Abstract

As optical communication systems have been developed for larger capacity (>10 Tb/s per filber) and bandwidth efficiency (>1 bit/s/Hz), optical filters play more and more important roles that perform functions of multiplexing different wavelength channels and demultiplexing them, background noise suppression, spectral filtering, dispersion compensation and optical signal processing. Among various optical filter techniques, fiber Bragg grating (FBG) has attracted much interest due to its all-fiber geometry, low insertion loss, potentially low cost, and easy fabrication. But the most distinguishing feature is the flexibility they offer for achieving desired spectral characteristics.

　Although FBG technologies have been developed well in recently years, making FBG with arbitrary phase shifts by using general phase masks is unavailable, which limits the possibility to obtain more high performance FBG based optical filters. This project focuses on solving this problem to make high performance FBG based optical filters with arbitrary phase shifts.

Multilevel phase-shifts phase sampling has been shown numerically to have higher diffraction efficiency than binary phase-shifts phase sampling. In order to fabricate grating structures with arbitrary phase shifts, displacing phase mask method has been proposed. Two kinds of phase sampling functions have been designed and corresponding FBG reflection spectra have been calculated numerically. Then, two kinds of phase sampled-FBGs (PSFBGs) have been fabricated to show the flexibility of displacing phase mask method for inducing phase change. The first has several binary phase shifts either 0 or π within a sampling period. The second has continuous phase variation between 0 to 2π within a desired sampling period. Experimental results verify that the developed fabrication method is suitable for making complex grating structures.

Multi-channel optical filters are cost effective for processing and manipulating optical signal in optical communication systems. Among various available technologies, phase-sampled FBGs have many advantages such as low insertion loss, all fiber geometry, compact and typically low cost. With the improved FBG fabrication method, two kinds of multilevel phase-sampled FBG (PSFBG) have been made with general uniform or chirped phase masks. Two applications based on fabricated multilevel phase-shifts PSFBGs are demonstrated. One is an 8-channel tunable dispersion compensator. The other is pulse repetition rate multiplier from 40 GHz to 160 GHz.

Conventionally, FBG-based optical filters are bandstop filters and need to be used conjunctionally with optical circulators, thus increasing the cost of devices. Although FBG-based tunneling filters with a flat passband have been demonstrated by inducing multiple π-phase-shifts along a uniform FBG, the stopband of uniform FBGs is generally narrower than 2 nm and such filters are still not suitable for many applications. We propose and demonstrate that direct-coupled resonators could be fabricated on LCFBGs, and that desired transmission characteristics could be designed by the amount and position of phase shifts. Since the stopband of LCFBGs can be very wide, bandpass filters based on LCFBGs are suitable for practical applications. In addition, center wavelengths of bandpass filters can easily be controlled inside the wide stopband.

In high-speed optical communication systems, relative timing drift between signal pulses and clock pulses severely affects the performance of optical switching. By shaping the signal pulse waveform into rectangular shape, this issue can be solved. Rectangular short pulse generation by using strong unchirped FBG has been demonstrated. The grating structure is synthesized by using inverse-scattering algorithm. With this improved FBG fabrication method, the FBG is fabricated and the measured reflection spectrum coincides with target reflection spectrum very well. From experimental results, it can be seen that ~18 ps rectangular pulses are generated successfully. We also propose a scheme to generate rectangular short pulses by using two linearly chirped FBGs (LCFBGs): one is used for pulse reshaping and the other is for dispersion compensation. As an example, the grating structure of the pulse reshaping LCFBG has been designed to generate 4-ps rectangular output pulses, and numerical calculations confirm the feasibility of the proposed scheme.

The current emphasis on optical communication systems is on increasing the spectral efficiency. One of the techniques that is used for achieving high spectral efficiency in WDM systems is vestigial sideband (VSB) filtering. VSB filtering increases spectral efficiency (the number of bits per unit bandwidth) and improves fiber dispersion tolerance. FBG based optical VSB filters for 40 Gb/s bit rate application have been studied. The principle of VSB optical filters and available techniques for realizing VSB optical filters are introduced. A FBG based VSB filter with a 3-dB bandwidth of 0.32 nm has been designed and fabricated. The fabricated VSB filter has been used in 40 Gb/s optical transmission system and experiment results have been analyzed in detail.

