Theoretical Analysis And Experimental Study On Imaging Mechanism Of Ptychography

Ptychography is a novel computational imaging technology that is developed in the last decade. It seeks the unique complex solution of a sample that is consistent with diffractive intensity measurements under overlapping scanning by phase retrieval iteration. The resolution of this technology is not subject to focusing devices, breaking through the physical diffraction limitation and achieving high-resolution detections. Ptychography has inherent advantages in some high-resolution and three-dimensional imaging fields that need recovering the diffractive information from intensity images whose phase component is missing, which brings potential applications of it in these fields. This thesis performed an extensively research on the imaging mechanism and applications of ptychography in coherent diffractive imaging, optical imaging and computational holography. And some solutions were proposed to overcome the defects and shortcomings of ptychography.The major content in this thesis is listed as follows:1) The theoretical background of ptychography was described briefly. First, on the basis of the theory of light-wave scalar diffraction and Fourier optics, some main diffractive phenomenons, Fourier transform operation of a lens and the imaging principle of optical systems were calculated numerically, and the limitation of the imaging resolution of a diffraction-limited optical system was analyzed. Next, the holography which records the complex amplitude of light-wave via coherent interference is introduced. Some computer generated holograms were produced and electro-optically reconstructed to confirm the diffraction calculation and holography. Finally, the concept of non-interferometric phase imaging and the theory of signal restorations by de-convolution filtering and Iterative searching were introduced briefly, which is the mathematical basis of phase retrieval algorithms. The incremental Wiener filter, the vary filter being employed in ptychography, was introduced subsequently.2) The coherent diffractive ptychography is investigated. Beginning with the principle of conversional coherent diffractive imaging, the restriction of isolated sample and the solution ambiguity were analyzed in details. Next, the procedure of ptychography was simulated, which involves the acquisition of diffractive intensity patterns of a sample under an optical probe overlapping scanning, and the reconstruction of the sample from these diffractive images via Iterative Fourier Transform (ITF) algorithm and incremental Wiener filter. Explained the reason why it can synthesize an arbitrary field-of-view and solve the inherent ambiguity of conventional coherent diffractive imaging. Third, the effects of some key parameters on the reconstruction quality were evaluated numerically. Finally, the mechanism of capturing the diffractive patterns of a sample which is in motility was simulated computationally. The ptychographical reconstruction of the sample indicated that due to the low acquisition efficiency of ptychography, long time for scanning sample and capturing images, motions bring distort, blur and noise in the reconstruction, and the reliability and resolution of recovery are inverse-proportional to the vibratory amplitude during the exposure time. A random phase modulated probe was employed in coherent diffractive ptychography to cure the degradation produced by the motions in a certain extent, which can enhance the ability of detecting samples being in motion, and partly solve the difficulty that only static or slow-changing samples can be imaged by coherent diffractive ptychography.3) The Fourier ptychography was studied. Firstly, the mechanism of Fourier ptychography was demonstrated. It makes a scan in spectral domain and obtains the intensity images of Fourier transform of the scanned spectrum. The scan is realized by relative shifts between the spatial spectrum of a sample and the Coherent Transform Function (CTF) of a finite aperture optical system. The shift is caused by angle-varied plane wave illuminations in Fourier ptychography. Using phase recovery, the complete spectrum of the sample is reconstructed from these low resolution intensity images, and transformed to a high resolution image furtherly. Next, a comparison between Fourier ptychography and coherent diffractive ptychography established their relationship and confirmed that it must submit to the overlapping constrain similarly. Third, rooting from synthetic aperture imaging, the virtual numerical aperture increase and the resolution improvement in Fourier ptychography were evaluated quantitatively. Finally, to improving the efficiency of image acquisition and processing and the robustness of system, an imaging approach was proposed and termed Coherent States Multiplexing Fourier Ptychography (CSMFP). This approach employs an electro-optical modulator to prove illuminations which contains multi coherent modals. This illumination makes the collected intensity images originating from multiple spectral fragments. A modified phase retrieval algorithm adding a separation of blended spectrum is used to restore a spectrum without aliasing, and a high-resolution image furtherly. The availability of this approach was tested by simulative and practical experiments. Experimental results proved that this imaging scheme can significantly reduce the amount of image acquired, relaxes the restriction of non-coherent multiple states in ptychography information multiplexing.4) The application of ptychography in integral-holography is investigated. This section begins with a calculation and a reconstruction of Fourier holograms of three-dimensional scenes, based on theory of digital holography. Next, Integral Imaging (Ⅱ) technique working in non-coherent illumination condition was introduced. A complete procedure of Integral-holography (IH), which is a combination of Ⅱ and holography, was simulated, involving capturing elemental images by Ⅱ, synthesizing multi-projections, generating integral-holograms by two existed IH schemes, executing numerical reconstructions. These experiments show that these existed schemes only obtain sparse sampling for light wave field, small size holograms, low-resolution and view angle crosstalk in reconstructed images. Third, the correlation between multi-view projection images and Fourier hologram was derived, which indicate that a projection image is the intensity of the reconstruction of a sub-hologram picked up from a big Fourier hologram, followed by determining the accurate position of sub-hologram in the entire hologram according the projective angel. Finally, a computational holographic approach named as Fourier Ptychographic Integral Holography (FPIH) was proposed. The approach recovers a high resolution hologram from multi-projection intensity images generated from Ⅱ through an iterative retrieval with Fourier ptychographic constrains. Finally, optical reconstructions were performed and the results show that this approach can overcome the defects of traditional IH, and reconstruct 3D scenes in high resolution.