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.     

基于Wollaston棱镜的大角度液晶光束偏转系统

设计了一种能够应用于空间光通信的大角度高精度光束偏转系统。系统采用液晶空间光调制器(LCSLM)实现小角度光束偏转,采用多Wollaston棱镜级联实现角度放大。介绍了LCSLM和Wollaston棱镜的光束偏转原理,并对大角度高精度光束偏转系统进行了仿真分析。仿真结果表明系统能够实现角度范围为±13.25°的光束偏转。国内首次提出运用Wollaston棱镜来实现大角度的光束偏转。     

Birefringent neutron prisms for spin echo scattering angle measurement

In the first decade of the 19th century, an English chemist, William Wollaston, invented an arrangement of birefringent prisms that splits a beam of light into two spatially separated beams with orthogonal polarizations. We have constructed similar devices for neutrons using triangular cross-section solenoids and employed them for Spin Echo Scattering Angle Measurement (SESAME). A key difference between birefringent neutron prisms and their optical analogues is that it is hard to embed the former in a medium which has absolutely no birefringence because this implies the removal of all magnetic fields. We have overcome this problem by using the symmetry properties of the Wollaston neutron prisms and of the overall spin echo arrangement. These symmetries cause a cancellation of Larmor phase aberrations and provide robust coding of neutron scattering angles with simple equipment.     

基于空间光调制器的非相干数字全息单次曝光研究

菲涅耳非相干相关全息术(Fresnel incoherent correlation holography,FINCH)利用在空间光调制器(spatial light modulator,SLM)上加载双透镜模式对同一物点光分束自相干,并通过改变加载的相位因子得到不同的相移全息图.本系统利用SLM可分区编码调制特性,将FINCH成像中SLM上分三次加载的0°,120°,240°相位双透镜掩模各提取1/3组成一幅复合相移模式加载,并研究了三种相位分布方式对FINCH成像质量的影响.结果表明:三个相位在SLM上分布间隔越大,再现像越清晰.在此基础上,提出了一种新的掩模加载方式,在SLM加载透镜阵列,每一个相位因子对应一个双透镜,具有一个光轴.实验表明,通过这种加载方式,通过SLM后形成的三个相移图能够一次在电荷耦合器上记录,并且三个相移图不重叠,然后通过MATLAB编程计算将不同相移角度的全息图分别提取出来,通过三步相移计算合成一幅包含有物光波的复值全息图,最后通过数值再现算法重建待测样品.此系统可用于对光源相干性较低的实时成像系统,也为微小形变测量、动态物体的观测提供了新方法,为非相干数字全息术的发展提供了新思路.     

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.     

LED光源数字全息技术研究

研究以发光二极管(LED)作为光源的部分相干光数字全息技术.首先研究LED的时间相干性和空间相干性,尽管LED的时间相干性较差,但空间相干性可以通过减小光源发光面积来提高.利用LED的时间相干性较差、相干长度短的特点,抑制相干噪声,改善数字全息重建质量.在同一全息记录系统,通过实验,比较了用激光和LED光源的数字全息重建图像质量.结果表明:基于LED光源的数字全息,完全消除了使用激光光源的散斑噪声和由光学元件引入的寄生干涉噪声,物光场的重现质量,包括振幅和相位都得到了很大提高.但由于LED光源的较低的空间相干性,一般只适用于同轴相移数字全息,待测物体的厚度在十几微米以内,应用受到一定限制.     

基于Wollaston棱镜的图像复分快照式成像光谱仪设计

设计一种以Wollaston棱镜为分光元件的图像复分快照式成像光谱系统,主要包括前置望远物镜、准直镜、Wollaston棱镜组、成像镜和补偿滤光片。此类光学系统可以一次曝光获取同一目标景物在不同波长下的二维信息。光束经过Wollaston棱镜组分光,为了使不同波长对应景物不至于重叠,要求分束角度比较大,这样进入成像镜的光线入射角度相对较大,无疑增加了成像镜的设计难度。分析了基于Wollaston棱镜的图像复分快照式成像光谱仪的原理及特点,设计了一套完整的成像光谱系统。全系统结构复杂,光学系统的光阑必须匹配好。为了使得不仅单个镜头成像质量良好,而且镜组之间能够良好的衔接,将前置望远物镜设计为像方远心结构,准直镜设计为物方远心结构。全系统采用多重结构,使得16个谱段在56线对处的MTF值均接近衍射极限,点列图中RMS值基本都在艾里斑以内,系统成像质量良好。     

