Patent ID: 8441717

Claim:
A Planar Lightwave Circuit (PLC)-type delay demodulation circuit comprising: a planar lightwave circuit that is formed on one PLC chip and demodulates a Differential Quadrature Phase Shift Keying (DQPSK)-modulated optical signal, wherein the planar lightwave circuit includes: an optical splitter that branches the DQPSK-modulated optical signal into two optical signals; and first and second Mach-Zehnder Interferometers (MZIs) that delay the branched optical signals by one bit so as to interfere with each other, wherein a wave plate is provided in central portions of first and second arm waveguides of the first MZI and first and second arm waveguides of the second MZI in such a manner that the wave plate intersects all of the four arm waveguides, the four arm waveguides being close to one another in a portion where the wave plate is provided, and in the planar lightwave circuit, the arm waveguides of the first MZI and second MZI are placed to overlap each other in the same region, the second arm waveguide of the first MZI and the first arm waveguide of the second MZI intersect each other at only two points, one of which is on one side of the wave plate and another of which is on another side of the wave plate, the first arm waveguide of the first MZI and the second arm waveguide of the second MZI do not intersect with other arm waveguides, and in the arrangement of the four arm waveguides in a portion where the wave plate is inserted, the first arm waveguide of the second MZI is placed between the two arm waveguides of the first MZI.