 will travel at different speeds, resulting in an overall spreading or smearing of the signal. As discussed earlier, chromatic dispersion comprises two subcategories: material dis- persion and waveguide dispersion. Material Dispersion Simply put, material dispersion occurs because different wavelengths of light travel at different speeds through 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 Chapter 7 an optical fiber. To minimize this particular dispersion phenomenon, two factors must be managed. The first of these is the number of wavelengths that make up the transmitted signal. An LED, for example, emits a rather broad range of wavelengths-between 30 and 180 whereas a laser emits a much narrower spectrum-typically less than 5 nm. Thus, a laser's output is far less prone to be seriously affected by mate- rial dispersion than is the signal from an LED. The second factor that affects the degree of material dispersion is a characteristic called the center operating wavelength of the source signal. In the vicinity of 850 nm, red, longer wavelengths travel faster than their shorter, blue counterparts, but at 1,550 nm, the situation is the opposite: Blue wavelengths travel faster. There is, of course, a point at which the two meet and share a common minimum dispersion level; it is in the range of 1,310 nm, often referred to as the zero-dispersion wavelength. Clearly, this is an ideal place to transmit data signals, since dispersion effects are minimized here. As we will see later, however, other factors crop up that make this a less desirable transmission window than it appears. Material dispersion is a particularly vexing problem in single- mode fibers. Waveguide Dispersion Because the core and the cladding of a fiber have slightly different indices of refraction, the light that travels in the core moves slightly slower 