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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to fiber optic interferometric
sensors and, more particularly, to extrinsic Fizeau interferometric fiber
optic sensors having particular application in hostile environments for
dynamic monitoring of strain, temperature or pressure in mechanical
structures. The extrinsic fiber optic sensors according to the invention
are capable of both relative and absolute measurement of strains and, with
suitable modification, can be used for the detection of the relative
polarity of dynamically varying strain, temperature or pressure. As used
herein, the word "strain" will be understood to mean strain, temperature,
pressure, magnetic fields (i.e., magnetostrictive materials), and other
like phenomena that can be translated into a displacement depending on the
specific application.
2. Description of the Prior Art
Fiber optic Fabry-Perot sensors reported in the literature have been highly
sensitive to temperature, mechanical vibration, acoustic waves and
magnetic fields. See T. Yoshino, K. Kurosawa, K. Itoh, and T. Ose, IEEE J.
Quantum Electron., QE-18, 1624 (1982). Techniques to create the
Fabry-Perot cavity have varied from the creation of Bragg gratings in or
on the fiber, as described by K. L. Belsley, J. B. Carroll, L. A. Hess, D.
R. Huber, And D. Schmadel, in Proc. Soc. Photo-Opt. Instrum., Eng. 566,
257 (1985), to the use of air-glass interfaces at the fiber ends as the
reflectors, as described by A. D. Kersey, D. A. Jackson, and M. Corke, in
Opt. Comm., 45, 71 (1983). A relatively new technique described by C. E.
Lee and H. F. Taylor, in Electron. Lett., 24, 193 (1988), involves
fabricating semireflective splices in a continuous length of fiber.
Applied strain at high temperature for intrinsic optical fiber sensors can
cause plastic deformation (i.e., creep), making such sensors unsuitable to
many hostile environments. The intrinsic Fabry-Perot interferometer
described by Lee and Taylor, ibid., exhibits stress-induced birefringence
in the air gap which causes polarization changes between the reference and
the sensing reflections. This, in turn, causes signal fading observed as a
decrease in fringe contrast. Extremely high magnetic fields can also cause
polarization changes inside an optical fiber which again would cause a
changing state of polarization between the reference and sensing
reflections in an intrinsic Fabry-Perot interferometric sensor.
Most Fabry-Perot sensor described in the prior art have been useful in the
measurement of quasi-static strain alone. That is, when the induced strain
changes its polarity, the Fabry-Perot interferometers would not be able to
detect this change if the switch in direction took place at a maximum or
minimum of the transfer function curve. Methods for obtaining directional
strain information by using thin-film or resistive gauges have been
reported by J. Putz, J. Putz, a. Wicks, and T. Diller, in "Thin-film shear
stress gauge", presented at the American Society of Mechanical Engineers
Winter Annual Meeting, Dallas, Tex., Nov. 26, 1990; however, no
corresponding capability has been demonstrated in Fabry-Perot optical
fiber sensors in the prior art.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an fiber
optic sensor which can be manufactured in micro-miniature form and capable
of withstanding extreme and hostile environments.
It is another object of the invention to provide an improved fiber optic
sensor which is immune to stress-induced, magnetic field or thermal
effects and other undesirable perturbations.
It is yet another object of the invention to provide an extrinsic optical
sensor which may be either surface mounted on or embedded in a mechanical
structure to be monitored.
It is a further object of the invention to provide an optical fiber sensor
capable of dynamically detecting strain and providing unambiguous
information on the direction of strain.
While the prior art sensors described above can be classified as intrinsic
sensors whereby the fiber itself plays a role in the sensing mechanism,
the sensor according to the invention is extrinsic in the sense that the
fiber acts as a conduit for optical power transmission to and from the
sensing element. In this sense, the invention is significantly different
from the prior art. The advantages are that low coherence sources, with
correspondingly low costs, can be used. Also, the fabrication process is
much simpler than earlier Fabry-Perot sensors. Another advantage is that
stable operation of the sensor with use of two wavelengths of light can be
implemented. Instability due to extraneous thermal drifts is a typical
problem in highly-sensitive phase-modulated fiber optic sensors.
