8 Degrees of Separation
The main difference between APC and UPC connectors is the fiber endface. APC connectors feature a fiber endface that is polished at an 8-degree angle, while UPC connectors are polished with no angle. UPC connectors are not exactly flat however; they have a slight curvature for better core alignment. Another more obvious difference is color. UPC adapters are blue while APC adapters are green.
What does the difference mean? With UPC connectors, any reflected light is reflected straight back towards the light source. The angled endface of the APC connector causes reflected light to reflect at an angle into the cladding versus straight back toward the source. This causes some differences in return loss, which is a measurement of reflected light that is expressed as a negative dB value (the higher the value, the better). Industry standards recommend that UPC connector return loss should be -50dB or greater, while APC connector return loss should be -60dB or greater.
Remember, return loss is different than insertion loss, which refers to the amount of optical power lost through a connector or cable length. Insertion loss is what we use to determine loss budgets. Achieving low insertion loss is typically easier with UPC connectors due to less air gaps than APC connectors. However, manufacturing techniques have improved significantly to create more precise angles on APC connectors and bring insertion loss down closer to that of UPC connectors. In fact, insertion loss on Belden’s FX Brilliance Universal Connectors is the same for both UPC and ACP singlemode connectors.
There are some applications that are more sensitive to return loss than others that call for APC connectors. For example, in higher optical wavelength ranges (above 1500 nanometers) like those use for RF video signals, reflected light can adversely impact the signal. That is why we see APC connectors being used by most cable companies and other FTTX providers in outside plant applications.
APC connectors are also commonly used in passive optical applications (both GPONs and passive optical LANs) due to the fact that many of these systems also use RF signals to deliver video. Future higher-speed passive optical networks and other WDM applications that will use higher wavelengths via singlemode fiber will also likely require the reduced return loss of APC connectors.
One thing that should be noted is that APC and UPC connectors cannot and should not be mated. Not only does it cause poor performance since the fiber cores will not touch, but it can also destroy both connectors. The last thing you want to do is cause permanent transmitter damage—especially with higher-cost singlemode equipment.