U.S. patent application number 10/952934 was filed with the patent office on 2005-03-03 for fiber optic sensing instrument and system with fiber of adjustable optical path length and method of using it.
Invention is credited to Cherpillod, Thierry, Manuelpillai, Gerald, Morison, W. Don, Rouse, Dominic, Tennyson, Roderick C..
Application Number | 20050046861 10/952934 |
Document ID | / |
Family ID | 25533256 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050046861 |
Kind Code |
A1 |
Morison, W. Don ; et
al. |
March 3, 2005 |
Fiber optic sensing instrument and system with fiber of adjustable
optical path length and method of using it
Abstract
An interoferometric fiber optic sensing system uses three
optical fibers. A sensing optical fiber is applied to a structure
to be monitored to detect displacement or the like by changing its
optical path length. A reference optical fiber has a fixed optical
path length. An adjustable length optical fiber is controllably
adjusted in its optical path length. The three optical fibers form
optical paths whose light outputs are caused to interfere. The
adjustable length optical fiber is adjusted until an interference
fringe appears. The quantity to be detected is derived from the
maximum of the interference fringe. Several sensing optical fibers
can be multiplexed; by staggering their optical path lengths, their
interference fringes can be separated sufficiently to resolve
them.
Inventors: |
Morison, W. Don;
(Mississauga, CA) ; Manuelpillai, Gerald; (Ajax,
CA) ; Tennyson, Roderick C.; (Toronto, CA) ;
Cherpillod, Thierry; (Maple, CA) ; Rouse,
Dominic; (Toronto, CA) |
Correspondence
Address: |
BLANK ROME LLP
600 NEW HAMPSHIRE AVENUE, N.W.
WASHINGTON
DC
20037
US
|
Family ID: |
25533256 |
Appl. No.: |
10/952934 |
Filed: |
September 30, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10952934 |
Sep 30, 2004 |
|
|
|
09987437 |
Nov 14, 2001 |
|
|
|
6804008 |
|
|
|
|
Current U.S.
Class: |
356/479 |
Current CPC
Class: |
G01D 5/35303
20130101 |
Class at
Publication: |
356/479 |
International
Class: |
G01B 009/02 |
Claims
1. A method for sensing a physical condition, the method
comprising: (a) generating light; (b) passing a first portion of
the light through a first optical path comprising a first optical
fiber, the first optical fiber being characterized by a first
optical path length which changes in response to the physical
condition; (c) passing a second portion of the light through a
second optical path comprising a second optical fiber, the second
optical fiber being characterized by a second optical path length
which changes in a controllable manner; (d) causing the first
portion of the light which has passed through the first optical
path to interfere with the second portion of the light which has
passed through the second optical path; (e) changing the second
optical path length within the second optical fiber until step (d)
results in an interference fringe having a maximum; and (f)
determining the physical condition in accordance with a value of
the second optical path length corresponding to the maximum of the
interference fringe.
2. The method of claim 1, wherein the second optical path length
does not change in response to the physical condition.
3. The method of claim 1, wherein the light has a coherence length,
and wherein the second optical path length has a maximum amount of
change which is greater than the coherence length.
4. The method of claim 1, wherein step (e) comprises changing a
physical length of the second optical fiber.
5. The method of claim 4, wherein the physical length of the second
optical fiber is changed through stretching the second optical
fiber.
6. (Canceled)
7. The method of claim 4, wherein the physical length of the second
optical fiber is changed through compressing the second optical
fiber.
8. The method of claim 1, wherein the first optical path further
comprises a third optical fiber having a third optical path length
which is constant.
9. The method of claim 8, wherein the second optical path further
comprises the first optical fiber.
10. The method of claim 9, wherein steps (b) and (c) are performed
by: (i) passing the first and second portions of the light through
the first optical fiber; (ii) passing the first portion of the
light through the third optical fiber; and (iii) passing the second
portion of the light through the second optical fiber.
11. The method of claim 10, wherein: step (i) comprises coupling
the light generated in step (a) into the first optical fiber
through a first coupler, reflecting the light back through the
first optical fiber and the first coupler, and coupling the light
from the first coupler into a second coupler to divide the light
into the first and second portions; step (ii) comprises coupling
the first portion of the light from the second coupler into the
third optical fiber; and step (iii) comprises coupling the second
portion of the light from the second coupler into the second
optical fiber.
12. The method of claim 11, wherein step (d) comprises reflecting
the first portion of the light back through the third optical
fiber, reflecting the second portion of the light back through the
second optical fiber, and recombining the first and second portions
of the light in the second coupler.
13. The method of claim 12, wherein: each of the second and third
optical fibers comprises a mirror for reflecting the light; and the
first optical fiber comprises a partial mirror and a mirror which
define the first optical path length between them.
14. The method of claim 12, wherein the interference fringe has the
maximum when the first optical path length equals a difference
between the second and third optical path lengths.
15. The method of claim 1, wherein the first optical fiber is
bonded to a structure in which the physical condition is to be
measured.
16. The method of claim 1, wherein the first optical fiber is
embedded in a structure in which the physical condition is to be
measured.
