Abstract
X-ray mirrors are widely used for synchrotron radiation, free-electron lasers, and astronomical telescopes. The short wavelength and grazing incidence impose strict limits on the permissible slope error. Advanced polishing techniques have already produced mirrors with slope errors below 50 nrad root mean square (rms), but existing metrology techniques struggle to measure them. Here, we describe a laser speckle angular measurement (SAM) approach to overcome such limitations. We also demonstrate that the angular precision of slope error measurements can be pushed down to 20nrad rms by utilizing an advanced sub-pixel tracking algorithm. Furthermore, SAM allows the measurement of mirrors in two dimensions with radii of curvature as low as a few hundred millimeters. Importantly, the instrument based on SAM is compact, low-cost, and easy to integrate with most other existing X-ray mirror metrology instruments, such as the long trace profiler (LTP) and nanometer optical metrology (NOM). The proposed nanometrology method represents an important milestone and potentially opens up new possibilities to develop next-generation super-polished X-ray mirrors, which will advance the development of X-ray nanoprobes, coherence preservation, and astronomical physics.
Highlights
Introduction Modern synchrotron radiation facilities andX-ray freeelectron lasers provide high-brilliance X-rays for cuttingedge scientific and industrial research, which explores the world through a refined understanding of the structure of matter
To overcome the above limitations of present metrology techniques, we propose a novel metrology instrument, the speckle angular measurements (SAM) optical scanning head. 2D random intensity patterns are generated by shining a laser through a diffuser and they can be treated as multiple pencil beams with different features
As described in earlier papers[43,44], a surface profile composed of parabolic sections is added on top of the ellipse, and a uniform top-hat-like power distribution is expected to be generated at the focus
Summary
Introduction Modern synchrotron radiation facilities andX-ray freeelectron lasers provide high-brilliance X-rays for cuttingedge scientific and industrial research, which explores the world through a refined understanding of the structure of matter. The successful exploitation and efficient utilization of X-ray beams depend on the quality of the optics. Among these X-ray optics, X-ray mirrors are critical optical components and are widely used for their exceptional characteristics of high efficiency and inherent achromaticity[2]. The extremely stringent tolerances on height error for X-ray mirrors are a consequence of Rayleigh’s quarterwavelength rule applied to X-ray wavelengths less than one-thousandth of those for visible light[2]. For the most demanding X-ray applications such as extreme energy resolution or nanofocusing, the required height error is often below 1 nm rms[3,4,5]. The manufacturing and metrology of X-ray mirrors poses major challenges
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