Short time-constant thermal self-defocusing of laser beams

Time constants as low as 20 μs have been observed when the beam of a helium-neon laser is focused on a small spot on a thin dye sample and the thermal self-defocusing effects observed. The short time constant arises mainly from the small spot size, but to some extent from axial heat flow to the cell ends. The analysis takes into account the axial heat flow, the variation of spot size within the cell, and diffraction defocusing. Agreement between analysis and experiment is good except for cases of the smallest spot sizes, in which case convective turbulence was observed.