Far-field Focal Spot Measurement Based On Plenoptic Camera

High power laser facility provides drive energy for inertial confinement fusion and other high energy physics experiments.Precisely controlled laser beam is focused onto and act upon the target material.The form and energy distribution of far-field focal spot are crucial to the experiment results.So accurate measurement of the focal spot at target is necessary.To achieve pretty high measurement dynamic range is the main challenge,as a single detector is far from completing the work.To solve the problem,schlieren method was proposed.Although it is effective and practical,it is short of flexibility and the optical path is some heavy.To improve the measurement dynamic range of single detector while keeping the optical path or measurement process simplified,plenoptic camera is introduced to focal spot measurement.To achieve effective application of plenoptic camera,it is necessary to go deep into the working principle and design technique,so the basic content on plenoptic camera was researched and clarified first.Starting from basic theory on light field imaging,the light filed capture rules of plenoptic camera were studied through paraxial ray tracing,and the principle of data processing along with the camera structuring method were explained as well.The paper compared the unfocused plenoptic camera and focused plenoptic camera,and the conclusion was verified by simulating the imaging of an on-axis point source.The primary problem of measuring focal spot by plenoptic camera is imaging.To find out how the irradiance property of focal spot which is different from natural scene affect the imaging process,beam propagation and imaging process was simulated based on physical optics,and the results show that the focal spot beam spread angle should be agreed with the NA of main lens of plenoptic camera to take advantage of plenoptic camera.To accommodate the tiny size of focal spot,structuring of plenoptic camera with specific spatial resolution was analyzed and verified by imaging simulation based on geometric optics.The feasibility of improving the measurement dynamic range of single detector based on plenoptic camera was analyzed from two perspectives,and complete simulation using Zemax and MATLAB was designed to verify the conclusions.First,image reconstruction process of plenoptic camera has positive effect on improving the SNR of measurement.Second,a novel method to improve the measurement dynamic range which is based on the “sub-aperture imaging” property of plenoptic camera was proposed.A certain degree of intensity attenuation is applied to the bottom half aperture of main lens,and increasing the integral time of detector by the degree which is agreed with the degree of attenuation at the same time.The measurement results from top half and bottom half aperture were reconstructed respectively,then correcting the saturated pixels of top half aperture result with corresponding pixels of bottom half result,and the two results were merged into the final result.A plenoptic detector was built based on an existing micro lens array.First,constructed with a 35 mm fixed focal length lens,the rotation angle and distance between the micro lens array and detector were calibrated.Results shows that the rotation angle is small but has an obvious negative effect on the image reconstruction and the distance agrees with the designed value in general.Then,a focal spot light field imaging system with a 5 ?m spatial resolution was built using the plenoptic detector and a 100 mm focal length objective lens.By comparing the reconstructed focal spot image with the result from a conventional detector which captured the same focal spot,it shows that the measurement dynamic range of the plenoptic detector is about 5 times the conventional detector,and from the result of the system,it is found that the micro lens will show some diffraction effect of rectangular aperture during the focal spot capturing which may affect the measurement accuracy.