Grazing-incidence test for cylindrical microlenses with high numerical aperture

Abstract

Cylinder symmetry of test objects allows grazing incidence tests. Here we propose the use of two diffractive optical elements (DOEs) instead of a single DOE in perpendicular incidence. The splitting of the optical function of the DOEs allows low spatial frequencies in the diffractive elements. In fact, the diffractive structures are curves parallel to the meridian of the cylinder lens. Here an interferometric test is described where identical DOEs serve as beamsplitter and combiner. The undiffracted wavefront serves as reference. Phase shifting evaluations are possible by axial translations of one of the DOEs. The effective wavelength can be chosen within wide limits. Actually, we used a pitch of 5µm, which is identical to the effective wavelength of the test. A surface deviation of 2.5µm corresponds to a fringe distance. The interferometric test enables rms values of better than 20 nm for any aspheric geometry of the meridian curve, even with lenses having numerical apertures up to 0.9.