OPTICAL QPSK SIGNAL QUALITY DEGRADATION DUE TO PHASE ERROR OF PUMP LIGHT

The influence of pump phase error on phase-sensitive optical amplifier (PSA) repeaters and the waveform degradation due to chromatic dispersion and fiber nonlinearities in the optical multi-relay transmission of quadrature phase-shift keying phase-conjugated twin waves are considered theoretically. First, the influence of noise from the pump phase error, optical local oscillator, receiver, and the amplified spontaneous-emission (ASE) in PSA repeaters is investigated with the assumption that transmission fibers are linear lossy channels. The bit-error rate (BER) is estimated as a function of the signal-to-noise ratio, and the relationship between the number of transmission relays and the fiber launch power is clarified. Waveform degradation due to chromatic dispersion and the optical fiber nonlinearities in transmission fibers are investigated with the noiseless condition, and the maximum repeatable number as a function of the fiber launch power is calculated. Finally, we show the relationship among the maximum repeatable number, standard deviation of pump phase error in PSA repeaters, and the fiber launch power to clarify the optimum transmission condition with consideration of the noise and the waveform degradation

All-optical Regeneration For Phase-shift Keyed Optical Communication Systems

All-optical signal processing techniques for phase-shift keyed (PSK) systems were developed theoretically and demonstrated experimentally. Nonlinear optical effects in fibers, in particular four-wave mixing (FWM) that occurs via the ultra-fast Kerr nonlinearity, offer a flexible framework within which numerous signal processing functions can be accomplished. This research has focused on the regenerative capabilities of various FWM configurations in the context of processing PSK signals. Phase-preserving amplitude regeneration, phase regeneration, and phase-regenerative wavelength conversion are analyzed and demonstrated experimentally. The single-pump phase-conjugation process was used to regenerate RZ-DPSK pulse amplitudes with different input noise distributions, and the impact on output phase characteristics was studied. Experiments revealed a limited range over which amplitude noise could effectively be suppressed without introduction of phase noise, particularly for signals with intensity pattern effects. Phase regeneration requires use of phase-sensitive amplification (PSA), which occurs in nonlinear interferometers when the pump and signal frequencies are degenerate (NI-PSA), or in fiber directly through single-stage (degenerate) or cascaded (non-degenerate) FWM processes. A PSA based on a Sagnac interferometer provided the first experimental demonstration of DPSK phase and amplitude regeneration. The phase-regenerative capabilities of the NI-PSA are limited in practice by intrinsic noise conversion (amplitude to phase noise) and to a lesser extent by the requirement to modulate the pump wave to suppress stimulated Brillouin scattering (SBS). These limitations are relaxed in novel materials with higher SBS thresholds and nonlinearities. Degenerate FWM provides PSA in a traveling-wave configuration that intrinsically suppresses the noise conversion affecting the NI-PSA, while providing stronger phase-matched gain. Experiments confirmed superior phase-regenerative behavior to the NI-PSA with simultaneous reduction of amplitude noise for NRZ-DPSK signals. Phase-regenerative wavelength conversion (PR-WC) provides the regenerative properties of PSA at a new wavelength, and was proposed and demonstrated for the first time in this research. The parallel implementation of two FWM processes, phase-conjugation and frequency conversion, provides two idlers which exhibit interesting and useful regenerative properties. These were investigated theoretically and experimentally. Ideal phase-regenerative behavior is predicted when the contributing FWM processes are equally phase-matched, which can be maintained over any interaction length or wavelength shift provided the pump powers are properly adjusted. Depleted-pump regime PR-WC provides simultaneous phase and amplitude regeneration. Experiments confirmed regenerative behavior for wavelength shifts of the idlers up to 5 nm. Two techniques for phase regeneration of 4-level PSK signals were developed and evaluated. The first is based on parallel operation of PSAs suitable for processing 2-level PSK signals, where phase projection and regeneration are combined to recover the input data. Analysis of this scheme outlined the conditions required for effective phase regeneration and for practical implementation using known PSAs. A novel process based on FWM (parallel phase-conjugation followed by PSA) was developed and analyzed, and demonstrated using numerical simulations. These studies provide a basis for further work in this area

