Fresnel diffraction method with object wave rotation for numerical reconstruction of digital hologram on tilted plane

We present a new method for numerically reconstructing holography image on tilted planes within the Fresnel domain. The method is based on the Fresnel diffraction algorithm and object wave rotation transform. By suitable rotation the object wave on recording plane and once fast Fourier transformation this method can be implemented. The proposed method can render the real image of the hologram not only on parallel plane but also on the tilted. Experimental results are presented to demonstrate the method for a single axis rotation.     

同轴全息术用于粒子场测量的数值模拟

 采用数值方法模拟了同轴全息术测量粒子场的过程,对两种不同的数值算法＿直接傅里叶变换算法和卷积算法,进行了分析和比较,结果表明卷积算法符合实际要求。分析了记录图像的空间频谱及其对图像采样频率的要求,得出了在记录波长、采样间隔等条件一定的情况下的最小记录距离。对于一幅512×512像素的数字图像,若像元尺寸为6.7 μm,所用光波长为532 nm,则最小记录距离为43.2 mm。在此基础上对实验记录的振幅和相位型静态粒子的数字全息图,均得到了满意的数值再现像。     

Contouring of diffused objects by using digital holography

A simple method for contouring of diffused objects by using lensless Fourier transform digital holography (LFTDH) and dual-index immersion method is presented. Depth contour interval up to minimum of 0.12 mm could be achieved without making the interference phase fringes over crowded.     

Image encryption based on extended fractional Fourier transform and digital holography technique

We present a new optical image encryption algorithm that is based on extended fractional Fourier transform (FRT) and digital holography technique. We can perform the encryption and decryption with more parameters compared with earlier similar methods in FRT domain. In the extended FRT encryption system, the input data to be encrypted is extended fractional Fourier transformed two times and random phase mask is placed at the output plane of the first extended FRT. By use of an interference with a wave from another random phase mask, the encrypted data is stored as a digital hologram. The data retrieval is operated by all-digital means. Computer simulations are presented to verify its validity and efficiency.     

Reduction of speckle noise in digital holography by using digital image processing

A fundamental problem in optical and digital holography is the presence of speckle noise in the reconstruction process. Many approaches have been carried out in order to overcome such a problem ranging from modifying the spatial coherence of the illumination (optical techniques) to image processing techniques (digital techniques). This work shows the merged use of digital image processing techniques in order to reduce the speckle noise in digital reconstruction of optically recorded Fresnel's holograms. The proposed filtering techniques are illustrated with experimental results.     

The influence of hologram aperture on speckle noise in the reconstructed image of digital holography and its reduction

Based on the whole process of the recording and reconstruction of digital holography, we study the formation cause of speckle noise in its reconstructed image and acquire the conclusion that the small size of hologram aperture diffraction aggravates the speckle noise of reconstructed image and the speckle noise has been one of primary noise sources in the reconstruction process. In order to reduce the speckle noise resulting from little hologram aperture diffraction, we set an appropriate aperture function matching the recording parameter and aperture size of hologram and deconvolve the reconstructed image with it. The validity has been proved in theory and experiment. Therefore, it offers a brand-new thought and practical way to reduce the speckle noise in the reconstructed image of digital holography.     

Curvature measurement of optical surface using digital holography

In this paper, an optical surface curvature measurement technique based on digital holography is proposed. Unlike the previous digital holographic methods, which only involve a small part of the measured data for parameter estimation, the proposed method adopts more of them using least square method. Hence, the analyzed result is not affected by data extraction direction and less sensitive to imperfect phase compensation. The proposed method is demonstrated with a sample of reflection mirror in laser resonator, and verified by comparing the measurement result with that of the white light interferometer.     

Improvement of signal-to-noise ratio in digital holography using wavelet transform

The basic problem in optical and digital holography is the presence of speckle noise in the reconstruction process, which reduces the signal-to-noise ratio (SNR). The presence of speckle noise is serious drawback in optical and digital holography since it substantially reduces the SNR in the reconstructed image. In this paper, we present wavelet filtering to improve SNR in the reconstructed images from digital holograms. Experimental results are presented.     

像面数字全息的重建相位误差分析和改善

像面数字全息是数字全息技术中常用的测量和成像方式,它通常采用离散傅里叶变换和频率滤波的方法进行物光波的重建.本文讨论了这些算法对重建相位的影响.首先分析了频谱泄露对于相位误差的影响,结果表明当采样周期为整数时,重建相位误差很小,因此具有极高的相位重建精度;而当不满足整周期采样时,相位重建误差有了明显的增加.为了改善频谱泄露所引起的相位误差,采用Hanning函数对数字全息图进行了预处理,结果表明Hanning窗的加入能够有效地提高重建相位的准确程度.     

