Troubleshooting Fiber

The fiber optic cable you just installed failed testing. What do you do next? How do you find the problem and fix it—fast? Fortunately, fiber optics is easy to install and experienced installers generally find that about 95 percent of all fibers they install will test good. But even the best installers sometimes have problems, and finding the cause can sometimes be easy, sometimes confusing. Let’s look at some common problems and their likely causes.

No Light At All

More often than not, this is not a fiber or cable problem but simply an identification problem, where you are not dealing with the same fiber at both ends. Since cables may have many fibers and color-coding is sometimes hard to see, it’s not unusual to have two techs working on each end of the cable connected to different fibers.

The solution is to use a visible light source—also called a fiber tracer or visual fault locator—to identify the fiber to be tested next. An inexpensive fiber tracer will pay for itself in no time, as it is also a continuity tester, confirming that the fiber is capable of transmitting light.

Sometimes, there really is a fiber fault. If the visual tracer does not shine through the fiber, you have several possible problems. First, make sure it is a problem with just that fiber, not the entire cable. Use the visible fiber tracer to check other fibers. If none are good, better plan on pulling in a new cable.

If only one fiber won’t transmit light, most likely a connector is bad. Diagnosing the faulty one can be a problem. Check the fiber in the end of the connector ferrule with a microscope, looking for dirt, cracks in the fiber or a really bad polish. If both ferrule connectors look OK, the problem is internal.

Unless you have a visual fault locator or OTDR (Optical Time Domain Reflectometer), it might be best to flip a coin to decide which connector to reterminate first. A wrong guess means reterminating both ends, but a visual fault locator (VFL) can help find the bad connector. It uses a bright-red laser coupled into the fiber. To troubleshoot, plug the suspect connector into the VFL and look at its back. If a lot of red light is visible, the connector is bad and should be replaced. If you look from the other end and see light coming only out of the fiber, that indicates a good connector. If the whole ferrule glows, it’s bad.

OTDRs can determine the bad connector if the cable is long enough. Use a long launch cable and the shortest pulse width for maximum resolution. Another trick is to connect the far end to a known good fiber, which allows testing both connectors at once.

Both the VFL and OTDR can find broken fibers, but the durability of the fiber optic cable makes that a less-likely event. If the break is near the cable end, often in the stripped portion of a distribution or loose-tube cable, use a VFL since it will be too close to the cable end for the OTDR to resolve the problem.

High Loss

Sometimes the problem will be that the fiber transmits light but the loss is too high. The possible problems are similar to the situations described above and troubleshooting processes are the same.

If all the fibers show high loss, the cable may have been overstressed during installation or have a kink. Test for stress losses at two wavelengths, 850 and 1,300 nm for multimode fiber and 1,310 and 1,550 nm for single-mode fiber. Fiber under stress shows significantly higher losses at higher wavelengths, not lower losses, as you would expect from the fiber specifications.

For example, if a multimode fiber does not show lower loss at 1,300 nm than 850, stress is the problem. It may be possible to check the fiber route and remove the stress, usually a tight bend, and the loss will go back to normal. If the stress cannot be removed, the cable is probably damaged and needs replacement. Checking with an OTDR beforehand is prudent to make certain that the problem is in the cable itself.

A Word Of Warning

Sometimes cables are damaged in transit. When you receive cable at a jobsite, inspect carefully for visible damage before accepting it. If it shows signs of damage, test it before acceptance with a loss test set or OTDR. Even if it looks OK, check continuity with a visual tracer before pulling it in, just to be safe. EC

HAYES is a VDV writer and trainer and the president of the Fiber Optics Association. Find him at

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 .

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