Performance evaluation of fiber Bragg grating sensors by digital holographic technique, strain gauge measurement

In view of the growing interest for non-destructive tests of materials, geodynamical monitoring and in general remote sensing, there is a great effort to bring practical optical sensors from research labs to industrial and environmental applications. In this paper, we employ digital holographic technique as an efficient tool for evaluating the strain measurement capability of fiber Bragg gratings (FBG). A cantilever beam has been employed as a test structure under loading test. The strain measurements results obtained by fiber-based sensors have been compared to those obtained by using full-field digital holographic technique and point-wise strain gauge sensors glued on the same cantilever beam. A simple theoretical model is also presented to interpret and compare the experimental results coming from different techniques.     

Recent contribution in color interferometry and applications to high-speed flows

Recent improvements brought to color interferometry for analyzing high-speed flows are described through different applications. First, the optical technique based on differential interferometry using a polarized white light and one or two Wollaston prisms allows to record high-speed interferograms of the flow downstream of a circular cylinder. Then, this technique has been applied to axisymmetric flows for studying an interaction between a supersonic hot jet and a coaxial supersonic flow. Another application concerns the study of hypersonic flows using Wollaston prisms with a large birefringence angle. Finally, the analysis of gaseous mixture and the evolution of two-gases interface submitted to an acceleration is presented. Interferograms analysis is made from a modeling of interference fringes versus the optical path difference which allows to easily extract quantitative information of the gas density. In order to obtain absolute measurements of the gas density, real-time holographic interferometry has been developed using a three-color laser source and a panchromatic holographic plate. The technique generates the achromatic white fringe which makes the zero order of interference fringes easy to identify. An application is presented in a 2D subsonic wind tunnel, in which the unsteady wake flow past a cylinder is recorded at high framing rate. In this optical setup, transmission holograms are used. As a conclusion, an approach is proposed to analyze the 3D flows from real-time color holographic interferometry using reflection holograms and the problems to solve are described.     

像面滤波技术在无透镜傅里叶变换数字全息中的应用

无透镜傅里叶变换数字全息波前重建主要采用全息图的一次快速傅里叶变换方法,重建图像不能充分占有重建平面.本文基于像平面滤波技术,提出对物体局部区域光波场进行放大重建并让重建图像布满重建平面的方法,给出具有精细结构物体的数字全息波前重建实例.此外,将数字全息光波场重建视为具有方形出射光瞳的光学系统的相干光成像过程,导出了物体放大图像的分辨率与光学系统相关参量的关系,并通过实验给予证明.     

像面滤波技术在无透镜傅里叶变换数字全息中的应用

无透镜傅里叶变换数字全息波前重建主要采用全息图的一次快速傅里叶变换方法,重建图像不能充分占有重建平面.本文基于像平面滤波技术,提出对物体局部区域光波场进行放大重建并让重建图像布满重建平面的方法,给出具有精细结构物体的数字全息波前重建实例.此外,将数字全息光波场重建视为具有方形出射光瞳的光学系统的相干光成像过程,导出了物体放大图像的分辨率与光学系统相关参量的关系,并通过实验给予证明.     

Quasi-common path three-wavelength holographic interferometer based on Wollaston prisms

This article presents a digital three-wavelength holographic interferometer based on the use of two Wollaston prisms. This provides an in-line setup with quasi common-path and as a consequence there is no additional independent reference wave to be added. Thus, immunity to external perturbations such as vibrations or thermal perturbations is achieved. Furthermore, the set-up exhibits a single shot and real time capability which is very useful to study dynamic events. By using the two Wollaston prisms in an astigmatic configuration, spatial carrier frequencies can be adjusted both in amplitude and orientation. The digital hologram processing is based on Fourier processing and filtering around the carrier spatial frequency so that phase shifting is not required. The use of three wavelengths leads to visualizing directly the zero order fringe and regions for which there is no air density variation in a dynamic flow. Experimental proof of concept is demonstrated with a supersonic jet when the injection pressure varies.     

