etely diminished residue from the manufacturing process. These impurities tend to absorb the energy of the transmitted signal and convert it to heat, resulting in an overall weakening of the optical signal. Hydroxyl absorption occurs at 1.25 and 1.39 u; at 1.7 u, the silica itself starts to absorb energy because of the nat- ural resonance of silicon dioxide. Dispersion As mentioned earlier, dispersion is the optical term for the spreading of the transmitted light pulse as it transits the fiber. It is a bandwidth- limiting phenomenon and comes in two forms: multimode dispersion, and chromatic dispersion. Chromatic dispersion is further subdivided into material dispersion and waveguide dispersion. Multimode Dispersion To understand multimode dispersion, it is first important to understand the concept of a mode. Figure 7-19 shows a fiber with a relatively wide Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies Figure 7-19 Multimode fiber: Note wide core diameter. Figure 7-20 Single-mode fiber: Note narrow core diameter. core. Because of the width of the core, it allows light rays arriving from the source at a variety of angles (three in this case) to enter the fiber and be transmitted to the receiver. Because of the different paths that each ray, or mode, will take, they will arrive at the receiver at different times, resulting in a dispersed signal. Now consider the system shown in Figure 7-20. The core is much nar- rower and only allows a single ray, or mode, to be sent down the fiber. This results in less end-to-end energy loss and avoids the dispersion problem that occurs in multimode installations. Chromatic Dispersion The speed at which an optical signal travels down a fiber is absolutely dependent upon its wavelength. If the signal comprises multiple wave- lengths, then the different wavelengths