High-Temperature Stability and Demodulation Techniques Analysis of Sapphire Fiber Bragg Grating Sensor

Sapphire fiber, renowned for their high melting point, mechanical robustness, and superior optical properties, are extensively utilized in high-temperature sensing applications. Sapphire fiber Bragg gratings (SFBG) fabricated from these fibers exhibit a high refractive index, a large mode field diameter, and a lack of cladding, which collectively result in significant multimode propagation. This multimodal behavior has the effect of broadening the reflection peak bandwidth, which in turn adversely impacts the precision of the measurements made by the sensor. To solve this problem, the present study employs a double-conical splicing technique to achieve quasi-single-mode SFBG, effectively suppressing the multimode effect and successfully realizing real-time demodulation up to 1,700 °C. Meanwhile, to address the impact of vibrational crosstalk and background radiation on measurement accuracy in high-temperature environments, this study introduces a PCA-KNN algorithm for demodulation, significantly improving demodulation accuracy in the high-temperature range. Experimental results demonstrate that the sensors exhibit high sensitivity (37 pm/°C) at elevated temperature, a high measurement limit (1,700 °C), and improved demodulation accuracy (2.42 °C), while successfully suppressing vibrational crosstalk and radiation interference. This research provides a novel solution for high-precision real-time monitoring and the practical application of SFBG sensors.