Single-Mode Helical Sapphire Bragg Grating for High-Temperature Sensing

Sapphire fiber Bragg grating (SFBG) is a potential high-temperature sensor, which can withstand 1900 °C. However, the broadband reflection spectrum of SFBG results from the multimode operation, hampering the sensing performance significantly. In this paper, we have reported on a single-mode SFBG based on the helical structure created by using a femtosecond laser direct writing technique. We found that the ring-shaped inscription pattern inscribed in sapphire fiber is irregular due to the focal-point distortion induced by the cylindrical geometry of the sapphire fiber. A slit beam shaping method was employed to solve this problem. By using the optimized slit width of 0.35 mm, the regular ring patterns can be inscribed within the sapphire fiber successfully. Such a structure has the maximum negative refractive index change of −8.8 × 10⁻³ and a width of 7.7 μm, which can serve as depressed cladding waveguide. Moreover, helical structures with various diameters have been created in sapphire fibers. The near-field profiles of the transmission mode and the spectra have been measured. The experimental results show that a single-mode helical SFBG with a diameter of 14 μm and a period of 1.78 μm can be achieved, exhibiting a narrow bandwidth of 0.18 nm and a high reflectivity of 66.3%. In addition, the temperature sensing performance of a single-mode SFBG was studied. Such a device shows increasing temperature sensitivity at elevated temperatures, i.e., 22.5 pm/°C at 20 °C, 27.5 pm/°C at 600 °C and 33.9 pm/°C at 1200 °C, which is similar to the conventional multimode SFBG. However, its temperature response has much better repeatability than that of the multimode SFBG, benefiting from the single-mode transmission with high stability. Hence, the proposed single-mode SFBG is a promising high-temperature sensor, that can be applied in many fields, for example, power plants, gas turbines, and hypersonic vehicles.