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Whereas the RJ-type connector is the most commonly used connector for twisted-pair copper data communications and voice connections, a variety of choices exist for fiber-optic connections you need to use.

This article focuses on the different types of fiber connectors and discusses how they are installed onto fiber-optic cable.

Fiber-optic connections use different terminology than copper based connectors. The male end of the connection in a fiber-optic system is termed the connector, in contrast to the plug in a copper-based system. The female end of the connection is termed the receptacle or adapter, in contrast to the jack in a copper-based system.

To transmit data up to 10 Gbps, two fibers are typically required: one to send and the other to receive. For 40 Gbps and 100 Gbps over multi-mode, as many as 24 fibers will be required. Fiber optic connectors fall into one of three categories based on how the fiber is terminated:

• Simplex connectors terminate only a single fiber in the connector assembly.
• Duplex connectors terminate two fibers in the connector assembly.
• Array connectors terminate more than two fibers (typically 12 or 24 fibers) in the connector assembly.

The disadvantage of simplex fiber optic connectors is that you have to keep careful track of polarity. In other words, you must always make sure that the connector on the “send” fiber is always connected to the “send” receptacle (or adapter) and that the “receive” connector is always connected to the “receive” receptacle (or adapter). The real issue is when normal working folk need to move furniture around and disconnect from the receptacle in their work area and then get their connectors mixed up. Experience has shown us that the fiber optic connectors are not always color coded or labeled properly. Getting them reversed means, at the least, that link of the network won’t work.

Array and duplex connectors and adapters take care of this issue. Once terminated, color coding and keying ensures that the connector can be inserted only one way in the adapter and will always achieve correct polarity.

Fiber optic connectors can be used for either single-mode or multi-mode fibers, but make sure you order the correct model connector depending on the type of cable you are using.

Of the four layers of a tight-buffered fiber (the core, cladding, coating, and buffer), only the core where the light is actually transmitted differs in diameter. The cladding, coating, and buffer diameters are mostly identical, allowing universal use of stripping tools and connectors.

The ST used to be the most widely deployed, but now the duplex SC and LC are the most widely used connectors. Other connector styles are allowed, but not specified. Other specifications, including those for ATM, FDDI, and broadband ISDN, now also specify the duplex SC.

This wide acceptance in system specifications and standards (acceptance in one begets acceptance in others), along with ease of use and positive assurance that polarity will be maintained, are all reasons why the duplex SC and LC are the current connectors of choice.

SC stands for "Subscriber Connector" or "Square Connector". It was developed by NTT. SC connector has a push-pull locking mechanism which is very flexible yet provides high repeatability and low insertion loss. It has been quickly replacing legacy connectors such as ST, SMA connectors and becoming the current most popular fiber connectors used in the fiber optic communication industry.

Because of its push-pull snap-in locking mechanism, SC connector has the advantage in keyed duplexibility to support send/receive channels in a single fiber connection unit. SC connector uses ceramic ferrules so they can provide accurate fiber alignment and high reliability. The ceramic ferrule is a 2.5 mm diameter cylindrical structure and the fiber is fixed in a hole at the center. The typical insertion loss of a SC connector is about 0.2 dB.

From outside, the SC connector is an unusual looking square-tipped connector that is slightly cone-shaped at the tip. It uses spring retention to hold the connector in place when mated. This square design and push/pull coupling mechanism make the it usable in high-density applications.

The SC connector may be coupled in duplex sets with a coupling receptacle or duplex clip. In a keyed duplex set, the SC connector easily implements a form of polarity matching with coupling adapters in fiber optic outlets or patch panels. SC connector has been adopted by TIA as the official recommended fiber connector in TIA/EIA 568-A. In the standard, it is called as 568SC connector.

Duplex version of SC connectors is gaining popularity in networks and other applications requiring full-duplex transmission. The connector is suitable for single mode and multi-mode fibers. The SC connector offers excellent packing density as well as exceptional performance and cost. SC connectors are widely used in Gigabit Ethernet, ATM, LAN, MAN, WAN, data communication, Fibre Channel, and telecommunication networks.

ST connector stands for "Straight Tip". It was developed by AT&T and is a registered trademark of AT&T. The formal name as defined in ISO/IEC standards is BFOC/2.5. ST connector was the first most popular and thus defacto standard connector in the fiber optic communication industry.

