91 research outputs found

    Developing coherent optical wavelength conversion systems for reconfigurable photonic networks

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    In future optical networks that employ wavelength division multiplexing (WDM), the use of optical switching technologies on a burst or packet level, combined with advanced modulation formats would achieve greater spectral efficiency and utilize the existing bandwidth more efficiently. All-optical wavelength converters are expected to be one of the key components in these broadband networks. They can be used at the network nodes to avoid contention and to dynamically allocate wavelengths to ensure optimum use of fiber bandwidth. In this work, a reconfigurable wavelength converter comprising of a Semiconductor Optical Amplifier (SOA) as the nonlinear element and a fast-switching sampled grating distributed Bragg reflector (SG-DBR) tunable laser as one of the pumps is developed. The wavelength conversion of 12.5-Gbaud quadrature phase shift keying (QPSK) and Pol-Mul QPSK signals with switching time of tens of nanoseconds is experimentally achieved. Although the tunable DBR lasers can achieve ns tuning time, they present relatively large phase noise. The phase noise transfer from the pump to the converted signal can have a deleterious effect on signal quality and cause a performance penalty with phase modulated signals. To overcome the phase noise transfer issue, a wavelength converter using tunable dual-correlated pumps provided by the combination of a single-section quantum dash passively mode-locked laser (QD-PMLL) and a programmable tunable optical filter is designed and the wavelength conversion of QPSK and 16-quadrature amplitude modulation (16-QAM) signals at 12.5 GBaud is experimentally investigated. Nonlinear distortion of the wavelength converted signal caused by gain saturation effects in the SOA can significantly degrade the signal quality and cause difficulties for the practical wavelength conversion of sig nal data with advanced modulation formats. In this work, the machine learning clustering based nonlinearity compensation method is proposed to improve the tolerance to nonlinear distortion in an SOA based wavelength conversion system with 16 QAM and 64 QAM signals

    Optical label-controlled transparent metro-access network interface

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    Reconfigurable multi-carrier transmitters and their application in next generation optical networks

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    With the advent of new series of Internet services and applications, future networks will have to go beyond basic Internet connectivity and encompass diverse services including connected sensors, smart devices, vehicles, and homes. Today’s telecommunication systems are static, with pre-provisioned links requiring an expensive and time-consuming reconfiguration process. Hence, future networks need to be flexible and programmable, allowing for resources to be directed, where the demand exists, thus improving network efficiency. A cost-effective solution is to utilise the legacy fibre infrastructure more efficiently, by reducing the size of the guard bands and allowing closer optical carrier spacing, thereby increasing the overall spectral efficiency. However, such a scheme imposes stringent transmitter requirements such as frequency stability, which would not be met with the incumbent laser-array based transmitters. An attractive alternative would be to employ an optical frequency comb (OFC), which generates multiple phase correlated carriers with precise frequency separation. The reconfigurability of such a multi-carrier transmitter would enable tuning of channel spacing, number of carriers and emission wavelengths, according to the dynamic network demands. This research thesis presents the work carried out, in the physical layer, towards realising reconfigurability of an optical multi-carrier transmitter system. The work focuses on an externally injected gain-switched laser-based OFC (EI-GSL), which is a particular type of multi-carrier source. Apart from the detailed characterisation of GSL OFCs, advances to the state of the art are achieved via comb expansion, investigating new demultiplexing methods and system implementations. Firstly, two novel broadband GS-OFC generation techniques are proposed and experimentally demonstrated. Subsequently, two flexible and compact demultiplexing solutions, based on micro-ring resonators and laser based active demultiplexers are investigated. Finally, the application of a reconfigurable multi-carrier transmitter, employed in access and data centre networks, as well as analog-radio over fibre (A-RoF) distribution systems, is experimentally demonstrated

    Optical Signal Processing for High-Order Quadrature- Amplitude Modulation Formats

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    In this book chapter, optical signal processing technology, including optical wavelength conversion, wavelength exchange and wavelength multicasting, for phase-noise-sensitive high-order quadrature-amplitude modulation (QAM) signals will be discussed. Due to the susceptibility of high-order QAM signals against phase noise, it is imperative to avoid the phase noise in the optical signal processing subsystems. To design high-performance optical signal processing subsystems, both linear and nonlinear phase noise and distortions are the main concerns in the system design. We will first investigate the effective monitoring approach to optimize the performance of wavelength conversion for avoiding undesired nonlinear phase noise and distortions, and then propose coherent pumping scheme to eliminate the linear phase noise from local pumps in order to realize pump-phase-noise-free wavelength conversion, wavelength exchange and multicasting for high-order QAM signals. All of the discussions are based on experimental investigation

