Reference Test Jumper Cables and Mating Adapters

reference cables

In order to test cables with a power meter and source or with an OTDR, one needs to establish test conditions. The test conditions are similar to how the actual cable plant will be used when communications equipment is connected (see below.)  For insertion loss testing, this requires reference launch jumper cables to connect the test source to the fiber in the cable under test and receive cables to connect the fiber optic power meter. For OTDR testing, this requires a reference launch cable to connect the OTDR to the fiber in the cable under test and a receive cable at the far end of the fiber.

measuring loss of fiber optic cable plant
Reference cables used with test equipment function similarly to the patchcords used connect the communications equipment to the cable plant. A fiber optic test source is similar to the transmitter of a datalink and an optical power meter is similar to a receiver, so the test made with the source and meter, called an insertion loss test, tests a fiber in the cable plant in a manner similar to how it is used when operating as a data link. Thus the reference cables can be considered substitutes for the patchcords. The launch cable can be used to set the test conditions, including modal conditions in multimode cable, and the reference connector on the launch cable mated to the connector on the fiber under test is used to test the quality of that connection. At the far end, the receive reference cable connector tests the connector on the cable under test and connects to the meter to measure the loss.

An OTDR tests the fiber differently, using the backscatter from the fiber to create a snapshot of the fiber. See below. (For more information on OTDR testing, read the section on the FOA Guide on OTDRs.)

Reference Cables For Insertion Loss Testing (see diagram above)

insertion loss DE

Insertion loss testing may require one, two or three reference cables, depending on the test performed and the appropriate mating adapters for the connectors.

3 ways for 0 dB reference

The 1 cable reference is used when the connector type on the instruments matches the connectors on the cable plant, e.g. both use SC connectors. After setting the 0 dB reference, the meter is disconnected and a receive cable attached to it. The source launch reference cable is connected to one end of the cable to be tested, the receive reference cable and meter to the other end, and a measurement of loss is made. This method is generally the preferred method of testing loss.

Note: Once the 0 dB reference is set, one should not disconnect the launch cable from the source. Doing that will change the 0 dB reference set and will require setting the 0 dB reference again.

The 2 cable reference can be used when the connectors on the instruments and the cable plant do not match, e.g. SC connectors on the instruments and LC on the cable plant. The reference cables will be hybrid cables, SC on one end and LC on the other end. After setting the reference, the 2 cables are disconnected at the middle connection, attached to the cable being tested and the loss measured.

The 3 cable method is used when the connectors on the instruments and the cable plant do not match and/or the connectors on the cable plant are plug/jack connectors such as the MPO connector with alignment pins on one connector and holes on the mating connector. Here the two cables on the ends are hybrid cables with connectors on one end to mate to the instruments and connectors on the other end to mate with the 3rd reference cable. The 3rd reference cable is a short cable with connectors that are identical to the cable plant being tested. After the 0 dB reference is set using all 3 cables, the center cable is replaced by the cable plant to be tested and the loss measured. This is sometimes called a "cable substitution test."

The cable plant loss you measure will depend on the reference method you use. The 2 cable method includes one connection between reference cables when you set the 0 dB reference and the 3 cable method includes two connections. The loss you measure with the 2 and 3 cable reference methods will be reduced by the loss of the connections included when setting the 0 dB reference.

Note: "1 c" is the loss of the connection when setting a 0 dB reference with 2 cables and "2c" is the loss of the 2 connections included when using the 3 cable reference method.

test 3 ways

If the test requires mode conditioning, such as with multimode fiber requiring a mandrel wrap or encircled flux (EF) launch condition, the mode conditioning is done in the launch cable attached to the source. Some multimode LED sources are specified for EF launch and do not require mode conditioning.

mode conditioning

Reference cables for insertion loss testing with a light source and power meter are typically 1-3 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 ~20-100 meters for multimode and ~1 km for singlemode.)

The launch and receive cables must be made with fiber that is the same as the fiber in the cable plant being tested and connectors matching the cables to be tested and terminated carefully to ensure low loss.

The accuracy of the insertion loss measurement will depend on 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 with them.

Reference Cables For OTDR Testing

OTDR test

OTDR testing can be done with one or two reference cables. Each cable has a different purpose.

The OTDR launch cable is used to overcome the "dead zone" of the OTDR caused by the high power test pulse overloading the receiver circuitry. So the launch cable must be long enough to allow the OTDR circuitry to recover. This varies according to the range of the OTDR, the width of the test pulse and the signal processing, but should be fairly long. For singlemode long distance tests, at least a 1 km launch cable is recommended. For short distance tests, like FTTH cables using high resolution OTDRs, the cable can be as short as 50-100 meters. Multimode cables are always short, so a cable of 50-100 meters is usually adequate.

