Hindawi Publishing CorporationJournal of SensorsVolume 2009, Article ID 607647, 12 pagesdoi:10.1155/2009/607647Review ArticlePlanar Bragg Grating SensorsFabrication andApplications: A ReviewI. J. G. Sparrow,1P. G. R. Smith,1G. D. Emmerson,1S. P. Watts,1and C. Riziotis21Stratophase Ltd, Unit A7, The Premier Centre, Premier Way, Romsey SO51 9DG, UK2Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation (NHRF),48 Vassileos Constantinou Avenue, 11635 Athens, GreeceCorrespondence should be addressed to I. J. G. Sparrow, ian.sparrow@stratophase.comReceived 5 April 2009; Accepted 13 July 2009Recommended by Valerio PruneriWe discuss the background and technology of planar Bragg grating sensors, reviewing their development and describing the latestdevelopments. The physical operating principles are discussed, relating device operation to user requirements. Recent performanceofsuchdevicesincludesaplanarBragggratingsensordesignwhichallowsrefractiveindexresolutionof1.9 106RIUandtemperature resolution of 0.03C. This sensor design is incorporated into industrialised applications allowing the sensor to beused for real time sensing in intrinsically safe, high-pressure pipelines, or for insertion probe applications such as fermentation.Initial data demonstrating the ability to identify solvents and monitor long term industrial processes is presented. A brief reviewof the technology used to fabricate the sensors is given along with examples of the exibility aorded by the technique.Copyright 2009 I. J. G. Sparrow et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.1. IntroductionAwiderangeofoptical sensingtechnologiesexistandaresubject to intensive development due to a number of drivingfactors. In the elds of process control and automation forexample, thereis adesiretomonitor concentrations andcompositions inreal time without the riskof damagingor contaminating high-value product. It is commonforsuchapplications tooccur involatileor ammableenvi-ronmentswherespark-freeorintrinsicallysafetechnologyis a prerequisite. Whilst there are many elds with disparatemotivations andsensing requirements, they doinmanyways share a common goal, that of rapid, accurate, and safedetection in a potentially harmful environment.Existing review papers discuss the broad range of opticalsensingtechnologies[1]andthemotivationsforusinganintegratedoptical format interms of compatibility withmicrouidics [2]. In this paper we concentrate on a reviewof results on planar Bragg grating sensors which are a recentaddition to the eld and oer attractive advantages.Adetailed description of the range of dierent techniquesand technologies available from the generic class of opticalsensors would form an extensive review and could include avast array of applications from particle counting to vibrationdetection. Therefore the various optical sensor technologiesmay be segmented in a wide number of ways, but it is helpfultodistinguishbetweensensorsinwhichthelightdoesnotphysically pass into the measurand material (such as a Bragggratingsensorforstrain)andsensorsinwhichtheopticalelddoes pass intothemeasurand. Inthis paper weareconcerned with the latter type of device. It is further useful toconsider the means by which the light interacts; this may beeither through the refractive index of the measurand or viaanabsorptionorotherenergyexchanginginteractionwiththe measurand. In that sense we can choose to characteriseboth types of linear properties of the light interacting in thematerial via a complex refractive index (n = n i) wheren is the real index at a particular wavelength and representsthe absorption. There are, of course, nonlinear interactionstoo, such as the Raman eectwhich are outside of the scopeof this paper.Bythinkinginterms of thecomplexrefractiveindexwecanchoosetoclassifytechniquesintoeitheronesthatmake use of the imaginary part (i) suchas absorption2 Journal of Sensorsspectroscopy, andthosethat makeuseof thereal part oftheindex(n)suchasrefractometry. Inthesetermsweseethat absorption spectroscopy can be viewed as investigatinghow (i) varies with wavelength. However, in this paper weareprimarilyinterestedinthosetechniquesthatmakeuseof the refractive indexproperties of the measurand, andparticularly techniques in which the light interaction in thesensor and measurand modies the modal properties of thelight. Amodal pictureis familiar inbreoptics whereamoderepresentsasolutiontothelawsofelectromagneticpropagationthat is constant informalong aninvariantrefractiveindexstructure. This modal concept is distinctfromrefractive index properties (such as are used inarefractometer)inwhichthelightpropagationiseectivelyfree space like with refraction occurring at a set of discreteboundaries. Thusfromthehugepossiblerangeof sensortechnologies we are led to consider rstly those that primarilysense the real part of the refractive index and then specialiseto those techniques in which modal interactions are used.Within this category of modal devices the most familiardevices make use of surface plasmonresonance [3] andprovide a well-established technology [4, 5]. Such plasmondevices are well established in literature; for a recent reviewofplasmonicsthebookbyMaier[6]providesawealthofinformation. Plasmon-basedsensorsareusedinanumberof commercial instruments producedbycompanies suchas Biacore, BiosensingInstruments Inc, Sensata, andICxNomadics.InaPlasmontypesensorthelightpropagationand modal properties are strongly dependent on the proper-ties of a thin lm of a metallic conductor (most commonlygold), in which the modal coupling properties are