Introduction to Fibre Optic Temperature Sensors

Fibre optic sensors are a family of sensors (temperature is not the only parameter that can be measured) that employ thin glass fibres as the sole means to excite and read the sensing element. The fibre is the same as used for communications.

Characteristics of fibre optic sensors

There are several characteristics of optical fibres that allow them to be used for sensors. These include micro bending, interferometric effects, refractive index change, polarization change, fibre length change, fibre diffraction grating effects, and the Sagnac effect (light traveling in opposite directions around a loop used to sense rotation).

For fibre optic temperature sensors, the sensing element is usually deposited directly on the cleaved end of an optical fibre and temperature is deduced from the reflected phase or spectrum.

Another approach is to use a bimetallic strip (or other temperature sensitive mechanism) to bend the glass fibre sufficiently to generate a measurable anomaly. This is crudely demonstrated in the following diagram:

Fibre optic temperature sensors and their associated measuring devices are expensive and hence applied only when they have a compelling advantage for specialist applications. For this reason they are not comprehensively covered in this document. It is possible that in the future these types of sensors will become significantly cheaper and more widely deployed as optical integrated circuits become a reality.

Great for measuring temperature in difficult or dangerous places

Sometime temperatures need to be measured in places that do not allow conventional sensors to be employed. For example when measuring the temperature of the windings of a high voltage oil cooled power transformer. The voltage may be as high as 500 kV peak. Wired sensors would be a health hazard to anyone near the measuring device. However non-contact sensors cannot be used, because the transformer windings are not visible. It is situations like these that fibre optic sensors become the only option.

Links to useful articles and manufacturers

P Fuhr; "Measuring with Light", Sensors Online 2000

R Nabiev, W Yuen; "Tunable Lasers for Multichannel fibre-Optic Sensors", Sensors Online 2003

Manufacture: FISO Technologies