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Fiber Connectors

Polishing Fiber Connectors

Fiber-optic installers started polishing fiber-optic connectors in the late 1970s. At that time they injected epoxy in the connector and let it set for 24 hours. Then the two or three steps of polishing began. Data rates were low and basically, if there was light, the system worked. Through the 30- to 100-power microscope, the connectors looked acceptable.

In the mid-1980s ovens were invented, as were new types of epoxy that would cure in 10 minutes at 100 degrees Celsius. This practice continued, and in the meantime innovative companies searched for new opportunities.

The Case for Prepolished

Somewhere along the line, someone decided that installers didn't like polishing connectors, and also noted the high cost of polishing film, alcohol and other consumables. The pitch made to the end-users was that the manufacturer could save them money by selling them a connector that already had a polished fiber stub within it. Figures were compiled, based on labor and material. And to the end-user, the numbers looked good. Had the manufacturers involved the installer in this equation, the results might have been different.

With these connectors-which came loaded with a prepolished stub of fiber-all the installer had to do was strip and clean the fiber, then cleave it to close to 90 degrees, put it in the connector and mechanically lock it in place. Some index-matching gel was in the connector to reduce insertion loss and back-reflection loss. At the time these types of connectors were emerging on the scene, about 1993, back reflection was not much of an issue.

The tool used to install these connectors was awkward, and the cleaver not so accurate. However, these connectors were pushed relentlessly to customers. In 1996 a second make of this connector type came on the market, and other revisions followed to the point where most companies have their own brand of what they call "field-installable connector."

As the years went on, cleavers improved as well as the geometry of the fiber and the connectors. Many companies accepted this cleave-and-crimp method as the way to install fiber-optic connectors, and did not seek additional opinions or options.

The Pigtail Tale

Despite the effectiveness of the APC connector, users began hearing about another fiber-connectivity option that was supposed to "come to the rescue"-the pigtail. The pigtail consisted of 6 feet of fiber attached to a prepolished factory connector. The connector had a physical contact (PC), ultra physical contact (UPC) or APC finish. The pigtail was spliced to the fiber, and the splice was protected in a splice tray.

Each of these connectivity approaches, the prepolished connector and the spliced pigtail, has its drawbacks. In the case of the prepolished, field-installable connector, we end up with two sources of loss rather than just the one that occurs with an epoxy/polish connector. Little is made of this fact, however, because the Telecommunications Industry Association (TIA) standard-development groups wrote specifications allowing for a loss of 0.75 dB on a mated pair of connectors. This level of allowable loss provides sufficient leeway for typical prepolished connectors to meet the standard.

With the pigtail approach, a splice tray is required, which occupies space and adds to cost. Also, the splice itself is both an additional source of loss and an additional potential point of failure.

Why are these fiber-optic connectivity methods so popular? We all have been told by providers of these products and technologies that we cannot expect a technician to hand-polish a connector with any success. Many end-user customers believe this and (literally) bought into the quicker, easier methods. Unfortunately we are ending up with connections that: 1) have additional loss, 2) change their loss values as a result of temperature cycling at the splice point in the connector, 3) include additional points of failure, 4) come at a higher finished cost, and 5) have the potential to require early replacement.

The solution is to use an epoxy connector and polish it by hand. An epoxy connector can be polished in about 30 seconds by a trained technician. This applies equally well to a PC, UPC or APC connector. Knowing how much to polish at each step, and when to stop, is critical for a low-loss, low-back-reflection connector.