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Fiber Optic Connectors Tutorial

SC fiber optic connector basic structure

More than a dozen types of fiber optic connectors have been developed by various manufacturers since 1980s.  Although the mechanical design varies a lot among different connector types, the most common elements in a fiber connector can be summarized in the following picture. The example shown is a SC connector which was developed by NTT (Nippon Telegraph and Telephone) of Japan.

A SC Connector Sample

SC Connector Structure

Elements in a SC connector

1. The fiber ferrule.

SC Connector Fiber Ferrule

SC connector is built around a long cylindrical 2.5mm diameter ferrule, made of ceramic (zirconia) or metal (stainless alloy). A 124~127um diameter high precision hole is drilled in the center of the ferrule, where stripped bare fiber is inserted through and usually bonded by epoxy or adhesive. The end of the fiber is at the end of the ferrule, where it typically is polished smooth.

2. The connector sub-assembly body.

The ferrule is then assembled in the SC sub-assembly body which has mechanisms to hold the cable and fiber in place. The end of the ferrule protrudes out of the sub-assembly body to mate with another SC connector inside a mating sleeve (also called adapter or coupler).

3. The connector housing

Connector sub-assembly body is then assembled together with the connector housing. Connector housing provides the mechanism for snapping into a mating sleeve (adapter) and hold the connector in place.

4. The fiber cable

Fiber cable and strength member (aramid yarn or Kevlar) are crimped onto the connector sub-assembly body with a crimp eyelet. This provides the strength for mechanical handing of the connector without putting stress on the fiber itself.

5. The stress relief boot.

Stress relief boot covers the joint between connector body and fiber cable and protects fiber cable from mechanical damage. Stress relief boot designs are different for 900um tight buffered fiber and 1.6mm~3mm fiber cable.

How fiber optic connectors mate

FC Connectors Mating

Unlike electronic connectors, most fiber optic connectors don’t have jack and plug design. Instead a fiber mating sleeve (adapter, or coupler) sits between two connectors. At the center of the adapter there is a cylindrical sleeve made of  ceramic (zirconia) or phosphor bronze. Ferrules slide into the sleeve and mate to each other. The adapter body provides mechanism to hold the connector bodies such as snap-in, push-and-latch, twist-on or screwed-on. The example shown above are FC connectors with a screwed-on mechanism.

Fiber Optic Connector Types and their applications

Both examples shown above are for single fiber cable (simplex) which is easy to install. However there are also duplex and multi-fiber connector designs. Below are loosely divided family types of fiber connectors which sometimes overlap.

Standard  fiber optic connectors – 2.5mm ferrule

ST connector – simplex only, twist-on mechanism. Available in single mode and multimode.

It is the most popular connector for multimode fiber optic LAN applications . It has a long 2.5mm diameter ferrule made of ceramic (zirconia), stainless alloy or plastic. It mates with a interconnection adapter and is latched into place by twisting to engage a spring-loaded bayonet socket.

ST Connector                          ST Adapter (mating sleeve)


FC connector – simplex only, screw-on mechanism. Available in single mode and multimode.

FC connector also has a 2.5mm ferrule (made of ceramic (zirconia) or stainless alloy) . It is specifically designed for telecommunication applications and provides non-optical disconnect performance. Designed with a threaded coupling for durable connections. It has been the most popular single mode connectors for many years. However it is now gradually being replaced by SC and LC connectors.

FC Connector

SC connector – simplex and duplex, snap-in mechanism. Available in single mode and multimode.

SC was developed by NTT of Japan. It is widely used in single mode applications for its excellent performance. SC connector is a non-optical disconnect connector with a 2.5mm pre-radiused zirconia or stainless alloy ferrule. It features a snap-in (push-pull) connection design for quick patching of cables into rack or wall mounts. Two simplex SC connectors can be clipped together by a reusable duplex holding clip to create a duplex SC connector.

Simplex SC Connector                                   Duplex SC Connector

Simplex SC Adapter                                         Duplex SC Adapter

FDDI connector – Duplex only, multimode only.

FDDI connector utilizes two 2.5mm ferrules. The ferrules are sheltered from damage because of the fix shroud that has been constructed in the FDDI connector. FDDI connector is a duplex multimode connector designed by ANSI and is utilized in FDDI networks. FDDI connectors are generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors.

FDDI Connector

ESCON connector

ESCON connectors derive their name from their original application, IBM's enterprise system connection (ESCON) for mainframe computers. ESCON connectors are similar to FDDI connectors, but contain a retractable shroud instead of a fixed shroud. ESCON connectors have two 2.55 mm ceramic ferrules and a robust strain relief design.

ESCON Connector

Small form factor fiber optic connectors

A number of small form factor fiber optic connectors have been developed since the 90s’ to fill the demand for devices that can fit into tight spaces and allow denser packing of connections. Some are miniaturized versions of older connectors, built around a 1.25mm ferrule rather than the 2.5mm ferrule used in ST, SC and FC connectors. Others are based on smaller versions of MT-type ferrule for multi fiber connections, or other brand new designs. Most have a push-and-latch design that adapts easily to duplex connectors.

LC connector – simplex and duplex – push and latch – 1.25mm ferrule. Available in single mode and multimode.

Externally LC connectors resemble a standard RJ45 telephone jack. Internally they resemble a miniature version of the SC connector. LC connectors use a 1.25mm ceramic (zirconia) ferrule instead of the 2.5mm ferrule. LC connectors are licensed by Lucent and incorporate a push-and-latch design providing pull-proof stability in system rack mounts. Highly favored for single mode applications.

