Interferograms obtained with a X-ray laser by means of a wavefront division interferometer

A wavefront division interferometer has been used for the first time with a soft X-ray laser (λ = 21.2 nm). The experiment aims to demonstrate X-ray laser interferometry in this configuration and to investigate the phase shifting measurement accuracy as well. The X-ray laser is generated in a neon-like zinc plasma in which it makes two passes thanks to a multilayer mirror half-cavity. The X-ray pulse duration is ≈ 50 ps. The beam has a very high brightness (≈ 4 X 10¹⁵ W cm^-2 sr^-1 in 0.01% bandwidth) which allows us to place the interferometer far from the source (2.8 m) and thus to benefit by a large transverse coherence width. The interferometer consists of a Fresnel bi-mirror which adds coherently one part of the X-ray laser beam section to the other one. Single laser-shot interferograms of a reflecting sample provided with a λ/2 dephasing step (51.7 nm height) have then been successfully recorded. The phase shifting accuracy resulting from the smallest observable fringe change is about λ/20.