Pulsed digital micro-holography of femto-second order by wavelength division multiplexing

Wavelength division multiplexing (WDM) used in a pulsed digital micro-holographic system to record ultra-fast processing of the order of femto-second is reported for the first time, where a BBO crystal is used to generate harmonic wave of the incident laser wave, and both of the basic and the frequency doubled waves are time-delayed and introduced to a Michelson’s interferometer to record two sub-holograms with different spatial frequencies on a single frame of a CCD by WDM. In the experiment, an ultra-fast dynamic process of air ionization induced by a single femto-second laser pulse is holographic recorded by this system, and both of intensity and phase difference images digitally reconstructed are obtained through Fourier transformation and digital filtering, which shows clearly the dynamic process of formation and propagation of the plasma, with a time resolution of the order of femto-second.     

Achromatization of Wollaston polarizing beam splitters

We describe the achromatization of Wollaston prisms to reduce the angular dispersion in the splitting angle. Analytical theory and ray-tracing modeling is presented. In an example application, a sixfold reduction in dispersion is achieved for wavelengths in the region 400 nm to 1.7 μm. Experimental proof of concept is demonstrated, and in an example application, the spectral dispersion of extended images recorded through cascaded Wollaston prisms is shown to be reduced by an order of magnitude.     

自适应双频激光准直系统的原理及实验

介绍了一种新型自适应双频激光准直系统，该系统利用两个完全对称的渥拉斯顿棱镜，五个作为测量元件，另一个作为补偿元件。对测量信号的处理，采用比相技术，测量元件可以暂时移出光路，因而能够进行同轴度的测量。两束干涉光基符合共光路原则，适应于长距离的准直测量。     

一种多棱镜组合设计优化图像干涉性能的新方法

为了在不影响干涉条纹成像质量的基础上通过增大双折射棱镜结构角的方法获得更细致的光谱成分,优化设计了双折射棱镜的结构.通过计算分束角与结构角之间的函数关系,分析得知在传统双折射棱镜后附加一个与第二个楔形同性质的等腰三角形棱镜,使出射棱镜的o光和e光平行于光轴,即分束角α为0°,从而干涉条纹的成像不再受分束角的变化所影响....     

Wollaston棱镜阵列干涉光谱仪的研制及其光通量的分析

利用空间调制干涉光谱技术研制了基于双层Wollaston棱镜组的干涉仪原理样机,用该样机进行了干涉光谱实验,采集了He-Ne激光器干涉图像,通过对图像进行数据处理,获得了光源的归一化光谱图。并对样机系统做了光通量分析。它与传统的使用单个Wollaston棱镜测量光谱的方法相比,光谱分辨率和光通量可以提高一倍以上。     

Digital color holography applied to fluid and structural mechanics

In various domains such as acoustics, vibrations or fluid mechanics, non-invasive metrological tools, giving full-field measurements with very high spatial and temporal resolutions, need to be developed to validate the models and/or numerical simulations of the studied phenomena. In this way, ONERA and LAUM have developed optical real-time methods based on three-wavelength digital holographic interferometry to analyze either unsteady wake flows or vibrating kinematics or complex fields. These digital color holographic methods aim at avoiding panchromatic holographic plates and their related constraints. In the domain of fluids mechanics, an interferometric set-up is presented to generate micro fringes from the observed area. The data are processed thanks to direct and inverse Fourier transforms to obtain a gas density field and to study its evolution in time. In structural mechanics, a versatile interferometric imaging set-up, based on digital Fresnel holography, is presented. These new optical imaging methods result in full-field measurements with a high spatial resolution. This research ultimately aims at analyzing, in time, unsteady complex wake flows, vibrating kinematics and acoustic fields. A few examples are given and the possibilities and limits of these methods are discussed.     

