ends on the interstage loss and divides two regions where completely different criteria have to be adopted for the amplifier opti- mization: a region for low values of 8 (below the threshold) where the optimum value of G2 is quite low and the single-stage solution is competitive and a region (above threshold) where the two-stage solution is really needed. In the considered case for an interstage loss of -10 dB, the threshold value is about 0.002 m while for the interstage loss of -13 dB the threshold value is 0.0011 m. TABLE 4.8 Parameters of the Two-Stage Amplifier Used in the Optimization Example of This Section Overall gain G (dB) Interstage loss a (dB) Y10 (adim.) Y20 (adim.) Input power (at the stage 1) (dBm) Local saturation first stage (dBm) Local saturation second stage (dBm) Active medium length (m) Telecommunication Networks 0.008 FIGURE 4.21 Noise power versus the saturated amplification of the second stage in the case of a realistic model of a 0.005 two-stage amplifier where the noise factor depends 8 = 0.002 on the amplification due to the characteristics of a simple material model. Different values of the mate- rial defect density parameter are considered. The presence of an optimum value of the gain for which the noise is minimum is evident. G2 (dB) a==13dB a==3 dB FIGURE 4.22 Optimum value of the gain of the second stage ver- sus the material defect density parameter for dif- ferent values of the interstage loss and an overall amplifier gain of 26dB. Defect parameter 8 (mm) Looking at the amplifier application, three situations that have practical meanings are represented in the figure: 1. A low value of the interstage loss (-3dB) that corresponds to the situation of a short metro system in which only a limited dispersion compensation is performed. 2. A interstage loss (-10 dB) corresponding to the length of DCF that is needed to compensate the dispersion of the span whose attenuation is compensated by the amplifier. As a matter of fact, if we consider a net loss of 0.3 dB/km for an SSMF in cable 