数字全息成像系统的景深和焦深分析

根据全息理论,分析了数字全息成像系统的景深和焦深.针对数字全息不同的记录光路结构,分别给出了焦深的近似表达式.结果表明：数字全息系统的景深和焦深不仅与记录波长以及记录时的数值孔径有关,还与记录时参考光波的偏置情况有关;在记录距离和CCD参量一定的条件下,离轴无透镜傅里叶变换全息对称偏置下的焦深比非对称偏置下的稍小;显微成像情况下离轴无透镜傅里叶变换全息系统的焦深大于同轴菲涅耳数字全息系统的焦深,计算机模拟表明了结果的正确性.     

Study on the simplified phase-shifting digital holographic microscopy

In this paper, a new simplified technique for effectively eliminating the zero order and the conjugate virtual image in digital holographic microcopy, which makes use of two-step phase-shifting method of just recording two holograms and an intensity image of object wave, is proposed. Meanwhile, combined with the principle of making full use of spatial bandwidth of the CCD sensor by in-line lens-less Fourier holographic recording geometry, the theory and experimental methods to increase the resolution of the reconstructed image in digital holography by using phase-shifting technique are detailedly analyzed. At end, the validity and availability of this technique has been demonstrated through the off-axis and in-line Fourier transform recording geometry. The study provides some theoretical and experimental guidance for the design and operation of a digital holographic microscopy system.     

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.     

Multiple-scattering speckle in holographic optical coherence imaging

We investigate the effect of multiple scattering on the image quality of holographic optical coherence imaging, which is a full-field coherence-domain imaging form of optical coherence tomography. The speckle holograms from turbid media and from multicellular tumor spheroids are characterized by high-contrast speckle on a multiply-scattered background caused by channel cross-talk. We quantify the multiple-scattered light that is accepted by the holographic coherence gate, and identify a cross-over from single-scattered to multiple-scattered light beyond 15 to 20 optical thicknesses. Speckle reduction relies on vibrating diffusers and on fast adaptive holograms in photorefractive quantum well devices. The high anisotropy factor for tumor tissue reduces multiply-scattered light contributions for biomedical tumor imaging.     

Digital holography particle image velocimetry for the measurement of 3Dt-3c flows

In this paper a digital in-line holographic recording and reconstruction system was set up and used in the particle image velocimetry for the 3Dt-3c (the three-component (3c), velocity vector field measurements in a three-dimensional (3D), space field with time history (t)) flow measurements that made up of the new full-flow field experimental technique—digital holographic particle image velocimetry (DHPIV). The traditional holographic film was replaced by a CCD chip that records instantaneously the interference fringes directly without the darkroom processing, and the virtual image slices in different positions were reconstructed by computation using Fresnel–Kirchhoff integral method from the digital holographic image. Also a complex field signal filter (analyzing image calculated by its intensity and phase from real and image parts in fast fourier transform (FFT)) was applied in image reconstruction to achieve the thin focus depth of image field that has a strong effect with the vertical velocity component resolution. Using the frame-straddle CCD device techniques, the 3c velocity vector was computed by 3D cross-correlation through space interrogation block matching through the reconstructed image slices with the digital complex field signal filter. Then the 3D-3c-velocity field (about 20 000 vectors), 3D-streamline and 3D-vorticiry fields, and the time evolution movies (30 field/s) for the 3Dt-3c flows were displayed by the experimental measurement using this DHPIV method and techniques.     

An approach for sensing marine plankton using digital holographic imaging

The goal of this paper is to use digital holographic imaging for sensing marine plankton in recording sampling volume. The process stage of this approach includes: wavefront recording using in-line holographic recording set up and numerical reconstruction using Fresnel approximation and convolution algorithm. So, by capturing hologram of marine plankton and reconstructing hologram, the recorded optical field of marine plankton is retrieved. Digital holographic imaging is an extremely powerful technique for the study of marine plankton fields as it allows instantaneous, noninvasive, high-resolution recording of substantial volumes. Finally, this paper presents that it is possible for digital holographic imaging system to sense marine plankton according to laboratory results.     

