OTDR Measurements

Now that we have covered what an optical time-domain reflectometer (OTDR) is and where to use it, let’s look at how to actually make measurements. Begin by setting up the instrument properly, as that is the key element in making good OTDR measurements.

The most valuable information you can have when testing any fiber optic link is good documentation on the cable plant. Knowing the length of the link and where connectors or splices are located makes testing much easier, whether you used an OTDR or light source and power meter. With documentation on the link, you can calculate a link-loss budget and have a good estimate of what the link loss should be. That gives you pass/fail loss value for the tests and helps to interpret the OTDR trace correctly.

Remember to use proper launch and receive reference cables with the OTDR. The reference cables must match the fiber in the cables being tested and have compatible connectors that are in good condition.
The OTDR requires first determining several measurement parameters. Start by choosing the wavelength for the test. For short multimode premises cables, use 850 nanometers (nm) as it gives the best resolution. Use 1,310 nm on single-mode cables, unless they are very long and require 1,550 nm. The range or distance measured should be set to about twice the length of the fibers being tested. Set signal averaging to about 64 times for short cables, up to 1,024 times for very long cables. The pulse width, which determines the resolution of the measurements as well as the distance capability of the OTDR, should be set at the shortest width that allows the OTDR to see the end of the fiber.

Acquire one trace, and look at it on the OTDR display. To be usable, the trace must clearly separate events. If the pulse width is too wide or events are too reflective, the trace will take a long time to return to the fiber level, making it difficult to interpret. Figure 2 shows good (blue) and bad (red) traces around an event, which in this case is a connector.

The red trace shows why OTDRs are often better at qualitative than quantitative testing. Looking at the trace tells you the event, a poor connector in this case, is so reflective that it saturates the OTDR as indicated by the flat top of the reflective event. The baseline recovery problem following the trace makes it impossible to measure the loss of that event. The blue trace, however, is ideal for analysis. The OTDR has markers (arrows in the figure) that you place on either side of the event, and the instrument calculates the loss.

Once you look at the trace and determine it has no events that make analysis difficult, you can go from end to end and analyze every event, check the fiber attenuation between events, and look for losses caused by stress on the cable. If you used both a launch and a receive cable, you can also use the OTDR to make end-to-end loss measurements by placing one marker before the first connector and one after the last connector.

The latest standards allow using an OTDR measurement of cable plant loss, but under some conditions, the OTDR measurement will be very different than insertion--loss measurements made with a light source and power meter or the actual loss seen by the transmission system. I don’t recommend using the OTDR for this purpose. Since you should be testing fibers with a light source and power meter anyway, you can make comparisons to see the differences for yourself.

When you have analyzed a few fibers and know what events to expect, you may be able to use the OTDR auto test function to check the rest of the fibers. Autotest can be a real time-saver if the traces are simple, but it can be confused easily by some traces. Never use autotest until you have analyzed several traces manually to ensure the OTDR auto test analysis is trustworthy. Blind use of auto test is the No.1 problem we receive calls about at The Fiber Optic Association (FOA).

Using an OTDR is more complicated than I can explain here, so we have created an OTDR course at the FOA’s online learning website, Fiber U, including a free OTDR simulator. You will find it at www.fiberu.org/OTDR.

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

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor
Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com .

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