Miniaturized GaN-Based Optical Hydrophones for Underwater Low-Frequency Acoustic Detection

In this letter, the fabrication and characterization of a miniaturized optical hydrophone for detecting underwater acoustic signals is demonstrated. A light emitter and detector are fabricated on a GaN-based device at chip scale through the monolithic integration approach. By incorporating a deformable PDMS film embedded with TiO2 nanoparticles, the sound vibration can be effectively converted into photocurrent changes. The millimeter-sized hydrophone can detect hydro-acoustic signals ranging from 0.1 Hz to 250 Hz, with an optimum sensitivity of about $2.3\times 10 ^{-14}$ A/ $\mu $ Pa at around 48 Hz and a minimum detectable pressure of about 4 Pa. It is capable of distinguishing waves originating from two distinct acoustic sources with a frequency difference as low as 0.2 Hz. This cost-effective, chip-scale design with high stability holds great potential for a variety of applications that necessitate identifying low-frequency acoustic signals in underwater environments.