The influence of gas pressure on spatial and spectral characteristics of a femtosecond laser pulse during its filamentation
D.V. Apeksimov, P.A. Babushkin, Yu.E. Geints, A.M. Kabanov, V.K. Oshlakov, A.V. Petrov, A.A. Udalov, E.E. Khoroshaeva
V.E. Zuev Institute of Atmospheric Optics of Siberian Branch of the Russian Academy of Science, Tomsk, Russia
Keywords: femtosecond laser radiation, filamentation, high-pressure gas, spectrum widening, supercontinuum, laser beam structure
Abstract
Filamentation of high-power femtosecond pulses in a gas is of great theoretical and practical interest with relation to study of large-scale spectral and temporal transformations of laser radiation in a medium and generation of extra-wide (supercontinuum) radiation, actual for the problems of nonlinear femtosecond diagnostics of the environment, transmission of optical information through an atmospheric channel, and modern optical technologies for material processing. This paper experimentally studies the influence of pressure of a gas medium (nitrogen) in an optical cell on the characteristics of femtosecond laser radiation propagating under filamentation conditions. It is shown that under conditions of high nitrogen pressure (up to 11 atm) and sharp geometric focusing of femtosecond radiation, its Kerr self-focusing is implemented, and the single filamentation made transforms into multiple post filamentation as the gas pressure increases. In this case, due to the phase self-modulation of a femtosecond pulse and plasma generation in the gas, there is a significant enrichment of the spectral composition of the radiation, and near-linear increase in the pulse spectrum width with gas pressure in the cell. It was established for the first time that the pulse spectrum is extended asymmetrically and mainly to the long-wave region with an increase in the sharpness of the initial focusing of a laser beam. In addition, the average size of intense light post-filaments formed inside a beam decreases and can be fractions of a millimeter as the gas pressure increases in the optical cell.
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