Marco Pisco

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.

A Novel Optochemical Sensor Based on

In this paper, a fiber optic sensing system, designed, and developed for the detection of ammonia in aqueous ambient at room temperature, is presented. The sensor is constituted by a standard silica optical fiber (SOF) coated by a tin dioxide sensitive layer. The SnO2 films have been transferred onto the distal end of the SOF by means of the simple and low-cost electrostatic-spray-pyrolysis deposition technique. The spectral characterization of the fabricated samples has been carried out in the wavelength range 400-1750 nm in order to estimate the thickness of the SnO2 fiber coatings. The morphology and the elemental composition of the deposited layers have also been investigated by means of scanning-electron-microscopy observation and energy-dispersive-spectrometer analysis, respectively. Single-wavelength reflectance measurements have been carried out to test the sensing performances of the realized sensors toward ammonia traces in water. A fiber-Bragg-grating temperature sensor has also been used for monitoring the temperature changes occurring inside the test ambient during the experimental measurements, in order to identify the effects of thermal drifts on the sensor response. The results here presented demonstrate that the developed refractometric chemical sensor is able to provide measurements of ammonia concentration in water and at room temperature with a high sensitivity, response times of few minutes, and a resolution as low as 2 ppm

\hbox{SnO}_{2}

In this paper, we present the study of the acousto-optic behavior of underwater-acoustic sensors constituted by fiber Bragg gratings (FBGs) coated by ring-shaped overlays. Via full-wave numerical simulations, we study the complex opto-acousto-mechanical interaction among an incident acoustic wave traveling in water, the optical fiber surrounded by the ring shaped coating, and the FBG inscribed the fiber, focusing on the frequency range 0.5-30 kHz of interest for SONAR applications. Our results fully characterize the mechanical behavior of an acoustically driven coated FBG, and highlight the key role played by the coating in enhancing significantly its sensitivity by comparison with a standard uncoated configuration. Furthermore, the hydrophone sensitivity spectrum exhibits characteristic resonances, which strongly improve the sensitivity with respect to its background (i.e., away from resonances) level. Via a three-dimensional modal analysis, we verify that the composite cylindrical structure of the sensor acts as an acoustic resonator tuned at the frequencies of its longitudinal vibration modes. In order to evaluate the sensor performance, we also carry out a comprehensive parametric analysis by varying the geometrical and mechanical properties of the coating, whose results also provide a useful design tool for performance optimization and/or tailoring for specific SONAR applications. Finally, a preliminary validation of the proposed numerical analysis has been carried out through experimental data obtained using polymeric coated FBGs sensors revealing a good agreement and prediction capability.

Sensitive Thin Film for ppm Ammonia Detection in Liquid Environment

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.

Opto-acoustic behavior of coated fiber Bragg gratings

In this paper, we report on recent experimental results obtained with fiber-Bragg-grating (FBG) hydrophones for underwater acoustic detection. The optical hydrophones under investigation consist of FBGs coated with ring-shaped polymers of different size and mechanical properties. The coating materials were selected and designed in order to provide mechanical amplification, via judicious choice of their acousto-mechanical properties and by exploiting selected resonances occurring in different frequency ranges. Our underwater acoustic measurements, carried out within the range 4–35 kHz, reveal the resonant behavior of these optical hydrophones, as well as its dependence on the coating size and type of material. These experimental data are also in good agreement with our previously published numerical results. By comparison with bare (i.e., uncoated) FBGs, responsivity enhancements of up to three orders of magnitude were found, demonstrating the effectiveness of polymeric coatings in tailoring the acoustic response of FBG-based hydrophones.

Structured Chirped Fiber Bragg Gratings

In this letter, a continuously variable optical delay line (ODL), useful for phased array antennas, is experimentally demonstrated. The ODL operates at a single optical wavelength and it is based on a linearly chirped fiber Bragg grating (LCFBG). Continuous true time delay is achieved by changing the temperature of the LCFBG. In order to limit the actuating temperature range, the grating was bonded to a metallic support and the effect of the thermal apparent strain was considered. The time-delay response of the system is measured with an optical signal modulated with double sideband technique at a frequency of 2 GHz. The experimental results demonstrate a time-delay resolution of 3.1 ps employing a temperature controller with a stability of plusmn0.1 degC and a maximum time delay of 96 ps over a temperature range of 16 degC-40degC. The time-delay-temperature characteristic offers a slope of 3.8 ps/degC with a mean-square deviation from the linearity of 3.5 ps. Such a delay line, by operating at a single optical wavelength and by using a simple actuating system, offers many advantages being low cost, compactness, and reliability. The main limitation to the ODL resolution is due to the amplitude ripple of the LCFBG used and a performance enhancement can be obtained with specifically tailored LCFBG

