Sensitivity-Enhanced Fiber-Optic Sensor Based on a Drilled PDMS Diaphragm for Ultrasound Imaging of Seismic Physical Model

An optical fiber Fabry–Perot interferometer (FPI) ultrasound sensor, based on a polydimethylsiloxane (PDMS) diaphragm with dual circular holes, is proposed and experimentally demonstrated. Two holes with central axis symmetry were drilled in the $20 \mu \text{m}$ -thick PDMS diaphragm using femtosecond micromachining. The dual holes eliminated the pressure difference between the two interfaces of the diaphragm, allowed the strain of the diaphragm vibration to concentrate around the center of the diaphragm, and increased the ultrasound sensing sensitivity. The center distance of dual holes was optimized to enhance high ultrasound response. The ultrasound response peak-to-peak voltage (PPV) of the PDMS diaphragm with a dual-holes center distance of $45.0 \mu \text{m}$ increased by 35% compared to that of the PDMS film without drilling. The FPI sensor was packaged using an acoustic plano-concave lens to facilitate the acoustic-to-sensor coupling efficiency and realize underwater ultrasound imaging. Experimental results showed that the proposed sensor realized the ultrasound imaging of an uplifted multilayer physical model.