d here. The relatively new L-Band has enjoyed some early success as the next effective operating window. Notice also that Rayleigh Scattering is shown to occur at or around 1,000 nm, while hydroxyl absorption by water occurs at 1,240 and 1,390 nm. Needless to say, network designers would be well-served to avoid transmitting at any of the points on the graph where Rayleigh Scatter- ing, high degrees of loss, or hydroxyl absorption have the greatest degree of impact. Notice also that dispersion, shown by the lower line, is at a minimum point in the second window, while loss, shown by the upper line, drops to a minimum point in the third window. In fact, dispersion is minimized in traditional single-mode fiber at 1,310 nm, while loss is at minimums at 1,550 nm. So the obvious question becomes this: Which one do you want to minimize-loss or dispersion? Luckily, this choice no longer has to be made. Today, dispersion-shifted fibers (DSF) have become common. By modifying the manufacturing 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 process, engineers can shift the point at which minimum dispersion occurs from 1,310 nm to 1,550 nm, causing it to coincide with the minimum loss point such that loss and dispersion occur at the same wavelength. Unfortunately, while this fixed one problem, it created a new and potentially serious alternative problem. DWDM has become a mainstay technology for multiplying the available bandwidth in optical systems. When DWDM is deployed over dispersion-shifted fiber, serious non- linearities occur at the zero dispersion point, which effectively destroy the DWDM signal. Think about it: DWDM relies on the ability to "chan- nelize" the available bandwidth of the optical infrastructure and main- tain some degree of separation between the channels. If dispersion is minimized in the 1,559 nm