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Figure5:Reflectionpatternsusingvariousconnectors(reducedFresnelmagnitudesinsideyellowbox)IntroductionOpticalTimeDomainReflectometry(OTDR)isacommontechniquefordetectingdamageinfiberopticcables.Theprocessinvolvestransmittingapulseoflightdowntheopticalfiber,analyzingtheamountoflightreflectedbacktothesource,anddisplayingthereflectionpatternsontheOTDRscreen.ConnectorTypeTheindexofrefractionofthepatchvs.thetestfiberwasalloweddifferbyupto10%,whichcreatedamismatchatthejunctionofthetwofibers.Fourtypesofconnectorsweresimulatedtodeterminewhichproducedthelowestreflectionmagnitude.PlottingthereflectionresponsepatternsfromallfourconnectiontypesshowsthattheAngledPhysicalContactconnectorproducedthelowestreflection(seeFigure6).Thoughmuchlessexpensive,IndexMatchingFluidonlyhasalifetimeof2years.Mostopticalfiberapplicationsrequire10yearslifeormore[3].References[1]Sadiku,N.O.Matthew.NumericalTechniquesinElectromagnetics[2]Newton,StevenA.NovelApproachestoOpticalReflectometry[3]Knapp,John.CharacterizationofFiber-OpticCablesUsinganOpticalTimeDomainReflectometer(OTDR)FiberopticcharacterizationusingasimulatedOpticalTime-DomainReflectometer(OTDR)RobbP.MerrillDepartmentofElectricalandComputerEngineering-UniversityofUtahDuringcharacterizationofshortfiberopticcablesofapproximately1meter,Fresnelreflectionsposeaseriouschallengetoaccuratedamagedetection.TheFresneltailobliteratesanysmallreflectionsthatareproducedbydamagedsectionsofcable,andthedamageisoverlooked.SimulationMethodTheFiniteDifferenceTimeDomainmethod[1]wasimplementedinMATLABtosimulateapulseoflighttravelingthroughthepatchandtestfibers.ThefollowingparametersusedinthesimulationwereobtainedfromanactualOTDRsystem:Indexofrefraction(n)oftestfiber=1.4525,Wavelength(λ)oflightpulse=850nanometers[3].PulseDuration1Abnormalitiesinthefiber,suchasbends,cracks,connectors,andotherabruptchangesintherefractiveindexcreatereflectionspikescalledFresnel(“Fre'-nel”)reflections[2].Afteraspikeisdetected,asignificantdelayoccurswhenthereflectometer‘settlesdown’fromitssaturatedstate.ThisdelayiscalledaFresneltail(Figure1).Figure3:SimulatedFresnelTailskews,thenobliterates,thedamagereflectionatlargerdurationsFigure4:Commontypesoffiberopticconnectorswithrelativereflectionmagnitudesshown11.522.533.544.5500.0050.010.0150.020.0250.030.035OTDRSaturationatIncreasedPulseDurationsTravelDistancefromSource(m)ElectricField(V/m)1second2seconds3seconds22.533.5051015x10-3TravelDistancefromSource(m)ElectricField(V/m)IdealReflectionCharacteristics(NoOTDRSaturation)Figure1:OTDRscreenshotshowingreflectionspikefromcableconnector,andresultingFresneltail(areamarkedbybracket)Figure2:Simulatedidealresponseshowingfiberdamage(smallreflectionbumps).DamageisvisiblebecausenoFres-neltailispresent.TodeterminetheeffectofthelightpulsedurationonthesaturationleveloftheOTDRunit,oneperiodofaraisedcosinepulsewastransmittedthroughthefiberatvariousfrequencies.Apulsedurationof1microsecondprovedtobethemostfavorablyresponsivefortheparametersofthesimulation(seeFigure3).Inreal-worldapplication,however,thedurationmustactuallybesmallerduetotherelativelyslowsimulationspeedvs.thephysicalspeedoflight.SummaryShortfiberopticcablespresentmanychallengesthatmustbeovercomeinordertoaccuratelydetectfiberdamageusingOTDR.Pulsedurationsshorterthan1microsecond,andAngledPhysicalContact(APC)fiberconnectorsarerecommendedtoprovidethegreatestreductioninFresnelreflection.ByperformingOTDRsimulations,anopticalsystemsengineercouldunderstandthebehaviorofafibernetworkanddetectpotentialproblemsbeforeactualproduction.