other types of sensors - university of washington

Other Types of Sensors

Wei-Chih Wang

Southern Taiwan University ofTechnology

In the weakly guiding approximation, single-mode circular core fiber can support two orthogonalpolarizations of the LP01-mode as well as the four degenerate LP11-modes when operated just belowthe single-mode cutoff wavelength. Two-lobe patterns can be obtained in the far field at the output ofthe fiber

Figure 6-2. Superposition of the LP 01, LP11 modes of an elliptical core fiber

+ ejφ

y y y y

intensityintensity intensity

LP01 LP11

φ = 0 φ = π/2 φ = π

+ ejφ

LP01 LP11

y polarization

x polarization

Elliptical Core Two-mode Fiber-OpticSensor

Excitation of first two LP mode - 2.405 < V < 3.832

Elliptical core -> eliminate circular symmetry of fiber -> only LP11even

mode (second order term) are propagating

unpolarizedHe-Nelaser(NEC

rotational

40x objective lens

linearpolarizer

detector

632nm filter

rotationallinear Pol.

10x objectivelens

sensing region

Polarizedmaintainingfiber

chopper

GLG-5261)

(analyzer)

Polarmetric Strain Sensor

Polarimetric SensorThe birefringence property arising from optical anisotropy is usedin the study of photoelastic behaviour . The anisotropy may be dueto naturally occuring crystalline properties or due to stress inducedbirefringence. It is the latter that is used in a photoelastic fiberoptic strain gauge. In a simple setup two lead fibers are used toilluminate and collect light passing through a photoelasticspecimen. A pair of linear polarizers is used in the crossed form toobtain a conventional polariscope. In such a case the intensity ateach point on the specimen is given by

where I0 is the total light intensity and θ is the angle that theprincipal stress directions make with respect to the axes of thepolarisers .

Due the stress birefringence the orthogonally polarized light wavestravel with a phase difference α given by

where λ is wave length , n is the index of refraction ,d is thethickness in the direction of light propagation ,C is stress-opticcoefficient and fa= (λ \C) is known as material fringe value.

When the polarisers are oriented 450 w.r.t. principal stressdirections equations (28 ) and (29) simplify to

with α = (2π /fa ) σ d where the applied load is assumed toproduce a uniaxial stress . By taking the derivative of theabove equation we obtain

Polarimetric Glucose Sensor

• Polarimetric measurement of glucoseconcentration is based on opticalrotatory dispersion (ORD) aphenomenon by which a solutioncontaining a chiral molecule rotates theplane of polarization for linearlypolarized light passing through it.


• The rotation is the result of a differencein refractive indices nL and nR for leftand right circularly polarized lighttraveling through the electron cloud of amolecule.


• The signal produced by the detector isproportional to the square of the E-fieldof the light incident on it and is given by:

Optical Setup

By Roger J. McNichols

Bragg Grating Sensor

• Features• Bragg Gratings have low insertion losses and are

compatible with existing optical fibers used intelecommunication networks.

• Bragg Gratings allow low-cost manufacturing of very highquality wavelength-selective optical devices.

• Phase masks used to photo-imprint the Gratings allowmanufacturing that is relatively simple, flexible, low-costand large-volume.

Bragg grating based sensor system is to monitor the shiftin wavelength of the returned bragg signal with the changesin the measurand (in this case strain). The bragg wavelengthor resonance condition of a grating is given by

λB = 2nΛ

Where Λ is the grating pitch and n is the effective index of the core.


The Bragg bandwidth of the reflected by the grating. The bandwidth of thereflected signal depends on several parameters, particularly the grating length.Perturbation of the grating results in a shift in the Bragg wavelength of devicewhich can be detected in either the reflected or transmitted spectrum.

