optical XOR and AND logic gates with low power consumption
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
OPTICS COMMUNICATIONS, 284, 14, 3528, 3533, ELSEVIER SCIENCE BV, Jul. 2011, [Peer-reviewed]
English, Scientific journal, We propose a novel ultra-compact all-optical XOR and AND logic gates without using nonlinear optics. In order to realize these devices, we adopt photonic crystal waveguides (PCWs) based on multi-mode interference devices. Numerical results show that the operating bandwidth of the ON to OFF logic-level contrast ratio of not less than 6.79 dB is 35 nm for XOR logic gate and 9 nm for AND logic gate. Proposed logic gates have the potential to be key components for an optical packet switching system due to their small feature sizes and low power consumption. Crown Copyright (c) 2011 Published by Elsevier B.V. All rights reserved.

Strong infrared radiation through passive dispersive wave generation and its control
Samudra Roy, Debashri Ghosh, Shyamal K. Bhadra, Kunimasa Saitoh, Masanori Koshiba
APPLIED OPTICS, 50, 20, 3475, 3481, OPTICAL SOC AMER, Jul. 2011, [Peer-reviewed]
English, Scientific journal, We observe strong infrared (IR) radiation as a result of passive dispersive wave generation for a realistic microstructured fiber having two zero-dispersion wavelengths. The IR radiation frequency can be suitably controlled by varying the operational wavelength, which falls in the first normal dispersion regime. The amplitude of the radiation can be significantly increased by introducing a suitable amount of chirp in the input pulse. This strong phase-matching radiation can be considered as an alternative solution for the IR laser for different applications. (C) 2011 Optical Society of America

Investigation on multi-core fibers with large Aeff and low micro bending loss
Katsunori Imamura, Yukihiro Tsuchida, Kazunori Mukasa, Ryuichi Sugizaki, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 19, 11, 10595, 10603, OPTICAL SOC AMER, May 2011, [Peer-reviewed]
English, Scientific journal, To realize large effective area (Aeff) multi-core fibers (MCFs), the design to suppress the cross-talk and the influence of the cladding diameter on the micro bending loss were investigated. As a result, the MCFs with large Aeff over 100 mu m(2) and low micro bending loss were successfully fabricated. The results indicate the importance of fiber design to realize large Aeff MCFs including fiber diameters, which largely affect the micro bending loss property. Additionally, MCF with large Aeff, low attenuation loss and suppressed cross-talk was successfully realized by optimizing the fiber design. The cross-talk properties could be estimated by the simulation based on the coupling power theory taking the influences of the longitudinal fluctuation of core diameter into account. (C) 2011 Optical Society of America

Dynamics of Raman soliton during supercontinuum generation near the zero-dispersion wavelength of optical fibers
Samudra Roy, Shyamal K. Bhadra, Kunimasa Saitoh, Masanori Koshiba, Govind P. Agrawal
OPTICS EXPRESS, 19, 11, 10443, 10455, OPTICAL SOC AMER, May 2011, [Peer-reviewed]
English, Scientific journal, We observe unique dynamics of Raman soliton during supercontinuum process when an input pulse experiences initially normal group-velocity dispersion with a negative dispersion slope. In this situation, the blue components of the spectrum form a Raman soliton that moves faster than the input pulse and eventually decelerates because of Raman-induced frequency downshifting. In the time domain, the soliton trajectory bends and becomes vertical when the Raman shift ceases to occur as the spectrum of Raman soliton approaches the zero dispersion point. Parts of the red components of the pulse spectrum are captured by the Raman soliton through cross-phase modulation and they travel with it. The influence of soliton order, input chirp and dispersion slope on the dynamics of Raman soliton is discussed thoroughly. (C) 2011 Optical Society of America

Crosstalk behavior of cores in multi-core fiber under bent condition
Shoichiro Matsuo, Katsuhiro Takenaga, Yoko Arakawa, Yusuke Sasaki, Shoji Tanigawa, Kunimasa Saitoh, Masanori Koshiba
IEICE ELECTRONICS EXPRESS, 8, 6, 385, 390, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Mar. 2011, [Peer-reviewed]
English, Scientific journal, Measurement results of bending diameter dependence of the crosstalk of each core in a multi-core fiber are presented. The crosstalk of cores shows different diameter dependence although the difference in core parameters is very small. The behavior is explained by taking into account the change of propagation constant caused by the fluctuation in the diameter and the refractive index of cores. The diameter dependence simulated with coupled-power theory shows the same tendency with the measured crosstalk.

Understanding formation of photonic bandgap edge for maximum propagation angle in all-solid photonic bandgap fibers
Tadashi Murao, Koyuru Nagao, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 28, 3, 453, 461, OPTICAL SOC AMER, Mar. 2011, [Peer-reviewed]
English, Scientific journal, We propose a simple formula that gives the effective refractive index of the first-order photonic bandgap (PBG) edge for the maximum propagation angle, which corresponds to the PBG floor in all-solid photonic bandgap fibers (PBGFs). In particular, based on an antiresonant reflecting optical waveguide (ARROW) theory incorporating a phase relation between resonant modes in low-index regions for perpendicular direction of the light propagation, we show that the effective index of the PBG edge at a wavelength does not depend on the diameter and refractive index of high-index rods, and is largely determined only by the pitch when normalized rod diameters are small. In other words, it has a constant value for the structural parameters with which the PBG emerges at the same wavelength normalized by the pitch. Moreover, the source of rod-diameter and refractive-index dependences on the condition for the formation of the PBG edge is demonstrated when the diameter and the refractive index of high-index rods are effectively large. Finally, the validity of the proposed simple formula, which can be applied for any structural parameters, is verified with the vectorial solver based on the finite-element method (FEM). (C) 2011 Optical Society of America

Polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes
Akira Nagasaki, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 19, 4, 3799, 3808, OPTICAL SOC AMER, Feb. 2011, [Peer-reviewed]
English, Scientific journal, We numerically investigate the polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes, through a full-vector modal solver based on the finite-element method (FEM). Firstly, we investigate the fundamental coupling properties between the core guided light and surface plasmon polaritons (SPPs) excited on the surface of metal wire. Secondly, we show that we can obtain highly polarization-dependent transmission characteristics in PCFs by introducing several metal wires closely aligned into the cladding, and reveal the strongly polarization-dependent coupling properties between the core guided modes and the SPP supermodes, which consist of discrete SPP modes. Finally, we show the importance of arranging the metal wires close to each other for high polarization-dependence. (C) 2011 Optical Society of America

An Investigation on Crosstalk in Multi-Core Fibers by Introducing Random Fluctuation along Longitudinal Direction
Katsuhiro Takenaga, Yoko Arakawa, Shoji Tanigawa, Ning Guan, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
IEICE TRANSACTIONS ON COMMUNICATIONS, E94B, 2, 409, 416, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Feb. 2011, [Peer-reviewed]
English, Scientific journal, The length dependence of the crosstalk in multi-core fibers has been investigated by introducing random fluctuation along longitudinal direction. The power coupling coefficients in the coupled-power theory in heterogeneous multi-core fiber with seven cores were estimated based on consideration of the power coupling coefficients of the homogeneous multi-core fiber. The crosstalk can be quantitatively evaluated by employing coupled-power theory instead of coupled-mode theory.

Design of one-dimensional photonic crystal coupled resonator optical waveguides embedded in air-slot waveguide
Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2011
English, International conference proceedings, We propose a design method of slow-light slot waveguide based on one-dimensional photonic crystal coupled resonator optical waveguides (1-D PC-CROWs). We show that slot waveguides proposed here realize small group velocity and low-loss simultaneously. © 2011 Optical Society of America.

A proposal of air-slot one-dimensional coupled resonator optical waveguides　　　　　　　　　　　　　　　
Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
16th Opto-Electronics and Communications Conference, OECC 2011, 697, 698, 2011
English, International conference proceedings, We propose air-slot waveguides based on one-dimensional photonic crystal coupled resonator optical waveguides (1-D PC-CROWs). Numerical results show that air-slot 1-D PC-CROWs can be a good candidate for nonlinear optical devices. © 2011 National Sun Yat-Sen Univ.

Length dependence of cutoff wavelength of trench-assisted multi-core fiber　　　　　　　　　　　　　　　
Yoko Arakawa, Katsuhiro Takenaga, Shoji Tanigawa, Yusuke Sasaki, Shoichiro Matsuo, Kunimasa Saitoh, Masanori Koshiba
16th Opto-Electronics and Communications Conference, OECC 2011, 256, 257, 2011
English, International conference proceedings, Fiber-length dependence of cutoff-wavelength of trench-assisted multi-core fiber is investigated. It is clarified theoretically and experimentally that the different cutoff-wavelength behavior between cores is associated with confinement losses of higher-order modes. © 2011 National Sun Yat-Sen Univ.

An architecture proposal of reconfigurable optical logic gates for BPSK signal　　　　　　　　　　　　　　　
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
16th Opto-Electronics and Communications Conference, OECC 2011, 699, 700, 2011
English, International conference proceedings, We propose architecture of reconfigurable optical logic gates for BPSK signal. The logic gates operate as NAND, OR, XOR, and XNOR by changing phase of input-lights appropriately and produce high contrast ratio. © 2011 National Sun Yat-Sen Univ.

Multiple resonant coupling mechanism for suppression of higher-order modes in all-solid photonic bandgap fibers with heterostructured cladding
Tadashi Murao, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 19, 3, 1713, 1727, OPTICAL SOC AMER, Jan. 2011, [Peer-reviewed]
English, Scientific journal, In this paper, we propose a novel mechanism for suppression of higher-order modes (HOMs), namely multiple resonant coupling, in all-solid photonic bandgap fibers (PBGFs) with effectively large core diameters. In an analogy to the well-known tight-binding theory in solid-state physics, multiple anti-resonant reflecting optical waveguide (ARROW) modes bound in designedly arranged defects in the cladding make up Bloch states and resultant photonic bands with a finite effective-index width, which contribute to the suppression of HOMs. In particular, contrary to the conventional method for the HOM suppression using the index-matching of the HOMs in the core of the PBGF and the defect mode arranged in the cladding, the proposed mechanism guarantees a broadband HOM suppression without a precise structural design. This effect is explained by the multiple resonant coupling, as well as an enhanced confinement loss mechanism which occurs near the condition satisfying the multiple resonant coupling. Moreover, we show that the proposed structure exhibits a lower bending loss characteristic when compared to the conventional all-solid PBGFs. The simultaneous realization of the single-mode operation and the low bending loss property is due to the novel cladding concept named as heterostructured cladding. The proposed structure also resolves the issue for the increased confinement loss property in the first-order photonic bandgap (PBG) at the same time. (C) 2011 Optical Society of America

Design of large-mode-area microstructured fibers with low bending loss for fiber laser applications
Kunimasa Saitoh
2011 IEEE Winter Topicals, WTM 2011, 187, 188, 2011, [Peer-reviewed]
English, International conference proceedings, Bending characteristics of microstructured fibers are investigated to achieve large-mode-area and effectively single-mode operation with a practically allowable bending radius for Yb-doped fiber applications, by enhancing differential bending loss between the fundamental and higher-order modes. © 2010 IEEE.

Case study of inter-core crosstalk of 7-core fiber depending on bending diameter
Yoshinari Awaji, Jun Sakaguchi, Naoya Wada, Kunimasa Saitoh
2011 IEEE Photonics Society Summer Topical Meeting Series, 135, 136, 2011, [Peer-reviewed], [International Magazine]
English, International conference proceedings, Multi-core fiber is one of powerful candidate for huge capacity optical link which can be adopted for LANPHY of terabit Ethernet. We characterized inter-core crosstalk depending on bending diameter for 7-core fibers, experimentally and numerically. Heterogeneous layout could suppress crosstalk at larger bending diameter, effectively. © 2011 IEEE.

Effective area limit of large-mode-area solid-core photonic bandgap fibers for fiber laser applications
Kunimasa Saitoh, Tadashi Murao, Lorenzo Rosa, Masanori Koshiba
OPTICAL FIBER TECHNOLOGY, 16, 6, 409, 418, ELSEVIER SCIENCE INC, Dec. 2010, [Peer-reviewed], [Invited]
English, Scientific journal, We investigate the bending characteristics of solid-core photonic bandgap fibers (SC-PBGFs) aiming to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for high-power Yb-doped fiber lasers and amplifiers. Through detailed numerical simulations based on the finite element method (FEM), we evaluate the impacts of the order of photonic bandgap on the bending performance and point out the limits of core size enlargement in the SC-PBGFs with a 1-cell core structure due to the increment of bending loss. In addition, under practical constraints, we find that the SC-PBGFs having a 7-cell core can achieve sufficient differential bending loss between the fundamental mode and the higher-order modes and a much larger effective area limit as compared with previously-reported index-guiding LMA fibers. (C) 2010 Elsevier Inc. All rights reserved.

Analysis of Leakage Losses in One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguide Using 3-D Finite Element Method
Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 28, 20, 2977, 2983, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2010, [Peer-reviewed]
English, Scientific journal, We have evaluated leakage losses of one-dimensional photonic crystal coupled resonator optical waveguides (1-D PC-CROW) by using three-dimensional finite element method (3-D FEM). We propose structure of 1-D PC-CROW with shifted position of air-hole, changed size of air-hole, and changed air-hole depth. We have also proposed air-bridge 1-D PC-CROW and evaluated group velocity and leakage losses of these CROWs. Our numerical results show that 1-D PC-CROW has large leakage losses, especially for vertical direction, and the proposed structure can reduce leakage losses. Leakage loss of 1-D PC-CROW with changed air-hole depth is two order of magnitude lower compared with normal structure.

Evaluation of Bending and Coupling Characteristics in One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides
KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J93-C, 7, 224, 230, The Institute of Electronics, Information and Communication Engineers, Jul. 2010, [Peer-reviewed]
Japanese, Scientific journal, 一次元フォトニック結晶結合共振器型光導波路（1-D PC-CROW）は,低群速度や製造が容易であるという特徴を有していることから, 超小型光波回路構成のための基本素子として注目されている. こうした1-D PC-CROWを用いて実際に光デバイスを構成する際には, 導波路曲がりや結合器が必要となるが, これまで, これらの曲げ特性や結合特性に関する評価は行われていない. そこで本論文では, 有限要素法に基づく数値解析によってこうした諸特性を明らかにする. 導波路曲がりを構成するパラメータを変化させた場合の透過特性を調べることにより, 高い透過率が得られる1-D PC-CROWの曲げ導波路構造の探索を行う. 更に, 1-D PC-CROWに基づく結合器についても検討を行い, 3dB方向性結合器の設計方法を示す.

A design method of a fiber-based mode multi/demultiplexer for mode-division multiplexing
Fumiya Saitoh, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 18, 5, 4709, 4716, OPTICAL SOC AMER, Mar. 2010, [Peer-reviewed]
English, Scientific journal, In the mode-division multiplexing (MDM) optical transmission system, a mode multi/demultiplexer is an important key device for excitation, multiplication, and separation of light signals which have distinct modes. In this report, we propose a fiber-type mode multi/demultiplexer based on selective phase matching between different cores/modes. Design method and device characteristics of 1x4 mode multi/demultiplexer are investigated through finite element analysis. In order to expand operating wavelength range, we reveal the structural parameters that satisfy phase matching conditions over wide wavelength range. Our numerical results demonstrate that the mode multi/demultiplexer with broadband, polarization-insensitive operation can be realized by applying the proposed fiber structure. (C) 2010 Optical Society of America

Three-dimensional vector finite element analysis of leakage losses in one-dimensional photonic crystal coupled resonator optical waveguides
Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2010
English, International conference proceedings, We evaluated leakage losses of one-dimensional photonic-crystal coupled-resonator-optical waveguides (1-D PC-CROW). We show design methods of low-loss 1-D PC-CROW and leakage losses of proposed structure are one order of magnitude lower than normal 1-D PC-CROW. © 2010 Optical Society of America.

Highly-nonlinear horizontal slot waveguides with low and flat dispersion　　　　　　　　　　　　　　　
Masa-Aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, 2010
English, International conference proceedings, We present an optimum design of highly-nonlinear horizontal slot waveguides with flat dispersion characteristics. Numerical simulations show that 6000 W-1m-1 nonlinear coefficient and flat dispersion on a 260-nm bandwidth can be achieved. © 2010 Optical Society of America.

Realistic squared-rods circular F-doped large-mode-area leakage channel fibers with low bending loss　　　　　　　　　　　　　　　
Lorenzo Rosa, Kunimasa Saitoh, Masanori Koshiba, Mrinmay Pal, Mukul Paul, Debashri Ghosh, Tarun Mahanty, Shyamal Bhadra, Luca Vincetti, Stefano Selleri
Optics InfoBase Conference Papers, 2010
English, International conference proceedings, We investigate realistic all-glass large-mode-area leakage channel fibers (LCFs) with a single down-doped-silica circular flattened-rod ring, engineering the silica bridge width for effectively single-mode behavior and reduced bending loss. © 2010 Optical Society of America.

Design of all-optical XOR and AND logic gates based on multi-mode interference devices
Yuhei Ishizaka, Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
Proceedings - 10th International Conference on Laser and Fiber-Optical Networks Modeling, LFNM 2010, 53, 55, 2010, [Peer-reviewed]
English, International conference proceedings, We propose ultra-compact all-optical XOR and AND logic gates based on multi-mode interference (MMI) without using nonlinear optics. Numerical simulations show that the least ON to OFF logic-level contrast ratio is 13.3 dB for XOR logic gate and 4.3 dB for AND logic gate in the whole C-band. © 2010 IEEE.

Generalized Simple Theory for Estimating Lateral Leakage Loss Behavior in Silicon-on-Insulator Ridge Waveguides
Kuniaki Kakihara, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 27, 23, 5492, 5499, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 2009, [Peer-reviewed]
English, Scientific journal, In this paper, we numerically investigate the lateral leakage loss behavior for TM-like modes in silicon-on-insulator ridge waveguides. If the waveguide width is set to satisfy a certain condition, the leakage loss of the TM-like mode leads to be a minimum. When utilizing TM-like modes in the ridge waveguides, it is the main drawback that the tolerance for the variations in the waveguide width and operation wavelength is too small. In this paper, we propose a novel ridge waveguide structure, where a dimple is introduced at the ridge region. It is shown from the finite-element analysis that the ridge waveguide with a dimple is both low loss and fabrication tolerant.

Design of Thermo-Tunable Couplers Based on Solid-Core Photonic Band-Gap Fibers
YOSHIDA Ryuichi, TSUCHIDA Yukihiro, MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J92-C, 11, 722, 730, The Institute of Electronics, Information and Communication Engineers, Nov. 2009, [Peer-reviewed]
Japanese, Scientific journal, 本論文では,屈折率が温度によって変化する高屈折率媒質をクラッド領域の空孔に注入した,ソリッドコアフォトニックバンドギャップファイバ(PBGF)を用いた低偏波依存性温度制御型分岐比可変カプラを提案している.はじめに,フォトニックバンドギャップ,及び基本モードの実効屈折率の温度依存性をフルベクトル有限要素法により評価し,ソリッドコアPBGFによる温度制御型分岐比可変カプラが実現可能であることを示している.更に,x偏波,y偏波における結合長,及び規格化出射パワーの構造パラメータ依存性を評価し,ソリッドコアPBGFによる低偏波依存性温度制御型分岐可変カプラの設計法を提案している.

Design of miniaturized silicon wire and slot waveguide polarization splitter based on a resonant tunneling
Masa-aki Komatsu, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 17, 21, 19225, 19234, OPTICAL SOC AMER, Oct. 2009, [Peer-reviewed]
English, Scientific journal, We propose an ultra-small polarization splitter based on a resonant tunneling phenomenon. This polarization splitter consists of two identical horizontally oblong silicon wire waveguides separated by a vertical slot waveguide. The structural parameters of the central resonant slot waveguide are designed to couple only the TM-like mode between the left and right side silicon wire waveguides. Results from numerical simulation with the full-vectorial beam propagation method show that a 16-mu m-long polarization splitter with extinction ratio better than - 20 dB on the entire C-band is achieved. (C) 2009 Optical Society of America

Design of Ultra-Small Polarization Splitter Based on Silicon Wire Waveguides
KOMATSU Masa-aki, KAKIHARA Kuniaki, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J92-C, 9, 509, 515, The Institute of Electronics, Information and Communication Engineers, Sep. 2009, [Peer-reviewed]
Japanese, Scientific journal, 本論文では,シリコン細線導波路を用いた新しいタイプの偏波スプリッタを提案している.ここで提案している共振結合型方向性結合器に基づく偏波スプリッタは,三つのシリコン細線導波路により構成され,左右の導波路は入出力導波路として,中央の導波路は共振器として用いられる.左右の導波路と中央の導波路において,準TMモードの実効屈折率を一致させるとともに,準TEモードの実効屈折率が大きく異なるように中央の導波路の高さと幅を設定することにより,消光比の優れた偏波スプリッタを構成することができる.フルベクトル有限要素法を用いてシリコン細線導波路の実効屈折率の構造依存性を評価するとともに,ベクトル有限要素ビーム伝搬法を用いてこの偏波スプリッタの偏波分離特性を評価し,12μm程度のデバイス長でCバンド帯全域において消光比20dB以上を有する超小形偏波スプリッタが設計可能であることを示している.

Design of Taper Structure for Highly Efficient Coupling Between 1-D Photonic Crystal Coupled Resonator Optical Waveguide and Straight Waveguide
Yuki Kawaguchi, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 27, 14, 2924, 2929, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2009, [Peer-reviewed]
English, Scientific journal, This paper presents a design method of a taper structure for highly efficient coupling between 1-D photonic crystal coupled resonator optical waveguides (1-D PC-CROWs) and input straight waveguides. We propose a new taper structure where not only air hole radius but also waveguide width are varied linearly in order to adjust the dispersion curves shift. By using the proposed tapered structure, we can connect each waveguide with high transmission over wide bandwidth. Our numerical simulation results show that a transmission of 98% around 1550 nm wavelength in a 6.6 mu m long taper can be obtained with a 42 nm bandwidth.

Genetic-Algorithm Assisted Design of C-Band CROW-Miniaturized PCW Interleaver
Lorenzo Rosa, Kunimasa Saitoh, Kuniaki Kakihara, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 27, 14, 2678, 2687, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2009, [Peer-reviewed]
English, Scientific journal, We investigate the design of a miniaturized C-band photonic-crystal waveguide (PCW) interleaver, employing coupled-resonator optical waveguide ( CROW) ring resonators. An optimization approach based on a genetic algorithm (GA) has been applied to the filter components, such as tapers and directional couplers, leading to low reflection and high phase linearity. To evaluate the interleaver transmittivity function, an matrix approach is worked out, based on the optimized building blocks. A sensitivity analysis is performed to evaluate the effect of parameter tolerances.

Photonic bandgap fibers with resonant structures for tailoring the dispersion
Z. Varallyay, K. Saitoh, A. Szabo, R. Szipocs
OPTICS EXPRESS, 17, 14, 11869, 11883, OPTICAL SOC AMER, Jul. 2009, [Peer-reviewed]
English, Scientific journal, Numerical simulations on different kinds of realistic photonic bandgap fibers exhibiting reversed dispersion slope for the propagating fundamental mode are reported. We show that reversed or flat dispersion functions in a wide wavelength range using hollow-core, air-silica photonic bandgap fibers and solid core Bragg fibers with step-index profile can be obtained by introducing resonant structures in the fiber cladding. We evaluate the dispersion and confinement loss profiles of these fibers from the Helmholtz eigenvalue equation and the calculated fiber properties are used to investigate the propagation of chirped femtosecond pulses through serially connected hollow core fiber compressors. (C) 2009 Optical Society of America

Design of S-Band Erbium-Doped Concentric Dual-Core Photonic Crystal Fiber Amplifiers With ASE Suppression
Shailendra Kumar Varshney, Kunimasa Saitoh, Masanori Koshiba, Bishnu P. Pal, Ravindra K. Sinha
JOURNAL OF LIGHTWAVE TECHNOLOGY, 27, 11, 1725, 1733, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 2009, [Peer-reviewed]
English, Scientific journal, In this paper, we theoretically design and numerically demonstrate a large mode area and single-mode erbium-doped photonic crystal fiber (PCF) amplifier operating in the S-band with a complete suppression of amplified spontaneous emission (ASE) and very low Raman gain coefficient at 980-nm pump. The proposed fiber design is based on a dual-concentric core refractive index profile and is solved through full-vectorial finite-element method. Numerical simulations reveal that more than 50 dB of gain can be achieved with a mean gain value of 27 dB over 70-nm bandwidth in a 7.2-m-long fiber. The effect of bending on the amplification characteristics has been noted. It has been shown that by a proper choice of the PCF profile parameters, the fiber amplifier can be made bend-insensitive for a bending radius as small as 5 cm. Further, our design guidelines should be useful to achieve a functional bend-insensitive fiber amplifier, where rack space is a premium and tight bending radius becomes inevitable.

Coupling Characteristics of Multicore Photonic Crystal Fiber-Based 1 x 4 Power Splitters
Shailendra K. Varshney, Kunimasa Saitoh, Ravindra K. Sinha, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 27, 12, 2062, 2068, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 2009, [Peer-reviewed]
English, Scientific journal, A new design of multicore photonic crystal fibers (PCFs) is proposed and investigated through full-vectorial finite-element method and finite-element beam propagation method. The fiber design comprises four identical cores surrounding a central core. The optical power launched into the central core is equally divided into other neighboring four cores with a 25% of coupling ratio. The coupled-mode analysis is also carried out to understand the supermode patterns and the coupling characteristics. Through numerical simulations, it is demonstrated that the optical power can be divided equally in a 5.8-mm-long multicore PCF. The power coupling characteristics obtained through coupled-mode analysis are in very good agreement with those calculated from beam propagation method solver.

Detailed theoretical investigation of bending properties in solid-core photonic bandgap fibers
Tadashi Murao, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 17, 9, 7615, 7629, OPTICAL SOC AMER, Apr. 2009, [Peer-reviewed]
English, Scientific journal, In this paper, detailed properties of bent solid-core photonic bandgap fibers (SC-PBGFs) are investigated. We propose an approximate equivalent straight waveguide (ESW) formulation for photonic bandgap (PBG) edges, which is convenient to see qualitatively which radiation (centripetal or centrifugal radiation) mainly occurs and the impact of bend losses for an operating wavelength. In particular, we show that cladding modes induced by bending cause several complete or incomplete leaky mode couplings with the core mode and the resultant loss peaks. Moreover, we show that the field distributions of the cladding modes are characterized by three distinct types for blue-edge, mid-gap, and red-edge wavelengths in the PBG, which is explained by considering the cladding Bloch states or resonant conditions without bending. Next, we investigate the structural dependence of the bend losses. In particular, we demonstrate the bend-loss dependence on the number of the cladding rings. Finally, by investigating the impacts of the order of PBG and the core structure on the bend losses, we discuss a tight-bending structure. (C) 2009 Optical Society of America

Design of effectively single-mode leakage channel fibers with large mode area and low bending loss
Kunimasa Saitoh, Yukihiro Tsuchida, Masanori Koshiba
IEICE ELECTRONICS EXPRESS, 6, 7, 412, 417, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Apr. 2009, [Peer-reviewed]
English, Scientific journal, Novel design of large-mode-area leakage channel fibers (LCFs) with a single air-hole ring surrounding a multi-unit-cell solid core is investigated for realizing robust single-mode and low bending loss characteristics. We perform detailed numerical simulations using a finite element method, and we show that the proposed LCFs with a solid core formed by multiple missing neighboring air-holes can achieve much lower bending losses compared with the reported LCF with six large air-holes.

Design of all-solid leakage channel fibers with large mode area and low bending loss
Kunimasa Saitoh, Yukihiro Tsuchida, Lorenzo Rosa, Masanori Koshiba, Federica Poli, Annamaria Cucinotta, Stefano Selleri, Mrinmay Pal, Mukul Paul, Debashri Ghosh, Shyamal Bhadra
OPTICS EXPRESS, 17, 6, 4913, 4919, OPTICAL SOC AMER, Mar. 2009, [Peer-reviewed]
English, Scientific journal, We investigate a novel design for all-solid large mode area (LMA) leakage channel fibers (LCFs) for high-power Yb-doped fiber lasers and amplifiers, based on a single down-doped-silica rod ring surrounding a seven-cell pure-silica core, aiming for effectively single-mode behavior and low bending loss characteristics. Through detailed numerical simulations based on the finite element method (FEM), we find that the proposed all-solid LMA-LCFs, having a seven-cell core and two different sizes of down-doped rods, can achieve sufficient differential mode loss and much lower bending loss, as compared with a previously-reported LCF with a one-cell core and six large down-doped-silica rods. (C) 2009 Optical Society of America

A Design Method for Thermo-Tunable Polarization Controllers Based on Photonic Crystal Fibers Filled with Liquid Crystals
YOSHIDA Ryuichi, TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J92-C, 2, 39, 46, The Institute of Electronics, Information and Communication Engineers, Feb. 2009, [Peer-reviewed]
Japanese, Scientific journal, 本論文では,液晶を注入したフォトニック結晶ファイバ(PCF)を用いた新しい温度制御型偏波コントローラを提案している.はじめに,コア周囲の一部の空孔に液晶を注入したPCFに対してフルベクトル有限要常法を適用し,コア周囲の空孔直径が偏波コントローラとしての特性を知る上で重要なパラメータであるハイブリッド性に与える影響を評価している.また,室温において高いハイブリッド性を有するPCFにおけるハイブリッド性の温度依存性,及び特定の直線偏波に変換するのに必要なデバイス長である半ビート長を評価することで,実効的に単一モードで動作するPCF型偏波コントローラの構造を特定している.更に,ベクトル有限要素ビーム伝搬法を用いて偏波変換特性を評価し,入射直線偏波の偏向角度の許容範囲が広いPCF構造を特定するとともに,偏波変換効率の波長依存性を評価することで,Cバンド全域をカバーし,かつ温度制御が容易なPCFを用いた高効率な温度制御型偏波コントローラの最適構造を提案している.

Effect of rod inaccuracy on genetic-algorithm-designed C-band photonic-crystal waveguide interleavers　　　　　　　　　　　　　　　
Lorenzo Rosa, Kunimasa Saitoh, Kuniaki Kakihara, Masanori Koshiba
Optics InfoBase Conference Papers, 2009
English, International conference proceedings, We investigate the effect of construction inaccuracies on C-band photonic-crystal waveguide (PCW) interleavers with flattened pass-band, designed through the employment of single-and multi-objective genetic algorithm (GA) approaches. © 2009 Optical Society of America.

Heterogeneous multi-core fibers: proposal and design principle
Masanori Koshiba, Kunimasa Saitoh, Yasuo Kokubun
IEICE ELECTRONICS EXPRESS, 6, 2, 98, 103, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Jan. 2009, [Peer-reviewed]
English, Scientific journal, A new type of optical fiber called heterogeneous multi-core fiber (heterogeneous MCF) is proposed towards future large-capacity optical-transport networks and the design principle is described. In the heterogeneous MCF, not only identical but also non-identical cores, which are single-mode in isolation of each other, are arranged so that cross-talk between any pair of cores becomes sufficiently small. As the maximum power transferred between non-identical cores goes down drastically, cores are more closely packed in definite space, compared to a conventional, homogeneous multi-core fiber (homogeneous MCF) composed of only identical cores.

