U.S. patent application number 11/945418 was filed with the patent office on 2008-04-10 for dynamic optical waveguide sensor.
Invention is credited to Richard T. Jones.
Application Number | 20080085073 11/945418 |
Document ID | / |
Family ID | 36241425 |
Filed Date | 2008-04-10 |
United States Patent
Application |
20080085073 |
Kind Code |
A1 |
Jones; Richard T. |
April 10, 2008 |
DYNAMIC OPTICAL WAVEGUIDE SENSOR
Abstract
Methods and apparatuses that sense physical parameters, such as
pressure and strain, using optical waveguide sensors are described.
A light source emits light at a predetermined wavelength along an
optical waveguide having a fiber Bragg grating optical sensing
element. That sensing element reflects light in accord with a
sloped -shape function of reflected light amplitude verses
wavelength. A receiver converts the reflected light into electrical
signals and an analyzer then determines a physical parameter based
on changes of amplitude of the reflected light. The analyzer also
maintains the wavelength of the light such that the wavelength
corresponds to a slope wavelength of the shape function.
Inventors: |
Jones; Richard T.; (Hamden,
CT) |
Correspondence
Address: |
William B. Patterson;MOSER, PATTERSON & SHERIDAN, L.L.P.
Suite 1500
3040 Post Oak Blvd.
Houston
TX
77506
US
|
Family ID: |
36241425 |
Appl. No.: |
11/945418 |
Filed: |
November 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11076706 |
Mar 10, 2005 |
7302123 |
|
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11945418 |
Nov 27, 2007 |
|
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Current U.S.
Class: |
385/12 |
Current CPC
Class: |
G01D 5/35303 20130101;
G01L 1/246 20130101 |
Class at
Publication: |
385/012 |
International
Class: |
G02B 6/00 20060101
G02B006/00 |
Claims
1. An optical sensor comprising: an optical waveguide with a first
fiber Bragg grating having a first Bragg wavelength, a second fiber
Bragg grating having a second Bragg wavelength, and a long period
grating disposed between said first fiber Bragg grating and said
second fiber Bragg grating; a light source for emitting light at
said first Bragg wavelength and at said second Bragg wavelength; a
first receiver for converting reflected light at said first Bragg
wavelength into first electrical signals; a second receiver for
converting reflected light at said second Bragg wavelength into
second electrical signals; a coupler for coupling said light into
said optical waveguide, for coupling reflected light at said first
Bragg wavelength to said first receiver, and for coupling reflected
light at said second Bragg wavelength to said second receiver; and
an analyzer for receiving said first and said second electrical
signals and for using said first and said second electrical signals
to determine a physical parameter applied to said long period
grating.
2. The optical sensor of claim 1 wherein said physical parameter
changes an amplitude of reflected light at said second Bragg
wavelength.
3. The optical sensor of claim 2 wherein said physical parameter is
stress.
4. The optical sensor of claim 2 wherein said physical parameter is
strain.
5. The optical sensor of claim 2 wherein said physical parameter is
pressure.
6. The optical sensor of claim 1, further including a filter
disposed between said coupler and said first receiver.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/076,706, filed Mar. 10, 2005, now U.S. Pat. No.
7,302,123, which is herein incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the present invention generally relate to
optical waveguide sensors, and more particularly to a fiber Bragg
grating optical waveguide sensors that dynamically senses strain
induced by a stimuli acting upon a transduction mechanism.
Description of the Related Art
[0004] A fiber Bragg grating (FBG) is an optical element that is
formed by a photo-induced periodic modulation of the refractive
index of an optical waveguide's core. An FBG element is highly
reflective to light having wavelengths within a narrow bandwidth
that is centered at a wavelength that is referred to as the Bragg
wavelength. Other wavelengths pass through the FBG without
reflection. The Bragg wavelength itself is dependent on physical
parameters, such as temperature and strain, that impact on the
refractive index. Therefore, FBG elements can be used as sensors to
measure such parameters. After proper calibration, the Bragg
wavelength acts is an absolute measure of the physical
parameters.
[0005] One way of using fiber Bragg grating elements as sensors is
to apply strain from an elastic structure (e.g., a diaphragm,
bellows, etc.) to a fiber Bragg grating element. For example, U.S.
