All-Optical Flip-Flops Based on Semiconductor Technologies

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Subsystems for High bit-rate Optical Networks

Questa tesi contiene parte del lavoro svolto negli ultimi tre anni presso i laboratori congiunti del CNIT e della la Scuola Superiore Sant'Anna di Pisa, dove ho lavorato nel gruppo di Sistemi Ottici sotto la supervisione del prof. Ernesto Ciaramella, ed in parte presso il Dipartimento di Fisica dell'Università di Pisa, sotto la supervisione del prof. Niccolò Beverini. Durante questi anni ho avuto l'opportunità di lavorare su vari filoni di ricerca (studio di sorgenti laser impulsate, esperimenti di processamento dei segnali tutto ottico, sistemi di protezione dei guasti di rete,...) inquadrati in differenti progetti di ricerca, ed anche in Università straniere (Massachussetts Intitute of Technology MIT di Boston, USA). In questa tesi verrà comunque descritta solo una parte dei risultati sviluppati. In particolare, verrà discussa la ricerca svolta mirata alla realizzazione di sotto-sistemi che possono essere impiegati nei sistemi di comunicazione ottica (o, più in generale nelle Reti Ottiche) basate su trassmissioni di dati alla frequenza di cifra di 40 Gb/s. Ogni sottos-sistema sarà presentato seguendo un ordine che riproduce quello in cui questi sotto-sistemi sono impegati effettivamente: inizierò descrivendo una sorgente di impulsi ottici ad altissimo bit-rate da impiegare nei sistemi ottici multiplati a divisione di tempo (OTDM); si passerà poi alla descrizione di vari convertitori di lunghezza d'onda che sono utilizzati nei nodi di rete; per concludere, verrà discussa una unità tutta ottica per i recupero del sincronisimo, che è tipicamente impiegata alla fine di un sistema di trasmissione ed è usata per affiancare i ricevitori veri e propri. Tutte queste funzionalità sfruttano le proprietà ottiche non lineari di dispositivi tra i più comunemente usati nei sistemi di comunicazione ottici: le fibre ottiche e gli amplificatori ottici a semiconduttore. Oltre che dalla natura "tutto-ottica", tutti questi dispositivi (o funzionalità) sono accumunati dalla ricerca di semplicità sia realizzativa che progettuale: come verrà mostrato caso per caso, ogni sotto-sistema è stato realizzato cercando di ricorrere al minor numero possibile di dispositivi per ridurre la complessità globale. Questo è un punto fondamentale per dimostrare che le tecnologie "tutto-ottiche" possono rappresentare un'alternativa all'elettronica. Ogni "sotto-sistema" verrà trattato separatamente in un capitolo. Ogni capitolo contiene una breve discussione sulle novità introdotte, rispetto a soluzioni simili presentate in letteratura o in commercio. Benchè il lavoro riportato in questa tesi è essenzialmente di carattere sperimentale, per migliorarne la comprensione e la completezza ogni capitolo contiene dei paragrafi in cui l'argomento viene illustrato dal punto di vista teorico. La tesi è divisa in 4 capitoli secondo lo schema seguente: Una panoramica sui sistemi di comunicazione basati su fibra ottica: un capitolo introduttivo per spiegare l'evoluzione e la struttura e possibili scenari delle Reti Ottiche ed introdurre i motivi fondanti della ricerca riportata nella tesi. Una sorgente solitonica, nel capitolo 2. Questo capitolo contiene una discussione sulla progettazione e la realizzazione di una sorgente laser da impiegare in sistemi OTDM. La sorgente è progetatta per produrre impulsi di durata inferiore al picosecondo ad una frequenza di ripetizione di 40 GHz repetition rate. La sorgente è studiata per essere utilizzata direttamente nei sistemi di comunicazione, senza la necessità di dover ricorrere a stadi di processamento successivi (come la risagomatura degli impulsi, la loro compressione o la rimozione di piedistallo). Gli impulsi sono generati tramite un fenomeno di propagazione in regime non-lineare controllato in una fibra ottica particolare. Benchè la sorgente sia stata progettata per essere impiegata in sistemi OTDM, dato il suo spettro ottico largo e periodico potrebbe essere utilizzata anche in altri ambiti, come verrà discusso più volte nel corso della tesi. Esperimenti di conversione di lunghezza d'onda (includendo anche espeimenti di conversione di lunghezza d'onda multipla), nel capitolo 3. In questo capitolo, la conversione di lunghezza d'onda (ovvero il trasferimento della modulazione contenuta su un segnale ottico ad uno su una portante a lughezza d'onda differente) è dimostrata attraverso diverse tecniche, principalmente ricorrendo alle dinamiche veloci non-lineari deli amplificatori a semiconduttore. In tutti questi esperimenti, verrà trattata in dettaglio anche la realizzazione della conversione di lunghezza d'onda simultaneamente su più canali: in particolare questa funzionalità è ritenuta molto importante per le reti di accesso di prossima generazione.. Uno circuito tutto ottico per l'estrazione del segnale di sincronia da un segnale modulato nel capitolo 4. Lo schema presentato in questo capitolo per i recupero del sincronismo rappresenta un notevole passo in avanti rispetto ai circuiti presentati precedentemente in letteratura, sia in termini di efficienza che di compattezza. Il dispositivo è basato sull'implementazione tutta-ottico del Tank-Circuit (largamente utilizzato in elettronica). Questo circuito si è dimostrato molto versatile: in particolare è stato dimostrato il suo impiego con diversi formati di modulazione, sia con traffico continuo che a pacchetti. Il circuito inoltre è adatto per un'integrazione fotonica ibrida

