Spatial evolution of multiple filaments created in air by femtosecond laser pulses

Abstract

The spatial evolution of filaments in air was experimentally investigated and compared with theoretical calculations. We used a new method to visualize the multi-filamentary-pattern along the laser propagation axis. This technique allows for the accurate determination of number and relative position of filaments across the beam cross section. The experiments have been performed with a Ti:Sapphire chirped pulse amplification laser system. The laser operates at a center wavelength of 800 nm, with an energy per pulse up to 15 mJ and a repetition rate up to 100 Hz. The pulse duration was maintained 50 fs during the experiments. The corresponding peak power is 0.3 TW, which is about 100 times the critical power for self-focusing in air. The measurements are in good agreement with numerical simulations of the pulse propagation starting from the measured intensity profile right behind the laser. Our nonlinear propagation equation comprises diffraction and self-focusing due to the optical Kerr effect. The defocusing effect of plasma generated by tunnel/multiphoton ionization is taken into account.