Research On Membrane-free Acoustic Sensing Based On Optical Fiber Mach-Zehnder Interferometers

The acoustic signal can carry information such as frequency and amplitude,so the acoustic detection technology has been widely used in the early warning of natural disasters such as earthquakes and volcanic eruptions,industrial control,biomedical imaging,oil and gas resources exploration,national defense and military,promoting the technological innovation.Compared with the traditional electronic acoustic sensors,optical fiber acoustic sensors can effectively solve the disadvantages of the traditional type,such as large size,low response sensitivity,electromagnetic interference,less suitable for high temperature and strong corrosion.In this thesis,a membrane-free acoustic sensing method is developed.The sensing principle relies on direct detection of sound-pressure-induced changes of the air refractive index in the open cavity,which can solve the problem that the sensitivity,frequency response and dynamic response range of traditional optical fiber acoustic sensors are mostly dependent on indirect coupling materials between the acoustic and optical signals.The membrane-free acoustic sensor based on optical fiber Mach-Zehnder interferometer is studied,then the integrated open cavity and sensitivity enhancement are investigated,and these sensors enable an inherently flat frequency response over a broad bandwidth.A Mach-Zehnder interferometer with a pair of collimators inserted into the sensing arm is used to verify the sensing mechanism.Simulation and experiment are carried out to verify and demonstrate the idea,and the results show that the sensing mechanism is independent of the frequency of sound source,and only linearly depends on the sound pressure of the external acoustic signal.Next,an open cavity fabricated by 3D printing technology is aimed to optimize the membrane-free MZI sensor.The open cavity is on the MZI sensing arm through a sleeve,which improves the miniaturization and integrity of the sensor.In order to further improve the air refractive index modulation efficiency of such open cavity structure,a reflective acoustic focusing device is made,which can integrate the structure,and gather the external acoustic signals to increase the air refractive index modulation.Using finite element method,the effect of different parameters of the device on the acoustic convergence are studied.The simulation results show that the device can effectively improve the acoustic sensitivity of the membrane-free acoustic sensor.