Statistics of a multimode laser beam under time-dependent thermal self-interaction conditions

The Monte Carlo method is used in a study of a time-dependent thermal self-interaction of the speckle field of a multimode laser beam. It is shown that representation of such a beam by a superposition of statistically independent transverse modes of an optical resonator provides a satisfactory description of the spatial coherence of the optical field across the beam and also of changes resulting from linear diffraction. The change in the spatial coherence due to the time-dependent thermal self-interaction is governed by the competition between the nonlinear divergence of the beam as a whole and fragmentation of the speckle field. The results obtained are in agreement with the experimental data. It is shown that the replacement of a statistical analysis by a solution of a determinate problem of the self-interaction of a single-mode beam with equivalent parameters underestimates the angular divergence.