Research progress and development tendency of sapphire fiber Bragg grating-based high-temperature sensors (invited)

In-situ measurement of ultra-high temperature above 1800 ℃ is critical in many key fields of national defense security and national economy, such as hypersonic vehicles, aero engines, and nuclear reactors. Conventional silica-based fiber sensors can not withstand a high temperature of 1000 ℃, which is limited by the material characteristics of silica. A single-crystal sapphire fiber is a promising candidate for high temperature sensing owing to its ultra-high melting temperature of ~2053 ℃ and low transmission loss. Sapphire fiber Bragg grating (SFBG) sensors, which have the advantages of outstanding temperature resistance, high measurement accuracy, and capability for multi-point measurement, are the most promising high temperature sensors and can be obtained by inscribing the grating in the single-crystal sapphire fiber. In this paper, we present the working principles and theoretical model of high temperature SFBG sensors. Then, three mainstream fabrication technologies, including femtosecond laser phase mask technology, femtosecond laser Talbot interferometry, and femtosecond laser direct writing technology, are introduced. Moreover, the advantages and disadvantages of these methods are discussed in preparation efficiency and spectral properties. It is concluded that the femtosecond laser direct writing technology is the best method for fabricating high temperature SFBG sensors. Moreover, the optimization methods of the reflection spectrum of SFBGs are summarized, including how to reduce 3 dB bandwidth and spectral noise. The temperature sensing characteristics, packaging process, and high temperature and strain sensing applications of SFBGs are further introduced. Finally, the future development trend of SFBG sensors is discussed. The rapid development and large-scale application of high temperature SFBG sensors will be helpful to solve the problem of containment in structural health monitoring of major equipment in aviation, nuclear power, and other fields.