Pulsed CO2 laser-induced effects on water droplets

Pulsed high-power CO2 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 CO2 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/cm2 (1.6-6.4 J/cm2 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 104 and 2 x 107 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.