Patent ID: 8285152

Claim:
A Differential Quadrature Phase Shift Keying (DQPSK) optical receiver to receive and demodulate a DQPSK-modulated optical signal, comprising: a first interferometer associated with a first arm; a second interferometer associated with a second arm; first optical-electrical converter to convert an optical signal output from the first interferometer to an electrical signal; second optical-electrical converter to convert an optical signal output from the second interferometer to an electrical signal; first and second low pass filters coupled to respective lines branching off from between the first optical-electrical converter and a first clock and data recovery circuit; third and fourth low pass filters coupled to respective lines branching off from between the second optical-electrical converter and a second clock and data recovery circuit; a first limiting amplifier to amplify a signal output from the first low pass filter and also to limit an amplitude of an output signal thereof; a second limiting amplifier to amplify a signal output from the third low pass filter and also to limit an amplitude of an output signal thereof; a first mixer to multiply the output signal from the first limiting amplifier by a signal output from the fourth low pass filter; and a second mixer to multiply the output signal from the second limiting amplifier by a signal output from the second low pass filter.