How to Specify Single-Mode and Multimode FO Connectors

By Contributed Article | June 16, 2014

As fiber optic technology continues to expand and evolve at an increasingly rapid rate, we asked the Fiber Optic Association to provide our readers with a simple guide on how to specify single-mode and multimode FO connectors.

Visit the Fiber Optic Association online for more valuable information about specifying these connectors, as well as for a guide to fiber optics and premises cabling.

As fiber optic technology developed over the last 30 years, many companies and individuals have invented the “better mousetrap” – a fiber optic connector that was lower-loss, lower-cost, easier to terminate, or solved some other perceived problem. In all, about 100 fiber optic connectors have been introduced to the marketplace, but only a few represent the majority of the market. Here is a rundown of the connectors that have been the leaders of the industry.


ST connectors

ST (an AT&T trademark) is one of the most popular connectors for multimode networks, such as those found in most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. Because they are spring-loaded, make sure they are seated properly. If there is high loss, reconnect to see if it makes a difference.


FC-PC Connectors

FC/PC has been one of the most popular single-mode connectors for many years. It screws on firmly, but the key must be aligned in the slot properly before tightening. These days it is often being replaced by SCs and LCs.


SC connector

SC is a snap-in connector that is widely used in single-mode systems for its excellent performance and in multimode systems because it was the first connector chosen as the standard connector for TIA-568 (now any connector with an FOCIS standard is acceptable). It’s a snap-in connector that latches with a simple push-pull motion.It is also available in a duplex configuration.


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The ST/SC/FC/FDDI/ESON connectors have the same ferrule size – 2.5mm or about 0.1 inch – so they can be mixed and matched with each other using hybrid-mating adapters. This makes testing convenient, since one set of multimode reference test cables with ST or SC connectors can adapt to all these connectors.


LC Connector

LC uses a 1.25mm ferrule, half the size of the ST. Otherwise, it’s a standard ceramic ferrule connector, easily terminated with any adhesive. It offers good performance and is highly favored for single-mode. It is also the connector of choice for multimode transceivers for gigabit speeds and higher, including multimode Ethernet and Fiber Channel.


MT RJ connector

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. It is multimode-only and field-terminated-only by pre-polished/splice method. It is basically obsolete.



Opti-Jack is a neat, rugged duplex connector cleverly designed around two ST-type ferrules in a package the size of an RJ-45. It has male and female (plug and jack) versions.



Volition is a simple, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. It is available in plug and jack versions, but one can only field-terminate jacks.



MU looks like a miniature SC with a 1.25mm ferrule. It’s more popular in Japan.



MTP/MPO is a 12-fiber connector for ribbon cable. Its main use is for pre-terminated cable assemblies.

Some Final Thoughts

  • The ST is still a popular multimode connector because it is inexpensive and easy to install.
  • The SC connector was specified as a standard by the old EIA/TIA 568A specification, but its popularity in premises applications was limited at first due to higher cost and the difficulty of installation. The newer SCs, however, are much better in both cost and ease of installation, so they have been increasingly adopted. It is now challenged by the LC, which is the connector of choice for transceivers for systems operating at gigabit speeds because of its small size and high performance.
  • Single-mode networks have used FC or SC connectors in about the same proportion as ST and SC in multimode installations. There are some D4s out there, too, but LCs have become the most popular, again for their performance and small size.
  • EIA/TIA 568 now allows any fiber optic connector as long as it has an FOCIS (Fiber Optic Connector Intermateability Standard) document behind it. This opened the way to the development of several new connectors, which are called small form factor (SFF) connectors, including AT&T LC, the MT-RJ, the Panduit “Opti-Jack,” 3M’s Volition, the E2000/LX-5, and MU. The LC has been particularly successful in the US.

For more information on fiber optic connectors and their suppliers, visit the Connector Buyers Guide.

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