How do fibre optic cables work?

Fibre optic cables – or optical fibre as some people call them - work by transmitting light. This makes them very different to traditional metal cables such as copper or aluminium, which work by conducting electricity. 

Fibre optic cables are made of extremely long and thin strands of glass or plastic – often comparable in thickness to a human hair – through which light is shone. These strands transmit data by sending binary code as pulses of light. At either end of the cable, transponders convert the data into light, and from light back into data again. 

The reason that glass or plastic is used to make the fibres instead of metal is because they exhibit a phenomenon known as total internal reflection. This means the signal in a fibre optic cable can travel much further than the signal in a copper cable without degrading. It’s also possible to send much more data down a fibre optic cable than a metal one – as much as 400 gigabits per second, while the Ethernet Standards state a category 5 cable will transmit around 1 gigabit per second.  

There are generally two types of fibre optic cable used: single mode and multi-mode. In single mode cables the diameter of the fibre is very small – under 10 micrometers (10μm) – and the light travels in a direct path. This enables single mode fibre to transmit data across distances in excess of 100km. Multi-mode fibre optic cables offer multiple paths for the light to travel, bouncing off along the way as it makes its way along the length of the cable. It means it can’t transmit data as far - only around 2km - but they are cheaper than single mode fibre and still more efficient than metal conductors, making them ideal for shorter connections such as those inside a data centre. 

As an aside, fibre optic cables are generally for data only - they aren’t as efficient as metal cables for power transmission. There are some circumstances though where metal isn’t suitable (such as near MRI machines, which have a strong electromagnetic field that can interfere with even shielded metal cables), so in these cases fibre optic cables can be used for power purposes, with a photovoltaic cell converting the light into electricity.