Research On The Measurement Method Of Inner Surface Defects Of Target Shot Based On Optical Microscope Zero-position Interferometry

The capsule,which is between a few hundred micrometers in diameter and a few millimeters,is one of the core devices used in the experiment with Inertial Confinement Fusion(ICF).Any isolated defect on the inner surface may cause asymmetric compression and lead to ignition failure.Null Interferometric Microscope is one of the most promising methods for detecting the inner surface defects of capsule.In this paper,the following researches were carried out from the perspective of optical space layered diffraction calculation,aiming at the problem of the surface morphology information of target shot inside and outside and the defect information of capsule inside defocusing:Based on the scalar diffraction theory,the mapping relationship between the diffraction surface and the observation surface is established through mathematical derivations,Kirchhoff's diffraction integral formula and Rayleigh-Sommerfeld formula are given in the spatial domain,and the plane angular spectrum diffraction formula is given in the frequency domain.The sampling problem of diffraction surface and the sampling problem of transfer function in the process of discretization are analyzed emphatically.Aiming at the problem of mixing inner and outer surface defect information of capsule,a model of restoring defect information of capsule was studied.The model stratifies the outer surface and inner surface of the capsule in the field of view in the object space with the depth of field as the step,and transforms the stratification of the object space into the stratification of the actual image plane in the image space by using the conjugation relation of the object image.On this basis,virtual lens is introduced to construct the virtual image surface with conjugate relation with the actual image surface,and the wave surface restoration at the detector surface is simplified to the inversion from the wave surface at the detector surface to the virtual image surface.On the basis of virtual lens,the original problem of aliasing information under point projection is transformed into the problem of aliasing information under parallel projection.By region division,the relationship between the complex amplitude of the detector surface and the virtual image surface function of the front inner surface and the front outer surface is established.Aiming at the calibration problem of axial distance,the self-focusing algorithm is studied.Diffraction calculation,surface stratification,self-focusing algorithm and virtual lens are combined into the final interlayer demodulation algorithm.In order to verify the reliability of the interlayer demodulation algorithm.The key steps of the interlayer demodulation algorithm are simulated and analyzed.Firstly,the influence of virtual lens on the reconstruction of complex amplitude is analyzed.Then,simulation analysis is carried out for the two methods of solving the interlayer modulation quantity.The first method is to solve the refocusing region image of each layer in the space layer,and the accurate topography information of the outer surface is combined by the splicing method.In the second method,the transfer function is improved by using the frequency-domain splicing method,so that only one diffraction calculation is needed to recover the entire front and outer surface defect morphology information map.Finally,the simulation results of interlayer demodulation algorithm under aberration are analyzed,and the results show that the recovery results are good.The Null Interferometric Microscope system is set up and the key parameters are analyzed.For the defect information of the front inner surface of the target shot with a diameter of 2mm,the interlayer demodulation algorithm was used to restore,and the distribution diagram of the defect on the inner surface of the restored single subaperture was compared with the centroid position of the original defocusing defect on the inner surface.Secondly,the internal surface defects restored by the algorithm under the same subaperture are compared with the results of internal surface defects obtained by direct imaging on the front internal surface.It is proved that the internal surface defects restored by the restoration algorithm proposed in this paper can eliminate the influence of external surface defect modulation and solve the problem of selfdefocusing.Finally,in the error analysis,the influence of the target surface roughness on the inner surface defect recovery is analyzed emphatically,and it is found that the inner surface defect recovery is good under the condition of less roughness.