WDM/TDM PON bidirectional networks single-fiber/wavelength RSOA-based ONUs layer 1/2 optimization

This Thesis proposes the design and the optimization of a hybrid WDM/TDM PON at the L1 (PHY) and L2 (MAC) layers, in terms of minimum deployment cost and enhanced performance for Greenfield NGPON. The particular case of RSOA-based ONUs and ODN using a single-fibre/single-wavelength is deeply analysed. In this WDM/TDM PON relevant parameters are optimized. Special attention has been given at the main noise impairment in this type of networks: the Rayleigh Backscattering effect, which cannot be prevented. To understand its behaviour and mitigate its effects, a novel mathematical model for the Rayleigh Backscattering in burst mode transmission is presented for the first time, and it has been used to optimize the WDM/TDM RSOA based PON. Also, a cost-effective, simple design SCM WDM/TDM PON with rSOA-based ONU, was optimized and implemented. This prototype was successfully tested showing high performance, robustness, versatility and reliability. So, the system is able to give coverage up to 1280 users at 2.5 Gb/s / 1.25 Gb/s downstream/upstream, over 20 Km, and being compatible with the GPON ITU-T recommendation. This precedent has enabled the SARDANA network to extend the design, architecture and capabilities of a WDM/TDM PON for a long reach metro-access network (100 km). A proposal for an agile Transmission Convergence sub-layer is presented as another relevant contribution of this work. It is based on the optimization of the standards GPON and XG-PON (for compatibility), but applied to a long reach metro-access TDM/WDM PON rSOA-based network with higher client count. Finally, a proposal of physical implementation for the SARDANA layer 2 and possible configurations for SARDANA internetworking, with the metro network and core transport network, are presented

