Femtosecond-laser-inscribed large-scale fiber Bragg grating arrays for distributed sensing in harsh environments

Ultra-weak fiber Bragg grating (UWFBG) arrays are key elements for constructing large-scale sensing networks for structural health monitoring. Conventional methods for creating UWFBG arrays rely on in-line UV exposure during fiber drawing. However, these UV-induced UWFBG arrays cannot withstand a high temperature above 450 °C. Here, we report a method for fabricating high-temperature-resistant UWFBG arrays using femtosecond laser direct writing technology. The influences of grating length, offset distance, and grating order on the UWFBGs were systematically investigated, and various UWFBGs with low peak reflectivity of less than -40 dB were successfully fabricated. After optimizing these parameters, two UWFBG arrays, consisting of 7000 and 72 UWFBGs, were successfully fabricated in single-mode silica fiber and multimode sapphire fiber, respectively. Additionally, a series/parallel UWFBG array with 140 UWFBGs was inscribed in a seven-core fiber (SCF). A distributed high-temperature sensing (DTS) up to 1000 °C was demonstrated by using optical frequency domain reflectometry and the fabricated UWFBG array. In addition, the UWFBG array was also used in distributed acoustic sensing (DAS) system to improve its performance. Moreover, 2D and 3D shape sensing were achieved with maximum reconstruction errors per unit length of 1.89% and 2.72% by using SCF UWFBG array. The proposed UWFBG array play an important role in promoting distributed sensing, especially in extreme environments.