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Figure 7-22 Erbium-doped fiber amplifier (EDFA)
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In optical amplifiers, this impurity is more often than not an element called erbium. Its role will become clear in just a moment. The pump laser shown in the upper left corner of Figure 7-23 generates 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, transmitted 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 prevent 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, therefore, still be a need at some point along the path of long-haul systems for regeneration, although far less frequently than in traditional copper systems. 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 transmission facility.
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Transport Technologies
Transport Technologies
Figure 7-23 The pump laser in the upper left corner injects light at the opposite direction of the transmitted optical signal. Bottom right, an actual EDFA.
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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 silica matrix that in turn emit photons that amplify the signal. The advantage of Raman amplification is that it requires no special doping: Erbium is not necessary. Instead, the silica itself gives off the necessary amplification. 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 amplification devices in some installations. In semiconductor optical amplifiers, the weakened optical signal is pumped into the ingress edge of a semiconductor optical amplifier. The active layer of the semiconductor substrate amplifies the signal and regenerates it on the other side. The primary downside to these devices is their size: They are small, and
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