Polarization Optimized PMD Source Applications …A PMD source is also necessary to evaluate the...
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INSTRUMENTS MODULES OCT PRODUCTS SPECIAL POLARIZATION COMPONENTS ACCESSORIES APPLICATION GUIDE FAQS INSTRUMENTS -197- General Photonics Corporation 909.590.5473 www.generalphotonics.com PMD mitigation in 40Gb/s systems As the bit rate of fiber optic communication systems increases from 10 Gbps to 40Gbps, 100 Gbps, and beyond, polarization mode dispersion (PMD) has more and more impact on system performance. PMD generally causes the two principal polarization components of a light signal to travel at different speeds and hence spreads the bit-width of the signal, as shown in Figure 1A. Consequently, it causes an increase in bit-error rate (BER) and service outage probability. Unlike other system impairments, such as chromatic dispersion (CD), the PMD effect on the system is random in nature and changes rapidly with time, making it difficult to mitigate. Figure 1 Illustrations of the PMD effect on an optical signal. A) The SOP of the input signal is aligned 45 degrees from the PSP of the fiber link, causing the worst-case signal distortion. In this case, if the DGD is larger than the width of one bit, then the DOP=0 because the two orthogonal polarization components have the same power, with no phase relationship. B) and C) The SOP of the input signal is aligned with the slow or fast PSP axis, respectively. In these cases, no signal distortion occurs except the slight late or early arrival, respectively, of the pulse. The DOP of the signal remains at 1. In all 3 cases, the PMD of the link remains the same, but the effect on the signal is different due to the different input polarization states. Several methods with varying success rates exist to mitigate PMD effects for 40G deployment. The first is to select fiber routes with low PMD coefficients. Such a method requires extensive survey of all available fiber routes to identify and cherry-pick those with sufficiently low PMD. However, such an approach has three potential drawbacks: First, the PMD of a fiber route changes with time as the environmental conditions around the fiber change, including land movements caused by earthquake, flood, or mud- slides. A route that is good today may become bad in the future. Second, interconnected fiber routes may always have bad fiber sections with high PMD, rendering the selection of low PMD fiber routes throughout the system impractical. Finally, the supply of fiber routes with low PMD values will eventually be exhausted as 40G deployments increase. The second method is to use bandwidth efficient modulation formats, such as DPSK or DQPSK. Such modulation formats reduce the effective bandwidth of a 40Gb/s channel to that of a 20Gb/s or even a 10Gb/s channel. Consequently, the impact of Polarization Optimized PMD Source Applications PMD on the signal is greatly reduced, though never eliminated. The third approach is electronic PMD compensation, in which forward error correction (FEC) or other algorithms are used to reduce the PMD effect on the signal after the optical signal is converted into an electrical signal. However, because the electrical signal does not contain the phase information of the corresponding optical signal, the resulting PMD effect reduction is limited. Optical PMD compensation is another attractive approach. An optical PMD compensator generally contains one or more polarization controllers, a PMD generation device, a PMD monitoring device, and a microprocessor based circuit, as shown in Figure 2. The circuit receives an error signal from the PMD monitoring device and instructs the PMD generation device to adjust the PMD values and the polarization controller to adjust the polarization state input to the PMD generation device to minimize the error signal from the PMD monitoring device. The PMD is compensated when the PMD generated by the PMD generating device is the same in value as that of the fiber link, but with its slow axis aligned with the fast axis of the fiber link. Figure 2 Optical PMD compensation. The DOP measured by the polarimeter is used as PMD effect indicator. The final approach is polarization multiplexed coherent detection. Because the detected electrical signals in this approach contain all of the amplitude, phase, and polarization information of the corresponding optical signal, any optical signal impairments, including PMD, can be compensated by digital signal processing. In addition, because of the quadrupled spectrum efficiency resulting from QPSK modulation and polarization multiplexing, the effective optical signal bandwidth of a 40G channel is about the same as that of a 10G TDM channel. Consequently, the PMD effect on such a narrow band signal is also greatly reduced even without PMD compensation. Although transceivers incorporating polarization multiplexed coherent detection may still not be cost-effective compared with other approaches for 40Gb/s transmission, they are most attractive for 100Gb/s and higher speed transmissions. Instruments required for PMD related tests PMD source for PMD tolerance test No matter which approach is taken, it is desirable to have an instrument capable of testing the performance of different methods in mitigating PMD effects. This test is called a PMD tolerance test. The general setup is shown in Figure 3A. The key instrument in this setup is the PMD source, which can generate precise 1st and higher order PMD values. The bit-error rate (BER) of the system, or another such performance indicator, is measured as the 1st order PMD (DGD) values generated by the PMD source are gradually increased until the BER reaches
Transcript of Polarization Optimized PMD Source Applications …A PMD source is also necessary to evaluate the...
