SPATIAL DISTORTIONS OF LASER-PULSES IN COHERENT ON-RESONANCE PROPAGATION - LARGE-SCALE SELF-FOCUSING

When cylindrical symmetry is assumed, a numerical study of coherent on -resonance self-focusing of laser pulses in atomic media reveals a wel l-defined dependence on the interaction's parameters for the focusing distance and the maximal on-axis energy density reached at the focus. We interpreted this focusing to be the result of a diaphragm effect at the edge of the pulse. This can be readily explained by considering t he transverse dependence of local self-induced transparency phenomena first described by McCall and Hahn [Phys. Rev. Lett. 18, 908 (1967); P hys. Rev. 183, 457 (1969)]. From this interpretation and the well-know n Maxwell-Bloch equations, we derived a theoretical quantitative model for coherent on-resonance large-scale self-focusing, that is to say, focusing of the beam as a whole. Our main results include the above-me ntioned exact parametric dependence for the focusing distance, as well as predictions about the ratios of on-axis energy densities and pulse transverse sizes between the input and focus planes. All of these pre dictions appear to corroborate the results of an experiment on a Tm-16 9 vapor.