c nonlinearity is that, when the pulse is present, the gain is heavily saturated and the output pulse is practically equal to the input one, while when there is practically no signal, the gain is high and the small residue power present when a zero is transmitted is amplified. In the case of the figure, that is quite extreme to better show the effect, the energy trans- mitted in the zero intervals is approximately multiplied by three, but the "one" energy increases only of 7%. The signal dynamic ratio (energy of a one divided energy of a zero) is thus reduced from 9.5 to 6.0 dB, causing a great degradation of the performances of a receiver that has to detect the signal and extract the binary sequence. A final effect that we would like to observe is the interference between frequency multi- plexed channels due to gain dynamics. This phenomenon is illustrated in the example of Figure 4.17 where two signals at differ- ent frequencies that are 10 GHz distant are injected into the amplifier; both the signals are amplitude modulated with a train of pulses at 100 Mbit/s. The plot reported in the figure shows the instantaneous power of the optical wave after narrowband filtering around the shortest wavelength signal. In Figure 4.17a, the result is reported without the second wavelength, and in Figure 4.17b, the second wavelength is present. The interference due to the dynamic ampli- fier gain is clear from the plots, and of course it would be a problem in case of DWDM systems. Up to now we have neglected the field phase, whose equation is very simple do =-n(z) (4.77) In the ideal case, this equation hides no problems, but in some practical amplifiers, the same mechanism that causes population inversion also causes the refraction index to depend on local gain and through it on the input signal. In this case, a further nonlinear effect is added to the nonlinear gain dynamics that creates a phase modulation in corre- spondence to a traveling amplitude-modulated signal. Technology for Telecommunications: Optical Fibers, Ampl