Compatibility defects of the fiber-optic liquid level and refractive index sensors based on modal interference

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.