TABLE OF CONTENTS Since somebody has to listen to what is being said, at the end of an optical transmission link there must be a receiver. The function of this device is to interpret the information contained in the optical signal. An optical receiver consists of a photodetector and various amounts of electronics. Depending on how sophisticated the receiving and associated signal processing functions are in the receiver, these electronics can range from some simple amplification functions for relatively strong, clean signals to hordes of complex circuitry if the receiver needs to interpret weak, distorted signals at high data rates.
In this chapter we first examine the performance characteristics of photodetectors without going deeply into the physics of their operation. We then look at how these devices are used in receivers by considering factors such as random noises associated with the photodetection process, the concept of signal-to-noise ratio, and the probability of errors occurring in a data stream.
The first element of the receiver is a photodetector. The photodetector senses the light signal falling on it and converts the variation of the optical power to a correspondingly varying electric current. Since the optical signal generally is weakened and distorted when it emerges from the end of the fiber, the photodetector must meet strict performance requirements. Among the most important of these are
A high sensitivity to the emission wavelength range of the received light signal
A minimum addition of noise to the signal
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