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A 3D near-field model for a uniform slanted fiber Bragg grating (FBG) is presented. This model, based on antenna theory, allows for the calculation of the radiated near field in the space surrounding a fiber that contains a tilted uniform fiber Bragg grating. Radiation patterns can also be obtained. The model has been successfully checked via experiments. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 428–432, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11081

In this work, a low-cost WDM interrogation system has been proposed and experimentally tested to obtain two independent channels of a fiber specklegram sensor. Two lasers of different wavelengths have been launched into two multimode Polymer Optical Fibers (POFs) and then combined through a coupler before their interrogation using a RBG camera. By analyzing each color of the video sequence, the two fiber channels can be independently obtained. Besides, the speckle sensitivity has been also studied by analyzing different properties of speckle patterns such as their contrast or the speckle size. The achieved results help to the development of new fiber specklegram sensors by allowing a direct comparison between two specklegrams of different properties.

Principal and Independent Component Analysis are used in this paper to provide a discrimination among those species participating in the plasma of welding spectra. This approach might be useful for spectral line identification for emission spectroscopy, especially for online welding diagnostics and laser induced breakdown spectroscopy. In this case, the feasibility of this proposal will be analyzed by means of arcwelding experiments where different plasma species will be separated by the proposed processing scheme.

Curved long-period fibre gratings are described in terms of a new model, based on a matrix method. The variations in the formulation of the main equations, which define the spectral behaviour of a long period fibre grating, are presented. Good agreement between experimental data that had previously been reported and results from the proposed model are obtained.

The effective index and fundamental mode width behavior on modified total internal reflection (MTIR) photonic crystal fibers is studied in a wide normalized frequency (Lambda/lambda) area. The sensitivities to the structural and opto-geometrical parameters are obtained and the higher sensitivity region is clearly identified. The fabrication tolerance expression is also shown. Conclusions from this letter will be useful for designing and fabrication processes, also for using MTIR fibers on optical devices such sensors.

A simple fiber optic accelerometer with large rotating equipment monitoring applications is presented. The sensor is optimized for detection of mechanical vibrations in the frequency range 0.2-140 Hz. The minimum detectable acceleration is 0.025 m/s/sup 2/ root mean square (rms), and it has low transverse sensitivity. The transducer operating principle, based on a fiber optic cantilever beam without any selamic mass on it, is simple and easy to construct, and the intensity-modulation and compensation techniques developed require simple electronic processing and provide very good stability and accuracy. Other advantages of the system are its robustness, compact size, and cost-effectiveness, which make it appropriate for industrial applications, specially for those where harsh environments are involved. The complete process of research and development of the sensor has been carried out, from the theoretical study of its operation principles, through the design, fabrication, experimental characterization and enhancement, to its starting up in a high-power electric station for hydrogenerator monitoring.

A processing scheme able to obtain any arbitrary intragrating strain distribution of a fiber Bragg grating (FBG) is proposed and demonstrated. The processing method employs just the intensity of the FBG reflection spectrum to obtain its deformation profile by combining a geometrical processing scheme with the particle swarm optimization technique. The technique has been evaluated using several spectra generated from very heterogeneous strain distributions and with a real spectrum obtained from a 5 mm length FBG under an asymmetric perturbation. The achieved results exhibit a good spatial resolution (Δ≈0.5 mm) and repeatability. This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403

On-line monitoring of an arc-welding process is performed in this paper by means of fiber Bragg grating transducers (FBGs) in two different configurations. FBGs were glued to the plate surfaces next to the weld joint, and a FBG transducer was designed to sense the perturbations in the air surrounding the welding plasma column. Sensitivity to both temperature and vibrations during the process was analyzed during several tungsten inert gas (TIG) welding tests, showing the feasibility of the proposed design to perform an on-line detection of different welding perturbations. To enhance the analysis, a setup based on plasma optical spectroscopy was also employed to allow a comparison among the response of both techniques to the appearance of these perturbations.

