n structures. They do differ in the way they emit light and in their performance characteristics. An LED is a forward-biased p-n junction that emits light through spontaneous emission, a phenomenon referred to as electroluminescence. LDs emit light through stimulated emission. LEDs are less efficient than LDs but are considerably more economical. They also have a longer operational life. The emitted light of an LED is incoherent with a relatively wide spectral line width (from 30 nm to 60 nm) and a relatively large angular spread, about 100°. On the other hand, a semiconductor laser emits a comparatively narrow line width (from <2 nm to 4 nm). Figure 9.29a shows the spectral line for an LED, and Figure 9.29b shows the spectral line for a semiconductor laser (i.e., a laser diode or LD). What is a spectral line? Many of us imagine that if we were to view a radio carrier (without modulation) on an oscilloscope, it would be a vertical line that appeared to be of infinitely narrow width. This thinking tends to be carried into the world of light in 9.4 FIBER-OPTIC COMMUNICATION LINKS Figure 9.29 Spectral distribution (line width) of the emission from (a) an LED and (b) a semiconductor laser (LD), where a is the optical wavelength and Ai is the spectral or line width. a fiber-optic light-guide. In neither case is this exactly true. The emission line or light carrier has a finite width, as does a radio carrier. The IEEE (Ref. 6) defines spectral width, full-width half-maximum as "The absolute difference between the wavelengths at which the spectral radiant intensity is 50% of the maximum." With present technology the LED is capable of launching about 100 uW 10 dBm) or less of optical power into the core of a fiber with a numerical aperture of 0.2 or better. A semiconductor laser with the same input power can couple up to 7 mW (+8.5 dBm) into the same cable. The coupling efficiency of an LED is on the order of 2%, whereas the coupling efficiency of an LD (semiconductor laser) is better than 50%. Methods of coupling a sour