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ApplicationNoteforPDLMeasurement

Introduction:

Polarization-dependent-loss (PDL)measurement is extremelysensitive to unwanted variations in the measurement system, including light source instability, connector back reflections, and even the layoutof the test fiber cables. Largemeasurementinaccuracies or fluctuations may occur if the test setup is not properly arranged, even if a high accuracy measurement instrument isused.ThisapplicationnotedescribesprecautionsforaccuratePDLmeasurementsingeneralandwaystominimizemeasurement inaccuracies using General Photonics’ PDLmultimeter(PDL-201).

PDLisdefinedas:

where Pmax and Pmin are the maximum and minimum output opticalpowerpassing throughadeviceunder test (DUT)whenthe input light is scanned over all possible polarization states, as showninFig.1.

Figure1IllustrationofPDLdefinition

Severalpossibleerrorsoruncertaintiesmaybeintroducedduringmeasurements, including:

1.Errorcausedbylightsourcefluctuation

As indicated in Eq. (1), if the power from the light sourcefluctuates with time, the measured Pmax and Pmin also fluctuate, resultinginmeasurementinaccuracies.Therefore,alightsourceusedforPDLmeasurementmustbehighlystable.

Even if the light source itself is highly stable, small backreflections from various positions in the measurement system may feed back to the laser, disturbing its operation and causing outputinstabilities.Therefore,itishighlyrecommendedthatanisolatorbeusedattheinputofthePDLmeasurementinstrumentto minimize back reflections, even though the laser source may alreadyhavean isolator at its output. In addition, all of theconnectorsbetweenthelightsourceandthePDLmetermustbeAPC type to minimize connector back reflection.

2.Errorcausedbydoublereflections

Theremaybesmallreflectionsfromsomecomponentsusedinthemeasurementsetup,includingtheconnectorsandtheDUTitself.AsshowninFig.2,reflectedlightfromacomponentmaybereflectedagainbyanothercomponent.Thedouble-reflected lighttravels inthe same direction as the main input light and can therefore interfere withit.Thetotaloutputopticalpoweristhus:

where Pin and Pdr are the powers of the main and double reflected

light beams, êin and êdr are the unit complex vectors of the statesofpolarization(SOP)ofthemainanddoublereflectedlightbeams, and Ø is the relative phase between them.

Figure2Illustrationofdoublereflection

Because the relativephaseand theSOPsofboth thedouble-reflected light and the main light beam change when the fiber is disturbed, their interference causes the total output power to fluctuate. Themagnitudeof therelative fluctuation, indB, fromEq.(2)is:

Although the double-reflected light is very weak, its contribution cannot be ignored because it interferes with a strong light signal (theinput).Forexample,ifalightbeamisfirstreflectedbyanopenPCconnectorwith the typical4%reflectivity,and is thenreflected again by a mated PC connector with a reflectivity of 0.01%(a return lossof40dB), thedetectedpower fluctuationcanbeaslargeas0.017dB.ThispowerfluctuationwillcauseaPDLmeasurementfluctuationof0.017dB.SuchavaluecannotbeignoredwhenmeasuringDUTswithsimilarPDLvalues.IfanAPCconnectorwithareturnlossof60dBisusedinsteadofthematedPCconnectorinthepreviousexample,thePDLfluctuationisreducedto0.0017dB,whichcanbeignoredinmostcases.

Tominimize thecontributionsfromdoublereflections,APCtypeconnectors with low reflections should be used, if possible. Alternatively,alightsourcewithashortcoherencelength(shorterthan the optical path difference between the double reflected lightandthemainlightbeam)canalsobeused.Thisway,theinterferencedescribed inEq. (2)cannot takeplace; therefore,theresultantinterferencefluctuationsdescribedinEq.(3)donotoccur.

