Intensity Clamping During Femtosecond Laser Filamentation

The femtosecond laser filamentation is commonly understood as the result of the dynamic balance between optical Kerr effect induced self-focusing and plasma defocusing effect. When the balance of these two processes is achieved, the laser intensity inside filament is not only stabilized along the propagation distance, but also almost invariable with the increase of the input laser energy. This special phenomenon is named intensity clamping. The intensity clamping represents one of the fundamental characteristics of the filamentation phenomenon. It governs the major evolution dynamics of the laser pulse propagation during the filamentation and impacts extensively the applications of the filamentation. This paper summarizes the main research results about the intensity clamping, including current understanding about the underlying mechanism of the intensity clamping, experimental evidences proving the intensity clamping and some recommended approaches to measure the clamped intensity. The paper also introduced some impressive effects brought by the intensity clamping in various applications, such as high stability of the fluorescence signal in remote sensing, beam-cleaning in short pulse production, etc. The effects are in fact in contrary to the common sense about the outcome of nonlinear interaction, which would be unstable under perturbation. Furthermore, the paper presents some methods to increase the clamped intensity inside the filament. At the end, some other effects that may arrest the beam collapse during the filamentation are briefly discussed. These effects mainly include the diffraction, the dispersion and high order Kerr effect.