Fibre Optic Connectors, Adaptors and Ferrules

Fibre Optic Connectors

Unlike with copper cabling where one style of connector, the RJ45, dominates, with fibre there are a number of different connectors that can be used.

ST Connectors

The ST connector has a round barrel and uses a bayonet fixing mechanism to secure it to the mating connector. It is not recommended for new installations and is only seen in legacy fibre deployments.

SC Duplex Connector

The ST connector has a square format and has a push/pull latching mechanism. The image shows two SC connectors that have been joined using a special clip to create a SC Duplex connector with the A and B legs identified in the moulding of the clip.

LC Duplex Connector

The LC connector is sometimes referred to as a Small Form Factor (SFF) connector as it has much smaller dimensions than the SC connector. In fact it is possible to get twice as many fibres terminated on LC connectors in the space used by SC connectors. LC connectors have a simple latch, similar to that used on a RJ45 plug, making it familiar in use for network technicians. This has become the connector of choice in most new network fibre installations.

MPO Connector

The MPO or MTP® connector has larger dimensions that the LC Duplex connector but can accommodate up to 24 fibres in a single ferrule, making it ideal for high density installation. This style of connector is also finding favour in multi-channel fibre applications such as 40G and 100G Ethernet. However, the high precision nature of the connector means that it is not suitable for field termination.

Fibre Optic Adaptors

The standard way of joining two fibres together at a patch interface is through the use of adaptors, sometimes also referred to as couplers. These devices consist of a plastic housing with precision alignment sleeves that are designed to ensure that the two fibre end faces are perfectly aligned when a fibre connector is inserted in to both sides of the adaptor. The adaptors are specific to the type of connector(s) to be mated. Some typical fibre adapters are shown below.

Fibre Adaptor Group

Fibre Optic Ferrules

When two fibre end faces are presented to each other it is important to minimise the air gap between the two ferrules as any gap will lead to back reflections, otherwise known as Optical Return Loss (ORL). When ceramic ferrules were first introduced they had a flat end face but the relatively large end face surface area allows for numerous slight but significant imperfections to gather on the surface. This lead to the development of Physical Contact (PC) connectors where the surface area of the ferrule end face was reduced by angling the corners of the ferrule.

Building on the convex end-face attributes of the PC, but utilising an extended polishing method, creates an even smaller ferrule end face, referred to as the Ultra Physical Contact (UPC) connector. This results in a lower back reflection (ORL) than a standard PC connector, allowing more reliable signals in digital TV, telephony and data systems, where UPC today dominates the market.

Although PC and UPC connectors have a wide range of applications, some instances require optical return losses in the region of one-in-a-million (60dB). Angled Physical Contact (APC) connectors can consistently achieve such performance. This is because adding an 8° angle to the ferrule end-face allows for even tighter connections and smaller end-face area.

Fibre Ferrules