Impact of Sinusoidal Tones on Parallel Decision-Directed Phase Recovery for 64-QAM

Coherent optical communications at Gbaud rates require parallelization in digital signal processing, however the feedback delay introduced by pipelining challenges phase recovery. Parallelized phase recovery creates a greater sensitivity to residual frequency offset induced by the presence of sinusoidal tones in the source. We experimentally investigate the impact of sinusoidal laser phase noise on parallel decision-directed phase recovery in a 5 Gb 64-quadrature amplitude modulation system, including the effects of frequency offset compensation and equalization.