Research On Absolute Inspection Technology Of ?300mm Optical Flat Crystal

Interferometry is a common method for detecting the surface accuracy of optical flats.However,this method is restricted by the accuracy of the reference flat of the interferometer.The measurement results inevitably carry the error of the reference flat.In order to solve this problem,break through the barriers in processing the large-aperture ultra-high-precision optical flats,and meet the higher precision requirements of optical components in the field of highprecision technology,the optical absolute testing technology is studied.This technology can separate the respective errors of the reference flat and the test flat,obtain the absolute surface distribution of the flat to be measured.It is a key technology in interferometer calibration and high-precision interferometry.First of all,two kinds of absolute testing methods suitable for large-aperture flats are mainly studied.The calculation process of the three-flat method based on single rotation oddeven function and the two-flat method based on image N rotation is theoretically deduced.The two algorithms are theoretically simulated by numerical calculations.The RMS value deviation of the restored surface is less than 0.05 nm.The influence of the rotation angle and eccentricity error on the three-flat method is analyzed.The results show that when the rotation accuracy is better than 0.3 ° and using spatial alignment method of the fork wire,the three-flat method can meet the high-precision requirements;the angle of rotation in two-flat method is improved,and the number of rotation is increased to 40 times,which effectively improves the detection accuracy.Secondly,the clamping support and deformation compensation scheme of the reference flat in the large-aperture vertical interferometer is designed.The finite element method is used to analyze the deformation of the large-aperture flat.The optimal aperture of the reference flat in the ?300mm vertical Fizeau interferometer is determined to be ?350mm,the thickness is90 mm,and the clamping method is three-layer adhesive support of 18 × 3 points sidely.The PV value of the deformation within the effective diameter range of flat ?300mm is 24nm;the deformation compensation method is studied,and the reliability of the force-reaction compensation method is analyzed by finite element analysis.The surface shape changes of the flat before and after the compensation are measured by the liquid reference method.The results show that the power term in the surface error is reduced from 0.048? to 0.013 ?,which proves the effectiveness of the force-reaction compensation method.Finally,the absolute test experiments of ? 100 mm,? 150 mm and ? 300 mm flats are completed by using the three-flat method based on single rotation odd-even function and the two-flat method based on image N rotation.The RMS value deviation of the same flat restored by the two different methods is less than 0.1nm.The horizontal and vertical absolute surface of the same flat obtained by the two methods are compared with the traditional three-flat method,the RMS value deviations are all less than 0.5 nm.The absolute surface of ? 300 mm reference flat in the vertical interferometer is calibrated by the liquid reference method,and the PV value and RMS value are superior to the relative detection results.The experimental results show the feasibility and effectiveness of the two optical flat absolute testing methods and the liquid reference method in the high-precision detection of large-aperture flats.