Non-adhesive optical connectors target multimode applications

Jun 1st, 1995

Non-adhesive optical connectors target multimode applications

By decreasing installation time but preserving performance, non-adhesive-type SC and ST connectors form reliable multimode fiber-optic links in local area networks

Joseph A. Paparella and nicholas A. Lee

3M Telecommunications Systems Division

Recent technological ad vances in fabrication enable non-adhesive connectors to achieve improved optical and mechanical performance levels and reinforce fiber-optic cable as the medium of choice for local area networks.

Although fiber-optic connectors primarily fall into two categories--adhesive and non-adhesive (crimp-on)--the major classification of connector styles implies the method of joining the housing. Five connector termination technologies are available--heat epoxy cure, ultraviolet epoxy cure, anaerobic epoxy cure, hot-melt adhesive and non-adhesive.

The connector styles usually include different versions for singlemode and multimode applications. In general, connectors range in price from $8 to $12 for singlemode and $5.50 to $7.50 for multimode. Adhesive connectors can be used in both singlemode and multimode environments, but non-adhesive connectors predominantly aim at multimode applications.

Four connector types--ST, SC, FDDI and FC/PC--apply to LANs and other short-haul communications networks. The ST and SC connectors are the only ones using the improved non-adhesive technology, however.

The four primary ferrule materials are zirconia ceramic, alumina ceramic, stainless steel and plastic or composite ferrule. The connector ferrule holds the fiber in place and provides fiber alignment, which are mandatory to ensure good performance. The ferrule material dictates the number of times connectors can be plugged and unplugged and how the connector reacts to temperature change. Industry studies confirm that zirconia ceramic ferrules provide the best performance.

Regardless of the type of connector being installed, the installation process includes three main areas--fiber retention, fiber end finishing and cable strain relief. Of these, the method used in fiber retention plays the biggest role in determining ease of connector installation. Connectors can be evaluated in terms of three types of fiber retention techniques--epoxy, which uses anaerobic, heat- or ultraviolet-curing; hot melt, which uses a heat-activated adhesive; and non-adhesive, which uses a mechanical latching technique.

Despite industry doubts regarding performance levels of non-adhesive connectors in the past, improved technologies now allow non-adhesive connectors to perform as effectively as adhesive connectors in multimode applications. The main concern with non-adhesive technology has been its rate of failure, which has ranged from 15% to 75% for the original connectors. Tests completed this year by 3M on the latest non-adhesive connector showed only a 4% failure rate, which is comparable to that of hot melt and epoxy connectors. Non-adhesive technology is new in the SC format, and as such, is not widely available. For comparison purposes, ST connectors are evaluated because they offer the widest array of termination options and are considered the industry`s most representative type.

Fiber retention

Fiber retention is critical to the performance of a fiber-optic connector, and the material used to anchor the fiber in the connector ferrule plays a crucial role. Restricting the movement of the fiber relative to the connector ferrule ensures the maintenance of intimate fiber contact at the connector interface.

For connectors that use epoxy, fiber retention is usually accomplished using pre-measured quantities of adhesive components, which are preloaded into single-use containers, or "bi-packs." The bi-pack serves two purposes in dispensing epoxy. First, it contains predetermined amounts of the two epoxy mixing components, thus eliminating the need for measuring and ensuring a proper ratio. Second, it places the two components within a single flexible container, simplifying adhesive mixing.

Once the adhesive is properly mixed, it is transferred from the bi-pack to a hypodermic syringe. Using the syringe, the installer applies the epoxy to the optical fiber, cable and connector. If the adhesive is applied to the wrong place, or in the wrong volume, the finished connector will perform poorly.

After the adhesive is properly applied and the fiber is threaded into the connector, the assembly is typically placed in a curing oven for 20 minutes to let the epoxy harden. Alternately, the connector can be cured at room temperature in approximately 18 hours. Ultraviolet and anaerobically curing systems perform faster than epoxy heat-curing systems.

Hot-melt connectors are factory preloaded with adhesive. Therefore, adhesive measuring, mixing and application become non-issues. To achieve fiber retention, a connector is placed in a heating fixture for 90 seconds to melt the preloaded adhesive. Once the adhesive is molten, the connector is removed from the heater, and the fiber is threaded into place. The connector is allowed to cool, and in approximately three minutes, the adhesive re-hardens. Unlike epoxies, the adhesives used in hot-melt connectors possess a long shelf life.

