BEHAVIOR OF FEMTOSECOND PULSES ON THE OPTICAL-AXIS OF A LENS - ANALYTICAL DESCRIPTION

The propagation of a femtosecond pulse along the optical axis of a lens is analytically described near the focus assuming homogeneous illumination. The results are compared with previous numerical calculations. The time dependence of the intensity in the focal point is also obtained. For small input beam size the diffraction, for large input beam size the pulse broadening causes the decrease of the intensity in the focal point. There is an optimal input beam radius for the highest intensity and an analytical expression is derived for it.