Optical Sources and Detectors

There are a number of light sources and photodetectors that could be for OWC systems. The most commonly used light sources used are the incoherent sources-light emitting diodes (LEDs) and coherent sources—laser diodes (LD). LEDs are mainly used for indoor applications. However, for short link (e.g., up to a kilometre) and moderate data rates, it is also possible to use LEDs in place of LDs. Lasers, because of their highly directional beam profile, are mostly employed for outdoor applications. ^{Particularly for long transmission links, it is crucial to direct the energy of the information to be transmitted precisely in the form of a well-collimated laser beam. This is to limit the often still very large channel power loss between the transmitter and the receiver. In order to limit the beam divergence, ideally, one should use a diffraction-limited light source together with a relatively large high-quality optical telescope. At the receiving end, it is also advantageous to use a high- directionality telescope not only to collect as much of the transmitted power as possible but also to reduce the background ambient light, which introduces noise and thus reduces the performance of the link. As for detectors, both the PIN and the APD photodetectors could readily be used. This chapter discusses the types of lights sources, their structures and their optical characteristics. The process of optical direct detection as well as coherent detection is also covered in this chapter. Different types of noise encountered in optical detection will be introduced and the statistics of the optical detection process is also discussed.}