Fiber Optic Cable TestingFiber Optic Cable Testing

Ch 17Ch 17

Fiber Optics Technician’s Fiber Optics Technician’s Manual, 3Manual, 3rdrd. Ed. Ed

Jim HayesJim HayesRevised 11-24-08

Testing RequirementsTesting RequirementsParameterParameter ExampleExample InstrumentInstrument

Optical powerOptical power Source output, Source output, receiver signal receiver signal levellevel

Power meterPower meter

Attenuation or lossAttenuation or loss Fibers, cables, Fibers, cables, connectorsconnectors

Power meter and Power meter and source, or Optical Loss source, or Optical Loss Test Set (OLTS)Test Set (OLTS)

Back reflection or Back reflection or Optical Return Loss Optical Return Loss (ORL)(ORL)

OTDR or OCWR OTDR 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 be The power meter by itself can be use to measure source poweruse to measure source power

With a source, it can measure the With a source, it can measure the loss of a cable plant, called loss of a cable plant, called insertion lossinsertion loss

Most power measurements are in Most power measurements are in the range +10 dBm to -40 dBmthe range +10 dBm to -40 dBm• Analog CATV (cable TV) or DWDM Analog CATV (cable TV) or DWDM

(Dense Wavelength Division (Dense Wavelength Division Multiplexing) systems can have Multiplexing) 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 three Power meters are calibrated at three standard wavelengthsstandard wavelengths• 850 nm, 1300 nm, 1550 nm850 nm, 1300 nm, 1550 nm

Typical measurement uncertainty is Typical 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 source Test your system with a source similar to the one that will be similar 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 source Power meter and source in a single unitin a single unit• Normally used in pairsNormally used in pairs• Automated, more complex Automated, more complex

and expensive than the and expensive than the combination of a source combination of a source and a power meterand a power meter

Rare in field testingRare in field testing• Image from aflfiber.comImage from aflfiber.com

OTDROTDROptical Time-Domain ReflectometerOptical Time-Domain Reflectometer

Image from exfo.comImage from exfo.com

OTDR UsesOTDR Uses

Measure lossMeasure loss Locate breaks, splices, and Locate breaks, splices, and

connectorsconnectors Produces graphic display of fiber Produces graphic display of fiber

statusstatus• Can be stored for documentation and Can 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 back A 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 larger Splices 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

Deadzone

OTDR AccuracyOTDR Accuracy

OTDR can give false loss values OTDR can give false loss values when coupling different fibers when coupling different fibers togethertogether• Splices can even show more light on the Splices can even show more light on the

other side “gainer”other side “gainer”• This is an illusion caused by increased This 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, shows Simplified, shows distance to a faultdistance to a fault

Links Ch 17c, d, eLinks Ch 17c, d, e

Visual Cable Tracers and Visual 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 get VFL 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 is To check to make sure the correct fiber is

connectedconnected• With bright sources, you can find the break by With 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 fiber through fiber• Images from links Ch 17 e & fImages from links Ch 17 e & f

Fiber IdentifiersFiber Identifiers

Bends the fiber to Bends the fiber to detect the lightdetect the light

Can be used on live Can be used on live fiber without fiber without interrupting serviceinterrupting service

Can detect a special Can detect a special modulated tone sent modulated tone sent down a fiberdown a fiber• Image from tecratools.com (link Image from tecratools.com (link

Ch 17d)Ch 17d)

Optical Continuous Wave Optical Continuous Wave Reflectometer (OCWR)Reflectometer (OCWR)

Measures optical return loss Measures optical return loss (reflectance) of connectors(reflectance) of connectors

Inaccurate on installed systems Inaccurate on installed systems because it includes backscatter and because 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 inspect Used to inspect fibers and fibers and connectorsconnectors• Particularly during Particularly during

epoxy-polish processepoxy-polish process Image from link Ch 17gImage from link Ch 17g

TalksetTalkset

Telephone calls Telephone calls over unused fibersover unused fibers

Rarely needed Rarely needed now that we have now that we have cellphonescellphones• See link Ch 17iSee link Ch 17i

AttenuatorsAttenuators

Simulates the loss of a Simulates the loss of a long fiber runlong fiber run

Variable attenuators Variable attenuators allow testing a network allow testing a network to see how much loss it to see how much loss it can withstandcan withstand

Can use a gap, bending, Can use a gap, bending, or inserting optical or inserting optical filtersfilters• Image from link Ch 17jImage from link Ch 17j

Reference CablesReference Cables

Test cables are needed to connect Test cables are needed to connect the cables to be tested to the test the cables to be tested to the test instrumentsinstruments

Must have correct connectors, be Must 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, or Detectors 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 the Instruments 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 with Although meters show a reading with hundredths of a decibel, they don’t mean hundredths 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 with Measure attenuation with cutback methodcutback method• Cut off Cut off

known known length, length, measure measure power power increaseincrease

Sources for Loss MeasurementsSources for Loss Measurements

Most multimode systems use LED Most multimode systems use LED sourcessources• High-speed multimode often uses High-speed multimode often uses

VCSELs (1 Gbps and higher)VCSELs (1 Gbps and higher)• See link Ch 17kSee link Ch 17k

