基于光纤干涉仪的振动测量技术

本文介绍了基于光纤Mach-Zchndcr干涉仪的振动测量系统,用线阵CCD测量条纹动态位移,实现了光纤干涉条纹图像的连续采集.针对干涉条纹图像噪声的特点完成了低通滤波器的设计、图像的平滑处理,消除了干涉条纹中心的定位误差.文章绘制了干涉条纹位移曲线并对其进行了离散傅里叶变换,给出了所测振动的频谱特性.最后讨论了测量系统的灵敏度以及影响测量精度的因素,为了能够辨别条纹的移动方向,指出了条纹最大允许的移动速度限制,给出了实验结果.     

Measurement of air turbulence for on-machine interferometry

There is increasing demand for in situ shape measurements performed on ultraprecision processing machines. One major source of error during interferometric measurements performed on machines is fringe displacement due to external disturbances. We have developed an interferometer equipped with an electro-optic phase modulator that measures the phase of interference fringes before they are displaced by air turbulence. The frequency characteristics of air turbulence induced by a heat source are derived from successive measurements of a test surface. Experimental results show that the phase of the interference fringes can be accurately measured in the presence of air turbulence when the intensity of the fringes is sampled at a speed of several hundred hertz.     

Diffraction-induced coherence levels

We examined the influence of complex diffraction effects on low-coherence fringes created for high-aspect depth-to-width ratio structures called trenches. The coherence function was analyzed for these micrometer-wide trenches and was registered with a white-light interference microscope. For some types of surface structure we observed that additional low-coherence fringes that do not correspond directly to the surface topology are formed near the sharp edges of the structures. These additional coherence fringes were studied by rigorous numerical evaluations of vector diffractions, and these simulated interference fields were then compared with experimental results that were obtained with a white-light interference microscope.     

Signal modeling for low-coherence height-scanning interference microscopy

We propose a computationally efficient theoretical model for low-coherence interferometric profilers that measure surface heights by scanning the optical path difference of the interferometer. The model incorporates both geometric and spectral effects by means of an incoherent superposition of ray bundles through the interferometer spanning a range of wavelengths, incident angles, and pupil plane coordinates. This superposition sum is efficiently performed in the frequency domain, followed by a Fourier transform to generate the desired simulated interference signal. Example applications include white-light interferometry, high-numerical-aperture microscopy with a near-monochromatic light source, and interference microscopy for thickness and topography analysis of thin-film structures and other complex surface features.     

Analysis of low-coherence interference fringes by the Kalman filtering method

Interferometers with low-coherence illumination allow noncontact measurement of rough-surface relief with a wide range of measurement definition by locating the visibility maxima of interference fringes. The problem is light scattering by the surface to be measured, which can cause distortion of low-coherence interferometric signals. We propose to use a stochastic fringe model and a Kalman filtering method for processing noisy low-coherence fringes dynamically. Prediction of the fringe's signal value at each discretization step is based on all the information available before this step; the prediction error is used for dynamic correction of the estimates of the fringe envelope and phase. The advantages of the Kalman filtering method consist in its immunity to noise, optimal fringe evaluation, and data-processing speed.     

Spatial resolution enhancement of fiber-optic scanning white-light interferometer by use of a Vernier principle

A Vernier principle is employed to improve the spatial resolution of a fiber-optic white-light interferometer to the accuracy of 0.2 microm. The Vernier principle is implemented by combination of interference fringes itself and a virtual fringe that is generated by means of software tracing the scanning mirror. Two rulers are read with respect to each other. This design is insensitive to intensity fluctuation of the interference fringe. The applications, submicrometer estimation for the quadrature-locking selection and the tolerance of the relative measurement, demonstrate its effectiveness.     

Development of a parallel demodulation system used for extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented, which is based on a Michelson interferometer and combines the methods of low-coherence interference and a Fourier-transform spectrum. The parallel demodulation theory is modeled with Fourier-transform spectrum technology, and a signal separation method with an EFPI and FBG is proposed. The design of an optical path difference scanning and sampling method without a reference light is described. Experiments show that the parallel demodulation system has good spectrum demodulation and low-coherence interference demodulation performance. It can realize simultaneous strain and temperature measurements while keeping the whole system configuration less complex.     

