 fiber (Figure 9.27b). Both step-index and graded-index light transmission are characterized as multimode because more than one mode propagates. (Two modes are shown in the figure.) Graded-index fiber has a superior bandwidth-distance product compared to that of step-index fiber. In other words, it can transport a higher bit rate further than step index. It is also more expensive. We can eliminate this cause of dispersion if we use single-mode fiber. Figure 9.28 shows a typical five-fiber cable for direct burial. 9.4.4 Splices and Connectors Optical fiber cable is commonly available in 1-km sections; it is also available in longer sections, in some types up to 10 km or more. In any case there must be some way of 9.4 FIBER-OPTIC COMMUNICATION LINKS Construction Construction Cladding 20-50 um 30-100 um Refractive index Refractive index profile profile Figure 9.27 Construction and refractive index properties for (a) step-index fiber and (b) graded- index fiber. Filling material Graded-index fibers Kevlar Polyurethane jacket Kevlar Polyurethane jacket Coated aluminum Black polyethylene outer jacket 6.73 mm 7.30 mm 9.40 mm Figure 9.28 Direct burial optical fiber cable. connecting the fiber to the source and to the detector as well as connecting the reels of cable together, whether in 1 km or more lengths, as required. There are two methods of connection, namely, splicing or using connectors. The objective in either case is to transfer as much light as possible through the coupling. A good splice couples more light than the best connectors. A good splice can have an insertion loss as low as 0.09 dB, whereas the best connector loss can be as low as 0.3 dB. An optical fiber splice requires CONCEPTS IN TRANSMISSION TRANSPORT highly accurate alignment and an excellent end finish to the fibers. There are three causes of loss at a splice: 1. Lateral displacement of fiber axes 2. Fiber end separation 3. Angular misalignment There are two types of splice now available: the mechanical splice and the fusion splice. With a mechan