Showing papers by "Ignacio R. Matias published in 2007"

Sensitivity optimization of tapered optical fiber humidity sensors by means of tuning the thickness of nanostructured sensitive coatings

Abstract: Electrostatic self-assembly has been used in this work to fabricate a new optical fiber humidity sensor. Said sensor consists of a single-mode tapered fiber coated with a [PDDA/Poly R-478] nanostructured overlay, in such a way that the thickness can be controlled in order to optimize the sensor sensitivity, by stopping the deposition process at the maximum slope of the transmitted optical power. The same tapered optical fiber tested with an overlay coating at the optimal working point achieves 26.8 times better sensitivity than with a double thickness overlay. A variation of 16 dB in optical power is achieved with responses time of 300 ms for changes in relative humidity from 75% to 100%. The high dynamic performance and low temperature cross-sensitivity allows this sensor to be used for human breathing monitoring.

Design of pH Sensors in Long-Period Fiber Gratings Using Polymeric Nanocoatings

Abstract: In this paper, two different pH sensors based on the deposition of nanometric scale polymeric films onto the surface of a long-period fiber grating (LPFG) have been studied and compared. An electrostatic self-assembled (ESA) method has been used to create sensitive films with an optimal overlay thickness. Two types of sensors have been designed: The first one is based on polyallylamine hydrochloride (PAH), polyacrylic acid (PAA), and the second one was done incorporating the pigment Prussian blue (PB) in the PAH/PAA matrix. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. Both sensors were tested and compared in terms of sensitivity and response time. A faster response was obtained with the introduction of PB particles in the polymeric matrix. Linear sensors in the pH range 4-7 were obtained, showing good repeatability and high sensitivity

Fiber-optic pH-sensors in long-period fiber gratings using electrostatic self-assembly.

Abstract: A novel pH sensor based on the deposition of electrostatic self-assembled polyallylamine hydrochloride and polyacrylic acid on the cladding of a long-period fiber grating has been designed. The overlay thickness is of the order of magnitude of the light wavelength used to interrogate the sensor. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. A study of the role that pH plays in the thickness of the sensitive overlay was carried out. Experimentally, a wavelength shift of the attenuation bands of up to 85 nm was obtained in the 4-7 range of pH units. The results are reproducible, which indicates the possibilities of a multiple-use sensor.

Evanescent Field Fiber-Optic Sensors for Humidity Monitoring Based on Nanocoatings

Abstract: The transmitted optical power of two different fiber optic based structures when a nanofilm is being deposited onto them is experimentally studied. The technique used to build the nanofilms is Electrostatic Self Assembly (ESA), which has been widely reported in the literature. For the shake of comprehensibility, the comparative analysis of this phenomenon is accomplished for a particular sensing measure, humidity. The two structures selected towards development of practical humidity evanescent field sensors are hollow core fibers and tapered optical fibers. Some preliminary experimental studies of depositing humidity sensitive thin films and demonstrating their feasibility are presented. Depending on the working point selected, up to 10dB of variation in the optical output power is obtained when the environmental humidity changes. Both configurations exhibit similar dynamic behavior and response times shorter than 300msec, making these evanescent field sensors good candidates to monitor human breathing

Response time enhancement of pH sensing films by means of hydrophilic nanostructured coatings

Abstract: This work describes the improvement in response time of thin film optical fluorescent sensors for pH detection based on the 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) pH-sensitive molecule. Two different kinds of structures were fabricated to demonstrate these concepts, using the electrostatic layer-by-layer (LbL) assembly method. These structures were built up on both glass slides and tapered ends of standard communications optical fiber substrates. The basic structures are based on a sensitive coating formed by the alternate deposition of the polymer poly(allylamine hydrochloride) (PAH) as the cationic monolayers and the polymer poly(acrylic acid) (PAA) and the fluorescent pH indicator HPTS as the anionic monolayers. These sensitive coatings showed a long response time when they were immersed in different pHs. In order to improve the performance of basic devices one approach was followed, which consisted of the deposition of a highly hydrophilic block before the sensitive coating. These enhanced devices were proved to decrease the response time dramatically. AFM and water contact angle measurements showed that properties of these structures including the hydrophilic block had higher roughness and lower contact angle (around 10°) whereas the ones without the hydrophilic block were smoother and had higher contact angles (around 50°). The response time of both devices was also measured while the pH was changed several times resulting in a remarkable reduction with the new devices. The most significant reduction was in rise time response between pH 3 and pH 7 which was five times minimized.

