Published In July 2001
There’s no doubt that termination of fiber optic cables is the most difficult part of the fiber optic installation process. Pulling cables is easy, as they are more rugged than Category 5 copper cables, and installing patch panels and other hardware is straightforward. However, working with those hair-thin strands of glass can be intimidating to installers. In addition, today’s high-speed networks need lower loss and lower back reflections than have been routinely achieved by field terminations in the past. Even experienced installers must be careful to produce only top-quality terminations every time. Connector manufacturers and trainers have been working overtime to make things simpler for the installer. Let’s look at some of the problems you may encounter and the products and techniques that can help you with your fiber optic installations. Definitions Connectors and splices: There are simple, but important differences between splicing and connecting. Connectors are used to couple two fibers together or to connect fibers to transmitters or receivers. They are also designed to be disconnected to allow patching to different locations and testing. Splices, however, are used to connect two fibers permanently. While they share some common requirements, like low loss, low back reflection and repeatability, connectors have the additional requirements of durability under repeated matings. Mechanical or fusion splicing: Mechanical splicing is done by having an alignment fixture for the two fibers, which are glued or clamped in place. Fusion splicing welds the two fibers together in an electric arc. Loss: At every joint between two fibers, some light will be lost because of misalignment or minute amounts of air or dirt between the two fibers. Back reflection: If any air is between the two fibers at the joint, there will be a slight reflection of light back toward the source. This can upset laser sources, especially in single-mode systems, or cause background noise that may affect receivers in any system. Common fiber optic connector types With multimode fiber for premises installations, the ST connector still dominates, but the SC connector specified in the EIA/TIA-568 documents is growing as cost decreases and new models make installation easier. The new small-form-factor (SFF) connectors like the MT-RJ, LC, Opti-Jack, and Volition offer advantages to both installer and user. For the user, the smaller size allows more cost-effective hardware to be used. For the installer, all but the LC are duplex connectors that terminate two fibers in one connector, with potential installation time savings. In single-mode fiber installations, SCs and FCs dominate, with a few STs used in data systems. Angled physical contact (APC with angle-polished ferrules) is gaining popularity for its lower back reflection. Of the SFF connectors, only the LC has made significant penetration of the single-mode market, but other types like the MU have become popular overseas and may make some inroads in the United States. Termination methods There are dozens of methods of attaching connectors to fiber optic cables, but a few predominate due to their ease of installation, reliability, and cost. These are epoxy adhesives, quick-curing adhesives, hot-melt adhesives, and prepolished/splice connectors. In addition, there are a number of non-adhesive connector termination methods, which represent a smaller portion of the market. All adhesive connectors use some type of adhesive to hold the fiber in place in the connector. The fiber is cleaved (carefully scored and broken at the end of the connector) and polished. The big difference is the adhesive used and the technique that adhesive requires. While some people may tell you that two-part epoxy connectors, usually heat cured, are obsolete, they actually represent the majority of all terminations. All factory-made patchcords and terminated cables use this method with ceramic ferrule connectors because it is the most reliable and least expensive. With practice, and using a curing oven to speed up the setting of the epoxy, a typical installer can terminate 10 to 20 fibers per hour and get losses of 0.1 to 0.3 dB routinely. Portable ovens are available, even battery-powered ones, to make this quite a viable termination method. Another method uses quick-curing adhesives, which are often called anaerobic adhesives but are technically known as acrylates. They can set in 30 seconds to five minutes and give equal loss figures. Some of the adhesives cure too fast, so fumbling during the process will lead to a discarded connector, and some use adhesives or “accelerator” solutions that are highly flammable. But with some practice, they are almost as cost effective as two-part epoxy and have fewer equipment needs. Hot-melt adhesives, used exclusively by 3M in their “Hot Melt” connectors, require heating the connector up to a very high temperature to soften the adhesive to a liquid so the fiber can be inserted. After the connector is allowed to cool so the adhesive sets, then the fiber can be polished. Hot Melt connectors require a very hot oven, but terminate fast and give equal losses to other adhesive types. Of the non-adhesive connectors, the most popular today use a prepolished/splice connector with a short fiber stub inside. All the new MT-RJ jacks terminate with this method. The fiber is terminated by cleaving it with a special tool and inserting it into the back of the connector, similar to making a mechanical splice. While this connector type is fast to terminate, each connector has a splice inside it, which causes an additional loss to the mating losses with another connector. Thus it is difficult to get losses as low as with regular