Research On A Dual-Sagnac-Structure Distributed Fiber Optics Disturbance Sensing System And Its Location Technology

Distributed optical fiber disturbance sensing(DOFDS)technology,which utilizes the optical fiber itself as both transmission medium and sensing unit,can continuously measure external disturbances on the optical fiber transmission path by using a single optical fiber.Due to the characteristics of long sensing distance,wide monitoring range,high sensitivity,and good real-time performance,it has a great promise in applications of power cable pipelines,oil and gas pipelines,railway and bridge construction,perimeter security,and so on.In the field of DOFDS,the c interference-based technology shows the unique technical advantages,such as simple structure,high sensitivity,low cost,and strong anti-interference ability.On the basis of advantages of traditional Sagnac-structure technology,we propose a novel distributed optical fiber disturbance sensing system based on dual-Sagnac structure,and also propose a new location algorithm named “spectrum peak ratio” for proposed sensor system,which provide a practical solution to the key technical problems of “polarization-induced fading”,“multi-point positioning”,and “multi-parameter detection”.The main contents are as follows:1?Based on a brief analysis of various types of DOFDS and known the characteristics and disadvantages of Sagnac-structure DOFDS technology,we propose a novel DOFDS system,which use a single source with single wavelength and utilizes two polarization modes in the polarization maintaining fiber to form a dual-Sagnac-structure interferometer system,and study the system composition and positioning principles.At the same time,the Jones matrix model of the proposed dual-Sagnac-structure DOFDS system is established,and the working principle of the system is analyzed in detail,which proves that the proposed sensor structure has strong "anti-polarization fading" characteristics.2?Based on the analysis of the common phase demodulation technology and the structure characteristics of proposed sensor structure,the application of the symmetrical demodulation method based on 3 × 3 coupler in the proposed sensor system is mainly studied.The phase noise during demodulation is studied,and the severe impact of phase noise on traditional time-domain-based positioning algorithms is analyzed.In order to improve the positioning accuracy,a new location algorithm named “spectrum peak ratio” based on frequency domain is proposed,and the theoretical method of multipoint positioning is derived and established,which provides a new and effective way for solving the problem of multi-point location.3?The theory and method of detecting the amplitude of disturbance based on the "spectrum-to-peak ratio" algorithm are analyzed,and it is experimentally proven that the sensor system in this paper can achieve quantitative measurement of multiple parameters such as the position,frequency,and amplitude of the disturbance signal.Experiments show that the new dual Sagnac structure distributed optical fiber sensor system proposed in this paper has the ability to simultaneously monitor multi-point and multi-parameter information for disturbance events.