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Sensors and Actuators B 122 (2007) 442–449

Sensitivity optimization of tapered optical fiber humidity sensors bymeans of tuning the thickness of nanostructured sensitive coatings

Jesus M. Corres ∗, Francisco J. Arregui, Ignacio R. MatıasDepartamento de Ingenierıa Electrica y Electronica, Universidad Publica de Navarra, Campus Arrosadıa s/n, 31006 Pamplona, Navarra, Spain

Received 7 February 2006; received in revised form 15 May 2006; accepted 2 June 2006Available online 24 July 2006

bstract

Electrostatic self-assembly has been used in this work to fabricate a new optical fiber humidity sensor. Said sensor consists of a single-modeapered fiber coated with a [PDDA/Poly R-478] nanostructured overlay, in such a way that the thickness can be controlled in order to optimize theensor sensitivity, by stopping the deposition process at the maximum slope of the transmitted optical power. The same tapered optical fiber tested

n optical power is achieved with responses time of 300 ms for changes in relative humidity from 75% to 100%. The high dynamic performancend low temperature cross-sensitivity allows this sensor to be used for human breathing monitoring.

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eywords: Tapered optical fibers; Humidity sensors; Fiber optic sensors; Ionic

. Introduction

Recently, much effort has been devoted to developing nanos-ructured sensitive materials to construct fiber optic evanescenteld-based sensors. Long period gratings, holey fibers, omnigu-

de fibers, hollow core fibers, tapered optical fibers (TOF) orlastic fibers have been used as transducers in order to getoated by these sensitive nanofilms for their use in sensingpplications [1–8]. Among these structures, biconically taperedections of monomode fibers are simple devices and very sen-itive to changes of the surrounding refractive index and theavelength of the input light [9]. When a fiber is tapered, the

ore/cladding interface is redefined in such a way that the singleode fiber in the central region of the taper acts as a multimodeber and the light becomes guided through the cladding of theber, which plays the role of the new core, and the new cladding

s the surrounding external medium. Theoretical aspects of this

∗ Corresponding author. Tel.: +34 948 169725; fax: +34 948 169720.E-mail address: jmcorres@unavarra.es (J.M. Corres).URL: http://www.unavarra.es.

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The shape that the fiber acquires after the tapering process,hich depends on the method employed in its fabrication, hashigh impact on the light transmission properties. The taperingf the fibers can be achieved by heat pulling or chemical etch-ng [15]. Usually chemically etched tapers are characterized byhe removal of part of the cladding, while heat pulling tapers

aintain the geometrical ratio between cladding and core. Heatulling tapers can be fabricated using a flame, a laser or an elec-rical arc. In this work an Ericsson FSU-905 splicing machineas been used for the tapering process. This method is differ-nt to those of other authors who either use a traveling flamer heating a 1-cm-wide platinum furnace to taper the fiber, get-ing waist lengths of 50 and 10 mm, respectively [10,11]; in theroposed method it is possible to get a waist length of onlymm yielding to a smaller region susceptible to be depositednd obtaining a smaller sensor easier to handle and less proneo mechanical perturbations. Using this technique on a standardommunications single-mode optical fiber (core and claddingiameters of 8.3 and 125 �m, respectively) it is possible to obtainaist diameters as thin as 10 �m; smaller diameters give devices

oo brittle. In order to fabricate the tapers a 2-m sample of this

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swit[cdtirhumidity sensing surface and for adjusting the working point ofthe sensor at the point of optimal sensitivity. If the transmittedoutput optical power is considered as a function of the overlaythickness, the slope of the transmission characteristic (TC) can

J.M. Corres et al. / Sensors an

eable but not degraded by heat, to give enough time to pullhe fiber.