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PMDmitigationin40Gb/ssystems
As the bit rate of fiber optic communication systems increases from 10 Gbps to 40Gbps, 100 Gbps, and beyond, polarization modedispersion (PMD)hasmoreandmore impactonsystemperformance. PMD generally causes the two principalpolarization components of a light signal to travel at different speeds and hence spreads the bit-width of the signal, as shown inFigure1A. Consequently, it causesan increase inbit-errorrate (BER)andserviceoutageprobability. Unlikeothersystemimpairments,suchaschromaticdispersion(CD),thePMDeffecton the system is random in nature and changes rapidly with time, making it difficult to mitigate.
Figure1 IllustrationsofthePMDeffectonanopticalsignal. A)TheSOPoftheinputsignalisaligned45degreesfromthePSPof the fiber link,causing theworst-casesignaldistortion. Inthiscase, if theDGD is larger than thewidthofonebit, thentheDOP=0becausethetwoorthogonalpolarizationcomponentshavethesamepower,withnophaserelationship.B)andC)TheSOPoftheinputsignalisalignedwiththesloworfastPSPaxis,respectively. In thesecases,nosignaldistortionoccursexcepttheslightlateorearlyarrival,respectively,ofthepulse.TheDOPof thesignal remainsat1. Inall3cases, thePMDof the linkremains the same, but the effect on the signal is different due to the different input polarization states.
SeveralmethodswithvaryingsuccessratesexisttomitigatePMDeffectsfor40Gdeployment.ThefirstistoselectfiberrouteswithlowPMDcoefficients. Suchamethodrequiresextensivesurveyof all available fiber routes to identify and cherry-pick those with sufficiently lowPMD. However, suchan approachhas threepotentialdrawbacks:First,thePMDofafiberroutechangeswithtime as the environmental conditions around the fiber change, including land movements caused by earthquake, flood, or mud-slides. A route that is good today may become bad in the future. Second, interconnected fiber routesmayalwayshavebad fibersectionswithhighPMD,renderingtheselectionoflowPMDfiberroutes throughout thesystemimpractical. Finally, thesupplyoffiberrouteswith lowPMDvalueswilleventuallybeexhaustedas40G deployments increase.
The secondmethod is to usebandwidth efficientmodulationformats, suchasDPSKorDQPSK. Suchmodulation formatsreduce the effective bandwidth of a 40Gb/s channel to that of a 20Gb/sorevena10Gb/schannel. Consequently, the impactof
PolarizationOptimizedPMDSourceApplications
PMDonthesignalisgreatlyreduced,thoughnevereliminated.
The thirdapproach iselectronicPMDcompensation, inwhichforwarderrorcorrection (FEC)orotheralgorithmsareused toreduce thePMDeffecton thesignalafter theopticalsignal isconvertedintoanelectricalsignal.However,becausetheelectricalsignal does not contain the phase information of the corresponding opticalsignal,theresultingPMDeffectreductionislimited.
OpticalPMDcompensation isanotherattractiveapproach. Anoptical PMD compensator generally contains one ormorepolarizationcontrollers,aPMDgenerationdevice,aPMDmonitoringdevice,andamicroprocessorbasedcircuit,asshown inFigure2.Thecircuitreceivesanerrorsignal fromthePMDmonitoringdeviceandinstructsthePMDgenerationdevicetoadjustthePMDvalues and the polarization controller to adjust the polarization state input to thePMDgenerationdevice tominimize theerrorsignalfromthePMDmonitoringdevice.ThePMDiscompensatedwhenthePMDgeneratedbythePMDgeneratingdevice isthesameinvalue as that of the fiber link, but with its slow axis aligned with the fast axis of the fiber link.
