Overcoming Phase Sensitivity in Real-Time Parallel DSP for Optical Coherent Communications: Optically Filtered Lasers

Implementation of real-time giga-Baud optical coherent systems for single-carrier higher-level modulation format such as 64-quadrature amplitude modulation (QAM) depends heavily on phase tracking. For offline digital signal processing, decision-directed phase recovery is performed at symbol rate with the best performance and the least computational effort compared among other best-known algorithms. However, in real-time systems, hardware parallelization and pipelining delay on feedback path pose stringer requirement on the linewidth, or the frequency noise spectral level of laser sources. This leads to the paucity of experiments demonstrating real-time phase tracking for 64- or higher QAM. In this paper, we experimentally investigate the impact of optically-filtered lasers on parallel and pipelined phase tracking in a single-carrier 5 Gbaud 64-QAM back-to-back coherent system. For parallelization levels higher than 24, the optically-filtered laser shows more than 2 dB improvement in optical signal-to-noise ratio penalty compared to that of the same laser without optical filtering.