adding The shortened path and higher speed obtainable because of the lower index toward the cladding results in the series of rays arriving at the receiver at approximately the same time. The lower portion of Figure 3.8 illustrates the flow of light through a multimode graded-index fiber. This type of fiber is more costly to man- ufacture than a step-index fiber. Popular graded-index fiber-optic cables have core diameters of 50, 62.5, and 85 um, with a cladding diameter of 125 um. The 62.5/125 fiber-optic cable is one of the most popular fibers used in data communications applications. Because a multimode grad- ed-index fiber minimizes modal dispersion, it supports a higher data transmission rate than does a multimode step-index fiber. Although multimode fiber is used primarily in LAN applications, this was not always the case. As communications carriers originally developed an optical backbone in the late 1970s or early 1980s, they first turned to the use of multimode fiber. However, because single-mode fiber supports much higher transmission rates, it is now the preferred medium for backbone and long-haul applications. SINGLE-MODE FIBER As mentioned above, a second general catego- ry of fiber is single-mode fiber. This type of fiber has a much smaller core diameter and allows only one mode of light at a time to propagate through the core; for this reason, there is no modal dispersion or pulse spreading. Thus, an optical signal can be transmitted over longer dis- tances. In addition, because of the lack of pulse spreading, the optical transmitter can reduce the gaps between pulses necessary when multi- mode fiber is used. This allows the use of a faster light source, which in turn supports a higher operating rate. Understanding Optical Fiber Figure 3.9 illustrates the transmission of a light ray through a single- mode fiber. The core of a single-mode fiber is typically 8 to 10 um, which means that it is more expensive to manufacture than multimode fiber. The outer cladding diameter of most single-mode fibers is 