Designing Fiber Optic Connectors

In last month’s Electrical Contractor, we wrote about how to improve your fiber optic termination techniques. Fiber optic connector manufacturers work hard to make terminations easier and less expensive, and with higher performance capabilities. Let’s look at what is necessary to design a good fiber optic connector. It can help you understand how to make better connections yourself. A fiber optic connector’s primary job is to connect two tiny glass fibers together so a minimal amount of light is lost at the junction. A human hair is about 75 microns (one one-millionth of a meter or three thousandths of an inch) in diameter, and the outside diameter of the optical fiber is only 125 microns (0.005 inch). The core of the fiber is smaller than the size of a hair, with a diameter of 50 or 62.5 microns for multimode fiber and 8 to 10 microns for single-mode fiber. The two fibers must be aligned by a connector to within a few microns to achieve low loss. Then the fibers must be protected from harm and extremes of temperature, humidity, etc. And, finally, the connector must be capable of surviving hundreds of mating-demating cycles without loss becoming significantly greater. Usually, fibers are held for alignment by using a “ferrule” or circular body with a hole precisely located down the center. The hole must also be of a precise diameter, large enough to allow the fiber to fit through it but small enough to align the fiber cores precisely. The material of the ferrule must be stable—it must maintain its dimensions with changes in temperature or other environmental conditions—to match the glass fiber that will be inside it. The most common connectors today use a ceramic ferrule. They are easily molded, can be made with precise dimensions, and are mechanically stable under all environmental conditions. They are also very hard, so polishing the fiber down to the ferrule is easily done. The earliest fiber optic connectors used metal ferrules. The higher metal expansion with temperature compared to the glass fiber caused problems, as did the softness of the metal, because it polished faster than the glass. There have been many types of molded plastic connectors, but only in the last few years have materials that offer high performance for fiber optic connectors been developed. Liquid crystal polymers (LCPs) can be used to make ferrules that offer virtually equal performance as ceramic ferrules at less cost. In Europe, they have been quite popular, but have only recently become available in the United States. Molded plastics allow noncircular ferrule designs like on the new duplex MT-RJ small-form-factor connector. With a ferrule connector, you must hold the fiber firmly in place. In most connectors, this is done with an adhesive. Factory-made connectors (and some field-installed ones also) are made with a two-part epoxy for the strongest bond. Epoxies can be cured in a few minutes using an oven. Quick-setting adhesives, generally acrylates, cure faster at room temperature but may not have the bond strength or tolerance of environmental changes of an epoxy. The connector ferrule must accommodate either type of adhesive. This means the hole in the ferrule must be large enough to allow a thin film of adhesive all around the fiber. Inserting the fiber into the connector after applying the adhesive may leave the fiber slightly offset in the hole, but rotating the fiber as it is inserted in the connector will float it to the center of the hole for a better connection. Other methods have been used to hold fibers in the ferrule, most of which involve crimping. Whenever you crimp on a fiber, you risk adding loss due to the stress of an inadequate grip on the fiber. These crimp-type connectors must be carefully made to prevent the fiber from moving in the ferrule, a process called “pistoning.” With all the emphasis on aligning the fibers, you may forget about the rest of the connector. The body of the connector holds the ferrules together and attaches to the cable securely. With the usual simplex cable or zipcord, the connector must be crimped onto the aramid fiber strength members (what most call “Kevlar,” the duPont trade name). Some connectors, like the ST, have the cable crimped directly onto the body of the connector and the ferrule. On these connectors, pulling on the cable will move the ferrule, causing variations in the loss of the connector. When using STs, it is important to keep patchcords dressed neatly to prevent stress on the connector and variable losses. Other connectors, with the SC being the best example, have the cable crimped onto the body of the connector, but the ferrule itself is floating. Pulling on the cable only stresses the snap-in body of the connector, so the ferrule remains unaffected. With either type of connector, it is important to crimp the strength members properly. If this is not done, any stress on the cable will be transmitted directly to the fiber inside the connector. Enough stress will crack the fiber, the connector will fail, and you will have a very hard time finding the problem. Next month, we will look at all the different types of connectors and where they are used. We will also speculate on what those funny names mean. HAYES is the founder of Fotec, the fiber optic test equipment company and the Cable U training programs. He can be contacted at

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