Abstract
Ptychography, a scanning coherent diffraction imaging method, can produce a high-resolution reconstruction of a sample and, at the same time, of the illuminating beam. The emergence of vacuum ultraviolet and X-ray free electron lasers (FELs) has brought sources with unprecedented characteristics that enable X-ray ptychography with highly intense and ultra-fast short-wavelength pulses. However, the shot-to-shot pulse fluctuations typical for FEL pulses and particularly the partial spatial coherence of self-amplified spontaneous emission (SASE) FELs lead to numerical complexities in the ptychographic algorithms and ultimately restrict the application of ptychography at FELs. We present a general adaptive forward model for ptychography based on automatic differentiation, which is able to perform reconstructions even under these conditions. We applied this model to the first ptychography experiment at FLASH, the Free electron LASer in Hamburg, and obtained a high-resolution reconstruction of the sample as well as the complex wavefronts of individual FLASH pulses together with their coherence properties. This is not possible with more common ptychography algorithms.
Highlights
Ptychography is a scanning coherent diffraction imaging (CDI) technique that allows the simultaneous imaging of a sample and an unknown illuminating beam [1,2]
We present a general adaptive forward model for ptychography based on automatic differentiation, which is able to perform reconstructions even under these conditions
We performed a reconstruction with an automatic differentiation (AD)-based ptychography model without any adaptations
Summary
Ptychography is a scanning coherent diffraction imaging (CDI) technique that allows the simultaneous imaging of a sample and an unknown illuminating beam (hereafter called probe) [1,2]. Scanning of a sample with overlap between adjacent scan positions builds the essence of a ptychography experiment by providing the required redundancy in the measured data [3] This redundancy prevents the ambiguities common to conventional CDI methods and leads to robust and stable reconstructions. Despite of all of these advances, a shot-to-shot spatially stable and ideally fully spatially coherent probe is still a prerequisite for successful ptychographic reconstruction Both conditions can be realized for synchrotron radiation at the cost of significant beam aperturing [8]. Successful application of ptychography at synchrotron facilities has drawn the attention towards implementation of ptychography at free electron lasers (FELs) These sources possess higher levels of spatial coherence and photon flux which, together with their ultra-short pulses, makes them ideal facilities for imaging [9]. FELs themselves benefit from ptychography as a beam characterization tool due to its ability to be applied to high-resolution reconstruction of unknown probe beams
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