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A fiber-optic liquid-level sensor is proposed and experimentaaaaally demonstrated.It is a Mach-Zehnder interferometer (MZI) composed of a thin-core single-mode fiber (TCSMF) without coating sandwiched between two single-mode fibers (SMFs).The transmission spectrum properties of the MZI modified by liquid around the TCSMF are used for detecting the liquid level.It is found that the sensor exhibits an excellent linear relationship between the level and the shift of interference dip wavelength and its achieved sensitivities are 0.160nm/mm and 0.288 nm/mm for liquid with an RI of 1.3330 and 1.3696,respectively.Employing the spectrum differential integration (SDI) method to analyze transmission spectra can increase the detecting resolution of the liquid level.Due to its advantages of an extremely easy fabrication process,high sensitivity and a large sensing range,the sensor is an ideal candidate for continuous liquid level sensing.