Stimulated Brillouin scattering and stimulated thermal scattering of microsecond pulses

Stimulated Brillouin scattering (STBS) and stimulated thermal scattering (STTS) of microsecond neodymium laser pulses were observed experimentally. It was found that in liquids having the absorption coefficient α ≈ 10^–2cm^–1, the lowest threshold is observed for parallel STTS that results in a sharp drop in the power within the angle of the diffractionlimited divergence and also in broadening of the angular spectrum of the radiation transmitted by the nonlinear medium. On the other hand, in liquids with α ≈ 10^–3cm^–1, and also in compressed gases with the same or lower optical absorption, stimulated Brillouin backscattering has the lowest threshold. Stimulated Brillouin scattering may be observed in liquids with α ≈ 10^–2cm^–1 by rotating the optical beams at an angular velocity such that the focal constriction is displaced over a distance equal to its radius within a few tens of nanoseconds.