Reliable and accessible fiber links are the very foundation of a sound optical network. So in order to assess the integrity of the infrastructure, we need accurate and faster methodologies and testing devices. OTDR (Optical Time-Domain Reflectometer) is such a powerful test instruments for fiber optic cable testing: when used properly, it not only simplifies testing requirements, but also help to increase the reliability and value of the network.
OTDR is used to test the performance of newly installed fiber links and detect problems that may exist in them. Its purpose is to detect, locate, and measure elements at any location on a fiber optic link. OTDR works like radar—it sends pulse down the fiber and looks for a return signal, creating a display called a “trace”or “signature” from the measurement of the fiber. Simply by connecting one end of the fiber, OTDR can calculate fiber attenuation, uniformity, splice and connector losses, then provides pictorial trace signatures (a graph of optical power in dB versus the length of the fiber). Its ability to locate and measure reflectance and loss makes OTDR the troubleshooting and fault locating equipment of choice. The picture below shows working principle and features of OTDR trace.
There seem to be a lot of confusion about where and how to properly use an OTDR. Given the two distinctly different fiber optic applications—outside plant (OSP) cabling and premises cabling, the functions of OTDR varies in different situation: In a long outside-plant cable with many splices, the OTDR is indispensable and often used to ensure the cable has not been damaged during installation and that each splice is properly made. And it is used for troubleshooting problems, such as finding locations of cable breaks. Premises cabling has short cable runs and almost never includes splices, so OTDR is poised as an alternative to insertion-loss testing with a light source and power meter—with OTDR price roughly 10 times higher than those fiber testers.
OTDR testing can either use one launch cable for testing, or use a launch cable together with a receive cable. The testing results, therefore, are different.
The high power test pulse of the OTDR overloads the instrument’s receiver, at this point, no measurements can be made, making the OTDR “blind” for that period of time. OTDR requires some time for recovery, which causes OTDR dead zone. In general, there are two types of dead zones—event dead zone (EDZ) and attenuation dead zone (ADZ).
Event dead zone: the minimum distance between the beginning of one reflective event and the point where a consecutive reflective event can be detected. Event dead zone is the location where the falling edge of the first reflection is 1.5 dB down from the top of the first reflection.
Attenuation dead zone: the minimum distance after which a consecutive non-reflective event being detected and measured. It is the location where the signal is within 0.5 dB above or below the back-scatter line that follows the first pulse. The attenuation dead zone specification is always larger than that of a event dead zone.
Dead zones can be overcome by connecting a long launch cable to the OTDR. Launch fibers place a necessary length of fiber between the OTDR and the actual fiber being measured, providing both the time and distance required for the OTDR to effectively measure the characteristics of the fiber being tested. Launch fibers do not interfere with the actual fiber, so they are very secure for identifying faults in the total length of fiber.
Launch and receive cables consist of spools of fiber with specific distances. They are usually connected to both ends of the fiber being tested, in order to qualify the front end and the far end connectors using an OTDR. The length of the launch and receive cables depends on the link being tested—generally between 300 m and 500 m for multimode testing and between 1000 m and 2000 m for single-mode testing. For very long haul, 4000 m of cable may be used. The larger the pulse width, the longer the launch cable and receive cables. Always notice that launch and receive cables must be the same type as the fiber under test.
The successful use of an OTDR requires knowing how to operate the instrument, choosing the proper measurement parameters and correctly interpreting the traces. So let’s look at how to set up the instrument properly, as that is the key element in making good OTDR measurements.
OTDR test range: the first OTDR parameter to set is the range, which is the distance the OTDR will measure. The range should be at least twice the length of the cable you are testing. Longer ranges will make the resolution of the trace poorer, and shorter ranges may create distortions in the trace.
OTDR test pulse width: 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.
OTDR test pulse width wavelength: Normally, it’s 850 nm on multimode fiber optic cable and 1,310 nm on single-mode—the shorter wavelength has more back-scatter, 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.
Number of averages for each trace: To improve the signal-to-noise ratio of the trace, the OTDR can average multiple measurements, but the more averaging, the more time it takes. Usually, 16 to 64 averages are adequate.
OTDRs are invaluable test instruments that can locate problems and fault in your optical fiber links, ensuring a reliable and sound network performance. Once you’re familiar with OTDR function and how to use it properly, you’ll be prepared to detect and eliminate your optical fiber events. FS provides OTDR with a variety of fiber types and wavelengths, including single mode fiber, multimode fiber, 1310nm, 1550nm, 1625nm, etc. For more details, visit www.fs.com.
Related Article: Working Principle and Characteristics of OTDR
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