nd high output saturation power at the expense of a high NF or obtaining lower G and Ps but with a much lower noise power. Moreover, when we have a loss in an amplification chain, we have to take into account that the overall shot noise does not change, SO that the traveling signal SN degrades. From a quantum point of view, every loss is like a four-way splitter that takes away from the drop port a part of the incoming signal and adds from the add port a proportional amount of vacuum state [44]. This model is represented in Figure 4.20 and has to be taken into account when analyzing the noise in multistage amplifiers. In order to optimize the amplifier with respect to the noise after detection, which is the important element in a communication system, we have to evaluate the SN at the optical Active Active medium medium Intrastage Power supply Control electr. FIGURE 4.19 Block scheme of a two-stage amplifier system with an interstage loss. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices (1-a)le)+al0) power FIGURE 4.20 Quantum scheme of a concentrated loss: it is equiva- lent to a quantum splitter eliminating part of the sig- (1-a) |0) nal and adding an equivalent part from the vacuum Field after the concentrated loss state. level. As a matter of fact, as we will see in Chapter 6, it is possible generally to express the error probability as a function of this parameter. The LP electrical field can be written as (4.78) where the random processes Hi(t, L) represent the noise terms at the output of the three sec- tions of our multisection amplifier and a the inter-stage loss. Thus, evaluating the noise term power, we get (4.79) where is the bilateral noise bandwidth. This means that if the noise bandwidth is deter- mined by a filter whose passband is W, it is = W. From Equation 4.79, it is clear that the expression of the overall noise factor of the two- stage amplifier is mainly dependent on the noise of the first stage, at least as far as the interstage loss is not SO high to over