Patent ID: 7884996

Claim:
A method for producing a delay-line demodulator of a planar lightwave circuit type for a differential quadrature phase shift keying (DQPSK) signal to be demodulated, comprising the following steps of: (a) forming individual Mach-Zehnder interferometers comprising a plurality of waveguides having different lengths therebetween, 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 an amount of one symbol as compared to a phase of the light signal propagating at another side of the waveguides; (b) arranging a half wavelength plate at a central part of the waveguide in each of the Mach-Zehnder interferometers; (c) forming a first heater (A) and a second heater (C) on the one side of the waveguides in the individual Mach-Zehnder interferometers as sandwiching the half wavelength plate therebetween; and forming a third heater (B) and a fourth heater (D) on the other side of the waveguides in the individual Mach-Zehnder interferometers as sandwiching the half wavelength plate therebetween; (d) adjusting a divergence amount of polarization by driving the first heater (A) and the fourth heater (D), or the third heater (B) and the second heater (C).