Optical fibres are devices which most people have heard of, or even come across in school physics lessons. They play an important part of our lives, from medical uses to carrying information for the Internet¹.

Who invented them?

In the 1840s, physicists Daniel Collodon and Jacques Babinet showed that light could be guided along jets of water. This was used for fountain sprays. By the turn of the Century, inventors discovered that bent quartz rods could carry light too, and in the 1920s John Logie Baird and Clarence Hansall patented the idea of using hollow pipes or transparent rods to carry images for television systems. The first true glass optic fibre was created in Germany by a medical student, Heinrich Lamm, who wished to use it to see inaccessible parts of the body. However, this was a poor fibre, as grease on the outer surface and damage to the core itself could cause light to leak out.

In the 1950s a further innovation was added. This was the addition of a cladding layer around the fibre (previously, all the fibres were core-only). This allowed better transmission of the light from one end of the fibre to the other, as it protected the important boundary region of the core from damage and contamination.

How do they work?

If you have ever sat at the bottom of a swimming pool and looked up, you will have noticed that you can only see out of the water in certain directions. In particular, there is a circular patch above your head which you can see through, but beyond this all you see is the reflection of the bottom of the pool. This is due to an effect called total internal reflection.

Total internal reflection comes about for the same reason that a straw in a glass of lemonade looks bent. A light ray that enters or leaves the glass is bent, thus making it look like the straw itself is bent. If light tries to leave the glass at too large an angle to the surface, the light is reflected back, as if the surface were a mirror. The same happens in the swimming pool. This is because the bending effect cannot take place since the angle with which it would leave the surface goes beyond 90 degrees. The cause of both effects is the same, based on a property of the materials known as the refractive index.

The total internal reflection effect is why light travels down an optical fibre. The ray enters the fibre and is effectively trapped in the core, being unable to leak out due to total internal reflection.

How are they used?

There are a number of uses for the modern optic fibre. Most of the fibre in the world today is used in the telecommunications industry to carry voice and data communications from exchange (or 'Switch,' in telecoms parlance) to exchange. There are also medical applications and some more esoteric uses as well.

Telecomms

Telecommunications technology has been revolutionised by the invention of the optic fibre. By using a pulsed laser to carry digital ones and zeros down a link, fibre optic communications have become truly global. They have a number of advantages compared to standard copper cabling. They are inherently safer, because there are no electrical connections en route - and hence no electrocution risk. They have a larger bandwidth capacity, allowing for more information to be sent down a single connection. The signals in a fibre suffer less attenuation over long distances, meaning less amplification is required, and with the invention of the erbium-doped fibre amplifier (EDFA) the amplification can take place without extracting the signal from the fibre, unlike electrical signals which must undergo amplification outwith the transmission system. In theory, fibre communications are more secure than ones sent using copper cabling, although it is still possible to tap into a fibre link. There is also less chance of cross-talk between channels, as the photons in the fibre do not interact, unlike the electrons in the copper cable.

Medical uses

Fibres are used in two different ways in a medical context. Firstly, they allow doctors to examine previously inaccessible parts of the body. For example, one of the first suggested uses of fibre optics was in the field of endoscopy. Here two fibres, or bundles of fibre, are used. One is used to deliver light to the area of interest as an illumination source, the other gathers reflected light from the area and brings it back up to the surgeon's eye.

There is a second use, and this is using the optic almost purely as a delivery device. There are certain drugs which are light-activated, and these can be targeted using illumination from a fibre optic. In a similar way, some illnesses can be treated by destroying tissue through heating. Here a laser beam is sent down the fibre to kill the targeted tissues.

Other uses

There are other uses for optical fibres. They can be used as a laser source themselves, by creating some feedback in a fibre loop. In a similar way to the delivery system used medically, some mechanical work can be done using a fibre delivery system to guide high energy laser pulses for cutting and welding materials. Fibres can also be used for decorative purposes (for example, those fibre-optic UFO things you see in shops these days). Finally there are uses for fibres in health and safety. By making a 'leaky' fibre one can make light 'ropes' - long, thin light sources. These can be used, for example, in low-level lighting systems in aircraft.