3 - Low frequency, short length link models

Summary

Introduction

The devices discussed in Chapter 2 are rarely used individually. More commonly a modulation device – either a diode laser or an external modulator – is combined with a photodetection device to form a link. In this chapter we begin to examine the performance of complete links by developing expressions for the gain of a link in terms of the modulation and photodetection device parameters. In subsequent chapters we develop analogous expressions for link frequency response, noise figure and dynamic range.

Recall from Chapter 1 that we defined a link as comprising all the components necessary to convey an electrical signal over an optical carrier. Since the definition of available power requires an impedance match, we expand the link definition slightly to include those passive electrical components needed to impedance match the modulation and photodetection devices to the electrical signal source and load, respectively. The impedance matching function is also required by the definitions of some of the link parameters we will be discussing. A more detailed version of the link block diagram is shown in Fig. 3.1.

Although the models we develop have applicability at any frequency, we choose to focus on relatively low frequencies here where lumped-element RLC passive elements are appropriate. This permits us to get the important concepts across without their being obscured by the myriad detailed effects that microwave models require.