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Sensors and Actuators B 123 (2007) 594–605

Fiber optic sensing of liquid refractive index

Argha Banerjee b, Sayak Mukherjee b, Rishi Kumar Verma c, Biman Jana d, Tapan Kumar Khan d,Mrinmoy Chakroborty d, Rahul Das d, Sandip Biswas b, Ashutosh Saxena a, Vandana Singh b,

Rakesh Mohan Hallen d, Ram Swarup Rajput b, Paramhans Tewari a, Satyendra Kumar b,Vishal Saxena a, Anjan Kumar Ghosh a,c, Joseph John a,c, Pinaki Gupta-Bhaya d,∗

a Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, Indiab Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, India

c Laser Technology Program, Indian Institute of Technology Kanpur, Kanpur 208016, Indiad Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

Received 10 April 2006; received in revised form 28 September 2006; accepted 28 September 2006Available online 13 November 2006

bstract

An optical fiber, partially stripped of its cladding is shown to sense refractive index of a liquid in which the uncladded sensing region is immersed,o a high degree of precision and over a wide range of refractive index. The slope of sensor response is found to be non linear, can have eitherign and can change sign at around refractive index of the fiber. The sensitivity of the sensor to refractive index change is dependent on cladding

hickness and is a maximum at an intermediate thickness value. It is insensitive to the presence of absorption at the wavelength at which refractivendex is being measured and to the chemical nature of the solute. Experiments designed to show that cladding modes are responsible for sensingre described.

2006 Elsevier B.V. All rights reserved.

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eywords: Liquid refractive index sensor; Fiber optic refractive index sensor; R

. Introduction

The measurement of refractive index (RI) is important in aariety of applications. Various methods for its measurementave been described [1,2]. Refraction of light by turbid colloidalispersions has been of interest to physical chemists for morehan 50 years [3–5]. Yet, only a few reports have appeared hith-rto on the systematic measurement of RI increment of colloidalpheres. The classical methods of RI measurement from criticalngle and Brewster angle data run into difficulty for absorbingnd turbid liquids [6–22]. The analysis of light scattering and tur-idity spectroscopic data of polymeric or particulate suspensionequires RI values of suspended particles [23]. RI determination

f suspended particles requires RI data of turbid suspensions.n bio-sensing there is a need to measure small RI changes inmall volumes of liquid. Traditional bulk refractometers are then

∗ Corresponding author. Tel.: +91 512 2597372; fax: +91 512 2597436.E-mail address: pinaki@iitk.ac.in (P. Gupta-Bhaya).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.09.063

tive index sensitivity of uncladded fiber; Uncladded optical fiber as sensor

ot appropriate. Their size and weight are inconvenient in thisnd other applications. In view of the above, development oflternative sensors of RI, viz., those based on optical fiber iselevant.

A fiber based RI sensor can be very compact in size and beade suitable for remote sensing. It is usable for liquids or poly-er composites, needs only a small volume and can be adapted to

e chemical sensors. Optical fiber has also been used as a sensorf absorption [24–27]. Metal-coated fibers using surface plas-on resonance [28,29] and in-fiber Bragg grating (FBG) [30,31]

ave been used as highly sensitive RI sensing devices. Taperingf fiber and stripping of the fiber cladding have been used toake optical fiber a sensitive sensor of RI [32–36]. Untapered,

ully cladded fibers with thin films deposited on them have alsoeen used as RI sensors [37,38]. A fiber stripped of cladding haseen layered with adsorbent material deposited by Sol–gel tech-

ique to make it a sensor of chemicals that get adsorbed on thedsorbent layer and modify its RI [39–45]. Optical fiber has beensed as a vehicle for carrying light in experiments designed toense RI, but not directly as a sensor of RI [46,47]. A few reports

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A. Banerjee et al. / Sensors an

efer to the significance of the cladding of a fiber in the contextf its use as a sensor [48–53].

In this report, we describe experimental results on RI sens-ng by a low cost plastic cladded plastic fiber and a silica fiberith a plastic coating that forms a protective layer on the sil-

ca cladding. This layer is not quite a formal cladding, but itsemoval shows a measurable sensitivity of output light inten-ity to the change of RI of the test liquid. In the rest of theaper, we refer to this plastic layer as the cladding of the sil-ca fiber. The cladding of these two fibers have been strippedo different thickness to make the fibers sensitive to RI of thenvironment. The effect of variation of cladding thickness andhe effect of mode scrambling highlight the role of cladding

odes in RI sensing. The influence of light absorption by solutend that of its chemical nature on RI measurement are alsonvestigated.

. Methods

.1. Measurement system

The experimental setup is shown in Fig. 1. A laser beamλ = 633 nm) obtained from a Melles-Griot (25LHP-213230)elium Neon Laser is chopped by a Stanford Research Systemptical chopper at ∼900 Hz. The chopped light beam falls on aeam splitter (Melles Griot) which splits the beam into a refer-nce and a sample beam that emerge in perpendicular directions.he sample beam passes through a sensor fiber, with claddingtched to different extents, immersed in the fluid of interest.he beam falls on two separate matched photodetectors whoseutput currents are converted into proportional voltages. Thehotodiode is appropriately biased to minimize noise. The twooltages are then fed into two separate Stanford Research Sys-em (SR 830) Lock-in-Amplifiers. The ratio of their outputs isalculated in a personal computer that is interfaced to the Lock-n-Amplifiers in a Labview environment. We have performedxperiments with a reference beam that directly falls onto a pho-odiode without passing through a fiber or has passed through

fully cladded fiber. Experiments performed with both theseptions give mutually consistent results. The system is verified

o be linear within a certain range of light intensity in whiche perform experiments. A typical measurement lasts for about5 min. Total number of data points over which averaging isone is 20/min.

