A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

A programmable, multi-format photonic transceiver platform enabling flexible optical networks

Development of programmable photonic devices for future flexible optical networks is ongoing. To this end, an innovative, multi-format QAM transmitter design is presented. It comprises a segmented-electrode InP IQ-MZM to be fabricated in InP, which can be directly driven by low-power CMOS logic. Arbitrary optical QAM format generation is made possible using only binary electrical signals, without the need for high-performance DACs and high-swing linear drivers. The concept enables a host of Tx-side DSP functionality, including the spectral shaping needed for Nyquist-WDM system concepts. In addition, we report on the development of an optical channel MUX/DEMUX, based on arrays of microresonator filters with reconfigurable bandwidths and center wavelengths. The device is intended for operation with multi-format flexible transceivers, enabling Dense (D)WDM superchannel aggregation and arbitrary spectral slicing in the context of a flexible grid environment

Equalização digital para sistemas de transmissão ópticos coerentes

This thesis focus on the digital equalization of fiber impairments for coherent optical transmission systems. New efficient and low-complexity equalization and mitigation techniques that counteract fiber nonlinear impairments are proposed and the tradeoff between performance and complexity is numerically assessed and experimentally demonstrated in metro and long-haul 400G superchannels-based transmission systems. Digital backpropagation (DBP) based on low-complexity split-step Fourier method and Volterra series nonlinear equalizers are experimentally assessed in an uniform superchannel system. In contrast with standard DBP methods, these techniques prove to be able to be implemented with larger step-sizes, consequently requiring a reduced number of multiplications, and still achieve a significant reach extension over linear equalization techniques. Moreover, given its structure, the complexity of the proposed Volterra-based DBP approach can be easily adjusted by changing the nonlinear filter dimension according to the system requirements, thus providing a higher flexibility to the nonlinear equalization block. A frequency-hybrid superchannel envisioning near-future flexible networks is then proposed as a way to increase the system bit-rate granularity. The problematic of the power-ratio between superchannel carriers is addressed and optimized for linear and nonlinear operation regimes using three distinct FEC paradigms. Applying a single FEC to the entire superchannel has a simpler implementation and is found to be a more robust approach, tolerating larger uncertainties on the system parameters optimization. We also investigate the performance gain provided by the application of different DBP techniques in frequency-hybrid superchannel systems, and its implications on the optimum power-ratio. It is shown that the application of DBP can be restricted to the carrier transporting the higher cardinality QAM format, since the DBP benefit on the other carriers is negligible, which might bring a substantially complexity reduction of the DBP technique applied to the superchannel.A presente tese foca-se na equalização digital das distorções da fibra para sistemas óticos de transmissão coerente. São propostas novas técnicas eficientes e de baixa complexidade para a equalização e mitigação das distorções não lineares da fibra, e o compromisso entre desempenho e complexidade é testado numericamente e demonstrado experimental em sistemas de transmissão metro e longa distância baseados em supercanais 400G. A propagação digital inversa baseada no método de split-step Fourier e equalizadores não lineares de séries de Volterra de baixa complexidade são testadas experimentalmente num sistema baseado em supercanais uniformes. Ao contrário dos métodos convencionais utilizados, estas técnicas podem ser implementadas utilizando menos interações e ainda extender o alcance do sistema face às técnicas de equalização linear. Para além disso, a complexidade do método baseado em Volterra pode ser facilmente ajustada alterando a dimensão do filtro não linear de acordo com os requisitos do sistema, concedendo assim maior flexibilidade ao bloco de equalização não linear. Tendo em vista as futuras redes flexı́veis, um supercanal hı́brido na frequência é proposto de modo a aumentar a granularidade da taxa de transmissão do sistema. A problemática da relação de potência entre as portadoras do supercanal é abordada e optimizada em regimes de operação linear e não linear utilizando paradigmas diferentes de códigos correctores de erros. A aplicação de um único código corrector de erros à totalidade do supercanal mostra ser a abordagem mais robusta, tolerando maiores incertezas na optimização dos parâmetros do sistema. O ganho de desempenho dado pela aplicação de diferentes técnicas de propagação digital inversa em sistemas de supercanais hı́bridos na frequência é tamém analizado, assim como as suas implicações na relação óptima de potência. Mostra-se que esta pode ser restringida à portadora que transporta o formato de modulação de ordem mais elevada, uma vez que o benefı́cio trazido pelas restantes portadotas é negligenciável, permitindo reduzir significativamente a complexidade do algoritmo aplicado.Programa Doutoral em Telecomunicaçõe

