High-sensitivity metal oxides-coated long-period fiber grating sensors for humidity monitoring in high-energy physics applications

This contribution deals with a feasibility analysis for the development of radiation tolerant fiber optic humidity sensors based on long period grating (LPG) technology to be applied in high-energy physics (HEP) experiments currently running at the European Organization for Nuclear Research (CERN). In particular, here we propose a high-sensitivity LPG sensor coated with a finely tuned titanium dioxide (TiO2) thin layer (similar to 100 nm thick) through the sol gel deposition method. The sensor characterization in the relative humidity (RH) range [0-75] % at four different temperatures (in the range -10 degrees C - 25 degrees C) was carried out to assess sensor performances in real operative conditions required in typical experiments running at CERN. Experimental results demonstrate the very high RH sensitivities of the proposed device (up to 1.4 nm/%RH in correspondence of very low humidity levels), which turned out to be from one to three orders of magnitudes higher than those exhibited by fiber Bragg grating (FBG) sensors coated with micrometer thin polyimide overlays. The radiation tolerance capability of the TiO2-coated LPG sensor is also investigated by comparing the sensing performances before and after its exposure to 1Mrad dose of gamma-ionizing radiation. Collected results demonstrate the strong potentialities of the proposed technology in light of its future exploitation in HEP applications as robust and valid alternative to currently used commercial hygrometers.