Pulsed CO2 laser-induced effects on water droplets

Abstract

Pulsed high-power CO 2 laser beams propagating through the atmosphere can be affected by such different linear or nonlinear phenomena as aerosol and molecular absorption, scattering, turbulence, thermal blooming, or optical breakdown, depending on the atmospheric characteristics and laser parameters. An experimental investigation was carried out to study the pulsed CO 2 laser-induced effects on water droplets. Water droplets with radii of 8-400 /im were irradiated. The average fluences used for the interaction were between 1 and 4 J/cm 2 (1.6-6.4 J/cm 2 on the droplet) for a pulse duration adjustable between 2 and 6 /is. Most measurements were made using an Imacon 790 converter camera allowing framing rates of between 10 4 and 2 x 10 7 frames/s with fast framing and a scanning rate of between 1 ns/mm and 1 /is/mm with streak photography. The experimental results presented in this paper identify four main mechanisms that can occur sequentially throughout the laser interaction, without specifying which dominate over the others: vaporization, deformation, shattering, and propulsion. In particular, with time, evolution of the hot vapor and shock wave produced by the vaporization of a single water droplet was observed. Evolutions of the droplet deformation with time is shown. Threshold values and characteristic shattering times of the liquid particles are given, as well as the drop velocities and ejection directions across the area illuminated by the laser pulse.

Effect of Liquid Transparency on Laser-Induced Motion of Drops

“…This optical breakdown results in the formation of plasma (Autric et al, 1988) that absorbs much of the incident laser radiation. The motion of the drop takes place nearly diametrically opposite to this hot spot or spark for transparent and near transparent drops due to the thermal gradient that the hot spot generates.…”

“…In USA too, the explosion of water drops under laser was studied (Singh and Knight, 1978;(Carls and Brock, 1987), so was the process of droplet heating under a continuous laser beam (Park and Armstrong, 1989) and also the hydrodynamics of a drop when exposed to pulsed laser (Zardecki and Pendleton, 1989). (Autric et al, 1988) also reported their findings on the effects of pulsed CO 2 on water droplets. The focus of the study of atmospheric laser propagation has also included cloud science, particularly cloud-clearing using high-energy laser beams and new applications such as wireless communication using laser beams.…”