In many of my previous articles on fiber optic testing, I have mentioned optical time-domain reflectometers (OTDRs). OTDRs are valuable fiber optic testers when used properly. Improper use, however, can be misleading and, in my experience, can contribute to expensive mistakes for the contractor. I have been personally involved in several instances where misapplication of OTDR testing has cost the contractor as much as $100,000 in wasted time and materials. Needless to say, it’s extremely important to understand how to use these instruments correctly.
An insertion-loss test made with a light source and power meter is simple and similar in principle to how a fiber optic link works. A light is placed on one end of the cable, and a power meter measures loss at the other end, just like a link transmitter and receiver use the fiber for communications.
An OTDR, however, works like radar. It sends a pulse down the fiber and looks for a return signal, creating a display called a “trace” or “signature” from the measurement of the fiber. Two factors contribute to the return signal from the OTDR test pulse: reflectance and backscatter.
Reflectance signals are the peaks in the OTDR trace that are produced by the polished fiber end at a connector or cleave where light is reflected back up the fiber.
Backscatter is a much smaller signal caused by the interaction of the light with the molecules in the fiber. When light strikes the molecules of the glass, some light is scattered like billiard balls, and a small amount (about 1 millionth) goes back up the fiber to the OTDR where it is amplified and measured. Both reflectance and backscatter are measured in decibels (dB) on the vertical axis of the OTDR trace.
Since the OTDR uses a pulse travelling down the fiber as its test signal, the signal varies over time depending on where the pulse is along the length of the fiber being tested. Knowing the speed of light in the glass fiber allows the OTDR to calculate the distance down the fiber at any point on the trace, making the trace a graph of optical power in decibels versus the length of the fiber.
When the OTDR signal travels down a length of fiber, the signal is attenuated by the loss in the fiber itself, which is caused by scattering and absorption. This is seen in a trace by a line sloping down, allowing for the measurement of the attenuation coefficient of the fiber. When the test signal pulse goes past a splice, the loss of the splice causes the signal to decrease, which is seen as a drop in the trace line on the OTDR at the distance where the splice is located. If the pulse goes through a connection (a joint made by two connectors), it will show a drop caused by the connection loss and a peak caused by reflectance in the connection. Even areas of high stress on the fiber, such as kinks in the cable, can be detected with the OTDR.
If the OTDR provides all this useful information, what could be the drawbacks of using them? For many contractors, cost is the first problem. OTDRs cost about 10 times the price of a light source and power meter, so you need to have a real need for one before making such a large investment. Otherwise, when you need them, you can rent them and charge the expense to the job where they are used.
OTDRs are also complex devices. The installer using one needs to know when OTDR testing is appropriate, how to set the instrument up properly and how to interpret traces. Some manufacturers have told contractors that they can use OTDRs anywhere, and all they need to know is “hit the autotest button.” Believing that is what cost several contractors I know so much time and money.
Finally, OTDR traces can have many errors that only the trained and experienced user will understand. Any contractor or installer using OTDRs needs to have proper training to recognize ghosts, gainers and other quirks of OTDR traces.
Over the next few months, we’ll explore where OTDR use is appropriate, how to use setup parameters to get the best results and how to properly interpret fiber traces. We’ll also look into how these complex devices can fool you and get you into big trouble!
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