How do the end-face shapes of PC, UPC, and APC polishes differ? How do these shapes affect reflected light?
In the field of optical fiber connectors, the terms PC, UPC, and APC refer to the physical shape and polishing grade of the fiber end-face. These shapes directly determine how light reflects back toward the source (Back Reflection/Return Loss).
1. Physical Shape Differences
- PC (Physical Contact): The fiber end-face is polished with a slight convex spherical surface. This ensures that the fiber cores are the first point of contact when two connectors are mated, minimizing air gaps.
- UPC (Ultra Physical Contact): An optimization of PC polishing. It uses extended polishing techniques to achieve a smoother surface finish and a more precise spherical radius. Visually, it looks similar to PC but with higher surface quality.
- APC (Angled Physical Contact): The end-face is polished at a 8-degree angle relative to the fiber axis. This is the most significant geometric difference compared to PC and UPC.
2. Impact on Reflected Light
The primary technical goal of these different shapes is to manage Return Loss (RL), which is the amount of light reflected back toward the light source.
- PC & UPC (Straight Reflection): Because the end-face is perpendicular to the fiber core, any light that is not transmitted is reflected directly back into the core of the fiber.
- PC typically achieves a return loss of about -40dB.
- UPC, due to its superior surface finish, achieves better performance, typically -50dB or lower.
- APC (Angled Reflection): The 8-degree angle ensures that reflected light is not directed back into the fiber core. Instead, it is reflected at an angle into the fiber cladding, where it is dissipated.
- APC provides the best performance, with a return loss of -60dB or lower.
3. Application in FBG Sensing
In Fiber Bragg Grating (FBG) sensing systems, managing reflections is critical. High back reflection from connectors can create noise that interferes with the FBG signal, especially in high-sensitivity or long-distance monitoring.
At OFSCN (Beijing Dacheng Yongsheng Technology Co., Ltd.), we typically provide customized connector options for our sensors and instruments. For applications requiring high signal-to-noise ratios, APC connectors (usually identified by green housings/boots) are recommended.
For more technical details on how these connectors integrate with sensing systems, you can refer to our product technical pages:
If you are designing a specific sensing network and need to determine which polishing grade is best for your environment, feel free to share your system requirements.