High-order fiber Bragg grating fabricated by femtosecond laser pulses for high-sensitivity temperature and strain sensing

Abstract

We have extensively investigated the characteristics of temperature and strain sensing for two high order (the 3rd and 4th order) fiber Bragg Gratings (FBGs). The FBGs were manufactured in a single mode fiber (SMF-28) without hydrogen-loading utilizing femtosecond laser based on a point-by-point direct writing method. The annealing process was studied under the response of temperature up to 900℃. With regarding to the 3rd-order gratings, the experimental sensitivity of temperature (T-sensitivity) was 15 pm/℃ in the range of 25–900N℃ and that of longitudinal strain (S-sensitivity) was 3.7 pm/ ℃ in the range of 0–1824 με. For the 4th-order gratings, T-sensitivity was 13 pm/℃ in the range of 22−900 ℃ and S-sensitivity was 1.1 pm/με in the range of 0–1824 με, the experimental value of which were lower than that of the 3rd-order FBG. During the isothermal annealing process, we achieved that the peak reflectivity of 98 % (-17.3 dB transmission loss) with stability in high-temperature at Bragg wavelength of 1548.5 nm for the 3rd-order gratings and the peak reflectivity of 93 % (-11.6 dB transmission loss) at Bragg wavelength of 1411.3 nm for the 4rd-order gratings. It indicated that the sensitivity of the FBGs depends on the mode order. The proposed FBGs have potential for high-temperature sensing and high-resolution strain sensing. The multi-parametric detection and hazardous environment monitor based on the FBGs can be accomplished in the near future.