Single-mode helical sapphire fiber Bragg grating sensors fabricated by femtosecond laser direct writing technology

Sapphire fiber Bragg grating (SFBG) is a potential high-temperature sensor. In this paper, we have reported on the fabrication of SFBGs using a femtosecond laser direct writing technique, including point-by-point, line-by-line and plane-by-plane. These approaches provide robust methods for producing SFBGs at various Bragg wavelengths with an acceptable reflectivity. However, the broadband reflection spectrum of SFBG results from the multimode operation, hampering the sensing performance significantly. Moreover, we have reported on a single-mode SFBG based on the helical structure. The near-field profiles of the transmission mode and the spectra have been measured, exhibiting a single-mode filed pattern with diameters of 15.4 μm × 12.4 μm, narrow bandwidth of 0.18 nm and a high reflectivity of 66.3%. In addition, the temperature sensing performance of SFBG was studied. Such a device can withstand a high temperature of 1900 °C. Its temperature uncertainty was ±2.9 °C. Hence, the proposed SFBGs are promising high-temperature sensors, that can be applied in many fields, for example, power plants, gas turbines, and hypersonic vehicles.