Hindawi Publishing CorporationJournal of SensorsVolume 2009, Article ID 979761, 12 pagesdoi:10.1155/2009/979761

Review Article

Surface Plasmon Resonance-Based Fiber Optic Sensors:Principle, Probe Designs, and Some Applications

B. D. Gupta and R. K. Verma

Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

Correspondence should be addressed to B. D. Gupta, bdgupta@physics.iitd.ernet.in

Received 26 January 2009; Accepted 2 June 2009

Recommended by Christos Riziotis

Surface plasmon resonance technique in collaboration with optical fiber technology has brought tremendous advancements insensing of various physical, chemical, and biochemical parameters. In this review article, we present the principle of SPR techniquefor sensing and various designs of the fiber optic SPR probe reported for the enhancement of the sensitivity of the sensor. Inaddition, we present few examples of the surface plasmon resonance- (SPR-) based fiber optic sensors. The present review mayprovide researchers valuable information regarding fiber optic SPR sensors and encourage them to take this area for furtherresearch and development.

Copyright © 2009 B. D. Gupta and R. K. Verma. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

1. Introduction

Surface plasmon resonance (SPR) is one of the most promis-ing optical techniques that find applications in differentfields. The first sensing application of SPR technique wasreported in 1983 [1]. Since then, numerous SPR sensingstructures for chemical and biochemical sensing have beenreported. In SPR technique, a TM (transverse magnetic) orp-polarized light causes the excitation of electron densityoscillations (known as surface plasmon wave, SPW) at themetal-dielectric interface. When the energy as well as themomentum of both, the incident light and SPW, match,a resonance occurs which results in a sharp dip in thereflected light intensity. The resonance condition dependson the angle of incidence, wavelength of the light beam,and the dielectric functions of both the metal as well as thedielectric. If the wavelength is kept constant and the angle ofincidence is varied, then the sharp dip appears at a particularangle and the method is called angular interrogation. Inanother method, called spectral or wavelength interrogation,the angle of the incident beam is kept constant and thewavelength is varied. In this method, resonance occurs at aparticular wavelength. The resonance parameter (angle orwavelength) depends on the refractive index of the dielectricmedium. Change in refractive index changes the value of the

resonance parameter. To excite surface plasmons, generally,a prism is used [2–9]. The prism-based SPR sensing devicehas a number of shortcomings such as its bulky size andthe presence of various optical and mechanical (moving)parts. Further, the prism-based SPR sensing device cannotbe used for remote sensing applications. These shortcomingscan be overcome if an optical fiber is used in place ofprism. The additional advantage of optical fiber is that theSPR probe can be miniaturized which can be advantageousfor samples which are available in minute quantity orare costly. Due to these advantages the surface plasmonresonance-based optical fiber sensors have drawn a lotof attention [10–23]. Both experimental and theoreticalinvestigations have been reported in literature on the SPR-based fiber optic sensors. The performance of these sensorsis, generally, evaluated in terms of sensitivity and signal-to-noise ratio (or detection accuracy). As is known, thehigher the values of these parameters, the better is thesensor.

The present review begins with a section on principle ofthe sensing technique. The section includes the descriptionof the performance parameters of the sensor: sensitivityand signal-to-noise ratio or detection accuracy. In the nextsection, we present the simplest SPR-based fiber optic sensorand discuss its modulation scheme. Section 4 of the present

2 Journal of Sensors

review deals with various designs of the SPR probe studiedto enhance the performance of the fiber optic sensor. Thedesigns include tapered, U-shaped and side-polished. Theadvantages of bimetallic coatings, addition of dopants infiber core and the choice of the metals for coating have beendiscussed in the same section. In the last section, we reviewsome of the SPR-based fiber optic sensors in details that havebeen reported in literature.

2. Principle

A metal-dielectric interface supports charge density oscil-lations along the interface which are called surface plasmaoscillations. The quantum of these oscillations is given thename as surface plasmon. The surface plasmons are accom-panied by a longitudinal (TM- or p-polarized) electric fieldwhich decays exponentially in metal as well as in dielectricmedium. The electric field has its maximum at metal-dielectric interface. The TM-polarization and exponentialdecay of electric field are found by solving the Maxwellequation for semi-infinite media of metal and dielectric withan interface of metal-dielectric. The propagation constant(KSP) of the surface plasmon wave propagating along themetal-dielectric interface is given by

KSP = ω

c

(εmεsεm + εs

)1/2

, (1)

where εm and εs are the dielectric constants of metal andthe dielectric medium, respectively, ω is the frequency ofincident light, and c is the velocity of light. From (1) it maybe noted that the propagation constant of surface plasmonwave depends on the dielectric constants of both the metaland the dielectric medium.

