提出一种基于光纤腔衰荡系统并以一小段包层腐蚀光纤作为传感头的液体挥发测量方法, 同时搭建相应的实验装置进行了实验研究.从模式理论出发分析单模光纤传输模场的能量分布, 并采用有限元法数值模拟光纤端面能量的分布, 结果表明: 包层越细的单模光纤对应越大的消光系数, 即泄露出来的倏逝波能量越大, 从而对外界环境更敏感, 但是较细包层的单模光纤存在的损耗更大.综合分析得出腐蚀光纤的最佳直径应为26 μm, 最佳腐蚀长度为1 cm.通过实时监测衰荡谱的衰荡时间变化, 得到了30 ℃恒温环境下, 乙醇与丙三醇混合溶液中乙醇浓度随挥发时间的变化曲线.实验结果表明, 混合溶液中乙醇的浓度随挥发时间呈单指数规律衰减, 该变化规律与理论分析相符.由于液体浓度与其折射率成正比, 随着乙醇的挥发, 混合溶液的折射率逐渐接近光纤包层的折射率, 使其对腐蚀光纤纤芯中泄漏的倏逝波能量的吸收程度逐渐增大, 环腔的衰荡时间也相应逐渐减小.此外, 该监测方法在常温下具有较低的温度交叉敏感性.

A method for measuring liquid volatilization by using fiber loop ring-down technique incorporating an etched fiber was analyzed and experimentally demonstrated. The transmissivity of the single mode fiber were deduced according to the coupled-mode theory; the energy distribution over the cross section of the fiber was numerically analyzed by using the finite element method. The results showed that, thinner the diameter of the single mode fiber, larger would be the extinction coefficient, thus leading to more energy loss in the fiber core. The etched fiber with length of 1 cm and diameter of 26 μm was chosen through analyzing. Through the relationship between ring-down time and concentration of ethanol, the changement of ethanol′s concentration in ethanol-glycerol solution was obtained during volatilization at 30℃. The experimental results show that the concentration of ethanol decreases exponentially with the time which is in good agreement with the theoretical derivation. Since the refractive index of liquid which varies linearly with the concentration increases with the volatilization of ethanol, leading to stronger evanescent wave absorption, as a result, the ring-down time decreases. In addition, this method has very low temperature cross-sensitivity.