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Low-coherence optical fibre speckle interferometry
Abstract
This paper describes the development of optical fibre low-coherence speckle interferometers capable of three-dimensional surface profiling with a resolution of 10–20 µm and a depth range of typically tens of centimetres. The technique is absolute, enabling the measurement of steps and through holes. The inclusion of optical fibres enables compact, flexible configurations to be realized, alleviating the experimental difficulties encountered with bulk interferometers, particularly when using long path lengths for measurements on large test objects. Sources including light-emitting and superluminescent diodes and multimode laser diodes are compared, and the use of a multimode laser diode source in pulsed mode is shown to improve depth resolution. Fibre-based systems using both single mode and polarization-maintaining fibre are described, and the results of experimental measurements on a stepped test object, a tilted plate and a coin are presented. A depth resolution of ±20 µm is obtained for the standard fibre system and ±14 µm for the system based on polarization-maintaining fibre.
... By using multi-wavelength digital speckle pattern interferometry (DSPI) the geometrical and refractive parameters of the lens were obtained and consequently the focal length was determined. Whole-field multi-wavelength interferometry has been successfully used for profilometry of diffusely scattering surfaces through DSPI [14,15] and for refractive index measurement in a holographic setup with a photorefractive crystal [16]. In this work the profilometry and refractometry methods are combined, thus enabling a complete description of the curvatures and the refractive index of the lens. ...
... Fig. 5c shows the shape of the output wavefront after traveling through the lens. The circles shows the experimental data and the solid curve is the fitting of C P according to Eq. (14), providing the value n = 1.52 ± 0.01. ...
Physical parameters of different types of lenses were measured through digital speckle pattern interferometry (DSPI) using a multimode diode laser as light source. When such lasers emit two or more longitudinal modes simultaneously the speckle image of an object appears covered of contour fringes. By performing the quantitative fringe evaluation the radii of curvature as well as the refractive indexes of the lenses were determined. The fringe quantitative evaluation was carried out through the four- and the eight-stepping techniques and the branch-cut method was employed for phase unwrapping. With all these parameters the focal length was calculated. This whole-field multi-wavelength method does enable the characterization of spherical and aspherical lenses and of positive and negative ones as well.
... This phenomenon can be used to create interference fringes with which to measure the displacements with a speckled field or a combination of fields [6,7]. This phenomenon is intrinsically interferometric and is applicable to cases where measurements are orders of magnitude of the wavelength of the laser used [6,8,9]. ...
A system was developed that allows different acoustic parameters to be measured inside the frequency audio range and that can be used in different propagation mediums. A simple configuration of fiber optic sensors based on multimodal interference (MMI) is used, due to the advantages they provide. To perform a spatial and frequency filtering according to the characteristics of the source of interest, a beamformer is applied to the data obtained and then phase sensitive detection is used to obtain the resulting signal. The simulations were made by replicating the existing conditions in the experiments subsequently made in the laboratory. The system showed that the combination of the proposed techniques increases the signal-to-noise ratio even in cases where the ambient noise exceeds the signal of interest several times.
... Low-coherence interferometry (LCI) is an optical technique for surface profiling [1][2][3], capable of spatial resolution down to a few μm and depth resolution down to around 1 μm, or considerably better for single surfaces smooth on the scale of the optical wavelength [4]. When the target is a semi-transparent, often biological, material and light penetrates below the surface to form an image of sub-surface structures, the adapted method is called optical coherence tomography (OCT) [5,6]. ...
Duct-profiling in test samples up to 25 mm in diameter has been demonstrated using a passive, low-coherence probe head with a depth resolution of 7.8 μm, incorporating an optical-fibre-linked conical mirror addressed by a custom-built array of single-mode fibres. Zemax modelling, and experimental assessment of instrument performance, show that degradation of focus, resulting from astigmatism introduced by the conical mirror, is mitigated by the introduction of a novel lens element. This enables a good beam focus to be achieved at distances of tens of millimetres from the cone axis, not achievable when the cone is used alone. Incorporation of the additional lens element is shown to provide a four-fold improvement in lateral imaging resolution, when compared with reflection from the conical mirror alone.