Figure2OpticalPMDcompensation.TheDOPmeasuredbythepolarimeterisusedasPMDeffectindicator.
Thefinalapproachispolarizationmultiplexedcoherentdetection.Because thedetectedelectricalsignals in thisapproachcontainall of the amplitude, phase, and polarization information of the corresponding optical signal, any optical signal impairments, includingPMD,canbecompensatedbydigitalsignalprocessing.In addition, because of the quadrupled spectrum efficiencyresulting fromQPSKmodulationandpolarizationmultiplexing,the effective optical signal bandwidth of a 40G channel is about thesameasthatofa10GTDMchannel.Consequently,thePMDeffect on such a narrow band signal is also greatly reduced even withoutPMDcompensation.Althoughtransceiversincorporatingpolarization multiplexed coherent detection may still not be cost-effective compared with other approaches for 40Gb/s transmission, they are most attractive for 100Gb/s and higher speed transmissions.
InstrumentsrequiredforPMDrelatedtests
PMDsourceforPMDtolerancetest
Nomatterwhich approach is taken, it is desirable to havean instrument capable of testing the performance of different methods inmitigatingPMDeffects. This test iscalledaPMDtolerancetest.ThegeneralsetupisshowninFigure3A.Thekeyinstrument in thissetup is thePMDsource,whichcangenerateprecise1st andhigherorderPMDvalues. Thebit-error rate(BER)of the system,or another suchperformance indicator,ismeasuredas the1storderPMD(DGD)valuesgeneratedbythePMDsourcearegradually increaseduntil theBERreaches
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is the self correlation function of the optical signal, which decreases as increases. Clearly, theDOPreachesaminimumvalue of when =½orwhen the inputSOP is45degreesfromthePSPoftheDGDmedium.AtthisSOP,thePMDhas its worst effect on the quality of the signal. Conversely, when =1orwhentheinputSOPisalignedwiththePSP,theDOPalwaysremainsatitsmaximumvalueof1,regardlessoftheDGDvalue.
When higher orderPMD is present, thePSP is different fordifferentwavelengthcomponentsofthesignalandEq.(1)maynolongerhold. Nevertheless, therealwaysexistsan inputSOPforwhichtheDOPofthesignalattheoutputendisataminimumandtherefore, forwhichthePMDhasthemostsevere impactonthequalityofthesignal.TherealsoexistsaninputSOPatwhichthePMDhastheleasteffectonthequalityofthesignal.
Coherentdetectionsystems:Mostcoherentdetectionsystemsusepolarizationmultiplexedsignals.Sincebothpolarizationcomponentsarepresent,theDOPofthesignalisnotareliableindicatoroftheseverityof thePMDeffect.Therefore, toensure that theworst-casePMDeffectisachieved,polarizationscramblingmustbeusedbeforethePMDgenerator.
PMDsourceforPMDcompensatorevaluation
APMDsource isalsonecessarytoevaluatetheperformanceofaPMDcompensator,whetheropticalorelectrical,asshown inFigure4.First,onecanevaluatethesystemimprovementmarginatdifferentPMDsettings. Second,bygradually increasing thePMDvalueswhilemeasuring theBERof the fiber link,onecanevaluate thePMDcompensation rangeof thecompensator. 3)Third,bychangingthePMDvaluerapidly,onecandeterminethespeedof thecompensator response toquickPMDvariations inthe link. Finally,bychanging theSOPrapidlyatdifferentPMDsettings, one can determine the response time of the compensator to thechange inSOPateachPMDsetting. Therefore,aPMDsourcewith fast PMD change and fast polarization controlcapabilitiesisattractiveforPMDcompensatorevaluation.
Figure4PMDsource forPMDcompensator evaluation. Forelectrical compensation, the compensator may be inside the receiver (RX). Theplotsshown inFigure3canbegeneratedherewithPMDcompensationatdifferentPMDsettingsof thePMDsource.
