Interferometric testing of aspheres using gracing incidence

为了精确测定光学表面的质量，可以采用干涉测量方法。该装置通常被安排为零测试，其中只测量样品与其理想表面形状的偏差。波前与表面的必要匹配是由所谓的零透镜完成的。对于非球面，衍射光学元件（DOE）是合适的。由于光刻生产的灵活性，结构可以很容易地调整到各种表面形状，而光刻工艺的精度，加上使用高质量的平面，确保了高质量的DOE。然而，当测试具有高数值孔径（NA）的样品时，由于所需的小周期，do受到限制。此外，表面只能在抛光后进行测试，因为粗糙表面上的相干照明会导致散斑噪声，从而影响测量。一种众所周知的测量任意孔径粗糙物体的几何方法是掠入射干涉法。为了使该方法适用于大孔径非球面和粗糙非球面的检测，本文提出通过DOE将波前调整为沿非球面子午曲线的特定切平面。对于球体/非球面，这导致在子午线曲线周围的小区域进行局部零测试。因此，非球面上的测试区域被限制在子午曲线的一个小区域内，然而，通过围绕对称轴旋转测试样品，可以测量整个表面。掠入射几何的独特特性使得反射和透射测量的优势结合在一起。表面偏差是确定的（如反射），但没有任何干扰的反向反射（如透射）。由于掠入射，波长增加到有效波长，通常比照明波长大得多。因此，可以在抛光前测量技术表面。此外，测试样品可以从任何角度测量（除了小的承载区域），从而可以测试具有高数值孔径的物体。本课题将研究一种基于衍射掠入射干涉仪的凸、粗糙（包括光滑）、旋转对称球面子午线的高精度测量方法。所描述的方法能够测量旋转对称的粗糙表面，其粗糙度在亚微米范围内。这导致光学表面的特征，甚至在最后的抛光过程之前。

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.