fiber bragg grating sensing
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Fiber Optic Bragg Grating-Based Sensing
Presented by Michael D. Todd, Ph.D.
Structural Engineering Department University of California San Diego
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 is the 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 mm) • 8-980 mm in diameter
• glass-based or plastic-based • n<1.44 • 125-1000 mm in diameter
cladding
coating/jacketing • plastic (acrylate, polyimide) • for protection, mechanical strength
• Optical fibers are characterized by the normalized frequency V:
�
V = 2πaλ
ncore2 − ncladding
2 V < 2.405 single mode V > 2.405 multi-mode
Component Integration: General Sensing System
optical source
sensing mechanism
photodetection
interferometry
intensity modulation
Bragg gratings electronic processing
(non-optical)
~30 cm
Intrinsic Local Sensor: Bragg Grating • A fiber Bragg grating is region of periodic refractive index perturbation inscribed in the core of an optical fiber such that it diffracts the propagating optical signal at specific wavelengths.
fiber core
refractive index modulation period, T
• Each time the forward-propagating light encounters a stripe (index mismatch), some is scattered (diffracted)
• Scattered light accrues in certain directions if a phase-matching condition is satisfied: in particular, at the resonant wavelength given by lr=2nT, light is reflected backward in phase with previous back-reflections such that a strong reflection mode at wavelength lr is generated
Bragg Grating Fabrication
optical fiber outer cladding
fiber core (Ge-doped)
• This photosensitivity occurs because electronic absorptions in silica materials are in this UV regime; this effect is enhanced with Ge-doping through Ge sub-oxide defect production • Defects leads to refraction index change (Kramers-Kronig relations)
grating period T
�
λref = 2nT = nλUVsin θ /2( )
modulation of refraction index (Bragg grating)
coherent ultraviolet beam at wavelength = 244 nm
�
λref
�
θ
Bragg Gratings Act as Optical Notch Filters
tran
smiss
ion
inte
nsity
wavelength
l = 2nT
broadband light inserted here
cladding core grating
typical LED source spectrum (input)
refle
ctio
n in
tens
ity
wavelength
l
• light at wavelength l is reflected
• FWHM of the reflection peak is typically 0.1-0.3 nm
• if the fiber is locally stretched or compressed, T changes, meaning l changes
• gratings may be multiplexed in the wavelength domain by initially writing each grating to reflect at a unique wavelength
• sensor system must track individual wavelength shifts
• #4 E
1550 SLED
4-ch
anne
l WD
M sp
litte
r
phase generated carrier/ active homodyne
carrier modulation signal (~20 kHz)
Mach-Zehnder interferometer
piezoelectric element
Grating Interrogation: WDM
Grating Interrogation: Tunable Filters 1550 SLED
tunable fiber Fabry-Perot
filter
tunable acousto-optic
filter
photodetector
Grating Interrogation: Tunable Filters
photodetector
tunable fiber Fabry-Perot
filter
d/dt
zero-crossing detector
driving signal
voltage wav
elen
gth
voltage to wavelength conversion
compare
�
tunable acousto-optic
filter
+
x
VCO
∫
counter
�
driving signal
driving signal
Grating Interrogation: CCD Array 1550
SLED sensing array
collimating lens (bulk optics)
plane grating (1200 lines/mm)
spec
trom
eter
linear CCD
scanning signal
�
centroid calculation
pixel array
Key Performance Results
-4
-2
0
2
4
det
ecto
r outp
ut
(V)
1.000.950.900.850.80
time (s)
-8
-4
0
4
dem
od
ulated
ph
ase (rad)
-12
-6
0
6
12
rad
ian
s
0.200.150.100.050.00
time (s)
-150
-100
-50
0
50
spec
tral
den
sity
(dB
re
rad/H
z1/2
)
0.012 4 6
0.12 4 6
12 4 6
10frequency (Hz)
-100
-50
0
50sp
ectral den
sity (d
B re !
!/Hz
1/2)
-1600
-800
0
800
1600
stra
in "!!#
3210time (s)
manual beam manipulations
free vibrations FBG RSG
(a) (b)
(c) (d)
-300
-150
0
150
300
stra
in !"!#
151050
time (hours)
compensated
uncompensated
-500
-250
0
250
stra
in !"!#
543210
time (hours)
compensated
uncompensated
60
40
20
0
tem
per
atu
re (
oC
)
-30
-15
0
15
30st
rain
!"!#
151050
time (hours)
compensated
uncompensated
(a)
(b)
(c)
Compensation Performance Results
Metric
Dynamic resolution (ne/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+
easy to build
filter limits
scan rate
pass- band
noise floor
hard to mu’x
overall perf.
parallel detection
drift comp.
drift. comp.
com- ponents
overall perf.
