PC, UPC and APC are the three ways to grind the inner collar of a fiber optic connector (as shown in the figure below). This guide explains the most common fiber optic connector. A fiber optic connector is a mechanical device used to align and join optical fibers, enabling light to pass through with minimal loss. When the. LC, SC, FC, ST, MPO/MTP compared: ferrule sizes, polishing types, insertion loss, and a decision flowchart to choose the right fiber connector for your application.