多模光纤中存在的多个自由度为大容量通信和多参量传感提供了可能, 然而多模光纤中存在的高阶模不仅不稳定、 易耦合和易辐射损耗, 而且导致了布里渊增益谱畸变、 谱宽展宽和布里渊增益峰值降低, 严重劣化了系统的测量精度与传感可靠度。 因此, 研究多模光纤布里渊增益谱的特性和整形优化尤为重要。 首先对多模光纤的布里渊频移和布里渊增益谱特性进行了理论研究, 并与少模光纤、 单模光纤进行对比。 结果表明： 多模光纤的布里渊频移与模式折射率及布里渊散射角有关, 当布里渊散射角不变时, 布里渊频移与模式群编号呈负相关; 当模式群编号不变时, 布里渊频移与布里渊散射角呈正相关。 相比单模光纤, 少模光纤与多模光纤由于受到高阶模的影响, 布里渊增益峰值和布里渊频移较低, 布里渊增益谱较宽。 多模光纤中的高阶模最多, 对应的布里渊增益峰值和布里渊频移最低, 布里渊增益谱最宽。 此外, 分析并设计了两种基于单模光纤的多模光纤布里渊增益谱整形优化方法。 搭建移频本地外差布里渊光时域反射系统, 通过测量比较两种整形优化系统的布里渊增益谱宽及抗弯曲性能, 以评估整形优化程度。 实验结果表明： 提出的两种整形优化方法不同程度地减小了多模光纤的布里渊增益谱宽, 获取的布里渊增益谱有着良好的Lorenz拟合度, 分别为0.974 47和0.987 89。 利用单模环形器结合单模光纤对准熔接多模光纤的方法有更好的整形优化效果和抗弯曲性能, 最小弯曲半径和布里渊增益谱宽分别为2.25 mm和53.12 MHz。

The presence of multiple degrees of freedom in multimode fiber allows high-capacity communication and multi-parameter sensing. However, the presence of high-order modes in multimode fiber is not only unstable, easy to couple and radiate loss but also leads to Brillouin gain spectrum distortion, spectrum width broadening and Brillouin gain peak reduction, which seriously deteriorates the measurement accuracy and sensing reliability of the system. Therefore, studying the characteristics and shaping optimization of Brillouin gain spectrum in multimode fiber is particularly important. Firstly, the Brillouin frequency shift and the multimode fibre gain spectrum characteristics are investigated and compared with few-mode fiber and single-mode fiber. The results show that the Brillouin frequency shift of multimode fiber is related to the mode refractive index and Brillouin scattering angle. When the Brillouin scattering angle is constant, the Brillouin frequency shift is negatively correlated with the mode group number. When the mode group number is constant, the Brillouin frequency shift is positively correlated with the Brillouin scattering angle. Compared with single-mode fiber, few-mode fiber and multimode fiber have lower Brillouin gain peak and Brillouin frequency shift and wider Brillouin gain spectrum due to the influence of high-order modes. Multimode fibers have the most high-order modes, corresponding to the lowest Brillouin gain peak and Brillouin frequency shift and the widest Brillouin gain spectrum. Furthermore, two Brillouin gain spectrum shaping optimization methods of multimode fiber based on single-mode fiber are analyzed and designed. A frequency-shifted local heterodyne Brillouin optical time domain reflectometry system is constructed to evaluate the degree of shaping optimization by measuring the Brillouin scattering spectrum width and bend-tolerant capacity of two. The experimental results show that the proposed two shaping optimization methods reduce the Brillouin gain spectrum width of multimode fiber to varying degrees, and the obtained Brillouin gain spectrums have good Lorenz fitting degrees, which are 0.974 47 and 0.987 89, respectively. Using a single mode optical circulator combined with a single mode fiber alignment fusion to an multimode fiber has better shaping optimization effect and bending tolerance. The minimum bending radius and Brillouin gain spectrum width are 2.25 mm and 53.12 MHz, respectively.