Fiber End Face
PC refers to physical contact, which is also close contact. According to the difference in return loss, connectors are divided into PC, SPC, UPC and APC.
PC, UPC, SPC
SPC refers to super physical contact, and UPC refers to ultra physical contact. The industrial standard of PC, SPC and UPC return loss are -35dB, -40dB and -50dB respectively (Return loss refers to the proportion of light reflected by the end face of the connector. The smaller the return loss, the better. Of course you can also say that the larger the return loss, the better, regardless of the negative sign in front).
Different connectors cannot be mixed in principle, but the fiber end faces of PC, SPC and UPC are all flat, and the difference lies in the quality of grinding. Therefore, the mixed connection of PC, SPC and UPC will not form a permanent physical damage to the connector.
APC is completely different. Its end face is ground to an angle to reduce reflection. Its industry standard return loss is -60dB. The APC connector can only be connected with APC. Since the structure of APC is completely different from that of PC, if an optical adapter is used to connect these two connectors, the fiber end face of the connector will be damaged.
The only way to connect APC to PC is realized by the fiber optic patch cord that converts from PC to APC. In addition, it should be noted that the APC connector is usually green, and the human eye can see the tilt of the fiber end face.
APC connectors are usually green, and UPC/PC connectors are blue. Common connectors are FC/PC, SC/PC, SC/APC, and LC/PC.