Reference Test Jumper Cables and Mating Adapters

reference cables

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, two or three 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 as 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.

Mode Conditioning
Mode conditioning is generally required for multimode and singlemode launch cables attached to a source. Multimode cables generally use a mandrel wrap mode conditioner (see here and here) and a LED source but special modal conditioning patchcords are available. Singlemode fiber just needs a 30mm loop to ensure the launch cable is singlemode.

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.

Loopback test OLTS

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!

Loopback test OLTS

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.

OTDR Loopback

OTDR Lookback 2

Additional Reading
Testing Installed Cable Plants  
Accuracy of fiber optic measurements  


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