Freeform mirror validation by interferometric techniques using a spatial light modulator

Abstract

The most widespread verification method for optical elements is interferometry but, in the case of freeform surfaces, a strong deviation of the slope along the surface can create areas in which the fringe density is too high for the interferometer to resolve them. The most desirable solution is to create a null or near null interferogram introducing compensating elements like a spatial light modulator (SLM) that provides the flexibility to accommodate the measurement of a wide range of free-form surfaces. This paper shows the process for a convex freeform mirror metrology. The method consists of inserting the SLM in the optical path to compensate the freeform component of the surface to be verified and to generate a null of aberrations in the interferometer. The system is previously modelled in an optical design software to calculate the required phase to be introduced in the SLM to generate the null. The arrangement of the SLM makes possible to keep its position fixed and use the same setup to measure a wide range of freeform surfaces, limited by the dynamic range of the SLM. For each specific surface, it is necessary to introduce suitable elements to compensate the base surface, reserving the SLM for the freeform component compensation. The method is illustrated with the verification of a convex freeform mirror whose freeform component is described by the astigmatism Zernike polynomial Z5.