Coherent optical detection: A thousand calls on one circuit: Dazzling applications for both long-distance and local networks are in the offing as experimental systems adapt long-successful radio techniques

The reasons why coherent detection of laser signals may well turn out to be the next great advance in optical communications are examined. The operation of coherent systems is described, and signal modulation and its effect on the ratio of separation to width are considered. Research on how to deal with the wide spectral lines of semiconductor lasers is discussed, and the outlook for increased sensitivity is considered.