  The effect of Kerr-induced optical fiber nonlinearities in C-band (5 THz)
EDFA and C+L-band (12.5 THz) Raman-amplified optical communication systems has
been studied considering the impact of third-order fiber dispersion. The
performance of digital nonlinearity compensation with single channel and
250-GHz bandwidth in both EDFA and Raman amplified systems has been
investigated, respectively. The achievable information rates (AIRs) and optimum
code rates in each individual transmission channel have been evaluated for the
DP-64QAM, the DP-256QAM and the DP-1024QAM modulation formats, both with and
without the use of the probabilistic shaping technique. It is found that, for
all considered modulation formats, the signal-to-noise ratios, AIRs and code
rates exhibit significantly asymmetric behavior about the central channel due
to the presence of the third-order dispersion. This provides a new insight that
the forward error correction schemes have to be optimized asymmetrically, on a
per-channel basis, to maximize the overall throughput.
