SATURATION OF THE SELF-INDUCED THERMAL DISTORTION OF LASER RADIATION IN A WIND

Abstract

When a laser beam propagates in an absorbing medium flowing transverse to the direction of propagation, the beam may be deflected and distorted by the self-induced thermal lens, which results from the absorbed laser beam energy. Experiments have been carried out examining the thermal bending and distortion of a cw CO2 laser beam in a 1-m-long wind tunnel. At the onset of the convection-controlled thermal distortion, the peak intensity of the laser beam is reduced and shifted into the wind. For experimental conditions leading to severe thermal distortion of the beam shape, it is observed that the deflection and decrease in the peak laser beam intensity tend to approach a constant value. This result is interpreted as a saturation of the convection-controlled, self-induced thermal lens which occurs when the thermal distortion is large and accompanied by a significant reduction in the minimum characteristic beam dimension. In the experiments reported, the path length and minimum characteristic beam dimensions associated with the thermal distortion were such that diffraction effects may be involved together with the thermal lens process in producing the observed saturation effect.