Abstract: An optical signal demodulator for demodulating an M-ary phase shift key (PSK) optical signal comprises a plurality of interferometers arranged such that the optical signal is divided between the interferometers. Each interferometer comprises a plurality of interferometer arms, each arm for transmitting a portion of the signal between an input and an output of the interferometer, the interferometer including an optical delay in one arm relative to another arm thereof. The optical delay between arms of an interferometer may be provided by an optical path length difference between the arms. The optical delay between arms of an interferometer may be different for one interferometer to that of another interferometer.