 has been shifted to the 1550-nm region. Such fiber, of course, is more costly, but the cost may be worth it. It is another tradeoff. The designer must select the most economic alternatives among the following factors: Fiber parameters: single mode or multimode; if multimode, step index or graded index; number of fibers, cable makeup, strength Transmission wavelength: 820 nm, 1330 nm, or 1550 nm Source type: LED or semiconductor laser; there are subsets to each source type Detector type: PIN or APD Use of EDFA (amplifiers) Repeaters, if required, and how they will be powered Modulation: probably intensity modulation (IM), but the electrical waveform entering the source is important; possibly consider Manchester coding 9.4.9.2 Loss Design. As a first step, assume that the system is power-limited. This means that our principal concern is loss. Probably a large number of systems being installed today can stay in the power-limited regime if monomode fiber is used with semiconductor lasers (i.e., LDs or laser diodes). When designing systems for bit rates in excess of 600 to 1000 Mbps, consider using semiconductor lasers with very narrow line widths (see Figure 9.33) and dispersion-shifted monomode fiber. Remember that "dispersion shifting" shifts the zero dispersion window from 1300 to 1550 nm. 9.4 FIBER-OPTIC COMMUNICATION LINKS For a system operating at these high bit rates, even with the attribute of monomode fiber, chromatic dispersion can become a problem, particularly at the desirable 1550-nm band. Chromatic dispersion is really a form of material dispersion described earlier. It is the sum of two effects: "material dispersion" and waveguide dispersion. As one would expect, with material dispersion, different wavelengths travel at different velocities of propagation. This is true even with the narrow line width of semiconductor lasers. Wave- guide dispersion is a result of light waves traveling through single-mode fibers that extend into the cladding. Its effect is more pronounced at the longer wavelengths becau