17. The method of claim 1, wherein the first optical fiber is
attached only at two end points of the first optical fiber to a
structure in which the physical condition is to be measured.
18. The method of claim 17, wherein the physical condition
comprises a displacement between the two end points.
19. The method of claim 1, wherein the physical condition comprises
displacement.
20. The method of claim 1, wherein the physical condition causes an
elongation of the first optical fiber relative to an initial state
of the first optical fiber.
21. The method of claim 1, wherein the physical condition causes a
contraction of the first optical fiber relative to an initial state
of the first optical fiber.
22. The method of claim 21, wherein the initial state is a
pre-tensioned state.
23. The method of claim 1, wherein: step (b) comprises passing at
least one further portion of the light through at least one further
optical path comprising at least one further optical fiber, the at
least one further optical fiber being characterized by at least one
further optical path length which changes in response to the
physical condition; step (d) comprises causing the at least one
further portion of the light which has passed through the at least
one further optical path to interfere with the second portion of
the light which has passed through the second optical path; step
(e) comprises changing the second optical path length until step
(d) results in a plurality of interference fringes, each having a
local maximum; and step (f) comprises determining the physical
condition experienced by the first optical fiber and each of the at
least one further optical fiber in accordance with a value of the
second optical path length corresponding to the local maximum of
each of the plurality of interference fringes.
24. The method of claim 1, wherein each of the first and second
optical fibers is a single-mode optical fiber.
25. A sensing system for sensing a physical condition, the sensing
system comprising: a source of light; a first optical path
comprising a first optical fiber, the first optical fiber being
characterized by a first optical path length which changes in
response to the physical condition; a second optical path
comprising a second optical fiber, the second optical fiber being
characterized by a second optical path length which changes in a
controllable manner; at least one coupler for causing first and
second portions of the light from the source to pass through the
first and second optical paths and for causing the first portion of
the light which has passed through the first optical path to
interfere with the second portion of the light which has passed
through the second optical path; a photodetector for detecting an
interference fringe between the first and second portions of the
light and for outputting a signal representing the interference
fringe; an actuator for changing the second optical path length
within the second optical fiber until the interference fringe has a
maximum; and a system, receiving the signal from the photodetector,
for permitting a determination of the physical condition in
accordance with a value of the second optical path length
corresponding to the maximum of the interference fringe.
26. The sensing system of claim 25, wherein the second optical path
length does not change in response to the physical condition.
27. The sensing system of claim 25, wherein the light from the
source has a coherence length, and wherein the second optical path
length has a maximum amount of change which is greater than the
coherence length.
28. The sensing system of claim 25, wherein the actuator changes a
physical length of the second optical fiber.
29. The sensing system of claim 28, wherein the actuator changes
the physical length of the second optical fiber through stretching
the second optical fiber.
30. (Canceled)
31. The sensing system of claim 28, wherein the actuator changes
the physical length of the second optical fiber through compressing
the second optical fiber.
32. The sensing system of claim 25, wherein the first optical path
further comprises a third optical fiber having a third optical path
length which is constant.
33. The sensing system of claim 32, wherein the second optical path
further comprises the first optical fiber.
34. The sensing system of claim 33, wherein the at least en one
coupler passes the first and second portions of the light through
the first optical fiber, passes the first portion of the light
through the third optical fiber and passes the second portion of
the light through the second optical fiber.
35. The sensing system of claim 34, wherein: the at least one
coupler comprises a first coupler and a second coupler; the first
coupler couples the light from the source into the first optical
fiber; the first optical fiber comprises a mirror for reflecting
the light back through the first optical fiber and the first
coupler; the first coupler couples the light reflected back through
the first optical fiber into the second coupler to divide the light
into the first and second portions; and the second coupler couples
the first portion of the light into the third optical fiber and
couples the second portion of the light coupler into the second
optical fiber.
36. The sensing system of claim 35, wherein: each of the second and
third optical fibers comprises a mirror for reflecting light back
through said each of the second and third optical fibers; and the
second coupler recombines the first and second portions of the
light.
37. The sensing system of claim 36, wherein the first optical fiber
further comprises a partial mirror, and wherein the mirror and the
partial mirror of the first optical fiber define the first optical
path length between them.
38. The sensing system of claim 36, wherein the interference fringe
has the maximum when the first optical path length equals a
difference between the second and third optical path lengths.
39. The sensing system of claim 25, wherein the first optical fiber
is bonded to a structure in which the physical condition is to be
measured.
40. The sensing system of claim 25, wherein the first optical fiber
is embedded in a structure in which the physical condition is to be
measured.
41. The sensing system of claim 25, wherein the first optical fiber
is attached only at two end points of the first optical fiber to a
structure in which the physical condition is to be measured.
42. The sensing system of claim 41, wherein the physical condition
comprises a displacement between the two end points.
43. The sensing system of claim 25, wherein the physical condition
comprises displacement.
44. The sensing system of claim 25, wherein the physical condition
causes an elongation of the first optical fiber relative to an
initial state of the first optical fiber.
