Optical deformation sensor

This invention relates to a lightweight and compact optical sensor that provides an indication of a deformation (e.g. fatigue, vibration, flex, torsion, bending, slippage) occurring at a monitored area. The sensor includes at least one light transmitting optical fiber and an optical detector that is responsive to the optical characteristics (e.g. amplitude, phase, polarization angle) of a supply of light signals being transmitted through the fiber. In the event that deformation occurs at the monitored region, the optical fiber undergoes a displacement which thereby causes a corresponding change in the optical characteristics of the light signals supplied to the detector. The optical characteristics of the light signals supplied to the detector relative to those of the light signals supplied to the optical fiber provide an accurate indication of the physical parameter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to low cost, compact optical sensors, wherein the 
displacement of an optical fiber in response to a force applied thereto 
provides an accurate indication of a deformation occurring in a monitored 
region. 
2. Description of the Prior Art 
In many environments, it is desirable to be able to sense the occurrence of 
local deformation over wide material areas. Such deformations include, for 
example, fatigue, vibration, flexing, slippage, torsion, and bending. 
Applications in which deformation monitoring is especially desirable 
include: automobile, ship, and aircraft manufacturing; testing structural 
members; molding forms in the construction industry; building pressure 
vessels, fabricating composite materials, etc. 
However, conventional deformation sensing typically requires the 
utilization of relatively cumbersome and expensive test equipment. More 
particularly, many wires and complex (mechanical-to-electrical) converters 
may be required to provide an accurate indication of deformation. 
Moreover, the size of such conventional equipment makes the deformation 
sensor generally unsuitable for permanent fixation to the area to be 
monitored. Accordingly, it becomes difficult to accurately monitor the 
test area for any long or continuous period of time. What is more, 
conventional sensing equipment may be undesirably susceptible to 
weathering or deterioration when exposed to certain environments for 
lengthy intervals. 
Examples of optical apparatus that include an optical fiber and that may 
provide an indication of a physical parameter (e.g. a pressure or force 
signal) can be found in the following U.S. patents: 
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U.S. Pat. No. Issue Date 
______________________________________ 
2,922,873 January 26, 1960 
3,051,003 August 28, 1962 
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SUMMARY OF THE INVENTION 
Briefly, and in general terms, an optical deformation sensor is disclosed. 
The optical sensor includes at least one optical fiber, the displacement 
of which in response to an applied force provides an indication of a 
deformation occurring at a monitored region. In a first preferred 
embodiment of the invention, a pair of optical fibers are arranged in 
relatively close proximity to one another at the region to be monitored. 
Incident light signals are transmitted through at least a first of the 
pair of the fibers. Some of the optical energy of the transmitted light 
signals is coupled to the second of the pair of the fibers. In the event 
of the occurrence of a deformation and a corresponding displacement of the 
first fiber in response thereto, the amount of optical energy transmitted 
by the first optical fiber and coupled to the second optical fiber 
changes. An optical detector is interfaced with each of the first and 
second optical fibers to monitor the relative change in the optical 
characteristics of the output light signals therefrom, whereby to provide 
an indication of the deformation. 
In a second preferred embodiment of the invention, a supply of polarized 
incident light signals is transmitted through an optical fiber. The 
incident light signals may be polarized by means of a first optical (e.g. 
diffraction) grating. A second optical (e.g. diffraction) grating is 
aligned with the first grating so that output light signals that have been 
transmitted through the optical fiber are also passed through the second 
grating. The first and second gratings may be positioned so as to be in an 
out of phase alignment with one another. In the event of the occurrence of 
a deformation and a corresponding displacement of the optical fiber in 
response thereto, the optical characteristics of the polarized incident 
optical signals are changed. An optical detector is interfaced with the 
optical fiber so as to receive the output light signals from the second 
grating. The optical characteristics of the output light signals relative 
to those of the incident light signals provides an indication of the 
deformation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A first preferred embodiment of a rugged, lightweight, and relatively 
inexpensive optical deformation sensor is shown in FIG. 1 of the drawings. 
The optical sensor comprises at least one pair of unclad, flexible fiber 
optic cores 1 and 2. Fiber optic cores 1 and 2 may be fabricated from any 
well known light conducting material, such as, for example, optical glass 
or plastic. Fiber cores 1 and 2 may be affixed (by means of adhesive, or 
the like) to the surface of a region that is to be monitored for 
deformation. It is to understood, however, that fiber cores 1 and 2 may 
also be embedded within the interior of any member to be monitored for 
deformation. Fiber cores 1 and 2 are positioned in relatively close 
proximity to one another. To enhance optical coupling therebetween and to 
increase the sensitivity of the optical sensor, fibers 1 and 2 are 
arranged to form curved, parallel paths (as shown). However, fibers 1 and 
2 can also be arranged to include straight path segments. 
