Self-focusing revisited: spatial solitons, light bullets, and soliton explosions

Abstract

Self-focusing is a subject that is almost as old as the field of nonlinear optics. Here we report on recent experiments at Bellcore that studied self-focusing in nonlinear planar glass waveguides. It is well known that self-trapping in an optical Kerr medium is unstable in three dimensions, but it is stable when diffraction is limited to one spatial dimension, such as in a planar waveguide. We have demonstrated that a beam that diffracts at low powers is self-trapped at high powers. The self-trapped beam can be described as a spatial soliton, a beam that retains its spatial profile as it propagates along the waveguide. We have also observed interactions between two bright spatial solitons. When the power levels are increased above those required to form the fundamental spatial solitons, we have observed the breakup of the beam shape into a triple-peak profile. This phenomenon is explained as the result of weak two-photon absorption in the waveguide. We will show through numerical modeling how two-photon absorption leads to the breakup of spatial and temporal solitons. The second part of the talk will take another look at three-dimensional self-focusing, a process that is known to be unstable and to lead to catastrophic collapse of the beam profile. We will show theoretically that in the presence of dispersion, a properly constructed pulse could undergo spatial-temporal collapse to generate extremely short and intense pulses of light. We will discuss the condition for such a process and the effects expected to limit it.