Ultra-precise characterization of LCLS hard X-ray focusing mirrors by high resolution slope measuring deflectometry

Helmholtz Zentrum Berlin, BESSY-II, Institute for Nanometre Optics and Technology, Albert-Einstein-Str. 15, 12489 Berlin, Germany.
Optics Express (Impact Factor: 3.49). 02/2012; 20(4):4525-36. DOI: 10.1364/OE.20.004525
Source: PubMed


We present recent results on the inspection of a first diffraction-limited hard X-ray Kirkpatrick-Baez (KB) mirror pair for the Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS). The full KB system - mirrors and holders - was under inspection by use of high resolution slope measuring deflectometry. The tests confirmed that KB mirrors of 350mm aperture length characterized by an outstanding residual figure error of <1 nm rms has been realized. This corresponds to the residual figure slope error of about 0.05µrad rms, unprecedented on such long elliptical mirrors. Additional measurements show the clamping of the mirrors to be a critical step for the final - shape preserving installation of such outstanding optics.

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    • "Slope measurement technologies like long trace profiler (LTP) use deflectrometry method to inspect the reflective optical surfaces without null lens or computer generate holograms (CGH). Some of these instruments have reported 0.05 µrad slope accuracy [2] [3]. However, LTP only measures a single line profile at a time and scanning mechanism is needed to obtain full field measurement of the test optics. "
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    ABSTRACT: SCOTS is a high precision slope measurement technology based on deflectometry. Light pattern on a LCD display illuminates the test surface and its reflected image is used to calculate the surface slope. SCOTS provides a high dynamic range full field measurement of the optics without null optics required. We report SCOTS tests on X-ray mirrors to nm and even sub nm level with precise calibration of the test system. A LCD screen with dots/check board pattern was aligned into the system at the test mirror position to calibrate camera imaging distortion in-situ. System errors were further eliminated by testing and subtracting a reference flat which was also aligned at the same position as the test mirror. A virtual reference based on the ideal shape of the test surface was calculated and subtracted from the test raw data. This makes the test a `virtual null' test. Two X-ray mirrors were tested with SCOTS. 0.1μrad (rms) slope precision and sub nm (rms) surface accuracy were achieved.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2013; 8848. DOI:10.1117/12.2024500 · 0.20 Impact Factor
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    ABSTRACT: In a Fizeau interferometer measurement, one of the factors that reduces long-term measurement reproducibility is variation in the shape of the reference flat. The purpose of the present work is to improve the measurement accuracy of a Fizeau interferometer by establishing the causes of the reference flat deformation. We discovered that the deformation is caused by a change in environmental humidity. We confirmed that a reference flat with a diameter of 152 mm deforms to a paraboloidal shape with a peak-to-valley height of 1 nm in response to a relative humidity change of 11%. The cause of the deformation can be interpreted as being a change in the film stress in an antireflective coating on the reverse side of the reference flat due to the adsorption of water molecules. We carried out the three-flat test for an absolute calibration of optical flats by considering the effect of a relative humidity change. The calibration results indicate that the uncertainty in the measurements of the Fizeau interferometer was successfully improved to 0.7 nm (σ: standard deviation) along a length of more than 145 mm. © 2012 Elsevier B.V. All rights reserved.
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    ABSTRACT: In a previous paper, the University of Arizona (UA) has developed a measurement technique called: Software Configurable Optical Test System (SCOTS) based on the principle of reflection deflectometry. In this paper, we present results of this very efficient optical metrology method applied to the metrology of X-ray mirrors. We used this technique to measure surface slope errors with precision and accuracy better than 100 nrad (rms) and ~200 nrad (rms), respectively, with a lateral resolution of few mm or less. We present results of the calibration of the metrology systems, discuss their accuracy and address the precision in measuring a spherical mirror.
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