Minimizing the photobleaching of self-assembled multilayers for sensor applications

Abstract: Layer-by-layer electrostatic self-assembled (ESA) multilayer nanostructures containing 1-hydroxypyrene-3,6,8-trisulfonate (HPTS), a fluorophore widely used for pH sensing, were built-up onto optical fiber substrates. The self-assembled structures are based on a basic matrix of alternating polyelectrolytes; poly(allylamine hydrochloride) (PAH) as polycation and poly(acrylic acid) (PAA) as polyanion. Doping the polyanionic solution with HPTS, the resultant (PAH/PAA + HTPS)n multilayer coatings exhibited a pH-dependent fluorescencent behavior appropriate for optical sensing applications. Unfortunately, these sensitive coatings show short lifetimes due to their high rate of photobleaching. In order to improve the performance of the devices, the fabrication atmosphere and some post-treatments, like thermal curing and the addition of an antifading agent, 1,4-diazabicyclo[2.2.2]octane, have been experimentally studied. After these modifications were introduced in the fabrication routine, the photobleaching rate was drastically reduced: the initial sensitive coatings show a decrease in fluorescence intensity due to photobleaching of 58% after 90 min of continuous illumination and the optimized sensitive coatings show a decrease of only 4.7% after 3 days of continuous illumination.

Sentry@Home - Leveraging the Smart Home for Privacy in Pervasive Computing

Abstract: This article introduces a privacy framework for Smart Homes, supporting individuals roaming freely in pervasive computing environments. Such environments typically are equipped with different kinds of sensors and tracking devices for context-aware service provisioning. While on the one hand, people want to take advantage of the comfort and added value of personalized context-aware services, privacy and traceability becomes a serious concern on the other hand. The question arises, how we can build up trust into inherently untrusted services in a potentially hostile environment? How can it be guaranteed that eventually all sensitive data is deleted or safely stored away? The Sentry@HOME concept, as part of our User-centric Privacy Framework, addresses these concerns. It is designed to become an integral part of the user’s home environment; seamlessly embedded into the Smart Home software infrastructure.. The Smart Home itself then can be leveraged to act as a privacy proxy for a tracked individual. On behalf of the user it constitutes the central privacy enforcement point for all privacy-relevant accesses to private or sensitive data. We are confident that our contribution, the combination of Smart Homes and a privacy-aware infrastructure, substantially adds to the success of personalized pervasive computing systems.

Fringe generation with non-uniformly coated long-period fiber gratings.

Abstract: In this work, the spectral characteristics of non-uniform symmetrically ring shaped coatings deposited on long-period fiber gratings (LPFGs) have been theoretically and experimentally investigated. To optimize the structure performances, the device was designed with a simulation tool based on vectorial analysis of modes in a multilayer cylindrical waveguide and coupled mode theory. Electrostatic self-assembling technique was selected to deposit with fine control uniform azimuthally symmetric coatings on the cladding of the LPFG. UV laser micromachining operating at 193nm was used to selectively remove the coating with high spatial resolution and with azimuthal symmetry. By locally and selectively removing portions of the overlay surrounding the LPFG from the middle of the grating, strong modifications of its spectral characteristics were observed. Phase-shift effects and multiple interference fringes have been observed for all the attenuation bands, strongly depending on the length of the uncoated region and the overlay features (thickness and optical properties). This provides a valid technological platform for the development of advanced photonic devices for sensing and telecommunication applications.