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Fig. 1. Schematic experim

uators B 123 (2007) 594–605 595

.2. Fiber specification

The plastic cladded plastic fiber (SH 4001, Super Eska Fiber,ber diameter: 1.00 mm and core diameter: 0.98 mm, RI ofore = 1.492 and of cladding = 1.402) was obtained from Mit-ubishi Rayon Co., Ltd., Tokyo, Japan. This fiber is a step indexber and is referred to as PCP fiber. A silica fiber was purchasedrom Electro-optical Products Division, International Telephonend Telegraph Corporation (ITT), Roanoke, VA, USA (T-1227).t is a graded index fiber, has a core thickness 50 (±5) �m, alass outer diameter (sum of core and cladding thickness) 125±6) �m, core axial RI 1.48 (nominal) and a numerical aperture.23–0.28. A second layer made of plastic forms a protectiveoating on top of the silica cladding. This fiber is referred to asCS fiber.

.3. Fiber etching

In experiments described in this paper, the optical fibers uncladded to different cladding thickness. As a result, thevanescent field in the core cladding interface interacts with theurrounding liquid. The PCP fiber is uncladded mechanicallyy careful use of a razor. The plastic coating of the PCS fibers removed by immersing a part of the fiber in concentrated sul-huric acid. The degree of cladding removal is dependent onhe length of the time of immersion of the fiber in acid and is

onitored by measurement of the fiber thickness by an opticalicroscope fitted with a digital camera. The magnified image of

he fiber is refined for more accurate edge detection by an imagerocessing software. The thickness of the fiber image is theneasured by counting the number of pixels between the two

dges using manufacturer provided thickness of the untreatedber as reference. A schematic diagram of an optical fiber whoseensing region is partially stripped of cladding is given in Fig. 2.ypical total length of fiber used is 30 cm. Typical length of thencladded region is half of the total length.

The PCS fiber has an outer buffer jacket which cannot beemoved by simple mechanical means. Sulphuric acid etchingemoves the buffer jacket first and then proceeds to etch the hardlastic layer that coats the silica cladding. It is not possible to

top etching right where the jacket alone has been removed andhe hard plastic layer is intact. As a result, we have not measuredhe thickness of the fiber with the plastic layer intact. A changen fiber thickness as a result of etching indicates a change in the

ental arrangement.

596 A. Banerjee et al. / Sensors and Actuators B 123 (2007) 594–605

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Fig. 3. Plot of change in intensity ratio �R as a function of change in refractiveiTc

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Fig. 7(a) displays data obtained on PCS fiber etched for16 h, a linear plot with (∂R/∂n) < 0 (henceforth called negative

ig. 2. Schematic diagram of an optical fiber sensor of refractive index whoseensing region is stripped to different cladding thickness.

hickness of the plastic layer. We find that the fiber treated in sul-huric acid for 16 h shows a fiber thickness of 120 �m which ishe thickness of silica core and silica cladding (the manufacturerpecification for this thickness is 125 ± 6 �m). Etching for 16 hemoves the plastic layer completely. In this report, observationsade as a function of fiber thickness are interpreted as having

een made as a function of thickness of the plastic layer.Data on known refractive indices of solutions of fructose,

agnesium sulphate, copper sulphate and sucrose are obtainedrom CRC Handbook of Chemical and Engineering Data [54].he data are reported at λ = 589 nm.

. Results and discussions

.1. Sensor response to RI change

Let Isig denote the output intensity from the uncladded fibermmersed in the liquid under test and let Iref be the intensity ofhe reference beam. The experimental indicator of the value ofhe RI of the liquid under test is the ratio

= Isig

Iref.

We have uncladded the sensing fibers by mechanical or chem-cal etching and experimentally determined the variation of Rith changes in RI of the liquid. The results of the experiments

re plotted in Figs. 3–11. The characteristics of the slopes ofhese plots are discussed in the next subsection.

.1.1. Non linearity and sign of slopeA plot of the intensity ratio as a function of RI of test liquids

hows a non linear functional dependence. The values of RI usedn these plots are literature values, determined by other moreirect techniques, viz., critical angle measurements. The refer-

nce liquids of known refractive indices that we use are aqueousaq.) sucrose solutions (n = 1.3359–1.3999) (54) and aq. glycerololutions (n = 1.3897–1.4735) (54). The plots of �R, the changen intensity ratio as a function of �n, the change in RI, with pure

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ndex �n, fitted with an exponential growth function. χ2 of fit are indicated.he origin with respect to which the �’s are calculated is sucrose solution ofoncentration 5% (w/v). Fiber: PCP.

ater or the most dilute solution in a given family of sampless reference, measured using PCP fibers, are given in Figs. 3–6.hese four plots differ from each other because the fiber prepa-

ations are different. The degree of cladding removal (a pointe elaborate on later) and the surface smoothness vary becausef mechanical scrapping used for cladding removal. However,n each of these plots the slope ∂R/∂n is positive (henceforthalled positive slope) and the experimental data points are fittedery well (with χ2 very close to unity) by an exponential growthunction, R = A exp(n/B) + C, where A, B, C are constants. Thealues of these constants for each figure are given in Table 1.hese numbers demonstrate that the sensitivity of the measured

ntensity ratio to RI change increases with increasing RI of test

ig. 4. Plot of change in intensity ratio �R as a function of change in refractivendex �n, fitted with an exponential growth function. χ2 of fit are indicated.he origin with respect to which the �’s are calculated is sucrose solution ofoncentration 3% (w/v). Fiber: PCP.

A. Banerjee et al. / Sensors and Actuators B 123 (2007) 594–605 597

Fig. 5. Plot of change in intensity ratio �R as a function of change in refractiveindex �n, fitted with an exponential growth function. χ2 of fit are indicated.The origin with respect to which the �’s are calculated is sucrose solution ofconcentration 2% (w/v). Fiber: PCP.

