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Absolute interferometric testing of an ultra-precise flat substrate with a liquid mirror
Author(s): Haiyang Quan; Xi Hou; Gaofeng Wu; Xiaochuan Hu
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Paper Abstract

The precise characterization of flat substrates is quite challenging for X-ray optics in synchrotron and free electron lasers. The surface requirements for the substrates are on the order of magnitude of few nanometers and sub-nanometers, which is also a great challenge for optical fabrication and testing. As for precise metrology, the core problem is to characterize the surface figure with high accuracy. And the key is to separate the errors of the measurement instrument from the intrinsic figure error of the surface under test. In addition, the surface figure of thin optics is largely affected by surface deformations due to gravity. In the paper, we presented an approach to achieve absolute planarity measurement of a thin x-ray mirror substrate through an interferometric method. With a liquid-flat reference using dimethyl silicone oil, the power term of the surface flatness of the interferometer transmission flat is retrieved. By floating the mirror on a heavy, high density liquid, deflections introduced by gravity are essentially eliminated. The unconstrained, floated x-ray mirror is tested through several rotational and translational shears. The absolute figure error is then calculated by iterative algorithm with pixel-level spatial resolution. By the proposed approach, both the interferometer transmission flat error and gravity-induced error are calibrated. Thus the unconstrained flatness of the x-ray mirror can be obtained. The method is described in detail and a measurement example of an x-ray mirror is provided in the paper.

Paper Details

Date Published: 26 April 2019
PDF: 6 pages
Proc. SPIE 11032, EUV and X-ray Optics: Synergy between Laboratory and Space VI, 110320J (26 April 2019); doi: 10.1117/12.2520894
Show Author Affiliations
Haiyang Quan, Institute of Optics and Electronics (China)
Xi Hou, Institute of Optics And Electronics (China)
Gaofeng Wu, Institute of Optics And Electronics (China)
Xiaochuan Hu, Institute of Optics And Electronics (China)


Published in SPIE Proceedings Vol. 11032:
EUV and X-ray Optics: Synergy between Laboratory and Space VI
René Hudec; Ladislav Pina, Editor(s)

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