... This was one of the first areas of speckle I investigated, in collaboration with Prof. Julian Jones who had recently moved to Heriot-Watt University from the AOG, and the optics research group for Rover cars at Gaydon (UK) [39, 40].Figure 19 shows contour fringes of a gas turbine compressor blade obtained by modulating the laser by 18 GHz170 GHz [38, 39, 41]. We explored the potential of using low coherence techniques combined with ESPI for measuring object shapes [42]. A fiber network (Fig. 21), implemented in birefringent optical fiber is used to couple light from a broadband source to the object and to provide a reference beam that is obtained from a mirror mounted on the test object and, via a mirror mounted on a translation stage, used to adjust the depth position on the object at which measurements are made and combined with the light scattered from the test object on a CCD camera. ...
This paper provides a short overview of the time I spent as a member of the Applied Optics Group at the University of Kent
(1985-1989) followed by a review of my research during my time at Cranfield University (1989 to date).
KeywordsOptical fiber sensors–optical instrumentation–laser velocimetry–speckle interferometry–fiber Bragg gratings–long period fiber gratings–optical gas sensing–optical coherence tomography–flow measurement–coherent fiber optic bundles
... Optical coherence tomography (OCT) is an adaptation of low-coherence interferometry (LCI). LCI was developed for noncontact surface profiling [1][2][3], whereas OCT refers to the same technique applied to subsurface imaging of semitransparent materials. The LCI technique comprises an interferometer, used with a broadband light source, in which one of the reflectors is replaced by the sample of interest and the other, known as the reference reflector, scans rapidly and linearly over a short distance parallel to the optical axis of the system, to modulate the interferometer path-length difference. ...
Fiber imaging bundles have been investigated for use in endoscopic optical coherence tomography (OCT) systems, to obviate the requirement for scanning components within the endoscope probe section. Images have been acquired using several optical configurations, two of which are common path in design. Configurations have been selected as having potential for miniaturization and inclusion in endoscopic-type systems, since the advantages of employing imaging bundles are most clearly seen in this type of system. The various types of bundle available are described, and the properties of the leached bundles used here are discussed in detail, with reference to their effect upon the performance of OCT systems. Images are displayed from measurements made on a range of samples.
... Due to the employ of fringe pattern evaluation procedures [1][2][3] such as phase shifting and phase unwrapping, whole-field interferometric methods, like holography [4][5][6] and DSPI [7,8] (digital speckle pattern interferometry), enable fast and precise reconstruction of wavefronts as a whole with a precision up to λ/100 in cases of low optical noise [9]. The rotation-source [10,11] and the heterodyne (two-color or multi-wavelength) methods [12][13][14] are among the most promising interferometric whole-field techniques for surface contouring. In both cases the interference fringes result from the intersection of the studied surface with uniformly distant and parallel planes of constant elevation. ...
This work studies both theoretically and experimentally the formation of the contour interference patterns generated by a two-wavelength real-time holographic interferometer. The resulting contour interference fringes are due to the intersection of the measured surface with parallel, equally spaced planes of constant elevation. The theoretical analysis describes how the spatial frequency of the elevation planes, their angular position, and the localization of the fringes depend on parameters of the optical setup. A theoretical model for fringe localization is developed and confirmed by the experiments, showing a strong dependence of the interferogram position on the slope of the studied surface. Due to the thick Bi12TiO20 crystal employed as the storage medium the Bragg selectivity of the holographic readout is also considered.
As high-precision measuring instruments have developed, interferometers have been widely applied in the measurement of lengths and of the shape of surfaces, with nanometer precision. The emergence of the laser is one of the revolutions that has led to a well-defined traceability route to the definition of the meter via interferometry. Another change is the ever-increasing adoption of detector arrays substituting for conventional methods of recording and analyzing interferograms. New applications have also arisen from the adoption of microscopes, optical fibers, chip-level components and diffractive optical elements, developing enhanced analogues of conventional interferometers, which have the advantages of high integration, low noise levels, and complete sets of measuring instruments with a high level of automation. Since the requirements for measurement parameters and the environment are becoming more complex, we expect that the related instruments will play a progressively significant role in the progress of advanced manufacturing processes and quality control. Multi-sensor integrated flexible measurement methods have been proposed to perform measurements with holistic, more accurate and reliable information. However, most of the proposed methods are not intelligent and are highly integrated, providing only specific solutions for given measuring tasks. In this paper, the principles, progress, prospects and development trends of interferometry are reviewed.
