Cascade Excitation of SRS Components
In conclusion, a brief mention should be made of peculiarities of SRS manifestation with regard for the cascade excitation of the Stokes components in the medium. The results of numerical solution of the first three equation of system (4.54) for the power and the effective radius of the beam are shown in Figure 4.12. The parameters of calculation were chosen to correspond to the OIL radiation propagating along the horizontal high-altitude path at G* = 200, L = 0.17, and an = 0.
It follows from Figure 4.12 that once some characteristic SRS length LR1, which corresponds to the threshold G1, is achieved, the marked growth of the power and radius of the first Stokes component begins with the simultaneous depletion of the main wave power. At the distance LR 2, the power of the Stokes signal at the frequency w_1 attains the saturation with the level close to the limiting value of the conversion coefficient pR1™. From this time, the amplification of the second Stokes component (o>_2) occurs due to nonlinear polarization induced by the field of the first Stokes component, which, in its turn, begins to deplete, and so on.
As a result, the final value of the conversion coefficient pR at the cascade amplification of the two Stokes components appears to be lower than that at the two- photon SRS due to energy losses in the medium to excitation of optical phonons first by the main radiation and then by the field of the first Stokes component. The limiting level of the SRS conversion, which can be achieved in this n-cascade process, is, obviously, pR^3* = wn /o>0. In the considered case of only two Stokes components and the oxygen laser radiation, we get that р^ < 41%.