Research On Key Technology Of Absolute Measurement Of Optical Plane Based On Conjugate Difference Method

Optical interferometry is the most commonly used method to measure the plane mirror’s surface shape with high accuracy. The essence of the method is to make a comparison test, namely, compare the surface under test with the reference surface by the interference of two beams and determine the deviation of the surface shape of the surface under test through the analysis of the interference fringes. The precision of the method is limited by the accuracy of the reference surface. When the precision of the surface shape of the surface under test is above λ/20, the absolute measurement method is needed to eliminate the deviation caused by the reference surface and achieve the absolute surface shape of the surface under test.The paper mainly studies the application of the conjugate differential method in measuring plane mirror’s absolute surface shape. The conjugate differential method is derived from the differential method and the main difference lies in employing the measurement scheme of the orthogonally bilateral symmetry position. The scheme changes the three-step tests of the traditional differential method into the four-step tests. However, it enhances the precision of the slope approximation of the surface under test and reduces the coupling between the multi-step tests, so the reliability of the tests has improved. What get in the experiment are the comparisons between the reference surface and the surface under test in the four orthogonally bilateral symmetry positions. Then figure out the differential datas of the surface shape of the surface under test in the orthogonal direction by subtracting the comparisons in the symmetric positions. In the process of wavefront reconstruction, this paper studies several different methods in detail, such as the Fourier transform, the Zernike polynomial fitting, the integration by oribit, the Hudgin structure slope method. In the experiment, this paper uses several algorithms to separately analyze the recovery on the conjugate differential method and the differential method, and compare the recovery with the result of the three-flat test. The comparison facts are that the difference of RMS value between the conjugate differential method and the three-flat test can be 0.1 nm and the difference of RMS value between the traditional differential method and the three-flat test can be 0.2nm. The RMS repeatability of the conjugate differential method can reach 0.2nm.