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Fiber Optic Cable Connectors

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Most popular connectors in use today have some common elements. Let's examine it below.

The most critical part, where the fiber is mounted, is the ferrule. Ferrule is a long, thin cylinder with the fiber mounted in the center hole. The center hole is sized to match fiber's cladding diameter which is usually 125um.

Fiber connector ferrules are made from several types of materials including ceramic(Zirconia), stainless steel and plastic.

The ferrule's job is to center and align the fiber and protects it from mechanical damage.The end of fiber is at the end of the ferrule, where the fiber end is polished smooth either flat or with a curvature.

The ferrule is mounted in the connector body and then the connector body is attached to the fiber optic cable structure. Finally, a strain-relief rubber boot protects the connector-cable juntion.

Unlike most electronic connectors, fiber optic connectors usually do not have the male-female polarity. Most fiber connectors are male only. Instead, fiber connectors mate to each other in fiber adapters, which are often called mating sleeves or coupling receptacles. Fiber optic adapters used to mate different connector types such as a FC connector to a SC connector are called hybrid adapters.

Although this approach requires the use of separate adapters, it otherwise reduces fiber connector inventory requirements since now you need to stock one type of connector only. Another advantage is that fiber adapters can be designed to mate one type of connector to another, which is a big plus compared to electronic connectors.

The fiber's plastic coating is stripped first before the fiber is inserted in the ferrule. The center hole through the ferrule is large enough to fit the fiber cladding (which is usually 125um after fiber coating stripped off) but tight enough to hold the fiber in a fixed position without any further moving.

Standard bore diameters are 126 +1/-0 um for single mode connectors and 127 +2/-0 um for multimode connectors. Because of fiber cladding diameter's variation from manufacturing, some fiber connector manufacturers also supply a range of ferrule bore sizes such as 124um, 125um, 126um and 127um.

Fiber optic epoxy or adhesive is injected into the ferrule hole before the fiber is pushed in to hold the fiber in place. The epoxy or adhesive is then cured with high temperature oven according to adhesive manufacturers' instruction. Finally the fiber end is polished to a smooth face on polishing films.

The ferrule is then slipped inside another hollow cylinder before it is mounted in the connector body. The connector body includes one or more pieces that are assembled to hold the cable and fiber in place. Connector body is made of metal or plastic.

The ferrule end protrudes beyond the connector body so it can slip into the mating sleeves (fiber adapters). A stain-relief rubber boot is finally slipped over the cable end of the connector to protect the cable-connector junction point.

In fiber optic cross connect boxes or fiber patch panels, an array of connector adpaters are mounted inside, ready for you to plug an input fiber cable in one side and an output cable in the other. Fiber connector adapters are also mounted in wall outlets, just like standard phone jack.

Fiber Optic Cable Connectors

Simplex connector means only one fiber is terminated in the connector. Simplex connectors include FC, ST, SC, LC, MU and SMA.

Duplex connector means two fibers are terminated in the connector. Duplex connectors include SC, LC, MU and MT-RJ. (Note: SC, LC and MU connectors have both simplex and duplex version)

Multiple fiber connector means more than two fibers (for up to 24 fiber) are terminated in the connector. These are usually ribbon fibers with fiber count of 4, 6, 8, 12 and 24. The most popular ribbon fiber connector is MT connector.

Now let's examine each type of connector closely.

:: FC Connector

FC stands for Fixed Connection and it was developed by Japanese company NEC (Nippon Electric Co.). It has a screw on locking mechanism and is more popular in single mode applications than in multimode applications. FC connectors are popular in test environments and long haul applications. FC was once the most popular connector but now is being replaced by SC and LC connectors. FC connector is available only in simplex version.

Advantages: lowest insertion loss, excellent repeatability

Disadvantage: Bulky, simplex version only, screw-on mechanism is hard to operate in a busy telecom closet.

:: ST Connector

ST stands for Straight Tip. ST connector has a bayonet twist locking mechanism and was developed by AT&T in the 1980s. ST connectors were predominant in the early 1990s but now they are being replaced by SC and LC connectors. ST connectors are most common for multimode applications. ST connectors are available in simplex version only.

Advantages: ST connector can be easily inserted and removed thanks to its spring-loaded bayonet locking mechanism.

Disadvantages: Its locking mechanism design are both a bless and a curse. You have to make sure the connector is seated in its socket securely and properly. It may be necessary to remove and reconnect your ST connector if you experience high insertion loss. But if you did it correctly, less than 0.3dB connection loss could be easily achieved.

:: SC Connector

SC stands for Subscriber Connector and was developed by Japanese company NTT. It is a square shaped push/pull type connector with a snap-in locking mechanism. Because of its easy locking mechanism and excellent repeatability, SC connector has become the dominant connector since late 1990s, especially in multimode applications. SC connectors are available in both simplex and duplex version.

Advantages: Easy push/pull snap-in locking mechanism, low cost, excellent repeatability, low insertion loss

Disadvantages: Bulky footprint. Modern high speed fiber optic networks need more compact connectors. LC connector is almost half the size of a SC connector and is becoming more popular, especially in single mode applications.

:: LC Connector

LC stands for Lucent Connector and it was developed by Lucent. This is a small form factor connector that occupies only half the size of a SC connector. LC connector looks much like a miniature SC connector. It also has the same push/pull snap-in type locking mechanism. LC has become the most favored connector for single mode applications.

Advantages: Easy push/pull snap-in locking mechanism, excellent repeatability, low insertion loss, small form factor

:: MT-RJ Connector

MT-RJ stands for Mechanical Transfer Registered Jack. MT-RJ is available in duplex only. It houses two fibers in half the size of a SC connector. MT-RJ connectors come in pair of male and female type. Male MT-RJ connector has two guiding pins and female type has two guiding holes. Male and Female MT-RJ connectors are mated together by plugging the guiding pins into the holes.

:: MT Connector

MT connector is designed for ribbon fiber applications. It can hold 4, 6, 8, 12, 24 and up to 72 fibers in a square shaped footprint. One MT compatible connector is called MTP connector which is manufactured by US Conec.

Advantages: MT connector provides much higher connection density than any other type of connectors. This makes it the excellent choice for patch panel, routing applications.

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Colin Yao has sinced written about articles on various topics from Computers and The Internet, Home Management and Computers and The Internet. Colin Yao is the sales manager at Fiber Optics For Sale Co. and an expert on fiber optic technologies and products. Learn even more about. Colin Yao's top article generates over 22200 views. to your Favourites.

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