35 NO 8 AUGUST 1999 ThreeDimensional Integrated Optics Using Polymers Sean M Garner SangShin Lee Vadim Chuyanov Antao Chen Araz Yacoubian William H Steier and Larry R Dalton Abstract Some of the key components are demonstrated to make threedimen ID: 31401
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IEEEJOURNALOFQUANTUMELECTRONICS,VOL.35,NO.8,AUGUST1999Three-DimensionalIntegratedOpticsUsingPolymersSeanM.Garner,Sang-ShinLee,VadimChuyanov,AntaoChen,ArazYacoubian,WilliamH.Steier,andLarryR.DaltonAbstractÐSomeofthekeycomponentsaredemonstratedtomakethree-dimensional(3-D)opticalintegratedcircuitspossibleusingpolymers.Fabricationtechniquesofshadowreactiveionetching,shadowphotolithography,andgray-levelphotolithog-raphytoproducecomplex3-Dintegratedopticstructuresaredemonstrated.Verticalwaveguidebendsexhibitexcesslosses Fig.1.Generaldesignof3-Dintegratedoptics.(Onlythewaveguidecoreisshownforclarity.)verticalwaveguideinterconnectsbetweenthelayers.Thethird 4vertical±horizontalstructure,andverticalwaveguidepo- etal.:THREE-DIMENSIONALINTEGRATEDOPTICSUSINGPOLYMERS (a) Fig.2.ShadowRIEtechniqueforslopeetching.(a)Schematicofetchingmethod.(b)Proleoftheetchedslope.photobleachingofpolymerswhichcontainchromophorescancreateadepth-dependentindexvariation[3]andhasbeenusedtofabricatetaperedwaveguides.Replicationmethodsofembossingandmoldingallowmassproductionofintegratedopticalcomponentsandhavebeenusedforlow-costmi-crostructures[4].Thesemethods,however,arenotsuitableforconstructingverticalwaveguideinterconnectsamongmultiplelayers.Tofabricate3-Dintegratedopticalstructures,weemployedreactiveionetching(RIE)inO andCF toetchlow-angleslopesinthepolymerlayers,eitherdirectlybyshadowmasksorthroughintermediatepatternedphotoresistA.ShadowMaskReactiveIonEtching(RIE)Thisapproachofdirectlyetchingaslopeinapolymerlayerinvolvesplacingashadowmaskonthepolymersurfacebeforeplacingthesampleintothereactingchamber.Fig.2illustratesboththeshadowRIEtechniqueandatypicalslopeasmeasuredwithaSloanDektakIIAprolometer.Thepresenceofthemaskoverhangproducesavariableetchrateacrossthesurfaceofthelm.Themasksconsistedofstandardmicroscopeslidessupportedattheedgesapproximately2mmabovethepolymerBycontrollingthegaspressure,RFpower,etchingtime,andphysicaldimensions(overhangoffsetandlength)ofthemask,predictableS-curvedverticalslopescanbeetchedbyusingcalibrationcurvesforeachparameter[5].Agoodempiricalcurvettothestretched-SshapedslopeproducedbytheRIEetchisprovidedby[5,eq.(1)].Thehorizontalandverticalfunctioncoordinatesare and ,while and aretheoverallslopeheightandlength,respectively: Theparameters and havebeenexperimentallymeasuredforavarietyofetchconditionsin[5].Forexample,toproducesmoothlyetchedsurfacesinthepolymerlms,weusedthefollowingconditionswithatypicalparallel-plateplasmaetcher:bothO andCF gases[6]at owratesof25and10sccm,achamberpressureof400mtorr,andanRFpowerdensityof0.15W/cm .A60-minetchtypicallyyields and valuesof5.5and1.4 m,respectively.Ingeneral,moredirectionaletches(higherpressure,etc.)andlargerspacingbetweenthemaskandthesurfacetobeetchedyieldlargerslopeanglesandshorterslopelengths.Withthismethod,wehaveproducedslopeanglesrangingfrom0.1 to3 withetchdepthsofupto15 m.Thisetchdepthwaslimitedonlybythethicknessofthepolymerlms.Slopelengthstypicallyrangedfrom0.1to2mm.Thiswidevarietyoftaperdimensionsenablesfastprototypingofnew3-Ddevicedesigns,anditavoidstime-consumingphotolithographysteps.B.PartialExposurePhotolithographyAnothermethodforproducingverticalstructuresinpoly-mersinvolvesrstcreating3-DfeaturesinaphotoresistoverlayerandthentransferringthemwithstandardRIEtech-niquestotheunderlyingpolymerlm.Toproducetheverticalfeatures,therststepreliesonthepartialexposureofthepho-toresisteitherbyshadowlithographyorgray-levellithographyandsubsequentdevelopmentofthephotoresist.TheverticalfeatureinthephotoresistcanbetransferredtotheunderlyingopticalpolymerlayerbyRIEbecausetheopticalpolymersandphotoresisthavesimilaretchrates,typicallyrangingfrom0.03to0.12 