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.
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 fibre 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.
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