Interferometric testing of aspheres using gracing incidence

为了精确地确定光学表面的质量，可以使用干涉测量方法。设置通常被安排为零测试，其中只测量试件与其理想表面形状的偏差。波前与表面的必要匹配是通过所谓的零透镜来完成的。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，以及高质量平面表面的使用，确保了高质量的DOE。但是，当测试具有高数值孔径(NA)的样品时，由于所需的小周期，DOE的精度受到限制。为了使该方法适用于大口径，甚至可能是粗糙非球面的检测，本文提出将DOE的波前调整为沿非球面子午线的特定切面。对于球面/非球面，这会导致在子午线周围的小区域内进行局部归零测试。因此，在非球面上测试的区域被限制在子午线的一个小邻域内，然而，通过测试样品绕对称轴旋转，可以测量整个表面。掠入射几何的独特性质导致了反射和透射测量的优点的结合。表面偏差是确定的(如反射)，但没有任何干扰的背反射(如在传输中)。由于掠射入射，波长被增加到有效波长，该有效波长通常比照明波长大得多。因此，在抛光之前测量技术表面是可能的。此外，测试样品可以从任何角度(除了小的支承面积外)进行测量，从而可以检测具有大数值孔径的物体。在本项目中，提出了一种高精度测量凸面、粗糙面(包括。光滑)，旋转对称非球面应基于衍射掠入射干涉仪开发。所描述的方法能够在亚微米范围内测量具有粗糙度的旋转对称的粗糙表面。这导致甚至在最终抛光过程之前就对光学表面进行了表征。

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.