OPTICAL DQPSK RECEIVER

PROBLEM TO BE SOLVED: To provide a DQPSK (Differential Quaternary Phase Shift Keying) optical receiver capable of suppressing a phase error even when an offset is present in a multiplier.SOLUTION: The optical DQPSK receiver includes: a first branch and a second branch provided with a delay interferometer, a balanced detector and a data reproducing circuit in the order, respectively; the multiplier to which the preceding stage signal and post stage signal of the data reproducing circuit are input in at least one of the first branch and the second branch; and an amplitude detection circuit for detecting the amplitude of the output signal of the multiplier. The optical DQPSK receiver may be provided with a phase adjuster for controlling the phase shift amount of the delay interferometer so as to reduce the amplitude detected in the amplitude detection circuit.