Optimization And Spectral Characteristics Of Femtosecond Laser Filaments In Air

The filaments formed by the nonlinear effect of intense femtosecond laser transmission in air have potential application in laser lightning induction,atmospheric remote sensing,white laser radar,plasma antenna and other fields.The optimal control of the length,lifetime,uniformity and spectral characteristics of filaments are the hot issues concerned by researchers at present.In this paper,the length,lifetime and uniformity of plasma filaments are optimized by adjusting the initial beam parameters such as the energy,polarization,the wavefront phase of laser beam,and the spectral characteristics of fluorescence radiation of plasma filaments are studied.1)In plasma filaments fluorescence radiation spectral characterization.The fluorescence radiation spectra of plasma filaments formed by focusing several focal lenses were measured and calibrated.The results show that the fluorescence spectra of plasma are mainly O atom and N atom and continuous spectrum under the focusing condition of f=10 cm.Under the focusing condition of 20 and 100 cm,the plasma radiation spectrum is mainly composed of N₂?N₂⁺?O₂and O₂⁺molecular and molecular ion lines.The spectral lines are identified and the first negative band of O₂⁺spectral line is relatively weak.2)In femtosecond laser filament lifetime optimization.The results show that the lifetime of plasma filaments ranges from 2.7 ns to 4.4 ns when the focal length is 10,30,50 and 100 cm.The lifetime of plasma filaments is the shortest when the focal length is100 cm.Through the combination of convex lens and vortex plate,the plasma filament lifetime can be controlled in the range of±20%.It is found that the lifetime of plasma filaments increases with the increase of the base Angle of the axicon.3)Optimization of femtosecond laser filament length.Respectively using the axicon base Angle of 1°,2°and 5°focus,analysis of the plasma filaments fluorescence radiation of spatial resolution spectrum,the study found femtosecond laser through the axicon focused can obviously prolong the length of the plasma filaments,and the larger the base angle of the conical lens,the better the uniformity of the plasma filament,The smaller the bottom angle,the longer the plasma filament;When the femtosecond laser pulse energy is constant,the linearly polarized laser is converted into circularly polarized laser through a quarter wave plate,the starting point of filaments is delayed and the length of filaments is shortened,which reflects that the laser polarization has a certain regulation effect on filaments.In this paper,the starting point and length optimization of the focusing filament of convex lens and axicon by vortex beam are mastered by spectroscopic method and imaging method.This study has certain reference value for the potential application of laser lightning and plasma antenna.