The recently-developed digital coherent receiver enables us to employ a variety of spectrally-efficient modulation formats such as M-ary phase-shift keying (PSK) and quadrature-amplitude modulation (QAM). Moreover, in the digital domain, we can equalize linear transmission impairments, which may stem from group-velocity dispersion (GVD) and polarization-mode dispersion (PMD) of fibers for transmission, because the phase information is preserved after coherent detection.
This paper reviews the history of coherent optical communications, the principle of coherent detection, and the concept of the digital coherent receiver. After that, we discuss digital signal processing (DSP) for mitigating transmission impairments, coherent transmission characteristics of multi-level optical signals, and future prospects of coherent optical communications.