More specifically, the subject invention is directed to extrinsic Fizeau
interferometric fiber optic sensors, as contrasted to intrinsic
Fabry-Perot interferometric fiber optic sensors known in the prior art.
Fabry-Perot interferometers are characterized by multiple reflections
within the cavity. The Fizeau interferometer operates on the principle of
a single reflection within the cavity. The principles of the Fizeau
interferometer are applied to a new fiber optic sensor to provide an
extrinsic sensor which is not subject to the problems associated with the
prior art intrinsic Fabry-Perot interferometric fiber optic sensors.
According to a preferred embodiment of the invention, the extrinsic Fizeau
interferometer comprises a single-mode fiber, used as an input/output
fiber, and a multimode fiber, used purely as a reflector, to form an air
gap within a silica tube that acts as a Fizeau cavity. The far end of the
multimode fiber is shattered so the reflections from the far end do not
add to the detector noise. The Fresnel reflection from the glass/air
interface at the front of the air gap (reference reflection) and the
reflection from the air/glass interface at the far end of the air gap
(sensing reflection) interfere in the input/output fiber. Although
multiple reflections occur within the air gap, the effect of reflections
subsequent to the ones mentioned above can be shown to be negligible. The
two fibers are allowed to move in the silica tube, and changes in the air
gap length cause changes in the phase difference between the reference
reflection and the sensing reflection. This phase difference is observed
as changes in intensity of the light monitored at the output arm of a
fused biconical tapered coupler.
The extrinsic Fizeau fiber optic sensor behaves identically whether it is
surface mounted or embedded, which is unique to the extrinsic sensor in
contrast to the intrinsic sensors of the prior art. See
"Phase-Strain-Temperature Model for Structurally Embedded Interferometric
Optical Fiber Strain Sensors with Applications" by Jim Sirkis, SPIE, vol.
1588, Fiber Optic Smart Structures and Skins IV (1991). The extrinsic
Fizeau optic fiber sensor according to the invention has been implemented
for temperature measurement at temperatures from .about.276.degree. C. to
1000.degree. C. The extrinsic Fizeau fiber optic sensor according to the
invention can be applied to a structure to be monitored by attaching or
embedding the single mode fiber at a single point such that no strain is
transferred to the fiber which avoids the possibility of plastic
deformation when combining strain and high temperature. Since the sensor
utilizes the air gap as the sensing mechanism, and not the fiber itself as
with intrinsic optical fiber sensors, there are no strain-optic or
thermo-optic effects for the extrinsic Fizeau interferometer sensor.
Therefore, the sensing reflection signal travels across the air gap and is
unaffected by high magnetic fields, temperature changes, or other
environmental conditions.
Immunity to polarization changes are another advantage of the extrinsic
Fizeau sensor over the intrinsic Fabry-Perot sensors of the prior art. Any
changes in polarization, due for example to stress-induced birefringence
or extremely high magnetic fields, take place before the reference
reflection in the extrinsic Fizeau interferometer optic fiber sensor.
The basic extrinsic Fizeau fiber optic sensor according to the invention
may be modified to provide two signals 90.degree. out of phase with
respect to each other. The phase shifted signals can be achieved by both
mechanical and optical means. More specifically, for mechanically obtained
quadrature phase shifted signals, two single-mode fibers are inserted into
one hollow silica tube and the gap-separations for the two fibers are
adjusted actively by moving the fibers in and out of the tube until a
90.degree. phase shift is achieved at the output. In a further variation,
two different silica tubes may be used. Since the two tubes have external
diameters on the order of a few hundred micrometers, the two sensors
monitor almost the same environmental perturbations. In one embodiment
employing an optical means of obtaining quadrature phase shifted signals,
two different sources are used in order to avoid interference effects
between the two signals returning to the coupler if only one source was
used to launch light into the single-mode fibers. In another embodiment, a
single source used with two different lengths of lead fibers for the two
sensors such that the difference in the lengths is greater than the
coherence length of the laser. Alternatively, the quadrature phase shift
may be approximated by using two laser sources of different wavelengths.