A modification of the optical deformation sensors of FIG. 1 is illustrated 
in FIG. 2. When it is desirable to integrate fiber cores 1 and 2 into a 
compact sensor cable 7 that provides protection from the environment of 
the monitored region, fiber cores 1 and 2 may be covered with a suitable 
cladding material 3. Alternatively, fiber cores 1 and 2 may be surrounded 
by either a liquid or a vaccum. The cladding material 3 may then be 
surrounded by a flexible, non-transparent material 4, such as, for 
example, silicon rubber, or the like. It is also to be understood that 
several pairs of active and passive optical fibers 1 and 2 could be 
integrated within a single protective cable, similar to that designated by 
the reference numeral 7. 
In operation, light signals are transmitted from the source 5 thereof, via 
active optical fiber 1, to optical detector 8. Prior to the occurrence of 
a deformation to a monitored region, a particular amount of light is 
initially coupled from the active optical fiber 1 to the passive optical 
fiber 2. In the event that a deformation (e.g. fatigue, vibration, flex, 
torsion, bending, slippage, and the like) occurs in the monitored region, 
at least one segment of active optical fiber 1 becomes increasingly bent 
relative to passive optical fiber 2. More particularly, any location (e.g. 
10) that is deformed along the length of fiber 1 in response to a physical 
parameter is moved either closer to (as shown) or farther away from fiber 
2. Incident light signals are split at the location 10 of the deformation, 
and, due to resulting bending losses, optical energy is transferred from 
active fiber 1 to the cladding material 3 (of FIG. 2). Moreover, the 
evanescent electromagnetic fields that occur within the cladding 3 become 
distorted between the fibers 1 and 2. Accordingly, an increase can be 
detected in the amount (i.e. the magnitude) of the light signals that are 
coupled from the active fiber 1 to the passive fiber 2 at any point 10 
where fiber 1 becomes deformed and the bending losses therefrom 
correspondingly increase. The change in the relative magnitudes of the 
output light signals measured by optical detectors 6 and 8 provides an 
indication of the movement of active fiber 1 and, accordingly, the rate 
and magnitude of the deformation sensed at the monitored region. After 
continuous use, the light attenuation in the active fiber 1 increases as a 
result of the introduction of scattering centers, microcracks, etc., so 
that the accumulated deformation of a monitored region can also be sensed. 
The technique by which fiber optic cores 1 and 2 sense deformation is 
explained when referring to FIG. 3 of the drawings. For the reason that 
will soon become apparent, fiber core 1 will hereinafter be referred to as 
the active optical fiber, and fiber core 2 will hereinafter be referred to 
as the passive optical fiber. A suitable source 5 of light is interfaced 
with a first end of the active optical fiber 1. For example, light source 
5 may be a light emitting diode that supplies incident light signals to 
fiber 1. The second ends of each of the active and passive optical fibers 
1 and 2 are interfaced with respective optical detectors 6 and 8. By way 
of example, each of optical detectors 6 and 8 may include a suitable 
semiconductor diode, such as a PIN diode, or the like. The optical 
detectors 6 and 8 are adapted to measure the intensity of the output light 
signals that are transmitted thereto from the second ends of active and 
passive optical fibers 1 and 2. 
Sensor modifications to maximize sensitivity or the size of the region that 
can be accurately monitored by the instant deformation sensor include 
surrounding an active optical fiber 1 of relatively large diameter by 
several passive optical fibers 2 of smaller diameter. What is more, a 
suitably sized array of active and passive fibers may be arranged in 
different planes in order to concurrently monitor several likely 
deformation points over a region. 
