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An optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due to reflection or misalignment of the fibers. In all, about 100 fiber optic connectors have been introduced to the market.
Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The basic connector unit is a connector assembly. A connector assembly consists of an adapter and two connector plugs. Due to the polishing and tuning procedures that may be incorporated into optical connector manufacturing, connectors are generally assembled onto optical fiber in a supplier’s manufacturing facility. However, the assembly and polishing operations involved can be performed in the field, for example, to make cross-connect jumpers to size.
Optical fiber connectors are used in telephone company central offices, at installations on customer premises, and in outside plant applications to connect equipment and cables, or to cross-connect cables.
Most optical fiber connectors are spring-loaded, so the fiber faces are pressed together when the connectors are mated. The resulting glass-to-glass or plastic-to-plastic contact eliminates signal losses that would be caused by an air gap between the joined fibers.
Every fiber connection has two values:
Measurements of these parameters are now defined in IEC standard 61753-1. The standard gives five grades for insertion loss from A (best) to D (worst), and M for multimode. The other parameter is return loss, with grades from 1 (best) to 5 (worst).
A variety of optical fiber connectors are available, but SC and LC connectors are the most common types of connectors on the market. Typical connectors are rated for 500–1,000 mating cycles. The main differences among types of connectors are dimensions and methods of mechanical coupling. Generally, organizations will standardize on one kind of connector, depending on what equipment they commonly use. Different connectors are required for multimode, and for single-mode fibers.
In many data center applications, small (e.g., LC) and multi-fiber (e.g., MTP) connectors are replacing larger, older styles (e.g., SC), allowing more fiber ports per unit of rack space.
Features of good connector design:
Outside plant applications may require connectors be located underground, or on outdoor walls or utility poles. In such settings, protective enclosures are often used, and fall into two broad categories: hermetic (sealed) and free-breathing. Hermetic cases prevent entry of moisture and air but, lacking ventilation, can become hot if exposed to sunlight or other sources of heat. Free-breathing enclosures, on the other hand, allow ventilation, but can also admit moisture, insects and airborne contaminants. Selection of the correct housing depends on the cable and connector type, the location, and environmental factors. Careful assembly is required to ensure good protection against the elements.
Depending on user requirements, housings for outside plant applications may be tested by the manufacturer under various environmental simulations, which could include physical shock and vibration, water spray, water immersion, dust, etc. to ensure the integrity of optical fiber connections and housing seals.
Many types of optical connector have been developed at different times, and for different purposes. Many of them are summarized in the tables below.
|Short name||Long form||Coupling type||Screw thread||Ferrule diameter||Standard||Typical applications||Image|
|Avio (Avim)||Aviation Intermediate Maintenance||Screw||Aerospace and avionics|
|ADT-UNI||Screw||2.5 mm||Measurement equipment|
|DMI||Clip||n/a||2.5 mm||Printed circuit boards|
|E-2000 (AKA LSH)||Snap, with light and dust-cap||n/a||2.5 mm||IEC 61754-15||Telecom, DWDM systems;|
|EC||push-pull type||n/a||IEC 1754-8||Telecom & CATV networks|
|ESCON||Enterprise Systems Connection||Snap (duplex)||n/a||2.5 mm||IBM mainframe computers and peripherals|
|F07||2.5 mm||Japanese Industrial Standard (JIS)||LAN, audio systems; for 200 μm fibers, simple field termination possible, mates with ST connectors|
|F-3000||Snap, with light and dust-cap||n/a||1.25 mm||IEC 61754-20||Fiber To The Home (LC Compatible)|
|FC||Ferrule Connector or Fiber Channel ||Screw||2.5 mm||IEC 61754-13||Datacom, telecom, measurement equipment, single-mode lasers; becoming less common|
|Fibergate||Snap, with dust-cap||n/a||1.25 mm||Backplane connector|
|FSMA||Screw||3.175 mm||IEC 60874-2||Datacom, telecom, test and measurement|
|LC||Lucent Connector, Little Connector, or|
Local Connector
|Snap||n/a||1.25 mm||IEC 61754-20||High-density connections, SFP transceivers, XFP transceivers|
|ELIO||Bayonet||n/a||2.5 mm||ABS1379||PC or UPC|
|Lucxis||1.25 mm||ARINC 801||PC or APC configurations (note 3)|
|LX-5||Snap, with light- and dust-cap||n/a||IEC 61754-23||High-density connections; rarely used|
