Patent ID: 7649678

Claim:
A delay-line demodulator of a planar lightwave circuit (PLC) type for a differential quadrature phase shift keying (DQPSK) signal to be demodulated, comprising: (a) individual Mach-Zehnder interferometers comprising a plurality of waveguides having different lengths therebetween and through which a light signal branched from the DQPSK signal propagates, wherein a phase of the light signal propagating at one side of the waveguides is delayed as compared to a phase of the light signal propagating at another side of the waveguides; (b) a half wavelength plate arranged at a central part of the waveguide in each of the Mach-Zehnder interferometers; (c) a first heater (A) and a second heater (C) formed on the one side of the waveguides in the individual Mach-Zehnder interferometers as sandwiching the half wavelength plate therebetween; and a third heater (B) and a fourth heater (D) formed on the other side of the waveguides in the individual Mach-Zehnder interferometers as sandwiching the half wavelength plate therebetween, (d) wherein a divergence amount of polarization is adjusted by driving the first heater (A) and the fourth heater (D), or the third heater (B) and the second heater (C).