ifiers, and Passive Devices From a qualitative point of view, this is an effect similar to Kerr nonlinearity in fibers and the related effects take the same name: self and cross phase modulation, FWM, and SO on. Naturally, since the amplifier dynamics is different from the fiber Kerr effect, the quanti- tative description of these phenomena will be different. We will present the results of this kind of analysis talking about semiconductor optical amplifiers in Chapter 10. 4.3.1.4 Amplifiers Functional Classification and Multistage Amplifiers In practice, an amplifier for use in telecommunication rarely is composed of the bare active medium and the pump: it is a complex system. An example of practical amplifier scheme is provided in Figure 4.18 [47]. Besides the pumped amplifying medium, the amplifier includes at least an isolator and a filter at the output; the isolator is needed to avoid the reflections from the line entering into the amplifier changing its behavior, the filter is needed to limit the ASE power propagating along the line and entering in the successive amplification stages. Depending on the position they occupy in a transmission system, amplifiers are special- ized as follows: Buster amplifiers: They are used generally after the receiver to enhance the power sent along the line; they are built to have a high saturation power and to amplify efficiently a relatively high input power signal coming out from the transmitter; in these amplifiers, the linear gain is not SO high since the noise factor NF has to be maintained within low values and the key parameter is the saturation power. Preamplifiers: They are generally used in front of the receiver; thus, they have to be designed with a very low noise factor, moreover, having a low input power and working generally in high saturation to provide stabilization of the signal arriving to the receiver, the saturation power can be lower than in booster amplifiers. The gain should not be huge, but has to be sufficient to send to the receiver a signal power hi