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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.

100 J of 10.6-μ laser energy produced in a 1.5-μs pulse was focused to a 2-mm-diam spot by a 50-cm-focal-length mirror. A plasma was created by breakdown in the 70-ns gain-switched peak of the pulse where the light flux becomes greater than 1010 W/cm2. The plasma column subsequently created in 5–20 Torr of hydrogen by the absorbed light energy was observed by 5-ns exposure and streak photographs. The aspect of beam trapping is demonstrated by measurements of the transmitted light intensity radial distribution and significant energy absorption is shown by the measurement of transmitted energy.

Results of an experimental study on air breakdown produced by radiation from a high-power CO2 laser are presented. From these measurements, the breakdown threshold over a flux range (1.5×108 <φ<7×108 W cm−2) is obtained as a function of the focal length varying from 1 to 55 m. The first of these experiments was carried out in the laboratory atmosphere (10<f/D<100), whereas the second part (f/D=135) was done outdoors under typical atmosphere conditions. The temporal evolution of the energy transmitted through the breakdown region for different laser parameters (5<Ei<220 J) was found.

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