Achieving ultralow detection limit in Fabry–Perot interferometric ultrasonic sensors with a high-finesse optical cavity

To address the pressing need for ultralow detection limits in ultrasonic detection, constructing a high-finesse optical cavity has become an effective approach to enhance the sensitivity of Extrinsic Fabry-Perot Interferometric (EFPI) acoustic sensors. This requires the simultaneous achievement of high beam collimation and high reflectivity of the Fabry-Perot (F-P) cavity. In this Letter, these two aspects are decoupled: a graded index (GRIN) lens is used to achieve beam collimation, while microelectromechanical system (MEMS) technology is employed to fabricate the high-reflectivity optical cavity. By accounting for diffraction and fabrication errors using the angular spectrum method in the design process, an EFPI acoustic sensor with an optical cavity finesse of 25 was fabricated. While maintaining a mechanical sensitivity of 26.46 nm/Pa, the sensor achieves a significantly enhanced sensitivity of 3.92V/Pa at the resonant frequency 32.7 kHz, with a minimum detectable pressure (MDP) as low as 1.03 μPa/√Hz, demonstrating leading-edge performance compared to reported EFPI sensors. By utilizing a high-finesse optical cavity to provide a further boost to the sensor sensitivity while preserving its mechanical sensitivity, the proposed method demonstrates significant application potential in areas such as partial discharge detection and industrial non-destructive testing.