Research On Low-frequency Acoustic Sensing Based On Optical Fiber-membrane Composite Devices

Low-frequency acoustic sensors have broad application prospects in the fields of early warning of natural disasters,industry,geological exploration,and military.Up to now,there are two types of low-frequency acoustic sensors in the market which are electrical acoustic sensor and optical acoustic sensor.And,optical fiber acoustic sensors occupy a major position in optical acoustic sensor.Optical fiber acoustic sensors have the advantages of anti-electromagnetic interference,high sensitivity,small size,low cost,long-term stability in harsh environments and other irreplaceable advantages.In this article,two different low-frequency acoustic sensors based on fiber-membrane composite devices have been proposed to achieve the goal of high-sensitivity,high-response flatness,and miniaturized low-frequency acoustic detection.Meanwhile,a new type of sensor which can be used for temperature-torsion dual parameter measurement has been designed in order to solve the problem that the optical fiber sensor is susceptible to temperature drift and external torsion stress,.The specific research content is as follows:(1)low-frequency acoustic sensor based on single-core six-hole special micro-structured fiber-PET film composite:a new type optical fiber curvature sensor whose sensitivity was 4.462 d B/m^-1 was prepared by welding a single-core six-hole special micro-structured fiber with multi-mode fiber.Next,the curvature sensor was attached to the diameter of the PET film for acoustic detection.The sound field information was modulated to the curvature of the optical fiber by the vibration of the PET film.The experimental results showed that a flat response to sound waves in the frequency range from 50 Hz to 3000 Hz was achieved by the method of light intensity demodulation.The voltage-sound pressure sensitivity at 1000 Hz was 120 m V/Pa,the minimum detectable sound pressure was 0.34 m Pa/Hz^1/2@1000Hz,and the equivalent noise sound pressure was 41.5 d B@1000Hz.(2)low-frequency acoustic sensor based on single-mode fiber-silicon nitride film composite:Based on the silicon nitride film and optical fiber end face,a Fabry-Perot non-intrinsic acoustic sensor was designed and experimentally demonstrated.The silicon nitride thin film whose thickness was 800nm was deposited on the silicon base by the method of PECVD(plasma enhanced chemical vapor deposition),and then the silicon wafer is etched by the micromachining process to form a thin film window of which radius was 1.75mm.The theoretically calculated resonance frequency of the silicon nitride film is 655.5Hz.The experimental results showed that the acoustic sensor has a high flatness and high sensitivity response to sound waves in the frequency range from 0.1Hz to 250Hz.The range of frequency response curve fluctuation is±0.6d B.The phase-sound pressure sensitivity at 100 Hz was 23.53mrad/pa,the minimum detectable sound pressure was 6.39m Pa/Hz^1/2@100Hz,the equivalent noise sound pressure was 42.5d B@100Hz.(3)The main structure of this twist sensor is a torsion perturbation point cascaded with single mode-multimode-single mode(SMS)fiber structure.The experimental results show that the twist sensor has ability to distinguish the twist direction with sensitivities of 34.8 pm/° at the chosen dips in the range from-100°(CCW)to 140°(CW).Furthermore,the temperature response characteristic of the sensor is also researched through experiment.Temperature sensitivities of 71.8pm/? can be obtained at the same dips in the range from 20? to 75?.In addition,the sensor can eliminate the temperature cross-sensitivity effect by monitoring the resonance wavelength shift and calculating through the cross-matrix.The theoretical resolutions of twist and temperature simultaneous measurement are±0.084° and±0.56?,respectively.