Thermal Blooming Effect Of Rotating Elliptical Gaussian Laser Beam And Radially Polarized Vector Beam Propagating Through The Atmosphere

With the rapidly development of free-space optical communication,laser weapon,energy transfer and imaging,high-power laser has become a research hot spot.Nonlinear thermal blooming effect is one of main problems for the high-power laser atmospheric propagation,i.e.,when a laser beam propagates through the atmosphere,a fraction of the laser energy is absorbed by the air along the propagation path,and then the laser beam spreads and is distorted due to thermally induced refractive index gradients so that the beam quality of high-power laser beam propagating through the atmosphere is seriously damaged.Therefore,it is of significance to study the thermal blooming effect of laser atmospheric propagation and how to reduce its influence on beam characteristics and beam quality.In practice,the astigmatism is often encountered.The general astigmatism Gaussian beam is also called the rotating elliptical Gaussian beam(REGB).However,the thermal blooming effect on REGB propagating through the atmosphere has not been reported.On the other hand,vector beams have been proved to have greater resistance to the turbulent atmosphere and better image quality in complex sea fog environment than the scalar beams due to its unique polarization distribution and structure.Radially polarized(RP)vector beam is one of the typical representatives of vector beams which attracts more and more attention due to its important applications in optical data storage,particle trapping and acceleration,microscopy and laser cutting,etc.Therefore,it is very important to study whether the RP vector beams can resist the thermal blooming.The main research contents and conclusions of this thesis include:1.The thermal blooming effect of rotating elliptical Gaussian beams propagating through the atmosphere is studied.The analytical formula of the distortion number of focused REGBs in the atmosphere is derived,which is an important parameter to describe the strength of thermal blooming.A time-dependent computer code for REGBs propagating through the atmosphere is developed to prove its applicability.It is found that at the same initial power,the thermal blooming effect of REGBs becomes weaker as the rotation parameter c decreases.Namely,a REGB with smaller value of c has greater resistance to the thermal blooming effect,and a REGB has greater resistance to the thermal blooming effect than an simple astigmatism Gaussian beam,which may be useful for laser applications in the atmosphere.On the other hand,the dependence of thermal blooming on the astigmatism parameter Q is not monotonic,there exists a maximum of the distortion number as Q varies,i.e.,the REGB is most affected by thermal blooming when a certain value of Q arrives,which should be avoided in practice.In addition,it is shown that the orbital angular momentum(OAM)and the gradient force of REGBs decrease because of thermal blooming effect in the atmosphere,which greatly weakens the ability of REGBs to capture particles.2.The thermal blooming effect of radially polarized vector vortex beams propagating through the atmosphere is investigated.A four-dimensional(4D)computer code of a high-power RP vortex beam propagating through the atmosphere is designed.The effects of thermal blooming on beam quality of RP vector vortex beams of different topological charges are investigated by numerical simulation.The effects of thermal blooming effect on Stokes vector and polarization structure of RP vector vortex beams are studied as well.It was found that,the RP vector vortex beam has better beam quality than the scalar vortex beam.With the same initial beam power and topological charge,compared with the scalar vortex beam,the RP vector vortex beams have smaller centroid shifts,β factor,beam width and larger peak intensity.That is,the RP vector vortex beam is not only less affected by thermal blooming than the scalar vortex beam,but also has better thermal blooming resistance.Further more,the higher the topological charge of RP vortex beams is,the smaller shift of the beam centroid position,the β factor and the beam width and larger Strehl ratio and peak intensity of RP vortex beams are.Besides,the thermal blooming effect can distort the Stokes vector of RP vector vortex beam so that the patterns of Stokes vector of RP vector vortex beam are like crescent.One also finds that the radial polarization structure of a RP vector vortex beam is destroyed during propagation,and the RP vector vortex beam become a partially polarized beam.