A high sensitivity fiber-optic method for registration of low intensity IR radiation is proposed. The method is based on the effect of variation of the speckle pattern in the far-field of a multimode fiber. IR radiation that falls on a lateral surface of the fiber leads to variation of the speckle image. Computer processing of the speckle image provides information on the amplitude of the perturbation that hits the fiber. An algorithm has been developed for processing of the speckle image and determining of the intensity of IR radiation. The results of computer simulation correlate sufficiently well with experimental ones.
Speckle noise is a major limitation for laser to be used in display technology. Experimental set-up and measurement parameters strongly affect the speckle characterization. In this paper we present different experimental settings for characterizing the laser speckle. The affects of the intensity of incident light and camera settings, such as gamma value, exposure time and F/#, on speckle measurement are experimentally analysed. The effect due to inappropriate intensity of incident light decreases as the ratio of the maximum intensity and the mean intensity of all intensity values in the speckle pattern increases. For a nonlinear charge-coupled device (CCD) camera, measured mean intensities are more sensitive to gamma settings of CCD than measured standard deviations. Achieved results can make a good contribution for setting up a standard for speckle measurement in laser display technology.
A high sensitivity speckle based fiber-optic method for registration of
low intensity IR radiation is described. The method is based on the
effect of variation of the speckle pattern in the far-field of a
multimode fiber. IR radiation that falls on a lateral surface of the
fiber leads to variation of the speckle image. Computer processing of
the speckle image provides information on the amplitude of perturbation
that interacts with the fiber. An algorithm was developed for processing
of the speckle image and determining of the intensity of IR radiation.
The results of the computer simulation correlate well with the
experimental ones.
A high sensitivity speckle based fiber-optic method for registration of low intensity IR radiation is proposed. The method is based on the effect of variation of the speckle pattern in the far-field of a multimode fiber. IR radiation that falls on a lateral surface of the fiber leads to variation of the speckle image. Computer processing of the speckle image provides information on the amplitude of the perturbation that hits the fiber. An algorithm has been developed for processing of the speckle image and determining of the intensity of IR radiation. The results of computer simulation correlate sufficiently well with experimental ones.
To evaluate the severity of airway pathologies, quantitative dimensioning of airways is of utmost importance. Endoscopic vision gives a projective image and thus no true scaling information can be directly deduced from it. In this article, an approach based on an interferometric setup, a low-coherence laser source and a standard rigid endoscope is presented, and applied to hollow samples measurements. More generally, the use of the low-coherence interferometric setup detailed here could be extended to any other endoscopy-related field of interest, e.g., gastroscopy, arthroscopy and other medical or industrial applications where tri-dimensional topology is required. The setup design with a multiple fibers illumination system is presented. Demonstration of the method ability to operate on biological samples is assessed through measurements on ex vivo pig bronchi.
In this article we present a method to achieve tri-dimensional contouring of macroscopic objects. A modified reference wave speckle interferometer is used in conjunction with a source of reduced coherence. The depth signal is given by the envelope of the interference signal, directly determined by the coherence length of the source. Fringes are detected in the interferogram obtained by a single shot and are detected by means of adequate filtering. With the approach based on off-axis configuration, a contour line can be extracted from a single acquisition, thus allowing to use the system in harsh environment.