Fig. 6. Plot of change in intensity ratio �R as a function of change in refractiveiTc

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Fig. 7. (a) Plot of change in intensity ratio �R as a function of change in refrac-tive index �n, fitted with an exponential decay function. χ2 of fit are indicated.The origin with respect to which the �’s are calculated is water. A straight linefit with the specified χ2 is obtained. Fiber: PCS. (b) Plot of intensity ratio at dif-ferent values of liquid refractive index (nl) to show the rise in intensity ratio athigh refractive index that follows an initial decrease with a minimum at n = ntd

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ndex �n, fitted with an exponential growth function. χ2 of fit are indicated.he origin with respect to which the �’s are calculated is glycerol solution ofoncentration 44% (w/v). Fiber: PCP.

lope). In this fiber preparation, the cladding is fully removed,he remaining fiber thickness being 120 �m. The plots in Fig. 7(and b) are obtained from two different experiments done with

wo different preparations of the PCS fiber under conditionsf etching time that give negative slope in both. The magni-udes of the intensity ratio and the slopes differ. Even though theCS fiber shows a negative slope when it is fully uncladded

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ig. 3 0.01518 (±0.00672)ig. 4 0.01077 (±0.00162)ig. 5 0.10544 (±0.05287)ig. 6 0.19912 (±0.02500)

he equation fitted is: y = A exp(x/t) + y0.

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he monotonic decrease at nl < nfiber is shown in (a), which is performed with aifferent fiber preparation of the same fiber. Fiber: PCS.

y sulphuric acid etching for 16 h (Fig. 7), a positive slopes found when etching time is shorter and the cladding thick-ess is larger (Fig. 11). These statements about positive andegative slopes refer to experiments with liquids whose refrac-

ive indices are below that of the fiber. According to Fresnelquation only a negative slope should be observed in theseiquids.

t y0

0.02729 (±0.00552) −0.01432 (±0.00874)0.03449 (±0.00240) −0.00969 (±0.00203)0.05574 (±0.02079) −0.10759 (±0.05432)0.03456 (±0.00160) −0.18521 (±0.04075)

598 A. Banerjee et al. / Sensors and Actuators B 123 (2007) 594–605

Fig. 8. Plot of intensity ratio as a function of refractive index of fructose andsdP

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ucrose solution pairs with very close refractive index. The continuous lineisplays the polynomial (degree 5) fit obtained with sucrose data points. Fiber:CP.

In order to extend the range of liquid RI above that of theber, we have to use non-aqueous solvents. Whereas, eitherCP or PCS fiber can be used with aqueous solutions, the lat-

er alone can withstand non-aqueous solvents, e.g., dimethylulfoxide (DMSO) and nitrobenzene. Fig. 7(a and b) displayata taken on aqueous solutions below the fiber RI and non-queous solvents above the fiber RI using a PCS fiber sensorhose cladding is completely removed by sulphuric acid etch-

ng for 16 h. We observe a monotonic decrease followed by anncrease in Fig. 7(a and b), the change in the sign of slope occur-ing (Fig. 7(b)) at a RI of the liquid that equals the RI of the fiber

n = 1.48).

To summarize, we find that ∂R/∂n, i.e., the slope of R versusplots, is a strong function of (a) the type of fiber used, PCP

r PCS, (b) the extent of cladding removal and (c) the range

ig. 9. Plot of intensity ratio as a function of refractive index of fructose andagnesium sulphate solution pairs with very close refractive index. The contin-

ous line displays the polynomial fit (degree 5) of fructose data points. Fiber:CP.

Fig. 10. (a) Plot of intensity ratio as a function of refractive index of aq. solu-tions of copper sulphate and fructose, to evaluate effect of optical absorption onrefractive index measurements. The continuous line displays the polynomial fit(degree 3) of the fructose data points. Fiber: PCP. (b) Plot of intensity ratio asa function of refractive index of aq. solutions of copper sulphate and fructose,to evaluate effect of optical absorption on refractive index measurements. Thecdt

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ontinuous line displays the polynomial fit (degree 3) of the copper sulphateata points. These data are obtained at significantly higher solute concentrationhan those of (a). Fiber: PCP.

f RI being measured. It can be either positive or negative andwitches sign at around fiber RI.

.1.2. Relation to literature reportsIn this subsection, we relate the observations summarized

bove to existing literature reports. Wong et al. [32] observensignificant response of a tapered fully cladded multimode plas-ic optical fiber to a RI change in the range 1.33–1.47 followedy a significant increase of normalized intensity from 1 to 14s RI changes from ∼1.47 to ∼1.50. Kumar et al. [33] observe

xperimentally that the output light intensity of a multimodeptical fiber whose sensing region is stripped of cladding and isapered, when plotted as a function of n2

l (nl is liquid RI beingensed) is a straight line with a negative slope. The measure-

A. Banerjee et al. / Sensors and Act

Fig. 11. Plot of sensitivity to refractive index change as a function of the thick-ness of the fiber (PCS). The thickness of the fiber (and of cladding) decreasesalong the positive X direction. For each thickness the ratio of the intensity ratiosin air to that in water is measured. This ratio is taken as a measure of sensitivitytfif

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o refractive index change and is plotted as relative sensitivity on the Y-axis. Theber with smallest thickness (16 h etching) registers a decrease in intensity ratiorom air to water. All other fibers register an increase.

ents cover a range of RI values from that of liquid water (1.33)o that of the fiber core (∼1.45). Villiatoro et al. [34] use a fiberith cladding intact and a tapered region that senses RI. The

ransmission shows a relatively steady light output upto a RIetermined by the waist diameter followed by a rapid fall at aigh RI (∼1.4). Afromowitz and Lam [35,36] use a fiber withtripped cladding to measure RI of a thin layer of epoxy resinhich is overlaid on it and find that the output light intensityf the sensor shows a negative slope. Spenner et al. [37] mea-ure RI of thin films deposited on a fully cladded fiber using theependence of the phase velocity of the guided modes on the RIf the film.

