Digital signal processing techniques for fiber nonlinearity compensation in coherent optical communication systems

The capacity of long-haul coherent optical communication systems is limited by the detrimental effects of fiber Kerr nonlinearity. The power-dependent nature of the Kerr nonlinearity restricts the maximum launch power into the fiber. That results in the reduction of the optical signal-to-noise ratio at the receiver; thereby, the maximum transmission reach is limited. Over the last few decades, several digital signal processing (DSP) techniques have been proposed to mitigate the effects of fiber nonlinearity, for example, digital back-propagation (DBP), perturbation based nonlinearity compensation (PB-NLC), and phase-conjugated twin wave (PCTW). However, low-complexity and spectrally efficient DSP-based fiber nonlinearity mitigation schemes for long-haul transmission systems are yet to be developed. In this thesis, we focus on the computationally efficient DSP-based techniques that can help to combat various sources of fiber nonlinearity in long-haul coherent optical communication systems. With this aim, we propose a linear time/polarization coded digital phase conjugation (DPC) technique for the mitigation of fiber nonlinearity that doubles the spectral efficiency obtained in the PCTW technique. In addition, we propose to investigate the impact of random polarization effects, like polarization-dependent loss and polarization mode dispersion, on the performance of the linear-coded DPC techniques. We also propose a joint technique that combines single-channel DBP with the PCTW technique. We show that the proposed scheme is computationally efficient and achieves similar performance as multi-channel DBP in wavelength division multiplexed superchannel systems. The regular perturbation (RP) series used to analytically approximate the solution of the nonlinear Schrödinger equation (NLSE) has a serious energy divergence problem when truncated to the first-order. Recent results on the transmission of high data-rate optical signals reveal that the nonlinearity compensation performance of the first-order PB-NLC technique decreases as the product of the transmission distance and launch power increases. The enhanced RP (ERP) method can improve the accuracy of the first-order RP approximation by partially solving the energy divergence problem. On this ground, we propose an ERP-based nonlinearity compensation technique to compensate for the fiber nonlinearity in a polarization-division multiplexed dispersion unmanaged optical communication system. Another possible solution to improve the accuracy of the PB-NLC technique is to increase the order of the RP solution. Based on this idea, we propose to extend the first-order solution of the NLSE to the second-order to improve the nonlinearity compensation performance of the PB-NLC technique. Following that, we investigate a few simplifying assumptions to reduce the implementation complexity of the proposed second-order PB-NLC technique

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