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Ch17 Ch17 Presentation Transcript

  • Fiber Optic Cable TestingFiber Optic Cable Testing Ch 17Ch 17 Fiber Optics Technician’sFiber Optics Technician’s Manual, 3Manual, 3rdrd . Ed. Ed Jim HayesJim Hayes Revised 11-24-08
  • Testing RequirementsTesting Requirements ParameterParameter ExampleExample InstrumentInstrument Optical powerOptical power Source output,Source output, receiver signalreceiver signal levellevel Power meterPower meter Attenuation or lossAttenuation or loss Fibers, cables,Fibers, cables, connectorsconnectors Power meter andPower meter and source, or Optical Losssource, or Optical Loss Test Set (OLTS)Test Set (OLTS) Back reflection orBack reflection or Optical Return LossOptical Return Loss (ORL)(ORL) OTDR or OCWROTDR or OCWR (Optical Continuous(Optical Continuous Wave Reflectometer)Wave Reflectometer) Source wavelengthSource wavelength Spectrum analyzerSpectrum analyzer BackscatterBackscatter Loss, length,Loss, length, fault locationfault location OTDROTDR Fault locationFault location OTDR, VFLOTDR, VFL Bandwidth/dispersionBandwidth/dispersion Bandwidth testerBandwidth tester
  • Power MetersPower Meters  The power meter by itself can beThe power meter by itself can be use to measure source poweruse to measure source power  With a source, it can measure theWith a source, it can measure the loss of a cable plant, calledloss of a cable plant, called insertion lossinsertion loss  Most power measurements are inMost power measurements are in the range +10 dBm to -40 dBmthe range +10 dBm to -40 dBm • Analog CATV (cable TV) or DWDMAnalog CATV (cable TV) or DWDM (Dense Wavelength Division(Dense Wavelength Division Multiplexing) systems can haveMultiplexing) systems can have power up to +30 dBm (1 watt)power up to +30 dBm (1 watt) Image from lanshack.com
  • WavelengthsWavelengths  Power meters are calibrated at threePower meters are calibrated at three standard wavelengthsstandard wavelengths • 850 nm, 1300 nm, 1550 nm850 nm, 1300 nm, 1550 nm  Typical measurement uncertainty isTypical measurement uncertainty is 5% (0.2 dB)5% (0.2 dB)
  • SourcesSources  Sources are either LED or laserSources are either LED or laser • 665 nm for plastic optical fiber665 nm for plastic optical fiber • 850 nm or 1300 nm for multimode850 nm or 1300 nm for multimode • 1310 nm or 1550 nm for singlemode1310 nm or 1550 nm for singlemode  Test your system with a sourceTest your system with a source similar to the one that will besimilar to the one that will be actually used to send dataactually used to send data Image from lanshack.com
  • Optical Loss Test SetOptical Loss Test Set  Power meter and sourcePower meter and source in a single unitin a single unit • Normally used in pairsNormally used in pairs • Automated, more complexAutomated, more complex and expensive than theand expensive than the combination of a sourcecombination of a source and a power meterand a power meter  Rare in field testingRare in field testing • Image from aflfiber.comImage from aflfiber.com
  • OTDROTDR Optical Time-Domain ReflectometerOptical Time-Domain Reflectometer  Image from exfo.comImage from exfo.com
  • OTDR UsesOTDR Uses  Measure lossMeasure loss  Locate breaks, splices, andLocate breaks, splices, and connectorsconnectors  Produces graphic display of fiberProduces graphic display of fiber statusstatus • Can be stored for documentation andCan be stored for documentation and later referencelater reference  Cable can be measured from one endCable can be measured from one end
  • BackscatterBackscatter  A small amount of light is scattered backA small amount of light is scattered back to the source from the fiber itselfto the source from the fiber itself  Splices or connector pairs cause a largerSplices or connector pairs cause a larger reflection of light back to the sourcereflection of light back to the source • Figure from techoptics.com (link Ch 17a)Figure from techoptics.com (link Ch 17a)
  • OTDR DisplayOTDR Display Dead zone
  • OTDR AccuracyOTDR Accuracy  OTDR can give false loss valuesOTDR can give false loss values when coupling different fiberswhen coupling different fibers togethertogether • Splices can even show more light on theSplices can even show more light on the other side “gainer”other side “gainer” • This is an illusion caused by increasedThis is an illusion caused by increased scattering on the other sidescattering on the other side • Splice loss uncertainty up to 0.8 dBSplice loss uncertainty up to 0.8 dB
