MOVING FOCUS IN THE PROPAGATION OF ULTRASHORT LASER-PULSES IN AIR

The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated b y use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplificatio n laser system. A filament contained only a small fraction of the puls e energy and always ended at the diffraction length of the beam (simil ar to 100 m), independently of the pulse energy. These features are ex plained by the moving-focus model, which is presented as an alternativ e to the self-channeling model. Computer simulations involving ionizat ion of the air also support the moving-focus model. (C) 1997 Optical S ociety of America.