6 m. Thus, 50/125 indicates that the fiber has a core 50 um in diameter while the cladding diameter is 125 um. Figure 3.7 shows how the core and cladding diameters are specified. As the diameter of the core of a fiber increases, more light can be cou- pled into the fiber from an external source. While this may appear to be welcome news, in reality it can be equivalent to a child let loose in candy store since too much of a good thing can be bad. In other words, too much light can result in saturation of a receiver. Thus, there are con- straints on the size of the core. Because one of those constraints is based on the general category of the optical fiber, let's discuss this topic. Cable Category There are two general categories of optical fiber: single-mode and multi- mode. Because multimode fiber was commercialized prior to single- mode, let's review their characteristics in their order of commercializa- tion. MULTIMODE FIBER Multimode fiber has a much larger core than single-mode fiber; therefore, multimode fiber permits hundreds to thou- sands of rays or modes of light to simultaneously propagate through the fiber. This type of fiber permits light to enter from different angles; as a result, the light bounces along the inside of the fiber, reflected back into the core when it hits the core-cladding boundary. This repeated bounc- ing continues until the light reaches the end of the fiber. The top portion of Figure 3.8 illustrates this light bouncing effect, which results in pulse spreading or modal dispersion. Because modal dis- Figure 3.7 cladding The size of a fiber- optic cable is specified in terms of the cable's X core core X and cladding Y diameters as X/Y in micrometers. Understanding Optical Fiber persion is caused by the difference in the propagation times of light rays, which take different paths along the fiber, a stretched light pulse at the end of the fiber results. That stretched light pulse is reduced in ampli- tude at the output of the fiber, making it more difficult for the receiver to detect. 