Contents I. Introduction
Optical differential quadrature phase shift keying (DQPSK) is a promising modulation format for 43-Gbit/s long haul core optical network systems because of its high spectral efficiency and large dispersion tolerance. Several investigations on optical DQPSK communication systems have been reported [1]–[4]. The optical DQPSK receiver consists of an optical DQPSK demodulator and two balanced photoreceivers, as shown in Fig. 1. An optical DQPSK demodulator converts a 43-Gbit/s optical quaternary phase shifted signal into two channels of 21.5-Gbit/s optical intensity modulated signals. The demodulated optical signals are received by balanced photoreceivers, which convert the optical signals into electrical signals. Therefore, the optical DQPSK receiver usually requires two balanced photoreceiver packages, which does not allow further reduction in the size of the receiver.
