refraction in the core of a fiber. Because different frequency compo- nents have different velocities of propagation in an optical fiber, this results in the broadening of the optical pulse. TRANSMISSION WINDOWS In the transmission properties of glass, there is an important relationship between attenuation and the wavelength of the light source. This relationship is well known and is plotted in Figure 3.10c. If you carefully examine the relationship between attenuation and wavelength shown in Figure 3.10c, you will note three so-called windows where transmission at certain wavelength ranges result in the lowest Chapter Three Transmitter Receiver Figure 3.12 Dispersion in an opti- cal fiber results in the spreading of pulses in the time domain. amount of attenuation. It is extremely important to design an optical transmission system to obtain the lowest possible attenuation. This is because a low level of attenuation permits optical amplifiers to be spaced father apart, in effect reducing the cost of the transmission system. The first generation of optical transmission systems operated in the 800- to 900-nm range. As indicated in the shaded area in the left portion of Figure 3.10c, this is equivalent to the first optical window and repre- sents the use of light-emitting diodes (LEDs). Because the lowest level of attenuation occurs at approximately 850 nm in this window, as you might expect, early optical systems used LEDs operating primarily at that wavelength. The second generation of optical fiber systems use lasers operating in the 1200- to 1300-nm range. As indicated in the shaded area in the mid- dle of Figure 3.10c, this represents the second optical window, in which attenuation is significantly lower than in the first window. Although most currently installed optical transmission systems oper- ate within the second optical window, this is rapidly changing. A third window, which is shown in the shaded area in the lower right of Figure 3.10c to extend from 1500 to 1600 nm, has a very low level of attenuation 