PMDanalyzerormonitorforlinkPMDdetermination
ThePMDvaluesof fiber routesgenerallymust bemeasuredbeforelinkdeployment.ThisrequiresPMDanalyzers.Ingeneral,aPMDanalyzerrequiresalarge-bandwidthlightsource,whetherit is tunable laser-based or broadband source-based. As mentionedpreviously, thePMDof fiberroutesmaychangewithtime,duetochangesinenvironmentalconditions.ForaROADMnetwork, after the fibers are lit, the wavelength channel for each transceiverpair isdeterminedand isconfinedwithina50GHzbandwidth. Therefore, PMD analyzers requiring broadbandlightsourcescannotbeusedtocharacterizesuchasystem. Inaddition, it is undesirable to disconnect the transceivers and insertaPMDanalyzer in the link,because thiswouldcausea
the limit set for the system, as shown in Figure 3B. ThecorrespondingDGDisthe1storderPMDtoleranceofthesystem.Both the1stand2ndorderPMD(SOPMD)valuescanalsobeincreasedas theBERof thesystem ismeasuredandplotted,asshown inFigure3C. Thesystemoutageprobabilitycanbecalculated from the data obtained.
Figure3A)PMDtolerancetestsetupusingaPMDsource.ThecomputerselectsPMDsettingsandcollectsBERreadings. B)BERvs.DGDcurve.TheDGD toleranceof thesystem for agivenBERthresholdcanbededucedusing thecurve. C)MapofBERvs.DGDandSOPMD. Eachcontour representsaBERvalue.ThePMDtolerancecanbeobtainedfromthecontourplot.
PMDtolerancetestresultscanbeusedbynetworkoperatorstocompare systems made by different vendors and to verify the PMDrelatedspecificationspromisedby thevendors. Theycanalsobeusedbysystemvendors to1)determinewhichPMDmitigationapproachismostsuitabletoadopt,2)tofinetunethePMDmitigation relatedparameters in the transceiversduringsystemdevelopment, and3) toperformoutputqualitycontrolof the finalsystem, including transceivers, for thePMDrelatedspecifications.
Importanceof polarizationoptimization:ThePMDeffect on asystemishighlypolarizationdependent.Whenthe inputstateofpolarization(SOP) isalignedorcounter-alignedwiththeprincipalstateofpolarization(PSP)ofthefiberlinkorthePMDsource,thePMDhasnoeffectonthesignal,andthereforenoeffectonsystemperformance,asshowninFigure1.Bycontrast,whenthe inputSOPisaligned45degreesfromthePSPofthefiberlinkorPMDsource,thePMDhasitsmaximumeffectonthetransmittedopticalsignal, and hence its worst-case effect on system performance. TheeffectofPMDondatacanbecharacterizedby thedegreeofpolarization(DOP)of theopticalsignal,becausePMDcausesdepolarizationof theopticalsignal,asshown inFigure1. ThesmallertheDOP,thelargerthePMDeffect.Ingeneral,theDOPofthesignalpassingthroughamediumwithnonzero1storderPMDcan be expressed as:
(1)
where istheDGDvalueofthemedium, is the power distribution ratio of the two polarization components with respect to the principal stateofpolarization(PSP)of theDGDelement( = ½ when the powers of the two polarization components are equal or when the inputSOPtothemediumisoriented45degreesfromitsPSP)and
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severeserviceinterruption.ItisthereforeattractivetobeabletodeterminethePMDvalueofalivelinkwhileboththetransmitterandreceiverareoperational.Thiscapabilityisimportantforthediagnosis of fiber routes with performance issues.
DrawbacksofPMDsourcescurrentlyonthemarket
CurrentlyavailablePMDsources,suchasthosefromNewRidge,areratherrudimentary.Theyarecapableofgeneratingboth1stand2ndorderPMD,buttheinputSOPtothePMDsourceisnotcontrolled and therefore changes with external disturbances on theinputfiber.Consequently,theeffectofthePMDonthesignalisunpredictableandchangesrandomly,becausethePMDeffectonthesignal isstronglydependentontheinputSOP,asshowninFigure1.Inordertoobtainpredictabletestresults,itmaybenecessarytoscramblethe inputSOPtoobtainaPMDeffectonthesignalthatisaveragedoveralargenumbersofinputSOPs.However,suchaprocessnotonlysignificantlyslowsdowneachtest point, but also adds the cost of a polarization scrambler to the test setup.