Tapered Optical Fiber Biosensor for the Detection of Anti-Gliadin Antibodies

Abstract: In this paper a new fiber optic sensor for the detection of antibodies to gliadin, in order to aid the diagnosis of celiac disease, is presented. Optical fibers of 200/230 mum (core and cladding diameters respectively) were tapered to a waist diameter in the range of 15-20 mum, and then the specific antigen was deposited using the Electrostatic Self-Assembly (ESA) method. Optimal deposition parameters have been selected using an in-situ interferometric characterization technique. The high sensitivity and continuous monitoring of the proposed scheme can reduce importantly the time and serum volume required for celiac disease tests. Performance time is shorter due to the absence of many washing and blocking steps as needed in conventional methods. Additionally, the ESA method allows the construction of nanometric scale recognition surfaces on the fiber optic, which helps to create fast response sensors for real time observation of the binding process.

Application of gold complexes in the development of sensors for volatile organic compounds

Abstract: Two different kinds of sensors have been developed by using the same kind of vapochromic complexes. The vapochromic materials [Au2Ag2(C6F5)4L2]n have different colours depending on the ligand L. These materials change, reversibly, their optical properties, colour and fluorescence, in the presence of the vapours of volatile organic compounds (VOCs). For practical applications, two different ways of fixing the vapochromic material to the optical fibre have been used: the sol-gel technique and the electrostatic self-assembly method (ESA). With the first technique the sensors can even be used to detect VOCs in aqueous solutions, and using the second method it has been possible to develop nanosensors.

Disappearing for a while - using white lies in pervasive computing

Abstract: The pervasive nature of future living environments, saturated with sensors and context-detecting services, pose a completely new challenge for computer science: the art of virtual disappearance. In many situations individuals do not want to be tracked by the environment and do not want their where abouts to be known publicly or even by their friends and relatives. Today's technology often allowsus to use white lies in such circumstances. The question we pose in this paper is: Can we achieve the same using pervasive computing technologies? In this paper we show how our User-centric Privacy Framework can be extended to allow users to pro-actively use white lies as a means to disguise their location or activity without sacrificing the use of context-services as a whole. As a result we are confident that also in the future we can perform some magic:disappearing for a while - when needed.

Towards personal privacy control

Abstract: In this paper we address the realization of personal privacy control in pervasive computing.We argue that personal privacy demands differ substantially from those assumed in enterprise privacy control. This is demonstrated by introducing seven requirements specific for personal privacy, which are then used for the definition of our privacy policy language, called SenTry. It is designed to take into account the expected level of privacy from the perspective of the individual when interacting with context-aware services. SenTry serves as the base for implementing personal privacy in our User-centric Privacy Framework for pervasive computing.

Fiber optic temperature sensor depositing quantum dots inside hollow core fibers using the layer by layer technique

Abstract: CdTe Quantum Dots (4 nm of diameter) have been successfully deposited on the inner part of hollow core fibers using the Layer-by-Layer Electrostatic Self-Assembly method. The architecture of the sensor consists on a short section of a hollow core fiber tapered at both ends and spliced to standard multimode optical fibers. Taking advantage of the dependence on temperature of the green fluorescent emission of the Quantum Dot sensitive nanofilms, optical fiber sensors were fabricated and experimentally demonstrated.

Study on White Light Optical Fiber Interferometry for pH Sensor Applications

Abstract: In this work we study the interferometric behavior of polymeric ultra-thin coatings fabricated by the Layer-by-Layer Electrostatic Self-Assembly (LbL) technique at the end of cleaved optical fibers. The study of such FPI structures is very important for sensing applications as far as the LbL fabrication technique allows to tailor the properties of the optical cavity trough a smart design. The experimental data held well with the simulations. Finally pH sensors were successfully fabricated. The interferometric nature of these sensors improve significantly the sensitivity and the robustness of these kind of sensors.