The surface plasmons can be excited by light withsame polarization state as that of surface plasmons. Thepropagation constant (Ks) of the light wave with frequencyω propagating through the dielectric medium is given by

Ks = ω

c

√εs. (2)

Since εm < 0 (for metal) and εs > 0 (for dielectric),for a given frequency, the propagation constant of surfaceplasmon (KSP) is greater than that of the light wave indielectric medium (Ks). To excite surface plasmons, twopropagation wave-vectors should be equal. Hence, the directlight cannot excite surface plasmons at a metal-dielectricinterface. To excite surface plasmons the momentum andhence the wave vector of the exciting light in dielectricmedium should be increased. This can be done if instead ofa direct light, evanescent wave is used to excite the surfaceplasmons. To obtain the evanescent wave for the excitationof surface plasmons, a prism with high dielectric constant isused.

When a light beam is incident through one of the twosides of the prism at an angle greater than the criticalangle at prism-air interface the total internal reflection oflight beam takes place. In the condition of total internalreflection light beam does not return exactly from the

p-polarized light Reflected light

SPW

EW

εp

εm

εs

Figure 1: Kretschmann configuration for the excitation of surfaceplasmon at metal-dielectric interface [23]. c© IEEE.

interface. Instead it returns after penetrating in the lowerrefractive index medium (air in this case). The field in thelower refractive index medium is called evanescent field andthe wave corresponding to this is called evanescent wave.The evanescent wave propagates along the prism-air interfaceand decays exponentially in the rarer medium (air). Thepropagation constant of the evanescent wave at prism-airinterface is given by

Kev = ω

c√εp sin θ, (3)

where εp represents the dielectric constant of the materialof the prism and θ is the angle of incidence of the beam.Increase in the dielectric constant of the prism increases thepropagation constant of the evanescent wave and hence thiscan be made equal to propagation constant of the surfaceplasmon wave to satisfy the surface plasmon resonancecondition. Kretschmann and Reather [24] devised a prism-based configuration, shown in Figure 1, to excite the surfaceplasmons using evanescent wave. In this configuration thebase of the glass prism is coated with a thin layer ofmetal (typically around 50 nm). The metal layer is kept indirect contact with the dielectric medium of lower refractiveindex (such as air or some other dielectric sample). Whena p-polarized light beam is incident through the prismon the prism-metal layer interface at an angle θ equal toor greater than the critical angle, the evanescent wave isproduced at the prism-metal interface. The excitation ofsurface plasmons occurs when the wave vector of evanescentwave exactly matches with that of the surface plasmonsof similar frequency. This occurs at a particular angle ofincidence θres. Thus the resonance condition for surfaceplasmon resonance is

ω

c√εp sin θres = ω

c

(εmεsεm + εs

)1/2

. (4)

The excitation of surface plasmons at metal/dielectric inter-face results in the transfer of energy from incident light tosurface plasmons, which reduces the intensity of the reflectedlight. If the intensity of the reflected light is measured as afunction of angle of incidence θ for fixed values of frequency,metal layer thickness and dielectric layer thickness then a

Journal of Sensors 3

Refl

ecta

nce

(R)

0

1

Angle (θ)

θres

Figure 2: SPR spectrum [23]. c© IEEE.

sharp dip is observed at resonance angle, θres, due to anefficient transfer of energy to surface plasmons as shownin Figure 2. The minimum of the reflected intensity can bequantitatively described with the help of Fresnel’s equationsfor the three-layer system.

For a given frequency of the light source and the dielectricconstant of metal film one can determine the dielectricconstant (εs) of the sensing layer adjacent to metal layerusing (4) if the value of the resonance angle (θres) is known.The resonance angle is experimentally determined by usingangular interrogation method. It is very sensitive to variationin the refractive index of the sensing layer. Increase inrefractive index of the sensing layer increases the resonanceangle.

Sensitivity and detection accuracy or signal-to-noiseratio (SNR) are the two parameters that are used to analyzethe performance of an SPR sensor. For the best performanceboth the parameters should be as high as possible. Sensitivityof an SPR sensor utilizing angular interrogation methoddepends on the amount of shift of the resonance angle witha change in the refractive index of the sensing layer. For agiven refractive index change if the shift in resonance angleincreases this means an increase in the sensitivity of thesensor.