With respect to the fabrication of sensitive films, there existeveral suitable coating techniques, such as spin-coating, dip-oating, physical and thermal evaporation, electrostatic self-ssembly, etc. The polymer spin coating deposition techniqueas been used to fabricate gas sensors [2]; it is based on the coat-ng of the fiber with a thin layer of polymer by placing a fiberolder on a photo-resist spinner set at a constant speed for a cer-ain period of time. This procedure produces a relatively smoothnd uniform coating whose thickness can be adjusted by regulat-ng the spinning period. However the rotation process does notermit to monitor the taper optical output power transmittivityuring the fabrication in order to adjust the sensor at an opti-al working point. Also, the use of sol–gel techniques to pro-

uce pH sensors, chemical sensors and biosensors is attractingonsiderable research interest [16,17]. Using both dip-coatingnd spin-coating, sol–gel layers are prepared by hydrolysis andondensation polymerization of the appropriate metal alkoxideolution, followed by a temperature program which controls theensification process. Although with dip coating the thicknessay be controlled via the withdrawal speed, a fine control is

ot possible. On the other hand, physical and thermal evapora-ion have been satisfactorily used for hydrogen sensing [1,7].he method consists in evaporating different “faces” of the fiber

n several steps to avoid shading zones, producing azimuthallysymmetrical devices [1]. The need for a multiple step methodomplicates the monitoring of the output optical power as aunction of the overlay thickness.

Electrostatic self-assembly (ESA) using polymeric elec-rolytes has the advantages of both the self-assembly and the

olecular control of the structure of the matter in three dimen-ions, molecular layer to molecular layer. The substrate is coatedy means of a layer-by-layer alternate adsorption on anionicnd cationic species from aqueous solutions. In contrast to otherhin film deposition techniques any size and shape of substratean be coated; the nanostructured material is optically homo-eneous with very low scattering losses and long-term stability18]. Also, the ESA technique does not require sophisticatedaterials, it can be carried out at room temperature and pres-

ure conditions and it can be easily automated and therefore thenal manufacturing process can be implemented at a low cost.ut the main reason for using this technique is that it allows

he control over film thickness with a precision of nanome-ers and because it is feasible to deposit azimuthally sym-

etric coatings onto the cylindrical surface of tapered opticalbers.

Furthermore, as the taper sensitivity depends on the modeoupling, which can be modified when depositing new lay-rs of material onto the fiber, high sensitivity devices can beonstructed if the deposition is stopped at the optimal coatinghickness. In this work, with the ESA technique a thin film ofDDA-Poly-R polymeric composite [18] is deposited onto a

aper to create a uniform layer at the same time than the out-ut optical power transmission is being monitored. The optimalhickness working points of the fabricated sensors will be inves-igated for the humidity sensing application.

uators B 122 (2007) 442–449 443

In a fabrication process, reproducibility is an important objec-ive. The tapering method used in this work introduces a degreef variability which can be excessive for a commercial purpose.n order to obtain devices with the same physical dimensionsapering robots should be used in order to provide devices withnough tolerances. Additionally, a low cost method is proposedere in order to set the devices at the optimal working point byonitoring the construction process.The main advantages of the proposed sensor are high sen-

itivity and short response time. Also, the optical nature of theensor gives high electromagnetic interference immunity and theossibility to make measurements at long distances with respecto the less expensive electronic sensors.

So, in this work a new optimized humidity sensor is pro-osed, taking as transducer a tapered fiber fabricated using theforementioned method. This taper is coated with a polymericumidity sensitive nanofilm using the ESA technique. The sen-itivity of the device will be optimized by tunning the thicknessf the sensitive layer. This fiber optic humidity sensor is stud-ed in the next sections. To our knowledge this is the first timehat the transmission of a tapered optical fiber is experimentallynalyzed as a function of both the wavelength and the overlayhickness. The paper is structured as follows: firstly, in Sec-ion 2, the working mechanism of the sensor is analyzed. Issueselated to the fabrication and test of these devices are discussedn Section 3. The transmission properties are given in Section. Experimental humidity response is commented in Section 5.inally, some concluding remarks are made in Section 6.

. Sensing mechanism

The optical fiber humidity sensor is shown in Fig. 1. It con-ists of a single-mode tapered fiber whose thinner part is coatedith a polymeric nanofilm sensitive to humidity. When the fiber

s tapered and the transmitted power is monitored, it is observedhat the power decreases to a small fraction of the initial power9]. The cladding modes that are the cause of the power lossan be modulated once the tapering process has finished by theeposition of an overlay surrounding material. Small changes inhe index of refraction or the thickness of this overlay greatlynfluence the transmission properties in the multimode centralegion. Here, the sensitive overlay is used both for creating a

Fig. 1. Humidity sensor structure. Taper profile with ESA overlay.

