The fiber-optic sensing structures based on modal interference have been widely applied in the measurement of refractive index, liquid level, temperature, gas concentration, and other parameters. In most studies, the sensor's sensing principles are summarized as the change of the effective refractive index of the cladding caused by the measured parameter, which will affect the sensing spectrum. However, the reason why the effective refractive index of the cladding changes with the measured parameter was not explained. In addition, due to the difference in the media's dispersion, this fiber-optic sensing structure has different sensing characteristics in liquid level or refractive index measurement for different media. But there are very little literature for relevant analysis. In this work, the relationship between the phase change caused by the total reflection and the external refractive index was proposed in order to better explain the sensor's sensing principle, and the difference of media's dispersion was fully considered. Theoretical analysis and experimental results proved that this fiber-optic sensing structure has compatibility defects in liquid level monitoring and refractive index measurement, which means it is only suitable for the measurement of its calibration medium.

基于模态干涉的光纤传感结构已广泛应用于折射率、液位、温度、气体浓度等参数的测量。在大多数研究中，传感器的传感原理概括为被测参数引起包层有效折射率的变化，从而影响传感光谱。然而，包层的有效折射率随测量参数变化的原因没有解释。此外，由于介质色散的差异，这种光纤传感结构对于不同介质在液位或折射率测量方面具有不同的传感特性。但相关分析的文献很少。在这项工作中，为了更好地解释传感器的传感原理，提出了全反射引起的相位变化与外部折射率的关系，并充分考虑了介质色散的差异。理论分析和实验结果证明，这种光纤传感结构在液位监测和折射率测量方面存在兼容性缺陷，仅适用于其校准介质的测量。