Design of resonant acoustic sensors using fiber Bragg gratings

Fiber Bragg gratings installed in structures have been used as acoustic sensors by using appropriate demodulation techniques capable of high-sensitivity detection of high-frequency waves. However, current sensor designs generate only non-resonant responses. Here we introduce two sensor configurations that enable novel resonant acoustic sensing using the fiber Bragg grating. The first sensor head is configured with one end free and one end fixed, and the second sensor head is configured with both ends fixed. Since the grating parts of the resonant sensors are not bonded to the structure, they cannot directly detect acoustic waves guided in the structure. Instead, the acoustic wave is converted into a fiber-guided acoustic wave (FAW) which is then detected by the fiber Bragg grating. In these configurations, the sensors resonate depending on the unbonded fiber lengths which become resonators of the standing FAWs. We compared the waveform characteristics of these new resonant sensors with those of non-resonant sensors in terms of repeatability and resonance spectrum bandwidth. Finally, we present methods for controlling the resonance frequency both experimentally and theoretically.