gh enough to neglect shot noise and receiver thermal noise; summarizing for preamplifier amplifiers, the key parameter is the noise factor. Fiber in Fiber out FIGURE 4.18 Block scheme of an amplifier system. AM, active medium; P, pump; EC, electronic control; DC, power supply; RS, residual pump; F, filter; I, isolator. Telecommunication Networks Line amplifiers: They are used along the line to compensate propagation losses; moreover, they are also used to compensate losses introduced by in-line opti- cal components like DCFs and optical add drop multiplexers; for these reasons, they must have a limited noise factor and, even more important, a high saturated gain; as a matter of fact, higher saturation gain allows longer spans to be realized, decreasing the transmission system CAPEX cost. In order to achieve this specialized design, generally, the structure of Figure 4.18 is not sufficient and a more sophisticated design is needed. Almost all line amplifiers and several boosters and preamplifiers are in fact multistage amplifiers with two or even three stages. Two-stage amplifiers can be designed in several ways, as an example, a possible scheme is provided in Figure 4.19. From the figure it is clear that a two-stage amplifier is the cascade of two optical ampli- fiers divided by a group of passive components (an isolator, a filter, and, e.g., a DCF) pro- viding an overall interstage loss. In order to analyze the design of such an amplifier, two elements have to be added to the theory we have developed up to now. First of all, as we have already underlined, even if the ideal ASE noise is simply almost proportional to the gain, in a practical amplifier, the noise factor NF depends, among the other things, on the efficiency of pump conversion; thus, it is higher if the gain is increased and if the output power is increased. Under this point of view, we can imagine that by tuning the pump power and the material characteristics (e.g., via doping) different amplifiers can be obtained, for example, achieving high gain a