The Optical Time Domain Reflectometer (OTDR) is an essential instrument for characterizing long outside plant fiber optic cables; it is the only instrument capable of verifying inline splices on concatenated fiber optic cables and locating faults. An outside plant installer will have an OTDR on hand to test every splice made as it is done, so bad splices can be fixed before the splice closure is sealed.
Why aren’t OTDRs more common in premises cabling? First of all, premises cabling is almost never spliced. Connectors that interconnect cables at patch panels terminate continuous cable runs. Secondly, the OTDR has limited distance resolution, making it hard to see cables. Finally, the OTDR uses an indirect measurement technique that gives different losses than a light source and power meter, which is the method that tests insertion loss according to all standards. Also, we are not even addressing the high cost of the OTDR, which has traditionally limited its use.
But some end-users and contractors persist in using OTDRs to test short premises cabling systems, in spite of the drawbacks. I am personally aware of several instances where contractors used OTDRs improperly and rejected perfectly good cable plants, once at enormous cost. If you find yourself in a situation where you must use an OTDR, we can at least give some advice on how to minimize the chance for making a costly mistake.
The successful use of an OTDR on premises cabling requires knowing how to operate the instrument, choosing the proper measurement parameters and correctly interpreting the traces. All OTDR manufacturers have an autotest function on their instruments, similar to Category 5e/6 UTP cabling certifiers. However, the OTDR does not always test the same cabling setup, so its operation cannot be as easily simplified. The OTDR user should never use the autotest function before analyzing one fiber trace from a cable to ensure the instrument is properly set up and the autotest function is giving valid data.
Let’s assume you are trained on the operation of the instrument you are using and have some basic understanding of OTDR traces. (See lennielightwave.com for more information on using OTDRs.) Connect the OTDR to the cable or cable plant you want to test with a reference launch cable at least 100 meters long for multimode cable and 1 km long for single-mode. The connectors on the launch cable should be tested occasionally to ensure they are in good condition, just like reference cables for insertion-loss testing with a light source and power meter.
The first OTDR parameter to set is the range, which is the distance over which the OTDR will measure. The range should be at least twice the length of the cable you are testing, usually 2 km for premises cabling. Longer ranges will make the resolution of the trace poorer, and shorter ranges may create distortions in the trace.
Then set the OTDR test pulse width to the shortest pulse width available, which will provide the highest resolution, giving the best “picture” of the fiber being tested. This is usually listed in nanoseconds (ns), with typical choices of 10 to 30 ns.
Next choose the wavelength. Normally, you start with 850 nm on multimode fiber and 1,310 nm on single-mode; the shorter wavelength has more backscatter, so the trace will be less noisy. After initial tests, you can make measurements at the longer wavelengths (1,300 nm on multimode and 1,550 nm on single-mode) and compare traces at the two wavelengths.
The final parameter is the number of averages for each trace. To improve the signal-to-noise ratio of the trace, the OTDR can average multiple measurements, but more averaging takes more time. Usually, 16 to 64 averages are adequate.
Now take a test trace, and look at the display. Is the trace noisy? If so, more averaging may be needed. Is the end of the fiber at the distance expected based on your knowledge of the length of the cable? If you don’t know the approximate length of the cable, it is easy to become confused by trace artifacts like “ghosts” (see my April 2006 column, “Ghost Busters”). Are the connections visible? Connectors should have high reflection peaks to identify their positions. Are there any peaks where connectors should not be? Those could be ghosts.
Depending on the answers to these questions, you may need to change some of the parameters and take another trace. Once the trace looks good, measure the lengths of individual fiber links and the losses of all the connectors. Save that trace to compare with other fibers in the same cable, to make sure all have similar traces.
Finally, try the autotest function on the OTDR. If the results are similar to the results obtained from the first test, you can feel confident in using it for the other fibers, saving lots of testing time. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.jimhayes.com.