Sensitivity-Enhanced Fiber-Optic Fabry–Perot Ultrasonic Sensor Based on Direct Laser Writing of Dual-Resonant Cavity

We report a dual-resonant cavity-based fiber-optic Fabry-Perot (FP) interferometer integrated on an optical fiber tip for ultrasound measurement, that is immediately printed through the utilization of two-photon polymerization technology employing direct laser writing. One of the resonant cavities is utilized for acoustic wave pressure amplification, while the other FP cavity is functionalized as an optical resonator. The combination of the two cavities can significantly enhance the vibration of the diaphragm, which can be used for the detection of weak acoustic waves. To improve its responsivity to acoustic waves, the designed structure is optimized by adjusting the parameters thickness of the upper diaphragm and the distance between the double diaphragms. The results show that the fabricated device can achieve low noise equivalent acoustic signal level of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$26.9~\mu $ </tex-math></inline-formula>Pa/Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\mathrm {1/2}}$ </tex-math></inline-formula> at 100 kHz. This value is much higher than that of FP devices with single uniform diaphragm. Such miniature sensor has broad prospects for development applicable to fields of structural health monitoring, photoacoustic imaging, and underwater acoustic sensing.