Quasi-lossless data transmission with ultra-long Raman fibre laser based amplification

The project consists of an experimental and numerical modelling study of the applications of ultra-long Raman fibre laser (URFL) based amplification techniques for high-speed multi-wavelength optical communications systems. The research is focused in telecommunications C-band 40 Gb/s transmission data rates with direct and coherent detection. The optical transmission performance of URFL based systems in terms of optical noise, gain bandwidth and gain flatness for different system configurations is evaluated. Systems with different overall span lengths, transmission fibre types and data modulation formats are investigated. Performance is compared with conventional Erbium doped fibre amplifier based system to evaluate system configurations where URFL based amplification provide performance or commercial advantages

Técnicas de monitorização e processameto totalmente óptico para redes transparentes

Doutoramento em Engenharia EletrotécnicaOptical networks are under constant evolution. The growing demand for dynamism require devices that can accommodate different types of traffic. Thus the study of transparent optical networks arises. This approach makes optical networks more "elegant" , due to a more efficient use of network resources. In this thesis, the author proposes devices that intend to form alternative approaches both in the state of art of these same technologies both in the fitting of this technologies in transparent optical networks. Given that full transparency is difficult to achieve with current technology (perhaps with more developed optical computing this is possible), the author proposes techniques with different levels of transparency. On the topic of performance of optical networks, the author proposes two techniques for monitoring chromatic dispersion with different levels of transparency. In Chapter 3 the proposed technique seems to make more sense for long-haul optical transmission links and high transmission rates, not only due to its moderate complexity but also to its potential moderate/high cost. However it is proposed to several modulation formats, particularly those that have a protruding clock component. In Chapter 4 the transparency level was not tested for various modulation formats, however some transparency is achieved by not adding any electrical device after the receiver (other than an analog-digital converter). This allows that this technique can operate at high transmission rates in excess of 100 Gbit / s, if electro-optical asynchronous sampling is used before the optical receiver. Thus a low cost and low bandwidth photo-detector can be used. In chapter 5 is demonstrated a technique for simultaneously monitoring multiple impairments of the optical network by generating novel performance analysis diagrams and by use of artificial neural networks. In chapter 6 the author demonstrates an all-optical technique for controlling the optical state of polarization and an example of how all-optical signal processing can fully cooperate with optical performance monitoring.As redes ópticas estão em constante desenvolvimento. A crescente demanda por dinamismo das mesmas requer cada vez mais, dispositivos que possam albergar diversos tipos de tráfego. Dessa forma surge o estudo sobre redes ópticas transparentes. Este tipo de abordagem torna as redes ópticas mais ’elegantes’, devido a um uso mais eficiente dos recursos da rede. Nesta tese o autor propõe dispositivos que pretendem formar alternativas quer no estado da arte destas mesmas tecnologias quer no encaixe das mesmas em redes ópticas transparentes. Dado que a transparência total é dificil de alcançar com as tecnologias atuais (talvez com computação óptica mais desenvolvida isso seja possível), o autor propõe técnicas com diferentes níveis de transparência. No âmbito da monitoria de desempenho óptico o autor propôe duas técnicas para a monitoria da dispersão cromática com diferentes niveis de transparência. No Capítulo 3 a técnica proposta parece fazer mais sentido para transmissões ópticas de longo curso e a elevadas taxas de transmissão, não só devido à sua moderada complexidade, mas também ao seu potencial custo. No entanto esta, é proposta para vários formatos de modulação, especialmente aqueles que têm uma componente de relógio bem saliente. No Capítulo 4, o nível de transparência não foi testado para vários formatos de modulação, no entanto alguma transparência é obtida através da não inclusão de nenhum dispositivo eléctrico após o receptor (excepto um conversor analógico-digital). Isto permite que esta técnica possa operar a elevadas taxas de transmissão acima de 100 Gbit~s, se for utilizada amostragem electro-óptica assíncrona antes do receptor. Dessa forma pode ser utilizado um fotodetector de baixo custo e de baixa largura de banda. No capítulo 5 é demonstrada uma técnica para monitoria em simultâneo de vários impedimentos da rede óptica atráves de um novo método para gerar diagramas de análise de desempenho com redes neuronais artificiais. No capítulo 6 o autor demonstra uma técnica para controlo da polarização, totalmente processada no domínio óptico assim como um exemplo de como o processamento totalmente óptico pode cooperar com o monitoria do desempenho óptico

