Effect of spherical aberrations in laser beam on quality of phase conjugation at SBS

Numerical calculations of phase conjugation at SBS are performed with using the model approximated to actual conditions and taking into account 3D medium, non-steady state effects, diffraction, effect of Stokes radiation on laser one (SBS saturation). SBS-mirror consists of focusing lens and SBS-cell. It is obtained that the negative spherical aberration, resulting in additional focusing of laser radiation, leads to decrease of SBS threshold, growth of reflection coefficient from SBS-cell, and the increase of phase conjugation quality. These effects take place under quasi-steady-state as well as non-steady state conditions of SBS. Thus, the influence of such an aberration is positive, if it will be possible to avoid parasitic non-linear effects (optical breakdown, self-action) with growth of laser beam intensity in a SBS-cell. It is found that the positive spherical aberration, resulting in additional defocusing of laser radiation, has negative effect on properties of a SBS-mirror. Such an aberration leads to a broadening of laser beam waist, the rise of SBS threshold and the decrease of reflection coefficient. The reducing of the reflection coefficient, in turn, results in deterioration of phase conjugation fidelity. These effects take place under quasi-steady-state as well as non-steady state conditions of SBS. The calculated data agree qualitatively with experimental results.