Resonant Hydrophones Based on Coated Fiber Bragg Gratings

In this letter, the feasibility of using hollow-core optical fibers (HOFs) integrated with carbon nanotubes for volatile organic compound (VOC) detection applications has been investigated. The Langmuir-Blodgett technique has been used in order to transfer single walled carbon nanotubes (SWCNTs) onto HOFs. Reflectance characterization of the obtained sensing probes has been carried out in the range 1520-1620 nm. Finally, the sensing capability of the HOF sensors has been investigated by exposure in a test chamber to traces of tetrahydrofuran. The preliminary results obtained demonstrate the capability of the novel HOF sensor to perform chemical detection of VOCs showing a good sensitivity and fast response times

Continuously Variable Optical Delay Line Based on a Chirped Fiber Bragg Grating

We report on a method for integrating sub-wavelength resonant structures on top of optical fiber tip. Our fabrication technique is based on direct milling of the glass on the fiber facet by means of focused ion beam. The patterned fiber tip acts as a structured template for successive depositions of any responsive or functional overlay. The proposed method is validated by depositing on the patterned fiber a high refractive index material layer, to obtain a ‘double-layer’ photonic crystal slab supporting guided resonances, appearing as peaks in the reflection spectrum. Morphological and optical characterizations are performed to investigate the effects of the fabrication process. Our results show how undesired effects, intrinsic to the fabrication procedure should be taken into account in order to guarantee a successful development of the device. Moreover, to demonstrate the flexibility of our approach and the possibility to engineering the resonances, a thin layer of gold is also deposited on the fiber tip, giving rise to a hybrid photonic-plasmonic structure with a complementary spectral response and different optical field distribution at the resonant wavelengths. Overall, this work represents a significant step forward the consolidation of Lab-on-Fiber Technology.

Novel Optochemical Sensors Based on Hollow Fibers and Single Walled Carbon Nanotubes

In this letter, experimental results on the capability of a tin dioxide (SnO2)-based silica optical fiber (SOF) sensor to detect sub-ppm ammonia concentrations in water environments, at room temperature, are presented. SnO2 sensitive layers have been deposited on the fiber end by using the simple and low cost electrostatic spray pyrolysis deposition technique. The surface morphology of the deposited SnO2 layers as well as its influence on the near field profile of the emergent electromagnetic field from the fiber coating have been investigated by means of atomic force microscopy and scanning near field optical microscopy. The room temperature adsorption measurements reveal the excellent sensor resolution of 80ppb, good recovery features, high repeatability, and fast response times (a few minutes). The results obtained demonstrate the strong potentiality of the proposed SnO2-based SOF sensor to be employed for water quality monitoring applications.

Optical fiber tip templating using direct focused ion beam milling.

We demonstrate the ability of Fiber Bragg Gratings (FBGs) sensors to protect large areas from unauthorized activities in railway scenarios such as stations or tunnels. We report on the technological strategy adopted to protect a specific depot, representative of a common scenario for security applications in the railway environment. One of the concerns in the protection of a railway area centers on the presence of rail-tracks, which cannot be obstructed with physical barriers. We propose an integrated optical fiber system composed of FBG strain sensors that can detect human intrusion for protection of the perimeter combined with FBG accelerometer sensors for protection of rail-track access. Several trials were carried out in indoor and outdoor environments. The results demonstrate that FBG strain sensors bonded under a ribbed rubber mat enable the detection of intruder break-in via the pressure induced on the mat, whereas the FBG accelerometers installed under the rails enable the detection of intruders walking close to the railroad tracks via the acoustic surface waves generated by footsteps. Based on a single enabling technology, this integrated system represents a valuable intrusion detection system for railway security and could be integrated with other sensing functionalities in the railway field using fiber optic technology.