Realization of an optical interferometer based on holographic optics for real-time testing of phase objects

The paper describes a simple and cost effective method for the realization of an optical interferometer based on holographic optics, which use minimal bulk optical components. The optical arrangement in the proposed method involves a very simple alignment procedure and inexpensive holographic recording material is used in the formation of holographic optical elements. The proposed interferometer set-up is quite suitable for performing optical test studies on phase (transparent) objects in real-time. Recording schemes for the formation of holographic optical elements and the related technique for the realization of the interferometer set-up along with the experimental results have been presented.     

Measurement of static and vibrating microsystems using microscopicTV holography

This paper describes a microscopic TV holographic arrangement to study the static and vibrating microsystems. In the optical setup, the object beam and the reference beam arms are provided with a phase shifting mirror and a bias phase modulation mirror to carry out the measurement of the out-of-plane deformation and the vibration amplitude fields, respectively. A long working distance microscope is used in the setup for magnifying and imaging the objects on to the CCD camera. For static fringe analysis, the system is used in double exposure subtraction mode of operation, while for vibration fringe analysis, it is used in the time average contrast reversal refreshing mode of operation. An improved approach for qualitative analysis of time averaged fringes helps in reducing the number of frames required for analysis. The usefulness of the system is demonstrated by examples of static and vibration measurements for different microobjects.     

光学扫描全息术研究进展

光学成像技术极大地拓展了人类的视觉极限,提高了人们观察和理解现实世界的能力。越多地获得目标的光学信息,对其的认识越充分。数字全息术是一种可以将样本的三维信息以二维全息图的形式编码记录下来的一种成像技术。通过获得由携带物体信息的物光波和参考光波叠加产生的干涉图案,可以以数字化的方式实现多种重建模态,例如图像恢复、相位成像和切片成像等。光学扫描全息术是一种独特的数字全息成像技术,通过主动式二维化扫描对三维物体进行成像,其完整的波前信息可以被单像素探测器记录,并基于光外差检测进行信号解调,从而恢复出复数全息图。对光学扫描全息术的最新进展进行介绍。首先,基于双光瞳成像系统,通过特殊的硬件和算法设计,提高光学成像系统的性能,如提高空间分辨率、缩短扫描时间。其次,基于计算成像原理,通过改进和优化全息像重建算法,实现高质量的图像恢复,主要涉及切片成像和三维成像等重建模态。第三,介绍光学扫描全息术的其他研究方向,并讨论该领域未来可能的发展方向。     

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.     

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.     

基于DMD的数字全息显示及其再现像质增强

采用DMD作为空间光调制器构建了一套数字全息显示系统.分析了数字全息图光学再现中影响再现图像质量的因素,提出了一种采用频域滤波重建高条纹对比度滤波全息图来改善其光学再现图像质量的新方法.采用信噪比及图像亮度作为评价参量,对原始全息图和滤波全息图的数值再现像进行定量分析表明,滤波后全息图的再现像质量明显优于原始全息图的再现像.基于DMD数字全息显示系统的光学再现实验也验证了理论分析结果.     

Holographic Three-Dimensional Display Using an Electron-Beam-Addressed Spatial Light Modulator

Three-dimensional (3-D) or holographic information extracted by two-dimensional active optical heterodyne scanning has been demonstrated recently. The technique is called optical scanning holography. To reconstruct the obtainable 3-D holographic information, digital techniques have been used and demonstrated. For real-time applications, we need to investigate spatial-light-modulator-based systems. In this paper, we first briefly review optical scanning holography, and then present experimental results of 3-D image reconstruction using an electron-beam-addressed spatial light modulator.     

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．     

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.