Vertically Coupled Femtosecond Phase-Shifted Fiber Bragg Grating for Evaluating Ultrasonic Guided Waves at High Temperatures

The operating temperature of conventional piezoelectric sensors is limited by their Curie temperature, typically up to 70 °C, though in some cases it may reach 400 °C. Consequently, conventional piezoelectric materials are unsuitable for detecting ultrasonic waves in harsh environments. Phase-shifted fiber Bragg gratings offer high sensitivity and broad bandwidth, while femtosecond fiber Bragg gratings exhibit excellent temperature resistance up to 1200 °C. This study integrates these two sensors to develop a femtosecond phase-shift fiber Bragg grating (FSPSFBG) and employs a vertically coupled technique to attach a steel-encapsulated FSPSFBG to a plate. The coupling mechanism between the out-of-plane displacement of ultrasonic guided waves in the plate and the axial displacement of waves in the vertically coupled FSPSFBG was analyzed using theoretical and finite element simulations. The sensor's frequency response was experimentally evaluated using tone-burst signals, demonstrating a broad 3-dB bandwidth of 513 kHz. Additionally, due to vertical coupling, the sensor exhibits circumferential sensitivity, distinguishing it from direct and remote coupling methods. The sensor's performance at 300 °C and 500 °C was validated through pencil lead break and ball drop experiments, respectively. Results indicate that, despite carbonized coating, the sensor maintained good performance at 500 °C, with only a 4.26 % reduction in sensitivity. The sensitivity of the sensor decreased only 1.4% after exposing to 8-hour high temperature, demonstrating exceptional thermal stability. This study demonstrates the feasibility of using vertically coupled FSPSFBG sensors for ultrasonic detection in high-temperature environments.