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This work demonstrates that the use of an unconventional optical fiber allows for generating lossy mode resonances (LMRs), without the need for an external high refractive index (RI) thin film. The basic idea consists of using a few centimeters of an optical fiber having a large core and cladding with higher RI, which is spliced between two multimode fibers. Here, the role of such kind of fiber is got by a commercially available double cladding fiber (DCF) having a W-type RI profile. The possibility to induce and tune the phenomenon is demonstrated, where the resonance wavelength of LMR peaks and mode order can be adjusted by varying the thickness of DCF outer cladding, e.g., through chemical etching. This novel sensing scheme becomes a valid alternative to thin-film coated optical fibers, where LMR-based sensors have been developed so far, due to advantages in terms of simplicity, cost, and stability. The response of fabricated LMR devices is characterized toward surrounding medium RI, demonstrating a sensitivity up to about 1700 nm/RIU in the RI range of 1.33–1.39, which makes this fiber sensor suitable for bio-chemical sensing applications. Low cross sensitivity to temperature is also found.

This paper reports the experimental assessment of an automated optical assay based on label free optical fiber optrodes for the fast detection of class C β-lactamases (AmpC BLs), actually considered as one of the most important sources of resistance to β-lactams antibiotics expressed by resistant bacteria. Reflection-type long period fiber gratings (RT-LPG) have been used as highly sensitive label free optrodes, while a higher affine boronic acid-based ligand was here selected to enhance the overall assay performances compared to those obtained in our first demonstration. In order to prove the feasibility analysis towards a fully automated optical assay, an engineered system was developed to simultaneously manipulate and interrogate multiple fiber optic optrodes in the different phases of the assay. The automated system tested in AmpC solutions at increasing concentrations demonstrated a limit of detection (LOD) of 6 nM, three times better when compared with the results obtained in our previous work. Moreover, the real effectiveness of the proposed optical assay has been also confirmed in complex matrices as the case of lysates of Escherichia coli overexpressing AmpC.

Flash sintering is a powerful tool for the ultrarapid consolidation of green ceramic compacts, although its activation mechanisms in electrically conductive PTC (Positive Temperature Coefficient for resistivity) materials' is poorly understood. It was argued that a flash event could be initiated and sustained for a transitory period in certain PTC ceramics because of an initial negative dependence of the green material resistivity with temperature. The thermal runaway phenomenon and its activation conditions on binderless tungsten carbide (WC) green bodies are investigated in the present work by numerical simulations using finite element methods (FEM). The flash event is recreated and studied within the COMSOL Multiphysics software at the macroscale, i.e., considering the flash as an electrical power surge driven by an increasing sample's conductivity. During the flash, very high temperatures in the range of 1800–2000 °C can be reached in the WC green sample in a few seconds. The accurate numerical simulation of such event results in heating rates exceeding 1000 °C/s, a condition that theoretically brings a powder compact at temperatures high enough to accelerate and prioritize sintering densifying mechanisms over non-densifying ones. Therefore, the sample's regions where the maximum sintering temperature is reached more slowly because of thermal contacts with the electrodes remain highly porous at the end of the process.

This contribution presents distributed and multipoint fiber-optic monitoring of cryogenic temperatures along a superconducting power transmission line down to 30 K and over 20 m distance. Multipoint measurements were conducted using fiber Bragg gratings sensors coated with two different functional overlays (epoxy and poly methyl methacrylate (PMMA)) demonstrating cryogenic operation in the range 300–4.2 K. Distributed measurements exploited optical frequency-domain reflectometry to analyze the Rayleigh scattering along two concatenated fibers with different coatings (acrylate and polyimide). The integrated system has been placed along the 20 m long cryostat of a superconducting power transmission line, which is currently being tested at the European Organization for Nuclear Research (CERN). Cool-down events from 300–30 K have been successfully measured in space and time, confirming the viability of these approaches to the monitoring of cryogenic temperatures along a superconducting transmission line.

Graphene oxide (GO) is a photoluminescent material whose application in optoelectronics has been strongly limited due to its poor emission intensity. In this work, a GO–porous silicon (GO–PSi) hybrid structure is realized in order to investigate the emission properties of GO infiltrated into a porous matrix. GO–PSi is characterized by Fourier transform infrared spectroscopy, spectroscopic reflectometry, and steady-state photoluminescence. A photoluminescence enhancement by a factor of 2.5 with respect to GO deposited on a flat silicon surface is demonstrated. Photoluminescence measurements also show a modulation of the emitted signal; this effect is attributed to the interference phenomena occurring inside the PSi monolayer.