    Optical code-division multiple access system and optical signal processing

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    This thesis presents our recent researches on the development of coding devices, the investigation of security and the design of systems in the optical cod-division multiple access (OCDMA) systems. Besides, the techniques of nonlinear signal processing used in the OCDMA systems fire our imagination, thus some researches on all-optical signal processing are carried out and also summarized in this thesis. Two fiber Bragg grating (FBG) based coding devices are proposed. The first coding device is a superstructured FBG (SSFBG) using ±π/2-phase shifts instead of conventional 0/π-phase shifts. The ±π/2-phase-shifted SSFBG en/decoders can not only conceal optical codes well in the encoded signals but also realize the reutilization of available codes by hybrid use with conventional 0/π-phase-shifted SSFBG en/decoders. The second FBG based coding device is synthesized by layer-peeling method, which can be used for simultaneous optical code recognition and chromatic dispersion compensation. Then, two eavesdropping schemes, one-bit delay interference detection and differential detection, are demonstrated to reveal the security vulnerability of differential phase-shift keying (DPSK) and code-shift keying (CSK) OCDMA systems. To address the security issue as well as increase the transmission capacity, an orthogonal modulation format based on DPSK and CSK is introduced into the OCDMA systems. A 2 bit/symbol 10 Gsymbol/s transmission system using the orthogonal modulation format is achieved. The security of the system can be partially guaranteed. Furthermore, a fully-asynchronous gigabit-symmetric OCDMA passive optical network (PON) is proposed, in which a self-clocked time gate is employed for signal regeneration. A remodulation scheme is used in the PON, which let downstream and upstream share the same optical carrier, allowing optical network units source-free. An error-free 4-user 10 Gbit/s/user duplex transmission over 50 km distance is reazlied. A versatile waveform generation scheme is then studied. A theoretical model is established and a waveform prediction algorithm is summarized. In the demonstration, various waveforms are generated including short pulse, trapezoidal, triangular and sawtooth waveforms and doublet pulse. ii In addition, an all-optical simultaneous half-addition and half-subtraction scheme is achieved at an operating rate of 10 GHz by using only two semiconductor optical amplifiers (SOA) without any assist light. Lastly, two modulation format conversion schemes are demonstrated. The first conversion is from NRZ-OOK to PSK-Manchester coding format using a SOA based Mach-Zehnder interferometer. The second conversion is from RZ-DQPSK to RZ-OOK by employing a supercontinuum based optical thresholder

    Investigation of performance issues affecting optical circuit and packet switched WDM networks

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    Optical switching represents the next step in the evolution of optical networks. This thesis describes work that was carried out to examine performance issues which can occur in two distinct varieties of optical switching networks. Slow optical switching in which lightpaths are requested, provisioned and torn down when no longer required is known as optical circuit switching (OCS). Services enabled by OCS include wavelength routing, dynamic bandwidth allocation and protection switching. With network elements such as reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects (OXCs) now being deployed along with the generalized multiprotocol label switching (GMPLS) control plane this represents the current state of the art in commercial networks. These networks often employ erbium doped fiber amplifiers (EDFAs) to boost the optical signal to noise ratio of the WDM channels and as channel configurations change, wavelength dependent gain variations in the EDFAs can lead to channel power divergence that can result in significant performance degradation. This issue is examined in detail using a reconfigurable wavelength division multiplexed (WDM) network testbed and results show the severe impact that channel reconfiguration can have on transmission performance. Following the slow switching work the focus shifts to one of the key enabling technologies for fast optical switching, namely the tunable laser. Tunable lasers which can switch on the nanosecond timescale will be required in the transmitters and wavelength converters of optical packet switching networks. The switching times and frequency drifts, both of commercially available lasers, and of novel devices are investigated and performance issues which can arise due to this frequency drift are examined. An optical packet switching transmitter based on a novel label switching technique and employing one of the fast tunable lasers is designed and employed in a dual channel WDM packet switching system. In depth performance evaluations of this labelling scheme and packet switching system show the detrimental impact that wavelength drift can have on such systems

