te, although it does SO indirectly, which requires a bit of expla- nation. As light flows through a fiber and encounters an impurity, the direction of flow of the photon hitting the impurity will change. This results in an effect referred to as scattering, which spreads the pulse and makes it more difficult to detect at the receiver. Chromatic Dispersion Another factor that limits the pulse rate of optical transmission is chro- matic dispersion, which results from the fact that the speed of an optical pulse traveling in a fiber changes as the pulse's wavelength changes. As photons flow through a fiber, the 2- to 5-nm spectral width of the laser results in a broadening of pulses, especially as photons scatter. The net effect of attenuation, scattering, and chromatic dispersion is illustrated in Figure 2.12. The top portion of the figure illustrates a sequence of "perfect" pulses generated by a laser. The middle sequence illustrates the effect of attenuation on the pulses, while the lower portion of the illus- tration indicates the effect of scattering and chromatic dispersion. In examining Figure 2.12, note that as the transmission distance increases, the pulses become smaller as attenuation increases and wider as the effect of scattering and chromatic dispersion increases. Operating Rates Currently, practical optical transmission systems operate at 10 Gbits/s. However, optical transmission at 40 Gbits/s is expected to become com- mercially available in the near future. For data rates of up to 2.5 Gbits/s, Chapter Two a. Perfect pulses originated at laser. Figure 2.12 The combined effect of attenuation, scat- tering, and chromatic distance dispersion on the flow of light pulses down a fiber. b. Effect of attenuation. distance C. Effect of scattering and chromatic dispersion and attenuation. distance most optical transmission systems use lasers that are switched on and off, representing digital 1s and Os. At higher data rates a modulation contain- ing a crystal lithium niobate waveguide is commonly used with a lase