Interferometer for demodulating differential M-phase shift keying signal

A device for demodulating a differential M-phase shift keying signal, comprising:

• an interferometer module in which the differential M-phase shift keying signal is inputted;
• a light reception module that receives optical output signals from the interferometer module;
• and
• an optical system located between the interferometer module and the light reception module that couples the optical output signals with the light reception module, the optical system including a lens which is attached to an end surface of the interferometer module for outputting optical signals, two optical output signals from the interferometer module being incident on the lens.

An interferometer for demodulating a phase shift keying signal, comprising:

• a planar lightwave circuit (PLC) type interferometer main body into which the phase shift keying signal is inputted;
• a heating portion that heats the PLC type interferometer main body;
• an intermediate member bonded to the PLC type interferometer main body and the heating portion while being sandwiched between the PLC type interferometer main body and the heating portion;
• and
• a light reception module that receives an optical output signal from the PLC type interferometer main body,
• the PLC type interferometer main body including two interferometers into which the phase shift keying signal is inputted, and
• a phase difference for one of the interferometers and a phase difference for the other of the interferometers being a combination of predetermined values.