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.     

Fringe interpolation by two-reference-beam holographic interferometry: Reducing sensitivity to hologram misalignment

Accurate fringe interpolation in holographic interferometry requires two-reference-beam holographic recording. Using two reference sources close together, the sensitivity to (angular) misalignement of the hologram is drastically reduced (10° instead of 0.01°). Overlapping of the cross-reconstructions can be tolerated, unless extreme precision and spatial resolution are demanded. Experimental results obtained by video processing of such holograms show an accuracy of better than $\text{1}\text{100}$ of a fringe. The optical setup becomes nearly as compact and simple as in classical holographic interferometry.     

Aberration-free lateral shear holographic interferometry

Lateral shear interferometry is considered as a possible means for optical correction of wave fronts reconstructed from singly and doubly exposed holograms. Optical aberrations are eliminated in a moiré pattern by superposing two lateral shear interferograms using waves diffracted by original holograms. It is shown that the singly exposed hologram allows compensation for only optical inhomogeneities due to the substrate, while the doubly exposed hologram also excludes aberrations due to the recording system. The experimental results supporting the efficiency of the method as applied to imperfect substrates of holograms are presented.     

电子学全息干涉术用于温度场测量

探讨了用电子学全息干涉术（双称数字全息干涉术）测量温度场分布及其变化的可行性，利用所设计的全息干涉实验光路，对一电烙铁头部周围温度场分布进行了实时全息记录，进而利用一维快速傅里叶变换及数字滤波处理再现出了反映温度场分布的全息干涉条纹图样，实验结果表明，与传统的光学全息干涉术相比，电子学全息干涉术借助于高分辨率CCD记录及高速计算机数字处理技术，从而可实现光学全息图的数字化记录、存储和重现。同时，利用再现物场相位倍增原理还可实现对干涉条纹数目的倍增，或利用物场相位分布的直接计算精确获取任意两点间的相位差，从而提高测量精度。此外，由于能够在不改变光路的前提下以较高的重复频率完成光学全息图的记录，电子学全息干涉术可以用于记录三维物场的变化并接近实时地再现和测量三维物场的变化规律，因此是一种极有发展前途的新型实时全息干涉计量技术。     

Real-time color holographic interferometry devoted to 2D unsteady wake flows

A new optical technique based on real time holographic interferometry in true colors has been implemented around the transonic wind tunnel of the ONERA-Lille center to analyze 2D unsteady wake flows. Tests realized in color interferometry, real time and double exposure, use simultaneously three wavelengths of a continuous waves laser (argon and krypton mixed) and holograms are recorded on silver-halide single-layer panchromatic Slavich PFG03c plates. The very principle of real-time true color holographic interferometry uses three primary wavelengths (red, green and blue) to record, under no-flow conditions, the interference among the three measurement beams and the three reference beams simultaneously on a single reference hologram. After the holographic plate is developed, it is placed on the test setup again in the position it occupied during exposure and the hologram is illuminated again by the three reference beams and three measurement beams. A flat, uniform color can then be observed behind the hologram. So a horizontal, vertical, or even circular fringe pattern can be formed and the achromatic central fringe can be made out very clearly. The single color is used to determine the path difference zero on the interferograms. The flow studied was the unsteady flow downstream of a cylinder placed crosswise in the test section. A sequence of hundred interferograms was recorded on the flow around the cylinder at Mach 0.37. The vortex formation and dissipation phases can be seen very clearly, along with the fringe beat to either side of the cylinder.     

Resolution improvement for enhancing interference fringes of digital holograms with near infrared illuminance

A new resolution improving method of enhancing the interference stripes, which can achieve better reconstruction of the recorded object of infrared digital holography has been proposed in this paper. The experiment is conducted under near infrared illuminance. We create poor quality holograms with very low-resolution interference stripes and enhance it with our new method. The processed holograms have much higher-resolution interference stripes. This method is based on interpolation theory, and we make it works fast, stable and easy to apply. Both the forward and backward interpolation of the oriental and portrait direction of the source pixels have been calculated, and have been used to realize the final aim of resolution improvement. During the experiment, since the responsivity of the CCD is not satisfied with the illuminance laser, which is by design, we also give specific analysis on the experimental setup of our work to make sure the object information can be fully recorded by the optical setup but cannot be reconstructed because of the low-resolution and unobvious of the interference stripes. After the resolution improvement process of the original hologram, the object information can then be fully reconstructed, which is very clear in the paper. Figures and dataflow give evidence and demonstrate the good performance of our method.     

