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So what is an optical fibre? An optical fibre is a strand of material (commonly glass fibre) through which light or infra–red radiation can travel. The diagram below shows thin strands of plastic optical fibres on the end of a torch. Optical fibres carrying light from a torch. As you can imagine, a long thin strand of glass the thickness of a hair is not very strong. For this reason, optical fibres require cladding. The cladding needs to be of lower refractive index than the strand of glass to ensure total internal reflection within the glass fibre. There are three main types of optical fibres used for telecommunications. Multimode optical fibre total internal reflection of several light impulses within the optic fibre cladding with lower refractive index than optical fibre optic fibre light entering cladded optical fibre Multimode optical fibre. 10 Information systems Gill Sans Bold Thousands of different digital transmissions may be sent along a multimode optical fibre. The light impulses are often sent in the infra–red band of the electromagnetic spectrum. The glass inside the cladding has a consistent refractive index throughout the fibre. Graded optical fibre optic fibre has a graded density causing light to travel faster on the edges of the fibre than the centre cladding with lower refractive index or optical density than optical fibre graded optic fibre Graded optical fibre. Imagine running 100 metres, but instead of running in a straight line, you zig–zagged across the track to the finish line. Would you have travelled further than if you had run in a straight line? Of course you would have. Light impulses bouncing off the edges of an optical fibre would also travel much further than light that barely touched the edges. This causes impulses to arrive at their destination at slightly different times. Graded optical fibres have a higher refractive index material towards the centre of the fibre. The refractive index gradually decreases towards the outer edges of the fibre. This causes light inside the optical fibre to appear to curve, although light is still travelling in a straight line. The result of graded optical fibres is all the light transmissions, regardless of how many times they are internally reflected, arrive at their destination at the same time. This prevents time delays in transmission. Quality of output information is better than for the multimode optic fibre. Part 6: Fibre optics 11 Small diameter core small diameter core optical fibre (3–5 mm) cladding Small diameter core. The glass strand inside this optical fibre is only 3–5 µm (3 –5 x 10–6 m) in diameter and only transmits a single impulse at any one time. Only one light wave can fit inside the inner core at one time. An impulse of light travels through the high refractive index material centre. Because there is no interference with other waves and the light contained inside the core cannot spread out due to the size of the core, the quality of the information transmitted is excellent and does not need to be boosted for 500 km. Small diameter optical fibres are therefore often used in long distance cables, however the precision lasers required to beam the impulses through these fibres and the technology required to align their tiny cores cause them to be more expensive than other optic fibre varieties. 1 What is the difference between a multimode optic fibre and a graded optic fibre? _____________________________________________________ _____________________________________________________ 2 Why is an optic fibre strand encased in cladding? ______________________________________________________ ______________________________________________________ 3 What problem does graded optical fibre attempt to overcome? ______________________________________________________ ______________________________________________________ 12 Information system Gill Sans Bold 4 Which type of optical fibre is more effective in long distance telecommunications and why? _____________________________________________________ _____________________________________________________ You have identified a glass strand and its cladding however optical fibres are surrounded by several more layers for strength and durability. 5 6 Label the diagram below with the following: • the thinnest tube is the glass core and cladding combined; together they form the optical fibre • the next layer is silicone • the next layer is the buffer jacket • the second last layer is the strength layer • the outside layer is polyurethane Label the above diagram as a single optical fibre. Check your answers. Even with the protective layers, optical fibres are quite flexible. Rarely is one optical fibre laid alone in telecommunications. Bundles of optical fibres such as the one above are usually encased inside a cable in telecommunications. Turn to Exercise 6.1 at the back of this part to summarise the properties of optical fibres. Part 6: Fibre optics 13