“…In environments that require ultra-high liquid level sensitivity like medical testing, the common capacitance, resistance, magnetostriction, and ultrasonic sensors have various defects and cannot meet the requirements of liquid level measurement [13] . In recent years, various optical fiber liquid level sensors were proposed, such as fiber Bragg gratings (FBGs) [14,15] , long period fiber gratings (LPFGs) [16,17] , Michelson interferometers (MIs) [18] , and Mach-Zehnder interferometers (MZIs) [19,20] . Especially, the sensor based on a multimode fiber (MMF) structure has attracted more and more attention because of its advantages such as simple structure, low cost, and convenient manufacture.…”