Wang Hongchang, Moriconi Simone, Sawhney Kawal
Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK.
Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK.
Light Sci Appl. 2021 Sep 22;10(1):195. doi: 10.1038/s41377-021-00632-4.
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.
X射线镜广泛应用于同步辐射、自由电子激光和天文望远镜。短波长和掠入射对允许的斜率误差施加了严格限制。先进的抛光技术已经制造出斜率误差低于50纳弧度均方根（rms）的镜子，但现有的计量技术难以对其进行测量。在此，我们描述一种激光散斑角度测量（SAM）方法来克服此类限制。我们还证明，通过使用先进的亚像素跟踪算法，斜率误差测量的角度精度可降至20纳弧度rms。此外，SAM能够在二维中测量曲率半径低至几百毫米的镜子。重要的是，基于SAM的仪器结构紧凑、成本低，并且易于与大多数其他现有的X射线镜计量仪器集成，如长跟踪轮廓仪（LTP）和纳米光学计量仪（NOM）。所提出的纳米计量方法代表了一个重要的里程碑，并有可能为开发下一代超抛光X射线镜开辟新的可能性，这将推动X射线纳米探针、相干性保持和天文物理学的发展。