The purpose of the receive reference cables is to test the connection with the connector on the end of the cable under test.  That leads to the requirement for a receive reference cable on the far end, a cable sometimes called a "tail cord," a strange name from international standards - does that make the launch cable the "head cord"?

Some OTDR tests are done with only a launch cable if the test is to verify splices in a cable being installed and segments spliced together and there is no connector on the far end..

Requirements For Reference Cables

  • Fiber type matches fiber in the cable plant being measured
  • Connectors can mate with instruments and connectors on cable plant which may require hybrid cables for 2 or 3 cable reference methods
  • Cables for insertion loss measurements should be 1-3 meters long.
  • Cables for OTDR testing should be sufficiently long for the launch cable to extend beyond the OTDR dead zone under test conditions and the receive cable be long enough to be easily seen by the OTDR.
  • Reference cables are low loss when measured against each other
  • Testing requires compatible connector mating adapters

Fiber Type

The fiber type must match the type of fiber in the cable plant. Singlemode reference cables will generally be G.652 fiber or G.657 bend-insensitive fiber. If the cable plant is very long and uses some version of dispersion shifted fiber, the only effect of using G.652 fiber for reference cables is a small mismatch loss at the connections on the ends. Multimode fiber is more sensitive, but while most cable plants use 50/125 fiber and any version of 50/125 fiber can be used (OM2/3/4/5), there are still cable plants using 62.5/125 multimode fiber which cannot be tested with reference cables with any other fiber due to the large mismatch losses, up to ~4 dB when going from 62.5/125 to 50/125 fiber.  See FOA Guide page on mismatched fiber losses.

Recent updates in standards have approved using bend-insensitive (BI) fibers for reference cables.With multimode cable plants, virtually all recent installations use BI fiber anyway, so using BI fiber reference cables is correct. Singlemode BI fiber is mostly used for patchcords and for new high-density microcables and high fiber count cables, but there appears to be no problem mating BI and regular fiber for testing purposes.

Connectors And Mating Adapters
Reference cables must have connectors that can mate to the connectors on the test equipment and cable plant under test. Some test equipment hs fixed connectors (e.g. SC) while others have modular connector adapters that allow changing the style of connectors. The big problem is often the test equipment has SC conenctors, or even modular connectors for any connector with a 2.5 mm ferrule, but many cable plants are now using LC connectors with 1.25 mm ferrules. One solution is to use hybrid test cables with SC connectors on one end and LC connectors on the other. Set the reference with 2 cables and record the test method with the loss data and/or compensate for the reference method.

Adapters that convert from SC to LC connectors are available but they have some problems when used with testing. Some a simple mechanical adapters, with mating sleeve with different hole sizes on each side, These cab be highly variable in loss, especially with singlemode fiber. Other adapters have fiber inside the adapter and are more consistent, but they require compensation when used with test equipment. Hybrid cables and a 2 or 3 cable reference method may provide better results.

"Special Quality" Reference Cables
Standards groups have tried to specify the quality of reference cables in terms of tight tolerance components like the fiber and connectors. Standards which call for special reference quality test cables  specify 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. For ferrule type connectors, the mating adapter is the critical alignment device for the ferrules and is an important component in the loss testing of a connection. 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.

How Long Do Reference Cables And Mating Adapters Last?
Multiple matings will cause connectors to wear on the end face and sometimes along the ferrules also. Most connectors are good for hundreds or even thousands of matings as long as they are cleaned regularly. Microscope inspection will show scratches or scuffing on the connector endface, indicating wear may be a problem. Testing with a single-ended loss test against each other with new mating adapters and comparing results taken when they were new or last tested will show if they are a problem. Mating adapters are harder to test, so they should also be cleaned regularly using the probe type cleaners recommended for jacks in patch panels and discarded after a number of matings - perhaps 500 for metallic alignment sleeves or several thousand for ceramic alignment sleeves. If you notice connector ferrules getting dirty on the side of the ferrule, change the mating adapters then. 

Caring For Reference Cables
It is very important to keep reference cables in good condition as they are a very important factor in making accurate and repeatable fiber optic measurements. Here are some guidelines for their care:

  • Inspect and clean regularly
  • Test against each other for loss
  • Keep protective caps on connectors
  • Store in safe place to prevent damage
  • Do not kink, twist or otherwise stress cables
  • Repair or replace when needed

Field Use - Creating Reference Cables When You Don't Have Them
You should always use the proper cables for reference cables, but if you find yourself in the field without a set of reference cables or your reference cables are worn and causing high loss measurements, you can always use patchcords or other cables, even fibers in a cable you are testing, as reference cables. Remember this adds uncertainty to the measurements due to the condition of the connectors and length of fiber.

OLTS Testing
For a insertion loss testing of a multi-fiber cable with a meter and source, you can use one fiber as a launch cable and loop-back 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 or use the 2-cable reference.  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  Testing
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  


(C) 2022, The Fiber Optic Association, Inc.