Research On Signal Demodulation Method Of Diaphragm Type Fiber Optic Acoustic Wave Sensor

As a dual-use technology with wide demand,acoustic wave detection technology has important applications in aerospace,national defense,industrial measurement,biomedical detection,environmental monitoring and other fields.Fiber optic Fabry-Perot(F-P)acoustic wave sensor is highly favored for its high sensitivity,anti-electromagnetic interference,corrosion resistance,good electrical insulation,simple structure,small size,and light weight.However,for the practical application of fiber optic F-P acoustic wave sensor,how to solve the problem of unstable demodulated signal due to fluctuation of light source and external environmental changes is the key of research.In addition,the signal-to-noise ratio is an important parameter of the sensor,and the research of fiber optic acoustic wave sensor with high signal-to-noise ratio is of great scientific value for the application in the field of weak acoustic signal detection.The most commonly used signal demodulation method is intensity demodulation.Although the traditional interference-intensity demodulation method has a simple structure,fast response speed and high demodulation sensitivity,the disturbance of the light source and the interference of the external environment will affect its demodulation stability,and the problem of operating point stability has been the difficulty of this demodulation technology.In this thesis,we propose a full-spectrum demodulation fiber optic F-P acoustic wave sensing system and signal demodulation algorithm for the commonly used intensity demodulation method,which is susceptible to light source perturbation and external interference leading to demodulation instability.The system utilizes a Distributed Feedback Laser(DFB)to perform a fast full-spectrum scan of the fiber optic F-P acoustic wave sensor.To achieve demodulation of the acoustic wave signal,the transient cavity length of the sensor modulated by the acoustic wave signal is demodulated by an algorithm based on a modified Fourier transform.This demodulation method belongs to wavelength demodulation,the demodulation result is not affected by the fluctuation of light source,and fundamentally solves the technical problem of working point stability in intensity demodulation.The specific work of this thesis is as follows:1.A full-spectrum demodulated fiber-optic F-P acoustic sensing system is proposed,and the general framework of the system is designed,and the selection and related designs are made regarding the instruments and functional modules used in the system.The sensitive diaphragm of the sensor is simulated and analyzed,the design and fabrication of the diaphragm type fiber optic F-P sensing probe is optimized by studying the relationship between the radius and thickness of the sensitive diaphragm and the intrinsic frequency as well as the sensitivity.2.In the research of fiber optic acoustic wave sensing system,the demodulation algorithm of sensor cavity length is optimized,and the phase demodulation step is added to the original Fourier transform demodulation and spectral interpolation to improve the demodulation accuracy of the demodulation algorithm,and the demodulation software of the system is developed based on Lab VIEW software development platform.3.An optical fiber F-P acoustic wave sensing test system was built.The stability and reliability of the signal demodulation of this system were verified by experiments.The experimental results show that the system has good time domain response at 500 Hz,1000 Hz,2000 Hz and 5000 Hz,and the demodulation sensitivity is about 11.39 m V/Pa.The signal-to-noise ratio of the system is 77.36 d B at the sound signal frequency of 1000 Hz,and has a good time stability.