Coherent Lightwave Systems

The increasing demand for higher data rates necessitates the need for high-order modulation formats that can modulate the amplitude, phase and polarization of a lightwave carrier. Coherent detection, where the carrier is combined with a local oscillator laser and simultaneously detected, allows the linear reception of such signals. The detected optical signals can be processed, using a digital coherent receiver, to compensate for transmission links impairments and carry out decoding of the received signal. This chapter reviews the basics of optical modulation, modulation formats, differential and coherent detection and the application of digital coherent receivers.