Cusano et al. [38] use a fully cladded single mode fiber toonitor RI of polymer-based composites deposited on the fiber.hey make measurements with liquid samples for purposes ofalibration. The quantity that monitors the RI of the liquid ishe intensity of reflected light at the liquid–fiber interface. Theybserve, just as we do (Fig. 7) a negative slope in the RI range1.37–1.48, the fiber RI and a positive slope when RI of test

iquid exceeds that of fiber (1.48). Their results, as well as oursre consistent with the prediction of Fresnel equation. In theirase the incident light falls on the fiber surface from outside,hereas in our measurement light is incident from within theber. Reflected light intensity as calculated by Fresnel equation

s insensitive to this difference in direction of incidence.Several experimental reports on fiber optic chemical sensors

se adsorbent layers deposited on the fiber core as sensors ofdsorbed chemicals which modify the RI of the adsorbent layers39–45]. Cherif et al. [39] report a rapid decrease of opticalower at a higher RI (∼1.4) of the adsorbent layer preceded

y a comparatively steady response at lower values of RI. Thewitchover from a steady response to a rapid decrease occurst a RI that depends on the angle of incidence. Abdelghani etl. [40] studied a PCS fiber coated with porous silica deposited

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uators B 123 (2007) 594–605 599

y sol–gel method as a sensor of chemicals that are adsorbed onhe porous silica layer. Chemicals with higher index of refractionhan porous silica, when adsorbed, lower its RI and an increase inutput light intensity is observed. Chemicals with lower valuesf RI show opposite behaviour. This report like those of Cherift al. [39] and Kumar et al. [33] show a negative slope of theensor sensitivity.

Durana et al. [48] in a simulation study on a fiber with varyingladding thickness find that the output optical power decreasest each cladding thickness when the medium changes from airo a high RI oil. The magnitude of decrease is cladding thicknessependent. The simulation is entirely based on Ray theory whichncludes tunneling rays. Kumar [55] in our laboratory simulatedight propagation in a flat fiber using wave theory and observed aharp increase in light output above a RI of ∼1.40, below whichhe sensor shows comparatively poor response.

Amongst the reports cited above, that of Wong et al. [32] ishe only one to report a positive slope in the range of liquid RIelow that of the fiber, in agreement with our results. They usefully cladded but tapered fiber. Their results are relevant to

ur observations because tapering and stripping of the claddingre qualitatively similar modifications. In an experiment [46]n which the fiber is not used directly as a sensor, two fibers,he same fibers cut into two parts, are aligned such that lightmerging from the first passes though the liquid under test andhen enters the second. Not all of the light intensity that leaveshe first enters the second. As the beam emerges out of the firstber, the light beam refracts, the angle of refraction depends on

he liquid RI. The light intensity output of the second fiber is thenmeasure of the RI of the intervening liquid. The experimentallyerified [46] prediction of the theory of this experiment is thathe output power, at the outlet of the second fiber, increases withncreasing RI of the liquid. The experiments with cladding fullytripped in the sensing region are very close to this experiment.e observe (Figs. 3–6, 8–10) in agreement with the theory and

xperiment of Brown et al. [46] an increase of light output withncreasing RI of liquid, a positive slope. Our observation of a noninear dependence of output light intensity on RI of test liquid isn agreement with those of Wong et al. [32], Kumar et al. [33],herif et al. [39] and Kumar [55].

.1.3. RangeThe measurements discussed above (Figs. 3–7) show that the

bers used in this report are sensitive RI sensors in the wholeange 1.33–1.56, below and above the fiber RI. This observedensitivity to a broad RI range is in agreement with the findingsf Kumar et al. [33] and Cusano et al. [38]. In contrast, Wongt al. [32] found no significant response to change of RI in theange 1.33–1.47 and a significant sensitivity only in the range= 1.47–1.50. The results of a recent theoretical study [55] inur laboratory are similar to those of Wong et al. [32].

.1.4. Precision

The relative standard error ((σ of �R)/�R) of a typical mea-

urement of intensity ratio is not significantly dependent on thealue of the ratio. It is typically 10−3 to 10−4 for the PCP fiberith a positive slope and is much larger, typically 10−1 for the

600 A. Banerjee et al. / Sensors and Actuators B 123 (2007) 594–605

Table 2Data on relative standard error of measurement of change in intensity ratio (�R) in different fibers identified by figure numbers in which data obtained with aparticular fiber is displayed

Figure Value of standard error (σ) Change in intensity ratio (�R) Range of refractive index �n ineach of the following is 0.0106

Value of (σ/�R)

Fig. 3 1.994E−4 0.00928 1.3403–1.3509 2.15E−20.03157 1.3796–1.3902 6.30E−3

Fig. 4 9.912E−5 0.00458 1.3403–1.3509 2.16E−20.01759 1.3893–1.3999 5.63E−3

Fig. 5 4.244E−5 0.02429 1.3403–1.3509 1.74E−30.03093 1.3600–1.3706 1.37E−3

Fig. 6 5.850E−4 0.59330 1.4829–1.4735 9.86E−4

Fig. 7(a) 2.230E−2 0.06812 1.3417–1.3523 3.27E−10.09076 1.3594–1.3700 2.45E−1

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CS fiber with a negative slope. Precision of �n measurementepends on this noise figure alone and is found to be about 10−5

o 10−6 (for �n = 1) for a fiber with a positive slope and 10−3

or a fiber with a negative slope. The precision is larger at large, because as pointed out earlier, �R/�n becomes larger as nncreases. The data on precision is given in Table 2. We con-lude that RI measurement to a precision in the fifth place ofecimal is possible with this sensor. The evaluation of precisions based on standard error of mean and not standard deviation.his is justified because the mean value measures the bulk RInd the calibration curve is based on bulk RI values of standardiquids. The reproducibility of the mean as given by standardrror of mean is therefore a measure of experimental precision.

