In this paper, the numerical and the experimental analyses of coated long-period fiber gratings (LPFGs) as a high-sensitivity optochemical sensor are presented. The proposed structure relies on LPFGs coated with nanoscale high refractive index chemical-sensitive overlays. The deposition of overlays with refractive index higher than the cladding one leads to a modification of the cladding-mode distribution. If the overlay features are properly chosen, a strong field enhancement within the overlay occurs, leading to an excellent sensitivity of the cladding-mode distribution to the coating properties. The effects of overlay thickness and cladding-mode order on sensor performances have been numerically and experimentally investigated. In order to provide a high-sensitivity and species-specific optochemical sensor, this mechanism has been proved with nanoscale overlays of syndiotactic polystyrene (sPS) in the nanoporous crystalline \delta form. The sensitive material has been chosen in light of its selectivity and high sorption properties towards chlorinated and aromatic compounds. Sensor probes were prepared by using dip-coating technique and an adequate procedure to obtain the \delta-form sPS. Experimental demonstration of the sensor capability to perform subparts-per-million detection of chloroform in water at room temperature is also reported.
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