Fiber-optic basics
Sep 21, 2008 8:00 AM
Fiber-optic cables come as singles or in bundles of two, six, 12 or more — they even come in ribbon cable.
Budget loss
Figure 2
Click to enlarge
When designing a fiber-optic system, the major factor to keep in mind is the budget loss, the difference between the amount of light the transmitter puts out and the threshold of the receiver. Typically, budget losses are in the order of 15dB-20dB, and for many shorter runs, the main problem will be too much light. Receiver overload can lead to data loss and possible damage to the receiver. (See Figure 2.)
Attenuators may need to be added to the system to keep the signal within the limits of the optical receiver. These can be purchased or made from the fiber-optic cable being wrapped around a pencil several times, but the cable manufacturer’s specification sheet should always be consulted first to avoid damaging the cable.
Installing fiber
For as strong as modern fiber-optic cable is, it still must be treated with care. Damage to a fiber-optic cable will result in signal loss, due to increased attenuation, or even total loss of light. Cables are constructed to ensure that the stress of being pulled does not damage the optical fibers, but always follow the cable manufacturer’s recommendations.
The bend radius should also be taken into consideration. Do not exceed the specifications for the cable being used. Bending the cable too tightly will harm the fibers inside and greatly reduce the cable’s lifespan, if not causing it to fail immediately.
Fiber-optic cable will be just fine in cable trays with no protection as long as they are laid flat on the bottom and supported all the way. If the cable is tangled up with other coax and audio cables that can be pulled (when the cable is being traced), the fiber cable can be stressed and bent. If being laid flat in a cable tray is not an option and it’s being installed in a existing cable environment or being run above drop ceilings, then an innerduct must be used to protect the fiber cable. An innerduct is a plastic flexible conduit intended for low-voltage wiring or fiber cable. The size and rigidity of the innerduct will prevent the fiber cable from being damaged in most cases. Be sure to use an innerduct of sufficient size for the cable and its connectors to be pulled through.
When fiber cable runs between floors, it is typical for junction boxes to be installed at each floor where each fiber bundle is terminated and jumper cables are used to interconnect them; this makes changes and troubleshooting much easier.
In most cases, fiber-optic cable will be purchased with the connectors preinstalled, so the cable can be pretested. Do not worry about ordering extra-long cables, because attenuation will not be a factor unless your cable run extends across town. In the case of multimode, fiber connectors can be field installed, but single-mode fiber requires a splice of a factory-made pigtail connector; this is because of the very small diameter of the fiber (about 9uM).
Splicing
Splicing fiber-optic cable can be accomplished in two ways. The first is a mechanical splice where the two ends are prepared and then held in place facing each other within a special alignment sleeve; the typical loss is 0.3dB. The second way is called a fusion splice, which requires a special fusion-splicing machine that is expensive and requires special training. The process involves preparing both ends and then fusing them together using an electric arc. These splices have a loss of about 0.1dB and are considered the best solution; of course, the best choice is to plan the install so splices are unnecessary.
Troubleshooting
The first and best tool for troubleshooting fiber-optic cable is a light power meter, which can measure and compare the amount of light. Fiber cable attenuation is measured in decibels per kilometer, with connector losses in the order of 0.3dB per connector. OTDRs (optical time-domain reflectometers) are also available, but be sure of the instrument’s dead zone specification. The dead zone defines the distance the can be measured after a strong reflection along a length of fiber optic cable, determining whether it will see the far end of a patch cord after the reflection of the closer end.
But the best tools when installing fiber-optic cable are care and cleanliness. Dirt is a factor in most faults in a fiber-optic system. Because the diameter of the fiber can be smaller than a human hair, any dirt in the connector or roughness of the fiber ends will cause a loss of light and possible failure.
Conclusion
Installing and maintaining fiber-optic cable is not so difficult as long as a few simple rules are followed, as outlined above. Taking the time to properly design and plan for a fiber-optic system will ensure a successful install. And once correctly installed, a fiber-optic system will provide years of service without the possibility of interference or noise.
Next Time
The next “Transition to Digital” will deal with the wide variety of picture monitoring technologies available today.
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