Structural Health Monitoring Using Statistical Pattern Recognition
Embedded Sensing:
Fiber Optics
Los Alamos Dynamics Structural Dynamics and Mechanical Vibration Consultants
Presented by
Michael D. Todd, Ph.D.
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Outline
• Optical fiber and photonics basics • fibers (optical waveguides)• photodetectors, couplers, filters, connections
• Fiber sensor approaches• intensity modulation• interferometry (phase difference modulation)• Bragg gratings (wavelength modulation)• hybrid approach based on Bragg gratings
• Applications• vibration, traffic monitoring on bridge• hull monitoring on composite boat• miscellaneous
• Fiber sensor advantages/disadvantages
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Some Fundamental Optics Ideas
• Optical radiation is an electromagnetic phenomenon and may described by electromagneticfield equations (electromagnetic waves)
• A waveguide is a dielectric (electrically non-conducting) material that is used to “guide” orpropagate these waves
• Optical propagation features:
• The refraction angle depends on the relative light wave speeds in the two materials; therefractive index (n) of a material is the ratio of light speed in a vacuum to light speed in the material (so always greater than 1)
medium 1
medium 2
incident ray reflected ray
transmitted or refractedray
i r
t
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Total Internal Reflection
n2 < n1
n1
refracted rays
reflected rays
total internalreflection
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Fiber: Cylindrical Optical Waveguide
• If medium 1 index is larger than medium 2 index, and the incident angle is large enough,then total internal reflection occurs: wave will not transmit into medium 2, and this isthe basis for how an optical waveguide works• Optical fibers are cylindrical dielectric waveguides:
core• glass-based (silica,fluoride, chalcogenide)• n~1.44 (1.31-1.55 m)• 8-980 m in diameter
• glass-based or plastic-based• n<1.44• 125-1000 m in diameter
cladding
coating/jacketing• plastic (acrylate, polyimide)• for protection, mechanical strength
• Optical fibers are characterized by the normalized frequency V:
V 2a
ncore2 ncladding
2 V < 2.405 single modeV > 2.405 multi-mode
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Optical Sources: Light-Emitting Diodes
Surface-emitting LED (SLED) Edge-emitting LED (ELED)
• LEDs are semiconductor devices that emit incoherent light, through spontaneous emission,when electrical current is passed through them• Fabrication materials are typically GaAs and AlGaAs (850 nm) and InGaAsP (1330-1550 nm)• SLEDs used for short-distance (0-3 km), lower bit rate (<250 Mb/s) systems, ELEDs forlarge distance, higher bit rate systems• ELEDs more sensitive to temperature fluctuations than SLEDs• optical bandwidth typically 30-70 nm FWHM, Gaussian profile• max power typically 15 W - 20 mW (superluminescent)
1550SLED
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Photodetector: Light to Volts
• photodetectors are devices through which optical power is converted to an electrical signalvia an absorption process
• photons are converted to electric charge carriers, and an electric field is applied to thephotodetection region to measure their effect
• most common types: PIN and avalanchephotodiodes
• APD has higher responsivity (internalgain) and higher shot noise than PIN
• PIN is cheaper, doesn’t require thermalcompensation
• typical InGaAs performance:
950-1650 nm operation, 1 A/W, 5 ns response time, 0.2 pW/Hz0.5 noise
3-4 cm
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Fiber Optic Components: Couplers
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture
• used to combine/split optical signals from differentfibers
• take advantage of evanescent field coupling: someof the field extends beyond core
• coupling lengths are usually a few millimeters
L
evanescent fieldP1
P2
P1 P1(0)cos2 kL
P2 P1(0)sin2 kL
input power
reflected power
transmitted power
coupled power
4-5 cm
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Fiber Optic Components: Tunable Filters
broad-band lightenters the filter...