OtherdrawbacksofpresentPMDsources includeaslowPMDchangespeedandlowPMDvaluerepeatability.OnePMDsourceon the market uses multiple birefringent crystals aligned linearly. Eachcrystalcanberotatedbyanelectricalmotor.DifferentPMDvalues are generated by rotating different crystals at different angles. Consequently, the speed and repeatability of PMDgeneration are limited by the speed and angular accuracy of the electrical motors and their associated control circuits.
DesirablefeaturesforaPMDsource
High repeatability and high speed of generation of 1st andhigherorderPMDvaluesaredesirableinaPMDsourcebecausethey enable fast, repeatable PMD tolerance tests and PMDcompensator evaluation.
AutomaticoptimizationofinputSOPisalsohighlyadvantageous.Asdescribedpreviously,PMDtolerancetestscanbemademuchsimpler if the inputSOP to thePMDelements inside aPMDsourcecanbecontrolledandoptimizedsuchthateachPMDvaluehas the most severe possible impact on the quality of the optical signal, regardless of polarization changes caused by disturbances tothefiberlink.Specifically,theinputSOPcanbeautomaticallymaintainedatanalignmentof45degrees fromthePSPof thePMDsource,ortheoutputDOPcanbeautomaticallymaintainedataminimumvaluebyactivelycontrollingtheinputSOP.
PMD compensation is also an extremely attractive function.First, system vendors and network operators need to knowhow much system performance can be improved by adding PMDcompensation inorder todecidewhether todeployPMDcompensatorsonalinkwithperformanceissues.Second,ifPMDcompensation significantly improves the system performance, it canbedetermined thatPMD is themajorcauseofperformancedegradation; otherwise, other impairments should be considered. Therefore,aPMDsourcewithPMDcompensationcapabilitycanbeusedforthediagnosisofsystemproblems.Finally,optimizedPMDcompensation canhelp todetermine thePMDvaluesofthe fiber link, because thePMDvalue used for optimalPMDcompensation isclose to the realPMDvalueof the fiber link.SuchafeatureisattractiveforfrequentPMDmonitoringofalivefiberlink,ataskthatcannotbeaccomplishedwithPMDanalyzerscurrently on the market.
ThepolarizationoptimizedPMDsource
GeneralPhotonics'polarizationoptimizedPMDsourcemeetsalloftherequirementsdescribedabove.ThisadvancedPMDsourceisconstructedwithapolarizationcontroller (PC),apolarimeterat the inputend,aPMDgenerator,asecondpolarimeterat theoutputend,andadigitalsignalprocessor(DSP)basedelectroniccircuit,asshowninFigure5.Thetwopolarimetersareusedtoprovide feedbacksignals to theDSPcircuit for fullpolarizationoptimization.Thecircuitreceivesthepolarizationmeasurementsfrom the two polarimeters, processes them, then sends commands to control thePMDgenerator and thepolarizationcontroller. Thecircuit canuse theSOP information from theinput polarimeter as the feedback to control the PC and generate anySOPtheuserprefers.Inparticular,itcanautomaticallyaligntheSOPto45degreesfromthePSPofthePMDgenerator.Thispolarizationstatecauses thePMDgenerator tohave theworst-case1storderPMD(DGD)effectontheopticalsignalatanyDGDsetting,and is therefore thepreferredSOP forDGD tolerancetests for transceivers and fiber systems. Alternatively, the circuitcanusetheDOPinformationfromtheoutputpolarimeterasthefeedbacktocontrolthePC.TheinputSOPthatminimizestheoutputDOPcausesthePMDgeneratortohavetheworst-casetotalPMDeffectontheopticalsignal,andisthereforetheoptimalpolarizationstate for totalPMD tolerance testsof transceiversandfibersystems.Finally,theoptimizedinputSOPobtainedbymaximizing theDOP fromthesecondpolarimeterenablesPMDcompensation.
