Page 117 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
7

Photodiodes and Receivers

Since somebody has to listen to what is being said, at the end of an optical trans-
mission 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 sophisti-
cated 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 photode-
tectors 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.

7.1. The pin Photodiode
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 cor-
respondingly varying electric current. Since the optical signal generally is weak-
ened 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
■ A fast response speed to handle the desired data rate

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