The FC connector is a fiber-optic connector with a threaded body, which was designed for use in high-vibration environments. It is commonly used with both single-mode optical fiber and polarization-maintaining optical fiber. What are the differences between them? Who is the most popular one? Find the answer in the article. Each type varies by shape, polish (APC, PC, or UPC), and return loss performance, which affect PC, UPC, and APC Polish Styles: What's the. A fiber optic connector is a mechanical device used to align and join optical fibers, enabling light to pass through with minimal loss. They directly affect insertion loss, return loss, reliability, and long-term network stability.