High power optically turbulent femtosecond light strings

Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air will be presented. Our model is built around the nonlinear Schrodinger equation and incorporates the main effects present in air including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The vectorial field equation is coupled to a Drude model that describes the plasma density evolution. We are interested in intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air. During propagation, multiple light filaments form and feed on the energy from a low-intensity background. High intensities in collapsing filaments generate plasma, which in turn causes strong defocusing and thereby regularizes the collapse events. A large part of the energy localized in a collapse is returned to the background reservoir and can be used for formation of other filaments: we term this process dynamic spatial replenishment. The vector version of the model is utilized to study the stability of the scalar solutions, dynamics of the polarization, and evolution of the polarization patterns along the propagation direction. Having in mind possible application to long-distance atmospheric propagation, we also investigate the effect of turbulence to assess the robustness of the above described dynamic spatial replenishment scenario. To this end, we compare linear and nonlinear regimes in the presence of disorder mimicking the fluctuations of the air density. Severe compression in space and time during a single self-focusing collapse event, makes numerical simulation extremely challenging. The situation is excabrated further when a large number of intensity filaments are seeded across a wide pulse via a modulational instability. Our results will be compared to recent experiments.