Phase Vortex Detection And Compensation Of Laser Beam Propagating Through The Near Groud Turbulent Atmosphere

The refractive index disturbance in turbulent atmosphere will cause fluctuations of both amplitude and phase.These reduce the performance of imaging and laser projection systems.In order to suppress the influence of atmospheric turbulence on laser propagation,adaptive optics system is used to correct the wavefront distortion and improve the performance of optical systems.When laser beam propagates through the near ground strong turbulent atmosphere,phase vortex arises.It has been confirmed that conventional wavefront reconstruction algorithms cannot recover phase vortex,making the correction of adaptive optics system decline.In order to improve the performance of adaptive optics system,a phase vortex reconstruction algorithm acclerated by cascadic multigrid method is proposed.In addition,phase vortex with 4 topological charge is recovered through experiment which verify the effectiveness of this algorithm.And the feasibility of near ground atmosphere compensation using phase vortex reconstruction algorithm is analyzed.The main research contents of this thesis are as follows:Firstly,the numerical simulation model of laser propagation through near ground atmosphere is established.The correction of adaptive optics system under different atmospheric conditions is analyzed.The origin of phase vortex in wavefront distortion under strong scintillation is given.Studies have shown that the performance of adaptive optics system is negatively correlated with the number of branch points,and the inaccurate reconstruction of phase vortex is the main limit factor of adaptive optics system.With the discontinuous phase containing phase vortex accurately restored,the correction of adaptive optics system is significantly improved.This investigation clearly pointed out that the recovery of phase vortex is one of the key problems in near ground atmosphere compensation.Secondly,this paper proposes a complex exponential reconstructor accelerated by cascadic multigrid method for phase vortex reconstruction.Wavefront reconstruction residual error and computational complexity of the proposed algorithm for recovering phase vortex and random continuous phase are analyzed.The number of float point multiplications required by the cascadic multigrid method is nearly 2 order of magnitude lower than that required by the direct iteration.With the number of subapertures increasing,the acceleration of the cascadic multigrid method is improved.In addition,the phase vortex measurement experiment based on Shack-Hartmann wavefront sensor is built.The effectiveness of the algorithm is verified by detecting and recovering the phase vortex with topological charge of 4 in the experiment.Thirdly,the feasibility of atmosphere compensation with complex exponential reconstructor algorithm in different beacon patterns is analyzed.For point source beacon pattern,the performance of adaptive optics system using complex exponential reconstuctor for wavefront reconstruction is significantly improved compared with that using the direct slope method.Furthermore,for active illumination beacon pattern,the complex exponential reconstructor can also recover the wavefront distortion and improve the correction of adaptive optics system.Those indicate that the proposed wavefront algorithm has potential applications in near ground atmosphere compensation.Above all,a phase vortex reconstruction algorithm accelerated by cascadic multigrid method is proposed,and the effectiveness of the wavefront reconstruction algorithm is verified by experiment,which lay the foundation for near ground atmosphere compensation.