m/min.Insomecases,thereisaslightdifferenceinetchrateswhichenableexpandingorcontractingtheverticalfeatureswhiletransferringthemtotheopticalpolymers.Inthistechnique,thephotoresistlayermustbeasthickasorthickerthantheverticalheightoftheslopebeingtransferred.WeusedphotoresistAZ5214E(HoeschtCelanese)forlmthicknessesofupto3.5 mandAZP4620forlmsupto6 m.Fordeepertopographies,weeitherspunmultiplephotoresistlayersormademultipleprocessingcycles.Tofullydevelopthesethickphotoresistlayersrequiressomeagitationwhileinthedevelopersolution.Theverticalfeaturesinthephotoresistcanbeperiodicallymeasuredandpreciselycontrolledbeforepermanentlyetchingtheopticalpolymer.SincetheslopecharacteristicsdependonboththeexposureanddevelopmenttimeswitheitherAZ400KorAZ421Kdevelopers,thefeaturedimensionscanbenetunedafterDektakmeasurements.Iftheopticalpolymeriscrosslinkedorotherwisehardenedtoberesistanttosolvents,misalignedorpoor-qualityphotoresistpatternscanbeeasilyremovedandtheprocessrepeated.Finally,theuseofastandardmaskaligner(KarlSuss-MJB3)allowsprecisioninboththealignmentandexposureofthephotoresistpatterns.Useofeithertheshadoworgray-levelphotolithographymask,describedbelow,produceshigh-qualityverticalphotoresist1)ShadowPhotolithography:Shadowphotolithographyconsistsofusingastandardstraight-edgemaskpattern,verticallyoffsetfromthesample,toproduceavariableexposureoftheunderlyingphotoresist.Fig.3(a)illustratesthistechnique.Thevariableexposureisduetothediffractionandre ectionsoftheultraviolet(UV)light.Fig.3(b)shows IEEEJOURNALOFQUANTUMELECTRONICS,VOL.35,NO.8,AUGUST1999 (a) Fig.3.Shadowphotolithographytechniqueforslopeetching.(a)Schematicofetchingmethod.(b)Proleofetchedslope. Fig.4.Controlofslopeanglesusingshadowphotolithography.Measuredslopeangleasafunctionofmaskalignerexposingenergydensity.Datashownfortwophotoresistthicknesses,atypicalslopeproduced.Ithasaheightof4 mandalengthof20 m.Becauseofthecollimatedlightfromthemaskalignersource,theresultingslopespossesslargeanglesrangingfrom1 to13 inphotoresistthicknessesofupto m.Forthistechnique,wetypicallyusedtheAZ5214Ephotoresistbecauseoftheshorterexposuretimesitrequired.Theresultingverticalslopedependsontheexposingenergydensity,photoresistthickness,andmaskverticaloffset.Vary-inganyoftheseparametersallowscontroloftheslopeangle,andthisexposureprocessmustbecalibrated.Fig.4showsthedependenceofslopeangleonexposingenergydensityforphotoresistthicknessesof3.5and4.2 m.Aconstantmaskverticaloffsetof600 mwasused.Withanexposingpowerdensityof5mW/cm ,weeasilyproducedawiderangeofrepeatableanglesbyvaryingtheexposingtimefrom1to2min.Becauseshadowphotolithographyproducesavarietyofangleswithasinglemask,itenablesfastprototypingofnewdevicedesigns.2)Gray-LevelPhotolithography:Gray-levelphotolithog-raphyconsistsofusingavariabletransmissionmasktopartiallyexposethephotoresist.Fig.5(a)illustratesthismethod.Theposition-dependenttransmissionofthemaskproducesthevariableUVexposureofthephotoresist.Thistechniqueallowsthetransferofverycomplexpatternstotheentirephotoresistlmwithasingleexposure.Fig.5(b)showsatypicalslopeproduced.Ithasaheightof5.5 manda (a) Fig.5.Gray-levelphotolithographytechniqueforslopeetching.(a)Schematicofetchingmethod.(b)Proleofetchedslope.taperlengthof100 m.Thelowerbendangleonlyappearssharpbecauseofthedifferentscalesofthetwoaxes.Usingthismethod,weproducedslopeanglesrangingfrom0.1 to3 inlmthicknessesofupto15 m.Additionally,theuseofeitherAZ5214orAZP4620producedhigh-qualityslopes.Wefabricatedthetransmissionmaskbytransferringacomputer-generatedgray-scaleimageontoaholographiclmplate(AGFA8E56HDNAH)[7].Themasksprovidedfeatureresolutionofabout10±15 mandproducedsmoothphotore-sistslopeswithtotallengthsof0.1±2mm.Thefabricationoftheselongslopelengthsdidnotrequirehigherresolutiongray-levelmasks.Alternativemasksourcesinclude:commercialfabricatingfromaspatiallylteredhalf-toneimage[8],anddirectelectronbeamwritingonadosagesensitivesubstrate[9].Theadvantageofgray-levelphotolithographyliesintheabilitytocreatearbitraryandcomplexsurfacetopographiesinphotoresistlms.Fabricationof32leveldiffractiveopticalelementswithoneexposurehavebeenreported[7].Asinglecomplexmaskcouldbeusedtofabricatemanyverticalfea-turesinoneprocessingstepinthemanufacturingofcomplex3-Dopticalcircuits.C.FabricationSummaryInsummary,threespecicmethodsexistforcreatingverti-calstructuresinopticalpolymers.TheseincludeshadowRIE,shadowphotolithography,andgray-levelphotolithography.ShadowRIEgenerallycreatessmallslopeanglesof 