A metal screw on connector, with a 2.5mm ferrule,. FC connectors are typical in test environments and for singlemode applications. FC connectors were designed for use in high-vibration environments.FC/PC has been one of the most popular singlemode connectors for many years. The ruggedness of this connector leads to its extensive use at the interfaces of test equipment. It is also the most common connector used for PM, polarization maintaining, connections. Please note that there are currently four different specifications for the key width on FC connectors and for the slot width on FC adapters. Therefore not all FC connectors will fit into all FC adapters