Fabry-Perot sensors for dynamic studies using spectrally based passive quadrature signal processing

This paper describes a simple, optically passive detection scheme for in-line Fabry-Perot etalon (ILFE) sensors that is also useful for intrinsic Fabry-Perot (IFP) and extrinsic Fabry-Perot interferometer (EFPI) sensors. This detection scheme is based on two spectrally shiffed Bragg wavelengths from in-line Bragg gratings to produce two quadrature phase shifted signals from the spectral transfer function of the Fabry-Perot sensor. Using the amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) as a low-coherence, high-power broad-band source, the passive detection technique is used to demonstrate the use of ILFE sensors in high strain rate dynamics studies. This study is designed to characterize the high strain rate response of optical fiber sensors and test their ability to monitor dynamic strain changes resulting from one-dimensional stress wave propagation.     

High-magnification moiré interferometer for crack tip analysis of steels

A high-magnification moiré interferometer, particularly suitable for near-tip field analysis in cracked materials, is described. It has a submillimeter field of view, a high-resolution image sensor (1.4 million pixels), X-Y-Z translation stage and an optical fiber light delivery system. These features enable the microscope head to observe the crack tip while the specimen is loaded in a standard tensile test machine. Automated fringe pattern analysis, using temporal phase shifting and spatial phase unwrapping, enables thex ory displacement component to be measured and the corresponding in-plane strain component computed. The displacement placement accuracy is better than 40 nm, and the effective strain gage dimension is ∼ 25 μm. Furthermore, the interferometer has a built-in white light microscope that allows the observation of the specimen granular microstructure in exact registration with the displacement field. The interferometer has hence been employed to investigate the near-tip fields of a precracked stainless steel specimen under load. The influence of the grain boundaries on the measured displacement fields was relatively minor. The near-tip strain field shows a significant asymmetrical behavior despite pure mode lloading conditions.     

Small attachable interferometric strain gages

Laser-based interferometry from two tiny reflective indentations can be used to measure in-plane strain/displacement over a very short gage length (on the order of 100 μm). If the specimen material is not reflective, then some other means of generating the interference patterns must be found. This paper describes two kinds of attachable gages: plated acetate replicas of indentations and reflective foils that are indented after application. In either case, the gage is applied with the techniques used for foil-resistance gages and the gage itself is very small. The manufacturing procedures are described. The results of experiments comparing the strain to that measured with foil-resistance gages are presented. Finally, the small interferometric gage is used to measure strain on one of the metal strips in a foil-resistance gage.     

Extended range pseudo-heterodyne demodulation for fiber optic sensors

This article describes a pseudo-heterodyne demodulation technique for interferometric fiber optic sensors that has a larger measurement range than is currently possible with pseudo-heterodyne demodulation. This sensor demodulation technique has a bandwidth of 30 Hz to 2.5 kHz, is capable of resolving optical phase angles as small as 5×10⁻⁴ rad, and has a maximum measurement range of tens of radians in a bandwidth of 30–500 Hz. A comparison between the response obtained from a resistance strain gage and a fiber optic sensor using this demodulation is favorable.     

Dynamic strain measurement with the interferometric strain gage

The interferometric strain gage has a short gage length, high-frequency response and the capability of measuring large plastic strains. Furthermore, the gage is easily ruled directly onto the specimen, and no mechanical or electrical contact needs to be made during the measurement. These features make the interferometric strain gage particularly suitable for dynamic plastic-strain measurement. In this paper, the details of an experimental setup for generating and measuring dynamic plastic strain are given. The photometric techniques of measuring the fringe motion of the interference patterns are describle as well as the data-reduction procedure. A typical result is presented, and the validity of the method is established.     

