Underwater Low-Frequency Acoustic Wave Detection Based on a High-Q CaF2 Resonator

Abstract

Whispering gallery mode (WGM) resonators with an ultra-high quality (Q) factor provide a new idea for high-precision underwater acoustic sensing. However, acoustic energy loss due to watertight encapsulation has become an urgent problem for its underwater application. In order to solve this problem, this paper proposes a hollowed-out array structure. The finite element simulation shows that the acoustic wave transmission loss is improved by 30 dB compared with that of the flat plate encapsulation structure. Using a calcium fluoride (CaF2) resonator with a Q factor of 1.2 × 108 as an acoustic sensitive unit, the amplitude and frequency of the loaded acoustic wave are retrieved by means of the dispersion coupling response mechanism. The resonator’s underwater experimental test range is 100 Hz–1 kHz, its acoustic sensing sensitivity level reaches −176.3 dB re 1 V/µPa @ 300 Hz, and its minimum detectable pressure can be up to 0.87 mPa/Hz1/2, which corresponds to a noise-equivalent pressure (NEP) of up to 58 dB re 1 µPa/Hz1/2.