This paper presents an interferometer based on a single-mode fiber-multimode fiber-thin-core fiber (SMF–MMF–TCF) Michelson interference structure that can be used for the measurements of coolant level and refractive index. Because of the different diameters of the cores of the individual fibers, optical excitation and coupling occur at the splicing points of the fibers. The multimode fibers are the couplers in the sensing structure, which allow the exciting light to enter the cladding of the thin-core fibers. The end face of the thin-core fiber is coated with a silver film to enhance the reflectivity of the light. The results show that the interference intensity first increases and then decreases with the length of TCF. When TCF is 4 cm, the interference light intensity is the strongest. The sensitivity of the sensor is 138.091 nm/RIU with the linearity of 0.977 over the refractive index of the coolant in the range of 1.3605–1.3880, and the temperature and time effects on the sensor are small. The proposed sensor has the advantages of simple fabrication, high repeatability, and good stability and it can be applied to the measurements of coolant level and refractive index in automotive engines.
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