Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1998-10-26
2001-09-18
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C250S227180, C356S340000
Reexamination Certificate
active
06289740
ABSTRACT:
BACKGROUND OF THE INVENTION
1.0 Field of the Invention
The present invention relates to a system, and a method of operation thereof, for measuring strain to which a structure having predetermined dimensions is subjected and, more particularly, to a system, and a method of operation thereof, for measuring strain using a technique for detecting the optical path difference (OPD) between first and second arms respectively of a sensing interferometer using a reading interferometer that is capable of being interrogated by changing the wavelength of the addressing light so as to change its optical path length. The reading interferometer can also be used for a variety of other low-coherence applications, in which the “sensing interferometer” can be used to measure strain, temperature or even be non fiber-optic, such as the cornea of a human eye. In the case of the human cornea, the reading interferometer with the wavelength based interrogation approach, described in this invention, can be used to measure the shape of the lens.
2.0 Description of the Prior Art
Strain gauges that detect the strain that stress produces in a body or structure are known. Strain gauges may consist of one or more fiber optic cables mated to the surface of the structure under test. In such arrangements, as the surface becomes strained, the optic fiber cable stretches, undergoing a change in length that is proportional to the change in strain. One such strain gauge system may be a long gauge sensor system which is attractive because its sensors can measure the average strain over long structures and which find usage in structural monitoring and damage assessment related to the effects of strain. Long gauge sensors have been proposed using low coherent differential interferometry techniques to sense strain over several meters of optical fiber. Two such proposals are described in the following two technical articles, one of Fan, N. Y., S. Huang and R. M. Measures, entitled “Localized Long Gauge Fiber Optic Strain Sensors,” published in
Smart Materials and Structures
, April, 1998, and the other of Inaudi, D., A. Elamari, L. Pflug, N. Gisin, J. Breguet, S. Vurpillot, entitled “Low-Coherence Deformation Sensors for the Monitoring of Civil-Engineering Structures,” published in
Sensors and Actuators A
, 1994, 44, pp. 125-130, and both of which technical articles are herein incorporated by reference. A more general reference on low coherence techniques is: “Recent Program in Fiber Optic Low Coherence Interferometer” of Y. J. Rae and O. A. Jackson published in
Meas. Sci. Techn
. (1998) pp. 981-999, and herein incorporated by reference.
The primary sensors described in the first two technical articles above require the use of mechanical activation to modify the path length of the interrogating interferometer, either by stretching the optical fiber, as described in the technical article of N. Y. Fan et al, or by moving a mirror, as described in the technical article of D. Inaudi et al. The technical article of N. Y. Fan et al also reports a technique which eliminates the need for mechanical activation based on the use of a tuneable laser and a fixed Fabry-Perot cavity. Although the proposal of N. Y. Fan et al to eliminate the mechanical activation is worthwhile, it is further desired that a system, and a method of operation thereof, be provided that not only eliminates mechanical activation of a sensor, but also improves the speed of response for detecting and decoding the sensed strain.
OBJECTS OF THE INVENTION
It is a primary object of the present invention to provide for a system, and a method of operation, that measures the strain to which a structure is being subjected without the need of any mechanical activation of any sensors, while at the same time providing a relatively rapid speed of response thereof.
It is another object of the present invention to provide for a system that measures the sensed strain and does so without the need for analyzing any unnecessary portions of the detected signal.
It is a further object of the present invention to provide for a system, and a method of operation thereof, that uses the technique for balancing the optical path difference (OPD) of a sensing interferometer using an interrogating interferometer, the OPD of which can be changed by changing the center wavelength of the addressing light.
SUMMARY OF THE INVENTION
Broadly, the invention concerns an apparatus and method employing a pair of unbalanced interferometers, each of which is driven by a tunable light source. One interferometer, which acts as a reference (and is also called a balancing or interrogating interferometer), has in one arm a broadband reflector, preferably a chirped fiber Bragg grating. This sort of grating is one in which the spacing of the gratings changes progressively along the optical path through the grating. This causes the center wavelength at which the Bragg grating reflects to change correspondingly along the grating's optical path (penetration depth), and thus different wavelengths of light will reflect from different places in the grating. The other interferometer, called a “sensing” interferometer, has one arm (or both arms) fixed to a body subjected to strain so that the optical fiber constituting that arm distends along with the body. Light from the tunable source enters the sensing interferometer and then the reading interferometer. Each of the interferometers, taken individually, has an optical path difference larger than the coherence length of the tunable source and, consequently, would not cause interference fringes to be produced at the output of either interferometer. However, when the two interferometers are combined in series (stacked one after another), and are such that the OPD of the reading interferometer is equal to that of the sensing interferometer, interference fringes are produced. This coherence matching condition is obtained by the proper selection of the wavelength of the tunable source, assuming that the lengths of the arms of the reading interferometer are designed such that this matching condition is possible with the available tuning range, determined by the length of the Bragg grating. Consequently, a change in the OPD of the sensing interferometer (such as would be due to stress in the body to which it is attached) can be tracked by changing the wavelength of the tunable source so as to keep maximum fringe visibility, which corresponds to matched OPDs between the interferometers. Under such conditions, change in wavelength corresponds to change in strain in the sensing interferometer. Thus, strain is tracked by following the center wavelength of the source that produces maximum fringe visibility.
More specifically, the invention is primarily directed to a long gauge fiber sensor system for sensing strain.
The system measures strain to which a defined structure having predetermined dimensions is subjected and comprises first and second interferometers, a broadband optical source, a tuneable bandpass filter, detectors and means for coherence detection.
The first interferometer has a first optical path difference. The first interferometer has an input and an output at the beginning and end respectively of the first optical path difference. The first interferometer further comprises an optical fiber cable arranged with the defined structure so that the length of the optical fiber changes in correspondence with any changes in the predetermined dimensions of the defined structure due to the structure being subjected to strain. The changes in the length of the optical fiber directly corresponds to changes in the length of the first optical path difference.
The second interferometer has a second optical path difference which is capable of being interrogated by bandpassed filtered light having a wavelength with the changes in the center wavelength of the bandpassed filtered light corresponding to changes in the length of the second optical path difference. The second interferometer has an input and an output at the beginning and end respectively of the second optical
Davis Michael A.
Kersey Alan D.
LeBlanc Michel J.
Posey, Jr. Ralph
Ferrett Sally A.
Karasek John J.
Noori Max
The United States of America as represented by the Secretary of
LandOfFree
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