Whether a ferrule end face is flat or angled at eight degrees — and why the two must never be mated.
What Is the Difference Between APC and UPC?
A UPC ferrule is polished flat with a slight dome, so that under spring pressure the two fibre cores touch and there is no air gap. Light that still reflects at the joint travels straight back down the core toward the transmitter.
An APC ferrule is polished at eight degrees. Reflected light strikes the tilted face and is deflected beyond the acceptance angle of the core, so it escapes into the cladding and is lost harmlessly rather than returning to the laser. The result is roughly 10 dB better return loss for the same connector body.
Why Reflection Matters More Than It Did
Reflected light re-entering a laser cavity perturbs it, adding intensity noise and jitter. A 10G DFB link tolerates that; a coherent system, where the information is carried in the phase, does not. Analogue and RF-over-fibre systems are similarly unforgiving.
Single-mode parallel optics are conventionally supplied APC for a related reason — no IEEE clause specifies the polish, but the module vendors budget for it. With eight or sixteen fibres in one ferrule, and short links where the reflection is barely attenuated on its way back, the accumulated return is significant. DR4 and DR8 are APC as a matter of course, as is essentially every PON deployment.
What Goes Wrong
Mating an APC to a UPC is a hard error, not a degradation. The angled face cannot seat flat against the domed one, so the cores stand apart, the loss is large and unstable, and the physical contact chips both ferrules. The connectors are usually scrap afterwards. The green and blue colour coding exists precisely so this is visible before it happens.
The subtler failure is mixing conventions inside one channel — an APC trunk landing on a UPC adapter panel because that is what was in the store. Adapters are keyed and coloured too, and a mismatched adapter will happily accept both.
How to Choose
Pick one convention per system and specify it on every component: cable, pigtail, adapter, cassette and jumper. Multimode is conventionally UPC because reflection at 850 nm is a much smaller problem. Single-mode data-centre parallel optics are APC. Legacy single-mode duplex is often UPC and there is rarely a reason to change it. See APC vs UPC and the connector polish article; our angled LC cords and MPO assemblies are built to whichever you specify.
APC and UPC reference
| APC angle | 8°, per IEC 61755-3-2 |
|---|---|
| UPC return loss | ≥ 50 dB single-mode |
| APC return loss | ≥ 60 dB single-mode |
| Multimode PC return loss | ≥ 30 dB |
| IEC 61753-1 return-loss grades | 1 = ≥60 dB, 2 = ≥45 dB, 3 = ≥35 dB, 4 = ≥26 dB |
| Bare glass-to-air interface | about 3.6% reflected — roughly 14.4 dB |
| Colour code | UPC blue, APC green, multimode PC beige or aqua |
| Where APC is the industry norm | single-mode parallel optics, PON, RF over fibre |
| Effect on insertion loss | none worth measuring — the polish changes reflection |