<title>Nonlinear distortions of high-power laser beams in the atmosphere from the data of optoacoustic measurements</title>

Abstract

Results of experimental and theoretical investigations into the characteristics of acoustic signals generated by highpower pulsed laser radiation (HPLR) propagating in under-breakdown regime through the atmosphere are presented. The amplitudes and shapes of acoustic signals are calculated for a triggering laser pulse, short laser pulse, and harmonically modulated laser radiation disregarding the effect of kinetic cooling in the laser beam propagation channel. An algorithm for reconstructing the spatial structure of laser beams from the data of wire bolometer sensors is suggested and realized. The nonlinear extinction coefficient of laser radiation in the atmosphere is retrieved from opto-acoustic measurements. It is demonstrated that acoustic signal shapes measured in summer reproduce well the cross-sectional laser energy distribution. In winter the measured acoustic signal is first negative, which can be explained by kinetic air cooling, when the air temperature in the laser beam propagation channel first decreases. The effective transverse beam diameter d is directly proportional to the positive half-period ofthe acoustic pulse. This provides the basis for opto-acoustic method of measuring the effective beam diameter. Results of opto-acoustic and bolometric measurements of the effective diameters of collimated and focused beams of C07-laser radiation versus the energy of emitted laser radiation and angles of laser beam divergence (self-focusing) are presented. A significant increase in the effective beam diameter with the increasing laser radiation energy is observed. Measurements of the divergence angles of collimated HPLR demonstrated that the effect ofself-focusing became more pronounced as the HPLR energy increases and can reach 4 mm of arc.Keywords: High-power pulsed laser radiation propagation, laser-induced acoustic signals, opto-acoustic effects in the atmosphere.