基于脉冲抽运的布里渊光时域反射计技术具有米数量级的空间分辩力。为了分析在其空间分辨力范围内的应变梯度增加对布里渊谱测量的影响,建立了后向布里渊散射谱与应变梯度的函数关系。数值分析结果表明应变梯度增大,不仅使布里渊频移随之线性增大,也会使后向布里渊散射谱峰值非线性降低,谱峰变得平坦。空间分辨力10 m的100 με/m应变梯度相对于零应变梯度,引起的布里渊频移测量误差增加2.04倍。传感光纤固定于悬臂梁上以模拟不同应变梯度,采用对频宽小于1 MHz的激光脉冲调制和基于相干平衡检测的布里渊光时域反射计系统,实验测得了沿传感光纤的后向布里渊散射谱数据和应变分布。布里渊谱数据经最小二乘法拟合后,得到的不同应变梯度的谱曲线变化与理论分析相吻合。

The spatial resolution of BOTDR (the Brillouin optical time domain reflectometer) based on pulse pump in fiber could reach meter order. The analytic relationship between Brillouin backscattering spectrum and stress gradient is set up to investigate the influence of the stress gradient within the resolution on the measurement accuracy of Brillouin frequency shift. The numerical results show that the increasing stress gradient results in the linear increasing of the Brillouin frequency shift, the nonlinear decreasing of the peak and the flattening of Brillouin spectrum. With the spatial resolution of 10 m and the stress gradient of 100 με/m, the measurement error of Brillouin frequency shift is 2.04 times larger than that without the stress gradient. Based on pulse modulation of lightwave with linewidth less than 1 MHz and BOTDR with balanced coherent detection, the Brillouin spectra and stress along the test fiber are experimentally measured at different stress gradients with the sensing fiber fixed on a cantilever beam. The Brillouin spectra of the different stress gradients are obtained from the experimental data by the least squares fitting. Experimental results are in agreement with the theoretical prediction.