Lucent Connector, Little Connector or Local Connector - snap-in connector with a 1.25 mm ferrule, half the size of the SC. Typically used for high
density connections. Good performance, highly favored for Single-mode. SFP modules and XFP transceivers usually require this connector type.
SC
Subscriber Connector, Square Connector or Standard Connector - snap-in connector with a 2.5 mm ferrule that is widely used for it's excellent performance. Commonly used for
FTTH (fiber to the home) applications, Datacom, and Telcom. GBIC modules require this connector type.
ST
Straight Tip connector with bayonet coupling and a 2.5 mm ceramic (usually) or polymer ferrule. Usually Multi-mode, but also used for Single-mode. Because STs are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.
FC
Ferrule Connector or Fiber Channel, with screw-thread coupling and a 2.5 mm ferrule. Make sure the key properly aligned before tightening. Commonly
used for datacom, telecom, measurement equipment and Single-mode lasers. Becoming less common, mostly replaced by SCs and LCs.
