Distributed Acoustic Sensing (DAS) is a highly promising technology to efficiently monitor assets for energy production and transportation, both off- and on-shore, such as boreholes, pipelines and risers. The aim of the hereby-presented measurements is to evaluate the sensitivity of the different optical fiber cables to acoustic signals in sand and water, independently from the DAS read-out unit type and manufacturer. Acoustic sensing cables specifically designed by BRUGG Cables are characterized and compared to standard telecommunication cables. The spectral response of each cable was quantified using an all-fiber Mach-Zehnder interferometer. The response was also measured with calibrated microphones in order to convert the measurements into absolute physical units (Pascal). The measurement campaign is part of an investigation program for a reliable DAS system, which comprises the sensing cable (including installation procedure), the interrogator unit and suitable software.
Speckle interferometry (SI) is used for the measurement of the shape change of x-ray mirrors. Initially flat under thermal equilibrium, the mirror, or "thermal bender", is deliberately and adaptively bent by means of well-controlled temperature gradients. As the deflection of an optically polished surface can be obtained by a number of methods, the choice of SI and its subsequent advantages are discussed. Quantitative results are reported, referring to four kinds of tests: conformity, stability, sensitivity and repeatability tests. SI is recognized to meet the expectations: it provides a simple, complete, sensitive and accurate control of the shape of the bent x-ray mirrors.
Supraconductive installations are now commonly used in large facilities, such as power plants and particle accelerators. This requires a permanent temperature control at very low temperature, but cryogenic temperature measurements in the 1-77K range requires expensive calibrated temperature probes. We report here the possibility to use stimulated Brillouin scattering in optical fibers for temperature sensing down to 1K. Such a technique offers the additional advantage to make possible distributed measurement, so that very large structures and systems can be controlled using a single fiber and a single analyzing instrument. In addition only one by-pass for the fiber is required as input to the cryogenic vessel, that is definitely a key advantage for the design and the energy loss. Brillouin scattering in optical fibers has never been investigated so far at temperature below 77K (nitrogen boiling point). This absence of interest probably results from the constant decrease of scattering efficiency that was observed while cooling the fiber down to 77K. Our measurements show the unexpected feature that scattering efficiency is significantly raised below 50K and is even much better than observed at room temperature. The relevance and the feasibility of the technique is demonstrated in real scale on the supraconductive magnets for the future world largest particle accelerator, namely the large hadron collider (LHC) at CERN Laboratory in Geneva.
Distributed strain measurements on synthetic fiber ropes and cable constructions are reported. The tension members incorporate single mode fiber-optic sensors for strain measurement. Brillouin-amplification-based distributed strain measuring system has been the measurement technique of choice to interrogate the fiber sensors incorporated into a parallel yarn aramid rope. Initial results are presented which conclusively demonstrate the technical feasibility of the approach.
Strain measurements on synthetic fiber rope constructions of the type used in marine applications are reported. The ropes are of a novel construction by virtue of incorporation of fiber-optic sensors for strain measurement. Two instrumentation approaches -- a Brillouin strain measuring system and a RF sub-carrier strain measuring system -- have been utilized to interrogate the fiber sensors.
Long-range distributed strain and temperature measurements along an optical fiber is presented, using a novel optical sensor based on stimulated Brillouin scattering. The optical effect only depends on the fiber material, so that the bare fiber itself acts as sensing element without any special fiber processing or preparation. The sensor accuracy is plus or minus 1 degree Celsius for temperature and plus or minus 20 (mu) (epsilon) for deformation. The spatial resolution is 1 meter and the sensor range is more than 20 km. Successful monitoring of a concrete dam element has been performed using an embedded standard cabled fiber. The temperature dynamics of lake waters have been also observed by simply laying a cable over the lake bed.
Long-range distributed strain and temperature measurements along an optical fiber is presented, using a novel optical sensor based on stimulated Brillouin scattering. The optical effect only depends on the fiber material, so that the bare fiber itself acts as sensing element without any special fiber processing or preparation. The sensor accuracy is +/- 1 degree C for temperature and +/- 20 (mu) e for deformation. The spatial resolution is 1 meter and the sensor range is more than 20 km. Successful monitoring of a concrete dam element has been performed using an embedded standard cabled fiber. The temperature dynamics of lake waters have been also observed by simply laying a cable over the lake bed.