The dual wavelengths necessary for quadrature phase shifting also can be
obtained by modulating the wavelength of laser diode using conventional
techniques.
A unique crack opening displacement monitor has also been demonstrated
using sensors according to the invention. The sensor is placed across the
crack and attached to either side such that a widening of the crack causes
the air gap within the sensor to widen. Such an application has particular
use in a critical aircraft part, for example. The same principle can be
used to monitor large displacements relative to the gage length, something
not possible with prior art intrinsic optical fiber sensors, which are
limited to the maximum allowable strains for optical fibers (3-4% strain).
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects and advantages will be better
understood from the following detailed description of a preferred
embodiment of the invention with reference to the drawings, in which:
FIG. 1 is a schematic block diagram showing the principle components of the
extrinsic Fizeau sensor according to the invention:
FIG. 2 is a cross-sectional view showing details of the construction of the
Fizeau sensor;
FIG. 3 is a graph showing the variation of output intensity with increasing
gap displacement of the sensor shown in FIG. 2;
FIG. 4 is a graph showing an oscilloscope trace of observed fringes for
increasing gap displacement;
FIG. 5 is a graph showing transfer functions illustrating the principle of
operation of the quadrature phase-shifted sensors according to the
invention;
FIG. 6 is a schematic block diagram showing an arrangement for obtaining
two signals 90.degree. out of phase with one another;
FIG. 7 is a cross-sectional view showing details of the construction of the
Fizeau sensor used in FIG. 6;
FIG. 8 is a cross-sectional view showing details of the construction of a
variation of the sensor shown in FIG. 7;
FIG. 9 is a graph of an oscilloscope trace of quadrature phase-shifted
sensors showing lead-lag phenomenon;
FIG. 10 is a block diagram showing a variation of the quadrature phase
shift sensor of FIG. 6 using but one laser light source;
FIG. 11 is a block diagram showing a system for multiplexing a plurality of
extrinsic sensors according to the invention;
FIG. 12 is a block diagram showing a system for cascading a plurality of
sensors which requires a modification of the structure shown in FIG. 2;
FIG. 13 is a block diagram showing a first system for making an absolute
measurement of strain; and
FIG. 14 is a block diagram showing a second system for making an absolute
measurement of strain.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 1, there is
shown a schematic block diagram of the principle components of the
extrinsic Fizeau sensor 10 according the invention. A single mode fiber 11
(.lambda..sub.0 =1300 nm) is used as the input-output fiber. The fiber 11
conducts light from a laser source 12 to the Fizeau sensor 13 and
reflected light back to a fused biconical tapered coupler 14. The
reflected light is coupled in coupler 14 to a single mode fiber 15 to a
detector 16.
FIG. 2 shows in more detail the construction of the Fizeau sensor 13 which
comprises a hollow-core silica fiber 21 into one end of which the single
mode fiber 11 is inserted. From the opposing end of the hollow-core silica
fiber 21, a multimode fiber 22 is inserted. The multimode fiber 22 is used
purely as a reflector, the far end of the multimode fiber being shattered
so the reflection from the far end does not add to the detector noise. The
space 24 between the end of the single mode fiber 11 and the end of the
multimode fiber 22 forms an air gap that acts as a Fizeau cavity.
The Fresnel reflection from the glass-air interface at the front of the air
gap (reference reflection) and the reflection from the air-glass interface
at the far end of the air gap (sensing reflection) interfere in the
input-output fiber 11. Although multiple reflections occur within the air
gap, the effect of reflections subsequent to the ones mentioned above can
be shown to be negligible. The two fibers 11 and 22 are allowed to move in
the silica tube 21, and changes in the air gap length cause changes in the
phase difference between the reference reflections and the sensing
reflection, thus changing the intensity of the light monitored at the
output arm of the coupler 14.
