A simple laser speckle dilatometer for thermal expansion measurements

A simple laser speckle method utilizing a 4 mW HeNe laser for thermal expansion measurements is described. The method utilizes the rotation of a probe kept on the sample and reference rods. For 5 cm long sample and reference rods kept 0.5 cm apart, a measurement sensitivity of 5 × 10^?6 for $\text{ΔL}\text{L}$ can easily be obtained.     

Determination of necking zone using laser speckle

This paper presents a whole field laser speckle method for the measurement of the necking zone on the surface of metallic specimens. The method employs an expanded laser beam to illuminate the test object. Diffracted images are recorded on a high resolution photographic film and subsequently filtered using point wise filtering. The intensity of the diffracted halo is measured with a photo-sensor and the intensity distribution on the specimen yields information on the necking zone of the test object. The technique is applied to aflat tensile specimen with two circular notches at the central portion. Measured necking zone is compared with results obtained by other conventional methods.     

Measurement of crack tip displacement field using laser speckle photography

It is shown how double exposure laser speckle photography, when used with an automatic image processing system, can provide detailed information on both in-plane displacement components around the tip of a crack. This paper describes the results of the analysis of six speckle photographs of sharp cracks in polymethylmethacrylate (PMMA), each on a mesh of 16 × 16 datapoints. Two numerical methods for calculation of the mode I stress intensity factor (K_I) from the measured displacement field have been investigated. Least-squares fitting of the theoretical displacement field gave good estimates of K_I, with low systematic and random errors (˜1%). Evaluation of the J-integral around square paths enclosing the crack tip was found to result in larger errors (up to 5%). It is thought that laser speckle photography together with J-integral calculations may find applications in nondestructive testing, as a method of automatically detecting the presence of a crack.     

Complete wavefront reconstruction using sequential intensity measurements of a volume speckle field

The recording of the volume speckle field from an object at different planes combined with the wave propagation equation allows the reconstruction of the wavefront phase and amplitude without requiring a reference wave. The main advantage of this single-beam multiple-intensity reconstruction (SBMIR) technique is the simple experimental setup because no reference wave is required as in the case of holography. The phase retrieval technique is applied to the investigation of diffusely transmitting and reflecting objects. The effects of different parameters on the quality of reconstructions are investigated by simulation and experiment. Significant enhancements of the reconstructions are observed when the number of intensity measurements is 15 or more and the sequential measurement distance is 0.5 mm or larger. Performing two iterations during the reconstruction process using the calculated phase also leads to better reconstruction. The results from computer simulations confirm the experiments. Analysis of transverse and longitudinal intensity distributions of a volume speckle field for the SBMIR technique is presented. Enhancing the resolution method by shifting the camera a distance of a half-pixel in the lateral direction improves the sampling of speckle patterns and leads to better quality reconstructions. This allows the possibility of recording wave fields from larger test objects.     

Peculiarities of using laser diodes in linear measurements

The position of the emitting region of a laser diode depends on the working current. This phenomenon, as well as the polarization of radiation, may influence the accuracy of linear measurements using such lasers.     

Diffusive media characterization with laser speckle

The statistical properties of laser speckle with partially coherent light are related to the scattering characteristics of an optically diffuse material. A diffusion equation model is shown to yield a speckle contrast ratio that agrees well with measurements of opaque plastics of varying thicknesses. We show that partially coherent light can be used to determine material parameters for highly scattering media. Measured data for stratified materials with differing scattering properties indicate that this technique may be useful in detecting inhomogeneities.     

Reconstructing an object image using the laser speckle pattern of the diffraction field

A method of reconstruction of the object image from a recorded speckle pattern of the scattered coherent radiation field has been developed and experimentally verified. In order to restore information about the phase of the diffraction field, which is lost during the recording of the speckle pattern, it is suggested to take the phase difference between adjacent speckles equal to p. The proposed method is applicable to objects for which the average intensity distribution is described by an even function of the coordinates.     

Interferometer using a laser pointer for remote microdisplacement measurements

A portable Michelson interferometer for remote optical sensing with imaging capability is developed based on a laser pointer and experimentally studied. The setup has an open sensing arm with a mirror that can be attached to any object for the measurement of responses such as microdisplacement, strain, stress, and temperature. A mirror in the reference arm is mounted on an encoded stepper motor, which can operate under remote control via Web/Internet for reset and calibration. Remote microdisplacement measurements precise to within 5 μm have been performed.     