To estimate the acoustic plasma energy in laser-induced breakdown spectroscopy (LIBS) experiments, a light collecting and acoustic sensing device based on a coil of plastic optical fiber (POF) is proposed. The speckle perturbation induced by the plasma acoustic energy was monitored using a CCD camera placed at the end of a coil of multimode POF and processed with an intraimage contrast ratio method. The results were successfully verified with the acoustic energy measured by a reference microphone. The proposed device is useful for normalizing LIBS spectra, enabling a better estimation of the sample’s chemical composition. This work was supported in part by the Science and Technology Ministry of the Spanish Government through the TEC2010-20224-C02-02 and FIS2010-19860 projects and grants AP2007-02230 and AP2009-1403.

Chemical analysis of dangerous materials entails a safety issue for the researchers. Laser Induced Breakdown Spectroscopy offers the possibility to analyze these materials away from them using Stand-Off Set-ups. To optimize the plasma induction, the remote focalization of the laser beam is of paramount importance. A custom Fiber Bragg Grating sensor system able to correct the laser beam focalization errors is proposed and experimentally checked. The optical transducer architecture and the preliminary obtained results are reported in this paper. This work has been co-supported by the Spanish Government through the project TEC2010-20224-C02- 02 and the grant AP2007-02230.

A structure based on a smart material and a PID control loop is presented in this paper. A glass fiber reinforced plastic material is instrumented with Fiber Bragg Gratings (FBG) and Shape Memory Alloy (SMA) actuators. The smart material and the smart structure are both successfully checked by being subjected to different operational conditions at the laboratory. Very good responses are obtained under both slow and quicker varying conditions. This work has been supported by the Spanish TEC2010-20224-C02-02 project and grant AP2009-1403

A fluid flow speed sensor based on Fiber Bragg Gratings (FBGs) is proposed and demonstrated. Two FBGs are attached to an arc-shaped plastic piece at the same distance from both edges. The fluid flow to be measured pushes the structure stretching one side and compressing the other one. The sensor has been successfully checked by subjecting the device, to different air flow controlled conditions inside of a pipe. Low energy impact detection has been also reported with the proposed device. This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403.

Laser-induced breakdown spectroscopy (LIBS) is a technique that provides an accurate in situ quantitative chemical analysis and, thanks to the developments in new spectral processing algorithms in the last decade, has achieved a promising performance as a quantitative chemical analyzer at the atomic level. These possibilities along with the fact that little or no sample preparation is necessary have expanded the application fields of LIBS. In this paper, we review the state of the art of this technique, its fundamentals, algorithms for quantitative analysis or sample classification, future challenges, and new application fields where LIBS can solve real problems.

In this paper, a technique to estimate the deformation profile of a Sampled Fiber Bragg Grating (SFBG) is proposed and experimentally verified. From the SFBG intensity reflection spectrum, any arbitrary longitudinal axis deformation profile applied to a SFBG is estimated. The synthesis algorithm combines a custom defined error metric to compare the measured and the synthetic spectra and the Particle Swarm Optimization technique to get the deformation profile. Using controlled deformation profiles, the proposed method has been successfully checked by means of simulated and experimental tests. The results obtained under different controlled cases show a remarkable repetitiveness (< 50 με) and good spatial accuracy (< 1 mm). This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403.

A new methodology for optimized and automatic spectral calibration is proposed. It is aimed to work in any spectroscopic system (linear or imaging) and comes to solve uncertainty problems in the quantification of transmittance and absorbance parameters when the detection sensor offers a poor signal to noise ratio. The goal is to automatically crop the spectral range removing those wavelengths where the application of the conventional spectral correction produces errors from a chemical point of view. Two cropping alternatives are presented and compared: one based on a factor related with the dark noise present in the black reference used for the spectral correction, and another related with the allowed variance in the measurement of the white reference of the spectral correction. The proposed procedures will enhance the performance of automatic wavelength and feature selection algorithms.

Structural Health Monitoring (SHM) has assumed a significant role in assessing the structures safety and integrity. SHM can be understood as the integration of sensing intelligence and possibly also actuation devices to allow the structure loading and damaging conditions to be recorded, analyzed, localized and predicted in such a way that non-destructive testing becomes an integral part of the structure. SHM sensing requirements are very well suited for fiber optic sensing technology. So in this paper, after a very brief introduction of the basic SHM concepts, the main fiber optic technologies for this application will be reviewed, several examples and the main current technical challenges will be addressed and, finally, the conclusions summarized. The authors thank the Spanish Education and Science Ministry for funding their R&D activities through the TEC2007-67987-CO2-01 and TEC2010-20224-C02-02 projects.

