Ultra-high-temperature Sensors Based on Sapphire Fiber Bragg Gratings Created with Femtosecond Laser Direct Writing Technique

We reported on a new method for fabricating sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser direct writing technique, i.e., line-by-line (LbL) scanning technique and Point-by-Point (PbP) method. At first, we fabricated a SFBG by using LbL scanning technique, and this SFBG exhibits a reflectivity of 6.3%. Moreover, we fabricated multi-layer SFBGs using this method, and the reflectivity was enhanced to 34.1 %. Furthermore, we proposed a filamentation process to enlarge the area of refractive index modulations (RIMs). An SFBG with a reflectivity of 2.3 % could be created by using PbP method, and the fabrication time only requires ~ 1.1 s. An SFBG array including five SFBGs was constructed successfully. In addition, we used an inert gas-sealed packaging method to suppress the lossy spots formed on surface of sapphire fiber after a high-temperature test. The long-term thermal stability of the SFBG was greatly enhanced. As a result, such an SFBG can be applied in many fields, including metallurgical, power, and aviation industries.