modiedbytherefractiveindexof thesurroundingdielectric(themeasurand).IncontrasttoSPRsensors, thetypesofdevicesinthispaper make use of dielectric waveguides in which there arenometallic elements andinwhichthemodal propertiesaredominatedbythereal part of therefractiveindexofthe waveguide andof the measurand. The most familiarformat for such a device is an optical bre sensor. A recentreviewarticle canbe found[7] whichcovers the wholearea of bre sensing. In this reviewwe are specicallyinterested in devices in which guidance occurs by totalinternal reectioninahigher indexcore, andwherethewaveguidestructureisprocessedtoallowlight tointeractwith a measurand uid. This interaction causes a change inoptical path length, which can be sensed in a number of waysbut typically is either interferometric or via a change in theresponseof agratingstructure. Morerecently, researchershave started to exploit the advantages of planar integrationas a way to allow enhanced functionality devices to be madeinwhichmicrouidics andmultiplesensor elements canbeincorporatedintoasingledevice. Suchdeviceshaveacommon physical operating principle, in that they all operatebyhavingadielectricwaveguideinwhichthepropagatingmodeisallowedtopartiallyinteract withthemeasurand,andwhere the optical pathchange associatedwiththatinteraction is measured. For example work by Heideman etal. [8]describestheoperationof aMach-Zehndersensorwhich measures fringe changes in the interferometer output;incontrast earlyworkbyTiefenthaler [9] usedasurfacegrating to measure water absorption on a planar waveguide.MorerecentlyworkbySchroederet al. [10]showedhowmultiple gratings at dierent operating wavelengths may beused to measure and correct for temperature variation andalsogaininformationonthe variationof refractivewithwavelength;however,thisdeviceusedaside-polishedbreembedded in a block, which is not simple to fabricate.A relatively small number of Bragg grating-based deviceshave been considered and implemented in planar form. Theyhavebeendemonstratedinavarietyof dierent materialplatforms such as in polymers [1114], Sol-gel systems [15],Silicon-on-Insulator SOI [16, 17], Lithium Niobate [18], andSilica-on-Silicon [19, 20].The wave-guiding and grating structures in Bragg-basedoptical sensors have been fabricated with a number ofapproachesleadingtosensingelementswithridgewaveg-uides [21], UVwrittenwaveguidesandgratings [19, 20],corrugated/etchedBragggratings [22, 23], or evenBragggratings through selective precipitation of nanoparticles[24].Howeveraverylimitednumberof designs havebeenproved viable in terms of commercialisation. Recently,Stratophase Ltd has commercialised a direct UV writ-ing technology following its original development at theUniversity of Southampton[19, 20]. The methodallowsthe inscription of waveguides and gratings onto planarsubstrates. This technique enjoys the benets of planarintegration and ease of applying microuidics and alsomakes useof telecommunicationgradesinglemodebrecomponents and measurement technology allowing fortremendous refractive index sensitivity while exploiting thetemperature compensation advantages rst demonstrated bySchroeder et al. [10].Movingonfromconsiderationof thephysical mecha-nisms, inthecontextoftheworkpresentedhere, perhapsthe most widelyusedtool is the benchtoprefractometerupon which samples taken fromprocessing steps are analysedin a lab to determine solution concentrations or sugar,alcohol, or solvents. Inline variants of this style of equipmenthave startedtoreachthe market inrecent years sothatoine measurements may be replaced with at-line or inlinemeasurementstosavetime, moneyandreducesafetyandcontaminationconcerns. Generallythesetools requireanelectricalsignalatthepointofmeasurementwhichcanbeproblematic in volatile environments which require intrinsi-cally safe equipment to minimise the risk of explosion.This paper presents a review of recent advances that havebeenmadeinthedevelopment of anoptical sensor thathas proven to be suitable in the application areas describedabove. The sensor, anevanescent wave device basedonplanar Bragggratings, oers boththerequiredsensitivityfor concentrationmeasurements andprocess monitoringandisalsosuitedtoindustrialenvironmentswhererobustand reliable devices must be installed. The all opticalmeasurementmeansthatthereisnoignitionrisk, makingthe technology highly suitable for volatile environments.Additionally, because the underlying principles are the sameasthoseusedintheeldoftelecommunications, multipleJournal of Sensors 3devicesmaybenetworkedandmultiplexedoververylargedistances. Multipledeviceslocatedat separateanddistantlocations can easily be monitored from a single analysis basestation to maximise convenience and minimise cost.To describe the sensing technology, an overviewofthe fabrication technique and its advantageous featuresis given. This is followed by an analysis of the devicesensitivity to refractive index and temperature. To completethe presentation two examples of industrial applications aregiven.2. BackgroundThecoretechnologyof thesensorsdiscussedhereisthatof the Bragg grating, a structure that has been knownfordecadesandhasalwaysbeenrecognisedashavingthepotential for use as a sensing element. Most commonlyemployed in optical bres, the Bragg grating reects opticalwavelengths according to the following relation:B = 2ne, (1)where Bis the Bragg wavelength at which maximumreectivityoccurs. providestheperiodof therefractiveindex modulationthat denes the grating. The eectiveindex of the waveguide that contains the Bragg grating, ne,is a combined refractive index