d emission due to the excited optical phonons. In the Stokes scattering process, the only process happening in guided propagation due to the overall momentum conservation, the energy of incident photon is reduced to lower level and energy is transferred to the molecules of silica in the form of kinetic energy, inducing stretching, bending or rocking of the molecular bonds. The Raman shift WR = Wp - WS (when P indicates the injected wavelength and S the Stokes wavelength produced by the nonlinearity) is dictated by the vibrational energy levels of silica. The Stokes Raman process is also known as the forward Raman process since the Stokes wave propagates in the same direction of the input wave and the energy conservation for the process is (4.47) where Eg and Ef are ground state and final state energies, respectively, and in silica fiber Wp - WS 2 12 THz (that in the third windows are about 100nm). Since Raman scattering is not only a potential cause of channel crosstalk in WDM sys- items, but also the principle of operation of Raman amplifiers that are used in several trans- mission system architectures, we will delay the discussion of a detailed physical model of the Raman amplification to the next section. Here it is interesting to note that the gain bandwidth of the Raman effect, whose experi- mental measure is reported in Figure 4.8 [24], is extremely wide and that, as all nonlinear effects generate new waves, the Stokes wave has an exponential growth in the limit of nondepleted pump, when the effect is thus very low. In correspondence to the slope change of the exponential, a Raman threshold can be defined whose value is about 600 mW in conventional standard single-mode fibers (SSMFs). 4.2.9 Brillouin Scattering Brillouin scattering is a nonlinear process that can occur in optical fibers at large field intensity. The large intensity produces compression in the material of the core of the fiber Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices through the process known as elec