WITH THE INCREASING TREND TOWARD THE USE OF FIBER instead of co-ax cable in a wide range of applications, most contractors are now required to understand the basics of terminating and laying fiber optic cable. Contrary to its reputation, fiber is actually quite easy to handle and use.

CABLE CONSTRUCTION

Fiber, at its most basic level, is a very pure strand of glass through which light can pass over great distances. All fiber optic cable has at its center a fiber core made of such glass, which is used for the actual signal transmission. The two most common techniques for protecting the fragile fiber are enclosing it in a loose-fitting tube and coating it with a tight-fitting buffer.

In the loose-tube method, the fiber is enclosed in a plastic buffer-tube that is larger in inner diameter than the outer diameter of the fiber itself. This tube is sometimes filled with a silicone gel to prevent the buildup of moisture. Since the fiber is basically free to “float” within the tube, mechanical forces acting on the outside of the cable do not usually reach the fiber.

In the tight buffer construction, a thick coating of a plastic-type material is applied directly to the outside of the fiber itself. This results in a smaller diameter of the entire cable and one that is more resistant to crushing and impact. However, because the fiber is not free to “float”, its tensile strength is not as great. (See Figure 1.)

Tight buffer cable is generally lighter and more flexible than loose-tube cable and is usually employed for less severe applications such as within a building or between individual pieces of equipment.

Like copper wire, fiber optic cable is available in many varieties. There are single and multiple conductor constructions, aerial and direct burial styles, plenum and riser cables and even ultra-rugged military-type tactical cables that will withstand severe mechanical abuse. The cable one chooses is, of course, dependent upon the application.

Both loose tube and tight-buffer constructed cables are available in single-mode and multimode versions. These terms refer to the diameter of actual glass fiber located within the core of the cable. More specifically, they refer to the number of light paths that may pass through the fiber. Single-mode fiber is so thin (8 to 10 microns, diameter) that only a single path of light can pass through its length. By contrast, multimode fiber, 65 microns in diameter, allows multiple paths of light to travel along its length simultaneously. Although it may seem counter-intuitive, single-mode fiber is able to carry more information over farther distances than multimode fiber.

TERMINATING FIBER OPTIC CABLE

The procedure for terminating fiber optic cable is a function of the type of connector being used, rather than the type of fiber. There are two types of connectors most frequently used today: ST and FCPC. As ST connectors may be used with either multimode or single-mode fiber and do not require any expensive, special equipment — unlike FCPC connectors — this article will focus exclusively on termination using ST-type connectors. All tools required for this type of termination can be purchased in standard fiber terminating kits available from fiber optic equipment manufacturers.

Similar to using electrical cable, the first step in terminating fiber cable is to strip it (see Figure 2). This involves stripping back the plastic coating of the fiber cable to reveal the glass core inside. A tool called a fiber-optic stripper, which looks like a small pair of pliers with jaws that grip the coating, is often used in this process.

Once this is done, the stripped material is trimmed back and inserted into a restraining grommet or sleeve, also called a boot.

After the cable is stripped, the ST connector must be prepared for use. Simply apply a dab of a quick-drying epoxy resin on the end of the optical connector. Once the resin is applied, immediately insert the fiber into a precision hole in the connector pin (see Figure 3 above).

At this point, with the fiber inserted through the connector hole, the fiber tip should be protruding from the front of the connector pin (see Figure 4 above). Apply a small bead of epoxy to this exposed end, and set the fiber/connector assembly aside to dry properly. Ideally, the epoxy should be allowed to dry overnight, but a 1-hour drying time is sufficient when time is not available.

Once the epoxy is completely dry, use a scribing tool, which looks similar to a paring knife, to score the fiber close to the epoxy bead. It is important that the fiber be cut flush with the end of the connector pin. Next the fiber tip must be ground down and polished. A sanding plate is used to smooth away any fiber that may be protruding through the epoxy. After sanding, you should see a very small black dot on the epoxy. This is the actual end of the fiber.

Last, the fiber must be polished. A polishing wheel coated in a finer grit micro-polish is used to remove any small particles that may still be on the tip of the fiber. After polishing, a compressed air hose is used to blow off any microscopic particles. Then a lint-free wipe with some rubbing alcohol is used to clean the optic tip.

Testing. The termination is now complete, but it is good practice to do some quick testing at this point. Otherwise, problems may arise during or after installation, at which point diagnosis will be more difficult. The first step is to examine the connector under a fiber-optic scope to make sure it is not exposed, broken, cracked or plucked (i.e., riddled with small holes made by particles as a result of scoring the fiber). Next, the connector should be attached to either a transmission unit or a test fixture that tests the loss in dB of the fiber cable. Fiber that has been correctly terminated should show no additional loss as a result of the added connector. If the termination shows no physical problems and the testing indicates an acceptable level of loss for length of cable, then the optical connector is ready for use. Wipe the tip of the fiber clean and place a protective dust cap on it. Now the process is complete.

