Heterodyne detection of 310-Mb/s quadriphase-shift keying using fourth-power optical phase-locked loop

The authors report an experimental synchronous heterodyne receiver for quadriphase-shift keying based upon a fourth-power optical phase-locked loop. The transmitter and local-oscillator sources are 1.521 mu m external-cavity semiconductor lasers, and the modulation is applied using a series combination of two traveling-wave LiNbO/sub 3/ phase modulators. In detection of 310-Mb/s quadriphase-shift keying (QPSK), a receiver sensitivity of 104 photons/b has been achieved. Using the same receiver, 155-Mb/s binary-phase shift keying (BPSK) with a sensitivity of 65 photons/b has been detected.>