Modelling strain dependence of fluorescence from doped optical fibres: application to neodymium

At the present time there is much interest in sensors for the simultaneous measurement of temperature and strain. To engineer strain or temperature sensors, based on the fluorescence lifetime or fluorescence intensity ratio techniques, with desirable characteristics requires detailed understanding of the physical origin of the strain dependency of these fluorescence effects. It has been suggested that the cause is slight shifts in the energy levels, since some of the important levels in rare-earth-doped crystals shift when subjected to considerable pressure. However, recent theoretical work analysing the two techniques, and outlined in the next section, does not support this idea. Instead, in that work, it was proposed that this sensitivity is due to a volumetric distortion of the energy transfer rates between the dopant ions. In this model an applied strain results in a slight decrease in concentration. To explore this latter idea, strain and temperature dependencies of Nd/sup 3+/-doped optical fibres of various concentrations have been analysed using this model. Existing data have been supplemented by new measurements, giving the sensitivities for the two techniques at a number of dopant concentrations.