Thermal defocusing of CO2laser radiation in gases

The steady-state thermal defocusing of CO2laser radiation (10.6-μ wavelength) in gases has been investigated experimentally and theoretically. The increase in the beam diameter and change in intensity caused by the self-induced index of refraction variations in absorbing gases has been studied as a function of laser beam power, the absorption coefficient of the gas, and the absorption pathlength. A Mach-Zehnder interferometer was used to examine directly the changes in the index of refraction of the absorbing gas, which cause the thermal defocusing. The experimental results compare favorably with a perturbation solution of thermal defocusing, taking into account the effects of thermal conduction and natural convection.