ABSTRACT
Semiconductor light sources, either light-emitting diodes (LEDs) or laser diodes (LDs), are ideal sources for fiber optics because of their small size. Light from LEDs consists of spontaneous emission (or amplified spontaneous emission) and is therefore incoherent, whereas light from LDs results from stimulated emission and is much more coherent. The degree of coherence of the light from LDs, in turn, can vary strongly from one type of LD to another. Which degree of coherence is required and hence, which type of light source is preferred, depends strongly on the application. Apart from coherence, there are a number of other properties which are important for application in optical communication. The possibility of obtaining high output power is a paramount object because it offers the prospect of spanning relatively long fiber-optic links without the need for intermediate regeneration. The possibility of high-speed direct modulation (i.e., modulation of the optical output power through the modulation of the injected current) is a significant development because it renders external modulators superfluous. A high-modulation bandwidth obviously allows high bit rates. Other factors of importance are cost, stability, lifetime, and temperature dependence. Furthermore, the noise on the output intensity and on the emission frequency should both be relatively low.
