Temperature sensing apparatus

Light is passed into an optical fibre 3 which is doped with holmium to make its light absorbancy highly dependent on temperature. As an alternative to holmium, other materials having a doublet ground state can be used. Another possibility is to use any material having energy states separated by 2.00 cm or less and having an absorption line in the range 700 nm to 1500 nm. Instead of being doped into the optical fibre, the holmium may be located between the abutting ends of two or more optical fibres. Temperature at two or more locations (e.g. 5, 6) may be sensed using detection of backscatterred light from a pulsed light source 1.