mulating the obtained structure with a numeri- cal solution of a more complete model (Figure 4.33). The first step to design a Raman amplifier is to optimize the pumps position. As far as the efficiency is concerned, configurations with counter-propagating pumps are more effi- cient with respect to configurations with copropagating pumps. This effect is evident from Figure 4.34, where the pump power needed to reach a certain maximum saturated gain in a single-pump configuration is compared for a copropagating and counter-propagating pump injected at 1480 nm into the fiber. The greater efficiency of the counter-propagating configuration is evident. If two pumps are used, as generally needed if a good gain is to be obtained in all the C or L band, the better configuration is bidirectional pumping, not only due to the higher global efficiency, but also to the opportunity to use the two pumps to flatten the gain. In principle, due to the nonlinear interaction among the pumps, this could be done only by simulation. In practice, the so-called pumps superposition rule holds: this means that it is possible with a very good approximation to superimpose the gains caused by different pumps to approximate the total gain with their products (their sum in dBs) [70]. Coprop. FIGURE 4.34 Pump power needed to reach a given maximum Counterprop. saturated gain in a single-pump configuration copropagating and counter-propagating pump injected at 1480 nm into the fiber. (After Islam, M.N., IEEE J. Sel. Top. Quantum Electron., 8(3), 548, 2002.) Saturation on-off gain T(dB) Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices TABLE 4.11 Parameters of a Raman Amplifier Used in the Numerical Examples of This Section DCF fiber length Dispersion parameter DCF -70ps/nm/km signal attenuation in DCF 0.35 db/km pumps attenuation in DCF 0.4dB/km Maximum local Raman gain 4.50E-03 1/m/W A2 (DCF, at 1550 mm) Pumps insertion loss To have an idea of what is possible to achieve with a lumped Raman amplifier using a DCF a