Thermal properties of fluoride fiber Bragg gratings at high to cryogenic temperatures.

The thermal sensitivity of fiber Bragg gratings (FBGs) is extensively employed in diverse industrial and scientific applications. FBGs lie 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 fibers within a temperature range of 4-373 K. Despite having higher thermal expansion coefficients, FBGs in the near-IR wavelength range do not exhibit high sensitivity at room or higher temperatures. However, the pronounced enhancement of their thermal sensitivity at longer Bragg wavelengths shows the potential for sensing applications in the light of the fluoride glass extended transmission range up to 4-5.5 µm. Most importantly, employing FBGs inscribed in fluoride fibers enables the further expansion of fiber-based sensors to cryogenic environments, as they exhibit a detectable sensitivity of 0.5-1.7 pm/K below 50 K. Overall, the exposure to low temperatures provides valuable information on glass stability and physical parameters, which is beneficial for the further development of photonic systems based on fluoride fibers.