  • Types of OTDRTypes of OTDR  Full-sizeFull-size • Complex, powerful,Complex, powerful, expensiveexpensive  Mini-OTDRMini-OTDR • Fewer featuresFewer features  Fault FinderFault Finder • Simplified, showsSimplified, shows distance to a faultdistance to a fault  Links Ch 17c, d, eLinks Ch 17c, d, e
  • Visual Cable Tracers andVisual Cable Tracers and Visual Fault LocatorsVisual Fault Locators  Cable tracer is just a flashlightCable tracer is just a flashlight  VFL uses an LED or Laser source to getVFL uses an LED or Laser source to get more light into the fibermore light into the fiber • Useful to test a fiber for continuityUseful to test a fiber for continuity • To check to make sure the correct fiber isTo check to make sure the correct fiber is connectedconnected • With bright sources, you can find the break byWith bright sources, you can find the break by looking for light shining through the jacketlooking for light shining through the jacket  Visible light only goes 3-5 kmVisible light only goes 3-5 km through fiberthrough fiber • Images from links Ch 17 e & fImages from links Ch 17 e & f
  • Fiber IdentifiersFiber Identifiers  Bends the fiber toBends the fiber to detect the lightdetect the light  Can be used on liveCan be used on live fiber withoutfiber without interrupting serviceinterrupting service  Can detect a specialCan detect a special modulated tone sentmodulated tone sent down a fiberdown a fiber • Image from tecratools.com (linkImage from tecratools.com (link Ch 17d)Ch 17d)
  • Optical Continuous WaveOptical Continuous Wave Reflectometer (OCWR)Reflectometer (OCWR)  Measures optical return lossMeasures optical return loss (reflectance) of connectors(reflectance) of connectors  Inaccurate on installed systemsInaccurate on installed systems because it includes backscatter andbecause it includes backscatter and all sources of reflectanceall sources of reflectance • See link Ch 17hSee link Ch 17h Cable to be Tested
  • MicroscopeMicroscope  Used to inspectUsed to inspect fibers andfibers and connectorsconnectors • Particularly duringParticularly during epoxy-polish processepoxy-polish process  Image from link Ch 17gImage from link Ch 17g
  • TalksetTalkset  Telephone callsTelephone calls over unused fibersover unused fibers  Rarely neededRarely needed now that we havenow that we have cellphonescellphones • See link Ch 17iSee link Ch 17i
  • AttenuatorsAttenuators  Simulates the loss of aSimulates the loss of a long fiber runlong fiber run  Variable attenuatorsVariable attenuators allow testing a networkallow testing a network to see how much loss itto see how much loss it can withstandcan withstand  Can use a gap, bending,Can use a gap, bending, or inserting opticalor inserting optical filtersfilters • Image from link Ch 17jImage from link Ch 17j
  • Reference CablesReference Cables  Test cables are needed to connectTest cables are needed to connect the cables to be tested to the testthe cables to be tested to the test instrumentsinstruments  Must have correct connectors, beMust have correct connectors, be clean, and high-quality (low loss)clean, and high-quality (low loss)  Use high-quality mating adaptersUse high-quality mating adapters • Ceramic or metal – not plasticCeramic or metal – not plastic • Singlemode rated are most accurateSinglemode rated are most accurate
  • Optical Power LevelsOptical Power Levels  Detectors are Silicon, Germanium, orDetectors are Silicon, Germanium, or Indium-Gallium-Arsenide semiconductorsIndium-Gallium-Arsenide semiconductors Network TypeNetwork Type WavelengthWavelength Power Range (dBm)Power Range (dBm) TelecomTelecom 1330, 15501330, 1550 +3 to -45+3 to -45 Telecom DWDMTelecom DWDM 15501550 +20 to -30+20 to -30 DataData 665, 790, 850,665, 790, 850, 13001300 -10 to -30-10 to -30 CATVCATV 1300, 15501300, 1550 +10 to -6+10 to -6
  • CalibrationsCalibrations  NIST is a standards laboratoryNIST is a standards laboratory • Offers power calibration services at 850,Offers power calibration services at 850, 1300, and 1550 nm wavelengths1300, and 1550 nm wavelengths • Instruments should be returned to theInstruments should be returned to the manufacturer for calibration annuallymanufacturer for calibration annually
  • UncertaintiesUncertainties  Absolute power: 5% or 0.2 dBAbsolute power: 5% or 0.2 dB  Insertion loss: 0.5 dB or moreInsertion loss: 0.5 dB or more  OTDR: up to several dBOTDR: up to several dB  Optical return loss: 1 dB or moreOptical return loss: 1 dB or more • Although meters show a reading withAlthough meters show a reading with hundredths of a decibel, they don’t meanhundredths of a decibel, they don’t mean anythinganything  A 2.13 dB loss might well re-measure as 2.54 dBA 2.13 dB loss might well re-measure as 2.54 dB
