DESIGN OF A FIBER BRAGG BASED MEASUREMENT SYSTEM FOR
STRAIN AND TEMPERATURE MONITORING
J. A. Smith, G. S. Glaesemann
Advanced Engineering, Corning Incorporated, Corning, NY 14831
ABSTRACT. The design of a fiber optic sensing system will be presented. The goal is to develop a
measurement system that will be deployed in a manufacturing or field environment. The sensing
elements will be formed from Bragg gratings and from Fabry-Perot interferometers constructed from
fiber Bragg gratings. The novelty of the method presented in this study lies in the simplicity and speed
in which stress and temperature can be simultaneously measured.
INTRODUCTION
To provide more efficiency in the design of systems involving engineering
structures, adaptive structures and manufacturing processes, the ultimate limits of the
engineering materials used in the systems are being approached. The reduced margins of
safety for the material used in the design of the systems will require the use of sensing
systems to measure strain and temperature. This will allow the designs to maintain safety
and reliability.
The purpose of this paper is to introduce a design for direct measurement of
dynamic stresses and temperatures in optical fiber sensors during processing, deployment,
and in-service lifetime. This method utilizes the well known stress dependence of fiber
Bragg gratings [1]. The novelty of the method presented in this study lies in the simplicity
and speed in which stress and temperature can be simultaneously measured and the ability
to actively monitor while the sensing fiber is being deployed. The goal is to develop a
measurement system that will be deployed in a manufacturing or field environment.
This paper will discus the sensing elements that are constructed from fiber Bragg
gratings and from Fabry-Perot interferometers built from fiber Bragg gratings. A
theoretical simulation studying the shifts in center wavelengths from the fiber Bragg and
the Bragg Fabry-Perot at various strains and temperatures will be presented that supports
the feasibility of separating strain changes from temperature changes. These results will
then be used to specify the instrumentation.
FIBER BRAGG SENSORS
Fiber Bragg gratings are naturally suited to handle the rigors of manufacturing
processing. When a grating is written into a fiber, the grating becomes part of the glass
with minimal or no loss in mechanical strength.. The Fiber Bragg grating reflects a
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
859
narrowband of wavelengths allowing for single sided and wavelength multiplexing
measurements.
The transduction mechanism for a fiber Bragg sensor is simple and reliable.
Variations in strain and temperature will cause a shift in the center wavelength of the
reflected light. The sensing schemes become complicated and expensive when
temperature effects must be separated from strain effects. The primary focus of this
report is the efficient separation of temperature and strain measurements.
DECOUPLING STRAIN AND TEMPERATURE EFFECTS
A strain measurement scheme must use a strain transducer that is totally thermally
insensitive or measure temperature and apparent strain in the fiber. The former is difficult
to achieve in fiber sensors and the latter is implemented in Figure 1. Two sensors are
incorporated into the same length of fiber: A hybrid Bragg Fabry-Perot (BFP) cavity and
a Fiber Bragg Grating (FBG) can be used to separate temperature and strain effects. Two
identical FBGs are used as partial mirrors which creates a FP sensor. A FBG sensor with
a center wavelength outside of the hybrid cavity bandwidth is placed within the FP cavity.
Thus the two sensors can operate independently.
Fiber Bragg Grating
A Fiber Bragg Grating (FBG) [1-3] is a wavelength selective device created by
forming a grating which modulates the index of refraction in a core of an optical fiber.
The FBG is a wavelength selective reflector. Figure 2 shows the simulated reflected
spectrum for a FBG. The grating period and the effective index of refraction [3]
determine the center wavelength of the reflected optical spectrum. The length of the
grating [3] determines the bandwidth of the reflected spectrum. Thus changes in the
location of the center wavelength in the reflected spectrum are caused by changes in the
grating period or the effective index of refraction. The center wavelength and effective
index of refraction within a FBG transducer are affected by changes in strain and
temperature. This cross sensitivity also makes decoupling temperature effects from strain
effects difficult.
The transmission spectrum is beneficial for wavelength addressing of a FBG in
transducer arrays. Each grating transducer would be allocated to operate within a center
frequency and bandwidth much like channel spacing in telecommunication systems. The
gratings used for sensing applications do not need to match the performance of
telecommunications gratings. The sensing gratings will be easier to manufacture and less
expensive.
Decoupling Strain and
Temperature Effects
ar
•L
•/'•
ar / as
Fabry-Perot Sensor
Fiber Bragg Sensor
FIGURE 1. Placing a FBG sensor within a hybrid BFP sensor can decouple temperature and strain if
the center wavelength sensitivity ratio with respect to temperature and strain for the BFP sensor is not
be equal to the identical ratio for the FBG sensor.