Optical encryption using phase-shifting interferometry in a joint transform correlator

We propose an optical encryption technique where the encrypted field and the decrypting key are obtained by three-step phase-shifting interferometry and registered as digital Fresnel holograms in a joint transform correlator architecture. Decryption can be achieved by digital or optical means. The technique allows the complete process to be achieved at high speed and data to be transfered via digital communication channels. Experimental implementation is performed in a system based on a programmable liquid-crystal TV display working in pure phase mode to represent the input data and to introduce the required phase shifts. A CCD is used to register the output data.     

Dual hologram interferometry

This report describes dual hologram interferometry, a new electro-optic technique used to calculate two-dimensional density distributions from measurable changes in index of refraction. Using this technique, two scenes recorded at different times are stored in separate holograms. One hologram contains reference data depicting the subject in an undisturbed state and the other hologram describes the same subject in a test or disturbed condition. Desired interference patterns are obtained by super-imposing the two separate scenes which are simultaneously reconstructed from each hologram. Specific interferograms, infinite fringe and various finite fringe patterns, are obtained by carefully regulating the optical attitude between the holograms and the reference beam during reconstruction of the scenes. In this project, dual hologram interferometry is used to describe quantitatively free convection from a uniformly heated right circular cylinder. Temperature distributions through the boundary layer and local heat transfer coefficients are determined at different azimuth positions. The results are compared both to experimental data obtained from another holographic technique and to a theoretical prediction of this specific test. This investigation shows that dual hologram interferometry accurately records changes in index of refraction which means the technique can be used to accurately measure experimental fluid flow data.     

Image encryption based on gyrator transform and two-step phase-shifting interferometry

A novel optical image encryption method is proposed, based on gyrator transform and phase-shifting interferometry. The input two-dimensional image to be encrypted is gyrator transformed two times, and two random phase masks are placed at the input plane and the output plane of the first gyrator transform. Two-step phase-shifting interferometry is used to record the digital holograms of the input image encrypted by use of double-random phase encoding technique in gyrator transform domain. The rotation angles of gyrator transform, the random phase mask in the gyrator plane and the arbitrary phase shift used for recording form the keys for decryption of the input image. Numerical simulations are presented to verify its validity and efficiency.     

Cutoff Wavelength Measurement for Single Mode Optical Fibers with Computer-generated Matched Filter

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One-step two-dimensional rainbow-holographic technique with a nonlaser light source

A technique for recording two-dimensional rainbow holograms by use of a nonlaser light source is described. Multicolor holograms are recorded in one step. The optical setup is lensless. The recording light source is an InGa lamp that provides a spectrum from UV to blue. The slit effect is generated by replication from the slit master plates.     

重铬酸盐明胶全息图的吸水退化模型

在有前明胶材料吸水机理的基础上，结合明胶全息图退化的有关实验结果，提出重铬酸盐明胶全息图的吸水退化模型，并对明胶全息图稳定性的实验结果从机理上进行解释。结果表明，这一模型对现有析实验有给出满足的解释，因此是进一步提高明胶全息图稳定性和具有一定的理论指导意义。     

Increase in the sensitivity of holographic reverse shearing interferometry due to the combination of object rotation with optical processing of holograms

It is shown for the holographic reverse shearing interferometry, which is used in investigations of phase objects with limited sizes, that combination of rotation of the object under study during hologram recording with subsequent optical processing of a pair of matched holograms makes it possible to obtain, with a much higher sensitivity, aberration-free interference patterns reflecting the changes in the wavefront induced by the object studied. The results of the experimental testing of the proposed method for the cases of optical processing of matched and optically conjugate reverse shearing holograms are reported.     