It evolved from copper cable connector designs using a half-twist bayonet lock mechanism and has a plug and socket. ST connector is standardized in EIA/TIA-604-02 chapter FOCIS 2. ST is similar to FC connector but has a quick release bayonet coupling mechanism instead of screwing on mechanism as in FC connectors.

It is available in both single mode and multi-mode types and are still widely used in premise wiring. ST connector has a key on an inner sleeve along with the outer bayonet ring. To make a connection, you line up the key on the inner sleeve of the ST plug with a corresponding slot on the ST receptacle. Then you push the connector in (it is spring loaded) and lock it in place by twisting the outer bayonet ring. This mechanism provides a tight connection with precise alignment between the two pieces of fiber optic cable being joined.

It is quick and simple to install, and for that reason it is very popular. The the problem is that it is also difficult to pair in dual-fiber installations. So ST connector is only available in simplex version and not in duplex version. Other similar simplex fiber optic connectors include SMA connector, FC connector and BNC connector. They all share the difficulty of pairing in dual-fiber installations. There is no practical way to bind two ST fiber connections into one unit. Now ST connector is considered a legacy fiber connector, as it has been around for quite some time and can still be found in many installation.

FC fiber optic connector is popular in test environments and mostly for single mode applications. FC stands for "Fixed Connection". It was the earliest connector and is now available in FC PC, FC APC, FC SPC and FC UPC types. It was originally devised by NTT (Nippon Telegraph and Telephone) for telecommunication applications. Thus it is popular in Japan and Europe. In the US, MCI used it in its fiber optic telephone networks in 1980s.

FC connector has a threaded coupling mechanism with adjustable keying to achieve minimum insertion loss. You can adjust the key in some degree in order to lock it at the minimum loss position. Although this tuning is useful in some applications where minimum loss is required, it is not used in most applications because of the small gain of 0.1~0.2dB in insertion loss. Even though both single mode and multi-mode type of FC connectors are available, it is mostly used for single mode applications.

FC connector offers very high precise positioning of the fiber cables that are being joined. Thus FC has set the standard for fiber optic connectors. But because of its threaded locking mechanism, FC connector cannot be paired in duplex version. This makes it less popular in LAN and premise fiber networks. It is being replaced by SC and LC connectors.

The threaded locking mechanism is constructed with a nickel-plated metal housing and a ceramic 2.5mm ferrule. This structure gives it secure connection even in high-vibration environments. The disadvantage is that it cannot be quickly connected and disconnected as with SC or LC connectors.

FC APC connector has a 8° angled ferrule endface. The purpose of this angled endface is to minimize reflection from the connector mating surfaces. FC APC connector provides minimum back reflection which is required to be lower than -60 dB in most cases.

As transmission rates increase and networks require the cramming in of a greater number of connections, the industry has developed small-form-factor (SFF) connectors and adapter systems for fiber-optic cables. The SC, ST, and FC connectors all take up more physical space than their RJ-45 counterparts on the copper side. This makes multimedia receptacle face-plates a little crowded and means that you get fewer terminations (lower density) in closets and equipment rooms than you can get with copper in the same space. The goal for the designers of the SFF connector was to create an optical-fiber connector with the same or lower cross-sectional footprint as an RJ-45-style connector in order to increase the number of connections per area (higher density).

The LC, the VF-45, and the MT-RJ SFF fiber optic connectors were initially developed to support the increase in density of fiber connections. The LC connector is gaining greater use and is regarded by many optical-fiber professionals as the superior simplex or duplex SFF connector.

The SFF connector was taken several steps forward in increasing connection density with the creation of the MPO array connector. This connector typically has 12 fibers lined up side by side (hence, use of the term array) in a single connector housing. These connectors may have multiple arrays stacked up on top of each other to produce connectors with as many as 48 or more fibers in a single connector housing. These types of connectors are typically used in data center and super-computing applications and to support the migration to parallel optical transmission systems for 40 Gbps and 100 Gbps.

With twisted-pair and coax cables, connectors are joined to the cable and conductors using some form of crimping or punch down, forcing the components into place. With fiber-optic cables, a variety of methods can join the fiber with its connector. Each manufacturer of connectors, regardless of type, specifies the method to be used, the materials that are acceptable, and sometimes, the specialized tools required to complete the connection. Connectors can be installed onto cable in the field or they can be factory-terminated.