Hollow-Core Photonic Bandgap Fibers with Broadband Negative Dispersion Slope
K. Saitoh, Z. Varallyay, K. Kakihara, M. Koshiba, R. Szipocs
2009 CONFERENCE ON LASERS AND ELECTRO-OPTICS AND QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2009), VOLS 1-5, 3093, +, IEEE, 2009, [Peer-reviewed]
English, International conference proceedings, Hollow-core photonic bandgap fibers exhibiting negative dispersion slope over a wide wavelength range around one micron for the fundamental air-core mode are presented by the introduction of partial reflector layer around the core. (C) 2009 Optical Society of America

Reversed dispersion slope photonic bandgap fibers and femtosecond pulse propagation
Z. Varallyay, K. Saitoh, A. Szabo, K. Kakihara, M. Koshiba, R. Szipocs
OFC: 2009 CONFERENCE ON OPTICAL FIBER COMMUNICATION, VOLS 1-5, 10, +, IEEE, 2009, [Peer-reviewed]
English, International conference proceedings, Reversed dispersion slope PBG fibers are presented around one micron by the introduction of partial reflector layer around the core resulting in a strongly wavelength dependent nonlinear phase shift on the propagating fundamental mode. (C)2008 Optical Society of America

Raman amplification characteristics of As(2)Se(3) photonic crystal fibers
Shailendra K. Varshney, Kunimasa Saitoh, Kento Iizawa, Yukihiro Tsuchida, Masanori Koshiba, R. K. Sinha
OPTICS LETTERS, 33, 21, 2431, 2433, OPTICAL SOC AMER, Nov. 2008, [Peer-reviewed]
English, Scientific journal, We present the dispersion and Raman amplification characteristics of As(2)Se(3) photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As(2)Se(3) fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 mu m in wavelength with an input power of 500 mW. (C) 2008 Optical Society of America

Loss Reduction Mechanism for Coupled Cavity Waveguides in One-Dimensional Photonic Crystals
Yuki Kawaguchi, Naoya Kono, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 26, 17-20, 3461, 3467, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2008, [Peer-reviewed]
English, Scientific journal, This paper presents a loss reduction mechanism for coupled cavity waveguides; (CCWs) relying on 1-D photonic crystals microcavities. The mechanism is based on structural modification in the vicinity of the cavity defects. With appropriate modifications, the tangential wavenumber components of the leaky guided modes, which can couple to cladding modes, can be decreased, resulting in the reduction of propagation losses due to diffraction. We numerically demonstrate the usefulness of this mechanism through the design of low loss CCWs. The diffraction losses in the CCWs, where air holes adjacent to the defects are changed in size, decrease dramatically compared with CCWs with air holes of constant size. This loss reduction mechanism can be applied even to the CCWs with an increased number of intercavity air holes, while the group velocity can be further decreased.

Reversed dispersion slope photonic bandgap fibers for broadband dispersion control in femtosecond fiber lasers
Z. Varallyay, K. Saitoh, J. Fekete, K. Kakihara, M. Koshiba, R. Szipocs
OPTICS EXPRESS, 16, 20, 15603, 15616, OPTICAL SOC AMER, Sep. 2008, [Peer-reviewed]
English, Scientific journal, Higher-order-mode solid and hollow core photonic bandgap fibers exhibiting reversed or zero dispersion slope over tens or hundreds of nanometer bandwidths within the bandgap are presented. This attractive feature makes them well suited for broadband dispersion control in femtosecond pulse fiber lasers, amplifiers and optical parametric oscillators. The canonical form of the dispersion profile in photonic bandgap fibers is modified by a partial reflector layer/interface placed around the core forming a 2D cylindrical Gires-Tournois type interferometer. This small perturbation in the index profile induces a frequency dependent electric field distribution of the preferred propagating higher-order-mode resulting in a zero or reversed dispersion slope. (C) 2008 Optical Society of America

Reduced lateral leakage losses of TM-like modes in silicon-on-insulator ridge waveguides
Masanori Koshiba, Kuniaki Kakihara, Kunimasa Saitoh
OPTICS LETTERS, 33, 17, 2008, 2010, OPTICAL SOC AMER, Sep. 2008, [Peer-reviewed]
English, Scientific journal, We numerically investigate the lateral leakage loss behavior for TM-like modes in silicon-on-insulator ridge waveguides. In order to improve the leakage loss properties, we propose a novel ridge waveguide structure where a dimple is introduced at the ridge center. It is shown that the ridge waveguide with a dimple is both low loss and fabrication tolerant. This behavior is predicted by not only an accurate finite-element-based analysis but also a simple, phenomenological effective-index-based analysis. (c) 2008 Optical Society of America.

Full-vectorial coupled mode theory for the evaluation of macro-bending loss in multimode fibers. application to the hollow-core photonic bandgap fibers
Maksim Skorobogatiy, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 16, 19, 14945, 14953, OPTICAL SOC AMER, Sep. 2008, [Peer-reviewed]
English, Scientific journal, In the hollow core photonic bandgap fibers, modal losses are strongly differentiated, potentially enabling effectively single mode guidance. However, in the presence of macro-bending, due to mode coupling, power in the low-loss mode launched into a bend is partially transferred into the modes with higher losses, thus resulting in increased propagation loss, and degradation of the beam quality. We show that coupled mode theory formulated in the curvilinear coordinates associated with a bend can describe correctly both the bending induced loss and beam degradation. Suggested approach works both in absorption dominated regime in which fiber modes are square integrable over the fiber crossection, as well as in radiation dominated regime in which leaky modes are not square integrable. It is important to stress that for multimode fibers, full-vectorial coupled mode theory developed in this work is not a simple approximation, but it is on par with such "exact" numerical approaches as finite element and finite difference methods for prediction of macro-bending induced losses. (C) 2008 Optical Society of America.

A Design Method for Single-Polarization Holey Fibers With Improved Beam Quality Factor
Yukihiro Tsuchida, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 26, 13-16, 2162, 2167, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2008, [Peer-reviewed]
English, Scientific journal, Recently, there has been an increasing interest in developing single polarization fibers for applications such as polarization splitters, polarizers, high power lasers, etc. Usually, the polarization maintenance can be achieved by form birefringence, such as by creating elliptical cores, or material anisotropy, or by stress-applying parts inside the fiber. However, this results into the non-Gaussian field distributions due to the distortion of fiber core, therefore leading to the degradation of beam quality factor M-2. In this study, we propose a design method for single-polarization holey fibers with improved beam quality by introducing the resonant coupling phenomenon between the central depressed core and the elliptical cores arranged on either side of the central core. The proposed single-polarization holey fiber with 1-m length has the extinction ratio of 30 dB over 10-nm wavelength range and a good beam quality factor M-2 = 1.31.

Strategies for realizing photonic crystal fiber bandpass filters
S. K. Varshney, K. Saitoh, N. Saitoh, Y. Tsuchida, M. Koshiba, R. K. Sinha
OPTICS EXPRESS, 16, 13, 9459, 9467, OPTICAL SOC AMER, Jun. 2008, [Peer-reviewed]
English, Scientific journal, Numerical design strategies are presented to achieve efficient broad or narrow band-pass filters based on index-guiding, solid-core, and single-mode photonic crystal fibers (PCFs). The filtering characteristics have been verified through BPM solver. By scaling the pitch constant, the bandpass window can be shifted accordingly. The fiber design constitutes a fluorine-doped central core, enlarged air-holes surrounding the down-doped core, and small air-holes in the cladding. The proposed bandpass filter is based on controlling the leakage losses, so one can tune filter characteristics simply by changing its length. From numerical simulations we show that for large values of air-hole diameter in the first ring, the bandpass window is narrow, while for low doping concentration and small sized air-holes in the first ring, bandpass window is very broad. We also simulate how the hole-size and number of rings in the PCF cladding affects the device characteristics. We find that a 5-cm long PCF with down-doped core and eleven rings of air-holes can result in similar to 440 nm 3-dB bandwidth with more than 90% of transmission. The longer device has reduced transmission and smaller 3-dB bandwidth. Tolerance analysis has also been performed to check the impact of fiber tolerances on the performance of the PCF bandpass filter. It has been observed that the decrement in cladding hole-diameter by 1% reduces the transmission to 21% from its peak value of 93%, however +/- 1% tolerance in the inner hole-diameter degrades the transmission to 75% from its peak. (C) 2008 Optical Society of America.

Structural optimization of air-guiding photonic bandgap fibers for realizing ultimate low loss waveguides
Tadashi Murao, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 26, 9-12, 1602, 1612, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2008, [Peer-reviewed]
English, Scientific journal, In this paper, we investigate the ultimate low loss property for several realistic core shapes in triangular-type air-guiding photonic bandgap fibers (PBGFs) through a full-vector modal solver based on the finite element method. We show that the surface mode free condition is expressed as a normalized silica-ring thickness T = 0.5 for any core size and the cladding structural parameters, regardless the core radius of the silica-ring is one of the main factors of dominating the surface mode's condition, and the wavelength range of the PBG changes on varying the structural parameters of the cladding. Moreover, we propose a novel type of PBGF without surface mode, which exhibits lower scattering losses caused by surface roughness of the silica-ring in comparison to 19 cell-core PBGFs and suppresses the mode coupling between fundamental-like and higher order modes when compared to 37 cell-core PBGFs.

Unique characteristic features of stimulated Brillouin scattering in small-core photonic crystal fibers
John E. McElhenny, Radha K. Pattnaik, Jean Toulouse, Kunimasa Saitoh, Masanori Koshiba
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 25, 4, 582, 593, OPTICAL SOC AMER, Apr. 2008, [Peer-reviewed]
English, Scientific journal, We present extensive stimulated Brillouin scattering (SBS) results from experiments and modeling for four different photonic crystal fibers (PCFs) with core diameters ranging from 8 to 1.7 mu m. These results reveal several SBS characteristic features of small-core PCFs, high thresholds, and acoustic peaks, which are due to their antiguiding nature and highly multimode acoustic character. The nature of what we believe to be new acoustic modes is examined in the light of the large variations observed in the Brillouin gain, Brillouin threshold, and Brillouin shift with decreasing core diameter and optical wavelength. (C) 2008 Optical Society of America.

Tunable photonic crystal fiber couplers with a thermo-responsive liquid crystal resonator
Kunimasa Saitoh, Nikolaos J. Florous, Shailendra K. Varshney, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 26, 5-8, 663, 669, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2008, [Peer-reviewed]
English, Scientific journal, We theoretically address the thermo-optical response of multicore photonic crystal fiber (PCF) couplers infiltrated with nematic liquid crystals (LCs). The proposed PCF coupler consists of two identical cores separated by a third one which acts as a liquid crystal resonator. With an appropriate choice of the design parameters associated with the liquid crystal core, phase matching at a single wavelength can be achieved, thus enabling thermo-tunable narrow-band resonant directional coupling between the input and the output cores. The verification of the proposed coupler design is ensured through an accurate PCF analysis based on finite element and beam propagation methods. The enhanced thermo-optical properties of LC-based PCF couplers are highly attractive for photo-thermal sensing applications.

Single-mode large-mode-area leakage channel fibers with octagonal symmetry
L. Rosa, K. Saitoh, Y. Tsuchida, S. K. Varshney, M. Koshiba, F. Poli, D. Passaro, A. Cucinotta, S. Selleri, L. Vincetti
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2008
English, International conference proceedings, A novel design for large-mode-area leakage channel fibers with a single ring of air- holes organized with octagonal symmetry has been proposed, obtaining lower bending loss and guided mode distortion with respect to hexagonal symmetry fibers. © 2008 Optical Society of America.

Design and optimization of devices for C-band photonic-crystal waveguide interleaver with flattened pass-band　　　　　　　　　　　　　　　
Lorenzo Rosa, Kunimasa Saitoh, Kuniaki Kakihara, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America, 2008
English, International conference proceedings, We investigate the optimization of a C-band photonic-crystal waveguide (PCW) interleaver with flattened pass-band, using coupled-resonator optical waveguides (CROW). A genetic algorithm (GA) is employed for optimizing the filter devices for high transmittivity and flatness. © 2008 Optical Society of America.

Bend-insensitive lasing characteristics of single-mode, large-mode-area Ytterbium-doped photonic crystal fiber
K. Iizawa, S. K. Varshney, Y. Tsuchida, K. Saitoh, M. Koshiba
OPTICS EXPRESS, 16, 2, 579, 591, OPTICAL SOC AMER, Jan. 2008, [Peer-reviewed]
English, Scientific journal, The bend-insensitive lasing characteristics of a newly designed ytterbium-doped photonic crystal fiber (YPCF) are evaluated numerically. The designed YPCF remains single-mode and possesses large-mode-area of 1400 mu m(2) at 1064 nm wavelength with the beam quality factor (M-2) of 1.15, suggesting a diffraction-limited and continuous-wave lasing operation. The doped-region size is optimized for maximum conversion efficiency and it is found through numerical simulations that the doped radius should be more than 21 mu m. The "mode expansion", which is the self-expansion of the fundamental mode within the doped region with wavelength increments on bending the fiber, is the basic physical mechanism to give the bend-insensitive lasing performances of YPCF. It leads to an unusual variation of overlap factor when the wavelength is increased. A 41 cm long piece of YPCF demonstrates more than 83% of slope efficiency with 75% of conversion efficiency when pumped with a 975 nm laser source delivering an input power of 1 W. (C) 2008 Optical Society of America.

Design and simulation of 1310 nm and 1480 nm single-mode photonic crystal fiber Raman lasers
S. K. Varshney, K. Sasaki, K. Saitoh, M. Koshiba
OPTICS EXPRESS, 16, 2, 549, 559, OPTICAL SOC AMER, Jan. 2008, [Peer-reviewed]
English, Scientific journal, We have numerically investigated the Raman lasing characteristics of a highly nonlinear photonic crystal fiber (HNPCF). HNPCF Raman lasers are designed to deliver outputs at 1.3 mu m and 1.48 mu m wavelengths through three and six cascades of Raman Stokes cavities when the pumps of 1117 nm and 1064 nm are injected into HNPCF module, respectively. A quantum efficiency of approximately 47% was achieved in a short length of HNPCF for 1.3 mu m lasing wavelength. The HNPCF design is modified further to operate in single-mode fashion keeping intact its Raman lasing characteristics. The modified HNPCF design consists of two air-hole rings where the higher-order modes in the central core are suppressed by enhancing their leakage losses drastically, thus ceasing their propagation in the short length of HNPCF. On the other hand, the fundamental mode is well confined to the central core region, unaffecting its lasing performances. Further, the lasing characteristics of HNPCF at 1480 nm are compared with conventional highly nonlinear fiber Raman laser operating at 1480 nm. It is found that one can reduce the fiber length by five times in case of HNPCF with nearly similar conversion efficiency. (C) 2008 Optical Society of America.

Design of thermo-tunable couplers based on solid core photonic band-gap fibers
YOSHIDA Ryuichi, TSUCHIDA Yukihiro, MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori
ITE Technical Report, 32, 127, 132, The Institute of Image Information and Television Engineers, 2008
Japanese, In this work, we propose low polarization-dependent thermo-tunable couplers based on solid-core photonic band-gap fibers (PBGFs) filled with high temperature-dependent liquid. Firstly, by employing a full-vectorial finite element method, the temperature dependence on photonic band-gap and effective indices of the fundamental modes are investigated, and we numerically confirm that thermo-tunable couplers based on solid-core PBGFs can be realized. In addition, by evaluating the structural parameters dependence on coupling length and normalized output power for x- and y-polarized waves, we present a design method for the low polarization-dependent thermo-tunable couplers based on solid-core PBGFs.

Analysis of bending loss in solid-core photonic bandgap fibers
MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori
ITE Technical Report, 32, 121, 126, The Institute of Image Information and Television Engineers, 2008
Japanese, Solid-core photonic bandgap fibers (SC-PBGFs) have gathered much attention because of its intriguing transmission band properties which can be understood by the theory for ARROW waveguides. In SC-PBGFs, it has been reported experimentally that the centripetal radiation as well as the centrifugal radiation occurs, when the fiber is bent at longer wavelength ranges in the PBG. This anomalous property was explained by considering the existence of the inner radiation caustic, which can not be seen in conventional optical fibers. However, the further analysis is required to understand the detailed properties of bent SC-PBGFs. In this paper, we investigate the bending loss properties for the SC-PBGFs through a full-vector modal solver based on the finite element method (FEM). In particular, we show that the cladding modes which induced by the bend cause the complete or incomplete leaky mode coupling with the core mode. Moreover, we show the structural and wavelength dependence of the bending losses for the SC-PBGFs.

Evaluation of induced form-birefringence and PMD in dispersion-compensating hole-assisted fibers
Shailendra K. Varshney, Kunimasa Saitoh, Masanori Koshiba
OFC/NFOEC 2008 - 2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference, 2008, [Peer-reviewed]
English, International conference proceedings, The birefringence and polarization-mode dispersion characteristics of dispersioncompensating hole-assisted fibers are analyzed numerically. It is revealed that distortion in airholes leads to high birefringence and also shifts the dispersion by ±2% to its optimum value. © 2008 Optical Society of America.

Fundamental and High-Order Mode Bending Loss in Leakage Channel Fibers
F. Poli, L. Vincetti, D. Passaro, A. Cucinotta, S. Selleri, L. Rosa, K. Saitoh, Y. Tsuchida, S. K. Varshney, M. Koshiba
2008 34TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC), IEEE, 2008, [Peer-reviewed]
English, International conference proceedings, Penalties induced by bending are investigated in large mode area polygonal leakage channel fibers for high-power applications. Fundamental and high-order mode losses are reduced considering decagonal symmetry fibers.

Dispersion, birefringence, and amplification characteristics of newly designed dispersion compensating hole-assisted fibers
Kunimasa Saitoh, Shailendra K. Varshney, Masanori Koshiba
OPTICS EXPRESS, 15, 26, 17724, 17735, OPTICAL SOC AMER, Dec. 2007, [Peer-reviewed]
English, Scientific journal, We propose a new design of hole-assisted fiber (HAF) that can compensate for the accumulated dispersion in single-mode fiber link along with dispersion slope, thus providing broadband dispersion compensation over C-band as well as can amplify the signal channels by utilizing the stimulated Raman scattering phenomena. The proposed dispersion-compensating HAF (DCHAF) exhibits the lowest dispersion coefficient of -550 ps/nm/km at 1550 nm with an effective mode area of 15.6 mu m(2). A 2.52 km long module of DCHAF amplifies incoming signals by rendering a gain of 4.2 dB with +/- 0.8 dB gain flatness over whole C-band. To obtain accurate modal properties of DCHAF, a full-vector finite element method (FEM) solver is employed. The macro-bend loss characteristics of the proposed DCHAF are evaluated using FEM solver in cylindrical coordinate systems of a curved DCHAF, and low bending losses (< 10(-2) dB/m for 1 cm bending radius) are obtained for improved DCHAF design while keeping intact its dispersion compensation and Raman amplification properties. We have further investigated the birefringence characteristics that can give significant information on the polarization mode dispersion of DCHAF by assuming a certain deformation ( eccentricity e = 7%) either in air-holes or in the doped core or in both at a same time. It is noticed that the distortion in air-holes induces a birefringence of 10(-5), which is larger by a factor of 10 than the birefringence caused due to the core ellipticity. A PMD of 11.3 ps/root km is obtained at 1550 nm for distorted air-holes DCHAF structure. (c) 2008 Optical Society of America.

Photonic Bandgap Fiber Filter Design Based on Nonproximity Resonant Coupling Mechanism
Kunimasa Saitoh, Nikolaos Florous, Tadashi Murao, Shailendra Varshney, Masanori Koshiba
IEEE Photonics Technology Letters, 19, 19, 1547, 1549, 01 Oct. 2007, [Peer-reviewed]
English, Scientific journal, The possibility of designing compact ultranarrow bandpass filters based on the phenomenon of nonproximity resonant tunneling in multicore photonic bandgap fibers is proposed and numerically demonstrated in this investigation by making use of versatile algorithms based on the finite-element method. The proposed multifunctional assembly exhibits bandpass transmission characteristics at four closed spaced wavelengths with a 3-dB bandwidth of about 1.2 nm and an insertion loss of about 0.8 dB. The isolation level between neighbor channels, ranging from 12 dB up to 15 dB, could be achieved at optical frequencies. The overall device performance offers some promising propagation characteristics for various all-fiber narrow bandpass filters. © 2007, IEEE. All rights reserved.

Realistic design of large-hollow-core photonic band-gap fibers with suppressed higher order modes and surface modes
Kunimasa Saitoh, Nikolaos John Florous, Tadashi Murao, Masanori Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 25, 9, 2440, 2447, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2007, [Peer-reviewed]
English, Scientific journal, This paper theoretically describes effective suppression of higher order modes (HOMs) in realistic large-hollow-core photonic band-gap fibers (PBGFs) and utilizes the use of this class of waveguides for low-loss data-transmission applications and high-power beam delivery systems. The proposed design strategy is based on the resonant-coupling mechanism of central air-core modes with defected outer core modes. By incorporating six 7-unit-cell air cores in the cladding of the PBGF with sixfold symmetry, it is possible by resonantly coupling the light corresponding to the HOMs in a central 19-unit-cell core into the outer 7-unit-cell core, thus significantly increasing the leakage losses of the HOMs in comparison to those of fundamental mode. We consider a realistic PBGF structure with hexagonal airholes having rounded corners and derive a surface-mode-free condition of a silica-ring thickness surrounding the hollow core for both 7-unit-cell and 19-unit-cell cores. Verification regarding the propagation properties of the proposed design is ensured with a PBGF analysis based on a finite element modal solver. Numerical results show that the leakage losses of the HOMs can be enhanced in a level of at least three orders of magnitude over 200-nm wavelength range in comparison to those of the fundamental mode, while in addition, we show that the incorporation of a realistic air core with optimized silica-ring thickness can eliminate surface modes and achieve strong confinement into the central core and very low,eta-factor for the fundamental mode.

Enhanced thermoplasmonic oscillations in metallic nanostructured particles for the realization of nanofluidic sensors
Nikolaos John Florous, Kunimasa Saitoh, Masanori Koshiba
IEEE TRANSACTIONS ON NANOTECHNOLOGY, 6, 5, 549, 555, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2007, [Peer-reviewed]
English, Scientific journal, We theoretically investigate the intense tunable nanofocusing of light in certain families of metallo-dielectric nanograin particles and topologically defected nanocylinders, mediated by thermoplasmonic resonances. By using versatile numerical algorithms, based on either exact 3-D electromagnetic scattering from canonical geometries, or discrete dipole approximation modeling for particles with arbitrary shapes, associated with the spectral as well as the thermooptical response of the proposed classes of plasmonic nanostructures, we identify the mechanisms responsible for the enhanced tunable thermoplasmonic resonances, by varying the ambient temperature. In addition it is found that the plasmonic resonances are also sensitive to the electric properties of the host medium, thus enabling this novel class of plasmonic nanoparticles to be used as fluidic (liquid/gas) sensors. Our investigation is expected to remove an essential obstacle in the development of nanosensing platforms with high sensitiveness to temperature fluctuations and ultracompact size, thus making the proposed plasmonic resonators excellent candidates for future nanoplasmonic sensing systems.

Numerical investigation and optimization of a photonic crystal fiber for simultaneous dispersion compensation over S+C+L wavelength bands
S. K. Varshney, N. J. Florous, K. Saltolt, M. Koshiba, T. Fujisawa
OPTICS COMMUNICATIONS, 274, 1, 74, 79, ELSEVIER SCIENCE BV, Jun. 2007, [Peer-reviewed]
English, Scientific journal, In this paper, we numerically investigate and optimize the profile of a photonic crystal fiber (PCF) that can eliminate the residual dispersion from the telecom link as well as can provide identical dispersion compensation over S + C + L bands. A full-vectorial finite element method combined with genetic algorithm is used to optimize the fiber's profile as well as to accurately determine its modal properties. The optimized PCF exhibits a dispersion of -98.3 ps/nm/km with a variance of +/- 0.55 ps/nm/km from 1.48 mu m to 1.63 mu m (i.e., over 150 nm bandwidth) and a zero dispersion slope. Macro-bending loss performance of the designed PCF is also studied and it is found that the fiber shows low bending losses for the smallest feasible bending radius of 5 mm. Further, sensitivity analysis has been carried out for the proposed fiber design and it has been found that a +/- 2% change in the fiber parameters may lead to a +/- 8% shift of the dispersion from its nominal value. (C) 2007 Elsevier B.V. All rights reserved.

Nonreciprocal microresonators for the miniaturization of optical waveguide isolators
Naoya Kono, Kuniaki Kakihara, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 15, 12, 7737, 7751, OPTICAL SOC AMER, Jun. 2007, [Peer-reviewed]
English, Scientific journal, By introducing nonreciprocal phase shifts into microresonators, we propose new designs for the miniaturization of optical waveguide isolators and circulators. We present detailed design procedures, and numerically demonstrate the operation of these magneto-optical devices. The device sizes can be reduced down to several tens of micrometers. The nonreciprocal function of these devices is due to nonreciprocal resonance shifts. Next, the operation bandwidth can be expanded by increasing the number of resonators ( the filter order). This is demonstrated by comparing the characteristics of a single-resonator structure with those of a three-resonator structure. This paper furthermore presents the nonreciprocal characteristics of three-dimensional resonators with finite heights, leading to a guideline for the design of nonreciprocal optical circuits. This involves a demonstration of how the resonators with selected parameters are practical for miniaturized nonreciprocal circuits. (c) 2007 Optical Society of America.

Authors' reply to "Comments on 'Thermooptical sensitivity analysis of highly birefringent polarimetric sensing photonic crystal fibers with elliptically elongated veins'''
Nikolaos J. Florous, Shailendra K. Varsheney, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 19, 9-12, 796, 797, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2007, [Peer-reviewed]
English

Design of effectively single-mode air-core photonic bandgap fiber with improved transmission characteristics for the realization of ultimate low loss waveguide
Tadashi Murao, Kunimasa Saitoh, Nikolaos J. Florous, Masanori Koshiba
OPTICS EXPRESS, 15, 7, 4268, 4280, OPTICAL SOC AMER, Apr. 2007, [Peer-reviewed]
English, Scientific journal, In this paper, we study the novel propagation properties of an improved triangular-type air-core photonic bandgap fiber (PBGF) structured with an anti-resonant silica core surround, through a full-vector modal solver based on the finite-element method (FEM). At first, to realize a single-mode operation over a wide wavelength range, the fiber whose core is constructed by removing 1 air-hole and expanded is proposed and structurally-optimized. In particular, the structural parameters for the fiber that prevent the narrow-band transmission due to the existence of the surface modes and enhance the confinement of the power in the air-core are presented. For the realization of an ultimate low loss transmission, a 7-unit-cell PBGF is analyzed and we show that the 7-unit-cell PBGF can achieve not only lower confinement loss than that of regular-type 7-unit-cell PBGF, but also lower power fraction in the silica-ring when compared with the regular 19-unit-cell PBGF with an anti-resonant core surround, exhibiting an effectively single-mode operation. (c) 2007 Optical Society of America.

Analysis of a realistic and idealized dispersion compensating photonic crystal fiber Raman amplifier
Shailendra K. Varshney, Kunimasa Saitoh, Masanori Koshiba, Peter J. Roberts
OPTICAL FIBER TECHNOLOGY, 13, 2, 174, 179, ELSEVIER SCIENCE INC, Apr. 2007, [Peer-reviewed]
English, Scientific journal, In the present paper, the highly efficient Raman amplification properties of a realistic and idealized dispersion compensating photonic crystal fiber (DCPCF) are described numerically. We have used an accurate full-vectorial finite element modal solver with hybrid curvilinear edge/nodal elements and anisotropic perfectly matched layers for an accurate modal characterization of the realistic as well as idealized DCPCF. A good agreement is observed between numerically evaluated and experimentally [P.J. Roberts, B.J. Mangan, H. Sabert, F. Couny, et al., J. Opt. Fiber Commun. Rep. 2 (2005) 435-461] measured dispersion values. A high peak Raman gain efficiency of 10.5 W-1 km(-1) is obtained at 13.1 THz frequency shift for a 1455 nm depolarized pump. A DCPCF module of 1-km length can compensate for the dispersion accumulated over 70-km of conventional single mode fiber link with a residual dispersion of +/- 50 ps/nm and can provide an average gain of 7.6 dB with +/- 0.9 dB gain ripples over C-band. (c) 2007 Elsevier Inc. All rights reserved.

Measurement of chromatic dispersion and Raman gain efficiency of a hole-assisted fiber: Influence of bend
S. K. Varshney, Y. Tsuchida, K. Sasaki, K. Saitoh, M. Koshiba
OPTICS EXPRESS, 15, 6, 2974, 2980, OPTICAL SOC AMER, Mar. 2007, [Peer-reviewed]
English, Scientific journal, We have experimentally measured the Raman gain efficiency (RGE) and chromatic dispersion (CD) of a hole-assisted fiber (HAF). The RGE of a HAF was characterized using standard pump on/off technique while the CD of the fiber was measured using optical network analyzer. Theoretical simulations of the modal characteristics and the RGE of HAF were carried out using an accurate full-vectorial finite element method. Further, the bending effects on the CD and the RGE of a HAF with a smallest feasible bending radius are demonstrated. It was found that the CD increases while the RGE is decreased by bending HAF in a smallest bending radius of 5 mm. Numerical predictions from the theory are shown to be in good agreement with the experimental results. (c) 2007 Optical Society of America.

Optimization of pump spectra for gain-flattened photonic crystal fiber Raman amplifiers operating in C-band
Kazuya Sasaki, Shailendra K. Varshney, Keisuke Wada, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 15, 5, 2654, 2668, OPTICAL SOC AMER, Mar. 2007, [Peer-reviewed]
English, Scientific journal, This paper focuses on the optimization of pump spectra to achieve low Raman gain ripples over C-band in ultra-low loss photonic crystal fiber (PCF) and dispersion compensating PCFs (DCPCFs). Genetic algorithm (GA), a multivariate stochastic optimization algorithm, is applied to optimize the pump powers and the wavelengths for the aforesaid fiber designs. In addition, the GA integrated with full-vectorial finite element method with curvilinear edge/nodal elements is used to optimize the structural parameters of DCPCF. The optimized DCPCF provides broadband dispersion compensation over C-band with low negative dispersion coefficient of -530 ps/nm/km at 1550 nm, which is five times larger than the conventional dispersion compensating fibers with nearly equal effective mode area (21.7 mu m(2)). A peak gain of 8.4 dB with +/- 0.21 dB gain ripple is achieved for a 2.73 km long DCPCF module when three optimized pumps are used in the backward direction. The lowest gain ripple of +/- 0.36 dB is attained for a 10 km long ultra-low loss PCF with three backward pumps. Sensitivity analysis has been performed and it is found that within the experimental fabrication tolerances of +/- 2%, the absolute magnitude of dispersion may vary by +/- 16%, while the Raman gain may change by +/- 7%. Through tolerance study, it is examined that the ring core's hole-size is more sensitive to the structural deformations. (c) 2007 Optical Society of America.

Numerical modeling of cryogenic temperature sensors based on plasmonic oscillations in metallic nanoparticles embedded into photonic crystal fibers
Nikolaos John Florous, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 19, 5-8, 324, 326, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2007, [Peer-reviewed]
English, Scientific journal, We perform a numerical investigation on the operation of an inline temperature sensor based on the inclusion of metallic nanoparticles into photonic crystal fibers (PCFs). This creates a simple, yet robust, platform which can be used to investigate the properties of nanoparticles, for sensing, spectroscopy, and optical switching applications. The optical response of gold nanoparticles embedded in the PCF matrix was evaluated as a function of temperature and the use of the structure as an inline fiber-optic temperature sensor is theoretically described. A blue-shift in the localized surface plasmon resonance related peak, as well as narrowing of the plasmon resonance, was observed upon decreasing the temperature of the nanoparticle embedded into the PCF at very low temperatures, thus permitting the operation of this sensing platform at cryogenic temperatures.

Design of single-moded holey fibers with large-mode-area and low bending losses: The significance of the ring-core region
Yukihiro Tsuchida, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 15, 4, 1794, 1803, OPTICAL SOC AMER, Feb. 2007, [Peer-reviewed]
English, Scientific journal, One of the major trends in optical fiber science is to be able to obtain fibers with large-mode-area ( LMA), optimized for various applications such as high power delivery, fiber amplifiers, and fiber lasers. In order to ensure the high beam quality and the ultimate controllability of the damage threshold in the fiber's material, it is required to have a LMA property and of course to operate in a single mode fashion. While the conventional fibers have some difficulties in providing simultaneously LMA, single mode operation, as well as low macro-bending loss characteristics, all-silica holey fibers are highly attractive candidates for realizing LMA single-mode fibers with low bending losses. In this paper, we present a novel type of effectively single-mode holey fibers with effective mode area of about 1400 mu m(2), small allowable bending radius as small as 5 cm, good beam quality factor of 1.15, and high confinement losses exceeding 1 dB/m for the higher-order mode at 1.064-mu m wavelength. (c) 2007 Optical Society of America.

Ultra-sensitive thermo-plasmonic oscillations in topologically-defected nano-cylinders: Merging photons and electrons for miniaturization of fluidic sensors
Nikolaos John Florous, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2007
English, International conference proceedings, Using an accurate analysis based on the discrete-dipole-approximation, we show that the enhanced tuning of the localized plasmonic resonances in corrugated nano-cylinders can form the basis for the miniaturization of future nano-fluidic sensing systems. © 2007 Optical Society of America.