Pat. No. 6,016,702, issued Jan. 25, 2000, entitled "High
Sensitivity Fiber Optic Pressure Sensor for Use in Harsh
Environments" by inventor Robert J. Maron discloses an optical
waveguide sensor in which a compressible bellows is attached to an
optical waveguide at one location while a rigid structure is
attached at another. A fiber Bragg grating (FBG) is embedded within
the optical waveguide between the compressible bellows and the
rigid structure. When an external pressure change compresses the
bellows the tension on the fiber Bragg grating is changed, which
changes the Bragg wavelength.
[0006] Another example of using fiber Bragg grating elements as
pressure sensors is presented in U.S. Pat. No. 6,422,084, issued
Jul. 23, 2002, entitled "Bragg Grating Pressure Sensor" by Fernald,
et al. That patent discloses optical waveguide sensors in which
external pressure longitudinally compresses an optical waveguide
having one or more fiber Bragg grating. The optical waveguide can
be formed into a "dog bone" shape that includes a fiber Bragg
grating and that can be formed under tension or compression to
tailor the pressure sensing characteristics of the fiber Bragg
grating. Another fiber Bragg grating outside of the narrow portion
of the dog bone can provide for temperature compensation.
[0007] While the foregoing pressure sensing techniques are
beneficial, those techniques may not be suitable for all
applications. Therefore, fiber Bragg grating techniques suitable
for dynamically sensing varying parameters such as pressure and
strain would be useful. Also useful would be fiber Bragg grating
techniques that provide for both static and dynamic measurements of
parameters.
SUMMARY OF THE INVENTION
[0008] Embodiment of the present invention generally provides for
optical waveguide measurement techniques that are suitable for
sensing dynamically varying physical parameters such as pressure
and strain. Furthermore, embodiments of the present invention also
provide for both static and dynamic measurements of physical
parameters.
[0009] The foregoing and other objects, features, and advantages of
the present invention will become more apparent in light of the
following detailed description of exemplary embodiments
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] So that the manner in which the above recited features of
the present invention can be understood in detail, more particular
descriptions of the invention, briefly summarized above, may be had
by reference to embodiments, some of which are illustrated in the
appended drawings. It is to be noted, however, that the appended
drawings illustrate only typical embodiments of this invention and
are therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
[0011] FIG. 1 illustrates an optical waveguide sensor having a
sequence of sensors disposed along the optical waveguide;
[0012] FIG. 2 illustrates a dog bone pressure sensor having both a
fiber Bragg grating pressure sensor and a fiber Bragg grating
temperature sensor;
[0013] FIG. 3 illustrates a swept frequency optical waveguide
measurement system that can be used for both dynamic and static
measurements;
[0014] FIG. 4 schematically illustrates parking a narrow line width
laser on the slope of a fiber Bragg grating; and
[0015] FIG. 5 schematically illustrates an optical waveguide AC
strain measurement system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0016] The present invention provides for optical waveguide
measurement systems that are suitable for sensing dynamically
varying physical parameters such as pressure and strain. Some
embodiments of the present invention enable both static and dynamic
measurements of physical parameters. Embodiments of the present
invention are suitable for use in harsh environments as found in
oil and/or gas wells, engines, combustion chambers, etc.
[0017] FIG. 1 illustrates an optical waveguide sensor system 100
having a sequence of sensors 102 disposed along an optical
waveguide 104. Each sensor 102 includes at least one fiber Bragg
grating 106. Depending on the application and the specific
configuration, the sensor system 100 can be operated in various
ways. For example, a tunable light source 108, such as a tunable
laser or a broadband light source mated with a tunable filter, can
inject light that is swept over a bandwidth into a coupler 110. The
coupler 110 passes the light onto the optical waveguide 104.
Reflections at the Bragg wavelengths of the various fiber Brag
gratings 106 occur. The coupler 110 passes those reflections into a
receiver 112. The fiber Bragg gratings 106 are disposed such that
the Bragg wavelengths depend on a physical parameter of interest.
The output of the receiver 112 is passed to an analyzer 114 that
determines from the Bragg wavelengths a measurement of the physical
parameter of interest sensed by the sensors 102. Alternatively, if
each sensor in a string has a different wavelength, then a
broadband light source without a tunable filter can be used as a
signal can still be received from each sensor at the receiver
112.
[0018] FIG. 2 illustrates an exemplary sensor 102 that is suitable
for measuring parameters such as pressure and strain. The optical
waveguide 104 includes a narrow core 202 that passes through a
relatively thick cladding layer 204. That cladding layer is thinned
around the fiber Bragg grating 106 to form a narrow section that
includes the fiber Bragg grating 106. Around the narrow section is
a shell 206 that is integrally mated with the cladding layer 204.
