Research On Application Of Dynamic Speckle Metrics In Beam Control Technology

Directed energy transmission of lasers is of vital importance in diverse fields,especially in industrial engineering and security and defense.To maximize the laser energy density on long-range targets and the efficiency of laser energy utilization,it is necessary to reduce or eliminate possible perturbations on the transmission path.To that end,more research had to be done for high-precision and high-efficiency wavefront sensorless closed-loop beam control technology,thus optimizing the laser energy distribution on the target by compensating the effects of various perturbations in the transmission path on the laser phase in real time.This paper mainly focuses on a closed-loop beam control technology based on the echo speckle metric and Stochastic parallel gradient descent(SPGD)algorithm.It demonstrates the design of a high-performance wavefront sensor-less beam control system.Additionally,a detailed analysis has been done to show the effects of target characteristics,turbulence disturbance and system parameters on the speckle.Furthermore,this study verified that the closed-loop system can be more efficient,simple and stable through feedback and control algorithm.The main research results are as follows:1.The paper reviews the latest research of the wavefront sensor-less adaptive optics technology and the method of monitoring target hit-spot based on speckle field.The theoretical model of closed-loop beam control system is established based on the echo speckle metric and SPGD algorithm.The methods and principles of static and dynamic speckle monitoring far-field spot are presented on the basis of the first-and second-order statistical properties.Meanwhile,the paper also summarizes the principles and influencing factors of the traditional SPGD algorithm and the SPGD algorithm combined with the Zernike mode,providing a theoretical basis for obtaining the ideal high energy density focusing spot at the target.2.The speckle field transmission model in target-in-the-loop system is established,then the effects of target characteristics(height standard deviation,autocorrelation length and illumination area)on the feedback accuracy of speckle metric are quantitatively analyzed.It turns out that when the height standard deviation is much greater than ,the speckle metric can accurately feedback the target spot change due to the complete scatter.In addition,the atmospheric turbulence phase screen is simulated by the power spectrum inversion method,and the reliability and failure of turbulence intensity on the feedback accuracy of speckle metric are analyzed in combination with the receiver aperture size.Moreover,combining with time history of the speckle pattern,the monotonicity between speckle metric and target spot size is studied.The results show that the temporal metric decreases with the increase of target hit-spot size and is more sensitive to the change of tiny beams,but has long measurement time;while the spatial metric increases along with target spot size and has high system bandwidth.3.The author proposes a multi-channel cooperative detection method that integrates the advantages of temporal and spatial speckle metric,and it improves the system bandwidth by (?) times and maintains the accuracy obtained based on channels.With this metric and the SPGD algorithm combined with the Zernike mode,a beam control simulation system is established,which improves the strehl ratio by 30 times and the power density by 19 times of the target beam.Compared with the results obtained by conventional SPGD algorithm,the strehl ratio is improved by 6.7 times,and the power density is improved by 5.1 times.In addition,it is verified that this metric has strong anti-disturbance ability in turbulent environment as the convergence speed is slightly improved and its effect remains stable with the increase of Greenwood frequency.4.It has been verified that the SPGD algorithm incorporating Zernike mode has faster and better convergence than the traditional SPGD algorithm in closed-loop beam control system,thus overcoming its weaknesses of tending to fall into the local extremum.The effects of system parameters(detection mode,transmission environment,target size and velocity)on the system evaluation factors(spatial speckle metric,multichannel cooperative metric and power-in-the-bucket)are analyzed in detail.And it is confirmed that the multi-channel cooperative metric has better feedback,anti-interference capability and target inclusiveness.This research proposes an optimized design and provides data support for applications of the closed-loop beam control system in industrial engineering.Furthermore,the trend of beam control system based on speckle metric and SPGD algorithm is analyzed.