FIBER OPTIC SENSORS

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FIBER OPTIC SENSORS

• Myoungsu Shin

Department of Civil Engineering University of
Illinois at Urbana-Champaign
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CONTENTS

• Definition of Fiber Optic Sensors
• Appearance of Fiber Optic Sensors
• Application (Usage) areas
• Advantages over Electrical Sensors
• Supporting Technology
• Types of Fiber Optic Sensors
• Introducing Several Products

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FIBER OPTIC SENSORS?

• Dictionary any device in which variations in the
  transmitted power or the rate of transmission of
  light in optical fiber are the means of
  measurement or control
• To measure physical parameters such as strain,
  temperature, pressure, velocity, and acceleration
• Optical fibers strands of glass that transmit
  light over long distances (wire in electrical
  systems)
• Light transmitted by continuous internal
  reflections in optical fibers (electron in
  electrical systems)

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What Does F.O.S. Look Like?

• Strain Gage
• Embeddable Strain Gage
• Pressure Transducer

• Displacement Transducer
• Temperature Transducer

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What Does F.O.S. Look Like? (Contd)

• Fiber Optic Sensor vs. Electrical Sensor

Various Fiber Optic Censors
Fiber Optic Shape Tape
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GENERAL USES

• Measurement of physical properties such as
  strain, displacement, temperature, pressure,
  velocity, and acceleration in structures of any
  shape or size
• Monitoring the physical health of structures in
  real time
• Damage detection
• Used in multifunctional structures, in which a
  combination of smart materials, actuators and
  sensors work together to produce specific action
• Any environmental effect that can be conceived
  of can be converted to an optical signal to be
  interpreted, Eric Udd, Fiber Optic Censors, John
  Wiley Sons, Inc., 1991, p.3

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Monitoring in Structural Engineering

• Buildings and Bridges concrete monitoring during
  setting, crack (length, propagation speed)
  monitoring, prestressing monitoring, spatial
  displacement measurement, neutral axis evolution,
  long-term deformation (creep and shrinkage)
  monitoring, concrete-steel interaction, and
  post-seismic damage evaluation
• Tunnels multipoint optical extensometers,
  convergence monitoring, shotcrete / prefabricated
  vaults evaluation, and joints monitoring Damage
  detection
• Dams foundation monitoring, joint expansion
  monitoring, spatial displacement measurement,
  leakage monitoring, and distributed temperature
  monitoring
• Heritage structures displacement monitoring,
  crack opening analysis, post-seismic damage
  evaluation, restoration monitoring, and old-new
  interaction

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ADVANTAGES

• Immunity to electromagnetic interference (EMI)
  and radio frequency interference (RFI)
• All-passive dielectric characteristic
  elimination of conductive paths in high-voltage
  environments
• Inherent safety and suitability for extreme
  vibration and explosive environments
• Tolerant of high temperatures (gt1450 C) and
  corrosive environments
• Light weight, and small size
• High sensitivity

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SUPPORTING TECHNOLOGY

• Kapron (1970) demonstrated that the attenuation
  of light in fused silica fiber was low enough
  that long transmission links were possible
• Procedure in Fiber optic sensor systems
• Transmit light from a light source along an
  optical fiber to a sensor, which sense only the
  change of a desired environmental parameter.
• The sensor modulates the characteristics
  (intensity, wave length, amplitude, phase) of the
  light.
• The modulated light is transmitted from the
  sensor to the signal processor and converted into
  a signal that is processed in the control system.
• The properties of light involved in fiber optic
  censors reflection, refraction, interference and
  grating

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TYPE OF FIBER OPTIC SENSORS

• Fiber optic censors can be divided by
• Places where sensing happens
• Extrinsic or Hybrid fiber optic sensors
• Intrinsic or All-Fiber fiber optic sensors
• Characteristics of light modulated by
  environmental effect
• Intensity-based fiber optic sensors
• Spectrally-based fiber optic sensors
• Interferometeric fiber optic sensors

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Extrinsic or Hybrid Fiber Optic Sensors

• Consist of optical fibers that lead up to and out
  of a black box that modulates the light beam
  passing through it in response to an
  environmental effect.
• Sensing takes place in a region outside the fiber.