A stepped voltagedrives a piezoelectricdevice which controlsthe mirror spacing
…but only a narrowwavelength band getspassed throughthe filter
• produced for wavelength operation 360-1600 nm• free spectral ranges between 40-60 nm• passband of ~0.1 nm (at 1550 nm)• losses below 3 dB
6-7 cm
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Fiber Optic Connections
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
ST SC
FC/PC or FC/APC
• keyed bayonet (like BNC)• MMF and SMF
• pop in/out connectorwith locking tab in plastic housing• SMF typically• durable and cheap
• position-tunable notch andthreaded receptacle• SMF only• very precise positioningand < -50 dB reflectivity
Typical performance: 0.2-0.5 dB insertion loss, <-40 dB reflectivity, temp. range -20 to 60 oC
E2000
• shutters provideprotection fromenvironment anddamage
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Fiber Optic Splicing
• Two fibers may be coupled together axially(spliced) by precise alignment of their cores
• Requires precise rectangular-edged cleave atthe fiber interfaces
cleaver
fusion splicer• Fusion splicers use an electric arc to weld thecleaved fiber faces together
• Use computer-controlled alignment using outerfiber contour lines
• Losses are about 0.02 dB
• Integrated cleaver, splicer, and recoatercommercially available ~$40K
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Component Integration: General Sensing System
opticalsource
sensingmechanism
photodetection
interferometry
intensitymodulation
Bragg gratings electronicprocessing
(non-optical)
~30 cm
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Optical Intensity Changes: Microbend Sensors
measurand
fiber
light modulatedlight
• measurand directly excites corrugated fiber clamp• localized bending in the fiber causes transmission power loss• for good sensitivity, typically requires multimode fiber• sensitivities 10-10 m/Hz1/2 reported• advantages: low insertion loss (light stays in fiber), fail-safe (total failure = no light),• disadvantages: requires compensation scheme (multiple sources of intensity fluctuations),
behavior highly dependent on modal properties (need optical source and insertion control)
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Optical Intensity Changes: Evanescent Field Interaction
• with cladding removed, evanescent wave can interact directly with measurand• typically, measurands are chemical or biological species or moisture (absorb light)• can be enhanced with specialized polishes, films, or layers• disadvantage: weak interaction with measurand due to small optical field penetration
into the cladding
evanescent waveinteraction
measurand
cladding
light modulatedlight
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• extrinsic architecture: light exits fiber to interactwith measurand (almost always mechanical)• fiber separation physically modulated by measurand,leading to transmission or reflection power loss• very simple devices (low cost), but suffer from nonlinearity, poor coupling efficiency,and high sensitivity to extraneous (undesirable) measurands
Optical Intensity Changes: Waveguide Coupling
measurand
light modulatedlight
measurand
light
modulatedlight
transmission configuration
reflection configuration
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Interferometric Sensing• An interferometer is a device in which two (or more) optical pathways are compared
• A sensor may be realized by coupling one of the optical paths to the measurand (signalarm) and isolating the other path (reference arm)
• If the measurand physically changes the length of the signal arm, then the relative difference ∆L between the path lengths creates an optical phase change ∆ø between thetwo signals when they are recombined:
I I0[1 M cos ] I0[1 M cos(2n
L)]
• When this recombined signal is photodetected, its intensity is given by
2n
L
where I0 is the mean signal level, M is the visibility of the interferometer, n is the corerefractive index, and is the wavelength of the light.
The detector signal directly encodes the measurand changes.
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Primary Interferometer Configurations
light inphotodetectioncouplercoupler
reference fiber
signal fiberMach-Zehnder
Michelson
light in
photodetection
coupler
signal fiber
reference fiber
reflectors
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Interferometer Phase Recovery
The phase difference to be extracted is buried inside a modulated waveform at the detector:what we see is I, but what we want is ∆ø, and these are related through a cosine function.
Homodyne approaches: lock the interferometer in quadrature by forcing the static phaseoffset between arms to be at π/2+Nπ (piezo stretcher on reference arm + control loop)
Heterodyne approaches: add an active carrier signal to the reference arm or modulate the optical wavelength and use a phase-locking technique to extract phase
time
dete
ctor
out
put Depending on the initial static
phase difference between thearms, the output signal varies in intensity.
