Bistable laser diodes (BLDs) have a long history. It has been well known that a continuous-wave operation of a semiconductor laser at the room temperature was first achieved in 1970, whereas the histories of BLDs were already found in 1964. First BLD had two separated p-contact regions called an absorptive BLD, which was constructed from a light emitting contact and an absorbing contact. Due to a carrier-density-depend absorption in the absorbing region, the absorptive BLD has two different stable states of on and off with a same DC injection. The absorptive BLD can be all-optically triggered to be the on state, so that there have been expectations to use BLDs as all-optical logic devices.

After demonstrations of the absorptive bistable lasers, two-mode bistable lasers were realized, which had two different lasing states in a laser cavity. The two-mode BLDs can be switched between each mode triggered in optical domain. The switched state is kept even after the triggered pulse owing to the bistability. Around the same time, additional waveguides enabled all-optical reset function to the absorptive BLDs. Under developments of these two types of BLDs, the BLDs have been expected to play all-optical memory roles by their latching function. An all-optical flip-flop using BLD is a keyword of this thesis, because the flip-flop supplies a memory function.

Another keyword is a photonic integration. Ideas of photonic integrated circuits (PICs) are already found in 1969. It has been claimed that the photonic integration has many advantages; it can reduce a number of components. Smaller number of components means less packaging cost. Smaller number of components uses less number of temperature controllers, which enables drastic diminishing of the power consumptions. In addition, monolithic integration approach realizes reduction of fiber-to-waveguide coupling, which is a major factor to determine optical loss through a chip. Using these advantages, the photonic integrations have been mainly applied to the transmitter and receiver chips, because they are the most important application of the photonics. Especially after the practical realization of wavelength division multiplexing (WDM), a number of elements in transmitters has been drastically increased in order to generate and detect multiplexed signals, so that the photonic integration becomes more important. Recently there have been many attempts to integrate multiple elements into a single chip using monolithic approach. Several venture companies start up and have succeeded.

Until this moment, all-optical memory element has not been photonic-integrated. Thus a PIC requires real-time processing. The BLDs have been expected to be integrated with other optical devices to act as all-optical memory elements in future PICs, owing to their latching abilities.

In this thesis, I present an all-optical flip-flop (AOFF) based on interferometric BLD to use it as an all-optical memory device in a PIC. Because the AOFF has waveguide structure and it does not require cleaved facet, the interferometric BLD can be integrated with other waveguide devices. I integrate the AOFF with a Mach-Zehnder interferometer (MZI) semiconductor optical amplifier (SOA) switch and demonstrate an application of all-optical packet switching on a single chip with an all-optical memory function.

Chapter 1 presents introduction and background of the research. Previous researches on AOFFs are introduced. Since the PIC is constructed from waveguides, methods for waveguide analysis are described in Chapter 2. The slab waveguide analysis and beam propagation method are mainly presented. Chapter 3 describes fabrication processes of the PIC. I used InGaAsP/InP based materials. The processes are divided into two steps: (1) crystal growth, active/passive and DBR integration, and (2) waveguides and electrodes formations.

From chapter 4 to 6, I describe experimental results. Chapter 4 treats a multimode interference (MMI) coupler based AOFF. It is based on the two-mode BLD using an active 2×2 cross coupler. Both static and dynamic demonstrations are shown with wavelength dependences, wavelength tunability, and polarization insensitive operations. Distributed Bragg reflectors (DBRs) are monolithically integrated with the AOFF. Single-mode lasing is obtained with a 3.1-nm tuning range. The MMI-BLD can be driven with a wavelength range of 48 nm. A rising and falling times of the AOFF are 280 and 146 ps, respectively.

Chapter 5 shows an AOFF based on a BLD with an MZI structure. The MZI-BLD has high tolerance to fabrication errors. I introduce demonstrations of both static and dynamic operations with two-mode lasing. The operable wavelength range is 58 nm. The device is driven by 2.5-pJ optical pulses with rising and falling time of faster than 319 and 68 ps, respectively. In order to discuss limiting factors of the device, a numerical model of the MZI-BLD is described based on coupled rate equations.

