Digital holographic microscopy

• Online Access: http://hdl.handle.net/2047/d10017185

A traditional microscope is not capable of imaging phase object or three dimensional object. Computational Microscopy is proposed to address these two limitations. In Computational Microscopy, the sample is under coherent illumination and the intensities of the diffracted light are recorded by a detector system (CCD camera in most cases). The recorded information is then numerically processed by computer to retrieve the complex field scattered by the object from which the quantitative properties of the object are reconstructed. Three dimensional imaging via Computational Microscopy is performed by illuminating the semi-transparent object from multiple directions and solving the inverse scattering problem via techniques of Optical Diffraction Tomography(ODT). Digital holographic microscopy(DHM) discussed in this thesis is a synthetic implementation of the Computational Microscopy and employs phase-shifting digital holography for phase retrieval. Different experimental schemes were tested and three numerical back-propagation methods were implemented for back-propagating the complex wave from measurement plane to near field. The most emphasized part of the thesis is the research in the algorithms for ODT. An inverse scattering algorithm based on distorted wave approximation for the geometry of ODT is introduced. We implement the algorithm to two Optical Diffraction Tomography configurations: free space and cylindrical symmetric background. For free space case both near field and far field inverse scattering problems are studied. For cylindrical symmetric background inverse scattering based on Distorted Wave Born Approximation and Distorted Wave Rytov Approximation are discussed and compared via computer simulation. In addition, an inverse scattering algorithm based on ray tracing is derived based on the similar idea. Experimental results are presented in which we have successfully reconstructed transmission functions of 2D semi-transparent objects and refractive index profile of a step index optical fiber.