High spectral efficiency superchannel transmission using a soliton microcomb

Optical communication systems have come through five orders of magnitude improvement in data rate over the last three decades. The increased demand in data traffic and the limited optoelectronic component bandwidths have led to state-of-the-art systems employing hundreds of separate lasers in each transmitter. Given the limited optical amplifier bandwidths, focus is now shifting to maximize the spectral efficiency, SE. However, the frequency jitter from neighbouring lasers results in uncertainties of the exact channel wavelength, requiring large guardbands to avoid catastrophic channel overlap. Optical frequency combs with optimal line spacings (typically around 10-50 GHz) can overcome these limitations and maximize the SE. Recent developments in microresonator-based soliton frequency combs (hereafter microcombs) promise a compact, power efficient multi-wavelength and phase-locked light source for optical communications. Here we demonstrate a microcomb-based communication link achieving state-of-the-art spectral efficiency that has previously only been possible with bulk-optics systems. Compared to previous microcomb works in optical communications, our microcomb features a narrow line spacing of 22.1 GHz. In addition, it provides a four order-of-magnitude more stable line spacing compared to free-running lasers. The optical signal-to-noise ratio (OSNR) is sufficient for information encoding using state-of-the-art high-order modulation formats. This enables us to demonstrate transmission of a 12 Tb/s superchannel over distances ranging from a single 82 km span with an SE exceeding 10 bits/s/Hz, to 2000 km with an SE higher than 6 bits/s/Hz. These results demonstrate that microcombs can attain the SE that will spearhead future optical networks

Superchannel transmission over flexible-grid optical networks

Superchannels have been proposed as a cost-effective solution to cope with the future data capacity demand in long-haul transport networks. However, in superchannels, the spectrum of the subchannels is tightly packed and the crosstalk between subchannels can become a performance constraint. Along its path in the optical network, a superchannel passes through several reconfigurable optical add-drop multiplexers (ROADMs). The cumulative filtering effect of the cascaded wavelength selective switches (WSSs), inside the ROADMs, reduces the available bandwidth and leads to signal distortion. The subchannels performance is significantly dependent on the intercarrier spacing. If too large, the edge subchannels suffer a higher distortion from the filtering cascade. If too narrow, it creates a considerable intercarrier crosstalk between the subchannels. In this work, we perform the exhaustive assessment of the optical signal-to-noise ratio (OSNR) penalties due to the optical filtering and intercarrier crosstalk between subchannels using different M-ary quadrature amplitude modulation (M-QAM) formats, symbol rates, roll-off factors, number of traversed WSSs, WSS bandwidths, number of subchannels and intercarrier spacing. We provide a procedure to obtain optimized values of these parameters that guarantee an OSNR penalty below 1.5 dB for all subchannels after 20 WSSs. Two WSS filter spectral models are compared, named analytical and super-Gaussian. We have shown the use of the computationally much faster performance metric, the error vector magnitude (EVM) penalty, and concluded that it provides a more pessimistic performance estimate than the OSNR penalty, to estimate the distortion due to the filters cascade.O uso de super-canais tem sido proposto como uma boa relação custo-benefício para suportar a futura procura de capacidade de dados nas redes de transporte a longa distância. Contudo, nos super-canais, o espectro dos sub-canais está muito compactado e a interferência entre sub-canais pode tornar-se uma limitação. Enquanto atravessa a rede óptica, um super-canal passa através de vários multiplexadores ópticos reconfiguráveis de inserção e extracção (ROADMs, em inglês). O efeito acumulado da filtragem de uma cascata de filtros baseados em comutadores selectivos no comprimento-de-onda (WSS em inglês) que se encontram dentro dos ROADMs, reduz a largura-de-banda disponível e causa distorção no sinal óptico. A performance dos sub-canais é significativamente dependente do espaçamento entre sub-canais, da interferência entre sub-canais e da filtragem nos WSS’s. Neste trabalho, avalia-se exaustivamente a penalidade na relação sinal-ruído óptica (OSNR, em inglês) devido a filtragem óptica e interferência entre sub-canais, usando diferentes formatos de modulação M-QAM, ritmos de símbolos, factores de excesso de banda, número de WSS’s, larguras-de-banda dos WSS’s, número de sub-canais e espaçamento entre portadoras. Fornece-se um procedimento para obter valores optimizados para estes parâmetros que garantem que a penalidade na OSNR é menor que 1.5 dB em todos os sub-canais depois de 20 WSS’s. Comparam-se também dois modelos espectrais para o filtro WSS, denominados analítico e super-Gaussiano. Investiga-se o uso da penalidade da EVM, computacionalmente muito mais rápida, como métrica de desempenho e conclui-se que fornece resultados mais pessimistas que a penalidade da OSNR para estimar a distorção da cascata de filtros

Building up low-complexity spectrally-efficient Terabit superchannels by receiver-side duobinary shaping

Recently, an increasing interest has been put on spectrally-efficient multi-carrier superchannels for beyond 100G. Apart from orthogonal frequency-division multiplexing (OFDM) and Nyquist wavelength-division multiplexing (WDM), another low-complexity WDM approach based on transmitter-side pre-filtering and receiver-side duobinary shaping is proposed to build up multi-carrier superchannels. This approach is referred to as receiver-side duobinary-shaped WDM (RS-DBS-WDM). Generation and transmission of a 1.232-Tbit/s 11-carrier superchannel is experimentally demonstrated. The superchannel signal can be well fit inside the passband of multiple 300-GHz reconfigurable optical add and drop multiplexers (ROADMs). In the superchannel scenario, the proposed RS-DBS-WDM is qualitatively compared with OFDM and Nyquist-WDM in terms of implementation complexity. In sum, the proposed RS-DBS-WDM approach features high transceiver analog-bandwidth efficiency, high spectral-efficiency, the absence of specific spectral manipulation, compatibility with conventional WDM technologies and coherent detection algorithms, and comparable implementation penalty