Guidelines On What Loss To Expect When
Testing Fiber Optic Cables
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.
A loss budget estimate can also be used to compare results from OTDR
testing, but the inherent uncertainties of OTDR testing make the estimate
less accurate. See OTDR Measurement Uncertainty in the OTDR
standards refer to the loss budget as the "attenuation allowance" but
there seems to be very limited use of that term.
The calculated loss budget is an estimate
that assumes the values of component losses and does not take into
account the uncertainty of the measurement. Be aware of this because
if measurements are close to the loss budget estimates, some judgement
is needed to not fail good fibers and pass bad ones!
Cable Plant Loss Budget
The cable plant "loss budget" is a function of the losses of the
components in the cable plant - fiber, connectors and splices, plus any
passive optical components like splitters in PONs.
Thus the loss budget of the cable plant is a major factor in the power
budget of the fiber optic link and is what one calculates to compare
against tested insertion loss (and even compares to OTDR loss
measurements) to determine if the cable plant is properly installed.
FOA has a free app for iOS smartphones and tablets that will calculate
loss budgets for the cable plant you are designing or testing. See the
Apple app store for your device for details.
to calculate a loss budget.
- For each
connector, we usually figure 0.3 dB loss for most adhesive/polish or
fusion splice-on connectors. The loss spec for prepolished/mechanical
splice connectors or multifiber connectors like MPOs will be higher
(0.75 max per EIA/TIA 568)
- When testing
cable plants per OFSTP-14 (double ended), include connnectors on both
ends of the cable when using the 1-cable reference For other options
see the note below. When testing per FOTP-171 (single ended), include
only one connector - the one attached to the launch cable.
- For each
splice, figure 0.3 dB (0.3 max per EIA/TIA 568)
- 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.
for the estimated loss of a cable plant, calculate the approximate
- (0.5 dB X #
connectors) + (0.2 dB X# splices) + (fiber attenuation X the total
length of cable)
- For more
information see calculate a loss budget.
about OTDR testing?
- OTDRs are used
for verifying individual 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 measurement
uncertainties, like connector or splice loss, where the OTDR can show
- 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
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
On Including Connectors On The Ends And Test Methods: Many
designers and technicians wonder when doing a loss budget whether the
connectors on the end of the cable plant should be included in the
loss budget. When the cable plant is tested, the reference cables will
mate with those connectors on the ends and their loss will be included
in the measurements but the results depends on the
method used to set the "0dB"
- All three of
these methods are approved in many standards, but it is important to
realize they will give different loss readings due to the connections
included when making the 0dB reference measurement. The
math of these methods is detailed here.
If the "0dB" reference for the insertion loss test was done with only
one reference test cable attached between the light source and power
meter which is the most common way, the connectors on the end of the
cable will be included in the loss so the loss budget should include
If the "0dB" reference for the insertion loss test was done with three
cables, the launch reference cable, a receive reference cable and a
third reference cable between them, a method used for many plug and
jack (male/female) connectors such as MPOs, the loss budget should not
incude the connectors on the end. When making the "0dB" reference with
three cables, two connections are included in setting the reference so
the measured value will be reduced by the value of those two
connections. If the loss budget is calculated without the connectors
on the ends, the value will more closely approximate the test results
with a 3-cable reference.
While the two-cable reference method is rarely used, it includes only
one connector. Thus you could use the same approach when calculating
loss budgets for this test method.
Whatever test method is presumed, it must be documented when the loss
budget is calculated.
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) 2004-18 The Fiber
Optic Association, Inc.
More detailed information can be found on the FOA
Online Reference Guide.
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