The effect of preformed plasma strings on the propagation of ultra-short laser pulse in air

In this investigation, propagation of collinear sequence of twin intense laser pulses is considered in air, numerically. Two sets of twin successive laser pulses were investigated. The first set includes two successive LG01 laser modes and the second one includes LG01 mode followed by a TEM00 mode. The physical effects that are considered include diffraction, nonlinear self-focusing, optical field ionization and associated energy depletion, plasma absorption and defocusing. The first laser pulse produces a ring plasma filament with inhomogeneous electron density in transverse direction. The numerical investigations show that the second laser pulse LG01 mode/TEM00 mode undergoes self-focusing producing plasma filament until joining to the head of the first filament. At this point, it diverges due to the plasma divergence effect and could not penetrate into the preformed plasma ring. The possibility of optical guiding of the second laser pulse through the ring plasma filament is considered, too.