Bragg Grating Sensors

λ

∆ λ Β

r e f le c te dw a v e le n g th

in d e x m a t c h in gf lu i d

c o u p le r2 x 2

b ro a d b a n dso u r c e

F a b r y - P e r o ts c a n n i n gin t e r f e ro m e te r

( s tr a in - in d u c e d s h if t)

λ Β

r e f l e c t e ds i gn a l

( w a ve le ng t h s h if t de te c t o r )

p o ly m e r b a s e dw a v e g u id e a n dg ra ti n g se n so r

p i e zo e le c tr ict ra n s d u c e r

λ B = 2 n Λ

Λ

Strain and Temperature Sensing

( )( ) )]))(

(2

1[2 121112

2

Tn

dTdn

nn zzB ∆++

+−

−Λ=∆ αρρνρελ

Where ρij are Pockel’s coefficients of the stress-optictensor, ν is poison’s ratio, εzz is the longitudinal strainand α is the coefficient of thermal expansion of thewaveguide, and ∆T is the temperature change. It is notpossible to separate the effect of the temperature from theeffect of the strain with only one sensor.

W.-C. Wang

Strain and Temperature Sensing

Strain response due to

• Physical change corresponding to pitch change in grating• index change due to photoelastic effect

Thermal response arise from

• Internal thermal expnasion• temperature dependent index change

W.-C. Wang

Strain Sensing

161078.01 −−= µε

δεδλ

λxB

B

For silica core fiber under constant temperature, the strainresponse is

W.-C. Wang

The response is 1nm per 1000 at 1.3µmµε

Temperature Sensing

161067.61 −−= Cx

ToB

B δδλ

λ

In silica fiber, thermal effect is dominated by δn/δT, whichaccount for 95% of the shift. The normalized thermalresponse at constant strain is therefore,

1pn is requires to resolve temperature change of 0.1oC

W.-C. Wang


• Applications• Bragg Gratings have proven attractive in a wide variety of optical fiber

applications, such as:• Narrowband and broadband tunable filters• Optical fiber mode converters• Wavelength selective filters, multiplexers, and add/drop Mach-Zehnders• Dispersion compensation in long-distance telecommunication networks• Gain equalization and improved pump efficiency in erbium-doped fiber

amplifiers• Spectrum analyzers• Specialized narrowband lasers• Optical strain gauges in bridges, building structures, elevators, reactors,

composites, mines and smart structures

Distributive feedback laser

Sacher Lasertechnik Group.

Using grating to select the desired operating wavelength

Surface Plasmon ResonatorSensor

Surface plasmon resonance sensor use surface plasmawaves to probe bimolecular interactions occurring at thesurface of a sensor.

SPR Theory

According to Maxwell’s theory, surfacePlasmons can propagate along a metallicSurface and have a spectrum of eigenfrequencies ω related to the wave-vector (k)by a dispersion relation,

Where ε2=ε2′+jε2´´ and ε1 are dielectric constants of metal andthe medium in contact with it.

Wave vector of light at frequency ω traveling through theMedium ε1 is described by:

If ε1 is air, the SP’s dispersion relation never intersect with thedispersion relation of light in air (k=ω/c), they cannot be exciteddirectly by a freely propagating beam of light incident upon themetal surface

Excite Plasmons via a Grating Coupler

For an angle of incident θo, the resonance condition is

The resonance can be observed at angle θo as minimumreflected intensity

If grating constant b, the light wave vector is increased by anAdditional term 2π/b, and the SP’s dispersion relation can bematched by light vector parallel to the surface

Prism coupling Method

The concept is based on the fact that the light line can belowered by a factor εo

0.5 if the beam is traveling through anoptical denser medium. Plasmons can be excited by TMpolarized light undergoing total internal reflection on prismsurface where evanescent wave penetrates to metal/air interface

For an angle of incident θo, the resonance condition is

Prism coupling introduced by Otto

Otto, A.Z., physik, 216 (1968), p398

Metal surface is separated from thePrism by an additional layer (air slit),ε1. SP resonance occurs at metal-dielectric interface.

