How To Assemble A Fiber Optic Connector - Fiber Optic Tutorial Series Five

By: Colin Yao

Since there are many different types of fiber connectors have been developed, we will talk about fiber connectors in fairly general terms.

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.

How To Tell The Difference Between Small Form-Factor Fiber Connectors

By: Colin Yao

Several types of small form-factor fiber connectors have been developed in recent years to fill the demand for fiber optic devices that can fit into tight spaces and allow denser packing of connections. These connectors include LC, MU, MT-RJ and VF-45, to name a few.

Most of these small form-factor connectors use push-latch locking mechanism just like the standard old SC connector. This mechanism has the advantage of easily adapt to duplex connectors such as duplex LC, duplex MU, etc.

Some of the small form-factor connectors use 1.25mm fiber ferrule while others are based on smaller versions of MT type ferrules. Now let's examine them in more details below.

:: MTRJ connector

MTRJ connector is derived from the MT connector design with a miniature two-fiber ferrule and an overall size about the same as a standard telephone jack (an RJ45 connector). Two pins are used for fiber alignment while the two fibers are between these two pins.

MTRJ connector is used for intrabuilding communication systems such as interior telephone wiring, and can fit into the same slot in a wall plate as the socket for an RJ45 telephone jack. MTRJ connectors are designed as plugs and jacks, like RJ45 telephone connectors.

:: LC connector

LC connector is a combined design with features from both RJ45 telephone connectors and the 2.5mm ferrule SC connectors. Its outside shape resembles a RJ45 telephone jack, while internally it is actually a miniature version of SC, with a 1.25mm ferrule holding the fiber instead of the 2.5mm ferrule as in SC connectors.

There is a connector very similar to LC, which is called LX.5. LX.5 looks very much like a LC connector, but it has a integral end cap that covers the ferrule when it is out of the mating adapter. When plugging it into the adapter, the end cap opens automatically.

:: MU connector

The MU connector is also a miniatured version of SC connector with a 1.25mm ferrule. But unlike LC connector, it has a push-pull snap-in latching mechanism instead the push-latch locking mechanism as in LC connectors.

There are also some other types of small form-factor connectors designed around the 1.25mm ferrules. They are pretty similar but differ in details such as latching mechanism and assembly procedures. Such an example is the Fiber-Jack connector which is built around a 2.5mm ferrule though.

:: VF-45 connector

3M designed its own VF-45 connector. Its special design is to insert the connector into the socket at a 45° angle instead of the straight face-to-face design in all other type of connectors. The 45° angle presses the pair of fibers into V grooves inside the plastic adapter. The pressure and 45° angle holds the fibers in place in the grooves.

The advantage of VF-45 connectors is that it doesn't need expensive ferrules, thus reducing fiber connector costs in a communication system.

:: Duplex connectors

Duplex version of small form-factor connectors are also available. Duplex connector has a pair of fibers and an internal key so they can be mated in only one orientation. This is necessary since most communication systems use separate fibers to carry signals in each direction. Attach the connector the wrong way and you have one transmitter sending signals to the other transmitter.