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宽光谱光源发出的光在单模光纤与空芯光纤的连接处会因为单模光纤与空芯光纤的纤芯直径不同而发生模式失配效应,从而产生不同高阶模式的传输光,这些高阶模式依据各自不同纵向传播常数在传感器中段的空芯光纤中继续传播,并且这些不同纵向传播常数的高阶模式在空芯光纤中会发生模间干涉。根据多模干涉理论可知,透射谱中的自映像峰值波长可以表达为[17]:
$ \lambda_{0}=P\left(\frac{n D_{\mathrm{HF}}{ }^{2}}{L}\right), (P=0, 1, 2, \cdots) $
(1) 式中, n为空芯光纤的有效折射率,P为自映像系数,DHF为空芯光纤有效模场直径,L为空芯光纤长度。由表达式可知, 自映像峰值波长λ0与空芯光纤的长度L、有效折射率n、有效模场直径DHF有关。当空芯光纤长度L不变,外界环境温度、折射率等发生变化时,均会改变空芯光纤的有效折射率n,从而造成自映像峰值波长发生变化[18]。所以当空芯光纤的部分被浸没在液体中时,峰值波长由露在空气中的部分和浸没在液体中的部分决定[19],其结构如图 2所示。
表达式为:
$ \begin{gathered} \lambda_{0}=P\left[\frac{n_{1} D_{\mathrm{HF}, 1}{ }^{2}}{L}\left(\frac{L_{1}}{L}\right)+\frac{n_{2} D_{\mathrm{HF}, 2}{ }^{2}}{L}\left(\frac{L_{2}}{L}\right)\right], \\ (P=0, 1, 2, \cdots) \end{gathered} $
(2) 式中,n1为空芯光纤暴露在空气中的有效折射率,DHF, 1为空芯光纤暴露在空气中的有效模场直径,L1为空芯光纤暴露在空气中的长度,n2为空芯光纤浸没在液体中的有效折射率,DHF, 2为空芯光纤浸没在液体中的有效模场直径,L2为空芯光纤浸没在液体中的长度。式中L=L1+L2, 所以表达式可以简化为:
$ \begin{gathered} \lambda_{0}=P\left[\frac{n_{1} D_{\mathrm{HF}, 1}{ }^{2}}{L}+\left(\frac{n_{2} D_{\mathrm{HF}, 2}{ }^{2}}{L^{2}}-\frac{n_{1} D_{\mathrm{HF}, 1}{ }^{2}}{L^{2}}\right) L_{2}\right], \\ (P=0, 1, 2, \cdots) \end{gathered} $
(3) 当液体介质均匀,折射率保持稳定时,即空芯光纤浸没在液体中的有效折射率n2与空芯光纤浸没在液体中的有效模场直径DHF, 2保持不变,干涉谱的自映像峰值波长λ0与空芯光纤浸没在液体中的长度L2成正比关系。
空芯光纤多模干涉型光纤液位传感技术研究
Research of the liquid level sensing technology based on a hollow fiber multimode interference optical fiber
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摘要: 为了测量液位高度变化,采用基于空芯光纤多模干涉效应的方法,研究了在外界介质影响下光源在空芯光纤中多模干涉所产生的干涉谱的变化,进行了基于空芯光纤中多模干涉效应的液位传感实验,研究了该液位传感器的干涉谱与液位变化的关系以及不同折射率液体对测量结果的影响,并分析了实验误差。结果表明,该光纤液位传感器的液位测量范围为0mm~55mm、液体折射率为1.33和1.35时,液位测量灵敏度分别为0.180nm/mm和0.224nm/mm。使用单模-空芯-单模结构的传感器进行液位变化测量是较为精准与可行的。Abstract: In order to achieve the purpose of measuring the height change of the liquid, the method based on the multimode interference effect of the hollow core fiber was used to study the change of the interference spectrum caused by the multimode interference of the light source in the hollow core fiber under the influence of the external medium. The liquid level sensing experiment of the multimode interference effect in the core optical fiber was carried out. The relationship between the interference spectrum of the liquid level sensor and the change of the liquid level and the influence of liquids with different refractive indexes on the measurement results were experimentally studied, and the experimental errors were analyzed. The results show that the liquid level measurement range of the fiber optic liquid level sensor is 0mm~55mm, when the liquid refractive index is 1.33 and 1.35, the liquid level measurement sensitivity is 0.180nm/mm and 0.224nm/mm, respectively. It is more accurate and feasible to use a sensor with a single-mode-air-core-single-mode structure to measure liquid level changes.
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Key words:
- fiber optics /
- fiber optic level sensor /
- multimode interference /
- hollow core fiber
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