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.
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 568)
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 plant:
Calculate
the approximate loss as:
(0.5 dB X
# connectors) + (0.2 dB X# splices) + (fiber attenuation X the total
length of cable)
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.