Coherent transmission systems

Coherent optical communications were studied extensively in the 1980s; however, their R&D activities have been interrupted for almost 20 years behind the rapid progress in high-capacity WDM systems using EDFAs. In 2005, the demonstration of the digital coherent receiver stimulated a widespread interest in coherent optical communications again. This is because such receiver enables us to employ a variety of spectrally-efficient multilevel modulation formats. In addition, we can realize post-processing functions such as compensation for GVD and PMD in the digital domain. In this lecture, after talking about the history, we discuss state-of-the-art coherent technologies and challenges for the future.