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Intrinsic or All-Fiber Optic Sensors

• Sensing takes place within the fiber itself.
• The sensors rely on the properties of the optical
  fiber itself to convert an environmental action
  into a modulation of the light beam passing
  through it.

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Intensity-based Fiber Optic Sensors

• Depend on the principle that light can be
  modulated in intensity (amount) by an
  environmental effect.

• Example1 Single fiber reflective sensor
• Light leaves the fiber end in a cone pattern, and
  strikes a movable reflector.
• The relationship between fiber-reflector distance
  and intensity of returned light
• Example 2 Bending the fiber
• As the deformer closes on the fiber, radiation
  losses increase and the transmitted light
  decreases.

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Spectrally-based Fiber Optic Sensors

• Depend on the principle that a light beam can be
  modulated in wavelength by an environmental
  effect.

• Example Black body radiation
• When the cavity rises in temperature, it starts
  to glow and act as a light source.
• Detectors in combination with narrow band filters
  are then used to determine the profile of the
  blackbody curve and in turn the temperature

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Interferometeric Fiber Optic Sensors

• The optical phase of the light passing through
  the fiber is modulated by the field to be
  detected.
• This phase modulation is then detected
  interferometerically, by comparing the phase of
  the light in the signal fiber to that in a
  reference fiber.
• Light is not required to exit the fiber at the
  sensor to interact with the field to be detected.
• In intensity based fiber optic censors, light has
  to leave the optical fiber to interact with the
  optical sensor at the end of the fiber, leading
  to substantial optical loss.
• Fabry-Perot, Sagnac, Mach-Zehnder and Nichelson,
  polarimetric, and grating interferometers

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Interferometeric Fiber Optic Sensors (Contd)

• Example Fabry-Perot interferometers (FPI)
• Constructed of two reflectors deposited on either
  side of an optically transparent medium, and on
  the tips of two optical fibers inserted into a
  micro-capillary
• Gage length the distance between the spots where
  the optical fibers are welded
• The transmittance of the interferometer changes
  with respect to spacing of the reflectors

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Fiber Optic Strain Gage

• Involved technology Fabry-Perot
  interferometer
• Strain range From -10000 to
  10000 microstrains (1 )
• Resolution Less than 0.01
• Transverse sensitivity Less than 0.1
• Operating temperature Up to 350 C (adhesive
  dependent)
• Gauge dimensions Diameter 180 mm, length
  1 to 10 mm
• Fiber optic cable Braided fiberglass,
  length 1.5 m, dia. 0.9 mm
• Special gages Embeddable gage,
  Surface-weldable gauge

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Displacement Transducer

• Involved technology Thin Film Fizeau
  Interferometer (TFFI)
• Linear Stroke 25 mm
• Resolution 0.002 mm (no
  averaging)
• 0.0002
  mm (averaging with signal condition)
• Operating temperature -150 C to 350 C (cable
  dependent)
• Transducer dimensions Length 103 mm, O.D. 13 mm
• Fiber optic cable Length 1.5 m, Custom
  length up to 5 km

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Pressure Transducer

• Involved technology Fabry-Perot
  interferometer
• Pressure range From 0-0.3 bar (5
  psi) up to 0-700 bar (1000 psi)
• Resolution 0.01 of FS
• Precision 0.1 of FS
• Operating temperature -20 to 350 C (650 F)
• Thermal sensitivity 0.01 of reading/ 1 C
• Gauge dimensions O.D. 19 mm, length 51 to
  102 mm
• depending
  on pressure range
• Fiber optic cable Length 10 m, Custom
  length up to 5 km

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Temperature Transducer

• Involved technology Fabry-Perot interferometer
• Temperature Range FOT-L -40 to 250 C, FOT-H
  -40 to 350 C
• Resolution 0.1 C
• Accuracy 1 C or 1 of FS
  (whichever is greater)
• Response time Less than 1.5 second
• Gauge dimensions Sensitive zone length 10
  mm, Probe O.D. 1.45 mm
• Fiber optic cable Length 1.5 m, Custom up
  to 5 km