Coherent lightwave techniques, when compared to direct detection techniques, offer nearly quantum noise limited sensitivity as well as fine tunability similar to that obtained at radio frequencies. These two aspects provide communication systems planners and engineers the means to better exploit the huge bandwidth of single mode optical fibers. Research activity in this field started in the early 1980s, and some laboratory experiments and field trials were performed by the end of the decade, showing that such techniques are suitable for transmitting multigigabit per second signals to distances well exceeding hundred kilometers. On the other hand, coherent multichannel, frequency division multiple access, local area networks have been proposed and experimented worldwide. This article will discuss the theoretical advantages and limitations of the various modulation and detection formats together with the state of the art. Moreover, some aspects, related to the introduction of coherent systems in local and metropolitan area networks, will be treated. Finally some experimental data will be provided and future evolution will be discussed.
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