Numerical Simulation And Experimental Study For Effects Of Thermal Blooming On Laser Propagation In The Atmosphere And Compensation With Adaptive Optics

When laser propagates in the atmosphere,it will interact with the atmosphere and produce a series of linear and nonlinear effects.Among them,the thermal blooming effect is one of the most serious nonlinear effects of laser propagation in the atmosphere,which causes the optical beam distorts,scatter and defocus.The transmission efficiency of laser in the atmosphere is seriously limited by the thermal blooming effect,which leads to the instability of laser transmission and affects the beam quality.Therefore,research on the thermal blooming effect and finding a method to reduce this effect are of great significance for the engineering application of lasers.Phase compensation of adaptive optics has proven to be a good correction for the thermal blooming effect.The dissertation studies the theoretical and numerical simulations of the thermal blooming effect,and theoretically and experimentally studies the AO technology of the phase compensation of the thermal blooming effect.In this dissertation,the feasibility of phase compensation for the thermal blooming effect of laser propagation in atmosphere using an adaptive optics?AO?system is verified.The main contents of this article include:Firstly,the constituent gases of the atmosphere are introduced,and the nonlinear thermal blooming effect of laser propagation in the atmosphere is summarized.The generation mechanism of the thermal blooming effect,the factors affecting the thermal blooming effect,and the classification of the thermal blooming effect are explained.A brief analysis of the thermal blooming effect in windy and windless conditions are given.And summarized some methods to reduce the thermal blooming effect.A brief overview of the linear effects of laser propagation in the atmosphere is given.Secondly,by combining the wave equation of paraxial approximation with the hydrodynamics equation,the equations which can describe the thermal blooming effect are obtained after some simplification.Then,the Bradley Hermann distortion number N_D,the collimated beam irradiance distortion parameter N_C and the generalized beam distortion parameters N are introduced.Thirdly,the horizontal propagation steady-state thermal blooming effects of laser beams with different intensity distributions,such as Gaussian beam,flat-top beam,and flat-top beam with center obscuration,have been investigated by numerical simulation.The impacts of the output power,the propagation distance,the beam diameter,and the wind velocity vertical to the propagation direction on the steady-state thermal blooming have been discussed for the above mentioned three kinds of beams.Furthermore,the steady-state thermal blooming induced Strehl ratio degradation and peak intensity offset versus the generalized thermal distortion parameter N after long-path horizontal propagation of laser beams with above mentioned three types of intensity distributions have been derived.The simulation results show that,for certain other parameters,the greater output power or longer propagation distance will induce the stronger thermal blooming,and the increment of the launch diameter or the convection wind velocity vertical to the propagation direction will weaken the thermal blooming oppositely.Furthermore,for laser beams with different intensity distributions,the impacts of the thermal blooming on the propagation are so different.Under the same generalized thermal distortion parameter N,the thermal blooming effect on the Gaussian beam is the most serious,followed by the flat-top beam,and flat-top beam with center obscuration is the smallest.The time-dependent thermal blooming is studied by numerical simulation.The numerical simulation shows the rules of the far-field spot of the focused Gaussian beam as time progresses.In order to evaluate the effect of time-dependent thermal blooming on the quality of the laser beam,far-field spot peak intensity offset?x/a,radius of encircling 83.8%energy and peak Strehl ratio SR calculations can be used to measure system performance as a function of time.The results show that,with the evolution of time,the quality of the laser beam affected by the thermal blooming will be degraded,which is manifested by the deflection of the beam,the far-field spot will be continuously expanded,and the peak Strehl ratio will be decreased,but eventually the spot pattern will be stable.Fourthly,the phase compensation of adaptive optics system for thermal blooming is studied by simulation.And the Zernike mode wave-front correction of steady-state thermal blooming effect is simulated and analyzed.Simulation results show that the closed-loop operation of the AO system can achieve a good effect on the phase compensation of thermal blooming when the Bradley-Hermann thermal distortion number N_D is about 50.In addition,when the N_D is less than 250,the AO system will be able to improve the beam quality of far-field spot.It is important to choose the appropriate Zernike reconstruction order for the correction efficiency and range of thermal blooming effect.Fifthly,the phase compensation of adaptive optics system for thermal blooming is studied by experiment.In the experiment,the adaptive optics system is used to compensate the thermal blooming phase distortion by laser with wavelength 1.064?m propagation in the ethanol?CH₃CH₂OH?has been done.In the experiment,use ethanol as absorption medium instead of horizontal atmosphere and transverse wind speed is generated by moving the platform which carry the absorbing medium.Also,experimental results showing the beam quality of far-field is improved,which is consistent with results obtained numerically.This research can provide a reference value for engineering application of high energy laser propagation efficiently in the medium.In this paper,the thermal blooming effect of laser propagation in the atmosphere is studied in-depth theoretically,and phase compensation is performed using adaptive optical system.A specific scheme and solution method for the thermal blooming effect correction problem are proposed.This research can provide reference value for the application of laser high-efficiency propagation in atmospheric medium.