  • Optical Fiber TestingOptical Fiber Testing  Before installationBefore installation • Test continuity with cable tracer or VFLTest continuity with cable tracer or VFL  Measure attenuation withMeasure attenuation with cutback methodcutback method • Cut offCut off knownknown length,length, measuremeasure powerpower increaseincrease
  • Sources for Loss MeasurementsSources for Loss Measurements  Most multimode systems use LEDMost multimode systems use LED sourcessources • High-speed multimode often usesHigh-speed multimode often uses VCSELs (1 Gbps and higher)VCSELs (1 Gbps and higher) • See link Ch 17kSee link Ch 17k  Singlemode systems use laserSinglemode systems use laser sourcessources  Test with the source you will reallyTest with the source you will really useuse • BUT Argilent says you should test allBUT Argilent says you should test all Multimode with LEDs (link Ch 17l)Multimode with LEDs (link Ch 17l)
  • Modal Effects in Multimode FiberModal Effects in Multimode Fiber  Mode scramblers mixMode scramblers mix modes to equalize power inmodes to equalize power in all modesall modes • Can be made with a sectionCan be made with a section of step-index fiberof step-index fiber  Mode filters remove higher-Mode filters remove higher- order modes to reachorder modes to reach equilibrium modalequilibrium modal distributiondistribution • Can be made with a mandrelCan be made with a mandrel wrapwrap
  • Modal Effects in Singlemode FiberModal Effects in Singlemode Fiber  Singlemode fibers shorter than 10Singlemode fibers shorter than 10 meters may have extra modesmeters may have extra modes • Use a launch cord to avoid that problemUse a launch cord to avoid that problem
  • OTDR Pulse WidthOTDR Pulse Width  Longer pulses can see further down the cableLonger pulses can see further down the cable because they have more lightbecause they have more light  But they have less accuracy finding locationsBut they have less accuracy finding locations • From link Ch 17aFrom link Ch 17a
  • OTDR UncertaintiesOTDR Uncertainties  Dead zoneDead zone • Nothing can be measured for the firstNothing can be measured for the first 100 meters or so100 meters or so  Distance ResolutionDistance Resolution • Two events too close together cannot beTwo events too close together cannot be resolvedresolved • Especially with long pulsesEspecially with long pulses
  • OTDR Distance ErrorsOTDR Distance Errors  Speed of light in fiberSpeed of light in fiber • May not be exactly what the OTDRMay not be exactly what the OTDR expects, distorting distancesexpects, distorting distances  Slack in fiberSlack in fiber • OTDR measures length along the fiber,OTDR measures length along the fiber, which is usually 1% - 2% longer thanwhich is usually 1% - 2% longer than the length along the cablethe length along the cable
  • OTDR Loss ErrorsOTDR Loss Errors  Joining two fibers with differentJoining two fibers with different backscatter coefficients will cause:backscatter coefficients will cause: • Too high a loss when measured in oneToo high a loss when measured in one directiondirection • Too low a loss in the other directionToo low a loss in the other direction  For accurate loss measurements,For accurate loss measurements, measure from both ends andmeasure from both ends and average the resultsaverage the results
  • OTDR GhostsOTDR Ghosts  Secondary reflection appears at double the realSecondary reflection appears at double the real cable lengthcable length  Using index-matching gel will eliminate ghostsUsing index-matching gel will eliminate ghosts
  • DispersionDispersion  Multimode fibers suffer fromMultimode fibers suffer from modalmodal dispersiondispersion  All fibers suffer fromAll fibers suffer from chromatic dispersionchromatic dispersion • Because different wavelengths travel atBecause different wavelengths travel at different speeds, and no source is completelydifferent speeds, and no source is completely monochromaticmonochromatic  In very long singlemode networks,In very long singlemode networks, polarization mode dispersionpolarization mode dispersion also mattersalso matters
  • Bandwidth TestersBandwidth Testers  There is a new unit available to testThere is a new unit available to test bandwidth in the field, but it is notbandwidth in the field, but it is not commonly done yet (link Ch 17 k)commonly done yet (link Ch 17 k) Input Output