Finally, I demonstrate a photonic integrated circuit for single-chip all-optical packet switching in chapter 6. The MMI-BLD type AOFF and MZI-SOA switch are monolithically integrated on a same chip without any additional processes. The AOFF acts as a memory device to drive the switch. 10-, 40-, and 160-Gb/s optical packet can be switched by the PIC with error-free operations in all data rates. It shows that the PIC is transparent to the data rate, and the PIC supports multi-color packets.

本論文は,"Photonic integrated circuits using all-optical flip-flops based on interferometric bistable laser diodes (干渉型双安定レーザによる全光フリップ・フロップを用いた光集積回路)"と題し,光デジタル情報処理に向けた全光フリップフロップ(AOFF)を,干渉計構造を有するInP系1.55μm帯双安定半導体レーザ(BLD)に依拠して試作,開発し,さらに,これらを集積化したより複雑な光集積回路を試作実証した結果について英文で纏めたもので,7章より構成されている.

第1章は序論であって,研究の背景,動機,目的と,論文の構成が述べられている.

第2章は"Waveguide analysis and device design"と題し,素子設計の基本となるスラブ導波路解析,多モード干渉解析,ビーム伝搬法解析,分布ブラッグ反射特性解析,の各手法について述べている.

第3章は"Device fabrication"と題し,素子試作に用いた個々の技術について詳細に記述している.能動素子と受動素子の集積技術全般について概説した後,本研究で実際に用いたプロセス技術,即ちフォトマスク作製,第一段階有機金属気相成長(MOVPE),能動・受動領域選択,分布ブラッグ反射鏡 (distributed Bragg reflector; DBR) 形成,第二段階埋め込みMOVPE,導波路形成,電極形成,実装が,順を追って論じられている.また,デバイス設計上の制限因子についても本章で述べている.

第4章は"Multimode-interference bistable laser diode"と題し,DBRを適用した多モード干渉結合器(MMI)-BLD型AOFFについて論じている.まず動作原理,素子設計に関し述べた後,試作素子の能動/受動界面の反射を評価している.次に静的AOFF動作とその波長依存性,波長チューニング特性を評価した結果,動作波長範囲48nm,チューニング波長範囲3.1nmという値を得た.さらに動的AOFF動作を測定評価し,立ち上がり時間,立ち下がり時間として各々280ps,146psを得ている.続いて伸張歪量子井戸活性層導入による偏光無依存化が図られ,偏光によらない閾値光入力-3dBmでのAOFF動作を達成したことが述べられている.

第5章は"Mach-Zehnder interferometer bistable laser diode"と題し,マッハツェンダー干渉計(MZI)構造を有するBLDによるAOFFについて論じている.まず素子設計に関し述べた後,試作素子の基本静特性と波長依存性を測定した結果につき記述している.ここでは動作波長範囲として58nmを得ている.次に動特性を評価し,2.5pJの入力光パルスに対し,夫々319ps以下,68ps以下の立ち上がり/立ち下がり時間を得た.最後に,素子性能の限界を知るため,結合レート方程式によるモデリングと動特性シミュレーションを行った結果が詳細に述べられている.

第6章は"Photonic integrated circuit using all-optical flip-flop"と題し,上記のMMI-AOFFと,半導体光アンプ(SOA)-MZI型全光スイッチをモノリシック集積化したワンチップ全光パケット処理回路を試作したことについて論じている.ここではAOFFが光ラベル情報を保持するメモリの役割を果たし,後段の全光スイッチを駆動している.10Gbps, 40Gbpsの単一波長光パケットと 160Gbpsの多波長光パケットを試作光集積回路で実際にルーティングし,全てのデータレートでエラーフリー動作を達成した.これより,全光ルーティング集積回路のデータレートおよび波長に対する透明性が実証された.

第7章は結論であって,得られた成果を総括するとともに将来展望について述べている.

以上のように本論文では,双安定分布ブラッグ反射半導体レーザと多モード干渉結合器またはマッハツェンダー干渉計を基に構成される半導体全光フリップフロップについて設計,特性解析,試作を行い,それらの性能および限界を明らかにするとともに,波長可変化,偏光無依存化の検討を行った.さらに,多モード干渉結合器型素子と全光スイッチをモノリシック集積化した全光パケット処理回路の試作に成功し,この過程を通じてデジタル光集積回路への道を拓いたもので,電子工学分野に貢献するところ多大である.

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.