444 J.M. Corres et al. / Sensors and Act

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e used as an indicator to decide the optimal working points.he sensing mechanism of this sensor is based on the changef the optical properties of the coating when the relative humid-ty increases. The hypothesis of the authors is that an increasef the relative humidity provokes a higher absorption of watern the sensitive overlay decreasing its index of refraction. Thishenomenon would induce a decrease of the equivalent opticalhickness of the coating, and therefore the transmitted opticalower would also change.

. Methods and materials

.1. Experimental set-up

Fig. 2 shows the experimental setup used for testing theumidity fiber optic-based sensors. Basically, the light fromither a broadband light source or a 1310 nm laser source (Rifocs65R) is launched through a lead-in optical fiber pigtail reachinghe tapered fiber and the transmitted power is measured using aP-86142A Optical Spectrum Analyzer or a Rifocs 675RE Opti-

al Power-meter depending on the experiment. A sealed chamber

mall hole into the sealed receptacle and suspended in the airbove different saturated salt solutions in order to measure dif-erent values of relative humidity [19]. A calibrated electronic

Fig. 3. Schematic of the ES

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umidity sensor (Honeywell, HIH-3610-001) was also intro-uced in the chamber for comparison purposes. All the electronicnd optical signals were saved simultaneously using a HP34970atalogger.

.2. Materials

The solutions selected for the experiments described in thisaper were Poly R-478 (anthrapyridone chromophore) as thenionic electrolyte and PDDA (poly-diallyldimethyl ammoniumhloride), as the cationic solution. In previous works it haseen demonstrated that this combination behaves as a homoge-ous optical film with interesting humidity sensing properties18,20]. All solutions were prepared using ultra pure waterupplied by a Barnstead nanopure diamond water purificationystem with resistivity of 18 M� cm. PDDA, 20 wt.% in water,

.W. 400,000–500,000, and PolyR-478 were supplied fromigma–Aldrich. Solutions were filtered using 0.45 �m PTFElters (Acrodisc).

.3. The ESA process

Once the tapered fibers have been fabricated, they are coatedsing the electrostatic self-assembled process technique. ESAs a technique used to build up coatings on a variety of differentubstrate materials such as ceramics, metals, and polymers ofifferent shapes and forms, including planar substrates, prisms,nd even convex and concave surfaces. This method is basedn the construction of molecular multilayers by the electro-tatic attraction between oppositely charged polyelectrolytes inach monolayer deposited, and it involves several steps [21,22].he ESA film deposition method is described schematically inig. 3. First, a substrate (in this case the optical fibers) is cleanednd treated to create a charged surface. Then, the substrate isxposed to a solution of a polyion of opposite charge for a shortime (minutes) and by adsorption a monolayer of polyions isormed on the surface. This way, the substrate is alternatelyipped into solutions of cationic and anionic polymers (or appro-riately charged inorganic clusters) to create a multilayer thin

s formed it is necessary to rinse the sample with pure watero remove the excess of molecules that are not bound and thato not contribute to the monolayer structure. This process is

A deposition process.

d Actuators B 122 (2007) 442–449 445

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Fig. 4. (a) Evolution of the optical output power spectrum of the proposed sensorwfiR

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epeated increasing, layer by layer, the thickness of the materialeposited.

The parameters of the ESA process are critical, especially theolution pH, in order to obtain repeatable results. The repeata-ility of the process is enhanced in good part by robotizing theSA process. Using a layer-by-layer deposition device (RieglerKirstein GmbH) which always positions the taper in the solu-

ions for the same periods and rinses with ultrapure water in theame way, a uniform and repeatable deposition is achieved. TheH is controlled periodically (each 10 layers) and adjusted if nec-ssary using hydrochloric acid or sodium hydroxide. Previousharacterization experiments using interferometric techniquesave given a refractive index of approximately 1.54 + 0.004i forhe nanocomposite under the conditions used in this work [4].