3 .Itdoesnotrequireanyextensiveprocessingsteps,soitisidealforprototypingnewdevicedesigns.Shadowphotolithography,ontheotherhand,generallycreateslargerslopeanglesof ±13 .Theuseofthemaskaligner,however,allowsprecisealignmentandtimingoftheexposurethatisnotpossiblewiththeRIEtechnique.Finally,gray-levelphotolithographyallowsthemostfreedominverticalstructurefabrication.Themaskalignerandtransmissionofthegray-levelmaskenableprecisecontrolofthedevelopedphotoresistfeatures.AlthoughtheySee,forexample,SinePatternsLLC,EastRochester,NY. etal.:THREE-DIMENSIONALINTEGRATEDOPTICSUSINGPOLYMERSoffervaryingdegreesofprocessingcontrol,eachofthethreefabricationtechniquesenableverticalstructuresforpractical3-Dintegratedoptics.Therestofthispaperdiscussesafewrepresentativeandkeyopticalelementsforthe3-Dintegrationofopticalwaveguidedevices.III.APPLICATIONSOF3-DSTheabilitytofabricateverticalfeatures,particularlygentleverticalslopes,makespossiblethekeyelementstorealizecomplex3-Dintegratedoptics.Thekeysingle-modeelementsthatwehavedemonstratedareverticalwaveguidebends,powersplitters,andpolarizationsplitterstoprovideroutingoftheopticalpowerbetweenmultipleverticallevels.Wehavealsodemonstratedtheintegrationofpolymerelectroopticswitchesandmodulatorswithpassivepolymerintegratedopticsusingthe3-Dapproach.Inallofthesestructures,additionallayersmustbespunontopoftheverticallypatternedslopestomaketheupperwaveguidinglayers.Forslopeanglesofafewdegrees,spincastingadditionallmsof 5- mthicknessespreservestheoriginalsurfacecontour.Thickerlmsorsteeperanglescauseaplanarizationeffectofthesurfacefeatureswhichmustbetakenintoaccountinthedevicedesign.A.VerticalWaveguideBendsSimilartothe2-Dcase,verticalwaveguidebendsareanecessarycomponentforpractical3-Dintegratedopticsforthelow-losstransferofpowerbetweenthelayers.Verticaldirectionalcouplers[10]havepreviouslybeenusedtotransferpowerbetweendifferentwaveguidecores.Thesedevices,however,areverysensitivetowavelengthandfabricationtolerancesofthewaveguidedimensions.Thedesignandfabricationoftheverticalwaveguidebendsdescribedbelowprovidesthebasisformorecomplicatedstructuressuchasverticalpowerandpolarizationsplitters.1)Fabrication:Thefabricationofverticalwaveguidebendsinvolvedrstspincastingan11- mUV15LV(MasterBond)lowercladdinglayerontoaSisubstrate.Thisthicknessrequiredtwosubsequentspinning/curingcycles.Toachievehigh-qualitylms,theUV15LVmustpassthrougha0.2- lterbeforespincasting.Next,a1.6 slopewithaheightof3.0 mwasetchedacrosshalfofthesamplewiththemethodsdescribedinSectionII.Thenalstepsincludedspincastinga2.0- mNOA-73(Norland)guidinglayer,etchingawaveguidewitha0.3- mridge,andthenspincastinganadditional4.6- mUV15LVuppercladdinglayer.AlllmswereUVcured.Thiscreatesstandardstraightwaveguidesandverticalwaveguidebendsonthesamesamplewithwaveguidewidthsvaryingfrom1to6 m.Forthecoreandcladdingmaterials,therefractiveindicesat mare1.542and1.510,respectively.Cuttingthesampleswithadicingsawtoalengthof1cmprovidedhigh-qualityendfacesforbercoupling.Fig.6illustratesthewaveguidebendtestsample.2)ExperimentalResults:Toevaluatetheverticalwave-guidebends,polarizedlight( m)fromastandardsingle-modeopticalberwaslaunchedintothewaveguide.Theoutputwascollectedwitha60 microscopeobjective Fig.6.Verticalwaveguidebendtestsample.Three-dimensionalwaveguidebendsfabricatedadjacenttostraightwaveguides.(Onlythewaveguidecoreisshownforclarity.) Fig.7.Propagationlossasafunctionoflengthforaslabverticalwaveguidebendatm.