We review the recent advancement in the system and device technologies for coherent optical communications. One major topic is high-dimensional modulation, and in particular the nonlinearity-tolerant modulation format family, based on four-dimensional 2A8PSK. This family, covering 5, 6, 7 bits/4D symbol, outperforms most known corresponding modulation formats in the linear and nonlinear region. We also review our recent progress on forward error correction including polar codes, and monolithic narrow linewidth semiconductor lasers.
Recent research in multidimensional modulation has shown great promise in long reach applications. In this work, we will investigate the origins of this gain, the different approaches to multidimensional constellation design, and different performance metrics for coded modulation. We will also discuss the reason that such coded modulation schemes seem to have limited application at shorter distances, and the potential for other coded modulation schemes in future transmission systems.