Fiber optic connectors are used to align and join two or more fibers together to provide a means for attaching to, or decoupling from, a transmitter, receiver, or any other fiber optic equipment. An adapter is a mechanical device used to align and join two or more fibers with different connection types.

Reliable connection requires that fiber ends be optically smooth and square. End-to-end positions must align precisely. This can be done with proper keying. A component within all fiber optic connectors is called the "ferrule." The ferrule ensures alignment during connector mating and is often made from a hardened material such as ceramic, stainless steel, plastic, or tungsten carbide. The end of the fiber is held within the ferrule by either adhesive or crimping so that it becomes a permanent component. The ferrule end is polished after insertion of the embedded fiber to provide the smooth interface required for coupling.

Parts of the fiber optic connectors include the connector body, the cable, and the coupling device. The connector body, also called housing, holds the ferrule. It is usually made of metal or plastic and includes one or more assemblies that hold the fiber in place. The ferrule extends past the connector body to slip into the coupling device. The cable is attached to the connector body and acts as a point of entry for the fiber. Extra strength is provided by a strain-relief boot added over the junction between the cable and connector body. A coupling device is used instead of a male-female configuration. The video below gives a great brief description of the types of connectors. A more detailed description can be found in the next section.

Video Credit: Corning Optical Communications / CC BY-SA 4.0

Types of Fiber Optic Connectors

Common types of fiber optic connectors include:

Biconic connectors have precision tapered ends for low insertion loss. It has a glass-filled plastic mold and started with the fiber being molded into the ferrule. Biconic connectors are becoming obsolete.

D4 connectors are made from a composite zirconia ceramic ferrule for durability. They have a high-performance thread mounting system and a keyed body for repeatability and intermateability. It uses a smaller ferrule than SCs or FCs. D4 connectors were widely used in telco networks in the 80s to early 90s, but they are becoming an obsolete technology.

ESCON’s name derives from its application in IBM's ESCON channel interface. It is similar to the FDDI connector—a duplex connector using 2.5 mm ferrules. The main difference is its retractable shroud as opposed to a fixed shroud. The shroud is over the ferrule and is spring-loaded and retracts. ESCON connectors are used to connect to the equipment from a wall outlet.

An FC, or fixed connection, connector is constructed of an all-zirconia ceramic ferrule for durability. They have a high-performance thread mounting system and a keyed body for repeatability and intermateability. They are primarily used with single mode fibers and used in telephone, instruments, and high-speed communication links. The style is similar to that of an ST connector, but it uses a threaded connection, which makes it better suited for high vibration environments.

FDDI connectors are designed by ANSI for use in FDDI networks. This connector is a duplex connector using two 2.5 mm ferrules and has a fixed shroud over the ferrules. This connector has a fixed shroud that protects the ferrules from damage. They are usually used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors.

LCs, also known as Lucent Connectors, Little Connectors, or Local Connectors, have precision PC polished zirconia ceramic ferrules. They have an RJ45 push-pull style housing and latching. They are half the size of standard connectors, resembling a smaller version of the SC. This design increases packaging density by 50% with its narrow footprint and push/pull snap-lock design. LC connectors are used in private and public networks, and are gaining in popularity due to their small form factor (SFF) optical transmitter/receiver assemblies.

Loopback connectors are used in testing transceiver systems. Loopback describes a way of routing electronic signals from their originating facility to the receiving end of the source without processing or modification. This is best used as a way to test the transmission of transportation infrastructure.

MTP, the abbreviation for Multi-Fiber Push On/Pull Off, connectors are a threaded type of fiber optic connector. They are ideal for high-density applications. MTP connectors is the commercial name for MT connectors, which are 12 fiber connectors for rubber cable. It is used for pre-terminated cable assemblies and cabling systems.

The MT-RJ uses a ferrule smaller than the standard MT to hold two fibers. MT stands for Mechanical Transfer and RJ stands for Register Jack. It also uses a press to release latch quite similar to that found on the modular jack to make its operation familiar. MT-RJs use pins for alignment and have male and female versions. They are multimode only, and are field terminated by prepolished/splice method.

An MU connector is a 1.25 mm diameter ferrule for compact multiple optical connectors and self-retentive mechanism for backplane applications. These "miniature units" are utilized in high-speed data communications, voice networks, telecommunications and high-density DWDM applications. MU connectors are more popular in Japan.

