Microstructured fibre Bragg gratings: analysis and fabrication

A microstructured fibre Bragg grating that relies on a partial and localised etching of the cladding layer along the grating region is proposed. The main effect is the formation of defect states inside the bandgap depending on surrounding refractive index. This leads to the possibility of realising novel optoelectronic devices for sensing and telecommunication applications.     

Structured Chirped Fiber Bragg Gratings

In this paper, a theoretical and numerical analysis of novel in-fiber photonic devices based on a structured chirped fiber Bragg gratings (CFBGs) for sensing and communication applications is presented. The investigated structure consists in a CFBG with single or multiple defects obtained by a deep and localized stripping of the cladding layer along the grating structure. The thinning of the cladding layer, partial or total, changes the core propagation features and thus leads to a significant modification of the grating spectral features. The effect of the local thinning, properly exploited, basically consists in the formation of one or more passbands within the original grating bandwidth and in one or more stopbands out of the original grating bandwidth. In addition, due to spatial encoding of the Bragg wavelength in CFBGs, the spectral position of each channel exclusively depends on the features of its own defect in a well defined location along the grating. Thus, the spectral properties of each channel are not affected by additional defects located elsewhere along the grating structure, enabling the possibility to develop independent multichannel devices by exploiting a single grating element. The spectral behavior exhibited by the microstructured device has been here numerically analyzed in dependence on the thinned region parameters. In addition a simple theoretical model has been extracted in order to easily design the device according to the desired spectral features for specific applications.     

Spectral behavior in thinned long period gratings: effects of fiber diameter on refractive index sensitivity

We report the experimental investigation of the sensitivity characteristics to the surrounding refractive index (SRI) in thinned long period gratings (LPGs) for a wide range of fiber diameters and different low-orders cladding modes. Wet chemical etching combined with microscopic and spectral analysis allow us to experimentally retrieve the SRI sensitivity characteristics of thinned LPGs. The obtained results allow us to identify accurately the dependence of the sensitivity characteristics on the fiber radius, taking into account the SRI range and the order mode. This provides a useful tool to identify the thinned structure able to fulfill the sensitivity requirements by maintaining an acceptable robustness level.     

Thinned fiber Bragg gratings as refractive index sensors

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.     

Micro-structured fiber Bragg gratings. Part II: Towards advanced photonic devices

In this paper, the authors present new and complex configurations involving micro-structured fiber Bragg gratings (MSFBGs) as advanced devices for sensing and communications applications. First, the spectral behaviour of multi-defect MSFBGs is investigated with particular regards to the possibility to carry out multi-parameter sensor by a single sensing element. More defects along the grating permit the formation of more defect states inside the band-gap ruled by the multiple interaction of the signal reflected by the unperturbed grating regions modulated by the phase delays of the perturbations. Here, a practical case based on a two-defect MSFBG is taken into account, revealing the potentiality to perform dual SRI measurements with a single MSFBG device. Finally, a novel configuration has been considered involving a single-defect MSFBG coated with nano-sized coatings with higher refractive index compared to the cladding one. The presence of a high refractive index coating along the thinned region would induce the mode redistribution and the transition from core modes to overlay modes if coating features are properly selected. Here, sensitivity characteristics of the proposed device are analysed and significant improvement in the sensing performances have been obtained, including the possibility to tune the maximum sensitivity in the desired range.     

Coated long-period fiber gratings as high-sensitivity optochemical sensors

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 /spl 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 /spl 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.