ST (an AT&T Trademark) is probably still the most popular connector for multimode networks (ca. 2005), like most buildings and campuses. It has a bayonet mount and a long cylindrical 2.5 mm ceramic (usually) or polymer ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. A mating adapter is used to mate two connectors (shown below.) And 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.
The ST/SC/FC/FDDI/ESON connectors have the same ferrule size - 2.5 mm or about 0.1 inch - so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST or SC connectors and adapt to all these connectors.
SC is a snap-in connector also with a 2.5 mm ferrule that is widely used for it's excellent performance. It was the connector standardized in TIA-568-A, but was not widely used at first because it was twice as expensive as a ST. Now it's only a bit more expensive and much more common It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration.
FC was one of the most popular singlemode connectors for many years. It also uses a 2.5 mm ferrule, but some of the early ones use ceramic inside stainless steel ferrules. It screws on firmly, but you must make sure you have the key aligned in the slot properly before tightening. It's been mostly replaced by SCs and LCs
Mating Dissimilar Connectors
The ST, SC and FC connectors share a 2.5 mm ferrule design so they can be mated to each other. To do so requires a hybrid mating adapter as shown here.
From the top:
LC is a small form factor connector that uses a 1.25 mm ferrule, half the size of the SC. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode.
The LC, MU use the same ferrule but cross-mating adapters are not easy to find.
FDDI - ESCON
Besides the SC Duplex, you may occasionally see the FDDI and ESCON* duplex connectors which mate to their specific networks. They are generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors. Since they both use 2.5 mm ferrules, they can be mated to SC or ST connectors with adapters.
FDDI - above - has a fixed shroud over the ferrules
ESCON - below - the shroud over the ferrules is spring-loaded and retracts
MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multimode only, field terminated only by prepolished/splice method.
MT-RJ, Volition and Opti-Jack (below) are difficult connectors to test, as most test sets do not allow direct adaptation to the connector. If you have to use hybrid (ST or SC to MT-RJ) reference cables, you cannot do a Method B (one jumper reference) insertion loss test. Usually the solution is to do a three cable (Method C) reference
MU looks a miniature SC with a 1.25 mm ferrule. It's more popular in Japan.
MPO is a 12 fiber connector for ribbon cable. It's main use is for preterminated cable assemblies and cabling systems. Here is a 12 fiber MT broken out into 12 STs.
MPO means multi-position optical. This connector is sometimes called a MTP which is a commercial name.
Military or Ruggedized Connectors
There are many styles of ruggedized connectors, mostly developed for tactical military use but are now widely used in field applications that call for rugged cables and connectors, applications such as oil and gas exploration or use at sporting events by TV networks. These connectors are generally multifiber connectors and some use expanded beam connector technology for its resistance to dirt and harsh environments.
Amphenol developed the SMA from the "Subminiature A" hence SMA, microwave connector. The model 905 had a machined ferrule exactly 1/8 inch in diameter that mated in a machined adapter. When the adapters were not precise enough for better fibers, a necked-down ferrule that mated with a Delrin adapter for better insertion loss performance. These connectors are still in use on some military and industrial systems.