Fiber Optic Cable Testing

7/31/2019 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



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



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



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 fromlanshack.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 Reflectometer Optical 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 andlater referencelater reference

Cable can be measured from one endCable can be measured from one end



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)



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 & f Images 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 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)



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



MicroscopeMicroscope

Used to inspectUsed to inspect

fibers andfibers and


•• 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



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



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

+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



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 method cutback method

•• Cut off Cut off knownknown

length,length,

measuremeasure

powerpowerincreaseincrease



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)



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



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



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 andaverage the resultsaverage the results



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



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



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



Connector Insertion Loss TestConnector Insertion Loss Test

This test gives the typical loss of aThis test gives the typical loss of aconnector typeconnector type



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


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 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



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 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



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



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



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 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)



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 Coupler M to N Coupler



Making CouplersMaking Couplers



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



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

Fib O ti D t li kFib O ti D t li k



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



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

D ’t U OTDR t MDon’t Use an OTDR to Meas re



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



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)



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



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

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