45. The sensing system of claim 25, wherein the physical condition
causes a contraction of the first optical fiber relative to an
initial state of the first optical fiber.
46. The sensing system of claim 45, wherein the initial state is a
pre-tensioned state.
47. The sensing system of claim 25, further comprising at least one
further optical fiber, the at least one further optical fiber being
characterized by at least one further optical path length which
changes in response to the physical condition.
48. The sensing system of claim 25, wherein each of the first and
second optical fibers is a single-mode optical fiber.
49. A sensing instrument for use with a sensor in sensing a
physical condition, the sensing instrument comprising: a source of
light; a reference optical fiber having a fixed optical path
length; an adjustable optical fiber having a controllably
adjustable optical path length; at least one optical coupler for
causing the light from the source to pass through the sensor,
receiving the light which has passed through the sensor, splitting
the light which has passed through the sensor to the reference
optical fiber and the adjustable optical fiber, and causing the
light which has passed through the reference optical fiber to
interfere with the light which has passed through the adjustable
optical fiber; a photodetector for detecting an interference fringe
between the light which has passed through the reference optical
fiber and the light which has passed through the adjustable optical
fiber; an actuator for changing the controllably adjustable optical
path length within the adjustable optical fiber until the
interference fringe has a maximum; and a system, receiving the
signal from the photodetector, for permitting a determination of
the physical condition in accordance with a value of the second
optical path length corresponding to the maximum of the
interference fringe.
50. The sensing instrument of claim 49, wherein the actuator
changes a physical length of the adjustable optical fiber.
51. The sensing instrument of claim 50, wherein the actuator
changes the physical length of the adjustable optical fiber through
stretching the adjustable optical fiber.
52. (Canceled)
53. The sensing instrument of claim 50, wherein the actuator
changes the physical length of the second optical fiber through
compressing the second optical fiber.
54. The sensing instrument of claim 49, wherein the light from the
source has a coherence length, and wherein the adjustable optical
path length has a maximum change which is greater than the
coherence length.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a fiber optic sensing
instrument for sensing deflections, displacements, or other
physical conditions and more particularly to such a sensing
instrument having a fiber of adjustable optical path length, and is
further directed to a system and method using such a sensing
instrument. The adjustable length can be used for spatial division
multiplexing, extending the range of displacements detectable by
the sensing instrument, or other purposes.
DESCRIPTION OF RELATED ART
[0002] The use of optical fibers to sense deflections,
displacements, temperatures and other physical conditions is well
known. Typically, such sensors operate by interferometry. An
interferometric fiber optic sensing instrument, in its simplest
form, operates by splitting light from a light source between two
fibers. The first fiber, a sensing fiber, is exposed to the
physical condition to be sensed, while the second fiber, a
reference fiber, is not. The light passing through the sensing and
reference fibers is recombined; if the difference between the
optical path lengths of the sensing and reference fibers is within
the coherence length of the light, an interference fringe dependent
on the phase difference between the light passing through the
sensing and reference fibers can be detected. The physical
condition to be sensed causes the sensing fiber to change its
optical path length, e.g., by changing its physical length or its
index of refraction. A change in the interference fringe allows the
computation of a change in the phase difference, which in turn
allows the computation of a change in the optical path length
experienced by the sensing fiber, which in turn allows the
computation of a quantity of the physical condition.
[0003] However, the simplest form has the following drawbacks.
First, while it can detect a quantity of the physical condition, it
is often relevant where along the sensing fiber the physical
condition occurs. For example, if a long fiber is used to sense
pressure or temperature over an extended area, the simplest form
cannot detect the location of the pressure or temperature in the
extended area. Second, the range of phase differences must fall
within 2.pi.; otherwise, the resulting phase ambiguity renders the
detection ambiguous or even meaningless.
[0004] To overcome the first drawback, various forms of
multiplexing are known. For example, U.S. Pat. No. 4,443,700 to
Macedo et al teaches an optical sensing apparatus with multiple
sensing fibers spaced along its length. Signals from the multiple
sensing fibers are distinguished by their time delays. However, it
is necessary to resolve such time delays on the order of a few
nanoseconds, thus complicating the device and requiring care in
selection of the optical fiber such that the pulse dispersion is
minimized.
[0005] To overcome the second drawback, U.S. Pat. No. 5,721,615 to
McBride et al teaches a fiber optic sensing instrument having a
sensor arm and a reference arm. The reference arm has a device
having a microscope stage for varying a path difference between the
sensor and reference arms. Alternatively, one of the arms can be
stretched by a clamp. An interferogram is generated when the path
lengths are equal. However, fairly complicated mathematics are used
to calculate strain and temperature from the group delay and
dispersion as determined from the interferogram.
