Regenerated Bragg gratings in Panda fibres for simultaneous temperature and force measurements at high temperatures

Simultaneous temperature and force measurements in an extended temperature range up to 500°C were shown with a regenerated fibre Bragg grating in a polarisation maintaining fibre of type Panda. Type I gratings in Panda fibres were regenerated under application of a high temperature annealing process. During the first temperature cycle, a distinct hysteresis of the fibre’s birefringence with temperature was observed, but reproducible dependencies of the birefringence on temperature were shown for subsequent temperature cycles. The birefringence of the fibre at room temperature was nearly doubled compared to the pristine fibre, and showed linear temperature dependencies below and above 500°C, but with different sensitivities in both temperature ranges. A change in the temperature dependence of the birefringence can be explained by the crossing of the transition temperature of the stress applying parts at around 500°C. Below this temperature, measurements of temperature and applied axial forces showed reproducible and approximately linear responses of the sensor element, allowing a 2x2 matrix approach for simultaneous temperature and longitudinal load measurements to be applied in this temperature range. Above 500°C, in contrast, the stress applying parts become viscous, resulting in a viscoelastic behaviour of the fibre.