s active fiber we will use the parameters of Table 4.11, which we will adopt also in Chapters 6 and 9 while analyzing WDM systems using Raman amplification. The shape of the Raman local gain in the chosen fiber, a DCF optimized for Raman effect, is reported in Figure 4.35. It is different from the Raman spectral gain in an SSFM due to the different modal area and to the particular shape of the refraction index. The Raman shift is with a good approximation the same as an SSMF, being dominated by the microscopic glass properties. The amplifier spectral gain in the C band is reported in Figure 4.36 for a two counter- propagating pumps scheme. From the figure, it results that with a specialized fiber a high gain can be achieved with a good flatness over all C band and with reasonable pumps power. For all the curves in Figure 4.26, the pumps are located at 1430 and at 1400 nm. In the case of the higher-gain curve, realizing an average net gain of 20.5 dB, the pump powers are 450 0.005 0.004 0.003 0.002 0.001 Wavelength displacement from the pump at -125 nm (nm) FIGURE 4.35 Normalized Raman gain of a highly nonlinear DCF manufactured for Raman amplification. Telecommunication Networks Single wavelength (nm) FIGURE 4.36 Gain of a Raman amplifier using a DCF and two pumps in positions designed to achieve the best possible uni- formity within the fixed design gain and maximum pumps power constrains. and 525 mW, respectively, with a pump insertion loss of 3 dB. The gain flatness in the C and is in this case 1.3 dB over all the C band. In the intermediate curve, the net average gain is 16.5 dB and the pump powers are 390 and 450 mW, while the gain flatness is 1.1 dB. Finally, the lower curve exhibits an average gain of 10.5 dB with a flatness of 0.7 dB using two pumps with a power of 340 and 300 mW. Globally, it is clear that there is a sort of trade-off between the flatness and the gain, due to the fact that in the adopted approximation, the local gain due to a pump is proportional to the pump power. It has to be noted als