The refracted ray hits the core-cladding interface and is refracted again. It represents the light-gathering capacity of an optical fiber. However, such fibers are not useful for the purpose of optical communications because of a phenomenon known as multipath dispersion, also called intermodal dispersion. From the figure 1.5, we can understand multipath dispersion as different rays travel along paths of different lengths. As a result these rays disperse in time at the output end of the fiber even if they were coincident at the input end and traveled at the same speed inside of the fiber.
L is the length of the fiber.
BL This only gives us a rough estimate of a fundamental limitation of step-index fibers. Such fibers can communicate data at a bit rate of 10Mbps over distances up to 10km and may be suitable for some local area networks. If we compare this inequality to the previous one, we can easily verify that the BL of such fibers is improved by nearly three orders of magnitude compared with the step-index fibers. And as matter fact, the first generation of lightwave systems used graded-index fibers. Further improvement is possible only by using single-mode fibers.