Influence of random initial amplitude perturbations on the filamentation of prefocused femtosecond laser pulses is numerically studied.Simulations are performed for the filamentation of laser pulses with different modulation degrees and initial geometric focusings.The transverse waist of the laser beam is 5 mm.The behavior of the filament is strongly affected by the initial non-uniformity of the laser pulse envelope for a short initial focal length(f=1 m).In this case,the filament and plasma channel can survive for a longer distance with a small initial amplitude perturbation.For a laser pulse focused by a lens with a long focal length(f=4 m),decrease in the initial perturbation does not contribute to the length of the filament and the plasma channel.
The propagation of picosecond deep ultraviolet laser pulse at wavelength of 193 nm in air is numericallyinvestigated. Long plasma channel can be formed due to the competition between Kerr self-focusing andionization induced defocusing. The plasma channel with electron density of above 10^(13)/cm^3 can be formedover 70 m by 50-ps, 20-mJ laser pulses. The fluctuation of laser intensity and electron density insideultraviolet (UV) plasma channel is significantly lower than that of infrared pulse. The linear absorption ofUV laser by air is considered in the simulation and it is shown that the linear absorption is important forthe limit of the length of plasma channel.
