Specific to single Mode applications, referring to the endface geometry of a Connector Ferrule as well as performance characteristics.

The UPC, ultra physical contact, connector - while not offering the superior optical return Loss performance of an APC connector - has RL characteristics that are acceptable for intraplant Serial Digital video or Data transmissions. When using UPC connectors, make sure your laser's specs can handle the return loss your UPC connectors will generate. Offering -55 dB RL, ultra physical Contact connectors rely on machine Polishing to deliver their low Optical Return Loss characteristics. Ultra physical contact polishing refers to the radius of the endface polishing administered to the ferrule, the precision tube used to hold a Fiber in place for alignment. The rounded finish created during the polishing process allows fibers to touch on a high point near the fiber Core where Light travels. Unlike APC connectors, UPC connectors can, with the proper tools and training, be repaired in the field.

See glossary definition for end face

A solution to the problems associated with a flat polish is to slightly curve the ends of the fiber. This eliminates much of the chance of an air gap being produced.