Patent ID: 8041228

Claim:
A transmitter for generating a DQPSK-modulated optical signal, comprising: 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, each being operated with an electrical modulation signal, wherein the first and second Mach-Zehnder interferometers are operated as phase modulators and wherein the electrical modulation signal corresponds to in-phase and quadrature-phase sequences of the system, a phase shifter in one of the branches generating a nominal phase shift of .pi./2, a combiner for combining the optical output signals of the two branches, a feedback circuit generating first and second bias signals for adjusting a bias of at least the first and second Mach-Zehnder interferometers, the feedback circuit comprising: a detector for generating first and second feedback signals from a sample signal extracted from the optical signal after the combiner indicative of a mean power of the sample signal, and for each of the first and second bias signals: a local oscillator, generating at least one auxiliary signal modulating each bias signal at a pre-defined frequency, a lock-in detector determining a phase difference between the at least one of a first and second feedback signal and the at least one auxiliary signal, and a bias circuit for generating the at least one of a first and second bias signal from an output signal of the lock-in detector, wherein the predefined frequencies of each of the auxiliary signals of the at least one auxiliary signal are different from one another, and the feedback circuit further comprises another detector which generates a third bias signal for adjusting a bias of the phase shifter, wherein said another detector comprises: a peak power detection unit, preferably a quadratic detector, for generating a third feedback signal from the sample signal, a local oscillator, generating at least one auxiliary signal, a lock-in detector determining a phase difference between the at least one of a first and second feedback signal and the at least one auxiliary signal, and a bias circuit for generating the third bias signal from an output signal of the lock-in detector wherein the third bias signal indicates a power of the sample signal exceeding a threshold value representing a nominal level of the amplitude of the sample signal.