of the core and cladding thatthe optical mode interacts with.Fromthisequationitcanbeseenthatasthematerialsurrounding the Bragg grating changes, variationintheeectiverefractiveindexcausesthereectedwavelengthtoshift. Thisformsthebasisof theuseof Bragggratingsassensors and is shown conceptually in Figure 1.Thespecicdevices that will beoutlinedinthesub-sequent discussionusethetechniqueof UVwriting. Thisapproach is highly exible and has been highly renedfor the creation and postfabrication trimming [25] ofoptical devices suitable for, amongst other applications, thetelecommunications industry.The sensors described here are fabricated using a uniqueextensiontotheUVwritingtechnologyknownas DirectGratingWritingwhichsimultaneouslycreatesawaveguideand a Bragg grating in a planar substrate.Early work on the conversion of the UV writtensubstrates into liquid sensors has been presented alongwith demonstrations of their use as tunable lters andrefractometers. Whilst this early work highlights some of theopportunities for such devices, the level of development wasnot initially suited towards full commercial exploitation.Such planar Bragg grating devices are appealing forsensing applications for several reasons.(1) Multiple wavelengths may be used oering thepossibility for analyte identication through opticaldispersion measurements and also providing a rangeof evanescent eldpenetrationdepths whichmayprovide additional information on the dimensions ofbiological entities.(2) Multipleseparatesensingregionsmaybeincorpo-rated onto a single sensor chip. This can be anBacteriaVirusToxinEach sensor pad can detect a different pathogenLiquid inputStratophasedetection chipOptical readoutMulti-spectraldata for each sensor padFigure1:RepresentationoftheinteractionbetweenliquidandaStratophase sensor chip that is mounted in a microuidic cell.Inthis case a functionalised chip for the purposes of specic biologicaldetection is shown.advantage particularly in immunoassay-based biode-tection where it is advantageous to test for multipledierent targets simultaneously without the need forduplication of equipment or time delays.(3) Themonolithicsiliconchip-baseddesignisrobust,requires noelectrical signal, is resistant toawiderangeofchemicals,andisthussuitablefordeploy-ment in a wide range of environments.In order to get the maximum possible performance fromaBraggsensortheopticalinterrogationmethodiscritical.Many industrial applications require multiple measurementsto be made 24 hours per day, and so relatively high capitalcost canbe toleratedas the cost is shared. For topendperformance the interrogation can come at a relativelyhigh nancial cost although technology improvements andcommercial competitionprovide a strong drive for costreduction over time. Additionally, the devices presentedhere are designedtooperate inthe telecommunicationswavelengthband. As suchit is possible tohave sensorspositionedat uptoseveral kilometre distances fromtheoptical source. More advantageous still, signals frommultiplesensors at dierent locations maybemultiplexedinsuchawayas tohavemanysensors all monitoredbyasingleread-out unit. Thus the cost per sensor, whichis oftenmore important thantotal cost, is dramatically reduced.Furthermore, the cost per measurement is lower still becauseoftheopportunitytoincorporatemultiplesensingregionson a single chip.In addition there is the possibility to integrate thesensing element with the interrogation system by deploying,for example,BragggratingorArrayedWaveguideGratings4 Journal of Sensors[2628]withinthesamechip, atechnologythatishighlycompatible and readily available in the Silica-on-Siliconplatform. This combination could lead to compact and self-contained sensing systems.3. Fabrication of Sensors3.1. UV Writing. In a method similar to prior work [29] andshown in Figure 2, Bragg gratings and waveguides are writteninto a three-layer silica on silicon substrate using the DGWtechnique.TheUVwritingprocessutilisesthephotosensitivityofGermaniumdoped silica. Exposure of the silica to UVlight at244 nm causes an increase in the refractive index. The threelayersamplesarefabricatedbyamehydrolysisdepositionsuchthat the silicais depositedontoasiliconsubstrate.Themiddlelayer is dopedwithgermaniumtoprovideaphotosensitive layer that may be exposed to UV to raise therefractive index.High-pressure hydrogen loading is used as a method ofenhancing the photosensitive response immediately prior toUV exposure. Similar techniques are used in the fabricationof breBragggratings prior toUVexposurefor gratinginscription. The planar samples are placed into hydrogen at120 bar for three days in order to allow hydrogen to diusethrough to the glass layer, that is, to act as the waveguide core.After hydrogen loading chips may be kept for many days indry ice before UV writing. Once removed from cold storagetheymaybekeptatroomtemperatureforinexcessofanhour before out-diusion of hydrogen becomes a signicantissue. The initial out-diusionof hydrogencanresult ina slight change of Bragg wavelengthdue toa reductioninphotosensitivity over time. This may be compensatedforwithawell-controlledUVwritingprocessif required.However in this application it is not the UV writing processthat determines sensitivity to refractive index. It is in fact thelaterprocessingstagesofetchingandover-layerdepositiondescribedinSection 3thatdetermineoverallperformance.Thus longUVwritingtimes maybe used, andcomplexstructuresmaybewrittenwithouttheneedforanyotherspecial precautions.Controlled, localised irradiation of the silica with asuciently high uence after hydrogen loading