ch17
Post on 31-Oct-2014
389 Views
Preview:
DESCRIPTION
TRANSCRIPT
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
top related