When the fiber connector is inserted into the receptacle, the fiber-optic core in the connector is placed in end-to-end contact with the core of a mating fiber in the adapter of a wall plate or transceiver. Two issues are of vital importance:

• The fiber-optic cores must be properly aligned. The end-to-end contact must be perfectly flush with no change in the longitudinal axis. In other words, they can’t meet at an angle.
• The surfaces must be free of defects such as scratches, pits, protrusions, and cracks.

To address the first critical issue, fiber connector systems must incorporate a method that both aligns and fixes the fiber in its proper position. The alignment is usually accomplished by inserting the fiber in a built-in sleeve or ferrule. Some technique—either gluing or crimping—is then applied to hold it in place.

Crimp-style connector systems for fiber-optic cable are always manufacturer-specific regarding the tools and materials required. Follow the manufacturer’s instructions carefully. With crimp connectors, the fiber is inserted into the connector, and the assembly is then placed in a crimping tool that holds the fiber and connector in proper position. The tool is then used to apply a specific amount of pressure in a controlled range of motion to crimp the connector to the buffer layer of the fiber.

To address the second critical issue, part of the connecting process usually involves a polishing step. With the fiber firmly established in the connector, the end of the fiber is rough trimmed. A series of abrasive materials is then used to finely polish the end of the fiber.

Use only the recommended cleaning and polishing kits designed to be used with optical fibers: do not use your shirt sleeve. Some 85% of all failures in the field are due to dirt or debris on the face of a fiber. Always clean the end face properly before connecting; otherwise, the fiber on the receiving side could be damaged as well.

Pre-polished fiber optic connectors are also available. They are connectors that have a fiber already installed in the connector: the end of the fiber at the ferrule end is polished and the other end terminates inside an alignment sleeve that is inside the connector. Following the recommended fiber preparation procedures of the manufacturer, the fiber from the cable is buttedup against the fiber that is already inside the connector, using a positive mechanical force to hold the ends of the fibers together. An index matching gel is often used to reduce the amount of reflection that can be caused at the interface of the two fibers. Such connectors are used primarily, if not exclusively, with multimode fibers because of the larger core diameter of multimode fiber-optic cable. Since the fiber is already inside a pre-polished connector, you must use the correct type of pre-polished multimode connector with the multimode fiber that you are installing. Use only 62.5 micron MMF with 62.5 MMF pre-polished connectors and 50 micron MMF with 50 micron MMF pre-polished connectors. Mixing MMF types will cause severe insertion loss where they are mated together.

Three types of adhesives can glue the fiber into position:

After the material is injected and the fiber is inserted into the connector assembly, it is placed in a small oven to react with the adhesive and harden it. This is time-consuming—heat-cured adhesives require as much as 20 minutes of hardening. Multiple connectors can be done at one time, but the time required to cure the adhesive still increases labor time, and the oven is, of course, extra baggage to pack to the job site.

Rather than hardening the material in an oven, an ultraviolet light source is used. You may have had something similar done at your dentist the last time you had a tooth filled. Only about a minute of exposure to the UV light is required to cure the adhesive, making this a more time-effective process.

This method relies on the chemical reaction of two elements of an epoxy to set up and harden. A resin material is injected in the ferrule. Then a hardener catalyst is applied to the fiber. When the fiber is inserted in the ferrule, the hardener reacts with the resin to cure the material. No extra equipment is required beyond the basic materials and tools. Hardening can take place in as little as 15 seconds.

Note that while you can use a single-mode optical fiber connector with a multimode fiber, you cannot use a multimode connector with a single-mode fiber. The reason is that the singlemode connecter is manufactured to tighter tolerances to permit the accurate matching of the cores of two single-mode fibers. Single-mode fiber connectors are much more expensive for this reason.

Data centers, generically referred to as main equipment rooms, are the heart and brains of a commercial enterprise network. More and more of the large Fortune 100 companies are consolidating their data centers in an effort to minimize operating costs. Since network uptime is critical, any interruption of ongoing systems can cause significant costs. Installing thousands of connectors in the field using adhesives can take a lot of labor time. To address this, a growing trend in data center cabling involves pre-connectorized cables. These are typically high-fiber-count cables that are pre-connected with multiple MPO connectors. The IT planning group would survey the data center and order specific lengths of these pre-terminated cables. On installation day, the cables can be simply plugged into factory-assembled patch panel systems. These MPO pre-terminated high-fiber-count cables are commonly called “plug-’n’-play” solutions. In designing and costing your networks, it would be wise to compare the material and labor cost of these cables to the cost of bare cables, which require connector installation in the field.