Research On Far-Field Laser Spot Monitoring Method Based On Statistical Characteristics Of Echo Speckle

Since the birth of laser,it has been widely used in various fields such as industry,medical treatment,national defense and so on.With the continuous development of laser application technology and the progress of science and technology,in many applications,it is necessary not only to have higher output power of laser beam,but also to obtain laser spot with high energy density after long-distance propagation to the target surface.In the course of beam propagation,due to the system jitter,the atmospheric turbulence,the thermal effect of the laser itself and other aspects,the focusing effect of the laser beam in the far field will be affected,which leads to the impossibility of obtaining the ideal high energy density focusing spot at the target.Therefore,in the laser beam long-distance focusing application,it is necessary to monitor the spot on target surface and feed the obtained spot information back to the focusing system,so as to improve the focusing effect of the laser beam.In this paper,the monitoring method of far-field laser spot based on statistical characteristics of echo speckle is selected as the main research content.This method overcomes the limitation that the traditional method of spot detection often needs to detect the spot at the target or close to the target.At the same time,compared with the far-field speckle imaging method that applies telescopic system,speckle monitoring can overcome the limitation of optical system parameters to a greater extent,and has a higher resolution accuracy.The evaluation factors obtained by the speckle monitoring method can reflect the scale change of the far-field focusing spot and help the control system to improve the focusing effect.Through the in-depth study of the statistical characteristics of echo speckle,the laser focusing spot evaluation method for spot on static target based on static speckle and for spot on high-speed moving target based on dynamic speckle are presented respectively.The feedback factors obtained from these monitoring methods can reflect the changing trend of the far-field focus spot size and have high detection accuracy.In this paper,the simulation experiments of several evaluation factors are carried out.The simulation analyses of the parameters involved in the system,the application constraints and other factors are conducted,and an experimental platform is built to verify the results of the simulation analyses.The main works of this paper are summarized as follows:1.Since the birth of laser,it has been widely used in various fields such as industry,medical treatment,national defense and so on.With the continuous development of laser application technology and the progress of science and technology,in many applications,it is necessary not only to have higher output power of laser beam,but also to obtain laser spot with high energy density after long-distance propagation to the target surface.In the course of beam propagation,due to the system jitter,the atmospheric turbulence,the thermal effect of the laser itself and other aspects,the focusing effect of the laser beam in the far field will be affected,which leads to the impossibility of obtaining the ideal high energy density focusing spot at the target.Therefore,in the laser beam long-distance focusing application,it is necessary to monitor the spot on target surface and feed the obtained spot information back to the focusing system,so as to improve the focusing effect of the laser beam.2.According to the statistical characteristics of speckle field,several far-field speckle monitoring methods based on static speckle are presented for rough target in relative static state.The integral evaluation factor on the edge of speckle is obtained based on the average scale of speckle,which decreases when the focus spot size decreases.The feasibility of the evaluation factor is verified by simulation analysis and experiment.Based on speckle correlation theory,the evaluation factor of speckle autocorrelation method is given,and the influence of rough target characteristics on the detection accuracy of evaluation factor is analyzed.The theoretical analysis results are verified by simulation and experiment.The above evaluation factors can be used to judge the change of the far-field spot size,but the theoretical results are not further analyzed and the theoretical constraints in application are not given.Then,according to the theory of speckle intensity threshold crossing,a speckle image sequential extraction method(SPSE)is proposed to obtain the speckle evaluation factor,which solves the above problems.The SPSE factor can not only reflect the scale change of the far-field focusing spot,but also quantitatively describe the spot size under certain conditions.The feasibility of SPSE factor is verified by simulation,and the influence of application constraints and receiving system caliber on the performance of evaluation factor is analyzed.Finally,the simulation results are verified by experiments.3.In this paper,the statistical characteristics of dynamic speckle are studied deeply,and the feasibility of applying dynamic speckle to evaluate the scale of far-field speckle is analyzed.The dynamic speckle model-scattering point motion model is established,and the rationality of the model is verified by simulation analyses.When analyzing the intensity distribution of dynamic speckle,the model only needs to calculate the intensity fluctuation in a specific position,which saves a lot of time in the simulation.In the follow parts of this paper,a method is proposed to obtain the far-field laser focus spot evaluation factor by means of double detectors cross-correlation.By observing the delay of the correlation peak,the change of the spot size is judged,and the time delay of the correlation peak increases when the focus spot size increases.The feasibility of the scheme is verified by simulation,and the influence of the distance between two detectors and the length of sampling data on the detection performance are simulated and analyzed.The simulation results are verified by experiments,and the experimental data show that the method is still effective under the condition of adding atmospheric disturbance.Finally,according to the time characteristics of the dynamic speckle field,a method to judge the scale change of the far-field focused spot is proposed based on the spectrum of the intensity fluctuation of the dynamic speckle.On the basis of the backscattering enhancement effect,a double-pass closed-loop(TIL)system is used to improve the signal-to-noise ratio of the received signal.In this method,the far-field spot can be monitored by a single-point detector,and the theoretical analysis results are verified by simulation and experiment.At the same time,the method to improve the monitoring accuracy of the system is given.