定义：利用法拉第效应的光学隔离器。法拉第隔离器是光学隔离器的一种，在特定方向的光可以通过而相反方向的光则不能通过。法拉第隔离器利用了法拉第旋转器，并且包含了光学隔离器最重要的技术。另一种类似的器件是光环形器，具有三个光学端口。光从端口1进入，会从端口2出来，而从端口2进去的光会传到端口3，而端口3进去的光则从端口1出来。
偏振敏感的法拉第隔离器最简单的偏振敏感的法拉第隔离器是只有特定方向线偏振的入射光束才能通过。这里，准直的线偏振入射光先通过第一个偏振器（图1），损耗非常小，然后经过45°法拉第旋转器，再经过第二个偏振器，其偏振方向相比于第一个偏振器也旋转了45°，于是光通过时的损耗仍然很小。
图1：偏振敏感的法拉第隔离器装置图。双箭头表示前向和后向传播光束的偏振方向。
图2：不同功率下法拉第隔离器的选择。如果光经过反射后以相同的偏振态进入隔离器的出射端口，仍然可以无损耗的通过第二个偏振器。然后通过法拉第旋转器其偏振态又旋转了45°，因此光不能通过第一个偏振器，或者说光会进入另一个单独的出射端口。出射偏振器在光被反射回去并且偏振状态改变的情况下非常重要。如果法拉第旋转器的旋转角度偏离45°（由于制备误差或者工作于非设计的波长处），也可以调节输出偏振器的偏振方向来得到最大的透射，但是隔离度会减小。最好优化偏振器的指向来得到最大的隔离度，前向传播时存在一定的插入损耗是可以接受的。
偏振不敏感的法拉第隔离器偏振不敏感的法拉第隔离器的入射光束可以处于任意的偏振态。这一器件在光纤光学中会用到，因为大多数光纤不是保偏的。尤其是光纤通信系统中的光偏振态通常不是确定的，因此法拉第隔离器及其它器件需要工作于任意偏振态。偏振不敏感的隔离器的工作原理为：首先将入射光中正交的两偏振分量采用某种偏振器在空间上分离，然后进入法拉第隔离器，再在第二个偏振器中合在一起。
图3：偏振不敏感的法拉第隔离器的示意图，前向透射率很高，而后向传播光束则在空间上被分离开。如果是光纤耦合的器件，还需要在两边各放置一个透镜用来准直和聚焦光束。图3是光纤光学中常用的一种偏振不敏感隔离器。这里，偏振器为双折射楔，所用的材料为TiO2。后向传播的任意偏振态的光都可以透射出来，但是光束位置发生偏移（图3），因此不能进入到入射光纤中。这一器件可以制作的非常小，如图4：
图4：光纤耦合的法拉第隔离器。还存在具有类似外壳的法拉第镜，只具有一个光纤端口，可以使反射光的偏振态旋转90°。偏振不敏感隔离器不需要保持偏振态，因为两正交偏振分量间的相对相位是任意变化的。相位变化与温度和波长有关。
插入损耗和隔离度
高功率工作
当隔离器工作于很高的功率时，需要考虑下列效应：
不仅法拉第隔离器，偏振器也需要满足高功率工作的要求。商用高功率法拉第隔离器可以在功率高达100W时工作，并且光束失真也不严重。有些应用中，尤其是结合高功率光纤激光器和放大器的应用需要更高的功率，目前正在研发更先进的装置可以在高于1kW的功率情况下工作。对于光纤耦合的装置，对功率有更大的限制，通常需要在100W以下。
光纤法拉第隔离器
玻璃光纤的维尔德常数可以很高。采用这种光纤可以制作光纤法拉第隔离器。尽管采用目前的技术得到的器件性能还不能与体装置比拟，但是其对未来的光纤激光器光源非常有用。
应用
法拉第隔离器和环形器在激光器技术中有许多应用：
单纯的法拉第旋转器除了用做隔离器，还有很多其它的用途。
Definition:opticalisolatorsbasedontheFaradayeffect
Moregeneralterm:opticalisolators
AFaradayisolatorisakindofopticalisolator,i.e.,adevicewhichtransmitslightinacertaindirectionwhileblockinglightintheoppositedirection.FaradayisolatorsarebasedonFaradayrotatorsandconstitutethetechnologicallymostimportanttypeofopticalisolators.
