Fiber Optic Cable Connectors

The primary task of a fiber optic connector is to minimize the optical loss across the interface of the coupled fibers. This loss is expressed in decibels (dB). High performance fiber optic connectors are classified (and required by TIA-568-A) as those with less than .75 dB of loss. Losses occur frominexact end-to-end mating of the fibers, and the surface condition of fiber ends. Secondly, the fiber optic connector is to provide mechanical and environmental protection and stability to the mated junction. And thirdly, the fiber optic connector design should permit rapid and uncomplicated termination of a cable in a field setting.

An ideal fiber optic connector would encompass the following features:
• Utilize a fiber alignment scheme yielding low loss;
• Be physically small;
• Be of rugged construction;
• Be easily field terminated;
• Be field repairable;
• Have good thermal characteristics;
• Offer excellent fiber/cable strain relief;
• Be of moderate cost;
• Be compatible with standard tooling and fiber cables.
(Factory-terminated cable assemblies provide users with the choice to field connectorize or to splice pigtail assemblies using fusion or mechanical splices).

CONNECTORS RECOGNIZED BY TIA-568-A
Two connector systems are most prevalent today: the SC and the ST. Both are excellent optical fiber connector systems. The TIA committees on building wiring have called out the SC as the connector of choice for new installations. But since the ST has a very large installed base in North America and Europe, it is allowed to remain in, or be added to, buildings and organizations which have an installed base of ST connectors. The SC is a push-pull connector standardized by the ISO/IEC in 1992, when it was recognized as the preferred optical connector for new installations in commercial buildings. The SC connector has since been recognized as the connector standard in the ANSIX3T9.3 Fiber Channel, ANSIX3T9.5 Low Cost FDDI, Broadband ISDN T1E1 (ATM) and by the TIA-568-A standards. The SC is available as a simplex or duplex connector. It is keyed to prevent cross-mating, and uses a push-pull design to mate and unmate.

The standard recommends it be color-coded beige for multimode, and blue for singlemode, but note that having these colors is not necessary for compliance. ST Connectors (BFOC/2.5): The de facto standard connector of the late 1980's and early 1990s, this bayonet connector style (uses a push/turn motion to attach to its mating sleeve) has been officially recognized by only one standard: IEEE 10BaseF. It is available in two versions,the ST and STII; both use a 2.5mm ferrule which is keyed. With the SC connector now preferred by most new standards, including the TIA-568-A Commercial BuildingWiring Standard, the use of the ST may decline except in pre-existing installations where its use is "grandfathered".

HYBRID CONNECTORS
Because of the evolution from ST to SC connectors,hybrid solutions (jumpers or adaptors) are coming on the market. These provide a path for current users (and manufacturers of network equipment) to migrate from the ST to the SC.

OTHER CONNECTORS - MULTIMODE
The following fiber connector styles are not recognized by the TIA-568-A standard but are included here for your information in case you run across them in your work, or need one for a particular application (such as the FDDI connector for FDDI applications). The FDDI connector (also called MIC or FSD) was established in 1986 as the ANSI-approved interface for the Fiber Distributed Data Interface (FDDI). The FDDI connector has two 2.5mm ferrules in a keyed plug.External keys allow for system topologies such as ring and star, preventing cross-mating from occurring. Key selection is very important in the design and constructionof both the Ring and Star topologies, for them to comply with the proper topology requirements. Two styles of sleeves (receptacles) allow for FDDI-to-FDDI connections, or hybrid ST-to-FDDI connections at patch panel interfaces. ESCON (Enterprise Systems Connection) connectors were developed by IBM with enhanced features such as a retractable shroud. The duplex connector uses 2.5mm ferrules and is similar in design to the FDDI/FSD connector.

High performance versions allow for systems use of both singlemode and multimode fibers. Hybrid ESCON/RSD to MIC/FSD sleeves are available. The SMA 905 AND 906 are threaded non-keyed connectors only available in multimode. They are not recommended for new designs. Delrin sleeves improve alignment but losses are high (1-2 dB). The Mini BNC is a bayonet style fiber optic connector used on older IBM systems and compatible products. The predecessor to ST types, this connector is keyed,has a 2.5mm ferrule and uses a bayonet coupling technique.

OTHER CONNECTORS - SINGLEMODE
The following connectors are common styles used for singlemode applications. Most can be used for multimode applications but are predominantly used in telecommunications systems.The FC is a simplex connector with keying features, using the 2.5mm ferrule. The FC continues to be a popular connector style in singlemode systems. The D-4 is predominantly used in singlemode systems, and offers high performance, but has a disadvantage in lack of users and availability. The Biconic, introduced in the 1990s, is a non-keyed connector which was generally used in the past by the telephone industry but its use is declining. Diamond connectors are the highest performance connectors available. Their design utilizes tungsten carbide for high repeatability and low losses. They are primarily used in test equipment and other high performance product.