RF-and-Microwave-Fiber-Optic-Design-Guide

RFandMicrowaveFiber-OpticDesignGuideApplicationNoteApril2001IntroductionAgereSystemsInc.,throughitspredecessors,begandevelopingandproducinglasersanddetectorsforlin-earfiber-opticlinksnearlytwodecadesago.Overtime,theseoptoelectroniccomponentshavebeencontinu-allyrefinedforintegrationintoavarietyofsystemsthatrequirehighfidelity,highfrequency,orlong-distancetransportationofanaloganddigitalsignals.Asaresultofthiswidespreaduseanddevelopment,bythelate1980s,theselinkproductswereroutinelybeingtreatedasstandardRFandmicrowavecomponentsinmanydifferentapplications.Thereareseveralnotableadvantagesoffiberopticsthathaveledtoitsincreasinguse.Themostimmediatebenefitoffiberopticsisitslowloss.Withlessthan0.4dB/kmofopticalattenuation,fiber-opticlinkssendsignalstensofkilometersandstillmaintainnearlytheoriginalqualityoftheinput.Thelowfiberlossisalsoindependentoffrequencyformostpracticalsystems.Withlaseranddetectorspeedsupto18GHz,linkscansendhigh-frequencysignalsintheiroriginalformwithouttheneedtodownconvertordigitizethemforthetransmissionportionofasystem.Asaresult,signalconversionequipmentcanbeplacedinconvenientlocationsoreveneliminatedaltogether,whichoftenleadstosignificantcostandmaintenancesavings.Savingsarealsorealizedduetothemechanicalflexibil-ityandlightweightfiber-opticcable,approximately1/25theweightofwaveguideand1/10thatofcoax.Manytransmissionlinescanbefedthroughsmallconduits,allowingforhighsignalrateswithoutinvestinginexpensivearchitecturalsupports.Theplacementoffibercableisfurthersimplifiedbythenaturalimmu-nityofopticalfibertoelectromagneticinterference(EMI).Notonlycanlargenumbersoffibersbetightlybundledwithpowercables,theyalsoprovideauniquelysecureandelectricallyisolatedtransmis-sionpath.Thegeneraladvantagesoffiber-opticsfirstledtotheirwidespreaduseinlong-hauldigitaltelecommu-nications.Inthemostbasicformoffiber-opticcom-munications,lightfromasemiconductorlaserorLEDisswitchedonandofftosenddigitallycodedinformationthroughafibertoaphotodiodereceiver.Bycomparison,inlinearfiber-opticsystemsdevel-opedbyLucent,thelightsentthroughthefiberhasanintensitydirectlyrelatedtotheinputelectricalcur-rent.Whilethisplacesextrarequirementsonthequalityofthelasersandphotodiodes,ithasbeenessentialinmanyapplicationstotransmitarbitraryRFandmicrowavesignals.Asaresult,tensofthou-sandsofAgereSystems’transmittersarecurrentlyinuse.Theinformationofferedhereexaminesthebasiclinkcomponents,providesanoverviewofdesigncalcu-lationsrelatedtogain,bandwidth,noise,anddynamicrangeanddistortion.Asectiononfiber-opticcomponentsdiscussesanumberofkeyparameters,amongthemwavelengthandloss,dis-persion,reflections,andpolarizationandattenua-tion.Additionalinformationevaluatesopticalisolators,distributed-feedbacklasersandFabry-Perotlasers,predistortion,andshort-vs.long-wave-lengthtransmission.TableofContentsAgereSystemsInc.ContentsPageFigurePageApplicationNoteRFandMicrowaveFiber-OpticDesignGuideApril2001Introduction............................................................................1BasicLinkApplicationsandComponents............................3TypicalLinearLinkApplications........................................3TypicalLinearLinkComponents.......................................3OpticalTransmitter............................................................4OpticalReceiver...............................................................5Fiber-OpticCable.............................................................5LinkDesignCalculations......................................................5Gain..................................................................................6DoublingtheOpticalLossTerm....................................7ResistivelyMatchedComponents.................................7ReactivelyMatchedLink...............................................8Bandwidth.........................................................................8Noise.................................................................................9LaserNoise...................................................................9ShotNoise..................................................................10ReceiverNoise............................................................10TotalLinkNoise..........................................................11CascadingNoiseFigures............................................12UnconvertedNoiseandSNR......................................12Noise-EquivalentBandwidth.......................................13DynamicRangeandDistortion.......................................131dBCompressionPoint.................................................13Third-OrderInterceptandSpur-FreeDynamicRange13LargeNumberofCarriers...........................................15PlacementofAmplifiers..............................................15Example..........................................................................15TransmitterandReceiverChoice...............................15Gain............................................................................16Noise.........