Encoding And Realizing Method Of Computer Generated Holograms For Aspheric Testing

In this paper, a kind of high precision phase-only encoding method of computerhologram has been researched based on the application background of testingaspheric surface through computer hologram method. The mathematic model hasbeen provided by mathematic modeling and simulating for different aspheric surface.Encoding precision of hologram has been improved effectively by encoding,analyzing of encode errors and correcting of encode errors. High precision asphericsurface shape has been generated by compensating double errors when LC-SLM(Liquid crystal spatial light modulator) been adopted as carrier.When applying phase-only CGH as a standard model of optical measurement incomputer-generated holography for aspheric surface testing, it has the advantages ofsimplifying optical path configuration and improving the diffraction efficiency of theincoming light. Thus, a kind of high precision phase-only CGH (computer generatedhologram) has been fabricated by reducing encoding errors of CGH based-on testingmethod of CGH combined with SLM. However, errors always exists during theencoding process of fabricating multiphase level CGHs and this kind of errors will beamplified level by level in the measurement and accumulated with other errors, theresulted errors can further reducing the testing accuracy. According to the analysis ofthe encoding error, the error of CGH increases linearly when its quantified periodincreases. In this paper, three optimization methods that respectively are datatracking based on deviation of minimum boundary value, matrix construction andcurve fitting optimization has been used to eliminate the encoding errors of CGH.Three optimization methods idealize the phase distribution of actual phase-only CGHby adjusting the phase of phase-only CGH to improve the testing accuracy. Thisrealized the destination of testing aspheric surface by fabricating high precision phase-only CGH.In the experiment, we built the optical path that loaded the CGH to LC-SLM toproduce the aspheric surface, and take the parabolic surface as an example to load theCGH before and after encoding error correction to the LC-SLM before and afterdistortion error correction to reconstruct the wave-front of a secondary parabolicsurface. The residual errors of before and after encoding error correction and theresidual errors of before and after encoding and distortion error correction have beencomputed compared with theory value by collecting and processing the reconstructedwave-front of secondary parabolic surface. The effectiveness of optimizationmethods have been verified by comparing the residual errors.