3.Errorfromconnectorandcablecontributions

Inaddition to theDUT, the fibercablesand/orconnectorsusedin the PDLmeasurement also have small PDL values. Forexample,opticalcableitselfmayhaveaPDLvalueontheorderof0.01dB, and thisvaluemay increasewhen it isbentwitha small radius of curvature. Connectorized fiber jumpers may alsohavea small amountofPDL,on theorderof0.01-0.02dB. Poorlyconnectorized jumperscanhaveevenhigherPDLvalues, probably caused by excess stress on the fiber during connectorization. Anglepolishedconnectors (APC)generallyhavehighPDL, especiallywhennotmatedwith anotherAPCconnector. Therefore, it ispossible thataconnectorized fiberjumperused in themeasurementcancontributeaPDLerrorof0.02dBorhighertothePDLmeasurementofaDUT.

4.VariationcausedbyPDLvectorsummation

PDLcanbeviewedasavector ina3Dspace,becausePmaxandPmin in Eq.(1) correspond to two orthogonal states ofpolarization of the input light, which can be represented on a

PDL = 10 Log (1)Pmax

Pmax

(2)

(3)

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Genera l Photon ics Corporat ion 909.590.5473 www.genera lphoton ics .com -210-

PoincaréSphere. Therefore,when twoormore componentsin themeasurementsetuphavenon-zeroPDLvalues, the totalPDL is thevector summationof thePDLvaluesof all of thecomponents. Forexample, for thesetupshown inFig.3, thetotalmeasured valueofPDL is the vector summationof thePDLsoffiberconnectorsA,B,C,D,andtheDUT(assumingforsimplicity‘ssakethatthefibercableshavezeroPDL):

where PDLT and êT are the value and complex unit vector of the totalPDL;PDLa, PDLb, PDLc, PDLd, and PDLDUTarethePDLvalues;and êa, êb, êc, êd, and êDUTare thecomplexPDLunitvectorsofconnectorsA,B,C,D,andtheDUT,respectively. Fig.4showsgraphically how thePDLvectors addup.ThemaximumPDLresultsifthePDLvectorsofallofthecomponentsareparallel:

TheminimumPDLresultsifthePDLvectorsofallconnectorsarealigned,butarecounter-parallelwiththePDLvectoroftheDUT,assumingthattheDUT’sPDLvalueislargerthanthesumoftheconnectorPDLvalues:

The direction of each component’s PDL vector relates tothe orientation of the component and to the stress induced birefringenceof the fibercable. When the fiberbetween twocomponents is disturbed, the relative directions of thePDLvectors are also changed, resulting in measurement variations. ThemaximumPDLvaluevariationistherefore:

If thePDLvalueof theDUT ismuch larger than thoseof theconnectors, the relative measurement error is small. A large relative errorwillresultiftheDUThasaPDLvaluesimilartothoseoftheconnectors.Therefore,foraccuratecharacterizationofDUTswithsmallPDLvalues(suchasfusedcouplers),thePDLvaluesofanyconnectorsandcablesconnectingtotheDUTmustbeextremelysmall. The limitingfactorformeasurementaccuracy isoftennottheaccuracyof the instrument itself,but theresidualPDLof theconnectorsandcablesconnectingtotheDUT.

TableI:TypicalPDLvaluesofcommonopticalcomponents:

Component TypicalPDLvalue

1 meter single mode fiber <0.02dB

10 km single mode fiber <0.05dB

PC type connector 0.005~0.02dB

APC type connector 0.02~0.06dB

50%fusedcoupler,single window

0.1~0.2dB

50%fusedcoupler,dual window

0.15~0.3dB

90/10 fused coupler,through path

0.02dB

90/10 fused coupler,-10dBpath

0.1dB

Isolator 0.05~0.3dB

3-port circulator 0.1~0.2dB

DWDM 0.05~0.15dB

Polarizer 30~50dB

Figure3PDLmeasurementsetupusingthePDL-201with4connectors:A,B,C,andD

Figure 4 I l lustration of the vector summation of PDLcontributionsfromallconnectorsandDUT.ThemeasuredPDLfluctuates between a maximum value and a minimum value, dependingontherelativeorientationsofeachPDLvector.