Distance measurements by speckle correlation of objective speckle patterns, structured by the illumination

Objective speckles produced by two beams overlapping and interfering on a rough object surface contain information about the angle of incidence of the two beams, and how well they overlap. We obtain the autocovariance function for such a speckle pattern, and demonstrate how the information carried by the objective speckles can be used to probe the distance between the object and the observation plane. From a distance of 75 mm to a distance of 150 mm, and using an angle of 0.3 deg between the two incident beams, we can measure the actual distance with an uncertainty of better than ±0.1% of the full range. As long as the beams overlap at the object surface, the proposed method can measure distance with an uncertainty inversely proportional to the spot size at the object.     

Transform method of processing for speckle strain-rate measurements

We have developed a highly sensitive method for measuring thermal expansion, mechanical strain, and creep rates. We use the well-known technique of observing laser speckle with a pair of linear array cameras, but we employ a data-processing approach based on a two-dimensional transform of the speckle histories from each camera. This technique can effect large gauge sizes, which are important in the assessment of the spatial statistics of creep. Further, the algorithm provides simultaneous global estimates of the strain rates at both small- and large-scale sizes. This feature may be of value in the investigation of materials with different short- and long-range orders. General advantages of our technique are compact design, modest resolution requirements, insensitivity to slow surface microstructure changes (as seen with oxidation), and insensitivity to zero-mean-noise processes such as turbulence and vibration. Herein we detail the theory of our technique and the results of a number of experiments. Thesetests are intended to demonstrate the performance advantages and limitations of the transform method of processing speckle strain-rate data.     

Wavelength-shift speckle interferometry for absolute profilometry using a mode-hop free external cavity diode laser

Abstract

In this paper we present an absolute surface topography measurement with a tuneable diode laser with an external cavity that has a tuning range of as much as 25 nm without mode hops and a very high tuning speed of less than 1 s for the whole range. With this laser, an absolute wavelength-shift speckle pattern interferometer was realized, capable of measuring optically smooth and rough surfaces. We briefly explain the principles of operation, and the importance of wavelength tuning without mode hops. We discuss the capabilities, possible measuring ranges and some results as well as calibration methods of wavelength-shift speckle profilometry.     

Random speckle modulation technique for laser interferometry

In spite of its obvious advantages over conventional contact and immersion techniques, laser interferometry has not yet become a practical tool in ultrasonic nondestructive evaluation since its sensitivity is insufficient in most practical applications. Part of the problem is that the maximum signal-to-noise ratio often cited in scientific publications and manufacturers' specifications cannot be maintained on ordinary diffusely reflecting surfaces. Although these surfaces reflect a fair amount (5–50%) of the incident laser light, this energy is randomly distributed among a large number of bright speckles. Unless the detector happens to see one of these bright speckles, the interferometer's signal-to-noise ratio will be much lower than the optimum. This adverse effect is almost completely eliminated by the suggested random speckle modulation technique. The conventional interferometric technique was modified to assure random occurrence of a few very bright speckles and to move the whole speckle pattern around at an appropriate speed. Random but frequent bright flashes detected from the surface of the specimen resulted. The bright periods are 0.1 ms or longer, sufficient to trigger the ultrasonic pulser and detect the transmitted signals before the flash subsides. As much as 5–10 times improvement of the optical sensitivity was achieved by this novel approach and close to maximum signal-to-noise ratio was maintained everywhere on the surface of a diffuse object.     

Displacement measurements using a self-mixing laser diode under moderate feedback

A semiconductor laser subject to moderate optical feedback has been used to design an interferometric displacement sensor. The autoadaptative signal processing presented in this paper has been computed in order to improve the accuracy of such a sensor. This setup has been successfully tested for both harmonic and aleatory displacements of a remote piezoelectric actuator with a maximum accuracy of 40 nm.     

Defocus measurement using speckle correlation

Abstract

A technique using correlation fringes is presented to estimate the out-of-plane displacement of the image plane. The width of the fringe is estimated using autocorrelation of the Young's fringes and the visibility of the autocorrelation of these fringes are used as an estimator of the focusing error. This technique allows us to estimate focusing errors of the order of few tens of microns.     

Non-contact strain measurement using metal foil gauges with the application of the laser speckle method

This paper presents a new method to measure cyclic strain with no contact using metal foil gauges assisted by the laser speckle method. When aluminium foil is pasted on a specimen and the specimen is loaded cyclically, slip bands are produced on the foil surface. There is a fixed relation between density of the slip bands and the strain amplitude or loading cycles depending on the foil material. Thus the fatigue strain of the base metal can be estimated by observing the surface change of the metal foil by the slip bands at a constant number of loading cycles. The method presented in this paper is intended to make a non-contacting strain measurement by the application of the laser speckle technique for the detection of the surface change. This method is based on observation of the changes in a laser speckle pattern depending on the surface roughness and surface property changes of the foil caused by fatigue. The laser speckle pattern can be analysed automatically and quantitatively using an image processing system.