FIG. 4 of the drawings shows a strain or deformation gauge 12 that can be 
used in conjunction with the optical deformation sensor of FIG. 3 to 
provide a signal that is representative or the deformation of a monitored 
region. Referring concurrently to FIGS. 3 and 4, the gauge 12 may comprise 
a bridge circuit having four resistive elements interconnected in a 
conventional fashion. One side of the bridge circuit includes the series 
connection of a photoresistor D.sub.1 and a fixed resistor R.sub.1. The 
other side of the bridge circuit includes the series connection of a 
photoresistor D.sub.2 and a variable resistor R.sub.2. Photoresistors 
D.sub.1 and D.sub.2 correspond to the resistances of the semiconductor 
diodes that form respective optical detectors 6 and 8. A suitable meter M 
is connected across the bridge circuit to indicate a change, such as in 
current, that occurs in either side of the bridge. Variable resistor 
R.sub.2 can be used to balance the bridge circuit (i.e. or null meter M) 
to account for the initial optical coupling of light signals from active 
optical fiber 1 to passive optical fiber 2. As previously disclosed, a 
deformation occurring at a monitored region changes the relative 
magnitudes of the optical signals coupled between optical fibers 1 and 2, 
which magnitude changes are detected by optical detectors 6 and 8. 
Accordingly, the photoresistances of detector elements D.sub.1 and D.sub.2 
change a corresponding amount, whereby to cause the bridge circuit of 
gauge 12 to become unbalanced. Hence, the output signal of meter M 
represents the magnitude (and the rate) of deformation that is sensed at a 
monitored region. 
A modification of the optical deformation sensors illustrated in FIGS. 1-4 
is shown in FIG. 5 of the drawings. The embodiment shown in FIG. 5 
represents a rugged and compact, phase modulated optical sensor which is 
characterized by a reduced dependency upon the changes in the relative 
intensities of output light signals that are sensed by an optical detector 
to indicate a deformation. That is, in the event that a deformation causes 
transmitted incident light signals to be coupled from an active to a 
passive optical fiber (as previously disclosed) and then coupled back to 
the active fiber at successive locations along the lengths of the fibers, 
the relative intensities of the output light signals may not provide a 
totally reliable indication of the deformation to be sensed. Therefore, in 
certain applications, a deformation sensor that is responsive to both the 
intensities and the phase of the detected output light signals is 
advantageously desirable. 
More particularly, the deformation sensor of FIG. 5 includes at least one 
pair of flexible fiber optic cores 14 and 15 (that may be surrounded by a 
common cladding material, similar to that described when referring to FIG. 
2). Fiber cores 14 and 15 may be either affixed to the surface or embedded 
within the interior of a region to be monitored for a deformation. To 
increase sensitivity, fiber cores 14 and 15 are typically arranged in 
relatively close proximity to one another and include generally curved 
segments. In the present embodiment of FIG. 5, each of the fiber cores 14 
and 15 can be regarded as an active optical fiber. Suitable light sources 
16 and 17 are interfaced with the respective first ends of optical fibers 
14 and 15. As described in the embodiments above, the light sources 16 and 
17 may be well known light emitting diodes that supply incident light 
signals to the first ends of fibers 14 and 15. The second ends of each of 
the optical fibers 14 and 15 are interfaced with respective conventional 
optical detectors 18 and 19, which detectors are adapted to measure both 
the amplitude and the phase of the output light signals transmitted 
thereto. 
In operation, incident light signals are transmitted from sources 16 and 
17, via optical fibers 14 and 15, to respective optical detectors 18 and 
19. In the event that a deformation occurs in the monitored region, the 
transmitted incident light signals are split at the location of the 
deformation, and, due to the resulting bending losses, optical energy is 
coupled (i.e. transferred) between the fibers 14 and 15. By way of 
example, should optical fibers 14 and 15 experience similar bending 
losses, whereby the amount of light energy coupled from fiber 14 to fiber 
15 approximates that coupled from fiber 15 to fiber 14, there will be 
little net change in the relative intensities of the output light signals 
measured by optical detectors 18 and 19. A deformation sensor of increased 
reliability is obtained by selecting light sources 16 and 17 which provide 
(e.g. sinusoidal) incident light signals that are out of phase with one 
another, for example, by 90.degree.. 
Referring concurrently to FIGS. 5-9 of the drawings, FIG. 6 represents the 
corresponding waveforms of the incident light signals that are supplied to 
respective optical fibers 14 and 15 from light sources 16 and 17. For the 
convenience of illustration, the waveforms of the two incident light 
signals have similar maximum amplitudes, designated A.sub.1 (but are out 
of phase from one another by +90.degree.). During the time prior to the 
occurrence of a deformation to a monitored region when little optical 
cross-coupling results, the waveforms (i.e. the relative intensity and 
phase) of the output light signals detected by optical detectors 18 and 19 
and represented by FIG. 7 of the drawings are substantially identical to 
those of the incident light signals (represented by FIG. 6). Therefore, 
prior to a deformation, the output light signals received by detectors 18 
and 19 have a maximum amplitude A.sub.1 and remain out of phase with one 
another by +90.degree.. 