large part of standard deviation arises from fluctuation of RIround the mean value (bulk RI) in the thin liquid layer sur-ounding the sensor region of the fiber. Literature reports onber optic sensors with overlaid thin layers [35–45] show that

hese sensors sense RI change very close to the fiber surface.umar [55] in our laboratory has also observed in his simula-

ion studies that the output power is significantly modified bythin layer of a different RI overlaid on the fiber surface. This

s understandable in view of the short range of evanescent fiberensing. We note that our experiments are as precise as the oneshat determine the standard values used to draw a calibrationurve. Any higher precision in our experiments would have beenseless.

.1.5. AccuracyThe technique described in this paper is not a direct mea-

urement of RI as is, for example, the methods that use criticalngle or Brewster angle. The accuracy of this method is entirelyependent on that of the reported RI values used in calibration.

.2. Sensor response to chemical nature of solute

It is necessary that we confirm an anticipated result that thentensity ratio (R) senses RI alone and is entirely independentf the chemical nature of the solute. This anticipation may notold in the presence of significant adsorption of solute onto the

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ensor surface. Fig. 8 shows the intensity ratio data of a solutionf fructose and that of sucrose of nearly equal RI [54]. In theolution pairs chosen for this comparative study, the refractivendices are very close, but not always identical. We have takenare of these non-identical values by fitting the values of R asfunction of RI for sucrose solution by a polynomial and plot-

ing the corresponding values of fructose solution on the sameraph. We find that the intensity ratio (R) as a function of RIf both the sucrose and fructose solutions are distributed withery small deviations around the plot of the polynomial that fitshe data of sucrose solutions best. The deviations of both theseamilies from the polynomial plot are small and comparablend do not increase with increasing solute concentration. Anyifference between fructose and sucrose solutions arising fromheir chemical differences would show dependence on soluteoncentration. We note that the concentrations of fructose solu-ions studied range from 0.112 to 2.613 M and those of sucroseolutions range from 0.059 to 1.375 M. The ratio of the highestnd the lowest concentration is 25 in both cases and the high-st concentrations are fairly large in magnitude. Any differencerising from the chemical nature of solutes would show up, if itndeed exists, at large concentration values used in these exper-ments. The deviations of the experimental intensity ratio (R)alues of sucrose solutions around the plot of the best-fit poly-omial obtained with data of sucrose solution are, in increasingrder of RI, within 3σ, 2σ, σ, σ, 3σ, σ and those of fructoseolutions are within σ, 3σ, σ, 2σ, 3σ, σ. The fact that the devia-ions around the plot of the best-fit polynomial are (i) small, (ii)andom, (iii) present for both fructose and sucrose solutions andiv) solute concentration independent at even high salt concen-rations, support the contention that the sensor senses RI alonend is not sensitive to the chemical nature of the solute. Identi-al conclusion is reached from measurements on matched pairf magnesium sulphate solution and fructose solution. The con-entrations of fructose solutions studied ranges from 0.340 to

.878 M and those of magnesium sulphate solutions range from.201 to 2.027 M. The ratio of the highest and the lowest con-entration is 6 and 14, respectively. The highest concentrationsre fairly large for both solutes. The maximum concentration

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A. Banerjee et al. / Sensors an

f magnesium sulphate studied is so large that ion adsorptions a possibility, but even if it is there, no significant effect isoticed in the sensing of RI. The experimental intensity ratioR) of Fructose solutions are fitted with a polynomial which isisplayed in Fig. 9 and the experimental values of magnesiumulphate solutions are plotted on the same graph. The devia-ions of the experimental intensity ratio values of magnesiumulphate solutions around the plot of the best-fit polynomial are,n increasing order of RI, within 2σ, σ, 2σ, σ, σ, σ and those ofructose solutions are within σ, σ, 2σ, 2σ, 3σ, σ.

.3. Effect of absorbance(n′′) on RI(n′) sensing

Meeten and co-workers [16,17,19–21] report errors in deter-ination of RI of optically absorbing and turbid fluids by critical

ngle and Brewster angle measurement, the classical methodsor determination of RI. The error in transmission mode is showno be much larger than that in the corresponding reflection mode.

method is described for analyzing the data of specular opticaleflectance at the interface between the fluid and the glass prisms a function of the angle of incidence (from the glass prismide) by detailed fitting with the theoretical equations to obtainhe real and imaginary refractive indices of the fluid. The methoducceeds for absorbing and some, not all, turbid fluids. In viewf this difficulty with the critical angle method, it is importanthat other sensors be investigated to find out about their useful-ess for absorbing and turbid fluids. We report our investigationn the effect of optical absorbance on RI measurement with theber sensor.