Summary of goodpractices forminimizingPDLmeasurementerrors

Keeping inmind the sources of PDLmeasurement errorsdescribedabove,here isasummaryofgoodpractices forPDLmeasurement,usingGeneralPhotonics’PDL-201multimeterasanexample.AtypicalmeasurementsetupisshowninFig.5.

1.The lightsourceused forPDLmeasurementmustbehighlystabilized, with a short-term stability similar to the intended PDLmeasurementaccuracy.Forexample,ifaPDLaccuracyof0.02dB is tobeachieved, the lasersource’sshort-termpowerstabilitymustbebetterthan0.02dB.

2.An isolator at the input of thePDL-201 is recommendedfor minimizing the amount of back-reflected light from the downstreamconnectorsandDUT thatgoesback into the lightsource. GeneralPhotonics’NoTail isolator,whichhasno fiberpigtails, is preferred.

3. Only APC connectors should be used between the light source and thePDL-201 input, inorder tominimizeback reflection tothe light source, and consequently to minimize back-reflection induced instability of the light source. The NoTail isolatordescribedinstep2shouldthereforebeAPCconnectorized.

4.Thepolarizationofthelightsourceshouldberelativelystable.Laserswith fastpolarization fluctuationwillcausemeasurementfluctuations.

5.Thewavelengthofthelightsourceshouldberelativelystable,because fast wavelength fluctuation of the light source will cause fast polarization fluctuation due to the birefringence of the fiber cable connecting the laser and the instrument.

6.FiberjumperswithlowPDLshouldbeusedforconnectingtotheDUT.

(4)

(5)

(6)

(7)

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Figure5TypicalsetupformeasurementofaconnectorizedDUTwiththePDL-201.

Figure6Productionfloorsetuptomeasureanunconnectorized,pigtailedDUTusingfusionsplicetoafreepigtailandbarefiberadaptertoconnecttothePDL-201.

Figure7Productionfloorsetuptomeasureanunconnectorized,pigtailedDUTusingonlyfusionspliceandbarefiberadaptertoconnecttothePDL-201.

7.Tominimizetheerrorcontributionfromdoublereflection,APCconnectors should be used for inputting light into the instrument andtotheDUT.ThePDL-201hasAPCbulkheadconnectors(inputconnector andconnectorA in Figs.5and6)mountedon thefrontpanelforthispurpose.Foraccuratemeasurementoflow-PDLDUTs(lessthan0.1dB),indexmatchinggelcanbeusedatconnectorsBandCtoreducebackreflectionandresidualPDL.

8.TominimizeconnectorPDLcontributions,aPCtypeconnectorshouldbeused tosend theoutput light fromtheDUT into theinstrument (connectorD inFigs.5and6). Light from thePCconnector will be directly incident onto a free space photodetector inside the instrument. If anAPCconnectorwereusedhere,aPDLerrorcontributionof0.03~0.04dBwouldresult.

9. There should be no sharp bending or tight coiling of thefibercablebetweenconnectorsAandD,becausebendingcanintroducenon-negligiblePDLinthefibercable.

10. For pre-connectorization measurement of pigtailedcomponentson theproduction floor, the setupshown in Fig.6 shouldbeused. In this setup, a fiberpigtailwith anAPCconnector at one end is connected to bulkhead connector A. Theotherendisleftfreeforfusionsplicing.Theoperatormayfusionsplice the freepigtail to thatof theDUT input,andusea removablePC typebare fiber adapter at theDUToutput toconnect tobulk-connectorD. IndexmatchinggelmaybeusedwithAPCconnectorAtominimizeitsPDLcontribution.

Note: Connector A in Figs. 5 and 6will also contribute aslight background PDL. Therefore, for pre-connectorizationmeasurement of pigtailed components (especially low-PDLcomponents),itcanbereplacedwithafiberpigtailwhichcanbefusionspliced to theDUT input (Fig.7).PleasecontactGeneralPhotonics for desired configuration before ordering.