To adjust the characteristics of the resulting sensor 102, when the
shell 206 is mated with the cladding layer 204 the optical
waveguide 104 could be under tension, under a slight compression (a
large compression would tend to buckle the narrow section), or,
more typically, unbiased. The result is a fiber Bragg grating
having a particular Bragg wavelength. When external pressure or
strain is applied to the shell 206, longitudinal tension or
compression occurs and the Bragg wavelength changes. A second fiber
Bragg grating 212 outside of the narrow section can be included to
provide a reference inside of the shell 206 for temperature
compensation.
[0019] FIG. 3 illustrates a tunable laser method of using optical
sensors 102 to provide dynamic (AC) measurements. In that method, a
tunable laser 302 produces a narrow line width laser pulse 304 that
is coupled by a coupler 110 into an optical waveguide 104 having at
least one optical sensor 102. The wavelength of the narrow line
width laser pulse 304 is swept through a wavelength band that
includes the Bragg wavelength of the fiber Bragg grating 106 in the
optical sensor 102. The shape function 306 of the fiber Bragg
grating 106, that is, its amplitude (Y-axis) verses wavelength
(X-axis) characteristics, is determined by a high frequency
receiver 112 and an analyzer 114. Referring now to FIG. 4, a
particular power level, say the 3 dB point down from the peak 402,
is selected by the analyzer. Then, the analyzer sets the wavelength
of the tunable laser 302 to the wavelength 404 that corresponds to
the selected power level. Thus, the wavelength of the tunable laser
302 is set at a specific wavelength that is on the shape function
306. Then the intensity of the reflected light is monitored.
Variations in the intensity correspond to dynamic pressure changes
impressed on the optical sensor 102. The high frequency receiver
112 and the analyzer 114 can provide wavelength and amplitude
information from the variations in intensity.
[0020] The foregoing method illustrated with the assistance of
FIGS. 3 and 4 can also provide static pressure measurements. Since
the position of the shape function 306 with respect to wavelength
(shown in X-axis) depends on static pressure, the analyzer 114 can
determine static pressure based on the wavelength position 409 of
the peak 410 fiber Bragg grating reflection. It should be
understood that while FIGS. 3 and 4 only illustrate one optical
sensor 102 the optical waveguide 104 could have numerous optical
sensors 102. PATENT
[0021] In addition to providing dynamic pressure measurements, the
principles of the present invention also provide for determining
dynamic (AC) strain. One technique of doing this is illustrated in
FIG. 5. As shown, a light source 500 launches light into port 1 of
a 4 port circulator 502. That light is emitted from port 2 of the
circulator 502 into an optical waveguide 104. That waveguide
includes a sensor 503 that is comprised of two fiber Bragg
gratings, 504 and 506. The gratings 504 and 506, which have
different Bragg wavelengths .lamda.1 and .lamda.2, respectively,
are separated by a long period grating 508 that is in a strain
sensing field. When the light reaches gratings 504 and 506 those
gratings reflect the Bragg wavelengths .lamda.1 and .lamda.2,
respectively. However, there is a strain induced loss within the
long period grating 508. Since .lamda.1 is reflected by grating 504
it signal is not attenuated by the long period grating 508, and
thus the power of wavelength .lamda.1 can act as a reference power.
However, the power of .lamda.2 depends on the loss within the long
period grating 508, which in turn depends on the applied strain.
Thus the ratio of the powers of .lamda.1 and .lamda.2 is a measure
of strain on the long period grating. The long period grating 508
can also be disposed to measure strain due to applied pressure or
some other stimuli.
[0022] Still referring to FIG. 5, the reflected light .lamda.1 and
.lamda.2 on the optical waveguide 104 enters the circulator 502.
Wavelength .lamda.2 passes through a wavelength filter 510, but
wavelength .lamda.1 is reflected. The passed wavelength .lamda.2 is
received and amplified by a first receiver 514. The output of
receiver 514 is passed to an analyzer 516. Meanwhile, .lamda.1 is
output from port 4 of the circulator 502. The wavelength .lamda.1
is received and amplified by a second receiver 518. The output of
the second receiver 518 is applied to the analyzer 516. The
analyzer 516 compares the ratio of the reflected wavelengths and
determines the dynamic (AC) strain applied to the long period
grating 508.
[0023] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
* * * * *