Coherent optical transmission systems require that the polarization of the received signal and the local oscillator are matched. Endless polarization control, polarization diversity, and data-induced polarization switching are the most promising solutions for this problem. However, in spite of a host of publications, no clear favorite has emerged, maybe because few laboratories have implemented more than one method and no direct comparison was ever made. In this paper we directly compare these three methods, and active, data-synchronous polarization switching, for the first time to our knowledge. We will see that endless polarization control is potentially the most powerful candidate, however, the choice of polarization control devices remains questionable. Polarization diversity is as versatile as polarization control and is potentially the fastest method; however, it yields lower receiver sensitivity. Endless control or a well-designed diversity receiver should be used for coherent trunk systems. Data-induced polarization switching is restricted to FSK systems. It promises a loss span similar to that of diversity but is far simpler which makes .it recommendable for FSK distribution systems.

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