Chapter 4: Optical Sources for Fibers

4.1 Introduction

Radio systems require electromagnetic energy sources in the frequency range of 1 MHz to 40 GHz, which must be coherent and stable in amplitude (Chapter 1). Recall that the spectrum of coherent sources appears as a single frequency with very little accompanying noise modulation. The spectrum of a noncoherent source is a band of noise, sometimes confined but usually quite broad. Because of their coherence and stability, RF sources can transmit signals through the modulation of their amplitude, phase or frequency. They also provide coherent and stable signals at receivers for subsequent demodulation.

Optical systems, on the other hand, require sources in the range of 100,000 GHz, which is in the spectrum of visible light and the infrared. Currently installed intensity-modulated optical fiber systems use light emitting diodes (LEDs) and lasers for sources. Both are fabricated from layered semiconductors. These light sources are not as coherent or stable as radio frequency oscillators. Light emitting diodes and lasers are usually modulated by varying the injected drive current. This results in an unfortunate characteristic: their performance is influenced by the energy level of the driving signal. However, LEDs provide a very good match for silica fiber systems, because of their low cost, reliability, small physical size and emission wavelengths. This chapter discusses only injection-modulated optical diodes, which are either light emitting diodes or lasers. When improved performance is needed, lasers are modulated externally to the lasing semiconductor cavity (Chapter 7).

The technologies used to obtain radio and optical...