Thermal Blooming Effect Of Hermite-Gaussian Beams And Hermite-Gaussian Array Beams Propagating Through The Atmosphere

Laser weapons are a kind of rapidly developing new concept weapon,which is an important application of high energy lasers.The beam quality of high energy lasers beam propagating through the atmosphere is seriously damaged due to the thermal blooming effect.In theory,high-order mode laser beams in Cartesian and cylindrical coordinate systems can be described by Hermite polynomials and Laguerre polynomials,respectively.However,the thermal blooming effect of Hermite-Gaussian(H-G)beams propagating through the atmosphere has not been reported.Because the output power of a single laser is limited,beams combination is one of the methods to obtain higher output power.It is very important to study the thermal blooming effect of H-G array beams propagating through the atmosphere.In addition,multiple mode laser beams are often encountered in practice,and they have higher light-to-light conversion efficiencies than single-mode laser beams.The study of the thermal blooming effect of multiple mode beams and multimode array beams propagating through the atmosphere is of great significance to the application of laser weapons.The main research contents and conclusions of this thesis include:1.The thermal blooming effect of H-G beams propagating through the atmosphere is studied.The analytical expression for the distortion parameter of steady-state thermal blooming of H-G beams propagating the atmosphere is derived.It is shown that,under the same beam power,the higher the mode order of H-G beams is,the weaker the thermal blooming effect is.In particular,the thermal blooming effect for a H-G beam is weaker than that for a Gaussian beam.A 4D computer code of a high-power H-G beam propagating through the atmosphere is designed.It is found that as the mode order increases,the beam width at the target may decrease due to the thermal blooming effect although the initial beam width increases.As the mode order of H-G beams increases,the shift of the beam centriod position decreases,but it takes more time to reach the steady-state thermal blooming.When the cross-wind velocity is large enough,the symmetry of asymmetric H-G beam may be improved by the thermal blooming if the astigmatism parameter is adopted to characterize the symmetry of H-G beams.2.The thermal blooming effect of incoherent H-G array beams propagating through the atmosphere is studied.By analyzing the distortion parameter of steady-state thermal blooming of incoherent H-G array beams propagating the atmosphere,it was found that,the higher the mode order of H-G array beams is,and the bigger inverse radial fill factor of H-G array beams is,the weaker the thermal blooming effect is.A 4D computer code of a high-power incoherent H-G array beam propagating through the atmosphere is designed.It is found that as the mode order of H-G array beams increases,and the inverse radial fill factor of H-G array beams increases,the shift of the beam centriod position decreases,but it takes more time to reach the steady-state thermal blooming.In addition,the focal shift in the atmosphere of incoherent H-G array beams is studied.The results show that: the higher the mode order of H-G array beams is,the closer the focus is to the target,and the smaller the beam width at the target is.3.The thermal blooming effect of multimode beams and multimode array beams propagating through the atmosphere is studied.The multimode beam is composed of several modes with the coaxial and incoherent combination.A 4D computer code of a high-power multimode beams and multimode array beams propagating through the atmosphere is designed.It is shown that a minimum beam spot on the target can be obtained if the optimal weighting factor of multimode beams and multimode array beams is adopted.In addition,the larger the weighting factor of GS beam in a multimode array beam is,the farther the focus is from the target.