Figure5SystemdiagramofpolarizationoptimizedPMDsource.TheDSPbasedcircuit isused to1)process information fromthepolarimetersandcontrolinstructionsfromusers,2)instructthePMDgeneratortoselectdesiredPMDvalues,3)controlthePCtooptimizetheinputSOPforvariousfunctions.
ThePMDgeneratorusedinthePMDsourceisbasedonGeneralPhotonics’ patent pending ternary polarization rotation technology. Thegeneratorisconstructedusingaseriesofbirefrigentcrystals,with a ternary polarization rotator sandwiched between each pair ofadjacentcrystalsegments,asshowninFigure6.Thecrystalsare arranged in descending order, with lengths decreasing sequentiallybyafactorof2.Eachrotatoriscapableofrotatingthestateofpolarization(SOP)by+45,0,or-45degrees.
Figure 6 The construction of a PMDgeneratormadewithbirefringent crystals and trinary polarization rotators which can rotateSOPin3ways.
WhentherotatorbetweentwoadjacentcrystalsrotatestheSOP+45°,theopticalaxesofthetwocrystalsarealignedtoproducethemaximumcombinedDGD.When the rotatorbetween two
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adjacentcrystalsrotatestheSOP-45°,theopticalaxesofthetwocrystals are counter-aligned to produce the minimum combined DGD. When the rotatorbetween twoadjacentcrystals rotatestheSOP0° (no rotation), theoptical axesof the twocrystalsarealigned45° fromeachother,producinghigherorderPMD.ThishigherorderPMDgenerationcapability isnotavailable inpreviousDGDgeneratordesignsinwhichthepolarizationrotatorscanonlygenerate±45°polarization rotations. The0degreepolarization rotation is essential for the generation of higher order PMD.
ThetotalnumberofPMDvaluesthatcanbegeneratedwithN+1sectionsofbirefringentmaterialandNrotatorpairs is3N. ForN=8,thetotalnumberofPMDstatesis6561.ThetotalnumberofDGD-only (1storderPMD)states is2N. ForN=8, the totalnumberofDGD-onlystatesis256.
UnliketheanalogcrystalrotatorsusedinPMDsourcesfromothervendors, which suffer from slow speed and low repeatability, the digitalternaryoperationmethodofthisPMDsourcemakesPMDvalue generation fast and highly repeatable.
In addition to the digital ternary rotators described above,GeneralPhotonics’PMDsourceusesaquasi-continuousrotatortoachieve independentDGDandSOPMDcontrol,allowing it touniformlycoverthePMDspace,asshowninFigure7.NotethattheSOPMDvaluesgeneratedusing thismethodarewavelengthindependent,sotherearenoFSRissues.
Figure7DGDandwavelengthindependentSOPMDrangeA)90psPMDPro™B)180psPMDPro™
Summary: Advantages and applications of the polarizationoptimizedPMDsource
ThepolarizationoptimizedPMDsource,constructedwithdigitalternary polarization rotators, offers the following attractive featuresforPMDrelatedtesting:
1)Highprecision,high repeatabilityPMDgeneration, resultingfrom the highly repeatable rotation angles of each ternary rotator. Initsdiscretedigitalcontrolmode,thePMDsourcecangeneratea total of6561differentPMDstates,ofwhich256areDGD-only,1024haveDGDandwavelength independentSOPMD,andtherestarewavelengthdependentPMD. In itsquasi-continuousoperation mode, it can generate states with independently controllableDGDandwavelength independentSOPMDvalues.UserscanselectindividualPMDstatesorscansequencesofPMDstates at user defined time intervals.
2)HighPMDgenerationspeed (minimum time intervalaround1ms), resulting from thehighspeedswitchingof the ternarypolarization rotators. ThishighspeedoperationcanspeedupPMDtolerancetestsandcanbeusedtotesttheresponsetimeofaPMDcompensatoragainstsuddenPMDchanges.
3)Automaticoptimizationof inputpolarization forworst-case1st order and2ndorderPMD tolerance tests, regardless ofrapidpolarizationchangesbefore the input to thePMDsource.Thepolarizationoptimizationeliminates test uncertaintiesandsignificantly reduces the timerequired tocomplete tests. It is,therefore,idealforPMDtolerancetestsfortransceiverproductionlines,asshowninFigure3.
