Research On Phase Demodulation And Compensation Method Based On Low Coherent Interference And AS-DCM For Optical Fiber Acoustic Vibration

Acoustic vibration signal is the carrier of information.It is of great significance to realize accurate sensing and correct demodulation of acoustic vibration signal in harsh environment by using optical fiber sensing technology.Aiming at the deterioration of the performance of optical fiber acoustic vibration sensor and demodulation system caused by environmental changes,a phase demodulation compensation method of optical fiber acoustic vibration based on low coherent interference and amplitude scaling differential cross multiplication(AS-DCM)algorithm is proposed in this paper,Birefringent crystal block is used as the main demodulation device to construct the demodulation system.This method has good ability to resist the sensor cavity length drift and optical power attenuation,and is less affected by temperature.It can realize the stable and accurate extraction of acoustic vibration signals under strong environmental interference.The main research contents include:1.Based on the principle of optical fiber Fabry-Perot acoustic vibration sensing and polarization low coherence interference,the mathematical model of low coherence interference signal including optical power attenuation coefficient,Fabry-Perot sensor cavity length and birefringent crystal optical path difference is established.The structure of demodulation system is proposed,and the effects of different device parameters on low coherence interference signal are analyzed.In addition,the problems of differential cross multiplication phase demodulation algorithm are analyzed,and the influence of external environment on the demodulation accuracy of differential cross multiplication algorithm is deduced theoretically.To solve this problem,an AS-DCM algorithm is proposed,which lays the foundation for real-time and accurate signal normalization.2.An optical fiber Fabry-Perot acoustic vibration phase demodulation compensation method based on low coherent interference and AS-DCM is proposed.The DC component of the signal is extracted in real-time by using the principle of low coherent interference,and the AC coefficient is eliminated in real-time by using ASDCM algorithm to realize high-precision and stable demodulation of acoustic vibration signal.The system parameters are designed and the influence of environmental changes from the sensor and demodulation system is analyzed.The cavity length drift of optical fiber Fabry-Perot acoustic vibration sensor,the thermal stability of birefringent crystal in demodulation system and the optical power attenuation of demodulation system are simulated.Through these works,the feasibility of optical fiber Fabry-Perot acoustic vibration phase demodulation system based on low coherent interference and AS-DCM algorithm is studied.3.The optical fiber Fabry-Perot acoustic vibration phase demodulation system based on the phase demodulation compensation method is built.The experiments on the cavity length drift of the optical fiber Fabry-Perot acoustic vibration sensor,the thermal stability of the birefringent crystal in the demodulation system,the optical power attenuation and long-time stability of the demodulation system are carried out.The experimental results show that when the cavity length offset is 4.0 μm,the maximum relative phase demodulation error is 0.694%;When the temperature rises from 25 ℃ to 45 ℃,the maximum relative demodulation error does not exceed 0.6%;When the optical power is reduced by ten times,the maximum relative demodulation error is only 0.205%;Within 8 hours of the system operation,the demodulation frequency error of 22 k Hz acoustic vibration signal is 0.023%,which verifies the stability of the compensation method and demodulation system.