A ceramic, plastic or stainless steel part of a fiber-optic plug that holds the end of the fiber and precisely aligns it to the socket. The fiber is inserted into the ferrule and cemented with an epoxy or adhesive, which gives it long-term mechanical strength and prevents contamination from the weather. Connectors may also use crimped ferrules that do not require cement. The ferrule is the most important and costly part of a fiber connector. If its length, hole centering and inside and outside diameters are not exact, a poor connection will result. See fiber optics glossary
With the naked eye, one cannot see the tiny opening for the optical fiber. It is only visible under a magnifying glass. The epoxy or adhesive is squeezed into the ferrule before the fiber is inserted (bottom right).