Abstract

Intense laser beams of usual shape (gaussian profile and circular symmetry) carrying more than a critical power are known to break up into several self-focused spots when propagating through homogeneous Kerr type nonlinear media. This chaotic instability prevents all the previously envisaged applications of the intensity controlled refactive index changed exhibited by Kerr materials. When the nonlinear propagation is restricted to two dimensions, for example in a planar waveguide, it has been shown, some years ago that soliton effects induce stable self guided beams at suitable intensities. This communication presents a new kind of geometrical intensity profiles that are stable as well for the two-dimensional as for the three-dimensional cases. These structures consist in the interference pattern of four (or at least two) copropagating beams, coming from the same laser sources and overlapping into the nonlinear medium. We report experimental results showing the use of this technic for (i) the increase of the intensity threshold before the growing of self-focusing instability and (ii) generating two-dimensional soliton beams. At higher intensity levels, distortions are introduced by the self-bending of beam trajectories due to the contribution of the enveloppe of the intensity input pattern to the refractive index profile. Intensity controlled self deflection has been observed and, when combined with a spatial filter, applied to the clipping of the more intense part (5 ps) of 30 ps pulses. The control of the beam spatial shapes during their nonlinear propagation offers a new way to avoid optical damages resulting from self-focusing, for example in very high power laser amplifiers. On an another side it opens the use of bulk Kerr media for achieving the optical processing of intense pulsed laser radiation. This processing can rest on geometrical as well as on temporal self phase modulation effects. The last limits come from the appearance of stimulated scatterings. Recent experiments on stimulated Raman scattering suggest to us that they could be probably overcome by giving to temporal pulses beatings the same role as the previously presented beam interferences.

© 1990 Optical Society of America