Potential of digital holography in particle measurement

This paper describes the potential power of digital holography in particle measurement and its expected development in the near future. In digital holography, image reconstruction is carried out numerically on a computer using observed hologram patterns and some quantitative information can be derived from the reconstructed images. In this paper, the basic concept and procedure of digital in-line holography are shown mainly for particle depth measurement and the performance test results obtained in numerical simulations and experiments are demonstrated to examine the potential of the present method.     

Method for eliminating zero-order diffraction in lensless Fourier transform digital holography

A method to induce phase shifting in lensless Fourier of digital holography system is presented. In this method, by computer simulation and theoretical analysis on the technology to eliminate the influence of zero-order diffraction in force, it can be found that, a reference light induced in a random non-2π integral number of phase shifting shooting to get the second hologram, and wave reconstruction can be got by the difference value image of the two holograms. And, the method of different digital holography record system with different phase shifting should be used. In this paper, theoretical analyses have been done in detail to discuss the problems that exist in the unsuitable phase-shifting methods. Furthermore, some experiments have been done to prove the reliability of this method. This method can significantly improve the image quality and give better resolution of the reconstructed image.     

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.     

Wave front reconstruction of Fresnel off-axis holograms with compensation of aberrations by means of phase-shifting digital holography

Off-axis holograms recorded with a CCD camera are numerically reconstructed in amplitude by calculating through the Fresnel–Kirchhoff integral. A phase-shifting Mach–Zehnder interferometer is used for recording four-quadrature phase-shifted off-axis holograms. The basic principle of this technique and its experimental verification are described. We show that the application of this algorithm allows for the suppression of the zero order of diffraction and of the twin image and that the contrast of the reconstructed images can be further enhanced by digital compensation of the aberrations introduced by the holographic recording system     

Information reduction using lensless Fourier transform digital composite holography

An optical hologram contains substantially more information than necessary for some specific applications. Practical methodology of handling huge information contents in these holograms for such applications is cumbersome. In this paper, a method of information reduction, which aims at efficient storage and transmission of holograms, is investigated using digital composite holography in lensless Fourier transform configuration. The maximum information reduction factor obtained in our experiment is 105. The advantages/disadvantages of this method over an earlier method used in optical holography [Lin LH. A method of hologram information reduction by spatial frequency sampling. Appl Opt 1968;7:545] are also discussed.     

一种实现单步相移无透镜傅里叶同轴数字全息的倒频谱技术

在数字全息技术中无透镜傅里叶同轴数字全息具有高分辨,再现速度快等优点,因此有着广泛的应用前景,但它也存在再现像与直透光场及其共轭像重叠,较难分离的缺点。针对这一问题,在分析了无透镜傅里叶同轴数字全息图记录和再现原理的基础上,提出了单步相移倒频谱技术,并在理论上分析了该技术对分离出清晰再现像的可能性和该技术适用的关键条件。通过仿真实验进行了验证,结果表明:单步相移倒频谱技术能够较好地解决无透镜傅里叶同轴数字全息技术中再现像与直透光场及其共轭像重合这一困难,得到清晰的再现像;同时给出了实际实验中单步相移倒频谱技术能够适用的条件参数,这为后面的进一步实验提供了参考依据。     

Joint approach of the sub-holograms in on-axis lensless Fourier phase-shifting synthetic aperture digital holography

In on-axis lensless Fourier phase-shifting synthetic aperture digital holography, to compose all of the phase-shifting sub-holograms to a large synthetic aperture digital hologram effectively, firstly, the cross-correlation algorithm of the object waves is presented to correct the joint misplacement of the sub-holograms. Secondly, to make the phase-shifting synchronization matching of different sequence phase-shifting holograms, the cross-correlation algorithm of the phase-shifting holograms is employed. Compared with the traditional cross-correlation algorithm of the sub-holograms, the proposed approach makes the joint precision of the sub-holograms reach sub-pixel accuracy, and the resolution of the reconstructed image is improved significantly. In general, the proposed approach is effective in restraining the quality degradation of the synthetic reconstructed image that comes from the joint misplacement of the sub-holograms and the phase-shifting non-synchronization of the phase-shifting holograms.     

无透镜傅里叶变换数字全息术中非共面误差的自动补偿算法

无透镜傅里叶变换数字全息术对活细胞进行显微相衬成像测量时,由于细胞处于运动状态导致记录物体与参考点源难以共面,从而引入成像误差.本文提出了一种有效可靠的基于位相分布的自动对焦再现算法,只需通过一幅全息图,即可确定出记录距离的优化数值解,并结合一次位相线性拟合,成功补偿掉非共面误差.将这种算法应用到活体宫颈癌细胞的相衬成像实验中获得了较为理想的实验结果,表明了这种自动补偿非共面误差算法的可行性. 关键词： 数字全息 无透镜傅里叶变换全息 自动对焦 位相畸变校正     

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.     

Lensless Fourier holography for digital holographic interferometry on biological samples

Digital holography as a tool for highly sensitive, interferometric non-destructive testing has several advantages compared to holographic measurements based on conventional storage media like an all-digital processing and a direct access to the phase of the object wave. Experimental results of interferometric investigations of heart valve bio-prostheses with a setup for lensless Fourier holography are presented which demonstrate that this technique is applicable to such biological samples with their wet and unstable surfaces. Limitations on size and resolution of the reconstructed object caused by the properties of the CCD sensor are discussed.     

