Modified phase-conjugate twin wave schemes for fiber nonlinearity mitigation.

Abstract

The conventional phase-conjugate twin wave (PCTW) scheme can effectively mitigate the fiber nonlinear distortions but reduce the spectral efficiency (SE) by half. In this paper, we show that by modulating one of the conjugated signals with additional bits, the information encoded in the conjugated variants can still suppress the nonlinear distortions in a similar manner as in the conventional PCTW method, while that encoded in the additional bits, representing the improved SE, is the dominant source limiting the system performance. We further introduce redundancy to the additional bits via error correction coding to overcome this performance bottleneck. The overall system performance can thus approach that of the conventional PCTW method while the SE is significantly enhanced. Simulations of a 25-Gbaud polarization-division-multiplexed quadrature phase shift keying system over 15,200 km show that the proposed scheme can increase the normalized SE from 50% to 80% for a similar performance as the conventional PCTW method. We also show that the proposed concept can be applied to higher-level formats.