Simultaneous Measurement of Liquid Level and Temperature Using High-Sensitivity Sensor Based on Etched Four-Core Fiber

This study proposes a dual-parameter sensor for liquid level and temperature based on etched four core fiber (FCF) structure. The sensor employs a FCF without a central core, and its interference mechanism superimposes both multimode interference (MMI) and Mach-Zehnder interference (MZI). Due to the lack of a central core, the sensor is structurally similar to a no-core fiber, enabling the excitation of multimode interference. Meanwhile, the compact inter-core enables Mach-Zehnder interference. The feasibility of this mechanism has also been verified theoretically. To enable liquid level measurement, the FCF cladding diameter was reduced by hydrofluoric acid solution to enhance its sensitivity to the surrounding refractive index. Owing to the coexistence of the two interference effects, different resonant dips exhibit distinct sensitivities to liquid level and temperature, a sensitivity matrix is employed to achieve the simultaneous measurement of the dual parameters. Experimental results demonstrate that within the liquid level range of 0 mm to 20.0 mm, the sensor exhibits a liquid level sensitivity of 0.704 nm/mm and a temperature sensitivity of 0.05 nm/℃ within the temperature range of 25℃ to 80℃. The highly sensitive dual-parameter optical fiber liquid level sensor proposed in this work demonstrates significant potential for applications in liquid level sensing.