Difference between PC, UPC, and APC Connectors
When joining fiber optic cables, connectors are everything. These highly sensitive networks live or die by how cleanly and efficiently the cables join inside of the connector. Because of that, there are many different connector designs, each catering to different uses and design philosophies.
One common philosophy is to focus on physical contact between the fibers. Doing so is difficult, as at least some air gap tends to exist inside the connector. To combat that and the signal disruption that comes with air gaps, three types of physical contact connectors are used in a wide range of systems. These are PC, UPC, and APC connectors.
If you are trying to determine which is best for your system, a quick comparison can help with your selection.
PC stands for “physical contact” connector. It was first developed in the 80s, so it has been around for some time. The premise of this connector is that it is a flat connector with a small taper around the endpoint. This taper is created by polishing the end, and it yields a spherical cone shape.
The purpose of this polished design is to decrease air gaps between fiber endpoints that frequently arise when flat connectors are used. Minimizing air gaps reduces optical return loss (ORL) and improves overall signal quality.
Common PC Use Cases
PC is most commonly found among telecom systems. Overall, telecom fiber optics have lower demands and are minimally affected by ORL. This enables the use of less-expensive connectors (PC) that still perform well enough for the system.
Fiber optics that suffer from increased ORL are not adequately served by PC connectors. It’s fair to think of this as an older connector for older systems.
UPC connectors take the PC design and expand on it and is rapidly becoming the mot popular and trusted type of fiber connector. UPC stands for “ultra physical contact” connector. The UPC design extends the polish you see on PC connectors to get a tighter shape on the end of the connector. This ultimately improves physical contact, which diminishes the air gaps and further lowers ORL. While UPC connectors are polished with no angle but they are not exactly flat, they do have a slight curvature for better core alignment. APC is polished at an eighth-degree angle which reflects light back to the cladding instead of to the source like UPC.
Despite the more extreme shape, UPC connectors can be joined with PC and other flat connectors (not APC connectors).
When to Use UPC Connectors
UPC connectors are the most common of the three. They can be found in all kinds of systems, and their low ORL connections are suited for high-demand systems that suffer from return loss. The most common places where you will find UPC connectors are digital TV, digital phone, and data systems.
In other words, the bulk of the fiber network that connects endpoint users to major phone, TV, and internet providers incorporates UPC connectors. They provide a good compromise between reduced optical return loss, affordability, and cross-compatibilies.
APC connectors take a more radical approach to dealing with back reflection. With an angled physical contact (APC) connector, an eight-degree angle is introduced into the endpoint design. This endpoint is kept flat, and beyond the contact joint, the connector is still polished. Despite the flat endpoint, APC connectors cannot be joined or mated with UPC or PC connectors, much less other flat connectors. This would result in extremely poor performance and the possibility of destroying both connectors.
This introduced angle exists solely to reduce back reflection. With that subtle angle, reflected light is diverted out of the fiber cladding. With this design change, APC connectors reduce total back reflection by an order of magnitude when compared to UPC connectors.
To put numbers on it, the return loss of each connector type is as follows:
- PC: -40dB
- UPC: -50dB
- APC: -60dB
Remembering that the decibel scale is logarithmic, these changes are significant for sensitive signals.
Leading Use Cases for APC
APC connectors are the best choice when back reflection is the greatest concern. A perfect example is analog fiber optics. Analog signals suffer more from back reflection, which is largely solved by the APC angle. Analog fiber optics are rising in popularity as a replacement for copper coaxial cables. Some applications are more sensitive to return loss than others which make APC a wiser choice for those. For example, in higher optical wavelength ranges (above 1500 nanometers) like those used for RF video signals, APC fiber connectors would be the appropriate choice.
So, how do you choose the right fiber connector? You pick the lowest-cost option that lives up to the specifications you need. In the majority of cases, it will be a UPC connector, but you might find exceptions, such as those listed above.
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