Patent ID: 8164824

Claim:
A demodulator, comprising: a splitter that splits a differential phase shift keying optical signal into a first split light beam and a second split light beam, outputs the first split light beam to a first optical path, and outputs the second split light beam to a second optical path; a first dielectric substance that is disposed in the first optical path and that has a refractive index higher than the average refractive index of the second optical path; and a combiner that combines the first split light beam traveling along the first optical path and the second split light beam traveling along the second optical path and causes the beams to interfere with each other, wherein a difference in length between the first and second optical paths and the refractive index of the first dielectric substance are set such that the first split light beam traveling along the first optical path is delayed by one bit with respect to the second split light beam traveling along the second optical path.