 be reduced by using a graded-index fiber; however, this type of fiber has relatively little if any commercial deployment. With this kind of fiber, high-angle rays, with a longer physical distance to travel, spend more time in regions of the silica/glass with a lower index of refrac- tion where the speed of light is faster. This compensating effect gives all rays almost the same transit time; the result is that graded-index fiber has a much higher information capacity than step-index fiber. The compensa- tion is never perfect, and a practical graded-index multimode optical fiber still cannot carry large data rates more than a few kilometers before dis- tortion makes data unintelligible. Furthermore, it is more difficult to man- ufacture this type of fiber. 2.1.4 Chromatic Dispersion Even in a single-mode fiber, all energy in a pulse does not reach the remote end at the same time. This phenomenon is called chromatic dispersion. Chromatic dispersion arises because, although there is only one mode, the light pulse is usually composed of a small spectrum of wavelengths, and dif- ferent wavelengths travel at different speeds in fiber. Chromatic dispersion is defined as the slope of the transit time versus the wavelength curve for a given fiber substance. Chromatic dispersion limits the bandwidth of single-mode fibers. It results from the wavelength dependence on the velocity at which the opti- cal signal travels, as shown in Figure 2-4. Optical rays with different wave- lengths travel along fiber at different velocities in different media. Because the optical sources (LDs) used produce signals over a range of wavelengths (for example, from LD chirping) and because each wavelength component of the pulse travels at a slightly different velocity, the pulse spreads in time as it travels down the fiber. In the visible spectrum of light, material disper- sion causes longer wavelengths to travel faster than shorter ones (see Fig- ure 2-12). However, in the near-infrared region around wavelengths of 1.1 Chapter 2 Figure 2-12 