Research On Performance Optimization Of Temperature And Strain Simultaneous Sensing System Based On Multi-peak BOTDR

In recent years,fully distributed optical fiber sensing technology based on Rayleigh,Raman and Brillouin scattering has been used in oil and gas pipeline monitoring,cable condition monitoring,urban infrastructure and structural health monitoring of large buildings,fire and landslide warning and It is widely used in damage monitoring of aircraft and ships.Among them,Brillouin scattering has attracted extensive attention of researchers because of its advantage that the Brillouin frequency shift is linearly related to both temperature and strain,but it also brings about the cross-sensitivity problem of simultaneous sensing of temperature and strain.In this paper,the sensitivity of different peaks to temperature and strain in the multi-peak Brillouin scattering spectrum is mainly used to construct a double-parameter matrix to solve the cross-sensitivity problem,so as to realize the simultaneous sensing of temperature and strain.Therefore,a large effective area fiber(LEAF)with a multi-fiber core layer and a refractive index rising ring that can generate multi-peak Brillouin scattering spectrum is selected for research,and the finite element analysis method is used to analyze the multi-peak Brillouin scattering spectrum and its The relationship between the sensing characteristics and the fiber structure,the Brillouin optical time domain reflectometry(BOTDR)was used to measure the Brillouin scattering signal and sensing performance in the experiment,and then the neural network was used to further optimize the design based on the simulation data.The new fiber structure with high performance,the new structure fiber maintains the multi-peak Brillouin scattering spectrum and has higher temperature and strain sensitivity.The main work of this paper is as follows:(1)Numerical simulation is used to analyze the Brillouin characteristics of the fiber structure with a multi-fiber core layer with a refractive index rising ring.Firstly,the optical mode and acoustic mode are analyzed by numerical simulation,the acousto-optic coupling coefficient and Brillouin frequency shift are calculated,and the multi-peak Brillouin gain spectrum is drawn.The Brillouin frequency shifts of the three sound fields obtained by simulation are 11.10GHz,11.19GHz,11.35GHz,compare the characteristics of the optical fiber when the core is layered,non-layered,with a refractive index rising ring,and without a refractive index rising ring,and explore the role of the multi-fiber core layer and the refractive index rising ring structure;finally,explore the multi-fiber core layer.The influence of the core thickness of each layer on the acousto-optic coupling coefficient and Brillouin beat frequency difference of the Brillouin scattering spectrum provides guidance for adjusting the structure and designing the fiber structure suitable for sensing.(2)Build a Brillouin Optical Time Domain Reflectometry(BOTDR)system to explore the temperature and strain sensing characteristics of multilayer core and large effective area single-mode fiber.The BOTDR system is built with single light source optical domain frequency conversion coherent detection technology.The Brillouin frequency shifts of the Brillouin scattering spectrum measured by the experiment are 11.11 GHz,11.18GHz,11.3 5 GHz,and the error of the Brillouin frequency shift from the simulation in the first part is less than 2%.In the experiment,the temperature and strain sensitivity of the first peak and the third peak with a large difference in Brillouin frequency were selected to construct a dual-parameter sensing matrix.The temperature and strain sensitivity of the first peak finally measured are 1.66MHz/℃,0.030MHz/με,and the temperature and strain sensitivity of the third peak are 1.21MHz/℃,0.042MHz/με.The determinant of the dual-parameter sensing matrix is not zero,which is consistent with the dual-parameter sensing condition.(3)A novel optical fiber structure with high temperature,high strain sensitivity and suitable for dual-parameter sensing is designed based on neural network optimization.Before the design,the influence of the multi-fiber core layer and the refractive index rising ring on the temperature and strain sensitivity is explored through simulation,and this guides the collection of data required for neural network training.After collecting the data,the neural network reverse design method was used to optimize the design of the optical fiber structure with high temperature and strain sensitivity.Through the design,the main peak temperature and strain sensitivity of the new optical fiber structure are 2.51MHz/℃,0.041MHz/με,and the third peak temperature and strain sensitivity are 2.05MHz/℃,0.052MHz/με,the measurement errors of temperature and strain are respectively 0.24℃ and 9.9με.Compared with the traditional single-mode fiber,the temperature and strain sensitivity are higher,and the measurement errors of temperature and strain are smaller.In this paper,the Brillouin characteristics of multilayer core and large effective area fiber are explored from the perspectives of simulation and experiment,and the fiber structure with high sensing performance is designed by reverse optimization of neural network.The multi-fiber core layer can generate a multi-peak Brillouin scattering spectrum capable of dual-parameter sensing,and the temperature and strain sensitivity of different Brillouin scattering peaks can be designed by adjusting the thickness of each core layer.The refractive index rising ring can adjust the field area of the fundamental mode and at the same time reduce the gain of the high-order mode of the fiber to protrude the fundamental mode,and ensure the single-mode transmission of the fiber is more conducive to the design and experimental measurement of multi-peak Brillouin spectra.The research results of this paper provide a reliable solution for exploring and designing optical fiber structures with higher sensing performance from both simulation and experiment in the future.