量子ドット半導体光増幅器を用いた光A/D変換の光量子化レベル向上

アナログ－ディジタル(A/D : Analog-to-Digital)変換は実世界に存在する連続的なアナログ信号をコンピュータなどで用いられているディジタル信号に変換する重要な信号処理技術であり、環境計測をはじめフォトニックネットーワークに至るまで幅広い分野で活用されている。しかし従来からの電気的なA/D変換では、電子デバイスの動作速度の限界やジッタの揺らぎなどが更なる大容量化・高速化のボトルネックとなることが予想される。そこで、光の広帯域性・高速性を生かした光A/D変換が注目されている。光標本化については既に多くの報告がされており実用段階である。しかし、光量子化については、光の領域で個々のパルスの強度を測定することが困難とされており、現在研究段階である。他の研究機関ではファイバ内で発生した非線形現象を用いたことによる報告がされているが、構成が複雑であり、高強度な入力パワーが必要などの課題があり、未だ実用段階ではない。半導体光増幅器(SOA : Semiconductor Optical Amplifier)は低消費電力かつ小型な光デバイスであり、増幅器としてだけでなく光信号処理デバイスとしても利用されている。SOAを用いた光量子化は一件だけ報告されているが、この研究においても構成が複雑であり、光デバイスの利得回復時間の影響による性能限界が存在する。近年、次世代のSOAとして注目されている量子ドット半導体光増幅器(QD-SOA : Quantum-Dot Semiconductor Optical Amplifier)は、従来の光増幅器と比較し、低雑音・高利得・広帯域・高速な利得回復時間という特徴を持つため、従来のSOA で対応できなかった光信号処理にも利用することが可能である。SOA では周波数チャープという増幅された出力信号の立ち上がり、立ち下がり時に搬送波周波数が瞬間的に変動する現象が発生し、信号品質の劣化や伝送距離の制限を招く。この現象はQD-SOAでも発生することが確認されており、周波数チャープ(レッドチャープ)は、入力パワーに依存することが報告されている。また、高速な利得回復時間により、データパターンに対して依存性が小さいことがわかっている。この特徴より、周波数チャープを強度－周波数変換に活用することで光量子化への応用が期待できる。実際、先行研究では5レベルの光量子化に成功している。本研究では、QD-SOAの周波数チャープを用いた光A/D変換を提案し、量子化レベルの向上を行う。提案方式における光量子化において、フィルタのロールオフの小ささが重要な要素であり、先行研究で用いた矩形波型フィルタは非常に急峻という特徴を持っていたが、細かいシフトが困難であり量子化レベルの向上が課題であった。そこで以前のフィルタよりも微細かつ精密なシフトが可能なプログラマブル矩形波型光フィルタ(PR-BPF : Programmable Rectangular-shape Bandpass Filter)を用いることで光量子化の多レベル化の実証実験を行った。光量子化特性の入力パワー依存性及び閾値依存性について評価を行い最適化することで9レベルの光量子化に成功した。光量子化特性の性能評価を行い、ファイバベースの光A/D変換と比較し、構成の簡素化及び低消費電力化を実現した。また、半導体光増幅器ベースの研究においては、構成の簡素化及び最多量子化レベルを実現し、本提案方式の有用性を示した。電気通信大学201

Ultrashort, High Power, And Ultralow Noise Mode-locked Optical Pulse Generation Using Quantum-dot Semiconductor Lasers