Singlemode systems use laser Singlemode systems use laser sourcessources

Test with the source you will really Test with the source you will really useuse• BUT Argilent says you should test all BUT 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 mix Mode scramblers mix modes to equalize power in modes to equalize power in all modesall modes• Can be made with a section Can be made with a section

of step-index fiberof step-index fiber Mode filters remove higher-Mode filters remove higher-

order modes to reach order modes to reach equilibrium modal equilibrium modal distributiondistribution• Can be made with a mandrel Can be made with a mandrel

wrapwrap

Modal Effects in Singlemode FiberModal Effects in Singlemode Fiber

Singlemode fibers shorter than 10 Singlemode 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 cable Longer 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 first Nothing can be measured for the first

100 meters or so100 meters or so Distance ResolutionDistance Resolution

• Two events too close together cannot be Two 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 OTDR May 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 than the which is usually 1% - 2% longer than the length along the cablelength along the cable

OTDR Loss ErrorsOTDR Loss Errors

Joining two fibers with different Joining two fibers with different backscatter coefficients will cause:backscatter coefficients will cause:• Too high a loss when measured in one Too 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 and measure from both ends and average the resultsaverage the results

OTDR GhostsOTDR Ghosts

Secondary reflection appears at double the real Secondary 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 from Multimode fibers suffer from modal modal dispersiondispersion

All fibers suffer from All fibers suffer from chromatic dispersionchromatic dispersion• Because different wavelengths travel at Because different wavelengths travel at

different speeds, and no source is completely different 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 test There is a new unit available to test bandwidth in the field, but it is not bandwidth 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 a This test gives the typical loss of a connector type connector 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 polished Used to inspect the ends of polished connectorsconnectors

Helpful to view the connector at an Helpful to view the connector at an angle while lighting it from the sideangle while lighting it from the side

Only defects over the core really Only defects over the core really mattermatter

Optical Return Loss in ConnectorsOptical Return Loss in Connectors

A pair of glass-air interfaces for A pair of glass-air interfaces for nonphysical contact connectors without nonphysical contact connectors without index-matching gelindex-matching gel• 4% reflectance – loss of 0.3 dB due to 4% 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 to Much less with Angled PC connectors – 40 to

60 dB60 dB Reflectance can be a problem in high Reflectance 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 the This 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 lowest It gives highest loss and lowest uncertaintyuncertainty

It tests both connectors on the cable It 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 connectors and It is not necessary to get connectors and

fiber with tighter specificationsfiber 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 or Use splice bushings with metal or ceramic alignment sleevesceramic alignment sleeves

Mismatched FibersMismatched Fibers

Coupling a smaller fiber to a larger Coupling a smaller fiber to a larger one causes only a small loss (0.3 dB one causes only a small loss (0.3 dB or so)or so)

Connecting large fiber to small fiber Connecting large fiber to small fiber causes a large losscauses a large loss• Both because of diameter and numerical Both because of diameter and numerical

apertureaperture

Testing the Installed Cable Plant Testing 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 850 Usually test multimode at both 850 and 1300 nm with LED sourcesand 1300 nm with LED sources

Singlemode test is usually at 1300 Singlemode 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 that For long-distance, and to show that

WDM can be performed laterWDM can be performed later• Also shows microbends – 1550 test is Also 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 into Splits 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, or You 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 two Light entering from the left containing two wavelengths is separated into the two wavelengths 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 and Requires special equipment and

techniques to testtechniques to test• Image from link Ch 17oImage from link Ch 17o

Fiber Optic AmplifiersFiber Optic Amplifiers

Boosts signal without Boosts signal without converting it to converting it to electricityelectricity

Complicated to test, Complicated to test, require special require 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 bidirectional Most networks will be bidirectional

((full duplexfull duplex) with two links working in ) with two links working in opposite directionsopposite directions

Bit Error RateBit Error Rate

The receiver power must be within The receiver power must be within the operating rangethe operating range• Too little power leads to high bit error Too little power leads to high bit error

rates (wrong data at receiver)rates (wrong data at receiver)• Too much power saturates the detector Too 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 connected Often the two fibers are connected backwards – check them with a backwards – check them with a visual tracervisual tracer

Check receiver power levelCheck receiver power level Check plant loss with double-ended Check plant loss with double-ended

methodmethod

Don’t Use an OTDR to Measure Don’t Use an OTDR to Measure Plant LossPlant Loss

OTDR does not see the loss of the OTDR does not see the loss of the end connectorsend connectors

Its power source is not the same as Its power source is not the same as normal LAN power sourcesnormal LAN power sources

OTDR measurements are affected by OTDR measurements are affected by backscatter coefficient which may backscatter coefficient which may not be the same for all the cables in not be the same for all the cables in a networka network

Back ReflectionBack Reflection

Back reflection can cause networks to Back reflection can cause networks to fail even though the loss is lowfail 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 patch Cure it by splicing in low-reflection patch

cords to replace high-reflectance cords to replace high-reflectance connectorsconnectors

• Angled PC connectors are designed to Angled PC connectors are designed to minimize reflectance for this reason (not minimize reflectance for this reason (not mentioned in textbook)mentioned in textbook)

ReliabilityReliability

Once installed, the fiber optics Once 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 cable Use orange or yellow jacket cable

indoorsindoors• A broken cable just behind a connector A 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 in LED in laser transmitter drops in power as it agespower as it ages

Laser sources are feedback-stabilized Laser sources are feedback-stabilized so they remain constant in power till so they remain constant in power till they failthey fail