Chapter 4
Optical Receivers
The role of an optical receiver is to convert the optical signal back into electrical form
and recover the data transmitted through the lightwave system. Its main component is
a photodetector that converts light into electricity through the photoelectric effect. The
requirements for a photodetector are similar to those of an optical source. It should
have high sensitivity, fast response, low noise, low cost, and high reliability. Its size
should be compatible with the fiber-core size. These requirements are best met by pho-
todetectors made of semiconductor materials. This chapter focuses on photodetectors
and optical receivers [1]–[9]. We introduce in Section 4.1 the basic concepts behind the
photodetection process and discuss in Section 4.2 several kinds of photodetectors com-
monly used for optical receivers. The components of an optical receiver are described
in Section 4.3 with emphasis on the role played by each component. Section 4.4 deals
with various noise sources that limit the signal-to-noise ratio in optical receivers. Sec-
tions 4.5 and 4.6 are devoted to receiver sensitivity and its degradation under nonideal
conditions. The performance of optical receivers in actual transmission experiments is
discussed in Section 4.7.
4.1 Basic Concepts
The fundamental mechanism behind the photodetection process is optical absorption.
This section introduces basic concepts such as responsivity, quantum efficiency, and
bandwidth that are common to all photodetectors and are needed later in this chapter.
4.1.1 Detector Responsivity
Consider the semiconductor slab shown schematically in Fig. 4.1. If the energyhνof
incident photons exceeds the bandgap energy, an electron–hole pair is generated each
time a photon is absorbed by the semiconductor. Under the influence of an electric field
set up by an applied voltage, electrons and holes are swept across the semiconductor,
resulting in a flow of electric current. The photocurrentIpis directly proportional to