Fiber Optic Communications: Fundamentals and Applications

Chapter 4Optical Modulators and Modulation Schemes

4.1 Introduction

To convey a message, the amplitude, frequency, and phase of an optical carrier are switched in accordance with the message data. For example, bits ‘1’ and ‘0’ can be transmitted by turning a laser diode on and off, respectively. Typically, the message signal is in the form of binary data in an electrical domain, and optical modulators are used to convert the data into an optical domain. Sections 4.2 to 4.5 review the various line coders, pulse shapes, and digital modulation schemes. Sections 4.6 and 4.7 deal with different types of optical modulators and generation of modulated signals using optical modulators. The benefit of adding a controlled amount of ISI is discussed in Section 4.8. Section 4.9 deals with multi-level signaling, which enables higher transmission data rates without having to increase the bandwidth.

4.2 Line Coder

Digital data can be represented by electrical waveforms in a number of ways. This process is called line coding. In the binary case, bit ‘1’ is sent by transmitting a pulse and bit ‘0’ is sent by transmitting no pulse. This line code is known as unipolar or on–off, as shown in Fig. 4.1(a). If a bit ‘1’ and bit ‘0’ are represented by and , respectively, such a line code is polar ...