Performance limits in optical communications due to fiber nonlinearity

In this paper, we review the historical evolution of predictions of the performance of optical communication systems. We will describe how such predictions were made from the outset of research in laser based optical communications and how they have evolved to their present form, accurately predicting the performance of coherently detected communication systems

Towards Practical Implementation of Phase-Sensitive Amplifier Based Transmission Systems

All commercially available optical amplifiers are so-called phase-insensitive amplifiers (PIAs) which degrade the signal-to-noise ratio (SNR) through the amplification process. This kind of amplifier has a quantum limited noise figure (NF) of 3 dB. Another category of amplifiers are phase-sensitive amplifiers (PSAs) which in theory are capable of noiseless amplification, i.e. amplification with a 0 dB NF. Successful implementation of PSAs in transmission systems would lead to significant performance improvements compared to using conventional PIAs. However, the implementation is challenging and no system with wavelength division multiplexing (WDM) compatibility has previously been demonstrated over a significant transmission distance. This thesis is dedicated to realizing and investigating the properties of PSA-amplified transmission links utilizing fiber optical parametric amplifiers (FOPA) and the so-called copier-PSA scheme. One of the main challenges on the way towards realization is to recover and amplify a weak phase-modulated wave with high fidelity. To handle this, a hybrid injection locking (IL)/Erbium-doped fiber amplifier (EDFA)-based pump recovery system was designed and thoroughly investigated experimentally. Other challenges include continuous phase-locking of several waves and high-precision wave tuning. A single-span PSA-amplified transmission link with WDM compatibility was demonstrated over 80 km of fiber. The link performance was compared against a conventional EDFA-based link for operation both in the linear and nonlinear transmission regime. The PSA-amplified system is shown to have capability to mitigate nonlinear distortions due to the Kerr effect and outperform the EDFA-amplified link in both regimes

Advanced techniques for the improvement of optical transmission systems

This thesis presents the experimental investigation into two novel techniques which can be incorporated into current optical systems. These techniques have the capability to improve the performance of transmission and the recovery of the transmitted signal at the receiver. The experimental objectives are described and the results for each technique are presented in two sections: The first experimental section is on work related to Ultra-long Raman Fibre lasers (ULRFLs). The fibre lasers have become an important research topic in recent years due to the significant improvement they give over lumped Raman amplification and their potential use in the development of system with large bandwidths and very low losses. The experiments involved the use of ASK and DPSK modulation types over a distance of 240km and DPSK over a distance of 320km. These results are compared to the current state of-the-art and against other types of ultra-long transmission amplification techniques. The second technique investigated involves asymmetrical, or offset, filtering. This technique is important because it deals with the strong filtering regimes that are a part of optical systems and networks in modern high-speed communications. It allows the improvement of the received signal by offsetting the central frequency of a filter after the output of a Delay Line Interferometer (DLI), which induces significant improvement in BER and/or Qvalues at the receiver and therefore an increase in signal quality. The experimental results are then concluded against the objectives of the experimental work and potential future work discussed

