摘要
为提高激光大气传输的靶面光束质量,开展了聚焦高斯光束在大气热晕效应影响下最佳焦距的定标研究。大气热晕效应会导致聚焦高斯光束发生焦移,使实际焦点移动到靶面之前,严重影响靶面光束质量。采用预散焦的方法进行调焦,得到透镜的最佳焦距,使得靶面峰值光强最大。通过物理分析以及大量的数值计算,给出了最佳焦距与广义热畸变参数(N)、靶面距离(z_(0))以及光束初始束宽(w_(0))的定标关系式。由此定标关系式可以预知不同大气条件以及光束参数下对应的透镜最佳焦距。该关系式在N<11、菲涅耳数N_(F)>4时适用,且平均相对误差为5.2%。另外,研究表明:广义热畸变参数N越大,调焦后的靶面峰值光强的提升越大,调焦效果越明显。研究结果对于激光大气传输的相关应用有重要意义。
Objective Laser beam propagating in atmosphere is affected by many effects such as turbulence,diffraction,selffocusing,and thermal blooming,which is an extremely complex physical process.Although the numerical simulation method can be used to predict the effects of different atmospheric conditions and beam parameters on laser propagation,its computational efficiency is insufficient for practical applications.Therefore,the study of scaling law becomes essential.However,on the one hand,most studies on the scaling law of laser atmospheric propagation focus on beam spreading.On the other hand,the thermal blooming effect causes the focus shift of a focused Gaussian beam,and the actual focus point is shifted to the position in front of the target plane,which severely degrades the beam quality at the target plane.In this paper,to achieve the maximum peak intensity at the target plane,the scaling law for the optimal focal length of a focused Gaussian beam under thermal blooming effect is investigated.Methods Thermal blooming effect of a laser beam propagating through atmosphere can be described by the paraxial wave equation and the hydrodynamic equation.A timedependent fourdimensional code to simulate the focused Gaussian beam propagating in atmosphere is developed by using the multiphase screen method,fast Fourier transform method,and difference method.In this paper,the method of predefocusing is employed to obtain the maximum peak intensity at the target plane.To predict the optimal focal length fopt under varying atmospheric conditions and beam parameters,this paper employs the controlled variable method to numerically analyze the effects of six scaling factors(initial laser power P,atmospheric wind speed v,atmospheric absorption coefficientα,initial beam width w0,target plane distance z0,and wavelengthλ)on fopt.Through physical analysis and extensive numerical calculations,a formula of the optimal focal length fopt relating with the six scaling factors is established.Results and Discussions In this paper,the method of predefocusing is employed to determine the optimal focal length of the lens,thereby achieving the maximum peak intensity at the target plane(Figs.1 and 2).First,three scaling factors(P,v,andα)are considered,which significantly affect the intensity of thermal blooming effect.The generalized thermal distortion parameter N is a physical quantity used to describe the intensity of thermal blooming effect.A scaling law between fopt and N is investigated[Fig.3 and Eq.(10)].In contrast,when the other three scaling factors(z0,w0,andλ)are changed,both the intensity of the thermal blooming effect and the intensity of the diffraction effect are altered(Figs.4‒6).Therefore,it is necessary to modify Eq.(10).The scaling law of fopt relating with N,z0 and w0 is modified on the basis of Eq.(10)[Fig.7 and Eq.(13)].This scaling law can predict the optimal focal length under various atmospheric conditions and beam parameters.When N<11 and Fresnel number NF>4,this scaling law is applicable,and the mean relative error is 5.2%[Fig.8 and Eq.(13)].Additionally,it is shown that the increase of peak intensity at the target plane after focusing is greater when N is larger(Fig.9).Conclusions In this paper,a scaling law for the optimal focal length of a focused Gaussian beam considering atmospheric thermal blooming effect is studied.The method of predefocusing is employed to determine the optimal focal length of the lens,thereby achieving the maximum peak intensity at the target plane.Using the controlled variable method,the influence of six scaling factors(P,v,α,z0,w0,andλ)on the optimal focal length fopt is obtained by varying them,respectively.Then,the scaling law of fopt relating with the generalized thermal distortion parameter N,the target plane distance z_(0)and the initial beam width w0 is established.This scaling law is applicable when N<11 and Fresnel number N_(F)>4.This scaling law can predict the optimal focal length under various atmospheric conditions and beam parameters,and the mean relative error is 5.2%.Additionally,it is shown that the increase of peak intensity at the target plane after focusing is greater when N is larger.It is worth noting that laser atmospheric propagation is a very complex physical process.In practical application,the beam quality at the target plane is also related to the beam quality at the source plane,beam truncation ratio,tracking accuracy,and atmospheric turbulence.The results obtained in this paper are useful for the application of laser atmospheric propagation.
作者
邓忠俊
李晓庆
万沛源
季小玲
Deng Zhongjun;Li Xiaoqing;Wan Peiyuan;Ji Xiaoling(College of Physics and Electronic Engineering,Sichuan Normal University,Chengdu 610068,Sichuan,China)
出处
《中国激光》
北大核心
2025年第11期175-182,共8页
Chinese Journal of Lasers
基金
国家自然科学基金(61775152,61505130)
四川省科技厅项目(2022NSFSC1836)。
关键词
热晕效应
调焦
最佳焦距
定标规律
thermal blooming effect
focusing
optimal focal length
scaling law