Avariantisanopticalcirculator,havingatleastthreeopticalports.Lightinjectedintoport1willexitatport2,whileinputatport2willbesenttoport3,andinputatport3toport1.
ThesimplesttypeofFaradayisolatorispolarization-sensitiveinthesensethatitworksonlywhentheinputbeamhasaprescribeddirectionoflinearpolarization.Here,aproperlypolarizedandcollimatedinputbeampassesafirstpolarizer(pol1inFigure1)withminimumloss,thena45°Faradayrotator,andfinallyanotherpolarizer(pol2)withitstransmittingaxisbeingrotatedby45°,suchthatthetransmissionlossesaresmall.
Figure1:Setupofapolarization-sensitiveFaradayisolator,containingaFaradayrotatorbetweentwopolarizers.Thedoublearrowsindicatethepolarizationdirectionsofaforwardandbackward-propagatingbeam.
Figure2:AselectionofFaradayisolatorsfordifferentpowerlevels.ThephotographwaskindlyprovidedbyGMPSwitzerland.
Whenlightisreflectedbacktotheoutputportoftheisolatorwithanunchangedpolarizationstate,itcanfullytransmittheoutputpolarizer(pol2).Then,however,itspolarizationdirectionisrotatedbyanother45°intheFaradayrotator,sothatthislightwillbeblockedattheinputpolarizer,orcanbesenttotheseparateoutputport.
Notethattheoutputpolarizer(pol2)isimportantiflightmaybereflectedbackwithamodifiedpolarizationstate.
IftherotationangleoftheFaradayrotatorsomewhatdeviatesfrom45°(eitherduetofabricationerrorsorduetooperationawayfromthedesignwavelength),theorientationoftheoutputpolarizermaystillbeadjustedformaximumtransmission,butinthatcasethedegreeofisolationisreduced.Itmaybebettertooptimizethatpolarizer'sorientationformaximumisolation,whileacceptingasomewhathigherinsertionlossinforwarddirection.
Apolarization-insensitiveFaradayisolatorisadevicewhichworksforarbitrarypolarizationoftheinputbeam.Suchdevicesareoftenrequiredinthecontextoffiberoptics,becausemanyfibersarenotpolarization-maintaining.Inparticular,opticalfibercommunicationsystemsareusuallyoperatedwithundefinedpolarizationstate,andFaradayisolatorsaswellasothercomponentsarethenrequiredtoworkwitharbitrarypolarizationstates.
Thebasicprincipleofapolarization-insensitiveisolatoristofirstspatiallyseparatetheorthogonalpolarizationcomponentsoftheinputbeamwithsomekindofpolarizer,thensendboththroughaFaradayrotator,andcombinethemagaininthesecondpolarizer.
Figure3:Setupofapolarization-insensitiveFaradayisolator,withhightransmittanceinforwarddirection(top)andaspatialoffsetforbackwardpropagation(bottom).(Thebeamanglesareexaggeratedforbetterclarity.)Inafiber-coupleddevice,thereisanadditionallensoneachsideforcollimatingandre-focusingthebeam.
AtypeofdeviceasfrequentlyusedinfiberopticsisshowninFigure3.Here,thepolarizersarebirefringentwedges,madee.g.ofrutile(TiO2).Backward-propagatinglightofanypolarizationdirectioncanbetransmitted,butwillappearwithanoffsetbeamposition(Figure3,bottom),andwillthusnotbelaunchedbackintotheinputfiber.Suchdevicescanbefittedintorathercompactpackages,asshowninFigure4.
Figure4:Afiber-coupledFaradayisolator.ThephotographwaskindlyprovidedbyMicrowavePhotonicSystems,Inc.TherearealsoFaradaymirrorsinsimilarhousings,havingonlyonefiberportandexhibitinga90°polarizationrotationforreflectedlight.
