The acronyms UPC and APC are quite common in the fiber optic industry. APC stands for Angled physical Contact while UPC stands for ultra-physical contact.
Even though these two terms are closely related, there are certain differences that will help you distinguish one from the other.
The main difference between UPC and APC is the fiber endface. The fiber endface of APC connectors is polished at an eight-degree angle while that of UPC connectors is polished at no angle. However, this does not mean that UPC connectors are flat, they are slightly bent for better core alignment.
Another difference in the two connectors is in their color. APC adapters are green while UPC adapters are blue.
The meaning of these difference
UPC connectors reflects all the reflected light towards the source of light while APC connectors reflects the reflected light at an angle to the cladding versus straight back toward the source.
This in turn creates some difference in return loss, which is the measure of the loss of power in the signal reflected by discontinuity in optical fiber and it is expressed as a negative dB value. The higher the dB value, the better.
The recommended return loss for UPC connectors is -50dB or greater while that of APC connectors is -60dB or greater.
Return loss is different from the insertion loss. Insertion power refers to the amount of optical power lost from the insertion of a device into an optical fiber and it is used to determine loss budget.
It is easier to achieve a low insertion loss with UPC connector than with APC connectors due to more air gaps.
But with the advancement in manufacturing techniques, it is now possible to create more precise angles on APC connectors which has in turn helped to lower APC insertion loss to close to that of UPC connectors.