09.21.2008 08:00 AM
Fiber-optic basics

Fiber optics provides a transmission medium that surpasses coax cable in many ways, especially for long transmission runs. But now with HD-SDI, the need for high-frequency capacity and low loss brings fiber optics into the broadcast plant. Coax cabling for HD is limited to 300ft-1000ft, depending on the receiver, while some fiber-optic cables can carry 3Gb HD is excess of 30km. The following tutorial presents the basics on fiber optics as well as some practical tips on installation and troubleshooting.

Fiber-optic history

The history of fiber optics goes back to 1841, but nothing came of it until the 1950s when the fiber scope was invented. In 1956, Narinder Kapany first coined the term “fiber optic.” The invention of the fiber scope encouraged scientists to develop fiber optics as we know it today. What they came up with was a cladding made of glass that surrounds the central core glass fiber; this cladding reflects the light that escapes the core and directs it back into it.

In 1962, the first solid-state laser was created and quickly found its way into the fiber-optics field. The first fiber-optic cable had an optical loss of 1000dB/km — obviously more research was needed. In 1970, scientists at Corning succeeded in developing a fiber-optic cable with an attenuation of 20dB/km; this was a turning point when it became possible to use fiber optics for transmission runs.

The first broadcast use of fiber optics was at the 1980 Winter Olympics, in Lake Placid, NY. Video engineers requested a fiber-optic backup link; soon they realized that the backup link was more reliable and provided a better quality signal, so they made the fiber-optic path the primary link. At the 1994 Winter Olympics in Lillehammer, Norway, fiber optics transmitted the first digital video signal. Since then, fiber optics has become more reliable and easier to install.

How it works

Today’s fiber-optic cable has come a long way since its inception. Attenuation has been reduced to 0.2dB/km and splices can be as transparent as 0.1dB (using a fusion splicer). Cables are stronger and less prone to aging than earlier ones. There are basically two different types of fiber-optic cables in use today: single-mode and multimode cables. Single-mode cable has a much smaller fiber-optic core at its center, resulting in a single ray of light (mode) traveling its length. The other is multimode cable, and it is comprised of a thicker core of fiber optics at its center made of multiple strands of optical fiber. The light is dispersed as it makes its way down the cable, which results in a lower-frequency bandwidth and higher attenuation as compared to single-mode fiber. Multimode is more common because the equipment is less expensive and the larger surface area of the fiber allows more light to be transferred into and out of the fiber.

What actually makes fiber optics work is the cladding, the material that surrounds the fiber-optic strands and causes total internal reflection, where all the light striking the boundary between the fiber and the cladding is reflected back into the fiber causing it to bounce back and forth down the cable. Today, the cladding is made of glass fiber similar to the fiber-optic strands but with a lower refractive index, which is the key. The entire assembly is then surrounded by a buffer material and a protective jacket.

Typical fiber-optic receivers and transmitters accept ASI, SD-SDI or HD-SDI signals, or even analog video and audio. The SD-SDI variety can handle either embedded or separate AES digital audio.

Fiber factors

Factors to keep in mind when using fiber-optic cabling are attenuation and dispersion. Attenuation is caused by the light being absorbed by impurities and by scattering due to irregularities in the glass. Attenuation is very low in fiber-optic cables, with single-mode fiber being as low as 0.25dB/km and multimode fiber somewhat higher. The frequency of the light plays a major factor in the amount of attenuation for a given fiber-optic cable. (See Figure 1.)

Dispersion is caused by various light rays taking different paths at different speeds. This causes a broadening of pulses sent down the cable, which affects the frequencies, or data rates, that can be used. Different cables have been optimized for use with certain light frequencies so they will have lower attenuation and dispersion for a given frequency.

Fiber-optic cables come as singles or in bundles of two, six, 12 or more — they even come in ribbon cable.

Budget loss

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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Wednesday 11:59 PM
Peer Profile: Tomaž Lovsin, STN, Slovenia
“Will there be a shift from coax to fibre? Or a mixture between the two which will require hybrid solutions to be implemented?”


 
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