Patent ID: 8477409

Claim:
A Planar Lightwave Circuit (PLC)-type delay demodulation circuit including a planar lightwave circuit that is formed on one PLC chip and demodulates a Differential Quadrature Phase Shift Keying (DQPSK)-modulated optical signal, the PLC-type delay demodulation circuit comprising: 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 two arm waveguides of the first MZI and two arm waveguides of the second MZI are formed so as to overlap each other in the same region of the planar lightwave circuit, optical paths of the first MZI and the second MZI are arranged such that the propagation directions of the optical signals are opposite to each other, a first half-wave plate is positioned in central portions of the two arm waveguides of the first MZI and central portions of the two arm waveguides of the second MZI, and a second half-wave plate is positioned at a position that deviates from the central portions of the two arm waveguides of the first MZI and a position that deviates from the central portions of the two arm waveguides of the second MZI.