When picking fiber, you are often faced with two options – UPC or APC connector. Is there any difference between them? Can you use either in your project?

Simple answer is – yes, they are different, and no, you shouldn’t use them together. 

UPC stands for Ultra Physical Contact connector. It refers to the connector with a flat polished end like in the image below. It achieves the best connection with another connector of this type. Light traveling through the fiber will pass the connection point like nothing happened, providing a reliable signal. However since the connector end is not perfectly polished there will be slight signal reflection in the point of the connection. The return loss of this type of connector is nearly -50dB or higher. 

APC stands for Angled Physical Contact connector. At the tip of that connector surface is slightly angled, specifically at 8 degree. This angle allows direct reflection in the connection point towards the outer shell of the fiber instead of inside fiber, reducing return losses even more. The optical return loss of APC connectors is -60dB or higher.

Now since we already know what UPC and APC stand for, let’s look closer into the differences and why you should use UPC together with APC. Most obvious – APC connector is not flat, and if you try connecting it with UPC, you will not achieve a clear seal. Which will result in signal degradation and in some cases destruction of both connectors.