emre fiber 1

8/3/2019 Emre Fiber 1

1/37

FIBEROPTIC

SENSORSEMRE SENTURK


2/37

Withtheinventionofthe laserin 1960s, a greatinterestinoptical systems fordata communications began. Theinventionof laser, motivated researchers to study thepotential offiberoptics fordata communications, sensing,and otherapplications.

Lasersystems could send a much largeramountof datathanmicrowave, and otherelectrical systems. Thefirstexperimentwiththe laserinvolved thefreetransmissionofthe laserbeaminthe air.

INTRODUCTION


3/37

Researchers alsoconducted experiments by transmitting

the laserbeamthrough differenttypes ofwaveguides.Glass fibers soon becamethepreferred mediumfortransmissionof light.

Initially, theexistenceof large losses inoptical fibersprevented coaxial cables from being replaced by opticalfibers. Early fibers had losses around 1000 dB/kmmakingthemimpractical forcommunications use.


4/37

In 1969, several scientists concluded thatimpurities in

thefibermaterial caused the signal loss inoptical fibers.By removing theseimpurities, constructionof low-lossoptical fibers was possible.In 1970, Corning Glass Worksmade a multimodefiberwith losses under20 dB/km. Thesamecompany, in 1972, made a high silica-coremultimodeoptical fiberwith a 4 dB/km loss.


5/37

Charles Kao at work in his laboratory at Harlow, England in 1966.


6/37

Recent advances infiberoptictechnology have

significantly changed thetelecommunications industry. Theability tocarry gigabits ofinformation atthe speed of lightincreased theresearchpotential inoptical fibers.Simultaneous improvements and costreductions inoptoelectroniccomponents led to similaremergence of

newproduct areas. Lastrevolutionemerged as designerstocombinetheproductoutgrowths offiberoptictelecommunications withoptoelectronic devices tocreatefiberoptic sensors.


7/37

Soonitwas discovered that, withmaterial loss almost

disappearing, and the sensitivity fordetectionofthe lossesincreasing, onecould sensechanges inphase, intensity, andwavelengthfromoutsideperturbations onthefiberitself.Hencefiberoptic sensing was born.


8/37

This fiber-optic sensoris electrical hazard free and its signal is not affected by anelectromagneticinterference.


9/37

Inparallel withthese developments, fiberoptic sensortechnology has been a significantuseroftechnology

related withtheoptoelectronic and fiberopticcommunicationindustry. Many ofthecomponentsassociated withtheseindustries wereoften developed forfiberoptic sensorapplications. Fiberoptic sensor

technology inturnhas often been driven by thedevelopment and subsequentmass productionofcomponents to supporttheseindustries. As componentprices have decreased and quality improvements havebeenmade, the ability offiberoptic sensors toreplace

traditional sensors have alsoincreased.


10/37


11/37

Robust, moreresistanttoharshenvironments.

High sensitivity.

Multiplexing capability toform sensing networks.

Remote sensing capability.

Multifunctional sensing capabilities such as strain,pressure, corrosion, temperature and acoustic signals.

Lightweight.


12/37

To date, fiberoptic sensors have beenwidely used tomonitora widerangeofenvironmental parameters suchas position, vibration, strain, temperature, humidity,

viscosity, chemicals, pressure, current, electricfield andseveral otherenvironmental factors.


13/37

Somefiberoptic sensortypes


14/37

OPTICAL FIBER BASICS

Anoptical fiberis composed ofthreeparts; thecore, the

cladding, and thecoating orbuffer.


15/37

*Thecoreis a cylindrical rod of dielectricmaterial and isgenerally madeof glass. Lightpropagates mainly along thecoreofthefiber.


16/37

*Thecladding layeris madeof a dielectricmaterial

with anindexofrefraction. Theindexofrefractionofthecladding material is less thanthatofthecorematerial. Thecladding is generally madeof glass orplastic. Thecladding executes suchfunctions asdecreasing loss of lightfromcoreintothesurrounding air, decreasing scattering loss atthe surfaceofthecore, protecting thefiberfromabsorbing the surfacecontaminants and addingmechanical strength.


17/37

*Thecoating orbufferis a layerofmaterial used toprotect

anoptical fiberfromphysical damage. Thematerial usedfora bufferis a typeofplastic. The bufferis elasticinnature and prevents abrasions.


18/37

The light-guiding principle along thefiberis based onthe total internalreflection. The angle atwhichtotal internal reflectionoccurs is called the

critical angleofincidence. At any angleofincidence, greaterthanthecritical angle, lightis totally reflected back intothe glass medium. Thecritical angleofincidenceis determined by using Snell's Law. Optical fiberis anexampleofelectromagnetic surfacewaveguide.


19/37

Optical fibers are divided intotwo groups called single

mode and multimode.Inclassifying theindexofrefractionprofile, we differentiate between stepindex and gradientindex. Stepindexfibers have a constantindexprofileoverthewholecross section. Gradientindexfibers have anonlinear, rotationally symmetricindexprofile, whichfallsofffromthecenterofthefiberoutwards.