In order to investigate the effect of absorbance of a liquidn the measurement of its RI at a wavelength where absorptions significant, one would have to make a comparative measure-

ent of the intensity ratio in two liquids with identical values ofI but one of them would have to be absorbing while the otherould not absorb, at the wavelength of interest. It is not possi-le to choose this liquid pair for the comparative measurementeferred to above, if we insist that they must have identical RI atwavelength where one of them absorbs. The values of RI of anbsorbing liquid within the absorption band are not reported [54]erhaps because of errors of measurements in standard methodshat use critical and Brewster angle data [16,17,19–21]. We havehosen a solution pair of copper sulphate and fructose in water,hose refractive indices at 589 nm are identical. This would

mply that at 633 nm the refractive indices would be nearly iden-ical, if not identical within errors of measurements. At 589 nm,opper sulphate solution does not absorb. The literature value ofI used for comparison with that of fructose solution at the sameavelength (589 nm) where neither of them absorb is there-

ore not complicated by the problem of having simultaneousbsorbance. We now describe experimental results of this com-arative study. The graphical display of the results are given inig. 10(a and b). As has been done in the comparison betweenructose and sucrose solutions, we fit the experimental values of

ntensity ratio (R) as a function of RI of one of the two liquidsfructose solutions in Fig. 10(a) and copper sulphate solutionsn Fig. 10(b)) and plot the experimental values of the other liq-id on the same graph. In Fig. 10(a) the deviations around the

uators B 123 (2007) 594–605 601

raph fitted with a polynomial are within, in increasing order ofI, σ, 2σ, 4σ for copper sulphate solutions and within 2σ, 3σ,for fructose solutions. In Fig. 10(b) the corresponding figures

re 2σ, σ, σ, σ for copper sulphate solutions and 2σ, σ, σ forructose solutions. We conclude that the measurement of RI byhe technique we describe is not significantly influenced by theresence of absorption.

Several reports have appeared in the literature on fiber opticvanescent field absorption sensor [24–27]. These papers, how-ver, do not refer to the influence of absorption on RI sensing.upta et al. [27] show the influence of launching condition andeometry of the sensing region on the sensitivity of the absorp-ion sensor. High sensitivity is achieved by launching power intofiber having tapered sensing region. Absorption sensitivity is

mall, ideally zero if light is launched horizontally into the fibers we do in our experiments. This is predicted theoretically byuddy [25] and is experimentally verified by Gupta et al. [27].

t is thus not surprising that we do not observe any differencen intensity ratio of an absorbing solution and its non-absorbingounterpart (measured at a wavelength within the absorptionand), which share the same RI value at a wavelength slightlyutside the absorption band. Any optical effect because of theedium is entirely due to refraction. It would be interesting to

o these experiments under different launching conditions, forhich absorption is sensed.

.4. Effect of cladding thickness on RI sensing

The experiments described below demonstrate the importantole played by the cladding of an optical fiber in sensing RI.

(a) The PCS fiber is etched with sulphuric acid for differentlengths of time in order to remove its cladding to differentextents. When the PCS fibers so prepared are used to senseRI, we find that their RI sensitivities depend significantly oncladding thickness. Fiber etched for 16 h shows a negativeslope in the plot of intensity ratio (R) as a function of RI.This fiber shows a steady response after being dipped inwater for a few days. The slope is found to be negativeright after etching and continues to be so, with change inmagnitude until a steady state is attained. Fiber etched for8 h shows an initial negative slope, which turns positive withtime and reaches a steady state with a positive slope. Fibersetched for 2, 4 and 6 h show positive slopes very early. Theslope remains positive in steady state. In all five cases, thetime taken to achieve steady state is around 72 h, beyondwhich the response changes very slowly. In the PCP fiber insharp contrast, a steady response is observed right after thecladding is removed by a razor. The fully cladded originalfiber is found not to sense RI.

b) The intensity ratios for five PCS fibers with differentcladding thickness (etched for different lengths of time 2,4, 6, 8 and 16 h) were compared in order to investigate the

effect of cladding thickness on sensitivity to RI sensing. Theratio of the intensity ratio in water to that in air is taken as ameasure of sensitivity. The plot (Fig. 11) of this measure ofsensitivity as a function of cladding thickness shows a max-

602 A. Banerjee et al. / Sensors and Act

Fig. 12. Plot of sensitivity to refractive index change as a function of claddingthickness for PCP fiber. The first point is for fully cladded fiber. Cladding thick-ness decreases along the positive X direction. The numbers on the X-axis indicatethe serial number of experimental points. For each thickness the ratio of the inten-sio

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ity ratio of 24% (w/v) to 20% (w/v) fructose solution is measured. This ratio isndicated as relative sensitivity on the Y-axis. It is a measure of the sensitivityf the fiber sensor to change in refractive index.

imum of positive sensitivity at an intermediate value. Wenote that the first four fibers show a positive slope whereasthe fifth fiber (etched for 16 h) shows a negative slope.

The same effect has been seen in a parallel study with the PCPber. A rise in RI sensitivity followed by a fall is observed as theber is subjected to successive removal of cladding and perhapslso the core by application of a razor. Here the ratio of thentensity ratio (R) of 24% aq. fructose solution to that of 20% aq.ructose solution is taken as a measure of sensitivity. In this study,owever, the diameter of fiber at different stages of fiber slicingas not monitored. Since the cladding thickness is only 2% of

he fiber diameter, we believe that part of the core was also slicedut in later slicing stages. Fig. 12 shows the results. The initialise in sensitivity, we believe in view of our experience with theCS fiber, owes its origin to an optimal removal of claddingaterial. We now refer to a literature report, a simulation study,hich when interpreted in terms of dependence of RI sensitivityn cladding thickness, gives results that are in agreement withhe experiments described above.

