Patent ID: 8041228
Filing Date: 2011-10-18
Classification: G02F,H04B

Abstract:
1. 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: 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.