There are three categories of optical fiber, as distinguished by their modal and physical properties:
1. Step index (multimode)
2. Graded index (multimode)
3. Single mode (also called monomode)
Step-index fiber is characterized by an abrupt change in refractive index, and graded index is characterized by a continuous and smooth change in refractive index (i.e., from n1 to n2). Figure 9.27 shows the fiber construction and refractive index profile for stepindex fiber (Figure 9.27a) and graded-index 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.
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