We introduce Endoscopic Low Coherence Interferometry to obtain topology of upper airways through commonly used rigid endoscopes. Quantitative dimensioning of upper airways pathologies is crucial to provide maximum health recovery chances, for example in order to choose the correct stent to treat endoluminal obstructing pathologies. Our device is fully compatible with procedures used in day-to-day examinations and can potentially be brought to bedside. Besides this, the approach described here can be almost straightforwardly adapted to other endoscopy-related field of interest, such as gastroscopy and arthroscopy. The principle of the method is first exposed, then filtering procedure used to extract the depth information is described. Finally, demonstration of the method ability to operate on biological samples is assessed through measurements on ex-vivo pork bronchi.
This thesis presents a novel approach for optical tri-dimensional contouring, requiring only one acquisition to extract a contour depth. It is based on an interferometric setup, using a reduced coherence light source, and is fully adaptable to endoscopic procedure commonly in used in ear-nose-throat (ENT) medicine. It has been demonstrated that the method can be successfully applied to imaging of porcine upper airways. Although this method uses interferometry to obtain depth range measurement, the phase signal is not directly evaluated, rather the location of the coherent superposition of two waves is extracted, and thus provides a robust technique even in harsh environment. The key feature is to extract the coherent area on the image by locating intentionally induced fringes created by off-axis superposition of a smooth reference wave and an object wave coming from a rough or diffusing sample through an optical system with limited aperture. A model for the propagation of the light through the optical system has been developed. It takes into account the broadened emission spectrum of the light source, as well as the statistical speckle effect induced by the illumination of a rough surface with partially coherent light. Simulated results have been carefully compared with measurements, both in the spatial domain and in the Fourier domain (spatial frequencies response), which has permitted to better understand the processes involved in the creation of the interference fringes, and improve the design of the device. Different filters have been investigated in order to extract the depth information from acquired interferograms. By using Gabor filterbanks, which do not need any a priori knowledge of the signal, selectivity of the extracted signal has been enhanced by a factor of 1.3 compared to a simple local Fourier spectrum evaluation, and time of the extraction procedure divided by 6, even when compared to fast iterative calculation in the Fourier spectrum. Moreover, by imposing the frequency and/or orientation of the signal to detect, which can be known by a simple calibration procedure, it has been proved that the selectivity can be further enhanced by a factor of 2.5, keeping the calculation time as short or even shorter. A compact prototype has been designed and built, with a specific multiple fibers arrangement for illumination. The system is separated in two parts: the first one is directly attached to the rigid endoscope, and contains imaging as well as recombining object/reference wave optics, and the detector. It fits into a box with overall dimensions of 115 × 83 × 94 mm. The second part contains the laser source, the scanning arm and the injection optics for the fibered system. It is designed to be left on a small cart. This device can potentially be brought to the bedside, and used during routine endoscopic check-up. Associated with the device, a complete framework has been elaborated in order to simplify the acquisition procedure as well as the signal processing. This results in a fully automated environment taking in charge hardware control, data storage and signal processing, up to the tri-dimensional scene rendering.
An interferometric technique for automated profilometry of diffuse objects has been proposed. It is based on the Fourier-fringe analysis of spatiotemporal specklegrams produced by a wavelength-shift interferometer with a laser diode as a frequency-tunable light source. Unlike conventional moiré techniques the proposed technique permits the objects to have discontinuous height steps and/or surfaces spatially isolated from one another. Experimental results are presented that demonstrate the validity of the principle.
We introduce a three-dimensional sensor designed primarily for rough objects that supplies an accuracy that is limited only by the roughness of the object surface. This differs from conventional optical systems in which the depth accuracy is limited by the aperture. Consequently, our sensor supplies high accuracy with a small aperture, i.e., we can probe narrow crevices and holes. The sensor is based on a Michelson interferometer, with the rough object surface serving as one mirror. The small coherence length of the light source is used. While scanning the object in depth, one can detect the local occurrence of interference within the speckles emerging from the object. We call this method coherence radar.
In this paper we study the potential of using multimode laser diodes as an alternative to low coherence length sources normally used white light fiber optic interferometric sensors. A simple theoretical model is introduced to demonstrate the autocorrelation function of such sources. An experimental setup of two interferometers in tandem was used to study the coherence properties of the multimode laser diode Mitsubishi ML-4406 and then to demonstrate the possibility of using it in coherence tuned multiplexing systems. The main advantage of using such sources is to launch more optical power into the monomode fiber and hence to improve the system resolution.
