Image Quality Optimization And System Implement For Digital Holographic Microscopy

Digital holographic microscopy (DHM) is a new type of microscopy developed on digital holographic imaging technique and a product of the combination of digital holography and microscopy. It can allow the extraction of both amplitude and phase information for a wave diffracted by a specimen. The phase information provides nanometric accuracy images of the optical path length through the specimen in transmission DHM, or topographic images in reflection DHM. Digital holographic microscopy has been an important microscopic imaging tool in recent years and has broad application prospect and application domains. The purpose of the dissertation is to improve the reconstructed image quality, which is the key issues affecting in the practical applications of DHM, and to realize the system design of DHM to make up the domestic blank in this field of technology.First, the basic theory in DHM is completely and systematically researched. The basic optical architecture, the recording principles and the recording conditions, the numerical reconstruction methods, phase unwrapping theory, as well as phase compensation methods, are deeply studied, which has laid a theoretical foundation for the research and the system implementation in DHM.On the basis of the theory, the dissertation carries out the in-depth researches on the improvement of reconstructed image quality in DHM and has achieved important results, which would lay a solid foundation for the system implementation. In order to acquire high-quality microscopic digital hologram, the impact of structure parameters on the reconstructed image quality is analyzed in detail. The conclusion is drawn that, under the assumption of perfect imaging without diffraction limit, the magnification and the recording distance among the structure parameters are the main factors affecting the reconstructed image quality. The analysis is of great significance for improving the reconstructed image quality in DHM. Then, based on the formation mechanism of speckle noise, a new method to achieve the complete elimination of speckle noise in DHM is proposed by choosing a microscope objective with large enough numerical aperture and designing large enough magnification. The method can overcome speckle noise in essence and solve the most difficult coherence noise problem in DHM. In the addition, two new methods to eliminate the zero-order diffraction and conjugate image are presented. One adopts a matched filter exactly selected according to the spectrum distribution of the hologram, which can completely filter out the other spectrum to obtain better reconstructed image. The other adopts a complex finite impulse response digital filter to filter the hologram directly in spatial domain instead of in the frequency domain. The method is simple and the computational work is greatly reduced. By these efforts, the reconstructed image quality in DHM can be improved and the key issue for DHM practical application has been solved.Then the study of system implementation in DHM is fully carried out. The instruments of reflection DHM and transmission DHM have been developed. The former is designed for the reflection digital holographic microscopic imaging and the latter for transmission imaging with transparent samples. Both of the system configurations are optimized. Particularly in the reflection system, in order to avoid fixed pattern noise due to unwanted reflection from optical elements, a new configuration is presented. And the system software for hardware control and data processing is developed. The systems for laboratory research for reflection DHM and transmission DHM have been completely built up.On the developed instruments for reflection DHM and transmission DHM, the experiments to prove the feasibility and stability of the systems are completed. In the reflection DHM. the accuracy of quantitative imaging is verified by a 1951 USAF resolution test target. And the reflection DHM is applied to detect the optical fiber connector end face and some important parameters, such as the radius of the end face, the height of the core, and the eccentricity of the vertex, are achieved. In transmission DHM, the accuracy of quantitative imaging is verified by a self-produced stepped transparent sample with known height and refraction. Some living biological samples such as onion epidermal cells, paramecium and red blood cells, are measured and quantitative phase images with high quality are obtained. And the movies of time series phase images for paramecium are achieved.In addition, a digital holographic microscopy with large field of view and high resolution is implemented. A sub-region phase image stitching technique is proposed which can effectively enlarge the testing area with high resolution. Each sub-hologram is recorded by in-line phase-shifting image plane holography to achieve maximum field of view. In order to accurately stitch sub-region phase images, the sub-holograms recording adjacent sub-regions must have common overlapped area. Correlation algorithm is used to determine the overlapped area and subtraction process, that is, the former subtracts the average of the difference between the common overlapped areas of adjacent sub-region phase images, is used to unite the coordinate. Thus, the testing area is enlarged by this technique. The system software for hardware control and data processing is developed and a 3×3 stitching experiment is successfully carried out.In the end, our work and achievements in the thesis are summarized and future work is proposed to solve the further problems.