Fiber optic sensors


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FIBER OPTIC SENSORS Myoungsu Shin Department of Civil Engineering University of Illinois at Urbana-Champaign


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


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)

What Does F.O.S. Look Like? : 

What Does F.O.S. Look Like? Strain Gage Embeddable Strain Gage Pressure Transducer Displacement Transducer Temperature Transducer

What Does F.O.S. Look Like? (Cont’d) : 

What Does F.O.S. Look Like? (Cont’d) Fiber Optic Sensor vs. Electrical Sensor Various Fiber Optic Censors Fiber Optic Shape Tape


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

Monitoring in Structural Engineering : 

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


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 (>1450 C) and corrosive environments Light weight, and small size High sensitivity


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


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

Extrinsic or Hybrid Fiber Optic Sensors : 

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.

Intrinsic or All-Fiber Optic Sensors : 

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.

Intensity-based Fiber Optic Sensors : 

Intensity-based Fiber Optic Sensors 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. Depend on the principle that light can be modulated in intensity (amount) by an environmental effect.

Spectrally-based Fiber Optic Sensors : 

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

Interferometeric Fiber Optic Sensors : 

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

Interferometeric Fiber Optic Sensors (Cont’d) : 

Interferometeric Fiber Optic Sensors (Cont’d) 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

Fiber Optic Strain Gage : 

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

Displacement Transducer : 

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

Pressure Transducer : 

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

Temperature Transducer : 

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

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