[0006] Smartec SA of Manno, Switzerland, advertises a technology
for fiber optic interoferometric measurement known as SOFO. Two
optical fibers are installed in the pipe or other structure to be
monitored; the first is in mechanical contact with the structure to
expand or contract therewith and functions as a sensing fiber,
while the second is free and functions as a reference fiber. An
analyzer for use with such a sensor also has two optical fibers,
one of which has a movable mirror to adjust its optical path
length. A modulated signal is obtained only when the difference in
optical path lengths between the two fibers in the structure is
compensated by die difference in optical path lengths between the
two fibers in the analyzer to better than the coherence length of
the light source. However, the SOFO system introduces an
undesirable complexity in that two fibers must be present in the
structure to be monitored. Also, the analyzer of the SOFO system
cannot demultiplex and analyze signals from multiple sensors
without the use of an optical switch to select the signal from one
of the sensors.,
SUMMARY OF THE INVENTION
[0007] It will be readily apparent from the above that a need
exists in the art for a simple way to overcome the above-noted
problems with the prior art. It is therefore a primary object of
the invention to provide a fiber optic sensing instrument capable
of detecting a wide range of phase differences between the sensing
and reference fibers.
[0008] It is another object of the invention to provide a fiber
optic sensing instrument, system and method capable of detecting a
wide range of phase differences so as to detect the location of the
physical condition being sensed as well as its quantity.
[0009] It is another object of the invention to provide a fiber
optic sensing instrument, system and method capable of detecting a
wide range of phase differences so as to detect a wide range of
displacements or other changes in the optical path length in the
sensing fiber.
[0010] To achieve the above and other objects, the present
invention is directed to a fiber optic sensing system incorporating
a fiber having an adjustable optical path length. A sensing optical
fiber is applied to a structure to be monitored to detect
displacement or the like by changing its optical path length. A
reference optical fiber has a fixed optical path length. An
adjustable length optical fiber is controllably adjusted in its
optical path length. The three optical fibers form optical paths
whose light outputs are caused to interfere. The adjustable length
optical fiber is adjusted until an interference fringe appears. The
quantity to be detected is derived from the maximum of the
interference fringe. Several sensing optical fibers can be
multiplexed; by staggering their optical path lengths, their
interference fringes can be separated sufficiently to resolve
them.
[0011] One embodiment provides a fiber optic interferometric
sensing system having a sensing fiber of any arbitrary length used
to measure deflections, or displacements, using a mirrored optical
fiber (single mode or multimode). This system can include a single
optical fiber bonded to, or attached at discrete points to, or
imbedded in a structure, of any shape or configuration.
Alternatively, the fiber can be fixed at both ends, with no
continuous attachment to a structure.
[0012] In addition, many such fiber sensors, acting as individual
strain sensors, can be optically coupled to a single backbone
fiber, provided each sensor length is different (according to
criteria described later) to provide a spatial division
multiplexing capability.
[0013] These sensors measure displacement, from which an average
value of strain can be calculated by dividing the measured
displacement by the length of the sensor. These sensors can be of
any length, typically ranging from a few centimeters to many
meters. The combination of sensor lengths that can be incorporated
on the same backbone can also vary from very short lengths (ie,
several centimetres) to very long gages (e.g.; up to 100 meters for
example).
[0014] An optical source of short coherence length (such as a light
emitting diode) produces a broadband light beam that is split
between the optical fiber sensor, a passive reference optical
fiber, and an adjustable length optical fiber which can be actuated
by various means to extend or contract its length (assuming an
initial pre-tensioned state). Each optical fiber has mirrored ends
to reflect the incident light beams. The light from the source thus
travels two paths that are recombined at a photodetector.
[0015] Upon activating the fiber optic sensor (a single mode fiber
is preferred due to losses associated with multimode fibers) by
means of structural loading, or any means that leads to extension
or contraction of the sensor (such as by temperature changes from
the installed reference state, for example), the displacement
difference between this sensor and its passive reference sensor is
measured by adjusting the adjustable optical fiber until an
interference pattern is detected by a photodetector. The peak in
the interference pattern occurs when the two optical paths are
equal.
[0016] The adjustable length optical fiber can be adjusted by any
suitable technique, such as a motor drive with die fiber wrapped
around cylindrical pulleys for example, or a piezoelectric cylinder
having the fiber wrapped around its circumference. The length of
this adjustable fiber determines the maximum displacement it can
measure, as limited by its tensile breaking strength, i.e., the
maximum strain or displacement it can undergo as limited by its
strength in tension. The longer the optical fiber, the greater the
magnitude of the displacement for a given ultimate strain for the
fiber material. For example, the typical maximum displacement for a
single mode optical fiber of 3 meters length is 60 mm. The rate at
which the adjustable optical fiber can be stretched or contracted,
determines the system's capability to measure dynamic displacement
profiles.
[0017] The total displacement range of the adjustable optical fiber
allows multiple fiber optic sensors of different lengths to be
monitored by the same optical light source and passive reference
fiber, provided that the sum of the changes in length of all of the
sensors is less than the maximum deflection length of the
adjustable optical fiber. Spatial (ie; different length fiber optic
sensors optically coupled to a single backbone fiber transmitting
the light beam from the light source) division multiplexing can be
achieved by altering the lengths of the fiber optic sensors in
increments, the sum of which is less than or equal to the length of
the passive reference optical fiber minus the sum of the
predetermined allowable measured deflections associated with the
application of each of the sensors coupled to the optical backbone
fiber.