Fibre-compatible modelocked lasers at 1.5μm

This thesis describes techniques for the generation of ultrashort optical pulses using semiconductor lasers. Active modelocking through gain modulation is applied to 1.?m InGaAsP semiconductor lasers which utilise microlensed optical fibres as external-cavity components, and tunable optical pulses of ~5ps duration have been generated. Particular attention has been given to suppression of the noisy subpulse features associated with low-frequency modelocked semiconductor lasers. This has led to two suppression methods applicable to symmetric (or balanced) and asymmetric cavity configurations, allowing the generation of clean, single feature short pulses of 7-10ps duration with peak powers in excess of 300mW. The physical phenomena relating to such processes will be outlined. Further amplification of these pulses using an erbium- doped fibre amplifier has resulted in output peak powers in the 5W range. A study of the phase-noise characteristics of these modelocked laser systems was undertaken using a high-speed photodetector and wide-band spectrum analyser arrangement. This characterisation allowed improvements in the rms pulse timing jitter from >2ps to below 300fs in the frequency range 50-5000Hz. Novel modelocked laser configurations were also constructed where the cavity components included both a semiconductor amplifier and an erbium-doped fibre amplifier. Linear and ring hybrid lasers were investigated with resultant pulse durations as short as 3ps with high peak power and excellent stability. CW and mechanical Q-switched doped fibre lasers were configured in diffraction grating tuned cavities with high output coupling (R=4%). Unprecedented output powers of up to 700mW tunable over ~100nm around 1.55mum, and 800mW tunable around 1.08mum were obtained. Also the value of 800W peak power for the 80ns output pulses from the Q-switched erbium-doped fibre laser amounts to the most intense pulse created thus far from any fibre laser device. Optimisation of the total tuning range attainable from erbium-doped fibre lasers via length tuning resulted in a variety of nonlinear phenomena, namely self Q-switching and optical bistability at the long wavelength tail of the tunable range

Rise Time Based Quality Analysis of Optical Networks

The optical network is becoming an important aspect for the present internet communication infrastructure due to good bandwidth provided by optical fiber. The throughputs provided by these networks are of the order of terabits per second, which is tremendously high. It also provides low error rates and low delays, and can also satisfy upcoming applications like supercomputer visualization, medical imaging, and distributed CPU interconnect. In optical networks, lightpath is defined as an all optical WDM channel which establishes a connection between source and destination node by network layer. Whenever the selected path doesn't satisfy the required criteria of either BER or Q-Factor or data rate, the connection will be blocked. Nowadays Wavelength division multiplexing (WDM) is used in optical networks in order to handle the increasing demand of network users. Nowadays, most of the network operators are very strict in terms of Quality of service parameters. The underlying routing systems determine the actual quality of transmission and the concept of Quality of service is based on this system parameters, can be defined as the overall service experience along with customer satisfaction of customer satisfaction. Lots of works has been done in finding Quality factor early using the effect of several parameters like bandwidth and delay, but this paper focuses mainly on finding quality of transmission using rise time and studies the effect of rise time on transmission data rate and other parameters. Here, we have taken a random network topology with 10 nodes and found out Q-factor for all possible paths and the shortest path for the given source and destination. The analytical model for Q-factor is linked with the rise time of various components in optical communication system and in turn system rise time

Monolithic integration of semiconductor ring lasers

The interest in semiconductor ring lasers (SRLs) has been steadily growing in the last few years because of several unique properties such as ultrafast directional bistability, stable single mode operation and potential for integration. However, most of the mode dynamical behavior as well as the optimum device design are still far from a complete understanding. This thesis reports on the design, technological development and characterization of SRLs emitting at 1.55 um, which are monolithically integrated with a number of other optical elements such as tunable couplers, optical amplifiers, Bragg reflectors and distributed feedback lasers (DFBs). A detailed analysis on the device design is presented with particular emphasis on its robustness with respect to fabrication tolerances and to the optical feedback from the output waveguides. The complete processing technology is developed with a focus on selective dry etching to achieve very accurate control of the waveguide bending losses. Three completely novel and monolithically integrated SRL devices are fabricated and characterized. The first is a master-slave device based on the monolithic integration of an SRL with a DFB that shows highly efficient cavity enhanced four-wave mixing up to detuning frequencies of 1.5 THz. In a second geometry, a Bragg reflector defined on one of the output waveguides selects the lasing mode of the SRL. The device shows world-record wavelength switching speeds as low as 450 ps and strong immunity to thermal fluctuations of the grating. The third device is an SRL with tunable couplers for active Q-switching applications. Pulses as short as 120 ps at a repetition rate of 1.8 GHz are obtained by injecting only a few mA of current into the tuning section

