The correct term for Ferrule is the metal ring that holds things together, such as the ring that holds the rubber onto a pencil, or the metal ring that hold the brush bristles on a paint brush.
In network, we use the term to describe the bit that sticks out of the end of of a fibre optic connector since it “goes over the top of the fibre” or holds the fibre in place. [holds the fibres in place being the joke... geddit... just like a paint brush fibres are held in place....jeesh, why do I bother]
Note that Ferrules have many shapes (at least, I thought it was interesting):
Connector Ferrule Shapes & Polishes ((Reference))
Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. early connectors, because they 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.
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. Reducing the air gap reduced the loss and back reflection (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 return loss of 20 dB, PC connectors had typical losses of 0.3 dB and a return loss of 30 to 40 dB.
Soon thereafter, it was determined that making the connector ferrules convex would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3dB and return loss 40 dB or better. The final 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.
