US Patent 7936999 Automatic gain control in a coherent optical receiver

In a coherent optical receiver, a method of at least partially compensating Polarization Dependent Loss (PDL) of an optical signal received through an optical communications system. A respective multi-bit sample stream of each one of a pair of orthogonal received polarizations of the optical signal is tapped, and used to derive a respective metric value indicative of a quality of each multi-bit sample stream. A gain of an analog front end of the coherent optical receiver is adjusted based on the derived metric values.