(Discontinuitycorrespondstoverticalslopeposition.)lensandfocusedontoacalibrateddetector.Comparingtheoutputpoweroftheverticalwaveguidebendswiththeadjacentstraightwaveguidesdemonstratedanexcesslossvalueaslowas0.2dB.Excesslossisdenedastheratioofthepoweroutofaverticalbendwaveguidetothatofanadjacentstraightwaveguideusingthesamelightinputandoutputconditionsasmuchaspossible.Theaccuracyofthismeasurementdependsontherepeatabilityoftheinputandoutputconditionsandisestimatedtobe0.1dB.Wewereabletoconrmthislowexcesslossforsimilarlyfabricatedslabwaveguidebendsbyusingtheliquidout-couplingmeasurementtechniquedevelopedbyTeng[11].Inthismethod,lightiscoupledintoaslabmodeandisoutcoupledatthesurfaceofahighindexliquidasthesampleisimmersedintheliquid.Theslopeofaplotoftheoutputpowerasafunctionofthedepthofimmersiongivesthepropagationloss,whichisindependentoftheinputcouplingefciency.Fig.7showsthisplotforaslabwaveguidewithaverticalbend.The0.5-dBdiscontinuityinthepropagationlosscorrespondstothepositionoftheverticalbend.Theaccuracyofthismeasurementislimitedbythelinearcurvettingofthepower uctuations.Typically,thiserrorisestimatedtobe0.3dB.Fig.8showstheTEandTMpolarizationinsertionlossesforvariouswaveguidewidths.Theseinsertionlossesincludebercouplingattheinput,materialloss,andpropagationlossinthewaveguide.Weobservedlossesindependentofboththepolarizationandpropagationdirection.Allwaveguidesweredesignedtobesinglemodeat1.31 m.Finally,viewingthewaveguidepatternswithascanningelectronmicroscopeshowedhigh-qualityphotolithographydenitionthroughoutthestructureeventhoughthelevelsdiffersubstantiallyin IEEEJOURNALOFQUANTUMELECTRONICS,VOL.35,NO.8,AUGUST1999 (a) Fig.8.Insertionlossforverticalwaveguidebendsandstraightwaveguidesm(allwaveguidesare1cminlength).(a)TEpolarization.(b)TMpolarization.verticalheight.Two-dimensionalbeampropagationsimula-tions(RSoft)predictedradiationlossesofonly0.05dBwhenrepresentingthebendasacosineS-curve[12],[13].Wethereforesuspectthatthemeasuredexcesslossesaredominatedbyscatteringlosswhichcouldpossiblybereducedbyimprovementsintheetchingprocedure.Theverticalbendsarecharacterizedbytherelativelylargeindexdifferencebetweenthecoreandthecladding,typically 0.03.Becauseofthelargeverticalindexdifference,theverticalbendscanhavemuchsmallerturningradiithanischaracteristicofhorizontalbendswhenthehorizontalpattern-ingisdonewithridgewaveguides.Forexample,tomovethemodeup5 minaverticalbendrequiredonlya110- mtransitionlength.Thesmallerhorizontaleffectiveindexdifference,however,requiresatotalbendlengthof 1600 mtohorizontallydisplacetheopticalmodethesamerelativedistancewithanequivalentradiationloss.B.VerticalPowerSplittersAnalogoustoasymmetricY-branches,verticalpowersplit-tersdividetheopticalpoweramongmultipleverticallayers.Theirdesignconsistsofoptimizingthecouplingoftheinputopticalmodetothemodesoftheoutputwaveguides,main-tainingtheopticalisolationoftheoutputs,andreducingtheradiationlosswithinthebranchingregion.Todividetheopticalpowerarbitrarilyamongtheoutputwaveguides,thesplittermustactasanonadiabaticdevice.Nonadiabaticstructuresarecharacterizedbyrelativelylargebranchingangles.Theresultisthattheratiooftheoutputopti-calpowersdependsonthedifferenceinpropagationconstantsofthetwooutputwaveguidesandtheincorporatedbranchangle[14].Incontrast,adiabaticdeviceshaverelativelysmallbranchinganglesandtheinputpoweriscoupledcompletely Fig.9.AsymmetricY-branchdesignforverticalpowersplitter.Theoutputpowersplittingratiodependsonthepropagationconstantsforthebranchwaveguidesandthebranchingangletotheoutputbranchwhosepropagationconstantmostnearlymatchesthatoftheinputwaveguide[14].Theverticalpowersplitterswehavefabricatedarenona-diabatic,andthepowersplittingratioiscontrolledbythepropagationconstantsofthetwosingle-modeoutputwaveg-uides.Duringfabrication,thesearecontrolledbyvaryingthespincastlmthicknesses.ForaY-branchtohaveequalpoweroutputs,theinputmodemustcoupleequallytothesuper-modesofthebranchingregion.Forsymmetricstructureswheretheoutputshavethesamecouplingangle,thisrequiresequaleffectiveindicesinthetwobrancharms.ForasymmetricY-branchesasshowninFig.9,however,adifferenceinthepropagationconstantsmustcompensateforthegeometricasymmetrypresent.Todeterminethepowersplittingdependenceonthebranch-ingangleandpropagationconstants,weperformed2-Dbeampropagationsimulations.Wevariedtheangleandpropagationconstantsbychangingthetransitionlengthandwaveguidethicknesses,respectively.Inthefabricationprocedurede-scribedinSectionIII-B1,theinputandtheupperoutputwaveguideshavethesamethickness.Wethereforemadethatassumptioninouranalysis.Theindexvaluesusedcorre-spondedtoUV15LVandNOA-73,andtheoutputwaveguideseparationwasaconstant10 