1. 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

2. 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

3. 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

4. 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)

5. 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

6. 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

7. OTDROTDR
Optical Time-Domain ReflectometerOptical Time-Domain Reflectometer
 Image from exfo.comImage from exfo.com

8. 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

9. 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)

10. OTDR DisplayOTDR Display
Dead
zone

11. 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

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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)

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. Double-Ended Loss TestDouble-Ended Loss Test
 Uses both a launch and receive cableUses both a launch and receive cable

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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)

47. 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

48. M to N CouplerM to N Coupler

49. Making CouplersMaking Couplers

50. 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

51. 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

52. Fiber Optic SwitchFiber Optic Switch
 See links Ch 17q and 17rSee links Ch 17q and 17r

53. 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

54. 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

55. 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

56. 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

57. 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)

58. 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

59. 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