Features of the femtosecond pulse propagation in air

The features of propagation of femtosecond laser pulses in the atmosphere are studied by numerical methods. The type of propagation is determined by the relation between processes of multiphoton ionisation and multiple small-scale self-focusing (filamentation). It is found that the process develops step by step and filamentation occurs without the pulse energy loss until the intensity threshold is achieved. After the achievement of the threshold, ionisation losses appear which are accompanied by the defocusing action of the electron plasma resulting in the intensity decrease. When the intensity becomes lower than the ionisation threshold, self-focusing can again appear. The characteristic features of the final stage of filamentation can be explained by the Bespalov–Talanov theory. By using the time-averaged equations and parallel methods, a program complex is developed for qualitative simulations of the propagation of high-power laser pulses over long distances.