云纹干涉载频调制技术及分析

在物体变形场的测量中,云纹干涉的载频调制技术有重要作用.在云纹干涉中,对称光照明产生的衍射光沿试件栅表面的法线传播并产生干涉.通过改变照明光的入射角度,可实现变形条纹场的空间调制.通过光程差的分析说明了载波条纹产生的机理,得出了载波频率与照明光入射角度变化之间的关系式.利用三点弯曲加载结合二维载频调制实验,给出了实验结果.证明了在初始条纹较少的情况下,载频调制技术能有效地测量物体的位移场.     

Quantitative three-dimensional holographic interferometry for flow field analysis

Holographic interferometry offers the potential for quantitative, wholefield analysis of three-dimensional compressible flows. The technique is non-intrusive, does not require the introduction of seeding particles, and records the entire flow information within the pulse duration of a Q-switched ruby laser (˜30ns). At present, however, holographic interferometry is mainly used qualitatively due to the practical restrictions of data recording, acquisition and processing.To address the potential of holographic flow analysis a prototype multi-channel interferometer has been designed and preliminary wind tunnel results have been obtained. The proposed configuration uses specular illumination which, unlike comparable diffuse systems, does not suffer from fringe localisation and speckle noise. Beam collimation and steering through the flow field is achieved in a single operation by the use of holographic optical elements (HOEs). The resulting design is compact, light efficient, has aberration compensation, and the recorded data are conducive to both tomographic analysis and direct comparison to computational fluid dynamics (CFD) predictions.Holograms have been recorded of simple two-dimensional and axisymmetric compressible flows, to compare the accuracy of holographic density measurements with data from conventional pressure sensors and CFD codes. Data extraction from the holograms, and the elimination of rigid body motion, was achieved using digital Fourier transform fringe analysis. The introduction of phase errors by image processing has been investigated by analysing simulated fringe patterns generated from a combination of experimental amplitude information and computer generated phase data.     

Low-complexity compression of holograms based on delta modulation

Past research has demonstrated that hologram of a real world object can be acquired by scanning it with a single two-dimensional optical scan. The technology commonly referred to as optical scanning holography (OSH), have found important applications such as in microscopy, pattern recognition, 3D holographic display, and optical remote sensing. As holograms are often employed to capture three dimensional objects with high resolution, large amount of data is required to represent them. In this paper we propose a method for compressing holograms based on Delta modulation. Specifically, we apply our proposed method for holograms captured by OSH. Experimental evaluation reveals that our proposed approach is capable of attaining a compression ratio of 64 times, and still preserves favorable fidelity on the reconstructed images. In addition, both the encoding and decoding processes of the proposed method are very low in complexity, hence enabling real-time operation.     

Moire processing of acoustic field holograms

A moiré method for processing optical image-plane holograms of acoustic fields is introduced. Derivatives of acoustic pressure with respect to space, time and other system variables can be obtained by this extension of holographic interferometry methods. The new method is employed to obtain simultaneous, independent measurements of acoustic pressure and particle acceleration. It is also demonstrated that moiré processes can improve the sensitivity of optical, whole field measurements at least 30 times.     

Estimating ellipticity of a fiber core using two types of interferometry

In the present work two types of low-coherence interferometry are used to estimate the ellipticity of the core of an optical fiber in the cross section. Using the time-domain low-coherence interferometry the group-index difference between the degenerate high-order modes is measured. Because of the usage of a short fiber sample the group-index difference between the degenerate fundamental modes is too small to measure. Secondly, a frequency-domain low-coherence interferometry is presented in details. Since it is an all-fiber setup, longer fiber is allowed to be used. The high-order modes are filtered. The group-index difference between the degenerate fundamental modes is obtained. For each of the methods the experimental results are used to estimate the ellipticity of the fiber core in numerical simulations.     

Alternative method of wavelength drift free dual-wavelength heterodyne interferometry for the absolute distance measurement

Based on the technique of dual-wavelength and principle of heterodyne interferometry, a modified method for measuring the absolute distance is proposed. Because two test lights suffer from the same influence of wavelength drift in the measurement setup, the minus effect coming from the wavelength drift can be offset. Therefore, the measurement accuracy can be significantly increased. The feasibility of this method was demonstrated with a measurement resolution of about 1.50 μm. This method provides the advantages of a simple optical setup, easy operation and rapid measurement.