Investigation Of The Performance Of Digital Holographic System When Used For Microscoptic Image

Recently the technological trends based on the miniaturization of mechanical devices to the microscopic scale, have lead to the development of communication, electronic engineering and mechanism. Biological medication and medical research also need to study in the micro-mechanism. The conventional optical holography could test the microstructure of objects, but the hologram of the tested object has to be operated chemically, which is not beneficial to the real-time reconstruction of holograms. In addition, it is not flexible enough for the holograms'transfer and storage. Digital holography is based on the optical holography. A hologram is recorded by CCD and transferred to a computer. The reconstruction is fulfilled digitally in the computer with the simulated reference wave. So, the whole procedure of the digital holography could be digitized, including the transmission of holograms, and have many potential applications. Based on the theoretical analysis and the corresponding experimental research, this paper addresses some basic issues of the lensless Fourier transform digital holography including record conditions, reconstruction algorithms and its application in the microscopic imaging. The main contents are as follows:(1) The record of the off-axis holograms is analyzed theoretically, and the recording condition of the lensless Fourier transform digital holography is investigated in detail. The serious recording limitations due to the finite size of CCD and space-bandwidth products of an off-axis system and a lensless Fourier transform system are discussed. As in the traditional optical holography, the lensless Fourier transform arrangement applies less constraint to the lateral size of the object and helps to relax the requirement on spatial resolution of CCD sensor, which is favorable for micromeasurement.(2) The reconstruction algorithms of the off-axis digital holography are studied theoretically and experimentally. Usually, there are three algorithms, namely Convolution Method (CM), Angular Spectrum Method (ASM) and Fresnel Transform Method (FTM). The formulas are presented. The relation and the characteristics of the three methods are analyzed. The Point Spread Function and the Frequency Transform Function in CM are studied. The relation of input and output in CM and FTM are compared. The lensless Fourier transform digital hologram obtained in the experiment is reconstructed by the three algorithms respectively. The reconstructed results indicate that the FTM is the best method for reconstructing the lensless Fourier transform digital holograms.(3) The imaging performance of the lensless Fourier transform digital holography has been studied experimentally. Following the theoretical analysis, the imaging system has been set up. In order to improve the resolving power, the record distance is shorten, the harmful light is get rid of, and the reference point source is placed as close to the center of the object as possible. Two conventional techniques for eliminating zero-order diffractive item have been studied.(4) The reconstruction of the 3D object in the lensless Fourier digital holography has been studied. The phase distortion has been analyzed in the reconstruction of lensless Fourier transform holography by reconstruction beams different from recorde reference wave. The corresponding formulation has been derived. With such formulation, the correctly reconstructed 3D image can be obtained by rectifying the phase distortion. Both the simulation results and the experimental results show that the theoretical prediction is proper.