Nonstationary self-action regime of intense tightly focused femtosecond laser pulse in air.
The problem on tightly focused propagation of high power femtosecond laser radiation in air is considered. On the basis of the numerical solution of nonlinear Schroedinger equation for complex envelope of light wave electric field the evolution of beam effective radius is investigated. The root-mean-square size of beam waist and possible achievable intensity of radiation in linear focus dependent on initial laser power are established as well. It is shown, that tight spatial focusing of an ultrashort laser pulse can lead to photoionization of medium and plasma formation in the region of beam maximum intensity. This results in intensity clumping, prevents it further growth in focal region, and transversal beam compression as a whole.