|MIC||Media Interface Connector||Snap||n/a||2.5 mm||Fiber distributed data interface (FDDI)|
|MPO / MTP||Multiple-Fiber Push-On/Pull-off ||Snap (multiplex push-pull coupling)||n/a||2.5×6.4 mm ||IEC-61754-7; EIA/TIA-604-5 (FOCIS 5)||SM or MM multi-fiber ribbon. Same ferrule as MT, but more easily reconnectable. Used for indoor cabling and device interconnections. MTP is a brand name for an improved connector, which intermates with MPO.|
|MT||Mechanical Transfer||Snap (multiplex)||n/a||2.5×6.4 mm||Pre-terminated cable assemblies; outdoor applications|
|MT-RJ||Mechanical Transfer Registered Jack or Media Termination - recommended jack ||Snap (duplex)||n/a||2.45×4.4 mm||IEC 61754-18||Duplex multimode connections|
|MU||Miniature unit ||Snap||n/a||1.25 mm||IEC 61754-6||Common in Japan|
|SC||Subscriber Connector  or|
square connector  or
|Snap (push-pull coupling)||n/a||2.5 mm||IEC 61754-4||Datacom and telecom; GPON; EPON; GBIC|
|SMA 905||Sub Miniature A||Screw||1/4"-36 UNS 2A||Typ. 3.14 mm||Industrial lasers, optical spectrometers, military; telecom multimode|
|SMA 906||Sub Miniature A||Screw||Stepped; typ. 0.118 in (3.0 mm), then 0.089 in (2.3 mm)||Industrial lasers, military; telecom multimode|
|SMC||Sub Miniature C||Snap||n/a||2.5 mm|
|ST / BFOC||Straight Tip/Bayonet Fiber Optic Connector||Bayonet||n/a||2.5 mm||IEC 61754-2||Multimode, rarely single-mode; APC not possible (note 3)|
|TOSLINK||Toshiba Link||Snap||n/a||most common is JIS F05||Digital audio|
|1053 HDTV||Broadcast connector interface||Push-pull coupling||n/a||Industry-standard 1.25 mm diameter ceramic ferrule||Audio & Data (broadcasting)|
|V-PIN||V-System||Snap (Duplex) Push-pull coupling||n/a||Industrial and electric utility networking; multimode 200 μm, 400 μm, 1 mm, 2.2 mm fibers|
|Short name||Long form||Coupling type||Screw thread||Ferrule diameter||Standard||Typical applications|
|D4 (NEC)||Screw||2.0 mm||Japanese telecom in the 1970s and 1980s; obsolete|
|Deutsch 1000||Screw||Telecom, obsolete|
|DIN (LSA)||Screw||IEC 61754-3||Telecom in Germany in 1990s, measurement equipment; obsolete|
|OPTIMATE||Screw||Plastic fiber; obsolete|
|OptoClip II||Snap (push-pull coupling)||n/a||None - bare fiber used||Proprietary Hüber & Suhner||Datacom and telecom; obsolete (last made in 2005)|
Field-mountable optical fiber connectors are used to join optical fiber jumper cables that contain one singlemode fiber. These assemblies can be separated into two major categories: single-jointed connector assemblies and multiple-jointed connector assemblies. According to Telcordia GR-1081, a single-jointed connector assembly is a connector assembly where there is only one spot where two different fibers are joined together. This is the situation generally found when connector assemblies are made from factory-assembled optical fiber connector plugs. A multiple-jointed connector assembly is a connector assembly where there is more than one closely spaced connection joining different fibers together. An example of a multiple-jointed connector assembly is a connector assembly that uses the stub-fiber type of connector plug.
Field-mountable optical fiber connectors are used for field restoration work and to eliminate the need to stock jumper cords of various sizes.
Glass fiber optic connector performance is affected both by the connector and by the glass fiber. Concentricity tolerances affect the fiber, fiber core, and connector body. The core optical index of refraction is also subject to variations. Stress in the polished fiber can cause excess return loss. The fiber can slide along its length in the connector. The shape of the connector tip may be incorrectly profiled during polishing. The connector manufacturer has little control over these factors, so in-service performance may well be below the manufacturer's specification.
Testing fiber optic connector assemblies falls into two general categories: factory testing and field testing.
Factory testing is sometimes statistical, for example, a process check. A profiling system may be used to ensure the overall polished shape is correct, and a good quality optical microscope to check for blemishes. Optical Loss / Return Loss performance is checked using specific reference conditions, against a reference-standard single mode test lead, or using an "Encircled Flux Compliant" source for multi-mode testing. Testing and rejection ("yield") may represent a significant part of the overall manufacturing cost.
Field testing is usually simpler. A special hand-held optical microscope is used to check for dirt or blemishes. A power meter and light source or an optical loss test set (OLTS) is used to test end-to-end loss, and an optical time-domain reflectometer may be used to identify significant point losses or return losses.
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