A good example of materials developed for use in the telecomms industry are the rare earth-doped glasses. The rare earth (Re) elements, or lanthanides, comprise numbers 57 (lanthanum) to 71 (lutetium) in the periodic table and although some, such as samarium and neodymium, are used in high intensity magnets and others including erbium and ytterbium are used in optoelectronics, the majority are not widely known.

Many research groups are studying Re-doped and co-doped optical fibres for temperature sensing applications. For example, sapphire fibres co-doped with erbium (Er) and ytterbium (Yb) have been shown to exhibit temperature-dependent red, blue and green upconverted emissions when pumped at 965nm and have been studied at temperatures from ambient to 1,000K. Upconversion is the visible wavelength luminescence that occurs in certain materials, particularly those containing Re elements, when excited at NIR (near infra-red) wavelengths. The ratio of the integrated upconversion intensities varies as a function of temperature and is approximately linear for the ⁴F _9/2/ ⁴S _3/2 transitions over the range 300-100K.

A different approach is based on measuring the temperature dependence of the ratio of the intensity of two closely-spaced fluorescence peaks. Research at the Australian Victoria University have studied silica and YAG (yttrium aluminium garnet) fibres doped with neodymium (Nd). The silica fibres have excellent sensitivities, with the fluorescence ratios falling from ten at 300 C to two at 50 C. A more widely studied approach involves measuring the temperature dependence of the fluorescent decay time in optical fibres,...