860
Bragg
Grating Reflectivity
Reflectivity vs.
vs. Wavelength
Wavelength
Bragg
Grating
Bragg
o0 Grating Reflectivity vs. Wavelength
Reflectivity(dB)
(dB)
Reflectivity
0
-20
-20
-20
-40
-40
-60
-60
-60
-80
-80h
-80
-100
-1
-100
1550
1550
1550
Strain:
—— Strain:
LL8
Strain: 0|0µε
0µε
O
Temperature:
Temperature:
25°C
Temperature: 25
25OC
C
1550.5
1551
1550.5
1551
1550.5
Wavelength
(nm)
Wavelength
Wavelength (nm)
1551.5
1551.5
FIGURE
2.2.Reflection
Reflection
spectrum
from
Bragg
grating
under
no
load
condition.
FIGURE
FIGURE2.
Reflectionspectrum
spectrumfrom
fromaaaBragg
Bragggrating
gratingunder
underaaano
noload
loadcondition.
condition.
Bragg
Faby-Perot
Reflectivity
Bragg
vs. Wavelength
Wavelength
BraggFaby-Perot
Faby-Perot Reflectivity
Reflectivity vs.
o00
Reflectivity (dB)
(dB)
Reflectivity
-20
-20
-40
-40
-60
-60
-80
-80
-100
-100
-100-120
-120
1550
1550
50
Strain:
0µε
Strain: 0(18
0µε
—— Strain:
Temperature:
Temperature:
25OOC
C
Temperature: 25
25°C
1550.5
1551
1550.5
1550.5
1551
Wavelength
(nm)
Wavelength
(nm)
Wavelength (nm)
1551.5
1551.5
FIGURE
3.3. Power
reflectivity
cavity. Note the
FIGURE3.
Powerreflectivity
reflectivityas
asaaafuction
fuction of
of wavelength
wavelength for
for aaa hybrid
hybrid BFP
FIGURE
Power
as
fuction
of
wavelength
for
hybrid
BFP cavity. Note the transmission
transmission
ofoflight
light
near
the
center
of
the
Bragg
grating
reflectance
peak
at
≈1550.4
nm. This
lightnear
nearthe
thecenter
centerof
ofthe
theBragg
Bragggrating
gratingreflectance
reflectance peak
peak at
at -1550.4
≈1550.4 nm.
of
This is
is the
the unloaded
unloaded state
state of
of
the
thesensor.
sensor.
the sensor.
861
Fabrv-Perot Cavity
A Fabry-Perot (FP) cavity [1, 4] is a wavelength selective device formed by
placing two partial mirrors in the optical path. The Fabry-Perot cavity selectively transmits
wavelengths that are integer multiples in value. The spacing between the transmitted
spectral peaks (free spectral range) [4] are determined by the effective cavity length
formed by the partial mirrors. The bandwidth and amplitude of the spectral peaks are
determined by the mirror reflectivity. Thus changes in the location of the spectral peaks
are only caused by changes in the effective cavity length.
The effective length of a FP cavity is affected by changes in both temperature and
strain. This cross sensitivity of the effective cavity length makes decoupling temperature
effects from strain effects difficult. The transmission spectrum also limits the use of FP
cavities in transducer arrays. The transmission of multiple wavelengths through the FP
cavity and the attenuation of off resonance wavelengths make addressing multiple
transducers by wavelength problematic. The use of Fiber Bragg Gratings can mitigate the
problems of cross sensitivity and wavelength addressing.
Hybrid Bragg Fabrv-Perot
By combining the FBG with a FP cavity, the hybrid sensor's response to strain and
temperature changes will be substantially altered from a FBG response. Figure 3 shows
the simulated reflected spectrum for a hybrid FP cavity created from FBGs. The hybrid
FP cavity transmits all light outside of the FBGs bandwidth. When the optical wavelength
approaches the FBG bandwidth, the FBGs begin to reflect the light. As the optical
wavelength approaches the resonant wavelength of the hybrid cavity, the light begins to be
transmitted. The hybrid cavity now behaves as a normal FP cavity until the wavelength of
light falls out side of the FBG bandwidth.
