Distributed acoustic sensing in harsh environments based on femtosecond laser-inscribed ultra-short fiber Bragg grating arrays

We propose and demonstrate a high-performance DAS system using ultra-short fiber Bragg grating arrays (USFBG) and a phase-sensitive optical time domain reflectometry (φ-OTDR). A USFBG with an ultra-short length of 30 μm was successfully fabricated in a single-mode fiber (SMF) by femtosecond laser point-by-point inscription, exhibiting a large full width at half maximum (FWHM) bandwidth of 24.6 nm. To the best of our knowledge, this is the largest grating bandwidth reported to date. The ultra-large bandwidth effectively avoids the mismatch between the wavelength of the system light source and the grating caused by temperature changes. Moreover, a USFBG array with 300 identical USFBGs and an interval of 5 m was fabricated along the SMF to enhance the backscattering signal and suppress fading noise. An optical pulse compression algorithm was also deployed in the heterodyne φ-OTDR system to improve the spatial resolution. Thanks to the combination of USFBG arrays and the pulse compression φ-OTDR system, a long-distance DAS with a sensing distance of 60 km, a spatial resolution of 5.9 m, and an improved strain resolution of 13.9 pɛ/√Hz was achieved. Then, long-term high-temperature annealing was carried out, and the results showed that the fabricated USFBGs can withstand a high temperature of 1000°C. A high-temperature DAS system capable of operating at up to 1000°C was also demonstrated. As such, the proposed DAS systems could be used in harsh environments, such as aerospace vehicles, nuclear plants, and oil and gas exploration.