Research On Spectral Characteristics Induced By Femtosecond Pulse Laser Filamentation

During the propagation of femtosecond laser pulses in transparent medium,a filament will be formed when the linear diffractive and dispersive properties of the medium and its nonlinear properties reach a dynamic equilibrium,that is,the dynamic equilibrium between the optical Kerr self-focusing effect and the defocusing caused by the plasma which generated by multi-photon ionization of molecules.The equilibrium propagation state is relatively stable,when the femtosecond laser pulses is propagating in air.The intensity inside of the filament core is clamped to about 5×10¹³W/cm².There are several nonlinear effects during the femtosecond laser filamentation,such as fluorescence emission,group velocity dispersion and photoionization,etc.The exploration of the mechanism for these effects is beneficial to the application of the femtosecond laser filamentation technology in the fields of remote detection,rainmaking and air pollutant detection.With the development of femtosecond laser technology,it has been combined with Laser Induced Breakdown Spectroscopy（LIBS）technology by researchers,then the femtosecond LIBS and the Filament Induced Breakdown Spectroscopy（FIBS）technology came into being in succession.Compared with the traditional LIBS technology,femtosecond LIBS technology has the unique advantages of low ablation threshold,small thermal damage,and can improve the sensitivity and stability of detection effectively.In addition to that,femtosecond laser filamentation can overcome the diffraction limit to achieve ablation of remote samples.Although FIBS technology has been successfully applied to remote detection,there are still some problems that need to be further researched.This paper discusses the influence of air pressure on nitrogen fluorescence emission induced by femtosecond laser and the influence of the propagation distance-resolved characteristics of copper plasma ablated by femtosecond laser filamentation under different experimental conditions.The major research contents are as follows:（1）Since the mechanism of femtosecond laser induced nitrogen fluorescence emission is uncertain,this paper experimentally investigates the effect of air pressure（0.1 Pa-100 k Pa）on the femtosecond laser induced nitrogen fluorescence emission and analyses the fluorescence emission mechanism.By observing the variation of nitrogen molecule（N₂）and molecular nitrogen ion（N₂⁺）characteristic fluorescence spectral lines’intensity as a function of air pressure,it is found that when the air pressure is between 10 Pa and 100 k Pa,the intensity of fluorescence signals fromN₂andN₂⁺shows opposite variation as a function of air pressure.According to this phenomenon,it can be deduced that dissociative recombination is the main mechanism for the generation of the C³Π_u excited state ofN₂.（2）The influence of axicon with different base angles（0.5°and 5.0°）on the propagation distance-resolved characteristics of copper plasma emission induced by femtosecond laser filamentation is experimentally investigated.It is found that the evolution of the spectral intensity,plasma temperature,and electron density along with the propagation distance.The experiment result shows that when the base angle of the axicon is 5.0°,the spectral intensity along with the propagation distance is more stable than that the base angle of the axicon is 0.5°.The changes of plasma temperature and electron density along with the propagation distance are basically consistent with the changes of spectral intensity.（3）The influence of different inter-pulse delays on the propagation distance-resolved characteristics of copper plasma induced by double pulse laser in air is experimentally studied by using the geometry of collinear filament and nanosecond laser pulse.It is found that the evolution of spectral intensities of copper plasmas induced by collinear filament-ns double LIBS along with the propagation distance at different inter-pulse delays.The result shows that compared with the longer inter-pulse delays（4,8 and 16μs）along with the propagation distance,the spectral intensities at short inter-pulse delays（1 and 2μs）are more stable.（4）The influence of laser pulse energy regulation with a circular iris or not on the propagation distance-resolved characteristics of copper plasma induced by femtosecond laser filamentation in air is compared.The distribution of the spectral intensity of copper plasma along with the propagation distance is obtained and the distribution of plasma temperature along with the propagation distance is calculated.The result shows that the length of filament when the circular iris is used to ablate Cu sample is significantly longer than that without the circular iris.