Fiber-optic terminations

Probably no fiber-optic component has been given greater attention than connectors. Searching for lower loss and easier installation, manufacturers have created more than 80 styles of connectors and about a dozen different ways to install them. Fortunately, only a few types are used for most applications. In the United States, those are the SC, ST and LC connectors. And they are now as easy to install as many copper connectors.

Optical loss is the most important characteristic of a fiber-optic connection. A single connector does not have loss per se, since loss is caused in the mating of two connectors, both of which contribute to the loss in the connection. The optical loss of a connection is a function of the precision of the alignment of the two fibers, both lateral and angular, and the quality of the finish on the end of the fiber. The reflectance of a connection, which is also important, especially in high-speed systems, is a function of how well the ends of the connector mate and the quality of the end finish. Connection performance is determined by both the precision of the connector itself and the quality of the polishing during the termination process.

Most connectors available today use ceramic ferrules to hold and align the fibers. Ceramic is used because it adheres well to glass, is easy to polish and has low thermal expansion like the glass fiber. The end of the ceramic ferrule is domed to ensure the fibers make good contact. These are known as physical contact (PC) connectors. Another type of connector used in single-mode systems, angled PC or APC, has the dome at a small angle to the axis of the fiber to further reduce reflectance. APC connectors are used on most CATV and many other high-speed systems.

Installing connectors

Most connectors are attached to the fiber by an adhesive and polished to a fine finish. Factory-made cable assemblies, patch cords and pigtails (a connector on one end of a simplex cable and bare fiber on the other end) are mostly made using a heat-cured epoxy adhesive and polished on automatic machines to get a consistent fine finish.

Field terminations can be made using similar adhesive/polish techniques, splicing preterminated pigtails onto the fibers or attaching prepolished/splice connectors. Some installers do no field terminations at all, preferring to purchase prefabricated cable systems, which require only installation and testing.

Splicing pigtails on cables

Most single-mode field terminations are made by splicing a factory-made pigtail onto each fiber in the cable being installed. Termination of single-mode fiber is more difficult than multimode fiber because the smaller core requires greater precision in components and processes. Reflectance is also a problem with single-mode systems, so the end of the fiber must be carefully polished, which is difficult with hand-polishing. Single-mode connectors are usually machine-polished using diamond polishing film and a polishing slurry to get the smooth finish needed. These precisely made connectors are then fusion-spliced onto each fiber for minimal loss.

Prepolished/splice connectors

Prepolished/splice connectors have a short fiber stub glued inside the connector that has been polished at the factory. (See Figure 1.) These connectors are like a short pigtail, except the end of the fiber that is spliced to the fiber being terminated is inside the connector. To attach the connector, one strips and cleaves the fiber, a process that involves creating a clean end on the fiber with a special tool, inserts it in the connector and crimps it in place. Having both a splice and a connection loss means these have higher loss than adhesive/polish connectors, but they can produce acceptable results.

Prepolished/splice connectors are more expensive due to the inclusion of a splice in the connector and the more complex manufacturing process. Toolkits to install them are also more expensive than adhesive connectors. However, they are quick to install so the labor savings may offset their higher costs.

Adhesive/polish connectors

Most connectors use adhesives to hold the fiber in the connector and polish the end of the fiber to a smooth finish. While this may sound like a time-consuming process, an experienced installer can install connectors in less than a minute and get extremely low optical loss. There are three common types of adhesives: epoxy, Hot Melt and anaerobic.

Most connectors, including virtually all factory-made terminations, are the epoxy/polish type where the fiber is glued into the connector with epoxy. The epoxy is injected into the connector before the prepared fiber is inserted. The adhesive can set overnight or be cured in several minutes in an inexpensive oven. The small bead of hardened epoxy that surrounds the fiber on the end of the ferrule makes the polishing processes much easier — practically foolproof. This process provides the most reliable connection, lowest losses and lowest costs, which is especially important if you are doing a lot of connectors.

Hot Melt is a 3M trade name for a connector that already has the adhesive (a heat set glue) inside the connector. The connector is placed in a hot oven, where the glue is melted in a few minutes. When the glue is melted, insert a prepared fiber into the connector, let it cool, and it is ready to polish. The Hot Melt is fast and easy, has low loss, but is not quite as cheap as the epoxy type. Hot Melt connectors require a special oven, as they need a much higher temperature than is used for curing epoxy.

Anaerobic adhesive connectors use a quick-setting anaerobic adhesive that cures faster than other types of adhesives. Various techniques of applying adhesive are used, including injecting it into the connector before inserting the fiber or simply wiping adhesive onto the fiber before inserting it in the connector. These adhesives dry in five minutes alone or in 30 seconds when used with a chemical accelerator.

The termination process

For all types of adhesive/polish connectors, the termination process is similar. (See Figure 2.) Start by preparing the cable, stripping off the outer jacket and cutting off strength members. Next, strip the fiber with a special tool that removes the plastic buffer coating without damaging the fiber. Then clean the fiber, and set it aside. Apply adhesive to the connector or fiber, and insert and crimp the fiber into the connector body.

After the adhesive is set, the fiber is then cleaved close to the end of the ferrule. Polishing takes three steps. First, air polish to grind down the cleaved fiber to near the end surface of the ferrule. Then polish on two different grades of abrasive film placed on a rubber pad using a polishing puck to keep the fiber perpendicular to the surface.

The process takes longer to read about than actually do. An experienced installer can terminate multifiber cables in about one minute per fiber, using the time required to cure the adhesive to prepare other connectors and reduce the time per connector.

It's important to follow termination procedures carefully, as they have been developed to produce the lowest loss and most reliable terminations. Use only the specified adhesives, as the fiber-to-ferrule bond is critical for low loss and long-term reliability. And, like everything else, practice makes perfect!

Jim Hayes is author of “The Fiber Optic Technicians Manual” and co-author of “Data, Voice and Video Cabling.” He currently serves as president of the Fiber Optic Association.

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