Durana et al. [48] in a paper on bending losses in fibers, reportimulation studies in a fiber of a given extent of bending, as aunction of cladding thickness in both air and oil. RI of oil equalshat of the fiber cladding. The output light intensity is small whenhe fiber is immersed in oil and is independent of cladding thick-ess. This is so because the equality of the refractive indices ofil and fiber cladding makes the cladding thickness virtually infi-ite. Since no optical inhomogeneity is encountered, light raysefracted out of the core propagate into the oil without under-

oing reflection. An output intensity whose magnitude is smallnd is independent of cladding thickness, is therefore expectednd is observed. In air, as the cladding thickness is increasedhe output light intensity initially increases and then decreases

uators B 123 (2007) 594–605

symptotically to the constant output intensity found in oil. Ashe cladding thickness is increased for a certain fiber length, theight rays undergo fewer reflections and thus fewer transmis-ion coefficients have to be considered. In addition, all of theseays are tunneling at the cladding-air interface and almost allf this power is reflected back to the cladding. After reachingmaximum, output power starts decreasing. Rays start being

efractive at the cladding-air interface, that is they are no longerunneling and a lot of power is transmitted to the air. This trans-ission of power to the air is stronger than the power gained by

he fiber as a consequence of reducing the number of transmis-ions and the overall energy balance is negative. These results,hen interpreted in terms of dependence of RI sensitivity (oilersus air) on cladding thickness, suggest that as cladding thick-ess increases, one will observe an increase in sensitivity whicheaches a maximum and then decreases.

Importance of cladding in determining the sensitivity of aber sensor has been referred to, in a few other publications.n a humidity sensor described by Khijwania et al. [49] orig-nal fiber cladding was replaced by a chemically synthesizedladding, The thickness of the latter was found to determine theensor sensitivity. An optimal thickness maximizes sensitivity.ower core diameter also enhances sensitivity. Deparis et al.

50] showed the dependence of the �-ray induced radiation lossf a fiber on cladding to core thickness ratio. Turan and Petrick51] studied the effect of fiber optic cladding on the sensitivityf phase modulated fiber optic sensor. Okamoto and Yamaguchi52] in a study of an optical wave guide sensor that uses a singleoupling prism to study optical absorption, demonstrated thathe sensitivity of the sensor depends on the thickness of bothladding and waveguide layer. Ansari et al. [53] studied sin-le mode optical waveguide with PbCl2 as a sensitive claddingayer for Chlorine sensing. They observed only slight changesn sensitivity with variation of cladding thickness.

.5. Cladding modes and RI sensing

(a) In order to investigate the role of cladding modes in RI sens-ing, we have studied the effect of mode scrambler on thissensitivity. This study has been done with both the PCS andthe PCP fibers. The mode scrambler was placed ∼10 cmbefore the sensing region. The mode scrambler increasescore-cladding coupling and radiation of cladding modes.The first effect will increase RI sensitivity, the second effectwill decrease it, if cladding modes are the dominant sensorsof RI. In Fig. 13, we compare the effect of mode scramblingon a PCS fiber etched for 4 h with that on a PCS fiber etchedfor 8 h. The ratio of the two experimental intensity ratios,that in water to that in air, is taken as a measure of sensi-tivity of the sensor to RI change. The first of this pair has alarger cladding thickness and a larger RI sensitivity as seenin Fig. 11. We therefore anticipate and also experimentallyobserve a larger effect of mode scrambling on RI sensitivity

of the first fiber (etched for 4 h). Also in both fibers we seean initial increase followed by a decrease in sensitivity asmode scrambler is progressively tightened. Fig. 14 showsthe result of change in sensitivity of the PCP fiber with a

A. Banerjee et al. / Sensors and Act

Fig. 13. Plot of sensitivity to refractive index change as a function of number ofturns on mode scrambler for PCS fibers, etched for 4 and 8 h. For a given numberoom

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f turns on mode scrambler, the ratio between the intensity ratio of water to thatf air is designated as relative sensitivity on the Y-axis. This ratio is taken as aeasure of the sensitivity of the sensor to refractive index change.

fixed cladding thickness as a function of the degree of modescrambling. In this experiment, the ratio of two experimen-tal intensity ratios, that in 20% aq. sucrose solution to that in12% aq. sucrose solution, is taken as a measure of sensitiv-ity. The results are qualitatively the same as those shown inFig. 13. In this experiment, the mode-scrambler was fabri-cated in-house in order to suit the fiber thickness of the PCPfiber, which was too thick for the Newport mode-scrambler.

In these experiments, the slope of the sensor response toincrease in RI is always positive, with and without the mode-scrambler.

ig. 14. Plot of sensitivity to refractive index change as a function of numberf turns on mode scrambler for PCP fiber for a given cladding thickness. Forgiven number of turns on mode scrambler, the ratio between the intensity

atio of 20% (w/v) sucrose solution to that of 12% (w/v) sucrose solution isesignated as relative sensitivity on the Y-axis. This ratio is taken as a measuref the sensitivity of the sensor to refractive index change.

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uators B 123 (2007) 594–605 603

b) The role of the cladding modes in determining the signof the slope of sensor signal to increase in RI, is high-lighted in the following experiment. As already noted inSection 3.4(a), the PCS fiber when etched in sulphuric acidfor 8 h, shows a negative slope initially but after remain-ing dipped in water, the slope eventually becomes posi-tive after passing through zero slope at some stage. Whenthe same experiment is repeated with a long fiber length(300 cm) preceding the etched sensing region, we find thatthe slope is not only negative immediately after etching,but changes very little and remains negative for a verylong time (∼100 h). A shorter fiber (30 cm) in the samelength of time reaches a steady state positive slope, startingfrom an initial negative slope. Since cladding modes dis-sipate during propagation through a long fiber length, thisexperiment relates the presence of cladding modes to theobservation of a positive slope of sensor response to increasein RI.

