Reference Test
Jumper Cables and Mating Adapters
- In order to test cables in an insertion loss test or with an OTDR,
one needs to establish test conditions. For insertion loss testing, this requires reference launch jumper
cables to connect the test source to the cable under test and
receive cables to connect the fiber optic power meter. For accurate
measurements, the launch and receive cables must be made with
fiber and connectors matching the cables to be tested and terminated carefully to ensure low loss.
- Insertion
loss testing requires one or two reference cables, depending on the
test performed and the appropriate mating adapters for the connectors.
Reference cables for insertion loss testing with a light source and
power meter are typically 1-2 meters long, with fiber and connectors
matching the cables to be tested.
- OTDR
reference/launch cables are also used to isolate the large reflectance
from the instrument connector asw well and need to be much longer,
typically ~100 meters for multimode and ~1 km for singlemode.
- The
accuracy of the measurement will depend on the quality of the reference
cables, since they will be mated to the cable under test and the
connection loss is measured against the connector on the reference
cable. To provide
reliable measurements, launch and receive cables must be in good
condition and kept very clean. They can easily be tested against each other to insure
their performance. Connector mating adapters are used
to connect the cables under test to the launch and receive cables.
Only the highest performance bulkhead splices should be used,
and their condition checked regularly, since they are vitally
important in obtaining low loss connections.
The
quality and cleanliness of the connectors on the launch and receive
cables is one of the most important factors in the accuracy of loss
measurements. Always test reference cables by the patchcord or single
ended method (FOTP-171, reversing the cables to test connectors on both
ends) to make sure they are in good condition before you start testing
other cables.
- Special Reference Cables
- Standards
groups have not been able to successfully specify the quality of
reference cables in terms of tightly toleranced components like the
fiber and connectors. Standards which call for special reference
quality test cables now specify cables with low loss connections. The
best recommendation for qualifying reference cables is to choose cables
with low loss, tested "single-ended" per cable test standard FOTP-171.
Mating Adapters
Connector-to-connector
mating adapters are used to connect reference cables to the cables
being tested. Only the highest quality mating adapters should be used
for testing, as they are a factor in loss also. Inexpensive adapters
generally have plastic mating sleeves to align the connector ferrules
which wear out quickly, causing high loss with even good connectors.
Use only mating adapters with metal or preferably ceramic mating
sleeves which are specified for both multimode and singlemode
connectors.
Field Use - Creating a "Launch Cable"
If
you find yourself in the field without a set of reference cables, you
can always test patchcords or other cables, even fibers in a cable you
are testing, against other cables and use any low loss cables as
reference cables.
OLTS Testing
For
a insertion loss testing with a meter and source, you can use one fiber
as a launch cable and loopback fibers from the far end of the
cable to test other fibers. You will need to take the meter to the far
end of the cable and calibrate the output of the launch cable.
It's easier to do this with two sets of meters and sources, but can be
done with one.
Continue
looping fibers back at the end of the cable. Remember each loopback
added includes the attenuation from two fiber lengths and two connector
losses!
OTDR Tests
If you are testing a multi-fiber cable that's long enough to test with
an OTDR and you have already tested it with an OLTS (like standards
require) so you know the fiber is good, you can use one of the fibers
in the cable as a launch cable. Set up as below and simply have another
tech connect up the fiber being used as the launch cable to the other
fibers to test each of them. If you want to test from one end by
yourself, loopback from the far end and then use the two fibers as the
launch cable to connect up to other fibers from the same end as the
OTDR (second drawing.) If you have access to both ends, you can
continue looping back and get many fibers on one trace, to the length
capability of the OTDR.
Additional Reading
Testing Installed Cable Plants
Accuracy of fiber optic measurements
