Femtosecond-Laser-Inscribed Fiber Bragg Gratings for High-Tempertature Sensing

We demonstrated the inscription of various fiber Bragg gratings (FBGs) for high-temperature sensing applications. Three different femtosecond laser inscription methods, including phase-mask method, point-by-point inscription method, and line-by-line scanning method, were proposed for inscribing FBGs. At first, we fabricated negative-index FBGs using femtosecond laser overexposure and thermal regeneration. A negative-index FBG was obtained with a reflectivity of 99.22%, an insertion loss of 0.08 dB, a blue-shift of 0.83 nm, and an operating temperature of up to 1000 °C. We also fabricated a type-II FBG in a suspended-core photonic microcell, which was post processed from a pure-silica photonic crystal fiber. These FBGs were made by using a femtosecond laser and a phase mask, and can withstand a high temperature of ~ 1100 °C. Subsequently, we fabricated FBGs with a short length of 500 m on fiber tips using femtosecond laser point-by-point inscription method. These fiber-tip FBGs can operate at 1000 °C, and were developed for the temperature measurements in high-frequency induction welding process. Moreover, we also successfully fabricated single-crystal sapphire FBGs (SFBGs) using a femtosecond laser line-by-line scanning technique. An SFBG with a reflectivity of 6.3 % was obtained and a serial array consisting of five different SFBGs was constructed. These SFBGs can withstand a high temperature of 1612 °C. Such SFBGs could be developed for promising high temperature sensors in aero engines.