nject- ing the pump field into a piece of polarization maintaining fiber with the polarization directed at /4 with respect to the polarization maintaining birefringence axis (see Section 4.2.6). In this way, at the output of the fiber, a depolarized pump is obtained. In order to better understand advantages and issues of current Raman technology and to look at future evolutions, the two important cases of Raman amplifiers have to be dealt separately; as a matter of fact, concentrated and distributed Raman amplifiers have differ- ent characteristics and problems. Conceptually, the block scheme of a concentrated Raman amplifier is given by Figure 4.18, where the active medium is a high Raman efficiency fiber. Frequently it is a DCF that is also used to compensate part of the transmission link dispersion. Raman-concentrated amplifiers frequently use more than one pump, due to the fact that multiple pumps assure gain flatness. The first point to analyze is the reduced pump efficiency that in a concentrated amplifier, designed to compete with EDFAs, is a particularly important point. In standard conditions, this is smaller than the efficiency of erbium-doped fibers, but there are particular and interesting cases in which this is not true. In Figure 4.32 [60], the efficiency of a Raman amplifier realized with a DCF is compared with the efficiency of a single-stage EDFA booster amplifier in a typical booster configura- tion, with an input power as high as 20mW. The input is a signal at 1529.5 nm with an input signal power of 20 mW. For the Raman amplifier curve, a Raman pump wavelength of 1433 nm was chosen, SO that the signal falls on the gain peak at 13.2 THz away from the pump wavelength. Raman FIGURE 4.32 Efficiency of a Raman amplifier realized with a DCF versus the efficiency of a single-stage EDFA booster amplifier in a typical booster con- figuration, with an input signal power of 20mW and different values of the pump power. (After Islam, M.N., IEEE J. Sel. Top. Quantum Electron., 8(3), 548, 2002.) Launched pump