Coherence scanning microscopy is a new technique in high resolution imaging. It shares many of the features of confocal microscopy but uses coherence effects to enhance the lateral and longitudinal resolution rather than physical apertures. This approach has two significant implications for profilometry: the longitudinal resolution is decoupled from the lateral resolution, and interference effects can be used to further enhance the longitudinal resolution. We detail the features of coherence scanning profilometry and give some examples.
Two-wavelength phase-shifting interferometry is applied to an interference phase-measuring microscope enabling the measurement of step features. The surface is effectively tested at a synthesized equivalent wavelength λeq = λ a λ b /|λ a − λ b | by subtracting phase measurements made at visible wavelengths λ a and λ b . The rms repeatability of the technique is λ/1000 at the equivalent wavelength. To improve the precision of the data, the phase ambiguities in the single-wavelength data are removed using the equivalent wavelength results to determine fringe orders. When this correction is made, a measurement dynamic range (feature height/rms repeatability) of 10⁴ is obtainable. Results using this technique are shown for the measurement of an optical waveguide and a deeply modulated grating.
La difference de phase entre les 2 faisceaux interferant varie de maniere connue et on fait des mesures de la distribution d'intensite a travers la pupille correspondant a au moins 3 dephasages differents
The feasibility of maintaining a single-mode optical fiber interferometer in quadrature is demonstrated using a servo driven piezoelectrically stretched coiled fiber. The controller has a range of ~10⁻⁵–1000 rad with a stress voltage coefficient of ~2π rad/V.
Phase-locked moiré systems have the advantage of rapid data acquisition and accuracy of better than one twentieth of the fringe spacing. In conventional moiré systems data acquisition is performed by making a photographic plate and taking measurements from the plate either visually or with a microdensitometer. This is slow, and the accuracy is about one-quarter of the fringe spacing. The phase-locked system is more accurate and permits a more favorable trade-off between accuracy and working depth. The equations governing moiré contouring are examined, and the theory of phase-locked moiré fringe analysis is presented. The design and operation of the phase-locked moiré contouring system are described, and the results of measurements made with the instrument are presented.
The contours of constant phases across sinusoidally vibrating objects have been mapped using reference wave phase modulation in time-average electronic speckle pattern interferometry.
It is shown that both single exposure and double exposure two-wavelength holography provide a good method of using visible light to obtain an interferogram identical to what would be obtained if a longer nonvisible wavelength were used. Both techniques provide for the real-time adjustment of defocus and tilt in the final interferogram. When both hologram exposures are made simultaneously, the sensitivity to air turbulence is essentially the same as if the longer nonvisible wavelength were used. Results are shown for testing both lenses and mirrors at equivalent wavelengths at 6.45 μ, 9.47 μ, 14.20 μ, 20.22 μ, and 28.50 μ obtained by using an argon laser for the visible light source.
Langmuir–Blodgett (LB) films of ω-tricosenoic acid are deposited onto side-polished optical fibers, resulting in wavelength-selective elements. The wavelength response is shown to be strongly dependent on film thickness, giving a shift in central wavelength with film thickness of Δλ0/Δ d = 2.4, in agreement with a simple phase-matching model. It is demonstrated that an alternative polishing technique, requiring no substrates or epoxy, allows more repeatable LB deposition and produces devices with similar characteristics. This is the only technique that will allow deposition of more complex electro-optic LB film-forming molecules.
A novel method of rapid three-dimensional optical metrology that is based on triangulation of a configration of color-coded light stripes is presented. The method exploits polychromatic illumination and a combined diffractive–refractive element, so the incident light is focused upon a stripe that is axially dispersed, greatly increasing the depth-measuring range without any decrease in the axial or the lateral resolution. The discrimination of each color stripe is further improved by spectral coding and decoding techniques. An 18-fold increase in the depth of focus was experimentally obtained while diffraction-limited light stripes were completely maintained.