LC Connector – Simplex and Duplex

LC Simplex Adapter                                                     LC Duplex Adapter

MU connector – simplex, duplex – snap in, 1.25mm ferrule.

MU connectors and adapters were developed by NTT, and have push-pull mechanism. They are called “mini SC” and are more popular in Japan. Applications include high-speed data communications, voice networks, telecommunications, and dense wavelength division multiplexing (DWDM). MU connectors are also used in multiple optical connections and as a self-retentive mechanism in backplane applications.

MU Connector – Simplex and Duplex

MU Adapter – Simplex                                   MU Adapter - Duplex

E2000 connector. 1.25mm ferrule, snap-in mechanism. Also called LX.5 connector. Available in single mode and multimode.

Externally a E2000 connector looks like a miniature SC connector. The connector is easy to install, with a push-pull latching mechanism which clicks when fully inserted.

It features a spring-loaded shutter which fully protects the ferrule from dust and scratches. The shutter closes automatically when the connector is disengaged, locking out impurities which could later lead to network failure, and locking in potentially harmful laser beams. When it is plugged into the adapter the shutter opens automatically.

E2000 Connector

E2000 Adapter

MT-RJ  connector, duplex only, multimode only. (not 1.25mm ferrule, but rather a two-fiber ferrule design derived from MT).  Overall size is about the same as a RJ45 connector.

MT-RJ connector is derived from MT ferrule design. It has a miniature two-fiber ferrule with two guide pins parallel to the fibers on the outside. The guides pins align ferrules precisely when mating two MT-RJ connectors. MT-RJ connectors are designed with male-female polarity which means male MT-RJ connector has two guide pins and female MT-RJ connector has two holes instead.

MT-RJ connectors are used in intrabuilding communication systems. Since they are designed as plugs and jacks, like RJ-45 telephone connectors, adapters can be used with some designs, but are not required for all.

MTRJ Connector

MTRJ Adapter

VF-45 connector (3M Volition), duplex only. No ferrules at all. Plug and jack versions.

VF-45 (3M Volition) is designed by 3M. VF-45 features plug and jack design without the need of adapters. VF-45 plug is inserted into the socket(jack) at a 45 degree angle, pressing a pair of fibers into V-grooves inside the socket. This design eliminates the need for expensive ferrules thus reduces connector costs.

3M VF45 (Volition) Connector and Adapter

Opti-Jack connector. Duplex only. 2.5mm ferrule. Plug and jack version.

Opti-Jack connector is a version of Fiber Jack connector design from Panduit Corporation, that provides a snap-lock plug and socket for a pair of fiber cables. It enables fibers to be quickly plugged and removed in a manner similar to RJ-45 connectors. It is designed around two ST-type 2.5mm ferrules in a package the size of a RJ-45.
Fiber Jack is a duplex fiber-optic connector standardized by the TIA (FOCIS-6). TIA stands for Telecommunications Industry Association. Fiber Jack allows two fibers to mate in a snap-lock type of plug and socket similar in size and convenience to an RJ-45 connector.

Opti-Jack(Panduit name Opto-Jack, TIA name “Fiber Jack”) Connector and Adapter

Ribbon Fiber Connector (multiple fiber connectors, 4~12 fibers). Available in single mode and multimode.

MTP and MPO connector

MTP and MPO are compatible ribbon fiber connectors based on MT ferrule which allow quick and reliable connections for up to 12 fibers. They are intended for installations that require many fiber connections. Up to 12 fibers in a ribbon are stripped to 125um cladding and inserted into 250um spaced parallel grooves. The ferrule also includes two 0.7mm diameter holes, running parallel to the fibers on the outer side of the ferrule. These two holes hold precision metal guide pins which align the fibers with tight tolerances.

MTP and MPO connectors feature male and female connector design. Male connectors have two guide pins and female connectors do not. Both connector types need an adapter to mate a pair of male and female connectors.
Because MTP and MPO connectors are trying to align so many fibers at once, their coupling loss are typically bigger than single fiber connectors.

MTP – MPO Connector

MTP – MPO Adapter

Other types

SMA 905 and SMA 906 connector . Simplex only. Multimode only.

SMA 905 and 906 connectors make use of threaded connections and are ideal for military applications because of their low cost multimode coupling.

SMA 905 and SMA 906 multimode connectors are available with stainless alloy or stainless steel ferrules. The stainless alloy ferrule may be drilled from 125um to 1550um to accept various fiber sizes.
SMA 906 ferrule has a step, as shown in the following picture, which requires a half sleeve to be installed when mating a SMA 906 connector with SMA 905 mating sleeves.

SMA 905 and 906 Connector

SMA 905 Adapter

Biconic Connector. Simplex only. Available in single mode and multimode.

Biconic connectors exhibit a cone-shaped ferrule that helps to align the optical fibers at the connection interface. Their robust design makes biconic connectors well-suited for military fiber optic applications. Fiber Connections terminates biconic connectors on either single mode or multimode fiber optic cable. The ferrule can be made with either polymer or metal.

Biconic connector’s applications include LAN and premises networks, data processing systems, medical instrumentation, remote sensing, telemetry, and cable television.

Biconic Connector

D4 connector. 2.5mm ferrule. Screw-on. Simplex only.

D4 connectors are made from a 2.5mm diameter ceramic (zirconia) ferrule for durability. They have a high-performance threading mounting system and a keyed body for repeatability and intermateability.

D4 Connector

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