Abstract:We investigate a digital back-propagation simplification method to enable computationally-efficient digital nonlinearity compensation for a coherently-detected 112 Gb/s polarization multiplexed quadrature phase shifted keying transmission over a 1,600 km link (20x80km) with no inline compensation. Through numerical simulation, we report up to 80% reduction in required back-propagation steps to perform nonlinear compensation, in comparison to the standard back-propagation algorithm. This method takes into account the correlation between adjacent symbols at a given instant using a weighted-average approach, and optimization of the position of nonlinear compensator stage to enable practical digital backpropagation.

The frequency domain equalizers (FDEs) employing two types of overlap-add zero-padding (OLA-ZP) methods are applied to compensate the chromatic dispersion in a 112-Gbit=s non-return-to-zero polarization division multiplexed quadrature phase shift keying (NRZ-PDM-QPSK) coherent optical transmission system. Simulation results demonstrate that the OLA-ZP methods can achieve the same acceptable performance as the overlapsave method. The required minimum overlap (or zeropadding) in the FDE is derived, and the optimum fast Fourier transform length to minimize the computational complexity is also analyzed. Keywords. Frequency domain equalizers (FDEs), overlapsave (OLS), overlap-add (OLA), chromatic dispersion (CD), quadrature phase shift keying (QPSK). PACS ® (2010). 42.25.Kb, 42.79.Sz.

PolMux QPSK has emerged as the solution of choice for the first commercial implementations of 100 Gb/s transmission systems. Thanks to coherent detection and digital signal processing (DSP), linear distortions such as chromatic dispersion (CD) and polarisation mode dispersion (PMD) can in principle be completely compensated for. And indeed, effective algorithms have been devised and extensively investigate that allow CD-and PMD-resilient transmission of 100 Gb/s over long distances, leaving optical noise accumulation and non-linear impairments as the factors ultimately limiting reach. In this paper, we present the evaluation of a simple algorithm to compensate for intra-channel Kerr non-linearity (both intra-and cross-polarisation) arising in the transmission of PolMux QPSK signals at 100 Gb/s.

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