High-Frequency Ultrasonic Sensor Arrays Based on Optical Micro-ring Resonators

High-frequency ultrasonic sensors are an important sensing technology in structural health monitoring applications. Compared with the traditional PZT transducer as ultrasonic sensors, novel ultrasonic sensors based on optical methods such as micro-ring resonators have gained increased attention. These micro-rings can be as small as a few microns in diameter, which improves their sensitivity to high-frequency ultrasound. In principle, acoustic waves irradiating the micro-ring induce strain, changing the dimensions and refractive index of the waveguides via the elasto-optic effect. This leads to a change of the guided whispering gallery modes (WGMs), which are extremely sensitive to change in the ring radius induced by the ultrasound strain field. Based on our prior research, here we present an integrated high-frequency ultrasonic sensor array based on optical micro-ring resonator array fabricated by direct laser writing. The fabrication has been optimized to provide high optical quality factor to ensure high detection sensitivity. The experiments demonstrate the potential of the polymer micro-ring resonator working as a high-performance ultrasonic sensor. Applications of the integrated ultrasonic sensor array for acoustic-emission ultrasound detection are shown.