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Among different interferometric fiber optic sensors a Relative Humidity sensor based on a singlemode-no-core-singlemode tapered fiber structure is proposed. The mathematical model of the interferometer structure is developed, simulated and validated as a refractive index sensor. Different materials for coating the No-core fiber section are analyzed for better Relative Humidity sensitivity. Agarose, Porous Silica and SiO2 nanoparticles were selected as coating materials for Pure Silica and dPMMA fibers. The d-PMMA fiber with Agarose coating presents the highest average sensitivity of 533.46pm/RH% in the 0-60% RH range, when the tapered section has zero length. Furthermore, variations in the dimensions of the structure are explored showing that we can tune the sensitivity according to the tapered length and radius. An extension of the tapered section up to 4000µm gives an improvement of 227% in the average sensitivity. Moreover, a reduction of the tapered radius from 15µm to 1.5µm gives an rise in the sensitivity of 295%.

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