Share Some Basic Knowledge About Fiber Optic Connectors

Remember that connectors are used as terminating fixtures for temporary non-fixed joints. As such, they are made to be plugged in and disconnected hundreds and possibly thousands of times. Since no one connector is ideal for every possible situation, a wide variety of connector styles and types have been developed over the short life of fiber communications. We can classify connectors by assigning them into five major categories:

1, Resilient ferrule 2. Rigid ferrule 3. Grooved plate hybrids 4. Expanded beam 5. Rotary

Of these types the rigid ferrule is by far the most common. Rigid ferrule types include the popular ST (compatible), FC, and SC, which use a single 2.5 millimeter cylindrical ferrule for fiber alignment. Other simplex connectors housing a single fiber, but no longer in common use today, include the SMA (905 and 906), D4, and the Biconic.

Duplex connectors contain two fibers allowing for a single connector body for both transmit and receive fibers. These connectors have come to the fore in recent years and are expected to gain popularity in the LAN arena. LAN hard ware manufacturers have already adopted these connectors since they offer a much smaller size, allowing more links per panel space on network equipment.

Early examples of duplex connectors include the FDDI and ESCON. These connectors are rather large and cumbersome. Newer duplex connectors are designed to fit in the same work area outlet space as a standard RJ45 telephone jack and include the MT-RJ, Opti-Jack, and Volition connectors. These are commonly referred to as small form factor (SFF) connectors.

Although some SFF connectors are duplex designs, several others are miniature simplex connectors that are similar in design to the SC. The LC, LX-5, and MTRJ connector use smaller 1.25-millimeter ferrules and miniature bodies to allow twice the panel density of the earlier simplex connector designs.

The end of an optical connector (Figure 6-4) can be either polished flat or with a PC finish, a slightly rounded, domed end to create a “physical contact,” hence the PC designation. Physical contact of the fibers reduces the back reflection caused by air between the fiber ends. Some singlemode connectors may also have an “angled PC” (APC) finish. The ends are angled at 8 degrees to minimize back reflections at the point of connection. These connectors cannot be mated with the normal flat or domed polish types (Figure 6-4).

Although few, if any, of the original designs were compatible, nowadays compatibility exists between the same types from different manufacturers (i.e., ST or SC designs), thanks to marketplace pressures and standards committees. Although not compatible with all other connector styles, most ferrules are 2.5 millimeters and will loose fit for temporary testing purposes. For example, by lightly inserting the ferrule of an ST into an FC coupler, a “quick-and-dirty” test can be made for continuity. Hybrid adapters to allow coupling of different types of connectors are generally available as either sleeve connectors or patch cords. Although no single connector is best for every application, Table 6-1 lists the currently popular connectors found in many different types for various applications.

Choosing a Fiber Connector

With all of the myriad selections of connector types, styles, and physical characteristics available on the market, choosing the specific connector for your job is often a mystifying task. One important criterion is connector performance. When selecting a connector, comparisons of performance are generally based on:

* Insertion loss, usually 0.10 to 1.0 dB per connection * Return loss (back reflection) varies from –20 (air gap like a SMA) to –60dB (the best APC angle polished connectors) * Repeatability of connection, usually specified at thousands of times

Your choice of fiber connector also may depend on whether you are mounting it onto singlemode or multimode fiber. Since singlemode connectors have a much tighter tolerance than multimode connectors, they may be used on either type of fiber. However the reverse is not true, that is, one may not use multimode connectors on single mode fiber because the loose tolerance will cause high loss with the very small singlemode core size. Generally multimode connectors are fitted onto multimode fibers because they are less precise and cost about one-half to one-third the cost of single mode connectors.

The accessibility of the fiber to casual users may cause you to anticipate rough handling. In this case, gripping strength of the connector on the cable becomes important to avoid pullouts by users. Gripping points of the connector may include the fiber itself, the primary plastic buffer coating (tight buffer), the loose tube buffer, the cable strength members (Kevlar), and/or the cable jacket itself.

Another reason for choosing a particular type of connector is the type of equipment already purchased or currently in use. If, for instance, you are adding to an existing system already equipped with ST fiber connector, you should continue to use ST connectors to ensure compatibility systemwide. If you are using previously purchased electronics with Biconic connectors installed, then that will be your choice, unless, of course, you want to change all of the connections on the patch panels and electronics.

Finally, your choice may be influenced by industry standards or new developments in the marketplace. The Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA) standards for premises cabling calls for the SC connector, although they are considering the new SFF connectors. Many of the newer connectors offer the promise of lower cost or higher performance, which can also influence the decision.