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.     

Reconstruction of digital hologram of small particles on arbitrarily tilted plane using digital holography

Digital holographic off-axis geometry is presented for the reconstruction of the digital hologram of small particles in an arbitrarily tilted plane. A single hologram is sufficient to obtain a well focused clear image. We can't obtain clear images of small particles in the case of plane tilted to the detector plane by ordinary reconstruction method because of the rotation of the hologram plane. Rotational transformation based on coordinate rotation in Fourier space makes it possible to reconstruct holographic images on any plane in the object space. The reconstruction of digital hologram is based on the plane wave expansion of the diffracted wave fields, using Fourier optics and the two-axis rotation of the wave vectors. With this method, the object-to-hologram distance can be any small distance because the minimum distance requirement does not apply.     

High resolution digital holography

In digital Fresnel holography, present specifications of charge-coupled device cameras require the incident beams to be quasi-parallel. That implies large speckle grain size and low lateral resolution in reconstructed images. Better lateral resolutions are demonstrated in our work, down to 8 μm, allowing the observation of sub-millimetre objects by digital holography. The experimental set-up built samples the incident light distribution with a definition of 500 pixels mm⁻¹. The maximum acceptable angle is then widened and the hologram recording and reconstruction distances are drastically reduced, as well as the speckle size. Interferometric holography was implemented for the measurement of the deformation of a sub-millimetre silicon cantilever and results in phase-contrasts imaging are also reported. The design of dedicated complementary metal-oxide-semiconductor active pixel sensor cameras is also discussed.     

数字全息技术的原理和应用

数字全息是随着现代计算机和CCD技术发展而产生的一种新的全息成像技术．文章主要介绍数字全息技术的基本原理，数字全息重建中的主要方法以及数字全息技术以其独特的优点在各个领域中的应用．     

Use of digital color holography for crack investigation in electronic components

This paper presents the experimental optical analysis of the crack inside an electronic component. The optical setup is used to carry out multidimensional deformation measurements using digital color holography and the spatial multiplexing of holograms. Since the Fresnel transform method depends on wavelength, a wavelength-dependent-zero-padding algorithm is described and results in a rigorous sizing of each reconstructed monochrome image. The criterion to optimize the parameters is presented and is based on minimizing the widening of the impulse response of the full recording/reconstruction process. The application of the proposed method is illustrated through the analysis of the mechanical deformation of the electronic component, and offers keys to understand its failure mode in industrial conditions.     

Studies on aperture synthesis in digital Fresnel holography

Aperture synthesis can improve image resolution in digital holography by increasing the numerical aperture of the system. In this paper, we show that both the lateral resolution and image field of view can be enhanced at the same time using a more general Fresnel holography setup and hologram stitching. The impact of aperture synthesis on the lateral resolution is investigated both theoretically and experimentally. In the experiment, the synthesis is executed by moving the compact digital holographic system in two directions. Nine holograms are recorded and stitched into one hologram. The reconstruction results show that expanding aperture improves the lateral resolution. The lensless Fresnel holography used in this paper is demonstrated to have the ability to provide a larger numerical aperture and can compress the object spectrum in recording process.     

Elimination of zero-order diffraction in digital off-axis holography

A simple experimental method of eliminating zero-order diffraction in the reconstructed image of off-axis digital holography is presented. Holographic diffraction grating acting as a beam splitter, an off-axis holography system is formed. The holograms of object with different recording parameters are obtained by adjusting the reflecting mirror in the recording optical system to vary the incidence orientation of the object beam in CCD to introduce a phase shift. The zero-order image can be eliminated by numerically processing the holograms of object with different recording parameters. The theoretical analyses have been done in detail and the experimental results are also given. The zero-order image eliminated, the area of reconstructed image increases remarkably, the image quality can then be significantly improved and the better resolution obtained. The experimental results show that the method presented in this paper is feasible, simple in optical system and easy in operation and data processing.     

Numerical reconstruction in in-line digital holography by translation of CCD position and gradient operator method

We propose an approach based on axial translation of a charge-coupled device (CCD) and gradient operator method for numerical reconstruction in in-line digital holography. The object wave in the hologram (or CCD) plane is determined using one position translation of CCD, and a phase-shifting device can be fully avoided. Gradient operator method is further applied to detect the focal plane of an object, and the object wave is reconstructed by an angular spectrum algorithm with the detected focal distance. The feasibility and effectiveness of the proposed method are demonstrated by numerical simulations.     

Green's formulation for robust phase unwrapping in digital holography

We present, for the first time to the best of our knowledge, a phase unwrapping method based on an algorithm which makes use of Green's first identity. This method aims at recovering the correct phase information encoded in a fringe pattern after digital holography (DH) numerical processing. DH provides a quantitative measurement of the three-dimensional surface profile of objects. The information about the profile can be obtained in principle from the phase-map. However, the measured phase-map provides the actual phase values wrapped mod.2π so that an unwrapping process is required in order to reconstruct the object profile.