. Transmission characteristics

In this section the transmitted optical power through the tapers studied when the sensitive nanocoating is being depositednto the thinner part of the tapered fiber. For this experi-ental study, step-index single-mode fibers (cladding diame-

er = 125 �m, core diameter = 8.3 �m) were tapered to obtain a0 �m waist diameter. Using the same instrumentation shownn Fig. 2 it is possible to observe the changes experienced byhe optical power due to the increase of the nanofilm thicknessn each monolayer deposited. After each new monolayer, theR transmission spectrum (1150–1650 nm) was logged using anP-86142A Optical spectrum analyzed and a broadband light

ource. The results are depicted in Fig. 4.The output optical power spectrum of tapered fibers exhibits

mportant wavelength dependence, showing several attenuationeaks [23]. Due to the evanescent field interaction with the exter-al medium, two interesting effects have been observed whenepositing the polymer overlay. On the one hand, at a givenavelength, there is a variation of the transmitted optical powerepending on the nanofilm characteristics (index of refractionnd thickness of the nanofilm). This fact is the consequencef the different phase modulation of the excited modes due tohe overlay and the effect in the phase coupling in the expan-ion zone of the taper (see Fig. 1). In Fig. 4(b), it is possibleo observe that in some wavelengths where the taper will be

ore sensitive to the variations on the thickness of the coating.ccording to the hypothesis of the authors that the variationf the surrounding relative humidity provokes a variation in thequivalent optical thickness of the coating, then the transmissionehavior along with the building-up deposition process couldive an idea of the taper performance as a sensor in order to selecthe operating working wavelength. At λw1 = 1300 nm the outputptical power variation is about �T(λw1) = −3 dB, and for theame overlay thickness, at λw2 = 1550 nm, a maximum changef �T(λw2) = −0.2 dB has been monitored. So, the wavelengtht which the sensor will be interrogated plays an important rolen the final sensor response.

On the other hand, as the thickness of the nanofilm isncreased, the spectrum attenuation peaks experience a dis-lacement to higher wavelengths, which can be seen in moreetail in Fig. 4(b). In the first nine monolayers the attenuation

hile the deposition process. (b) Zoom for the first eight monolayers. Taperedber waist diameter: 20 �m. Taper length: 2.2 mm. Nanofilm: [PDDA+/Poly-−].

eaks located initially at λP1 and λP2 experiences an increasef �λP1 ≈ �λP2 ≈ 20 nm. These two variables, thickness andavelength, will be studied in the next section.

. Experimental results

.1. Wavelength dependence of the sensitivity

In Fig. 5, the spectral response to relative humidity in theange 50–100% of a TOF-based humidity sensor when 23 bilay-rs have been deposited is shown. Measurements of the opticalower transmitted through the PDDA/Poly-R coated taper wereerformed when they were exposed to different levels of relativeumidity. In Fig. 5, it is possible to observe that when the rela-ive humidity increases, the optical output power increases in theame proportion as if the number of bilayers were also decreased,his is, as if the equivalent optical thickness decreased. Theseesults corroborate the hypothesis of the authors. Moreover, theost sensitive wavelengths are those that experienced higher

ariation when the coating was deposited, just as was com-

he relative humidity of the sealed chamber was slowly variedetween 50 and 100%. The output of the electronic calibrated

446 J.M. Corres et al. / Sensors and Actuators B 122 (2007) 442–449

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ig. 5. Optical output spectrum dependence with respect to the relative humidityith 23 bilayers deposited. Tapered fiber waist diameter: 20 �m. Taper length:.2 mm. Nanofilm: [PDDA+/Poly-R−].

H sensor was monitored at the same time that the optical spec-rum of the tapered optical fiber sensor (Figs. 6a and 7a). Againhe sensor exhibited a higher sensitivity for those wavelengthshat showed a higher slope throughout the deposition of the

rum shows a variation with the wavelength. The spectra evo-ution with the relative humidity is shown in Fig. 7. The direct

ig. 6. (a) Relative humidity variation applied to the sensors. (b) Evolution ofhe transmission espectrum with the relative humidity.

