Recently, there have been several efforts to build optical heterodyne detection systems in the Gbit/s range.1–3 However, it is becoming difficult to achieve high receiver sensitivity in this range, because the heterodyne receiver requires a broad IF bandwidth. An optical quadriphase shift keying (QPSK) heterodyne system can achieve the same receiver sensitivity as the biphase shift keying (BPSK) system, requiring a half-bandwidth of the BPSK system. Therefore, the QPSK system has advantages in the Gbit/s range. The key factors, modulation method, and linewidth requirement for a QPSK coherent detection system are discussed. The feasibility of a Gbit/s range QPSK system with external cavity LDs was confirmed through theoretical calculation and the simulation experiment.

© 1988 Optical Society of America

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