COHERENT OPTICAL QPSK INTRADYNE SYSTEM - CONCEPT AND DIGITAL RECEIVER REALIZATION

One major advantage of a coherent optical transmission system in relation to a direct system is the frequency selectivity which makes it suitable to coherent optical multichannel systems, e.g., for HDTV-broadcasting. In order to keep the subscriber front-end cheap, optical realization expense should be low and be replaced as far as possible by an electrical realization. Moreover, it should be aimed at a digitalization of the receiver functions to make an integration possible. The paper presents a receiver concept based on optical quaternary phase shift keying (QPSK) and a digital realization of synchronous demodulation including phase synchronization. To keep the signal processing bandwidth low a phase diversity receiver-in the following called an intradyne receiver-with an orthogonal electrical demodulation is proposed. Basic principles of the synchronous orthogonal and digital demodulation are described. After the evaluation of the shot noise limit some aspects of the digital phase-locked loop are presented. In a realized 100-Mb/s transmission system a receiver sensitivity of -51.6 dBm has been measured. The loss in relation to the shot noise limit of -66.3 dBm (18 photons/bit) is mostly due to the low local laser power and the influence of the receiver input noise.