ht flows within an optical fiber. Light Flow As discussed previously, the ability to ensure that light flows or propa- gates the length of an optical fiber is based on ensuring that the ray is totally refracted whenever it reaches the core-cladding boundary. This is indicated by the numeral 1 in Figure 3.6. To accomplish this, light rays must be injected into the fiber at an angle less than the critical angle, resulting in total internal reflection (TIR). Figure 3.6 illustrates the potential flow of a light ray within an optical fiber. As noted by the numerals 1, 2, and 3, three potential scenarios can govern a light ray's flow. First, the ray can be injected and focused direct- ly into the core. This requires the use of a laser as the light source and a special type of optical cable referred to as a single-mode cable. The term sin- gle mode reflects the fact that there is no refraction and the pulses that are injected into the fiber travel in one mode. As we will note later in this chapter, the ability to accomplish single-mode light propagation requires the use of a relatively small core, which makes the fiber more expensive to manufacture. A second method of light flow results in the reflection of light at the core-cladding boundary. As indicated by the numeral 2 in Figure 3.6, the rays are reflected when the angle that the ray forms with the boundary is less than the critical angle. This results in total internal reflection. Because the use of light-emitting diodes (LEDs) results in a spread of rays entering an optical fiber, the result is a series of reflected rays that are received at the opposite end of the fiber that "spreads" the original injected light pulse. This situation, referred to as modal dispersion, requires the transmitter to slow its light pulse rate to a pace slower than that of the dispersion at the receiver to allow pulses to be recognized. Thus, the bouncing effect also affects the data transfer rate. As we will discuss later in this chapter, two types of optical fiber allow multiple modes