Abstract
Characterization of the wavefront of an X-ray beam is of primary importance for all applications where coherence plays a major role. Imaging techniques based on numerically retrieving the phase from interference patterns are often relying on an a-priori assumption of the wavefront shape. In Coherent X-ray Diffraction Imaging (CXDI) a planar incoming wave field is often assumed for the inversion of the measured diffraction pattern, which allows retrieving the real space image via simple Fourier transformation. It is therefore important to know how reliable the plane wave approximation is to describe the real wavefront. Here, we demonstrate that the quantitative wavefront shape and flux distribution of an X-ray beam used for CXDI can be measured by using a micrometer size metal-coated polymer sphere serving in a similar way as the hole array in a Hartmann wavefront sensor. The method relies on monitoring the shape and center of the scattered intensity distribution in the far field using a 2D area detector while raster-scanning the microsphere with respect to the incoming beam. The reconstructed X-ray wavefront was found to have a well-defined central region of approximately 16 µm diameter and a weaker, asymmetric, intensity distribution extending 30 µm from the beam center. The phase front distortion was primarily spherical with an effective radius of 0.55 m which matches the distance to the last upstream beam-defining slit, and could be accurately represented by Zernike polynomials.
Highlights
Wavefront sensors are important tools used in optics, perhaps most notably in conjunction with adaptive optics for imaging extraterrestrial objects through the distortions caused by turbulence in the Earth’s atmosphere [1,2]
We demonstrate that the quantitative wavefront shape and flux distribution of an X-ray beam used for Coherent X-ray Diffraction Imaging (CXDI) can be measured by using a micrometer size metal-coated polymer sphere serving in a similar way as the hole array in a Hartmann wavefront sensor
The method relies only on the use of a 2D detector, a sphere in the size range of 1 μm and a setup for raster scanning the sphere with respect to the incoming wave field
Summary
Wavefront sensors are important tools used in optics, perhaps most notably in conjunction with adaptive optics for imaging extraterrestrial objects through the distortions caused by turbulence in the Earth’s atmosphere [1,2]. The visible light can be magnified by an ordinary microscope objective and measured by a CCD camera [8] This scintillator-based method has been proven to work well for measuring the wavefront for soft X-ray beams [8,9,10], it has the disadvantage that only relatively large beams can be measured, because multiple holes, typically in the micrometer range, must be present across the beam [8]. The resolution of these methods are generally good, and has proven to be accurate to 1/120 part of the wavelength in a study with photon energy of 92.5 eV [7]. The technique has the advantages of being conceptually easy and not relying on slowly converging iterative reconstruction algorithms, and works for X-rays of high energy (> 6 keV), provided that the transverse coherence length is bigger than the sphere diameter (~3 μm)
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