There are many applications where monitoring of physical quantities such as strain, acoustic pressure, vibration or temperature over large distances would be beneficial, for example condition monitoring along railway lines, pipelines or subsea cables. An array of single point sensors is often costly and impractical due to amount of cabling required, so ideally a sensor in the form of a long cable continuously monitoring the spatial distribution of a disturbance would be a desirable solution. Optical fibres are excellent candidates due to the low cost of the fibre; moreover, the opto-electronic technology is well established due to use of optical fibres in telecommunications.
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