The purpose of a receiver in an electronic communication system is to extract the information sent by the corresponding transmitter with as minimum a carrier power level as possible. The primary function of an optical receiver in an optical fiber communication link is to convert the received optical signal into an equivalent electrical signal and recover the data. One of the main components of an optical receiver is a photodetector that converts incident optical signals into electric signals using photoelectric effects. High Sensitivity, dynamic range, fast response (i.e., acquisition time), high reliability, low noise, compatible size with that of fiber, and low cost are some of the important requirements of a photodetector. These requirements are best met by semiconductor photodetectors that convert an optical signal transmitted via optical fiber cables to equivalent electrical signals for further processing to achieve the desired output. The type of photodetectors suitable for three optical spectrum ranges of 800–900 nm, 900–1100 nm, and 1100–1600 nm vary in the material used for their fabrication as well as assembly techniques. A p–i–n photodiode is an ideal semiconductor photodetector device, because it can provide high quantum efficiency, fast response and capability to operate at higher modulation frequencies. The minimum received optical power that can be detected by a photodetector is limited by noise. A fully integrated single beam optical receiver comprises of a semiconductor photodiode, preamplifier in the electric domain, digital logic circuits, and an off-chip electronic driver circuit. This chapter discusses all the important aspects of photodetectors and optical receivers. The discussion begins with basic concepts behind the photo detection process, followed by description of different types of photodetectors usually used by optical receivers. Next, the components used in an optical receiver unit are explained. Finally, different types of noise sources in optical receivers that limit the signal-to-noise ratio, the receiver sensitivity parameter and its degradation are covered in sufficient detail.
Requirements for a Photodetector
In an optical fiber communication system, a semiconductor photodetector is the basic component in an optical receiver, as shown in Fig. 4.1.