Patent ID: 7864892

Claim:
An apparatus for receiving an optical signal that receives and demodulates an optical signal subjected to differential quadrature phase shift keying (DQPSK) modulation, the apparatus comprising: a demodulator that includes a plurality of delay interferometers and photoelectric conversion elements, the interferometers and the photoelectric conversion elements converting an in-phase signal and a quadrature signal of a DQPSK optical signal into electrical signals of each; a recovering unit that recovers a clock signal and data based on the electrical signals; a multiplexer that multiplexes the electrical signals from the recovering unit; a parallelizing unit that parallelizes the signals multiplexed by the multiplexer; a processor that performs a frame process including a frame synchronization process on the signals parallelized by the parallelizing unit; and a judging unit that judges whether a bit delay is present based on a phase difference between a phase of the clock signal at the recovering unit and a phase of the clock signal at the parallelizing unit, wherein the processor includes a logic processing circuit that executes a bit delay process according to a result of judgment by the judging unit, wherein the judging unit includes a phase comparing circuit that outputs a phase difference between a phase of the clock signal at the recovering unit and a phase of the clock signal at the parallelizing unit; and a phase judging circuit that judges whether the phase difference is larger than a threshold.