Patent ID: 7974540

Claim:
A communication system comprising: a transmission path for serving as a transmission medium of light; a first station which emits time-divided optical pulses into the transmission path, returns optical pulses modulated at a second station into the transmission path, and measures a phase difference between the optical pulses returning from the transmission path; and the second station including, a polarization beam splitter splitting the time-divided optical pulses into first orthogonally polarized components and second orthogonally polarized components, a first phase modulator receiving the first of the split optical pulses and producing the phase difference corresponding to a value of a random number bit between the time-divided optical pulses, and a second phase modulator receiving the second of the split optical pulses after polarization direction of the second of the split optical pulses is rotated by 90 degrees, producing a same phase difference as the first phase modulator between the first of the optical pulses, and modulating the orthogonally polarized components of each optical pulse to have a phase difference of 180 degrees therebetween, wherein an output of the first phase modulator is combined with an output of the second phase modulator after the polarization direction is rotated by 90 degrees, and then the combined output returns into the transmission path.