| In recent years,distributed optical fiber sensing technology has been widely used in large-scale civil engineering,petroleum and petrochemical,tunnel traffic,high-voltage transmission lines and other fields.Brillouin frequency shift has a linear relationship with temperature,strain and other environmental factors.The distributed optical fiber sensing system based on Brillouin scattering is based on this principle.Because temperature and strain can affect the Brillouin frequency shift of optical fiber at the same time,if there is only one peak in Brillouin spectrum,it is impossible to measure the temperature and strain information at the same time.For complex index profile fiber,there is more than one Brillouin scattering mode in the fiber,so there are multiple peaks in the Brillouin spectrum.The Brillouin frequency shift of each peak has different dependence on temperature and strain.The temperature and strain information can be obtained simultaneously by using the frequency shift of two peaks.The bending resistant low loss fiber designed in this paper belongs to complex index profile fiber with solid core,which adopts the structure of multi-layer core and groove.Subsequently,the performance of anti-bending low loss fiber is optimized by combining with neural network,and its Brillouin scattering characteristics are analyzed.The main work is as follows(1)The finite element simulation software is used to design the anti-bending low loss fiber,and the influence of multi-layer core and groove parameters on the effective mode field area and bending loss of the fundamental mode is explored.Through the optimization of the fiber structure,the effective mode field area of the anti-bending low loss fiber can reach 212.8 μm2,and the bending loss can be reduced to 8.2 ×10-8dB/m.Finally,the effective area of fundamental mode is 199.9μm2,and the bending loss is only 1.4 × 10-7 dB/m.(2)Brillouin characteristics of anti-bending low loss fiber are studied.The finite element simulation method is used to analyze the optical mode and acoustic mode of anti-bending low loss fiber.The Brillouin frequency shift and peak gain of anti-bending low loss fiber are calculated when the core refractive index and size are different.The Brillouin spectrum is drawn to explore the trend of spectrum change.Finally,by comparing the Brillouin power measurement errors under different core parameters,the advantages and disadvantages of anti-bending low loss fiber are judged.When the core radius of anti-bending low loss fiber is 6.0 μm and the refractive index at the axis is 1.4660,the frequency shifts of the three peaks are 10.790,10.953 and 11.110 GHz respectively.The three peak power is close to each other,and the Brillouin power measurement error is small.(3)The Brillouin optical time-domain reflectometry(BOTDR)is built by using dual light source coherent detection technology to explore the temperature Brillouin frequency shift variation of single-mode fiber and non-zero dispersion shifted fiber used in the laboratory.The temperature sensitivity of single-mode fiber is about 1.5 MHz/℃,and that of non-zero dispersion shifted fiber is 5.5 MHz/℃ and 6.0 MHz/℃.This paper realizes the design of anti-bending low loss fiber from the perspective of simulation,and puts forward the idea of neural network reverse design,which provides an efficient and reliable scheme for the fast simulation and performance optimization of fiber in the future.As a kind of fiber with complex refractive index distribution,the bending resistant low loss fiber solves the problem of cross sensitivity of temperature and strain.At the same time,the bending resistant low loss fiber has higher sensitivity of temperature and strain due to its low loss and high Brillouin scattering power,providing a more excellent scheme for the double parameter sensing technology based on Brillouin scattering. |
