Double Grating Shearing Interferometry for X-ray Free-Electron Laser beams

The advancements in bright and coherent X-ray free-electron lasers (XFEL) have in the last decade revolutionized diverse fields of study, leading to an ever-increasing demand in more intense X-ray pulses. The accurate knowledge of the wavefront and the focal profile of such pulses ultimately dictates much of the experimental outcome. Here we present a single-shot shearing interferometry method to measure the wavefront of a focused hard XFEL pulse. Two identical transmission gratings are placed near the focus, and the interference pattern of overlapping diffraction orders is analyzed, resulting in a two-dimensional reconstruction of the X-ray wavefront. The spatial resolution and wavefront sensitivity can be tuned in-situ during the measurement. The method is non-invasive (i.e., the zeroth order or transmitted pulse is not modified in intensity or profile), allowing for either a simultaneous intensity measurement at high resolution or a fully characterized transmitted pulse for general experimental use.