The right connectors for simplex and duplex optical fibres

21st August 2019
Anna Flockett

Optical fibre has becoming increasingly popular, providing high-speed, reliable data transfer and communications for a huge range of applications. As well as the fibre itself, it’s important to pick the right connectors for the job, but navigating through the multiple options that are available can be confusing. In fact, there are three types of fibre connectors in common usage: SC, ST (bayonet-twist) and LC (push-pull locking). 

The most frequently specified is the LC connector, because it is much more compact than the other two, making it suitable for space-constrained applications, and also because it provides a secure clip connection, so there is less chance of the fibre becoming accidentally disconnected.

Simplex or duplex?
With that choice out of the way, there is an additional complication: fibre optic LC connectors are available in simplex and duplex options. What does that mean?

Put simply, a simplex optical fibre has a single strand of glass or plastic fibre, and is only capable of transmitting data in one direction. This makes it suitable for applications that only need this uni-directional capability, for example transmitting data from a sensor in an Internet of Things (IoT) system.

A duplex optical fibre, on the other hand, consists of two strands of fibre, and can therefore transmit data in both directions. It may be ‘half duplex’, that can only send data in one direction at a time, or ‘full duplex’, that can handle simultaneous, bi-directional communications – for example, for an IP telephony application.

As you’d expect, simplex fibres cost less than duplex, as there is just one fibre not two. Bear in mind that simplex and duplex fibres are both available as either single mode or multimode versions, depending on the needs of any particular use case.

Connectors for harsh environments
Once the type of fibre is chosen, you then need to specify the appropriate LC connector that matches the fibre – simplex or duplex. For some applications, a standard LC connector will be fine, but fibre is increasingly being used in outdoor installations, and in harsh environments – where the delicate optical fibre is at risk from moisture or dirt, which can affect the transmission of light and hence the data rates achieved.

The fibres themselves are protected by an acrylic layer, but the connectors can be vulnerable in outdoor usage. In extreme situations, water can gain access to a connector and then freeze, causing damage to the fibre and potentially halting any communication.

Standard LC connectors are not rugged enough for harsh environments, and there is no guarantee that moisture will not make its way into the connector.  One solution is to use a custom enclosure to protect the connector, but this is typically an expensive option – and will be too big for many use cases.

A better answer is to specify a rugged LC connector, such as the 4000 series fibre range from Bulgin, which provides an industry-standard LC interface as specified by IEC 61754-20. This connector protects the ends of the optical fibre from dirt and damage, and provides a seal to prevent any ice forming.

The fibre connection provided by the 4000 series is UV resistant, salt spray resistant and sealed to IP66, IP68 and IP69K. It also provides a secure quick twist bayonet connection, ensuring durable mechanical mating. The connector operates over a wide operational temperature range of -25 to 70ºC, with an average insertion loss of less than 0.1dB and a maximum insertion loss of 0.2dB.

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