Deformation of the laser beam profile as a result of thermal defocusing and its influence on the stimulated Brillouin scattering threshold for long pulses

Broadening of the caustic of a focused pulsed single-mode light beam when its energy W exceeds the critical value for thermal self-defocusing reduces [proportionally to ~ (1 + W/W_cr ) ^– 1/2 the integral along the longitudinal coordinate of the axial intensity of the same beam (this integral characterizes the efficiency of excitation in the stimulated scattering field). A derivation is given of a universal normalized dependence of the stimulated Brillouin scattering threshold of an absorbing medium on the excess above the threshold energy (above the critical value W_cr ) for a similar but nonabsorbing medium.