are caused by the optical signal itself, the signal is seen to cause the phenomenon; this is called Stimulated Brillouin Scattering. To summarize: Because of backscattering, SBS reduces the amount of light that actually reaches the receiver and causes noise impairments. The problem increases quickly above the threshold and has a more dele- terious impact on longer wavelengths of light. One additional fact: In-line optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) add to the problem significantly. If there are four optical amplifiers along an optical span, the threshold will drop by a factor of four. Solutions to SBS include the use of wider-pulse lasers and larger effec- tive area fibers. 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 Stimulated Raman Scattering (SRS) SRS is something of a power-based crosstalk problem. In SRS, high- power, short-wavelength channels tend to bleed power into longer wave- length, lower power channels. It occurs when a light pulse moving down the fiber interacts with the crystalline matrix of the silica, causing the light to (1) be backscattered and (2) shift the wavelength of the pulse slightly. Whereas SBS is a backward-scattering phenomenon, SRS is a two-way phenomenon, causing both backscattering and a wavelength shift. The result is crosstalk between adjacent channels. The good news is that SRS occurs at a much higher power level- close to a watt. Furthermore, it can be effectively reduced through the use of large effective area fibers. Optical Amplification As long as we are on the subject of Raman Scattering, we should intro- duce the concept of optical amplification. This may seem like a bit of a non sequitur, but it really isn't; true optical amplification actually uses a form of Raman Scattering to amplify the transmitted signal! Traditional Amplificati