A LIDAR technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air

We demonstrate a new method for measuring the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique. The LIDAR involves a detector with a fast response time. The back-scattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament is measured, from which the length of the filament can be determined. We find good agreement between the measured filament length and the length estimated from burn patterns on paper. In addition, good qualitative agreement between the experimental measurement and numerical simulations is obtained for the signal features of the filament. We propose that this new method can be used to quantitatively determine filamentation at longer distances.