Ferrule is the name given to the high-precision mechanical guide tubes that hold the optical fiber in fiber optic connectors. They are made of a material with a high degree of hardness; chrome-plated metal, hard metal, ceramic or plastic, and are characterized by the lowest guide tolerances and thus by the most precise manufacturing.
The smallest tolerance deviations in the ferrule have a direct effect on the axial offset of the optical fibers to be connected, which has a direct influence on the insertion loss. Also, the outer diameter must not have any deviations along the entire length of the ferrule. It is important that ferrules do not cause axial mis alignment whenever possible, even under pressure, temperature effects, or other mechanical stress.
In addition to high manufacturing precision, the accuracy of the bore hole is another crucial point for the quality of ferrules. This hole must have a high degree of accuracy, both in terms of its centricity and the hole diameter and its roundness, because the fiber is pushed into this hole and must not be crushed. The diameter of the drilled hole is only 130 µm for all fiber types, which corresponds to the outer diameter of the fiber and a tolerance of 10 µm.
Before optical fibers are inserted into the ferrule, their coating is removed. Then their end faces are ground and polished. Depending on the end face coupling, a distinction is made between convex polished end faces, those with Physical Contact( PC), and angled polished convex( APC), which differ in their characteristic properties.