Integrating the Sensors
The requirement to uncouple strain and temperature effects from the two types of
sensors is given by the Equation in Figure 1. The equation states that the center
wavelength sensitivity ratio with respect to temperature and strain for the BFP sensor must
not be equal to the identical ratio for the FBG sensor. The greater the inequality, the
greater the achievable measurement accuracy of strain and temperature.
Since the bandwidth of the FBG sensor is outside of the bandwidth of the FP
cavity, the two sensors do not interfere with each other and are uniquely wavelength
addressable. Their close proximity also assures that the two sensors are affected by the
same local environmental conditions. However, the sensitivities of the two sensors with
respect to strain and temperature are significantly different. The FP cavity is most
sensitive to changes in the effective cavity length and the FBG is most sensitive to
changes in the grating spatial frequency.
SENSOR SYSTEM MODELING
The grating sensors are modeled using fundamental models which are straight
forward and simplistic. The models are linear in nature and will not accurately follow
nonlinear changes in the fiber due to mechanical and thermal loading. The effects of
loading are measured by following the changes in the center wavelength of the reflected
spectrum from a Bragg grating and by monitoring changes in the transmission
wavelengths within the reflective spectrum in the hybrid BFP cavity. The models have
862
been tested and have been shown to provide for a linear change in center wavelength for
strains from 0 to 0.02, for temperatures from 25° to 250 °C, and for the combined effects
of temperature and strain.
The FBGs are modeled using the classical equation for reflection as shown in
Equation 1. The small r denotes amplitude reflectivity. The notation can be found in
Reference 3. The small r denotes amplitude reflectivity.
The Fabry-Perot cavity is modeled using the classical FP equation shown as
Equation 2 [4]. The uppercase Rs denote the power reflectivity of the cavity (R), partial
mirror 1 (Ri) and partial mirror 2 (R2). 0 is the optical path length as defined in reference
4.
_
a cosh(o£) - i
_
sin2 0
(i - 4R\Ri )2 + *4R\Ri sin2 °
The Bragg Fabry-Perot is modeled by substituting the reflectivity calculated by the
FBG Equation 1 into the FP Equation 2 for the reflectance of the partial mirrors. Thus the
hybrid BFP cavity will have the wavelength selective properties of a Bragg Grating along
with the narrow band transmission of a FP cavity as shown in Figure 3.
Temperature and Strain can potentially be separated by only using the hybrid BFP.
The reflective peak caused by the FBG can be followed as well as the narrow transmission
lines due to the FP. However to make the processing easier due to the distortion of the
reflective peak by the FP transmission lines, the response to a separate FBG sensor has
been followed.
Figure 3 shows the reflected spectrum from an unloaded BFP sensor. Note that
there is only one transmission line in the return spectrum. The monitoring of only one
transmission line within the reflected spectrum will work well for small temperature and
strain variations. However with large temperature and strain variations, the center
wavelength changes will vary in excess of the FP free spectral range. Therefore, the BFP
sensor should be designed such that the bandwidth of the FBG is slightly larger than the
free spectral range of the interferometer. This will allow continuous monitoring of the
changing transmission line wavelengths by accounting for the transition into and out of
multiple FP free spectral ranges.
Even though separating temperature effects from strain effects is difficult, the
interaction between temperature and strain is linear. This is demonstrated in Figure 4 by
the two planes created by the two types of sensors. The changes in measured wavelength
will be a linear function of loading in the sensors. Figure 4 also clearly shows that
calibration of the sensors with respect to temperature and strain can be accomplished
independently. This greatly simplifies the calibration procedure. Since the two planes
intersect along a line, the wavelength combination for the FBG and the BFP sensors will
be unique. Thus a lookup table can be created to determine temperature and strain from
the measurement of wavelength provided by the two types of sensors or the sensing matrix
can be inverted [1,6].
SENSING SYSTEM DESIGN
The modeling has shown promise for the two sensors to separate temperature and
strain. It is now appropriate to consider the design of the measurement system. The
863
measurement
gives aa
measurementsystem
systemmust
must be
be able
able toto meet
meet the
the specified
specified requirements.
requirements. Table
Table 11 gives
listing
based
listingofofthe
therequirements
requirementsand
andthe
thecorresponding
corresponding implicit
implicit measurement
measurement requirements
requirements based
ononthe
themodels
modelsdeveloped
developedininthis
thispaper.
paper.
