Patent ID: 7411725

Claim:
A demodulator for demodulating an optical signal on which DPSK (Differential Phase Shift Keying) is applied, the demodulator comprising: a Michelson interferometer that includes: a half-mirror for splitting the optical signal into a first split light and a second split light, emitting the first split light to a first optical path, and emitting the second split light to a second optical path; a first reflector for reflecting the first split light to the half-mirror, the first reflector being disposed on the first optical path; and a second reflector for reflecting the second split light to the half-mirror, the second reflector being disposed on the second optical path, wherein the half-mirror recombines the first split light incident from the first reflector and the second split light incident from the second reflector, and emits a recombined optical signal while splitting the recombined optical signal in two branches; and a balanced optical detector that includes: two light receiving elements for receiving the recombined optical signals being emitted in two branches from the Michelson interferometer, wherein the balanced optical detector generates a demodulated signal based on the recombined optical signals, wherein an optical path length difference between the first optical path and the second optical path is set so that the second split light has a delay time which is equal to a one-bit period of the optical signal, with respect to the first split light.