Phase Noise Tracking and Compensation in Coherent Optical Systems Using Kalman Filter

Abstract

The performance of phase-modulated coherent optical communication systems is often limited by laser and nonlinear phase noises (PN and NLPN). The laser PN and the NLPN at high launch powers degrade system performance severely. In this letter, we investigate the use of Kalman filter (KF) to estimate and track both the laser PN and the NLPN in 100-Gb/s single channel coherent optical phase-modulated systems. Initially, the KF is operated in the training mode with pilot symbols; the filter is switched to decision directed mode after training. Simulation results for single-polarization, 100-Gb/s QPSK transmission over $20\times 80$ km fiber with in-line dispersion compensation show that KF can successfully mitigate NLPN at 8-dBm launch power with 1-MHz laser linewidth. Comparison with digital backpropagation algorithm (one step/span) shows that KF provides 6-dB improvement in $Q$ -factor. Kalman estimates corresponding to the time evolution of PN and NLPN are also provided, which shows optimal tracking of both the noise processes, thereby validating our proposed scheme.