Initial tests of a new X-ray Michelson interferometer

A Michelson interferometer for X-rays of wavelength ∼1 Å has been proposed to test the temporal coherence of FEL radiation. Such a device has been made at HASYLAB from a silicon single crystal. It is based on the BBB interferometer of Bonse and Hart. One of its mirrors can be pushed by a piezo to introduce the desired beam path difference. For the preliminary tests described in this paper, the crystal was also equipped with an LLL interferometer to check the quality of the crystal and of its preparation. The rotation of a phase-shifting object (a Plexiglas plate) in one of the LLL's beam paths has been found to produce a fringe pattern which can be shown to yield the phase-shifting material's index of refraction. The interferometer crystal's front section was held fixed with a compression spring. At the same time, the interferometer was bathed in viscous silicone oil to reduce vibrations.