[0018] Application examples include, but are not limited to,
surface bonding the sensors to pipes, pressure vessels, bridge
structures of steel or concrete, or imbedding the sensors in
concrete or composites. In these embodiments, the sensors can
measure displacements in the form of elongation or contraction,
which can be converted to strains in tension or compression. It is
envisaged that to measure compression or contraction, the sensors
are bonded under a pretension load. This will be important for
measuring temperature fluctuations for example, which can be below
that of the installation temperature, thus leading to possible
thermal contraction of the substrate material, depending on its
thermal coefficient of expansion.
[0019] Other applications that do not require a continuous
attachment to a structure include using the pre-tensioned sensors
as deflection measuring sensors between two or more fixed points.
Spatial division multiplexing can also be used in this
configuration.
[0020] A very long gage mirrored fiber optic sensor (consisting of
a single mode or multimode fiber) capable of measuring average
displacements over any gage length, typically varying from about
one meter to over a hundred meters, can be implemented.
Applications of very long gage fiber optic displacement sensors
include bonding them to long pipeline sections to measure changes
in the pipe geometry due to such factors as pressure changes,
corrosion leading to wall thinning and radial expansion, cracks or
leaks leading to gas/fluid loss. Other applications as long gage
displacement measuring devices include monitoring the movement of
large structures such as dams, due to movement in the
earth/concrete foundation over large distances, vibration and creep
behaviour of bridges and buildings. These sensors can be used where
electrical and semiconductor based strain gages and vibrating wire
gages are too small in length to provide displacement information
over long distances, exceeding typically many meters for
example.
[0021] The invention further includes the instrument itself and the
method of using the system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] A preferred embodiment of the present invention will be
described in detail with reference to the drawings, in which:
[0023] FIGS. 1A and 1B show a schematic diagrams used for
explaining principles of operation of the preferred embodiment;
[0024] FIG. 2 shows a flow chart used for explaining the principles
of operation of the preferred embodiment;
[0025] FIGS. 3A-3C show various installations of the sensing
optical fiber of the preferred embodiment;
[0026] FIGS. 4A and 4B show two actuators for varying the optical
path length of the adjustable length optical fiber;
[0027] FIGS. 5A and 5B show two configurations of multiple sensing
fibers; and
[0028] FIGS. 6 and 7 show graphs of experimental data.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] A preferred embodiment of the present invention and
variations thereon will be described in detail with reference to
the drawings, in which like reference numerals refer to like
elements throughout.
[0030] FIG. 1A shows a schematic diagram of a sensor for use with
the instrument according to the preferred embodiment. The sensor 1
includes a lead fiber 3, preferably a single-mode optical fiber,
terminated by a lead ceramic ferrule 5 and a lead mirror 7 formed
as a partially mirrored surface on the end of the lead fiber 3. A
ceramic sleeve 9 is used to attach the ferrule 5 to another ceramic
ferrule 11, which is in turn attached to a sensor fiber 13, which
is also preferably a single-mode optical fiber. The sensor fiber 13
ends in a sensor mirror 15, which, like the lead mirror 7, is
formed as a partially mirrored surface on the end of the sensor
fiber 13. The sensor 1 is connected to an instrument 100 which
functions as both a light source and a demodulator.
[0031] The sensor 1 is installed such that a change in a physical
condition (e.g., deformation) of a structure affects the optical
path length of the sensor fiber 13, but not that of the lead fiber
3. Thus, light introduced from the instrument 100 into the sensor 1
takes one of two paths: one from the instrument 100 to the lead
mirror 7 and back, and the other from the instrument 100 to the
sensor mirror 15 and back. The two paths differ in their optical
path lengths by twice the optical path length of the sensor fiber
13. Thus, detection of a change in the optical path lengths
provides a determination of the change in optical path length of
the sensor fiber 13 and thus of the physical condition of the
structure. Accordingly, the optical path length of the sensor fiber
13 is the gage length of the sensor 1.
[0032] The instrument 100 will be explained in detail with
reference to the block diagram of FIG. 1B and the flow chart of
FIG. 2. In the instrument 100 of FIG. 1B, light exits a source
having a short coherence length, such as a light emitting diode
(LED) 102, and travels via an optical fiber 104 having an optical
path length A to a 50/50 coupler 106. The coupler 106 splits the
light in two, such that fifty percent of the light is directed
through an optical fiber 108 and ultimately discarded and the other
fifty percent is directed through the lead fiber 3 of the sensor
1.
[0033] The light travels along the lead fiber 3 and through an
optional connector 112 and encounters the lead mirror 7. The
optical path length up to the lead mirror 7 is B, which, as noted
above, is constant. The lead mirror 7 reflects a portion (e.g., 3%)
of the light back through the lead fiber 3 and transmits the
remainder of the light along the sensor fiber 13, where it
encounters the sensor mirror 15 separated from the lead mirror 7 by
an optical path length C, which, as noted above, defines the gage
length of the sensor 1. Thus, the light returning along the sensor
1 has two components whose optical path lengths are 2B and 2B+2C,
respectively; that is, they differ by 2C.
