Abstract

In coherent optical transmission systems using conventional single mode fibers, the received state of polarization (SOP) is unknown and changing [1]. Polarization scrambling of the transmitted signal [2] or the local oscillator [3] overcomes signal fading by forcing a signal to undergo very large polarization changes during each bit cycle. The power penalty is ideally 3 dB because on average the polarization match between signal and local oscillator is only one-half. If the transmitted signal is scrambled, only one scrambler is needed even if several subscribing receivers are used. One reported scrambler [2] has an integrated optical phase shifter as the active element and switches a certain class of input SOPs alternately between themselves and their orthogonal states. This particular scheme suffers from two notable deficiencies. First, some input SOPs are not scrambled at all. Second, a receiver bandwidth of many times the scrambling frequency is required to capture all spectral components produced by the scrambler, which amounts to an additional sensitivity penalty in a practical system.

© 1988 Optical Society of America