QuickTime™ and a Motion JPEG A decompressor are needed to see this picture.
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Intrinsic Local Sensor: Bragg Grating
• A fiber Bragg grating is region of periodic refractive index perturbation inscribed in the coreof an optical fiber such that it diffracts the propagating optical signal at specific wavelengths.
fibercore
refractive index modulation period, T
• Each time the forward-propagating light encounters a stripe (index mismatch), some isscattered (diffracted)
• Scattered light accrues in certain directions if a phase-matching condition is satisfied: in particular, at the resonant wavelength given by rnT, light is reflected backward inphase with previous back-reflections such that a strong reflection mode at wavelengthr is generated
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Bragg Grating Fabrication
optical fiberouter cladding
fiber core(Ge-doped)
reflection =nUV
sin/22 n T =
coherent ultraviolet beam
at wavelength UV=244 nm
modulation ofrefraction index(Bragg grating)grating period T
• This photosensitivity occurs because electronic absorptions in silica materials are in this UVregime; this effect is enhanced with Ge-doping through Ge sub-oxide defect production• Defects leads to refraction index change (Kramers-Kronig relations)
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Bragg Gratings Act as Optical Notch Filters
tran
smis
sion
inte
nsi
ty
wavelength
nT
broadband lightinserted here
cladding coregrating
typical LED source spectrum (input)
refl
ecti
onin
ten
sity
wavelength
• light at wavelength is reflected
• FWHM of the reflection peak istypically 0.1-0.3 nm
• if the fiber is locally stretched orcompressed, T changes, meaningchanges
• gratings may be multiplexed in thewavelength domain by initiallywriting each grating to reflect at a unique wavelength
• sensor system must track individualwavelength shifts
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1550SLED
4-ch
ann
el W
DM
sp
litt
er
phase generated carrier/active homodyne
carrier modulation signal (~20 kHz)
Mach-Zehnderinterferometer
piezoelectricelement
Grating Interrogation: WDM
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Grating Interrogation: Tunable Filters1550
SLED
tunable fiberFabry-Perot
filter
tunableacousto-optic
filter
photodetector
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Grating Interrogation: Tunable Filters
photodetector
tunable fiberFabry-Perot
filter
d/dt
zero-crossingdetector
driving signal
voltagewav
elen
gth
voltage towavelengthconversion
compare
�
tunableacousto-optic
filter
+
x
VCO
∫
counter
�
driving signal
driving signal
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Grating Interrogation: CCD Array1550
SLED sensing array
collimating lens(bulk optics)
plane grating(1200 lines/mm)
spec
trom
eter
linear CCD
scanning signal
�
centroidcalculation
pixelarray
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Al G
compensationgratings
...
