Grating-based Wavefront Metrology
The ever increasing demand on X-ray optical elements to preserve the wavefront shape and coherence requires new instruments that can measure the wavefront shape in-situ, at-wavelength and under experimental conditions. Due to its high angular sensitivity on the order of 10 nrad and its single-shot capability, hard X-ray grating interferometry is ideally suited to observe the wavefront downstream of X-ray optical components and assess the aberrations they introduce.
The first applications of X-ray grating interferometry to optics metrology were in the metrology of an X-ray multilayer mirror  and in analyzing the distortions of a single beryllium focusing lens of long focal length . The technique has also been applied to in-situ observations of heatload induced distortions on a double crystal monochromator . As no reference scan can be recorded, a high placement accuracy of the microfabricated grating structures is essential.
We have implemented a grating interferometer to analyze the wavefront at a hard X-ray free electron laser, the Linac Coherent Light Source (LCLS)  and the japanese XFEL SACLA . The experimental setup is outlined in figure 1. Like the future free electron laser SwissFEL at PSI, LCLS produces x-ray pulses of unprecedented intensity, high spatial coherence and extremely short pulse duration. This enables new and exciting experiments in many areas of science. Many of these experiments rely fundamentally on a clean, well-defined and reproducible wavefront from one pulse to the next.
Using grating interferometry, we have shown that the harmonic offset mirrors (figure 1) are slightly collimating the radiation in horizontal direction. The experimental data from a grating interferometer also enables a spatially resolved analysis of the wavefront distortions introduced by the optical elements, such as the aspherical components of the mirror profile shown in figure 2.
Due to its high sensitivity, even the location of the source point in the 130 m long undulator of LCLS can be determined with grating interferometry . Absolute radius of curvature measurement are possible, once the grating alignment angles have been determined . These source point position measurements are illustrated an observation of the source position of LCLS as it is driven into saturation (figure 3).
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