Sensitivity enhancement of optical fiber vibration sensor through encapsulation of acoustic Helmholtz resonator

Optical fiber sensors should be encapsulated to prevent the possible damage in practical applications. Moreover, the encapsulation should be optimized to increase the sensitivity of the sensor as much as possible. Herein three different Helmholtz resonators are designed to encapsulate the singlemode-multimode-singlemode (SMS) fiber vibration sensor. Finite element analyses exhibit the acoustic pressure distributions in the resonators. The SMS fiber structures are fixed in the resonators with different schemes. Experiments demonstrate that the encapsulations increase the sensitivity of the sensors, which are determined by the fixing schemes, the acoustic pressure distributions, and the structures of the resonators. The combined resonator with two coaxially attached cavities renders the sensor a response range from 20 to 1000 Hz, which possesses a higher sensitivity and wider frequency response range than the sensor with a single cavity of resonator. The sensitivity of the encapsulated sensor with the combined resonator is 2.11 times of the sensitivity of the un-encapsulated sensor at 120 Hz. The SMS sensor encapsulated by the combined resonator exhibits a higher sensitivity than a commercial microphone, which indicates a favorable practicability of the Helmholtz resonator to the encapsulation of various kinds of fiber vibration sensors.