Introduction

The device slope efficiencies that we developed in Chapter 2, and that were cascaded to form links in Chapter 3, explicitly ignored any frequency dependence. In this chapter we remove that restriction. As we shall see, virtually all modulation and photodetection devices have an inherently broad bandwidth. Digital links require broad bandwidth, which is one of the reasons for the numerous applications of fiber optic links to digital systems. A few analog link applications also require the full device bandwidth. However, it is far more common for analog links to need only a portion of the devices' inherent bandwidth. Consequently most analog link designs include some form of RF pre- or post-filtering to reduce the bandwidth.

For completeness we address bandpass and broad bandwidth impedance matching for three electro-optic devices: PIN photodiode, diode laser and Mach–Zehnder modulator. We then combine the bandpass impedance matched cases to form both direct and external modulation links. However, the same analytical approach is used for both impedance matching methods and both modulation techniques. Therefore those readers desiring a less exhaustive treatment can obtain a complete introduction to the subject by studying only one of the impedance matching methods and one of the modulation techniques.

One may be tempted to ask: why bother with bandwidth reduction, since this adds components and complicates the design? There are at least two key reasons for implementing bandwidth reduction.