Research On Brillouin Scattering Characteristics Of Multiple Modes Of Fibers

Brillouin scattering is a nonlinear light scattering process caused by the interaction of light waves and sound waves during transmission in optical fibers.Through experimental research on Brillouin scattering,it is found that the type of fiber and the temperature and strain of the fiber's environment affect the characteristics of Brillouin frequency shift(BFS),Brillouin linewidth,and Brillouin gain peak.Using these characteristics can realize multi-parameter,high-precision and high-spatial-resolution distributed sensing.Therefore,it is of great significance to systematically study the Brillouin scattering characteristics of multiple modes of fibers.Starting from the principle of optical fiber sensing based on Brillouin scattering,the changes of the four sensing coefficients of Brillouin under the simultaneous action of temperature and strain are theoretically deduced,and the corresponding quantitative formulas are derived.The results show that the strain and temperature changes are positively correlated with the BFS temperature coefficient and strain coefficient,respectively,and negatively correlated with the Brillouin power temperature and strain coefficient.When using the uncorrected and corrected Brillouin coefficient to measure temperature and strain at the same time,it is found that there is a certain difference in the amount of temperature change and strain obtained by the two measurement methods.The finite element simulation software is used to solve the optical modes and acoustic modes transmitted in different optical fibers,and it is found that the Brillouin parameters corresponding to different optical modes are different.As the mode order increases,the BFS,linewidth,and gain peak of different optical modes gradually decrease.After multiple modes superposition,the Brillouin gain spectrum(BGS)broadens compared to the linewidth of a single mode,and the gain peak significantly reduced.The BFS and gain peaks of the few-mode fiber and the multi-mode fiber are smaller than that of the single-mode fiber,and the linewidth of the BGS is larger than that of the single-mode fiber.The influence of acousto-optic coupling effect on ordinary single-mode fiber is analyzed,and the influence of core doping and core radius on BGS is further studied,and the optimal multi-peak BGS in this thesis is obtained.The results show that with the increase of the germanium doping concentration or the increase of the core radius,the overall BFS corresponding to different acoustic modes shows a slight decreasing trend,while the gain peak value shows an increasing trend;and the BGS will appear multi-peak phenomenon,and the BFS corresponding to the main peak gradually decreases with the increase of the germanium doping concentration;when the core radius is 2 ?m and the germanium doping concentration is 3 mol%,the optimal multi-peak BGS can be obtained.The research in this thesis provides a theoretical basis and technical reference for further improving the measurement accuracy of the distributed optical fiber sensing system based on Brillouin scattering and realizing the simultaneous measurement of temperature and strain.