Figure 3 shows a plot of reflectance as a function of angleof incidence of the light beam for sensing layer with refractiveindices ns and ns + δns. Increase in refractive index by δnsshifts the resonance angle by δθres. Thus the sensitivity of anSPR sensor utilizing angular interrogation method is definedas

Sn = δθres

δns. (5)

The detection accuracy or the SNR of an SPR sensordepends on how accurately and precisely the sensor candetect the resonance angle and hence, the refractive index ofthe sensing layer. The narrower the width of the SPR curve,the higher is the detection accuracy. Therefore, if δθ0.5 is theangular width of the SPR curve corresponding to reflectance0.5, the detection accuracy of the sensor is assumed to be

Refl

ecta

nce

(R)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

θ

δθres

δθ0.5

ns ns + δns

Figure 3: SPR spectra for two different refractive indices of thesensing layer [23]. c© IEEE.

inversely proportional to δθ0.5 (Figure 3). The SNR of theSPR sensor with angular interrogation is, thus, defined as[25]

SNR = δθres

δθ0.5. (6)

3. Fiber Optic SPR-Based Sensors

In the case of a prism-based SPR sensor evanescent waverequired to excite surface plasmons is resulted due to the totalinternal reflection taking place at the prism-metal interfacewhen the angle of incidence of the beam is greater thanthe critical angle. The evanescent wave is also present inan optical fiber because the light guidance in an opticalfiber occurs due to the total internal reflection of theguided ray at the core-cladding interface. In the case ofoptical fiber the evanescent wave propagates along the core-cladding interface. Therefore, to design an SPR-based fiberoptic sensor, the prism can be replaced by the core of anoptical fiber. To fabricate an SPR-based fiber optic sensor,the silicon cladding from a small portion of the fiber,preferably from the middle, is removed and the unclad coreis coated with a metal layer. The metal layer is further,surrounded by a dielectric sensing layer as shown in Figure 4.In an SPR-based fiber optic sensor, all the guided raysare launched and hence, instead of angular interrogation,spectral interrogation method is used. The light from apolychromatic source is launched into one of the ends of theoptical fiber. The evanescent field produced by the guidedrays excites the surface plasmons at the metal-dielectricsensing layer interface. The coupling of evanescent fieldwith surface plasmons strongly depends on wavelength, fiberparameters, probe geometry, and the metal layer properties.Unlike prism-based SPR sensor, the number of reflections formost of the guided rays is greater than one in SPR-based fiberoptic sensor geometry. The smaller the angle of incidenceat the interface, the larger is the number of reflections perunit length in the fiber. In addition, the number of reflectionsfor any ray also depends on the length of the sensing region

4 Journal of Sensors

SPW

Cladding

Core

Metal layer

Sensing layer

Input light

Transmitted light

θ

Figure 4: A typical probe of an SPR-based fiber optic sensor [23].c© IEEE.

and the fiber core diameter. The number of reflections isone of the important parameters that affect the width ofthe SPR curve. The intensity of the light transmitted afterpassing through the SPR sensing region is detected at theother end of the fiber as a function of wavelength. The SPRspectrum thus obtained is similar in shape to that shownin Figure 2. The sensing is accomplished by observing thewavelength corresponding to the dip in the spectrum (calledresonance wavelength). A plot of resonance wavelength withthe refractive index of the sensing layer is the calibrationcurve of the SPR sensor. The sensitivity and the detectionaccuracy are determined in the same way as determined inthe case of angular interrogation. The angles are replacedby wavelengths in the definitions of sensitivity and detectionaccuracy.

4. SPR Probe Designs

Sensitivity, detection accuracy, reproducibility, and operatingrange of a sensor are the important parameters to compareit with other sensors. The best sensor is the one that hashigh sensitivity, detection accuracy and operating range,in addition to giving reproducible results. To achieve thisvarious modifications have been carried out in the design offiber optic SPR probe. We review some of these modificationsin what follows.

4.1. Bimetallic Coating. For metallic coating on prism baseor fiber core either silver or gold is used. Gold demonstratesa higher shift of resonance parameter to change in refractiveindex of sensing layer and is chemically stable. Silver, onthe other hand, displays a narrower width of the SPR curvecausing a higher SNR or detection accuracy. The sharpnessof the resonance curve depends upon the imaginary partof the dielectric constant of the metal. Silver having thelarger value of the imaginary part of the dielectric constantshows narrower width of the SPR curve causing a higherSNR or detection accuracy. On the other hand, the shiftof the resonance curve depends on the real part of thedielectric constant of the metal. The real part of the dielectricconstant is large in the case of gold than the silver and hencegold demonstrates a higher shift of resonance parameter tochange in refractive index of sensing layer. The chemicalstability of silver is poor due to its oxidation. The oxidationof silver occurs as soon as it is exposed to air and especiallyto water, which makes it difficult to give a reproducibleresult and hence the sensor remains unreliable for practical