[0034] As noted above, the optical path length C varies in
accordance with a value of the physical condition. The goal is to
determine C and thus the quantity of the physical condition.
[0035] Both of those components of the light pass again through the
coupler 106. Fifty percent of the light travels toward the LED 102,
where it is lost. The other fifty percent travels along an optical
fiber 118 having an optical path length D to a second 50/50 coupler
120, which divides the light between an optical fiber 122 having an
optical path length E and an optical fiber 124 having an optical
path length F.
[0036] The optical fiber 122 is called the adjustable-length
optical fiber because the optical path length E of the optical
fiber 122 is controllable. In some embodiments, the optical path
length E is changed by changing the physical length of the fiber
122. For instance, the fiber 122 can be stretched between an anchor
126 and an actuator 128. Other techniques for changing the optical
path length E will be described below. The optical fiber 122 ends
with a mirror 130, so that the light passing through the fiber 122
travels by an optical path length 2E. The total variation in E is
greater than the coherence length of light from the light
source.
[0037] The optical fiber 124 is called die passive reference fiber
because its optical path length F is fixed and is used as a
reference value in the determination of C. Since the optical fiber
124 ends with a mirror 134, the light passing through the fiber 124
travels by an optical path length 2F. The fiber 124 can also
include a connector 134, which should preferably have optical
characteristics identical to those of the connector 112.
[0038] The light reflected by the mirrors 130 and 132 is recombined
in the coupler 120. The recombined light travels through an optical
fiber 136 having an optical path length G to a photodetector
138.
[0039] The sensor 1 and the instrument 100 provide four possible
paths for the light emitted by the LED 102, since the light can be
reflected from the mirror 7 or 15 and then from the mirror 130 or
132. The first path involves the mirrors 7 and 130 and has an
optical path length A+2B+D+2E+G. The second path involves the
mirrors 15 and 130 and has an optical path length A+2C+2B+D+2E+G,
or the first optical path length plus 2C. The third path involves
the mirrors 7 and 132 and has an optical path length A+2B+D+2F+G.
The fourth path involves the mirrors 15 and 132 and has an optical
path length A+2B+2C+D+2F+G, or the third optical path length plus
2C.
[0040] The terms A+2B+D+G are common to all four paths. If those
terms are eliminated, the remaining terms are:
[0041] First path, 2E
[0042] Second path, 2C+2E
[0043] Third path, 2F
[0044] Fourth path, 2C+2F.
[0045] Thus, the goal becomes that of deriving changes in C from
the known quantity F and the controllable quantity E.
[0046] Only the second and third paths are involved in producing an
interference fringe at the photodetector 138. The interference
fringe is maximized when those path lengths become equal, namely,
when 2C+2E=2F, or 2C=2F-2E. The adjustable length optical fiber 122
is adjusted until that condition is reached, as determined by
observation of the interference fringe detected by the
photodetector 138. Then, changes in C can be derived from the fixed
quantity F and the value of E needed to maximize the interference
fringe:
C=F-E. (Eq. 1)
[0047] In other words, the actuator 128 is actuated until the
difference in path lengths between the mirrors 130 and 132 equals
the difference in path lengths between the mirrors 7 and 15.
[0048] For instance, when the sensing fiber 13 is bonded to a
structure (FIG. 2, step 202), it will be at a rest position in
which C has an initial value LS1. As the structure deforms, C
reaches a new value LS2. In other words, the sensing fiber 13
experiences a displacement LS2-LS1. Similarly, when the adjustable
fiber 122 is in its rest position, the difference F-E between the
optical path lengths of the fibers 124 and 122 has an initial value
LR1. During sensing, light is applied to the fibers 3, 13, 122 and
124 (FIG. 2, step 204), and an output is detected at the
photodetector 138. The actuator 128 adjusts the optical path length
of the fiber 122 (FIG. 2, step 206) until the interference fringe
is detected (FIG. 2, step 208). The interference fringe is
maximized when the path difference between the fibers 124 and 122
reaches a new value LR2 which is equal to LS2. Then, in FIG. 2,
step 210, C can easily be determined. Then the sensing process ends
in step 212.
[0049] FIG. 1B also shows a plot of the signal output by the
photodetector 138, showing the resultant interference fringes.
[0050] The above is easily generalized to a sensor having multiple
sensing fibers with path lengths C, C1, C2, etc. A single
adjustable fiber and a single reference fiber can be used, and as
the adjustable fiber is adjusted, a series of interference fringes
will appear, one for each of die multiple sensing fibers. The
fringes have peaks when LR2-LR1=0, C1-C, C2-C, . . . . If those
peaks occur in mutually exclusive ranges, spatial division
multiplexing is possible, and the various sensing fibers can be
resolved without the need in the prior art to resolve nanosecond
differences in time between pulses or to provide an optical switch
to select a signal from one of the sensors.