Spectral characteristics in long-period fiber gratings with nonuniform symmetrically ring shaped coatings

Abstract: In this work, the spectral characteristics of nonuniform symmetrically ring shaped coatings deposited on long-period fiber gratings have been theoretically and experimentally investigated. Electrostatic self-assembling technique was used to deposit with fine control uniform azimuthally symmetric coatings at nanoscale level. UV laser micromachining operating at 193nm was applied to selectively remove the coating with high spatial resolution on ring shaped geometry. Phase shift effects and multiple interference fringes have been observed for all the attenuation bands, strongly depending on the length of the uncoated region and the coating features (thickness and optical properties). The ability of the proposed structure to exhibit wavelength selective operation, combined with the effects of high refractive index coatings on the cladding modes distribution, provides a valid technological platform for the development of advanced photonic devices for sensing and telecommunication applications.

Optical fiber sensors based on nanostructured coatings fabricated by means of the layer-by-layer electrostatic self-assembly method

Abstract: The Layer-by-Layer Electrostatic Self-Assembly (ESA) method has been successfully used for the design and fabrication of nanostructured materials. More specifically, this technique has been applied for the deposition of thin films on optical fibers with the purpose of fabricating different types of optical fiber sensors. In fact, optical fiber sensors for measuring humidity, temperature, pH, hydrogen peroxide, glucose, volatile organic compounds or even gluten have been already experimentally demonstrated. The versatility of this technique allows the deposition of these sensing coatings on flat substrates and complex geometries as well. For instance, nanoFabry-Perots and microgratings have been formed on cleaved ends of optical fibers (flat surfaces) and also sensing coatings have been built onto long period gratings (cylindrical shape), tapered fiber ends (conical shape), biconically tapered fibers or even the internal side of hollow core fibers. Among the different materials used for the construction of these sensing nanostructured coatings, diverse types such as polymers, inorganic semiconductors, colorimetric indicators, fluorescent dyes, quantum dots or even biological elements as enzymes can be found. This technique opens the door to the fabrication of new types of optical fiber sensors.

Optical fiber pH sensors based on self-assembled multilayered neutral red coatings

Abstract: The Electrostatic Self Assembly Layer-by-Layer (ESA) technique has been already used for the fabrication of optical fiber pH sensors, the properties of the resultant pH sensitive coatings has been found to be strongly dependent on the fabrication parameters, especially on the pH of the polyelectrolyte solutions used for the deposition of the coatings. In this work the resulting properties of the sensitive coatings have been optimized by changing the parameters of fabrication. Techniques as AFM, for studying the thickness and morphology of the sensing films, and spectroscopy, for studying the composition of the films, were used for the characterization of the devices. Optical fiber pH sensors with response from pH 3 to pH 7 were successfully fabricated.

Optical fiber pH sensor based on poly (p-phenylene vinylene)

Abstract: Poly(p-xylene tetrahydrothiophenium chloride) was successfully deposited on tapered ends of optical fiber by means of the Layer-by-Layer Electrostatic Self-Assembly method. A posterior thermal curing at low temperatures (around 100°C) is necessary for the synthesis of the conjugated polymer poly (p-phenylene vinylene), (PPV). The fluorescence of this polymer has shown dependence to pH. Optical fiber pH sensors were experimentally demonstrated for the working range from pH 3 to pH 8.

Non-uniform nano-coated long-period fiber gratings for sensing applications

Abstract: This work numerically and experimentally investigates the spectral modifications due to non-uniform azimuthally symmetric high refractive index (HRI) nano-coatings deposited on long-period fiber gratings (LPFGs). First, a uniform overlay was deposited on a LPFG by using electrostatic self-assembling technique, which guarantees a fine control on the coating thickness. Successively, UV (l=193nm) laser micromachining was applied to locally and selectively remove the coating with high spatial resolution and preserving the azimuthal symmetry of the structure. As the overlay removal was performed starting from the middle of the grating, strong modifications of the LPFG transmission spectrum occurred. Phase-shift phenomenon and selective fringes generation in correspondence of all the attenuation bands can be observed. The two-effects occurring are ruled by the longitudinal length of the uncoated region and the overlay features (thickness and optical properties). The wavelength selectivity, combined with the effects of the HRI coatings on the cladding modes distribution, can be successfully applied for the development of advanced and high performances sensing devices.