requently. Optical amplifiers represent one of the technological leading edges of data networking. Instead of the O-E-O process, optical amplifiers receive the optical signal, amplify it as an optical signal, and then retransmit it as an optical signal-no electrical conversion is required. Like their elec- trical counterparts, however, they also amplify the noise; at some point, signal regeneration is required. Optical Amplifiers: How They Work It was only a matter of time before all-optical amplifiers became a real- ity. It makes intuitively clear sense that a solution that eliminates the electrical portion of the O-E-O process would be a good one; optical amplification is that solution. You will recall that SRS is a fiber nonlinearity that is characterized by high-energy channels pumping power into low-energy channels. What if that phenomenon could be harnessed as a way to amplify optical signals that have weakened over distance? Optical amplifiers are actually rather simple devices that, as a result, tend to be extremely reliable. The optical amplifier comprises the follow- ing: an input fiber, carrying the weakened signal that is to be amplified; a pair of optical isolators; a coil of doped fiber; a pump laser; and the out- put fiber that now carries the amplified signal. A functional diagram of an optical amplifier is shown in Figure 7-22. The coil of doped fiber lies at the heart of the optical amplifier's func- tionality. Doping is simply the process of embedding some kind of func- tional impurity in the silica matrix of the fiber when it is manufactured. 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 Figure 7-22 Erbium-doped LASER fiber amplifier (EDFA) Optical Doped Optical input Isolator Fiber Isolator signal or signals In optical amplifiers, this impurity is more often than not an element called e