Study on stitching interferometry for synchrotron mirror metrology

With the quick development and progress of the synchrotron radiation and free electron laser facilities, the figure error requirement for X-ray delivering and focusing mirrors is getting higher. To preserve the wave-front without introducing additional wave-front error, the mirror surface figure error is typically specified approaching the sub-nm root mean square level. This kind of ultimate specification challenges the X-ray mirror metrology and inspection. In addition to the profile scanners based on the angular measurement, such as the long trace profilometer, which have been widely used in the synchrotron light source in various countries, stitching interferometry has also been developed as an effective method for synchrotron mirror metrology. In this work, the dedicated stitching metrology platform for X-ray mirror metrology was developed. Several stitching methods with varying stitching parameters were investigated at the proposed stitching interferometric system. In this paper, we focused on the principle and measurements of using the software stitching mode. The measurement results were compared with those from several different instruments in different research institutions to verify the effectiveness of the stitching interferometry for the synchrotron mirror inspection and to demonstrate the performance of the stitching platform. According to the measurement data, the repeatability of measuring an X-ray flat mirror in the software stitching mode is at 0.1 nm RMS level. When measuring a curved mirror with its radius of curvature changing from 50 m to 130 m, the repeatability is around 0.2-0.3 nm RMS. Basically, it meets the requirement for the routine inspection and fabrication feedback of flat and near-flat (radius of curvature larger than 50 m) X-ray mirrors.