Thermal Properties of Fluoride Fibre Bragg Gratings at High to Cryogenic Temperatures

The thermal sensitivity of fibre Bragg gratings (FBGs) is extensively employed in diverse industrial and scientific applications. Notably, it lies at the core of flexible, low-cost and highly precise sensors, featuring stability in harsh environments and distributed sensing capability. This study assesses the thermal properties of FBGs in fluoride fibres within a temperature range of 4—373 K. Despite having higher thermal expansion coefficients, these fibres do not exhibit significantly higher sensitivity at room or higher temperatures. Still, the pronounced dependence of their thermal sensitivity on the Bragg wavelength in the light of the extended transmission range up to 4—5.5 µm shows the potential for sensing applications. Most importantly, employing FBGs inscribed in fluoride fibres enables the further expansion of fibre-based sensing systems to cryogenic environments, as they exhibit a sensitivity of 1—1.35 pm/K in a temperature range of 4—50 K. Moreover, exposure to low temperatures provides valuable information on glass stability and physical parameters, which is beneficial for further development of photonic systems based on fluoride fibres.