allows well-denedwaveguidestructures tobecreated. Fortheworkreportedhere,afrequencydoubledargon-ionlaserisusedto produce the UVbeamfor irradiation. The beamisconditionedandfocusseddowntoavemicrondiameterspot in a xed location. Samples for UV writing are mountedon an air-bearing translation stage which allows threedimensional movement of the photosensitive substrate to aspatial resolution better than 10 nm.Themost direct methodof creatingawaveguideistosimplytranslatethesampleataconstantspeedundertheUV beam at a constant power. However, to allow the inscrip-tionof bothBragg gratings andwaveguides this simplearrangement is extended. Using a beam splitter and carefullypositioned mirrors an interferometer is created such that theUVbeamisrecombinedtocreateaninterferencepattern.This pattern, when incident on a photosensitive sample, will(a)Two interfering,focussed UVbeams crossingin the core layer(b)Figure2: Photographicanddiagrammaticrepresentationof theUVwritingprocess withcrossingUVbeams focussedontothephotosensitive substrate.createacorrespondingrefractiveindexmodulationinthecore layer. The period of the interference pattern is set to be530 nm, a period which when combined with the refractiveindex of the core layer can be used to create a Bragg gratingreecting at 1550 nm.The interference patterncannot simply be translatedacross the sample to create a Bragg grating. A static exposurewould,ineect,createaveryshortBragggratingofjustafew grating planes in length. However, upon translating thesample this refractive index modulation would be smearedout in the direction of translation. This destroys the periodicrefractive index modulation resulting in a uniform increasein the refractive index. Thus, translation of the samplebeneath a continuous UV spot results in the formation of anunmodulated waveguide. To create, a Bragg grating the UVbeam must be modulated as the sample is translated underthe UV beams. Monitoring the position of the sample as it istranslated and briey turning on the UVbeamevery time thesample has moved by one grating period causes the patternof refractive index modulation to be repeatedly written intothesample. This occurs insuchawaythat eachpatternis shiftedbypreciselyoneperiodrelativetothepreviousone. Inother words, as thesampleis translatedaBragggratingiswrittenintothesample. TochangefromBragggrating to waveguide the UV beam is simply returned to itsformernonmodulatingstate. Inthisway, asingleprocessmaybeusedtowritebothwaveguidesandBragggratingsinto a sample in a single but highly exible process. Accuratecontrol oftheUVswitchingisachievedbyusingthehighprecision position information from the translation stages todrive an acousto-optic modulator.3.2. Grating Parameters. A number of parameters maybe controlled in order to tailor the Bragg gratings andwaveguides. The primary factor in UV writing is the uenceused to write the waveguide and grating structures. A higherUVpower (or a lower translationspeed) gives a higheruence, resulting in larger changes in refractive index.Journal of Sensors 5The relationship between translation speed and the rateat which the UV beam is modulated plays an important rolein the creation of Bragg structures. Variation of the duty cyclecanbeusedtocontrol thestrengthof theBragggrating.Dutycycle refers tothe percentage of time that the UVbeam is turned on in the process of writing a Bragg grating.Previously, the modulation of the UVbeamwhilst the sampleis translated was discussed. The duration of this modulationasapercentageoftheBraggperiodgivesthedutycycle.Adutycycleof 100percent results inawaveguidewithnorefractive index modulation whilst a duty cycle of 0 percentresults in no refractive index change being written into thesample whatsoever.It is desirable for the average refractive index of a Bragggratingtobecloseto, ifnotidenticalto, thewaveguideateitherendof thegrating. AlthoughthelinkbetweenUVpowerandinducedrefractiveindexchangeisnotperfectlylinear, to a close approximation uence and duty cycle maybe used to index match waveguides and gratings very simply.A grating written with a 50-percent duty cycle with a givenUVpowermustbetranslatedundertheUVbeamathalfthe speed used for a waveguide written with the same power.This gives the same uence andtherefore approximatelythesameaveragerefractiveindexinboththegratingandwaveguide. Similarly a 90-percent duty cycle grating wouldbe translatedat 90percent of the waveguide translationspeedtouencematchthetwostructures. This simplerelation is possible because the waveguides and gratings arebeing written into a blank substrate where no waveguideexists beforehand. In the case of bre Bragg gratings this isslightly dierent as the gratings are added to a pre-existingwaveguide,andsotheaverageindexofthewaveguideandbre cannot be the same unless special extra steps aretaken.Dutycyclemayalsobeusedtochangethestrengthofthe Bragg gratings written. Generally speaking, a higher dutycycle results in a lower contrast between the grating planesresulting in a more response with a more narrow bandwidththan one written with lower duty cycles.Anadditional degreeof exibilitymaybeachievedbymodulating the UV beam at a rate very slightly dierent totheintrinsicperiodof theUVintensitymodulation. Thecumulative eect of the multiple UVexposures used to createagratingproducesagratingwithaperiodthatisequaltotheperiodbetweenexposures, not theintrinsicperiodoftheinterferencepattern.