Generation of a CW Local Oscillator Signal Using a Stabilized Injection Locked Semiconductor Laser

In high speed-communications, it is desirable to be able to detect small signals while maintaining a low bit-error rate. Conventional receivers for high-speed fiber optic networks are Amplified Direct Detectors (ADDs) that use erbium-doped fiber amplifiers (EDFAs) before the detector to achieve a suitable sensitivity. In principle, a better method for obtaining the maximum possible signal to noise ratio is through the use of homodyne detection. The major difficulty in implementing a homodyne detection system is the generation of a suitable local oscillator signal. This local oscillator signal must be at the same frequency as the received data signal, as well as be phase coherent with it. To accomplish this, a variety of synchronization techniques have been explored, including Optical Phase-Lock Loops (OPLL), Optical Injection Locking (OIL) with both Fabry-Perot and DFB lasers, and an Optical Injection Phase-Lock Loop (OIPLL). For this project I have implemented a method for regenerating a local oscillator from a portion of the received optical signal. This regenerated local oscillator is at the same frequency, and is phase coherent with, the received optical signal. In addition, we show that the injection locking process can be electronically stabilized by using the modulation transfer ratio of the slave laser as a monitor, given either a DFB or Fabry-Perot slave laser. We show that this stabilization technique maintains injection lock (given a locking range of ~1GHz) for laser drift much greater than what is expected in a typical transmission system. In addition, we explore the quality of the output of the slave laser, and analyze its suitability as a local oscillator signal for a homodyne receiver

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

Integrated Optoelectronic Devices and System Limitations for WDM Passive Optical Networks

This thesis puts focus on the technological challenges for Wavelength Division Multiplexed Passive Optical Network (WDM-PON) implementation, and presents novel semiconductor optical devices for deployment at the optical network unit (ONU). The first-ever reported L-band Reflective semiconductor optical amplifier (RSOA) is presented based on InP-base material. A theoretical model is developed to estimate the optical gain and the saturation power of this device compared to a conventional SOA. Experiments on this device design show long-range telecom wavelength operation, with polarization-independent gain of greater than 20 dB, and low saturation output power of 0 dBm suitable for PON applications. Next, the effect of the amplified spontaneous emission noise of RSOA devices on WDM-PON system is investigated. It is shown through theoretical modeling and simulations that the RSOA noise combined with receiver noise statistics increase probability of error, and induce considerable power penalties to the WDM-PON system. By improving the coupling efficiencies, and by distributing more current flow to the input of these devices, steps can be taken to improve device noise characteristics. Further, in spectrally-spliced WDM-PONs deploying RSOAs, the effect of AWG filter shape on system performance is investigated. Simulation modeling and experiments show that deployment of Flat-band AWGs is critical for reducing the probability of error caused by AWG spectral shape filtering. Flat-band athermal AWGs in comparison to Gaussin-shape counterparts satisfy the maximum acceptable error probability requirements, and reduce the power penalty associated with filtering effect. In addition, detuning between two AWG center wavelengths impose further power penalties to the WDM-PON system. In the last section of this thesis, motivated by RSOA device system limitations, a novel injection-locked Fabry-Perot (IL-FP) device is presented which consists of a gain section monolithically integrated with a phase section. The gain section provides locking of one FP mode to a seed source wavelength, while the phase modulator allows for adjusting the wavelength of the internal modes by tuning bias current to maintain mode-locking. This device counters any mode drifts caused by temperature variations, and allows for cooler-less operation over a wide range of currents. The devices and the performance metrics subsequently allow for a hybrid integration platform on a silicon substrate and integrate many functionalities like reflective modulator with thin film dielectric filter and receiver on a single chip for deployment at the user-end of future-proof low cost WDM-PONs