applications. Therefore, the treatment of silver surface bya thin and dense cover is required. In this regard, a newstructure of resonant metal film based on bimetallic layers(gold as outer) on the prism base with angular interrogationmethod was reported [25]. The new structure displayeda large shift of resonance angle as gold film, and alsoshowed narrower resonance curve as silver film along withthe protection of silver film against oxidation. The samestructure with spectral interrogation was extended to SPR-based optical fiber sensor for the selected and all guidedrays configurations [19]. The sensitivity and SNR wereevaluated numerically for different ratios of the thicknessesof silver and gold layers. Figures 5(a) and 5(b) show thevariations of SNR and sensitivity with percentage of silverin bimetallic combination, respectively [19]. As expected theSNR increases with the increase in the silver thickness. Thevariation is almost the same for both kinds of launching butthe values of SNR for all guided rays launching are about1.5 times higher than those corresponding to selected rayslaunching. As far as sensitivity is concerned, it decreases asthe silver layer thickness increases or gold layer thicknessdecreases. Its variation with silver thickness is almost samein the two cases but in terms of values, the selected raylaunching has higher value than the all guided rays launchingcase.

4.2. Choice of Metals. The capability of other metals such ascopper (Cu) and aluminium (Al) for SPR sensor applicationshas also been analyzed. Both metals have the ability to beused for an SPR sensor. The copper has some limitationslike silver. It is chemically vulnerable against oxidation andcorrosion, therefore, its protection is required for a stablesensing application. The SPR sensing capabilities of differentbimetallic combinations made out of Ag, Au, Al, and Cuwere theoretically investigated for the design of SPR-basedfiber optic sensors [26]. Figures 6(a) and 6(b) show thevariation of sensitivity and SNR with the ratio of inner layerthickness to total bimetallic thickness for different bimetalliccombinations, respectively [26]. The figure predicts thatthe sensor with single gold layer is the most sensitivewhereas the sensor with single aluminium layer is theleast. Further, Cu-Al combination provides the minimumsensitivity for any ratio of their corresponding thicknessvalues. In all the combinations with gold, Ag-Au and Cu-Au combinations provide good sensitivity for the smallthickness of the inner layer while the Au-Al combinationprovides larger sensitivity than all other combinations forthe larger thickness of inner gold layer. It implies that athick Au layer with very thin cover of Al layer (around 2–4 nm) provides quite a large sensitivity. As far as variationof SNR with inner layer fraction is concerned, Cu-Al isbetter among all the bimetallic combinations while the Ag-Au combination provides the minimum values of SNR.To achieve highest SNR, one should choose a thin Culayer and a much thicker covering of Al layer. This studyimplies that there is no single combination of metals thatprovides high values of both SNR and sensitivity simultane-ously.

Journal of Sensors 5

Sign

al/n

oise

rati

o

0.70.80.9

11.11.21.31.41.51.61.71.81.9

2

Percentage of silver thickness

0 20 40 60 80 100

Selected rayAll guided rays

(a)

Sen

siti

vity

(mm

/RIU

)

5

5.2

5.4

5.6

5.8

6

6.2

6.4

Percentage of silver thickness

0 20 40 60 80 100

Selected rayAll guided rays

(b)

Figure 5: Variation of (a) signal-to-noise ratio and (b) sensitivity with percentage of silver in bimetallic layer for two different kinds of lightlaunching [19]. c© Elsevier.

Ag-Au

Ag-Al

Cu-Au

Cu-Al

Au-Al

Sen

siti

vity

(μm

/RIU

)

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3

Inner layer thickness/total bimetallic thickness

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

(a)

Ag-Au

Ag-Al

Cu-Au

Cu-Al

Au-AlSNR

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Inner layer thickness/total bimetallic thickness

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

(b)

Figure 6: Variation of (a) sensitivity and (b) SNR with inner layer fraction for different bimetallic combinations [26]. c© AIP.

4.3. Effect of Dopants. It may be noted from (4) that the SPRcondition depends upon the refractive index of the materialof the fiber core. Therefore, if an optical fiber is fabricated byadding dopants in the fiber core the sensitivity of the sensorcan be enhanced or tuned. Generally, an optical fiber withpure silica core is used for SPR-based sensors. Sharma et al.[27] carried out theoretical modeling and analysis of SPR-based fiber optic sensor to evaluate the effect of dopants onthe sensitivity and SNR. Germanium oxide (GeO2), boronoxide (B2O3), and phosphorus pent-oxide (P2O5) were usedas dopants for pure silica. Figure 7 shows the variation ofsensitivity with the refractive index of the sensing layer fordifferent dopants with concentrations [27]. The simulationpredicts an increase of about 50% in sensitivity betweenB2O3 (5.2) and GeO2 (19.3) dopants. Moreover, as thedoping concentration of GeO2 is increased from a low of6.3 mole % to a high of 19.3 mole %, a noticeable decrease in

sensor’s sensitivity is obtained. Further, the effect of dopantson the sensitivity of the fiber optic SPR sensor was reportedto be same irrespective of whether a single metal layer or abimetallic configuration is used.