Broadband dispersion compensating hole-assisted fibers
Kunimasa Saitoh, Shailendra K. Varshney, Masanori Koshiba
2007 33rd European Conference and Exhibition of Optical Communication, ECOC 2007, Institute of Electrical and Electronics Engineers Inc., 2007
English, International conference proceedings, A novel design of a bradband disperion compensating hole-assisted fber is demonstrted numercaly. The prposed strctur exhibits the larest negative dispersion of -552psnmm at 1550nm with matched RDS. Furher, its Raman amplication prperies ar investigated.

Nonreciprocal microdisk resonators utilizing magnetic bubble domains in garnet film for ultra-compact optical waveguide isolators
Kunimasa Saitoh, Naoya Kono, Kuniaki Kakihara, Masanori Koshiba
2007 33rd European Conference and Exhibition of Optical Communication, ECOC 2007, Institute of Electrical and Electronics Engineers Inc., 2007
English, International conference proceedings, The three-dimensional characteristics of magneto-optical microresonators are presented, leading to a guideline for the design of nonreciprocal circuits. This involves a demonstration of how the resonators with selected parameters are practical for miniaturized nonreciprocal circuits.

Fluidic sensors based on photonic crystal fiber gratings: An emerging technology for realizing ultra-low thermo-responsive sensing platforms
Nikolaos John Florous, Kunimasa Saitoh, Shailendra Varsheney, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2007
English, International conference proceedings, We propose the use of photonic crystal fiber gratings (PCFGs) as fluidic sensors. The thermo-optical sensitivity response of PCFG-based sensors is found to be superior in comparison to conventional fiber Bragg gratings. © 2007 Optical Society of America.

1.3 μm photonic crystal fiber Raman laser
Shallendra K. Varshney, Kazuya Sasaki, Kunimasa Saitoh, Nikolaos J. Florous, Masanori Koshiba
Conference on Lasers and Electro-Optics Europe - Technical Digest, 2007, [Peer-reviewed]
English, International conference proceedings

Inline cryogenic temperature sensors based on photonic crystal fiber Bragg gratings infiltrated with noble gases for harsh space applications
Nikolaos J. Florous, K. Saitoh, S. K. Varsheney, Y. Tsuchida, T. Murao, M. Koshiba
Conference on Lasers and Electro-Optics Europe - Technical Digest, 2007, [Peer-reviewed]
English, International conference proceedings

Large-mode-area single-mode holey fiber with low bending losses: Towards high power beam delivery systems
Yukihiro Tsuchida, Kunimasa Saitoh, Masanori Koshiba
OFC/NFOEC 2007 - Optical Fiber Communication and the National Fiber Optic Engineers Conference 2007, 2007, [Peer-reviewed]
English, International conference proceedings, We propose a novel design method for large-mode-area single-mode holey fiber with effective mode area of 1400 μm2, small allowable bending radius of 5 cm, and good beam quality factor of 1.15 at 1.064-μm wavelength. © 2007 Optical Society of America.

Proposal for miniaturized interleaver with flat-top passbands utilizing coupled-resonator optical waveguide rings in photonic crystals
Naoya Kono, Nobuhiro Yokoi, Kunimasa Saitoh, Masanori Koshiba
OFC/NFOEC 2007 - Optical Fiber Communication and the National Fiber Optic Engineers Conference 2007, 2007, [Peer-reviewed]
English, International conference proceedings, The introduction of coupled-resonator optical waveguides to ring resonators in the IIR niter configuration enables a miniaturized interleaver with wide flat-top passbands. A wavelength-insensitive photonic crystal directional coupler with two decoupling points is also presented. © 2007 Optical Society of America.

Enhancement of the stimulated brillouin scattering of higher-order acoustic modes in hole-assisted fibers
Kunimasa Saitoh, Hironori Nagano, Nikolaos Florous, Masanori Koshiba
OFC/NFOEC 2007 - Optical Fiber Communication and the National Fiber Optic Engineers Conference 2007, 2007, [Peer-reviewed]
English, International conference proceedings, By using an accurate numerical method we show that both the higher-order peaks and main peak of the SBS in hole-assisted-fibers can be enhanced by more than 3-dB, by appropriate selection of the design parameters. ©2007 Optical Society of America.

Single-mode air-guiding photonic bandgap fiber with improved broadband transmission characteristics: The benefits of an anti-resonant core design
Tadashi Murao, Kunimasa Saitoh, Nikolaos J. Florous, Masanori Koshiba
OFC/NFOEC 2007 - Optical Fiber Communication and the National Fiber Optic Engineers Conference 2007, 2007, [Peer-reviewed]
English, International conference proceedings, The condition for single-mode operation in photonic band-gap fibers (PBGFs) is investigated. If the silica-ring satisfies an anti-resonant condition, the PBGF with a realistic core exhibits a single-mode operation over a wide wavelength range. © 2007 Optical Society of America.

Design of large hollow-core photonic band-gap fibers with suppressed higher-order modes
Kunimasa Saitoh, Nikolaos Florous, Tadashi Murao, Masanori Koshiba
OFC/NFOEC 2007 - Optical Fiber Communication and the National Fiber Optic Engineers Conference 2007, 2007, [Peer-reviewed]
English, International conference proceedings, We present the suppression of higher-order modes in realistic large-hollow-core photonic band-gap fibers, free of surface modes, based on the index-matching mechanism of central air-core modes with defected outer-core modes for an effectively single-mode operation. ©2007 Optical Society of America.

Experimental Demonstration of Raman Gain Efficiency and Chromatic Dispersion of Hole-Assisted Fiber: Influence of Bend
S. K. Varshney, Y. Tsuchida, K. Sasaki, K. Saitoh, M. Koshiba
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5, 1896, 1897, IEEE, 2007, [Peer-reviewed]
English, International conference proceedings, The Raman gain efficiency and chromatic-dispersion of a hole-assisted fiber with and without minimum allowable bending radius are measured. Numerical predictions from the theory are shown to be in good agreement with the experimental results. (C) 2007 Optical Society of America

Approximate analytical solutions for nonlinear photonic crystal fibers
Takahto Kirihara, Takeshi Fujisawa, Kunimasa Saitoh, Masanori Koshiba
ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS, 90, 5, 19, 26, SCRIPTA TECHNICA-JOHN WILEY & SONS, 2007, [Peer-reviewed]
English, Scientific journal, This paper proposes an analytical approximation theory for evaluation of the transmission characteristics of a nonlinear photonic crystal fiber. A photonic crystal fiber with a cornplex cross-sectional structure is replaced equivalently with a conventional step-index fiber so that an analylical approximation theory developed for the conventional nonlinear theory developed for theconventional nonlinear optical fiber can be applied directly to the nonlinear photonic crystal fiber. In order to derive an effective index of refraction of the clad region of the photonic crystal fiber with holes, an empirical formula for the normalized frequency of the photonic crystal fiber proposed recently is introduced. Hence, the fundamental quantities describing the transmission characteristics of the nonlinear photonic crystal fiber, specifically the effective index of refraction and the effective core cross section, can easily be derived without numerical calculations. By using the proposed method, the optical power dependence of the transmission characteristics of the nonlinear photonic crystal fiber is analyzed. Validity of the proposed method is verified by comparison of the results with those by calculations based on the full-vectorial finite element method. (C) 2007 Wiley Periodicals, Inc.

Thereto-Plasmonic Resonances in Hybrid Metallo-Dielectric Nano-Particles: Towards Tunable Standalone Nano-Sensors
Nikolaos John Florous, Kunimasa Saitoh, Tadashi Murao, Masanori Koshiba
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5, 2556, 2557, IEEE, 2007, [Peer-reviewed]
English, International conference proceedings, We theoretically investigate the intense nano-focusing of light through a novel family of metallo-dielectric nano-particles mediated by thermo-plasmonic resonances. The results show a significant sensitivity of the plasmonic resonance to temperature and environmental fluctuations. (C) 2007 Optical Society of America

All-Fiber Integrated Assemblies Based on the Resonant Tunneling Effect in Multi-Core Photonic Band-Gap Fibers
Kunimasa Saitoh, Nikolaos John Florous, Shailendra Kumar Varshney, Masanori Koshiba
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5, 1914, 1915, IEEE, 2007, [Peer-reviewed]
English, International conference proceedings, We demonstrate the possibility of designing compact ultra-narrow band-pass filters to split four different wavelengths based on the phenomenon of non-proximity resonant tunneling in multi-core photonic band-gap fibers for resonance-sensing and wavelength-selective filtering applications. (C)2007 Optical Society of America

Leakage losses of quasi-phase-matched second-harmonic-generation devices with air gap
Yoshiki Ishikawa, Kunimasa Saitoh, Takeshi Fujisawa, Masanori Koshiba
ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS, 90, 5, 11, 18, SCRIPTA TECHNICA-JOHN WILEY & SONS, 2007, [Peer-reviewed]
English, Scientific journal, This paper theoretically evaluates the leakage loss characteristics of a quasi-phase-matched second-harmonic generation device using a ridge-type MgO:LiNbO3/LiNbO3 waveguide with a gap proposed as an efficient low-loss wave conversion device. The full-vectorial finite element method for an analysis of the anisotropic waveguide is used. The effects of the distance from the MfO:LiNbO3 surface to the gap and the width and dept of the gap on the leakage loss are studied in detail. Also, it is found that there are cases where the leakage loos of the polarization related to wavelength conversion does not become less than a certain level even if the depth of the gap is made sufficiently deep. Its mechanism is found and a provision for further reduction of such a leakage loss is described (C) 2007 Wiley Periodical, Inc.

Full-vectorial finite element method in a cylindrical coordinate system for loss analysis of photonic wire bends
Kuniaki Kakihara, Naoya Kono, Kunimasa Saitoh, Masanori Koshiba
OPTICS EXPRESS, 14, 23, 11128, 11141, OPTICAL SOC AMER, Nov. 2006, [Peer-reviewed]
English, Scientific journal, This paper presents a new full-vectorial finite-element method in a local cylindrical coordinate system, to effectively analyze bending losses in photonic wires. The discretization is performed in the cross section of a three-dimensional curved waveguide, using hybrid edge/nodal elements. The solution region is truncated by anisotropic, perfectly matched layers in the cylindrical coordinate system, to deal properly with leaky modes of the waveguide. This approach is used to evaluate bending losses in silicon wire waveguides. The numerical results of the present approach are compared with results calculated with an equivalent straight waveguide approach and with reported experimental data. These comparisons together demonstrate the validity of the present approach based on the cylindrical coordinate system and also clarifies the limited validity of the equivalent straight waveguide approximation. (c) 2006 Optical Society of America.

Highly Efficient Coupling between Photonic Crystal Circuits and Si-Wire Waveguides
TORIHATA Akihiro, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J89-C, 11, 933, 940, The Institute of Electronics, Information and Communication Engineers, Nov. 2006, [Peer-reviewed]
Japanese, Scientific journal, ピラー型フォトニック結晶光波回路とシリコン細線導波路の高効率な接続法を提案し,その結合効率を理論的に評価している.シリコン細線導波路については,その先端をテーパ構造とすることによって,波長1550nmを中心とした200nmの帯域で90%以上の結合効率が得られることを示している.更に,先端テーパ型シリコン細線導波路をCWDM用フォトニック結晶波長合分波回路に適用し,シリコン細線導波路を単純突合せ接続した場合に比べて,分波特性が大幅に改善されることを確認している.

Fluidic sensors based on photonic crystal fiber gratings: Impact of the ambient temperature
Nikolaos John Florous, Kunirnasa Saitoh, Shailendra K. Varsheney, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 18, 21-24, 2206, 2208, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2006, [Peer-reviewed]
English, Scientific journal, The spectral and thermooptical responses of photonic crystal fiber gratings (PCFGs) infiltrated with gaseous solutions, Are investigated theoretically using an accurate semivectorial modal solver combined with exact equations for the reflection response of fiber Bragg gratings. We demonstrate numerically perhaps for the first time, that by an appropriate selection of the design parameters, it is possible to obtain compact sensing platforms based on the shift of the calculated reflectance spectra of the PCFG. Thus, our investigation adds evidence to the potential use of PCFGs as gas/liquid sensors. In addition, we show through a thermooptical sensitivity analysis that this type of sensors can easily meet the requirements for channel allocation in wavelength-division-multiplexing systems with relatively low temperature sensitivity.

Leakage Losses of Quasi-Phase-Matched Second-Harmonic-Generation Devices with Air Gap
ISHIKAWA Yoshiki, SAITOH Kunimasa, FUJISAWA Takeshi, KOSHIBA Masanori
The transactions of the Institute of Electronics, Information and Communication Engineers. C, J89-C, 10, 700, 707, The Institute of Electronics, Information and Communication Engineers, Oct. 2006, [Peer-reviewed]
Japanese, Scientific journal, 高効率で低損失な波長変換素子として提案された空げきを有するリッジ型MgO:LiNbO_3/LiNbO_3導波路を用いた擬似位相整合第二高調波素子の漏れ損特性を理論的に評価している.異方性光導波路解析のためのフルベクトル有限要素法を適用し,MgO:LiNbO_3表面から空げきまでの距離,空げきの幅並びに深さが漏れ損に与える影響を詳細に調査している.また,波長変換に関与する偏波の漏れ損が,空げきの深さを十分深くしても,あるレベル以下にならない場合があることを見出し,そのメカニズムを明らかにするとともに,こうした漏れ損を更に低減する方策について述べている.

Approximate Analytical Solutions for Nonlinear Photonic Crystal Fibers
KIRIHARA Takahito, FUJISAWA Takeshi, SAITOH Kunimasa, KOSHIBA Masanori
The IEICE transactions on electronics C, J89-C, 10, 692, 699, The Institute of Electronics, Information and Communication Engineers, Oct. 2006, [Peer-reviewed]
Japanese, Scientific journal, 非線形フォトニック結晶ファイバの伝送特性を評価するための解析的近似理論を提案している.複雑な断面構造を有するフォトニック結晶ファイバを,等価的に通常のステップインデックスファイバに置き換えることによって,従来の非線形光ファイバに対して開発された解析的近似理論を,非線形フォトニック結晶ファイバにそのまま適用することを可能にしている.また,フォトニック結晶ファイバの空孔を有するクラッド領域の等価的な屈折率を求めるために,ここでは,最近提案されたフォトニック結晶ファイバの正規化周波数に対する経験式を導入しているので,非線形フォトニック結晶ファイバの伝送特性を記述する基本量,具体的には実効屈折率と実効コア断面積を,数値計算を一切行うことなく,容易に算出できる.ここで提案した手法を用いて,非線形フォトニック結晶ファイバの伝送特性の光パワー依存性を解析し,フルベクトル有限要素法に基づく精密計算による結果と比較することによって,本手法の妥当性を検証している.

Nonlinear photonic crystal fibres: pushing the zero-dispersion towards the visible
Kunimasa Saitoh, Masanori Koshiba, Niels Asger Mortensen
NEW JOURNAL OF PHYSICS, 8, 207, 1, 9, IOP PUBLISHING LTD, Sep. 2006, [Peer-reviewed]
English, Scientific journal, The strong waveguide dispersion in photonic crystal fibres (PCFs) provides unique opportunities for nonlinear optics with a zero-dispersion wavelength lambda(0) far below the limit of similar to 1.3 mu m set by the material dispersion of silica. By tuning the air hole diameter d, the pitch Lambda, and the number of rings of air holes N, the strong waveguide dispersion can in principle be used to extend lambda(0) well into the visible, albeit to some extent at the cost of multi-mode operation. We study in detail the interplay of the zero-dispersion wavelength, the cutoff wavelength lambda(c), and the leakage loss in the parameter space spanned by d, Lambda, and N. As a particular result, we identify values of d (similar to 500 nm) and Lambda (similar to 700 nm) which facilitate the shortest possible zero-dispersion wavelength (similar to 700 nm), while the fibre is still in single-mode for longer wavelengths.

Design of photonic band gap fibers with suppressed higher-order modes: Towards the development of effectively single mode large hollow-core fiber platforms
Kunimasa Saitoh, Nikolaos John Florous, Tadashi Murao, Masanori Koshiba
OPTICS EXPRESS, 14, 16, 7342, 7352, OPTICAL SOC AMER, Aug. 2006, [Peer-reviewed]
English, Scientific journal, The objective of the present investigation is to propose and theoretically demonstrate the effective suppression of higher-order modes in large-hollow-core photonic band gap fibers (PBGFs), mainly for low-loss data transmission platforms and/or high power delivery systems. The proposed design strategy is based on the index-matching mechanism of central air-core modes with defected outer core modes. By incorporating several air-cores in the cladding of the PBGF with 6-fold symmetry it is possible to resonantly couple the light corresponding to higher-order modes into the outer core, thus significantly increasing the leakage losses of the higher-order modes in comparison to the fundamental mode, thus making our proposed design to operate in an effectively single mode fashion with polarization independent propagation characteristics. The validation of the procedure is ensured with a detailed PBGF analysis based on an accurate finite element modal solver. Extensive numerical results show that the leakage losses of the higher-order modes can be enhanced in a level of at least 2 orders of magnitude in comparison to those of the fundamental mode. Our investigation is expected to remove an essential obstacle in the development of large-core single-mode hollow-core fibers, thus enabling them to surpass the attenuation of conventional fibers. (c) 2006 Optical Society of America.

Impact of structural deformations on polarization conversion in high index contrast waveguides
Kuniaki Kakihara, Naoya Kono, Kunimasa Saitoh, Takeshi Fujisawa, Masanori Koshiba
OPTICS EXPRESS, 14, 16, 7046, 7056, OPTICAL SOC AMER, Aug. 2006, [Peer-reviewed]
English, Scientific journal, The objective of this paper is the detailed study of polarization conversion in deformed high index contrast (HIC) waveguides. The type of deformation considered here is the slanted sidewalls of buried channel waveguides. Polarization conversion of HIC waveguides are investigated for possible core refractive indices ranging from 2 (SiNx) to 3.5 (Si), by using numerical schemes based on the finite-element and beam propagation methods. The numerical results show that polarization conversion can be greatly magnified in HIC channel waveguides. For example, in Si-wire waveguides, complete polarization conversions can occur within just tens of micrometers. (c) 2006 Optical Society of America

Realization of single-moded broadband air-guiding photonic bandgap fibers
Tadashi Murao, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 18, 13-16, 1666, 1668, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2006, [Peer-reviewed]
English, Scientific journal, In this letter, we propose a novel type of air-guiding photonic bandgap fiber, which exhibits an effectively single-mode operation with a low confinement loss over a wide wavelength range, extended from 1460 to 1840 run. For several realistic structural parameters, the wavelength dependence of the dispersion as well as the confinement loss properties are investigated through a full-vector modal solver based on the finite-element method. In particular, we optimize the structure so as to exhibit the following performance: effectively single-mode operation from 1460 to 1840 rim, with corresponding confinement loss of the fundamental air-guided mode of 8.8 x 10(-5) dB/km, and enhancement of the confinement loss of the higher order mode of 43 dB/m for ten-ring structure.

Thermooptical sensitivity analysis of highly birefringent polarimetric sensing photonic crystal fibers with elliptically elongated veins
Nikolaos J. Florous, Shailendra K. Varsheney, Kunimasa Saitoh, Masanori Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 18, 13-16, 1663, 1665, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2006, [Peer-reviewed]
English, Scientific journal, The spectral and thermooptical sensitivity responses of highly birefringent photonic, crystal fibers (PCFs) with elliptically elongated veins integrated in their profile are being investigated using a novel sensitivity analysis based on an accurate semivectorial modal solver combined with temperature-dependent Sellmeier equations for pure silica and dry air. We demonstrate that by an appropriate selection of the design parameters it is possible to optimize the sensitivity of the modal birefringence or even to obtain null response at specific wavelengths. Thus, our investigation adds evidence to the potential use of highly birefringent PCFs as polarimetric sensors. Optimized overall temperature sensitivity could be obtained up to 7.5 center dot 10(-7) K-1 at operational wavelength of 1.55 mu m.

Approximate empirical relations for nonlinear photonic crystal fibers
Kunimasa Saitoh, Takeshi Fujisawa, Takahito Kirihara, Masanori Koshiba
OPTICS EXPRESS, 14, 14, 6572, 6582, OPTICAL SOC AMER, Jul. 2006, [Peer-reviewed]
English, Scientific journal, Approximate empirical relations for nonlinear photonic crystal fibers (PCFs) are newly proposed. Replacing a PCF with a conventional step-index fiber, closed form expressions for the effective refractive index and the effective core area of nonlinear PCFs are derived. To define the equivalent cladding index, the effective index of the so-called fundamental space-filling mode, which is calculated using empirical relations for the effective normalized frequency, is introduced, and thus, nonlinear guided waves propagating in PCFs can be easily characterized without the need for numerical computations. The validity of the method proposed here is ensured by comparing the calculated results with those obtained by a full-vector finite-element method. (c) 2006 Optical Society of America.

Raman amplification properties of photonic crystal fibers　　　　　　　　　　　　　　　
S.K. Varshney, K. Saitoh, N.J. Florous, M. Koshiba
International Journal of Microwave and Optical Technology, 1, 1, 173, 180, Jun. 2006, [Peer-reviewed]
English, Scientific journal

Synthesis of polarization-independent splitters based on highly birefringent dual-core photonic crystal fiber platforms
NJ Florous, K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 18, 9-12, 1231, 1233, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2006, [Peer-reviewed]
English, Scientific journal, The objective of this letter is to introduce and numerically investigate the operation of a novel type of polarization-independent splitter based on highly birefringent dual-core photonic crystal fiber (PCF), which allows wavelength multiplexing at 1.3 and 1.55 mu m. The design procedure follows a rigorous synthesis algorithm based on exact equations for describing the wavelength decoupling mechanism and on full-vector finite element as well as beam propagation methods for accurate modeling of PCFs. Typical characteristics of the newly proposed PCF splitter are coupling length of 9.08 mm and available optical bandwidths defined at a level of -20 dB of 5.5 and 2 nm around 1.3 and 1.55 mu m, respectively.

Non-proximity resonant tunneling in multi-core photonic band gap fibers: An efficient mechanism for engineering highly-selective ultra-narrow band pass splitters
NJ Florous, K Saitoh, T Murao, M Koshiba, M Skorobogatiy
OPTICS EXPRESS, 14, 11, 4861, 4872, OPTICAL SOC AMER, May 2006, [Peer-reviewed]
English, Scientific journal, The objective of the present investigation is to demonstrate the possibility of designing compact ultra-narrow band-pass filters based on the phenomenon of non-proximity resonant tunneling in multi-core photonic band gap fibers ( PBGFs). The proposed PBGF consists of three identical air-cores separated by two defected air-holes which act as highly-selective resonators. With a fine adjustment of the design parameters associated with the resonant-air-holes, phase matching at two distinct wavelengths can be achieved, thus enabling very narrow-band resonant directional coupling between the input and the two output cores. The validation of the proposed design is ensured with an accurate PBGF analysis based on finite element modal and beam propagation algorithms. Typical characteristics of the proposed device over a single polarization are: reasonable short coupling length of 2.7 mm, dual bandpass transmission response at wavelengths of 1.339 and 1.357 mu m, with corresponding full width at half maximum bandwidths of 1.2 nm and 1.1 nm respectively, and a relatively high transmission of 95% at the exact resonance wavelengths. The proposed ultra-narrow band-pass filter can be employed in various applications such as all-fiber bandpass/bandstop filtering and resonant sensors. (c) 2006 Optical Society of America.

Apodized photonic crystal waveguide gratings
N Yokoi, T Fujisawa, K Saitoh, M Koshiba
OPTICS EXPRESS, 14, 10, 4459, 4468, OPTICAL SOC AMER, May 2006, [Peer-reviewed]
English, Scientific journal, Apodized photonic crystal (PC) waveguide gratings are proposed to suppress sidelobes which appear in reflection spectra of usual PC waveguide gratings. By using specific functions (Gauss and Gauss-cosine functions) for the longitudinal refractive index distribution, it is possible to suppress sidelobes in the reflection spectra of PC waveguide gratings efficiently. The apodization is realized by simply changing diameters of dielectric pillars adjacent to the PC waveguide core. It is shown that by using Gauss-cosine functions for the apodization, Bragg frequency of the waveguide grating becomes insensitive to the magnitude of perturbation leading to the possibility of designing waveguide gratings with arbitrarily Bragg frequency and bandwidth by modulating geometrical parameters only. (c) 2006 Optical Society of America.

Light-wave guidance through stratified photonic crystal metamaterials synthesized by superinductive layers of metallic nanostrips
NJ Florous, K Saitoh, M Koshiba
OPTICS LETTERS, 31, 9, 1226, 1228, OPTICAL SOC AMER, May 2006, [Peer-reviewed]
English, Scientific journal, The electrodynamic properties of a novel class of photonic-crystal metamaterial platform entirely synthesized by ultralow-refractive-index suspended nanostrips embedded in a dielectric matrix are described and investigated. The optical properties of the homogeneous metamaterial platform can be understood on the basis of equivalent distributed circuit extraction from an electromagnetic model that has negatively distributed superinductive properties. Low propagation loss, temperature-insensitive characteristics, and ultracompact size are the main advantages of the proposed technology, making this new type of metamaterial an excellent candidate for use in compact multilayer nanophotonic integrated systems. (c) 2006 Optical Society of America.

Design and analysis of a broadband dispersion compensating photonic crystal fiber Raman amplifier operating in S-band
Shailendra K. Varshney, Takeshi Fujisawa, Kunimasa Saitoh, Masanori Koshiba
Optics Express, 14, 8, 3528, 3540, Optical Society of American (OSA), 17 Apr. 2006, [Peer-reviewed]
English, Scientific journal, This paper presents an optimized design of a dispersion compensating photonic crystal fiber (PCF) to achieve gain-flattened Raman performances over S-band using a single pump. Genetic algorithm interfaced with an efficient full-vectorial finite element modal solver based on curvilinear edge/nodal elements is used as an optimization tool for an accurate determination of PCF design parameters. The designed PCF shows high negative dispersion coefficient (-264 ps/nm/km to -1410 ps/nm/km) and negative dispersion slope, providing coarse dispersion compensation over the entire S-band. The module comprised of 1.45-km long optimized PCF exhibits ±0.46 dB gain ripples over 50 nm wide bandwidth and shows a very low double Rayleigh backscattering value (-59.8 dB). The proposed module can compensate for the dispersion accumulated in one span (80-km) of standard single mode fiber with a residual dispersion of ±700 ps/nm, ensuring its applicability for 10 Gb/s WDM networks. Additionally, the designed PCF remains single mode over the range of operating wavelengths. © 2006 Optical Society of America.

Theoretical prediction of thermooptical and structurally disordered sensitivities in metallo-dielectric photonic crystals
NJ Florous, K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 18, 5-8, 898, 900, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2006, [Peer-reviewed]
English, Scientific journal, In this letter, we introduce a novel computational technique for predicting thermal and structural sensitivities in metallo-dielectric photonic crystals (PCs). The computational scheme is based on a hybrid formalism of the scattering matrix technique combined with the adjoint network method. The proposed technique can predict with high accuracy the impact of temperature fluctuations or structural disorders, to the light-wave propagation in PCs, without additional computational effort. Numerical simulations show that PC circuits based on metallic-metamaterial platforms are significantly less sensitive to temperature variations in comparison to usual dielectric PCs.

Design of air-guiding modified honeycomb photonic band-gap fibers for effectively single-mode operation
T Murao, K Saitoh, M Koshiba
OPTICS EXPRESS, 14, 6, 2404, 2412, OPTICAL SOC AMER, Mar. 2006, [Peer-reviewed]
English, Scientific journal, We investigate photonic band-gap (PBG) profiles of a modified honeycomb lattice structure and we identify the structural parameters that possess the largest band-gap. By incorporating the identified profile into the cladding, the wavelength dependence of the dispersion properties and confinement losses of air-guiding modified honeycomb PBG fibers (PBGFs) is investigated through a full-vector modal solver based on finite element method. In particular, we find that broadband effectively single-mode operation from 1450 nm to 1850 nm can be achieved using a modified honeycomb PBGF with a defected core realized by removing 7 air holes. (c) 2006 Optical Society of America.

Low-temperature-sensitivity heterostructure photonic-crystal wavelength-selective filter based on ultralow-refractive-index metamaterials
NI Florous, K Saitoh, M Koshiba, M Skorobogatiy
APPLIED PHYSICS LETTERS, 88, 12, 1, 3, AMER INST PHYSICS, Mar. 2006, [Peer-reviewed]
English, Scientific journal, We propose and numerically investigate the thermal-insensitive properties of a wavelength-selective filter based on heterostructure photonic crystals with ultralow-refractive-index metallic nanowires. The operational principle of the proposed device is based on the photon trapping by total external reflections between the ultralow refractive index metamaterial claddings and the guiding air cores. The low propagation losses, the ultracompact size as well as the temperature-insensitive operation are the main advantages of the proposed metamaterial technology, making the proposed de-multiplexer an excellent candidate for applications in nanophotonic-integrated systems operating in the visible frequency spectrum.

The impact of elliptical deformations for optimizing the performance of dual-core fluorine-doped photonic crystal fiber couplers
SK Varshney, NJ Florous, K Saitoh, M Koshiba
OPTICS EXPRESS, 14, 5, 1982, 1995, OPTICAL SOC AMER, Mar. 2006, [Peer-reviewed]
English, Scientific journal, In this paper we study the impact of elliptically-deformed features such as cladding air-holes and elliptically-modulated cores, as ingredients for optimizing the coupling characteristics of dual-core fluorine-doped photonic crystal fiber (PCF) couplers. We provide a detailed numerical investigation by using a trial and error approach for optimizing the propagation characteristics of fluorine-doped PCF couplers. Typical characteristics of the newly proposed PCF coupler structure are: wavelength-flattened coupling characteristics between 0.7 mu m and 1.6 mu m wavelength range, coupling efficiency of 50 +/- 1% from 0.9 mu m to 1.6 mu m, and a reasonably small coupling length of 1.3 cm. In addition we have elaborately derived the design parameters so that our proposed dual-core PCF coupler exhibits polarization-insensitive characteristics verified by using a full-vectorial beam propagation method. The proposed dual-core PCF can be effectively used as a 3-dB coupler, over a wide wavelength range. (c) 2006 Optical Society of America.

Transverse lightwave circuits in microstructured optical fibers: resonator arrays
M Skorobogatiy, K Saitoh, M Koshiba
OPTICS EXPRESS, 14, 4, 1439, 1450, OPTICAL SOC AMER, Feb. 2006, [Peer-reviewed]
English, Scientific journal, Novel type of microstructured optical fiber couplers is introduced where energy transfer is enabled by transverse resonator arrays built into a fiber crossection. Such a design allows unlimited spatial separation between interacting fiber cores which, in turn, eliminates inter-core crosstalk via proximity coupling, thus enabling scalable integration of many fiber cores. Moreover, in the limit of weak inter-resonator coupling, resonator arrays exhibit moderate polarization dependence. (c) 2006 Optical Society of America.

Thermo-optical sensitivity analysis in photonic crystal circuits based on semiconducting or metallic metamaterial constituents
NJ Florous, K Saitoh, M Koshiba
OPTICS LETTERS, 31, 3, 404, 406, OPTICAL SOC AMER, Feb. 2006, [Peer-reviewed]
English, Scientific journal, We introduce a novel analysis technique for predicting thermo-optical sensitivities in photonic crystal (PC) circuits composed of either dielectric-semiconducting or metallic constituents. The proposed numerical analysis is based on a hybrid formalism of the scattering matrix technique combined with the adjoint network method. The proposed computational scheme can, with modest computational resources, predict with high accuracy, the effect of the temperature fluctuations to the light-wave propagation in PCs. Numerical simulations show that PC circuits based on metallic metamaterial platforms are significantly less sensitive to temperature variations than the usual dielectric or semiconducting PCs. (c) 2006 Optical Society of America.

