o that the coupling loss between the pumps and the active fiber is a key in determining the Raman gain. Considering very wide band amplifiers, a flat gain over C+L band, a band of about 60nm, can be achieved by three or four pumps and gain in excess of 21 dB has been reported in literature as shown in Figure 4.37 [71]. Advantages can also be achieved by the so-called double pass architecture, where the signal is reflected at one amplifier length to pass through the amplifying fiber two times in opposite directions [72]. This design, quite Input 4.1 dBm FIGURE 4.37 Spectral gain of a Raman booster amplifier using Input 7.6 dBm a DCF and four pumps, whose power (always the same for each pump) is tuned to attain different values of the gain, and whose position was opti- mized to minimize gain ripple in a 60nm band. (After Lewis, S.A.E. et al., Electron. Lett., 35(14), 1178, 1999.) Wavelength (nm) Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices FIGURE 4.38 Gain of a lumped Raman amplifier designed to work in Wavelength (nm) the S band. interesting since suitable for low levels of input signals, has been experimentally imple- mented observing a maximum gain of 22 dB gain with a noise figure smaller than 5 dB over all the C band [72]. Finally, there is to consider another possible application of Raman amplifiers: amplifica- tion in the S band, where EDFAs cannot be used. Very good results have been achieved in this spectral area using high Raman efficiency fibers, as reported in [60] and shown in Figure 4.38 where a few experimental data reported in [60] are interpolated with the model presented in this section by adopting the parameters of a DCF and four pumps, whose power (always the same for each pump) is tuned up to a maximum of 330 mW, to attain different values of the gain and whose position was optimized to minimize gain ripple in the S band (from 1493 to 1523nm). Let us now consider the noise characterization of Raman amplifiers. Starting from the ASE noise, we have to conside