Non-adhesive connectors require no epoxy, which means no measuring, mixing, application, heating or curing. Because no adhesive is used, no heating or cooling is needed. Fiber retention is accomplished nearly instantaneously by mechanically locking the fiber within the connector after the fiber is threaded into place. Non-adhesive connectors do not require a curing oven or hot-melt heater, the bench space to support them or the electricity to power them.

The next process step--fiber end finishing--accomplishes a dual purpose. First, it precisely controls the length of the optical fiber relative to the surrounding connector. Second, it provides the end of the fiber with an optical finish, thus ensuring acceptable lightwave transmission.

The two-step fiber end finishing procedure for epoxy connectors calls for two grades of polishing film (coarse and fine), a single backing pad and a one-piece polishing tool. One unfortunate aspect is the potential for contamination caused by the coarse abrasive.

This contamination usually takes the form of scratches on the optical interface. Although these scratches can be removed with careful repolishing, the extra work increases the cost of installing the connector. Another work issue involves when to switch between the coarse and fine grades of polishing film. Switching too soon results in spending additional time to slowly remove hardened epoxy with the fine grade film. Switching too late can result in deep scratches in the face of the fiber that can be difficult to remove once all the epoxy has been polished off the connector face.

The hot-melt connector offers an advantage in fiber end finishing. Because the adhesive used in a hot-melt connector is not molecularly crosslinked as a cured epoxy is, the adhesive on the face of the connector is easier to remove abrasively. Thus, a single grade of polishing film can be used to remove the adhesive and provide an optically smooth finish on the fiber end. This one-step polishing procedure eliminates the potential for fiber scratching because of contamination, and it eliminates the risk of over-polishing on a coarse grade of polishing film.

Non-adhesive connectors employ a one-step polishing procedure with an added benefit. Unlike epoxy and hot-melt connectors, the latest non-adhesive connector uses a simple, handheld polishing fixture rather than a bench-top polishing pad. This polishing approach does not need bench space and can be used to polish connectors in such awkward places as the top of a ladder or underneath a desk.

The third step in the connector installation process, cable strain relief, secures the cable strength members or jacketing to the connector. This process isolates the fiber from forces exerted on the cable and reduces the potential for fiber damage. The same epoxy that secured the fiber is used by the epoxy connector to secure the cable strain relief members and jacketing. For additional support, the backbone of the connector is crimped around the cable.

For hot-melt connectors, the adhesive within the connector aggressively grips the cable strain relief members and jacketing so no crimping operation is required. Non-adhesive connectors require a crimp to lock the cable components to the connector body, but they do not need an adhesive.

Total assembly time for connectors varies depending on the installation method. The installation of epoxy connectors generally requires 30 minutes (with a curing oven), but 18 hours at room temperature. Hot-melt connectors can be installed in six minutes, and installation of non-adhesive connectors can be accomplished in less than two minutes.

Each termination option has its benefits. Although adhesive connectors require more time and a higher level of expertise for installation, they cost less than non-adhesive connectors. Epoxy and hot-melt connectors can be inexpensive options if time, electricity and trained installers are readily available. On the other hand, a slightly more expensive non-adhesive connector is useful in cramped and confined installation conditions.

The ST and SC connectors are the only connectors to date that use the improved non-adhesive technology. Epoxy and hot-melt termination options are available for ST, SC and FC/PC connectors; FDDI connectors are available with epoxy only.

The ST connectors are popular for multimode applications, and were the first connectors to feature the latest non-adhesive termination option. The crimped ST connector provides low prices, installation ease, high reliability and low light loss for premises and LAN applications.

The SC connector, which is popular in singlemode applications, has a rectangular push-pull coupling mechanism that provides easy insertion and high repeatability. These connectors are usually less expensive than ST connectors in singlemode but more expensive in multimode. However, multimode prices are expected to decrease as production volume increases. The SC connectors have just recently become available with a non-adhesive termination option for multimode applications. u

Joseph A. Paparella is advanced product development engineer at the 3M Telecommunications Systems Division in Austin, TX, and Nicholas A. Lee is research specialist at the 3M Sector Fiber Optics Laboratory in St. Paul, MN.

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