Science: Physics: Uses of Optical Fibres

In this section we will show you how optical fibres are used. As you will be able to see when you read further, optical fibres are revolutionising fields like communications and medicine.

Telecommunications Industry

Until the optical fibre network was developed, telephone calls were mainly sent as electrical signals along copper wire cables. As demand for the systems to carry more telephone calls increased, simple copper wires did not have the capacity, known as bandwidth, to carry the amount of information required.

Systems using coaxial cables like TV aerial leads were used but as the need for more bandwidth grew, these systems became more and more expensive especially over long distances when more signal regenerators were needed. As demand increases and higher frequency signals are carried, eventually the electronic circuits in the regenerators just cannot cope.

Optical fibres offer huge communication capacity. A single fibre can carry the conversations of every man, woman and child on the face of this planet, at the same time, twice over. The latest generations of optical transmission systems are beginning to exploit a significant part of this huge capacity, to satisfy the rapidly growing demand for data communications and the Internet.

The main advantages of using optical fibres in the communications industry are:

- A much greater amount of information can be carried on an optical fibre compared to a copper cable.

- In all cables some of the energy is lost as the signal goes along the cable. The signal then needs to be boosted using regenerators. For copper cable systems these are required every 2 to 3km but with optical fibre systems they are only needed every 50km.

- Unlike copper cables, optical fibres do not experience any electrical interference. Neither will they cause sparks so they can be used in explosive environments such as oil refineries or gas pumping stations.

- For equal capacity, optical fibres are cheaper and thinner than copper cables which makes them easier to install and maintain.

Case Study: British Telecom

In the UK at the moment there are three million kilometres of optical fibre cable in the BT network. Most of BT's trunk network now uses optical fibre cables. In a recent trial in Bishop's Stortford optical fibres were actually laid into homes. This allowed customers to receive cable TV and stereo radio as well as phone and information services.

Optical fibres could be put into all homes but currently the cost of the system including lasers and detectors would be too high for simple telephone calls. Some companies have a direct optical fibre link if they need to send large quantities of information by phone, for example between computers at different business centres.

Optical fibre submarine links are in use all around the world. Because of the low loss and high bandwidth of optical fibre systems they are ideal for submarine systems where you want to minimise the amount of complex electronics in regenerators sitting on the sea bed. In fact, the link from the UK to the English Channel Islands is achieved directly without any submerged regenerators.

The world's first international optical fibre submarine cable was laid by BT in 1986 between the UK and Belgium. It is 112Km in length and has only 3 regenerators. BT was a major partner in the first transatlantic optical fibre cable system - TAT 8 (Transatlantic Telecommunications cable no 8) which was capable of carrying 40,000 telephone calls at once, or the equivalent in data, facsimile, or TV pictures.

The second transatlantic cable, TAT 9, which came into service in 1992, has twice that capacity and links five separate landing points in the UK the USA, Canada, France and Spain. The transatlantic optical fibre cable network, completed in 1996, spanned 14,000Km and linked the UK, France and the USA . It could handle up to 320,000 phone calls at one time.

Undersea cables consist of fibres with a copper coated steel conductor which are covered in protective layers of steel and polypropylene. In shallow waters, e.g. the continental shelf, a submersible remote-controlled plough is used to bury the cables one metre below the seabed, to protect them from damage by trawling and ships' anchors.

Medicine Industry

The advent of practicable optical fibres has seen the development of much medical technology. Optical fibres have paved the way for a whole new field of surgery, called laproscopic surgery (or more commonly, keyhole surgery), which is usually used for operations in the stomach area such as appendectomies. Keyhole surgery usually makes use of two or three bundles of optical fibres. A "bundle" can contain thousands of individual fibres". The surgeon makes a number of small incisions in the target area and the area can then be filled with air to provide more room.

One bundle of optical fibres can be used to illuminate the chosen area, and another bundle can be used to bring information back to the surgeon. Moreover, this can be coupled with laser surgery, by using an optical fibre to carry the laser beam to the relevent spot, which would then be able to be used to cut the tissue or affect it in some other way.

Other Uses

- Optical fibres can be used for the purposes of illumination, often carrying light from outside to rooms in the interiors of large buildings.

- Another important application of optical fibres is in sensors. If a fibre is stretched or squeezed, heated or cooled or subjected to some other change of environment, there is usually a small but measurable change in light transmission. Hence, a rather cheap sensor can be made which can be put in a tank of acid, or near an explosion or in a mine and connected back, perhaps through kilometres of fibre, to a central point where the effects can be measured.

An advantage of fibre-optic sensors is that it is possible to measure the data at different points along the fibre and to know to what points the different measurements relate. These are the so-called distributed sensors.

- Fibre optics are also used to carry high power laser beams from fixed installations within factories to the point of use of the laser light for welding, cutting or drilling. The fibre provides a flexible and safe means of distributing high power laser radiation around a factory so that robots or machine tools can be provided with laser machining capability.

- Optical fibres can also be used as simple light guides. At least one fancy modern car has a single high intensity lamp under its bonnet, with optical fibres taking the light to a series of mini-headlamps on the front. Less high tech versions carry light from bulbs to the glove compartment etc.

- A research group at the Clarendon Laboratory, Oxford, is designing a laser installation at the William Hershel Telescope on La Palma to help astronomers make an 'artificial' star in the layer of atomic sodium which exists at a height of 100km above the Earth's surface.

The Earth's atmosphere is a big problem for astronomers. It is a gas that is constantly moving which makes the light traveling through it from distant starts flicker. If astronomers could use a reference 'star' whose brightness they knew, then they could allow for this twinkling.

The telescope will look at how the atmosphere is effecting the artificial star second by second and adjust the telescope's mirror to compensate. This should allow astronomers to capture pictures of astronomical objects of a quality previously only obtainable from the Hubble Space Telescope. The optical fibre in this case is used to pipe the laser power needed to create the artificial star from the lasers to the telescope itself.

- As light is not affected noticeably by electromagnetic fields. It also does not interfere with other instruments that do use electricity. For this reason, fibre-optics are also becoming very important for short-range communication and information transfer in applications situations like aircraft. This application is now being extended into motor cars, and plastic optical fibres will soon (say in 5-8 years time) be very common for transmitting information around the car.



So we can see that optical fibres are not just passive light pipes. Researchers are finding ways in which they can make the fibres become the active elements of the circuit, e.g. amplifiers or filters. This means that the information could remain in light form from one end of a link to the other, removing the limitations of the electronics in circuits and enabling more of the theoretical information carrying capacity to be used.

Engineers of the future can look forward to designing and using telecommunications systems that have no loss, infinite bandwidth and high reliability. New services for customers, such as 3D high definition TV and virtual reality information and entertainment systems, could be more easily provided as well as giving them the benefits of lower costs and greater flexibility - an exciting future.