  • Connector Insertion Loss TestConnector Insertion Loss Test  This test gives the typical loss of a connectorThis test gives the typical loss of a connector typetype
  • Modal DistributionModal Distribution  The insertion loss testThe insertion loss test • FOTP-34 by the TIAFOTP-34 by the TIA  Three options of modal distributionThree options of modal distribution • EMD or steady stateEMD or steady state  After a mandrel wrapAfter a mandrel wrap • Fully filledFully filled  After a mode scramblerAfter a mode scrambler • Any other specified conditionsAny other specified conditions
  • MicroscopesMicroscopes  Used to inspect the ends of polishedUsed to inspect the ends of polished connectorsconnectors  Helpful to view the connector at anHelpful to view the connector at an angle while lighting it from the sideangle while lighting it from the side  Only defects over the core reallyOnly defects over the core really mattermatter
  • Optical Return Loss in ConnectorsOptical Return Loss in Connectors  A pair of glass-air interfaces forA pair of glass-air interfaces for nonphysical contact connectors withoutnonphysical contact connectors without index-matching gelindex-matching gel • 4% reflectance – loss of 0.3 dB due to4% reflectance – loss of 0.3 dB due to reflectancereflectance  PC connectors can have a reflectance ofPC connectors can have a reflectance of 1% or an ORL of 20 dB1% or an ORL of 20 dB • Much less with Angled PC connectors – 40 toMuch less with Angled PC connectors – 40 to 60 dB60 dB  Reflectance can be a problem in highReflectance can be a problem in high bitrate singlemode systemsbitrate singlemode systems
  • Basic Cable Loss TestBasic Cable Loss Test  Test FOTP-171Test FOTP-171 • Measure power through launch cableMeasure power through launch cable • Then add cable to testThen add cable to test  This tests only one connector – turn theThis tests only one connector – turn the cable around to test the other endcable around to test the other end
  • Double-Ended Loss TestDouble-Ended Loss Test  Uses both a launch and receive cableUses both a launch and receive cable
  • Single-Cable ReferenceSingle-Cable Reference  Refer to this conditionRefer to this condition  Test this wayTest this way • EIA/TIA 568 requires thisEIA/TIA 568 requires this • See link Ch 17mSee link Ch 17m
  • Why Use Single-Cable Reference?Why Use Single-Cable Reference?  It gives highest loss and lowestIt gives highest loss and lowest uncertaintyuncertainty  It tests both connectors on the cableIt tests both connectors on the cable to testto test
  • Choosing a Launch Cable forChoosing a Launch Cable for TestingTesting  Choose cables with low lossChoose cables with low loss • It is not necessary to get connectorsIt is not necessary to get connectors and fiber with tighter specificationsand fiber with tighter specifications  Handle the launch cables carefullyHandle the launch cables carefully  Inspect them with a microscopeInspect them with a microscope  Keep them cleanKeep them clean • Use splice bushings with metal orUse splice bushings with metal or ceramic alignment sleevesceramic alignment sleeves
  • Mismatched FibersMismatched Fibers  Coupling a smaller fiber to a largerCoupling a smaller fiber to a larger one causes only a small loss (0.3 dBone causes only a small loss (0.3 dB or so)or so)  Connecting large fiber to small fiberConnecting large fiber to small fiber causes a large losscauses a large loss • Both because of diameter and numericalBoth because of diameter and numerical apertureaperture
  • Testing the Installed Cable PlantTesting the Installed Cable Plant  Can use one-cable reference, or two-Can use one-cable reference, or two- cable, or three-cable, but the type ofcable, or three-cable, but the type of reference must be documentedreference must be documented
  • WavelengthsWavelengths  Usually test multimode at both 850Usually test multimode at both 850 and 1300 nm with LED sourcesand 1300 nm with LED sources  Singlemode test is usually at 1300Singlemode test is usually at 1300 nm onlynm only • 1550 nm is sometimes required also1550 nm is sometimes required also • For long-distance, and to show thatFor long-distance, and to show that WDM can be performed laterWDM can be performed later • Also shows microbends – 1550 test isAlso shows microbends – 1550 test is much more sensitive to bending lossmuch more sensitive to bending loss