20/37


21/37


22/37

FIBER OPTIC SENSOR PRINCIPALS

The general structureof anoptical fibersensorsystemisshowninfigure.Itconsists of anoptical source (Laser,LED, Laserdiodeetc), optical fiber, sensing ormodulatorelement (whichtransduces themeasurand to anopticalsignal), anoptical detectorand processing electronics

(oscilloscope, optical spectrum analyzeretc).

SOURCE

MEASURAND

TRANSDUCER

DETECTOR

ELECTRONICPROCESSING

Optical fiber Optical fiber


23/37

Fiberoptic sensors can beclassified underthreecategories:

*The sensing location

*The operating principle

*The application


24/37

*Based onthe sensing location, a fiberoptic sensorcan be

classified as extrinsicorintrinsic.


25/37

In anextrinsicfiberoptic sensor, thefiberis simply

used tocarry lightto and from anexternal opticaldevicewherethe sensing takes place.Inthis cases, thefiberjust acts as a means of getting the lighttothesensing location.


26/37

Based ontheoperating principleormodulation and

demodulationprocess, a fiberoptic sensorcan beclassified as anintensity, a phase, a frequency, orapolarization sensor. All theseparameters may besubjecttochange duetoexternal perturbations. Thus,by detecting theseparameters and theirchanges, the

external perturbations can be sensed.


27/37

Based onthe application, a fiberoptic sensorcan be

classified as follows:

* Physical sensors: Used tomeasurephysicalproperties liketemperature, stress, etc.

* Chemical sensors: Used forpH measurement, gasanalysis, spectroscopic studies, etc.

* Bio-medical sensors: Used in bio-medical applications

likemeasurementof blood flow, glucosecontentetc.


28/37

FIBER OPTIC SENSOR TYPES

Intensity Based FiberOptic Sensors

Intensity-based fiberoptic sensors rely on signalundergoing some loss. They aremade by using anapparatus toconvertwhatis being measured into aforcethat bends thefiberand causes attenuationofthe

signal. Otherways to attenuatethe signal is throughabsorptionorscattering of a target. Theintensity-basedsensorrequires more light and thereforeusually usesmultimode largecorefibers. There are a variety ofmechanisms such as microbending loss, attenuation,

and evanescentfields thatcanproduce a measurand-induced changeintheoptical intensity propagated byanoptical fiber.


29/37

The advantages ofthese sensors are: Simplicity of

implementation, lowcost, possibility of beingmultiplexed, and ability toperform as realdistributed sensors. The drawbacks are: Relativemeasurements and variations intheintensity ofthe light sourcemay lead tofalsereadings, unless areferencing systemis used.


30/37

Wavelength Modulated FiberOptic Sensors

Wavelengthmodulated sensors usechanges inthewavelengthof lightfordetection. Fluorescencesensors, black body sensors, and the Bragg gratingsensorareexamples ofwavelength-modulated

sensors. Fluorescent based fibersensors are beingwidely used formedical applications, chemicalsensing and physical parametermeasurements suchas temperature, viscosity and humidity.


31/37


32/37

Phase Modulated FiberOptic Sensors

Phasemodulated sensors usechanges inthephaseoflightfordetection. Theoptical phaseofthe lightpassingthroughthefiberis modulated by thefield to be detected.This phasemodulationis then detectedinterferometerically, by comparing thephaseofthe lightinthe signal fibertothatin a referencefiber.In aninterferometer, the lightis splitintotwo beams, whereone beamis exposed tothe sensing environment andundergoes a phase shift and theotheris isolated fromthe

sensing environment and is used foras a reference. Oncethe beams arerecombined, they interferewitheachother.


33/37

Polarization Modulated FiberOptic Sensors

The directionoftheelectricfield portionofthe lightfield is defined as thepolarization stateofthe lightfield. Differenttypes ofpolarization states ofthe lightfield are linear, elliptical, and circularpolarization

states. Forthe linearpolarization state, the directionoftheelectricfield always keeps inthe same lineduring the lightpropagation.For theellipticalpolarization state, the directionoftheelectricfieldchanges during the lightpropagation. Theend ofthe

electricfield vectorforms anelliptical shape; hence, itis called elliptical polarized light.


34/37


35/37

Applications of FiberOptic Sensors

*

Measurementofphysical properties such as strain,displacement, temperature, pressure, velocity, andaccelerationin structures of any shapeorsize.

*Monitoring thephysical healthof structures inreal

time.

*Buildings and Bridges: Concretemonitoring duringsetting, crack (length, propagation speed)monitoring,prestressing monitoring, spatial displacementmeasurement, neutral axis evolution, long-termdeformation (creep and shrinkage)monitoring,concrete-steel interaction, and post-seismic damageevaluation.


36/37

*Tunnels: Multipointoptical extensometers, convergence

monitoring, shotcrete / prefabricated vaults evaluation,and joints monitoring damage detection.

*Dams: Foundationmonitoring, jointexpansionmonitoring, spatial displacementmeasurement, leakage

monitoring, and distributed temperaturemonitoring.

*Heritage structures: Displacementmonitoring, crackopening analysis, post-seismic damageevaluation,restorationmonitoring, and old-newinteraction.


37/37

THE END

emre fiber 1

Documents