Primary FBG System Performance Comparison
Transducers: Measuring Things Other Than Strain Fiber interferometers and Bragg gratings may be coupled with mechanical transducers to detect other measurands besides strain:
interferometric accelerometers
interferometric magnetic field sensor
Bragg grating accelerometer
biological agent setection sensor
Deployment Examples
I-10 bridge Norwegian surface-effect ship
• 78 sensors • 9-month continuous monitoring • data remote link
instrumented span
my rental car
I-10 Traffic/Bridge Monitoring
1 and 2 sensorconfiguration
3 sensorconfiguration
underside of bottom flange(all configurations)
web(except in 1 sensor config.)
underside of top flange
web
7 0
0
1 0
2 0
3 0
4 0
5 0
6 0
6 .00
0 .00
1 .00
2 .00
3 .00
4 .00
5 .00
3 00
200 .0
- 200 . 0
- 100 . 0
0 . 0
100 .0
3 00 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8
200 .0
- 200 . 0
- 100 . 0
0 . 0
100 .0
3 00 5 1 0 1 5 2 0 2 5
200 .0
- 200 . 0
- 100 . 0
0 . 0
100 .0
3 00 5 1 0 1 5 2 0 2 5
2.5 Hz
3.68 Hz
8.2 Hz
3.92 Hz
4.72 Hz
I-10 Results: Time/Frequency and Modal Analysis
2000
1500
1000
500
0
8070605040
speed (mph)
vehi
cle
coun
t
72 day period; Nov. to Jan. posted speed limit = 55 MPH
veh
icle
wei
ghts
day count 12K-33K lbf
load level
coun
t
I-10 Traffic Monitoring
Final system deployment on the KNM Skjold fast patrol boat
• 56 sensor system
• mounted on inner hull and on waterjet
• Real-time local strain and global load monitoring
Surface-effect fast patrol boat
Instrumentation of Surface-Effect Fast Patrol Boat
400 410 420 430Time (s)
-4000
-2000
0
2000
4000
µεWave slamming event
280 284 288 292 296 300Time (s)
-1000
0
1000
strai
n (m
icro
stra i n
)
sagg/hogg motion
whippingA1
C1
A3
a)
fa,b,e = (Tnormal)-1ETnormal
fc,d = (Tshear)-1ETshear
measured time series
stress calculations
hull planar strain state
wave impact event
Real-Time Hull Loads Display
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)
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.
Fiber is ~125 microns, adding negligible weight and space to application
Built-in telemetry eliminates invasive wiring
Fiber Sensor Advantages
Bragg grating rosette
Resistive gage rosette
composite hull
Fiber Sensor Advantages
Fiber sensors are immune to electromagnetic interference and won’t create a spark source.
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
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: Systems And Applications, Vol. 2, Arctech House (Norwood, MA), 1989.
T. S. Yu and S. Yin (eds.), Fiber Optic Sensors, Marcel Dekker Inc. (New York), 2002.
Extra Slides
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 for large 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 mW - 20 mW (superluminescent)
1550 SLED
Photodetector: Light to Volts
• photodetectors are devices through which optical power is converted to an electrical signal via an absorption process
• photons are converted to electric charge carriers, and an electric field is applied to the photodetection region to measure their effect
• most common types: PIN and avalanche photodiodes
• APD has higher responsivity (internal gain) and higher shot noise than PIN
• PIN is cheaper, doesn’t require thermal compensation
• typical InGaAs performance:
950-1650 nm operation, 1 A/W, 5 ns response time, 0.2 pW/Hz0.5 noise
3-4 cm
Fiber Optic Components: Couplers
• used to combine/split optical signals from different fibers
• take advantage of evanescent field coupling: some of the field extends beyond core
• coupling lengths are usually a few millimeters
L
evanescent field P1
P2
�
P1 = P1(0)cos2 kL
P2 = P1(0)sin2 kL
input power
reflected power
transmitted power
coupled power
4-5 cm
Fiber Optic Components: Tunable Filters
broad-band light enters the filter...
A stepped voltage drives a piezoelectric device which controls the mirror spacing
…but only a narrow wavelength band gets passed through the 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
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 (signal arm) 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 the two signals when they are recombined:
�
I = I0[1+ M cosΔφ]= I0[1+ M cos(2πnλ
ΔL)]
• When this recombined signal is photodetected, its intensity is given by
�
Δφ = 2πnλ
ΔL
where I0 is the mean signal level, M is the visibility of the interferometer, n is the core refractive index, and l is the wavelength of the light.
The detector signal directly encodes the measurand changes.
Primary Interferometer Configurations
light in photodetection coupler coupler
reference fiber
signal fiber Mach-Zehnder
Michelson
light in
photodetection
coupler
signal fiber
reference fiber
reflectors
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 phase offset 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 the arms, the output signal varies in intensity.
Fiber Optic Connections
ST SC
FC/PC or FC/APC
• keyed bayonet (like BNC) • MMF and SMF
• pop in/out connector with locking tab in plastic housing • SMF typically • durable and cheap
• position-tunable notch and threaded receptacle • SMF only • very precise positioning and < -50 dB reflectivity
Typical performance: 0.2-0.5 dB insertion loss, <-40 dB reflectivity, temp. range -20 to 60 oC
E2000
• shutters provide protection from environment and damage
Fiber Optic Splicing • Two fibers may be coupled together axially (spliced) by precise alignment of their cores
• Requires precise rectangular-edged cleave at the fiber interfaces
cleaver
fusion splicer • Fusion splicers use an electric arc to weld the cleaved fiber faces together
• Use computer-controlled alignment using outer fiber contour lines
• Losses are about 0.02 dB
• Integrated cleaver, splicer, and recoater commercially available ~$40K
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