While this procedure does get easier with practice, it is not difficult to master and can be done relatively quickly, even by a novice. In fact, once you are completely familiar with the finishing steps, the most time-consuming aspect of the entire process is waiting for the epoxy to dry. And, for those who may still have reservations, there are “quick-crimp” connectors that eliminate the epoxy and finishing steps altogether. While these “quick-crimps” are more convenient in the field, the connection has slightly more optical signal loss.

OPTICAL SPLICES

While optical connectors can be used to connect fiber optic cables together, splicing — the process of terminating one fiber directly to another without use of a connector — is often more desirable because it provides lower signal loss. Two of the most common types of splices are the mechanical splice and the fusion splice.

In a mechanical splice, the ends of two pieces of fiber are cleaned and stripped, then carefully butted together and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remain aligned.

A fusion splice involves melting (fusing) together the ends of two pieces of fiber. The result is a continuous fiber without a break. Fusion splices require special, expensive splicing equipment but can be performed very quickly, so the cost becomes reasonable if done in quantity. Because fusion splices are fragile, protectors and a plastic coating called shrink tubing are usually placed around the spliced area to protect it from breakage.

INSTALLATION

Fiber optic cable offers the installer a great deal of freedom and flexibility during the actual installation process. For starters, fiber is light and easy to handle, and much less of it must be laid than the amount of co-ax required to provide an equal level of transmission capacity. The specifics of how and where fiber can be laid is mostly a function of the type of fiber being used. As discussed in the “Cable Construction” section of this article, fiber is available in a wide range of constructions, each designed to withstand certain types of environmental conditions and application challenges. In general, all fiber uses less duct space than co-ax and, in fact, may often be laid without ducts — simply passing between walls and flooring wherever convenient. It can also accommodate structural curves and turns, although any tight bends must have a turning radius of at least 1 inch.

Fiber is virtually unaffected by outdoor atmospheric conditions and electrical interference; it can be lashed directly to telephone poles or electrical cables without concern for extraneous signal pickup. Because it is so resistant to the environment, fiber is ideal for connecting systems between buildings when cable must be laid outside, underground. In fact, if the proper type of fiber cable is used, it can be laid directly in the ground with no concern for exposure to moisture or humidity. And if a cable is accidentally severed, there is no risk of a spark causing a fire or endangering personnel.

AMPLIFIERS AND REPEATERS

Although fiber optic cable is often chosen over co-ax because it can transmit signals over longer distances, there are certainly limitations to how far fiber transmission systems can carry a signal without amplification. When the desired transmission distance exceeds the maximum distance that a system is designed to support, amplifiers or repeaters are required.

In an AM- or FM-based system, amplifiers are used to boost the strength of an attenuated signal so that it can be transmitted along an additional length of fiber. Fiber-optic amplifiers are very similar to their traditional electrical counterparts. The transmitted light beam is captured by the amplifier, converted back to a voltage for amplification purposes, and then relaunched as light for transmission over the next span of fiber. As in copper-based systems, fiber optic amplifiers do pass on any distortions and interference that have been acquired by the signal throughout the transmission, and those distortions are amplified along with the signal. Therefore, if a signal is amplified enough times, it will become greatly distorted.

This problem is eliminated when using a digital transmission system, as transmission length is extended through the use of repeaters instead of amplifiers. When a fiber optic system uses digital signaling techniques, a repeater converts the transmitted light beam back into its electrical equivalent, in digital format, and then launches a brand new fiber-optic signal based on the regenerated digital electrical signal. (Note that the signal does not return to its baseband format until it reaches its final destination.) Because of the digital nature of the transmitted signal, no distortions are picked up by the repeater or passed on in the repeating process. Therefore, theoretically, digital repeaters could be used to transmit a signal over an infinite length of fiber. This is a significant advantage over traditional AM and FM systems and is not limited to systems designed for the transmission of digital baseband signals. Today, there are fiber-optic systems that use all-digital signaling and processing to transmit traditional analog video, audio and data signals, and do so at a competitive price.

SUMMARY

If you are a system installer who has shied away from using fiber optic transmission systems because the installation process was unfamiliar and intimidating, we hope this article has made you reconsider. With a little practice, laying and terminating fiber cable should become just as simple as using co-ax, and the advantages are innumerable. For more information on fiber optic technology, read the educational guides Introduction to Fiber Optics, Fiber Optic Cables and Connectors and Advantages of Digital Fiber Optic Systems. These are available, along with more information, at Communications Specialties' Web site.
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John Lopinto is president, CEO and co-founder of Communications Specialties Inc. He has held various technical and managerial positions at CBS, HBO and Time Warner Inc.

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