Etched single-crystal sapphire fiber Bragg gratings for simultaneous temperature and strain sensing at 1500℃

Sapphire fiber Bragg grating (SFBG) is promising for temperature and strain sensing in high temperature environments. An SFBG sensor has a cross-sensitivity of the strain and temperature, which significantly hampers its sensing performance. Here, we propose and demonstrate a compact sensing device for simultaneous temperature and strain measurements, which consists of two cascaded SFBGs with different fiber diameters. The SFBGs were inscribed with a femtosecond laser line-by-line scanning technique in a single sapphire fiber, in which a section was wet etched by using hot acid. The strain sensitivity of the etched SFBGs was measured. The strain sensitivity of the cascaded SFBGs is inversely proportional to its cross-sectional area of the fiber. The regulation of strain sensitivity is also related to the length scale of each part in etched sapphire fiber. Furthermore, a temperature and strain experiment of the etched SFBGs were performed and the strain sensitivities were 4.2 pm/με and 0.9 pm/με at 1500 °C.