adhesive connectors. Hints to improve your termination process with adhesive connectors All adhesive connectors have the same basic termination process: glue the fiber in the ferrule, cleave the fiber, “air polish” to remove the fiber stub left after cleaving so it barely protrudes from the ferrule and final polish. Each adhesive type has a few unique aspects to its process. The secret to making epoxy-polish connectors with the highest yield and lowest loss is to leave a small bead of epoxy on the end of the connector ferrule when you inject the epoxy. It will form a small fillet around the fiber which, when cured, supports the fiber during cleaving and polishing. This prevents breaking the fiber off below the surface of the ferrule, and ruining the connector. The colored epoxy also visible indicates when the air polishing is complete. The epoxy will leave only the faintest film on the end of the ferrule, showing that the connector is ready for final polish. For quick-curing adhesives, you can’t get a cured bead of adhesive on the end of the ferrule, because the adhesive only cures inside the connector. You must be extra careful in cleaving and air polishing to make sure you do not break the fiber. Let the adhesive cure a bit longer than the specifications to make certain that the fiber will not move during the polishing process. For Hot Melt connectors, insert the fiber into the heated connector, then pull it back about 1/16 inch before letting it cool. Then, if you break the fiber during cleaving or polishing, the connector can be reheated and the fiber pushed in further to allow a second try. For all polish connectors, we recommend air polishing with a 12-micron aluminum oxide film, then final polishing on a rubber pad. Although many connector instructions say to polish on a hard plate, a rubber pad will yield a better end finish (which means lower loss and back reflection) and reduce the possibilities of over polishing, which will cause higher loss and back reflection. Remember that multimode and single-mode connectors use different connectors, polishing films, and processes, so make certain you have the proper parts and instructions before beginning. The normal process in single-mode termination is to buy factory-terminated pigtails and splice them on. If field termination is done, it must use these special procedures or the results will be about 1 dB average loss! Do not overpolish the connectors. Today’s connectors are designed to be polished in about eight to 10 strokes, but we find installers often believe more polishing is better. This just wears away the fiber, cutting it back from the ceramic ferrule, causing higher loss! Learn to barely polish the ends with little pressure. After every connector is polished, clean it and inspect it with a good microscope. We recommend viewing at two angles. Straight-on viewing will show cracks, while angle viewing is better for inspecting the end polish. Prepolished/splice connector tricks The prepolished/splice connectors are very sensitive to both tools and techniques. Most manufacturers supply their own toolkits, which include a cleaver that looks like a stapler. Properly used, this cleaver can provide good cleaves that will yield low-loss connectors. If you do not practice with it and develop the proper technique, it’s only good for crushing fiber and making a mess of the connectors. Cleavers require technique. If you have problems with one type, try another. If you are cleaving a large quantity of connectors, there are expensive cleavers that can produce perfect cleaves every time, rapidly paying back their high initial costs. Unlike adhesive/polish connectors, where a microscope inspection can show if you have made a good connection even before you test it with a power meter and source, you cannot look inside the prepolished/splice connectors to see how well they are made. However, a simple tool can show you the inside of the connector. A visual fault locator (VFL) is a bright red laser focused into a fiber. It is used to trace fibers and find breaks and high-loss connectors or splices. At points of high loss, the light lost is visible to the human eye, making fault finding easy. With the prepolished/splice connectors, the VFL makes the internal splice visible if it is not properly made. With most of these connectors, you can use the VFL to optimize the splice as you are making the termination, ensuring that a bad connector is never found during later testing. The hints given here for these prepolished/splice connectors also apply to making mechanical splices in the field. A proper cleaver and VFL will ensure fast, low-loss splices every time. Cleanliness is mandatory Dirt is the biggest problem installers face in the field. Airborne dirt is almost as big as the light-carrying core of the fiber. If it gets on the fiber ends, it causes loss by absorbing the light. Two connectors crushing a piece of dirt between them may scratch or crack the polished end surfaces of the fibers. If dirt gets on the polishing film, it may be impossible to get a good end finish. A clean environment is best, but we’ve found that even working under heating vents can be a big problem, as the dust is blown down on the work surface. Terminating fiber optic cables is not difficult if you understand the process and follow it carefully, using the proper tools and components. As with any other skill, it improves with practice. When you have time, we recommend you make up your own patchcords just to perfect your technique, which will lead to faster work and better quality in field installations. HAYES is the founder of Fotec, the fiber optic test equipment company and the Cable U training programs. He can be contacted at Jh@jimhayes.com.