Comparison of Probabilistically Shaped 64QAM With Lower Cardinality Uniform Constellations in Long-Haul Optical Systems

In this paper, we compare the performance of probabilistically shaped 64QAM with uniform 16QAM and 32QAM modulation formats at the same net data rate in long-haul coherent optical communications systems. Experimental results at 16 GBaud are shown, with offline postprocessing of the data performed using either an ideal or a realistic carrier phase estimation (CPE) scheme. We show that the choice of the CPE algorithm at the receiver is crucial, since, as predicted by current models, most of the additional nonlinear noise introduced by the shaping is nonlinear phase noise (NLPN). Thanks to the use of probabilistic shaping (PS), maximum reach gains ranging from 15.5% and $\text{34}\%$ are obtained over pure silica-core fiber, where the NLPN can be efficiently compensated for by standard CPE algorithms, while over nonzero dispersion-shifted fiber, the gain of PS is drastically reduced, due to residual short-correlated NLPN.