Research On Aspheric Surface Testing Technology Based On Computer Generated Hologram(CGH)

Design and fabrication of diffractive optical elements with high precision and diffraction efficiency is until now one of the most promising trends in development of optics. These elements are thin relief phase structures and have been already widely used in optical metrology.A kind of diffractive optical elements called computer generated hologram(CGH) provide new methods for high precision aspheric and freeform surface testing.The basic principle of this method is that CGH use diffraction to create wavefronts of light with desired amplitude and phase variations and realize phase compensation.At first, The latest development in fabrication technology of CGH and its application for aspheric surface testing is reviewed at this dissertation.Then the basic theory of interferometry for aspheric surface testing and the Offner Null Corrector for testing actual aspheric mirror surface quality are introduced. The detailed design of Offner Null Corrector for the F/#=2, D=400mm, conic constant k=-1 paraboloid mirror is given.Next,the phase function of CGHs are analyzed through the concrete example. CGHs may be described through Zernike polynomials(Zernike wavefront description) or any other aspheric phase profile because such lenses applied to aspheric surface testing are analytic diffractive lenses or phase plates designed through ray tracing rather than calculated iteratively as typical numeric CGHs. Additionally a CGH phase function using an exact geometric model is built. A parametric model that can be used for optimizing the CGH design to give good diffraction efficiency and limited sensitivity to manufacturing errors is provided and it could give a guide for phase quantization.Next,The detailed design of CGH Null Corrector is described.In contrast, the optimal design result of CGH is achieved through analysis. And We also design the Hindle sphere and CGH for testing the convex mirror the difficulty of fabricating the CGH is assessed respectively.Then the technological process for CGH fabrication is reviewed.Critical dimension and groove depth and the layout formats developed for the same IC industry to represent the shapes to be patterned on the photomask for the CGH fabrication is analyzed in detail. CGH for the F/#=2, D=400mm, conic constant k=-1 paraboloid mirror is fabricated and relief phase structures are tested and certified.Finally, Experiments showed that CGH aspherical testing system could achieve very high precision and it could complete large-diameter aspherical mirror and freeform surface mirror alignment that could not be tested by conventional approach.