The interference of the two-wave interferometer is evaluated in terms of a
plane-wave approximation. A coherent, approximately plane wave detected at
the output of the sensor can be represented in terms of its complex
amplitude U.sub.i (x,z,t), given by
U.sub.i (x,z,t)=A.sub.i exp{j.phi..sub.i }, i=1,2, (1)
where the variable A.sub.i can be a function of the transverse coordinate x
and the distance traveled z and the subscripts i=1,2 stand for the
reference and the sensing reflections, respectively. Assuming that the
reference reflection coefficient A.sub.1 =A, the sensing reflection
coefficient A.sub.2 can be approximated by the simplified relation
##EQU1##
where a is the fiber core radius, t is the transmission coefficient of the
air-glass interface (.apprxeq.0.98), s is the end separation, and NA is
the numerical aperture of the sing mode fiber 11, given by
NA=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2. n.sub.1 and n.sub.2 are the
refractive indices of the core and the cladding, respectively. Equation
(2) is described by G. Keiser in Optical Fiber Communications, McGraw-Hill
(1983), at page 134. The observed
I.sub.det =.vertline.U.sub.1 +U.sub.2 .vertline..sup.2 =A.sub.1.sup.2
+A.sub.2.sup.2 +2A.sub.1 A.sub.2 cos (.phi..sub.1 -.phi..sub.2),(3a)
which can be rewritten as
##EQU2##
where it is assumed that .phi..sub.1 =0 and .phi..sub.2
=2s(2.pi./.lambda.) and .lambda. is the wavelength of operation in free
space. The simplified loss relation expressed in Equation (2) for the
misalignment of two fibers is sufficient for understanding the drop in the
output intensity of the sensor as a function of the gap displacement. For
a strain sensor, it is useful to plot the detected intensity versus gap
separation s, as shown in FIG. 3. FIG. 3 shows that the fringe contrast
drops as the displacement increases. This is to be expected since the
relative intensity of the sensing reflection starts dropping with respect
to the reference reflection.
The extrinsic Fizeau interferometer has been tested as a displacement
sensor by attaching one fiber, the single mode fiber 11, to a stationary
block and the second fiber, the multimode fiber 22, to a micropositioner
which produces a known displacement between the fiber ends. FIG. 4 is an
oscilloscope trace of the continuously monitored output intensity of the
sensor for s=0 to s=203 .mu.m and shows the experimentally counted number
of fringes for the displacement to be 310.5, which corresponds to a
theoretically calculated displacement of 202 .mu.m.
FIG. 5 is a graph showing a typical sinusoidal variation of the output
intensity with respect to changes in the phase difference between the
reference and the sensing reflections. If the phase difference .phi.
varies sinusoidally with time and the peak-to-peak variation is large
enough to push the sensor out of its linear range, fringes are observed at
the output of the detector, as indicated by FIG. 4. The basic principle of
operation of the detection scheme for a dynamic strain measurement system
can now be described by considering two sinusoidal transfer functions out
of phase by 90.degree.. Assuming that the transfer function for the first
detector (D.sub.1) leads that of the second detector (D.sub.2), the output
waveform for D.sub.1 leads that of D.sub.2 until time t.sub.1. At time
t.sub.1, the phase .phi. changes direction because of a change in
direction of the strain, and the output waveforms switch their lead-lag
properties. The output from D.sub.2 now leads that of D.sub.1 until time
t.sub.2, when the strain changes direction again. Keeping track of the
lead-lag phenomenon between the two detectors provides unambiguous
information about the relative direction of the strain.
If only one detector is used, the switch in direction would not be
noticeable if the strain changes direction at a peak of the transfer
function curve. With two signals out of phase by 90.degree., if the
direction change occurs at one peak (of either D.sub.1 or D.sub.2), the
other transfer function curve will provide information about the direction
change.
Practical methods of obtaining two signals 90.degree. out of phase with
respect to one another are shown in FIGS. 6, 7 and 8. FIG. 6 shows the
basic components as comprising the extrinsic Fizeau sensor shown in FIG. 1
and a replication of light source, detector, coupler, and single mode
optic fiber. More particularly, the quadrature phase-shifted, extrinsic
Fizeau sensor 30 according to one embodiment of the invention comprises
first and second single mode fibers 31 and 32 used as input-output fibers.
The fiber 31 conducts light from a laser source 33 to the Fizeau sensor 34
and reflected light back to a fused biconical tapered coupler 35.