Methodology for analysis of displacement using digital holography

Digital holography is applied to make microdisplacement determination of a steel bar and an aluminum sheet. The displacement of aluminum sheet is made under immersion conditions in water and air. An experimental installation is presented for digital hologram registration with plane reference beam, using a CCD camera as detector. The “HOLODIG” computing system was developed for digital reconstruction of the image using the direct method, the intensity and phase image and interferograms were obtained. Taking into account the phase interferogram and making the unwrapping, the phase was determined for every point of the study surface. As examples of method application, the displacement of a square section steel bar, tested in air, and an aluminum sheet, tested in air and submerged in water, was determined. Experimental values of the displacement obtained agree with the applied ones. In the case of aluminum plate submerged in water, the magnitude of the measured displacement is proportional to water refraction index.     

Examination of the upper measurement limit in displacement measurement using fringe compensation in digital holography

Digital holography is a widely used method for displacement measurement in coherent optical metrology. An obvious limit of the method is that too large displacements result in dense fringes, so the fringes are practically invisible. The maximum number of contour fringes in displacement measurement is limited, because the cameras are discrete devices and sampling theory plays an important role. Because of the limited measurement range, compensation methods are promising tools for practical measurements. It can be shown that the practical measurement range can be extended above the Nyquist sampling limit. Compensation methods can be digital, because digital holographic interferometry operates with images recorded with a digital camera. In our research work the upper measurement range of fringe compensation method was examined. Our goal was to perform automatic compensation even if the displacement is higher than the measurement range of the basic method. The operation of the automatic fringe compensation method was based on the combination of two types of out-of-plane displacement measurements with different sensitivities.     

Lensless digital holography with short-coherence light source for three-dimensional surface contouring of reflecting micro-object

In this paper, a new lensless digital holography system with short-coherence light source is reported for recording three-dimensional surface contouring of reflecting micro-objects. In the experiment, each of the layers on the inwall of a conical pore is respectively recorded by changing the path length of the object beam, instead of changing that of the reference beam, which can reduce the recording complexity and errors. In addition, the least-square-polynomial-fitting is used for the first time to carry out three-dimensional reconstruction with a series of two dimensional intensity images of a micro-object, which can be used not only to reduce obviously the complication of the three-dimensional reconstruction, but also to carry out three-dimensional reconstruction of a micro-object with strong laser speckle noise, of which the phase images can not be obtained from the conventional phase unwrapping process.     

Design of the spatial filter window for digital holographic convolution reconstruction of object beam field

In the convolution reconstruction process of digital holographic object beam field, the object beam fields with different magnifications can be obtained when the reconstructing beams are spherical waves with different wave curvature radii. This paper presents a theoretical and experimental discussion on the useful spatial frequency spectrum of the hologram numerically illuminated by the spherical wave. The result shows that there would be an image in the spatial frequency spectrum of digital hologram, which is completely the same as the object, if the wave surface radius of the spherical wave is equal to the distance from the object to the CCD sensor. Taking this image as a reference and designing a filter, the position of the reconstructed images with different magnifications can be predicted correctly in the reconstructed plane. Additionally, since the spectrum distribution of zero-order diffraction can be forecasted accurately in theory, its contribution can be effectively eliminated through changing the spatial filter shape; then a reconstructed object field containing more high-frequency information can be obtained.     

Sectional image reconstruction and three-dimensional microscopy of small particles by angular spectrum method

In this letter, we demonstrate sectional image reconstruction and three-dimensional microscopy of small particles. We demonstrate sectional image reconstruction and holographic methods to obtain 2D and 3D images of small particles. A single hologram is sufficient to obtain a section containing only the focused parts of the reconstructed image. One can obtain images of different plane sections of a specimen in addition to its 3D display. The reconstruction of a digital hologram is based on the plane-wave expansion of the diffracted wave fields using Fourier optics (this method is also known as the angular spectrum method). With this method, the object-to-hologram distance can be quite small because the minimum-distance requirement does not apply. Furthermore, numerical reconstruction of transparent objects by this method may be interesting for micro-structure measurement.     