Bragg Fabry-Perot and Bragg Sensors Wavelength
Response to Temperature and Strain Variations
1554
150
100
50
Temperature (°C)
FIGURE 4. The calculated values for center wavelength in
FIGURE 4. The calculated values for center wavelength in
wavelength in a BFP sensor for temperature and tensile loadings.
wavelength in a BFP sensor for temperature and tensile loadings.
to the FBG and the bottom plane is the response due to the BFP.
to the FBG and the bottom plane is the response due to the BFP.
a FBG sensor and transmission
a FBG sensor and transmission
The top plane is the response due
The top plane is the response due
Grating Based Measurement System
Grating Based Measurement System
Broadband Light
Source
Payout Spool
Payout Spool
j
FBG & BFPJ
FBG
& BFP j
Sensors
Sensors
3 dB Coupler
Manufacturing Process
Manufacturing Process
t
rr>\j oi, r>rr
FBG
& BFP
Sensors
Sensors
—
j
FBG & BFP
FBG
& BFP
Sensors
Sensors
Grating
Spectrometer
Computer
Linear Array
Camera
Computer
liillfli
FIGURE 5. Schematic of the measurement system designed to make measurements of the return
spectra from
the Fiber
Bragg
Grating Sensors
and the to
Bragg
sensors.
The
FIGURE
5. Schematic
of the
measurement
system designed
makeFabry-Perot
measurements
of the return
measurement
will be
an instantaneous
Optical and
Spectrum
Analyzer.
spectra
from system
the Fiber
Bragg
Grating Sensors
the Bragg
Fabry-Perot sensors. The
measurement system will be an instantaneous Optical Spectrum Analyzer.
864
TABLE
wavelength measurment
measurmentsystem.
system.
TABLE 1.1. Specifications
Specifications for
for aa wavelength
Requirement
Requirement
Physical
Physical
Specification
Specification
Strain
Strain Range
Range
Strain
Strain Resolution
Resolution
Strain
Strain Rate
Rate
Acquisition
Acquisition Rate
Rate
Temperature
Temperature Range
Range
Temperature
Temperature
Resolution
Resolution
00 -- 5,000
5,000 µε
us
10
10 µε
U£
ε/s
100
lOOe/S
77kHz
kHz
O
-55
200 °C
C
-55 -- 200
Wavelength
Wavelength
Detection
Detection
Specification
Specification
9.5 nm
nm
9.5
0.02 nm
nm
0.02
µ
190
m/s
190 Um/s
7 kHz
7kHz
3.6 nm
3.6nm
O
22°C
C
0.03 nm
nm
0.03
A
meet the
the needed
needed
A wavelength
wavelength measurement
measurement system has been developed
developed to meet
specifications given
given in
in Table
Table 11 [5].
[5]. A schematic
schematic diagram of the system
specifications
system is shown
shown in
in Figure
Figure
5. The
The measurement
measurement system
system is
is based
based on
on an
an instantaneous
instantaneous Optical Spectrum
Spectrum Analyzer.
5.
Analyzer. A
A
broadband light
light source
source isis used
used to
to interrogate
interrogate the grating based sensors as they are being
broadband
fed through
through the
the fiber
fiber processing
processing equipment. The light spectra which is reflected
fed
reflected by
by the
the
sensors isis guided
guided to
to the
the optical
optical spectrometer.
spectrometer. The reflected light is incident upon a grating
sensors
grating
inside the
the spectrometer
spectrometer which spatially
spatially disperses the light based on wavelength.
inside
wavelength. The
The
linear array
array measures
measures the light intensity as a function
linear
function of wavelength
wavelength along
alongthe
the linear
lineararray.
array.
Since aa broadband
broadband wavelength
wavelength spectrum is taken at an instant in time,
Since
time, multiple
multiple
sensors can
can be
be monitored
monitored at
at the same
same time (wavelength multiplexed)
sensors
multiplexed) as long
long as the
the
individual spectra
spectra from
from the sensors
sensors do not overlap. Sensors can be added
individual
added or
or dropped
dropped
without reconfiguring
reconfiguring the measurement system. The system
without
system can
can be
be optimized
optimized for
for
broadband
wavelength
(dispersion)
coverage
or
for
wavelength
resolution
by
broadband wavelength (dispersion)
wavelength resolution by changing
changing the
the
spectrometer's grating
grating pitch. The
The center
center wavelength of the spectrometer
spectrometer can be
spectrometer’s
be changed
changed
to provide
provide broad
broad band
band coverage
coverage at maximum wavelength resolution.
to
resolution. The
The spectral
spectral output
output
will be
be useful
useful when
when trying to use the sensors for other types
will
types of
of fiber
fiber loading
loading such
such as
as
bending, lateral
lateral compression,
compression, and to diagnose sensor failure
bending,
failure mechanisms.
mechanisms.
