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Transport Technologies
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Optical Amplifiers: How They Work
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It was only a matter of time before all-optical amplifiers became a reality. 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 following: 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 output fiber that now carries the amplified signal. A functional diagram of an optical amplifier is shown in Figure 6-21. The coil of doped fiber lies at the heart of the optical amplifier s functionality. Doping is simply the process of embedding some kind of functional impurity in the silica matrix of the fiber when it is manufactured. 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 6-21 generates a light signal at a particular frequency, often 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, and so on) and then
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Figure 6-21 EDFA.
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Doped Fiber
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Weak input signal or signals
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Transport Technologies
Transport Technologies
falls back down, giving 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 erbiumdoped 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. Therefore, a need will still exist 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.
Other Amplification Options
At least two other amplification techniques in addition to EDFAs 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 is 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 one 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
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