Filamentation of ultrashort pulse laser beams resulting from their propagation over long distances in air

The propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations. Filaments are formed after 5-10 m and their propagation over distances in excess of 200 m is reported for the first time. The lateral dimensions of the filaments are found to range from about 100 mu m to a few millimeters in diameter. The early values of plasma electron density have been inferred to be a few times 10(16) cm(-3) using longitudinal spectral interferometry. For 500 fs pulses and a wavelength of 1053 nm, the energy in the filament can be quite high initially (similar to 8 mJ) and is found to stabilize at about 1.5-2 mJ, after about 35 m. A simple model based on the nonlinear Schrodinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well. (C) 1999 American Institute of Physics. [S1070-664X(99)00705-3].