Transverse light guides in microstructured optical fibers
M Skorobogatiy, K Saitoh, M Koshiba
OPTICS LETTERS, 31, 3, 314, 316, OPTICAL SOC AMER, Feb. 2006, [Peer-reviewed]
English, Scientific journal, A novel class of microstructured optical fiber coupler is introduced that operates by resonant, rather than proximity, energy transfer by means of transverse light guides built into a fiber cross section. Such a design permits significant spatial separation between interacting fibers, which, in turn, eliminates intercore cross talk owing to proximity coupling. A controllable energy transfer between the cores is then achieved by localized and highly directional transmission through a transverse light guide. The main advantage of this coupling scheme is its inherent scalability, as one can integrate additional fiber cores into the existing fiber cross section simply by placing the cores far enough from the existing optical circuitry to prevent proximity cross talk and then making the necessary intercore connections with transverse light wires, in direct analogy with on-chip electronics integration. (c) 2006 Optical Society of America.

Transmission characteristics of laterally illuminated photonic crystal fibers
Keiichi Morikawa, Takeshi Fujisawa, Kunimasa Saitoh, Masanori Koshiba
IEICE ELECTRONICS EXPRESS, 3, 4, 70, 73, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, Feb. 2006, [Peer-reviewed]
English, Scientific journal, Transmission characteristics of laterally illuminated photonic crystal fibers for both Gamma-K and Gamma-M directions are investigated. From the numerical results obtained, it is found that a stop band is formed in the transmission spectrum for TE modes and light illumination towards Gamma-K direction, even though a photonic band gap (PBG) in Gamma-K direction does not exist. Such a pseudo-PBG can be used for photonic crystal fiber sensing applications as well as a true PBG.

Realization of polarization-independent splitters based on highly-birefringent dual core photonic crystal fibers
Kunimasa Saitoh, Nikolaos Florous, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2006
English, International conference proceedings, We propose and numerically demonstrate the operation of a novel type of polarizationindependent splitter based on photonic crystal fiber architecture. The proposed splitter is an excellent candidate for performing wavelength de-multiplexing operation at 1.30 μm and 1.55 μm. © 2006 Optical Society of America.

Light-wave guidance through stratified photonic crystal metamaterials synthesized by super-inductive layers of metallic nano-strips
Nikolaos John Florous, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2006
English, International conference proceedings, We theoretically demonstrate the possibility of strong light-wave guidance through a novel class of photonic crystal metamaterial platform entirely synthesized by ultra-low refractive index suspended nano-strips. Basic waveguiding structures can be realized on the nano-scale. Optical Society of America. © 2006 Optical Society of America.

Hetero-structure photonic crystal demultiplexer based on ultra-low refractive index nanowires: Towards temperature-insensitive metamaterial platforms
Nikolaos John Florous, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2006
English, International conference proceedings, We propose and numerically investigate the thermal-insensitive properties of a novel type of wavelength selective filter based on heterostructure photonic crystals entirely synthesized by ultra-low refractive index metallic nanowires, for efficient demultiplexing of visible frequencies. © 2006 Optical Society of America.

Design of twin core fluorine-doped photonic crystal fiber directional coupler incorporating elliptical air-holes
Shailendra K. Varshney, K. Saitoh, N. Florous, M. Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2006
English, International conference proceedings, Using an accurate simulator based on full-vectorial finite element method, we demonstrate the operation of a novel type of photonic crystal fiber coupler with inherently flattened wavelength (900-nm) and polarization-insensitive coupling characteristics. © 2006 Optical Society of America.

Optical characterization of planar photonic crystal structures with elliptically-elongated veins: A generalized fourier-mathieu multipole expansion technique　　　　　　　　　　　　　　　
Nikolaos Florous, Kunimasa Saitoh, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America, 2006
English, International conference proceedings, An efficient analytical approach based on an extension of the scattering-matrix technique is presented for characterizing planar photonic crystal resonant-cavities composed of elliptical veins. Extensive numerical results for various lattices are presented. © 2006 Optical Society of America.

Thermo-optical sensitivity analysis of highly-birefringent photonic crystal fibers with elliptically elongated air holes: Towards polarimetric-sensing fiber platforms　　　　　　　　　　　　　　　
Nikolaos John Florous, Kunimasa Saitoh, Shailendra Varsheney, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America, 2006
English, International conference proceedings, We theoretically investigate the thermo-optical sensitivity response in highly birefringent photonic crystal fibers (PCFs), and we identify novel mechanisms that will lead to the potential use of PCFs as polarimetric sensing platforms. © 2006 Optical Society of America.

The role of artificial defects for engineering large effective mode area, flat chromatic dispersion, and low leakage losses in photonic crystal fibers: Towards high speed reconfigurable transmission platforms
N Florous, K Saitoh, M Koshiba
OPTICS EXPRESS, 14, 2, 901, 913, OPTICAL SOC AMER, Jan. 2006, [Peer-reviewed]
English, Scientific journal, The present paper describes a novel systematic solution to the challenging task of realizing photonic crystal fibers (PCFs) with flat chromatic dispersion, low leakage losses, and large mode area, mainly for applications as information carriers in wide-band high speed optical transmission systems. The proposed design strategy is based on the existence of an artificially-defected air-hole ring in the cladding and on the modulation of the refractive index of the core by assembling additional defected air-holes in the central core region of the fiber. The validation of the proposed design is carried out by adopting an efficient full-vectorial finite element method with perfectly matched layers for accurate characterization of PCFs. The remarkable flat chromatic dispersion as well as the large mode area and the low leakage losses are the main advantages of the proposed PCF structure, making it an ideal candidate for performing wavelength division multiplexing operation in reconfigurable optical transmission systems or as an information delivering platform in high speed optical communication systems. Typical characteristics of the newly proposed PCF are: flattened chromatic dispersion of 6.3 +/- 0.5 ps/km/nm in the S+C+L telecommunication band, and effective mode area as large as 100 mu m(2) in the same wavelength range. We additionally provide numerical data about the performance of the proposed PCF in splicing mode as well as during macrobending operation and we give qualitative information regarding the sensitivity of the proposed transmission platform to structural disorders of the design parameters. (c) 2006 Optical Society of America.

Theoretical realization of holey fiber with flat chromatic dispersion and large mode area: an intriguing defected approach
K Saitoh, NJ Florous, M Koshiba
OPTICS LETTERS, 31, 1, 26, 28, OPTICAL SOC AMER, Jan. 2006, [Peer-reviewed]
English, Scientific journal, We present a novel design approach for realizing holey fibers (HFs) with flat dispersion characteristics and large mode area based on the existence of an artificially defected air-hole ring in the cladding, and on the inclusion of additional defected air holes in the core of the fiber. This unique type of HF can be used for achieving remarkable flat dispersion characteristics as well as a large mode area, which are particularly useful for high-speed data transmission. The validation of the proposed design is done by adopting an efficient full-vectorial finite element method for optical characterization of HFs. The proposed fiber can be employed in reconfigurable optical transmission systems for performing wavelength division multiplexing operation. Typical characteristics of the proposed HF are a flattened dispersion of 6.3 +/- 0.5 ps/km/nm from 1.45 to 1.65 mu m and an effective mode area as large as 100 mu m(2) in the same frequency range. (c) 2006 Optical Society of America.

Chromatic dispersion profile optimization of dual-concentric-core photonic crystal fibers for broadband dispersion compensation
T Fujisawa, K Saitoh, K Wada, M Koshiba
OPTICS EXPRESS, 14, 2, 893, 900, OPTICAL SOC AMER, Jan. 2006, [Peer-reviewed]
English, Scientific journal, Chromatic dispersion profile of dual-concentric-core photonic crystal fibers is optimized for broadband dispersion compensation of single mode fibers (SMFs) by using genetic algorithm incorporated with full-vector finite-element method. From the numerical results presented here, it is found that by increasing the distance between central core and outer ring core, larger negative dispersion coefficient and better dispersion slope compensation are possible. There is a trade off between the magnitude of negative dispersion coefficient and dispersion slope compensation due to the concave dispersion profile of dual-concentric-core photonic crystal fibers. In spite of the tradeoff, dual-concentric-core photonic crystal fibers having larger negative dispersion coefficient as well as compensating for dispersion slope of SMFs in the entire C band with large effective area can be designed. (c) 2006 Optical Society of America.

Non-proximity resonant tunneling in multi-core photonic band gap fibers: A revolutionary technology for all-fiber integrated assemblies
Kunimasa Saitoh, Nikolaos J. Florous, Tadashi Murao, Masanori Koshiba, Maksim Skorobogatiy
2006 European Conference on Optical Communications Proceedings, ECOC 2006, 2006, [Peer-reviewed]
English, International conference proceedings, We demonstrate the operation of a novel class of narrow-band pass filters based on the revolutionary technology of non-proximity resonant tunneling in multi-core photonic band gap fibers.

Transverse lightwave circuits in MOFs: Waveguides and resonator arrays
M. Skorobogatiy, K. Saitoh, M. Koshiba
Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006, 2006, [Peer-reviewed]
English, International conference proceedings, Microstructured optical fiber couplers are proposed that operate by resonant rather than proximity energy transfer via transverse waveguides and resonator arrays in a fiber crossection, allowing significant inter-fiber separations, eliminating inter-core crosstalk by proximity coupling. © 2005 Optical Society of America.

A design method for single-mode holey fibers with low bending losses
Y Tsuchida, K Saitoh, M Koshiba
ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS, 89, 2, 1, 7, SCRIPTA TECHNICA-JOHN WILEY & SONS, 2006, [Peer-reviewed]
English, Scientific journal, With increased demand for larger capacity and lower cost for optical communication systems, high-density wiring and indoor wiring using optical fibers that can be installed or stored with a small bending radius are required. In response to this requirement, the holey fiber (HF), which has a structure with several air holes around the usual optical fiber core, has drawn attention. For the HF, the bending losses can be reduced by enlarging the air holes. However, if a large air hole diameter is used, there is a possibility that multimode operation may occur. Like the conventional optical fiber, the bending losses can be reduced in the HF if the mode field diameter (MFD) is made smaller. In this case, it becomes difficult to connect the HF with a standard single-mode fiber without increasing the loss. In this paper, an HF with two layers of air holes around a core doped with germanium is considered. It is reported that an HF with an allowable bending radius of less than 6.5 mm, a cutoff wavelength of less than 1.25 mu m, and an MFD of about 9 pm at a wavelength of 1.55 mu m can be designed if the core diameter, the hole spacing, and the hole diameter in each layer are appropriately selected. (c) 2006 Wiley Periodicals, Inc.

Design of gain-flattened highly nonlinear photonic crystal fiber Raman amplifier using a single pump: A leakage loss approach
Shailendra K. Varshney, K. Saitoh, T. Fujisawa, M. Koshiba
2006 OPTICAL FIBER COMMUNICATION CONFERENCE/NATIONAL FIBER OPTIC ENGINEERS CONFERENCE, VOLS 1-6, 461, 463, OPTICAL SOC AMERICA, 2006, [Peer-reviewed]
English, International conference proceedings, We present a novel inherent gain-flattened design of highly nonlinear photonic crystal fiber Raman amplifier exhibiting +/- 0.85-dB gain-ripples over 28-nm bandwidth using a single pump in C-band employing the leakage loss property with integrated dispersion compensation functionality. (C) 2005 Optical Society of America.

Multi-Core Photonic Crystal Fibers and Their Applications to Fiber Devices
KOSHIBA MASANORI, SAITO KUNIMASA
レーザー研究, 34, 1, 31, 36, Jan. 2006, [Peer-reviewed], [Invited]
Japanese

Endlessly single-mode holey fibers: the influence of core design
K Saitoh, Y Tsuchida, M Koshiba, NA Mortensen
OPTICS EXPRESS, 13, 26, 10833, 10839, OPTICAL SOC AMER, Dec. 2005, [Peer-reviewed]
English, Scientific journal, In this paper we evaluate the cut-off properties of holey fibers (HFs) with a triangular lattice of air holes and the core formed by the removal of a single (HF1) or more air holes (HF3 and HF7). With the aid of finite-element simulations we determine the single-mode and multi-mode phases and also find the air hole diameters limiting the endlessly single-mode regime. From calculations of V and W parameters we find that in general HF1 is less susceptible to longitudinal non-uniformities compared to the other designs for equivalent effective areas. As an example we illustrate this general property for the particular case of a macro-bending induced loss. (c) 2005 Optical Society of America.

Design of narrow band-pass filters based on the resonant-tunneling phenomenon in multicore photonic crystal fibers
K Saitoh, NJ Florous, M Koshiba, M Skorobogatiy
OPTICS EXPRESS, 13, 25, 10327, 10335, OPTICAL SOC AMER, Dec. 2005, [Peer-reviewed]
English, Scientific journal, The objective of the present paper is to introduce and numerically demonstrate the operation of a novel band-pass filter based on the phenomenon of resonant tunneling in multi-core photonic crystal fibers (PCFs). The proposed PCF consists of two identical cores separated by a third one which acts as a resonator. With a fine adjustment of the design parameters associated with the resonant-core, phase matching at a single wavelength can be achieved, thus enabling very narrow-band resonant directional coupling between the input and the output cores. The validation of the design is ensured with an accurate PCF analysis based on finite element and beam propagation algorithms. The proposed narrow band-pass filter can be employed in various applications such as all fiber bandpass/bandstop filtering.

Novel design of inherently gain-flattened discrete highly nonlinear photonic crystal fiber Raman amplifier and dispersion compensation using a single pump in C-band
SK Varshney, T Fujisawa, K Saitoh, M Koshiba
OPTICS EXPRESS, 13, 23, 9516, 9526, OPTICAL SOC AMER, Nov. 2005, [Peer-reviewed]
English, Scientific journal, In this paper, we report, for the first time, an inherently gain-flattened discrete highly nonlinear photonic crystal fiber (HNPCF) Raman amplifier (HNPCF-RA) design which shows 13.7 dB of net gain (with +/- 0.85-dB gain ripple) over 28-nm bandwidth. The wavelength dependent leakage loss property of HNPCF is used to flatten the Raman gain of the amplifier module. The PCF structural design is based on W-shaped refractive index profile where the fiber parameters are well optimized by homely developed genetic algorithm optimization tool integrated with an efficient vectorial finite element method (V- FEM). The proposed fiber design has a high Raman gain efficiency of 4.88 W-1. km(-1) at a frequency shift of 13.1 THz, which is precisely evaluated through V- FEM. Additionally, the designed module, which shows ultra-wide single mode operation, has a slowly varying negative dispersion coefficient (-107.5 ps/nm/km at 1550 nm) over the operating range of wavelengths. Therefore, our proposed HNPCF-RA module acts as a composite amplifier with dispersion compensator functionality in a single component using a single pump. (c) 2005 Optical Society of America.

Numerical modeling of photonic crystal fibers
K Saitoh, M Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 23, 11, 3580, 3590, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2005, [Peer-reviewed], [Invited]
English, Scientific journal, Recent progress on numerical modeling methods for photonic crystal fibers (PCFs) such as the effective index approach, basis-function expansion approach, and numerical approach is described. An index-guiding PCF with an array of air holes surrounding the silica core region has special characteristics compared with conventional single-mode fibers (SMFs). Using a full modal vector model, the fundamental characteristics of PCFs such as cutoff wavelength, confinement loss, modal birefringence, and chromatic dispersion are numerically investigated.

Modeling of two-dimensional photonic crystal resonant cavities incorporating elliptically shaped dielectric cylinders
NJ Florous, K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 17, 11, 2316, 2318, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2005, [Peer-reviewed]
English, Scientific journal, Modeling of photonic crystal structures is an ongoing challenge. This letter is devoted to the theoretical modeling of photonic crystal resonant cavities, composed of elliptically shaped dielectric pillars. Through this rigorous study, we can examine the impact of the elliptical-shape deformations to important quantities related to the photonic crystal cavity performance, such as the quality factor and the resonance frequency.

Theoretical investigation of photonic crystal waveguide splitters incorporating ultralow refractive index metallic nanowires
NJ Florous, K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 17, 11, 2313, 2315, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2005, [Peer-reviewed]
English, Scientific journal, The present letter theoretically investigates the optical properties, and provides design guidelines for optimum performance of a photonic crystal (PC) waveguide splitter composed of ultralow refractive index metamaterials, by utilizing a rigorous analytical modeling based on the scattering matrix technique. We believe that this type of nanocomposite metallic PC waveguide can bring additional degrees of design flexibility to ultradense optical integration, because of its compact size and relatively low-loss propagation characteristics, mainly for powersplitting applications in the visible frequency spectrum.

A novel design for dispersion compensating photonic crystal fiber Raman amplifier
SK Varshney, K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 17, 10, 2062, 2064, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2005, [Peer-reviewed]
English, Scientific journal, This letter presents a novel design for dispersion compensating photonic crystal fiber (DCPCF) which shows inherently flattened high Raman gain of 19 dB ( +/- 1.2-dB gain ripple) over 30-nm bandwidth. The proposed design module has been simulated through an efficient full-vectorial finite element method. The designed DCPCF has a high negative dispersion coefficient (-200 to -250 ps/nm/km) over C-band wavelength (1530-1568 nm). The proposed fiber module of 5.2-km length not only compensates the accumulated dispersion in conventional single-mode fiber (SMF-28) but also compensates for the dispersion slope. Hence, the designed DCPCF module acts as the gain-flattened Raman amplifier and dispersion compensator.

Three-color photonic crystal demultiplexer based on ultralow-refractive-index metamaterial technology
NJ Florous, K Saitoh, M Koshiba
OPTICS LETTERS, 30, 20, 2736, 2738, OPTICAL SOC AMER, Oct. 2005, [Peer-reviewed]
English, Scientific journal, We numerically demonstrate the operation of a novel class of wavelength-division demultiplexing circuit based on photonic crystal waveguides that are entirely synthesized by ultralow-refractive-index metallic nanopillars. The operational principle of the newly proposed device is based on the phenomenon of total external reflections in ultralow-refractive-index metallic photonic crystal structures (metamaterials). In addition we provide detailed design guidelines for optimum device performance. The low propagation losses and compact size, as well as temperature-insensitive operation over a wide temperature range, are only a few of the advantages of the proposed technology, making this new type of demultiplexer an excellent candidate for applications in the visible spectrum. (c) 2005 Optical Society of America

Ultra-flattened chromatic dispersion controllability using a defected-core photonic crystal fiber with low confinement losses
K Saitoh, N Florous, M Koshiba
OPTICS EXPRESS, 13, 21, 8365, 8371, OPTICAL SOC AMER, Oct. 2005, [Peer-reviewed]
English, Scientific journal, The present paper describes a novel systematic solution to the problem of controlling the chromatic dispersion and dispersion slope in photonic crystal fibers (PCFs), using a structurally-simple PCF with a defected-core. By adjusting the size of the central air-hole defect we can successfully design an ultra-flattened PCF with low confinement losses, as well as small effective mode area. The design strategy is based on the mutual cancellation between the waveguide and the material dispersions of the PCF, by varying the size of the central defected region in the core. The verification of the ultra-flattened chromatic dispersion property of the proposed PCF is ensured with an accurate full-vector finite element method with anisotropic perfectly matched layers. The ultra-flattened dispersion feature, as well as the low confinement losses and the small effective mode area are the main advantages of the proposed PCF structure, making it suitable as a chromatic dispersion controller, dispersion compensator, or as candidate for nonlinear optical applications. (c) 2005 Optical Society of America.

Transverse lightwave circuits in microstructured optical fibers: waveguides
M Skorobogatiy
OPTICS EXPRESS, 13, 19, 7506, 7515, OPTICAL SOC AMER, Sep. 2005, [Peer-reviewed]
English, Scientific journal, Novel class of microstructured optical fiber couplers is introduced that operates by resonant, rather than proximity, energy transfer via transverse lightguides built into a fiber cross-section. Such a design allows unlimited spatial separation between interacting fibers which, in turn, eliminates inter-core crosstalk via proximity coupling. Controllable energy transfer between fiber cores is then achieved by localized and highly directional transmission through a transverse lightguide. Main advantage of this coupling scheme is its inherent scalability as additional fiber cores could be integrated into the existing fiber cross-section simply by placing them far enough from the existing circuitry to avoid proximity crosstalk, and then making the necessary inter-core connections with transverse light "wires" - in a direct analogy with an on chip electronics integration. (c) 2005 Optical Society of America.

A novel approach for designing photonic crystal fiber splitters with polarization-independent propagation characteristics
N Florous, K Saitoh, M Koshiba
OPTICS EXPRESS, 13, 19, 7365, 7373, OPTICAL SOC AMER, Sep. 2005, [Peer-reviewed]
English, Scientific journal, We propose and numerically investigate the operation of a novel class of polarization-independent splitters based on the photonic crystal fiber (PCF) technology. The proposed polarization-independent feature of the PCF splitter is realized by uniformly distributed elliptically-shaped airholes in the cladding of a dual-core PCF. The design procedure follows a rigorous synthesis protocol based on exact equations for describing the wavelength de-coupling mechanism, and on full-vectorial finite element as well as beam propagation methods for optical characterization of PCFs. The compact de-coupling lengths as well as the low cross-talk over appreciable optical bandwidths are the main advantages of the proposed PCF splitter. The proposed device can be employed in reconfigurable optical communication systems for performing wavelength de-multiplexing operation, especially for fiber-to-the-home applications, as well as the emerging passive optical network applications. (c) 2005 Optical Society of America.

Simple evaluation of confinement losses in holey fibers
M Koshiba, K Saitoh
OPTICS COMMUNICATIONS, 253, 1-3, 95, 98, ELSEVIER SCIENCE BV, Sep. 2005, [Peer-reviewed]
English, Scientific journal, An easy-to-evaluate method is described for the prediction of confinement losses in a photonic crystal fiber with a triangular air-hole lattice, also called a holey fiber (HF). First, a HF with a finite number of holes is replaced by a doubly clad fiber with a depressed-index inner cladding and then, a simple expression for leakage losses in the doubly clad fiber is revived for the evaluation of confinement losses in the HF. Calculated confinement losses agree approximately well with those of numerical simulations. (c) 2005 Elsevier B.V. All rights reserved.

Bending-insensitive single-mode hole-assisted fibers with reduced splice loss
K Saitoh, Y Tsuchida, M Koshiba
OPTICS LETTERS, 30, 14, 1779, 1781, OPTICAL SOC AMER, Jul. 2005, [Peer-reviewed]
English, Scientific journal, We propose and demonstrate a novel type of bending-insensitive single-mode hole-assisted fiber that has a doped core and two layers of holes with two different airhole diameters. The fiber has a 9.3 mu m mode field diameter, a bending loss of 0.011 dB/turn at 1.55 mu m for a bending diameter of 10 mm, and a cutoff wavelength below 1.1 mu m. The fiber can be fusion spliced to a conventional single-mode fiber with low loss. (c) 2005 Optical Society of America.

低曲げ損単一モードホーリーファイバの一設計　　　　　　　　　　　　　　　
土田幸寛, 齊藤晋聖, 小柴正則
電子情報通信学会論文誌, J88-C, 3, 65, 72, Jul. 2005, [Peer-reviewed]
Japanese, Scientific journal

A Design Method for Single-Mode Holey Fibers with Low Bending Losses
TSUCHIDA YUKIHIRO, SAITO KUNIMASA, KOSHIBA MASANORI
電子情報通信学会論文誌 C, J88-C, 7, 512, 518, The Institute of Electronics, Information and Communication Engineers, Jul. 2005, [Peer-reviewed]
Japanese, 光通信システムに対する大容量化と低コスト化の要求が高まるにつれて, 小さな曲げで敷設や収納が可能な光ファイバを用いた高密度配線や宅内配線が求められている.この要求にこたえるために, 最近, 通常の光ファイバのコア周辺に複数の空孔を設けた構造のホーリーファイバ(HF)が注目されている.HFにおいては, 空孔径を大きくすることにより, 曲げ損を低減することができるが, 大きな空孔径を用いると, マルチモードファイバとなる問題がある.また, 通常の光ファイバと同様に, モードフィールド径(MFD)を小さくすれば, 曲げ損を低減できるが, この場合, 標準的なシングルモードファイバと低損失で接続することが難しくなる.ここでは, ゲルマニウムを添加したコアの周囲に空孔を2層設けたHFを考え, コア径と空孔間隔, 及び各層の空孔直径を適切に設定することで, 許容曲げ半径が6.5mm以下, カットオフ波長が1.25μm以下, MFDが波長1.55μmにおいて約9μmのHFが設計可能であることを報告する.

Design and characterization of single-mode holey fibers with low bending losses
Y Tsuchida, K Saitoh, M Koshiba
OPTICS EXPRESS, 13, 12, 4770, 4779, OPTICAL SOC AMER, Jun. 2005, [Peer-reviewed]
English, Scientific journal, As the fiber-to-the-home network construction increased, optical fiber cables are demanded to be easier to handle and require less space. Under this situation, a single mode fiber (SMF) permitting small bending radius is strongly requested. In this paper, we propose and demonstrate a novel type of bending-insensitive single-mode holey fiber that has a doped core and two layers of holes with different air-hole diameters. The fiber has a mode field diameter of 9.3 mu m at 1.55 mu m and a cutoff wavelength below 1.1 mu m, and shows a bending loss of 0.011 dB/turn at 1.55 mu m for a bending radius of 5 mm and a low splice loss of 0.08 dB per fusion-splicing to a conventional SMF. (C) 2005 Optical Society of America.

Fundamental characteristics of localized acoustic modes in photonic crystal fibers
Enomori, I, K Saitoh, M Koshiba
IEICE TRANSACTIONS ON ELECTRONICS, E88C, 5, 876, 882, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, May 2005, [Peer-reviewed]
English, Scientific journal, Propagation characteristics of acoustic waves in photonic crystal fibers (PCFs) have been theoretically investigated in details. In order to evaluate acoustic band structures and guided modes for out-of-plane propagation in PCFs, analysis methods based on the finite element method are newly formulated. It is shown through numerical results that complete acoustic band-gaps (ABGs) exist in the cladding region of PCFs and that acoustic guided modes could be localized in the defect region of PCFs by the ABG effect. Furthermore, it is shown that acoustic guided modes could also be localized in the defect region of PCFs by the total internal reflection. These confinement mechanisms of acoustic waves propagating along the fiber length are completely different to those of lightwaves.

High group birefringence in air-core photonic bandgap fibers
MS Alam, K Saitoh, M Koshiba
OPTICS LETTERS, 30, 8, 824, 826, OPTICAL SOC AMER, Apr. 2005
English, Scientific journal, A numerical investigation of group birefringence is carried out on a recently reported highly hirefringent hollow-core photonic bandgap fiber by use of an efficient vector finite-element method. The hollow fiber core has an area as large as that of approximately four airholes in the cladding region and assumes a rhombic shape with round corners, and the airholes in the cladding region are hexagonal and provide a high airfilling fraction. Numerical results show very high group birefringence of the order of 10(-2) and phase birefringence of the order of 10(-3). (c) 2005 Optical Society of America.

Numerical investigation of raman amplification properties in photonic crystal fibers
Shailendra K. Varshney, K. Saitoh, M. Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2005
English, International conference proceedings, A full-vectorial finite-element method is used to investigate Raman amplification properties of photonic crystal fibers. Raman gain of 9 dB is obtained in 4-km length of PCF with a high optical signal-to-noise ratio. © 2005 Optical Society of America.

Novel bandpass filter based on a three-core photonic crystal fiber
Kunimasa Saitoh, Yuichiro Sato, Masanori Koshiba
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2005
English, International conference proceedings, The wavelength-selective coupling of three-core photonic crystal fibers is investigated numerically. It is found that, with suitable choice of air-hole diameters, it is possible to obtain allfiber bandpass filtering devices for various applications. © 2003 Optical Society of America.

Directional coupling in hollow Bragg fiber bundles　　　　　　　　　　　　　　　
Maksim Skorobogatiy, Kunimasa Saitoh, Masanori Koshiba
Conference on Optical Fiber Communication, Technical Digest Series, 2, 249, 251, 2005
English, International conference proceedings, Coupling between TE01 modes of two collinear hollow-core Bragg fibers is characterized as a function of the inter-fiber separation and wavelength in a view of designing a directional coupler. © 2004 Optical Society of America.

A design method for bending-insensitive single-mode holey fibers　　　　　　　　　　　　　　　
Kunimasa Saitoh, Yukihiro Tsuchida, Masanori Koshiba
Conference on Optical Fiber Communication, Technical Digest Series, 1, 5, 7, 2005
English, International conference proceedings, We propose a novel design method for bending-insensitive single-mode holey fibers with a mode field diameter of 9 μm, a small allowable bending radius of 6.4 mm, and a cutoff wavelength of 1.24 μm. © 2004 Optical Society of America.

Empirical relations for simple design of photonic crystal fibers
K Saitoh, M Koshiba
OPTICS EXPRESS, 13, 1, 267, 274, OPTICAL SOC AMER, Jan. 2005
English, Scientific journal, In order to simply design a photonic crystal fiber (PCF), we provide numerically based empirical relations for V parameter and W parameter of PCFs only dependent on the two structural parameters - the air hole diameter and the hole pitch. We demonstrate the accuracy of these expressions by comparing the proposed empirical relations with the results of full-vector finite element method. Through the empirical relations we can easily evaluate the fundamental properties of PCFs without the need for numerical computations. (C) 2005 Optical Society of America.

Raman amplification properties of ultralow loss photonic crystal fibers
SK Varshney, K Saitoh, M Koshiba
2005 PACIFIC RIM CONFERENCE ON LASERS AND ELECTRO-OPTICS, 343, 344, IEEE, 2005, [Peer-reviewed]
English, International conference proceedings, A full-vectorial finite element method is used to investigate Raman amplification performances of ultralow loss photonic crystal fibers. More than 10 dB of net Raman gain can be achieved regardless large scattering losses.

Localized acoustic modes in photonic crystal fibers
Enomori, I, K Saitoh, M Koshiba, T Matsui
ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS, 88, 3, 27, 35, SCRIPTA TECHNICA-JOHN WILEY & SONS, 2005, [Peer-reviewed]
English, Scientific journal, A computational method based on the finite element method is proposed for the first time to evaluate the band structure and the guided mode of an acoustic wave propagating in a photonic crystal fiber (PCF). By using this method, it is found that an acoustic band gap (ABG) is formed within a PCF and that there exists a propagating guided acoustic mode localized at the defects in the PCF due to the trapping effect of the ABG. (C) 2005 Wiley Periodicals, Inc.

Bending-insensitive single-mode hole-assisted fibers with reduced splice loss
Tsuchida, V, K Saitoh, M Koshiba
2005 Conference on Lasers & Electro-Optics (CLEO), Vols 1-3, 384, 386, OPTICAL SOC AMERICA, 2005, [Peer-reviewed]
English, International conference proceedings, We propose a novel type of bending-insensitive single-mode hole-assisted fiber with a mode field diameter of 9.3 mu m and a bending loss of 0.011 dB/turn at 1.55 mu m for bending diameter of 10 trim. (c) 2005 Optical Society of America.

Coupling between two collinear air-core Bragg fibers
M Skorobogatiy, K Saitoh, M Koshiba
Photonic Crystal Materials and Devices III, 5733, 206, 213, SPIE-INT SOC OPTICAL ENGINEERING, 2005, [Peer-reviewed]
English, International conference proceedings, We characterize coupling between two identical collinear hollow core Bra fibers. assuming TE01 launching condition. Using multipole method and finite element method we investigate dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the inter-fiber separation, fiber core radius and index of the cladding. We established that coupling is maximal when fibers are touching each other decreasing dramatically during the first tens of nanometers of separation. However, residual coupling with the strength proportional to the fiber radiation loss is very long range decreasing as an inverse square root of the inter-fiber separation. and exhibiting periodic variation with inter-fiber separation. Finally. coupling between the TEO, modes is considered in a view of designing a directional coupler. We find that for fibers with large enough core radii one can identify broad frequency ranges where intermodal coupling strength exceeds super-mode radiation losses by an order of magnitude, thus opening a possibility of building a directional coupler. We attribute such unusually strong inter-mode coupling both to the resonant effects in the inter-mirror cavity as well as a proximity interaction between the leaky modes localized in the mirror.

