Femtosecond Laser Inscribed Sapphire Fiber Bragg Grating for High Temperature and Strain Sensing

In this letter, a sapphire fiber Bragg grating (SFBG) is fabricated by a femtosecond laser line-by-line scanning method. A third-order fiber Bragg grating is obtained in a single crystal sapphire fiber with a diameter of 60 μm. Compared to the SFBG written by a point-by-point method, the SFBG written by the line-by-line method has a higher reflectivity and grating with about 15% reflectivity is achieved when the track length is about 40 μm. The temperature sensing characteristics of this SFBG from a room temperature to 1600 °C are studied, and the temperature sensitivity of SFBG is 34.96 pm/°C from 1000 °C to 1600 °C. The strain characteristics are also tested at the temperature of 26 °C, 500 °C, 1000 °C, and 1600 °C, and the strain sensitivities are 1.42, 1.42, 1.44, and 1.45 pm/με, respectively. These characteristics show that this SFBG has potential applications in a harsh environment structural health monitoring.