rbium. Its role will become clear in just a moment. The pump laser shown in the upper left corner of Figure 7-23 gener- ates a light signal at a particular frequency-oftentimes 980 nm-in the opposite direction than the actual transmitted signal flows. As it turns out, erbium becomes atomically excited when it is struck by light at that wavelength. When an atom is excited by pumped energy, it jumps to a higher energy level (those of you who are recovering physicists will remember classroom discussions about orbital levels-1S¹, 1S2, 2S¹, 2S², 2P6, etc.), then falls back down, during which time it gives off a photon at a certain wavelength. When erbium is excited by light at 980 nm, it emits photons within the 1,550 nm region - coincidentally the wavelength at which multichannel optical systems operate. So, when the weak, trans- mitted signal reaches the coil of erbium-doped fiber, the erbium atoms, now excited by the energy from the pump laser, bleed power into the weak signal at precisely the right wavelength, causing a generalized amplification of the transmitted signal. The optical isolators serve to pre- vent errant light from backscattering into the system, creating noise. EDFAs are highly proletariat in nature: They amplify anything, including the noise that the signal may have picked up. There will, there- fore, still be a need at some point along the path of long-haul systems for regeneration, although far less frequently than in traditional copper sys- tems. Most manufacturers of optical systems publish recommended span engineering specifications that help service providers and network designers take such concerns into account as they design each transmis- sion facility. 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 Figure 7-23 LASER The pump laser in the upper left corner injects light 