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In situ strain sensing at high-temperature environment is crucial in the aerospace field. Silica fibers will soften at 700 ℃, which can hardly be utilized for strain sensing at higher temperatures. Notably, single crystal sapphire fiber is a promising material for high-temperature sensing due to the high melting point (~2045 ℃). Here, we report the strain sensing at 800 ℃ of sapphire fiber Bragg gratings (SFBG) inscribed by a femtosecond laser lineby- line scanning technique. At first, a line-by-line sapphire fiber grating was inscribed using femtosecond laser direct writing technique. The sapphire fiber ends were polished into bevels to reduce Fresnel reflections, and the signal-to-noise ratio of the SFBG was improved from 9 dB to 17.2 dB. And then, strain characteristics of the SFBG were investigated at room temperature. It was found that the maximum strain of SFBG was decreased to 3600 με (64% reduction) comparing with 9714 με of the pristine sapphire fiber, which is due to the micro-damage introduced by femtosecond laser pulses. In addition, the strain sensitivity of the SFBG is 1.42 pm/με. Subsequently, a strain sensing experiment of the SFBG was carried out at 1100 ℃ using a high-temperature tensile testing system. After annealing at 1100 ℃ for 4 h to improve the high-temperature stability, the SFBG exhibited a strain sensitivity of 1.6 pm/με (R2=0.998) at 1100 ℃. As a result, strain sensing at 1100 ℃ environment was realized based on the SFBG, which indicates a promising application in the aerospace field, especially in strain sensing for structural safety monitoring of hypersonic aircraft at high-temperature.
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