Theoretical And Experimental Study On The Reconstruction Of The Digital Holograms

In digital holography, the holographic information is directly recorded on a charge-coupled device (CCD,CMOS) camera and the optical reconstruction is replaced by a computer numerical reconstruction, so the holographic full process including recording, memory and reconstruction is entirely digital. With the rapid developments of the megapixel CCD sensors and the computers, digital holography have recently became feasible and have been used in many fields, such as 3-D profilometry, grain size analysis, holographic microscopy, information security etc. Lower space-bandwidth product (SBP) of the CCD array than conventional photographic materials, however, limits the use of an off-axis recording geometry for separating the reconstructed image from the conjugate replica and the zero diffraction order. Digital in-line holography is often adopted for reducing the SBP of the holograms and a phase shifting algorithm is introduced to remove the influence of the zero-order autocorrelation and the conjugate image noises. In this situation, a phase shifting device with high precision is required and the stability of the phase shifting device will influence the quality of the reconstructed image. Based on theoretic analysis, computer simulation and the corresponding experimental research, this paper gives some analysis about fundamental issue of recording method, the choice of the reconstruction algorithms and the location of the exact image plane. By annlysis above problems, some important research results are obtained. In this paper, the main innovative researches are as follows:1. Based on the two in-line holograms recorded in two planes, a method to reconstruct the Laplacian second-order differentiation of the object wave in transverse direction is proved. We analyze the diffraction properties of the picture obtained by subtraction of two in-line holograms recorded in two planes. Our theoretical analysis reveals that the reconstructed field at the object plane is approximately equal to the Laplacian second-order differentiation of the object wave in transverse direction. The technique can reduce the influence of the zero-order autocorrelation and the conjugate image noises. We further investigate the dependence of the edge-enhancement quality on the longitudinal differential distance and find that the reconstructed images have the sharpest edge-enhancement and high signal-to-noise ratio at the same time under our experimental condition.2. Three auto-focus algorithms of digital imageing process are compared and analyzed. And these three algorithms are gray variance method, gray entropy methed and the Laplacian of Gaussian (LoG) method. The gray variance method and the gray entropy method cann't detect the image plane exactly in ordinary in-line digital holography, while the LoG method can detect the image plane exactly. A new focus detection technique based on Laplacian second-order differentiation is developed for ordinary in-line digital holography. It's different from the LoG method in that the Laplacian differentiation of the object wave is obtained by the reconstructing the subtraction of two in-line holograms recorded in two planes at different distances and the critical function is got by accumulating the amplitudes of the reconstructed iamge. It can detect the image plane exactly in ordinary on-line digital holography, because it can reduce the influence of the zero-order autocorrelation and the conjugate image noises. It can get the sharpness refocus curve than the method LoG and strongly save computationally time.