Fourier transform holography is a coherent, lensless imaging method. In the most common soft X-ray implementation of Fourier transform holography, the source of the phase encoding reference beam is a small pinhole which is integrated into a metal film adjacent to an object aperture that defines the field-of-view. By illuminating this integrated mask with coherent X-rays, a hologram can be recorded in the far-field. The reconstruction of the exit wave in the object aperture is retrieved by Fourier transforming the hologram. While this configuration is very robust against sample drifts and vibrations, the efficiency is limited by the reference pinhole. Smaller reference pinholes improve the resolution whereas the reference signal on the detector and ultimately the image contrast in the reconstruction diminishes. The choice of the reference pinhole size is therefore a compromise that trades image resolution for photon efficiency. The first part of this thesis contains a theoretical discussion of the imaging properties for X-ray Fourier transform holography (FTH) and derives a model for the determination of reasonable pinhole diameters that ensure photon efficient imaging. Typical imaging artifacts are characterized and strategies for their prevention are discussed. Two of these strategies can additionally be utilized to enhance the spatial resolution for certain configurations of reference pinhole based FTH. The findings are successfully verified by a proof-of-principle experiment. The second part presents a new approach that replaces the reference pinhole by a Fresnel zone plate lens (FZP). The method drastically enhances the signal strength of the reference beam and effectively decouples the spatial resolution in the reconstruction from image contrast. Related to the FZP diffraction orders, the method yields several differently strong defocused reconstructions which superpose along the beam axis. The FZP diffraction efficiency is much improved by combining several focal order reconstructions which are individually refocused by numeric wave field propagation. To separate a focused reconstruction of a particular FZP focal order from the remaining defocused reconstructions, a novel algorithm is proposed. The superiority of the FZP based reconstruction over the conventional pinhole reconstruction is experimentally demonstrated. The extraction of depth information from three-dimensional specimen imaged by conventional FTH with pinhole references is discussed and experimentally demonstrated in the third part. The approach exploits the holographically encoded phase information from a hologram of an artificial three-dimensional test structure. Object features outside the depth-of-field are numerically refocused by propagating the reconstructed object wave along the beam axis into focus. Contrary to tomographic approaches, a three-dimensional model of the test structure is obtained from a single view measurement.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Optics, Electrical engineering|
|Keywords:||Fresnel zone plate lens|
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