Welding processes are one of the most widely spread industrial activities, and their quality control is an important area of research. The presence of residual traces from the protective antioxidant coating, is a problematic issue since it causes a significant reduction in the welding seam strength. In this work, a solution based on a Laser Induced Breakdown Spectroscopy (LIBS) setup and a Support Vector Machines (SVMs) classifier to detect and discriminate antioxidant coating residues in the welding area without destroying the sample before the welding procedure is proposed. This system could be an interesting and fast tool to detect aluminium impurities. This work has been co-supported by the Spanish Government through the project TEC2007-67987-C02-01 and the grant AP2007-02230.

In a previous paper a new approach was explored where the output parameters of a welding monitoring system based on plasma spectroscopy were the participation profiles of plasma ions and neutral atoms. They were obtained by the generation of synthetic spectra and the use of an optimization algorithm, showing correlation to the appearance of defects on the seams. In this work a feature selection algorithm is included in the model to determine the most discriminant wavelengths in terms of defect detection, thus allowing to reduce the spectral range where the synthetic spectra are generated. This should also give rise to an improvement in the overall computational performance of the algorithm. Alternatives to the use of controlled randomn search algorithms will be also explored, and the resulting model will be checked by means of experimental and field tests of arc-welding processes. This work has been co-supported by the Spanish TEC2007-67987-C02-01 projects.

Because of increased worldwide competition, efficiency improvements and establishing processes that are suitable for manufacturing high-end, high-quality products are primary commercial aims. In the agri-food industry, raw-material online characterization during the input stages of processing chains is of key importance for quality control. Classification into different quality categories and separation of unwanted spurious materials are traditionally done based on time-consuming machine-vision techniques. In industrial settings, however, welding-quality monitoring plays a major role, particularly in aeronautics and the nuclear industry, and the appearance of flaws (such as porosities or inclusions) must be prevented. Using x-rays, ultrasound, or other offline nondestructive evaluation techniques is associated with both economic and productivity costs, however. Optical spectroscopy (OS)1 has been widely used in process manufacturing, since each substance is uniquely identifiable from its spectral properties. However, traditional OS techniques generate information about the bulk properties of (or part of) a sample,2 and, in commonly used applications, many aspects of the derived sample properties result from composition heterogeneity. Imaging spectroscopy simultaneously determines the optical spectral components of a particular light-matter interaction (such as absorption) and its spatial location. It can successfully monitor sample heterogeneity. Hyperspectral imagers allow data collection in hundreds or even thousands of spectral bands. Thus, multicomponent information and high sensitivity to minor components are achieved, thus allowing enhanced quality control. Of the different types of imaging spectrometers,3 Figure 1. Global block diagram of our hyperspectral-imaging system for agri-food and industrial applications. LOS: Line of sight. a.u.: Arbitrary units. : Wavelength.

In this paper a new spectroscopic monitoring parameter is proposed for the on-line monitoring of welding processes, the plasma RMS signal, which is determined by considering the contribution from the spectral samples over a particular spectral window. This parameter is directly related to the heat input that can be estimated by measuring both welding voltage and current, but it exhibits a higher sensitivity to the appearance of weld defects. A comparison between the results obtained from the different spectroscopic parameters will be presented, with data from both experimental and field arc-welding tests. This work has been co-supported by the Spanish TEC2007-67987-C02-01 projects.

