Thermal Blooming Effect Of Vortex Beam Propagation In The Atmosphere

High-power laser has many advantages such as good directivity,high energy fast reaction speed strong anti-electromagnetic interference ability and so on.It has shown great application value in military,material processing and other fields.When the high-power laser propagation through the atmosphere,the atmospheric thermal blooming effect will cause the destructive distortion,energy attenuation and beam quality decline and other negative effects,which seriously limits the development and application of high-energy laser.At present,the study of thermal blooming effect is mostly based on Gaussian beam,the theoretical simulation work on thermal blooming effect of vortex beam is relatively few.And vortex beam has special properties such as spiral wavefront and topological charge,which has shown attractive application prospects in integrated optics,quantum security system,laser communication and information transmission.Therefore,the study of thermal blooming effect of vortex beam propagation in the atmosphere has important guiding significance for the development and application of high-energy vortex beam in the fields of atmospheric optics,laser weapons,free space optical communication and so on.This paper will mainly study the thermal blooming effect of vortex beam transmission in the atmosphere,which is divided into the following three aspects:Based on the paraxial scalar wave equation,hydrodynamic equations and Lorentz equation,and based on the fast Fourier transform,the steady-state thermal blooming effect of a vortex beam propagation in the atmosphere is investigated using the multiple phase-screen simulation method.The variations of propagation distance,initial power,transverse wind speed,atmospheric absorption coefficient,initial radius and wavelength are demonstrated to illustrate the change in the thermal blooming effect.The results show that the thermal blooming effect can be enhanced by increasing the initial power,propagation distance and atmospheric absorption coefficient of a vortex beam propagation in the atmosphere without changing other parameters.And increasing the transverse wind speed,initial radius,and wavelength can reduce the thermal blooming effect.Based on the perturbation method of geometrical optics,the analytical expressions for the thermal blooming of vortex beams with no wind and the thermal blooming with wind are derived,and the thermal blooming of vortex beams with no wind and the thermal blooming effect with wind are studied.The results show that when there is no wind,the annular structure of the vortex beam changes into a double-annular structure due to the thermal lens effect.When other parameters are fixed,the increase of initial power,propagation distance and atmospheric absorption coefficient will increase the thermal blooming caused by thermal effect.In the case of x direction transverse wind,the intensity distribution of vortex beam distorts obviously along the wind direction,and the thermal blooming effect increases with the increase of thermal distortion parameters.On the other hand,the thermal blooming distortion and transverse centroid offset of the vortex beam propagated in the atmosphere based on the multi-phase screen method and the perturbation method are compared and analyzed with different parameters.The results show that the centroid offset of the spot obtained by the two methods is basically the same in the near field transmission,but with the increase of propagation distance,initial power and absorption coefficient,the results obtained by the two methods will be significantly different.Based on the random phase screen of atmospheric turbulence generated by power spectrum inversion method,the far field and phase distribution of vortex beam under the influence of turbulence and thermal blooming are numerically simulated by using the multi-phase screen method and Fourier transform.The effects of turbulence intensity,propagation distance,wavelength,wind speed and topological charge on vortex beam under the combined action of turbulence and thermal blooming are analyzed in detail.The results show that turbulence distorts the intensity distribution and the isophase lines of the vortex beam.The thermal blooming causes the peak intensity of the vortex beam to shift,forming an asymmetrical intensity distribution,and the equal-phase line to bend and rotate.Under the combined effect of turbulence and thermal blooming,the distortion of vortex beam is the superposition of the distortion of intensity distribution and phase change distribution caused by both of them.On the other hand,when other parameters remain unchanged,the distortion degree of vortex beam will be increased by increasing turbulence intensity and propagation distance alone,while the distortion degree will be reduced by increasing wind speed and wavelength alone.From the intensity distribution,the structure of the original dark core can still be maintained despite the distortion.From the perspective of phase distribution,when m=1,the original spiral wavefront is destroyed except that the phase singularity remains at the center,and with the increase of topological charge,the phase singularity at the center will also be destroyed.