S. Campopiano

Mode transition in high refractive index coated long period gratings

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.

High-sensitivity optical chemosensor based on coated long-period gratings for sub-ppm chemical detection in water

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 p...

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.

Silicon micromachined hollow optical waveguides for sensing applications

Novel micromachined optical waveguides useful for sensing applications are proposed. The waveguide is designed as hollow-core antiresonant reflecting optical waveguide (ARROW) and can be easily fabricated using standard silicon micromachining techniques. The hollow structure permits to use the core to confine simultaneously the light and the substance to be probed, leading to an increase of the interaction efficiency. Numerical simulations, performed using finite element method technique, show that with a suitable design these waveguides can be used in sensing applications, where the substances under test can be gases or liquids.

Refractive index sensor based on microstructured fiber Bragg grating

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/.

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.

Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors

A critical issue in practical structural health monitoring is related to the capability of proper sensing systems integrated within the host structures to detect, identify, and localize damage generation. To this aim, many techniques have been proposed involving dynamic measurements such as modal analysis, acoustic emission, and ultrasonics. This paper relies on the use of embedded fiber Bragg grating sensors for performing an experimental modal analysis on a wing of an aircraft model. Time domain response of the embedded fiber-optic sensors induced by hammer impacts were acquired and transformed into the frequency domain. Using a classical technique based on the frequency transfer function, the first displacement and strain mode shapes of the wing have been retrieved in terms of natural frequencies and amplitudes. Experimental results confirm the excellent performances of this class of sensing devices to determine the modal behavior within complex structures compared with conventional accelerometer-based detection systems.

Optical chemo-sensor based on long period gratings coated with /spl delta/ form syndiotactic polystyrene

In this work, a long period grating (LPG) is coated with a thin film of syndiotactic polystyrene (sPS) in the nanoporous crystalline /spl delta/ form in order to obtain a species-specific chemical sensor. The combination of the intrinsic and high refractive index sensitivity of the LPGs with the high sorption properties toward chlorinated and aromatic compounds of sPS /spl delta/ form allows detecting very low concentrations (in the range of few parts per million) of chloroform in water. Chemical detection is carried out by measuring the wavelength shift and the amplitude changes in the attenuation band of the LPG due to analyte sorption.

Nonuniform thinned fiber Bragg gratings for simultaneous refractive index and temperature measurements

In this work, a novel method for simultaneous measurements of temperature and refractive index employing fiber Bragg gratings is presented. The principle of operation relies on the use of a single nonuniform thinned grating, where the cladding layer is partially or completely removed only in a part of the grating. The perturbation leads to a wavelength-splitting of the spectral response of the unperturbed structure in two separate peaks. The first one, corresponding to the thinned region, is sensitive to the local temperature and the surrounding refractive index changes, while the other one, related to the unperturbed part, would respond only to thermal changes. Here, wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization of the sensitivities to the external refractive index and temperature, varying in the range 1.33-1.45 and 15/spl deg/C-50/spl deg/C, respectively, is presented for a 7.6-/spl mu/m etched cladding sensor. The same approach can be efficiently used as key element for the development of sensors array for chemical sensing in practical applications.

Chirped fiber Bragg gratings for electrically tunable time delay lines

In this paper, a continuously variable delay line for phased array antennas is presented. The proposed delay line operates at a single wavelength and is based on properly designed linearly chirped fiber Bragg gratings. Continuous true time delay can be achieved by changing the temperature or strain along the grating region. Numerical results show that the delay line can be used for wide-band beamforming at radio frequencies up to 30 GHz with 1 ps as minimum time delay.