How Does Fibre Optic Cabling Work?

Fibre optic cabling has brought about a major change to the way we make phone calls, link to the internet and create office networks. But exactly how does fibre optic technology work?

Using the medium of glass

At the heart of fibre optic technology is – as you might expect – a fibre. This is made of glass and is as thin as a human hair. It’s also optically pure, which is key to its ability to transmit data in the form of light pulses, which travel through the fibre. Optical cables are usually made up of a bundle of these fibres, allowing them to handle large bandwidths.

Each individual fibre has three elements. There’s an inner core where the light travels, this is surrounded by an outer cladding which bounces light back into the core. Finally, there is an outer coating, known as a buffer, which helps protect the fibre from damage and moisture.

As we’ve said, optical cables are usually made up of bundles of fibres and these are protected by an outer jacket usually made of plastic. Just to make things a little more complicated though, there are three types of optical fibre:

• Single mode fibres have a core of about nine microns in diameter and are designed to transmit laser light.
• Multi mode fibres are larger – around 62 microns – and are designed to transmit infrared light from LEDs.
• Plastic fibres have a diameter of around one millimetre, designed to transmit visible wavelength light from LEDs.

On reflection

Okay, so how do optical fibres transmit data? Well in much the same way that ships used to transmit messages in Morse code to each other from long distances using pulses of light, the transmission of data using optical fibre operates on a similar principle. The difference being that the rate of the pulses of light in fibre optic cable is many millions that of those once used by sailors and seaman.

Signalling in morse code using pulses of light requires you to have a clear line of sight to the person receiving the signal, because light travels in straight lines. One way to get around this would be to position a mirror to reflect the light around the corner. This same principle is used by fibre optics except at very very short distances.

The light pulses in a fibre optic cable travels through the core of the cable, but by bouncing off the cladding at an angle as it goes (fig 1). So in our example above, the cladding represents the mirror, allowing the signal to travel around bends in the cable.

Due to slight imperfections in the glass of the fibre, there is a slight degradation of the signal over long distances, although this is much less than the signal loss experienced with copper cables. To get around this, optical networks use a signal booster – called an optical regenerator – to allow the signal to cover longer distances without problems.

Advantages

Now we know how fibre optics work, we can begin to see why they have become so popular. They offer less signal loss than copper cables and because the glass fibres are so much thinner than their copper cousins, many more can be bundled into a given space, allowing the network to carry more traffic (higher bandwidth).

There’s also no interference between fibres as there can be with copper cables, so the integrity of the data is preserved. Finally, since no electricity passes through the fibre, it can be used in hazardous environments without risk.