Similarly, the fiber 32 conducts light from a laser source 36 and
reflected light back to a fused biconical tapered coupler 37. The
reflected light coupled in coupler 36 is conducted by a single mode fiber
38 to a detector 39 (D.sub.1), and the reflected light coupled in coupler
37 is conducted by a single mode fiber 41 to a detector 42 (D.sub.2). The
outputs of detectors 39 and 42 are supplied to a data analyzer 43, which
may be implemented with a personal computer (PC) programmed to track the
phase reversals and provide a measure of the polarity of the sensed strain
as well as the magnitude of the strain.
In FIG. 7, the two single-mode fibers 31 and 32 are inserted into one end
of a single hollow silica tube 43. A multimode optic fiber 44, similar to
multimode fiber 22 in FIG. 2, is inserted in the opposite end of hollow
silica tube 43. The gap-separations for the two fibers 31 and 32 versus
the fiber 44 are adjusted actively by moving the fibers 31 and 32 in and
out of the tube 43 until a 90.degree. phase shift is achieved at the
output detectors 39 and 42.
FIG. 8 shows as system using two different silica tubes 45 and 46 and two
different multimode fibers 47 and 48, the two multimode fibers 47 and 48
being movable in unison. In this construction, the quadrature phase shift
is also adjusted actively. Since the two tubes have external diameters on
the order of a few hundred micrometers, the two sensors monitor almost the
same environmental perturbations.
In FIG. 6, two different sources are used in order to avoid interference
effects between the two signals returning to the coupler if only one
source was used to launch light into the single-mode fibers. It is also
possible to use a single source and two different lengths of lead fibers
for the two sensors such that the difference in the lengths is greater
than the coherence length of the laser.
To test the validity of the quadrature phase-shifted sensors, the sensor
was attached, using the scheme shown in FIGS. 6 and 7, to a cantilever
titanium beam with an epoxy. The fiber sensor was attached along the
length of the beam and beam vibrations were monitored. A typical
oscilloscope trace is shown in FIG. 9 which clearly shows the shift in the
lead/lag properties of the two signals as the relative direction of the
strain in the beam changes form increasing to decreasing. Sensitivities of
5.54.degree. phase shift/microstrain-cm have been obtained.
Thus, the simple, extrinsic Fizeau sensor according to the invention can be
operated in a quadrature phase-shifted mode to obtain dynamically varying
strain information. Limitations of the frequency range of operation will
be set by the signal processing electronics at the output. The
signal-to-noise ratio of the sensor decreases if a large air gap is
introduced since the fringe contrast starts to drop. Hence, the sensor
according to the invention will be useful for applications in which
maximum displacements to be measured are of the order of a few hundred
micrometers. Minimum displacements of 5 nm have been detected in a
laboratory environment.
FIG. 10 shows a modification of the quadrature phase shift sensor shown in
FIG. 6 wherein a single laser light source is used instead of the two
shown in that figure. With reference to FIG. 10, the single diode laser 51
is modulated by two current levels output by a function generator 52 to
provide light at two wavelengths, .lambda..sub.1 and .lambda..sub.2. These
wavelengths are sufficiently close that a range within the beat length of
the two light signals can be chosen that, within the small range of the
gage length, approximate phase quadrature signals. The light from the
laser 51 is coupled via a single mode optical fiber 53 to a coupler 54.
The coupler 54 passes this light to a single mode optical input/output
fiber 55 to the sensor 56. The sensor 56 is constructed as shown in FIG.
2. The reflected light from sensor 56 is passed via input/output fiber 55
back to the coupler 54 which, in turn, couples the reflected light via
single mode optical fiber 56 to a detector 57. Detector 57 generates an
electrical output signal which is supplied to an electronic signal
processor 58. The electrical signal from the detector carries information
which reflects the pulsed modulation of the laser diode. Therefore, the
electronic signal processor 58 is synchronized to the function generator
52 to sample the detector signal at points intermediate the leading and
trailing edges of the pulsed output from function generator 52.
