Fibre optic splicing

- an overview or tutorial covering fibre optic splicing (fiber optic splicing) - the way in which it is done and why it is used instead of fibre optic connectors.

Rather than using optical fibre connectors, it is possible to splice two optical fibres together. An fibre optic splice is defined by the fact that it gives a permanent or relatively permanent connection between two fibre optic cables. That said, some manufacturers do offer fibre optic splices that can be disconnected, but nevertheless they are not intended for repeated connection and disconnection.

There are many occasions when fibre optic splices are needed. One of the most common occurs when a fibre optic cable that is available is not sufficiently long for the required run. In this case it is possible to splice together two cables to make a permanent connection. As fibre optic cables are generally only manufactured in lengths up to about 5 km, when lengths of 10 km are required, for example, then it is necessary to splice two lengths together.

Fibre optic splices can be undertaken in two ways:

  • Mechanical splices
  • Fusion splices

The mechanical splices are normally used when splices need to be made quickly and easily. To undertaken a mechanical fibre optic splice it is necessary to strip back the outer protective layer on the fibre optic cable, clean it and then perform a precision cleave or cut. When cleaving (cutting) the fibre optic cable it is necessary to obtain a very clean cut, and one in which the cut on the fibre is exactly at right angles to the axis of the fibre.

Once cut the ends of the fibres to be spliced are placed into a precision made sleeve. They are accurately aligned to maximise the level of light transmission and then they are clamped in place. A clear, index matching gel may sometimes be used to enhance the light transmission across the joint.

Mechanical fibre optic splices can take as little as five minutes to make, although the level of light loss is around ten percent. However this level of better than that which can be obtained using a connector.

Fusion splices form the other type of fibre optic splice that can be made. This type of connection is made by fusing or melting the two ends together. This type of splice uses an electric arc to weld two fibre optic cables together and it requires specialised equipment to perform the splice. The protective coating from the fibres to be spliced is removed from the ends of the fibres. The ends of the fibre optic cable are then cut, or to give the correct term they are cleaved with a precision cleaver to ensure that the cuts are exactly perpendicular. The next stage involves placing the two optical fibres into a holder in the fibre optic splicer. First the ends if the cable are inspected using a magnifying viewer. Then the ends of the fibre are automatically aligned within the fibre optic splicer. Then the area to be spliced is cleaned of any dust often by a process using small electrical sparks. Once complete the fibre optic splicer then uses a much larger spark to enable the temperature of the glass in the optical fibre to be raised above its melting point and thereby allowing the two ends to fuse together. The location spark and the energy it contains are very closely controlled so that the molten core and cladding do not mix to ensure that any light loss in the fbre optic splice is minimised.

Once the fibre optic splice has been made, an estimate of the loss is made by the fibre optic splicer. This is achieved by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side of the splice.

The equipment that performs these splices provides computer controlled alignment of the optical fibres and it is able to achieve very low levels of loss, possibly a quarter of the levels of mechanical splices. However this comes at a process as fusion welders for fibre optic splices are very expensive.

Mechanical and fusion splices

The two types of fibre optic splices are used in different applications. The mechanical ones are used for applications where splices need to be made very quickly and where the expensive equipment for fusion splices may not be available. Some of the sleeves for mechanical fibre optic splices are advertised as allowing connection and disconnection. In this way a mechanical splice may be used in applications where the splice may be less permanent.

Fusion splices offer a lower level of loss and a high degree of permanence. However they require the use of the expensive fusion splicing equipment. In view of this they tend to be used more for the long high data rate lines that are installed that are unlikely to be changed once installed.

By Ian Poole

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