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What Are Optical Fibres

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What are Optical Fibers ? Optical Fibres are fibres of glass, usually about 120 micrometres in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data. History Interest in the use of light as a carrier for information grew in the 1960's with the advent of the laser as a source of coherent light. Initially the transmission distances were very short, but as
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  What are Optical Fibers ?   Optical Fibres are fibres of glass, usually about 120 micrometres in diameter, which areused to carry signals in the form of pulses of light over distances up to 50 km without theneed for repeaters. These signals may be coded voice communications or computer data. History   Interest in the use of light as a carrier for information grew in the 1960's with the adventof the laser as a source of coherent light. Initially the transmission distances were veryshort, but as manufacturing techniques for very pure glass arrived in 1970, it becamefeasible to use optical fibres as a practical transmission medium. At the same timedevelopments in semi-conductor light sources and detectors meant that by 1980 worldwide installation of fibre optic communication systems had been achieved. Advantages   Capacity Optical fibers carry signals with much less energy loss than copper cable and with a muchhigher bandwidth. This means that fibers can carry more channels of information over longer distances and with fewer repeaters required.   Size and Weight  Optical fibre cables are much lighter and thinner than copper cables with the same bandwidth. This means that much less space is required in underground cabling ducts.Also they are easier for installation engineers to handle.  Security Optical fibres are much more difficult to tap information from undetected; a greatadvantage for banks and security installations. They are immune to Electromagneticinterference from radio signals, car ignition systems, lightning etc. They can be routedsafely through explosive or flammable atmospheres, for example, in the petrochemicalindustries or munitions sites, without any risk of ignition.  Running Costs The main consideration in choosing fibre when installing domestic cable TV networks isthe electric bill. Although copper coaxial cable can handle the bandwidth requirementover the short distances of a housing scheme, a copper system consumes far moreelectrical power than fibre, simply to carry the signals. Disadvantages    Price In spite of the fact that the raw material for making optical fibres, sand, is abundant andcheap, optical fibres are still more expensive per metre than copper. Having said this, onefibre can carry many more signals than a single copper cable and the large transmissiondistances mean that fewer expensive repeaters are required. Special Skills Optical fibers cannot be joined (spliced) together as a easily as copper cable and requiresadditional training of personnel and expensive precision splicing and measurementequipment  Areas of Application   Telecommunication's Optical fibres are now the standard point to point cable link between telephonesubstations.  Local Area Networks (LAN's) Multimode fibre is commonly used as the backbone to carry signals between the hubsof LAN's from where copper coaxial cable takes the data to the desktop. Fibre links to thedesktop, however, are also common. Cable TV  As mentioned above domestic cable TV networks use optical fibre because of its verylow power consumption. CCTV  Closed circuit television security systems use optical fiber because of its inherentsecurity, as well as the other advantages mentioned above. Optical Fibre Sensors Many advances have been made in recent years in the use of Optical Fibres as sensors.Gas concentration, chemical concentration, pressure, temperature, and rate of rotation canall be sensed using optical fibre. Much work in this field is being done at theUniversity of Strathclyde. Reflection and Refraction of Light When light travelling in a transparent material meets the surface of another transparentmaterial two things happen1.some of the light is reflected2.some of the light is transmitted into the secondtransparent materialThe light which is transmitted usually changes direction when it enters the secondmaterial. This bending of light is called refraction and it depends upon the fact that light  travels at one speed in one material and at a different speed in a different material. As aresult each material has its own Refractive Index which we use to help us calculate theamount of bending which takes place. Refractive index is defined as:-where n is the refractive index C  is the speed of light in a vacuum V  is the speed of light in the materialTwo possible cases exist.These are:-1.where light goes from a material with a low refractive index to onewith a high refractive index, OR 2.where light goes from a material with a high refractive index toone with a low refractive index.These two cases are shown in the diagrams below.
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