FIG. 11 shows another modification to the basic invention which allows
multiplexing a plurality of extrinsic Fizeau fiber optic sensors of the
type shown in FIGS. 1 and 2. For purposes of illustration, there are three
such sensors 61, 62 and 63 shown, but the principle may be applied to an
unlimited number of sensors depending only on the specific application.
Light is supplied from a laser 64 via a single mode optical fiber 65 to
the first of a plurality of cascaded couplers 66, 67 and 68, each of which
is coupled to the next preceding coupler via a single mode optical fiber
69, 70 and so forth. Each coupler supplies light to a respective one of
the sensors 61, 62 and 63 via corresponding single mode optic input/output
fibers 71, 72 and 73. The reflected light from sensors 61, 62 and 63 is
coupled via respective optical fibers 71, 72 and 73 back to corresponding
couplers 66, 67 and 68 and thence, via respective single mode optical
fibers 74, 75 and 76, to a common detector 77. The electrical output of
the common detector 77 is coupled to an electrical spectrum analyzer 78.
The laser 64 is modulated by a stepped function generator 79 which provides
a synchronizing signal to electrical spectrum analyzer 78. The modulating
frequencies, .omega..sub.1, .omega..sub.2 and .omega..sub.3, sequentially
modulate the light output of laser 64. While each of the sensors 61, 62
and 63 respond to laser light modulated by each of these frequencies, the
electrical spectrum analyzer displays only the response from sensor 61 for
the modulating frequency .omega..sub.1, the response from sensor 62 for
the modulating frequency .omega..sub.2 and the response from sensor 63 for
the modulating frequency .omega..sub.3. This is accomplished by providing
suitable delays in the signals from couplers 66, 67 and 68 to the detector
77 so that the spectrum analyzer 78, triggered by the synchronizing signal
from function generator 79, responds to only those sensor responses. The
delays are provided by choosing lengths of optic fibers 74, 75 and 76 so
that the desired responses arrive simultaneously at the detector 77. The
resulting electrical signal is itself modulated, and it is the frequencies
of this modulation which are detected and displayed by the spectrum
analyzer 78. The amplitudes of the respective frequency peaks provide a
measure of strain sensed by each of the sensors 61, 62 and 63.
A variation of the system shown in FIG. 11 provides for quadrature phase
techniques as described above are applied so that the polarities of the
strains sensed by each of the sensors may be monitored. More particularly,
two lasers 64 and 81, having wavelengths of .lambda..sub.1 and
.lambda..sub.2, respectively, are controlled by the function generator 79.
Thus, each of the two light wavelengths .lambda..sub.1 and .lambda..sub.2
are modulated by the modulating frequencies .omega..sub.1, .omega..sub.2
and .omega..sub.3. The modulated light outputs of the lasers 64 and 81 are
combined in a wavelength division multiplexer 82 and output onto the optic
fiber 65. In addition, two detectors 77 and 83, tuned to wavelengths of
.lambda..sub.1 and .lambda..sub.2, respectively, are coupled via a second
wavelength division multiplexer 84 to each of optical fibers 74, 75 and
76. The output of detector 83 is connected to a second electrical spectrum
analyzer 85. The desired linearized electrical output proportional to
strain can be obtained by supplying a signal processor 86 with the two
electrical spectrum analyzer outputs. The signal processor tracks the
lead-lag phenomena shown in FIG. 9, thereby providing a measure of
polarity of the sensed strain.
Another system which uses the extrinsic Fizeau interferometer fiber optic
sensors according to the invention is shown in FIG. 12. Whereas the
sensors in the system of FIG. 11 were connected to couplers which were
connected in cascade, the sensors 91, 92 and 93 are themselves connected
in cascade. This is accomplished by a simple modification of the sensor
structure shown in FIG. 2 wherein the multimode fiber 22 is replaced by a
single mode fiber which acts as the input/output optical fiber for the
next succeeding sensor in the cascaded string. Thus, for example, sensor
91 is connected to sensor 92 via a single mode input/output optical fiber
94, sensor 92 is connected to sensor 93 via a single mode input/output
optic fiber 95, and so forth. The entire string is connected to a single
mode optical fiber 96 which, in effect, acts as the input/output fiber for
the string.
