In this work, the numerical and experimental analysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for outer refractive index around 1.45 and 1.333, respectively.

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, highly sensitive refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating (FBG) sensors. When the cladding diameter is reduced, significant changes in the effective refractive index occur due to surrounding medium refractive index modifications, leading to Bragg wavelength shifts. Uniformly thinned FBGs have been obtained by using wet chemical etching in hydrofluoric acid solutions. In order to prove sensor sensitivity, experimental tests have been carried out by using glycerine solutions with well-known refractive indices. Obtained results agree well with the numerical analysis carried out by using the three-layer fiber model. If the cladding layer is completely removed, resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for the outer refractive index around 1.450 and 1.333, respectively, are possible. Finally, a novel approach based on the selective etching along the grating region has been analyzed, leading to high-sensitivity refractive index sensors based on intensity measurements.

Fiber optic label-free biosensors are currently experiencing wide diffusion, as they combine the performance of optical measurements with the advantages of optical fiber. This paper presents a comprehensive review about the design, development and testing of biosensor solutions based on in-fiber long period gratings (LPG). The attention is focused on optical transducers and methodologies allowing a significant enhancement of the sensitivity. Moreover, coating nanomaterials, biofunctionalization strategies and recognition elements providing the selectivity to the target analyte are reported in detail. The aim is to provide a detailed review concerning the state of the art about LPG-based label-free biosensors and to outline the future research trends.

In this work, an all-fiber high-resolution refractive index sensor based on a microstructured fiber Bragg grating is presented. The proposed structure relies on a partial and localized etching of the cladding layer along a standard grating. The main spectral changes of the structured grating are the increasing of the stopband and the formation of a narrow allowed band strongly dependent on the etching features and the surrounding refractive index. A sensor prototype has been fabricated by using wet etching and a proper masking procedure; experimental results reveal the possibility to carry out low-cost refractive index measurements with a resolution of 4/spl middot/10/sup -5/.

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.