Coupling between two collinear air-core Bragg fibers
M Skorobogatiy, K Saitoh, M Koshiba
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 21, 12, 2095, 2101, OPTICAL SOC AMER, Dec. 2004, [Peer-reviewed]
English, Scientific journal, We characterize coupling between two identical collinear hollow-core Bragg fibers, assuming a TE01 launching condition. Using a multipole method and a finite-element method, we have investigated the dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the interfiber separation, the fiber core radius, and the index of the cladding. We established that coupling is maximal when the fibers are touching each other and decreases dramatically during the first hundreds of nanometers of separation. However, residual coupling with the strength proportional to the fiber radiation loss decreased over a long range as an inverse square root of the interfiber separation and exhibited periodic variation with interfiber separation. Finally, we considered coupling between the TE01 modes with a view to designing a directional coupler. We found that for fibers with large enough core radii one can identify broad frequency ranges in which the intermodal coupling strength exceeds supermode radiation losses by 1 order of magnitude, thus opening the possibility of building a directional coupler. We attribute such unusually strong intermode coupling both to the resonant effects in the intermirror cavity and to proximity interaction between the leaky modes localized in the mirror. (C) 2004 Optical Society of America.

Resonant directional coupling of hollow Bragg fibers
M Skorobogatiy, K Saitoh, M Koshiba
OPTICS LETTERS, 29, 18, 2112, 2114, OPTICAL SOC AMER, Sep. 2004, [Peer-reviewed]
English, Scientific journal, Coupling between the lowest-loss TE01 modes of two touching hollow-core Bragg fibers is considered with a view to designing a directional coupler. We have found that for fibers with core radii larger than 10 mum one can identify broad frequency ranges in which intermodal coupling strength exceeds supermode radiation losses by an order of magnitude, thus opening the possibility of building a directional coupler. We attribute such unusually strong intermode coupling both to the resonant effects in the intermirror cavity and a proximity interaction between the leaky modes localized in the mirror. (C) 2004 Optical Society of America.

Polarization splitter in three-core photonic crystal fibers
K Saitoh, Y Sato, M Koshiba
OPTICS EXPRESS, 12, 17, 3940, 3946, OPTICAL SOC AMER, Aug. 2004, [Peer-reviewed]
English, Scientific journal, A novel design of polarization splitter in three-core photonic crystal fibers (PCFs) has been proposed. The three-core PCF consists of two given identical cores with two-fold symmetry separated by a core with high birefringence. The polarization splitter is based on the phenomenon of resonant tunneling. Numerical simulations with a full vectorial beam propagation method demonstrate that it is possible to obtain a 1.9-mm-long splitter with the extinction ratio better than -20 dB and a bandwidth of 37 nm. (C) 2004 Optical Society of America.

Applicability of classical optical fiber theories to holey fibers
M Koshiba, K Saitoh
OPTICS LETTERS, 29, 15, 1739, 1741, OPTICAL SOC AMER, Aug. 2004, [Peer-reviewed]
English, Scientific journal, We discuss the applicability of well-established classical optical fiber theories to holey fibers. By appropriately defining the V parameter, we can easily estimate the fundamental properties of holey fibers, such as effective index, group-velocity dispersion, mode field diameter, beam divergence, and splice loss, through simple empirical expressions without the need for heavy numerical computations. We confirm the validity of the V parameter defined here by comparing the calculated results with the earlier experimental and numerical results. (C) 2004 Optical Society of America.

Highly nonlinear dispersion-flattened photonic crystal fibers for supercontinuum generation in a telecommunication window
K Saitoh, M Koshiba
OPTICS EXPRESS, 12, 10, 2027, 2032, OPTICAL SOC AMER, May 2004, [Peer-reviewed]
English, Scientific journal, We propose a new structure of highly nonlinear dispersion-flattened (HNDF) photonic crystal fiber (PCF) with nonlinear coefficient as large as 30 W-1 km(-1) at 1.55 mum designed by varying the diameters of the air-hole rings along the fiber radius. This innovative HNDF-PCF has a unique effective-index profile that can offer not only a large nonlinear coefficient but also flat dispersion slope and low leakage losses. It is shown through numerical results that the novel microstructured optical fiber with small normal group-velocity dispersion and nearly zero dispersion slope offers the possibility of efficient supercontinuum generation in the telecommunication window using a few ps pulses. (C) 2004 Optical Society of America.

Dispersion-Flattened Photonic Crystal Fibers with an Extra Air-Hole Located at the Core-Center
TSUCHIDA Yoshihiro, SAITOH Kunimasa, KOSHIBA Masanori
The Transactions of the Institute of Electronics, Information and Communication Engineers C, J87-C, 3, 320, 327, The Institute of Electronics, Information and Communication Engineers, Mar. 2004, [Peer-reviewed]
Japanese, Scientific journal, フォトニック結晶ファイバ(PCF)は,その格子定数や空孔径を変えることによって,波長分散特性をかなり自在に制御でき,既に通信波長帯において低分散かつ分散フラット特性を実現するための設計指針もいくつか報告されている.しかしながら,従来型のPCFで低分散かつ分散フラット特性を実現しようとすると,空孔径が小さいために,閉込め損失を抑制するには非常に多くの空孔リング数が必要となる.そこでここでは,クラッド領域に大きな空孔と,コア中心に微小空孔を有するPCFを用いて,より少ない空孔リング数で,広帯域な低分散・分散フラット特性を実現するための一方法を提案する.

Localized Acoustic Modes in Photonic Crystal Fibers
ENOMORI Ikumi, SAITOH Kunimasa, KOSHIBA Masanori, MATSUI Takashi
The Transactions of the Institute of Electronics, Information and Communication Engineers C, J87-C, 3, 312, 319, The Institute of Electronics, Information and Communication Engineers, Mar. 2004, [Peer-reviewed]
Japanese, Scientific journal, フォトニック結晶ファイバ(PCF)内を伝搬する弾性波のバンド構造及び導波モードを評価するための有限要素法に基づく計算手法を初めて提案している.本手法を用いてPCF内にアコースティックバンドギャップ(ABG)が形成されていること,更に,ABGの閉込め効果によってPCFの欠陥部に局在して伝搬する弾性波の導波モードが存在することを見出している.

Air-core photonic band-gap fibers: the impact of surface modes
K Saitoh, NA Mortensen, M Koshiba
OPTICS EXPRESS, 12, 3, 394, 400, OPTICAL SOC AMER, Feb. 2004, [Peer-reviewed]
English, Scientific journal, We study the dispersion and leakage properties for the recently reported low-loss photonic band-gap fiber by Smith et al. [Nature 424, 657 (2003)]. We find that surface modes have a significant impact on both the dispersion and leakage properties of the fundamental mode. Our dispersion results are in qualitative agreement with the dispersion profile reported recently by Ouzounov et al. [Science 301, 1702 (2003)] though our results suggest that the observed long-wavelength anomalous dispersion is due to an avoided crossing (with surface modes) rather than band-bending caused by the photonic band-gap boundary of the cladding. (C) 2004 Optical Society of America.

Hollow Bragg fiber bundles: when coupling helps and when it hurts
M Skorobogatiy, K Saitoh, M Koshiba
APPLICATIONS OF PHOTONIC TECHNOLOGY, CLOSING THE GAP BETWEEN THEORY, DEVELOPMENT, AND APPLICATION, PT 1 AND 2, 5577, 561, 567, SPIE-INT SOC OPTICAL ENGINEERING, 2004, [Peer-reviewed]
English, International conference proceedings, We characterize coupling between two identical collinear hollow core Bragg fibers, assuming TE01 launching condition. Using multipole method and finite element method we investigate dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the inter-fiber separation, fiber core radius and index of the cladding. We established that coupling is maximal when fibers are touching each other decreasing dramatically during the first tens of nanometers of separation. However residual coupling with the strength proportional to the fiber radiation loss is very Iona range decreasing as an inverse square root of the inter-fiber separation, and exhibiting periodic variation with inter-fiber separation. Finally, coupling between the TE01 modes is considered in a view of designing a directional coupler. We find that for fibers with large enough core radii one can identify broad frequency ranges where intermodal coupling strength exceeds super-mode radiation losses by an order of magnitude, thus opening a possibility of building a directional coupler. We attribute such unusually strong inter-mode coupling both to the resonant effects in the inter-mirror cavity as well as a proximity interaction between the leaky modes localized in the mirror.

Coupling characteristics of dual-core photonic crystal fiber couplers
K Saitoh, Y Sato, M Koshiba
OPTICS EXPRESS, 11, 24, 3188, 3195, OPTICAL SOC AMER, Dec. 2003, [Peer-reviewed]
English, Scientific journal, Coupling characteristics of dual-core photonic crystal fiber (PCF) couplers are evaluated by using a vector finite element method and their application to a multiplexer-demultiplexer (MUX-DEMUX) based on PCF is investigated. The PCF couplers for 1.48/1.55-mum, 1.3/1.55-mum, 0.98/1.55-mum, and 0.85/1.55-mum wavelength MUX-DEMUX are designed and the beam propagation analysis of the proposed PCF couplers is performed. It is shown from numerical results that it is possible to realize significantly shorter MUX-DEMUX PCFs, compared to conventional optical fiber couplers. (C) 2003 Optical Society of America.

Leakage loss and group velocity dispersion in air-core photonic bandgap fibers
K Saitoh, M Koshiba
OPTICS EXPRESS, 11, 23, 3100, 3109, OPTICAL SOC AMER, Nov. 2003, [Peer-reviewed]
English, Scientific journal, The wavelength dependence and the structural dependence of leakage loss and group velocity dispersion (GVD) in air-core photonic bandgap fibers (PBGFs) are numerically investigated by using a full-vector finite element method. It is shown that at least seventeen rings of arrays of air holes are required in the cladding region to reduce the leakage losses to a level of 0.1 dB/km in 1.55-mum wavelength range even if using large air holes of the diameter to pitch ratio of 0.9 and that the leakage losses in air-core PBGFs decrease drastically with increasing the hole diameter to pitch ratio. Moreover, it is shown that the waveguide GVD and dispersion slope of air-core PBGFs are much larger than those of conventional silica fibers and that the shape of air-core region greatly affects the leakage losses and the dispersion properties. (C) 2003 Optical Society of America.

Finite-Element Analysis of Birefringence and Dispersion Properties in Actual and Idealized Holey-Fiber Structures
M. Koshiba, K. Saitoh
Applied Optics, 42, 31, 6267, 6275, Nov. 2003, [Peer-reviewed]
English, Scientific journal

Finite-element analysis of birefringence and dispersion properties in actual and idealized holey-fiber structures
M Koshiba, K Saitoh
APPLIED OPTICS, 42, 31, 6267, 6275, OPTICAL SOC AMER, Nov. 2003, [Peer-reviewed]
English, Scientific journal, Using a full-vector finite-element method, we calculate birefringence and dispersion in index-guiding photonic crystal fibers, also called holey fibers. Through real-model simulations the polarization-dependent dispersion in actual fiber structures is numerically demonstrated, for the first time to our knowledge. Furthermore the transverse-electric-field vector distributions in the fabricated holey fibers are also clarified for the two linearly polarized fundamental modes. (C) 2003 Optical Society of America.

Single-polarization single-mode photonic crystal fibers
K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 15, 10, 1384, 1386, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Oct. 2003, [Peer-reviewed]
English, Scientific journal, A new structure of single-polarization single-mode (SPSM) photonic crystal fiber (PCF) is proposed and analyzed by using a full-vector finite element method with anisotropic perfectly matched layers. From the numerical results it is confirmed that the proposed fiber is low-loss SPSM-PCF within the wavelengths ranging from 1.48 to 1.6 mum, where only the slow-axis mode exists and the confinement loss is less than 0.1 dB/km.

Structural dependence of effective area and mode field diameter for holey fibers
M Koshiba, K Saitoh
OPTICS EXPRESS, 11, 15, 1746, 1756, OPTICAL SOC AMER, Jul. 2003, [Peer-reviewed]
English, Scientific journal, A rigorous full-vector finite element method is effectively applied to evaluating the effective area A(eff) and the mode field diameter (MFD) of holey fibers (HFs) with finite cross sections. The effective modal spot size ( a half of MFD), w(eff), is defined with the help of the second moment of the optical intensity distribution. The influence of hole diameter, hole pitch, operating wavelength, and number of rings of air holes on A(eff) and weff is investigated in detail. As a result, it is shown that A(eff) and weff are almost independent of the number of hole rings and that the relation A(eff) = piw(eff)(2), which is frequently utilized in the conventional optical fibers, does not always hold, especially in smaller air-filling fraction and/or longer wavelength regions. In addition, we find that for HFs with large air holes operating at longer wavelengths, the mode profiles of the two linearly polarized fundamental modes are significantly different from each other, even though they are degenerate. Using the values of A(eff) and weff obtained here, the beam divergence and the nonlinear phase shift are calculated and are compared with the earlier experimental results. (C) 2003 Optical Society of America.

Polarization-dependent confinement losses in actual holey fibers
M Koshiba, K Saitoh
IEEE PHOTONICS TECHNOLOGY LETTERS, 15, 5, 691, 693, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2003, [Peer-reviewed]
English, Scientific journal, Through the real-model simulations, the polarization-dependent confinement losses in actual holey fibers are, for the first time, numerically demonstrated. Furthermore, it is confirmed that the interstitial holes caused by the packing of the capillaries p lay a role to significantly reduce the confinement losses.

Chromatic dispersion control in photonic crystal fibers: application to ultra-flattened dispersion
K Saitoh, M Koshiba, T Hasegawa, E Sasaoka
OPTICS EXPRESS, 11, 8, 843, 852, OPTICAL SOC AMER, Apr. 2003, [Peer-reviewed]
English, Scientific journal, In order to control the dispersion and the dispersion slope of index-guiding photonic crystal fibers (PCFs), a new controlling technique of chromatic dispersion in PCF is reported. Moreover, our technique is applied to design PCF with both ultra-low dispersion and ultra-flattened dispersion in a wide wavelength range. A full-vector finite element method with anisotropic perfectly matched layers is used to analyze the dispersion properties and the confinement losses in a PCF with a finite number of air holes. It is shown from numerical results that it is possible to design a four-ring PCF with flattened dispersion of 0 +/- 0.5 ps/(km.nm) from a wavelength of 1.19 mum to 1.69 mum and a five-ring PCF with flattened dispersion of 0 +/- 0.4 ps/(km.nm) from a wavelength 1.23 mum to 1.72 mum. (C) 2003 Optical Society of America.

Confinement losses in air-guiding photonic bandgap fibers
K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 15, 2, 236, 238, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 2003, [Peer-reviewed]
English, Scientific journal, The wavelength dependence and the structural dependence of confinement losses in air-guiding photonic bandgap fibers; (PBGFs) are, for the first time, investigated by using a full-vector finite element method. It is confirmed from computed results that one of the main causes of transmission losses in air-guiding PBGFs is the confinement loss and that even if using considerably large air holes of the diameter to pitch ratio of 0.9, at least twenty rings of arrays of air holes are required in the cladding region to reduce the confinement loss to a level of 0.01 dB/km.

Unique dispersion properties of photonic crystal fibers
K Saitoh, M Koshiba
ICICS-PCM 2003, VOLS 1-3, PROCEEDINGS, 171, 175, IEEE, 2003, [Peer-reviewed]
English, International conference proceedings, In conventional photonic crystal fibers (PCFs), the cladding structure is usually formed by air holes with the same diameter arrayed in a regular triangular lattice. However, in index-guiding PCFs, the periodicity in the cladding region is not essential to confine the guiding light into the high-index core region. In this paper, we report that it is possible to control chromatic dispersion of index-guiding PCFs in wide wavelength range by varying the hole diameter of each air-hole ring along the radius. Using this design principle, a PCF with ultra-low and ultra-flattened dispersion is designed through a full-vector finite element method with anisotropic perfectly matched layers. Effective mode areas and leakage loss properties are also investigated.

Highly birefringent photonic bandgap fibers and its dispersion properties
K. Saitoh, M. Koshiba
International Topical Meeting on Microwave Photonics (MWP), Institute of Electrical and Electronics Engineers Inc., Nov. 2002, [Peer-reviewed]
English, International conference proceedings, A highly birefringent air-guiding photonic bandgap fiber is proposed and its dispersion properties are investigated. It is shown that the fiber has birefringence of the order of 10-3 and a large amount of group velocity dispersion.

Photonic bandgap fibers with high birefringence
K Saitoh, M Koshiba
IEEE PHOTONICS TECHNOLOGY LETTERS, 14, 9, 1291, 1293, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 2002, [Peer-reviewed]
English, Scientific journal, A highly birefringent air-guiding photonic crystal fiber which provides guidance by a full two-dimensional photonic bandgap (PBG) effect is proposed. The fiber core is formed by introducing a large air hole that has an area of four unit cells of the cladding materials. It is shown from computed results by a full-vector finite element method that the proposed PBG fiber has birefringence of the order of 10(-3).

Full-vectorial imaginary-distance beam propagation method based on a finite element scheme: Application to photonic crystal fibers
K Saitoh, M Koshiba
IEEE JOURNAL OF QUANTUM ELECTRONICS, 38, 7, 927, 933, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2002, [Peer-reviewed]
English, Scientific journal, A full-vectorial imaginary-distance beam propagation method based on a finite element scheme is newly formulated and is effectively applied to investigating the problem of leakage due to a finite number of arrays of air holes in photonic-crystal holey fibers (HFs). In order to treat arbitrarily shaped air holes and to avoid spurious solutions, a curvilinear edge/nodal hybrid element is introduced. Furthermore, in order to evaluate propagation characteristics of not only bound modes but leaky modes in HFs, an anisotropic perfectly matched layer is also employed as a boundary condition at computational window edges. It is confirmed from numerical results that the propagation loss increases rapidly with increasing wavelength, especially for HFs with one ring of smaller air holes, and that the propagation loss is drastically reduced by adding one more ring of air holes to the cladding region.

Numerical verification of degeneracy in hexagonal photonic crystal fibers
M Koshiba, K Saitoh
IEEE PHOTONICS TECHNOLOGY LETTERS, 13, 12, 1313, 1315, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 2001, [Peer-reviewed]
English, Scientific journal, Modal birefringences in photonic crystal fibers with six air holes symmetrically arranged around the core region and with multiple air holes in hexagonal lattice are numerically investigated in detail. It is confirmed from computed results obtained toy a full-vector finite element method that both the hexagonal holey fibers are not birefringent, namely, the two fundamental modes are degenerate.

Approximate scalar finite-element beam-propagation method with perfectly matched layers for anisotropic optical waveguides
K Saitoh, M Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 19, 5, 786, 792, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, May 2001, [Peer-reviewed]
English, Scientific journal, The perfectly matched layer boundary condition for arbitrary anisotropic media is incorporated into the approximate scalar beam propagation method, The procedure is based on a finite-element method for three-dimensional anisotropic optical waveguides with off-diagonal elements in a permittivity tenser. In order to treat a wide-angle beam propagation, the Fade approximant operator is employed, To show the validity and usefulness of this approach, numerical results are presented for Gaussian beam propagation in free space and Gaussian beam excitation on a three-dimensional anisotropic optical waveguide.

Full-vectorial finite element beam propagation method with perfectly matched layers for anisotropic optical waveguides
K Saitoh, M Koshiba
JOURNAL OF LIGHTWAVE TECHNOLOGY, 19, 3, 405, 413, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Mar. 2001, [Peer-reviewed]
English, Scientific journal, Perfectly matched layer (PML) boundary conditions are incorporated into the full-vectorial beam propagation method (BPM) based on a finite element scheme for the three-dimensional (3-D) anisotropic optical waveguide analysis. In the present approach, edge elements based on linear-tangential and quadratic-normal vector basis functions are used for the transverse held components. To shove the validity and usefulness of this approach, numerical examples are shown for Gaussian beam propagation in proton-exchanged LiNbO3 optical waveguides, Numerical accuracy of the present PML boundary condition is investigated in detail by comparing the results with those of the conventional absorbing boundary condition (ABC).

Unified software for the design of acoustooptic devices
K Saitoh, M Koshiba
IEEE TRANSACTIONS ON MAGNETICS, 36, 4, 1779, 1783, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2000, [Peer-reviewed]
English, Scientific journal, A numerical approach based on the finite element method (FEM) and the beam propagation method (BPM) is newly formulated for the design and modeling of static strain optic (SSO) and acoustooptic (AO) devices, and unified software for SSO and AO devices is presented. In this software, the static stress and the surface acoustic wave (SAW) analysis methods are linked to the optical waveguide analysis method. Numerical examples are shown for strain-induced optical waveguides on LiNbO3 substrates and AO tunable filters with weighted coupling using focused SAW.

Beam propagation method for three-dimensional surface acoustic waveguides
K Saitoh, M Koshiba
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, 39, 5B, 2999, 3003, JAPAN J APPLIED PHYSICS, May 2000, [Peer-reviewed]
English, Scientific journal, A beam propagation method (BPM) based on the finite element method (FEM) is newly formulated for the analysis of longitudinally varying three-dimensional (3-D) surface acoustic waveguides. In order to avoid nonphysical reflections from computational window edges, a transparent boundary condition (TBC) for acoustic waves is proposed. To show the validity and usefulness of this approach, numerical results for a Rayleigh-wave beam compressor using Delta upsilon/upsilon-type guidance are presented.

Stress analysis method considering piezoelectric effects and its application to static strain optic devices
Kunimasa Saitoh, Masanori Koshiba, Yasuhide Tsuji
Journal of Lightwave Technology, 17, 9, 1626, 1633, Sep. 1999, [Peer-reviewed]
English, Scientific journal, A stress analysis method considering piezoelectric effects based on the finite-element method (FEM), which can be applied to arbitrarily anisotropic material-based optical waveguide devices, is newly formulated. To produce a two-step analysis of static strain optic (SSO) and electrooptic (EO) modulations of optical waveguide devices, this stress analysis is linked to the guided mode analysis and the beam propagation analysis taking into account the refractive index changes. Numerical examples are shown for strain-induced optical waveguides and strain-induced polarization mode converters on LiNbO 3 substrates.

Stress analysis method for elastically anisotropic material based optical waveguides and its application to strain-induced optical waveguides
K Saitoh, M Koshiba, Y Tsuji
JOURNAL OF LIGHTWAVE TECHNOLOGY, 17, 2, 255, 259, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Feb. 1999, [Peer-reviewed]
English, Scientific journal, A numerical approach for the stress analysis of elastically anisotropic material-based optical waveguides is newly formulated with the finite element method (FEM). The stress analysis method developed here is linked to the guided mode analysis method to produce a two-step analysis of acoustooptic modulation of optical waveguides. Numerical examples are shown for strain-induced optical waveguides on LiNbO3 substrates.

Numerical Analysis of Integrated Acoustooptic Tunable Filters with Weighted Coupling
Kunimasa Saitoh, Masanori Koshiba, Yasuhide Tsuji
Journal of Lightwave Technology, 17, 2, 249, 254, Feb. 1999, [Peer-reviewed]
English, Scientific journal, A numerical approach based on the finite element method (FEM) and the beam propagation method (BPM) is newly formulated for design and modeling of integrated acoustooptic tunable filters (AOTF's) with weighted coupling on a piezoelectric substrate. First, acoustical field profiles in the substrate region and on the substrate surface are separately calculated with FEM and BPM, respectively. These two-dimensional (2-D) profiles are used for constructing three-dimensional (3-D) strain and electric field profiles. Then, from the strains and electric fields the refrac-tive index changes due to acoustooptic (AO) and electrooptic (EO) effects are derived. Finally, by analyzing modified optical waveguides with AO- and EO-induced changes in refractive indexes using again BPM, optical filter characteristics of AOTF's can be evaluated. Numerical examples are shown for AOTF's with a tapered acoustical directional coupler on a LiNbO3 substrate.

Stress-analysis method for optical waveguides composed of elastically anisotropic materials and its application to strain-induced optical waveguides
K Saitoh, M Koshiba, Y Tsuji
ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS, 81, 5, 16, 23, SCRIPTA TECHNICA-JOHN WILEY & SONS, May 1998, [Peer-reviewed]
English, Scientific journal, In this paper we propose a new stress analysis method that can evaluate the residual thermal stress distribution in an optical waveguide with an elastically arbitrary anisotropy. We analyze the optical waveguide by taking into account the refractive index change obtained from stress analysis. A rigorous evaluation of the propagation characteristics in a strain-induced optical waveguide is now possible. Since the finite-element method is used for both the stress analysis and the optical waveguide analysis, the complex refractive index change obtained from stress analysis can be faithfully reflected in the optical waveguide analysis. Strain-induced optical waveguides with a SiO2/GaAs structure and a SiO2/LiNbO3 structure are chosen. The stress and strain in the waveguide and the change of the refractive index profile are obtained numerically and their effects on the transmission characteristics of the optical wave are studied. To the best of our knowledge, this is the first time that an optical waveguide analysis has been carried out from stress analysis taking into account an anisotropy. (C) 1998 Scripta Technica.

弾性的に任意の異方性を有する光導波路の応力解析法とそのひずみ誘起光導波路解析への応用　　　　　　　　　　　　　　　
齊藤晋聖, 小柴正則, 辻 寧英
電子情報通信学会論文誌, J81-C-I, 1, 17, 23, Jan. 1998, [Peer-reviewed]
Japanese, Scientific journal

Numerical analysis of integrated acoustooptic tunable filters with weighted coupling
K Saitoh, M Koshiba, Y Tsuji
1998 IEEE ULTRASONICS SYMPOSIUM - PROCEEDINGS, VOLS 1 AND 2, 1293, 1296, IEEE, 1998, [Peer-reviewed]
English, International conference proceedings, A numerical approach is newly formulated for design and modeling of integrated acoustooptic tunable filters (AOTF's) with weighted coupling. First, acoustical field profiles in the substrate region and an the substrate surface are separately calculated with the finite element method (FEM) and the beam propagation method (BPM), respectively. These two-dimensional (2-D) profiles are used for constructing three-dimensional (3-D) strain and electric field profiles. Then, from the strains and electric fields the refractive index changes are derived. Finally, by analyzing modified optical waveguides with acoustooptic (AO) and electrooptic (EO) induced changes in refractive indices using again BPM, optical filter characteristics of AOTF's can be evaluated.

Special Invited Talk : 1-Pb/s Dense Space Division Multiplexing (DSDM) Transmission Technology Using 100Gbaud-class 16QAM Channels : Review of OFC2017 Postdeadline Paper Presentation
小林 孝行, 中村 政則, 濱岡 福太郎, 芝原 光樹, 水野 隆之, 佐野 明秀, 川上 広人, 磯田 暁, 長谷 宗彦, 山崎 裕史, 宮本 裕, 安間 淑通, 佐々木 雄佑, 竹永 勝宏, 愛川 和彦, 齋藤 晋聖, ジュン ヨンミン, リチャードソン デイビッド, プルヴァラー クラウス, ボーン マーク, 盛岡 敏夫, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 117, 26, 25, 27, 12 May 2017
電子情報通信学会, Japanese

Impulse Response Analysis of Coupled Multicore Fibers Having Random Twisting with Macrobending
千田 泰之, 藤澤 剛, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 456, 5, 10, 16 Feb. 2017
電子情報通信学会, Japanese

A proposal of low-loss and compact Si wire 90° waveguide bend using clothoid curves
牧野 俊太郎, 菅 将大, 藤澤 剛, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 388, 257, 262, 18 Jan. 2017
電子情報通信学会, Japanese

Optical Fibers for Ultra Large Capacity Space Division Multiplexing Transmission
齊藤 晋聖, 光学 = Japanese journal of optics : publication of the Optical Society of Japan, 45, 12, 448, 453, Dec. 2016
日本光学会, Japanese

Theoretical investigation of highly efficient connecting structure between Ge-rib and GeSn waveguides and extinction characteristics of GeSn/SiGeSn multiple-quantum-well waveguide for mid-infrared photonics
秋江 みなみ, 佐藤 孝憲, 牧野 俊太郎, 荒井 昌和, 藤澤 剛, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 274, 55, 60, 27 Oct. 2016
電子情報通信学会, Japanese

A Proposal of Broadband Mach-Zehnder Mode Multi/Demultiplexer Based on 3dB Mode Divider with Taper Waveguide
太多 惇, 牧野 俊太郎, 藤澤 剛, 坂本 泰志, 松井 隆, 辻川 恭三, 中島 和秀, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 273, 97, 102, 27 Oct. 2016
電子情報通信学会, Japanese

Fabrication of PLC-based 2-mode multi/demultiplexer designed by wavefront matching method and the reason of its broadband characteristics
田口 恵梨, 山下 陽子, 牧野 俊太郎, 藤澤 剛, 半澤 信智, 坂本 泰志, 松井 隆, 辻川 恭三, 中島 和秀, 山本 文彦, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 274, 91, 96, 27 Oct. 2016
電子情報通信学会, Japanese

A Proposal of Broadband Mach-Zehnder Mode Multi/Demultiplexer Based on 3dB Mode Divider with Taper Waveguide
太多 惇, 牧野 俊太郎, 藤澤 剛, 坂本 泰志, 松井 隆, 辻川 恭三, 中島 和秀, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 274, 97, 102, 27 Oct. 2016
電子情報通信学会, Japanese

C-3-45 A study on a PLC-based 6-mode multi/demultiplexer
Yamashita Yoko, Ishizaka Yuhei, Hanzawa Nobutomo, Fujisawa Takeshi, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Yamamoto Fumihiko, Nakajima Kazuhide, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 1, 183, 183, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

BCI-1-2 Evaluation Methods for Multicore/Multimode Fibers
Sasaki Y., Saitoh S., Maruyama R., Takenaga K., Aikawa K., Saitoh K., Nakajima K., Ohashi M., Proceedings of the IEICE General Conference, 2016, 2, "SS, 145"-"SS-146", 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-21 Measurement of Bragg Gratings in FM-MCF Using All Fiber-Optic Few-Mode Input/Output Module
Omichi Koji, Uemura Hitoshi, Sasaki Yusuke, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 2, 442, 442, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-28 Investigation of fused type fan-in/fan-out devices with dual step-index profile for 4LP-mode MCF
Fushimi Naoto, Nishimoto Shoko, Fujisawa Takeshi, Sasaki Yusuke, Uemura Hitoshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 2, 449, 449, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-24 Impulse Response Analysis of Homogeneously-Coupled Six Core Fiber
Chida Yasuyuki, Tojo Ren, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 2, 445, 445, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-25 Design of 6 mode multi/demultiplexer based on fused type multicore fiber coupler
Nishimoto S., Fujisawa Takeshi, Uemura H., Sasaki Y., Matsuo Shoichiro, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 2, 446, 446, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-26 3-LP mode multiplexer/demultiplexer using a 5-core fiber
Uemura Hitoshi, Sasaki Yusuke, Nishimoto Shoko, Omichi Koji, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2016, 2, 447, 447, 01 Mar. 2016
The Institute of Electronics, Information and Communication Engineers, Japanese

Development of wavefront matching method based on full-vectorial finite-element beam propagation method and its application to polarization control components
牧野 俊太郎, 藤澤 剛, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 115, 276, 91, 96, 29 Oct. 2015
電子情報通信学会, Japanese

Quantum confined Stark effect analysis of Ge(Sn)/SiGe(Sn) quantum wells based on many-body theory
藤澤 剛, 齊藤 晋聖, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 115, 276, 23, 28, 29 Oct. 2015
電子情報通信学会, Japanese