A new computer-based technique for automatic 3-D shape measurement is proposed and verified by experiments. In contrast to the moire contouring technique, a grating pattern projected onto the object surface is Fourier-transformed and processed in its spatial frequency domain as well as in its space-signal domain. This technique has a much higher sensitivity than the conventional moire technique and is capable of fully automatic distinction between a depression and an elevation on the object surface. There is no requirement for assigning fringe orders and interpolating data in the regions between contour fringes. The technique is free from errors caused by spurious moire fringes generated by the higher harmonic components of the grating pattern.
The high resolution surface profile of a 3-D diffuse object is obtained by measurement of the phase distribution across the image of a projected sinusoidal grating deformed by the surface. A shearing polarization interferometer is used for projection. Deformed grating images are detected by an array camera and processed by a microcomputer. Surface height resolutions of better than 10 μm have been attained, limited essentially by electronic quantization noise. In contrast to direct deformed grating analysis, this method, based on phase-shifting interferometric techniques, is capable of accurate measurement even with coarse projected gratings and low density image sensing arrays. Areas of application include industrial quality control, biostereometrics, robotics, and solid modeling for computer graphics.
The performance of Fourier-transform profilometry is enhanced by a new technique that is based on spatial frequency multiplexing combined with the Gushov–Solodkin phase unwrapping algorithm. The technique permits the three-dimensional shape measurement of objects that have discontinuous height steps and/or spatially isolated surfaces, which has not been possible by conventional Fourier-transform profilometry. An important feature of the technique is that it requires only a single fringe pattern; the single-shot recording makes possible the instantaneous three-dimensional shape measurement of discontinuous objects in fast motion. Experimental results are presented that demonstrate the validity of the principle.
Holographic and speckle interferometry are optical techniques which use lasers to make non-contracting field view measurements at a sensitivity of the wavelength of light on optically rough (i.e. non-mirrored) surfaces. They may be used to measure static or dynamic displacements, the shape of objects, and refractive index variations of transparent media. As such, these techniques have been applied to the solution of a wide range of problems in strain and vibrational analysis, non-destructive testing (NDT), component inspection and design analysis and fluid flow visualisation. This book provides a self-contained, unified, theoretical analysis of the basic principles and associated opto-electronic techniques (for example Electronic Speckle Pattern Interferometry). In addition, a detailed discussion of experimental design and practical application to the solution of physical problems is presented. In this new edition, the authors have taken the opportunity to include a much more coherent description of more than twenty individual case studies that are representative of the main uses to which the techniques are put. The Bibliography has also been brought up to date.
Fringe scanning techniques, now renamed heterodyning or phase shift interferometry, are covered in a completely rewritten chapter. New chapters have been added to cover wavefront fitting and evaluation as well as holographic and Moire methods. The chapter on parameter measurements has been completely rewritten and an appendix added suggesting appropriate tests for typical optical surfaces.
Since the initial concept of speckle interferometry was published more than 30 years ago the subject has matured to the point where it is now an engineering tool used in a wide range of applications with instrumentation available from a number of companies. The development of the technique to this level has been accomplished by making use of parallel developments in a range of technological areas. These are the laser, the PC, and the CCD camera. More recently the incorporation of optical fiber technology has also had a major impact. This paper reviews recent developments in the optoelectronic technology used in speckle interferometry instrumentation and the use made of it in a range of application areas.
Shearography is a full field non-contact optical technique usually used to measure the gradient of the displacement of a surface subjected to thermal or mechanical loading. This paper describes the use of shearography for surface slope and shape measurement. Interferometric speckle patterns obtained before and after displacement of the optical source are correlated to yield correlation fringes which are, in general, a mixture of slope fringes and carrier fringes. This paper contains a full treatment of the sensitivity of slope fringes to the parameters; the illumination and the imaging geometry and the magnitude and the direction of the source displacement. The slope fringes are corrected for distortion, which is due to the necessary off-axis illumination, and are scaled using parameters calculated using a mathematical model. Experimentally generated phase-stepped slope fringes are unwrapped and integrated to recover the object shape.
