In fiber optics, there seems to be a lot of confusion about where and how one properly uses an optical time-domain reflectometer (OTDR).

A big factor causing confusion is what the operator is trying to test. In a long outside-plant cable with many splices, the OTDR is used to ensure the cable has not been damaged during installation and that each splice is properly made. Results are archived with other documentation so that the information is available if restoration is necessary in the future. Later OTDR testing may be used for troubleshooting problems, such as finding locations of cable breaks caused by dig ups.

Premises cabling, however, has short cable runs and almost never includes splices, so the requirement of OTDR testing appears to be as an alternative to insertion-loss testing with a light source and power meter. New international testing standards, in fact, include both OTDR and insertion-loss testing. The differences in the measurement techniques used by OTDRs and a light source and power meter means that OTDR testing may not be comparable to measured insertion loss or the actual loss the communications system will experience, especially on longer cable plants with higher loss.

For this reason alone, it is recommended that insertion-loss testing be conducted even when OTDR testing is required by installation contracts. In my experience, OTDR testing of premises cabling systems is often required by users who do not really understand when OTDR testing is appropriate or even what an OTDR is. A knowledgeable contractor should be able to educate the user on proper testing requirements.

From a more technical standpoint, the first and most important consideration for OTDR use is the length of the fibers to be tested. Most OTDRs, especially single-mode ones, are designed for long cable plants and may be inappropriate for short cables. Some multimode OTDRs are now usable for short length multimode premises cables but only if they are properly set up before use.

The high power test pulse of the OTDR overloads the instrument’s receiver, requiring some time for recovery, making the OTDR “blind” for that period of time. The width of the test pulse limits how closely two connections or splices can be resolved. Since the OTDR converts time into distance, the test pulse determines the distance resolution of the instrument.

Close to the OTDR, this overload causes a “dead zone” where no measurements can be made. This dead zone can be overcome by connecting a long launch cable to the OTDR, long enough to allow the instrument to fully recover. Launch cables are typically about 100 meters long for multimode and around 1 kilometer for single-mode fiber. The launch cable also allows the OTDR to check the first connector on the fiber being tested. An OTDR must always be used with a launch cable that matches the fibers being tested.

Along the length of the fiber, the test pulse limits the resolution of the OTDR for finding and measuring closely spaced events, such as patchcords or broken fibers in splice closures. For best resolution, the OTDR test pulse should be set as narrowly as possible. If connections on the fiber are highly reflective—typical of many connectors—the OTDR may be overloaded again, causing an effect similar to the initial dead zone and making measurements unreliable.

Most OTDRs have an option to control the power of the test pulse using pulse width. The test pulse needs to have a lot of power for long cable runs, typical of outside-plant telecom or utility systems. A wide pulse width is generally not a problem there, since long cables are spliced several kilometers apart and resolution of close events is not a big problem.

In premises cable plants, it’s common to find cable lengths of less than a few hundred meters, 1 meter patchcords and short fibers inside breakout modules on prefabricated cabling systems. These combine to limit the use of an OTDR to special instruments optimized for premises applications with extremely narrow pulse widths for maximum resolution. Even those specialized OTDRs require careful setup and operation to give reliable test data.

Next month, this column will cover the correct setup and use of OTDRs.


HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.