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Split-And-Delay Unit for FEL Interferometry in the XUV Spectral Range

Photon Science Division, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
The Hamburg Centre for Ultrafast Imaging CUI, 22761 Hamburg, Germany
Department of Physics, University of Hamburg, 22761 Hamburg, Germany
Faculty of Electrical Engineering, Helmut Schmidt University, 22043 Hamburg, Germany
Author to whom correspondence should be addressed.
Academic Editor: Kiyoshi Ueda
Appl. Sci. 2017, 7(6), 544;
Received: 29 March 2017 / Revised: 5 May 2017 / Accepted: 22 May 2017 / Published: 25 May 2017
(This article belongs to the Special Issue X-Ray Free-Electron Laser)
PDF [3108 KB, uploaded 25 May 2017]


In this work we present a reflective split-and-delay unit (SDU) developed for interferometric time-resolved experiments utilizing an (extreme ultraviolet) XUV pump–XUV probe scheme with focused free-electron laser beams. The developed SDU overcomes limitations for phase-resolved measurements inherent to conventional two-element split mirrors by a special design using two reflective lamellar gratings. The gratings produce a high-contrast interference signal controlled by the grating displacement in every diffraction order. The orders are separated in the focal plane of the focusing optics, which enables one to avoid phase averaging by spatially selective detection of a single interference state of the two light fields. Interferometry requires a precise relative phase control of the light fields, which presents a challenge at short wavelengths. In our setup the phase delay is determined by an in-vacuum white light interferometer (WLI) that monitors the surface profile of the SDU in real time and thus measures the delay for each laser shot. The precision of the WLI is 1 nm as determined by optical laser interferometry. In the presented experimental geometry it corresponds to a time delay accuracy of 3 as, which enables phase-resolved XUV pump–XUV probe experiments at free-electron laser (FEL) repetition rates up to 60 Hz. View Full-Text
Keywords: white light interferometry; split-and-delay unit; XUV optics characterization; pump-probe white light interferometry; split-and-delay unit; XUV optics characterization; pump-probe

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Usenko, S.; Przystawik, A.; Lazzarino, L.L.; Jakob, M.A.; Jacobs, F.; Becker, C.; Haunhorst, C.; Kip, D.; Laarmann, T. Split-And-Delay Unit for FEL Interferometry in the XUV Spectral Range. Appl. Sci. 2017, 7, 544.

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