For the end face coupling of optical fibres, the aim is to connect the optical fibres to each other with the lowest possible attenuation and few reflections. To achieve this, the end faces of the optical fibres in the FO connectors
are given a specific shape and polished.
A simple surface shape is the convex one, which is ground and polished to achieve the lowest possible insertion loss (IL) and high return loss. Depending on how fine the grinding and polishing are, convex ground end faces are classified as Physical Contact (PC), Super Physical
Contact (SPC) and Ultra Physical Contact
The UPC face is more rounded than the PC face and is extremely finely polished. It has the best values as far as curved end faces are concerned: the return loss is above 50 dB and the insertion loss is below 0.3 dB.