Long UV Pulse Propagation in the Atmosphere

Abstract

Summary form only given. The propagation of intense laser pulses in the atmosphere is relevant to a wide range of applications: Lidar, lightening protection, wave guides in air, directed energy. Due to the nonlinear part of air refraction index, high power laser pulses can self-focus during their propagation through the atmosphere. Beyond critical power, self-focusing overcomes diffraction and the beam collapses. When self focusing is balanced by creation of plasma, a stable filament can form. Much of the theoretical work on filaments has been conducted in the infrared (IR), typically with wavelengths near 800 nm and pulse durations of the order of hundreds of fs. Since the peak intensity in these filaments exceeds 100 TW/cm2, nonlinear effects can be substantial. Some researches have begun to investigate the possibility of creating self-guided pulses in the ultraviolet (lambda ~ 250 nm ). In this case, peak intensity is only of the order of 1 TW/cm2. Consequently most of the higher order nonlinear effects can be neglected. It has been recently proposed by Schwarz and Diels that the UV filaments should scale with respect to increasing the pulse duration. This should result in long distance propagation of long duration pulses carrying a high energy. The present paper addresses the long UV pulses (tau ~ 1 ns ) propagation. It appears that the values of two of the parameters chosen by Schwarz and Diels in their model are wrong. It has been shown that attachment must be taken into account. The new equations obtained to describe the propagation of the filament tend to those published by Schwarz and Diels when the attachment mechanism is neglected.1 We will compare the results obtained with the new parameters and equations to those obtained by Schwarz and Diels. The domain of validity for a steady state analysis will be discussed. The feasibility of long UV filaments is also being studied experimentally. The laser system used to create UV filaments is based on a ND:YAG mode-locked oscillator. It produces up to 300 ps pulses at 1064 nm that are amplified in several Nd:YAG amplifiers. Their wavelength is then converted from 1064 to 266 nm by two second harmonic generations. The resulting duration is close to 200 ps, and will be further increased by using a Michelson interferometer with polarization separation. Our very first results should be presented at the conference.