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ig. 7. (a) Relative humidity variation applied to the sensors. (b) Evolution ofhe transmitivity with the relative humidity for several working wavelengths.

ependence between the deposition slope and the sensor sen-itivity can be appreciated here. In this case the curves at theottom had the higher slopes with respect to the upper curvest higher wavelengths. Therefore, the humidity TOF-based sen-or responses will have higher sensitivities when interrogated atower wavelengths with a maximum around the second telecom-

unication window, as is corroborated in these experimentalesults.

.2. Overlay-thickness dependence of the sensitivity

The dependence of the sensitivity on the overlay thicknesss studied in this section. In Fig. 8, the optical transmitivity athe second telecommunication window of a fabricated humidityensor consisting of a 20 �m waist diameter taper, coated withhe humidity sensitive polymer [PDDA+/Poly-R−] is shown.epending on the number of bilayers at which the deposition

ig. 8. Optical fiber humidity sensor transmitivity as a function of the sensitiveeposited nanofilm thickness. Tapered fiber waist diameter: 20 �m. Taper length:.2 mm. Nanofilm: [PDDA+/Poly-R−]. λ = 1310 nm.

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ig. 9. Experimental response of 20 �m waist diameter TOF-based humidityensors to RH corresponding to three working points of coating thicknesses: (a)3 bilayers, (b) 26 bilayers and (c) 62 bilayers.

trated next. Prior to the deposition of the sensitive coating, thentrinsic sensitivity of the taper without coating was measured as.008 dB/%RH. For comparison purposes the deposition processas stopped at three different working points and the responsehat these three points was tested. After each test the depositionas restarted. As previously commented, the hypothesis of the

uthors is that if the humidity increases, then, the sensor wouldehave as if the working point located in the optical output depo-ition curve of Fig. 8 experienced a backward movement in theirection of a virtual reduction of the overlay thickness. This

he adsorbed layer of water molecules.In Fig. 9, the experimental response to relative humidity of

he three sensors corresponding to the three working points is

ig. 10. Repeatability of the TOF humidity sensor in the range 75–100% RH.

hown. The first sensor was fabricated stopping the building uprocess at the first working point (the thinner one, on the left inig. 8) and its response to RH is shown in Fig. 9(a). By observing

he transmitivity characteristic experimental curve of Fig. 8, itan be appreciated that it had a slope of −2 dB/bilayer. After theumidity test, the sensor sensitivity was experimentally demon-trated to be 0.2 dB/%RH, as shown in Fig. 9a. In Fig. 9(b),he working point of the second sensor had a −6.6 dB/bilayerlope. This is the maximum slope point of the transmission char-cteristic and corresponds to the highest sensor humidity sen-itivity, +0.885 dB/%RH. In Fig. 9(c), the coating process wastopped when the thickness of the overlay was the double than inig. 9(b). The slope of this working point was −0.4 dB/bilayer,nd the sensitivity to humidity obtained is +0.033 dB/%RH. Allhese results are in good agreement with the assumption that aigher slope of the TC when coating the taper results in sen-ors with a higher sensitivity. The linearity of the experimentalesponse depends on the number of excited modes. By fabricat-ng a higher waist diameter tapered optical fiber sensor, generally

ore linear responses and an extended RH range can be obtainedt the cost of a lower sensitivity. But in any case, it is necessaryo consider first the TC of the resultant TOF-based sensor. Fur-hermore, it has been demonstrated that using the experimentalutput optical transmission characteristics curve it is possibleo find out the maximum slope zone of the potential sensor inrder to fabricate an optimized sensitivity sensor with a coat-ng thickness tuned around this working point. According to thenitial hypothesis, it is experimentally corroborated again thats the humidity increases, the sensors response follows a virtualecrease of the optical thickness of the nanocoating in their TCurves.

In Fig. 10, the repeatability of the humidity sensor is shown.n this figure, the dispersion of the measured relative humidityas been represented using error bars. Measurements were taken

ondensation effects.

