Blue Road Research Fiber Optic Grating Sensors and Applications Blue Road Research Session 1 Session 5 Session 4 Session 3 Session 2 Session 6 Session

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Fiber Optic Grating Sensors and Applications

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Session1

Session5

Session4

Session3

Session2

Session6

Session8

Session7

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Criteria for a Successful Fiber Optic Sensor Application

Blue Road Research Session 6, Page 1

• Meets and important application need

• Is unique, superior solution

• Is economically compelling

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Fiber Grating Sensors


• Key parameters - strain and temperature

• Competitive technology - electrical resistive strain gauges and thermocouples/low cost ($20) - well known, difficult to embed and successfully operate

• Today - fiber gratings are high cost items ($200 each) being used to measure strain and temperature in embedded materials

• The future - cost competitive with electrical strain gauges with options for 3 axis strain measurement, environmentally superior performance

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Fiber Grating Sensor Prospects


• Fiber gratings are likely to drop into the $25 to $40 range for small quantity buys in two to three years enabling direct competitiveness with electrical strain gauges.

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Fiber Grating - Holographic Method


Inducedgratingpattern

Fiber

Laser beams

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Fiber Grating - Phase Mask Method


Inducedgratingpattern

Fiber

Laser beam

Phase mask

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Means to Write Fiber Gratings


• Long exposure side imaged interference pattern» United technology / operates to approximately 500

deg C

» Good spectral characteristics /reflectance

• Short pulse side imaged interference pattern» Naval Research Lab / operates to 800 deg C

» Demonstrated manufacture during draw

» First gratings low of quality

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Means to Write Fiber Gratings(continued)


• Phase masks» Moderate temperature» Good performance» Canadian Communication Research Center

• Line by line» Higher temperature to 800 deg C» Canadian Communication Research Center

• Phase masks / bending fiber» Brown University / Bragg Technology» Wider bandwidth, good performance

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Major Historical Milestones


• K.O. Hill, CRC 1978, discovery of photosensitivity.• Lam and Garside, McMaster U. 1981, observed

photosensitivity as a two photon effect.• Meltz, Morey, and Glenn, UTRC 1989, side writing

technique with UV laser.• Hill and Snitzer, CRC and Rutgers 1993, mask

technique for writing fiber gratings.• LeMaire, AT&T 1993, Hydrogen loading.• Archambault, Reekie, Russell, Southampton U. 1993,

single pulse, type 2 grating, and draw tower exposure.

(Source - 3M Bragg Grating Technologies)

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Bragg Grating Exposure

Blue Road Research Session 6, Page 11(Source - 3M Bragg Grating Technologies)

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Photoinduced Index Change


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0.1% Chirp, Peaks at 0.25 and 0.75L


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Grating Reflection with 1nm Bandwidth and Reduced Sidelobes


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Coupling to Cladding Modes with 4o Blaze


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Wavelength-Selective Light Coupling from Fibers with Bragg Gratings


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Bandpass Filter with Fiber Gratings in Michelson Arrangement


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Transmission of Fiber Bragg Grating Pair as Fabry-Perot Interferometer


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External Cavity Laser Diode


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External Cavity Laser Diode


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Temperature Relation for Grating Sensors


/ = ( + ) T

= expansion coefficient

= 0.55x10-6 oC-1 for silica

= thermooptic coefficient for fiber core material

= 8.31x10-6 oC-1 estimated for GeO2 doping*

/ = 8.86x10-6 T

= 0.0073 nm/oC at 820 nm

* From S. Tahahashi and S. Shibata,

Journal of Non-Crystalline Solids 30(1979) 359-370

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Strain Relation for Grating Sensors


/ = (1 - pe)

pe = photoelastic constant

= (n2 / 2)[p12 - (p11 + p12)] = 0.22 for silica

/ = 0.78 = 6.4 nm / 1% at 820 nm

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Fiber Grating Wavelength Shift


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Reflectivity Over Test Period at 650 oC


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Bandwidth Over Test Period at 650 oC


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Transmission Plots Before and After High Temperature Exposure


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Temperature and Strain Cycling


• Use preannealed FBG• 4 hour cycles, 21 oC to

427oC• 512 cycles over 2048

hr..• No change measured in

FBG spectrum

• Apply dynamic strain in tension load

• Maximum strain 2500 microstrain

• 1.4 million cycles• No change measured in

FBG spectrum

Temperature Cycle Test Strain Cycle Test

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Fiber Grating Demodulators


• Open loop versus closed loop

• Open loop single grating approach requires broadband grating

• Brown University is working on broadband chirped gratings

• PZT stacks and designs for adequate modulation exist at modest voltages

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Low Cost Approaches


• Overcoupled coupler

• Miniature Mach-Zehnder

• Fiber grating spectral filter

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Overcoupled Beamsplitter Layout


Light sourceFiber grating

Ratioed outputDetectors

Overcoupledbeamsplitter

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Overcoupled Coupler Issues


• Thermal drift more severe with

higher sensitivity

• Polarization mixing issues

• Packaging of sensitive devices» very long overcoupled couplers are fragile

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Miniature Mach-Zehnder


• More rugged than overcoupled coupler

approach with comparable sensitivity

• Superior thermal and polarization

properties to overcoupled coupler

• Smooth spectral profile

• Needs further thermal and polarization

improvements - close to ready

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Grating Sensor with Fiber-Interferometric Wavelength Discriminator


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Fiber Grating Spectral Filter


• Can be tailored to match desired dynamic

range and sensitivity

• Athermal package for operation over -40

to 80 oC (less than 0.1 nm drift)