[30].Inthisway,gratingsmaybewrittenspanninghundredsofnanometerusingexactlythesame process.Renement of theUVwritingprocesshasallowedanimmensely exible technique for waveguide and gratingproduction to be developed. Using specially developedsoftware packages it becomes a simple matter to create scriptswhich when loaded into the UV writing system can producestraight andcurvedwaveguideswithsophisticatedgratingstructuresatmultiplewavelengthsinasingleprocess. Notonly can bespoke waveguide and grating designs be rapidlycreatedwithout the needfor expensive phase masks butalso no clean room facilities are needed, making the overallinfrastructure requirements low.6055504540353025201520 1525 1530 1535 1540 1545 1550Wavelength (nm)Reflected power (dB)Apodised gratingUniform gratingFigure 3: Comparison of reection spectra of uniformandapodised gratings.Someexamplesofgratingspectrathatmaybewrittenusing this process are given below. All structures were createdusingthesameproprietarysoftwarepackagetocreatetherequired grating responses.Figure 3 compares a straightforwarduniformgratingwith another grating of similar properties but which has beenapodised using a cosine squared function. The reduction insidelobesisclearalbeitattheexpenseofaslightlybroaderreection peak.Asthegratingsare writteninamannerthat isclosetobeingplanebyplane, itisstraightforwardtoachievehighlevels of control of the grating structure along its length. Forexample, phase shifts may be inserted in order to achieve asharp dip in the reection response which may in some casesbe advantageous when determining the centre wavelength ofpeaks. Such a spectrum is shown in Figure 4.Periodic spacing of phase shifts can be used to generatemore elaborate grating structures which provide multiwave-lengthresponses. Anexampleof thisisgiveninFigure 5,which is a demonstration of a superstructured gratingproviding over 15 wavelengths that may be used for sensingpurposes over a 130 nm wavelength span. This grating, just4 mmin length, opens up opportunities to measure refractiveindex over suciently wide ranges to allowdispersioncharacterisation and thus material ngerprinting to beperformed.This approach may be extended, as shown in Figure 6, touse multiple superstructured gratings that are interlaced suchthat they allow a greater spectral density of measurements tobe performed from a compact device which is again writtenin a single step onto a single device that is just millimetres indimensions.Drawingonthetechnologyof breBragggratings itshould be possible, if required, to fully control the spectralproperties of Bragg gratings through the use of novel designstrategies employed in bres [31] and then implement themthroughthe DGWfabricationtechnique. This advantagegives this particular planar platformfurther exibilityto6 Journal of Sensors65605550454030351571Wavelength (nm)Reflected power (dB)1551 1556 1561 1566(a)Wavelength (nm)5553514947454139373543Reflected power (dB)1561.4 1561.5 1561.6 1561.7 1561.8(b)Figure 4: Example of a phase-shifted Bragg grating with a deep transmission dip written by DGW. The grating used here has a total lengthof 3 mm.706560555045403530251480 1500 1520 1540 1560 1580 1600Wavelength (nm)Reflected power (dB)Figure 5: Wide spanreectionspectrumof a superstructuredgrating written into a planar sensor chip.developcustomisedpurpose-orientedsensorscombinedina compact integrated form.3.3. Creating a Sensor. Intheir as-writtenstate, the UVwritten samples are intrinsically sensitive to temperature orstressandcanbeusedassensorsinamannercomparableto that widely usedinbre sensing [7]. To exploit theadvantages of the planar geometry this temperature sensingability may be combined with multiple liquid refractive indexsensing regions. Conversion to a sensor uses relatively simpleprinciples. The Bragg wavelength of a grating is determinedby the eective index of the waveguiding structure in whichthe grating is dened. In other words the combined refractiveindicesofthewaveguidecoreandcladdingplayakeyrolein setting the wavelength or wavelengths at which the Bragggrating operates. If we remove the upper cladding and replace454035302520151051510 1520 1530 1540 1550 1560 1570 1580 1590Wavelength (nm)Reflection (dB)Figure 6: Reection spectra of two interlaced superstructure Bragggratings on separate waveguides. These allow multiple wavelengthsto be measured in a single sensing region on a single chip.it with something else, the eective index and thus the Braggwavelengthischanged. Inthisway, thedevicecannowbemade into a sensor. Liquid that is used to replace the uppercladding in the vicinity of the Bragg grating will control theBragg wavelength. As the properties of that liquid change, sothe Bragg wavelength changes.The cladding over the sensor gratings is removed using awet etch. The etchant is delivered to the silica surface usinga microuidicow cell which allows the chemical to comeinto contact with only the areas of the chip where etching isrequired.TheBraggresponseismonitoredthroughouttheetching process to ensure that the etch is allowed to proceeduntil suciently deep to ensure that the cladding is removedbut that it is stoppedbeforetheresponseis degradedbyJournal of Sensors 7Figure 7: An etched sensor chip before bre pigtailing. The sensingregion is seen as a small oval towards the right hand end of the chip.removalofthewaveguidecore. Temperaturemeasurementgratings areleft unetchedsuchthat theyareimmunetochanges in the refractive index of any liquids above them.Followingetching,thepenetrationdepthoftheopticalmode into the liquid analyte is relatively low. To extend thepenetrationfurther,ahigh-indexoverlayermaybeappliedtotheetchedsurface[32].Thishastheeectofliftingtheoptical