Abstract

Nonlinear wave collapse is an inherent phenomenon to several areas of physics including hydrodynamics, plasma physics, and nonlinear optics. In the latter case, the process responsible for collapse is the self-focusing of light in a medium with an intensity-dependent refractive index. The nature of the collapse has been theoretically analyzed using a variety of techniques including the variational principle [1], modulation theory [2], and numerical simulations [2,3]. Although the technique based on the variational principle is simple to apply, the other techniques have demonstrated that the collapse dynamics are too complicated to be fully captured by the ansatz that the beam maintains a Gaussian profile. In particular, modulation theory and numerical simulations predict that regardless of the shape of the input beam profile a collapsing beam transforms its radial profile into a focused Townes soliton. In this report, we present experimental evidence demonstrating that the spatial profile of a collapsing femtosecond laser pulse does indeed evolve into the cylindrically symmetric profile described by the Townes soliton for the case in which the input beam is either elliptically or irregularly shaped. These results could lend further understanding to recent experiments [4] that have observed extended light filaments in air with femtosecond laser pulses.

© 2002 Optical Society of America