4.4. Tapered Probe. Several groups have worked on theimprovement of the sensitivity of a fiber optic SPR sensorby changing the shape or the geometry of the fiber opticSPR probe. Tapering the fiber optic SPR probe was one ofthe modifications reported in literature [18, 28]. A typicaltapered fiber optic SPR probe is shown in Figure 8 [29]. Theuses of dual-tapered and tetra-tapered fiber optic SPR probesfor gas and liquid sensing have also been reported [18].Changing the profile of the tapered SPR probe also affects thesensitivity of the sensor. Surface plasmon resonance-basedtapered fiber optic sensor with three different taper profiles,

6 Journal of SensorsSe

nsi

tivi

ty(μ

m/R

IU)

1.8

2.2

2.6

3

3.4

3.8

4.2

4.6

5

Sensing layer refractive index (ns)

1.333 1.338 1.343 1.348 1.353 1.358 1.363 1.368

B2O3 (5.2)

Pure SiO2

P2O5 (10.5)

GeO2 (6.3)

GeO2 (19.3)

Figure 7: Variation of sensitivity with sensing layer refractive index.A: Silica doped with GeO2 (19.3); B: Silica doped with GeO2 (6.3);C: Silica doped with P2O5 (10.5); D: Pure silica with no doping; andE: Silica doped with B2O3 (5.2). Numbers in brackets are the molarconcentrations of dopant in mole % [27]. c© Elsevier.

Gold layer

Sensing region

Cladding

Core2ρiθ φ

Ω

Ω

2ρ0

L

Figure 8: A typical SPR-based fiber optic sensor with tapered probe[29]. c© Elsevier.

namely, linear, parabolic, and exponential-linear, shown inFigure 9, was analyzed theoretically [29]. Figure 10 shows thevariation of the sensitivity of the tapered fiber optic SPRprobe with taper ratio for these three taper profiles [29].Theoretical analysis predicts an increase in the sensitivitywith the increase in the taper ratio. The study further showsthat, for a given taper ratio, the exponential-linear taperprofile provides the maximum sensitivity. The increase insensitivity occurs because of the decrease in the angle ofincidence of the guided rays with the normal to the core-cladding interface in the tapered region.

To further enhance the sensitivity, an SPR probe ofuniform core (with metallic coating) sandwiched betweentwo unclad tapered fiber regions, shown in Figure 11, wasproposed [30]. The taper region 1 brings down the anglesof the bound rays in the fiber close to the critical angle ofthe unclad tapered region while the taper region 2 reconvertsthe angles of these rays to their initial values so that all theguided rays can propagate up to the output end of the fiber.This was achieved by choosing the minimum allowed value

Exponential-linear

Linear

Parabolic

ρ(z)

ρ0

ρi

Z

L

Figure 9: Three different taper profiles showing the linear,parabolic, and exponential-linear [29]. c© Elsevier.

Sen

siti

vity

(μm

/RIU

)

2.8

3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

Taper ratio (ρi/ρ0)

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Exponential-linearParabolicLinear

Figure 10: Variation of sensitivity with taper ratio for threedifferent taper profiles [29]. c© Elsevier.

of the radius of the uniform core in the sensing region. In thesensing region rays propagate close to the critical angle of theregion. Figure 12 shows the variation of sensitivity with taperratio for this kind of probe [30]. The sensitivity increaseswith an increase in the taper ratio as reported in otherstudies [29]. However, a significant sensitivity enhancementof more than 5 times was obtained for a taper ratio of 2.0 incomparison to conventional (TR = 1) fiber optic SPR sensor.

4.5. U-Shaped. The angle of incidence of the ray with thenormal to the core-cladding interface can be brought closeto the critical angle by using a U-shaped probe. An SPR-based fiber optic sensor with uniform semimetal coated U-shaped probe, shown in Figure 13, was analyzed using a bi-dimensional model [31]. To make the analysis simpler, all

Journal of Sensors 7

Sensing region

Taperedregion 2

Taperedregion 1

Au layer

SPW

Core

Cladding

EW2ρi

θiθ

θaφ

Ω

2ρ0

z = 0 z = L z = 2L z = 3L

Figure 11: A novel SPR-based fiber optic sensor. Sensing probe of uniform core radius is sandwiched between two tapered fiber regions[30]. c© IEEE.

