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In this work a new relative humidity sensor based on the deposition of a subwavelength thickness overlay of alumina (Al2O3) and poly(Sodium 4-StyreneSulfonate) (PSS) on the cladding of a Long-Period Fiber Grating (LPFG) is presented. This material has a lower refractive index compared to the refractive index of the optical fiber. This characteristic limits the sensitivity of the sensor. In order to enhance the sensitivity to humidity it has been intercalated a high refractive index layer, which permits to improve the sensitivity of LPFG sensors by a ratio of four. The method proposed here widens the design of LPFG based sensors characterized by overlays of low refractive index, and could be used for other applications such as the design of optical filters.