Fly-cutting and testing of freeform optics with sub-μm shape deviations

Abstract

Optical designs in various applications profit from the increasing use of freeform elements. However, freeform optics always challenges the manufacturing process. The complexity of the fabrication derives from the missing symmetry in freeform surfaces. Ultra-precision machining is an appropriate method to realize complex optical freeforms. Surface deviations can be reduced in a deterministic process by a test and correction loop to achieve shapes with sub-μm deviations. But freeform elements do not only require the optical performance, they also depend on tight tolerances of the surface position with respect to reference structures. Due to the absence of rotation symmetry in freeform elements, all six degrees of freedom have to be constrained. Diamond machining allows to machine reference structures on the optical part. They can be used for alignment while testing or during the assembly processes. This paper shows a deterministic approach to manufacture optical freeform surfaces with sub-μm surface deviations by fly-cut-machining and servo assisted diamond turning. Reference structures are included at the edge of the element in order to support the following measurement and assembly processes. The reference structures are manufactured within the machining process of the optical surface. This procedure ensures tight tolerances between reference structures and optical surface /1/. The complex optical surface is measured with respect to the references with the tactile measurement system UA3P. The reference structures are used to locate the coordinate system of the element and hence to constrain the alignment parameter. After fitting the data, a revised tool-path is used to improve the shape deviation to sub-μm accuracies.