A commercial
commercial spectrometer
spectrometer will meet the needed specifications
A
specifications given
given in
in Table
Table 1.
1.
The
resolution
of
the
spectrometer
with
a
50
Jim
aperture
(pixel
width)
will
be
close
The resolution of the spectrometer with a 50 µm aperture (pixel width) will be close to
to 0.1
0.1
nm and
and have
have 20
20 nm
nm bandwidth
bandwidth for
for aa 1.33
1.33 m
m cavity
cavity length.
length. The
nm
The spectrometer
spectrometer comes
comes with
with
a
256
x
1
pixel
line
camera
which
will
acquire
the
entire
wavelength
spectrum
a 256 x 1 pixel line camera which will acquire the entire wavelength spectrum at
at aa rate
rate of
of
7.0 kHz.
kHz.
7.0
CONCLUSIONS
CONCLUSIONS
A measurement
measurement system
system design
design based
based on
on Fiber
Fiber Bragg
Bragg Grating
Grating sensors
A
sensors and
and aa grating
grating
spectrometer
measurement
device
has
been
presented.
The
measurement
spectrometer measurement device has been presented.
The measurement system
system is
is
designed to
to simultaneously
simultaneously measure
measure dynamic
dynamic strains
strains and
designed
and temperatures.
temperatures. This
This will
will provide
provide
more efficiency
efficiency in
in the
the design
design of
of systems
systems requiring
requiring the
more
the ultimate
ultimate limits
limits of
of engineering
engineering
materials. The
The stresses
stresses and
and temperatures
temperatures in
in these
these materials
materials can
can now
now be
materials.
be efficiently
efficiently and
and
dynamically monitored
monitored to
to assure
assure that
that the
the ultimate
ultimate limits
limits of
dynamically
of the
the materials
materials have
have not
not been
been
exceeded in
in aa manufacturing
manufacturing or
or field
field environment.
environment.
exceeded
The
design
contains
sensing
elements
that are
are constructed
constructed from
from Bragg
The design contains sensing elements that
Bragg gratings
gratings and
and
from
hybrid
Fabory-Perot
interferometers
built
from
fiber
Bragg
gratings.
A
from hybrid Fabory-Perot interferometers built from fiber Bragg gratings. A theoretical
theoretical
simulation confirms
confirms that
that the
the shifts
shifts in
in center
center wavelengths
wavelengths from
simulation
from the
the fiber
fiber Bragg
Bragg and
and the
the
Bragg
Fabry-Perot
sensors
at
various
strains
and
temperatures
can
be
used
to
the
separate
Bragg Fabry-Perot sensors at various strains and temperatures can be used to the separate
865
strain changes from temperature changes. The results of the study have been used to
specify a commercial spectrometer and line scan camera as the wavelength monitoring
device.
The grating based sensors can be wavelength multiplexed in a single fiber and
monitored at the same time. Sensors can be added or dropped without reconfiguring the
measurement system. The system can be optimized for broadband wavelength coverage or
for wavelength resolution. The center wavelength of the spectrometer can be changed to
provide broad band coverage at maximum wavelength resolution. The spectral output will
be useful when trying to use the sensors for other types of fiber loading such as bending,
lateral compression, and to diagnose sensor failure mechanisms. The novelty of the
measurement system designed in this study lies in the simplicity and speed in which stress
and temperature can be simultaneously measured.
ACKNOWLEDGEMENTS
The authors would like to thank Vikram Bhatia for his insightful comments and
discussions. The authors would also like to thank Corning Incorporated for proving the
support necessary to write this paper.
REFERENCES
1.
2.
3.
4.
5.
Culshaw, B., Smart Structures and Materials, (Artech House, Boston, 1996).
Agrawal, G. P., Fiber-Optic Communication Systems, (John Wiley & Sons, Inc.
New York, 1997).
Kashyap, R., Fiber Bragg Gratings, (Academic Press, New York, 1999).
Verdeyen, J. T., Laser Electronics, (Prentice Hall, Englewood Cliffs, 1989), Chap.
6.
Capouilliet, S., Smith, J. A., Walter, D. J. , Glaesemann, G. S., Kohnke G. E., and
Irion, R. D., A Fiber Bragg Based Measurement System for Monitoring Optical
Fiber Strain, 50th International Wire & Cable Symposium, Disney's Coronado
Springs Resort, Lake Buena Vista, Florida, USA, pp 240-248, November 12-15,
2001.
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