A simple electronic speckle shearing interferometer

A new speckle shearing interferometer with simple setup is proposed. The interferometer can be converted easily from a speckle referenced speckle pattern interferometer into a speckle shearing interferometer and vice versa. A beamsplitter and a mirror are used to generate the shear. The amount of shear can be adjusted by simply rotating the mirror. The costs involved are low and it is particularly useful for fast inspection in non-destructive testing of structural integrity. Both the theoretical studies of the method of measurement and the experimental results are presented.     

用于多参数同时测量的散斑干涉仪

本文描述了一种新的散斑干涉仪,它可以同时对表面应变物体的面内位移、离面位移、位移梯度和莫尔曲率进行测量;由于利用了取向滤波方法,在滤波系统输出平面同时得到了与这些参数对应的散斑条纹.还利用散斑空间运动规律较好地解释了散斑干涉与散斑剪切干涉之间的关系.     

An in-plane insensitive multiaperture speckle shear interferometer for slope measurement

A modified two-aperture speckle shear interferometer that eliminates the contribution of the in-plane component and its derivative to the phase change, and yields a fringe pattern corresponding to the first-order partial derivatives of the out-of-plane displacement component, is reported in this note. In this method, two laterally sheared object points are viewed axially. The wave fields from these points are independently combined at the image plane of the imaging system.     

激光散斑验光仪新进展

本文介绍了散斑验光的原理及激光散斑验光仪的新进展,较好地解决了散斑验光的技术问题。     

A new laser displacement sensor based on triangulation for gauge real-time measurement

A new laser displacement sensor has been designed, constructed, and used in a train running at a speed of 64 km/h. The laser displacement sensor is insensitive to ambient light and to temperature. The parameters of the laser displacement sensor are as follows: the measurement range is 20 mm, and the laser displacement sensor resolution is 0.3 mm. All the results show that this new displacement sensor meets the requirement for real-time gauge measurement.     

Application of iteration and finite element smoothing technique for displacement and strain measurement of digital speckle correlation

In this paper, an improved method for measuring displacement in digital speckle correlation technology, which is based on an iterative and spatial-gradient algorithm, is developed. After obtaining full-field displacement, both finite element method and 2D generalized cross-validation (GCV) algorithm are adopted for smoothing the displacement field, and then the strain field can be obtained from the smoothed displacement field. The method is estimated by simulated speckle patterns and three-point bending experiment. All the results show the improved method can obtain a reasonable estimation of displacement and strain fields in digital speckle correlation method.     

电子散斑归一化互相关法测位移

把粗糙物体表面受到相干光照明时产生的散斑场看作是一个稳态随机信号,在此基础上,本文提出了一种能做到大量程,高精度位移测量的新方法——电子散斑归一化互相关法测位移或称电子散斑照相法测位移.该法实质上是借助数字图像处理技术,通过物体位移前后二个散斑场之间的相关性来进行位移测量方法的发展.文中给出了理论分析和实验系统.实验结果表明,该法有诸如准实时,大量程(毫米级),高精度和线性可靠等优点.     

Effect of primary aberrations on transverse displacement of laser speckle patterns

The effect of primary aberrations on the transverse displacement of laser speckle patterns in the real image plane due to transverse displacement of an object is studied in the approximation of Fourier optics. Primary aberrations cause: (i) a complementary displacement of the speckle pattern; and (ii) its decorrelation. The main features of complementary displacements are as follows: (a) the overall complementary displacement is the sum of complementary displacements caused by each one of the primary aberrations; (b) the complementary displacement caused by the spherical aberration is uniform in the image plane; (c) spherical aberration and curvature of field cause complementary displacements that are parallel to the object's displacement; (d) the complementary displacement caused by distortion is always zero on the optical axis; (e) the complementary displacement caused by distortion is free of decorrelation; (f) complementary displacements are non-proportional to the object's displacement; and (g) complementary displacements depend on the position of the illuminating source with the exception of the complementary displacement caused by distortion.     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

Fibre position effects on the operation of opto-pair fibre displacement sensors

In this article, two optical-fibre probes used in displacement measurements are reported. In one arrangement, two optical fibres with protective buffer jackets are placed together, while in the second arrangement, the protective buffer jackets are removed and the two fibres are placed with the claddings in contact, with a concomitant smaller centre-centre distance. As the core and cladding diameters are the same in both probes, the effect of the centre-centre distance on double-fibre designs can be investigated. The reflected output powers are recorded for the probes as a function of the axial distance from a reflective surface. A theoretical model is also implemented, which considers the effect of centre-to-centre separation of the optical fibres on the sensor performance, and the experimental results are compared with the theoretical predictions. The computed response curve includes a dead region where there is no response to displacement, an active region with an increasing reflection power with increasing displacement and a decaying region in which the reflected power decreases with increase in displacement. The operational performance criteria of the two probes are also investigated. The hysteresis, reproducibility and time stability of the probes are investigated. The hysteresis is approximately 1%, reproducibility approximately 1.1% and time stability is better than 1.5% (600 s time scale) in both probes.     

