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A photonic crystal fiber sensor is created by infiltrating liquid with higher refractive index than background silica into two adjacent air holes situated in different layers. When the refractive index of the liquid is decreased to that of silica by heating, the two liquid rods and the solid core form a three-parallel-waveguide structure, which enables efficient mode energy coupling between the two eigenmodes and results in an interference fringe pattern. Theoretical analysis has been carried out to reveal the modes involved in the interference. Moreover, the temporal stability of the device is enhanced by inserting short sections of ultraviolet curable glue at both ends of the liquid, followed by solidification, to prevent the liquid-air interface fluctuation. The temperature sensitivity achieved is ~ -8.8 nm/°C, in the range between 45 and 50°C.