 depends on the signal traveling through it. To make an example, let us consider our model of Chapter 4 of the two level amplifiers and let us consider an amplifier with center wavelength at 1550 nm, microscopic satura- tion power of 10 mW microscopic bandwidth 155 nm and microscopic gain 5m-1. With these parameters the linear gain is 21 dB and the macroscopic input saturation power is 0.1 mW. This amplifier is in deep saturation if a signal of 0.5mW power is injected at the wave- length of 1540 nm, as a matter of fact the gain of the signal is 16.7 dB. In Figure 6.36, the gain of the signal at 1540 nm is shown versus the power of a second signal at 1460nm, thus quite far from the first. It is evident that, increasing the second signal input power, the gain of the first signal decreases, up to compress greatly when the second is more powerful. This behavior is due to the fact that, even if far from a spectral point of view, both the sig- nals leverage on the same population of optically activated molecules; thus, they enter into competition inside the amplifier and the presence of the second signal de facto reduces the inverted population on which the gain of the first is built. This phenomenon naturally also changes the spectral behavior of the macroscopic gain, depending on the power distribution of the input channels. In Figure 6.37, the spectral gain of the amplifier is shown for different values of the two injected signals and it is clear that the percentage difference between the gain peak and the value, for example, at 1550 nm changes greatly, passing from 7.7 to 3 dB. This behavior, even if obtained from a greatly simplified model, well represents what happens with EDFA amplifiers. Since when a ring is deployed, it is not a priori known what will be the order in which the channels will be switched on and what will be the route of each channel; a static gain flattening cannot be used in optical rings. Two strategies are available to face this problem: either the use of a dynamically adjustable flattening f