IR broadbandsource sensing gratings
1 2 m
3x3coupler
V1
V2
V3
Mach-Zehnderinterferometer
photodetectorvoltages
scanningFabry-Perot
filter
Optics Module
+
V1
V2
V3
one shot
peak detector
peak detector
peak detector
d/dt
VTH
multi-function
board
Electronics Module
�
Demodulation/Display Module
New Hybrid System: SFP + MZI+3x3
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Optics Module
1550SLED
tunable fiberFabry-Perot
sensingarray
2x2Mach-Zehnderinterferometer
2x2 3x3
photodetectors
compensationarray
Compensation Module
QuickTime™ and aAnimation decompressor
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G = i + GTA = i + AT
• two gratings on thermallymismatched substrates andplaced in sealed package
• may be interrogated seriallywith sensing FBGs byplacing them at spectrum edge
• interferometer drift and thermal shifts are detectedin this way
thermaldrift
interferometerdrift
Optics and Grating Compensation
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Key Performance Results
-4
-2
0
2
4
dete
ctor
out
put (
V)
1.000.950.900.850.80time (s)
-8
-4
0
4
demodulated phase (rad)
-12
-6
0
6
12
radi
ans
0.200.150.100.050.00time (s)
-150
-100
-50
0
50
spec
tral
den
sity
(dB
re
rad/
Hz1/
2 )
0.012 4 6
0.12 4 6
12 4 6
10frequency (Hz)
-100
-50
0
50
spectral density (dB re με/H
z1/2)-1600
-800
0
800
1600
stra
in (μ
ε)
3210time (s)
manual beam manipulations
free vibrations FBG RSG
(a) (b)
(c) (d)
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-300
-150
0
150
300
stra
in (μ
ε)
151050time (hours)
compensated
uncompensated
-500
-250
0
250
stra
in (μ
ε)
543210time (hours)
compensated
uncompensated
60
40
20
0
tem
pera
ture
(o C
)
-30
-15
0
15
30
stra
in (μ
ε)
151050time (hours)
compensated
uncompensated
(a)
(b)
(c)
Compensation Performance Results
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Metric
Dynamic resolution(n/Hz1/2)
Scanning rate(Hz)
Mu’xing capability
Main advantage
Main disadvantage
SFP AOTF WDM 3x3 MEMS CCD
100 <200 <5 <10 <10 50
0-360 0-40K 100-20K 0-20K 0-100K 0-20K
High Med Low High High+ High+
easyto build
filterlimits
scanrate
pass-band
noisefloor
hardto mu’x
overallperf.
paralleldetection
driftcomp.
drift.comp.
com-ponents
overallperf.
Primary FBG System Performance Comparison
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Transducers: Measuring Things Other Than Strain
Fiber interferometers and Bragg gratings may be coupled with mechanical transducers to detectother measurands besides strain:
interferometricaccelerometers
interferometricmagnetic field sensor
Bragg gratingaccelerometer
biological agentsetection sensor
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Deployment Examples
I-10 bridge Norwegian surface-effect ship
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• 78 sensors• 9-month continuous
monitoring• data remote link
instrumentedspan
my rentalcar
I-10 Traffic/Bridge Monitoring
1 and 2 sensor configuration
3 sensor configuration
underside of bottom flange (all configurations)
web (except in 1 sensor config.)
underside of top flange
web
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70
0
10
20
30
40
50
60
6.00
0.00
1.00
2.00
3.00
4.00
5.00
300
200.0
200.0
100.0
0.0
100.0
300 2 4 6 8 10 12 14 16 18 20 22 24 26 28
200.0
200.0
100.0
0.0
100.0
300 5 10 15 20 25
200.0
200.0
100.0
0.0
100.0
300 5 10 15 20 25
2.5 Hz
3.68 Hz
8.2 Hz
3.92 Hz
4.72 Hz
I-10 Results: Time/Frequency and Modal Analysis
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2000
1500
1000
500
08070605040
speed (mph)
veh
icle
cou
nt
72 day period; Nov. to Jan.posted speed limit = 55 MPH
veh
icle
wei
ghts
day count12K-33K lbf
load level
cou
nt
I-10 Traffic Monitoring
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Final system deploymenton the KNM Skjold fast patrol boat
• 56 sensor system
• mounted on inner hull andon waterjet
• Real-time local strain and global load monitoring
Surface-effect fast patrol boat
KNM Skjold
Instrumentation of Surface-Effect Fast Patrol Boat
400 410 420 430Time (s)
-4000
-2000
0
2000
4000
Waveslammingevent
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280 284 288 292 296 300Time (s)
-1000
0
1000st
rain
(mic
rost
rain
)
sagg/hogg motion
whippingA1
C1
A3
a)
fa,b,e = (Tnormal)-1ETnormal
fc,d = (Tshear)-1ETshear
measuredtime series
stresscalculations
hull planarstrain state
wave impactevent
Real-Time Hull Loads Display
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Other Application Areas
• SPIE Smart Structures/NDE Conference (March, San Diego)
always has sessions on composites and aerospace applications
In 2003: 68 papers on fiber optic sensors/applications
In 2004: 76 papers on fiber optic sensors/applications
• Composite materials area
• measuring crack-bridging forces (EPFI, NC State)
• delamination identification (lots of people)
• impact load detection/identification (lots of people)
• transverse load and strain gradient monitoring (Blue Road,
UK, Sweden)
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Other Application Areas (Continued)
• Aerospace structures and embedded sensing• corrosion monitoring (China, USA)• CFRP wing monitoring (Airbus, DaimlerChrysler)• MEMS accelerometers, pressure, temperature sensors
(USA, Japan)• FRP aircraft tail monitoring (Airbus, DaimlerChrysler)• composite component process monitoring
• These examples taken from these references:
[1] Daniele Inaudi and Eric Udd (eds.), Proc. SPIE Smart Sensor Technology and Measurement Systems,
vol. 4694, Int. Soc. for Optical Engineering (Bellingham, WA), 2002.
