Study On The Application Of Gas Lens In Ultra-high Strength Parallel Laser Beam Generation

Gas lens is controlled by the variation of the gas temperature distribution and the refractive index distribution,to obtain the parabolic phase delay distribution similar to the traditional glass lens.Then,the laser beam passing through the gas lens will gain an approximation spherical wavefront,resulting in beam divergent or focusing effect.The damage threshold of gas lens is much higher than the traditional gas lens.Furthermore,the gas lens has stable quality,long service life,safety and economic benefits,and ignorable scattering.the gas lens,may replace the glass lens is used to control the high power and high energy laser beam in the future.In this paper,a gas negative lens device was designed in order to replace the glass negative lens optical elements.Firstly,the numerical solution results of the distribution of temperature,density,refractivitye of a gas lens in gas were simulated by using finite element software.The key performance parameters of gas lens system were analyzed to study the equivalent of the negative lens focal length;in addition,the laser beam divergent effect was verified by experiment,and the equivalent focal length was deduced,which was almost agreed with theoretical analysis.This study is helpful to understand the principle of gas lens,and to provide some reference for the design and optimization of gas lens.The main research contents and results included were as the follows:(1)Using the finite element software,the three-dimensional model of the gas negative lens was constructed and simulated,and the numerical solution of the temperature,density and refractive index distribution in the gas negative lens.The results showed that the temperature or density distribution on the central axis of rotational symmetry,isothermal surface or surface density as a series of concentric cylinders around the optical axis of the surface;approximation,the cross section of the refractive index distribution and radial position meet parabola relationship;through numerical integral to obtain the phase axial radial gradient index caused by the propagation of light the delay distribution of phase delay distribution by least squares fitting,the fitting results show that with the parabola function.(2)The equivalent focal length of gas negative lens is obtained based on the curvature of phase delay distribution.The key parameters of the equivalent focal length,such as the length of the negative gas lens,the temperature of hot air flow,the axial velocity of the hot gas flow,the temperature of the coolant and the heat transfer coefficient,are simulated.Results show:a).The shorter the length of the structure,the larger the equivalent focal length is.The reason for this phenomenon is that the hot air temperature gradient formed in a very short tube is very small,resulting in reduced thermal lens effect.However,the equivalent focal length kept constant as the length of the structure increased to a certain value.b).With the increase of the heat flow temperature and axial velocity of the proposed gas filled negative lens,the equivalent focal length overall decreased.As the hot air flow axial velocity increased from 0 to 10m/s,the equivalent focal length rapidly decreased.When the axial velocity is greater than 20m/s,the equivalent focal length almost reached to a steady value.c).The improvement of the heat exchange conditions of gas negative lens,such as increasing the flow rate of cooling liquid or reducing the coolant temperature,will result in enhancing the effect of thermal lens and reducing the equivalent focal length.(3)We have used the CCD method to measure the parallel laser beam passing through the spot size of the divergent degree before and after the gas negative lens.Based on the optical transmission matrix and the ABCD rule of the Q parameter,the equivalent focal length of the len has been deduced,according to the relation between the object and the Gauss transform.The experimental results are in good agreement with the theoretical analysis,and the ultra high strength parallel laser beam generation system is summarized.