On the calibration of diffractive nulls for transmission tests of aspheric components

Aspheric testing is commonly carried out by using diffractive nulls. This well introduced technique requires more and more calibration methods to reach high accuracies or to get rid off the errors of interferometric components and especially also the deviations of the diffractive nulls. Here, we will discuss transmitted light tests of aspheric components which are lacking on their part stigmatic properties because they are only components of more complex optical systems. The discussion will be restricted to rotationally symmetric components because the proposed calibration procedure assumes inversion symmetry. The deviations of the diffractive null from ideal can be split into substrate deviations and lithographic errors of the diffractive structure. To make the interferometer suitable for calibrations it is proposed to use a COMBO-DOE which is a sliced version of a DOE (diffractive optical element) carrying a spherical as well as an aspheric wave for the test of the aspheric component. Beside the special calibration proposal also the differences and the advantages of this approach in comparison to the better known reflected light surface test using DOE-nulls are discussed. Although the main application of the transmitted light test concerns the aspheric lens component as a whole it is also possible to measure the surface deviations in transmitted light if the second boundary surface of the aspheric component is a plane surface or if the back surface is of lower quality a high quality plane plate can be immersed to the plane back plane of the plano-aspheric lens in order to reduce the error contribution of the back plane. Due to the compensation philosophy the test wave front is only aspheric between DOE and surface under test. Outside the space between the sample and the DOE only plane waves are propagating being tolerant to the interferometer errors.