Fiber connectors: PC, UPC, APC
The three polish geometries of mated fiber connectors. PC and UPC polish the end-face flat-convex to increasing surface quality; APC angles it 8° so residual reflections leave the core. Polish sets return loss; the connector body (LC, SC, FC) is a separate choice.
Two decisions define a fiber connector, and they are independent: the body/form factor (LC, SC, FC, and the multi-fiber MPO, which govern latching and density) and the polish geometry of the ferrule end-face, which governs optics. The polish taxonomy:
PC (Physical Contact). The ferrule end is polished to a slight convex dome so the fiber cores touch physically, eliminating the air gap of early flat connectors. Return loss ~35–45 dB. Legacy grade.
UPC (Ultra Physical Contact). The same geometry taken to a finer polish; RL ≥ 50 dB when mated. The standard for digital telecom and datacom. Color code: blue.
APC (Angled Physical Contact). The end-face is polished at 8°. Whatever the interface still reflects departs at an angle that exceeds the core's acceptance cone and dies in the cladding. Even an unmated APC connector stays quiet (≥60 dB mated, ~55–65 dB even open). The standard wherever reflections are intolerable: PON/FTTH plant, analog RF-over-fiber and CATV, interferometric sensing, narrow-linewidth laser delivery. Color code: green.
The cardinal rule: never mate APC to UPC/PC. The angled and flat domes touch at a point that isn't the core; the result is a large air gap (dB-scale loss, poor RL) and, with force, cracked ferrule domes. Green mates to green, blue to blue. The color coding exists because the damage is real and common.
Practical notes that decide link quality: cleanliness beats polish grade (a dust particle at the contact point costs more than any spec difference, so inspect and clean every mate; scopes exist for a reason); mating cycles wear the polish (specs assume hundreds, not thousands, of cycles); and every connector pair budgeted into a link contributes both insertion loss (0.1–0.3 dB typical per mated pair) and its RL term, which is why permanent joints are spliced instead.