Sen

siti

vity

(μm

/RIU

)

2.75

4

5.25

6.5

7.75

9

10.25

11.5

12.75

14

15.25

Taper ratio (ρi/ρ0)

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

Figure 12: Variation of sensitivity of the SPR-based fiber opticsensor with taper ratio [30]. c© IEEE.

the guided rays of the p-polarized light launched in thefiber and their electric vectors were assumed to lie in theplane of bending of the U-shaped probe. Figure 14 showsthe variation of the sensitivity of the probe with bendingradius for different values of the sensing length of the probe.Increase in sensitivity with the decrease in the bending radiuswas obtained. The increase in sensitivity was up to a certainvalue of the bending radius below that it starts decreasingsharply. The decrease in sensitivity occurs because the angleof incidence of the last ray becomes less than the critical anglerequired for a ray to be guided in the bent region. Thus,there exists an optimum value of the bending radius at whichthe sensitivity of the SPR sensor based on U-shaped probeacquires a maximum value. The trend of variation was samefor all the three values of the sensing length used. This is dueto the independence of the angle of incidence on the sensinglength. For a given bending radius the sensitivity increasesas the sensing length increases. The increase in sensitivity,as mentioned earlier, is due to the decrease in the angle ofincidence. The decrease in angle of incidence increases the

Sensingmedium

Gold-layer

Cladding

Core2ρ

θ h

R0

Φ

δ

Figure 13: A typical U-shaped fiber optic SPR probe [31]. c© IOP.

number of reflections which causes the broadening of SPRcurve and hence the decrease in the detection accuracy orSNR of the sensor. The enhancement in sensitivity obtainedwas much more compared to the decrease in the detectionaccuracy and hence the decrease in detection accuracy canbe tolerated. In fact the maximum sensitivity achieved wasseveral times more than that reported for an SPR-based fiberoptic tapered probe.

4.6. Side-Polished Fiber. The SPR probes reported above usemultimode optical fibers. A single mode fiber has also beenused to fabricate fiber optic SPR probe. Surface plasmonresonance sensor using side-polished single mode opticalfiber and a thin metal over layer is shown in Figure 15[32]. The design of the probe is slightly different from thatshown in Figure 4. In this configuration, the guided modepropagating in the fiber excites the surface plasmon waveat the interface between the metal and a sensing medium.The resonance occurs if the two modes are closely phasematched. Such single-mode optical fiber based SPR sensoris more sensitive and more accurate in comparison to thosewith multi-mode optical fibers. However, their fabrication ismuch more complex and sophisticated compared with thosethat use multi-mode fibers. The advantage of side-polishedhalf block SPR sensor is that it requires a very small amountof sample for measuring the refractive index.

8 Journal of Sensors

Sen

siti

vity

(μm

/RIU

)

0

510

1520

2530

3540

4550

5560

6570

75

Bending radius (cm)

0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

L = 1 cmL = 2 cmL = 3 cm

Figure 14: Variation of sensitivity with bending radius for threedifferent values of the sensing length [31]. c© IOP.

Gold thin film

Core

Cladding

Silica block

of a fiber

Figure 15: Side-polished single mode fiber optic SPR probe [23].c© IEEE.

Recently SPR-based side-polished multimode opticalfiber sensors have been reported [33, 34]. In these sensorsthe fiber was side-polished until half the core was closed. Thisincreased the sensing area which is an advantage. Apart fromside-polished single mode fiber, D-type single mode opticalfibers have also been used for sensing applications utilizingSPR technique [35, 36]. These fibers also improve sensitivity.The other designs for SPR-based fiber optic sensor includesSPR probe at one of the ends of the fiber with the reflectingend face [37, 38] and a fiber tip [39, 40]. The photonic band-gap fiber based SPR sensors have also been reported veryrecently [41, 42].

4.7. Effect of Skew Rays. Most of the theoretical studieson fiber optic SPR sensors using ray optics and reportedin literature do not consider skew rays in the analysis.These studies consider the propagation of only meridionalrays in the fiber that makes the analysis simpler. However,apart from meridional rays, skew rays also exist in thefiber depending on the light launching conditions. Theserays follow a helical path inside the fiber. To specify thetrajectory of a skew ray, a second angle (known as skewnessangle), in addition to the inclination of the ray with the

axial direction of the fiber, is required. Recently, the effectof skew rays on the sensitivity and the SNR of a fiberoptic SPR sensor were studied using spectral interrogationmethod [43]. Figures 16(a) and 16(b) show the variation ofsensitivity and SNR with skewness parameter, respectively[43]. Both the sensitivity and the SNR decrease as the valueof skewness parameter increases irrespective of the metalused for coating. The sensitivity is better in the case ofgold whereas silver demonstrates better SNR as expected.The decrease in sensitivity for highest value of skewnessparameter is about 6% in the case of both the metals. Theeffect of skew rays is more on SNR than on the sensitivity. Inthe case of gold film it is about 40% while in the case of silverit is around 30%.

