Analysis Of Optimization Of Optical Structural Parameters And Wavefront Error In Point Diffraction Interference System

The rapid development of optical technology put forward the high quality of the optical system and the optical elements. The accuracy of existing commercial interferometer is limited by the quality of the standard surface, and root mean square error of the surface shape generally only can reaches lambda/50to lambda/100which is unable to reach higher accuracy. As a kind of high precision measuremental tool, the point diffraction interference technology enables the measurement accuracy in the order of subnanometer. In the point diffraction interference system, apart from the diffraction wavefront error, the intensity of diffraction light and the uniformity of light intensity also has great influence on the measurable accuracy of the system. Non-uniform diffraction light intensity and insufficient diffraction intensity will cause non-ideal contrast and signal-to-noise ratio of the interference pattern, which will affect the test results of the system.Smaller wavefront error, a reasonable diffraction light intensity and uniform diffraction intensity can contribute to the improvement of measurement accuracy of the point diffraction interference system. However, in the pinhole point diffraction interference system, there are several key optical structure parameters, including numerical aperture of the objective lens, the size of the pinhole, and the measurement numerical aperture, all have effect on the three aspects, and restrict each other. So it is necessary to select reasonable optical parameters of structures to make three aspects to be a balanced state, so as to the point diffraction interference system meet the test requirements.Simulation models of two-dimensional and three-dimensional to analyze the pinhole diffraction wavefront quality are established based on the finite difference time domain method (FDTD) respectively. The key factors of the point diffraction system, including the numerical aperture of objective lens, the size of the pinhole and the measurement numerical aperture, to the point diffraction wavefront error, the intensity of the diffraction light and the uniformity of the diffraction intensity, is analyzed in detail. And according to the specific measurement requirements, optimizing the parameters of key optical structures, get the structure parameters matching with different measurement requirement. All of the analysis is of great significance for choosing the parameters of the point diffraction system The alignment error between the convergence spot and the center of pinhole, such as pan, tilt, and the focal alignment error, to quality of pinhole diffraction wavefront is analyzed. The effect of the oval pinhole which is caused by fabrication error on pinhole to the quality of the diffraction wavefront is also studied in this paper.Applying the simulation results to the point diffraction experiment system to measure the spherical surface, a good contrast and brightness of interference fringes can be obtained, and compare with the measuring results by Zygo interferometer, and the difference of root mean square (RMS) of wavefront error is smaller than0.0008λ, so as to verify the reasonability of choosing the parameters of the optical structures.