m.ThecontourplotsinFig.10showthesplittingratio,whichisdenedastheoutputpowerratiooftheuppertolowerwaveguides,asafunctionofbranchingangleandupperwaveguidethickness.Ineachplot,thelowerwaveguidethicknessisheldtoitsinitialvalue.Fromthecontourplots,threedesignfeaturesbecomeappar-ent.Firstofall,comparingthethreeplotsshowsthatahighertoleranceforfabricationerrorsexistsinthethickerwaveguidedesigns.Asthewaveguidethicknessincreases,theareaforeachsplittingvalue( 10%)becomeslarger.Second,3-dBpowersplitterspossessanoptimumbranchinganglevalueforxedwaveguidedimensions.Forlargerangles,theinputmodepredominantlycouplestothelowerwaveguidebecauseofthegeometricalasymmetryandhigherradiationlossintheupperbranch.Forsmallerangles,themodeadiabaticallyselectstheoutputwithahighereffectiveindex.Finally,changingtheupperwaveguidethicknessvariesthepowersplittingratiobyaffectingthedifferenceinwaveguideeffectivein-dices.Therefore,neadjustmentsofthetransitionlengthandwaveguidethicknessesallowcontroloftheverticalpower etal.:THREE-DIMENSIONALINTEGRATEDOPTICSUSINGPOLYMERS (a) (b) Fig.10.Y-branchpowersplittingratioasafunctionofupperbranchwaveguidethicknessandranchingangle.Lowerbranchwaveguidethicknessheldtotheconstantvalueindicatedforeachgraph.Thecontourlabelsindicatethepowersplittingratiovalues.(a)2-mlowerwaveguidethickness.(b)3-lowerwaveguidethickness.(c)4-mlowerwaveguidethickness.splittingratiothroughawiderangeofvalues.Waveguidethicknessescloseto3 mprovidethebestperformancecontrolwhenconsideringpracticalspincastthicknessesandmodeMaintainingopticalisolationoftheoutputwaveguides,anotherdesignconsideration,requiresaminimumthicknessoftheintermediatecladdinglayer.FortheUV15LVandNOA-73indexvalues,waveguidethicknessesof3 m,outputwaveguidelengthsof1cm,anda6.5- mverticalwaveguideseparationyieldsanisolationbetweentheoutputsof 30dB.Finally,verticalpowersplittersmustminimizeradiationloss.Themajorsourceofradiationlossistypicallytheradiationintothewaveguideslabmodesinthebranchingregion.Inthisregion,thewaveguideverticalthicknessbe-comesgreaterwhiletheridgeheightremainsconstant;thisreducesthehorizontalconnementandincreasesthecouplingtotheslabmodes.Topreventthis,wedesignedadualridge/channelwaveguidestructurewhichmaximizesopticalconnementthroughoutthestructure.Fig.11illustratesthedualridge/channelstructureandalsoshowscorecrosssections Fig.11.Verticalpowersplitterschematicandcorecrosssections.Crosssectionsshowthetransformationoftheinputridgewaveguideintoadualridge/channeloutputstructurewithhighmodalconnement.(Onlythewave-guidecoreisshownforclarity.)withinthebranchingregion.Thepresenceofboththeridgeandthechanneloftheupperandlowerwaveguidesinthejunctionallowsthecoreslabregiontoverticallyexpandwhilemaintainingthesameamountofhorizontalconnement.Experimentalresultsshownlatersupportthisconclusion.1)Fabrication:FabricationofverticalpowersplittersisillustratedinFig.12.TherststepinvolvespatterningaAu-coatedSiwaferwiththehorizontalwaveguidepatterntoprovidethenecessarymarksforaligningtheverticallystackedwaveguides.Thisallowshorizontalalignmentofthewaveguidesonmultipleverticallevelswithin 2 m.Next,a9.5- mUV15LVlowercladdingwasspun,and0.6- deep,6- m-widechannelwaveguidepatternswereetched.AfterspincastingNOA-73fora3.1- mlowercorethickness[Fig.12(a)]anda9.5- mUV15LVintermediatecladdinglayer,a0.5 slopewithadepthof12.6 mwasetcheddownthroughthedenedlowerchannel[Fig.12(b)].Wecontrolledtheetchdepthbymonitoringthelmthicknessinaportionoftheetchedregion.Basedonmonitoredlowercoreandslopedimensions,theuppercorethicknessnecessaryfora3-dBpowersplitterwasdeterminedfromthecomputeranalysis.Finally,a3.2- muppercorewasspincastanda0.4- mupperridgewaveguidepatternetchedintothelm[Fig.12(c)].Thistransformsthesingle-ridgewaveguideinputintoadualsingle-moderidge/channeloutputstructure,anditcausestheinputanduppercorewaveguidestohavethesamethickness.AlllmswereUVcured.Thenalstepsincludespincastinga