[0051] The sensor 1, in combination with the instrument 100 or
another suitable instrument, can be used to detect static or
dynamic conditions. For dynamic conditions, the frequency which can
be detected is limited only by the time needed to adjust the
adjustable fiber.
[0052] The optical fibers are preferably single-mode fibers to
reduce losses. The actuator 128 has a suitable way of determining
the degree of actuation so that E can be determined. For example,
if the actuator 128 is mechanical, a mechanical encoder can be
used, while if the actuator 128 is piezoelectric, E can be derived
from the voltage required to maximize the interference fringe.
[0053] Any suitable electronics 140 can be used to receive the
signals from the photodetector and locate the interference maximum.
The electronics can locate the maximum automatically or operate
under manual control.
[0054] The sensor 1 can be implemented as a long-gage optical fiber
sensor. The sensor 1 can be placed in or on an object whose
physical condition (e.g., displacement) is to be sensed.
[0055] FIGS. 3A-3C show some examples. In FIG. 3A, the sensing
fiber 13 is bonded to the exterior of a structure 301. As the
structure 301 degrades over time, the sensing fiber 13 experiences
a displacement. In FIG. 3B, the sensing fiber 13 is embedded in a
structure 303, such as a dam. In FIG. 3C, the sensing fiber 13 is
attached to two anchors 305, 307 on a structure 309, but is
otherwise free from the structure 309. The distance between the
anchors 305 and 307 defines the gage length C of the fiber 13. In
any of FIGS. 3A-3C, the fiber 13 can be positioned in a
pre-tensioned state such that contraction does not cause the fiber
13 to collapse if it is anticipated that a contraction of the
structure 301, 303 or 309 may have to be detected.
[0056] The gage length of the fiber 13 typically varies from less
than ten centimeters to over a hundred meters. To permit
measurements over such a gage length, the adjustable length optical
fiber 122 (not shown in FIGS. 3A-3C) can be adjusted in any of
several ways. For example, as shown in FIG. 4A, the fiber 122 is
wrapped around a piezoelectric cylinder 401 which can be actuated
to expand. Alternatively, as shown in FIG. 4B, the fiber 122 is
suspended between an anchor 403 and a pulley 405 controlled by a
motor 407. Other actuators, such as a linear motor, could be used
instead.
[0057] The maximum displacement which the sensor can measure is
determined by the length of the adjustable fiber 122 and the
tensile breaking strength, i.e., the maximum strain or displacement
which the fiber 122 can undergo. The longer the optical fiber, the
greater the magnitude of the displacement for a given ultimate
strain for the fiber material. For example, a typical single-mode
optical fiber three meters in length has a maximum displacement of
60 mm. The rate at which the adjustable optical fiber 122 can be
stretched or contracted determines the capacity of the sensor to
measure dynamic displacement profiles.
[0058] The sensor can be modified to include multiple sensing
fibers whose signals can be resolved through spatial division
multiplexing. Each of the sensing fibers can be a long-gage fiber
and can be used as in the first preferred embodiment. As an
alternative, short gage lengths, such as a few centimeters, can be
used. A single sensor can incorporate sensing fibers having long
and short gage lengths.
[0059] Two configurations of sensing optical fibers are shown in
FIGS. 5A and 5B. FIG. 5A shows a configuration 501 in which a
backbone optical fiber 503 is coupled through a coupler 505 to
multiple sensing fibers 507 having different optical path lengths
determined in a manner to be described shortly. FIG. 5B shows a
configuration 511 in which a backbone optical fiber 513 is coupled
through couplers 515 along its length to multiple sensing fibers
517. In the configuration 511, the spacing between adjacent ones of
the couplers 515 supplies an optical path difference which can be
used in multiplexing. Still other configurations could be devised;
for example, a single optical fiber could have multiple
semi-reflecting mirrors spaced along its length so that the
interval between each two adjacent ones of the mirrors serves as a
sensor. In either of the configurations, the coupler or couplers
should provide some back reflection of the light, so that they
either incorporate or take the place of the lead mirror 7. The
sensing fibers 507 or 517 can be configured like the sensor fiber
13, ending in mirrors like the sensor mirror 15.
[0060] In the configuration of FIG. 5A, the sensing fibers 507
should have different optical path lengths, so that as the length
of the adjustable length optical fiber 122 is adjusted, the
interference fringes produced by light from the various sensing
fibers 507 will not coincide or overlap. Thus, the values of the
physical condition detected by the various sensing fibers 507 can
be distinguished by spatial division multiplexing, so that neither
time resolution on the order of a few nanoseconds nor an optical
switch is required as in the prior art. In the configuration of
FIG. 5B, the separation of the couplers 515 provides the necessary
difference in optical path lengths.
[0061] The manner of determining the differences among the optical
path lengths of the fibers 503 of FIG. 5A will now be described. As
noted above, if a number N of sensing optical fibers area attached
to a single backbone optical fiber, the sensing optical fibers have
different optical path lengths to permit spatial division
multiplexing. The difference in optical path length is given by
.DELTA.=(.DELTA..sub.max-.SIGMA..sub.iL.sub.i.epsilon..sub.i)/(N-1),
[0062] where .DELTA..sub.max is the maximum displacement of the
adjustable length fiber 122, .epsilon..sub.i is the expected strain
of the ith sensing fiber, L.sub.1 is the optical path length of the
ith sensing optical fiber, and i assumes integer values from 2 to
N. The values of L.sub.i start with the length of the reference
optical fiber and increase in increments of .DELTA..
