Optical turbulence and 'minimal' thermal self-defocusing of a vertical laser beam in liquids and gases

An analysis is made of an optical turbulence of a new type due to filamentation of a vertical laser beam in thermal self-defocusing in liquids and gases. It is shown that light-induced convective flow along the beam selects the optimal self-defocusing scale and the optimal (nonlinear) coherence radius of the laser radiation. This ensures the minimal beam divergence and creates a granular fluctuating structure of the radiation. The optical turbulence precedes hydrodynamic turbulent convection and is unrelated to convective instabilities or convective structures in a liquid. The results obtained are compared with the available experimental data.