Coherent Lightwave Systems The lightwave systems discussed so far are based on a simple digital modulation scheme in which an electrical bit stream modulates the intensity of an optical carrier inside the optical transmitter and the optical signal transmitted through the fiber link is incident directly on an optical receiver, which converts it to the original digital signal in the electrical domain. Such a scheme is referred to as intensity modulation with direct detection (IM/DD). Many alternative schemes, well known in the context of radio and microwave communication systems [1]–[6], transmit information by modulating the frequency or the phase of the. | Fiber-Optic Communications Systems Third Edition. Govind P. Agrawal Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-21571-6 Hardback 0-471-22114-7 Electronic Chapter 10 Coherent Lightwave Systems The lightwave systems discussed so far are based on a simple digital modulation scheme in which an electrical bit stream modulates the intensity of an optical carrier inside the optical transmitter and the optical signal transmitted through the fiber link is incident directly on an optical receiver which converts it to the original digital signal in the electrical domain. Such a scheme is referred to as intensity modulation with direct detection IM DD . Many alternative schemes well known in the context of radio and microwave communication systems 1 - 6 transmit information by modulating the frequency or the phase of the optical carrier and detect the transmitted signal by using homodyne or heterodyne detection techniques. Since phase coherence of the optical carrier plays an important role in the implementation of such schemes such optical communication systems are called coherent lightwave systems. Coherent transmission techniques were studied during the 1980s extensively 7 - 16 . Commercial deployment of coherent systems however has been delayed with the advent of optical amplifiers although the research and development phase has continued worldwide. The motivation behind using the coherent communication techniques is two-fold. First the receiver sensitivity can be improved by up to 20 dB compared with that of IM DD systems. Second the use of coherent detection may allow a more efficient use of fiber bandwidth by increasing the spectral efficiency of WDM systems. In this chapter we focus on the design of coherent lightwave systems. The basic concepts behind coherent detection are discussed in Section . In Section we present new modulation formats possible with the use of coherent detection. Section is devoted to synchronous and asynchronous demodulation .
