The Story Of Legacy Fiber Optic Connectors

by : Colin Yao

:: Choices of connectors

Over the years, a large variety of fiber optic connectors have been designed for most fiber optic installations. Lightwave equipment manufacturers have never standardized on any one connector type, they actually provide a selection of connectors for their equipment interface.

It is very important to choose the fiber connector type for your applications. You should maintain a connector standard throughout a fiber optic project.

:: The components of a fiber connector

For most fiber optic connectors, the major components are ferrule, connector body, connector cap and strain-relief boot.


The ferrule is the center part of the connector. It contains the optical fiber in a center hole. Ferrule can be made from ceramic (Zirconia), stainless steel or composite material.

Ceramic ferrule is the most popular material since it has stable temperature characteristics. The most common ceramic is Zirconia which is a white hard ceramic material. Ceramic ferrule is used for both single mode and multimode connectors. It offers the lowest insertion loss and the best repeatability.

Connector Body

Fiber optic connector body can be steel or plastic. The body design differs from connector to connector. The connector body provides the mechanism to hold the ferrule and other parts together. It also provides the locking mechanism for connector mating. The most common locking mechanisms are push-pull, snap-in, thread on and twist-on.

Connector Cap

Connector cap often has the same material as the body: either steel or plastic. The cap can be screw on, twist lock or snap-on for connector mating.

Strain-relief Boot

Strain-relief boot is the part connecting the connector body and the fiber cable. It is often made from rubber. Although it looks minimal, it is actually one of the most critical parts in a connector. It provides strain relief on the optical fiber to prevent micro-bending, breakage and provides mechanical support for fiber and connector handling.

:: Legacy Fiber Connector Types

The following list shows the legacy connector types that have been deployed in numerous fiber optic networks for many years.

ST Connector

ST connector was once the most popular fiber connector. It was well-liked for both single mode and multimode fiber connections. The average insertion loss for a ST connector is about 0.5dB.

ST connector has a twist-on locking mechanism which is not susceptible to loosening in vibrating environment. It is most often used in LAN networks.

FC Connector

FC connector is popular for single mode applications. It has the lowest insertion loss and the best repeatability. It has a thread-on locking mechanism.

SMA connector

SMA connector is a pretty old connector type which is mostly used for multimode applications. Today it is still being using on medical equipment.

There are two types of SMA connectors on the market: SMA 905 and SMA 906. The only difference between them is that SMA 906 has a straight ferrule while SMA 905 has a step-down ferrule.

D4 Connector

The NEC D4 was probably the first connector to use ceramic or hybrid ceramic/stainless steel ferrules. It uses a smaller ferrule than SCs or FCs. It was widely used in telco networks in the 80s to early 90s and some may still be in use.

FDDI connector

Besides the SC Duplex, you may occasionally see the FDDI 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.

Biconic Connector

Biconic connector's yellow body indicating a SM version - mutlimode versions were usually black. Biconic connector was developed by a team led by Jack Cook at Bell Labs in Murray Hill, NJ.