角位移Fabry-Perot干涉测量

提出了一种基于Fabry-Perot板干涉的角位移测量新方法。此方法采用函数近似,只需将初始入射角确定在40°到50°之间,即可由角位移与干涉信号条纹数变化间的函数关系,高精度测量角位移。解决了采用F-P板干涉法测量角位移需精确确定入射光初始角的问题。使用计算机处理采集的干涉信号,对干涉条纹进行细分,实现干涉信号相位测量的高分辨力。理论模拟和实验结果得出本方法可以实现精度为10-5rad数量级的角位移测量。该方法的测量装置采用带尾纤的半导体激光作为光源,由自聚焦透镜准直,出射光束直径为0.5mm,使探测头为小光斑。该装置结构简单,能实现小型化。     

High-speed and high-sensitivity displacement measurement with phase-locked low-coherence interferometry

A novel high-speed and high-sensitivity displacement measurement sensing system, based on the phase-locked low-coherence interferometry, is presented. The sensing system is realized by comprising the Michelson fiber-optic interferometer. In order to obtain quadrature signals at the interferometer outputs, a 3×3 fused silica fiber-optic directional coupler is used. Therefore, the usage of the interferometer phase modulation as well as the usage of the lock-in amplification has been avoided. In this way, the speed of such a realized sensing system is significantly increased in comparison with the standard phase-locked interferometric systems that can be found elsewhere in the literature. The bandwidth of the realized sensing system is limited by the first resonance frequency of the used piezo actuator to 4.6 kHz. The estimated noise floor in the displacement measurement is approximately 180 pm/√Hz.     

Simulation method for interference fringe patterns in measuring gear tooth flanks by laser interferometry

We present a ray-tracing-based method for simulation of interference fringe patterns (IFPs) for measuring gear tooth flanks with a two-path interferometer. This simulation method involves two steps. In the first step, the profile of an IFP is achieved by means of ray tracing within the object path of the interferometer. In the second step, the profile of an IFP is filled with interference fringes, according to a set of functions from an optical path length to a fringe gray level. To examine the correctness of this simulation method, simulations are performed for two spur involute gears, and the simulated IFPs are verified by experiments using the actual two-path interferometer built on an optical platform.     

High-precision shape measurement by white-light interferometry with real-time scanner error correction

White-light interferometric techniques allow high-precision shape measurement of objects with discontinuous structures by detecting the peak of the coherence envelope. These techniques assume a specific change in the optical path difference (OPD) between the interfering beams; however, the scanning device effecting that change often introduces OPD errors that are carried over to the measurements. We present a technique for measuring OPD changes from the collected interference fringes during each measurement. Information about the scan is directly fed into the algorithm, which compensates for the errors, resulting in improved measurement accuracy. The method corrects not only the scanner errors but also slowly varying vibrations. In addition, this technique can be easily adapted to any existing low-coherence interferometer because no large data storage or postprocessing is required.     

平晶检定时干涉条纹的快速调整方法与平面度计算

在平面等厚干涉仪上检定平晶工作面平面度时,需要在被检区域调整出3到5条干涉条纹,依据测量原理和计算公式求出被检平晶工作面的平面度.文章重点分析干涉条纹的快速调整方法与标准平晶平面度的正确引用方法.     

Intermodal interferometer for strain and temperature sensing fabricated in birefringent boron doped microstructured fiber

We present a compact in-line fiber interferometric sensor fabricated in a boron doped two-mode highly birefringent microstructured fiber using a CO(2) laser. The intermodal interference arises at the fiber output due to coupling between the fundamental and the first order modes occurring at two fiber tapers distant by a few millimeters. The visibility of intermodal interference fringes is modulated by a polarimetric differential signal and varies in response to measurand changes. The proposed interferometer was tested for measurements of the strain and temperature, respectively, in the range of 20-700?°C and 0-17?mstrain. The sensitivity coefficients corresponding to fringe displacement and contrast variations are equal respectively for strain -2.51 nm/mstrain and -0.0256 1/mstrain and for temperature 16.7 pm/°C and 5.74×10(-5) 1/°C. This allows for simultaneous measurements of the two parameters by interrogation of the visibility and the displacement of interference fringes.     

一种基于Matlab的干涉条纹自动处理方法

对接触式干涉仪量块检定时产生的两种干涉条纹进行了研究,图像整体采用傅里叶级数曲线拟合法,实现干涉暗条纹大致定位.局部采用最小二乘二次曲线拟合法,确定干涉暗条纹中心的精确位置及暗条纹个数.利用Matlab和VC平台进行编程,快速有效地实现了干涉条纹中心位置的自动判读. 该方法已应用在中国计量科学研究院接触式干涉仪的量块检定中.     

