Fiber Optic Temperature Sensors

About the fiber optic temperature sensors

Fiber optic cables can be used as temperature sensors in various ways, although special attention has to be paid to providing a suitable protective coating when high temperatures are measured. Two out of many examples of fiber optic temperature sensors are the fiber optic cross-talk sensor and special types of fiber where the temperature-sensitive refractive indices of the core and cladding are almost equal. Further examples are the phase-modulating fiber optic temperature sensor, and forms of monomode fiber which modulate the polarization of the light transmitted according to the temperature.

One extremely accurate form of extrinsic fiber optic sensor is a device known as the Accufibre temperature sensor. This is a form of radiation pyrometer which has a black-box cavity at the focal point of the lens system. A fiber optic cable is used to transmit radiation from the black-box cavity to a spectrometric device which computes the temperature. This has a measurement range of 500C to 2000C, a resolution of 10-5C and an inaccuracy of only 0.0025% of full scale.

Fiber optic cables are also frequently used to transmit light into standard radiation pyrometers from a remote targeting lens. The cables can be used in all types of radiation pyrometer, including the two-color version. However, it is not possible to use fiber optics for measuring very low temperatures because of the attenuation along the fiber of the very small radiation levels which exist at low temperatures. The minimum temperature which can be measured is about 50C, and the light guide for this must not exceed 600 mm in length. At temperatures exceeding 1000C, lengths of fiber up to 20 m can be successfully used as a light guide.

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