Processamento ótico de sinal para sistemas de comunicações óticas

Doutoramento em Engenharia FísicaO processamento ótico de sinal é uma alternativa possível para melhorar o desempenho e eficiência de sistemas de comunicações óticas, mas o seu estágio atual de desenvolvimento é ainda insuficiente para aplicações em sistemas reais. De forma a inverter esta situação, novas estratégias e pos-sibilidades para processamento ótico de sinal são aqui investigadas, com ênfase em conversão de comprimento de onda, regeneração de fase e amplificação sensível à fase em dispositivos de niobato de lítio com inversão periódica dos domínios ferroelétricos e fibras fortemente não-lineares. Um novo método para o desenho do perfil de inversão dos domínios fer¬roelétricos nos dispositivos de niobato de lítio de acordo com um espetro de conversão alvo é investigado nesta tese. O método proposto é validado numericamente e através da produção de um dispositivo real com largura de banda de conversão de 400 GHz. O dispositivo produzido é utilizado para conversão de onda multicanal de oito sinais modulados em fase, com a possibilidade adicional de sintonizar o comprimento de onda dos sinais con¬vertidos. Observa-se a existência de um compromisso entre elevada largura de banda de conversão e eficiência do dispositivo. São também investigadas nesta tese conversão e permuta de comprimento de onda tolerantes ao ruído de fase adicionado por fontes de bombeamento. Demonstra-se neste trabalho que a utilização de fontes de bombeamento coerentes permite evitar a adição de ruído de fase aos sinais convertidos. Nesta tese é também analisada analítica e numericamente amplificação sensível a fase baseada em dispositivos de niobato de lítio com inversão periódica dos domínios ferroelétricos para configurações de amplificadores de um, dois ou quatro modos. É ainda avaliada a possibilidade de ge¬rar ondas correlacionadas e de realizar amplificação sensível a fase num único dispositivo com propagação bidirecional. Com base neste esquema, demonstra-se regeneração de fase de sinais modulados em fase, porém com ganho limitado devido à baixa eficiência de conversão dos dispositivos e com desempenho afetado por instabilidades térmicas e foto refrativas. Mo¬tivado por estas limitações, demonstra-se amplificação de elevado ganho num amplificador sensível à fase de quatro modos, construído com uma fibra fortemente não-linear em vez de um dispositivo de niobato de lítio. Por fim, é efetuada uma análise numérica do impacto de utilizar amplifica¬dores sensíveis à fase em vez de amplificadores de fibra dopada com érbio no alcance em transmissão ponto a ponto de sinais e na amplificação e regeneração em redes óticas. Demonstra-se que amplificadores sensíveis à fase são mais vantajosos para formatos de modulação avançados e siste¬mas compostos por ligações óticas longas. As simulações assumem mode¬los simplificados para o ganho e ruído dos amplificadores, bem como uma versão modificada do modelo de ruído Gaussiano para estimar a potência das distorções não-lineares em sistemas com compensação total da dispersão cromática no final de cada segmento de fibra entre amplificadores.All-optical signal processing techniques are a possible way to improve the performance and efficiency of optical communication systems, but the cur¬rent stage of development of such techniques is still unsatisfactory for real- world implementation. In order to invert this situation, new strategies and possibilities for all-optical signal processing are investigated here, with a particular focus on wavelength conversion, phase regeneration and phase- sensitive amplification in periodically poled lithium niobate waveguides and highly nonlinear fibers. A new and flexible method to design the poling pattern of periodically poled lithium niobate devices according to a target conversion spectrum is inves¬tigated in this work. The proposed method is validated through numerical simulations and by producing a real device with broad conversion bandwidth of 400 GHz. The device is then used for multichannel wavelength conversion of eight phase-modulated signals, with the additional possibility to tune the wavelength of the converted signals. A trade-off between high conversion bandwidth and conversion efficiency is observed. Advanced wavelength conversion and wavelength exchange tolerant to the phase noise added by the pump lasers are also investigated. It is shown that the additional phase noise transferred to the converted signals is eliminated by using coherent pumps, generated from the same light source. Phase-sensitive amplification based on periodically poled lithium niobate devices is also investigated in this thesis by numerically comparing the gain properties for one-, two- and four-mode configurations. The possibility to si¬multaneously generate correlated waves and observe phase-sensitive amplifi¬cation in a single device with bidirectional propagation is also demonstrated. Using such scheme,"black-box" phase regeneration of phase-encoded sig¬nals is experimentally demonstrated, albeit with limited net gain due to the low conversion efficiency of the device, and the limited reliability due to thermal and photorefractive instabilities. Motivated by such limitations, high-gain amplification in a four-mode phase-sensitive amplifier built with a highly nonlinear fiber instead of a periodically poled lithium niobate is demonstrated. Finally, the impact of using phase-sensitive amplifiers instead of common erbium-doped fiber amplifiers on the reach in point-to-point transmission and on the amplification and regeneration requirements in optical transport networks is numerically investigated. The calculations show that phase- sensitive amplifiers are particularly advantageous when considering high- order modulation formats and for transport networks comprised by long links. The numerical simulations are performed using simplified models for the gain and noise properties of the amplifiers, and a modified enhanced Gaussian noise model to estimate the power of the nonlinear distortions in systems with full dispersion compensation at the end of each span of fiber

