Study of the refractive properties of the transparent region in a water-droplet aerosol by transverse illumination with a laser probe

Abstract

We have studied the refractive displacement of the energy center of a laser probe when it intersects at a small angle the region where a cw CO 2 laser beam interacts with a waterdrop aerosol. The experimental results are compared with results calculated from an approximate optical model describing the change in refractive index when the aerosol is evaporated by the CO 2 laser. Under certain conditions the propagation of an intense COz laser beam in a water aerosol is accompanied by the formation of a transparent region characterized by greater transparency to visible and infrared radiation as compared with the unperturbed aerosol. The passage of a signal through the bleached region has been studied [1-3] as a function of the CO 2 laser power and the initial properties of the aerosol. There are a number of works devoted to experimental studies of the thermal self-stress or refraction of laser light in water aerosols [1, 4-6]. Most of these studies have drawn conclusions about changes in the medium's refractive index from the refraction of a secondary, low-power beam passing through the interaction region rather than from changes in the phase front of the CO 2 laser beam interacting with the aerosol. The unique aspect of the method used in [1, 4, 6] is that no optical elements are introduced into the CO 2 laser beam. This eliminates additional distortions of the laser beam and increases accuracy while allowing us to measure the refraction over a wide range of intensity variations (over 60 dB) in the probe beam corresponding to variations in the optical thickness of the aerosol during bleaching. Thus, intensity changes do not influence the results of measurements of the position of the energy center of the probe. 1. Optical Model of Refraction in the Bleached Region Formed When a CO 2 Laser Interacts with a Water Aerosol Our model assumes that the refractive index changes in the region where a CO~ laser beam interacts with an aerosol because of heating due to heat flushes and the flux of heated vapor from the surface of the aerosol particles and also because of the molecular absorption of laser radiation in the air. Part of the laser radiation is absorbed by the aerosol drops, causing them to warm up and evaporate. Thermal flux at the drop surface heats the surrounding medium. We assume that thermal conductivity is primarily responsible for heat transfer fromthe drop. Evaporation of the aerosol also produces diffusion currents of vapor* heated near the surface. The total heat flux from the drop surface is given by the equation w = ~v Fw~, (I)