C-3-2 A study on a PLC-based LP_<11> mode rotator with a curved trench
Yamashita Yoko, Makino Shuntaro, Fujisawa Takeshi, Saitoh Kunimasa, Hanzawa Nobutomo, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Nakajima Kazuhide, Yamamoto Fumihiko, Proceedings of the Society Conference of IEICE, 2015, 1, 76, 76, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-52 A Reduction of Bending Loss of Silicon Wire Waveguide using Clothoid Curve
Suga Masahiro, Makino Shuntaro, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2015, 1, 126, 126, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-5 A Study on a Miniaturization of a Polarization Splitter-Rotator Using Wavefront Matching Method
Makino Shuntaro, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2015, 1, 79, 79, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-9 Inter-core crosstalk of higher-order modes in a few-mode MCF
Saitoh S., Sasaki Y., Takenaga K., Ohashi M., Saitoh K., Nakajima K., Matsuo S., Proceedings of the Society Conference of IEICE, 2015, 2, 173, 173, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-8 Quasi-Single-Mode Homogeneous 31-core Fiber
Sasaki Y., Saitoh S., Amma Y., Takenaga K., Matsuo S., Saitoh K., Morioka T., Miyamoto Y., Proceedings of the Society Conference of IEICE, 2015, 2, 172, 172, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-27 An Investigation of Air-Hole Added Coupled 6-Core Fibers for Mode Division Multiplexing Transmission
Tojo Ren, Chida Yasuyuki, Fujisawa Takeshi, Saitoh Shota, Matsuo Shoichiro, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2015, 2, 305, 305, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-20 Impulse Response Analysis of Homogeneously-Coupled Three Core Fiber
Chida Yasuyuki, Tojo Ren, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2015, 2, 298, 298, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-35 Double-core fiber based fused taper type fan-in/fan-out device for few-mode multi-core fiber
Uemura H., Masumoto K., Takenaga K., Omichi K., Matsuo S., Fujisawa T., Saitoh K., Proceedings of the Society Conference of IEICE, 2015, 2, 313, 313, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-21 A Study on Few-Mode Hole Assisted 12-Core Graded Index Fiber for Large Capacity Transmission
Amano Sho, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2015, 2, 299, 299, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-49 A Study on Calculation Method of Nonlinear Parameter γ for Periodic Optical Waveguides
Sato Takanori, Makino Shuntaro, Ishizaka Yuhei, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2015, 1, 173, 173, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-52 Wavefront matching method based on full-vectorial finite-element beam propagation method
Makino Shuntaro, Ishizaka Yuhei, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2015, 1, 176, 176, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-50 A study on bandwidth expansion of PLC-based 2 mode multi/demultiplexer using wavefront matching method
Yamashita Yoko, Masumoto Kohei, Makino Shuntaro, Fujisawa Takeshi, Saitoh Kunimasa, Hanzawa Nobutomo, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Yamamoto Fumihiko, Proceedings of the IEICE General Conference, 2015, 1, 174, 174, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-25 Design of 2-LP-mode Heterogeneous GI-MCF with Low DMD
Tobita Yuki, Igari Akira, Fujisawa Takeshi, Saitoh Kunimasa, Amma Yoshimichi, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the IEICE General Conference, 2015, 2, 508, 508, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-10 Investigation of fused type fan-in/fan-out devices with dual step-index profile for two-LP-mode MCF
Masumoto Kohei, Fujisawa Takeshi, Saitoh Kumimasa, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the IEICE General Conference, 2015, 2, 493, 493, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-16 High-density 30-core fiber with four kinds of cores
Saitoh S., Amma Y., Sasaki Y., Takenaga K., Matsuo S., Tu J., Saitoh K., Koshiba M., Morioka T., Miyamoto Y., Proceedings of the IEICE General Conference, 2015, 2, 352, 352, 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

BCI-2-7 Mode multiplexer/demultiplexer for 2LP mode fiber
Takenaga K., Matsuo S., Saitoh K., Proceedings of the IEICE General Conference, 2015, 2, "SS, 133"-"SS-134", 24 Feb. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

An analysis of transmission characteristics of homogeneously coupled-three-core fiber based on full-vector finite-element method and coupled field theory
Igari Akira, Fujisawa Takeshi, Saitoh Kunimasa, Takenaga Katsuhiro, Matsuo Shoichiro, Technical report of IEICE. OCS, 114, 451, 87, 92, 19 Feb. 2015
We analyze the transmission characteristics of homogeneously coupled-three-core fiber for mode-division multiplexing transmission. Supermode properties of the fiber (effective refractive index and group delay) are evaluated by full-vector finite element method. It is shown that by adding an air-hole to the center of the fiber, the effective refractive index difference between different supermodes is reduced, leading to stronger mode coupling and smaller differential modal group delay (DMGD). Also, the core-to-core distance can be reduced by adding air-hole. Modal coupling caused by micro-bending in the fiber is assessed by the coupled field theory. In the strong mode coupling regime, it is found that the DMGD of the fiber having smaller effective refractive index difference becomes small further since the modal coupling occurs for smaller perturbation due to micro-bending., The Institute of Electronics, Information and Communication Engineers, Japanese

An analysis of transmission characteristics of homogeneously coupled-three-core fiber based on full-vector finite-element method and coupled field theory
Igari Akira, Fujisawa Takeshi, Saitoh Kunimasa, Takenaga Katsuhiro, Matsuo Shoichiro, Technical report of IEICE. OPE, 114, 453, 121, 126, 19 Feb. 2015
We analyze the transmission characteristics of homogeneously coupled-three-core fiber for mode-division multiplexing transmission. Supermode properties of the fiber (effective refractive index and group delay) are evaluated by full-vector finite element method. It is shown that by adding an air-hole to the center of the fiber, the effective refractive index difference between different supermodes is reduced, leading to stronger mode coupling and smaller differential modal group delay (DMGD). Also, the core-to-core distance can be reduced by adding air-hole. Modal coupling caused by micro-bending in the fiber is assessed by the coupled field theory. In the strong mode coupling regime, it is found that the DMGD of the fiber having smaller effective refractive index difference becomes small further since the modal coupling occurs for smaller perturbation due to micro-bending., The Institute of Electronics, Information and Communication Engineers, Japanese

Input/output devices for multicore and multimode fibers
齊藤 晋聖, 半澤 信智, 植村 仁, オプトロニクス, 33, 12, 54, 58, Dec. 2014
オプトロニクス社, Japanese

Material gain analysis of GeSn/SiGeSn quantum wells based on many-body theory
Fujisawa T., Saitoh K., Technical report of IEICE. OCS, 114, 281, 49, 54, 30 Oct. 2014
Material gain of GeSn/SiGeSn quantum wells, which can be grown on Si substrate by using a buffer layer, is analyzed based on many-body theory (MBT). MBT can take into account a gain spectrum broadening associated with scattering phenomena, such as Coulomb scattering, based on quantum field theory, and does not need any fitting parameters, such as a relaxation time, determined by experiment. Not only Γ- but also carrier distributions in L-point are considered for the gain analysis. By using MBT, the quantum well structures maximizing the material gain and the differential gain are investigated in terms of the well thickness, the strain, and the energy difference of quantum states between Γ- and L-point., The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-44 Investigation about the taper shape of fused type fan-in/fan-out devices for two-mode multi-core fibers
Masumoto Kohei, Fujisawa Takeshi, Saitoh Kunimasa, Uemura Hitoshi, Omichi Koji, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the Society Conference of IEICE, 2014, 2, 331, 331, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-40 An Investigation on Reduction of DMD in Dual Concentric Core Few-Mode Fibers
Tojo Ren, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2014, 2, 327, 327, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-45 LP_<01>/LP_<11a>/LP_<11b> mode multiplexer/demultiplexer using a multi-core fiber
Uemura H., Sasaki Y., Nishimoto S., Uematsu T., Takenaga K., Omichi K., Goto R., Matsuo S., Saitoh K., Proceedings of the Society Conference of IEICE, 2014, 2, 332, 332, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-39 A Study on Few-Mode Hole Assisted 12-Core Fiber for Large Capacity Transmission
Amano Sho, Igari Akira, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2014, 2, 326, 326, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-11 Three-mode 12-core Multicore Fiber with Multi-step-index Profile
Amma Yoshimichi, Sasaki Yusuke, Takenaga Katsuhiro, Matsuo Shoichiro, Saitoh Kunimiasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2014, 2, 185, 185, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-11 Design of Photonic-Plasmonic Mode Converters based on Hybrid Plasmonic Waveguides
Nagai Masaru, Ishizaka Yuhei, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2014, 1, 108, 108, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-19 Investigation of Excitation Characteristics of a 4LP-mode Optical Fiber Connected to a Silica-Based OpticalWaveguide
Ishizaka Yuhei, Chida Yasuyuki, Fujisawa Takeshi, Saitoh Kunimasa, Sakamoto Taiji, Hanzawa Nobutomo, Matsui Takashi, Tsujikawa Kyozo, Yamamoto Fumihiko, Proceedings of the Society Conference of IEICE, 2014, 1, 116, 116, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-43 Investigation of a taper waveguide for a highly-efficient connection between 1-D PC-CROW and straight waveguide
Makino Shuntaro, Ishizaka Yuhei, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2014, 1, 140, 140, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-53 Design of a Polarization Beam Splitter Based on S-bend Directional Coupler
Kitayama Tetsuya, Ishizaka Yuhei, Fujisawa Takeshi, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2014, 1, 150, 150, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-20 Design of 3-mode multiplexer and demultiplexer using a multicore fiber
Nishimoto S, Fujisawa T, Takenaga K, Uemura H, Sasaki Y, Omichi K, Goto R, Matsuo S, Saitoh K, Proceedings of the Society Conference of IEICE, 2014, 1, 117, 117, 09 Sep. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

Crosstalk increase in tightly bent multi-core fiber mediated by cladding modes
HAYASHI Tetsuya, TARU Toshiki, NAKANISHI Tetsuya, SASAKI Takashi, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OCS, 114, 195, 1, 6, 28 Aug. 2014
We newly derived a simplified coupled-power equations including cladding modes for estimating the inter-core crosstalk of the multi-core fiber, and found, in tightly bent conditions, the crosstalk can be increased from the existing predictions due to indirect coupling mediated by the cladding modes. The crosstalk increase was confirmed, both in the theory and experiment, to be proportional to the square of the bend loss coefficient. We also investigated the dependence of the crosstalk increase on core layouts experimentally., The Institute of Electronics, Information and Communication Engineers, Japanese

フューモード・マルチコアファイバー技術 (特集 空間多重の極限追及を目指す光ファイバー通信技術)
松尾 昌一郎, 齊藤 晋聖, O plus E : Optics・Electronics, 36, 7, 781, 787, Jul. 2014
アドコム・メディア, Japanese

Investigation of Nonlinear Characteristics in Photonic Crystal Coupled Resonator Optical Waveguides
SATO Takanori, MAKINO Shuntaro, ISHIZAKA Yuhei, SAITOH Kunimasa, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 114, 14, 45, 50, 17 Apr. 2014
Photonic crystal coupled resonator optical waveguides have attracted attention because they increase light-matter interactions owing to the slow-light effect and enhance the nonlinear effects. In this paper, we develop a three-dimensional finite-element mode-solver for nonlinear periodic optical waveguides and investigate nonlinear characteristics in one-dimensional photonic crystal coupled resonator optical waveguides., The Institute of Electronics, Information and Communication Engineers, Japanese

Invited Talk : Multicore fibers for large capacity transmission
SAITOH Kunimasa, MATSUO Shoichiro, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 114, 14, 27, 32, 17 Apr. 2014
In this report, the relationship between crosstalk characteristics and spatial multiplicity in multicore fibers (MCFs) for large capacity transmission is presented. In addition, future prospects on further enhancement of core multiplicity factor in MCFs are introduced., The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-11 LP_<01>/LP_<11> mode multiplexer/demultiplexer using a multi-core fiber
Uemura H., Sasaki Y., Nishimoto S., Uematsu T., Takenaga K., Omichi K., Goto R., Matsuo S., Saitoh K., Proceedings of the IEICE General Conference, 2014, 2, 507, 507, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

BCI-1-8 PLC based mode multi/demultiplexer for mode-division-multiplexed transmission
Saitoh Kunimasa, Uematsu Takui, Hanzawa Nobutomo, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Yamamoto Fumihiko, Proceedings of the IEICE General Conference, 2014, 1, "SS, 56"-"SS-57", 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-39 A Proposal of 1-D PC-CROWs Based on Horizontal Slot Waveguides
Makino Shuntaro, Ishizaka Yuhei, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2014, 1, 194, 194, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-41 Investigation of Nonlinear Characteristics in 1-D PC-CROW using 3-D Finite-Element Mode-Solver
Sato Takanori, Makino Shuntaro, Ishizaka Yuhei, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2014, 1, 196, 196, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-40 Proposal of Photonic-Plasmonic Mode Converters based on Hybrid Plasmonic Waveguides
Nagai Masaru, Ishizaka Yuhei, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2014, 1, 195, 195, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

Hole-assisted 6-core Fiber with Cladding Diameter of 125μm
Sakamoto Taiji, Saitoh Kunimasa, Hanzawa Nobutomo, Tsujikawa Kyozo, Ma Lin, Koshiba Masanori, Yamamoto Fumihiko, Technical report of IEICE. OPE, 113, 448, 141, 146, 27 Feb. 2014
We report 125μm-cladding multi-core fiber for high-density space-division multiplexing transmission. We show numerically that a hole-assisted structure can reduce both crosstalk and leakage loss. We achieved 125μm-cladding 6-core fiber experimentally with a low crosstalk of less than -30 dB/100km and a high normalized core multiplicity of 6.0., The Institute of Electronics, Information and Communication Engineers, Japanese

A Study of Fused Type Fan-in/Fan-out Devices for Two-Mode Multi-Core Fibers
Masumoto Kouhei, Uematsu Takui, Saitoh Kunimasa, Uemura Hitoshi, Omichi Koji, Takenaga Katsuhiro, Matsuo Shoichiro, Technical report of IEICE. OPE, 113, 448, 77, 80, 27 Feb. 2014
Few-mode multi-core fibers (FM-MCF) which are combinations of multi-core fibers (MCF) and few-mode fibers (FMF) have been investigated in recent years to further increase the transmission capacity. In order to realize the transmission system using FM-MCF, fan-in/fan-out (Fi/Fo) devices which connect the FM-MCF to the FMF are required. In this work, we propose fused type Fi/Fo devices for two-mode MCF (TM-MCF) and investigate the structural parameters to maintain an effective area (A_) in the input and output of the proposed Fi/Fo devices., The Institute of Electronics, Information and Communication Engineers, Japanese

Hole-assisted 6-core Fiber with Cladding Diameter of 125 μm
Sakamoto Taiji, Saitoh Kunimasa, Hanzawa Nobutomo, Tsujikawa Kyozo, Ma Lin, Koshiba Masanori, Yamamoto Fumihiko, Technical report of IEICE. OCS, 113, 446, 101, 106, 27 Feb. 2014
We report 125μm-cladding multi-core fiber for high-density space-division multiplexing transmission. We show numerically that a hole-assisted structure can reduce both crosstalk and leakage loss. We achieved 125μm-cladding 6-core fiber experimentally with a low crosstalk of less than -30 dB/100km and a high normalized core multiplicity of 6.0., The Institute of Electronics, Information and Communication Engineers, Japanese

Multicore Fiber for Large Capacity Transmission
小柴 正則, 齊藤 晋聖, 佐々木 雄佑, フジクラ技報, 125, 5, 11, Feb. 2014
フジクラ, Japanese

A study on effective cutoff wavelength in few-mode fiber
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyouzo, Koshiba Masanori, Yamamoto Fumihiko, Technical report of IEICE. OFT, 113, 265, 63, 66, 24 Oct. 2013
Recently, a mode division multiplexing (MDM) transmission with a few-mode fiber has been studied. We consider that realizing a larger capacity requires the increasing number of modes in few-mode fiber. Thus, we consider that a cutoff wavelength measurement is important in order to suppress a mode crosstalk. In this study, we measured the cutoff wavelength by using conventional measurement in three kinds of fibers. In addition, we report a mode selective excitation method for measuring cutoff wavelength., The Institute of Electronics, Information and Communication Engineers, Japanese

A Study on Large-Mode-Area 19-cell-core All-Solid Photonic Bandgap Fibers
Saitoh Shota, Saitoh Kunimasa, Kashiwagi Masahiro, Matsuo Shoichiro, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 2, 367, 367, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

A Note on a Plasmonic Mode Coupled with Non-major Component of Electric Field
Ishizaka Yuhei, Nagai Masaru, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 1, 186, 186, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Expanding the bandwidth of planar waveguide mode multi/demultiplexer for mode multiplexed transmission
Uematsu Takui, Saitoh Kunimasa, Hanzawa Nobutomo, Matsui Takashi, Tsujikawa Kyozo, Masumoto Kouhei, Ishizaka Yuhei, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 1, 200, 200, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

AProposal of a Mode Converter using Hybrid Plasmonic Waveguide with Uniform Silica Layer
Nagai Masaru, Ishizaka Yuhei, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2013, 1, 187, 187, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

A Study on 3 Mode Ring-Core Fibers
Kasahara Motoki, Saitoh Kunimasa, Sakamoto Taiji, Hanzawa Nobutomo, Matsui Takashi, Tsujikawa Kyozo, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 2, 385, 385, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Design of an Optical AND Logic Gate Based on Phase Information
Kitayama Tetsuya, lshizaka Yuhei, Saitoh Kunimasa, Proceedings of the Society Conference of IEICE, 2013, 1, 124, 124, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

investigation ofNonlinear Characteristics in Coupled Resonator Optical Waveguides Based on Slotted Nanobeam Cavities
Makino Shuntaro, Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 1, 129, 129, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Fused taper type fan-in/fan-out device using double-core fibers
Uemura H., Takenaga K., Omichi K., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2013, 2, 251, 251, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Low Crosstalk Trench Assisted Two-Mode Seven-Core Fiber
Sasaki Y., Amma Y., Takenaga K., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2013, 2, 226, 226, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

A Study on Heterogeneous Coupled-Three-Core Fiber
Igari Akira, Kasahara Motoki, Saitoh Kunimasa, Takenaga Katuhiro, Matsuo Shoichiro, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2013, 2, 227, 227, 03 Sep. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Investigation of Fundamental Characteristic of a Hybrid Plasmonic Waveguide with Uniform Silica Layer
NAGAI Masaru, ISHIZAKA Yuhei, SAITOH Kunimasa, IEICE technical report. Component parts and materials, 113, 188, 159, 162, 29 Aug. 2013
Hybrid plasmonic waveguides, which is one of the plasmonic waveguides, have the advantage of redusing the propagation loss compared with the other plasmonic waveguides. In this paper, we propose a hybrid plasmonic waveguide with a uniform silica layer and evaluate its propagation length, effective area, coupling characteristics, and bending characteristics., The Institute of Electronics, Information and Communication Engineers, Japanese

Multicore Fibers for Extremely Large Capacity Transmission
MATSUO Shoichiro, TAKENAGA Katsuhiro, SAITOH Kunimasa, KOSHIBA Masanori, レーザー研究, 41, 6, 399, 405, 15 Jun. 2013
レーザー学会, Japanese

Mode Division Multiplexed Transmission with Waveguide Mode Multi/Demultiplexer
HANZAWA Nobutomo, SAITOH Kunimasa, SAKAMOTO Taiji, MATSUI Takashi, TSUJIKAWA Kyozo, KOSHIBA Masanori, YAMAMOTO Fumihiko, レーザー研究, 41, 6, 432, 436, 15 Jun. 2013
レーザー学会, Japanese

Simulation Techniques for Optics and Their Applications to the Analysis of Optical Waveguides and Devices
SHIBAYAMA Jun, SAITOH Kunimasa, The Journal of the Institute of Electronics, Information, and Communication Engineers, 96, 6, 406, 410, 01 Jun. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Design of Broadband Hollow Core Photonic Bandgap Fibers
Matsubara Kazuki, Saitoh Kunimasa, Matsuo Shoichiro, Technical report of IEICE. OFT, 113, 55, 41, 44, 23 May 2013
Hollow core photonic bandgap fibers (PBGF) have gathered much attention in recent years because of their intriguing propagation properties. By the photonic bandgap (PBG) effect, light can be guided into the air-core with low nonlinearities and an ultimate low loss property. However, the relatively narrow transmission bandwidth of conventional hollow-core PBGFs has limited their use in some applications. In this work, we propose an improved triangular lattice of rods (ITLR) for their cladding structure and show that the ITLR generates wider PBG than the conventional triangular lattice of holes (TLH) and square lattice (SL)., The Institute of Electronics, Information and Communication Engineers, Japanese

CS-5-6 Analysis of Crossing Slot-Waveguides Using Three-Dimensional Vector Finite Element Method
Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2013, 1, "S, 21", 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-51 Coupling Characterization of Hybrid Plasmonic Waveguides With Uniform Silica Layer
Nagai Masaru, Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2013, 1, 211, 211, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-47 Investigation of Nonlinear CharacteriStics in Slot Waveguides Composed of 1-D PC-CROWs
Makino Shuntaro, Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2013, 1, 207, 207, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-7 Effect ofbending on cro sstalk behavior o f heterogeneous multi-core fiber with homo geneous A eff
Amma Y., Sasaki Y., Takenaga K., Matsuo S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 2013, 2, 338, 338, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-11 Fused tapeku-in/fan-out devices for multi-core fiber
Uemurai H., Takenagal K., Ori T., Matsuol S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 2013, 2, 342, 342, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-11 A Study on 2 Mode Holey Fibers with Large Effective Area
Kasahara Motoki, Saitoh Kunimasa, Sakamoto Taiji, Hanzawa Nobutomo, Matsui Takashi, Tsujikawa Kyozo, Koshiba Masanori, Proceedings of the IEICE General Conference, 2013, 2, 476, 476, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-5 Stu.dy on Transmission Bandwidth Expansion of Hollow Core Photonic Bandgap Fibers
Matsubara Kazuki, Chiba Masaki, Saitoh Kunimasa, Koshiba Masanori, Matsuo Shoichiro, Proceedings of the IEICE General Conference, 2013, 2, 470, 470, 05 Mar. 2013
The Institute of Electronics, Information and Communication Engineers, Japanese

Mode-division multiplexing technology for realizing ultra-large capacity transmission
Sakamoto Taiji, Hanzawa Nobutomo, Mori Takayoshi, Saitoh Kunimasa, Yamamoto Takashi, Masanori Koshiba Masanori Koshiba, Yamamoto Fumihiko, Technical report of IEICE. OCS, 112, 447, 39, 44, 28 Feb. 2013
Recently, mode-division multiplexing transmission has been intensively investigated because the transmission capacity can be increased greatly by using this technology. In this paper, we report mode MUX/DEMUX and transmission fiber for realizing ultra-large capacity transmission system with mode and conventional multiplexing technologies such as WDM or PDM. Finally we experimentally achieved 2-mode MDM transmission in S-band by using fabncated mode-MUX/DEMUX and PCF., The Institute of Electronics, Information and Communication Engineers, Japanese

Mode-division multiplexing technology for realizing ultra-large capacity transmission
Sakamoto Taiji, Hanzawa Nobutomo, Mori Takayoshi, Saitoh Kunimasa, Yamamoto Takashi, Koshiba Masanori, Yamamoto Fumihiko, Technical report of IEICE. OFT, 112, 448, 39, 44, 28 Feb. 2013
Recently, mode-division multiplexing transmission has been intensively investigated because the transmission capacity can be increased greatly by using this technology. In this paper, we report mode MUX/DEMUX and transmission fiber for realizing ultra-large capacity transmission system with mode and conventional multiplexing technologies such as WDM or PDM. Finally we experimentally achieved 2-mode MDM transmission in S-band by using fabricated mode-MUX/DEMUX and PCF., The Institute of Electronics, Information and Communication Engineers, Japanese

3-D vector finite element analysis of surface plasmon cavity sandwiched between metal wall and photonic crystal
ISHIZAKA Yuhei, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. LQE, 112, 400, 151, 155, 24 Jan. 2013
Surface plasmon cavities have attracted much attention for light sources m the electronic-photonic integrated circuits and high-efficiency single photon sources. In this paper, we propose a surface plasmon cavity sandwiched between metal wall and photonic crystal. We evaluate Q-value and mode volume of the proposed structure using a 3-D vector finite element solver developed by our research group., The Institute of Electronics, Information and Communication Engineers, Japanese

Mode division multiplexed transmission with asymmetric mode coupler
HANZAWA Nobutomo, SAITOH Kunimasa, SAKAMOTO Taiji, MATSUI Takashi, TOMITA Shigeru, KOSHIBA Masanori, 電気学会研究会資料. CMN, 通信研究会, 2012, 42, 73, 76, 21 Nov. 2012
Japanese

Longitudinal Power Decay along a Weakly Coupled Multi-Core Fiber
HAMAGUCHI Hiroki, ISHIDA Itaru, TAKENAGA Katsuhiro, MATSUO Shoichiro, SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. CMN, 通信研究会, 2012, 42, 61, 66, 21 Nov. 2012
Japanese

Mode division multiplexed transmission with asymmetric mode coupler
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tomita Shigeru, Koshiba Masanori, Technical report of IEICE. OFT, 112, 311, 37, 40, 21 Nov. 2012
We realized mode multi/demultiplexer with an asymmetric mode coupler, which we designed and fabricated with interaction parameters to multiplex the LP_<01> and LP_<11> modes. In addition, we achieved a 2 mode x 4 wavelength MDM-WDM transmission over 9 km-long two-mode fiber., The Institute of Electronics, Information and Communication Engineers, Japanese

Longitudinal Power Decay along a Weakly Coupled Multi-Core Fiber
HAMAGUCHI Hiroki, ISHIDA Itaru, TAKENAGA Katsuhiro, MATSUO Shoichiro, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OCS, 112, 310, 47, 52, 14 Nov. 2012
An estimation method of inter-core crosstalk along a multi-core fiber by fitting bi-directional optical time domain reflectometry signature with theoretical longitudinal power decay is presented. The validity of estimated coupled-power coefficient is confirmed by simulation results derived from solving coupled-power equation. The results indicate that the difference of longitudinal power decay depending on the position of excited cores originates not from the different coupled-power coefficient but from the different effective crosstalk due to the number of adjacent cores., The Institute of Electronics, Information and Communication Engineers, Japanese

A Study on Large-Mode-Area All-Solid Photonic Bandgap Fibers
SAITOH Shota, CHIBA Masaki, SAITOH Kunimasa, KOSHIBA Masanori, KASHIWAGI Masahiro, MATSUO Shoichiro, Technical report of IEICE. OFT, 112, 261, 17, 20, 18 Oct. 2012
In order to realize high power fiber lasers with good beam quality, suppression of nonlinear effects is strongly needed. Therefore, large-mode-area (LMA) fibers have been investigated because of their large core and effective area (A_). All-solid photonic bandgap fibers (AS-PBGFs) are expected to be applied as LMA fibers. In this work, employing numerical simulations based on the finite element method, we optimize the structure of AS-PBGFs and show the limits of A_ enlargement. In addition, we investigate the relationship between the limits of A_ enlargement and the assumed bending radii., The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-3 Longitudinal Power Decay along a Weakly Coupled Multi-Core Fiber
Hamaguchi Hiroki, Ishida Itaru, Takenaga Katsuhiro, Matsuo Shoichiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 2, 185, 185, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-4 Large-Effective-Area Two-mode Multi-core Fiber
Sasaki Y., Takenaga K., Guan N., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2012, 2, 186, 186, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-2 Relationship between Crosstalk and Core Density of Circularly Arranged Multi-Core Fibers
Okuyama Kazuhiro, Saitoh Kunimasa, Koshiba Masanori, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the Society Conference of IEICE, 2012, 2, 184, 184, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-3 Polygonal Arranged Multi-Core Fibers for Large-Mode-Area Fibers
Murai Ryota, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 2, 302, 302, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-22 An Investigation of Filtering Characteristics in Ytterbium-Doped All-Sorid Photonic Bandgap Fibers
Chiba Masaki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 2, 321, 321, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-28 TE/TM-pass Lithium Niobate on Insulator Ridge Waveguide Polarizer
Saitoh Emi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, 145, 145, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-85 Demonstration of multi-demultiplexing with asymmetric parallel waveguide for MDM transmission
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tsujikawa Kyozo, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, 202, 202, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-44 Three-dimensional Vector Finite Element Method for Analyzing Photonic Crystal Microcavities
Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, 161, 161, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-46 Investigation of Nonlinear Characteristics of 1-D Photonic Crystal Coupled Resonator Optical Waveguides
Makino Shuntaro, Ishizaka Yuhei, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, 163, 163, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-87 Limitation of Effective Area Enlargement in All-Solid Photonic Bandgap Fibers
Saitoh Shota, Chiba Masaki, Saitoh Kunimasa, Koshiba Masanori, Kashiwagi Masahiro, Matsuo Shoichiro, Proceedings of the Society Conference of IEICE, 2012, 1, 204, 204, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-29 Design of a Broadband Polarization Beam Splitter with Point-symmetrical MZI
Uematsu Takui, Ishizaka Yuhei, Komatsu Masa-aki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, 146, 146, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

CI-1-5 Ultra-Compact Mode Evolution Type Polarization Rotator Based on Surface Plasmon Polariton
Komatsu Masa-aki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2012, 1, "SS, 9"-"SS-10", 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

A Large Effective Area Two-Mode Multi-Core Fiber
TAKENAGA Katsuhiro, SASAKI Yusuke, GUAN Ning, MATSUO Shoichiro, KASAHARA Motoki, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OCS, 112, 193, 25, 30, 23 Aug. 2012
A design concept of large effective area (A_) two-mode multi-core fiber (MCF) is presented. Cores with a step-index profile are designed to support two LP modes over the C band and the L band with large A_ larger than 110 μm^2. The pitch of cores is determined regarding the inter-core crosstalk related to the LP_<11> mode. The two LP modes MCFs with four cores and seven cores show relative core multiplicity factor (RMCF) of 7.3 and 9.2, respectively. The RCMF of proposed two LP modes MCF is larger than that of previous reported single-mode MCFs. Finally, characteristics of fabricated fibers based on the design are shown., The Institute of Electronics, Information and Communication Engineers, Japanese

A bend-insensitive heterogeneous trench-assisted multi-core fiber with large effective area
TU Jiajing, SAITOH Kunimasa, KOSHIBA Masanori, TAKENAGA Katsuhiro, MATSUO Shoichiro, Technical report of IEICE. OCS, 112, 193, 19, 24, 23 Aug. 2012
A kind of heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) with 12 cores is proposed to achieve large effective area (A_) and high density of cores. As bending radius becomes larger than 30 mm, the crosstalk value at 1550-nm wavelength of the Hetero-TA-MCF is about -42 dB after 100-km propagation and the A_ of this Hetero-TA-MCF can reach 100 μm^2., The Institute of Electronics, Information and Communication Engineers, English

Development of a 3-D Vector FEM Solber for Designing Photonic Crystal Microcavities
ISHIZAKA Yuhei, SAITOH Kunimasa, KOSHIBA Masanori, 電子情報通信学会技術研究報告. EST, エレクトロニクスシミュレーション = IEICE technical report. EST, Electronic simulation technology, 112, 157, 167, 171, 19 Jul. 2012
Photonic crystal microcavities that can confine the light to ultra-small volumes for a long time have attracted much attention due to having feasibility of all optical processing. In this paper, we develop a 3-D vector FEM solver for designing photonic crystal microcavities. We report formulation of the 3-D vector FEM for cavity problem and performance evaluation of the implemented solver., The Institute of Electronics, Information and Communication Engineers, Japanese

Development of a 3-D Vector FEM Solber for Designing Photonic Crystal Microcavities
ISHIZAKA Yuhei, SAITOH Kunimasa, KOSHIBA Masanori, 電子情報通信学会技術研究報告. MWP, マイクロ波・ミリ波フォトニクス = IEICE technical report. MWP, Microwave and millimeter-wave photonics, 112, 158, 167, 171, 19 Jul. 2012
Photonic crystal microcavities that can confine the light to ultra-small volumes for a long time have attracted much attention due to having feasibility of all optical processing. In this paper, we develop a 3-D vector FEM solver for designing photonic crystal microcavities. We report formulation of the 3-D vector FEM for cavity problem and performance evaluation of the implemented solver., The Institute of Electronics, Information and Communication Engineers, Japanese

Mode-division multiplexing transmission with DMD-independent low complexity MIMO processing
SAKAMOTO Taiji, MORI Takayoshi, YAMAMOTO Takashi, HANZAWA Nobutomo, TOMITA Shigeru, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OCS, 112, 86, 57, 62, 14 Jun. 2012
We propose differential mode delay (DMD) independent low computational complexity MIMO processing for a mode-division multiplexing (MDM) transmission system. The functional imperfection of the mode MUX/DEMUX can be fully compensated by using this technique. We achieved experimentally a WDM-MDM transmission using PLC based mode MUX/DEMUX over the C-L band with low MIMO processing complexity even when using a few-mode fiber with a high DMD., The Institute of Electronics, Information and Communication Engineers, Japanese

