in the same window. However, minimal dispersion has a rather severe, unintended 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 consequence when it occurs in concert with DWDM: Because it reduces dispersion to near zero, it also prevents multichannel systems from exist- ing because it does not allow proper channel spacing. Four-wave mixing, then, becomes a serious problem. Several things can reduce the impact of FWM. As the dispersion in the fiber drops, the degree of four-wave mixing increases dramatically. In fact, it is worst at the zero-dispersion point. Thus, the intentional inclu- sion of a small amount of chromatic dispersion actually helps to reduce the effects of FWM. For this reason, fiber manufacturers sell NZDSF, which moves the dispersion point to a point near the zero point, thus ensuring that a small amount of dispersion creeps in to protect against FWM problems. Another factor that can minimize the impact of FWM is to widen the spacing between DWDM channels. This, of course, reduces the efficiency of the fiber by reducing the total number of available channels and, therefore, is not a popular solution, particularly since the trend in the industry is to move toward narrower channel spacing as a way to increase the total number of available channels. Already, several vendors have announced spacing as narrow as 5 GHz. Finally, large effective area fibers tend to suffer less from the effects of FWM. Intermodulation Effects In the same way that cross-phase modulation results from interference between multiple simultaneous signals, intermodulation causes sec- ondary frequencies to be created that are cross-products of the original signals being transmitted. Large effective area fibers can alleviate the symptoms of intermodulation. Scattering Problems Scattering within the silica matrix causes the se