• Relatively polarization independent

• Suitable for a demodulator with

approximately 100 microstrain sensitivity

and +/- 5000 microstrain range

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Fiber Grating Spectral Filter Demodulator


Fiber grating

Fiber gratingspectral filter

Light source

Receivers

Beamsplitters

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Chirped Fiber Grating Spectral Filter


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1550 nm Grating Demodulation Kit


• 1550 nm ELED light source

• (2) 3 dB beamsplitters

• Chirped fiber grating filter

• (2) receivers

• Patch cords

• FC connector cleaner

• 1 single axis grating sensor

• Data CD with manual

• Optional DAQ card & software

• Optional Carrying case

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High Speed Grating Demodulators


• Stand-alone configuration

• Three bandwidth options

» 1kHz, 10 kHz, 2 MHz

• Flexible design for varied

applications

• Integrated light source and spectral

filters

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Fiber Grating System


Modulated reference fiber grating

Detector

Light source Fiber gratings

1 2

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Fiber Fabry-Perot Tunable Filters


Air gap1-2 microns

PZT actuator Mirrors

Cavity length 20microns for 50 nm FSR

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Fabry-Perot Detector/Fiber


Bimorphactuator

Capillary tubeFiberSilicon

detector

Silicondioxide

Siliconlayer

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Fiber Bragg Grating Sensor Array with Fiber Fabry-Perot Demodulator


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Shift in FFP Control Voltage and Bragg Wavelength with Applied Strain to FBG Sensor Element


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Bragg Grating Axial Strain and Temperature Sensor


• Measures T and 1 for surface mounted applications

• Overlaid Bragg gratings at two wavelengths» 850 and 1300 nm

• Output spectrum contains two peaks b1 = f1(1,T) and b2 = f2(1,T)

b1

b2

.K 1

K 2

K T1

K T2

1

T

1

T

.K1

b1

b2

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Multi-Parameter Bragg Grating


• Two overlaid Bragg gratings created in birefringent fiber1, 2

• Birefringent fiber can transmit two orthogonal polarization modes» p,q

• Reflected spectrum will contain four peaksp1, q2, p2, q2

• Four peaks can be used to determine three axis of strain and temperature 1, 2, 3, T

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Bragg Grating in Birefringent Fiber


• Two polarization modes with different values of n (np, nq)

• Two distinct Bragg peaks

p

q

p

q

p0 = 2d0np0

q0 = 2d0nq0

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Response of Birefringent Fiber to Applied Strain and Temperature


• When the fiber is subjected to or T, will shift due to change in d (elongation) and n (stress-optic effect)

p0..2 d 0 n p0

q0..2 d 0 n q0

p

p0

n p

n p0

d

d 0

q

q0

n q

n q0

d

d 0

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Response of Birefringent Fiber to Applied Strain and Temperature


• If we assume 23=0, the equations are linear in and T

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3 Axis Strain and Temperature


Polarization preserving fiber axes

Dual overwritten fiber gratings

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Two Overlaid Gratings in Birefringent Fiber


• If we add a second grating to the fiber at a different wavelength, we will obtain two additional peaks in the reflected spectrum

• The response of these new peaks will be different due to the wavelength dependence of fiber properties (pij, etc.)

• The response of the four peaks to strain and temperature can be expressed as:

p1

q1

p2

q2

.

K 11

K 21

K 31

K 41

K 12

K22

K 32

K 42

K 13

K 23

K 33

K 43

K 14

K 24

K 34

K 44

1

2

3

T

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Determining Three Axes of Strain and Temperature


• Provided K is well conditioned, we can determine the strains (1, 2, 3) and temperature (T) from the change in wavelength of the four peaks using:

1

2

3

T

.K1

p1

q1

p2

q2

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Experimental Setup to Test the Three Axis Strain and Temperature Sensor


3 Axis strainand temperaturesensor

couplerWDM1300 nm light source

1550 nm light source

Variable FPetalon

Etalon controllerReceiver

Dataacquisitionunit

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Experimental Setup to Test the Three Axis Strain and Temperature Sensor


WDM

1550 nm

1300 nm

Z

50/50 splitter

Fiber grating sensor

FC/PC connector sleeveOptical Spectrum

Analyzer

PC running Blue Road Research software

GPIB interface

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Axial Loading of 3 Axis Fiber Grating Sensor


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Transverse Loading of 3 Axis Fiber Grating Sensor


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3 Axis Demodulation Kit


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Possible Applications of 3 Axis Sensor


• Aerospace

• Biomedical

• Geotechnical

• Civil structures

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Civil Structure Applications for 3 Axis Sensor


Fiber OpticStrain Sensors

Suspension Bridge

Fiber Grating Load Cells

Drawbridge

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Civil Structure Applications for 3 Axis Sensor (continued)


Scouring Sensors

Load cellsScouringScouring

Fiber OpticStrain Sensor

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Civil Structure Applications for 3 Axis Sensor (continued)


Embedded Fiber Optic Strain Sensors

Roadside Demodulation box

Speed and Weight

Embedded Fiber Optic Strain Sensors

Traffic Control

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Long Period Fiber Gratings


• Temperature dependence 0.04 to 0.05 nm/ oC (short period grating is about 0.01nm/oC)

• Strain dependence very fiber specific, examples: Grating A - 0.7 nm/m, Grating B - 1.5 nm/m (short period gratings are 1.0 to 1.8 nm/m for the 1.3 to 1.5 micron range)

• Extremely sensitive to bending which can override the grating

Reference: Vengsarker et al, JLT, p.53, Jan 96

Documents

Blue Road Research Fiber Optic Grating Sensors and Applications Blue Road Research Session 1 Session 5 Session 4 Session 3 Session 2 Session 6 Session