A Study on Design of Few-Mode Fibers for Mode-Division Multiplexing Transmission
KASAHARA Motoki, SAITOH Kunimasa, SAKAMOTO Taiji, HANZAWA Nobutomo, MATSUI Takashi, KOSHIBA Masanori, TOMITA Shigeru, Technical report of IEICE. OFT, 112, 54, 9, 14, 17 May 2012
Since the data traffic continues to increase with 40〜70% per year, large transmission capacity will be highly required for optical fiber communication systems. Recently, space-division multiplexing (SDM) transmission by mode-division multiplexing (MDM) has been attracted as new generation optical fiber transmission system in order to realize large transmission capacity. In this work, we investigate the fiber parameters which meet design conditions for low bending loss and effectively few mode operation, the limit of A_ enlargement, and the differential mode group delay (DMGD) on few-mode fibers for MDM transmission., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-88 Evaluation of two mode multi/demultiplexer based on MMI waveguides
Uematsu Takui, Ishizaka Yuhei, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 1, 237, 237, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-28 Study on Three-Dimensional Crossing Slot Waveguides with Vertical Coupler
Ishizaka Yuhei, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 1, 177, 177, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-73 Investigation of Coupling Characteristics in 1-D Photonic Crystal Coupled Resonator Optical Waveguides
Makino Shuntaro, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 1, 222, 222, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-24 Proposal of TE/TM-pass Polarizer Based on Lithium Niobate on Insulator Ridge Waveguide
Saitoh Emi, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 1, 173, 173, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-81 Ultra Compact Polarization Rotator Based on Surface Plasmon Polariton
Komatsu Masa-aki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 1, 230, 230, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-20 Large-Effective-Area Uncoupled 10-Core Fiber
Sasaki Y., Takenaga K., Arakawa Y., Tanigawa S., Matsuo S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 2012, 2, 343, 343, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-44 Low Crosstalk Heterogeneous Multi-Core Fibers under Bending Condition
Tu Jiajing, Saitoh Kunimasa, Koshiba Masanori, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the IEICE General Conference, 2012, 2, 548, 548, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, English

B-13-47 Demonstration of mode-division multiplexing transmission using asymmetric parallel waveguide
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tomita Shigeru, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 2, 551, 551, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-43 Investigation on Circularly Arranged Multi-Core Fibers for Increasing the Number of Cores
Okuyama Kazuhiro, Saitoh Kunimasa, Koshiba Masanori, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the IEICE General Conference, 2012, 2, 547, 547, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-23 Superiority of the Filtering Characteristics of the First-Order Photonic Bandgap in Ytterbium-Doped All-Sorid Photonic Bandgap Fibers
Chiba Masaki, Nagao Koyuru, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 2, 527, 527, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-41 Tapered-Index Coupled Multi-Core Fibers for Large-Mode-Area Fibers
Murai Ryota, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2012, 2, 545, 545, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

Multi-Core Fiber for Space-Division Multiplexing
小柴 正則, 齊藤 晋聖, 竹永 勝宏, フジクラ技報, 121, 1, 7, Feb. 2012
フジクラ, Japanese

Evaluation of Structural Dependence of Bending Loss in Slot Waveguide
SAITO Yu, KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, 電子情報通信学会技術研究報告 : 信学技報, 111, 412, 253, 258, 26 Jan. 2012
Slot waveguide is expect to be used in ultra small devices and also used in devices which have optical nonlinear effect, because of its ability of strong field confinement. In order to integrate these devices in high density, ultra small bending structures with low losses would be required. In this work, we evaluate structural dependence of bending loss in slot waveguide using three-dimensional finite element method, and investigate low bending loss slot waveguide structures., The Institute of Electronics, Information and Communication Engineers, Japanese

Mode division multiplexing transmission with fiber coupler and long-period fiber grating
半澤信智, 齊藤晋聖, 坂本泰志, 松井隆, 冨田茂, 小柴正則, 電子情報通信学会技術研究報告, 112, 261(OFT2012 32-45), 2012

Measurement and simulation of bending characteristics of homo- and hetero-geneous seven core fibers
AWAJI Yoshinari, SAKAGUCHI Jun, WADA Naoya, SAITOH Kunimasa, 電気学会研究会資料. CMN, 通信研究会, 2011, 43, 35, 38, 17 Nov. 2011
Japanese

A Large Effective Area Multi-Core Fibre
ARAKAWA Yoko, TAKENAGA Katsuhiro, SASAKI Yusuke, TANIGAWA Shoji, MATSUO Shoichiro, SAITOH Kunimasa, KOSHIBA Masanori, 電子情報通信学会技術研究報告. OCS, 光通信システム, 111, 297, 65, 69, 10 Nov. 2011
Japanese

Measurement and simulation of bending characteristics of homo- and hetero-geneous seven core fibers
AWAJI Yoshinari, SAKAGUCHI Jun, WADA Naoya, SAITOH Kunimasa, 電子情報通信学会技術研究報告. OFT, 光ファイバ応用技術, 111, 298, 21, 24, 10 Nov. 2011
Japanese

Structural Dependence of Group Velocity and Leakage Losses in 1-D Photonic Crystal Coupled Resonator Optical Waveguide Based on Mode-Gap Confinement
KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. LQE, 111, 267, 27, 32, 20 Oct. 2011
One-dimensional photonic crystal coupled resonator optical waveguide(1-D PC-CROW) based on mode-gap confinement has a possibility for reducing leakage losses compared with conventional 1-D PC-CROW constructed by placing defects periodically. However, transmission characteristics of 1-D PC-CROW based on mode-gap confinement have not been investigated so far. In this report, we evaluate the structural dependence of group velocity and leakage losses of 1-D PC-CROW by using three-dimensional vector finite element method for periodic waveguide analysis. We show a design method of 1-D PC-CROW satisfying small group velocity and low leakage losses simultaneously by investigating dispersion relationships of CROW with changed structural parameters., The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-27 Micro-bending Loss of Large Effective Area Multi-core Fibers
Sasaki Y., Takenaga K., Arakawa Y., Tanigawa S., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2011, 2, 240, 240, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-26 Relationship between Large A_ and Low Crosstalk of Trench-Assisted Multi-Core Fiber
Okuyama Kazuhiro, Saitoh Kunimasa, Koshiba Masanori, Takenaga Katsuhiro, Matsuo Shoichiro, Proceedings of the Society Conference of IEICE, 2011, 2, 239, 239, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

BI-7-7 Demonstration of mode-division multiplexing transmission using LP_<01> and LP_<11> modes
Hanzawa Nobutomo, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Tomita Shigeru, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 1, "SS, 65"-"SS-66", 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

BI-7-3 Trench-Assisted Multi-Core Fiber
Takenaga K., Arakawa Y., Tanigawa S., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2011, 1, "SS, 57"-"SS-58", 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-33 A Study on Few-Mode Fibers for Mode-Division Multiplexing
Kasahara Motoki, Saitoh Kunimasa, Sakamoto Taiji, Hanzawa Nobutomo, Matsui Takashi, Koshiba Masanori, Tomita Shigeru, Proceedings of the Society Conference of IEICE, 2011, 2, 370, 370, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-6 Effectively single-mode solid photonic bandgap fiber with low bending loss and large mode area
Kashiwagi Masahiro, Saitoh Kunimasa, Takenaga Katsuhiro, Tanigawa Shoji, Matsuo Shoichiro, Fujimaki Munehisa, Proceedings of the Society Conference of IEICE, 2011, 2, 343, 343, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-34 Coupled Multi-Core Fibers Applied to Large-Mode-Area Fibers
Murai Ryota, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 2, 371, 371, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-3 Design of Single-Mode Hole-Assisted Fibers with Low Bending-Loss
Matsumori Kyosuke, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 2, 340, 340, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-61 Evaluation of Bending Losses in Lithium Niobate Photonic Wires
Saitoh Emi, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 1, 183, 183, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-23 Structural Dependence of Leakage Losses in One-Dimensional Photonics Crystal Coupled Resonator Optical Waveguides
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 1, 145, 145, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-54 Characterization of Crossing Slot Waveguides Using Three-Dimensional Vector Finite Element Method
Ishizaka Yuhei, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 1, 176, 176, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-55 Design of Slot Waveguides with Low Bending Loss
Saito Yu, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2011, 1, 177, 177, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

Characteristics of Trench-Assisted Multi-Core Fiber
TAKENAGA Katsuhiro, ARAKAWA Yoko, TANIGAWA Shoji, GUAN Ning, MATSUO Shoichiro, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 111, 181, 7, 10, 18 Aug. 2011
Multi-core fiber (MCF) is regarded as one of expected candidates for increasing the capacity of optical fibers. Using the MCF as a transmission line, crosstalk between each core is an issue to be solved. We propose a trench-assisted MCF to achieve high dense MCF design with a solid structure. In the case of the similar MFD, core pitch and cutoff wavelength characteristics, the crosstalk of the trench-assisted MCF was 20 dB smaller than that of the MCF with step-index profile. Also, we have investigated the cutoff wavelength of trench-assisted MCF., The Institute of Electronics, Information and Communication Engineers, Japanese

Suppression of Leakage Losses in LNOI Ridge Waveguides
SAITOH Emi, KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. EMT, 電磁界理論研究会, 2011, 57, 171, 176, 21 Jul. 2011
Japanese

Study on Large Effective Area Hole-Assisted Fibers for Optical Transmission
IWATA Kosei, SAITOH Kunimasa, SAKAMOTO Taiji, MATSUI Takashi, HANZAWA Nobutomo, KOSHIBA Masanori, TOMITA Shigeru, IEICE technical report, 111, 69, 71, 76, 19 May 2011
Because of rapid network traffic increase, high capacity optical fibers for next-generation transmission are highly required. Hole-assisted fibers (HAFs) have several advantages such as low bending loss characteristics and high fiber fuse threshold as compared with conventional fibers. Therefore HAF is expected to be a good candidate for large effective area (A_) transmission fibers. In this work, we investigate the limit of A_ enlargement in HAFs which are compliant with ITU-T G655. It is shown that A_ of HAF can be enlarged to 240 μm^2 by designing air-hole size and position as well as Ge-doped core region appropriately., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-76 Evaluation of Transmission Characteristics in Right-Angle Slot Waveguide Bend Using Three-Dimensional Vector Finite Element Method
Saito Yu, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 1, 251, 251, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-75 Design of an Optical NAND Logic Gate for Signal Processing of BPSK Labels
Ishizaka Yuhei, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 1, 250, 250, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-49 Evaluation of Leakage Losses in LNOI Ridge Waveguides
Saitoh Emi, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 1, 224, 224, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-44 Evaluation of Leakage Losses of One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides Based on Mode-Gap Confinement
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 1, 219, 219, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-4 Effect of bending on crosstalk behavior of multi-core fiber
Matsuo S., Takenaga K., Arakawa Y., Sasaki Y., Tanigwa S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 2011, 2, 313, 313, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-2 Reduction of Crosstalk by Trench-Assisted Multi-Core Fiber
Arakawa Y., Takenaga K., Tanigawa S., Guan N., Matsuo S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 2011, 2, 311, 311, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-9 Study on Enlargement of Effective Area of Hole-Assisted Fibers for Optical Transmission
Iwata Kosei, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Hanzawa Nobutomo, Koshiba Masanori, Tomita Shigeru, Proceedings of the IEICE General Conference, 2011, 2, 485, 485, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-10 A Study on Effective Area Enlargement of Large-Mode-Area Leakage Channel Fibers under Bent Condition
Sasaki Kaori, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 2, 486, 486, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-15 Identifying Structural Parameters for Reduction of Bending Losses in All-Solid Photonic Bandgap Fibers
Nagao Koyuru, Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2011, 2, 491, 491, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

LP₀₁とLP₁₁モードを用いたモード多重伝送
半澤信智, 齊藤晋聖, 坂本泰志, 松井隆, 冨田茂, 小柴正則, 電子情報通信学会大会講演論文集, 2011, 2011

Multiple resonant coupling mechanism for higher-order-modes' suppression in all-solid photonic bandgap fibers with heterostructured cladding
MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 110, 292, 15, 19, 11 Nov. 2010
In this work, we propose a novel mechanism for suppression of higher-order modes (HOMs), namely multiple resonant coupling, in all-solid photonic bandgap fibers (PBGFs) with effectively large core diameters. In particular, contrary to the conventional index-matching method for the HOM suppression, the proposed mechanism guarantees a broadband HOM suppression without a precise design of the defected cores set in the cladding. Moreover, we show that the proposed structure exhibits a lower bending loss characteristic when compared to the conventional all-solid PBGFs and the simultaneous realization of the single-mode operation and the low bending loss property is also due to the novel cladding concept named as heterostructured cladding. The proposed structure also resolves the issue for the increased confinement loss property in the first-order photonic bandgap (PBG) at the same time., The Institute of Electronics, Information and Communication Engineers, English

Multiple resonant coupling mechanism for higher-order-modes' suppression in all-solid photonic bandgap fibers with heterostructured cladding (放送技術)
Murao Tadashi, 齊藤 晋聖, Koshiba Masanori, 映像情報メディア学会技術報告, 34, 46, 15, 19, Nov. 2010
映像情報メディア学会, English

B-13-21 Limitation of Effective Area of Large-Mode-Area Leakage Channel Fibers
Sasaki Kaori, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 2, 328, 328, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-7 Study on Enlargement of Effective Area of Holey Fibers for Optical Transmission
Iwata Kosei, Saitoh Kunimasa, Sakamoto Taiji, Matsui Takashi, Hanzawa Nobutomo, Koshiba Masanori, Tomita Shigeru, Proceedings of the Society Conference of IEICE, 2010, 2, 314, 314, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-1 Degradation of Bending Properties in Single-Mode All-Solid Photonic Bandgap Fibers with a Large Mode Field Diameter
Nagao Koyuru, Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 2, 308, 308, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-2 Proposal of Approximate Formula for Photonic Bandgap Depth in All-Solid Photonic Bandgap Fibers
Murao Tadashi, Nagao Koyuru, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 2, 309, 309, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

BS-6-6 Quasi-Homogeneous Multi-Core Fibers
Takenaga K., Arakawa Y., Tanigawa S., Guan N., Matsuo S., Saitoh K., Koshiba M., Proceedings of the Society Conference of IEICE, 2010, 2, "S, 35"-"S-36", 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-5 Design of Optical AND Logic Gate Based on Multi-Mode Interference Devices Using Photonic Crystal Waveguides
Ishizaka Yuhei, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 1, 126, 126, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-33 3-D Vector Finite Element Analysis of 2-D Photonic Crystal Coupled Resonator Optical Waveguides
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 1, 154, 154, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-32 Three-Dimensional Finite Element Beam Propagation Analysis of One-Dimensional Photonic Crystal Microcavites
Saito Yu, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2010, 1, 153, 153, 31 Aug. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

Evaluation of Bending Characteristics in Silicon Wires Using Three-Dimensional Frequency-Domain Vectorial Finite Element Method
KAISE Satoshi, KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 110, 155, 205, 210, 22 Jul. 2010
Silicon wires can achieve a strong optical confinement according to the high refractive index difference between the core and cladding material. Therefore, it is expected that they can reduce the size of optical circuits based on conventional silica waveguides. In general, optical circuits are composed of numbers of bent waveguides connected with straight waveguides. In order to evaluate propagation characteristics of such waveguides, we have to consider mismatch of the mode field between bent waveguides and straight waveguides. In this work, we evaluate bending characteristics of silicon wires with a newly developed three-dimensional vectorial finite element method., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-7 Design of Low-Loss One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides Using Three-Dimensional Vector Finite Element Method
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2010, 1, 180, 180, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-12 Horizontal Slot Waveguides with High Nonlinearity and Flat Dispersion
Komatsu Masa-aki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2010, 1, 185, 185, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-26 A Study on Optimization of Structure for Thulium-Doped Depressed-Index-Core Photonic Crystal Fiber Amplifiers
Tanaka Eiichi, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2010, 1, 199, 199, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-28 Multi-Core Hole-Assisted Fibers for High Core Density SDM
Saitoh Kunimasa, Matsui Takashi, Sakamoto Taiji, Koshiba Masanori, Tomita Shigeru, Proceedings of the IEICE General Conference, 2010, 2, 523, 523, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-23 Higher-order mode suppression in a bent large-mode-area multi-cladding fiber
Ichii K., Saito K., Sugimoto Y., Tanigawa S., Matsuo S., Fujimaki M., Proceedings of the IEICE General Conference, 2010, 2, 518, 518, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-30 Design Principle for Solid-Core Photonic Bandgap Fibers with Low Bending Losses
Murao Tadashi, Saitoh Kunimasa, Nagao Koyuru, Koshiba Masanori, Proceedings of the IEICE General Conference, 2010, 2, 525, 525, 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

A Design Method of a Fiber-Type Mode Multi/Demultiplexer for Mode-Division Multiplexing
SAITOH Fumiya, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 109, 377, 43, 48, 14 Jan. 2010
In the mode-division multiplexing optical transmission system, a mode multi/demultiplexer is an important key device for excitation, multiplexing, and separation of light signals which have distinct modes. In this report, we propose a fiber-type mode multi/demultiplexer based on selective phase matching between different cores/modes. Design method and device characteristics of 1x4 mode multi/demultiplexer are investigated through finite element analysis. In order to expand operating wavelength range, we reveal the structural parameters that satisfy phase matching conditions over wide wavelength range. Our numerical results demonstrate that the mode multi/demultiplexer with broadband and polarization-insensitive operation can be realized by applying this fiber structure., The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-19 Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber
Arakawa Y., Takenaga K., Tanigawa S., Guan N., Matsuo S., Saitoh K., Koshiba M., Proceedings of the IEICE General Conference, 357, 357, 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-49 Evaluation of Bending Characteristics in Silicon Wires Using Three-Dimensional Vectorial Finite Element Method
Kaise Satoshi, Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2009, 1, 204, 204, 01 Sep. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-47 3-D Vector Finite Element Analysis of 1-D Photonic Crystal Coupled Resonator Optical Waveguides
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2009, 1, 202, 202, 01 Sep. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

BS-7-8 Photonic Band-Gap Fibers and Their Future Prospects
Saitoh Kunimasa, Murao Tadashi, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2009, 2, "S, 44"-"S-45", 01 Sep. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

Propagation Characteristics of Photonic Crystal Fibers with Metallic Nanowires
NAGASAKI Akira, TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 109, 179, 1, 6, 20 Aug. 2009
In a photonic crystal fiber (PCF) where the cladding air holes have been filled with metallic nanowires, the core guided mode can be coupled with surface plasmon polaritons (SPPs) on the metallic nanowires. A study on metal-filled polarization-maintaining PCFs has been reported for optical fibers which have polarization-dependent charasteristics, and the PCFs are expected to be applicable as polarizers, however, the reported PCFs do not have sufficient performance for polarizers. Moreover, there has not been any reports of concrete design methods for polarizers based on metal-filled PCFs. In this work, we investigate the propagation characteristics of PCFs selectively filled with metallic nanowires into cladding air holes through a full-vector modal solver based on the finite element method (FEM). We show that it is possible to design metal-filled polarizers with sufficient performance by introducing several metallic nanowires in lines., The Institute of Electronics, Information and Communication Engineers, Japanese

Bend-Insensitive and Single-Mode Hetero-Cladding Structured Solid-Core Photonic Bandgap Fibers
MURAO Tadashi, SAITOH Kunimasa, TARU Toshiki, NAGASHIMA Takuji, MAEDA Kuniaki, SASAKI Takashi, KOSHIBA Masanori, IEICE technical report, 109, 59, 31, 34, 21 May 2009
Solid-core photonic bandgap fibers (SC-PBGFs) guide light along the fiber length in the low-index defected core region by the photonic bandgap (PBG) effect, where the transmission band properties are understood by the theory for ARROW waveguides. Due to its fibre-type functionality of alternate transmission and transmission-inhibited bands, the fiber can be applied to several filter devices. In general, SC-PBGFs suffer from the trade-off problem between the low bending losses and low confinement losses, because the use of 3rd PBG leads to low confinement losses while causing high bending losses. Exploiting 1st PBG, on the other hand, leads to low bending losses while causing high confinement losses. Moreover, reducing bending losses and realizing a single mode operation are also in the trade-off relation because the requirement of the large gap of the effective indexes between the core mode and the PBG floor to decrease the bending losses causes the higher-order-modes' guidance in general. In this work, we propose a novel concept of cladding structure which resolves the trade-off issues. The mechanism is based on the new concept of "heterostructured cladding"., The Institute of Electronics, Information and Communication Engineers, Japanese

超広帯域化と光パワー・光非線形限界打破を目指すフォトニック結晶ファイバ (特集 我が国基礎・基盤研究の現状--NICT EXAT研究会)
齊藤 晋聖, 松尾 昌一郎, ITUジャーナル, 39, 5, 12, 15, May 2009
日本ITU協会, Japanese

C-3-81 Structural Dependence of Nonlinearity in Slot Waveguides with Silicon Nanocrystal
Komatsu Masa-aki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 1, 232, 232, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-82 Thermally Tunable Band-Rejection Filter Based on Hole-Assisted Fiber
Saito Nozomu, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 1, 233, 233, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-6 Coupling Characteristics of a Directional Coupler Based on 1-D Photonic Crystal Coupled Resonator Optical Waveguides
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 1, 157, 157, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-73 Three-dimensional Finite Element Analysis of 90° Slot Waveguide Bends
Kakihara Kuniaki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 1, 224, 224, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-30 All-Optical Logic Gates Using a Slot-Defected-Core Bismuth Microstructured Optical Fiber Coupler
Saitoh Fumiya, Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 1, 181, 181, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-10 Design of a PCF-Based Polarizer with Metallic Wires
Nagasaki Akira, Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 2, 498, 498, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-32 Analysis of Structural Dependence of Bandgap Depth in Solid-Core Photonic Bandgap Fibers
Maeda Kuniaki, Murao Tadashi, Hatakeyama Hiroki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 2, 520, 520, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-31 Structural Dependence of Bending Losses in Solid-Core Photonic Bandgap Fibers
Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 2, 519, 519, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

B-13-21 Effectively Single-mode LMA Optical Fibers with Fluorine-doped Silica Rods
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2009, 2, 509, 509, 04 Mar. 2009
The Institute of Electronics, Information and Communication Engineers, Japanese

Bending Characteristics of One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides
KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. EMT, 電磁界理論研究会, 2009, 22, 41, 44, 30 Jan. 2009
Japanese

Bending Characteristics of One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides
KAWAGUCHI Yuki, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 108, 418, 141, 144, 22 Jan. 2009
One-dimensional photonic crystal coupled resonator optical waveguide(1-D PC-CROW) has attractive features such as small group velocity and ease in fabrication. Recently, we have proposed a low-loss 1-D PC-CROW structure and reported the possibility of its application for ultra compact optical integrated circuit. In order to construct various optical functional devices, waveguide bend structure will be required, however, bending characteristics of 1-D PC-CROW have not been investigated so far. In this report, we investigate the transmission characteristics of the bent 1-D PC-CROW by using finite element method and show that the broadband high transmission as high as that in a conventional silicon wire waveguide bend can be obtained., The Institute of Electronics, Information and Communication Engineers, Japanese

Design of thermo-tunable couplers based on solid core photonic band-gap fibers
YOSHIDA Ryuichi, TSUCHIDA Yukihiro, MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 108, 310, 71, 76, 13 Nov. 2008
In this work, we propose low polarization-dependent thermo-tunable couplers based on solid-core photonic band-gap fibers (PBGFs) filled with high temperature-dependent liquid. Firstly, by employing a full-vectorial finite element method, the temperature dependence on photonic band-gap and effective indices of the fundamental modes are investigated, and we numerically confirm that thermo-tunable couplers based on solid-core PBGFs can be realized. In addition, by evaluating the structural parameters dependence on coupling length and normalized output power for x- and y-polarized waves, we present a design method for the low polarization-dependent thermo-tunable couplers based on solid-core PBGFs., The Institute of Electronics, Information and Communication Engineers, Japanese

Analysis of bending loss in solid-core photonic bandgap fibers
MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 108, 310, 65, 70, 13 Nov. 2008
Solid-core photonic bandgap fibers (SC-PBGFs) have gathered much attention because of its intriguing transmission band properties which can be understood by the theory for ARROW waveguides. In SC-PBGFs, it has been reported experimentally that the centripetal radiation as well as the centrifugal radiation occurs, when the fiber is bent at longer wavelength ranges in the PBG. This anomalous property was explained by considering the existence of the inner radiation caustic, which can not be seen in conventional optical fibers. However, the further analysis is required to understand the detailed properties of bent SC-PBGFs. In this paper, we investigate the bending loss properties for the SC-PBGFs through a full-vector modal solver based on the finite element method (FEM). In particular, we show that the cladding modes which induced by the bend cause the complete or incomplete leaky mode coupling with the core mode. Moreover, we show the structural and wavelength dependence of the bending losses for the SC-PBGFs., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-45 Photonic Crystal Fiber and Its Applications
Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2008, 1, 167, 167, 02 Sep. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-11 Design of Ultra-Small Polarization Splitter Based on Slot Waveguides
Komatsu Masa-aki, Kakihara Kuniaki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2008, 1, 133, 133, 02 Sep. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

A Design Method for Band-Pass Filters Based on Photonic Crystal Fibers with Depressed-Index Core
SAITOH Nozomu, VARSHNEY Shailendra K., TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 108, 190, 71, 76, 28 Aug. 2008
In this paper, we propose a design method of all-fiber bandpass filters based on index-guiding and single-mode photonic crystal fibers (PCFs). This proposed PCF design constitutes a fluorine-doped central core, enlarged air-holes surrounding the central core, and small air-holes in the cladding. It has been predicted from numerical simulations that by varying the design parameters, we can achieve the conditions (short- and long-wavelength cut-offs) where the fundamental mode becomes a leaky-mode, allowing a control on the leakage loss characteristics of the fundamental mode. By tailoring the short- and long-wavelength cut-offs through fiber's structural parameters, doping concentration, and fiber length, we can obtain the desired bandpass filter characteristics., The Institute of Electronics, Information and Communication Engineers, Japanese

Leakage Loss Reduction of TM-Like Modes in Silicon-On-Insulator Ridge Waveguides with a Dimple
KAKIHARA Kuniaki, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 108, 189, 1, 6, 28 Aug. 2008
We numerically investigate the lateral leakage loss behavior for TM-like modes in silicon-on-insulator (SOI) ridge waveguides. If the waveguide width is set to satisfy a certain condition, the leakage loss of the TM-like mode leads to be a minimum. When utilizing TM-like modes in the ridge waveguides, it is the main drawback that the tolerance for the variations in the waveguide width and operating wavelength is too small. In this paper, we propose a novel ridge waveguide structure, where a dimple is introduced at the ridge region. It is shown from the finite element analysis that the ridge waveguide with a dimple is both low-loss and fabrication tolerant., The Institute of Electronics, Information and Communication Engineers, Japanese

A design method for polarization controllers based on photonic crystal fibers infiltrated with liquid crystals
YOSHIDA Ryuichi, TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 107, 513, 27, 32, 07 Mar. 2008
In this paper, we propose a novel type of temperature-controlled polarization controller based on photonic crystal fibers (PCFs) filled with liquid crystals. Firstly, the effect of air hole diameter changes on hybridness is investigated using full-vectorial finite element method, where hybridness is a key parameter to understand the polarization conversion properties. By evaluating the temperature dependence on not only hybridness but also half-beat length, the optimum structures for the PCF-based polarization controller are determined. In addition, by employing full-vectorial finite element beam propagation method, we numerically confirm that the highly efficient temperature-controlled polarization controller based on PCFs can be realized., The Institute of Electronics, Information and Communication Engineers, Japanese

Structural optimization of air-guiding photonic bandgap fibers for realizing ultimate low-loss waveguides
MURAO Tadashi, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 107, 513, 21, 26, 07 Mar. 2008
The existence of surface modes is the main drawback and the reduction of the fiber losses is not sufficient in 6-fold symmetry air-guiding photonic bandgap fibers (PBGFs). We show that the surface mode free condition is expressed as a normalized silica-ring thickness T=0.5 for any core size and the cladding structural parameters, regardless the core radius of the silica-ring is one of the main factors of dominating the surface mode's condition, and the fact that the wavelength range of the photonic bandgap (PBG) changes on varying the structural parameters of the cladding. Moreover, we propose a novel type of PBGF, namely 3-fold symmetry 27 cell-core PBGF without surface mode, which exhibits lower scattering losses caused by surface roughness of the silica-ring in comparison to 6-fold symmetry 19 cell-core PBGFs and suppresses the mode coupling between fundamental-like and higher-order modes when compared to 6-fold symmetry 37 cell-core PBGFs., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-48 Reduced Losses of TM-Like Modes in Silicon-On-Insulator Ridge Waveguides
Kakihara Kuniaki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 229, 229, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-94 Design of Single-Mode Leakage Channel Fibers with Large-Mode-Area and Low Bending Loss
Iizawa Kento, Tsuchida Yukihiro, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 275, 275, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-99 Highly Efficient Transmission Between 1-D Photonic Crystal Coupled Cavity Waveguides and Straight Waveguides
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 280, 280, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-4 Structural Optimization of Low-Loss Solid-Core Photonic Bandgap Fibers
Maeda Kuniaki, Murao Tadashi, Saitoh kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 185, 185, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-3 Analysis of Bend Loss in Solid-Core Photonic Bandgap Fibers
Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 184, 184, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-27 Design of Ultra-Small Polarization Splitter Based on Silicon-Wire Waveguides
Komatsu Masaaki, Kakihara Kuniaki, Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2008, 1, 208, 208, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-2 Design of Thermo-Tunable Solid-Core Photonic Band-Gap Fiber Couplers
Yoshida Ryuichi, Tsuchida Yukihiro, Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 1, 183, 183, 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-21 Design of Polarization Controllers Based on Photonic Crystal Fibers
Yoshida Ryuichi, Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 144, 144, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-19 Cascaded Raman Photonic Crystal Fiber Laser Operating at 1480-nm Wavelength Range
Sasaki Kazuya, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 142, 142, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-20 Impact of Bending on Lasing Performance of Yb^<3+>-Doped Photonic Crystal Fibers
Iizawa Kento, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 143, 143, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-30 Design methodologies for combinational logic circuits consisting of photonic crystal devices
Yamada Kazuhito, Asai Tetsuya, Saitoh Kunimasa, Hirose Tetsuya, Amemiya Yoshihito, Proceedings of the Society Conference of IEICE, 2007, 1, 153, 153, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-17 A design method of singl-polarization holey fibers with improved beam quality
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 140, 140, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-10 Design of Si-Wire Waveguides with Low-Loss Coupled Resonator Structures
Kawaguchi Yuki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 133, 133, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-18 Structural Optimization of Air-Guiding Photonic Bandgap Fibers for Ultimate Low Loss Waveguides
Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2007, 1, 141, 141, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

A design method for single-polarization holey fibers with improved beam quality factor
TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 107, 198, 1, 6, 16 Aug. 2007
Recently, there has been an increasing interest in developing single polarization fibers for applications such as polarization splitters, polarizers, high power lasers, etc. Usually, the polarization maintenance can be achieved by form birefringence, such as by creating elliptical cores, or material anisotropy, or by stress-applying parts inside the fiber. This results into the non-Gaussian field distributions due to distortion of fiber core, therefore leading to the degradation of beam quality factor M^2. In this study, we propose a design method for single-polarization holey fibers with improved beam quality by introducing the resonant coupling phenomenon between the central core and the elliptical cores arranged on either side of the central core., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-53 Cascade Raman Fiber Laser Using Highly-Nonlinear Photonic Crystal Fiber
Sasaki Kazuya, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2007, 1, 209, 209, 07 Mar. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-42 Three-Dimensional Finite Element Analysis of Bent Silicon-Wire Waveguides
Kakihara Kuniaki, Kono Naoya, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2007, 1, 198, 198, 07 Mar. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-51 Design of effectively single-mode air-core photonic bandgap fibers with improved transmission characteristics for realization of ultimate low loss waveguides
Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2007, 1, 207, 207, 07 Mar. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