角色散FP干涉测速技术应用与分析

发展了固定腔结构的角色散FP干涉测速系统,干涉仪结构紧凑,采用固定腔标准具,实现了干涉 条纹永久免调,并且条纹常数的标定非常简单。该系统可用于靶面反射光强动态变化很大的场合,在电炮驱 动Mylar膜飞片实验中,光强变化达100倍时仍然获得了很好的结果。分析了标准具厚度误差、标准具端面 不平行、干涉条纹动态展宽和扫描图像畸变等因素对系统测量精度的影响以及系统的速度和时间分辨能力。     

Whole-field strain measurement in a pin-loaded plate by electronic speckle pattern interferometry and the finite element method

The strain field in an epoxy plate loaded in tension through a steel pin is determined using electronic speckle pattern interferometry (ESPI) and the finite element method (FEM). In a dual-beam illumination speckle interferometer, the in-plane component of the displacement at the plate's surface is accurately measured using a four-step phase-shifting algorithm. Digital image processing algorithms have been developed for noise reduction and strain calculation directly in the computer from the phase map with a strain gage length of about 0.4 mm. A whole-field strain map is obtained, as well as distributions of strain concentration factor, in critical regions near the hole of the plate. FEM is used to perform a nonlinear contact analysis accounting for friction effects at the pin/hole interface. The agreement between experimental results and numerical predictions is good. In terms of speed, accuracy and ease of use, dual-beam ESPI appears to be a superior method of whole-field strain analysis.     

The interferometric strain gage

The interferometric strain gage consists of two very shallow grooves ruled on a highly polished surface. The grooves are cut with a diamond and are 4×10^?5 in. deep and 5×10^?3 in. apart. Coherent, monochromatic light from a He?Ne gas laser incident upon these grooves will produce fringe patterns. A fringe pattern with the fringes parallel to the grooves is formed on each side of the impinging beam. The position of these patterns in space is related to the distance between the two grooves. As this distance changes, the fringes shift. Measurement of these fringe shifts enables one to determine the local strain of the specimen. In this paper, the theory of the measurement is developed first. The strain, ∈, is given by ∈=ΔFλ/d _o sin α _o where ΔF is the average fringe shift of the two patterns, λ is the wavelength of light,d _o is the initial distance between grooves, and α _o is the angle between the incident light beam and the fringe patterns. A procedure for making static measurements with the interferometric strain gage is presented. The sensitivity for these measurements is 0.5 percent strain per fringe shift, and the maximum strain is 4 percent. The method is evaluated by comparing its results with other accepted means of measuring large plastic strain. These other techniques are: post-yield foil gages, a 2-in. clip gage, and an Instron testing machine. The average percent difference among these techniques is less than 0.4 percent based on a full-scale measurement of 4-percent strain. The interferometric strain gage has the following features: a gage integral with the specimen surface, a very short gage length, relatively easy application, and the ability to measure large strains.     

Differential interferometry with adjustable spatial carrier fringes for turbine blade cascade flow investigations

 Digital evaluated differential interferometry using adjustable spatial carrier fringes was applied to flow measurements in a transonic turbine blade cascade. The interferograms were evaluated using a two-dimensional Fourier Analysis. This evaluation provided density gradient maps of the flow field in a digital form. A specially designed interferometer was used allowing adjustment of sensitivity and superposition of carrier-fringe system separately. This type of interferometer is also highly insensitive to vibration noise. For a turbine blade cooling film a comparison of this type of differential interferometry with holographic interferometry and a Schlieren visualisation is also given. As a result differential interferometry using adjustable spatial carrier fringes and digital fringe analysis gave density-gradient maps similar to Schlieren recordings but of quantitative nature, thus enabling compensation of wave front distortions. Integration of these density gradient maps resulted in density maps which were then compared to recordings done with pulsed holographic interferometry. Received: 27 December 1996/Accepted: 30 April 1997     

The application of Moire interferometry in the measurement of displacement field and strain field at notch-tip and crack-tip

This paper presents the application of Moire interferometry in measuring the displacement and strain field at notch-tip and crack-tip before and after crack propagation. The experiment is carried out using a three point bending beam with a notch. TheN _x andN _y fringe patterns representing displacement field, and the ΔN _x/Δx and ΔN _γ/Δγ fringe patterns representing the strain field are obtained. The sensitivity of the measured displacement is 0.417 μm per fringe order. The displacement and strain distribution along the sectionx=0 have been worked out according toN _x andN _γ fringe patterns. The project supported by Chinese Academy of Sciences and National Natural Science Foundation of China     

