Patent ID: 8331802

Claim:
A synchronous circuit for use in an optical homodyne receiver for synchronizing a received local oscillation light with a received optical BPSK (Binary Phase-Shift Keying) modulated signal, comprising: a synchronizing signal generator for receiving the optical BPSK signal and the local oscillation light, combining either of the received optical BPSK signal and the received local oscillation light with a phase-shifted signal to generate a plurality of different optical signals, at least one of which for use in producing a signal demodulated from the optical BPSK signal is square-law detected, calculating the square-law detected optical signal to convert a resultant optical signal into a corresponding electric signal, and producing an electric phase-locking signal which will be a demodulated signal from the optical BPSK signal on a basis of the converted electric signal; an intensity-modulating circuit for using the phase-locking signal as a modulating signal to intensity-modulate an incident continuous light into an optical intensity-modulated signal; an optoelectric converting circuit for optoelectrically converting the optical intensity-modulated signal and square-law detecting the converted signal; and an optical VCO (Voltage Controlled Oscillator) circuit for using the square-law detected signal as a phase error signal to generate the local oscillation light with a phase or a frequency adjusted to output the local oscillation light to said synchronizing signal generator.