Fiber Connector Ferrule Shapes and Polishes

Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as end finish or polish types. Early connectors, which did not have keyed ferrules and could rotate in mating adapters, always had an air gap between the connectors to prevent them rotating and grinding scratches into the ends of the fibers. The ends of the ferrules were polished on hard,flat surfaces. They are sometimes referred to as NC or "Non-Fiber Contact" styles.

Beginning with the ST and FC which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors. These connectors were still polished flat on the end. Reducing the air gap reduced the loss and reflectance (very important to laser-based singlemode systems ), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflected back up the fiber. While air gap connectors usually had losses of 0.5 dB or more and a reflectance of -20 dB, PC connectors had typical losses of 0.3 dB and a reflectance of -30 to -40 dB. PC connectors required polishing on a flat surface with a soft rubber pad to allow the end to be polished convex.

Soon thereafter, it was determined that polishing the connector ferrules to a convex end face would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3dB and reflectance -40 dB or better. 

The ultimate solution for singlemode systems extremely sensitive to reflections, like CATV or high bitrate telco links, was to angle the end of the ferrule 8 degrees to create what we call an APC or angled PC connector. Then any reflected light is at an angle that is absorbed in the cladding of the fiber, resulting in reflectance of >-60 dB.

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