CS-3-8 Vector Finite Element Analysis of Bent Silicon-Wire Waveguides
Kakihara Kuniaki, Kono Naoya, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, "S, 43", 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-35 Numerical Investigation of Stimulated Brillouin Scattering in Hole-Assisted Fibers
Nagano Hironori, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 157, 157, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-37 Design of Low-Loss Photonic Band-Gap Fibers with Broadband Single-Mode Operation
Murao Tadashi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 159, 159, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-39 Gain-Flattened Dispersion-Compensating Photonic Crystal Fiber Raman Amplifiers Operating in C-Band
Sasaki Kazuya, Varshney Shailendra K., Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 161, 161, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-82 Proposal of CROW-Based Ring Resonators Used for Miniaturizing Interleaves
Yokoi Nobuhiro, Kono Naoya, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 204, 204, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-38 Equifrequency Surfaces and Transmission Characteristics of Laterally Illuminated Photonic Crystal Fibers
Morikawa Keiichi, Kono Naoya, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 160, 160, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-36 Optimization of Broadband-Dispersion-Compensating Holey Fibers
Wada Keisuke, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 158, 158, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-34 A Design Method for Large-Mode-Area Single-Mode Holey Fibers with Low Bending Losses
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2006, 1, 156, 156, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-84 Structural Dependence of SBS Spectrum in Pure-Silica Holey Fibers
Nagano Hironori, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 219, 219, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-86 Low-Bending-Loss Single-Mode Holey Fibers with Large Mode Area
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 221, 221, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-79 Gauss-Cosine-Type Photonic-Crystal Waveguide Gratings
Yokoi Nobuhiro, Fujisawa Takeshi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 214, 214, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-82 Thermo-Optical Sensitivity of Polarimetric-Sensing Photonic Crystal Fibers
Florous Nikolaos John, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 217, 217, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, English

C-3-85 Optimization of Dual-Concentric-Core Holey Fibers for Broadband Dispersion Compensation
Wada Keisuke, Saitoh Kunimasa, Fujisawa Takeshi, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 220, 220, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-87 Multi-Core Photonic Band Gap Fiber Splitters Based on Highly-Selective Non-Proximity Resonant Coupling
Saitoh Kunimasa, Florous Nikolaos John, Koshiba Masanori, Skorobogatiy Maksim, Proceedings of the IEICE General Conference, 2006, 1, 222, 222, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, English

C-3-83 Transmission Characteristics of Laterally Illuminated Photonic Crystal Fibers
Morikawa Keiichi, Fujisawa Takeshi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2006, 1, 218, 218, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

Approximate Analytical Solutions for Nonlinear Photonic Crystal Fibers
KIRIHARA Takahito, FUJISAWA Takeshi, SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2006, 1, 75, 80, 31 Jan. 2006
Japanese

Leakage Losses of Quasi-Phase-Matched Second-Harmonic-Generation Devices with Air Gap
ISHIKAWA Yoshiki, SAITOH Kunimasa, FUJISAWA Takeshi, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2006, 1, 167, 172, 31 Jan. 2006
Japanese

Highly Efficient Coupling between Photonic Crystal Circuits and Si-Wire Waveguides
TORIHATA Akihiro, SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2006, 1, 43, 48, 31 Jan. 2006
Japanese

Leakage Losses of Quasi-Phase-Matched Second-Harmonic-Generation Devices with Air Gap
Ishikawa Yoshiki, Saitoh Kunimasa, Fujisawa Takeshi, Koshiba Masanori, Technical report of IEICE. LQE, 105, 593, 45, 50, 26 Jan. 2006
Leakage losses in a quasi-phase-matched second-harmonic-generation device based on a ridge-type MgO:LiNbO_3/LiNbO_3 waveguide with air gap, which is expected to be a highly efficient, low-loss wavelength converter, are theoretically estimated. Using a full-vector finite element method, the influence of the distance between MgO:LiNbO_3 surface and air gap, and the air-gap width and depth on the leakage losses is investigated in details. It is found that the leakage losses of modes associated with the wavelength conversion are not reduced with increasing air-gap depth. The mechanism of this phenomenon is clarified and a method for further reduction of the leakage losses is discribed., The Institute of Electronics, Information and Communication Engineers, Japanese

Approximate Analytical Solutions for Nonlinear Photonic Crystal Fibers
KIRIHARA Takahito, FUJISAWA Takeshi, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. LQE, 105, 592, 33, 38, 25 Jan. 2006
An approximate analytical method for the solutions of nonlinear photonic crystal fibers (PCFs) is newly proposed. Replacing a PCF with a conventional step-index fiber, closed form expressions for the effective refractive index and effective core area of nonlinear PCFs are derived. To define the equivalent cladding index, the effective index of the so-called fundamental space-filling mode, which is calculated from the empirical relation for the effective normalized frequency, is introduced, and thus, nonlinear guided waves propagating in PCFs can be easily characterized without any need for numerical computations. The validity of the method proposed here is confirmed by comparing the calculated results with those obtained by a full-vector finite-element method., The Institute of Electronics, Information and Communication Engineers, Japanese

Highly Efficient Coupling between Photonic Crystal Circuits and Si-Wire Waveguides
TORIHATA Akihiro, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. LQE, 105, 592, 1, 6, 25 Jan. 2006
An outstanding technique for efficient coupling between pillar-type photonic crystal circuits and Si-wire waveguides is proposed. In order to improve the coupling efficiency, Si-wire waveguide tapers with nanometer-sized tips are introduced and theoretically optimized, resulting in coupling efficiencies higher than 90% for 200-nm bandwidth around an operational wavelength of 1550nm. The tapered Si-waveguide couplers are placed at the input and output ports of a wavelength multiplexer-demultiplexer based on pillar-type photonic crystal circuits for coarse wavelength division multiplexing systems. It is confirmed though numerical simulations that the demultiplexing characteristics are drastically improved, in comparison to the conventional butt coupling technique., The Institute of Electronics, Information and Communication Engineers, Japanese

Chromatic dispersion profile optimization of dual-concentric-core photonic crystal fibers for broadband dispersion compensation
Fujisawa Takeshi, Saitoh Kunimasa, Wada Keisuke, Koshiba Masanori, Technical report of IEICE. OPE, 105, 589, 13, 18, 25 Jan. 2006
Chromatic dispersion profile of dual-concentric-core photonic crystal fibers is optimized for broadband dispersion compensation of single mode fibers (SMFs) by using genetic algorithm incorporated with full-vector finite-element method. From the numerical results presented here, it is found that by increasing the distance between central core and outer ring core, larger negative dispersion coefficient and better dispersion slope compensation are possible. There is a tradeoff between the magnitude of negative dispersion coefficient and dispersion slope compensation due to the concave dispersion profile of dual-concentric-core photonic crystal fibers. In spite of the tradeoff, dual-concentric-core photonic crystal fibers having larger negative dispersion coefficient as well as compensating for dispersion slope of SMFs in the entire C band with large effective area can be designed., The Institute of Electronics, Information and Communication Engineers, English

A novel approach for designing photonic crystal fiber splitters with polarization-independent propagation characteristics
FLOROUS Nikolaos, SAITOH Kunimasa, KOSHIBA Masanori, IEICE technical report, 105, 382, 15, 20, 04 Nov. 2005
We propose and numerically investigate the operation of a novel class of polarization-independent splitters based on the photonic crystal fiber (PCF) technology. The proposed polarization-independent feature of the PCF splitter is realized by uniformly distributed elliptically-shaped air-holes in the cladding of a dual-core PCF. The design procedure follows a rigorous synthesis protocol based on exact equations for describing the wavelength de-coupling mechanism, and on full-vectorial finite element as well as beam propagation methods for optical characterization of PCFs. The compact de-coupling lengths as well as the low cross-talk over appreciable optical bandwidths are the main advantages of the proposed PCF splitter. The proposed device can be employed in reconfigurable optical communication systems for performing wavelength de-multiplexing operation, especially for fiber-to-the-home applications, as well as the emerging passive optical network applications., The Institute of Electronics, Information and Communication Engineers, English

C-3-116 Structural Dependence of Core-Shape on Bending Losses in Holey Fibers with Large Mode Areas
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 260, 260, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-113 Approximate Analytical Solutions for Saturable Nonlinear Photonic Crystal Fibers
KIRIHARA Takahito, FUJISAWA Takeshi, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 257, 257, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-78 Design of Photonic Crystal WDM Filters for B-PON Systems
Torihata Akihiro, Fujisawa Takeshi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 222, 222, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-112 Numerical Investigation of Stimulated Brillouin Scattering in Holey Fibers
Nagano Hironori, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 256, 256, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-109 Transmission Characteristics of Laterally Illuminated Photonic Crystal Fibers
Morikawa Keiichi, Fujisawa Takeshi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 253, 253, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-117 Broadband Dispersion Compensation by Using Dual-Concentric-Core Holey Fibers
Wada Keisuke, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 261, 261, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-24 Leakage Losses of QPM-SHG Device with Air Gap
Ishikawa Yoshiki, Saitoh Kunimasa, Fujisawa Takeshi, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, 168, 168, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

CS-2-2 A Novel Type of Holey Fiber with Dispersive Core
Saitoh Kunimasa, Koshiba Masanori, Proceedings of the Society Conference of IEICE, 2005, 1, "S, 15"-"S-16", 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

Structural Dependence of Core-Shape on Bending Losses in Holey Fibers with Large Mode Area
TSUCHIDA Yukihiro, SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. LQE, 105, 251, 13, 17, 19 Aug. 2005
Large-mode-area (LMA) single-mode fibers are vital component for many applications such as high power delivery, fiber amplifier, data transmission, and so on. Although standard optical fibers have difficulties in obtaining both large mode area and single-mode operation, all-silica photonic crystal fibers, also called holey fibers, could be considered as good candidates for realizing LMA single-mode fibers because of their endlessly single-mode properties. In this paper, we numerically demonstrate the possibility of improved LMA endlessly single-mode holey fibers. We compare a standard holey fiber with a core formed by one missing air hole (HF1) with other types of holey fibers that have three, six, and seven missing neighboring air holes, and we find that HF1 shows the lowest macro-bending loss property among these four types of endlessly single-mode holey fiber structure with the same effective area., The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-71 A Novel Holey Fiber with Highly Dispersive Core
Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2005, 1, 241, 241, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-109 Highly Efficient Excitation Method for Photonic Crystal Circuits Composed of Dielectric Rods
Torihata Akihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2005, 1, 279, 279, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-102 A Simple Design Method for Quasi-Phase-Matched Second-Harmonic Generation Devices
Ishikawa Yoshiki, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2005, 1, 272, 272, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-74 Low Bending Loss Holey Fiber with Reduced Splice Loss
Tsuchida Yukihiro, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 2005, 1, 244, 244, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-69 Apporoximate Analytical Solutions for Nonlinear Photonic Crystal Fibers
KIRIHARA Takahito, FUJISAWA Takeshi, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2005, 1, 239, 239, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-26 Analysis of SBS in optical fiber based on FEM-based acoustic and optical mode solvers
Hasegawa Takemi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of the IEICE General Conference, 196, 196, 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

P1-7 Highly-Efficient Acousto-Optic Devices Based on Photonic Crystal Fibers
Enomori Ikumi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of Symposium on Ultrasonic Electronics, 25, 25, 26, 27 Oct. 2004
Steering committee of symposium on ultrasonic electronics, Japanese

Empirical Relations for Simple Design of Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, レーザー学会研究会報告 = Reports on the Topical meeting of the Laser Society of Japan, 326, 47, 52, 26 Aug. 2004
Japanese

C-3-141 Polarization Splitter Based on Photonic Crystal Fibers
SATO Yuichiro, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2004, 1, 315, 315, 08 Mar. 2004
The Institute of Electronics, Information and Communication Engineers, Japanese

C-3-135 Applicability of Classical Optical Fiber Theory to Holey Fibers
Koshiba Masanori, Saitoh Kunimasa, Proceedings of the IEICE General Conference, 2004, 1, 309, 309, 08 Mar. 2004
The Institute of Electronics, Information and Communication Engineers, English

C-3-136 Modeling of Realistic Air-Core Photonic Band-Gap Fibers
Saitoh Kunimasa, Mortensen N. A., Koshiba Masanori, Proceedings of the IEICE General Conference, 2004, 1, 310, 310, 08 Mar. 2004
The Institute of Electronics, Information and Communication Engineers, English

C-3-137 Fundamental Characteristics of Localized Acoustic Modes in Photonic Crystal Fibers
ENOMORI Ikumi, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2004, 1, 311, 311, 08 Mar. 2004
The Institute of Electronics, Information and Communication Engineers, Japanese

A-1 Localized Acoustic Modes Propagating in Photonic Crystal Fibers
Enomori Ikumi, Saitoh Kunimasa, Koshiba Masanori, Proceedings of Symposium on Ultrasonic Electronics, 24, 1, 2, 12 Nov. 2003
Steering committee of symposium on ultrasonic electronics, Japanese

A New Controlling Technique of Chromatic Dispersion in ,Photonic Crystal Fibers and Its Application to Ultra-Flattened Dispersion
SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OFT, 103, 257, 63, 68, 14 Aug. 2003
In order to control dispersion and dispersion slope of index-guiding photonic crystal fibers (PCFs), a new controlling technique of chromatic dispersion in PCF is reported. Moreover, our technique is applied to design PCF with both ultra-low dispersion and ultra-flattened dispersion in wide wavelength range. A full-vector finite element method with anisotropic perfectly matched layers is used to analyze the dispersion properties, the effective mode areas, and the leakage losses in PCFs with finite number of air holes. It is shown from numerical results that it is possible to design a PCF with flattened dispersion of-0.08 ± 0.01 ps/(km-nm) from 1.45 μm to 1.62 um wavelength range and that this dispersion-flattened PCF is suitable for efficient super-continuum generation., The Institute of Electronics, Information and Communication Engineers, English

Dispersion-Flattened Photonic Crystal Fibers with Central Air-Hole
TSUCHIDA Yoshihiro, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2003, 1, 231, 231, 03 Mar. 2003
The Institute of Electronics, Information and Communication Engineers, Japanese

Localized Acoustic Modes in Square-Lattice Photonic Crystal Fibers
MATSUI Takashi, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2003, 1, 221, 221, 03 Mar. 2003
The Institute of Electronics, Information and Communication Engineers, Japanese

Single-Polarization Single-Mode Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2003, 1, 226, 226, 03 Mar. 2003
The Institute of Electronics, Information and Communication Engineers, Japanese

Confinement Loss in Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the Society Conference of IEICE, 2002, 1, 152, 152, 20 Aug. 2002
The Institute of Electronics, Information and Communication Engineers, Japanese

Localized Acoustic Modes in Photonic Crystal Fibers
MATSUI Takashi, SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the Society Conference of IEICE, 2002, 1, 154, 154, 20 Aug. 2002
The Institute of Electronics, Information and Communication Engineers, Japanese

Full-Vectorial Analysis of Confinement losses in Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OFT, 102, 269, 7, 12, 15 Aug. 2002
The finite element method (FEM) with anisotropic perfectly matched layer (PML) is applied to analyzing transmission properties of photonic crystal fibers (PCFs). Using a vector FEM (VFEM), the wavelength dependence and the structural dependence of confinement losses in holey fibers (HFs) are investigated. Furthermore, the real-model simulation results for confinement losses are compared to the earlier experimental results and the polarization-dependent confinement losses in actual HF structures are numerically demonstrated., The Institute of Electronics, Information and Communication Engineers, Japanese

A-2 Analysis Methods for Acoustic Band Structures and Propagation Characteristics of Sonic Crystal Waveguides
MATSUI Takashi, SAITOH Kunimasa, KOSHIBA Masanori, Symposium on ultrasonic electronics, 22, 3, 4, 07 Nov. 2001
Steering committee of symposium on ultrasonic electronics, Japanese

Full-Vector Analysis of Photonic Crystal Fibers by the Finite Element Method
SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2001, 62, 45, 50, 31 Oct. 2001
English

Full-Vectorial Analysis of Transmission Properties of Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, Technical report of IEICE. OFT, 101, 331, 43, 48, 29 Sep. 2001
The finite element method(FEM)is applid to analyzing transmission properties of photonic crystal fibers(PCFs). Using a vector FEM(VFEM), the modal birefringence, single-moded operation, and group-velocity dispersion of holey fibers(HFs)are investigated. The degeneracy in hexaganal HFs with six-fold rotational symmetry, the maximum hole diameter for single-moded operation, and the group-velocity dispersion agreed with experiment are shown., The Institute of Electronics, Information and Communication Engineers, Japanese

Space Filling Modes and Single-Mode Transmissions in Photonic Crystal Fibers
SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the Society Conference of IEICE, 2001, 1, 133, 133, 29 Aug. 2001
The Institute of Electronics, Information and Communication Engineers, Japanese

Nondegeneracy in Circular-Hole Photonic Crystal Fibers
KOSHIBA Masanori, SAITOH Kunimasa, Proceedings of the Society Conference of IEICE, 2001, 1, 130, 130, 29 Aug. 2001
The Institute of Electronics, Information and Communication Engineers, Japanese

Fundamental Characteristics of Sonic Crystal Waveguides
SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2001, 50, 39, 44, 08 Jun. 2001
Japanese

An Anisotropic PML for Finite Element Beam Propagation Analysis of Anisotropic Optical Waveguides
SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 2000, 65, 93, 98, 25 Oct. 2000
English

Full-Vectorial FE-BPM for Anisotropic Optical Waveguides
SAITOH Kunimasa, KOSHIBA Masanori, 電気学会基礎・材料・共通部門大会講演論文集 = Proceeding of Annual Conference of Fundamentals and Materials Society, IEE Japan, 2000, 371, 371, 07 Sep. 2000
Japanese

An Anisotropic PML for Scalar FE-BPM
SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the Society Conference of IEICE, 2000, 1, 266, 266, 07 Sep. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

An Anisotropic PML for Full-Vectorial FE-BPM
SAITOH Kunimasa, KOSHIBA Masanori, Proceedings of the IEICE General Conference, 2000, 1, 341, 341, 07 Mar. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

A Design Method for AOTF's Taking into Account of 3-D SAW Distributions
TAKAHASHI Michiaki, KOSHIBA Masanori, SAITOH Kunimasa, TSUJI Yasuhide, Proceedings of the IEICE General Conference, 2000, 1, 344, 344, 07 Mar. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

C-1 Beam Propagation Analysis of Three-Dimensional Surface Acoustic Waveguides
KOSHIBA Masanori, SAITOH Kunimasa, Symposium on ultrasonic electronics, 20, 21, 22, 17 Nov. 1999
Steering committee of symposium on ultrasonic electronics, Japanese

Beam Propagation Method for 3-D Surface Acoustic Waveguides
SAITOH Kunimasa, KOSHIBA Masanori, 電気学会研究会資料. : The Papers of Technical Meeting on Electromagnetic Theory, IEE Japan. EMT, 電磁界理論研究会, 1999, 88, 55, 62, 09 Nov. 1999
Japanese

Characterization of Acoustooptic Devices by Using Stress and Beam Propagation Analysis Methods
SAITOH Kunimasa, KOSHIBA Masanori, TSUJI Yasuhide, Proceedings of the Society Conference of IEICE, 1999, 1, 158, 158, 16 Aug. 1999
The Institute of Electronics, Information and Communication Engineers, Japanese

Efficient Analysis of SAW Waveguides for Design of Acoustooptic Devices
TAKAHASHI Michiaki, KOSHIBA Masanori, SAITOH Kunimasa, TSUJI Yasuhide, Proceedings of the IEICE General Conference, 1999, 1, 308, 308, 08 Mar. 1999
The Institute of Electronics, Information and Communication Engineers, Japanese

A Stress Analysis Method Considering Piezoelectric Effects and Its Application to Strain Induced Optical Waveguides
SAITOH Kunimasa, KOSHIBA Masanori, TSUJI Yasuhide, Proceedings of the IEICE General Conference, 1999, 1, 307, 307, 08 Mar. 1999
The Institute of Electronics, Information and Communication Engineers, Japanese

A Design Method for Acoustooptic Tunable Filters
SAITOH Kunimasa, KOSHIBA Masanori, TSUJI Yasuhide, Proceedings of the IEICE General Conference, 1998, 1, 361, 361, 06 Mar. 1998
The Institute of Electronics, Information and Communication Engineers, Japanese

Optical Fiber Telecommunications Volume V1B, Sixth Edition: Systems and Networks (Optics and Photonics)　　　　　　　　　　　　　　　
SAITOH Kunimasa, Chapter 13: Transmission Systems Using Multicore Fibers
Academic Press, May 2013, [Contributor]

光通信技術の飛躍的高度化－光通信の新たな挑戦－　　　　　　　　　　　　　　　
齊藤 晋聖, 第2 節 光ファイバ技術の現状と今後の展望
オプトロニクス社, 2012, [Contributor]

Frontiers in Guided Wave Optics and Optoelectronics　　　　　　　　　　　　　　　
Zoltan Varallyay, Kunimasa Saitoh, Chapter 10: Photonic Crystal Fiber for Dispersion Controll
INTEC, 2010, [Contributor]

Suppression of Group-Delay Spread in Coupled Two-LP-Mode Four-Core Fiber　　　　　　　　　　　　　　　
T. Sato, K. Yoshida, T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
IEEE Photonics Conference (IPC 2018), Oct. 2018, English, Oral presentation
[International presentation]

120 Spatial Channel Few-mode Multi-core Fibre with Relative Core Multiplicity Factor Exceeding 100　　　　　　　　　　　　　　　
T. Sakamoto, K. Saitoh, S. Saitoh, Y. Abe, K. Takenaga, A. Urushibara, M. Wada, T. Matsui, K. Aikawa, K. Nakajima
European Conference on Optical Communications (ECOC 2018), Sep. 2018, English, Invited oral presentation
[Invited], [International presentation]

1.2 Pb/s Transmission Over a 160 mm Cladding, 4-Core, 3-Mode Fiber, Using 368 C+L band PDM-256-QAM Channels　　　　　　　　　　　　　　　
R. S. Luis, G. Rademacher, B. J. Puttnam, T. A. Eriksson, H. Furukawa, A. Ross-Adams, S. Gross, M. Withford, N. Riesen, Y. Sasaki, K. Saitoh, K. Aikawa, Y. Awaji, N. Wada
European Conference on Optical Communications (ECOC 2018), Sep. 2018, English, Oral presentation
[International presentation]

Iterative Unreplicated Parallel Interference Canceler for MDL-tolerant dense SDM (12-core × 3-mode) Transmission over 3000 km　　　　　　　　　　　　　　　
K. Shibahara, T. Mizuno, D. Lee, Y. Miyamoto, H. Ono, K. Nakajima, Y. Amma, K. Takenaga, K. Saitoh
European Conference on Optical Communications (ECOC 2018), Sep. 2018, English, Oral presentation
[International presentation]

Microscopic Gain Analysis of Modulation-Doped GeSn Quantum Well: Epitaxial Design toward High-Temperature Lasing　　　　　　　　　　　　　　　
T. Fujisawa, M. Arai, K. Saitoh
International Conference on Group IV Photonics (GFP), Aug. 2018, English, Oral presentation
[International presentation]

Few-Mode Multi-Core Fiber Technologies for Repeated Dense SDM Transmission　　　　　　　　　　　　　　　
T. Sakamoto, K. Saitoh, S. Saitoh, K. Shibahara, M. Wada, Y. Abe, A. Urushibara, K. Takenaga, T. Mizuno, T. Matsui, K. Aikawa, Y. Miyamoto, K. Nakajima
IEEE Photonics Society Summer Topicals Meeting Series (SUM), Jul. 2018, English, Invited oral presentation
[Invited], [International presentation]

PLC-based Mode Controlling Devices for Mode-Division-Multiplexing　　　　　　　　　　　　　　　
T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Invited oral presentation
[Invited], [International presentation]

Mode Dependent Loss Reduced Few-Mode Multi-Core Fiber Design for Repeated Dense SDM Transmission　　　　　　　　　　　　　　　
T. Sakamoto, K. Saitoh, S. Saitoh, K. Shibahara, M. Wada, Y. Abe, A. Urushibara, K. Takenaga, T. Mizuno, T. Matsui, K. Aikawa, Y. Miyamoto, K. Nakajima
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Invited oral presentation
[Invited], [International presentation]

A Broadband Silicon Two-Mode Multiplexer Designed by Wavefront Matching Method　　　　　　　　　　　　　　　
Y. Sawada, T. Sato, T. Fujisawa, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Oral presentation
[International presentation]

Proposal of Si Four-Wavelength Multiplexer using Higher-Order Mode for 100GbE　　　　　　　　　　　　　　　
J. Takano, T. Sato, Y. Sawada, T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Oral presentation
[International presentation]

Novel All-Optical Diode based on Single-Port and Four-Port Photonic Crystal Cavities　　　　　　　　　　　　　　　
T. Sato, T. Fujisawa, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Oral presentation
[International presentation]

Dependence of Cladding Diameter on Inter-Core Crosstalk in Heterogeneous Multi-Core Fibers　　　　　　　　　　　　　　　
Y. Amma, K. Takenaga, T. Fujisawa, K. Saitoh, M. Koshiba, K. Aikawa
OptoElectronics and Communications Conference (OECC), Jul. 2018, English, Oral presentation
[International presentation]

DMD-unmanaged long-haul SDM transmission over 2500-km 12-core × 3-mode MC-FMF and 6300-km 3-mode FMF employing intermodal interference cancelling technique　　　　　　　　　　　　　　　
K. Shibahara, T. Mizuno, L. Doowhan, Y. Miyamoto, H. Ono, K. Nakajima, S. Saitoh, K. Takenaga, K. Saitoh
Optical Fiber Communication Conference (OFC 2018), Mar. 2018, English, Oral presentation
[International presentation]

Beam propagation analysis of optical activity and circular dichroism in helically twisted photonic crystal fiber”, The 22nd Microoptics Conference　　　　　　　　　　　　　　　
S. Nakano, T. Fujisawa, T. Sato, K. Saitoh
The 22nd Microoptics Conference (MOC 2017), Nov. 2017, English, Oral presentation
[International presentation]

Proposal of compact three-mode exchanger based on symmetric and asymmetric directional couplers with integrated mode rotator　　　　　　　　　　　　　　　
T. Fujisawa, E. Taguchi, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
The 22nd Microoptics Conference (MOC 2017), Nov. 2017, English, Oral presentation
[International presentation]

A Mach-Zehnder mode multi/demultiplexer based on Si/Silica hybrid PLC platform for WDM/MDM transmission　　　　　　　　　　　　　　　
M. Kudo, S. Ohta, E. Taguchi, T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
IEEE Photonics Conference (IPC 2017), Oct. 2017, English, Oral presentation
[International presentation]

Development of fully three-dimensional wavefront matching method and its application to the design of ultrasmall Si mode converters　　　　　　　　　　　　　　　
Y. Sawada, S. Makino, T. Fujisawa, K. Saitoh
IEEE Photonics Conference (IPC 2017), Oct. 2017, English, Oral presentation
[International presentation]

High spatial density six-mode seven-core fibre for repeated dense SDM transmission　　　　　　　　　　　　　　　
T. Sakamoto, K. Saitoh, S. Saitoh, K. Shibahara, M. Wada, Y. Abe, A. Urushibara, K. Takenaga, T. Mizuno, T. Matsui, K. Aikawa, Y. Miyamoto, K. Nakajima
European Conference on Optical Communications (ECOC 2017), Sep. 2017, English, Oral presentation
[International presentation]

Group delay spread analysis of few-mode, coupled 3-core fibres: Optimum index profile and maximum transmission distance for strong coupling regime　　　　　　　　　　　　　　　
K. Yoshida, T. Fujisawa, T. Sato, K. Saitoh
European Conference on Optical Communications (ECOC 2017), Sep. 2017, English, Poster presentation
[International presentation]

Mach-Zehnder mode/wavelength (de)multiplexer for WDM/MDM optical transmission　　　　　　　　　　　　　　　
S. Ohta, T. Fujisawa, S. Makino, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
European Conference on Optical Communications (ECOC 2017), Sep. 2017, English, Oral presentation
[International presentation]

High-forward-transmission all-optical diode based on cascaded L3 photonic crystal cavities　　　　　　　　　　　　　　　
T. Sato, T. Fujisawa, K. Saitoh
The 24th Congress of the International Commission for Optics (ICO-24), Sep. 2017, English, Oral presentation
[International presentation]

A proposal of Mach-Zehnder mode multi/demultiplexer for WDM/MDM optical transmission system　　　　　　　　　　　　　　　
S. Ohta, S. Makino, T. Fujisawa, T. Sakamoto, T. Matsui, K. Tsujikawa, K. Nakajima, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2017, English, Poster presentation
[International presentation]

A compact and low-loss GeSn electroabsorprtion modulator using vertical multimode interference for mid-infrared Ge-on-Si platform　　　　　　　　　　　　　　　
M. Akie, T. Sato, M. Arai, T. Fujisawa, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2017, English, Poster presentation
[International presentation]

Coupled W-type four-core fiber with low differential mode group delay for C+L band　　　　　　　　　　　　　　　
D. Cheng, J. Tu, X. Zhou, K. Long, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2017, English, Poster presentation
[International presentation]

Weakly-coupled and strongly-coupled multicore fibers　　　　　　　　　　　　　　　
K. Saitoh, T. Fujisawa, T. Sato
OptoElectronics and Communications Conference (OECC), Jul. 2017, English, Invited oral presentation
[Invited], [International presentation]

Multicore fiber-based 5-mode multiplexer/demultiplexer　　　　　　　　　　　　　　　
Y. Sasaki, H. Uemura, S. Nishimoto, K. Takenaga, K. Aikawa, S. Matsuo, T, Fujisawa, K. Saitoh
OptoElectronics and Communications Conference (OECC), Jul. 2017, English, Oral presentation
[International presentation]

Design and analysis of weakly- and strongly-coupled multicore fibers　　　　　　　　　　　　　　　
K. Saitoh, T. Fujisawa, T. Sato
OSA Advanced Photonics Congress (APC), Jul. 2017, English, Invited oral presentation
[Invited], [International presentation]

Material analysis of GeSn/SiGeSn quantum wells based on many-body theory　　　　　　　　　　　　　　　
T. Fujisawa, K. Saitoh
IEEE Photonics Society Summer Topicals Meeting Series (SUM), Jul. 2017, English, Invited oral presentation
[Invited], [International presentation]

Highly efficient GeSn electroabsorprtion modulator using higher-order-mode for mid-infrared Ge-on-Si platform　　　　　　　　　　　　　　　
T. Sato, M. Akie, M. Arai, T. Fujisawa, K. Saitoh
Conference on Lasers and Electro-Optics (CLEO 2017), May 2017, English, Oral presentation
[International presentation]

1-Pb/s (32 SDM/46 WDM/768 Gb/s) C-band dense SDM transmission over 205.6-km of single-mode heterogeneous multi-core fiber using 96-Gbaud PDM-16QAM channels　　　　　　　　　　　　　　　
T. Kobayashi, M. Nakamura, F. Hamaoka, K. Shibahara, T. Mizuno, A. Aano, H. Kawakami, A. Isoda, M. Nagatani, H. Yamazaki, Y. Miyamoto, Y. Amma, Y. Sasaki, K. Takenaga, K. Aikawa, K. Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, Md. Nooruzzaman, T. Morioka
Optical Fiber Communication Conference (OFC 2017), Mar. 2017, English, Oral presentation
[International presentation]

Ultimately low-loss and compact Si wire 90° waveguide bend composed of clothoid and normal curves for dense optical interconnect PICs　　　　　　　　　　　　　　　
S. Makino, M. Suga, T. Sato, T. Fujisawa, K. Saitoh
Optical Fiber Communication Conference (OFC 2017), Mar. 2017, English, Oral presentation
[International presentation]

One chip, PLC three-mode exchanger based on symmetric and asymmetric directional couplers with integrated mode rotator　　　　　　　　　　　　　　　
T. Fujisawa, E. Taguchi, T. Sakamoto, T. Matsui, Y. Yamashita, K. Tsujikawa, K. Nakajima, K. Saitoh
Optical Fiber Communication Conference (OFC 2017), Mar. 2017, English, Oral presentation
[International presentation]

Scrambling-type three-mode multiplexer based on cascaded Y-branch waveguide with integrated mode rotator on PLC platform　　　　　　　　　　　　　　　
T. Fujisawa, Y. Yamashita, T. Sakamoto, T. Matsui, S. Makino, K. Tsujikawa, K. Nakajima, K. Saitoh
Optical Fiber Communication Conference (OFC 2017), Mar. 2017, English, Oral presentation
[International presentation]

Heterogeneous 32-core fiber with square-lattice layout for high-density transmissions　　　　　　　　　　　　　　　
X. Xie, J. Tu, K. Long, K. Saitoh
Asia Communications and Photonics Conference (ACP 2016), Nov. 2016, English, Oral presentation
[International presentation]

Ultrasmall silicon mode converters designed by wavefront matching method developed for waveguide discontinuity problem　　　　　　　　　　　　　　　
S. Makino, T. Fujisawa, K. Saitoh
IEEE Photonics Conference (IPC 2016), Oct. 2016, English, Oral presentation
[International presentation]