taking into account connectors, patch panels, bending, and margins for aging, the amplifier is able to compensate a span of 86 km with its 26 dB of overall gain. If we imagine a DCF with a dispersion parameter D = =-72.5ps/nm/km and a loss of 0.5 dB/km, we see that to compensate the dispersion of 86km SSMF with D = 17 ps/nm/km, we need km 20 of DCF, whose loss is exactly -10 dB. 3. An interstage loss of -13 dB, allowing to allocate also an OADM in the amplifier site to construct a branching point. In the first case, the optimum value of G2 is really small and it is a very good solution to use a single-stage amplifier before the concentrated loss. In both the other cases, the concentrated loss is sufficiently high to justify a second stage with a relevant gain. In condition of very high defect parameter (when the approximations Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices we have done are at the working limit), the second stage has to gain more than the first, inverting what happens in the ideal case. 4.3.2 Erbium-Doped Fiber Amplifiers The optical amplifier most used in telecommunication systems is a fiber amplifier: the erbium-doped fiber amplifier (EDFA) [48,49]. This amplifier uses as gain medium a fiber optics doped with erbium ions that are excited via optical pumping. The use of a fiber optic as amplification medium allows the pump and the signal fields be confined in a very small area (on the order of 60 um ², thus achieving high pump power density, which is a condition for a good quantum efficiency, and a very good matching between the area in which population inversion and the signal mode distribution happen. All this elements cooperate to increase the gain and reduce the noise. In order to amplify signals in the third fibers transmission window (i.e., around 1550 mm), the dopant has to have a suitable transition and must be compatible with the glass matrix. The first condition allows stimulated emission at the chosen wavelength, the second is key to manufacture dope