A displacement measurement system, utilizing a Wollaston interferometer

A dual interferometric displacement measurement system is presented where a Wollaston prism interferometer is employed in conjunction with a normal Michelson interferometer. The system operates without the use of external polarizers, apart from those associated with the Wollaston prism interferometer itself. It is shown that an optical path difference induced in the Michelson interferometer can be detected using the Wollaston prism in a normal interferometer arrangement. Further, the interference pattern produced by the Wollaston prism interferometer changes in a measurable, linear fashion as the optical path difference from the Michelson interferometer alters. A simple theoretical analysis of the system is presented and used to derive a computer model of the optical arrangement. Results from an experimental implementation of the system, using a Wollaston prism with a beam separation of 0.5 degrees and a superluminescent diode, of wavelength 825 nm, as a light source, are included and compared to the results from the computer model.     

激光散斑与随机激光器件制作

提出了一种基于大学物理实验教学中激光散斑的随机激光器件的制作,分析了随机激光的分类标准和产生机理,系统考察了不同泵浦条件下该激光器件的输出特性.该实验方案为大学物理实验提供了与科技前沿接轨的素材.     

Whole field surface roughness measurement by laser speckle correlation technique

This paper describes a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm. Instead of moving the laser beam, the specimen is rotated to achieve angular speckle correlation (ASC) in the far-field plane. The technique is simple and requires minimum optical alignment. The experimental results show a good agreement with the standard specimen of known roughness. An error analysis on the experiment has been carried out. Together with the theoretical curves, the roughness values can be easily related to the change of incidence angle at a particular visibility of the correlation fringes between two speckle patterns.     

A stereoscopic digital speckle photography system for 3-D displacement field measurements

Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.     

A sinusoidal phase-modulating fiber-optic interferometer insensitive to the intensity change of the light source

In a conventional laser-diode sinusoidal phase-modulating (LD-SPM) interferometer, the wavelength of the LD is modulated by varying its injection current. The intensity modulation concurrent with the wavelength modulation leads to measurement errors. A photothermal-modulation method has been proposed to decrease the intensity change of the LD; however, this method cannot be used to measure vibration with a high frequency, and the beam diameter is too large to be used to measure minute objects. In this paper, we propose LD-SPM fiber-optic interferometer, in which the effect of the intensity change of the light source on measurement is eliminated. The diameter of the light beam is less than 0.5 mm. Using this interferometer, we measured displacements of a mirror driven by a piezoelectric transducer. The measurement repeatability is less than 1 nm.     

Separation of 3-D displacement components in the white light speckle method

Measurement of in-plane and out-of-plane displacement components using the white light speckle method is considered for three different cases. The first is applicable when in-plane components are small and stresses are directly related to the second differential of the out-of-plane component. The other cases involve components of the same order of magnitude but require recording of either one or two specklegrams. Evidence of experimental demonstration of each case is given.     

Numerical simulation of multiple scattering for modeling speckle roughness analyses on vertical surface regions of silicon wafers

Laser speckle measurement methods enable the roughness analysis also for vertical regions of silicon wafers. However, we have to calculate upon disturbing intensities that are generated by multiple reflection from the surrounding surfaces. A numerical simulation will be described in this study by which the higher spatial frequency of multiple scattered speckle intensities can be shown. Since focused laser beam is generally used for this purpose the influences of convergent illumination will also be taken into account. The results confirm the feasibility of speckle measurement methods (speckle-contrast, speckle-correlation) for roughness analysis of inaccessible surface regions of microstructures (side walls of grooves, inside surfaces of borings). Also, practically undetectable effects can be analysed through the calculation algorithm.     

High stability interleaved fiber Michelson interferometer for on-line precision displacement measurements

A self-reference fiber Michelson interferometer measurement system, which employs fiber Bragg gratings (FBGs) as in-fiber reflective mirrors and interleaves together two fiber Michelson interferometers that share the common-interferometric-optical path, is presented. One of the fiber interferometers is used to stabilise the system by the use of an electronic feedback loop to compensate the influences resulting from the environmental disturbances, while the other one is used to perform the measurement task. The influences resulting from the environmental disturbances have been eliminated by the compensating action of the electronic feedback loop, this makes the system suitable for on-line precision measurement. By means of the homodyne phase-tracking technique, the linearity of the measurement results of displacement measurements has been very high.