How do fibre optic cables work?

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Manuela Schewe-Behnisch / EyeEm / Getty Images

When you flip a light switch, light is projected from a bulb and transmitted into the room, illuminating the surrounding space. What if you could take this transmission of light and use it to send data thousands of kilometres away in an instant?

This is the principle behind the fibre optic cable – a system that uses highly purified glass, a laser and reflection and refraction to load a cat video from a server halfway around the world.

To make it easier to understand let’s break it down into its two main components: the cable itself and the light transmitted through it.

The fibre optic cable, while a critically important piece of hardware, works with very simple physics.

If you’ve ever been in direct sunlight and wearing a watch, you might have noticed that you could bounce light off the watch face onto another surface, such as a table top.

Light reflection is the redirection of light using a reflective surface. In this case, the light is redirected once from the Sun to the table via the watch face. In a fibre optic cable, this may happen a million or more times.

The cable consists of two key parts: core and cladding. The core is made from highly purified glass such as silicon dioxide. Surrounding it is yet more glass – the cladding – but it’s infused, or “doped”, with tiny amounts of elements such as germanium and boron.

Doping the cladding reduces its index of refraction. In the same way that a light shone down a stream of water appears to be “contained” within the stream, because water has a higher refraction index than the surrounding air, light shone  at the right angle down a pure glass tube within cladding will also keep a light beam bouncing inside.

The core and cladding are surrounded by a plastic coating and shroud made from strong fibres such as Kevlar or metals to hold it all together. 

Finally, an outer protective layer keeps the outside environment, such as water or dirt, from interfering with the cable. The entire cable may be only a few centimetres thick, with the core just five to 10 millionths of a metre in diameter.

But how does this beam of light transmit a video or phone call?

Light can exist in two main states – on or off. Computers speak in a language with two digits – 0 or 1. So rapidly blocking and then unblocking the beam of light, this can then be translated into packets of data comprising a series of 0s (on) and 1s (off). 

It’s this packet of data that contains the cat video you just loaded.

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