Data Acquisition And Analysis Of Optical Fiber Vibration Based On Mach-Zehnder Interference

The distributed optical fiber sensing system uses optical fiber as the sensing and transmission medium.Compared with the point and quasi-distributed optical fiber sensor,it can implement distributed measurement and extend the monitoring range.The phase modulation distributed fiber sensor,based on the Mach-Zehnder fiber interferometer,is very sensitive to the strain and vibration.That system commonly applied to monitoring occasions such as the embankments construction,the change of the tunnel's structure and the leakage of oil-gas pipelines.Compared with traditional monitoring method of manual patrol,pressure and flow,the Mach-Zehnder distributed fiber sensor system with high sensitivity,strong anti-electromagnetic interference and wide monitoring range,become the focus of leakage monitoring.The existing dual Mach-Zehnder distributed optical fiber sensing system uses the acquisition card to collect data and the host computer to demodulate vibration information.This approach is easy to implement.While in the practical application,the lack of sampling leads to the low-position accuracy and too much threads of the host computer cause poor stability.This paper deeply studies the principle of phase modulation and optical path difference positioning,and the cross-correlation time difference demodulation algorithm.Then the appropriate components are selected to build the optical system.Aiming at the problems of traditional system,the photoelectric conversion and signal conditioning system are designed to reduce the sampling error.Simulation by MATLAB proof that the digital filter and the discrete sequence cross-correlation demodulation algorithm are validated.The design of digital signal demodulation system based on FPGA is the key point of this paper.Finally the vibration waveform and information are sent to the computer for display.The performance of the system monitoring is verified by repeated vibration simulation experiment at a single point.When the sampling frequency is set to 50 MHz,the offset error of a single point is 2 meters,and the average error in actual measurement is 5.6 meters.The modified system that based on FPGA digital signal acquisition and demodulation module is improved on the positioning accuracy and stability.So it will be used in project widely.