Using Dark Optical Fiber: Pros and Cons

Integrators want to know: Can we put dark fiber to use


Optical fiber has been one of the better signal transmission solutions introduced to the security industry. About 20 years ago when the potential of fiber optic transmission was realized, miles and miles were installed everywhere. Fiber optic transmission delivers extremely high bandwidth transmission. During the fiber optic boom, it was everywhere. Even without planned use, it was installed in buildings, campuses, stadiums, factories and more. The thinking was it was cheaper in the long run to put it in during the construction phase than try to install it later.

Years later this optical fiber, having never been utilized, is known as dark fiber. Dark, not in color, but because light has never passed through it or it has not been used. It may be just installed or to the point of the connectors; LC, ST or SC may have even been installed. But just because the fiber has never been used doesn't mean it's not good.

The most substantial benefit to using dark fiber is cost savings. The cost to pull new optical fiber is not inexpensive. Typical optical fiber costs range from $0.08 per foot per strand for single mode fiber to $0.12 per foot per strand for multimode. Realistically fiber is pulled in multiple strands jacketed together and the general estimate is that it is usually 10 to 20 times the cost of the medium for installation based on location and specification challenges. So the obvious benefit to using dark fiber is the tremendous cost savings.

There is a downside to optical fiber that has never been utilized. The potential disadvantages include the lack of information about the fiber, fiber path or the fiber history. Some of the questions that arise regarding dark fiber include:

Was the fiber pulled correctly or was excessive stress or strain applied? If too much force is used, damage to the optical fiber can occur. Trying to track a damaged fiber is extremely difficult and in reality should any damage be identified the fiber is usually abandoned.

Does the fiber cable contain the proper jacket for the environment? The type of jacket used dictates where it can be used. There are water-resistant jackets, temperature resistant jackets, etc.

What type of fiber is installed?

If the fiber is multimode, could the fiber be old, relatively useless step-index multimode fiber; graded index 50/125 æm or 62.5/125 æm multimode fiber; or LASER-optimized 50/125 æm multimode fiber?

If the fiber is terminated, was it terminated properly?

What is the nature of the optical path? Are there patch panels and splices along the way? If so, how many, where are they and what is their condition? What is the end-to-end optical path loss? Was the minimum bend radius considered when installing fiber around corners or bends? Is documentation available that would show parameters such as optical attenuation coefficients, bandwidth-distance products or dispersion?

Confirm the optical path of the fiber

If the above conditions are satisfied, the next step is to determine the continuity of the fiber. That is, is the dark fiber suitable to be used for transmission? To determine the suitability of a fiber for transmission, the optical path must be confirmed.

First a warning: Never look into an optical fiber or into an inspection device that is connected to a fiber unless you can guarantee that nothing is connected to the other end of the fiber or to any access points along the optical path such as path panels. Irreversible damage could be done to your eye before you could have the chance to react.

A fiber optic microscope, an optical power meter kit, and an Optical Time Domain Reflectometer (OTDR) would allow complete inspection of the optical path. A fiber optic microscope is used to inspect an optical connector for debris or damage.

An optical power meter kit and an OTDR can be used to determine the status of a fiber path. A power meter kit is more accurate than an OTDR for measuring optical loss but an OTDR is required to determine the location of potential faults in the optical path.

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