When talking about fiber optic patch cords, pigtails and adapters, it will involved in the fiber connectors. When describing fiber connector, we usually use terms LC/APC, SC/UPC. So what is APC and UPC? What is the difference between them?
Fiber connectors with different polishing types have different back reflections. With the development of technology, four polishing types are available: flat-surface, Physical Contact (PC), Ultra Physical Contact (UPC), and Angled Physical Contact (APC).
Flat Fiber Connector
The primary issue with Flat Fiber connectors is that when two of them are mated it naturally leaves a small air gap between the two ferrules; this is partly because the relatively large end-face of the connector allows 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 um, hence the necessary evolution to Physical Contact (PC) Connectors.
Physical Contact Connector
The PC is similar to the Flat Fiber connector but is polished with a slight 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 towards 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: bringing us the Ultra Physical Contact (UPC) connector. This results in a lower back reflection (ORL) than a standard PC connector, allowing more reliable signals in digital TV, telephony and data systems, where UPC today dominates the market.
Most engineers and installers believe that any poor performance attributed to UPC 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
So what the industry needed was a connector with low back reflection, that could sustain repeated matings/unmatings without ORL degradation. Step forward the Angled Physical Contact (APC) connector.
Although PC and UPC 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 that is redirected back towards the source is actually reflected out into the fiber cladding, again by virtue of the 8° angled end-face.
It is true that this slight angle on each connector brings with it rotation issues that Flat, PC and UPC connectors simply don’t have. It is also the case that 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?
The uses of APC connectors
The best feedback examples from my previous blog came from people experienced with FTTX and Radio Frequency (RF) applications. The advance in analogue fiber optic technology has driven demand for it to replace more traditional coaxial cable (copper). Unlike digital signals (which are either ON or OFF), the analogue 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 you have to remember that 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 that whilst 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 typical reflectance loss of -65dB or less. The signal from an unmated UPC connector however, will be sent straight back towards the light source, resulting in disastrously high loss (more than 14dB), massively impeding the splitter module performance.
Differences Between UPC and APC
Currently, with UPC and APC connectors dominating the market, these two types are the most commonly seen connectors employed in the fiber optic industry. In this part, we will mainly explain the difference between them. Besides the more obvious difference that UPC connectors are blue while APC connectors are green, the main difference actually lies in the fiber endface.
APC connectors feature a fiber endface that is polished at an 8-degree angle, while UPC connectors are polished with no angle. UPC connectors are not exactly flat however, they have a slight curvature for better core alignment. With UPC connectors, any reflected light is reflected straight back towards the light source. However, the angled endface of the APC connector causes reflected light to reflect at an angle into the cladding versus straight back toward the source. This causes some differences in return loss, which is a measurement of reflected light that is expressed as a negative dB value (the higher the value, the better). Industry standards recommend that UPC connector return loss should be -50dB or greater, while APC connector return loss should be -60dB or greater.
Picking the right physical contact connector
Looking at current technology, it’s clear that all of the connector end-face options mentioned in this blog post have a place in the market. Indeed, 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 UPC or APC will depend on your particular need. With those applications that call for high precision optical fiber signaling, APC should be the first consideration, but less sensitive digital systems will perform equally well using UPC.