Integrating fiber optics
Dec 1, 2008 12:00 PM, By Phil Cianci
It may be time to abandon coax.
Figure 4. Fiber-optic cable performance is degraded by macrobends, a failure to observe
minimum bend radius. Smaller microbends, caused by cable ties or improper handling,
can also impair the optical light path.
Select figure to enlarge.
Connectors:
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Keep dust caps on all connectors.
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Clean all connectors after removing dust caps and before connecting to transceivers or connectors. Solvents specifically developed for fiber cleaning are now available.
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Don't touch the terminations.
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Mate the cable and the connector as soon as possible.
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Place the dust caps in a small plastic bag and tape inside the cabinet for future use.
Testing:
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Measure transmitter power, receiver sensitivity and optical loss.
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Use tested patch cords of the same fiber type as the installed cabling with a known attenuation at the appropriate wavelength.
Safety is no accident
Working with fiber optics requires strict adherence to safety practices. Many of the hazards are not obvious. Even a microsecond of exposure to a fiber light beam can cause permanent eye damage. Never work on an active fiber.
Trimming, stripping or cutting fibers can create small glass fragments that are light enough to float in air. They can penetrate the skin and become embedded, causing irritation, and if ingested, fibers can cause internal damage.
Chemicals and solvents used in cleaning and splicing fiber optics can also be hazardous.
Safety practices:
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Workers performing splicing or termination should wear safety glasses with side shields.
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All food and beverages should be kept out of the work area. Wear disposable aprons to keep fiber particles off clothing. Check clothing for pieces of stray fiber; remove them with double-sided tape.
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Wash hands thoroughly before touching eyes, especially if wearing contact lenses.
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Read all instructional material before handling chemicals.
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All cut fiber pieces should be disposed of properly along with any used chemicals and containers. A disposable container that can be tightly closed should be used for fiber scraps.
Maintaining fiber installations
Fiber-optic networks generally do not require periodic maintenance. Inspection or maintenance can cause damage or allow dirt to get into components; however, outdoor installations are susceptible to mechanical and environmental damage and should be carefully inspected regularly.
Fiber installation documentation packages should include transmission wavelength, cable color, cable number, section length and the location of splices or terminations. The optical loss, measured during testing, should be recorded on the as-builts, and spare fibers should be noted. Power figures, both calculated and measured, along with installer contact information, should be included.
A new, improved cable
With the increased use of fiber for telco TV installations, the need arose for fiber cables that can withstand outdoor environments and tight residential installation spaces. The result is the development of bend-optimized fiber.
Specifically designed to minimize the effects of increased attenuation resulting from cable bends, bend-optimized fiber, as shown in Figure 4 on page 53, can be coiled into loops and tied in bundles with a 10mm loop radius; wavelengths between 1260nm and 1625nm are supported.
Benefits
The need for higher data rates will increase. One of the problems with coaxial cables is that as signal data rates have increased, the usable length of cable runs has decreased. Because optical fiber can support higher data rates, its use can help provide some measure of future-proofing for a facility.
Also, because fiber-optic cables are smaller in diameter, they require less cable tray space. This additional space allows “dark fiber” to be pulled during cable installation and reserved for future use.
The amount of fiber cables required for a given number of signals is significantly less than for an equivalent coax installation. For example, in campus and metro network topologies, a CDWM fiber can carry 16 HD-SDI signals over a single fiber for up to 40km. Similar reduction of cable numbers can be realized by using fiber instead of coax in broadcast centers.
With 1000ft of 1694A and 1000ft of dual-strand fiber both priced at about $300 and 1000ft of 12-fiber cable at $550, fiber deployments are viable, affordable solutions — despite the added cost for optical connectors, patch cables and skilled installation technicians.
Finally, total cost of ownership for fiber installation may be less than that of coax. An “enlightened” design philosophy should evaluate the long-term benefits of using fiber-optic technology in any new system design. BE
Phil Cianci is a design engineer for Communications Engineering, Inc., in Newington, VA.
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