Several high-performance FBG based optical filters with arbitrary phase shifts have been realized by using developed fabrication method with just general phase masks. It is anticipated that these high performance optical filters will play important roles in contemporary optical communication systems.

　本論文は"High-performance Optical Filters based on Fiber Bragg Gratings with Arbitrary Phase Shifts (任意位相シフト光ファイバブラッグ回折格子に基づいた高性能光フィルタ)"と題し，英文で執筆され8章からなる。

　光ファイバ伝送システムの急速な発展とともに，光デバイス技術も著しく進歩しているが，この中で光フィルタは特に重要な役割を果たしている。例えば，光波長多重および分離，光増幅器の雑音除去，狭帯域スペクトルフィルタリング，分散補償などの機能は，すべて光フィルタを用いて実現される。 光ファイバブラッググレーティング(FBG)は，光フィルタの一種である。FBGを用いた 光フィルタの利点の一つは，回折格子の振幅と位相特性を任意に設計でき，任意の伝達関数を実現できることにある。しかしこれまで，位相シフトのないFBGまたはπ位相シフトしか含まないFBGが作製されているのみで，任意の位相特性を実現する技術は未開拓であった。 本研究では，上記のFBGの利点を最大限に引き出すことを目標にして，任意の位相シフトをFBGに付与する技術を開発している。さらにこの技術に基づき，高性能なFBG光フィルタを種々提案し，その作製と動作確認に成功している。

　第一章は"Introduction"であり，主に本研究の背景について述べている。

　第二章は"Background of Fiber Bragg Gratings"と題し，本論文を理解するための必要な基礎知識を述べている。FBGの動作原理，解析手法としての結合モード理論，FBGの作製方法などが要約されている。

　第三章は"Arbitrary Phase Shifts in Fiber Bragg Gratings"と題し，任意の位相シフトを持つFBGの利点について説明している。 FBGの位相シフトと複素伝達特性との関係を明らかにし，任意の位相シフトを与えることにより，伝達関数設計の完全な自由度が得られることを示した。 さらに，任意の位相シフトを実現する位相マスク移動法という新しいFBG作製法を提案し， 実験でこの方法の有効性を確かめた。

　第四章は"Fiber Bragg Grating Based Multi-channel Optical Filters"と題し，マルチレベル位相サンプルFBGを用いたマルチチャンネル光フィルタについて述べている。マルチレベル位相サンプリングの原理と設計法を説明したのち，実際に作製した8チャンネル可変分散補償器と40 GHzから160 GHzへのパルス繰り返し逓倍器の動作を示した。

　第五章は"Fiber Bragg Grating based Optical Tunneling Filters"と題し，チャープ型FBGによるマルチレベル位相シフト直接結合共振器を提案している。 チャープ型FBGにマルチレベル位相シフト技術を用いて直接結合共振器を実現するための設計法を示した後， 提案したFBGを作製することにより，実用的な光トンネリングフィルタを実現した。

　第六章は"Fiber Bragg Grating based Rectangular Short Pulse Generation Optical Filter"と題し，方形光パルスを発生するための光フィルタ について述べている。超高速光スイッチング回路における方形光パルスの必要性について論じた後，非チャープ型FBGを用いた光フィルタの設計と試作結果について述べている。 さらに，時間幅を狭くするためにチャープ型FBGを用いることを提案し，その有効性を数値計算により確認している。

　第七章は"Fiber Bragg Grating based Optical Vestigial Sideband Filters"と題し，FBGに基づく残留側波帯(VSB)フィルタの設計および作製について述べている。。40Gbit/s用VSBフィルタの設計と作製について述べ， 作製したFBGが設計通りのVSB伝達関数をもつことが示している。

　第八章は本研究のまとめである。

　以上のように本研究では，FBGに任意の位相シフトを与えることにより， 任意の伝達特性を実現できることを明確化し，このようなFBGの作製法を新たに開発した。この方法を用いて，マルチチャンネル光フィルタ，直接結合光共振器，方形光パルス発生用光フィルタ，残留側波帯光フィルタなど，新規な高機能光フィルタを設計・試作しており，光信号処理技術の発展に大きく寄与し，電子工学への貢献が多大である。

　よって本論文は博士(工学)の学位請求論文 として合格と認められる。