ptical fiber. Because an LED emits incoherent light over a broad spectrum, its coupling loss is higher than that when a laser is used as a light source. Thus, the spectral width governs the effec- tive signal bandwidth since a larger spectral width requires a larger por- tion of the optical fiber bandwidth. This is illustrated in Figure 4.7, where the bandwidths of a typical LED and laser diode are compared. The area under each curve shown in Figure 4.7 represents the optical power generated by each device. Although our first inclination is to note that the LED generates more power than a laser diode does, as a famous radio announcer would say, "That's part of the story." This is because the spectral width, which is the half-power spread, results in a much higher coupling loss when an LED is used. In an attempt to limit coupling loss, LEDs may include a focusing lens bonded to the semicon- ductor and, as noted previously, can also result in the bonding of the LED to a fiber. However, for most applications the use of LEDs requires a Chapter Four Figure 4.7 Comparison of the spectral width of an LED and laser diode. Laser diode Wavelength thicker diameter fiber and results in a maximum drive distance of a few kilometers. In addition, because the LED does not instantly generate and stop producing light in tandem with applying and removing a forward bias, the obtainable data transmission rate is also limited. LEDs are com- monly used for data rates under 20 Mbits/s even though they can oper- ate at a data rate up to approximately 300 Mbits/s. Modulation Methods As with analog modems, more than one method can be used to convey information via an optical fiber. Different methods that were developed for the modulation of information include intensity modulation, fre- quency-shift keying, phase-shift keying, and polarization modulation. Because of the short drive distance associated with the use of LEDs and the simplicity of intensity modulation, this technique is used by the transmitter section of an LED that receives a