4) Integratedpolarizationscrambler forPMD tolerance testsofcoherentdetectionsystems:Sincecoherentdetectionsystemsuse polarization multiplexed signals, automatic polarization optimizationcannotbeused.Therefore, the inputpolarizationmustbescrambled toensure that theworst-casePMDeffectisachieved.The integratedpolarizationcontrollerhasabuilt-inscrambling function for this purpose.
5)PMDcompensationusingeitheranautomaticallyoptimizedPMDvalueorauserselectedPMDvalue.ThePMDcompensationis accomplished bymaximizing the DOP detected by thepolarimeterat theoutputport. BothPMDandDOPvalueswillbeshownonthefrontpanelLCDdisplay.BysteppingthePMDvaluesupanddownand lookingat themaximizedDOPvalues,the user candirectly seehow thePMDvalue chosen affectsPMDcompensation. WhenoptimizedPMDmode is selected,the instrumentwillgo throughallPMDstatesandsearch forthemaximumDOP. ThePMDstatewith themaximumDOP isselectedastheoptimizedPMDforPMDcompensation.
6) PMD value determination of an in-service fiber link. TheoptimizedPMDcompensationmodecanbeused todeterminethePMDinthelink,asshowninFigure8,becausetheoptimizedPMDvalue forcompensationshouldbeclose to thePMDvalueof the fiber link. Therefore, thePMDvalueofa fiber linkcanbedeterminedbysimply insertingapolarizationoptimizedPMDsource in the light path andenabling thePMDcompensationfunction. TheoptimizedPMDvalue identifiedby the instrumentis then thePMDvalueof the link. Itmayalsobenecessary tocharacterizethePMDofaparticularchannelrouteinanin-serviceROADMnetwork todetermine its feasibility for40Goperationbefore installing 40G transmitters and receivers. In such asituation,anASEsourcecanbeplacedat the transmitterend
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andapolarizationoptimizedPMDsourceat thereceiverend toperformPMDcompensation.TheoptimizedPMDvalueidentifiedby thePMDsource is then thePMDvalueof the fiber route.Withthisinformation,itispossibletodecidewhethertherouteissuitablefor40GtransmissionandwhetheraPMDcompensatorisrequired.AnEDFAmaybeusedbeforethePMDsourcetoboostthe signal level.
7)Systemimpairmentdiagnosis.Itcanbedifficulttoidentifythecauseofperformanceproblemsinafiberlink.Signaldegradationcan be caused by PMD, chromatic-dispersion (CD), signal-to-noise ratio (SNR) issues, or other problems. PerformingPMDcompensationcanhelpdeterminewhether theproblem isprincipallyduetoPMD:IfPMDcompensationsubstantiallysolvesthe transmissionproblem, it canbededuced thatPMD is theprincipalcauseoftheproblem.Ifnot,itmaybepossibletoruleoutPMD.Withsuchadiagnosis,itispossibletodecidewhetherPMDcompensationisrequiredforthefiberlink.
8)PMDemulation. ThePMDgeneratorcangeneratestatisticalPMDdistributionstoemulatePMDvariationsinfibersystems.
Figure8PolarizationoptimizedPMDsource inan in-serviceWDM link for thedeterminationof itsPMDanddiagnosis of performance issues.
9)Polarizationcontrol functions. Using itsbuilt-inpolarizationcontroller and polarimeters, this instrument can perform all polarization control functions, including deterministic SOPgeneration, polarization scrambling, and polarization trace generation. It can therefore be used as a general purposepolarization synthesizer/controller for all polarization control needs.
Polarization optimization is used in the following three functions.
a. DGD tolerance test: Optimize input SOP using the SOPinformation from the first polarimeter as feedback to obtain the worst-casesignaldegradationcausedbyDGD.
b.PMDtolerancetest:Optimize inputSOPtominimizetheDOPdetected by the second polarimeter to obtain the worst-case signaldegradationcausedbybothDGDandSOPMD.
c. PMD compensation: Optimize input SOP bymaximizingDOPdetected by the second polarimeter tominimize signaldegradationcausedbyDGDandSOPMD.
