Fibre connectors

As we saw in previous chapters, optical fibres transmit light signals. While copper cables simply need contact between metal pins to enable the conveyance of their electrical signals, fibre requires extremely precise alignment for optical communication to be possible. Therefore, connectors are integral to the process. There are many different types of connectors, each appropriate for a particular application.

The first thing to know about connectors is that they can be either single-fibre or multi-fibre, depending on how many fibres they group together. Single-fibre connectors are further separated into simplex and duplex versions. Simplex fibre connectors hold a lone fibre, while duplex fibre connectors hold a pair of fibres. Multiple-fibre connectors can hold up to 72 fibres together.

Single-fibre:

Simplex:

Duple:

Multi-fibre:

Physical Components

Next, fibre connectors are made up of the following:

• Ferrule: Thin, hollow structure that forms a tight grip on the glass fibre. They are usually made from ceramic, metal, or high-quality plastic. Ferrules are generally either 1.25 mm or 2.5 mm in diameter for single-fibre connectors.
• Connector Body: Plastic or metal structure that links the ferrule to the jacket and actually protects the extremity of the fibre.

• Coupling Mechanism: Part of the connector body that holds it in place when it gets attached to another device. This mechanism can be screw-type, bayonet-type, etc.
• Boot: Extension from the connector along the fibre which provides strain relief to the cable as well as rigidity to the entire apparatus.

• Heat Shrink: Sock-like plastic coating meant to provide added support or act as a canvas for serial numbers.

• Protective Cap: Plastic cover designed to protect the tip of the optical fibre, which is very sensitive to dust and even contact with human skin.

Connector Types and Applications

LC is one of the most popular connector types available. It was developed by Lucent Technologies and thus stands for Lucent Connector. It features a 1.25 mm ferrule. Its body is square and it uses a clip-like coupling mechanism. Due to their small form factor, these connectors are very popular in high-density applications such as datacoms and telecoms. Both simplex and duplex versions are available.

ST stands for a straight tip and was one of the first prominent fibre cable connectors. At 2.5 mm, its ferrule’s diameter is double that of the more modern LC connector’s. It boasts a cylindrical body shape and a twist-on/twist-off bayonet style mechanism. While ST connectors were formerly popular across a range of different employments, they are now used mostly in multimode applications such as LANs. ST connectors only come in simplex versions.

SC connectors, developed in Japan, are similar to LC connectors in that they house a single fibre, use a push-on/push-off mating mechanism and their shape is square. The main difference between the two is the diameter of the ferrule: at 2.5 mm wide, SC connectors are twice as thick as LC connectors. SC connectors are ideal for datacoms, telecoms, and fibre to the home (FTTH) applications such as point to point and passive optical networking. SC connectors are available in simplex and duplex models.

FC connectors are similar in appearance to ST connectors and have the same 2.5 mm ferrule diameter. However, their round body uses a screw-type fitment rather than a bayonet-style coupling mechanism. This feature increases its installation time compared to other connectors but makes it especially popular in high-vibration environments. FC connectors only come in simplex versions.

MPOs are multiple-fibre connectors and as such is different from the connectors previously discussed. They can hold up to 12 fibres in a single ferrule and use a push-on/push off coupling mechanism. Their ability to save space makes them popular in high-density applications such as datacentres.

End Faces

The end face of a fibre connector refers to the structure of the ferrule tip, which affects the precision and quality of the connection. The different types of end-faces evolved over time to reduce the amount of reflectance – or light lost by being reflected back down the core towards its source. Various end-faces have tried to compensate for this phenomenon, which stems from the existence of a minuscule air bubble between two connected ferrules.

Polish types:

• Flat: The first ferrule tips simply had a flat design, which did not try to circumvent the air bubble conundrum in any way. As a result, many imperfections gathered on the edge of the fibre which diminished the quality of the connection.

• Physical Contact (PC): Next, PC end-face polishing was introduced. This version featured a convex end-face so that physical contact occurs in the central core area.

• Ultra Physical Contact (UPC): UPC end-face polishes are essentially higher quality PC end finishes, as the name suggests. These allow for even less reflections back towards the light source.

• Angled Physical Contact (APC): Finally, APC end-face polish was developed. This version boasts the least amount of reflectance of all thanks to its innovative way of tackling the issue. The finish is similar to a UPC end-face, but with an 8° offset. Instead of light being reflected back down the core of the fibre from which it originated, the angled face pushes most rays back into the cladding.