Carrier fringe method of moiré interferometry for tiny strain measurements in micro-field

In this paper, we demonstrate a new optical method for tiny strain measurements based on the principle of carrier fringes of moire interferometry. A cross-line grating with frequency of 1200 lp/mm is replicated on the specimen surface, and the strain can be deduced from the changes in carrier fringes before and after the deformation of an object. Four coherent laser beams are used to obtain the carrier fringe patterns of field U and V. Both theoretical analysis and numerical simulation indicate that the ideal accuracy of strain can be controlled within a range of ±1με. Case study of a plane extension experiment shows that the measurement accuracy of strain can be controlled within the range of ±10με. The average strain values of every row of field U and every column of field V can be obtained by using this method, and approximated strain of every pixel in the whole-field can be further acquired, and thus it is possible to measure tiny strains occurred in a micro-field. The technology in this paper can provide comprehensive information for analyzing related mechanical content in the field of MEMS.     

Dynamic response of strain gages up to 300 kHz

Dynamic response of foil strain gages is investigated in the ranges of up to approximately 300 kHz in frequency, 2000 in strain and 750 s^–1 in strain rate. Impulsive input is applied to the gages using elastic wave pulses in a circular bar. The input is determined by measuring the velocity of the bar end with a laser interferometer. It was confirmed that deviations from the static gage factors do not exceed–5 percent within the frequency range up to 45 kHz, 110 kHz and more than 290 kHz for 20-mm, 10-mm and 3-mm gages, respectively, and that the gage length is the dominant factor in the gages' frequency characteristics. This paper also mentions self-generated voltages.     

基于激光干涉测量的液体高脉冲压力校准

针对压电式高压传感器等的幅值灵敏度脉冲压力校准中的量值溯源问题,将一种基于牛顿第二定律的液体脉冲压力激光干涉测量方法用于落锤式液体高脉冲压力校准装置。通过质量块的动力学建模以及激光干涉测量质量块的加速度,得到脉冲压力幅值大小,使脉冲压力幅值能溯源到时间、长度与质量等基本量。通过对压力和加速度分布不均、摩擦力等影响进行理论与实验分析,压电式压力传感器幅值灵敏度校准不确定度得到了完整评估。激光干涉法液体脉冲压力校准装置压力幅值覆盖（10~500）MPa,扩展不确定度在1.8%以内。     

Experimental investigation of phase-strain-temperature models for structurally embedded interferometric fiber-optic sensors

Analytical models relating the optical phase change in a structurally embedded optical fiber sensor to the strain and temperature state integrated along that sensor path is investigated in this paper. Generalized plane strain elasticity solutions are combined with two phase-strain-temperature models to predict the thermomechanically induced optical retardation of the light propagating in the fiber sensor. These predictions are compared with experimental data obtained from Mach-Zehnder and Fabry-Perot sensors embedded in transverse compression and uniformly heated specimens respectively. Paper was presented at the 1992 SEM Spring Conference held in Las Vegas, NV on June 8–11.     

