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| Optical Communication - Optical Fibres |
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| An optical fiber is a dielectric wave guide that transports light signals from one place to another just as a twisted-wire pair or a coaxial cable transports electrical signals. It consists of a central core within which the propagating electromagnetic field is confined and which is surrounded by a cladding layer, which is itself surrounded by a thin protective jacket. The core and cladding are both made of pure silica glass, whereas the jacket is made of plastic. Optical fibers have unique characteritics that make them highly attractive as a transmission medium. In particular, they offer the following unique characteristics: |
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| (i) Enormous potential bandwidth, resulting from the use of optical carrier frequencies around 2 x 1014 Hz ; with such a high carrier frequency and a bandwidth roughly equal to 10 per cent of the carrier frequency. The theoretical bandwidth of a light wave system is around 2 x 1013 Hz, which is very large indeed. |
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| (ii) Low transmission losses, as low as 0.1 db/km. |
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| (iii) Immunity to electromagnetic interference, which is an inherent characteristic of an optical fiber viewed as a dielectric wave guide. |
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| (iv) Small size and weight, characterized by a diameter, which is no greater than that of a human hair. |
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| (v) Ruggedness and flexibility, exemplified by very high tensile strengths and the possibility of being bent or twisted without damage. |
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| Last, but by no means least, optical fibres offer the potential for low-cost line communications since they are fabricated from sand, which, unlike the copper used in metallic conductors, is not a scarce resource. The unique properties of optical fibres have fuelled phenomenal advances in light wave systems technology, which have, in turn, revolutionised long-distance communications and continue to do so. |
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| History |
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| In 1870 John Tyndall, a natural philosopher living in England, demonstrated one of the first guided light systems to the Royal Society. His experiment involved using water as a medium to prove that light rays bend. He filled a container with water and allowed the water to escape through a horizontal orifice at the bottom. The water escaping from the bottom formed a natural curve (parabolic) as it descended to a container located some distance below the first. During the movement of the water from one container to the other, Tyndall directed a beam of light into the orifice through which the water was escaping. The light followed a zigzag path in the water and then followed the curve to the container below. This experiment established some of the fundamental rules that we follow in optical communication. |
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| During the early 1950s, researchers experimented with flexible glass rods to examine the inside of the human body. By 1958, Charles Townes and Arthur Schawlow of Bell Laboratories had theorized the use of the laser as an intense light source. In 1960, Theodore Maiman of Hughes Research Laboratory operated the first laser. In 1962, the first semiconductor laser was in its infancy. |
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 The use of water to guide light based on John Tyndall's 1870 experiment |
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| Throughout the 1960s and 1970s, major advances were made in the quality and efficiency of optical fibres and semiconductor light sources. Today this emerging field of communications competes with its more established wire conductor counterpart. One notable achievement was an experiment carried out by the U.S. Air Force. In 1973 the Airborne Light Optical Fiber Technology (ALOFT) program replaced 302 cables, which weighed 40 kg by a fiber system, which weighed only 1.7 kg. |
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| By the late 1970s and early 1980s, every major telephone communications company was rapidly installing new and more efficient fiber systems. |
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| Because of rapidly increasing demands for telephone communications throughout the world, multiconductor copper cables have become not only very expensive but also an inefficient way to meet these information requirements. The frequency limitations inherent in the copper conductor system (approximately 1 MHz) make a conducting medium for high-speed communications necessary. The optical fiber, with its low weight and high-frequency characteristics (approximately 40 GHz) and its imperviousness to interference from electromagnetic radiation, has become the choice for all heavy-demand long-line telephone communication systems. |
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