optical amplification becomes necessary as follows. First, we would reduce the 20 dBm of allowable light by 0.10 dBm X 2 for the couplers on each end. Thus, we have 19.8 dBm of adjusted allow- able attenuation. Since the cable attenuation is 0.20 dBm/km, this means that the permissible long-haul distance becomes 19.8 dBm, which at 0.20 dBm/km, results in 99 km. Now that we have an appreciation for the major components associated with an optical transmission system and how they can govern the obtain- able transmission distance before optical amplification becomes necessary, let's devote the remainder of this chapter to identifying the primary com- ponents of an optical transmission system other than the optical cable that was discussed in Chapter 3. Light Sources As mentioned earlier in this chapter, two basic types of devices are used to convert electronic signals into light signals for fiber-optic transmis- sion: light-emitting diodes (LEDs) and lasers. In this section we will examine each device separately. Light-Emitting Diodes The light-emitting diode (LED) represents a semiconductor device that emits incoherent optical radiation in the form of electromagnetic waves Light Sources and Detectors when biased in the forward direction. Optical radiation in the form of light is referred to as "incoherent" as it radiates in all directions from the surface of an LED. This is illustrated in Figure 4.2, which shows how light generated by an LED flows in many directions. Although we will discuss LED operation in detail in the next subsec- tion, we can note several interesting general operational characteristics of an LED-based optical transmission system: (1) because of the dispersion of light, we need to use a thicker core fiber; (2) because of light disper- sion, it is extremely important to attempt to couple the LED as best as possible to the fiber to minimize light loss; and (3) because of the inco- herent optical radiation, the transmission distance associated with LEDs is considerably less than that obtainable wit