ABSTRACT
Since the invention of optical —bers, the modulation of light waves for transmission in optical communication systems has been based mainly on the on-off switching of the intensity of the waves, rather than manipulating the phase or frequency of the —eld of the waves. In the 1980s, interest in extending the repeater-less distance from 40 km to longer than 60 km using coherent reception and transmitting techniques attracted the use of modulation in the carrier wave. However, suf—ciently narrow line width laser and integrated optical components that would facilitate the development of such techniques were not available. Then, the invention of optical ampli—cation devices diverted the attentionof coherent communications, till recently when the bit rate was extended to 40 Gb/s. With that the necessity of narrowing the signal bandwidth to minimize the dispersion effects has encouraged the exploitation of modulation techniques for direct detection systems.Several modulation techniques have been studied. Coherent communications have also been revived in this trend and are associated with electronic processing for estimationof the phase of the demodulated signals. Digital signal processing (DSP) associated with coherent receptions has overcome several hurdles confronting the ‘analog’ coherent reception techniques employed in the 1980s, such as the laser linewidth and the frequency offset between the local oscillator and the channel carrier, the phase and clock recovery, and, most importantly, the compensation of the distortion effects due to linear chromatic dispersion as well as nonlinear effects described in previous chapters. It is noted that coherent reception can be either self-coherent detection, in which the signals are beating themselves, or non-self-coherence, indicating the mixing between the signals and alocal oscillator to boost the amplitudes of the resultant signals. This chapter and subsequent chapters discuss both of these coherent reception techniques.