. Conclusion

The light output of an optical fiber stripped of its cladding ishown to be a sensitive indicator of the RI of a liquid in whichhe uncladded sensor region is immersed. The characteristics ofhe fiber sensor, in particular the slope of the sensor output signalo RI change have been investigated. As a result of these inves-igations, we conclude that these sensors are easy to fabricatend handle, are highly sensitive to RI change (precision to fifthlace of decimal), can be used with a small liquid volume, areensitive over a wide range of RI (1.33–1.56) and are insensitiveo light absorption at the frequency at which RI is being mea-ured. These features make them useful in some applicationshere bulk refractometers are inconvenient. Cladding modes

re closely related to the sensing of RI. Their manipulation bydjustment of cladding thickness and fiber length preceding theensor region modifies sensor response.

cknowledgements

We are indebted to Drs. O.P.Katyal, S.C. Agarwal, Ban-ilal, A. Pradhan, David, Krishna Kumar, K.K. Sharma, P.upta, P.K. Chatterji, R. Sharan, V.K. Singh, Ms. Nina Joseph,essers Maharaj Singh, Sudhansu Kumar, A. Sivabalan, Bablooumar, Peeyush Sahay and A.K. Verma of Indian Institute ofechnology Kanpur, Kanpur, India and Drs. T.K. Alex and K.anakaraju of the Laboratory for Electronic and Optical Sys-

ems (LEOS), Indian Space Research Organisation, Bangalore,ndia. This work is based on a part of the Ph.D. thesis of T.K.K,he M.Tech thesis of R.K.V. and M.Sc. theses of AB, SM, BJ,KD, MC and SB, submitted to Indian Institute of Technologyanpur.

Financial support from Indian Space Research Organisation

ISRO-IITK programme). Defence Research and Developmentrganisation, New Delhi, Ministry of Human Resource Devel-pment, New Delhi and Indian Institute of Technology Kanpurre gratefully acknowledged.

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iographies

rgha Banerjee has obtained MSc degree in Physics from Indian Institute ofechnology (IIT), Kanpur. He is Graduate student in Theoretical Physics fromata Institute of Fundamental Research, Mumbai, India.

ayak Mukherjee has obtained MSc degree in Physics from IIT, Kanpur. He israduate student in Theoretical Physics from Virginia Tech, Blacksburg, USA.

ishi Kumar Verma has obtained BE degree in Electronics and Communica-ion from MMMEC Gorakhpur, India. He is MTech in Laser Technology fromIT, Kanpur. He is Scientist at Defence Bio-engineering and Electromedicalaboratory (DEBEL), Bangalore, India; area: fiber optic sensors.

iman Jana has obtained MSc degree in Chemistry from IIT, Kanpur. He israduate student in Theoretical Physical Chemistry, Solid State and Structuralhemistry Unit, Indian Institute of Science, Bangalore.

apan Kumar Khan has obtained PhD degree in Chemistry from IIT, Kanpur.e is Assistant Professor in Blanchette Rockefeller Neurosciences Institute,ockville, MD, USA.

rinmoy Chakroborty has obtained MSc degree in Chemistry from IIT, Kan-ur. He is Graduate student in Chemistry from Carnegie Mellon University, USA;rea: electronic structure of high valent complexes of biological relevance.

ahul Das has obtained MSc degree in Chemistry from IIT, Kanpur. He israduate student in Chemistry from Rice University, USA; area: stochastic

rocesses in complex systems.

andip Biswas has obtained MSc degree in Physics from IIT, Kanpur. He israduate student in Theoretical Particle Physics from University of Hawaii,SA.

PbUBD

uators B 123 (2007) 594–605 605

shutosh Saxena has obtained BTech degree in Electrical Engineering (EE)rom IIT, Kanpur. He is Graduate student in EE from Stanford University,SA.

andana Singh has obtained PhD degree in Physics from G.B.P. University ofgriculture and Technology, Pantnagar, India. She is Senior Project Scientist

rom Samtel Center for Display Technology, IIT Kanpur; area: organic flexiblerintable electronics.

akesh Mohan Hallen has obtained PhD degree in Chemistry from IIT, Kanpur.

am Swarup Rajput is a member in Technical Staff IIT, Kanpur.

aramhans Tewari is a member in Technical Staff IIT, Kanpur.

atyendra Kumar has obtained MSc degree in Physics from University ofoorkee. He has obtained PhD degree in Physics from IIT, Delhi. He has gainedesearch experience at Ecole Polytechnic, Paris and Pennsylvania State Univer-

ity, USA. He is Professor of Physics in IIT, Kanpur; area: thin films.

ishal Saxena has obtained BTech and MTech degrees in EE from IIT, Kanpur.e is Principal Research Engineer in Department of EE, IIT, Kanpur; area:

nstrumentation.

njan Kumar Ghosh has obtained BTech degree in EE from IIT, Khargpurnd MS in EE from SUNY, StonyBrook, USA. He has obtained PhD degreen EE from Carnegie Mellon University, USA. He is a member in Technicaltaff, Bell Labs, Taught at Iowa State University, USA; Nanyang Technologi-al University, Singapore. He is a Professor in EE Department in IIT, Kanpur.urrently he is Visiting Professor in Department of ECE, Oklahama Univer-

ity. Tulsa, USA; senior member of IEEE, area: optical information processing,ptical communications, photonic sensors, instrumentation.

oseph John has obtained BSc in Engineering from College of Engineering,hiruvananthapuram, University of Kerala, India. He has obtained MTech inE, IIT, Kanpur; PhD degree in EE, Birmingham University, UK. He is Scholarf the Commonwealth Fund; Professor in EE Department, IIT, Kanpur; area:hotonics, instrumentation.

inaki Gupta-Bhaya is Natural Sciences Tripos, BA (Hons), MA from Cam-ridge University, England. He has obtained PhD in Chemistry from Columbianiversity, New York. He is Post Doctoral Fellow from Max Planck Institute furiophysikalische Chemie, Gottingen, Germany. He is Professor in Chemistryepartment, IIT, Kanpur; area: physical chemistry.