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Ultrasonic wave is a powerful tool for many applications, such as structural health monitoring, medical diagnosis and partial discharges (PDs) detection. The fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensor has become an ideal candidate for detecting weak ultrasonic signals due to its inherent advantages, and each time with a performance enhancement, it can bring great application potential in broadened fields. Herein, an EFPI ultrasonic sensor for PDs detection is proposed. The sensing diaphragm uses a 5-μm-thickness and beam-supported structure to improve the responsive sensitivity of the sensor at the resonant frequency. Furthermore, the ability of the sensor to detect characteristic ultrasonic signal of PDs is further enhanced by assembling a Fresnel-zone-plate (FZP)-based ultrasonic lens with the sensing probe to amplify the ultrasonic wave before it excites the sensing diaphragm. The final testing results show that the originally developed sensor owns the sensitivity of − 19.8 dB re. 1 V/Pa at resonant frequency. While, when the FZP is assembled with the probe, the sensitivity reaches to − 12.4 dB re. 1 V/Pa, and leads to a narrower frequency band, which indicates that the proposed method has a great potential to enhance the detection ability of sensor to characteristic ultrasonic wave of PDs.

Fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensors are ideal candidates for on-line partial discharges (PDs) monitoring due to their inherent advantages, such as immunity to electromagnetic interference (EMI), highly compact sensing probes, and remote signal transmission. However, up to date, the design and fabrication of high-performance sensing diaphragms still remain challenging, and most of the reported diaphragms utilize circular structures with the peripheral sidewalls completely fixed. Herein, a novel EFPI ultrasonic sensor for on-line PDs monitoring is demonstrated. The proposed sensing diaphragm combines a silicon beam-supported diaphragm and a fixed boundary ring with a thickness of 5 µm, which was optimized through the multi-objective genetic algorithm (MOGA) revealing its high design flexibility and manufactured by using the microelectromechanical systems (MEMS) processing technology on a silicon-on-insulator (SOI) wafer. Compared with the circular and beam-supported diaphragm, the developed structure exhibits a higher sensitivity. The testing results show that the developed sensor owns the sensitivity and noise-limited minimum detectable ultrasonic pressure (MDUP) of -10 dB re. 1V/Pa and 63 µPa/sqrt(Hz) at its designed resonant frequency, respectively. Finally, the sensor's ability to detect PDs is validated in a temporary built PDs experimental environment, further proving its great potential to perform the on-line PDs monitoring.