However, in the event of the occurrence of a deformation, increased bending 
of optical fibers 14 and 15 results in a shift in phase and an attenuation 
of the output light signals received by optical detectors 18 and 19 
relative to the incident light signals (of FIG. 6) supplied by light 
sources 16 and 17. FIG. 8 of the drawings represents the corresponding 
waveforms of the output light signals that are detected when approximately 
50% of the incident light signal transmitted by optical fiber 14 is 
coupled to optical fiber 15, and vice versa. In the present example of 
FIG. 8, each of the output light signals are attenuated to a maximum 
amplitude designated A.sub.2, and the phases thereof are shifted by 
90.degree. relative to one another, so that phase difference between the 
output light signals is reduced to approximately 0.degree. (i.e. the 
waveforms of the output light signals are essentially in phase with one 
another.) 
With the continued passage of time, optical fibers 14 and 15 may experience 
increased bending and corresponding bending losses. Accordingly, the 
waveforms of the output light signals received by optical detectors 18 and 
19 are further shifted in phase and attenuated relative to the waveforms 
of the incident light signals (of FIG. 6) that are supplied by light 
sources 16 and 17. FIG. 9 of the drawings represents the corresponding 
waveforms of the output light signals that are detected when approximately 
100% of the incident light signal transmitted by optical fiber 14 is 
coupled to optical fiber 15, and vice versa. In the present example of 
FIG. 9, the intensities of each of the output light signals are attenuated 
to a maximum amplitude designated A.sub.3, and the phases thereof are 
shifted by an additional 90.degree. (from that of FIG. 8) relative to one 
another, so that the difference in phase between the output light signals 
is -90.degree.. 
In each of the examples of FIGS. 6-9, it was assumed, for convenience, that 
the maximum intensities of the incident light signals (FIG. 6) and the 
corresponding amount of optical energy cross-coupled between optical 
fibers 14 and 15 (FIGS. 7-9) were substantially identical to one another. 
Therefore, the difference in phase between the detected output light 
signals relative to that between the incident light signals is suitable to 
provide an accurate indication of a deformation sensed at a monitored 
region. However, it is also to be understood that the respective maximum 
intensities of the incident light signals and of the output light signals, 
as well as the corresponding amounts of energy cross-coupled between 
fibers 14 and 15, may vary. In this way, the difference between the 
intensities of the output light signals after the occurrence of a 
deformation relative to that detected prior to the deformation may also be 
utilized (according to the technique described when referring to FIGS. 
1-4) to provide an indication of the deformation. 
A second preferred embodiment of an optical deformation sensor is shown in 
FIG. 10 of the drawings. Similar to the deformation sensors previously 
disclosed when referring to FIGS. 1-9, the deformation sensor 20 of FIG. 
10 includes (a clad) optical fiber rod 22, the displacement of which in 
response to a deformation provides an accurate indication of the 
deformation occurring within a monitored region. Each end 24 and 26 of 
optical fiber rod 22 is suitably shaped into a hemisphere, or the like 
configuration. The hemispheres may be formed by fusing and polishing ends 
24 and 26 into the desired shape. An optical polarizer 28, such as a 
diffraction grating, or the like, is applied to the flat face of one 
hemispherical fiber end 24. A suitable light source (not shown) is 
interfaced with polarizer 28 in order to supply incident light signals 
thereto. A reflecting surface 30, such as an aluminum, or the like, 
covered substrate, is applied to the flat face 27 of the opposite 
hemispherical fiber end 26. Reflecting surface 30 is adapted to reflect 
incident light signals that are supplied thereto from polarizer 28 via 
optical fiber rod 22. The inclusion of reflecting surface 30 is 
advantageous to maximize the sensitivity of sensor 20, inasmuch as light 
is conveyed twice (i.e. transmission and reflection) through fiber rod 22, 
so as to essentially double the number of output optical signals. An 
optical detecting and polarizing surface 32, such as a diffraction 
grating, is also applied to the flat face 25 of hemispherical end 24, 
adjacent polarizer 28. Defraction gratings 28 and 32 and reflecting 
substrate 30 may be applied to their respective flat faces 25 and 27 by 
means of conventional material deposition and/or photolithographic 
interferometric techniques. The disclosed hemispherical shape into which 
the ends 24 and 26 of fiber optic rod 22 are formed is advantageous for 
providing both a suitable flat glass substrate for receiving gratings 28 
and 32 and a shape for maximizing the optical coupling of light signals 
therewith. 
