- The Fiber Optic Association - Tech
Guidelines On What Loss To Expect When
Testing Fiber Optic Cables For Insertion Loss With A Meter and
Source or OLTS
To be able to judge whether a fiber optic
cable plant is good, one does a insertion loss test with a light source
and power meter and compares that to an estimate of what is a reasonable
loss for that cable plant. The estimate, called a "loss budget" is
calculated using typical component losses for each part of the cable
plant - the fiber, splices and/or connectors. If the measured loss
exceed the calculated loss by a significant amount (remembering the
inherent uncertainty in all measurements), the system should be tested
segment-by-segment to determine the cause of high loss.
FOA has a free app for smartphones and
tablets that will calculate loss budgets for the cable plant you are
designing or testing. See the app store for your device for details.
- How to
calculate a loss budget.
- Connector Loss:
- For each connector,
figure 0.3 dB loss for most adhesive/polish connectors. The loss spec
for prepolished/splice connectors will be higher (0.75 max per EIA/TIA
- When testing cable plants per OFSTP-14
(double ended), include connnectors on both ends of the cable. When
testing per FOTP-171 (single ended), include only one connector -
the one atached to the launch cable.
- Splice Loss
- For each splice,
figure 0.3 dB (0.3 max per EIA/TIA 568)
- Fiber Loss:
- For multimode fiber,
the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300
nm. (3.5 and 1.5 dB/km max per EIA/TIA 568)
This roughly translates into a loss of 0.1 dB per 100 feet (30 m) for
850 nm, 0.1 dB per 300 feet(100 m) for 1300 nm.
- For singlemode fiber,
the loss is about 0.5 dB per km for 1310 nm sources, 0.4 dB per km for
1550 nm. (1.0 dB/km for premises/0.5 dB/km at either wavelength for
outside plant max per EIA/TIA 568)This
roughly translates into a loss of 0.1 dB per 600 (200m) feet for 1310
nm, 0.1 dB per 750 feet (250m) for 1300 nm.
- So for the estimated loss of a cable
- Calculate the approximate loss as:
- (0.5 dB X # connectors) + (0.2 dB X#
splices) + (fiber attenuation X the total length of cable)
- For a better estimate, calculate
a loss budget.
- What about OTDR testing?
- OTDRs are used for verifying indivudual
events like splice loss on long links with inline splices or for
troubleshooting. All standards require an insertion loss test for
qualification of the link loss. In MM fibers, the OTDR will
underestimate the loss considerably - as much as 3 dB in a 10 dB link
- but the amount is unpredictable. In long distance SM links, the
difference may be less, but there are other measurment uncertainties,
like connector or splice loss, where the OTDR can show a gain.
- What happens when you test
with an OTDR with its limited distance resolution?
Specifically, if you have singlemode fiber terminated with
fusion spliced pigtials, you cannot see the both splice
and the connector losses. Or what if you have a patch
panel with connections using short patchcords?
For insertion loss testing, you simply sum up all the loss
contributors and get a total for the cable run. In
the case of an OTDR, you are analyzing each event.
So if you have a connection point where both fibers were
terminated with spliced-on pigtails, you should analyze
the event as the sum of 2 fusion splices and one
connection, not each individually. A patchcord termination
would be two connection losses, plus splices if the
termination was by splicing on pigtails.
- For more on OTDRs, see the
FOA Online Reference Guide on Testing or Lennie
- Will the network run on that link?
- Here is a table
showing the loss margin for most fiber optic LANs and links. If
the loss of the cable plant is less than the maximum loss allowed for
the link, it should run (but you really want a little bit of margin!)
(C) 2004-9 The Fiber Optic Association, Inc.
More detailed information can be found on
the FOA Online Reference Guide.
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