Abstract

We report on an experimental synchronous heterodyne receiver for quadriphase-shift keying (QPSK)based on a fourth- power optical phase-locked loop. In detection of 310-Mbit/s QPSKwe have achieved a receiver sensitivity of 104 photons/bit. An optical QPSK system with homodyne or heterodyne detection requires, for a given bit rate, approximately half the optical and electrical bandwidth of its BPSK counterpart.¹ QPSK systems are far less sensitive to degradation by chromatic dispersion than are BPSK or MSK systems of the same bit rate.² Heterodyne receivers for QPSK combine the signal and local- oscillator (LO) light in a 3-dB coupler and require only a single balanced detector, whereas homodyne receivers³ require a 90° optical hybrid and two balanced detectors.

© 1993 Optical Society of America