Interferometric-fiber-optic strain sensor

An interferometric-fiber-optic sensor and an efficient fringe-detection scheme are described. The fiberoptic interferometer consists of two fibers; they are labeled the reference fiber and the sensing fiber. The reference fiber is arranged in a circular pattern, whereas the sensing fiber is arranged in an ‘S’ pattern. These fibers are exposed to the same strain field and each experiences a strain-induced phase shift. A difference in the phase shift between the two fibers indicates a change in strain. The strain-induced phase difference causes the interferometrically produced fringes to shift spatially. Analysis shows that the number of fringes passing an arbitrary point on a screen (the detection point) is linearly related to the strain in the fiber. In this analysis, the strain sensor is assumed to be perfectly bonded so that the fibers experience the same strain field as the specimen. It is further assumed that the sensor covers a sufficiently small area so that the strain can be considered constant over the entire strain sensor. Also, the phase change produced by transverse strain components (with respect to the fiber) induced by the specimen is assumed negligible compared to the phase changes attributable to the axial strain components. A cantilever beam was used as a specimen. Experimentally determined strains correlated well with the strains predicted by beam theory. The fringe-detection scheme described is a high-speed fringe counter. The speed of this counter is necessary to detect vibrational phase noise which is invisible to the human eye. Two photodiodes detect the fringes, and a logic circuit counts the fringe shifts, both strain and noise induced. Since noise is random in nature, it can be averaged out. This fringe detector exhibits good sensitivity and is the key to moving the sensor from the laboratory to the field.     

Grating holographic interferometry

A holographic interferometric technique, combining an image hologram with a grating approach, is proposed for three-dimensional deformation measurements on opaque planar object surfaces. In this technique, the holographic plate is brought close to the object surface, onto which a high-frequency crossed-line diffraction grating has been replicated. The grating surface produces multiple object waves rather than the usual diffusely reflected object waves. The double-exposed single holograms can be reconstructed at multiple off-axis angles. Four independent high-contrast fringe patterns are extracted simultaneously. Displacement vectors over the entire measurement area are separated in three orthogonal directions. The resultant displacements are presented as three-dimensional meshed plots and topographic contour maps. The optical system for both recording and reconstruction of the holograms has been simplified compared to conventional holographic interferometry. Experimental errors associated with fringe readout and system geometry are reduced because of the sharp images and the well-defined spatial orientation in the reconstruction system.     

Nano-meter Displacement Measurement by Phase Analysis of Fringe Patterns Obtained by Optical Methods

0Introduction Inordertomonitorthehealthofstructures,straingagesanddisplacementtransducersare usuallyused.Thereasonisthatmanyrulesorcodesforinspectionofstructuresrequiretousestrain gagesanddisplacementtransducers,anditiseasytousethem.However,thesemethodsarebasically one pointmeasurementmethods.Theyareexpensiveandtime consumingfordistributionanalysis.Opticalmethodssuchasgrating projection,geometricmoir啨,moir啨interferometry,holographic interferometryandspeckleinterferometryareusefultomeasuredi…     

Recent high-resolution resonant refractivity studies of a sodium-seeded flame

A new experimental technique, namely, the combination of a Fabry-Perot etalon with a Mach Zehnder interferometer (MZI) makes it possible to deduce the index of refraction resonant dispersion curve near the center of an atomic line from the superimposed fringe system. A tunable pulsed dye laser as light source and a sodium seeded flame as test flow enabled us to verify the very large enhancement of sensitivity for refractive flow-visualization methods such as shadowgraph, schlieren and interferometry. The specific refractivity is a direct measure of the sensitivity, and that quantity increases from a non-resonant value of 4 × 10^?3 m'/kg to a peak value of 189 m3/kg at the half-power points of the sodium D₂-line.     

Instantaneous phase-stepping interferometry using polarization imaging with a micro-retarder array

An instantaneous phase-stepping and subsequent phase analysis method, using a CCD camera with a form-birefringent micro-retarder array, is proposed for interferometry. An optical setup of a polarization interferometry using a Twyman-Green interferometer with two polarizers is constructed to analyze the distribution of out-of-plane displacement. Light emerging from the interferometer is recorded using a CCD camera that has micro-retarder array on the CCD plane. This micro-retarder array has four different principal directions. That is, an image obtained by the CCD camera contains four types of data corresponding to four different optical axes of the retarder. The four images separated from the image recorded by the CCD camera are reconstructed using gray-level interpolation. Then, the distributions of the Stokes parameters that represent the state of polarization are calculated from the four images. The phase distribution of the interference fringe pattern produced by the Twyman-Green interferometer is then obtained from these Stokes parameters. This method is applicable to time-dependent phenomena because multiple exposures are unnecessary for sufficient data acquisition in the completion of phase analysis.