When picking fiber optic cable, you are often faced with two options – UPC or APC connector. What is the difference between them? Why you need to understanding them?

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.

Fiber optic cable typically follows an industry-standard color code: a yellow jacket denotes single mode, an aqua jacket denotes multimode OM3, an orange jacket denotes multimode OM2, etc. But what about the connectors? What's the difference between blue connectors and green connectors? After all, both find their way onto single mode and, selectively, multimode cable.

The answer has to do with the connector endface polish, or the angle of connection, and the good news is connectors also follow industry-standard color codes. Aqua and blue denote a straight through (or UPC) polish and green denotes an angled (or APC) polish.

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MPO-12 Optical Connectors UPC vs. APC

Some of the light sent into a fiber reflects backwards from the fiber end face. Slower speed electronics and optics are less sensitive to back reflection created inside the optical fiber. Hence, 25G-NRZ (4x25G-NRZ SR4) and some 50G-PAM4 transceivers (4x50G-PAM4 SR4) use the MPO-12/UPC or Ultra-flat Polished Connector.

8x 50G-PAM4 400G and all 100G-PAM4 transceivers are more sensitive to back reflections and use the MPO-12/APC or Angled Polished Connector. This has an 8-degree polish on the end that causes the back reflections to be diverted into the fiber side cladding and away from the transmitter.

MPO/UPC (flat) cannot be mated together with MPO/APC (angled) connectors. When fiber optic cables are correctly color-coded, it is much easier for installers to identify which cable goes where. This can save a lot of time and resources during the installation process, ultimately making for a more cost-effective project.

LightOptics offers cables, connectors and installation accessories in both APC and UPC formats. Please visit www.lightoptics.co.uk for additional information.