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

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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 µm to a few millimeters in diameter. The early values of plasma electron density have been inferred to be a few times 10¹⁶ 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 (~8 mJ) and is found to stabilize at about 1.5–2 mJ, after about 35 m. A simple model based on the nonlinear Schrödinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well. ©1999 American Institute of Physics.