This Letter deals with a feasibility analysis for the development of radiation-tolerant fiber-optic humidity sensors based on long-period grating (LPG) technology to be applied in high-energy physics (HEP) experiments currently running at the European Organization for Nuclear Research (CERN). In particular, here we propose a high-sensitivity LPG sensor coated with a finely tuned titanium dioxide (TiO_2) thin layer (∼100  nm thick) through the solgel deposition method. Relative humidity (RH) monitoring in the range 0%–75% and at four different temperatures (in the range −10°C–25°C) was carried out to assess sensor performance in real operative conditions required in typical experiments running at CERN. Experimental results demonstrate the very high RH sensitivities of the proposed device (up to 1.4 nm/% RH in correspondence to very low humidity levels), which turned out to be from one to three orders of magnitude higher than those exhibited by fiber Bragg grating sensors coated with micrometer-thin polyimide overlays. The radiation tolerance capability of the TiO_2-coated LPG sensor is also investigated by comparing the sensing performance before and after its exposure to a 1 Mrad dose of γ-ionizing radiation. Overall, the results collected demonstrate the strong potential of the proposed technology with regard to its future exploitation in HEP applications as a robust and valid alternative to the commercial (polymer-based) hygrometers currently used.

The thermal degradation and the fire behavior of a polyester resin containing phosphate-based fire-retardant additives and its corresponding glass fiber composites were investigated. An unsaturated commercial polyester resin was modified by the addition of three phosphate-based fire retardants: ammonium polyphosphate, silane-coated ammonium polyphosphate, and melamine pyrophosphate, at 35% w/w. The effects of the fire retardants on resin thermal decomposition and small-scale fire behavior were studied using dynamic thermogravimetric tests at different heating rates and microcalorimetric measurements according to ASTM D7309-07. Different modes of degradation with different activation energy levels for the neat resin and the phosphate-loaded resins were identified by analyzing the thermogravimetric data through the Kissinger method. Since the ammonium polyphosphate-containing resin showed greater thermal and fire performance than the other systems, it was used to manufacture unidirectional glass fiber composites by a vacuum infusion process. The oxidative pyrolysis and fire behavior of the composites produced were studied using thermogravimetric and cone calorimeter tests that demonstrated improvement of their thermal stability and fire performance.

A calibration method for fiber Bragg grating (FBG) cryogenic temperature sensors based on look-up-table was proposed and demonstrated experimentally. The operating principles of different kind of FBG cryogenic temperature sensors are introduced. A statistical characterization of the data was carried out to verify the quasi-static condition of the measurement system by checking the stability of the measurement. Once verified this condition, the sensitivity curves of the sensors were determined by correlating the wavelength shift of the FBGs with the reference sensor measurements. On the basis of the sensitivity curves, the look-up-table (LUT) have been determined. The experimental data shows that the LUT fitting approach provides good and reliable performance in terms of accuracy and processing time.

The development of fiber optic chemical sensors based on the integrated design of novel sensitive nanocoatings combined with advanced sensing configurations is reported in the present study. Different sensitive coating materials have been exploited togheter with their deposition techniques, i.e nano-porous polymers deposited by using dip coating, carbon nanotubes deposited by Langmuir-Blodgett method and metal oxides, prepared by electrostatic spray pyrolysis. In accordance to the chemo-optical properties of these materials novel sensing schemes based on nano-coated Long Period Fiber Gratings, modified Fabry-Perot interferometers involving near field effect and photonic bandgap modification in Hollow-core Optical Fibers, have been proposed, respectively, to provide the best sensing performance. Experimental results reveal the potentiality of the integated approach that simultaneously accounts for the selection of the coating materials and of the fiber optic sensing scheme in developing optical devices for a wide range of applications related to the environmental monitoring either in air and water environments.

Shape memory alloys (SMA) exhibit functional properties associated with the shape memory effect, responsible of the SMA shape recovery after a cycle of deforming-heating and of a simultaneous generation of mechanical work. Composite systems incorporating SMA wires have the ability to actively change shape and other structural characteristics. The functional properties of such adaptive composites are related to the martensitic transformation in the SMA elements and to the constraining behaviour that the composite matrix has on the SMA wires. In this work the behaviour of a shape memory alloy hybrid composite (SMAHC) is numerically and experimentally investigated. A plate was fabricated using prestrained SMA wires embedded in an epoxy resin pre preg glass fibres composite system. Upon calorimetric and mechanical material characterization, a finite element model was used in order to predict the structural behaviour of the SMAHC. In the experimental tests, the plate was clamped at one side and actuated via electrical heating. Temperature and displacement data were collected and compared with the prediction of the finite element model. The results show that the model is able to capture the shape change in the actuation region, although a thorough description of the SMAHC behaviour requires further modelling work, including the simulation of the SMA loading history during composite manufacturing.

