Research On Zero-order-elimination Spectrum Filtering And Diffraction Imaging Auto-focusing In Off-axis Digital Holographic Reconstruction

Digital holography is a three-dimensional imaging technology,by which both the amplitude and the phase information of an object can recorded in holograms.Due to its advantages of fast,sensitive and non-destructive imaging,digital holography is widely used in various imaging applications,such as medical imaging,particle localization,deformation analysis and surface profiling.Digital holograms can be categorized as the in-line holograms and the off-axis holograms structures according to the configuration of recording optical path.In Fourier spectrum domain of off-axis digital holograms,the centers of the zero-order spectrum,positive first-order spectrum and negative first-order spectrum are separated one another.In the numerical reconstruction imaging,it is usually necessary to filter the positive first-order spectrum or the negative first-order spectrum to reconstruct the amplitude and phase images.The spectrum separation feature of off-axis digital holograms is suitable to the spectrum interception for filtering,but also limits the utilization of the spatial bandwidth of the CCD in off-axis digital holography.This is why the imaging resolution of off-axis digital holographic reconstruction is lower than that of in-line digital holography under the same conditions.In order to improve the spatial bandwidth utilization in off-axis digital holography,researchers have proposed many methods in terms of algorithms and optical system design.In this thesis,there include that the zero-elimination hologram is synthesized by multiplexing light intensity ratio to improve the image resolution of off-axis digital holographic reconstruction,and that the fast self-focusing in off-axis digital holographic imaging by using image evaluation functions are explored.Details of the research are as follows:In the first part,based on the linear variation of off-axis digital holograms,a zero-order elimination method in off-axis digital holography by recording three holograms with different light intensity ratios and generating one zero-order elimination hologram is investigated.The zero-order elimination method for off-axis digital holography by using multiplexed light intensity ratios is described in principle,and the effectiveness of the zero-order elimination method by multiplexed light intensity ratios is verified via analytical simulations.The off-axis holographic recording system based on the Mach-Zender interference structure is constructed,and the reconstruction imaging of the zero-order-elimination hologram with multiplexed light intensity ratios is performed.In the experiment,by using the combination of one half-wave plate and one polarized beam-split prism in the optical system,three off-axis digital holograms with the different intensity ratios of the object and reference waves are recorded in turn by arbitrarily turning the half-wave plate.Then,the spatial-frequency spectrums of the three holograms are obtained via Fourier transform,and then linearly fitted in their Fourier spectrum domain to determine the two fitting coefficients₁ηandη₂.By multiplying the second and the third holograms with coefficients ₁ηandη₂ respectively,the both holograms with the coefficients are subtracted from the first hologram to obtain one resultant hologram.The zero-order spectrum of the resultant hologram is eliminated.Further,the amplitude and phase images are reconstructed after performing the spectrum filtering on the positive first-order spectrum.The reconstruction imaging results both in simulation and in experiment demonstrate that the zero-order spectrum of the resultant hologram is effectively eliminated by multiplexing intensity ratios.Due to the elimination of the zero-order spectrum,the filtering interception window can reach to the spectrum center,which can intercept more high-frequency spectrum and result in the improvement of reconstructed image resolution.In particular,the amplitude distribution of a resolution test target and the phase distribution of dragonfly wing are reconstructed by the zero-order-elimination filtering.Compared with the conventional filtering method,the resolution and image quality are improved significantly.In the experiment,three holograms with different intensity ratios of object wave and reference wave can be recorded just by rotating the half wave plate,so it is easy to operate.In the second part,the use of image evaluation functions for fast autofocusing in off-axis digital holographic reconstruction is explored.First,several common image evaluation functions are introduced,and the method for finding a focusing position by using image evaluation functions is described.Three characteristics of the focusing evaluation curve as criteria are described,which can be used for selection of the evaluation functions.Secondly,for the nine existing image evaluation functions,the image evaluation functions suitable for focusing evaluation are determined at the different scales according to the three features of a focusing evaluation curve.Several image evaluation functions have been used in the diffraction propagations of the pattern and of Fresnel hologram to search their focusing positions.By analyzing their focusing results,the autocorrelation function（Vollath）and the amplitude modulus analysis function（AMS）are selected as focusing evaluation function.The focusing curves of the two evaluation functions show the excellent focusing characteristics,indicating the high robustness for focusing evaluation.Further,the fast autofocusing either with Vollath or AMS as focusing evaluation function is performed in the experiment.The autofocusing time with the focusing evaluation functions is about 19.59sec in the case of no decrease of focusing accuracy.