The determination of material parameters of microcomponents using digital holography

Progresses in microsystem technology promise a lot of new applications in industry and research. However, the increased complexity of the microsystems demand sensitive and robust measurement techniques. Fullfield and non invasive methods are desirable to get access to spatially resolved material properties and parameters.This contribution describes a simple and fast interferometric method for the analysis of shape and deformation of small objects by optical means. These quantities together with a well defined loading of the components can be the starting point for the determination of material parameters like Poisson-ratio, Young's modulus or the thermal expansion coefficient. Holographic interferometry and multiple wavelength contouring as well as multiple source point contouring are precise enough to fulfill the requests for precision and resolution in microsystem technology even on complex shaped structures with steps or gapsA new adaptive, iterative algorithm is developed and applied to the measured results that allows the numerical evaluation of the phase data to get absolute shape and deformation information in Cartesian coordinates. Surfaces with holes, gaps and steps can be registered without any ambiguities. Digital holography as the underlying holographic recording mechanism is extremely suitable for small objects and lead to simple and compact setups in which the objects’ shape as well as their deformation behavior can be recorded. Experiments using silicon microbeams and an object from fine mechanics are described to show the great potential of these fast and robust measurement techniques with respect to the determination of material parameters.     

Proposal for underwater structural analysis using the techniques of ESPI and digital holography

The purpose of this article is to study the application of the holographic interferometry techniques in the structural analysis of submarine environment. These techniques are widely used today, with applications in many areas. Nevertheless, its application in submarine environments presents some challenges. The application of two techniques, electronic speckle pattern interferometry (ESPI) and digital holography, comparison of advantages and disadvantages of each of them is presented. A brief study is done on the influence of water properties and the optical effects due to suspended particles as well as possible solutions to minimize these problems.     

Optical encryption of gray-level image using on-axis and 2-f digital holography with two-step phase-shifting method

In this paper we propose an encryption/decryption technique of gray-level image information using an on-axis 2-f digital holographic optical encrypting system with two-step phase-shifting method. This technique reduces the number of holograms in phase-shifting digital holography and minimizes the setup of the encryption system more than multistep phase-shifting technique. We are able to get the complete decrypted image by controlling the K-ratio which is defined as the reference beam intensity versus the object beam intensity. We remove the DC-term of the phase-shifting digital hologram to reconstruct and decrypt the original image information. Simulation results show that the proposed method can be used for encryption and decryption of a 256 gray-level image. Also, the result shows some errors of the decrypted image according to K-ratio.     

Study on the reduction of speckle noise in the reconstructed image of digital hologram

As one of the most important 3-D display technique, reduction of speckle noise in the reconstructed image of digital holography should be grounded on the digital hologram itself. Based on the whole process of the recording and reconstruction of digital holography, the optical distributions of recorded object and reconstructed image of digital holography have been studied. It has been proposed that the root formation cause of speckle noise in its reconstructed image is the speckle noise formed on the recorded object surface when illuminated by coherent light because of its optical roughness. A novel approach has been presented to reduce speckle noise in digital holography by changing the interference structure of hologram itself. First, by reducing the speckle noise in the reconstructed image, the distribution of ideal reconstruction light with reduced speckle noise is acquired. Then in turn, taking the ideal reconstruction light with reduced speckle noise as ideal object light, a new hologram can be rebuilt, which can reconstruct the ideal object light. The experimental results are given to confirm the proposed method. Therefore, it offers a brand-new thought and practical way to reduce the speckle noise in the reconstructed image of digital holography.     

全数字全息术在图像信息隐藏中的应用

提出了全数字全息术在计算机图像信息隐藏中的应用。待隐藏的秘密信息以及公开的承载信息均为灰度级静止图像。首先对待隐藏的计算机信息图进行全息变换,即通过计算全息图的制作步骤制作出全息图的图像文件,然后再进行Arnold对称性变换。在此基础上,将秘密图像嵌入到承载图像的DCT域的中频系数中。全数字全息术的引入进一步增强了隐藏图像的透明性,通过控制制作全息图时的参数可起到加密的作用,相当于增加了一把秘密钥匙,具有较高的保密性和稳健性。     

Application of wavelet transform to hologram analysis: three-dimensional location of particles

The wavelet analysis provides an efficient tool in numerous signal-processing problems and has been implemented in optical processing techniques, such as in-line holography. When the diffraction pattern recorded on a hologram is analyzed by means of a wavelet transform, the 3-D location of small particles can be determined very accurately. The diffraction process can, in fact, be interpreted as a convolution with a family of wavelet functions, or, merely, as a wavelet transform. The scale parameter of the wavelet family is related to the axial distance z that the wave propagates. The original field is then reconstructed by searching for the optimum value of the scale parameter which produces a maximum of the wavelet transform modulus. The technique proposed is implemented and experimental results are presented.