 window, then the channels will effectively overlay each other in DWDM systems. Specifically, a problem called four- wave mixing creates "sidebands" that interfere with the DWDM chan- nels, destroying their integrity. In response, fiber manufacturers have created nonzero dispersion-shifted fiber (NZDSF) that lowers the disper- sion point to near zero and makes it occur just outside of the 1,550 nm window. This eliminates the nonlinear four-wave mixing problem. Fiber Nonlinearities A classic business quote, imminently applicable to the optical networking world, observes "in its success lie the seeds of its own destruction." As the marketplace clamors for longer transmission distances with minimal amplification, more wavelengths per fiber, higher bit rates, and increased signal power, a rather ugly collection of transmission impairments, known as fiber nonlinearities, rises to challenge attempts to make them happen. These impairments go far beyond the simple concerns brought about by loss and dispersion; they represent a significant performance barrier. The "special relationship" that exists between transmission power and the refractive index of the medium gives rise to four service-affecting optical nonlinearities: self-phase modulation (SPM), cross-phase modu- lation (XPM), four-wave mixing (FWM), and intermodulation. Self-Phase Modulation (SPM) When SPM occurs, chromatic dispersion kicks in to create something of a technological double whammy. As the light pulse moves down the fiber, its leading edge increases the refractive index of the core, which causes 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 Transport Technologies a shift toward the longer wavelength, blue end of the spectrum. The trail- ing edge, on the other hand, decreases the refractive index of the core, causing a shift toward the shorter wavelength, 