Due to the complex microstructure and phase transformations taking place in Shape Memory Alloys (SMA), the behaviour and properties of these materials are deeply influenced by their loading history in terms of stress, strain and temperature. Indeed, a thorough understanding of the effects of the loading history on SMA properties is fundamental for a correct modelling and design of SMA applications, in particular in the case of complex loading. An example are shape memory alloys embedded in composite systems, in which stress, strain and temperature vary simultaneously depending upon the properties of the SMA and composite. This work presents the first results of an experimental investigation on the effects of the loading history on SMA transformation temperatures. Nitinol wires of untrained material were considered. Specimens consisting of Martensite and R-phase were subjected to different loading histories, keeping in turn stress, strain or temperature at a constant value. Transformation temperatures at zero stress of these samples were measured via Differential Scanning Calorimetry. Contrarily to most constitutive models assumptions, the DSC results highlight a dependence of the transformation temperatures on the loading history, influencing in particular the Martensite to Austenite phase transformation.

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.

Composites of carbon nanotubes (CNT) in polymeric matrices have attracted considerable attention in the research communities due to their good electrical conductivity, high stiffness and high strength at relatively low CNT contents. Effective utilization of CNT in composites depends primarily on the ability to disperse them homogeneously throughout the polymer matrix, avoiding the formation of bundles due to van der Waals interactions existing between the nanotubes. In this work composites of polystyrene at various percentages of SWNT were fabricated using Latex Technology technique, a polymer type‐independent method based on using a surfactant as a dispersing agent. An electrical characterization of SWNT composites was performed both in DC and AC modes. From the analysis of DC data a percolative behavior was found for the conductivity as function of SWNT content. The innovative contribution of this work consists in the modeling of the composite material upon its electrical properties. AC measurements and the analysis of impedance as function of angular frequency lead to the formulation of an equivalent circuit able to model the composite material in correspondence of the percolative threshold.

Due to the complex microstructure and phase transformations taking place in Shape Memory Alloys (SMA), the behaviour and properties of these materials are deeply influenced by their loading history in terms of stress, strain and temperature. Indeed, a thorough understanding of the effects of the loading history on SMA properties is fundamental for a correct modelling and design of SMA applications, in particular in the case of complex loading. An example are shape memory alloys embedded in composite systems, in which stress, strain and temperature vary simultaneously depending upon the properties of the SMA and composite. This work presents the first results of an experimental investigation on the effects of the loading history on SMA transformation temperatures. Nitinol wires of untrained material were considered. Specimens consisting of Martensite and R-phase were subjected to different loading histories, keeping in turn stress, strain or temperature at a constant value. Transformation temperatures at zero stress of these samples were measured via Differential Scanning Calorimetry. Contrarily to most constitutive models assumptions, the DSC results highlight a dependence of the transformation temperatures on the loading history, influencing in particular the Martensite to Austenite phase transformation.

Shape memory alloys (SMA) exhibit functional properties associated with the shape memory effect, responsible of the SMA shape recovery after a cycle of deforming-heating and of a simultaneous generation of mechanical work. Composite systems incorporating SMA wires have the ability to actively change shape and other structural characteristics. The functional properties of such adaptive composites are related to the martensitic transformation in the SMA elements and to the constraining behaviour that the composite matrix has on the SMA wires. In this work the behaviour of a shape memory alloy hybrid composite (SMAHC) is numerically and experimentally investigated. A plate was fabricated using prestrained SMA wires embedded in an epoxy resin pre preg glass fibres composite system. Upon calorimetric and mechanical material characterization, a finite element model was used in order to predict the structural behaviour of the SMAHC. In the experimental tests, the plate was clamped at one side and actuated via electrical heating. Temperature and displacement data were collected and compared with the prediction of the finite element model. The results show that the model is able to capture the shape change in the actuation region, although a thorough description of the SMAHC behaviour requires further modelling work, including the simulation of the SMA loading history during composite manufacturing.

In this work, recent developments of a new class of all-fibre and high-sensitivity refractive index sensors based on thinned and micro-structured fibre Bragg gratings (FBGs) are reported. Uniform and non-uniform thinned FBGs (ThFBGs) have been proposed for refractive index measurements providing self temperature referencing with performances adequate to the state of the art. Furthermore, micro-structured FBGs have been developed as advanced refractive index sensors able to provide low-cost and high-sensitivity refractive index sensors for chemical sensing. Here, analysis, fabrication and characterization of the proposed optoelectronic devices are presented. Finally preliminary studies on two-defect micro-structured gratings are presented.

We describe a novel noninvasive optical technique to monitor the refractive-index variation in an epoxy-based resin that is due to the polymerization process. This kind of resin is widely used in polymer matrix composites. It is well known that the process of fabricating a thermoset-based composite involves mass and heat transfer coupled with irreversible chemical reactions that induce physical changes. To improve the quality and the reliability of these materials, monitoring the cure and optimization of the manufacturing process are of key importance. We discuss the basic operating principles of an optical system based on angle deflection measurements and present typical cure-monitoring results obtained from optical characterization. The method provides a flexible, high-sensitivity, material-independent, low-cost, noninvasive tool for monitoring real-time refractive-index variation.