An optical analyzer 34 is interfaced with detecting surface 32, so as to 
receive for processing the output light signals that are transmitted 
thereto from reflecting surface 30 via optical rod 22. By way of example, 
analyzer 34 may be a holographic processor such as that described in 
detail in U.S. Pat. No. 4,296,994 issued Oct. 27, 1981, and incorporated 
herein by reference. 
In operation, and in a reflecting mode of operation, incident light signals 
are supplied to polarizer 28. Polarized incident light signals are 
transmitted via optical fiber rod 22, to reflecting surface 30, and then 
to detecting surface 32. The spacings between the gratings which form the 
polarizing and detecting surfaces 28 and 32 may be chosen to match the 
wavelength of a monochromatic light source, and the shape and periodicity 
of the opaque regions of the surfaces 28 and 32 may be chosen to provide 
particularly recognizable grating patterns or characteristics in a manner 
that is well known to those skilled in the art. Or, as is more 
particularly shown in FIG. 11 of the drawings, the grating alignment of 
the polarizing and detecting surfaces 28 and 32 at the hemispherical fiber 
end 24 can be mechanically positioned, so as to be out of phase with one 
another. By way of example, the grating patterns illustrated in FIG. 11 
are 90.degree. out of phase from one another. The aforementioned 
positioning is advantageous, because the ratio of the intensities of the 
output light signals prior to and after a deformation can provide an 
indication of the deformation. Therefore, an optical deformation sensor 
such as that illustrated in FIG. 11 is relatively insensitive to the 
effects of attenuation of the light signals that are transmitted by fiber 
optic rod 22. 
Accordingly, prior to the occurrence of a deformation, the output light 
signals that are provided to analyzer 34 via detecting surface 32 have an 
initial set of optical characteristics. In the event that a region to be 
monitored experiences a deformation, such as one of those deformations 
mentioned above, optical rod 22 will undergo a twisting or a bending in 
response to the magnitude and direction of the deformation. Accordingly, 
the initial optical characteristics of the polarized light transmitted via 
fiber rod 22 may be modified during transmission and reflection from 
polarizer 28 to detecting surface 32. More particularly, the relative 
change in intensity at detecting surface 32, the shift in polarization 
angle, and the extent of depolarization of the output light signals can be 
compared by analyzer 34 with the corresponding initial optical 
characteristics sensed prior to deformation, so that an accurate 
indication of the magnitude and rate of deformation at a monitored region 
can be provided. 
By way of example, if fiber optic rod 22 is subjected to a temperature 
change, a corresponding dilation of the grating spacings will alter the 
optical characteristics of the transmitted (or reflected) light signal. If 
rod 22 is twisted in response to the application of a torque, the relative 
angles of orientation of the diffraction gratings 28 and 32 is changed. 
What is more, fatiguing of the fiber rod 22 caused by repeated flexing 
will affect the intensity and polarization of the output light signals. 
An alternate embodiment of the optical deformation sensor of FIG. 10 is 
shown in FIG. 12. The deformation sensor 40 of FIG. 12 is similar to that 
illustrated in FIG. 10, except that a reflecting surface is omitted from 
the present embodiment. More particularly, an optical polarizer 28 is 
applied to the flat face 25 of hemispherical fiber end 24, and a detecting 
surface 32 is applied to the flat face 27 of hemispherical fiber end 26. 
Rather than reflect incident light signals back to the hemispherical end 
24 (as in the deformation sensor 20 of FIG. 10), in a transmitting mode of 
operation, polarized incident light signals are, otherwise, transmitted 
via fiber optic rod 22 directly through detecting surface 32 to analyzer 
34. Therefore, reflection and absorption losses (at the cladding 
surrounding fiber rod 22) are minimized. Accordingly, the relative changes 
of the optical characteristics of the output light signals can be 
externally compared and processed by analyzer 34, so that an accurate 
indication can be provided of the various physical parameters and/or 
mechanical stresses that are imposed on the fiber rod 22 at the region to 
be monitored. 
Additional sensor modifications include the following. It is to be 
understood that if a polarized source of incident light signals is 
utilized, optical polarizer surface 28 may be eliminated from the input 
end 24 of the optical rod 22. Also, the flat faces 25 and 27 of the 
hemispherical fiber ends 24 and 26 may be canted at any suitable angle 
with respect to the longitudinal axis of the rod 22.