In order to make the end face of two fibers contact better, the end face of the insert core is usually ground into different structures. PC, APC and UPC represent the front end structure of ceramic insert core. PC is physical contact, physical contact. PC is a micro sphere surface grinding and polishing, the surface of the insert core is ground into a light sphere, and the fiber core is located at the highest bending point, so that the two optical fiber ends reach physical contact. APC (angled physical contact) is called the physical contact of the inclined plane, and the optical fiber end surface is usually ground to an 8 ° bevel. The 8 ° angle bevel makes the fiber end face closer and reflects the light through its slant angle to the cladding rather than directly back to the light source, which provides better connection performance. UPC (ultra physical contact), super physical end. UPC is based on PC, which optimizes the surface polishing and surface finish, and the end surface looks more dome like. Connector connection needs to be of the same end structure, such as APC and UPC can not be combined together, which will cause the connector performance to decline.