One of the negative aspects of high tech is its habit of using abbreviations and acronyms for practically everything. It gets so bad, we joke about all the TLAs or three-letter acronyms. It can certainly be confusing for those of us in fiber optics, for it seems we have more TLAs than words to describe our technology.
Connectors are particularly bad, since most names of connectors are acronyms and the many versions of connector specifications are often described in some obscure multi-letter fashion. Fiber optics has even been around so long that most people have forgotten what some of them mean. Let’s look at some of them.
Connectors become known by either some description or a manufacturer’s designation. One of the first connectors, the SMA, was named after its microwave ancestor, the “Sub-Miniature A-type” microwave connector, which shared its basic design with a screw-on 1/4-36 nut. The fiber version of the SMA substituted a 1/8-inch machined fiber alignment ferrule for a center contact on its microwave sibling.
A few years later, manufacturers introduced a series of connectors using ceramic 2.5mm ferrules. These connectors had excellent fiber alignment, mostly due to the precision of the ceramic ferrule, and were keyed to prevent variations in loss due to rotation of the connector in the mating adapter. The first one of these connectors, the FC from Japan, used a screw thread nut for holding the connector on. FC stands for “Fiber Connector,” as named by its Japanese inventors.
AT&T introduced the ST connector shortly thereafter. ST is rumored to mean “straight terminus,” referring to the straight ferrule in contrast to the Biconic, the first AT&T connector. The ST uses a bayonet-locking nut like a BNC connector (which itself is supposed to mean “bayonet nut connector.”)
Japan responded by introducing the SC connector, still using the 2.5mm ferrule, but with a molded plastic body that conveniently snaps in and out. The SC was designed for use in high-density racks for fiber to the home, so SC is rumored to mean “subscriber connector.”
Two duplex connectors were designed in the late 1980s that took their names from the networks, which spawned their creation. The FDDI connector was chosen as the standard connector for “Fiber Distributed Data Interface,” a 100 Mbps token ring network. The ESCON connector took its name from the IBM “Enterprise System CONnection,” a network that connected peripherals to mainframe computers.
Recently, we have seen a number of small form factor (SFF) connectors introduced that are attempting to reduce the size of a fiber optic connection to the size of a copper connector, typically, but erroneously, called the RJ-45. It’s really a “modular eight-pin connector terminating Category 5/5e cable,” since the real RJ-45 is the same connector with USOC pinouts for telephones. (RJ refers to “Registered Jack” and comes in many varieties, such as RJ-11, RJ-14, and RJ-45.
USOC stands for “Universal Service Ordering Code,” the AT&T standard for connecting to the public telephone network.)
The SFF connector with the most vendor support, the MT-RJ, has a predictable name. MT stands for “Multiple Termination” and RJ brings to mind its copper nemesis, the RJ-45. The only other one with a letter designation, the MU, is relatively unknown in America. We suspect its name refers to the design goal of mounting in “Multiple Units,” since two fit into the same space as one SC.
Connector termination types
Another confusion factor in fiber optic connectors is the type of end finish used on the connector ferrule end face when it is polished. You have probably noticed connectors like the SC referred to as SC-PC, SC-UPC, or SC-APC. All these designations refer to the method used in the polishing of the connector.
Early fiber optic connectors were designed to not touch each other when mated to prevent scratching of the fibers. This gap caused these connectors to have a high loss, since reflections at the gap could cause 0.3 dB loss in itself, as well a high back reflections. Once connectors had hard ceramic ferrules and were keyed to prevent rotation, scratching was no longer a problem. Connectors could be made to touch by polishing the end face convex, reducing mated loss. These connectors came to be known as “Physical Contact” or PC connectors.
Competition in the marketplace to produce better connectors than the competitors, especially in back reflections, caused some manufacturers to refer to their connectors as Super PC (SPC) or Ultra PC (UPC) connectors. These names never caught on in the marketplace, so they have tended to refer to only a few manufacturers’ products.
APC connectors, however, are a widely accepted standard. For single-mode connectors, angling the end face 8 degrees effectively eliminates back reflection. APC, therefore, means “Angled Physical Contact.” APC connectors are the most widely used version of the SC connector for high bandwidth applications like CATV or 10 Gbps telephone systems (OC-192, where OC stands for Optical Channel, could lead to another complete column on telecommunications acronyms.)
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.