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Electrical Engineering, Measurement and Control Technology
The first chromatic hard x-ray interferometer with a large and variable path length difference (PLD), based on a variation of the all-Bragg Graeff–Bonse x-ray interferometer has been built and successfully tested. See W. Graeff and U. Bonse, Z. Phys. B 27, 19 (1977). Such a capability can be used for better beam coherence measurements and also opens up the possibility of Fourier transform spectroscopy in the x-ray regime. Going through this interferometer, the beam is split in two and then recombined, by a set of four Bragg reflections in a (+−−+) geometry. In order to incorporate a large variable PLD between the two beams, a small angle (1°) was partly cut into the mirror faces (middle two reflections) in one of the arms of the interferometer. The PLD thus varies depending on the incident beam location on the beamsplitter surface. In this design, a very large PLD (millimeters) is easily achievable and is limited only by the available size of the silicon crystal. A larger angular cut in the mirror surfaces will also increase the achievable PLD. The test consisted of measuring the interference fringes visibility as a function of the PLD. The fringes are obtained by measuring the exit beam intensity through small slits (25 μm), while a plastic wedge is inserted in one of the beam paths and translated in the direction of its thickness gradient. Based on these measurements, fringe visibility analysis was performed to give the transmitted beam coherence lengths. The results agree very well with the expected coherence values based on the interferometer's angular and spectral acceptances. © 2002 American Institute of Physics.
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