    Dynamically reconfigurable long-reach PONs for high capacity access

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    Fibre-to-the-Premises (FTTP) is currently seen as the ultimate in high-speed transmission technologies for delivering ubiquitous bandwidth to customers. However, as the deployment of network infrastructure requires a substantial investment, the main obstacle to fibre deployment is that of financial viability. With this in mind, a logical strategy to offset network costs is to optimise the infrastructure in order to capture a greater amount of customers over larger areas with increased sharing of network resources. This approach prompted the design of a long-reach passive optical network (LR-PON) in which the physical reach and split of a conventional PON is significantly increased through the use of intermediate optical amplification. In particular, the LR-PON architecture effectively integrates the metro and access networks enabling the majority of local exchange sites to be bypassed resulting in a substantial reduction in field equipment requirements and power consumption. Furthermore, the extension in physical reach and split can be coupled with an increased information capacity through the use of time- and wavelength division multiplexing (TWDM) which serve to exploit the large bandwidth capabilities offered by single-mode fibre. In this project, reconfigurable TWDM LR-PON architectures which dynamically exploit the wavelength domain are proposed, assembled and characterised in order to establish an economically viable ‘open access’ environment that is capable of concurrently supporting multiple operators offering converged services (residential, business and mobile) to support diverse customer requirements and locations. The main investigations in this work address the key physical layer challenges within such wavelength-agile networks. In particular, a range of experimental analysis has been carried out in order to realise the critical component technologies which include low-cost, 10G-capable, wavelength-tuneable transmitters for mass-market residential deployment and the development of gain-stabilised optical amplifier nodes to support the targeted physical reach (≄ 100km) and split (≄ 512). Finally, the feasibility of the proposed dynamically reconfigurable LR-PON configurations as a flexible and cost-effective solution for future access networks is verified through full-scale network demonstrations using an experimental laboratory test-bed

    Optical packet networks : enabling innovative switching technologies

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    Les rĂ©seaux informatiques avec une grande capacitĂ© nĂ©cessitent des liaisons de transmission de donnĂ©es rapides et fiables pour prendre en charge les applications web en pleine croissance. Comme le nombre de serveurs interconnectĂ©s et la capacitĂ© de stockage des mĂ©dias ne cessent daugmenter, les communications optiques et les technologies de routage sont devenues intĂ©ressantes grĂące au taux binaire Ă©levĂ© et Ă  lencombrement minimum offert par la fibre optique. Les rĂ©seaux optiques Ă  commutation de paquets (OPSNs) offrent une flexibilitĂ© accrue dans la gestion de rĂ©seau. OPSNs exploitent les convertisseurs de longueur donde accordables (WC) pour minimiser la probabilitĂ© de blocage et fournir une allocation dynamique des longueurs donde. Les Ă©metteurs optiques basĂ©s sur des sources multi-longueurs donde se prĂ©sentent comme une solution intĂ©ressante en termes de coĂ»t, dencombrement et defficacitĂ© Ă©nergĂ©tique par rapport aux autres types de lasers. Les convertisseurs de longueurs donde doivent permettre des taux binaires Ă©levĂ©s et une transparence Ă  une grande variĂ©tĂ© de formats de modulation, tout en offrant une rĂ©ponse rapide, des niveaux de puissance modĂ©rĂ©s et un rapport de signal Ă  bruit optique (OSNR) acceptable Ă  la sortie. Plusieurs technologies de conversion de longueur donde ont Ă©tĂ© proposĂ©es dans la littĂ©rature. Lutilisation du mĂ©lange Ă  quatre ondes (FWM) dans les amplificateurs optiques Ă  semi-conducteurs (SOA) permet lutilisation de faibles niveaux de puissance dentrĂ©e et offre une bonne efficacitĂ© de conversion ainsi que la possibilitĂ© dintĂ©gration photonique. Les SOAs offrent donc un excellent compromis par rapport aux autres solutions. Pour couvrir une plus large bande de conversion, nous utilisons le schĂ©ma exploitant le FWM avec doubles pompes dans les SOAs. Pour la stabilitĂ© de phase, les pompes viennent d’un laser en mode bloquĂ© (QDMLL) qui sert comme source multi-longueurs donde. Deux modes du QDMLL sont sĂ©lectionnĂ©s par un filtrage accordable et servent comme doubles pompes. Un filtre accordable placĂ© Ă  la sortie du SOA sert Ă  sĂ©lectionner le produit du FWM pour le signal final. Nous Ă©tudions le convertisseur de longueur donde proposĂ© et comparons sa performance pour diffĂ©rents formats de modulation (modulation dintensitĂ© et de phase) et Ă  diffĂ©rents dĂ©bits binaires (10 et 40 Gbit/s). Le taux derreur binaire, lefficacitĂ© de conversion et la mesure de lOSNR sont prĂ©sentĂ©s. Nous dĂ©montrons aussi la possibilitĂ© de simultanĂ©ment convertir en longueurs donde les donnĂ©es et l’étiquette. Les donnĂ©es Ă  haut dĂ©bit et l’étiquette Ă  faible dĂ©bit se retrouvent dans une seule bande de longueurs d’onde, et ils sont convertis ensemble avec une bonne efficacitĂ©. Notre dĂ©monstration se concentre sur les performances de conversion, donc les donnĂ©es et lĂ©tiquette sont des signaux continus plutĂŽt que de paquets optiques. Des mesures de taux derreur binaire ont Ă©tĂ© effectuĂ©es Ă  la fois pour les donnĂ©es et pour lĂ©tiquette. Nous proposons aussi lutilisation de QDMLL comme source de transmetteurs WDM pour deux applications diffĂ©rentes: unicast et multicast. Nous dĂ©montrons aussi sa compatibilitĂ© avec le format de transmission DQPSK Ă  haut dĂ©bit binaire. Nous Ă©valuons la performance du DQPSK en terme de taux derreur binaire et comparons sa performance Ă  celle dune source laser Ă  cavitĂ© externe.Large scale computer networks require fast and reliable data links in order to support growing web applications. As the number of interconnected servers and storage media increases, optical communications and routing technologies become interesting because of the high speed and small footprint of optical fiber links. Furthermore, optical packet switched networks (OPSN) provide increased flexibility in network management. Future networks are envisaged to be wavelength dependent routing, therefore OPSN will exploit tunable wavelength converters (WC) to enable contention resolution, reduce wavelength blocking in wavelength routing and switching, and provide dynamic wavelength assignment. Optical transmitters based on multi-wavelength sources are presented as an attrative solution compared to a set of single distributed feedback lasers in terms of cost, footprint and power consumption. Wavelength converters should support high bit rates and a variety of signal formats, have fast setup time, moderate input power levels and high optical signal-to-noise ratio at the output. Several wavelength conversion technologies have been demonstrated. The use of four wave mixing (FWM) in semiconductor optical amplifiers (SOAs) provides low input power levels, acceptable conversion efficiency and the possibility of photonic integration. SOAs therefore offer excellent trade-offs compared to other solutions. To achieve wide wavelength coverage and integrability, we use a dual pump scheme exploiting four-wave mixing in semiconductor optical amplifiers. For phase stability, we use a quantum-dash mode-locked laser (QD-MLL) as a multi-wavelength source for the dual pumps, with tunability provided by the frequency selective filter. We investigate the proposed wavelength converter and compare its performance of wavelength conversion for different non-return-to-zero (NRZ) intensity and phase modulation formats at different bit rates (10 and 40 Gbit/s). Bit error rate, conversion efficiency and optical signal-to-noise ratio measurements are reported. We demonstrate the possibility of tightly packed payload and label wavelength conversion at very high data baud rate over wide tuning range with good conversion efficiency. Our demonstration concentrates on conversion performance, hence continuous payload and label signals were used without gating into packets. Bit error measurements for both payload and label were performed. We propose the use of QD-MLL as multi-wavelength source for WDM unicast and multicast applications and we investigated its compatibility with DQPSK transmission at high bit rate. We quantify DQPSK performance via bit error rate measurements and compare performance to that of an external cavity laser (ECL) source

