Page 208 - Fiber Optic Communications Fund

5

Optical Receivers

5.1 Introduction

In the past few decades, there have been tremendous advances in optoelectronic integrated circuits (OEICs),
primarily because of their widespread use in optical communication systems. Among OEICs, some of the
key drivers have been high performance, low cost, and small size of photoreceivers. And in photoreceivers
and optical receivers, the photodetector and preamplifiers are critical components. The photodetector’s
function is to convert light (photons) or radiant energy into charge carriers, electrons and holes, which can
then be processed, stored, or transmitted again [1]. Further, a monolithically integrated photoreceiver has
several advantages–low parasitics, compact size, and low cost. To date, various designs and structures of
photodetectors, transistors, and integrated circuits have been used to produce high-performance integrated
photoreceivers. In the design of integrated photoreceivers, various devices and circuit parameters are
involved. To obtain the best possible photoreceiver performance, the parameters of both the photodetector
and the preamplifier should be optimized. Therefore, we concentrate on describing some important pho-
todetector structures and optical receivers. An example of a typical optical detection system is shown in
Fig. 5.1, [1–4]. In an optical communication system, the photodetector can be configured either as a direct
or incoherent detector, or as a coherent detector.
In direct or incoherent detection, the “direct” detector converts the incident radiation into an electrical
signal (sometimes called the photo-signal) that is proportional to the power of the incident light. There is
no phase or frequency information and the photo-signal is then processed electronically using a low-noise
preamplifier followed by signal processing circuits. The preamplifier should have very low noise and wide
enough bandwidth to accurately reproduce the temporal characteristics of the input signal, which may be a
10 or 40 Gb/s pulse stream. Minimization of noise in an optical direct detection system is a critical issue. In
particular, the various sources of noise from the background, the photodetector itself, biasing resistors, and
other additional noise sources such as the signal processing circuits must be minimized if the optical detection
system is to have an acceptable signal-to-noise ratio and low bit-error rates for a given input signal power.
A coherent detector, in contrast, is one in which the output electrical signal is related to the phase of the
input as well as the input power. The coherent detector requires a local oscillator whose phase is “locked”
onto the phase of the received signal or the phase difference between the two should be corrected dynamically
using digital signal processing (DSP). More details on these two types of photodetection system will be
presented later.
In this chapter, we will discuss various types of photodetector. We will describe photodetectors without
internal gain, such as pn photodiodes, pin photodetectors (pin-PDs), Schottky barrier photodetectors, and

Fiber Optic Communications: Fundamentals and Applications, First Edition. Shiva Kumar and M. Jamal Deen.
© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.