Patent ID: 8213806

Claim:
A Differential Quaternary Phase Shift Keyed (DQPSK) modulator, comprising: a differential pre-coder for providing a plurality of drive signals in response to a plurality of input baseband data streams; and an optical phase modulator arrangement for generating a differential phase shift key-modulated optical signal in response to the plurality of binary digital baseband drive signals, wherein the optical phase modulator arrangement comprises: a splitter for splitting an optical carrier signal into a plurality of component signals, a first Mach-Zehnder interferometer optical phase modulator for phase modulating one of the plurality of component signals in response to one of said plurality of drive signals, the first optical phase modulator generating a first phase modulated signal, a second Mach-Zehnder interferometer optical phase modulator for phase modulating another one of said plurality of component signals in response to another one of the plurality of drive signals, the second optical phase modulator generating a second phase modulated signal, a phase shifter optically coupled in series with at least one of the first and second optical phase modulators for imparting a fixed relative phase shift of pi/2 between the component signals, and a recombiner for combining the first shifted modulated signal and the second modulated signal into an output signal.