Sensitivity investigation of new fiber optic read-out system for guided waves based on Pi Shifted Fiber Bragg Gratings and Mode Locked Lasers

Ultrasonic monitoring of large structures can be used to optimize maintenance scheduling and predict their remaining life-time, particularly for difficult to reach structures such as windmills. Often, piezoelectric systems are used, but these could be difficult to install and require local pre-amplification to deal with the signal loss due to long cable lengths. Furthermore, they are vulnerable to lightning strikes which is significant for offshore wind farms. Fiber Bragg Grating (FBG) based sensors could be a more efficient alternative since an array of multiple FBG sensors can be realized in a single optical fiber. Moreover they are easy to mount and insensitive to electromagnetic interference. However, for guided wave inspection of composite structures traditional FBG-based systems show poor sensitivity compared to piezoelectric transducers. Here, we investigate the sensitivity of a newly developed Fiber Optic Photonic Ultrasound Transducers (FOPUT) system for guided waves measurement based on the interferometric read-out of Pi Shifted Fiber Bragg Gratings (PS-FBGs) in combination with a Mode Locked Laser (MLL) light source. The improvement of three different PS-FBGs with notch widths of 1, 3, and 25 pm compared with a standard FBG is also shown.