This dissertation explores various aspects and potential of optical pulse generation based on active, passive, and hybrid mode-locked quantum dot semiconductor lasers with target applications such as optical interconnect and high speed signal processing. Design guidelines are developed for the single mode operation with suppressed reflection from waveguide discontinuities. The device fabrication procedure is explained, followed by characteristics of FP laser, SOA, and monolithic two-section devices. Short pulse generation from an external cavity mode-locked QD two-section diode laser is studied. High quality, sub-picosecond (960 fs), high peak power (1.2 W) pulse trains are obtained. The sign and magnitude of pulse chirp were measured for the first time. The role of the self-phase modulation and the linewidth enhancement factor in QD mode-locked lasers is addressed. The noise performance of two-section mode-locked lasers and a SOA-based ring laser was investigated. Significant reduction of the timing jitter under hybrid mode-locked operation was achieved owing to more than one order of magnitude reduction of the linewidth in QD gain media. Ultralow phase noise performance (integrated timing jitter of a few fs at a 10 GHz repetition rate) was demonstrated from an actively mode-locked unidirectional ring laser. These results show that quantum dot mode-locked lasers are strong competitors to conventional semiconductor lasers in noise performance. Finally we demonstrated an opto-electronic oscillator (OEO) and coupled opto-electronic oscillators (COEO) which have the potential for both high purity microwave and low noise optical pulse generation. The phase noise of the COEO is measured by the photonic delay line frequency discriminator method. Based on this study we discuss the prospects of the COEO as a low noise optical pulse source

Advanced progress on χ(3) nonlinearity in chip-scale photonic platforms

χ(3) nonlinearity enables ultrafast femtosecond scale light-to-light coupling and manipulation of intensity, phase, and frequency. χ(3) nonlinear functionality in micro-and nano-scale photonic waveguides can potentially replace bulky fiber platforms for many applications. In this Review, we summarize and comment on the progress on χ(3) nonlinearity in chip-scale photonic platforms, including several focused hot topics such as broadband and coherent sources in the new bands, nonlinear pulse shaping, and all-optical signal processing. An outlook of challenges and prospects on this hot research field is given at the end

Advances in Optical Amplifiers

Optical amplifiers play a central role in all categories of fibre communications systems and networks. By compensating for the losses exerted by the transmission medium and the components through which the signals pass, they reduce the need for expensive and slow optical-electrical-optical conversion. The photonic gain media, which are normally based on glass- or semiconductor-based waveguides, can amplify many high speed wavelength division multiplexed channels simultaneously. Recent research has also concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhanced functions. Advances in Optical Amplifiers presents up to date results on amplifier performance, along with explanations of their relevance, from leading researchers in the field. Its chapters cover amplifiers based on rare earth doped fibres and waveguides, stimulated Raman scattering, nonlinear parametric processes and semiconductor media. Wavelength conversion and other enhanced signal processing functions are also considered in depth. This book is targeted at research, development and design engineers from teams in manufacturing industry, academia and telecommunications service operators

State-of-the-art InAs/GaAs quantum dot material for optical telecommunication

This thesis reports on the characterization of the state-of-the-art In(Ga)As/GaAs quantum dot (QD) material grown by molecular beam epitaxy for optical telecommunication applications. A wide variety of characterization methods are employed to investigate the material properties and characteristics of a number of QD-based devices enabling future device optimization. The motivation that prompted this study was predicated mainly upon two technological advantages. First, that the QDs gain spectra exhibits a symmetric gain shape and thus the change of refractive index with respect to gain is negligible at the lasing wavelength. This is therefore expected to result in a zero or a very small linewidth enhancement factor (LEF), which is desirable for instance, for high-speed modulation purposes where frequency chirp under modulation, which is directly proportional to the LEF, may be substantially reduced. Second, the fact that not only QDs exhibit a damped frequency response attributed to the carrier relaxation dynamics but also as the resilience of a laser to optical feedback is inversely proportional to the fourth power of the LEF, QD lasers are expected to demonstrate a relatively higher feedback insensitivity. This bodes well for operating these devices isolator free, which would be greatly cost-effective. The absorption and gain spectra of the QD active material are investigated in chapters 2 and 3, respectively. The LEF of QD lasers at a range of temperatures is studied in chapter 3, which confirms the expectation for the first time for In(Ga)As/GaAs QD lasers from -10 oC to 85 oC. Subsequently, the findings of chapters 2 and 3 are employed in chapter 4 with an electro absorption modulator device in mind which would be able to operate with chirp control. In chapter 5, the modulation response of QD lasers is investigated through examining the relative intensity noise (RIN) spectra in the electrical domain. The resilience of the devices to optical feedback is subsequently studied through the RIN characteristics at a range of temperatures. Chapter 6 provides a summary of the thesis findings and possible future works that may be carried out as continuation to this project, which fell outside of the remit of this work