The theory behind fibre optics

On December 12, 2006, Sri Lanka Telecom (SLT) announced the inauguration of the Dhiraagu - SLT Optical Fibre Submarine Cable System between Sri Lanka and the Maldives.

This cable system aims to enhance economic growth via superior telecommunications in South Asia.

The cable line between Colombo and Male, which will span 850 kilometres, has a capacity of 10 gigabits per second.

It will enable Dhiraagu to be connected with the rest of the world, through Sri Lanka, and will allow for super highway level bandwidth between Sri Lanka and the Maldives.

Dhiraagu is SLT's second bilateral regional tie-up on optical fibre cable systems, following the commissioning of the Bharat-Lanka Optical Fibre Submarine Cable System.

Through this cable system, SLT can connect the Maldives to SLTNet, Sri Lanka Telecom's Internet Service Provider (ISP), with a complete range of Internet-related services.

What is fibre optics?

After reading this news item, you must be wondering what fibre optics are. Let us enlighten you on the history of fibre optics. In 1870, John Tyndall demonstrated that light follows the curve of a stream of water pouring from a container. It had been this simple principle that had led to the study and development of applications for this phenomenon of fibre optics.

In the 1950s, more research and development into the transmission of visible images through optical fibres

led to some success in the medical world, as they began to be used in remote illumination and viewing instruments.

In 1966, Charles Kao and George Hockham proposed the transmission of information over glass fibre, and they also realised that to make it a practical proposition, much lower losses in the cables were essential.

This had been the driving force behind the development to improve the optical losses in fibre manufacturing.

The advantages of using fibre optics

Because of the low loss, high bandwidth properties of fibre cables, they can be used over greater distances than copper cables. In data networks, that can be as much as 2km without the use of repeaters. Their light weight and small size also make them ideal for applications where running copper cables would be impractical. By using multiplexors, one fibre could replace hundreds of copper cables.

The real benefits of using this in the data industry is its immunity to Electro Magnetic Interference and the fact that glass is not an electric conductor, because fibre is non-conductive, it can be used where electrical isolation is needed, for instance between buildings where copper cables would require cross-bonding to

 eliminate differences in earth potentials.

Fibres also pose no threat in dangerous environments such as chemical plants where a spark could lead to an explosion. The biggest advantage is that no one can tap into a fibre cable to read the data signals.

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How it works

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 known 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, 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 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 running right across Australia, you could use a torch to flash a message to a person watching in the other end. Also, as light travels at about 300,000 km per second, you could use your torch to flash a message around the world in next to no time.

Of course, actually doing it is much more complicated, but that is the principle on which fibre optics work.

Compiled by Janani Amarasekara