5. Some Applications

The surface plasmon resonance-based fiber optic sensorshave large number of applications for quantitative detectionof chemical and biological species. These include foodquality, medical diagnostics and environmental monitoring.These are detected by changing the refractive index ofthe medium around the metallic coating. The measurandchanges the refractive index of the medium directly orindirectly. Here we present few SPR-based fiber optic sensors.

5.1. Temperature. Since the refractive index of a mediumdepends on its temperature, the SPR technique can beapplied to sense the temperature of a medium. The tem-perature sensor based on surface plasmon resonance wasproposed using a coupling prism and angular interrogationmode of operation [44]. It was suggested that for SPRtechnique to be used in temperature sensing, the sensinglayer of large thermo-optic coefficient (such as titaniumdioxide or silicon acrylate) should be used. Further, thepenetration of surface plasmon wave should be restrictedonly to metal and sensing layers by taking an appropriatethickness of the two layers. The analysis was later extendedto SPR-based fiber optic remote sensor for temperaturedetection [45]. The sensing layer was assumed to be ofTiO2 (titanium dioxide). The outermost ambient mediumwas chosen as air, which adds to the flexibility of thesensor’s design. Figure 17 shows the variation in resonancewavelength with temperature for silver and gold [45]. Forboth the metals, resonance wavelength shifts to shorter sidewith the increase in the temperature. However, there is noappreciable difference in resonance wavelength (and hence,in temperature sensitivity) for two metals used. This wassuggested to be due to the similarity in the variation ofphysical properties of both the metals with temperature.Figure 18 depicts the corresponding variation in SPR curvewidth with temperature [45]. The SPR curve width increaseswith the increase in the temperature. This was reportedto happen because the imaginary (absorption) part of themetal dielectric function increases with temperature. Theeffect of other parameters such as numerical aperture andthe ratio of sensing region length to fiber core diameter onthe sensitivity and detection accuracy were also reported.

Journal of Sensors 9

Silver

Gold

Sen

siti

vity

2.5

2.6

2.7

2.8

2.9

3

3.1

3.2

Skewness parameter

0 0.2 0.4 0.6 0.8 1

(a)

Silver

Gold

SNR

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Skewness parameter

0 0.2 0.4 0.6 0.8 1

(b)

Figure 16: Variation of (a) Sensitivity and (b) SNR of an SPR-based fiber optic sensor with skewness parameter for two different metals[43]. c© OSA.

Res

onan

cew

avel

engt

h(n

m)

557

558

559

560

561

562

563

564

565

566

Temperature (◦K)

300 330 360 390 420 450 480 510 540 570 600

SilverGold

Figure 17: Variation of resonance wavelength with temperature fortwo metals: silver and gold [45]. c© Elsevier.

It was also reported that the fiber with smaller numericalaperture provides larger detection accuracy without affectingthe temperature sensitivity. Similarly, the small values of theratio of sensing region length to fiber core diameter give highdetection accuracy. For SPR-based fiber optic temperaturesensor smaller sensing region and highly multimode opticalfiber was recommended.

5.2. Naringin. The processing of citrus fruit juice has facedformidable problems in terms of bitterness, thereby affectingits consumer acceptability. The bitterness is caused by exces-sive naringin contents in fruit juice. The presence of naringinin fruit juice can be detected using an SPR-based fiber opticsensor. An SPR-based fiber optic sensor relying on spectralinterrogation method was reported for the detection of

SPR

curv

ew

idth

(nm

)

56

57

58

59

60

61

62

63

Temperature (◦K)

300 330 360 390 420 450 480 510 540 570 600

SilverGold

Figure 18: Variation of SPR curve width with temperature for twometals: silver and gold [45]. c© Elsevier.

naringin [21]. The SPR probe was prepared by immobilizingthe enzyme naringinase on the silver-coated core of theoptical fiber. To immobilize, gel entrapment technique wasused. The experimental set up for the characterization ofthe SPR-based fiber optic sensor is shown in Figure 19 [21].Light from a broadband source (tungsten halogen lamp)was focused on the input face of the fiber using a circularslit and a microscope objective. The probe was mountedin a flow cell to keep the sample around the probe. Thespectral distribution of the transmitted power for the samplearound the probe was determined using a monochromator,a silicon detector and a power meter. The spectral distri-bution of transmitted light so obtained for a sample wasdivided by the corresponding spectral distribution obtainedwithout any sample around the probe. The resultant SPRspectrum so obtained does not depend on the source spectral

10 Journal of Sensors

Enzymes immobilized fiber

Si detector

Monochromator

Flow cell

LampMicroscope

objective

Figure 19: Experimental set up of the SPR-based fiber optic sensor [21]. c© Elsevier.