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ig. 11. Relative humidity step response of tapered fiber sensor vs. commercialapacitive RH sensor.

.3. Dynamic performance

The design method proposed here allows the building upf high sensitivity humidity sensors with a fast response. Theapered optical fiber-based sensor has been compared in termsf response time with a Blue box humidity sensor T12000/6,rom Philip Harris, creating a RH step by introducing theensors in a chamber with a 100% RH from ambient con-itions (Fig. 11). It has been experimentally demonstratedhat the optical sensor is at least ten times faster in reachinghe stationary state than the capacitive based sensor, becausef the thin film sensing layer method used [18]. The stabi-ization time that the tapered fiber sensor apparently needs,s due to the small perturbation to the humidity parameterhen opening the small hole of the chamber to introduce the

ensors.In order to measure more precisely the speed response of the

roposed sensor, it was exposed to quick changes of environmentumidity. The human breathing contains more water vapor thanhe normal room environment. As a result, one possible way of

easuring the response time is to expose them to a mouthful ofir. According to this, the sensors were set 3 cm from a subject’south. The results obtained are shown in Fig. 12. The observed

ise response time of the TOF humidity sensor is 0.3 s, and theall time is 1.7 s.

Finally the cross-sensitivity to temperature was also tested.he transmitted optical power when the environment air tem-erature is slowly increased from 25 to 65 ◦C was monitored. In

Fig. 12. Experimental response to the human breath.[

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his experiment the maximum peak to peak oscillation was only.1 dB.

. Conclusions

In this work it has been proved that the ESA deposition tech-ique can be used to fabricate fiber optic-based sensors in suchway that the overlay thickness can be controlled in order to

ptimize the sensor sensitivity. The power transmission spec-rum of the axis-symmetric nanometric coating has been studieds a function of the thickness overlay, exhibiting high amplitudescillations that have a decisive impact in the selection of the sen-or working point. It has been found out that the highest sensorensitivity can be adjusted with the coating thickness by stoppinghe deposition process at the maximum slope of the transmittedptical power. The humidity sensor designed with the proposedechnique consists in a cladded single-mode tapered optical fiberoated with a thin [PDDA/Poly R-478] layer. The fast responsebtained with the proposed sensor can make it suitable for breathnalysis, the control of highly humidity dependent chemical pro-esses or weather prediction, among others [24–26]. In addition,he proposed method is susceptible to be applied to other sensi-ive coatings or even to other transmission optical transducers,uch as long period gratings, hollow-core fibers, etc.

cknowledgments

This work was supported by Spanish CICYT Research GrantsIC2003-00909 and Gobierno de Navarra research grants.

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iographies

esus M. Corres received the MS degree in electrical engineering from the Pub-ic University of Navarra, Pamplona, Spain, in 1996 and the PhD degree from theublic University of Navarra, Pamplona, Spain in 2003. He has been a memberf the Department of Electrical and Electronics Engineering of the Public Uni-ersity of Navarre for eight years, and has been involved in different projectsith industry including power systems design and motion control. His main

esearch interests include optical fiber sensors and nanostructured materials.

rancisco J. Arregui received the MS degree in electrical engineering fromhe Catholic University of Navarra, San Sebastian, Spain, in 1994 and the PhDegree from the Public University of Navarra, Pamplona, Spain in 2000. Since995 he has been working at the Public University of Navarra, (Pamplona, Spain)here currently he is a Permanent Associate Professor. During 1998, 2000 and004 he was a Visiting Scientist at the Fiber & Electro Optics Research Center,irginia Polytechnic Institute and State University, (Blacksburg, VA, USA).is main research interests include optical fiber sensors, sensor materials andanostructured materials. Francisco J. Arregui is a member of SPIE and IEEE.

gnacio R. Matias received the MS degree in electrical and electronic engineer-ng and the PhD degree in optical fiber sensors from the Polytechnic University

Sensitivity optimization of tapered optical fiber humidity sensors by means of tuning the thickness...

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Transcript of Sensitivity optimization of tapered optical fiber humidity sensors by means of tuning the thickness...

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