A former blog of ours about the different types of fiber connectors, demonstrates how important the subject is to both fiber installers and network planners alike.
In that blog, we discussed the difference between SC, LC, FC, ST, MTP/MPO, HFOC and VSFF connectors, and we thought it would be beneficial to focus on one area the original post deliberately didn’t cover – the differences between Angled Physical Contact (APC) and Ultra Physical Contact (UPC) fiber connectors.
Besides one connector type having a green body and the other being blue, the different ways they both treat light are crucial in planning a network.
To help us understand all this jargon, let’s look back at why the original Flat Fiber connector evolved into the Physical Contact (PC) connector, then we'll talk about UPC and APC connectors.
Flat Fiber Connector
The primary issue with Flat Fiber connectors is when two of them are mated, they naturally leaves a small air gap between the two ferrules. This is partly because the relatively large end-face of the connectors allow for numerous slight, but significant imperfections to gather on the surface. This is not much use for single-mode fiber cables with a core size of just 8-9 µm, leading to the necessary evolution to Physical Contact (PC) connectors.
Physical Contact Connector
PC connectors are similar to the Flat Fiber connector but are polished with a slightly spherical (cone) design to reduce the overall size of the end-face. This helps to decrease the air gap issue faced by regular Flat Fiber connectors, resulting in lower Optical Return Loss (ORL), with less light being sent back toward the power source.
Ultra Physical Contact Connector
Building on the convex end-face attributes of the PC, but utilizing an extended polishing method creates an even finer fiber surface finish, we have Ultra Physical Contact (UPC) connectors. This extended polishing results in a lower back reflection ORL than standard PC connectors, allowing more reliable signals in digital TV, telephone, and data systems, where UPC today dominates the market.
Most engineers and installers believe that any poor performance attributed to UPC fiber connectors is not caused by the design, but rather poor cleaving and polishing techniques. UPC connectors do have a low insertion loss, but the back reflection ORL will depend on the quality of the fiber surface and, following repeat matings/unmatings, it will begin to deteriorate.
Angled Physical Contact Connector
What the industry needed was a connector with low back reflection that could sustain repeated matings/unmatings without ORL degradation – the Angled Physical Contact (APC) connector.
Although PC and UPC fiber connectors have a wide range of applications, some instances require return losses in the region of one-in-a-million (60dB). Only APC connectors can consistently achieve such performance. This is because adding a small 8° angle to the end-face allows for even tighter connections and smaller end-face radii. Combined with that, any light redirected back toward the source is reflected out into the fiber cladding, again by virtue of the 8° angled end-face.
This slight angle on each connector brings with it rotation issues that Flat, PC and UPC connectors simply don’t have. The three aforementioned connectors are all inter-mateable, whereas the APC isn’t. So, why then is the APC connector so important in fiber optics?
Important Uses of APC Connectors
The best feedback examples come from those experienced with FTTx and Radio Frequency (RF) applications. The advancement in analog fiber optic technology has driven demand for it to replace the more traditional coaxial cable (copper). Unlike digital signals (which are either ON or OFF), the analog equipment used in applications such as DAS, FTTH and CCTV is highly sensitive to changes in signal and therefore requires minimal back reflection ORL.
APC ferrules offer return losses of -65dB. In comparison, a UPC ferrule is typically not more than -55dB. This may not sound like a major difference, but remember the decibel scale is not linear. To put that into context, a -20dB loss equates to 1% of the light being reflected back, -50dB leads to nominal reflectance of 0.001%, and -60dB (typical of an APC ferrule) equates to just 0.0001% being reflected back. This means while a UPC polished connector will be okay for a variety of optical fiber applications, only an APC will cope with the demands of complex and multi-play services.
The choice is even more important where connector ports in the distribution network might be left unused, as is often the case in FTTx PON network architectures. Here, optical splitters are used to connect multiple subscriber Optical Network Units (ONUs) or Optical Network Terminals (ONTs). This is not a problem with unmated APC connections where the signal is reflected into the fiber cladding, resulting in a typical reflectance loss of -65dB or less. The signal from an unmated UPC connector, however, will be sent straight back toward the light source, resulting in disastrously high loss (more than 14dB), massively impeding the splitter module performance.
|Connector Polish||Nominal Reflectance (dB)||Nominal Reflectance (%)|
|APC||-60 dB or higher||0.0001%|
Picking the Right Physical Contact Connector
Looking at current technology, it's clear all of the connector end-face options mentioned in this blog have a place in the market. If we take a sidestep across to Plastic Optical Fiber (POF) applications, this can be terminated with a sharp craft knife and performance is still deemed good enough for use in the high-end automotive industry.
When your specification also needs to consider cost and simplicity, not just optical performance, it’s hard to claim that one connector beats the others. Therefore whether you choose a UPC or APC fiber connector will depend on your particular need. With applications that call for high-precision optical fiber signaling, APC should be the first consideration. Less sensitive digital systems will perform equally well using UPC.
This post was originally published on July 28, 2016, and updated on February 16, 2023.