Research On Detection And Control Of Beam Direction In Beam Cleanup Of Solid-state Slab Lasers

Beam cleanup adaptive optics systems are widely employed in high-power solidstate lasers for improved beam qualities.Beam stabilization is one of the most important roles of an adaptive optics systems,in which accurate detections of the beam directions are critical.There are generally two well accepted methods to detect the direction of a laser beam in an adaptive optics system.One method is by averaging the wavefront slopes from each sub-apertures of a Shack-Hartmann wavefront sensor(the mean slope method).The other one is by detecting the far-field spot location of the entire beam(the far-field spot method).An additional far-field camera and its real-time signal processing system are necessary for the far-field spot method,thus the mean slope method is employed in many adaptive optics systems for decreased costs and complexities.However,owing to the edge effects of the high-power pumped slabs,and the diffraction effects of the hard-edged scrappers of unstable resonators,the wavefront slopes at the edge of the beam can be very large with fluctuating intensities.This would lead to reduced accuracy of the mean slope method and deteriorated beam stabilization.A series of numerical simulations are proposed to study this problem.As the ultimate expectation of beam stabilization is generally reducing the spreading of the beam on the target to enable high power intensities,the far-field method it considered as the basis of evaluations throughout this dissertation.A numerical model of studying the aberrated rectangular beam from a solid-state slab laser is established,whose phase distortions are generated using the first 64 two-dimensional Legendre polynomials.Both methods are applied to calculate the direction of the beam containing the aberrations.The mean slope method is found to bring in errors owing to limited spatial sampling frequencies of the Shack-Hartmann wavefront sensor,especially for high order aberrations.Then the aberrations of the laser beam from a high-power solid-state slab laser is detected using a Shack-Hartmann wavefront sensor,and the direction of the beam is calculated using both methods.The measurements indicate that the large wavefront slopes and fluctuating intensities at the edge of the beam could lead to detection errors of the beam direction.An approach of removing the wavefront data from the edge subapertures in the average slope method(average slope method with removed edge subapertures)is proposed.Simulations with the measurements data show that this method would further improve the accuracy of beam direction detections.A verification experiment system is established.Large wavefront slopes and intensity fluctuations are added to the edge of the beam,and the average slope method with removed edge sub-apertures is tested.Experiments show that this method significantly improved the beam stabilization.Finally,the proposed method is applied to the high-power solid-state slab laser.The RMS of the beam jitter is decreased from 1.53 ?rad(horizontal)and 1.30 ?rad(vertical)achieved by the mean slope method to 1.12 ?rad and 0.87 ?rad.The above results demonstrate the proposed method is effective for solid-state slab laser beam cleanup adaptive optics systems.