modeuptowardstheanalyte, resultinginamuchhigherpenetrationdepthandamuchgreatersensitivitytorefractive index. A number of methods and materials may beusedtoachievethiseectdependingonthedesireduppersurface material and the required refractive index sensitivity.To date several materials have been utilised including, silica,silicon nitride, silicon oxynitride, uorinated polymers,titania, zirconia, andalumina. The dierent materials allrequire dierent processing steps and results in dierent endproducts.Toperformmeasurements theetchedandoverlayeredchips are pigtailed using single mode bre, and opticalspectra are obtained with the use of commercially availableBragg grating interrogators.Figure 7 shows a photograph of a typical sensor chip afterfabricationbut beforeoptical brepigtailing. Theetchedwindow containing the sensing region can be seen as a smalloval tothe right of the chip. Temperature measurementgratings are embedded in the left-hand side of the chip.It should be noted that the process of etching the chip andsubsequentlyaddingahigh-indexoverlayerbringsaboutavery high level of birefringence in the Bragg grating section ofthe device. In many applications this would not be tolerableas it would cause large amounts of polarisation dependence.Here, the eect is suciently large that TE and TM modescanbeindependentlyresolved. Thismeansthatinsteadofrequiringpolarisationcontroltoobtainareliablemodeofoperation, themoresimplerouteof usinganunpolarisedoptical source can, if desired, be taken.Similarly, whilst overall optical loss would in manyapplications be required to be minimal, it is of less concernhere. It is critical to measure changes in Bragg wavelength asaccurately as possible but this is not dependant on the opticalpower that is reected.02468101214161.28 1.3 1.32 1.34 1.36 1.38 1.4 1.42Refractive indexNormalised wavelength shift (nm)RI liquidsSolvents WaterMethanolEthanolIPAAcetoneFigure 8: Sensitivity curve showing variation in Bragg wavelengthwith refractive index.4. Refractive Index SensitivityThe ability to detect changes in refractive index depends ontwokeyaspectswhenusingaBragggratingdevice. Firstlyand most obviously, the rate at which the Bragg wavelengthvarieswithrefractiveindex,theintrinsicsensorsensitivity,mustbeconsidered. Secondly, theresolutionandaccuracyto which the Bragg wavelength can be determined must betaken into account.Toaddress the rst of these considerations, a simplesensitivitycurvemaybegenerated. SuchacurveisshowninFigure 8whereCargillerefractiveindexliquidsareusedonthesensor chips, andtheresultant Braggwavelengthsaremeasured. Itmust benotedthatthereisaveryslightuncertainty inthis data as the RI liquids are quotedat589 nm, notat1550 nmusedhere. Asaresultofmaterialdispersion, there will be small but currently unknown osetbetweentheRIvaluesquotedinthisgraphandtheirtruevalues.It is easy to see that over a refractive index span of 1.30to 1.40 the Bragg wavelength shifts by approximately 15 nm,giving rise toa particularly highsensitivity torefractiveindex. The sensitivity of this device increases at higherrefractive indices as expected [20]. The gradient of the curveprovidesthesensitivitymeasuredinrefractiveindexunits(RIUs)pernanometre. Atanindexof 1.31thesensitivityshown is 92 RIU/nmrising to 155 RIU/nmat an indexof 1.36. Theupper endof thecurveshownheregives asensitivityof 193 RIU/nmatarefractiveindexof 1.39. Atanalyterefractiveindices muchabove1.40this particulardeviceceases toact as awaveguide. Thecombinedeectofthehigh-indexoverlayerandtherefractiveindexoftheanalyte means signicant modal mismatch occurs as the lighttravels into the etched window region, and all of the opticalmode is lost to the overlayer and analyte with the result thatthe Bragg signal is completely lost.InadditiontotheRI liquiddatashownonFigure 8,datapoints associatedwithseveral solvents are included.These data points were obtained by simply using the relevant8 Journal of Sensors45678910111213140 100 200 300 400 500 600 700 800Time (seconds)Wavelength shift (pm)Figure9: Steady-stateBraggwavelengthdeterminationshowingsubpicometre stability.solvent astheanalyteonthesensor. ThemeasuredBraggwavelength allows the solvent data points to be interpolatedonto the existing RI curve such that the measured RI of thesolvents can be determined. Clearly the ve solvents, namely,water, methanol, acetone, ethanol, and isopropyl alcohol, canall beeasilyresolvedandthusidentiedbymeasurementof the Bragg wavelength. These liquids represent just asmall subset of theliquidsthat maybemonitoredinthisway.Thesecondfactorindeterminingrefractiveindexres-olutionis that of themeasurement technique. Firstly, anoptical spectrum must be obtained in an easy to manipulateformat.WithinthisspectrumtheBraggreectionmustbeidentied and the centre wavelength determined. A numberof Bragg grating interrogators are available in an o the shelfconguration, typicallyoperatinginwavelengths between1500and1600 nm. The stabilityandrepeatabilityof theinterrogatorsisof obviousimportanceasisthereliabilityof the centre wavelength determination from the reectionspectrum.Using a simple weighted average calculation on a reec-tion peak a quick and easy measurement of centre wavelengthmay be made. Figure 9 shows the result of such a calculationon a single Bragg reection from a sample made accordingtothe techniques outlinedearlier. The chipwas heldataconstant 35CusingaPIDtemperaturecontroller inastableenvironmentsuchthattheBraggwavelengthsofthepeakswouldremainconstantasnoexternal changeswerepermitted. Measuring the Bragg wavelength for a period oftime allows an estimate of the stability and the