Direct Measurement of Laser Fluence of Femtosecond Laser Filament in Air

Laser filaments appear spontaneously during the propagation of intense ultrashort laser pulses with a power exceeding a critical value Pcr (Pcr = 5 GW at 800 nm in air) [1]. The intense beam tends to collapse due to the optical Kerr self-focusing effect until the intensity is high enough to ionize the medium, giving rise to a defocusing plasma. Thereafter, a dynamic competition between these two effects takes place leaving a thin and weakly ionized plasma channel in the wake of the pulse. Femtosecond laser filamentation in air attracts considerable interest because of its wide range of applications in remote-sensing, light frequency conversion, laser-based weather control and so on [1, 2].