The incorporation of laser diodes and monomode fibre optics into an Electronic Speckle Pattern Interferometer (ESPI) has led to the development of a novel holographic surface contouring system. Height contours are obtained by switching between two laser wavelengths. Contour intervals from 0.5–5 mm have been generated by modulating the injection current of a single laser diode source and are demonstrated on an automotive disc brake hub.
High performance, short coherence length light sources with broad bandwidths and high output powers are critical for high speed, ultrahigh resolution OCT imaging. We demonstrate an all-fiber continuous-wave Raman light source based on a photonic crystal fiber, pumped by a continuous-wave Yb-fiber laser, which generates 330 mW output power and 140 nm bandwidths. The light source is compact, robust, turnkey and requires no optical alignment. In vivo high speed, ultrahigh resolution OCT imaging of tissues with < 5 µm axial resolution at 1.3 µm center wavelength is demonstrated.
We first provide an overview of 3D shape measurement using various
optical methods. Then we focus on structural light techniques where
various optical configuration, image acquisition techniques, data
post/processing and analysis methods and advantages and limitations are
presented. Several industrial application examples are presented,
important areas requiring further R&D are discussed. Finally, a
comprehensive bibliography on 3D shape measurement is included, although
it is not intended to be exhaustive.
Intensity data obtained with white light and an achromatic phase shifter in an interferometric surface profiler can be used to locate the peak of the visibility curve as the test surface is moved in steps along the height axis. We show that the same intensity data can also be used to find the fractional interference order at each point on the test surface, at the position of the test surface nearest to the visibility peak and, hence, its height at that point, with a high degree of accuracy.
Video speckle interferometry of small objects are discussed. A major problem is posed by the small depth of focus in object space. At high magnifications the exit aperture of most microscopic lenses is too small for optimum recordings. Recording areas smaller than about 0.2 × 0.2 mm2 by a standard CCD does not improve the resolution, but the oversampling of the speckle pattern improves the signal-to-noise ratio. Under optimum conditions, we may theoretically detect defects down to about 4 μm in size. A microscopic TV -holography system based on optical fibers has been constructed to test theoretical results.
Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase and gradient of displacement amplitude of a vibrating object using continuous wave illumination in a path length imbalanced speckle shearing interferometer. The unwanted mismatch in the magnification of the two images in the shearing interferometer and sensitivity to radial shear introduced by a path length imbalance is overcome by the use of a transparent block of high refractive index in one path in the interferometer.
A technique is presented that allows simple and potentially automated determination of the optical path-length imbalance in ESPI systems that utilize source-wavelength modulation to obtain phase shifting. The method is based on modulating the emission wavelength of the laser synchronously with the framing rate of the CCD/image processing board and recording the average speckle intensity of the image. The injection current is increased until a , or multiple of , change in the phase has been achieved. Repeating this measurement for two different optical path-length imbalances of known difference allows determination of the laser diode coefficient and the optical path-length imbalance without the use of additional spectrometers.
Measurement of plane-strain surface displacement with ESPI requires at least two in-plane illumination geometries. For static loading conditions it is acceptable to record these two interferograms sequentially. However, for time-dependent strain fields, it is necessary to use both illumination geometries simultaneously so that a recording is made with identical strain conditions existing for both. The authors describe a new interferometer that has been devised to measure two in-plane interferograms at the same time. The determination of the method of operation and experimental verification of the technique are given.
Modulation of the emission wavelength of a laser diode is demonstrated as a heterodyning technique for measuring the phase and amplitude of a vibrating object using continuous wave (cw) illumination in a time-averaged electronic speckle pattern interferometer. The technique is demonstrated with an interferometer constructed from single mode optical fibre.
A novel optical fibre system, for out-of-plane vibration analysis, comprising the complementary techniques of laser velocimetry and electronic speckle pattern interferometry, is described. The laser velocimeter (LV) and the electronic speckle pattern interferometer (ESPI) share a common reference arm and phase modulator. The LV locks to the motion of a point on the vibrating surface and since the ESPI and LV share a common reference arm, automatic heterodyning of the time-averaged ESPI fringes is achieved. The experimental results are described.
