Each optical fibre element is most usually coated with layers of plastic and held within a tube which is designed to work well in the environment in which it is used.
Fibres are formed into bundles or ribbons within the cable and generally coated with polyimide or acrylate polymer.
Although the coating protects fibres from any damage it does not affect their functionality. Once fibres have been formed in to bundles or ribbons they are then encased within a layer of durable resin or a core tube which forms the centre of the cable.
After this the core is covered in a number of layers of sheathing, which can be made from various materials. This then forms the cable. In some cases, ‘dark’ glass is placed between the fibres. This helps to absorb any light and stop any light leaking from one cable into another. This is designed to reduce ‘flare’ or ‘cross talk’ between fibres which helps them to function more efficiently.
Indoor Cables
When utilised indoors, enclosed, jacketed fibres are generally used. These contain a flexible fibrous polymer bundle such as Aramid. This is held within a lightweight covering of plastic which produces a simple cable. The end of each cable will be fitted with a terminator which means the cable is easy to connect and disconnect form any appliance it is being used with.
Outdoor or Robust Cables
When a fibre optic cable is being used outdoors it needs to be more robustly constructed in order to withstand wear and tear. One of the main types of cable construction is known as ‘loose-tube construction’. Fibre is placed into semi rigid tubing meaning the cable is able to stretch but the fibre won’t stretch.
This means that the fibre is protected from the elements, such as heat stretching during hot weather and protected from being stretched out during the laying process. To protect the cable from water, copper tubing and jelly or powder that repels water can be used.
Strength and Reliability
Fibre optic cables are in general extremely strong as well as being extremely powerful and efficient, which is why fibre optic cable has become widely used in all types of application since its invention. However, the strength of a cable can be compromised within the manufacturing process by tiny surface flaws. These are unavoidable within the manufacturing process. This is taken into account when cables are fitted to ensure that the correct cable strength is used. The three main factors which can lead to cable degradation, damge or failure are known as ‘zero stress aging’, static fatigues’ and ‘dynamic fatigues’.
Modern Cable Use
Modern fibre optic cables are available with a variety of different coverings depending on what they are to be used for. Uses can include being buried within trenches to create lines for broadband internet connection, use as power lines, telephone lines, or within submarines or aeroplanes.
Dark Fibre
It is estimated that only one percent of fibre optic cable that has been buried within the past few years is actually being used. Many companies have realised the unused potential of spare ‘dark fibre’ and have begun selling unused fibre to other providers. Other providers are now adding more fibres than necessary when fitting new cables with the intention of selling cable space in the future as demand grows.