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Transcript of Fiber Optic Cable Testing
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Fiber Optic Cable TestingFiber Optic Cable Testing
Ch 17Ch 17
Fiber Optics Technician’sFiber Optics Technician’sManual, 3Manual, 3rdrd. Ed. Ed
Jim HayesJim Hayes
Revised 11-24-08
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Testing RequirementsTesting Requirements
Bandwidth testerBandwidth testerBandwidth/dispersionBandwidth/dispersion
OTDR, VFLOTDR, VFLFault locationFault location
OTDROTDRLoss, length,Loss, length,
fault locationfault locationBackscatterBackscatter
Spectrum analyzerSpectrum analyzerSource wavelengthSource wavelength
OTDR or OCWROTDR or OCWR
(Optical Continuous(Optical Continuous
Wave Reflectometer)Wave Reflectometer)
Back reflection orBack reflection or
Optical Return LossOptical Return Loss
(ORL)(ORL)
Fibers, cables,Fibers, cables,
connectorsconnectors
Source output,Source output,receiver signalreceiver signal
levellevel
ExampleExample InstrumentInstrumentParameterParameter
Power meter andPower meter and
source, or Optical Losssource, or Optical LossTest Set (OLTS)Test Set (OLTS)
Attenuation or lossAttenuation or loss
Power meterPower meterOptical powerOptical power
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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, calledinsertion 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 fromlanshack.com
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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)
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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 fromlanshack.com
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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
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OTDROTDR
Optical Time-Domain Reflectometer Optical Time-Domain Reflectometer
Image from exfo.comImage from exfo.com
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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 andlater referencelater reference
Cable can be measured from one endCable can be measured from one end
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Backscatter Backscatter
A small amount of light is scattered backA small amount of light is scattered back
to the source from the fiber itself to the source from the fiber itself
Splices or connector pairs cause a largerSplices or connector pairs cause a largerreflection 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)
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OTDR DisplayOTDR Display
Dead
zone
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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
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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
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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 & f Images from links Ch 17 e & f
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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 withoutinterrupting serviceinterrupting service
Can detect a specialCan detect a special
modulated tone sentmodulated tone sentdown a fiberdown a fiber•• Image from tecratools.com (linkImage from tecratools.com (link
Ch 17d)Ch 17d)
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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 tobe
Tested
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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
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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
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Attenuators Attenuators
Simulates the loss of aSimulates the loss of a
long fiber runlong fiber run
Variable attenuatorsVariable attenuators
allow testing a networkallow testing a networkto 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
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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
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Optical Power LevelsOptical Power Levels
Detectors are Silicon, Germanium, orDetectors are Silicon, Germanium, or
Indium-Gallium-Arsenide semiconductorsIndium-Gallium-Arsenide semiconductors
+10 to -6+10 to -61300, 15501300, 1550CATVCATV
-10 to -30-10 to -30665, 790, 850,665, 790, 850,
13001300DataData
15501550
1330, 15501330, 1550
WavelengthWavelength Power Range (dBm)Power Range (dBm)Network TypeNetwork Type
+20 to -30+20 to -30Telecom DWDMTelecom DWDM
+3 to -45+3 to -45TelecomTelecom
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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
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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
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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 method cutback method
•• Cut off Cut off knownknown
length,length,
measuremeasure
powerpowerincreaseincrease
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Sources for Loss MeasurementsSources for Loss Measurements
Most multimode systems use LEDMost multimode systems use LEDsourcessources
•• 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 lasersourcessources
Test with the source you will reallyTest with the source you will reallyuseuse
•• 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)
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Modal Effects in Multimode Fiber Modal Effects in Multimode Fiber
Mode scramblers mixMode scramblers mixmodes to equalize power inmodes to equalize power inall modesall modes
•• Can be made with a sectionCan be made with a sectionof step-index fiberof step-index fiber
Mode filters remove higher-Mode filters remove higher-order modes to reachorder modes to reachequilibrium modalequilibrium modaldistributiondistribution
•• Can be made with a mandrelCan be made with a mandrel
wrapwrap
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Modal Effects in Singlemode Fiber Modal 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
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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
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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
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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
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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 andaverage the resultsaverage the results
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OTDR GhostsOTDR Ghosts
Secondary reflection appears at double the realSecondary reflection appears at double the realcable lengthcable length
Using index-matching gel will eliminate ghostsUsing index-matching gel will eliminate ghosts
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DispersionDispersion
Multimode fibers suffer fromMultimode fibers suffer from modal modal
dispersiondispersion
All fibers suffer fromAll fibers suffer from chromatic dispersionchromatic dispersion
•• Because different wavelengths travel atBecause different wavelengths travel atdifferent 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 dispersion polarization mode dispersion also mattersalso matters
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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)
InputOutput
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Connector Insertion Loss TestConnector Insertion Loss Test
This test gives the typical loss of aThis test gives the typical loss of aconnector typeconnector type
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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
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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
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Optical Return Loss in ConnectorsOptical Return Loss in Connectors
A pair of glass-air interfaces forA pair of glass-air interfaces fornonphysical contact connectors withoutnonphysical contact connectors withoutindex-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 of PC 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 highbitrate singlemode systemsbitrate singlemode systems
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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
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Double-Ended Loss TestDouble-Ended Loss Test
Uses both a launch and receive cableUses both a launch and receive cable
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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
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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
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Choosing a Launch Cable for Choosing 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
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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 fibercauses a large losscauses a large loss
•• Both because of diameter and numericalBoth because of diameter and numerical
apertureaperture
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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 of cable, or three-cable, but the type of
reference must be documentedreference must be documented
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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
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Optical Splitter Optical 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)
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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
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M to N Coupler M to N Coupler
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Making CouplersMaking Couplers
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Wavelength Division MultiplexersWavelength Division Multiplexers
Light entering from the left containing twoLight entering from the left containing twowavelengths 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 andtechniques to testtechniques to test
•• Image from link Ch 17oImage from link Ch 17o
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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
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Fiber Optic SwitchFiber Optic Switch
See links Ch 17q and 17rSee links Ch 17q and 17r
Fib O ti D t li kFib O ti D t li k
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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 duplex full duplex ) with two links working) with two links working
in opposite directionsin opposite directions
Bit E R tBit E R t
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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
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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
D ’t U OTDR t MDon’t Use an OTDR to Meas re
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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 ina networka network
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Back ReflectionBack Reflection
Back reflection can cause networksBack reflection can cause networksto 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-reflectionCure it by splicing in low-reflection
patch cords to replace high-reflectancepatch cords to replace high-reflectanceconnectorsconnectors
•• Angled PC connectors are designed toAngled PC connectors are designed tominimize reflectance for this reason (notminimize reflectance for this reason (notmentioned in textbook)mentioned in textbook)
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ReliabilityReliability
Once installed, the fiber opticsOnce installed, the fiber opticsshould 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 cableindoorsindoors
•• A broken cable just behind a connectorA broken cable just behind a connectorin a patch panel is hard to findin a patch panel is hard to find
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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 tillthey failthey fail