Research On Performance Optimization Of Grating Array Distributed Acoustic Sensing Based On Optical Pulse Coding

Vibration sensors are widely used in border security,oil and gas pipeline safety maintenance,geological exploration,underwater acoustic detection and other fields.Compared with electrical sensors,fiber optic sensors are more advantageous due to their higher sensitivity,anti-electromagnetic interference,and suitability for corrosion and other harsh environments.Among them,due to the characteristics of high sensitivity and distributed measurement,fiber distributed acoustic sensing(DAS)technology are widely used.However,with the increase of the sensing distance of the system,the reflected optical power of the system gradually decreases,resulting in a decrease in the signal-to-noise ratio of the demodulated signal,which requires high incident optical energy of the system.Enhancing the pulsed light energy of the DAS system can be achieved by increasing the pulse peak power or pulse width: when the pulse peak power increases to a certain level,the nonlinear effect of stimulated Brillouin scattering will occur in the system,which affects the system demodulation.The effect of increasing the incident light energy is limited by this method;increasing the pulse width will lead to a decrease in the spatial resolution of the system.Combining these two points,the spatial resolution and sensing distance of the long-distance DAS system are restricted,and the optical pulse coding technology can effectively solve this problem.At present,the coding DAS is mainly concentrated in the fiber-based DAS,and there are still some limitations: due to the influence of interference fading,the signal-to-noise ratio improvement performance achieved by the coding is degraded;times to dozens of times.In view of the above two problems,this paper uses the Fiber Bragg Grating(FBG)array as the sensor to study the Fiber Bragg Grating DAS encoding and decoding method,and applies the Orthogonal Frequency Division Multiplexing(OFDM)technology to The fiber grating type DAS coding system shortens the measurement time.The main work contents are as follows:(1)Research on the encoding and decoding method of fiber grating DAS.The structure,mathematical characteristics and encoding and decoding principles of Golay codes are analyzed,the Golay encoding and decoding methods based on fiber grating DAS,data resampling and signal processing methods in the decoding process are studied,and the performance of encoding systems under the influence of different factors is analyzed.(2)Research on OFDM encoding and decoding method of Fiber Bragg Grating DAS.The basic principle of OFDM technology to improve the demodulation speed of fiber grating DAS is analyzed,and the influence of pulse width,carrier frequency and pulse shape on frequency reuse is discussed.The signal-to-noise ratio gain and system response bandwidth under the method are analyzed,the influence of each factor on the decoding effect is analyzed,and the decoding signal processing flow is formulated;(3)Coding experimental research and performance analysis.According to the system requirements,the device selection and performance analysis of the components of the optical circuit are carried out,and the demodulation optical circuit is designed.Design the demodulation experiment of the OFDM coding mode,compare the demodulation results of the time division multiplexing mode to verify the improvement of the demodulation signal-to-noise ratio and the improvement of the demodulation speed;Long-distance coding experiment,testing the performance of coding system under long-distance.