Embedded Sensing: Fiber Optics · Optical Intensity Changes: Microbend Sensors measurand fiber light modulated light • measurand directly excites corrugated fiber clamp • localized

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.

Los Alamos Dynamics Structural Dynamics and Mechanical Vibration Consultants 2

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


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


Total Internal Reflection

n2 < n1

n1

refracted rays

reflected rays

total internalreflection


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


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


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


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


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

QuickTime™ and aBMP decompressor

are needed to see this picture.


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


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


Component Integration: General Sensing System

opticalsource

sensingmechanism

photodetection

interferometry

intensitymodulation

Bragg gratings electronicprocessing

(non-optical)

~30 cm


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)


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



• 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


measurand

light

modulatedlight

transmission configuration

reflection configuration


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.


Primary Interferometer Configurations

light inphotodetectioncouplercoupler

reference fiber

signal fiberMach-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 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.


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


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)


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


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


Grating Interrogation: Tunable Filters1550

SLED

tunable fiberFabry-Perot

filter

tunableacousto-optic

filter

photodetector


Grating Interrogation: Tunable Filters

photodetector


filter

d/dt

zero-crossingdetector

driving signal

voltagewav

elen

gth

voltage towavelengthconversion

compare

�

tunableacousto-optic

filter

+

x

VCO

∫

counter

�

driving signal

driving signal


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


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


Optics Module

1550SLED


sensingarray

2x2Mach-Zehnderinterferometer

2x2 3x3

photodetectors

compensationarray

Compensation Module

QuickTime™ and aAnimation decompressor


QuickTime™ and aAnimation decompressor


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


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)


-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


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


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


Deployment Examples

I-10 bridge Norwegian surface-effect ship


• 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


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


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


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


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


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.


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


Fiber is ~125 microns,adding negligible weightand space to application

Built-in telemetryeliminates invasivewiring

Fiber Sensor Advantages


Bragg grating rosette

Resistive gage rosette

compositehull

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: 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.

Documents

Embedded Sensing: Fiber Optics · Optical Intensity Changes: Microbend Sensors measurand fiber light modulated light • measurand directly excites corrugated fiber clamp • localized