Multifocal Filamentation of High-Power Femtosecond Laser Pulses in Compressed Gases. Numerical Simulation

We report the results of numerical simulations of the propagation of high-power ultrashort laser radiation in self-focusing, filamentation, and postfilamentation channeling regimes in air with varying pressure in the propagation medium. The latter regime is of interest for the formation of high-intensity, low-divergence light channels (postfilaments). The simulations are based on a reduced (time-integrated) nonlinear Schrödinger equation for the optical field envelope. The propagation of high-power femtosecond pulses from a Ti:sapphire laser is considered under conditions of changing air pressure. Use is made of scaling laws, which establish a relationship between the pressure in the propagation medium and the initial parameters of high-power ultrashort laser pulses. This study examines the conditions for the formation of a nonlinear focus during self-focusing of femtosecond laser pulses and the formation of a filamentation region along real atmospheric paths extending over hundreds of meters. The formation of a multifocal structure within the filamentation region, which is particularly evident under varying pressures in the propagation medium, is examined in detail. The obtained results expand our understanding of the complex and multifactorial filamentation dynamics of high-power ultrashort laser radiation along long paths, as well as promising areas for optimizing and expanding the range of applications based on this phenomenon. These include remote diagnostics of atmospheric components and energy delivery along long paths.