  • Optical SplitterOptical Splitter  Splits light signal from one fiber intoSplits light signal from one fiber into two fiberstwo fibers • Figures from tpub.com (link Ch 17n)Figures from tpub.com (link Ch 17n)
  • Couplers Can Split or CombineCouplers Can Split or Combine  You can also split one to M, orYou can also split one to M, or combine M to 1combine M to 1
  • M to N CouplerM to N Coupler
  • Making CouplersMaking Couplers
  • Wavelength Division MultiplexersWavelength Division Multiplexers  Light entering from the left containing twoLight entering from the left containing two wavelengths is separated into the twowavelengths is separated into the two fibers on the rightfibers on the right  Combining the two signals is also possibleCombining the two signals is also possible  Requires special equipment andRequires special equipment and techniques to testtechniques to test • Image from link Ch 17oImage from link Ch 17o
  • Fiber Optic AmplifiersFiber Optic Amplifiers  Boosts signal withoutBoosts signal without converting it toconverting it to electricityelectricity  Complicated to test,Complicated to test, require specialrequire special equipmentequipment • Image from link Ch 17pImage from link Ch 17p
  • Fiber Optic SwitchFiber Optic Switch  See links Ch 17q and 17rSee links Ch 17q and 17r
  • Fiber Optic DatalinksFiber Optic Datalinks  The diagram shows a single linkThe diagram shows a single link  Most networks will be bidirectionalMost networks will be bidirectional ((full duplexfull duplex) with two links working) with two links working in opposite directionsin opposite directions
  • Bit Error RateBit Error Rate  The receiver power must be withinThe receiver power must be within the operating rangethe operating range • Too little power leads to high bit errorToo little power leads to high bit error rates (wrong data at receiver)rates (wrong data at receiver) • Too much power saturates the detectorToo much power saturates the detector and also leads to high bit error ratesand also leads to high bit error rates  Use an attenuator in this caseUse an attenuator in this case
  • What Goes Wrong?What Goes Wrong?  Often the two fibers are connectedOften the two fibers are connected backwards – check them with abackwards – check them with a visual tracervisual tracer  Check receiver power levelCheck receiver power level  Check plant loss with double-endedCheck plant loss with double-ended methodmethod
  • Don’t Use an OTDR to MeasureDon’t Use an OTDR to Measure Plant LossPlant Loss  OTDR does not see the loss of theOTDR does not see the loss of the end connectorsend connectors  Its power source is not the same asIts power source is not the same as normal LAN power sourcesnormal LAN power sources  OTDR measurements are affected byOTDR measurements are affected by backscatter coefficient which maybackscatter coefficient which may not be the same for all the cables innot be the same for all the cables in a networka network
  • Back ReflectionBack Reflection  Back reflection can cause networksBack reflection can cause networks to fail even though the loss is lowto fail even though the loss is low  Power meter can’t measure reflectionPower meter can’t measure reflection • Use an OTDR or OCWRUse an OTDR or OCWR • Cure it by splicing in low-reflection patchCure it by splicing in low-reflection patch cords to replace high-reflectancecords to replace high-reflectance connectorsconnectors • Angled PC connectors are designed toAngled PC connectors are designed to minimize reflectance for this reason (notminimize reflectance for this reason (not mentioned in textbook)mentioned in textbook)
  • ReliabilityReliability  Once installed, the fiber opticsOnce installed, the fiber optics should work for a long timeshould work for a long time  People break the cable by accidentPeople break the cable by accident • Mark where cables are buriedMark where cables are buried • Bury a marker tape above the cableBury a marker tape above the cable • Use orange or yellow jacket cableUse orange or yellow jacket cable indoorsindoors • A broken cable just behind a connectorA broken cable just behind a connector in a patch panel is hard to findin a patch panel is hard to find
  • Source FailureSource Failure  LED in laser transmitter drops inLED in laser transmitter drops in power as it agespower as it ages  Laser sources are feedback-stabilizedLaser sources are feedback-stabilized so they remain constant in power tillso they remain constant in power till they failthey fail