Kretschmann Configuration

Kretschmann, O.Z., physik 241 (1971), p313

ε0

ε2

ε1

Metallic layer is formed on the prism surface and acts as thespacer. for the correct film thickness, the evanescent filedexpanding through the metal may couple to the SP on theopposite (ε2/ε1) metal surface.

Reflected Intensity as a function of θ

SPR curve measured for silver using Kretschmann Configuration

Resonant angle θr is a function ofthe dielectric constants of the twocontacting media. Due to thisproperty, the surface plasmonresonance can be utilized inmonitoring surface reactions, asevery new adlayer formed on themetal surface causes changes indielectric function of medium ε1,establishing new resonance angle θr

Fixed angle SPR detection• Using Kretschmann geometry

measurement-1. Recording whole resonance curve

by turning the prism2. Fixed angle when prism is stopped

near its resonance dip.

∆n=10-6 for sample expose to air∆n=10-5 for liquid sample

Optical sensitivity depends on metal usedNoise suppression and light fluctuationcompensation

The angle of incidence of light isfixed and chosen to be in the middleof the slope of the reflectance dip

θ

Int

Focused Beam SPR detection

Idea of forming simultaneously more thanone angle of incidence and thus being ableto record the whole SPR curve without thenecessity of rotating the prism.

The SPR curve and possible changes of itsshape can be followed in real-time by aCCD array.

Commercial model of the focused SPR combined with multi channel flow cell systemWith immunological studies are currently available on the market.

SPR lightpipe sensor developed at the University ofWashington to detect biological toxins

PH Sensor Operating Principle

The sensor consists of three main parts: light source, optrode and detector. The main partof the sensor, so-called optrode, contains an appropriate indicator which changes its opticalproperties in dependence on the analyte. In most cases, it is necessary to use an indicatorbecause the analyte does not give or exhibit changes of optical properties. The indicatorcan change, for example, absorbance or fluorescence intensity. The light source is matchedto the so-called analytical wavelength of the indicator then the best sensitivity of the sensorcan be obtained. Detector, usually photodiode or PMT, converts optical signal into electricone which is next electronically processed.

Operating principle of apH sensor based on

absorbance indicator

Sensor based onabsorbance indicator atsolution of different pH

Sensor based on fluorescenceindicator

Sensor based onfluorescence indicator atsolution of different pH

Smart Structure

Embedded Structure: concrete, metal, polymer, composite

Applications: monitor and correct for structural changes in flight

Several schemes of damage detection using optical fiber sensors arebeing investigated

School of Mechanical and Production EngineeringNanyang Technological University

Fiber reinforced polymer composites are becoming increasinglypopular, damage detection in these materials has become an importantissue.

School of Mechanical and Production EngineeringNanyang Technological University

Concrete monitoring during setting, old-newinteraction evaluation, bending / torsionmeasurement, curvature analysis, long-termmonitoring, automatic and remote monitoring, loadtesting, more than 100 fiber optic sensors installed

Versoix bridge Project

To monitor the behavior of the bridge during theworks and in the long term, it was decided toinstrument it with more than 100 sensors allowing themeasurement of the curvatures at 13 differentsections and the calculation of the bridge's horizontaland vertical deformations by double integration of thecurvatures. Sensors pairs were also used to verifythe adherence between old and new concrete.


Image of the Fiberoptic SensorDamage Detection System

Nippon Telegraph and Telephone Corp.The University of TokyoGH Craft Ltd.

1) Feedback on hull structural design

This system produces three-dimensional strain data on the entirehull on the basis of the continuous strain measurement results bythe fiberoptic sensors attached to the yacht rather than the pointdata by conventional sensors such as strain gauges[3]. Moreover,the system not only detects damage but allows checks on hulldeformation.

2) Establishment of damage (cracks or debonding) detectionmethods

The point where cracks or debonding have actually occurred canbe detected from the continuous strain distribution measured bythe fiberoptic.

other types of sensors - university of washington

Documents