UPC connectors have a domed end face that meets its partner square on; APC connectors are polished at an 8 degree angle so reflected light leaves the fibre core. APC gives much better return loss, typically 60 dB or more against about 50 dB for UPC, and is used in PON, FTTH and RF video. UPC is normal for Ethernet data links. Never mate the two.
PC, UPC and APC end faces
Every physical contact connector polishes the fibre end face so that the two cores touch inside the adaptor. The difference between UPC vs APC, and the older PC, is the shape of that polish.
PC (physical contact) has a slightly domed end face, so the cores meet at the highest point of the dome. UPC (ultra physical contact) uses the same dome with a finer, longer polish, which leaves a smoother surface and less reflection at the joint. APC (angled physical contact) is domed and then angled at 8 degrees to the fibre axis. Light that reflects at the joint strikes an angled surface and is sent into the cladding, where it is lost, instead of travelling back down the core towards the transmitter.
An APC connector is keyed so the two angled faces line up when mated.
UPC vs APC compared
| Property | PC | UPC | APC |
|---|---|---|---|
| End face | Domed | Domed, finer polish | Domed, 8 degree angle |
| Return loss (typical industry values, single-mode) | About 40 dB | 50 dB or better | 60 dB or better, often 65 dB |
| Insertion loss (good mated pair) | Typically 0.3 dB or less | Typically 0.3 dB or less | Typically 0.3 dB or less |
| Usual connector colour | Blue (single-mode), beige (multimode) | Blue (single-mode), beige or aqua (multimode) | Green |
| Fibre | Single-mode and multimode | Single-mode and multimode | Single-mode; multimode APC is uncommon |
| Typical use | Older plant, multimode | Ethernet, data centres, most digital links | PON and FTTH, RF video, CATV, high-power and analogue links |
| Mates with | PC or UPC | PC or UPC | APC only |
By common practice blue identifies single-mode UPC, green identifies APC, beige identifies older multimode and aqua identifies laser-optimised 50 µm multimode such as OM3 and OM4. Colour is a convention, not a guarantee: check the cable legend and the drawing before mating anything in a mixed rack. The fibre connector types guide covers the connector bodies themselves.
Insertion loss and return loss, explained simply
Insertion loss is the light lost as the signal passes through a mated pair, measured in dB. Lower is better. It depends mostly on how well the two cores line up and how clean they are, so UPC and APC connectors of the same quality have similar insertion loss. ANSI/TIA-568 allows up to 0.75 dB per mated pair when a channel is planned, and a fibre loss budget adds these values along the link.
Return loss measures how much light is reflected back towards the source. It is also in dB, but here a higher number is better, because it means less reflection. At 50 dB, roughly one part in 100,000 of the light comes back; at 60 dB, roughly one part in a million.
Where APC matters
Reflected light causes trouble when it reaches a laser or when it bounces between two connectors and interferes with the main signal. Systems that are sensitive to this use APC.
- PON and FTTH. Passive optical networks use SC/APC almost everywhere in the optical distribution network.
- RF video overlay. Many PONs carry broadcast video as an analogue RF signal at 1550 nm alongside data. Analogue receivers turn reflection noise into visible picture faults.
- CATV and RF over fibre. Analogue links, including antenna remoting, need high return loss for the same reason.
- High-power links. Amplified and high-power systems reflect more absolute power at each joint, which can destabilise lasers and amplifiers.
At high power any contamination can burn an end face, whatever the polish.
Where UPC is normal
Most digital data links use UPC. Ethernet optics are specified to tolerate the reflection of ordinary UPC connectors, and a digital receiver only needs to tell a one from a zero, so a small amount of reflected noise does not change the result. Almost all SFP, SFP+ and duplex QSFP transceivers have LC/UPC ports, and enterprise and data centre duplex cabling follows them.
Single-fibre multimode connectors are PC or UPC as a matter of course.
Why you never mate APC to UPC
An APC plug and a UPC plug of the same family fit the same adaptor, which is why mistakes happen. When they meet, the angled face touches the domed face at its edge and leaves a wedge-shaped air gap over the cores. Insertion loss rises to several dB, return loss falls well below either rated value, and the contact force sits on a small area that can chip or scratch both end faces.
Where UPC equipment has to connect to APC plant, use a cord with the right polish at each end, for example LC/UPC to SC/APC. The LC to SC patch cord page describes how such a cord is specified.
MPO end faces
Multi-fibre MPO connectors follow the same rule. Multimode MPO connectors are normally flat polished to PC or UPC. Single-mode MPO connectors are normally angle polished at 8 degrees, and many single-mode parallel optics, such as 400G DR4 modules, use an MPO APC interface. Single-mode MPO UPC exists but is less common, so state the polish on every single-mode MPO line. An angled MPO and a flat MPO must not be mated. The MPO connector guide covers keys, pins and fibre counts.
What to specify
- Polish at each end, taken from the equipment port or panel adaptor, not from the colour of an old cord.
- Connector family at each end, such as LC, SC, FC or MPO.
- Fibre type, OS2 single-mode or the multimode grade.
- Minimum return loss, for RF video, CATV or other analogue systems, as stated by the equipment maker.
- Maximum insertion loss per connector or per mated pair.
- End-face inspection to the IEC 61300-3-35 criteria, and test wavelengths, usually 1310 and 1550 nm for single-mode.
Radix builds single-mode and multimode cords and assemblies in UPC, APC or mixed polish; list the details for each end in a quote request.
Questions buyers ask
Is APC better than UPC?
APC has better return loss, so it is the right choice for PON, RF video and other reflection-sensitive links. For ordinary Ethernet data links UPC performs just as well and matches the transceiver ports. The correct polish is the one the equipment and plant already use.
What colour is an APC connector?
APC connectors are green by convention. Single-mode UPC connectors are blue, older multimode connectors are beige and laser-optimised multimode connectors are often aqua. Confirm the polish from the drawing or the port label in any rack where both types are present.
What happens if I connect APC to UPC?
The angled and domed faces meet with an air gap over the cores. Loss rises to several dB, reflection increases and both end faces can be scratched or chipped. Inspect both connectors with a fibre microscope before using them again.
Do SFP transceivers use UPC or APC?
Standard SFP and SFP+ data transceivers have LC/UPC ports. Some PON and video optics use SC/APC or LC/APC instead, and the datasheet states which. Match the cord to the port, then match the far end to the panel.
What is the difference between PC and UPC?
Both have a domed end face. UPC is polished longer and more finely, which gives a smoother surface and a typical return loss of 50 dB or better against about 40 dB for PC. The two mate with each other.