Fiber Optic Grating Sensors and Applications

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Fiber Optic Grating Sensors and Applications
Blue Road Research
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Criteria for a Successful Fiber Optic Sensor
Application

• 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
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Fiber Grating - Phase Mask Method
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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, ATT 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
(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
(Source - 3M Bragg Grating Technologies)
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External Cavity Laser Diode
(Source - 3M Bragg Grating Technologies)
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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
(Source - 3M Bragg Grating Technologies)
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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
(Source - 3M Bragg Grating Technologies)
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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

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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
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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
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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
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Fiber Fabry-Perot Tunable Filters
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Fabry-Perot Detector/Fiber
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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)

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Multi-Parameter Bragg Grating

• Two overlaid Bragg gratings created in
  birefringent fiber
• ?1, ?2
• Birefringent fiber can transmit two orthogonal
  polarization modes
• p,q
• Reflected spectrum will contain four peaks
• ?p1, ?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

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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)

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Response of Birefringent Fiber to Applied Strain
and Temperature

• If we assume ?230, the equations are linear in ?
  and ?T

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3 Axis Strain and Temperature
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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

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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

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Experimental Setup to Test the Three Axis Strain
and Temperature Sensor
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Experimental Setup to Test the Three Axis Strain
and Temperature Sensor
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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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Transverse 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
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Civil Structure Applications for 3 Axis Sensor
(continued)
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Civil Structure Applications for 3 Axis Sensor
(continued)
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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