When a surface is illuminated with laser light it has a speckled appearance due to random interference. If two speckle patterns are superimposed the distribution of intensity in the resultant pattern depends on the relative phases of the component patterns. By measuring the correlation between the resultant patterns at two different times a change of relative phase is detected. This leads to a versatile method of measuring either the normal or in-plane components of displacement over the whole of a surface at one time.
An interferometer without moving parts is proposed for measurement of distances to, or distance differences between, diffuse surfaces. The interferometer, which is a variation of the non-scanning white-light interferometer, utilizes the limited coherence of the light source. The light is divided by a beamsplitter and reflected from two surfaces, one reference and one object surface. After recombination it is spatially filtered. A distance difference causes a delay between the reflections. The delay can be measured with the non-scanning white-light interferometer, by finding the positions of the correlation peaks. Since there are no moving parts, the interferometer can be used to measure rapidly moving objects.
Un interféromètre sans sections mobiles est proposé pour la mesure de distance jusquaux surfaces diffusantes ou la différence de distance entre les surfaces diffusantes. L'interféromètre proposé utilise la cohérence limitée de la source de lumière. La lumière est divisée par un diviseur de faisceaux et devient alors reflectée d'une part de la surface de l'objet et de l'autre part de la surface de référence. Apres la recombinaison la lumiere devient filtrée spatiale. Une différence des distances apporte un retard entre les réflections. Ce retard est mesuré avec un interféromètre pour la lumière blanche, de façon à localiser les sommets des corrélations. Du moment que l'interféromètre n'a pas de sections mobiles il peut être utilisé pour la mesure des objets qui se déplacent rapidement.
A basic foundation for holography, interferometry and speckle measurement is provided to assist those readers who do not have an optics background. The inclusion of this introductory material is also intended to reduce the amount of repetitive material that would have been required by the subsequent authors. The principles of many optical diagnostics measurements can be understood simply and with very little mathematics, by using graphics and moiré analogies—these are used extensively here. The physical and mathematical analogy between interferometry and moiré is exploited to provide a simple, easily demonstrable explanation of virtually all of the phenomena in interferometry and holography.
A new method for phase-stepping grating profilometry of complex objects is proposed and verified by experiments. Both, the discrete phase distribution and the modulation depth distribution are calculated. To construct binary control masks the discrete values and the histogram of the modulation intensities are inspected first. Then the blocking lines, which are connecting the pairs of opposite poles, are traced down through the local minimums of the modulation intensities. The control masks are used to identify valid and invalid discrete phase distributions during phase unwrapping and phase interpolation, and to control the path of phase unwrapping. The method could equally well be used in grating profilometry or in interferometry.
Some of the benefits of holographic interferometry in metrology are explored and related to difficulties experienced in practical methods. A new type of speckle pattern application is introduced which effectively samples the wavefront phase, enabling the interferometric measuring process to be carried out with low density data recording.
Two broadband sources can be used in combination in white light
interferometry to simplify the problem of central fringe identification.
In this paper, a method for selecting the optimum wavelength
combinations of such sources is presented. The results are obtained from
a theoretical analysis and computer simulations, verified by experiment.
It has been shown that the minimum signal-to-noise ratio required by the
system to identify the central fringe position can be greatly reduced by
using the optimum wavelength combination
The distortion and noise characteristics of semiconductor lasers in connection with optical fibers are reviewed. In particular, the intrinsic distortions and noise of semiconductor lasers together with the partition noise are discussed followed by a discussion on the influence of reflections. Modal noise phenomena due to the interference pattern at the endface of optical fibers are treated with respect to noise and distortions. Finally, the influence of polarization coupling in single-mode fibers on the resulting transmission behavior is discussed.
Combined fibre-optic laser velocimeter and electronic speckle pattern [46] Jackson D A, Priest R, Dandridge A and Tveten A B 1980 Elimination of drift in a singlemode optical fiber interferometer using a piezoelectrically stretched coiled fiber
- J D Valera
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