Speckle interferometry has been successfully used in many cases where a non-contact analysis of a surface's deformation state was required. This technique has been mostly applied in favorable environment, where the absence of vibrations and a reduced ambient illumination could be assured, and for the observation of areas being not in excess of some dm2. When inspecting larger surfaces, all the problems related to the spatial resolution, to the limited sensitivity of the acquisition system, to the amount of illumination and to the extension of the observed area, should be taken into account. If the observed surface belongs to a structure directly positioned in-situ, problems of vibration, air turbulence, or ambient light saturation, become very restrictive for the application of the method. After a brief introduction about the theoretical principles of speckle interferometry, some main problems related to the inspection on quite large surfaces in-situ discussed. The problem is treated with the help of some simulations, possible solutions are proposed and some applications are presented. This work is the result of a joint collaboration between may institutes and research centers in the framework of a European project named VISILAS. Special fields of application foreseen for this project are civil engineering, aircraft industry, ship building and power plants for energy production.
In this paper the application of classical tools for the characterization of building materials is criticized and the applicability of optical techniques in this field is investigated. Optical diagnostic techniques are particularly attractive for a non destructive evaluation of a surface's state and the detection of incipient damage. Non contact, high precision measurements and full-field of observation are features that can bring enormous advantages in experimental tests. Electronic Speckle Pattern Interferometry has ben preferred because it allows a real- time inspection of surfaces and is less sensitive to ambient conditions. The observation of fracture propagation, the full-field evaluation of deformations, the early detection of defects can help for a better understanding of the materials. Some experimental results obtained by optical tests are here discussed.
The characterization of materials used in civil constructions is needed and for this purpose many parameters can be obtained from experimental tests. In the last years optical interferometric techniques have acquired more and more importance in laboratory tests and can be proposed as alternative or complementary means to classical techniques. On the basis of previous calibration tests, in the paper the authors describe the application of ESPI method to the determination of the crack tip position and of the strain field surrounding the crack that propagates in a specimen of clay material.
Many engineering applications of interferometry, such as fracture mechanics and stress analysis, require the extraction of the first derivative of some measured function g(x,y) which is in turn represented by a 2 dimensional, digitized fringe pattern I(x,y). The conventional approach is to obtain an intermediate function (phi) (x,y) which represents the modulo 2(pi) interferometric phase change related to some change in optical path. It is first necessary to `unwrap' this phase information to obtain a smooth and continuous function, which may then be numerically differentiated to obtain the desired quantity of g(x,y). This analysis process, while reasonably straightforward, may be computationally expensive and in any case sensitive to noise, bad data, and quantization error. In this paper, we investigate the use of gradient operators for the direct extraction of the derivative of the interferometric phase function. Our goals are to be able to apply local gradient operators directly to the wrapped phase data, thus removing the `unwrapping' part of the process, and to reduce the susceptibility to noise of such operators.
Optical interferometric techniques are very fascinating tools for a full-field quantitative evaluation of displacements. While in some applications this information is sufficient, in many cases it is necessary to determine stresses that arise in the inspected object. These quantities are closely related to deformation and can be obtained from the partial derivatives of displacement. In this paper maps of displacement derivatives are retrieved by a direct manipulation of interferograms obtained with the phase-shifting method. Three main advantages are thus achieved: no need of prior displacement evaluation, no need of unwrapping procedures, short processing time.
Holographic and Speckle Interferometry provide full-field inspection of surfaces with high accuracy: these techniques offer some useful advantages (e.g. speed, high-precision,...) over other optical nonintrusive inspection techniques. Although the performance and precision of Holographic and Speckle Interferometry are affected by ambient conditions (temperature gradients, air turbulences, ...), the development of special experimental methods allows the use of these techniques directly in-field. An application of these interferometric methods can be foreseen, for example, in traditional testing laboratories or for a direct industrial production control. A great effort is currently in progress at ENEA-Frascati and at JRC-Ispra in order to develop new methods and computerized optoelectronic systems aiming at the application of Holographic and Speckle Interferometry to structural testing. Our most recent experimental results using time-average holographic interferometry and pulsed holographic interferometry for dynamic displacement measurements are presented. A theoretical background of the time-average ESPI procedure is described and some experimental results are shown.
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