Theoretical and Experimental Studies on Phase-shifting Digital Holographic Microscopy
|School||Xi'an Optics and Fine Mechanics|
|Keywords||Digital Holography Digital Holographic Microscopy Phase-Shifting Polarization Phase-Shifting Common-Path Dual-wavelength Autofocusing|
Digital Holographic Microscopy (DHM) is a recently well developed microscopictechnique, and has been becoming a hot research spot in optic science. DHM uses acharge coupled device (CCD) camera, intead of the traditional hologram recordingmedium, to store the hologram in computer digitally, and then simulate the opticaldiffraction process with corresponding algorithms for reconstruction of the originalobject field to realize three-dimensional microscopic imaging and measurement of thespecimen. DHM combines the characteristics of optical interferometic technology andmodern digital image processing technology, and has the features of whole-field,nondestructive, high-speed, real-time3D measurement. It has been widely used inbiomedical diagnosis, vibration analysis, deformation detection, particle field analysis,micro-/nano device inspection, and so on.Although the common off-axis DHM can totally eliminate the affection of zeroorder and twin images in real-time measurement process, the low resolution ofreconstructed image is the major drawback of off-axis DHM, due to the limitedresolution of current CCDs. In order to improve the DHM’s resolution, in-linePhase-Shifting Digital Holographic Microscopy (PSDHM) has been invented. Bychanging the phase of the reference wave with respect to the object wave, multiplephase-shifted holograms are captured during the recording process. Then the complexamplitude of the object field is reconstructed by using corresponding algorithms,which can eliminate zero order and twin images effectively. Compared with theoff-axis DHM, in-line PSDHM makes full use of CCD’s space bandwidth, thus theresolution of the reconstructed image is improved. Aimed at constructing a highlystable DHM setup for practical application, this thesis focused on PSDHM, andaccomplished the following main contents:1. The history and progress of digital holography (DH) and DHM are reviewed.The characteristic of PSDHM and its development are introduced andanalyzed comprehensively. The basic knowledges of PSDHM are introduced,including the concept of phase-shifting, the methods to realize thephase-shifting, recording and reconstruction of digital holograms, phase-shifting algorithms, and so on. All these knowledges are the theoreticalbasis for the next research.2. Based on the principle of polarization phase-shifting, step-by-stepphase-shifting DHM and parallel phase-shifting DHM have been built up,respectively, and the configuration, performance and application scope ofthem are discussed and compared. The physical formation of phase aberrationintroduced by condenser lens and microscope objective and the effect ofphase-shifting error are theoretically analyzed. In order to eliminate theadditional phase aberration and phase-shifting errors to improve the accuracyof measurement, a fitting method based on least square ellipsoidal model anda simple algorithm based on the statistical property of object wave for blindextraction of real phase-shift have been proposed.3. Two kinds of common-path PSDHM have been built up by combining thespatial beam splitting technique and the point-diffraction interferometry. Withbeamsplitters or gratings to split the light beam in space, the reference wave isreproduced by a pinhole filter to ensure the object wave and the referencewave to share the same path. The common-path configuration makes thedistributions of the two beams counteracted by each other in recordingprocess, and improves DHM’s stability and practicability.4. A dual-wavelength DHM with a slightly off-axis configuration has beenproposed based on the imaging characteristic of color CCD anddual-wavelength illumination. To realize the slightly off-axis recording andimprove the resolution of image, two single wavelength holograms withorthogonal spatial carrier frequencies can be extracted from the recordedcolor hologram with one exposure, and the dc term can be suppressed bysubtracting one from the other in property. Meanwhile, the real-timeunwrapped phase measurement is achieved by a synthetic wavelengthΛ12=λ1λ2/|λ1-λ2|, which is produced by beat frequency of two wavelengths λ1and λ2, which provides a useful measuring tool for moving object or dynamicprocess detection.5. Two kinds of autofocusing schemes are proposed, which are based on thediffraction difference of dual-wavelength and two-angle off-axis illuminations of single wavelength, respectively. The image plane can be automaticallydetermined in the reconstructing process by seeking the minimal difference ofreconstructed images of two wavelengths or in different illuminationdirections. Both of algorithms can realize autofocusing exactly, and areapplicable to specimens without chromatic absorption and birefringence.