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In my previous article, we took a first look at fibre optics. The use of fibre to transmit data is the backbone of our modern communications network. Thousands of kilometres of optical fibre are spread across the world’s oceans and landmasses to provide the data highway on which the internet and telecommunications depend.
The trouble comes when the time comes to join two optical fibres together. As we saw in the previous article, light travelling through a fibre does not behave in the same way as an electrical circuit. For a circuit to conduct electricity, all it requires is physical contact between two conductive surfaces, sufficient for electrons to flow from one to the other. The larger the surface area is, the greater the conductivity will be and therefore the lower the resistance. An electrical circuit does not care about the orientation of the two conductive parts, nor does it matter how they are connected. All that matters is that there is that physical contact.*
In contrast, light carries through optical fibre travels in a very particular manner. An optical fibre is constructed of two types of glass, each with a different refractive index. The narrow core is the element through which optical signals travel. The surrounding cladding has a lower refractive index, and the change in refractive index between the two layers causes the light to remain within the core by a process known as total internal reflection.
For signals to pass from one fibre to another, it is necessary to bring the ends of both together. If we use a plumbing analogy, if we wish to join two identical pipes together, we need to ensure that they are aligned so that there is no resistance that might prevent the free flow of water from one to the other. The same is true of optical fibre, although the task is somewhat harder due to the small size of fibres – the core of a single-mode fibre is just 9 microns across. Unlike a pipe, an optic fibre is not hollow but instead is solid glass.
The light uses glass as its medium, so when connecting two fibres together, the glass needs to be brought into contact with great precision. To ensure that the fibres are aligned, they are held within a tube or ferrule. This ferrule is a hard cylinder of ceramic and features a cavity through the centre that is only marginally larger than the fibre itself. To fix the fibre in place, it is pushed through the ferrule and anchored with epoxy, ensuring that it cannot move.
The next action is to polish the face of the ferrule. The goal is to create a surface that has no blemishes or marks that might affect the optical signals that pass through it. To make the connection, two of these ferrules are brought together into physical face-to-face contact, with the intention that the light passes from one fibre to the other with the minimum signal loss.
It is this polishing process that presents one of the greatest challenges when terminating fibre-optic connectors. The need for cleanliness, precision and a mirror-smooth finish is such that this is not something that can be easily accomplished in the field. Electrical connections can be made with a crimp or solder joint in confined circumstances. However, preparing a fibre for use in a connector requires polishing tools, inspection microscopes and sophisticated measuring equipment to ensure the performance of the complete system.
Once in use, fibre optic terminals with their highly-polished ferrules need to be protected from the rigours of everyday service. The connector into which the terminal is fixed performs a number of roles. The first is to ensure protection for the sensitive fibre surfaces. If dust and dirt are allowed to build up on the surfaces of the ferrule, the resulting loss of light through the termination may be sufficient to prevent effective communications from one to the other.
A fibre optic connector performs another role. As optical fibres require direct physical contact between surfaces to deliver effective communication, it is important to prevent vibration and shock that might momentarily force the two ferrules apart. To prevent this, the terminals are provided with springs to ensure positive contact. The body of the connector provides the anchor for the terminals.
Fortunately, the connector industry has spent many decades developing robust, waterproof and reliable connections for electrical connections. Many of these are ideally suited for modification to include fibre-optic terminals. For this reason, alongside dedicated connector designs such as the popular LC, there is a range of specialist connectors that provide reliable performance across an array of different industries.
In the final part of this series, we will take a look at some of these specialist types, and see how different manufacturers provide connectors that take fibre optics into some of the toughest environments.