Research On Quasi-distributed Optical Fiber Vibration Sensing Based On Acousto-optic Modulation

Interferometric fiber optic sensors have the advantages of anti-electromagnetic interference,corrosion resistance,high-sensitivity detection,small size and easy integration,and intrinsically passive probes,which make them more and more important in the fields of underwater acoustic detection and early warning of earthquakes and tsunamis.In the vibration sensing system,combined with optical multiplexing technology,a large-scale interferometric sensor array can be formed,and the quasi-distributed sensor structure can be realized.At the same time,the pulse signal generated based on the acousto-optic modulator has a high extinction ratio,which helps to improve the overall performance of the quasi-distributed system.In addition,the phase demodulation methods of the interference signal also play a key role in the detection process.Therefore,using the quasi-distributed interferometric optical fiber sensing technology,the recovery and detection of the vibration signal can be achieved by analyzing the phase information of the interference signal.A series of research and analysis on fiber-optic interference type vibration sensing system are carried out in this article,and the main contents are as follows:(1)In order to solve the phenomenon of random phase fading in the interferometric sensor system,two techniques of phase generation carrier homodyne detection and heterodyne detection are studied respectively.And in view of the influence of modulation depth drift in the phase generation carrier technology,an improved algorithm with better performance is proposed.Compared with traditional demodulation algorithms,the improved algorithm uses differential cross division to eliminate the influence of modulation depth on system performance during the demodulation process.The algorithm has been verified through related simulation work,and the simulation results show that when the modulation depth changes within the range of 0.5-3.5rad,the demodulation performance of the improved algorithm always remains stable.At the same time,comparing homodyne detection and heterodyne detection in terms of spectrum structure and dynamic range influencing factors,the results show that heterodyne detection has obvious advantages in large signal detection and reduction of low-frequency noise.(2)On the basis of analyzing the performance of the acousto-optic modulation signal,combining time division multiplexing technology with interferometer structure,a quasi-distributed vibration sensing structure based on heterodyne detection is proposed.On the basis of the above,the acousto-optic modulated pulse signal timing and sensor primitives are simulated respectively,and the key parameters of the system such as the coupler splitting ratio,system loss and crosstalk are analyzed.In addition,the circuit of the photodetection part of the system is designed.The circuit structure can not only convert the photocurrent signal into voltage signal,but also realize the function of signal amplification.And the simulation results show that the bandwidth of the photodetection peripheral circuit can reach 30 MHz,and the gain can reach 120 d B.The circuit board of the photodetection module was fabricated,and then the sinusoidal signal was modulated onto the optical carrier and detected.The experimental results show that the circuit can realize the functions of photoelectric conversion and signal amplification.Finally,the link of the vibration sensing system is designed and built,and the detection of 100 Hz and 1KHz vibration signals are realized respectively,which has certain reference significance for the application and development of the vibration sensor array.