Notethatapolarization-insensitiveisolatorwillingeneralnotpreservethepolarizationstate(orsimplyrotateit),sincethereisanarbitraryrelativephasechangebetweenthetwopolarizationcomponents.Thatphasechangedependsontemperatureandwavelength.
Formanyapplications,alowinsertionloss,i.e.,alowlossforforward-propagationlight,isessential.Theinsertionlossisdeterminedbyseveralfactors:thetypeandqualityofthepolarizersused,residualreflectionlossesonanti-reflectioncoatings,andtheprecisionanduniformitywithwhicha45°rotationangleisobtainedintherotator.
Anotherimportantperformancefigureofanopticalisolatoristhedegreeofisolation,i.e.,theattenuationachievedforback-reflectedlight,alsocalledreturnloss.TypicalFaradayisolatorsachieveareturnlossoftheorderof30to40dB.Ahighdegreeofisolationisgenerallymoredifficulttoachieveforhigh-powerdevices,wherethebeamintheFaradaymediumcoversarelargerareaandisthusmoresensitivetofieldinhomogeneities.Thequalityandalignmentofthepolarizersusedisalsoimportant.Alowerdegreeofisolationmayresultfromoperationatanon-optimizedwavelength,fromimproperalignmentorlargedivergenceoftheinput,orfromthermaleffectswhentheisolatorisoperatedwithahighopticalaveragepower.
Ifthereturnlossachievablewithasingleisolatorisinsufficient,acombinationoftwo(orevenmore)isolatorsmaybeused.Therearedevicesavailablewheretwoisolatorsarepackagedintoonehousing(double-stageisolators).Suchacombinationmayalsobeusedtowidentheopticalbandwidthwithsufficientisolation.
Some“tunable”isolatorsallowtheusertoadjusttheangularorientationofthepolarizers,andthustooptimizetheisolationfordifferentwavelengths.Inthatway,asingledevicemaybeusedtocoverabroadwavelengthregion,e.g.from500nmto750nm,limitedonlybythebandwidthoftheanti-reflectioncoatings.Foroptimizingthebandwidthofafixedsetting(asrequirede.g.forusewithatunabletitanium–sapphirelaser),onerequiresaFaradayrotatorbasedonamediumwithalowrelativewavelengthdependenceoftheVerdetconstant.
Insomecases,asufficientlyhighdegreeofisolation(e.g.60dB)canbeachievedonlywithadual-stageisolator.Essentially,thismeansthatoneusestwoFaradayisolatorsinsequence;ofcourse,onlyasinglepolarizerisneededbetweenthetworotators.
Thetwoisolatorsmayeitherrotatethepolarizationinthesamedirection,resultinginatotalrotationangleof90°,ortheyrotateinoppositedirections,generatingnonetrotation.
Theinsertionlosswilltypicallybesomewhathigherwithadual-stagedevice.
Foroperationofanisolatoratveryhighopticalpowerlevels,variouseffectshavetobeconsidered:
Ofcourse,notonlytheFaradayrotator,butalsothepolarizershavetobesuitableforhigh-poweroperation.
Commerciallyavailablehigh-powerFaradayisolatorscanhandleopticalaveragepowersuptotheorderof100Wwithnottoostrongbeamdistortions.Particularlysomeapplicationsincombinationwithhigh-powerfiberlasersandamplifiersdemandhigherpowers,andmoreadvanceddevicesforpowersoftheorderof1kWappeararebeingdeveloped.Forfiber-coupleddevices,thepowerlevelsaremorelimited,usuallytowellbelow100W.
ItispossibletofabricateopticalfibersfromglasseswithahighVerdetconstant[1,4,10].Withsuchfibers,onecanmakeentirelyfiber-based(notonlyfiber-coupled)Faradayisolators.Althoughtheperformanceachievedsofarwiththatmethoddoesnotreachthatofbulkdevices,thisdevelopmentmaybeusefulforfuturefiberlasersources.
Faradayisolatorsandcirculatorsfindmanyapplicationsinlasertechnology:
TherearealsovariousapplicationsofpureFaradayrotators,notfunctioningasisolators.