at Optical Doped Optical input the opposite Isolator Fiber Isolator signal or direction of the signals transmitted optical signal. Bottom right, an actual EDFA. Other Amplification Options There are at least two other amplification techniques in addition to EDFAs that have recently come into favor. The first of these is called Raman amplification, which is similar to EDFA in the sense that it relies on Raman effects to do its task, but different for other rather substantial reasons. In Raman amplification, the signal beam travels down the fiber alongside a rather powerful pump beam, which excites atoms in the sil- ica matrix that in turn emit photons that amplify the signal. The advan- tage of Raman amplification is that it requires no special doping: Erbium is not necessary. Instead, the silica itself gives off the necessary amplifi- cation. In this case, the fiber itself becomes the amplifier! Raman amplifiers require a significantly high-power pump beam (about a watt, although some systems have been able to reduce the required power to 750 mw or less) and even at high levels the power gain is relatively low. Their advantage, however, is that their induced gain is distributed across the entire optical span. Furthermore, it will operate within a relatively wide range of wavelengths, including 1,310 and 1,550 nm, currently the two most popular and effective transmission windows. Semiconductor lasers have also been deployed as optical amplifica- tion devices in some installations. In semiconductor optical amplifiers, the weakened optical signal is pumped into the ingress edge of a semi- conductor optical amplifier. The active layer of the semiconductor sub- strate amplifies the signal and regenerates it on the other side. The primary downside to these devices is their size: They are small, and 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. Tr