Optical receiving apparatus

An optical receiving apparatus for receiving an optical differential quaternary phase shift keying signal, comprising:

• an optical interferometer that includes a delay section to give a delay time to one arm of two arms through which part of inputted optical differential quaternary phase shift keying signal passes, a phase section to give a previously determined phase difference between signals of the two arms, and a phase adjustment section to adjust a phase of the signal of the arm different from the phase section;
• a photoelectric conversion element to convert an optical signal from the optical interferometer into an electric signal and to output the electric signal;
• a discriminator to reproduce a digital signal based on a level of the electric signal and to output the digital signal;
• a difference unit to obtain a difference signal between an input to the discriminator and an output of the discriminator;
• and
• a control circuit to adjust the phase given by the phase section to reduce an amplitude of the obtained difference signal.