Profile Extraction Algorithm Researches On Confocal Scanning Measurement Of Curved Substrate Diffractive Elements

With the structure of modern optical elements becoming more and morecomplex and the surface becoming more and more accurate, traditional plane orspherical optical elements cannot meet the needs of modern optical system anymore. Besides, a large number of aspheric surface, free-form surface and curvedsubstrate diffractive elements have been applied in some large scale high-precisionoptical engineering. Moreover, it is a great challenge for non-contact measurementof modern optical elements to reach the requirement of surface accuracy ofnano-scale.Using confocal microscopy measurement which has unique property of axialtomography measurement and high lateral resolution, this paper titled―profileextraction algorithm researches on confocal scanning measurement of curvedsubstrate diffractive elements‖has been proposed. It can achieve the measurementof the diffraction element on curved-surface substrate. This subject focuses on theSinc~2peak extraction algorithm of confocal axial response and profile extractionalgorithm for curved substrate diffraction elements. Besides, software based onLabVIEW has been designed and experimental verification has been executed, themain content of the subject including:(1) Based on Debye diffraction integral theory, through approximate calculation,it is proved that axial response can also be expressed as Sinc~2function model in ahigh NA system. And the model of Sinc~2peak extraction has been developed.(2) Carry out the research of Zernike polynomial fitting method for profileextraction. Using two methods to solve the Zernike polynomial coefficients: first,using Householder transform to realize orthogonal triangulation of the coefficientmatrix of contradictory equations, directly obtaining the fitting coefficient; second,utilizing the covariance matrix as solving linear equations augmented matrix to gainthe solution of Zernike polynomial coefficients.(3) Carry out the research of Gauss profile extraction method which pointed outthat close loop Gauss filter causes non-adaptive in handling open boundary effects.With regression analysis algorithm, to attain dynamic weight function, thenon-parametric fitting open loop Gauss filter method was put forward to solve theproblem of boundary effects.(4) Design the confocal scanning measurement software with LabVIEW. Thesoftware has a nice man-machine interface, and can achieve well switch betweenconfocal scan measurement and data processing, galvanometer control synchronization data acquisition and reconfiguration of3D measurement result.(5) Carry out relevant experimental with the confocal scan measurement software.To verify the effect of Sinc~2peak extraction method, experiments with a planarmirror and step edge has been done, and comparison with the direct extractionmethod and the centroid method has been made. Experiment results show thatstandard deviation of the Sinc~2peak extraction method is smaller. To verify theeffect of profile extraction algorithm, the planar-convex lens and aspheric substratediffractive element were measured respectively, using Zernike polynomial fittingand open-loop Gauss filtering to deal with measurement data. The results show thatthese two methods are effective in suppressing high-frequency noise and extractingthe profile of optical elements.