应用波分复用技术实现飞秒级超快动态过程的脉冲数字显微全息记录

采用波分复用方法实现飞秒级超快动态过程的脉冲数字显微全息记录和再现。在全息记录过程中,利用BBO倍频晶体对入射激光进行倍频,将基波和谐波分光,并经时间延迟后进入迈克耳孙干涉仪,从而可应用波分复用技术,先后在CCD的一帧图像上记录两张具有不同空间频率的子全息图。采用这种方法,实现了对单脉冲飞秒激光激发空气电离的超快动态过程的全息记录,并通过数字傅里叶变换和数字滤波的方法,分别再现出每张子全息图记录的波前,从而获得了具有飞秒时间分辨的空气等离子体形成和传播过程的动态振幅和相位图像,其单次曝光时间为50 fs,曝光时间间隔为400 fs,相应的曝光频率约为2.5×1012f/s。     

基于斯托克斯矢量的偏振成像仪器及其进展

偏振成像技术将景物的偏振信息转化为二维图像信息,从而可以和灰度图像一样对目标景物进行场景特征分析。介绍了基于斯托克斯矢量的偏振成像原理。按照获取斯托克斯矢量方法的不同,分别介绍了偏振片起偏、偏振棱镜分光和可变延迟波片调制三类分时偏振成像方式,分振幅、分孔径、分焦平面三种同时偏振成像方式。结合我国的制造水平,设计了双CCD渥拉斯顿棱镜同时偏振成像实验系统,并采集了实验图像。分析了各种偏振成像方式的优缺点。指出了偏振激光照明主动偏振成像、光谱偏振成像是偏振成像仪器进一步的发展方向。     

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.     

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.     

一种有效解决离轴数字全息相图倾斜畸变的数字参考平面方法

离轴数字全息中使用倾斜的平面参考波以消除成像中的零级衍射和共轭像是一种简捷和常用的方法,然而该方法遇到的一个困扰是,由于倾斜参考波引入了附加的载波频率并很难通过实验测量准确地获得附加的载波频率值或倾角,因此会导致重建的相图出现一定的倾斜畸变而无法完全修正.本文提出了一种数字参考平面算法以解决这一问题.该算法利用重建相图的平坦区域选点构建一个能准确表征相图倾斜的数字参考平面,并建立该平面参量与参考波载波频率的数学关系和作为随后相图畸变修正迭代计算的判据.该算法简单有效,不仅能实现对倾斜相位畸变的准确修正,而且能准确地获得倾斜平面参考波的附加载波频率.由于在相位解包裹重建中结合了抑噪处理,因此该方法在环境和系统噪声的影响下仍然有效,实验结果验证了理论设计的有效性.     

数字全息技术中双CCD镜像重叠位置的调节与标定

CCD像素数和动态范围的不足极大地限制了数字全息技术的发展与应用,针对这一问题提出基于信息融合的双CCD镜像重叠布置方案。该方案首先给出了利用2个球面波干涉的方法来判断2个CCD的空间相对位置,并证明了该方法的可行性。然后在实验中利用球面波与平面波进行离轴干涉并结合数字干涉技术来实时地调节2个CCD的位置,使其相对位置初步达到镜像重叠。最后再利用计算机数字图像处理技术进行更加精确的标定,得出2个CCD之间镜像重叠位置的标定参数。实验结果表明,利用上述方法能够较精确地调节和标定2个CCD的相对镜像重叠位置,其误差仅为14 m。     

记录飞秒级超快动态过程的脉冲数字全息技术

采用脉冲数字全息技术实现对飞秒级超快动态过程的数字显微全息记录.其中，全息记录系统将单脉冲分割成具有飞秒至皮秒量级时间延迟的角度相同的物光子脉冲序列和具有同样时间延迟的角度不同的参考光子脉冲序列，并以空间角分复用的方式在普通CCD的一帧图像上记录下包含多张子全息图的复合全息图，然后通过数字傅里叶变换和数字滤波的方法，分别再现出每张子全息图所记录的图像.通过对飞秒激光激发空气电离过程的全息记录，获得了具有飞秒时间分辨的等离子体形成和传播过程的动态图像. 关键词： 超短脉冲激光 飞秒全息 脉冲数字全息 空间角分复用