APC vs UPC connectors
Both are single-mode connector polishes, they look almost identical, and mixing them damages the end faces and kills the link. Here is how they differ and where each belongs.
The short answer
| UPC | APC | |
|---|---|---|
| End face | Domed, flat across the core | Angled 8° |
| Color | Blue | Green |
| Return loss, typical spec | 50 dB or better | 60 dB or better (65 premium) |
| Insertion loss | Similar: about 0.2 to 0.3 dB for a good pair | |
| Where used | Ethernet and data links, much enterprise and data center gear | PON/FTTH, RF video, CATV, high-speed and long-haul links |
Why the angle matters
Where two connectors meet, a small amount of light reflects back toward the transmitter. On a UPC connector the reflected light travels straight back down the core. Polishing the end face at 8° sends that reflection out at an angle, into the cladding, where it is lost harmlessly. That is the whole difference, and it is why APC's return loss is so much better.
Reflections hurt analog video and RF over fiber the most, and they matter in PON, where one OLT shares a fiber tree with many ONTs and video is often carried on 1550 nm. That is why FTTH networks are built with SC/APC.
Never mate APC with UPC
An angled ferrule against a domed one touches only at the edge. That leaves an air gap over the core, which causes high loss and strong reflection, and the pressure on the edges can chip or scratch both end faces. A link with one mismatched mating may come up, but it will be marginal, and the damaged connectors stay damaged.
The trap is that the bodies are the same shape: an SC/APC fits an SC/UPC adapter perfectly. Go by color (green against blue), by the printed legend on the jumper, and by the adapter color in the panel. When in doubt, look at the ferrule from the side; the angle is visible on an APC.
Testing with the right leads
- Your launch cable, receive cable and test jumpers must match the link: APC leads for APC panels.
- Many OTDRs have a UPC port. Use a UPC-to-APC hybrid launch cable rather than an adapter that mates the two directly.
- On a power meter, the adapter cap does not care about polish, but the reference jumper does. Set your reference with jumpers of the same type you will test with.