SPR

wav

elen

gth

(nm

)

500

520

540

560

580

600

620

640

Concentration of naringin (gm/100 mL)

0 0.05 0.1 0.15 0.2

Figure 20: Variation of resonance wavelength with the concentra-tion of naringin in buffer solution [21]. c© Elsevier.

output, spectral sensitivity of photodetector and the spectralabsorbance of the fiber. From the SPR spectrum resonancewavelength was determined. The calibration curve of theSPR-based fiber optic sensor obtained for the detection ofnaringin is shown in Figure 20 [21]. As the concentration ofnaringin increases the resonance wavelength increases. Theincrease in resonance wavelength was reported to be dueto an increase in the refractive index of the immobilizedlayer which may occur due to the formation of a complexof naringin with the enzyme naringinase in the film.

5.3. Pesticides. Surface plasmon resonance-based fiber opticsensor for the detection of organophosphate pesticide, chlor-phyrifos, was also reported recently using similar experimen-tal method as used for the detection of naringin [46]. Theprobe was prepared by immobilizing acetylcholinesteraseenzyme on the silver-coated core of the fiber. The principleof detection of pesticide was slightly different from thatof naringin. It was based on the principle of competitivebinding of the pesticide (acting as an inhibitor) for thesubstrate (acetlythiocholine iodide) to the enzyme. Figure 21shows SPR spectra for pesticide concentration of 1.0 μMand a substrate concentration of 2.5 mM for the two cases

A BNor

mal

ised

outp

ut

pow

er

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

SPR wavelength (nm)

450 550 650 750 850

Figure 21: Variation of normalized output power as a functionof wavelength for a substrate concentration (2.5 mM) and 1.0 μMconcentration of pesticide (A) without enzyme and (B) withenzyme (B) [46]. c© Elsevier.

(A) without enzyme (control experiment) and (B) withenzyme in the film [46]. The dip in control experimentoccurs due to the refractive index of the gel. The differencein two spectra implies that the enzyme is playing a role inchanging the refractive index of the gel in the presence ofpesticide. The resonance wavelength was obtained around590 nm for 1.0 μM pesticide concentration. Figure 22 showsthe calibration curve of the sensor [46]. The trend is oppositeto that reported for the detection of naringin. In this case,the SPR wavelength decreases with the increase in theconcentration of the pesticide for the fixed concentration ofthe substrate. The results appear to be due to the decreasein the refractive index of the film as the concentration ofpesticide increases.

Surface plasmon resonance-based fiber optic sensorshave also been reported for other chemical species. Theseinclude measurements of salinity in water [22, 37], refractiveindices of alcohols [17], BSA [34], vapor and liquid analyses[18]. In addition a single-mode waveguide surface plasmonresonance sensor has been developed for biomolecular inter-action analysis [47]. Recently, fiber optic SPR sensors havebeen reported that uses either a series of long period grating[48] or Bragg grating [49]. The advantage of these designs isthat these offer multiple sensing channels capability.

Journal of Sensors 11SP

Rw

avel

engt

h(n

m)

550

560

570

580

590

600

610

620

630

Log (concentartion of pesticide) (moles/liter)

−8 −7 −6 −5 −4 −3 −2

Figure 22: Calibration curve of SPR-based fiber optic pesticidesensor [46]. c© Elsevier.

6. Summary

In this review article we have first described the principleof surface plasmon resonance technique. The technique iswell established and will remain unchanged for years. Inthe beginning, the technique was used for prism-based SPRsensors but later when the advantages of optical fiber wererealized, it was applied to optical fiber based sensors. Recentlythere have been significant advancements in both the designof fiber optic SPR sensors and strategies for the enhancementof the performance of the sensors particularly sensitivity. Theprobe designs include tapered, U-shaped, and side-polishedfibers. These developments are likely to drive future trends inthe research and development of optical fiber sensors. Theunique optical properties of metal nanoparticles have alsoattracted the sensor community to develop localized surfaceplasmon resonance- (LSPR-) based sensors. The LSPR-basedsensors have few advantages over bulk SPR-based sensors.However, the pace of collaboration of LSPR with optical fiberfor sensing is at present slow but may gain momentum incoming years.

Acknowledgment

The present work is partially supported by the Departmentof Science and Technology (India).

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