SCs, or Subscriber Connectors, are high-precision ceramic ferrules. They feature a snap-in locking mechanism for positive latching. SC connectors use a push-pull design and can be used for simplex or multiplex applications. They have a keyed body for repeatability and intermateability and are ideal for high-density applications. SCs are referred to as premises connectors since they are used for premises installations. Although it has a compact hard plastic design, they do not have the smallest footprint, requiring 0.378 square inches.

SMA connectors have a low cost multimode coupling with approval for military applications. SMA, which stands for Subminiature A, is a microwave connector.

ST, or Straight Tip, connectors are composed of a precise zirconia ceramic ferrule. They have an easy-to-assemble, one-piece bayonet mounting system. They can be used for both single mode and multimode fibers. This type of connector is very popular in communication applications, such as LANs and CCTV systems. ST connectors are also referred to as a BFOC (Bayonet Fiber Optical Connector). They have a ceramic tip that extends past the connector body. This is also known as a simplex bayonet-style twist-lock connector and it requires special care since the ferrule is unprotected. While the ST connector is still popular in many industrial applications, it is losing favor to more compact, keyable form factors.

Shown in actual size: A connector size comparison between ST, SC, FC, and LC styles. The LC connector advantages are precision keying, low interface losses, and about 50% space savings.

Features

Fiber applications for fiber optic connectors can be single mode or multi mode.

Single mode describes a fiber with a small core that only allows one mode of light to propagate. Modes define the way that the wave travels through space. Single-mode fibers have the same mode but different frequencies. This means that they are distributed in space in the same way to give a single ray of light. Single mode is also called transverse mode since, although the light is running parallel with the fiber, the electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber. The size of the core is only a few times the wavelength of light transmitted. It is commonly used with laser sources for high speed, long distance links.

Multi mode describes a fiber with a core diameter much larger than the wavelength of light transmitted. This allows many modes of light to propagate through the fiber. It also gives multi-mode fiber a higher "light-gathering" capacity than single-mode fiber. Each mode, or ray, is transmitted at a slightly different frequency, which means that the channels can be split and multiple signals can be sent at the same time. The larger core size simplifies connections and allows the use of lower-cost electronics such as light-emitting diodes. Commonly used with LED sources for lower speed, short distance links, such as within a building or on a campus.

Fiber types for fiber optic connectors can include simplex, duplex, and multifiber.

Simplex—Simplex cables are fiber optic cables with a single optical fiber. They are used in applications that only require one-way data transfer. Simplex is available in single mode and multi mode.

Duplex—Duplex cables are fiber optic cables with two optical fibers. They are usually set up side-by-side and can be used for applications that require simultaneous, bi-directional data transfer. Duplex fiber is available in single mode and multi mode.

Multifiber—Multifiber cable is a fiber optic cable with several optical fibers.

For more information on Fiber Optic Cables, please read the How to Select Fiber Optic Cables Selection Guide.

Specifications

When selecting a fiber optic connector, it is important to consider alignment accuracy, ruggedness, repeatability, and loss specifications.

• Maximum cable diameter—The maximum fiber optic cable diameter allowed for the connector.
• Operating temperature—The full required range of ambient operating temperature.
• Loss:

Insertion loss is a measure of the attenuation of a device by determining the output of a system before and after the device is inserted into the system. For example, a connector causes insertion loss across the interconnection (in comparison to a continuous cable with no interconnection). Insertion loss can be calculated by adding up all the connector interfaces and multiplying by the expected loss in dB per connector. That number is added to the total cable loss for the run length calculated by the critical upper frequency. The sum of these two sets of losses yields the basic system loss due to transmission line and connector losses.

Return loss refers to energy reflected back to the signal source due to endface reflectivity. Reflected energy returns down the cable in the form of a delayed signal. Poor return loss performance at each connector junction is a cumulative effect that will degrade receiver decoding even though signal level may be within nominal boundaries.

• Ferrule material—The connector or adapter ferrule can be constructed from ceramic, polymer, or stainless steel.

Related Information

Electronics360—Board-to-Board Connectors Tackle Multi-Gigabit Data Rates

Resources

A Fiber Optic Connector Primer

The Fiber Optic Association—Tech Topics

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