Method to obtain a clear fringe pattern with a zone-plate interferometer

When a zone-plate interferometer is used, a bright spot appears at the center of the image plane. The spot makes it difficult to analyze the interference fringes. A simple technique that is based on the principle of fringe-intensity reversal is proposed to analyze the fringes efficiently. A zone plate with a phase fraction of π/2 or 3π/2 is used in this technique to diminish the bright spot. Unlike the masking technique, no part of the data on the fringes is lost. The fringes can, therefore, be analyzed completely. The technique is described in detail, and the results of an experiment in which the shape error of a concave mirror was measured with the proposed zone plate is presented. The experimental results agree well with the results obtained with the Fizeau interferometer.     

Multiple-beam wedge-plate shear interferometer for collimation testing

We describe a modification of the wedge-plate shear interferometer for collimation testing. The surface of the wedge plate is coated to increase the reflectivity such that multiple-beam interference takes place resulting in sharp fringes. In addition to sharpening the fringes also tend to split when the test beam is noncollimated. This splitting has been used as a test criterion for collimation testing. Experimental results are presented.     

Approach for the evaluation of speckle deformation measurements by application of the wavelet transformation

We introduce a new, to our knowledge, method using wavelets and probability theory for the evaluation of speckle interference patterns for quantitative out-of-plane deformation measurements of rough surfaces of nontransparent solids. The experiment uses a conventional Twyman-Green interferometer setup. The speckle interference patterns are obtained by the common method of subtraction of images taken before and after a surface deformation. The data are processed by a wavelet transformation, which analyzes the image structures on different length scales. Thus it is possible to separate the interference fringes from the noise. From the locations of the interference fringes, the deformation of the surface can be reconstructed by means of probability theory.     

Nonlinear Mach–Zehnder interferometer with ultrabroadband squeezed light

We study both theoretically and experimentally the interference pattern in a nonlinear Mach–Zehnder interferometer formed by two aperiodically-poled crystals, where broadband squeezed light is generated by both crystals via parametric down-conversion with a common quasi-monochromatic pump. This configuration is important for measuring the squeezing produced by the first crystal and also for measuring a small phase shift introduced by a sample between the crystals. On the basis of the approximate quantum Rosenbluth formula for each crystal we develop an analytic model for the field evolution in the interferometer. We report an experimental observation of the interference fringes, caused by the dispersion of the generated PDC waves in both crystals forming the interferometer. We observe a displacement of the interference pattern caused by a sample between the crystals and infer the phase shift within a band of 20?nm. The experimental data are in a good agreement with the predictions of the developed model, up to imperfections of the samples.     

Probing of a random phase object by a focused spatially modulated laser beam. Integral scanning method

A telescope system incorporating an illuminating spatially modulated laser beam, a diffuser in the entry plane, and a random phase screen in the spatial frequency plane was used to analyze the formation of average-intensity interference fringes in the image plane of the diffuser. It is shown that the system can operate as a shift interferometer where the contrast of the fringes is independent of the diffuser characteristics. Analytic expressions are obtained for the contrast of the fringes as a function of the parameters of the screen and the illuminating beam and it is established that the statistical anisotropy of the screen influences the contrast of the fringes. Pis’ma Zh. Tekh. Fiz. 25, 5–10 (December 26, 1999)     

Compact interferometric optical tweezer for patterned trapping and manipulation of polystyrene spheres and SWCNTs

We demonstrate a simple and compact optical interferometric unit combined with a conventional optical tweezer system for simultaneous multiple trapping and micromanipulation of mono-dispersed polystyrene spheres and aggregation of small-floating clusters of single-walled carbon nanotubes (SWCNTs). The interferometric unit was made compact by means of coating a thin layer of aluminum oxide on one side of the cubic beam splitter (CBS) which works as a static reference mirror and an adjustable mirror was mounted on a XYZ translational stage facing the other adjacent side of the CBS. Thus, the developed interferometric unit is quite analogous to a Michelson interferometer but is compact. Sinusoidal interference fringes with variable carrier frequency and orientations were generated. The interference fringes were then used for multiple trapping of polystyrene spheres along bright fringes resulting in pattern formation and also the aggregation of tiny floating clusters of SWCNTs. The proposed system is compact and easy to align because of its common-path geometry.