Study On Laser Interferometric Measurement Of Femtosecond Filamentation In Air

High peak power femtosecond laser pulses in air,will form a optical structure which has a length more than its Rayleigh distance–filamentation.Filamentation and its variety of nonlinear effect shown both in basic physics,or filamentation induced lightning,condensation of water in the air and the formation of snow,analysis of air pollutants many fields have extremely important value.Therefore,for the characteristics of femtosecond laser filamentation,including plasma electron density,size and length of core diameter is very important to the application of the filament in the above-mentioned fields.In this paper,the Mach-Zehnder laser interference method is used to measure the space-time distribution of filamentation,and high quality shadow and interferogram is obtained.The ionization wavefront velocity of filamentation is obtained by adjusting the delay of the detection light.The Mach-Zehnder interferogram is processed by fast Fourier transform and phase unwrapping.The phase shift distribution and electron density distribution are obtained,and the decay of the electron density with delay time is studied.First of all,three kinds of tightly focused laser induced filamentation are investigated.The mechanism of interaction of filamentation is analyzed,and the transmission process of femtosecond laser induced filamentation is discussed.The results show that the length of filamentation increases exponentially with the increase of focal length.One can change the focal length of the lens to control the length of filamentation.The length of the filamentation induced by three cases of tight focusing is exponential fitted and the ionization front velocity of filamentation is measured.Then,through the establishment of the diagnosis system of laser Mach-Zehnder interferometer,we for the first time use femtosecond laser interferometer as a diagnostic tool to study femtosecond laser filamentation.The interferograms of filamentation at different delay times are obtained,and the obvious bending of the interference fringes is observed.Two dimensional fast Fourier transform and phase unwrapping are used to extract the phase shift map and the electronic density map from the interferograms.The results show that the electron density of filamentation induced by f=75mm reaches 7×10¹⁸cm^-3 and as the delay time increases,the phase shift and the electron density will decrease.It is found that the phase shift of the center of the filamentation plasma forms a groove distribution.Compared with case of f=100mm focusing,the plasma electron density decreass more quickly in the early stage for the f=75mm case.Filamentation in the case of f=100mm focal length,the electron density is relative low(3.5×10¹⁸cm^-3).The changes of phase shift and electron density for the case of f=100mm are basically the same as that of f=75mm focusing.Due to the rapid expansion of the filamentation plasma,the forward mutation of the phase shift is found around the plasma.The filamentation diameters are estimated for the cases of f=75mm and f=100mm,which are about 110?m and 90?m respectively.