ristic of Raman scattering and due to the nonresonant nature of the Raman interaction. This means that in order to obtain a certain gain, a Raman amplifier needs much more pump power with respect to an EDFA amplifier [60,61]. Besides pump efficiency, also the stimulated radiation buildup is less effective, due to the weakness of the nonlinear effect. This means that a concentrated Raman amplifier needs a much longer fiber with respect to a similar EDFA. This last issue however has been overcome in two different ways, SO that the charac- teristic of Raman amplifiers to need a long fiber is at present days nearly reverted to an advantage. In concentrated Raman amplifiers, the Raman effect is obtained in the DCF [62,63], which in any case should be installed along the line to control chromatic dispersion. If this solution is adopted, no interstage loss has to be designed into the Raman amplifier (differently from what happens in EDFAs), thus achieving a better gain and lower noise. An alternative solution is to use as active Raman fiber the same transmission fiber, thus designing a distributed Raman amplifier [64]. This last solution is particularly effective when used either in systems where in-line amplifiers are not foreseen (e.g., short undersea systems) or in conjunction with EDFAs in ultra-long-haul systems. A third concern that has to be taken into account when designing a Raman amplifier for application in telecommunication systems is that the Raman lifetime is very short: on the typical telecommunication time scale, Raman effect is almost instantaneous. In this condition, distortion related to gain dynamics can affect a signal amplified by Raman effect (compare with Section 4.3.1.3) both as pulse dynamics reduction and inter- ference among channels at different wavelengths [65]. A large effort has been made in the last 10 years to overcome these limitations and to device practical Raman amplifiers and currently there are Raman amplified DWDM sys- tems installed in the field. The first key development has be