The coherent optical receiver can linearly convert an optical electric field to an electrical signal with the highest receiver sensitivity. Using the coherent receiver, a number of research groups challenged long-distance optical transmission experiments in the 1980s. However, the invention of EDFAs made the shot-noise limited receiver sensitivity of the coherent receiver less significant, and further R&D activities in coherent optical communications had almost been discontinued after 1990.

On the other hand, with the tremendous capacity increase of WDM transmission systems during the 1990s, coherent technologies have started to regain a large interest after 2000. In 2005, using the phase-diversity homodyne receiver and the succeeding digital signal processing (DSP), our group has demonstrated demodulation of the 20-Gbit/s QPSK signal for the first time. Currently, 100-Gbit/s digital coherent transponders using the QPSK modulation format and polarization-division multiplexing have been introduced into commercial transmission systems. 

After summarizing the history of coherent optical communications, I will discuss the advantages of coherent optical communication systems from viewpoints of the quantum optics and classical information theory in this lecture. Next, I explain the impact of the combination of analog optics and digital electronics. Owing to high-speed DSP at the receiver, we can track random changes in the carrier phase and state of polarization of the transmitted signal and compensate for group-velocity dispersion (GVD) and polarization-mode dispersion (PMD) of fibers for transmission.

The born-again digital coherent technology becomes a part of the next generation of optical communication systems and provides new capabilities that were not possible without detection of the optical phase.