:: What Are APC Connectors?
APC Connector is a type of fiber connector that minimizes backreflection due to a 5° to 15° angle-polish applied to end faces. Like illustrated in the following picture.
Because of the angle, the reflected light does not stay in the fiber core but instead leaks out into the cladding.
Angle-polished connectors should only be mated to other angle-polished connectors. Mating to a non-angle polished connector causes very high insertion loss. Generally angle-polished connectors have higher insertion loss than good quality straight physical contact ones.
“Ultra” quality connectors may achieve comparable back reflection to an angled connector when connected, but an angled connection maintains low back reflection even when the output end of the fiber is disconnected.
:: How to Tell an APC connector?
Angled-polished connections are distinguished visibly by the use of a green strain relief boot, or a green connector body. The parts are typically identified by adding “/APC” to the name.
For example, an angled FC connector may be designated FC/APC, an angled LC connector may be designated LC/APC, etc.
FC/APC Connector SC/APC Connector LC/APC Connector
:: Choosing the Right Connector – APC vs. UPC
The choice between angle-polished connectors (APC) and ultra-polished connectors (UPC) can make a significant impact on how a particular network will perform. There are several considerations to make, including the network design or purpose and the types of services that will be transported over the fiber.
1. Physical Appearance
The generally accepted color code for connector bodies and/or boots is beige for multimode fiber, blue for single mode fiber, and green for APC connectors. UPC connectors are easily identified by their blue color on the connector boot.
The major physical difference between APC and UPC connector is the endface geometry. The APC ferrule endface radius is polished at an 8° angel while UPC connectors are polished at a 0° angle. The significance of this 8° angle becomes apparent when addressing return loss issues.
Today, the SC connector type is a standard in both inside and outside plant applications. Telcordia GR-326-CORE, issue 3 “Generic Requirement for Singlemode Optical Connectors and Jumper Assemblies” is the guiding document for fiber optic standards.
3. Insertion Loss
In the past, low insertion loss using APC connectors was difficult to achieve due to air gaps in the apex offsets which caused substantial loss. However, due to improved connector designs and manufacturing processes, insertion loss differences between APC and UPC connectors have diminished. Most advertised typical insertion loss characteristics range from 0.14 dB to 0.18 dB for both connector types.
Today, the factors that relate to insertion loss are the same for both connector types. All four of these factors are identical for today’s APC and UPC connectors.
- Outside diameter (OD) of the fiber
- Concentricity of the fiber core
- Inside diameter (ID) of the ferrule
- Concentricity of the ferrule’s ID
4. Return Loss
Return loss is a measurement of the light reflected back to the source at an optical interface.
APC connectors are superior to UPC connectors in this performance category because of their angle-polished endface geometry.
The value of return loss for mated APC connectors in the field will typically be greater than -65 dB. For a UPC connection, it is normally -55 dB.
When connectors are unmated—such as unused ports in an FTTP distribution frame—the return loss for APC connectors is -65 dB or greater, compared to UPC connectors that will be in the neighborhood of 14 dB.
This is an important consideration for building today’s FTTX architectures.
For systems such as RF video, the APC connector is preferred because these particular systems are extremely sensitive to any back reflections from connectors within the network.
In general, UPC connectors are deployed in transport systems designed for digital signal transport, while APC connectors are preferred for RF video signal transport. APC connectors are also preferred where there are open ports at the other end of splitters, very typical in FTTP network designs.