Download - OTDR > Back To Basics
Martin WarneSenior Technical Sales Engineer
OTDR > Back To Basics
Tuesday 5th June 2018
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The Issues
All of these issues make OTDR testing as much of a headache as it was 25 years ago
Fiber Everywhere Set UpTrainingHigh speed Analysis
FTTH, DC, 5G, etc.
10G FTTH 400G Core. Reduced Needs to be
correct.Right or Wrong?
Remove macrobendsNot accounted for in loss budget and can go undetected. Impact DWDM and future FTTH technologies.
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Verify the network length as per loss budget
Verify connector loss AND reflectance.Dirty or damaged connectors have a high impact on loss budget and are often overlooked.
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1 Verify splicing standard is acceptable and within limits.Resplice if necessary
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Why OTDR?
Reflectometry TheoryOTDR launches short duration light pulses.
The pulses encounter scattering (Rayleigh) and Reflective (Fresnel) events. A fraction of the pulse returns to the launching port. The returned signal is proportional to pulse power and varies as a function of the event.
Measuring the difference between the launching time and the time of arrival of the returned signal, one determines the distance between the launching point and the event.
Lord John Strutt Rayleigh
English Physicist
1842 -1912
Augustin Jean Fresnel
French Physicist
1788 -1827
Reflectometry TheoryRayleigh Backscattering
Comes from the “Natural” reflection of the fiber
The OTDR will use the Rayleigh back reflections to measure fiber attenuation (dB/km)
Back reflection level around -77dB
Higher wavelength will be less attenuated by the Rayleigh Backscatter
Consider splices have ‘natural reflection’ also. Source
Ray of light
Silica particles
Fresnel back reflections
Will come from abrupt changes in the IOR, ex: (glass/air)Fiber break, mechanical splice, bulkheads.
Will show as a “spike” on the OTDR trace
UPC reflection is typically -55dB and APC -65dB (as per ITU)
Fresnel reflections are higher than natural backscatter
Will create a « Dead Zone » after the reflection
Reflectometry Theory
Distance calculation
Time is correlated to distance :D = (c t) / 2n
D : Distance along the fiberc : Speed of light (in a vacuum)t : Time of the round trip of the lightn : IOR
Reflectometry Theory
Corning SMF-28e+ ULL (G.652D) Corning LEAF (G.655)
A Successful Test Strategy
We do see:
Fiber Networks now everywhere, not just core, now -> 4G/5G, FTTH, DC, etc. Training reduced or even stopped altogether
Unskilled technicians working on the network Timescales are tight
Resulting in: Incorrect test sequences Misinterpretation of results Networks passed that should have failed Repeat truck rolls at some stage required
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OTDR: 2 Steps
Set UpOur result will only be as good as our set up. An educated set up will greatly assist in giving a more accurate picture of the
link we want to measure.
Incorrect set up or a dependency on the unit to make a decision for us:
AUTOWill give us a false sense of security that
the OTDR is making the optimum selection of Distance and Pulse Width
Reading the ResultHere lies the greatest cause of
issues, an inability to understand the result in front of you.
Many issues or problems are missed due to misinterpretation
of the result displayed.
Assistance is given in trying to detect macrobends, high losses, high reflectance but it ultimately is the users responsibility to P/F
the link.
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Step 1: Setting up the OTDRWavelengths
Use at least 2 wavelengths to capture macrobends. For long distances use
only 1550 and 1625nm if available.
Pulse WidthProbably the most
important setting. Will determine how much of the link we can see and in what
detail.
Distance/RangeUse a minimum of Launch Cable + Fibre Under Test +
25% for the distance to use.
Averaging/TimeWill allow a cleaner trace to
more accurately see the events. The minimum should
be to see clearly the final event before the end of fiber.
SettingsIoR, BsC, splice,
reflectance and EoFthresholds as well as
P/F thresholds.
Distance*
Pulse Width*
Averaging Time
FUT + 25% ≈Min 15s or 30s
???? > Loss Dependent
Step 2: Reading The Result - Tips
Reminder: Our OTDR Result is ONLY as good as our set up
Inspect and Clean ALL optical connectors on OTDR & ODF Use a Launch/Dummy Fibres to characterise Connector at ODFs Spend 5 – 10 minutes experimenting with settings to optimise
If necessary use AUTO as a first pass and adjust Review your OTDR trace from the START and not the END
Tendency is to look at END, anything lost at START….. Give result a correct naming convention Save in raw or .SOR format
Repeat for Opposite Direction if required (B>A).
Our Result
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1 > Launch/Dummy Fibre: Loss and Reflectance at ODF:
Our Result
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2 > Splices in the link: Loss > Losers and Gainers (due to MFD mismatch)
Our Result
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3 > Missing Splices in the link: Under Threshold Set
Our Result
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4 > End of Fiber: Reflectance only, no Receive/Dummy fiber
Our Result – But We Missed?
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1 > Patch at start of 5m2 > Missing Splices in the link
Pulse Width too large to capture
Ugly Stuff - Bends
Ugly Stuff - Ghosts
Inability to clean connectors leads to high reflectance and ghosting
FTTH NetworksDue to close nature of high loss splitters, may be necessary to do 2 traces to see both and close events.
Multiple Pulse Solutions Use multiple PW depending upon topology Allows sight of near and far end events
Self setting unit conditions Advanced algorithms to consolidate into one file Results displayed as icons and interpretation of trace taken by software not
individual. Can store as Multiple PW file, .SOR and .PDF.
How to See More?
How to See More > P2P Example
Now we can see the near end events and a macrobend > Analysed by the unit, not the individual
How to See More > FTTH Example
OTDR Test Summary?
OTDR Requires KnowledgeSetting Up The Unit
Correct Distance, PW and TimeUse AUTO under conditions!Interpretation
Multiple Pulse Width SolutionsNo set-up – self settingMultiple pulse widths/wavelengthsEasy/intuitive icon results displayReduced training requirements/experience
Q&A
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