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