Fiber optic connectors offer a disconnectable method for joining fibers to transmitters and receivers, other fibers and devices such as couplers and multiplexers. Most connectors are in the form of a cable-mount plug in which plugs mate with a feed-through receptacle that resembles a cylindrical coupling bushing. The inner diameter of the receptacle often has a split sleeve to provide a snug alignment of the mated plugs.
Because connector size is a primary factor in port density, cable connectors continue to get smaller. Most of this gain comes from the use of smaller ferrules. The newer MU and LC UPC connector use a 1.25 mm diameter ferrule, exactly half the size of a 2.5 mm diameter ferrule used in traditional FC, ST and SC connectors. Also, MT ferrule connectors are being combined in high-density arrays. Table 1 gives typical values for popular styles of connectors, grouped by ferrule size.
Popular Fiber Optic Connector Types
LC Connectors: The fully standardized LC connector utilizes a 1.25-mm ceramic ferrule and the versatile, pull-proof RJ-45 latching mechanism. Available in single-mode, multimode, simplex and duplex versions, they offer low insertion loss, low back reflectance and repeatable performance.
MU Connectors: The MU connector is one half the size of the standard SC connector, and is sometimes referred to as the mini-SC. Featuring a push-pull latching mechanism similar to the SC, the MU connector is easy to connect and disconnect. It provides a single point of disconnect in high-density applications, a feature often required by telecom operators.
SC Connectors: These emerged in the early 1990s as the general-purpose connectors of choice and are the recommended interfaces for premises cabling, ATM, Fibre Channel and low-cost FDDI. SC connectors use a 2.5 mm ferrule, push-pull locking mechanism and pull-proof design that prevents a slight pull on the cable from pulling the ferrule out of optical contact. Single connector plugs can be snapped together to form multiposition connectors.
ST Connectors: Originally developed by AT&T, they use quick-release bayonet couplings. A key ensures consistent, repeatable mating with the coupling bushing. They are available in a range of variations including ceramic, polymer or stainless steel ferrules and epoxy or epoxyless style termination. ST connectors remain the most widely and broadly used connector type.
FC Connectors: Used mainly by the telecommunications industry from which they derive, they use threaded couplings and 2.5 mm ferrules. Some variations of the connector use tunable keying to achieve the lowest loss. Tuning allows one ferrule to be rotated in relation to the other to minimize losses. The connector is keyed so that connectors will always mate in the tuned position.
FDDI-MIC Connectors and ESCON Connectors: Designed to meet the specifications of the ANSI X3.166 FDDI PMD (physical medium dependent) document, this duplex connector uses a side-latching mechanism and two 2.5 mm ferrules, as well as a fixed protective shroud to protect the ferrules. The connectors can be keyed according to fiber distributed data interface (FDDI) specifications, and also can be used for non-FDDI applications.
ESCON connectors, used in IBM ESCON channel interfaces, are similar to the FDDI-MIC connectors, but use retractable shrouds.
Plastic Fiber Connectors: These are available at low cost for fast termination to the cable, even at the expense of low loss. Most connectors require no epoxy, allow end finish to be achieved by trimming the fiber with a hot knife, and require little or no polishing.
Performance is in the 1 to 3 dB range. Many designs tend to be proprietary, although several standards have evolved using plastic fibers and connectors. Standardized applications include MIDI and digital audio, automotive and industrial automation. Many plastic fiber connectors incorporate LEDs and detectors directly as a means of reducing cost, simplifying the system and providing standardized parts.
MT-RJ Connectors: The MT-RJ is a two-fiber connector that resembles a standard telephone plug. The resemblance is intentional, as the connector is aimed at replacing the ST and SC types in wiring closets and at the desk. The connector fits in the same cutout as an RJ-45 jack, allowing fiber to be installed in network equipment, patch panels and wall plates without space penalties. The connector features a single, snagless latch. Rather than the typical fiber mating scheme that uses two plugs joined in a coupling adapter, the MT-RJ connector offers a true plug-to-receptacle mating technique. Plugs on the patch cable plug into a jack on the panel or faceplate. Fibers are terminated directly to the back of the jack with an epoxyless and no-polish termination.
High-Density Array Connectors: Multifiber array connectors such as LIGHTRAY MPX and MPO connectors, and fiber-ribbon cable have been gaining in popularity (Figure 4). MT-style connectors, which use a small ferrule to hold 4, 8, 12 or 72+ fibers, provide the high-density interface required for high-bandwidth communications.
These connectors are used mainly in fiber arrays that replace the rat’s nest of coaxial cable associated with mainframe computers. They offer a neat, compact and orderly method to connect equipment.These arrays are typically fanned out with jumper cables terminated in standard connectors, such as an ESCON duplex connector, to form the interface to the system equipment.
Fiber optic connectors are available in a wide range of configurations. The choice of connector typically depends on the applications.
Connectors offer low insertion losses well within the reasonable demands of applications. Application speed, the required interface and special environmental or mechanical requirements may narrow the selection, but overall connector selection should be neither difficult nor tricky.