Highly Accuracy Aspheric Surface Testing Based On Computer-generated Hologram

Computer-generated hologram used in optical interferometric system makesthe process of aspheric testing easier. Any complex wavefront can be generated by CGH.The application of CGH in aspheric surface testing can achieve high detection accuracywithout using expensive reference surfaces or null lenses.Aspheric surface testing based on CGH is a main development direction ofmodern high accuracy aspheric surface testing technology which can meet therequirement of low cost but high accuracy. The development of micro-electronicsindustry makes the fabrication of CGH with high accuracy become reality which hasgreatly promoted the application of CGH in high accuracy aspheric surface testing.High accuracy aspheric surface testing technology based on CGH has importantpractical significance and application value to ensure the development and productionof high precision instruments and optical equipments. Errors in the aspheric surfacetesting system can classified into CGH fabrication errors, interferometer errors, spatialfilter errors and aspheric suface errors. To improve the detection accuracy of asphericsurfaces, the design method of CGH, the measurement methods of CGH fabricationerrors and the absolute testing method for aspheric sufaces based on Twin-CGH areproposed in this paper.The purpose of the study is to improve the accuracy of asphericsurface testing.The main research work of this paper can be summarized as follows:1. Design CGH including main CGH, alignment CGH and fiducial CGH. Analyzethe necessity of designing alignment CGH and fiducial CGH and the factors of CGHlayout.2. Errors and uncertainties during CGH fabrication processes result in errors inthe diffraction wavefronts created by the finished hologram. The influences of CGHfabrication errors and aspheric surface curvature radius errors on aspheric surfacetesting accracy are analyzed. The method of measuring phase depth errors andamplitude errors and the experimental method of eliminating ghost image are proposed.3. The method of measuring and eliminating the effect of substrate errors in aCGH is proposed. Using this method, the detection accuracy of aspheric surfaces can begetting better. Two problems are encountered in this method: residual wavefront errorsproblems and spherical aberrations problems. The selection principle of beam for measuring substrate errors is also proposed.4. The method of measuring and eliminating the effect of pattern distortion errorsin a CGH is proposed. This method also has great contribution to improve the detectionaccuracy of aspheric surface testing. The equal inclination interference circular gratingand the circularly symmetric type CGH grating are equally spacing that is required inthis method.5. This paper presents a fast Fourier transform method to analyze Twin-CGH.Twin-CGH may be written as the linear combination of two phase fuctions. Phaseoptimization method of Twin-CGH is proposed in this paper which has provided thepowerful support for the absolute testing method for aspheric sufaces. Theoptimization results by this method are very good.6. Using Zygo’s Verifire Asphere interferometer as aspheric surfacetesting equipment, Φ135mm、R144mm and Φ78mm、R93.86mm aspheric surfaces aremeasured. The measurement method of CGH substrate errors and the selection of beamfor measuring substrate errors are made experimental verification.