Аннотация:We study, both experimentally and numerically,
the variations of the refractive index at the filamentation
of a femtosecond laser pulse in a fused silica. Using
the polarigraphy and shadowgraphy techniques, we investigate
the spatio-temporal variations of the refractive index
induced by Kerr and plasma nonlinearities. The induced
variations of the refractive index play the role of dynamic
lenses: focusing for Kerr nonlinearity and defocusing for
plasma nonlinearity. We show that intensity saturation
occurs at the distance, where the total optical force of Kerr
and plasma nonlinear lenses reaches its maximum.