Optical dqpsk receiver and optical phase monitor apparatus for use in optical dqpsk receiver

An optical DQPSK receiver comprising;

• a first branch circuit comprising a first delay interferometer having a first phase-shift element, a first photo detector for detecting output light from the first delay interferometer, and a first recovery circuit for recovering data from an output signal from the first photo detector;
• a second branch circuit comprising a second delay interferometer having a second phase-shift element, a second photo detector for detecting output light from the second delay interferometer, and a second recovery circuit for recovering data from an output signal from the second photo detector;
• a demultiplex circuit for demultiplexing data recovered by the first and second recovery circuits;
• a common adjustment unit for adjusting the first phase-shift element and the second phase-shift element;
• an individual adjustment unit for adjusting the second phase-shift element;
• and
• a control unit for controlling the common adjustment unit in accordance with a first signal output from the first photo detector and a second signal obtained by demultiplexing, in the demultiplex circuit, an output signal from the second recovery circuit, and for controlling the individual control unit in accordance with the first signal and a third signal output from the second photo detector.