calculation tobe made.This data shows that over the 12 minute sectioninthegraphthewavelengthvariesbyamaximumof0.7 pmrepresenting excellent stability. Each data point shown is themean wavelength of 5 seconds worth of data collected fromsingle Bragg reection peak. A common method of quotingthe stability is to multiply the standard deviation of the dataset by three, a calculation which provides a stability of just0.36 pm.Withthe assumptionthat any change inwavelengthgreater than 0.36 pm should be the smallest change in wave-lengththatcanberesolvedwiththiscombinationofchip,interrogator,andanalysistechnique,wecandeterminetherefractiveindexresolution. Figure 8allowedthesensitivityto refractive index to be measured at 193 RIU/nm; so with awavelength resolution of 0.36 pmwe obtain a refractive indexresolutionof just 1.9 106RIU. Furthertime-averagingmay allow this to be further improved but this would come atthe expense of data update rate which in many applicationsis an important parameter.5. Temperature MeasurementAs mentionedearlier, theUVwrittenchips maybeusedas temperaturesensingdevices as well as refractiveindexsensors by choosing toleave one or more of the Bragggratings unetched. In this case thermal expansion of the silicabrings about a change in the Bragg period and therefore inthe Bragg wavelength.Usingatemperaturecontrolledandstabilisedenviron-ment a sensor chipwas rampedupanddownbetweenmultiple temperatures and allowed to stabilise at ve degreeintervals. BoththeBraggwavelengthandtheactual chiptemperaturewererecordedthroughout theprocess. Tem-perature was measured using a 100 PRT monitored usingaMeasurementComputingUSB-Tempmodule. ThisdataisshowninFigure 10withthedatapointsbetweenstabletemperatures being removed for clarity. The steps in outputas the temperature was ramped up and down are easily seen.Inordertoseetherelationshipbetweentemperatureandwavelength the data is replotted in Figure 11 where a linearrelationshipbetweentemperatureandwavelengthisclear.Whilst thisgraphappearstoshowonlyninedatapoints,there is in fact over twenty ve thousand points plotted. Thepointsareverycloselyclusteredduetothestabilityofthetemperature and therefore wavelength.The data in Figure 11 yields a temperature coecient forthis unetched grating of 0.011 nm perC, which when usingthewavelengthresolutionof 0.36 pmderivedearliergivesatemperatureresolutionofjust0.03C. Thisresolutioniseasilyequal tothatobtainablebymanyelectrical methodsandis mademoreattractiveas theoptical natureof themeasurement will not be compromised in electrically noisyenvironments whichmay degrade the signals fromPRTand thermistor type devices. Additionally, this temperaturemeasurement technique is entirely compatible with therefractive index sensing technology as well as being safe foruse in volatile or ammable environments.6. Real World Measurement TechniquesOne of the great challenges in creating a sensor technologyis the conversion from development laboratory experimentintoarobust, easytousepackagesuitableforreal worldapplications. The packaging needs to meet end-user require-ments andbe suciently robust toallowinstallationbyJournal of Sensors 910152025303540450 5000 10000 15000 20000 25000Data pointMeasured temperature (C)(a)0 5000 10000 15000 20000 25000Data point1531.351531.41531.451531.51531.551531.61531.651531.71531.75Bragg wavelength (nm)(b)Figure 10: Variation in temperature (left-hand graph measured using PRT) and corresponding Bragg wavelength (right-hand graph) overtime. Data points recorded at a rate of 2 Hz.1531.351531.41531.451531.51531.551531.61531.651531.71531.75Bragg wavelength (nm)14 19 24 29 34 39 44Measured temperature (C) = 0.011T + 1531.2R2 = 0.9998Figure11: Relationshipbetweentemperature andBraggwave-length. Note that over twenty ve thousand data points are plottedonthisgraphbut that highprecisiontemperaturemeasurementresults in data points being closely clustered.unskilled workers but retain the performance of the labora-tory prototypes. To this end, the sensor technology discussedabove has been integrated into a variety of applicationspecicpackageswhichallowtheliquidsforanalysistobeeasily delivered to the sensing region.6.1. High-FlowIndustrial ProcessControl FlowCell. Moni-toringpipelinesinaprocesscanbeaninvaluabletool forprotecting the overall process integrity in many applications.An unknown or uncontrolled concentration of reagentsenteringaprocessat anypoint mayseverelycompromiseor totally write-o a complete processing batch. In order toallowpipelinecontentstobemeasuredinsituandinrealtime, Stratophase has dened a series of sensor chip products(Figure 12)thathavebeendesignedintoarobuststainlessFigure 12: Industrial sensor cell for high-volume ow applications.steel unit designedforincorporationintohigh-ow, highpressure pipelines. The industrial process control (IPC) owcells are designed to accept standard 1/4

