Study On Influence Of Atmospheric Thermal Blooming Effect On The Propagation Characteristics Of Elliptical Gaussian Beams And Vortex Beams

High-energy laser beam plays an important role in optical communication,laser weapon and other fields because of its good directivity,high energy and long action distance.When a high-energy laser beam propagates through the atmosphere,it will encounter nonlinear thermal blooming effect,which will result in spreading and distortion of the high-energy laser beam.The beam quality of laser will become worse because of thermal blooming effect,which severely limits the energy propagation efficiency.In practice,the beam produced by high-energy laser is more nearly elliptical in cross section,but the effect of thermal blooming on an elliptical beam propagating in the atmosphere has not been reported.On the other hand,vortex beam has a wide application prospect in the fields of laser communication,particle manipulation and information transmission due to the particular properties of carrying orbital angular momentum(OAM)and topological charge,which is one of the hotspots of current research.However,there are few reports on the effect of thermal blooming on atmospheric propagation characteristics of vortex beams.In this thesis,we investigate the influence of atmospheric thermal blooming on the propagation characteristics of elliptical Gaussian beams(EGB)and vortex beams.The main research results are summarized as follows:1.The thermal blooming effect of EGB propagating through the atmosphere is discussed.The analytical expressions of the intensity at the target for steady-state thermal blooming and the thermal distortion parameter of an EGB propagating in the atmosphere are derived.It is found that,as the beam width in the windward direction of the source plane is larger i.e.,the beam width along wind direction on the source plane is smaller(same power and spot area),the thermal blooming effect on EGB is weaker.Moreover,a four-dimensional(4D)computer code is designed to simulate propagation of a focused EGB in the atmosphere,It shows that for a focused EGB,the beam width in the windward direction of the source plane is larger,the beam symmetrical spot is better,and the energy focusability is higher at the target,which means better beam quality.2.The thermal blooming effect of vortex beams propagating through the atmosphere is discussed.Taking Laguerre Gaussian(LG)vortex beam as an example,a four-dimensional(4D)computer code is designed to simulate the propagation of a vortex beam in the atmosphere.It is found that,at the same power,the thermal blooming effect for a vortex beam is weaker than that for a GS beam.The shift of the beam centriod position and beam width of vortex beam are smaller than GS beam at the target.Due to the atmospheric anisotropy,the thermal blooming will lead to the singularity splitting of the vortex beam,and the number of tilted side lobes near the center of the beam spot is exactly equal to initial topological charge,which propose a method to detect the topological charge of the source laser.Moreover,the essential mechanism of the thermal blooming effect of a vortex beam propagating through the atmosphere is studied by using the Poynting vector and the density distribution of OAM.It is found that the thermal blooming effect will lead to the decrease of OAM of the vortex beam.As the wind velocity decreases and the propagation distance increases,the thermal blooming effect becomes more serious so that the beam vortex structure is destroyed more seriously,which also makes the OAM decreases.Furthermore,due to nonlinear thermal blooming effect,the OAM does not always increase with the beam power,but there is a critical power to maximize OAM.