APC
This article discusses what APC connectors are, outlines the differences between APC and UPC connectors, and provides guidance on how to choose between them based on application needs.
What is APC?
In order to make the end faces of the two component ferrules contact better, the end faces of the component jumpers are usually polished into different structures. Common polishing methods are PC/APC/UPC, which represent the end face structure of the ceramic ferrule. APC (Angled Physical Contact) fiber end face is usually polished into an 8° bevel. The 8° angle bevel makes the fiber end face tighter and reflects the light to the cladding through its bevel angle instead of returning directly to the light source, providing better connection performance.
UPC vs APC, What's the Difference?
The main difference between APC and UPC connectors is the end face of the fiber.The APC connector fiber end face is polished at an angle of eight degrees, while the UPC connector is polished at no angle. However, the end face of the UPC connector is not completely flat, but is slightly curved to allow for better alignment of the fiber core. Another more noticeable difference is the color. The UPC connector is blue, while the APC connector is green.
What does this difference mean in terms of performance? In a UPC connector, any reflected light is reflected directly back to the light source. The beveled end face of an APC connector causes the reflected light to be reflected at an angle to the cladding, rather than directly back to the light source. This leads to some differences in return loss, which is a measure of reflected light and is expressed as a negative dB value (the higher the value the better). Industry standards recommend a return loss of -50 dB or greater for UPC connectors and -60 dB or greater for APC connectors. Remember that return loss is different from insertion loss, which is the amount of optical power lost through the length of a connector or cable. We use insertion loss to determine the loss budget. Since UPC connectors have a smaller air gap than APC connectors, it is often easier to achieve low insertion loss. However, today's manufacturing techniques have greatly improved, allowing APC connectors to achieve more precise angles and bring their insertion loss closer to that of UPC connectors.
UPC vs APC, How to Choose?
Some applications are more sensitive to return loss than others and therefore require the use of APC connectors. For example, in the higher light wavelength range (over 1500 nanometers) used for things like RF video signals, reflected light can adversely affect the signal. This is why we see most cable companies and other FTTX vendors using APC connectors for outside plant applications.
APC connectors are also commonly used in passive fiber optic applications (including GPON and passive fiber optic LANs) because many of these systems also use RF signals to transmit video. Future high-speed passive fiber optic networks and other WDM applications using higher wavelengths over single mode fiber may also require APC connectors that reduce return loss.
It is important to note that APC and UPC connectors cannot and should not be mated. Since the cores are not touching, mating will not only result in poor performance, but will also damage both connectors. The last thing you want to happen is to cause permanent damage to the transmitter, especially with higher-cost single-mode equipment.
It's also important to note that cable loss is usually the smallest factor affecting system attenuation. The greatest loss comes from the connectors. Higher-performance connectors can mitigate loss more dramatically, more measurably, and more permanently.
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