535.42In the paraxial approximation, it has been shown that the quasi-stationary SRS amplification coefficient in the near-threshold region is determined by the product of the pump intensity in the beam center and the overlap integral of the transverse structure of the pump and the Stokes component. Expressions for the overlap integral and the scattering powers have been obtained and analyzed for the following cases: the pump and the Stokes field are Gaussian beams; the pump and the Stokes field are Bessel beams; the pump is a Bessel beam and the Stokes field is an axial Gaussian beam. Relations between the SRS excitation thresholds in different cases have been found.Introduction. The question of using stimulated Raman scattering (SRS) for laser radiation frequency conversion is receiving much attention. With the aim of upgrading the conversion efficiency and lowering the thresholds, various schemes of realization of the SRS process are being studied and their more or less adequate variants of the theory are being developed . In the last few years, a prominent place in the stream of such investigations has been occupied by works devoted to SRS in the Bessel pump field [1-10]. An important feature of these works is the fact that their topic is the interaction of fields having a fine transverse structure strongly influencing the energy exchange between them. For example, depending on the pump field parameters the first Stokes component of scattered radiation can have the form of a Bessel flux (conical wave in the far field), a near-axis flux, or both taken together. Knowledge of the conditions for obtaining a Stokes beam with a certain transverse structure is of practical interest. However, such conditions have thus far not been conclusively established.The Bessel field is often called a raylike field. This actually emphasizes that its fine structure (the central maximum and the rings) is a consequence of the interference of waves with a comparatively large cross section. The axial Stokes beam of SRS in such a field is apparently not raylike, and diffraction may undoubtedly have a marked effect on its development. The diffraction also plays an important part in the formation of a Stokes flux under SRS in strongly focused fields. Under such conditions the plane-wave approximation is inapplicable in the SRS theory and analysis of experimental data should be carried out on the basis of more general approaches, many of which were proposed in [11,12,[15][16][17][18][19][20]. However, the problem of describing the SRS with allowance for the transverse structure of the field and the action on it of the processes of SRS amplification and diffraction cannot thus far be considered to be solved.In the present paper, one more approach in describing the near-threshold SRS with allowance for the transverse structures of the pump and the Stokes flux is proposed. This approach is based on the equation for the power of the Stokes flux that arises as a result of the SRS amplification from spontaneous scattering. This equation follow...