[2] Richard Claus and William Sillman, J. (eds.), Proc. SPIE Sensory Phenomena and Measurement Instrumen-
tationfor Smart Structures and Materials, vol. 3986,Int. Soc. for Optical Engineering (Bellingham, WA), 2000.
[3] G. Mignani and H. C. Lefevre (eds.), Proc. 14th Int. Conf. on Optical Fiber Sensors, SPIE vol. 4185, CNR
(Florence, Italy), 2000.
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NASA Efforts in Fiber Optic Sensors
• NASA NDE working group
• composite pressure vessel SHM (customer: RLV)
• tunable laser for FBG demu’xing: (customer: RLV, SS)
• micromachined accelerometer (testing FY04 aboard SRA)
• FBG pressure sensor (NASA Glenn, ISCO project)
• fiber optic gyroscopes (SAMS)
• strain measurements for X-33 Vehicle Health Management
• over 1500 hits on NASA’s main web page using “fiber optic
sensor”
RLV=Reusable Launch Vehicle, SS=Space Shuttle, SRA=F15B Systems ResearchAircraft, ISCO=Intelligent Systems Controls and Operations,SAMS=Space Acceleration Measurement System
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Fiber is ~125 microns,adding negligible weightand space to application
Built-in telemetryeliminates invasivewiring
Fiber Sensor Advantages
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Bragg grating rosette
Resistive gage rosette
compositehull
Fiber Sensor Advantages
Fiber sensors are immune to electromagnetic interference and won’t create a spark source.
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Fiber Sensor Disadvantages
• lack of commercialization, particularly at the system level (a “stand-alone” box that’s “plug-and-play”)
• cost per sensor is high for FBGs (~$100 per sensor), BUT cost per channel is competitive
• fiber size (128 micron or even 80 micron) may lead to possible delamination sites for embedded applications -56 micron single mode fiber now available!
• for FBGs, severe strain gradients over gage length may cause chirping leading to loss of signal
• serialization causes risk: loss of one FBG sensor in an array leads to loss of all “downstream” sensors -can be partially compensated for in design
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Further Reading
Jose Miguel Lopez-Higuera (ed.), Handbook of Optical Fibre Sensing Technology,John Wiley and Sons Ltd. (Chichester, UK), 2002.
Eric Udd (ed.), Fiber Optic Sensors: An Introduction for Scientists and Engineers, Wiley Interscience (New York), 1991.
Alan Kersey et al., “Fiber Grating Sensors,” Journal of Lightwave Technology, 15,1442-1463, 1997.
Ken Hill and Gerry Meltz, “Fiber Grating Technology Fundamentals and Overview,”Journal of Lightwave Technology, 15, 1263-1276, 1997.
Brian Culshaw and John Dakin (ed.), Inteferometers in Optical Fiber Sensors: SystemsAnd Applications, Vol. 2, Arctech House (Norwood, MA), 1989.
T. S. Yu and S. Yin (eds.), Fiber Optic Sensors, Marcel Dekker Inc. (New York), 2002.