Polarization-insensitive technique for coherent optical communication

Abstract

To exploit the potential advantages of coherent optical communications systems, the polarization states of the received optical signal and the local oscillator waves must be matched. A mismatch may severely degrade detection performance. These mismatches occur because the received signal state of polarization changes with time and along the fiber. We first review several existing techniques for handling this problem, such as polarization-maintaining fibers, polarization-state controllers and polarization-diversity receivers. The insensitive technique reduces the problem of polarization mismatch by forcing the polarization state of either the transmitted signal or local oscillator to vary with time in a non-adaptive manner so that polarization-insensitive performance is obtained. The proposed scheme adopts a completely new approach which uses high-birefringence single-mode fibers to implement polarization switching. These techniques require only a single photodetector and give a fixed level of detection performance, with a power penalty relative to ideal of 3 dB. A specific technique is presented in which polarization-insensitive heterodyne detection is achieved through data-induced polarization switching. The polarization switching is brought about by inserting a passive, birefringent optical device in the path of the transmitted FSK signal.