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Fibre Optic Thermometers Properties of Optical Fibres Dielectric opticalfibre, which is also referred to as a light guide, is composed of a rod core surrounded by a sheath as shown in Figure . The core, which conducts the electromagnetic wave, has a refractive index, nj , while the sheath, which contains the wave within the core, has a refractive index, n2 . To ensure that the wave is contained in the core it is necessary that the condition nj n2 is satisfied. The electromagnetic waves incident on the front part of the optical fibre, within the angle cone, 2a,. | Temperature Measurement Second Edition L. Michalski K. Eckersdorf J. Kucharski J. McGhee Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-86779-9 Hardback 0-470-84613-5 Electronic 6 Fibre Optic Thermometers Properties of Optical Fibres Dielectric optical fibre which is also referred to as a light guide is composed of a rod core surrounded by a sheath as shown in Figure . The core which conducts the electromagnetic wave has a refractive index ri while the sheath which contains the wave within the core has a refractive index n2 To ensure that the wave is contained in the core it is necessary that the condition tij n2 is satisfied. The electromagnetic waves incident on the front part of the optical fibre within the angle cone 2a corresponding to the critical angle 0cr enter the fibre core. After multiple reflections from the core-sheath interface these waves leave the other end of the fibre. The waves from outside the 2 -cone leave the sheath to environment. Smolinski 1985 has shown that the value of critical angle 0cr and of a are given by cos0cr n2 i sin a - n2 where j is the refractive index of the core material and n2 is the refractive index of the sheath material. In addition to the condition n I n2 0 optical fibre materials whose core material has a low absorption coefficient at the transmitted wavelength should be chosen. Optical fibres are used for wavelengths in the range from ultraviolet through visible up to infrared radiation. The most commonly used optical fibres have a core and sheath made of quartz CORE SHEATH PROTECTIVE TUBE Figure Propagation of electromagnetic waves in optical fibre 126 FIBRE OPTIC THERMOMETERS glass with different chemical composition. Usually sheath materials are made entirely of plastics. High temperature optical fibres have cores made of quartz and sapphire. Some optical fibres are covered by layers which reduce optical losses and provide good mechanical strength for protective purposes. In Table which gives

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