muppercladdinglayerandcuttingtheendfaceswithadicingsawtoalengthof1.5cm.2)ExperimentalResults:Toevaluatetheverticalpowersplitterweusedthesameexperimentalsetupasthatfortheverticalbends.Launchinglightintothesinglewave-guideendshowedsingle-modeperformanceofthetwooutputwaveguides.Likewise,launchinglightintoeitherofthetwobranchwaveguidesexcitedonlyasinglemodeofthebasewaveguide.Fig.13showstheoutputpatternfromeachofthestructureends.Addingtheoutputpowerofthetwobranchwaveguidesshowedaninsertionlossof4.0 dB.Similarlyfabricatedstraight2-Dwaveguidesresultedininsertionlossesof4.0 0.2dB.Theseinsertionlossesincludeinputbercoupling,waveguidepropagation,andexcesslossduetotheverticalslope.Comparingtheseinsertionlossvaluesgivesanindicationoftheexcesslossduetotheverticalsplitting.Thislowexcesslossof 0.3dBexistedforboth IEEEJOURNALOFQUANTUMELECTRONICS,VOL.35,NO.8,AUGUST1999 (a)(b)(c)Fig.12.Fabricationprocedureofverticalpowersplitters.(a)Channelwaveguidedenedinlowercladdingandcore.(b)Middlecladdingspincastanverticalslopeetcheddownthroughlowercorelayer.(c)Uppercorespincastandridgewaveguidedenedthroughoutstructure. (a) Fig.13.Verticalpowersplitteroutputpatternsfor3-D12structureatm.(a)Singlewaveguideendoutput.Lightlaunchedintoeitherbranchwaveguide.(b)Dualwaveguidebranchedoutput.Lightlaunchedintosinglebasewaveguide.polarizations.Additionally,weobservednoscatteringfromabovewithaninfraredcamerawhenlaunching40mWintothewaveguides.Furthermore,performancedidnotdegradefor m-widewaveguideshorizontallymisalignedbyupto2 ascanbeseenfromtheoutputpatternofFig.13(b).Wemeasuredthepowersplittingratio(theratioofthepowerfromtheupperwaveguidetothatfromthelower)byfocusingtheoutputpatternonadetector50cmawayandusingaknifeedgetoblocktheoutputfromeithertheupperorlowerwaveguidecores.Wemeasuredtheoutputpowerratiotobe1.6 0.3.Simulationsbasedonmonitoredfabricationdimensionspredictedasplittingratioof1:1.Thedeviationfromtheexpectedoutputratiomayresultfromuncertaintyinthecorethicknesses,uncertaintyinthebranchingangle,andfromthedifferentphotolithographyandetchingconditionsthateachoutputbrancharmmayexperience.Tightercontrolofthefabricationconditionsshouldimprovethepredictability.VerticallystackingtwohorizontalY-branchesresultedina3-D1 4splitter.Thesingle-basewaveguiderstbranchedverticallyandthen,immediately,horizontallytoproducethefourbranchwaveguides.Fig.14showsthedeviceandtheoutputpatternsforboththesingleandbranchedwaveguide (a) (b)(c)Fig.14.Three-dimensional14powersplitterwaveguidedesignandoutputpatternatm.(a)14powersplitterdesignincorporatingoneverticalbranchandtwohorizontalY-branches.(Onlythewaveguidecoreisshownforclarity.)(b)Singlewaveguideendoutput.Lightlaunchedintoanyofthefourbranchwaveguides.(c)Branchedwaveguideendoutput.Lightlaunchedintosinglebasewaveguide.endsofthestructure.Combiningthemeasuredbranchoutputpowersshowedinsertionlossesof4.8 0.1dB.Forcom-parison,similarlyfabricated2-D1 2horizontalY-branchesresultedininsertionlossesof4.3 0.6dB.Thisincreasedinsertionlossofthe3-Dsplitterisduetotheextrawaveguidebranchingintheverticaldirection.Thefabricated1 splittershadpowervariationsamongthe4outputwaveguidesof33%.Zerovariationcorrespondstoa1 4splitterwithequalpowerinalloutputs.Asmeasuredinthefabricated2-D 2Y-branches,horizontalsplittingresultsinoutputpowervariationsof12%.Theslightlyincreasedinsertionlossandpowervariationofthe3-D1 4splittersmayresultfromradiationlossduetothecloseproximityoftheverticalandhorizontalwaveguidebranches,andthesemaybereducedwithlongerdevicelengths.Itisimportanttoconsiderhowfabricationerrorsaffectthepredictabilityofthepowersplittingratio.Theerrorscanbeinthebranchwaveguidedenition(thicknessandridge/channelheight)whichvariestheeffectiveindexoftheoutputguides,andtheerrorscanalsooccurintheslopedenition.From etal.:THREE-DIMENSIONALINTEGRATEDOPTICSUSINGPOLYMERSrepeatedmeasurements,webelievetheerrorsinlmthicknessandridge/channelheightcanbeheldto 0.1 m,andthevariationintheindexofrefractionofthepolymersistypically 10 .Fromabeampropagationanalysis,theseerrorscanresultina 13%variationinthepowersplittingratio.Thelargestsourceofthevariationisduetotherelativethicknessandridge/channelheightvariationoftheoutputwaveguides.Intheetchingoftheslope,boththeslopeangleanddepthoftheetchcouldvaryfromtheexpectedvalue.VariationsinslopeangleoccurduetoRIEorphotolithographyrepeata-bilitieswhenprocessing 