The system shown in FIG. 12 contemplates the quadrature phase shift
principles described above for measuring polarity as well as relative
amplitude of strain sensed by the sensors; however, it will be understood
by those skilled in the art that this technique of connecting sensors in
cascade does not depend on quadrature phase sensing. That is, this
technique could be applied to a sensing and monitoring system in which the
polarities of the sensed strains is not important. In the system
illustrated in FIG. 12, an optical time division reflectometer 97 provides
light of two wavelengths, .lambda..sub.1 and .lambda..sub.2, to a
wavelength division multiplexer 98. The output of multiplexer 98 is
coupled via single mode optic fiber 101 to coupler 102, and the output of
coupler 102 is coupled to the input/output optical fiber 96. The light
received by coupler 102 from input/output optical fiber 96 is in turn
coupled via single mode optical fiber 103 to a second wavelength division
multiplexer 99. The output of multiplexer 99 is divided into the two
wavelengths .lambda..sub.1 and .lambda..sub.2 and input to the
reflectometer 97. The reflectometer 97 generates two electrical signals,
corresponding to the received wavelengths .lambda..sub.1 and
.lambda..sub.2, and these electrical signals are connected to respective
digital oscilloscopes 104 and 105. The optical pulse from the optical time
division reflectometer is on the order of 50 pico seconds in width and is
repeated at a Khz rate. Fibers 94, 95 and 96 must be long enough relative
to the pulse width such that the returned reflections from each sensor are
distinguishable and are not overlapped in time. The intensity of the
reflected signal is an indication of the interference between the
reference and sensing signals of each sensor. The oscilloscope displays
the reflected intensity of the individual sensors in time.
Optionally, the outputs of the digital oscilloscopes 104 and 105 are
connected to a data analyzer 106, which may be implemented with a personal
computer (PC) programmed to analyze the digital data outputs from the
oscilloscopes. More specifically, the data analyzer will monitor the
relative changes in intensity of the individual peaks reflected in time
from each of the multiplexed sensors to allow the detection of strain and
its polarity.
The systems thus far described measure relative strain. It is also possible
to measure absolute strain using the extrinsic Fizeau fiber optic sensors
according to the invention. FIG. 13 shows a first approach which comprises
a sensor 111 of the type shown in FIG. 2. Light from a broad spectrum
source 112, such as a light emitting diode or with white light, such as
from a tungsten bulb, is coupled via a single mode optic fiber 113 to a
coupler 114 and thence, via an input/output single mode optical fiber 115,
to the sensor 111. The reflected light from the sensor 111 is coupled via
coupler 114 and single mode optical fiber 116 to an optical spectrum
analyzer 117. The output of the optical spectrum analyzer 117 is a display
of optical peaks which represent wavelengths of constructive interference
caused by a unique air gap displacement of the extrinsic Fizeau
interferometer sensor according to the invention. A data analyzer 118 is
used to process the signal from the optical spectrum analyzer into the
desired form or display.
Another system for measuring absolute strain is shown in FIG. 14 wherein a
laser 121 is modulated by a sawtooth waveform so that the frequency of the
laser light increases linearly over a time period, t. The thus modulated
light from laser 121 is coupled via single mode optic fiber 122 to a
coupler 123 and thence, via a single mode input/output optic fiber to a
sensor 124. The sensor 124 is of the type shown in FIG. 2. The modulated
light from the laser is also coupled via coupler 123 and a single mode
optic fiber 125 to a first, or reference, detector 126. The reflected
light from sensor 124 is, in turn, coupled via coupler 123 and a single
mode optic fiber 127 to a second detector 128. The electrical signals from
the two detectors 126 and 128 are divided in analog divider 129 to
generate a quotient signal which is supplied to an electrical spectrum
analyzer 130. The output of the electrical spectrum analyzer 130 can be
supplied to a data analyzer 131, said as a personal computer (PC), to
manipulate the data into the desired form.
While the invention has been described in terms of a single preferred
embodiment with variations in construction, those skilled in the art will
recognize that the invention can be practiced with modification within the
spirit and scope of the appended claims.
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