In this work, the numerical and experimental investigation of the cladding modes re-organization in high refractive index (HRI) coated Long Period Gratings (LPGs) is reported. Moreover, the effects of the cladding modes re-organization on the sensitivity to the surrounding medium refractive index (SRI) have been outlined. When azimuthally symmetric nano-scale HRI coatings are deposited along LPGs devices, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the SRI. In particular, if layer parameters are properly chosen, the transition of the lowest order cladding mode into an overlay mode occurs. As a consequence, a cladding modes re-organization can be observed leading to relevant improvements in the SRI sensitivity in terms of wavelength shift and amplitude variations of the LPGs attenuation bands.

In this work, the use of fiber long-period gratings (LPGs)—coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline δ form as specific and highly sensitive chemical sensors for in water monitoring—is proposed. The approach presented here, combines the excellent sorption properties of δ form sPS as a chemosensitive layer with the excellent refractive index sensitivity of LPG-based sensors as ideal transducers. In particular, when overlays with a high refractive index compared with the cladding one are deposited along the grating region, as in this case, the refraction-reflection regime at the cladding-overlay interface occurs. As result of this mechanism, the attenuation bands of coated LPGs would respond to the optical changes induced in the sensitive overlay due to chemical sorption by a significant modification of the peak central wavelength and intensity. The sensitivity depends strongly on the overlay thickness and the grating coupled cladding mode. Here, sensor probes were prepared by using a dip coating technique and a proprietary procedure to obtain the δ form sPS. An experimental demonstration of the sensor capability to perform sub-ppm detection of chloroform in water at room temperature is reported here. Also, the effects of the overlay thickness and the cladding mode order on sensor sensitivity and response time have been numerically and experimentally investigated.

We theoretically and experimentally investigated the reorganization of cladding modes in high-refractive-index- (HRI-) coated long-period gratings (LPGs), focusing on the influence of refractive-index sensitivity. When azimuthally symmetric nanoscale HRI coatings are deposited along LPG devices, a significant modification of the distribution of cladding modes occurs, depending on the layers' features (refractive index and thickness) and on the external refractive index. In particular, if these parameters are properly chosen, a transition between cladding modes and overlay modes occurs. Numerical and experimental effects of the mode transition on the sensitivity of the surrounding refractive index are described.

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 δ-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 δ-form sPS. Experimental demonstration of the sensor capability to perform subparts-per-million detection of chloroform in water at room temperature is also reporte

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