Single-mode helical sapphire fiber Bragg grating for high-temperature sensing

High-temperature sensing is in great demand in the aviation, nuclear power and petroleum industries. Single-crystal sapphire fiber is a promising candidate for the fabrication of ultra-high temperature sensor due to its high melting temperature of 2045 °C. However, sapphire fiber usually exhibits multimode operation owing to it having no cladding. We demonstrate a new method for fabricating single-mode helical Bragg grating waveguides (HBGWs) in a multimode sapphire fiber based on femtosecond laser direct writing technique. Such a helical Bragg waveguide can be obtained by using merely one fabrication step. The negative refractive index changes region works as a depressed cladding waveguide, and the periodical structure yields Bragg resonance. And hence, a single-mode HBGW created in sapphire fiber was successfully fabricated by using the proper parameters, such as a diameter of 10 μm and a single-pulse energy of 29.9 nJ, and the bandwidth of its reflection spectrum was merely 0.68 nm. Subsequently, the temperature response of the fabricated HBGW created in sapphire fiber was tested and it could withstand the high temperature of 1800 °C and its temperature sensitivity was 41.2 pm/°C.