Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2000-01-11
2001-01-09
Lee, John D. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C250S227280, C356S033000, C356S035500, C356S329000, C356S329000
Reexamination Certificate
active
06173091
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical strain gauges and more particularly to a fiber optic Fabry-Perot sensor specifically configured to measure both the magnitude and direction of strain applied thereto.
BACKGROUND OF THE INVENTION
Strain gauges for measuring strain in various different structures and materials are well known. Such strain gauges typically utilize various different types of transducers in which a change in resistance or capacitance is indicative of a corresponding change in strain.
Although such electrical strain gauges are generally suitable for measuring strain, those skilled in the art will appreciate that such electrical transducers are not suitable for use in some particular applications. For example, it is generally not desirable to utilize electrical transducers in explosive environments, where it is possible that an electrical spark may initiate an undesirable explosive reaction. Further, in some applications the presence of electricity may undesirably interfere with sensitive electronic equipment and the like. Further, in some applications ambient electrical fields may undesirably effect the performance of such electrical transducers. Further, the electricity associated with such electrical transducers may cause the undesirable generation of heat.
Optical strain sensors are known for eliminating such undesirable characteristics of electrical strain gauges. Such optical strain sensors typically comprise Fabry-Perot interferometers wherein the cavity thereof is disposed along the length of an optical fiber which may either be attached to or embedded within a material or structure for which strain measurement is desired.
However, one problem commonly associated with the use of simple Fabry-Perot optical strain sensors is that no indication of the direction of the strain, i.e., compressive or tensile, is indicated thereby. Contemporary simple Fabry-Perot optical strain sensors provide only an indication of the magnitude of the strain applied thereto and do not provide any indication of absolute strain. As used herein, the term absolute strain indicates a strain measurement with which a direction is associated. Thus, a measurement of absolute strain provides both the magnitude of the strain and an indication as to whether the strain is compressive or tensile in nature.
In an effort to provide a measurement of absolute strain, various different prior art devices have been developed. Such prior art devices utilized dual Fabry-Perot interferometers wherein the signal output of each of the interferometers are in quadrature with one another. Thus, an indication of whether the etalon is decreasing or increasing in length is provided as the Fabry-Perot interferometer experiences either compression or tension. One example of such a prior art dual interferometer strain sensor is provided in U.S. Pat. No. 5,301,001, issued on Apr. 5, 1994 to Murphy et al and entitled EXTRINSIC FIBER OPTIC DISPLACEMENT SENSORS AND DISPLACEMENT SENSING SYSTEMS.
However, as those skilled in the art will appreciate, prior art devices which facilitate the measurement of absolute strain are comparatively complex. Two separate fiber optic signal cables are required. Additionally, two separate optical sensors and their related electronics must also be utilized. The complexity of such devices inherently reduces their reliability and also makes them more difficult to use. This is particularly true since two separate optical fibers must be imbedded, mounted, and/or routed. Such prior art dual interferometer strain sensors are also inherently more expensive, due to the increased number of components thereof.
In view of the foregoing, it is desirable to provide a fiber optic strain sensor which measures both the magnitude and direction of strain applied thereto and which is simple in construction, so as to enhance the reliability and ease of use thereof, while also reducing the cost thereof.
SUMMARY OF THE INVENTION
The present invention addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method and apparatus for measuring absolute strain. The fiber optic strain sensor of the present invention utilizes a single Fabry-Perot interferometer to provide a measurement of absolute strain. Thus the present invention does not require the use of plural optical fibers, and consequently is less expensive, more reliable, and easier to install and use than contemporary devices.
The method of the present invention comprises the steps of providing a coherent beam of light, separating the coherent beam of light into first and second beams having different polarization angles, combining the first and second beams in a manner which maintains the differing polarizations thereof so as to form a combined beam, applying the combined beam to a Fabry-Perot strain sensor to form a reflected combined beam, and then separating the reflected combined beam into first and second reflected beams having different polarization angles. The intensity of each of the first and second reflected beams is sensed and a determination of the change in etalon length of the Fabry-Perot strain sensor is thus determined from the sensed intensities. Thus, according to the present invention, the change in etalon length is indicative of absolute strain.
The coherent beam of light is preferably provided via a laser, preferably a laser providing a polarized light output. If the output of the laser is not adequately polarized, then the coherent beam of light is polarized prior to being separated into first and second beams.
The coherent beam of light is separated into first and second beams having different polarization angles by separating the coherent beam of light into first and second beams and then rotating the polarization angle of the second beam relative to the first beam. The phase of one of the beams is delayed by 90°, so as to place the two beams in quadrature with one another.
The first and second beams are then combined into a single beam in a manner which maintains the respective polarizations thereof. Thus, the combined beam comprises two orthogonally polarized light beams which are in quadrature with one another.
The combined beam is applied to the Fabry-Perot strain sensor via a polarization maintaining fiber, so as to prevent undesirable interaction of the first and second orthogonally polarized beams thereof.
The first beam of the combined beams is polarized along either the fast or slow axis of the polarization maintaining fiber and the second beam is polarized along the other axis thereof. In this manner, two separate coherent laser beams are effectively provided so as to facilitate the measurement of absolute strain with the present invention.
The first and second reflected beams are in quadrature, i.e., 90 degrees out of phase with respect to one another, regardless of the length of the etalon of the Fabry-Perot interferometer. As those skilled in the art will appreciate, it is possible to determine the direction of strain, i.e., compression or tensile, from the two return signals from the Fabry-Perot interferometer. A detailed explanation of the methodology utilized to determine the direction of strain is provided in U.S. Pat. No. 5,301,001, issued on Apr. 5, 1994 to Murphy et al, the contents of which are hereby incorporated by reference.
These, as well as other advantages of the present invention, will be more apparent from the following descriptions and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.
REFERENCES:
patent: 4773753 (1988-09-01), Hirose et al.
patent: 4777358 (1988-10-01), Nelson
patent: 4840481 (1989-06-01), Spillman, Jr.
patent: 4842403 (1989-06-01), Tarbox et al.
patent: 4928004 (1990-05-01), Zimmermann et al.
patent: 4958929 (1990-09-01), Kondo
patent: 5094527 (1992-03-01), Martin
patent: 5187983 (1993-02-01), Bock et al.
patent: 5301001 (1994-04-01), Murp
Anderson Terry J.
Hoch, Jr. Karl J.
Lee John D.
Northrop Grumman Corporation
LandOfFree
Fiber optic Fabry-Perot sensor for measuring absolute strain does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fiber optic Fabry-Perot sensor for measuring absolute strain, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fiber optic Fabry-Perot sensor for measuring absolute strain will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2461689