Grating based high-frequency ultrasonic sensors

Damage in civil, aerospace, and mechanical structures caused by crack growth and impact loading generate transient ultrasonic waves whose frequency and amplitude can reveal the underlying structural health condition. Hence, it is necessary to find a useful tool based on ultrasonic detection for structural health monitoring. Recently, smart sensors based on gratings such as fiber Bragg gratings (FBGs) have been shown to be suitable to detect such acoustic waves in structural health monitoring applications. However, the fiber-based gratings as the ultrasonic sensor has limited sensitivity to high frequency ultrasound detection due to a specific grating length and a finite spectrum width. To eliminate this limitation, one improvement has been made by using phase shift FBGs due to their special filtering characteristics. The phase shift FBGs can have a narrower spectral width, which will significantly improve the detection sensitivity. Another big improvement, for example Bragg grating waveguide (BGW) sensor, is to optimize the grating structure using different materials. In this work, we describe a 3D printed-polymer BGW sensor for ultrasound detection fabricated through a two-photon polymerization process. The design and fabrication have been optimized for high detection sensitivity. The results demonstrate the potential application of BGW devices for high-sensitivity ultrasound detection.