10- mfeatureheights.Fromourmeasurements,anglestypicallyfallwithin 15%ofthetargetvalueandetchdeptherrorsare 0.1 m.Basedonabeampropagationanalysis,theseerrorscanresultina 7%variationinthepowersplitting,withthelargestvariationduetoslopeangleerrors.Theverticaldepthfabricationerrorindeningthesloperesultsinavariationoftheinputbasewaveguidethickness.Theslighteffectthisthicknesshasonthepowersplittingperformanceallowstheverticalpowersplitterstobebuiltwithouttheneedforanetchstop[15].Itmaybepossible,duringthefabricationprocess,tocom-pensateforaccumulatederrorsinthelowercoreorslopedenitionbytheappropriateadjustmentoftheuppercoredimensions.Theselmthicknessorslopeangleerrorscanbedetectedusingtheprolometer.Also,incorporationofbleach-ablechromophoresshouldallowinsitutrimmingofdeviceperformanceinawaysimilarto2-Dstructures[16],[17].C.VerticalPolarizationSplittersPolarizationsplittersareusedtoseparatetheorthogonalpolarizationcomponentsoftheguidedwaves.Theseareessen-tialcomponentsforcoherentreceiversandltersemployingapolarizationdiversitydesign.Three-dimensionalintegrationmayndapplicationinthesesystemstoreducethesubstratesizerequiredandaverticalpolarizationsplitterwouldbeakeycomponent.1)Design:Thedesignofverticalpolarizationsplitters[19]followscloselythatofthedevicesdescribedpreviously,buttheyrelyonthebirefringencesobtainableinpolymerstocreateapolarizationdependentsplitting.Fig.15(a)illustratesthisdevice.Itsoperationisbasedontheadiabatictransformationinthejunction.TheTEmodethenpropagatestotheoutputwiththehighestTEeffectiveindex,andtheTMmodepropagatestotheoutputwiththehighestTMeffectiveindex.Commerciallyavailablebirefringentandisotropicpolymerscreatetherequiredpolarization-dependentdifferenceinef-fectiveindex.ThebirefringentpolyimidesUltradel9020Dand7501(Amoco)composedthelowercladdingandcore,respectively.TheisotropicUV15LVandNOA-73madeuptheremainingcladdingandcorelayers,respectively.At m,9020Dhasanindexvalueof1.522and1.495forTEandTMmodes,respectively.7501hasindexvaluesof1.562and1.526forTEandTMpolarizations.Beampropagationsimulationsdeterminedtheoptimumwaveguidedimensionsandbranchingangle.Thefabricationprocesswassimilartothatfortheverticalpowersplitter.In[18],thedetailsofthedesign,fabricationprocedure,andamorecompleteanalysisoftheverticalpolarizationsplitterarereported. (a) (b)(c)(d)Fig.15.Verticalpolarizationsplitterdesignandoutputs.Alloutputpatternshavelightlaunchedintosinglebasewaveguideatm.(a)Generalwaveguidestructure.Polarizationextinctionratiodependsonwaveguidebirefringenceandbranchingangle.(Onlythewaveguidecoreisshownforclarity.)(b)OutputpatternwithanalyzersetforTEpolarization.(c)OutputpatternwithanalyzersetforequalTEandTMpolarizations.(d)OutputpatternwithanalyzersetforTMpolarization.2)ExperimentalResults:Lightat m,polarizedwithequalpowerintheTEandTMmodes,wascoupledintotheinputfromastandardopticalber.Theoutputpassedthroughapolarizerwitha50-dBextinctionandcollectedbya microscopeobjectivelensontothedetector.TheTEandTMcrosstalk,denedastheratioofthepowerintheexpectedpolarizationatanoutputtothepowerofthatpolarizationintheoppositeoutput,were17 5dBand13 4dB,respectively.Todemonstratethesingle-modeperformanceofthesplitter,wefocusedtheoutputpatternontoascreenandvieweditwithaninfraredcamera.Fig.15(b)±(d)showstheoutputwithdifferentanalyzersettings.TheTEmodeoutputisfromthelowerwaveguide,andtheTMisfromtheupperwaveguide.Weobservednohigherordermodeswhilevaryingthelaunchconditionsoftheinputber.D.3-DIntegrationofActiveandPassivePolymerDevicesOneoftheadvantagesofopticalpolymertechnologyistheabilitytousedifferenttypesofpolymerswithinthesameintegratedopticalcircuittoperformspecicfunctions.Forexample,electroopticpolymersorlightamplifyingpolymerscouldbeintegratedwithlowlosspassivepolymerswhichprovidethelowlossinterconnections.Oneexampleofthisistheintegrationofahighspeedelectro-opticpolymermodulatorwithaDragonewavelengthmultiplexer[19]fabricatedfrompassivepolymermaterials.Whiletheadhesionandpatterningproblemscansometimesbedifcult,thegreatestdifcultyisofteninachievinganopticalmodematchbetweenthewaveguidesmadefromdifferentpolymers[20].The3-Dconceptprovidesapromisingmethodtointegratedifferentpolymerswhileeasilysolvingthemodematchproblem.In IEEEJOURNALOFQUANTUMELECTRONICS,VOL.35,NO.8,AUGUST1999 Fig.16.Polymerelectroopticmodulatorintegratedontopoflow-losspas-sivepolymerwaveguide.