Swagelok ttingsand pipes and allow liquid pressures up to 6 bars to be used.6.2. Reaction Vessel Insertion Probe. Many applications, par-ticularly those requiring fermentation processes for pharma-ceutical,biofuel,orfoodandbeveragemustmonitortheirreactionsinlarge reactionvesselsorinmultiplebench-topbioreactors.Intheseapplications, aowcellcongurationis not always suitable as it requires a portion of theliquidtobetappedofromthemaintankandpumpedthroughaseparatesensingloop. Instead, insertionprobesare commonly used to allow sensors to be pushed throughthe walls or lids of the reaction vessels. A common solutionusesastandardform-factor,typicallywithaPg13.5threadtoallowsensors tobescrewedintostandardports ttedtothefermentersorreactors.Aprobe-housingcompatiblewithstandardportshasbeendesignedtoallowtheplanar10 Journal of SensorsFigure 13: Probe sensor for in-vessel monitoring applications.Bragg sensors discussedhere tobe usedinaninsertionprobe format (Figure 13). The probes may be used in staticor retractable housings andrequire onlyanoptical breconnection to access the sensor data.7. Example ApplicationsUsingtheinlineowcellandinsertionprobeformats,theplanar Bragg grating devices are capable of monitoring manyaspectsof processevolutionovermanyweeks. Whilst thefocus of this paper has sofar beenonthe sensitivitytorefractiveindex, it ismorecommontorequireaninsightinto changes in a process due to concentration variation orbiological growthratherthaninabsoluterefractiveindex.Eventhesmallestchangeinthepropertiesofaliquidwillresult ina refractive indexchange regardless of whetherthechangeisduetotemperature, concentration, orotherreactionevents. By measuring bothtemperature andRIsimultaneously and detailed picture of process evolution maybe obtained.7.1. Real Time Monitoring of FermentationProcesses. Theprocess of fermentationusing yeast to convert sugar toalcohol is as old as it is commercially important. Thepresenceofsugars,nutrients,andnalalcoholcontentarethe key factors of interest, and the variation of these may beobservedthroughrefractiveindexchange. Todemonstratethis capability the IPCowcell described above was plumbedinto a standard fermentation process using a pump tocirculatethefermentingliquidoverasensor. Trackingthevariation in Bragg wavelength over a number of weeks revealsclear events that correlate to distinct phases of the process.Figure 14 shows howthe sensor output varies with theprocessevolutionoverseveral weeks. Therefractiveindexoutputofthesensorisshownontheleft-handaxisandisplottedinunitsofnanometresandprovidesthechangeinBragg wavelength over time. These units are used rather thanconversion to refractive index units as it is overall insight intothefermentationprocessthatisdesirable,nottheabsoluterefractive index at any given moment. In contrast, the outputof the sensor that provides temperature has been calibratedinto degrees C.During the initial stages it is clear that the Braggwavelengthdrops steadily due tothe reductioninsugarconcentration as it is consumed by the yeast. Following thisinitial activity the rate of change of concentration decreasesas more and more of the sugar is consumed, and thesensor output indicates that a steady state has been achieved.Betweenabout500and700hoursofprocesstimeseveralningagentsareaddedtothebrewingliquidinordertoassist in separating out the yeast and from the liquid productof interest. Thiscausesstepchangesinbothpositiveandnegative directions as the decrease in suspended particles isreduced and as further chemicals are added, respectively. Inthenal stages, additives toimproveavourandmouth-feel are added to the liquid, causing a distinct change in thesensor output.The lower trace on the graph, which corresponds to theright-handaxis,showstheopticallymeasuredtemperaturevariationandallows events inthe liquidsensor trace to bedecoupled from temperature eects. For example, at about500hoursofprocessingtimeafaultintheenvironmentaltemperature controls caused an oscillation of several degreesinthetemperature. Thisisreectedintheupperfermen-tationtrace. Other key events inthe fermentationtraceareknowntoberealeectsoftheprocessandnotduetoenvironmental changesastheyarenotmirroredbythetemperature reference data.Thesestepsshowthattheprocessesmaybemonitoredcontinuouslyinreal time. Perhapsmoreimportantly, theyalso showthat additive or sugar concentration may bemeasured throughout the process such that remedial actionmay be immediately taken should the process deviate fromthe required path.7.2. Industrial Feedstock Monitoring. Another example appli-cationis inthe protectionof pharmaceutical productionfromcontamination of the feedstock. Asolvent that iscontaminated with another product may signicantly aectprocessyieldandeciency, ultimatelycostinglargesumsof money. Bymonitoringthesupplyof liquidspriorandthroughout the process, errant steps may be avoided.Figure 15showstheoutput of asensormountedintoan IPC ow block and connected to an ethanol supply line.Contamination of the ethanol with water at regular intervalscausestheclearstepsinsensoroutputtojump. Thelargechanges shown here make it clear that very small amounts ofwater contamination (approximately 0.1% by volume) maybe detected.8. SummaryInthis paper wehaveprovidedareviewof thecapabili-ties of planar waveguideBraggsensors. Startingwiththebackground physical principles we have shown how dierenttypes of refractive index sensor may be considered to operateoncommonprinciplesandthenindicatedtheadvantagesJournal of Sensors 11Time (hours)0 200 400 600 800FlavouringaddedMixingMixedFermentation in progress7891011122010010203040Temperature (C) RI output (nm) 90 50 100 150 2009.28.5785 790 795 800 805 8109.510.511.591011129.49.69.81010.210.410.610.811Figure 14: Sensor measurement of refractive index and temperature over several weeks during fermentation process. Close-up sections showthe initial consumption of sugars and the addition of chemical avouring.3.23.253.33.353.43.453.53.553.63.653.77600 8100 8600 9100 9600 10100 10600 11100Time (seconds)RI output1540 nm sensor1560 nm sensorWater content0%1% 2% 3% 4% 5% 10%Figure15: Sensor output as ethanol withsmall variable watercontent ows over sensing region.ofusingaBragggrating-baseddeviceusingamodal lightconnement approach. 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