Abstract
To reconstruct the wavefront in a single-lens coherent diffraction imaging (CDI) system, we propose a closed-loop cascaded iterative engine (CIE) algorithm based on the known information of the imaging planes. The precision of diffraction distance is an important prerequisite for a perfect reconstruction of samples. For coherent diffraction imaging with a lens, autofocus is investigated to accurately determine the object distance and image distance. For the case of only the object distance being unknown, a diffuser is used to scatter the coherent beam for speckle illumination to improve the performance of autofocus. The optimal object distance is obtained stably and robustly by combing speckle imaging with clarity evaluation functions. SSIM and MSE, using the average pixel value of the reconstructed data set as a reference, are applied on two-unknown-distance autofocus. Simulation and experiment results are presented to prove the feasibility of the CIE and proposed auto-focusing method.
Highlights
In computational imaging, phase retrieval (PR) is a tool to reconstruct a wavefront with diffraction images [1,2,3,4,5,6]
The amplitude-phase retrieval (APR) [16] is a parallel iterative algorithm that renews the complex amplitude of the target with the average of calculated data
The system is mainly composed of four parts: fiber-optic laser with collimating lens, diffuser, single lens, and scientific CCD
Summary
Phase retrieval (PR) is a tool to reconstruct a wavefront with diffraction images [1,2,3,4,5,6]. The iterative process is only carried out on the diffraction plane without considering the modulation effect of the lens and the unknown object and image distances in the system. For a case in which image distance is the only unknown parameter, the diffraction patterns are recorded from the back focal plane of the lens. In the case that both the distances are unknown, we use two error functions, mean squared error (MSE) and structural similarity (SSIM) as CEF to obtain the object distance and image distance simultaneously. This method realizes the accurate acquisition of the auto-focusing curves in three-dimensional space. Simulations and experiments have been performed to test the performance of the auto-focusing scheme
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