Tightly focused propagation of high-power femtosecond laser radiation in air is considered. Based on numerical solution of the nonlinear Schrodinger equation for complex envelope of light wave electric field, evolution of the beam effective radius is studied. Dependence of the rms size of a focal spot and the maximally attainable intensity of radiation at the spot focal waist on the initial laser power is established. It is shown that tight spatial focusing of ultrashort laser pulse can lead to photoionization of the medium and plasma generation in the region of maximal beam intensity. This may prevent the light wave intensity from its further growth in the focal region and retard the transversal contraction of the beam as a whole.
ultrashort laser radiation, filamentation, multi-photon ionization