cond major impairment phenomenon. Two significant nonlinearities result: Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS). 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 Stimulated Brillouin Scattering (SBS) SBS is a power-related phenomenon. As long as the power level of a transmitted optical signal remains below a certain threshold, usually on the order of three milliwatts, SBS is not a problem. The threshold is directly proportional to the fiber's effective area, and because dispersion- shifted fibers typically have smaller effective areas, they have lower thresholds. The threshold is also proportional to the width of the origi- nating laser pulse: As the pulse gets wider, the threshold goes up. Thus, steps are often taken through a variety of techniques to artificially broaden the laser pulse. This can raise the threshold significantly, to as high as 40 milliwatts. SBS is caused by the interaction of the optical signal moving down the fiber with the acoustic vibration of the silica matrix that makes up the fiber. As the silica matrix resonates, it causes some of the signal to be reflected back toward the source of the signal, resulting in noise, signal degradation, and a reduction of overall bit rate in the system. As the power of the signal increases beyond the threshold, more of the signal is reflected, resulting in a multiplication of the initial problem. It is interesting to note that there are actually two forms of Brillouin Scattering. When (sorry, a little more physics) electric fields that oscillate in time within an optical fiber interact with the natural acoustic reso- nance of the fiber material itself, the result is a tendency to backscatter light as it passes through the material. This is called Brillouin Scatter- ing. If, however, the electric fields 