(Claddinglayersarenotshownforclarity.)thisapproach,theinterconnectwaveguidepatternisrstfabricatedinalow-losspassivepolymersystem.Theactivepolymeristhenplacedontopofthislayerandpatternedintotheareawhereneeded.Verticalcouplingstructuresarethenfabricatedtochannelthelightupintotheactivepolymerandthenbackdownagainintothepassivewaveguides.Thestructuresdiscussedpreviouslyprovidethree-dimensionalroutingtotransferthebeambetweenthepassiveandactivepolymerlayers.Todemonstratethefeasibilityoftheapproach,wehaveintegratedapoledpolymermodulatorwithpassivepolymerwaveguides.1)Design:ThedesignofthepolymermodulatorintegratedontopofapassivewaveguideisshowninFig.16.Theuppercladdingimmediatelybelowtheupperelectrodeandthelowercladdingbelowthelowerelectrodearenotshownforclarity.Thepassivewaveguidewasdesignedtoprovideaclosemodematchtothestandard8- mcoreberandforbercoupling.Theetchedverticaltaperwasdesignedtoadiabaticallycouplelighttothehigherindexuppercorelayerwhichismadeofapoledelectroopticpolymer.Voltageappliedtotheelectrodeswillphasemodulatethelightor,ifconguredasaMach±Zehnderinterferometer,willamplitudemodulatethelight.Inthemodulatorsection,thepassivecorelayerservesasthemodulatorlowercladding.Aftermodulation,thepoweragaintransferstothepassivecoreforfurtherrouting.Boththeadiabaticslopesandthelowerelectrodeserveasinherentmodeltersinthedesigntominimizestraylightthatexistsinthedevice.Whilethemodeinthepassivewaveguidewasdesignedtobesymmetricforgoodbercoupling,themodeinthemodulatorwasdesignedtobetightlyconnedtotheactivelayerforgoodmodulatorefciency.In[21],thedesignconsiderationsandfabricationprocedurearereportedindetail.Fig.17(a)showsthenaldevicedimensionsandscaledcrosssectionsofthepassiveandactivewaveguidesegments.ThepassivecoreandcladdinglayersconsistedofNOA-73andUV15LV,respectively.Polyurethanecontainingatricyanochromophore[22]composedtheactiveuppercore.Thepolyurethanelayerwaspoledbyanelectriceldtoobtaintheelectroopticeffect.The6.5-cm-longdeviceswerefabricatedon3-inSiwafersasasubstrate.Thepassivecorewaveguidesinbothsectionsweredesignedtobesinglemodeat1.31 2)ExperimentalDemonstration:Anelectroopticphasemodulatorintegratedontopofapassivewaveguidewasdemonstratedat m.Theinputlightwasbutt (a) Fig.17.Integratedmodulatordimensionsandopticaloutput.(a)Schematicandscaledcrosssectionsofpassiveandactivewaveguidesegments.(b)OpticaloutputpatternwhenlightlaunchedintopassivewaveguidecoreatcoupledfromaberintothelowerwaveguideandcontainedequalamountsofTEandTMpolarization.ThenearcircularmodeoutputprolefromthelowerwaveguideisshowninFig.17(b).Thepolarizationoftheoutputwasmodulatedbylow-frequencysignalsappliedtotheelectrodesandthemodulationdetectedbyviewingtheoutputthroughacrossedpolarizer.Themodulationmeasuredcorrespondedtoanelectroopticcoefcientof pm/V.Boththecontrastobservedinthemodulationandtheinsertionlossofthedeviceindicatedthatessentiallyallofthetransmittedlightcoupledupintothemodulatorandbackdownagain.Anylightthatremainedinthelowerwaveguideishighlyattenuatedbythelowermetalelectrode.Wealsomeasuredtheinsertionlossofseveralsampleswithdifferentlengthsofpassiveandactivewaveguideregions.Estimatingthelossesinthepassiveandactivematerialtobe0.5and1.5dB/cm,respectively,wewereagainabletoconrmthatthelightcouplesalmostentirelyupintotheelectroopticpolymeranddownagain.Also,fromtheknownpropagationlossesinthepassiveandactivematerials,wewereabletoestimatethelossinthetransitionregiontobe 1dB.Frombeampropagationstudies,weexpecttheradiationlossesinthetaperstobesmalland,therefore,believethelossisduetoscatteringfromthesurfaceroughnessoftheetch.Betterfabricationtechniquesshouldreducethislosssignicantly.IV.CThisresearchshowsthat3-Dintegratedopticsinpolymercanbeaviableapproachtosignicantlyincreasingthedensityofintegratedoptics.Therecouldbenumerousapplications 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