Single-shot ptychograpy based on Damman grating as a tool for imaging and beam characterization at FELs

Ptychography is a scanning coherent diffraction imaging technique capable of simultaneous imaging of extended samples and beam characterization. The scanning structure of the ptychographical measurement results in a powerful optimization constraint that ensures the high robustness of the reconstructions. Because of that, ptychography can simultaneously diffraction-limited imaging of the sample and reconstruction of the illumination wavefield, making it a tool of choice for characterizing X-ray and XUV optics. However, the scanning nature of ptychography prevents its use for single-shot imaging and makes possible only average beam characterization. The search for the robust single-shot diffraction motivated the development of the single-shot that can be performed by collecting the diffraction patterns of multiple overlapping beams in one shot, thus measuring the whole dataset at once and removing the need for scanning. A setup realizing this principle was proposed for visible light, with a similar setup used for the single-shot beam characterization. However, none of them can be straightforwardly applied to X-rays due to the use of refractive optics. We solved this problem by using a single-shot ptychography setup based on a combination of X-ray focusing optics and beam-splitting grating and a corresponding forward model that facilitates single-shot imaging of extended samples at soft X-ray wavelengths. The setup was tested during the proof of concept experiment at the free-electron laser FLASH at DESY and allowed us to obtain a reconstruction of a test sample and probe wavefield from the data measured with a single pulse of FLASH. However, the fidelity and resolution of the reconstruction were limited by the low performance of the diffraction grating and the inability of the forward model to fit the inter-beamlet interference. Here, we present further progress in the single-shot psychography at FELs. We used an improved experimental setup based on the Damman grating with higher diffraction efficiency and diffraction order uniformity, and an improved forward model. These improvements allowed us to perform single-shot ptychographical imaging and beam characterization during the beamtime at FLASH. This technique further improved and adapted for harder X-rays, will allow the high-resolution single-shot imaging of extended dynamical samples as well as the single-shot beam characterization at X-ray free-electron lasers.