Fiber optical transmission system, transmitter and receiver for dqpsk modulated signals and method for stabilizing the same

Abstract

A transmitter () for generating a DQPSK-modulated optical signal, including: a splitter () for dividing an optical carrier signal into a first and second branch (), a first and second Mach-Zehnder interferometer () in the first and second branch (), respectively, a phase shifter () in one of the branches () generating a nominal phase shift of .pi./2, and a combiner (') for combining the optical output signals of the two branches (). The transmitter () has a feedback circuit () generating at least a first and second bias signal (to) for adjusting a bias of at least the first and second Mach-Zehnder interferometers (), the feedback circuit () includes a detector for generating at least a first and second feedback signal from a sample signal extracted from the optical signal after the combiner (′), and for each bias signal: a local oscillator generating an auxiliary signal modulating the bias signal (to) at a pre-defined frequency (f, f, f), a lock-in detector determining a phase difference between the feedback signal and the auxiliary signal, and a bias circuit for generating the bias signal (to) from an output signal of the lock-in detector, wherein the pre-defined frequencies of the auxiliary signals (f, f, f) are different from one another (f.noteq.f.noteq.f) and preferably no integer multiples of each other.