Speckle-based curvature optical metrology

Advanced metrology methods are continually being developed and refined to meet the demanding quality standards of high-performance X-ray mirrors. Among these, interferometric techniques are the most widely used for measuring the height maps of optical surfaces. However, they often encounter limitations when applied to strongly curved or freeform surfaces, primarily due to high fringe density caused by steep slope. To address these challenges, we have developed a laser Speckle-based Curvature Optical Metrology instrument (SCOM) for measuring the two-dimensional surface curvature maps. This technique offers an alternative for characterizing complex optical surfaces by using a digital image correlation algorithm to extract curvature information from the speckle pattern, which effectively acts as a set of wavefront markers. We have demonstrated the effectiveness of this method for measuring strongly curved mirrors with a radius of curvature from 10 m down to 100 mm. Additionally, we have applied this technique to online deterministic figuring of optical surfaces, in-situ stress measurements during multilayer deposition processes, and the characterization of deformable mirrors. This technique shows great promise not only for high precision metrology of X-ray mirrors used in synchrotron radiation facilities, free-electron lasers, and space and astronomical observatories, but also for freeform optical components in advanced industrial applications.