We report an original design approach based on the modal dispersion curves for the development of long period gratings in transition mode near the dispersion turning point exhibiting ultrahigh refractive index sensitivity. The theoretical model predicting a giant sensitivity of 9900 nm per refractive index unit in a watery environment was experimentally validated with a result of approximately 9100 nm per refractive index unit around an ambient index of 1.3469. This result places thin film coated LPGs as an alternative to other fiber-based technologies for high-performance chemical and biological sensing applications.

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.

Fiber optic sensors emerged from the low loss fiber optic technology developed in the 70s and incredibly increased their growth in both research and applications over the last two decades. Cost effectiveness of fiber optic sensors comes out from the rapid developments in telecommunications optics, cost reduction in laser diodes and optical fibers. Actually primary fiber optic sensors markets are oil wells, security, smart structures and seismic detection in oil industry with a market grew of about 90 in 2005 achieving revenue of about 130 million . Today, market opportunities for sensors are developing and expanding due to the increase of environmental global modifications, homeland security needs and elderly population. Fiber optic based technology can respond to these needs due to the inherent networking efficiency, the optimal cost/performance ratio and the demonstrated ease of integration of optical fibers with sensitive materials for physical, chemical and biochemical sensors. A wide choice of discrete and distributed fiber optic sensors are commercial available based on well assessed optical transduction schemes such as Fabry Perot cavities and Fiber Bragg Gratings or Raman and Brillouin scattering techniques that cover most of the relevant applications in physical sensing (temperature, strain, stress, vibration). Moreover applications in chemical, biological, environmental and medical fields requires a new generation of devices which principle of operation relies on the sensing functions added by specific designed coating materials. In fact fiber optic sensing technology still continues to be the subject of significant basic research effort not only investigating novel phenomena that can be utilized in sensing but also addressing the integration of novel materials and nano/micro technology in the optical sensor design. In particular, the ongoing interest is stimulated by an ever increasing portfolio of technologies through which light may be caused to interact with chemical or biological conditions which surround it and, in recent years, materials nano/micro technology has provided new opportunities for chemical and biological applications. This issue will contribute toward encapsulating recent exciting developments in the integration of new transduction mechanisms and novel materials whilst in parallel covering the continually expanding world of field trials and application assessments, with special attention to explore new perspectives and outline technological challenges. A novel generation of fiber optic devices for chemical and biological sensing is approaching based on the concurrent addressing of the issues related to the different aspects of their global design, such as: dielectric properties definition and optical modeling, materials identification, functionalization and activation, novel optical transduction principle development. Up to now, great effort has been carried out by the scientific community to develop photonic devices, however, the weak integration of competencies required to address this challenge, intrinsically multidisciplinary, limits the capability to achieve high performances devices. A highly integrated approach involving continuous interactions of different backgrounds aimed to optimize each single aspect with a continuous feed-back, would enable the definition of an overall and global design concept. This special issue presents relevant developments in the field of fiber optic chemical and biological nano/micro sensors starting from a last five years review presented in chapter 1 including gas optodes (oxygen, hydrogen, carbon dioxide and ammonia), humidity sensors, monitors for pH, cations and anions, sensors for organic compounds. Also, biosensors based on enzymes, antibodies, nucleic acids and whole microorganisms are described to illustrate the state-of-the-art in this active area. Chapter 2 illustrates advanced interdisciplinary approaches to in-fibre chemical sensing where nano structured materials (nanocristalline polymers, carbon nanotubes and microstructured metallic oxides) and innovative optical transducers schemes (near field optical transducers, photonic bandgap fibers and nano-coated long period gratings) are concurrently developed. Addressing molecular recognition through the mimic of biological receptors is the theme of chapter 3 where the development of optical biosensors based on molecularly imprinted polymers is reviewed and recent innovations presented with special emphasis on their analytical applications.......

A low-cost all-fiber Bragg grating sensing system for temperature and strain measurements is presented. Broadband interrogation and grating-based optical filtering are used to detect a strain- or temperature-induced Bragg wavelength shift with a resolution of 1 μ&Vegr; and 0.1°C. The passive nature of the grating demodulation enables dynamic measurements, limited only by the electronic circuitry involved in the system read out. The same system is also used for dynamic strain detection. A fiber grating is bonded to a piezoceramic actuator in different configurations. A laser-triangulation-based system is used as a reference measurements up to 10 kHz, while dynamic strain measurements by a Fiber Bragg grating (FBG) are carried out up to 50 kHz, demonstrating a dynamic resolution of ≈40 n&Vegr;/(Hz)1/2.