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IT: Information

5.2.2 Optical fibre

In all developed countries, long-distance communication links (which used to be called 'trunks', by analogy to 'trunk road') nearly always use optical fibre. It is only where the terrain makes it difficult to lay a cable (such as in mountains or, sometimes, between islands) or when a new link is needed quickly and there isn't time to lay a cable that microwave links are used instead.

An optical fibre is a strand of glass or plastic, not much thicker than a human hair, which guides light from one end to the other (Figure 26). The guidance comes about because of an effect known as total internal reflection. This means that light shone in one end of the fibre doesn't come out from the sides of the fibre even if the fibre is bent around corners. The light just travels inside the fibre until it comes to the far end. Because the fibre is so thin it is flexible and, from the outside of a cable with protective plastic coverings, looks and feels much the same as an electrical wire. Signals are conveyed by changing the 'brightness' of the light injected into the fibre and measuring it at the far end. Bits are sent by 'on-off keying': 1s are represented by light on and 0s by light off.

Optical fibre of the type used for communications. The bare glass fibre is vulnerable to scratches and will break if bent too tightly, so the cable shown has several layers of protective coverings, shown here stripped off layer by layer.
Figure 26 Optical fibre of the type used for communications. The bare glass fibre is vulnerable to scratches and will break if bent too tightly, so the cable shown has several layers of protective coverings, shown here stripped off layer by layer.

The attraction of optical fibre is that it can be used for very high data rates over long distances. It is this combination – high data rates and long distances – that distiguishes it from wires.

When electrical signals are transmitted over wires the attenuation increases with increasing data rate, so the higher the data rate, the greater the attenuation and therefore the shorter the distance that can be used. So, for example, although signals at 1 gigabit/s can be carried around on wires inside a computer, it is difficult to transmit electrical signals even a few metres at data rates that high. With optical fibre the attenuation is not dependent on the data rate, and the attenuation is anyway very low, so that signals at tens or even hundreds of gigabit/s can be sent for tens of kilometres with the right sort of fibre.

There are other factors to consider, including, as mentioned above, that higher data rates require higher power (which is the case whatever the medium used to carry the signal). Nevertheless, even though optical fibre is generally more expensive to use than wires, optical fibre is the transmission medium to use for high data rates over long distances.

Activity 28

When optical fibre was first developed as a communications medium, it was initially used only for long-distance transmission between cities. More recently it has been used for shorter distances, including many new local area networks (LANs) within office buildings. There is also a debate about how and when it should be used in the links between private homes and the local telephone exchange. From the discussion above, can you suggest why it is now finding applications for shorter distances, where metallic wires were previously used?

Discussion

In recent years the data rates required of many communication links have been increasing. Since the attenuation of wires increases with increasing data rates, many links which previously could use wires cannot do so any more, because the data rates are too high. Where data rates have got too high to use wires, optical fibre is often used instead.

Also, although this was not discussed in the text, the equipment needed for optical fibre transmission has been getting cheaper, making its use more economical in a variety of applications.

T175_8

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