Optical Communications, 2nd Edition

1.2 OPTICAL SOURCES, MODULATORS, AND BEAM FORMERS

The key element in any optical communication system is the availability of a light source that can be easily modulated. Such a source should produce energy concentrated in a narrow wavelength band. The primary sources of light in modern optical systems are the light-emitting diodes (LED), the laser, and the laser diode (LD) [2–4]. Although the physical descriptions of these devices are beyond our scope here, their output properties and characteristics will be important in assessing the performance when used in an optical communication system.

An LED is formed from semiconductor junctions that interact when subjected to external current so as to radiate light energy, as indicated by the diagram in Figure 1.7a. A detailed theory of band energy is needed to describe this interaction and is not pursued here. The choice of the junction materials determines the emitted wavelength. Light-emitting diodes are typically formed from compounds of gallium arsenide, and produce light in the 0.8–0.9-μm wavelength bands. An LED is small in size (centimeters), relatively inexpensive, and can produce radiation with low-current drive levels. However, they are limited in output power (1 to 10mW), and the emitted light tends to be unfocused. Table 1.1 summarizes these basic characteristics.

Figure 1.7. Light sources. (a) Light emitting diode (LED), ( ...