To detect guided ultrasonic waves within large structures, one important tool is a fiber Bragg grating (FBG) sensor, a type of optical fiber-based sensor. When passing through areas of damage in a structure, these waves provide information on the type and extent of damage in the structure. The sensors offer multiple advantages for use in extreme environments, including high-temperature applications or corrosive environments such as salt water. However, since their sensitivity to ultrasonic waves is actually quite low, in order to obtain accurate information on damage, it’s often necessary to amplify the measured signal.

However, researchers Kara Peters and Jee Myung Kim, professors of mechanical and aerospace engineering at North Carolina State University in Raleigh, noticed over the course of multiple experiments that the optical fiber could actually capture ultrasonic waves and transmit them. 

“This is counterintuitive, as the optical fibers are designed to transmit optical signals,” Peters said. “This means that the ultrasonic wave can be measured at any point along the optical fiber. Therefore, the [FBG] sensor does not have to be in the same environment, which can make its sensitivity to the ultrasonic wave much higher.”