[0063] A numerical example will now be given. In the example, the
maximum value of the deflection of the adjustable optical fiber is
30 mm. Five sensing fibers 507 are coupled to a single backbone
fiber 503. The measured strain expected on the structure on which
the configuration 501 is used is 200 microstrain, or in other words
200.times.10.sup.-6 mm/mm, and is expected to be the same for all
of the sensing fibers 507. The reference optical fiber has an
optical path length of 1000 mm, so that the base line sensor is
also 1000 mm in optical path length. The resulting difference
.DELTA. required to determine which of the sensing fibers 507 is
associated with any particular interference peak is .DELTA..about.7
mm. Accordingly, the optical path lengths of the five sensing
fibers 507 are 1000 mm, 1007 mm, 1014 mm, 1021 mm and 1028 mm.
[0064] Similar considerations apply to configurations such as that
of FIG. 5B. Those skilled in the art who have reviewed the present
disclosure will readily be able to design such configurations to
permit spatial division multiplexing.
[0065] Test data from a sensor made in accordance with an
embodiment of the present invention will now be set forth. The test
data demonstrate the ability of the sensor to measure deformations
in a pipe, such as pre-buckling, buckling, internal pressure in the
axial and circumferential directions and plastic deformations
caused by loads exceeding the yield stress of the steel from which
the pipe is made. Three sensor lengths are used: 10 cm, 1 m and 10
m. In addition, the ability of a long spiral wrap sensor is proved
in terms of its ability to measure pressure loads in pipes.
[0066] The geometry and material properties of the steel pipe
tested and the loads under which it was tested are set forth in the
following table:
1 Test loads applied to steel pipe Internal pressure (p)
1550.about.1560 PSI Axial compression (P) 6 .times.
10.sup.5.about.8 .times. 10.sup.5 lbs Bending moment (M) 3 .times.
10.sup.6.about.14 .times. 10.sup.6 in-lbs Steel pipe properties
Length .about.72 in Diameter 19.4 in Thickness 0.46 in Modulus of
elasticity 30 .times. 10.sup.6 PSI Poisson's ratio 0.30
[0067] The following table summarizes the strains measured by two
different lengths of fiber optic sensors: 1 m and 10 m (spiral
wrap) under two different load conditions involving combinations of
internal pressure and axial compression. The data show a good
agreement with theoretical predictions based on well known
stress/strain equations for pipes under those load conditions. The
data show that both tension and compression can be accurately
measured using the sensors under different load conditions.
2 Test results of internal pressure and axial compression
Circumferential strain (10.sup.-6) Spiral gages Axial strain
(10.sup.-6) 1.0 m Strain (10.sup.-6) Load Predicted 1.0 m avg
Predicted avg 10 m avg p = 1547 PSI +202 +258 +929 +934 +933 P =
12780 lbs p = 1547 PSI -500 -471 +1140 +943 n/a P = 6 .times.
10.sup.5 lbs
[0068] In the above data, a positive (+) value indicates tension,
while a negative (-) value indicates compression. The spiral gages
measure primarily circumferential strain.
[0069] FIG. 6 shows a graph of data collected from two sensor
lengths (1 m and 10 cm) measuring strains in the axial
(longitudinal) direction of a pipe which is under internal pressure
of 1547.about.1558 PSI and a pre-load of axial compression of
2690.about.3580 kN. The strains are plotted as a function of jack
loads, i.e., a measure of the bending load applied to the pipe. The
results show linear elastic behavior up to buckling, i.e., local
collapse of a portion of the cylinder wall, associated with high
compressive axial stresses due to the pre-load and jack-induced
bending load. The results also show that upon unloading of the jack
load, there was a permanent plastic deformation in the pipe,
located in the buckled region.
[0070] FIG. 7 shows the hoop strain, i.e., the tensile strain in
the circumferential direction, as measured by a 1 m sensor on a
pipe which is under preloads of internal pressure and axial
compression. The strain is plotted as a function of the jack load.
Once again, the data show linear elastic behavior up to the
buckling of the pipe. As the jack load is removed, the pipe evinces
plastic deformation at the buckle location.
[0071] While a preferred embodiment and variations thereon have
been described above in detail, those skilled in the art who have
reviewed the present disclosure will readily appreciate that other
embodiments can be realized within the scope of the present
invention. For example, the optical path length of the adjustable
fiber can be varied by electro-optic or magneto-optic techniques
without a need for any moving parts. Also, the sensor 1 and the
instrument 100 can be formed as a unit or can be separable; in the
latter case, the instrument 100 can include a ferrule and connector
for attachment to any sensor 1 or to multiple sensors 1. Therefore,
the present invention should be construed as limited only by the
appended claims.
* * * * *