A Narrow-Band Sapphire Fiber Bragg Grating High-Temperature Sensor Based on Decreasing-Ring Structure

Although sapphire optical fibers exhibit excellent tolerance to extremely high temperature environments, their multimode characteristics and the resulting fluctuations in the wide reflection spectrum limit their sensing applications. To address this issue, we propose and demonstrate a novel sapphire optical fiber high-temperature sensor that utilizes a decreasing-ring grating structure fabricated through a femtosecond laser direct writing process. This design effectively suppresses the higher-order modes of the sapphire optical fiber. By employing arc discharge technology to create the fusion points between sapphire fibers and single-mode fibers, we generate a single, narrow reflection peak with a full width at half maximum (FWHM) of approximately 0.27 nm at 1530.67 nm, measured at room temperature (approximately 24°C). This sensor exhibits high temperature sensitivity and can maintain stable operation above 1500°C in harsh high-temperature environments. Due to its exceptional temperature tolerance, narrow bandwidth, and resistance to electromagnetic interference, this sensor holds significant potential for precise temperature sensing in extremely challenging conditions.