Performance Evaluation of Raman Amplifiers in Fibre Optic Communication Systems

This thesis presents an overview of Raman amplifiers in fibre optic transmission systems. Detailed analysis of the nonlinear accumulated noise and relative intensity noise (RIN) induced penalties are evaluated in discrete and distributed Raman amplifiers. In addition to these the thesis also includes different architectures of Raman amplifiers enabling multiband transmission. The parametric dependency of fibre chromatic dispersion (CD) on the accumulated nonlinear noise in discrete Raman amplifiers (DRAs) was studied both theoretically and experimentally. Analytical modelling was performed over different fibre types that are widely used as a gain medium in DRAs. It was found that systems using Raman gain fibres with a positive value of CD induce lower accumulated nonlinear noise in comparison to systems using Raman gain fibres with a negative value of CD. The results obtained from the analytical model were then validated experimentally over a long-haul transmission system with DRAs as an inline amplifier using a recirculation loop. RIN-induced penalties in distributed Raman amplifiers (DiRAs) were experimentally studied in two standard single-mode fibre (SSMF) G.654.E©TXF and G.652.D with different pumping schemes. Signal RIN for G.654.E© TXF was found to be lower in comparison to its counterpart G.652.D. The impact of RIN on the short-haul system was validated using both the test fibres pumped in a forward-pumped distributed Raman. Similarly, backward and bidirectional pumping was performed over a long-haul transmission system using a recirculation loop. It was experimentally observed that RIN-induced transmission penalties for G.654.E are lower in comparison to G.652.D making it a better choice of SSMF type for distributed amplification. Experiments on novel architectures such as cascaded dual-stage and dual-band designs were demonstrated over a coherent transmission system with S-, C- and L-band signals. It was observed that the dual-stage design requires a guard band of ~10 nm to prevent overlapping of the pumps and signal, reducing the overall transmission capacity. In contrast, for dual-band design, no such guard band was required, but this benefit comes at a cost of the additional pump requirement increasing the overall amplifier power consumption. The performances of novel multistage Raman amplifier structures were also evaluated over the E-, S-, C- and L-band. Experimental studies were performed independently using DRAs only, hybrid bismuth-DRA and hybrid distributed-DRA. The E- and S-band signals were seen to have higher performance penalties in comparison to C- and L-band signals in the case of DRAs only and hybrid bismuth-DRA. In contrast, for the hybrid distributed-discrete design, the E-band signals were seen to have a similar penalty as C- and L-band signals