Stabilized Ultra-High-Temperature Sensors Based on Inert Gas-Sealed Sapphire Fiber Bragg Gratings

In situ measurement of high temperature is critical in aerospace, petrochemical, metallurgical, and power industries. The single-crystal sapphire fiber is a promising material for high-temperature measurement owing to its high melting point of ∼2045 °C. Sapphire fiber Bragg gratings (SFBGs), which could be inscribed in sapphire fibers with a femtosecond laser, are widely used as high-temperature sensors. However, conventional SFBGs typically exhibit a significant deterioration in their spectra after long-term operation at ultra-high temperatures, resulting from the formation of some unwanted microstructural features, that is, lossy spots and micro-etched lines, on the surface of the sapphire fiber. Here, we report for the first time, to the best of our knowledge, a thermally stabilized ultra-high-temperature sensor based on an SFBG created by femtosecond laser inscription, inert gas-sealed packaging, and gradient temperature-elevated annealing. The results indicate that the lossy spots are essentially aluminum hydroxide induced by high-temperature oxidation, and the inert gas-sealed packaging process can effectively insulate the sapphire fiber from the ambient air. Moreover, the formation of micro-etched lines was suppressed successfully by using the gradient temperature-elevated annealing process. As a result, the surface topography of the SFBG after operating at high temperatures was improved obviously. The long-term thermal stability of such an SFBG was greatly enhanced, showing a stable operation at 1600 °C for up to 20 h. In addition, it could withstand an even higher temperature of 1800 °C with a sensitivity of 41.2 pm/°C. The aforementioned results make it promising for high-temperature sensing in chemical, aviation, smelting, and power industries.