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.
Polarimetric Glucose Sensor
• 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.
Polarimetric Glucose Sensor
• The signal produced by the detector isproportional to the square of the E-fieldof the light incident on it and is given by:
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.
Bragg Grating Sensor
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
Bragg Grating Sensor
• 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.
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.
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.
Versoix bridge Project
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.