Interferometric testing of aspheres using gracing incidence

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For the precise determination of the quality of optical surfaces, interferometric measurement methods can be used. The setup often is arranged as a null test, where only deviations of the specimen from its ideal surface shape are measured. The necessary matching of the wave front to the surface is done by so called null lenses. For aspheric surfaces, diffractive optical elements (DOE) are suitable. Due to the flexibility of the lithographic production, the structures can easily be adjusted to various surface shapes, while the precision of the lithographic process, together with the use of high quality planar surfaces, ensures a high quality of the DOE.However, DOEs are limited when testing samples with high numerical aperture (NA), due to the required small periods. Furthermore, the surfaces only can be tested after polishing, because coherent illumination on rough surfaces leads to speckle noise, impairing the measurements.A well known geometry to test rough objects with arbitrary aperture is grazing incidence interferometry. To adapt this procedure for the testing of high aperture, and maybe rough aspheric surfaces, it is proposed here to adapt the wave front via DOE to the particular tangent plane along the meridian curve of the asphere. For spheres/aspheres this leads to a local null test in a small area around the meridian curve. The region under test on the asphere is therefore limited to a small neighbourhood of the meridional curve, nevertheless by rotation of the test sample around the axis of symmetry the whole surface can be measured.The unique properties of the grazing incidence geometry leads to a combination of the advantages of measurements in reflection and transmission. The surface deviations are determined (as in reflection) but without any disturbing back reflections (as in transmission). Due to the grazing incidence, the wavelength is increased to an effective wavelength, which, in general, is much greater compared to the illumination wavelength. Therefore it is possible to measure technical surfaces before polishing. In addition, the test sample can be measured from any angle of view (besides a small bearing area), so that objects with high numerical aperture can be tested.In this project, a method for the high-precision measurement of the meridian curve of convex, rough (incl. smooth), rotationally symmetric aspheres shall be developed based on a diffractive grazing incidence interferometer. The described method enables the measurement of rotationally symmetric, rough surfaces with roughnesses in the sub-micrometer regime. This leads to a characterisation of optical surfaces even before the final polishing process.