Two-Photon 3D Printing Hexagon-Beam-Supported Diaphragm Type Fiber Fabry–Pérot Interferometer for Ultrasonic Detection

Fiber Fabry–Pérot interferometers (FPIs) fabricated via two-photon polymerization with submicron precision enable tunable resonant frequencies through customizable cavity designs. Their monolithic architecture and intrinsic immunity to electromagnetic interference support broadband ultrasonic sensing applications, ranging from structural health monitoring to high-frequency material diagnostics. This study presents the design and experimental validation of an FPI featuring a hexagon-beam-supported circular diaphragm structure. Guided by finite element modeling (FEM), the structure was successfully fabricated at the tip of an optical fiber using two-photon 3D printing. Experimental results show that the sensor achieves an acoustic pressure sensitivity of 298.73 mV/kPa at a resonant frequency of 200 kHz, a sound pressure level (SPL) of 51.66 dB at 200 kHz, a noise-equivalent pressure (NEP) of 11.25 Pa, and a -6 dB bandwidth of 718 kHz. The structural configuration significantly enhances the sensor's output characteristics, indicating strong potential for elastic solid density measurements.