High-Sensitivity Low-Frequency Fabry-Perot Ultrasonic Hydrophone With Chitosan Diaphragm

Herein we demonstrate a miniature flexible optical fiber Fabry-Perot (F-P) underwater ultrasonic sensor with chitosan diaphragm coated onto the tip of a capillary with an inner diameter of $50~\mu $ m. Due to its low Young's modulus and high surface finish, the chitosan film with a thickness of ~955 nm, formed by low-speed spin-coating of a 3% solution of biotech chitosan dissolved in 2% acetic acid, enables a high sensitivity to low-frequency ultrasonic sound pressure for the developed sensor. Because of the molecular permeability of chitosan, this type of sensor contributes to effectively reducing the working point shift of the F-P sensor caused by static pressure in underwater ultrasonic measurement. Then the ultrasonic test in a water tank offered a high sound pressure sensitivity of 52.6 mV/kPa in the tested ultrasonic range of 100 kHz to 800 kHz with a noise-limited minimum detectable pressure level of ~2.37 mPa/Hz ^1/2 . Also, the directivity measurement showed the maximum signal deviation of 2.47 dB in response to ultrasound pressure in the angle range of −90° – 90°. The proposed technique greatly simplifies the miniaturized integration of all-optical ultrasonic detectors, thus suggesting the great potential in ultra-high sensitivity hydrophone in biomedical photoacoustic measurement.