    Optical multicarrier sources for spectrally efficient optical networks

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    During the last 30 years the capacity of commercial optical systems exceeded the network traffic requirements, mainly due to the extraordinary scalability of wavelength division multiplexing technology that has been successfully used to expand capacity in optical systems and meet increasing bandwidth requirements since the early 1990’s. Nevertheless, the rapid growth of network traffic inverted this situation and current trends show faster growing network traffic than system capacity. To enable further and faster growth of optical communication network capacity, several breakthroughs occurred during the last decade. First, optical coherent communications, which were the subject of intensive research in the 1980’s, were revived. This triggered the employment of advanced modulation formats. Afterwards, with the introduction of orthogonal frequency division multiplexing (OFDM) and Nyquist WDM modulation techniques in optical communication systems, very efficient utilisation of the available spectral bandwidth was enabled. In such systems the spectral guard bands between neighbouring channels are minimised, at the expense of stricter requirements on the performance of optical sources, especially the frequency (or wavelength) stability. Attractive solutions to address the frequency stability issues are optical multicarrier sources which simultaneously generate multiple phase correlated optical carriers that ensure that the frequency difference between the carriers is fixed. In this thesis, a number of optical multicarrier sources are presented and analysed, with special focus being on semiconductor mode-locked lasers and gain-switched comb sources. High capacity and spectrally efficient optical systems for short and medium reach applications (from 3 km up to 300 km), based on optical frequency combs as optical sources, advanced modulation formats (m-QAM) and modulation techniques (OFDM and Nyquist WDM) have been proposed and presented. Also, certain optoelectronic devices (i.e. semiconductor optical amplifier) and techniques (feed-forward heterodyne linewidth reduction scheme) have been utilised to enable the desired system performance
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