Fibre Optics

Since the earliest times people have wanted to send messages to each other over distances. Once you get too far away to shout or see hand signals you have to use something else. All kinds of things have been tried – banging on hollow trees or drums and even tapping on walls. You know the kind of thing: one bang for 'yes' and two bangs for 'no'. Smoke signals by day and fires by night have used light to transmit information. In places like Egypt where sunshine was reliable, people developed a system of signalling using mirrors to reflect the sun and send 'flashes'. The most advanced version of this device (called a heliograph) had two moving mirrors so that one could pick up sunlight and the other could flash signals in any direction.

The lights used to guide ships at sea are another way of using light as a signal. Each light gives out flashes of different timing and length so that sailors can tell which light they are seeing many kilometres away – just by timing the light and looking it up in a guide book.

But there are some major problems with using light this way to send messages. For one thing, even very powerful lights such as those used at sea (which can have millions of candlepower) can only be seen from a relatively short distance. This is partly because the Earth is curved, and partly because as light radiates out from a source it gradually becomes weaker because of the effects of dust and smoke in the air and because of the inverse square law of radiation.

Pushing light through a pipe

A cleverer way of using light to send messages is to push it through a pipe – much in the same way as water is moved around through pipes. This is where 'light pipes' – better know as optical fibres – come in. Optical fibres are long strands of transparent material which let the light pass through the middle. Of course, the light tries to get out (left to itself, light will always travel in a dead straight line) but the outer walls of the optical fibres act like a continual tube of mirror. So the light travels along the fibre bouncing off the mirror-like outer casing until it arrives at the other end of the fibre.

These optical fibres – which are thinner than a human hair – work when bent around corners, laid underground or even laid on the ocean floor. And because the light is contained within the walls of the fibre and can't disperse or radiate away, it takes very little light energy to send a signal over a long distance. In theory, if you had a single optical fibre that ran right across Australia, you could use a torch to flash a message to a person watching the other end! Also, as light travels at about 300,000 kilometres per second, you could use your torch to flash a signal right around the world in next to no time.

Of course, actually doing it is much more complicated than that, but that is the principle on which it works.

Three questions to consider

Three questions had to be answered when optical fibres were considered for use in telecommunications.

• How does light behave when it is sent through a long fibre?
• What sort of physical equipment do you need to make it work?
• How are you going to use the flashes of light to carry a message?

• How light behaves

  When light passes down an optical fibre, it continues to travel in straight lines – until it hits the mirror-like side of the fibre and bounces off. It then travels in a straight line again until it bounces off another part of the fibre wall. Because of the angle of reflection, the light cannot go back on itself so it must always go in the same direction, bouncing its way along until it reaches the end. Various things affect the way the light is transmitted – including the quality of the original light source, the exact composition of the transmitting fibre, and the material used for the walls of the fibre. Each of these qualities must be understood in order to predict what the light will do under differing circumstances.

• What sort of equipment do you need?

  To send a flash or 'pulse' of light along an optical fibre you need something to generate the light in the first place. You could use a hand-held torch but this would be very slow and inefficient. The optical fibre communications network uses lasers to generate a suitable light source. Lasers can produce very tightly focused pulses of light – and they can do it many times a second. The pulses are then picked up at the other end of the fibre by a light-sensitive cell which can convert the pulses of light into pulses of electricity. These pulses of electricity are then fed into a computer and decoded to reveal the message.

• Using the flashes of light to carry a message

  Simply flashing messages down an optical fibre on the 'one flash for yes, two flashes for no' principle would take a very long time. So, complex digital codes have been worked out to take advantage of the very high speed and volume of data that can be sent through an optical fibre. Using a standard commercial system, it is possible to send the entire contents of the 32 volumes of the Encyclopaedia Britannica through an optical connection in less than one second! In fact, using a combination of codes, many messages can be sent along an optical fibre at the same time.

Optical fibres are now used in many telecommunications systems, so the next time you pick up a phone to speak – or use a computer modem to send a message – you may well be using an optical fibre system to do it.