Research On Key Technologies Of High Sensitivity Distributed Optical Fiber Acoustic Sensing

Due to the advantages of long sensing distance and low cost of laiding,the distributed fiber optic acoustic sensing system has widely used in the fields of railway detection,transportation pipeline monitoring,boundary prevention and control,and invasion testing.It is a research hotspot today.Phase-Sensitive Optical Time Domain Reflectometry((37)-OTDR)is an important branch in the field of distributed fiber sensing technology.It has the advantages of rapid response and high sensing sensitivity.In the optical fiber acoustic sensor system.Based on the(37)-OTDR system,this article aims at improving the detection sensitivity of the acoustic signal.Based on the relationship between the rear transformation of the loose light phase and external disturbance of the(37)-OTDR system,strives to improve the signal to noise ratio of the system.Established two types of detection systems based on phase-based demodulation,which can solve the external acoustic signals.Finally,the algorithm is optimized to optimize the acoustic signal,so that the signal’s signal to noise ratio is improved.The main work content of this article is as follows:1.The mechanism of Rayleigh scattering and external disturbance in the optical fiber is Introduced,and the sensor principle of the(37)-OTDR system is explained.Then the important performance indicators of(37)-OTDR is introduced,and finally the decline of the signal of the performance of the(37)-OTDR system is introduced.2.Based on the theory of the scattering unit,the backscattered model of the(37)-OTDR is established,and the linear relationship between the differential phase and the disturbance is analyzed.The I/Q orthogonal adjustment algorithm used for phase demodulation was discussed,and the principles of the external differential detection system and the bipolar external detection system were introduced.Using the software for numerical simulation,verifying the ability of these two demodulation schemes to disturb signal demodulation.3.Conducting heterodyne coherent detection and double-pulse heterodyne detection experiments,and two experimental systems for heterodyne detection are built,applying sine wave disturbance to the optical fiber and detect the backscattered optical signal,and then use the phase demodulation algorithm to demodulate Out of the sine wave signal,the power spectral density of the acoustic wave signal of the two systems finally reflects that the ratio of the amplitude of the signal to the noise is 9.9d B and 14.3d B,respectively.Then,the signal of knocking on the table is used to represent the external disturbance,and the signal is demodulated at 10 km,20km,25 km and 30 km optical fibers.Compared with the currently widely used intensity demodulation system,the signal to noise ratio is significantly improved.4.For the noise existing in the acoustic signal,the original signal of the collected backscattered light is first preprocessed,and the influence of the noise in the signal is reduced before the demodulation phase demodulation.Then use the wavelet noise reduction and EMD algorithm to reduce the noise of the acoustic signal collected at 30 km,so that the signal to noise ratio of the acoustic signal is effectively improved,and the signal to noise ratio of the signal is increased by 4.3d B and 2.9d B respectively,making the acoustic signal easier be discerned.