Optical spectroscopy has been utilized in various fields of science, industry and medicine, since each substance is discernible from all others by its spectral properties. However, optical spectroscopy traditionally generates information on the bulk properties of the whole sample, and mainly in the agri-food industry some product properties result from the heterogeneity in its composition. This monitoring is considerably more challenging and can be successfully achieved by the so-called hyperspectral imaging technology, which allows the simultaneous determination of the optical spectrum and the spatial location of an object in a surface. In addition, it is a nonintrusive and non-contact technique which gives rise to a great potential for industrial applications and it does not require any particular preparation of the samples, which is a primary concern in food monitoring. This work illustrates an overview of approaches based on this technology to address different problems in agri-food and industrial sectors. The hyperspectral system was originally designed and tested for raw material on-line discrimination, which is a key factor in the input stages of many industrial sectors. The combination of the acquisition of the spectral information across transversal lines while materials are being transported on a conveyor belt, and appropriate image analyses have been successfully validated in the tobacco industry. Lastly, the use of imaging spectroscopy applied to online welding quality monitoring is discussed and compared with traditional spectroscopic approaches in this regard. This work has been co-supported by the project TEC2007-67987-C02-01.

Plasma optical spectroscopy is a technique widely used for on-line welding diagnostics, given the rich information to be found within the plasma spectra generated during the process. One of the key factors in this regard is the computational performance of the whole monitoring system, as it determines the resulting spatial resolution. The electronic temperature of the welding plasma is typically used as the output monitoring parameter, but it requires an identification of the emission lines, what implies additional processing stages. In this paper we propose the use of the line-to-continuum method to generate the system output profiles, previously choosing the required spectral band by means of the SFFS algorithm.

An online welding quality system based on the use of imaging spectroscopy is proposed and discussed. Plasma optical spectroscopy has already been successfully applied in this context by establishing a direct correlation between some spectroscopic parameters, e.g., the plasma electronic temperature and the resulting seam quality. Given that the use of the so-called hyperspectral devices provides both spatial and spectral information, we propose their use for the particular case of arc welding quality monitoring in an attempt to determine whether this technique would be suitable for this industrial situation. Experimental welding tests are presented, and the ability of the proposed solution to identify simulated defects is proved. Detailed spatial analyses suggest that this additional dimension can be used to improve the performance of the entire system.

A new spectroscopic parameter is used in this paper for on-line arc-welding quality monitoring. Plasma spectroscopy applied to welding diagnostics has typically relied on the estimation of the plasma electronic temperature, as there is a known correlation between this parameter and the quality of the seams. However, the practical use of this parameter gives rise to some uncertainties that could provoke ambiguous results. For an efficient on-line welding monitoring system, it is essential to prevent the appearance of false alarms, as well as to detect all the possible defects. In this regard, we propose the use of the root mean square signal of the welding plasma spectra, as this parameter will be proven to exhibit a good correlation with the quality of the resulting seams. Results corresponding to several arc-welding field tests performed on Inconel and titanium specimens will be discussed and compared to non-destructive evaluation techniques.

In CCD-spectrometers, the relation between the CCD-pixel number and the associated wavelength is established by means of a calibration polynomial, whose coefficients are typically obtained using a calibration lamp with known emission line wavelengths and a regression procedure. A recalculation of this polynomial has to be performed periodically, as the pixel number versus wavelength relation can change with ambient temperature variations or modifications in the optics attached to the spectrometer connector. Given that this calibration procedure has to be performed off-line, it implies a disturbance for industrial scenarios, where the monitoring setup must be altered. In this paper an automatic wavelength calibration procedure for CCD-spectrometers is proposed. It is based on a processing scheme designed for the in-process estimation of the plasma electronic temperature, where several plasma emission lines are identified for each spectral capture. This identification stage involves the determination, by means of a sub-pixel algorithm, of the central wavelength of those lines, thus allowing an on-line wavelength calibration for each single acquired spectrum. The proposed technique will be demonstrated by means of several experimental arc-welding tests.

In this paper a new spectroscopic analysis technique is proposed for on-line welding quality monitoring. This approach is based on the estimation of the wavelength associated with the maximum intensity of the background signal (continuum) of the welding plasma spectra. It will be demonstrated that this parameter exhibits a clear correlation with the welding quality of the seams, as it also happens with the traditional spectroscopic approach based on the determination of the plasma electronic temperature, thus allowing an identification of the appearance of weld defects. This technique offers a relevant improvement in terms of computational performance, what enables to detect smaller defects within the seam.

The search for an efficient on-line monitoring system focused on the real-time analysis of the welding quality is an active area of research, mainly due to the widespread use of both arc and laser welding processes in relevant industrial scenarios such as aeronautics or nuclear. In this work, an improvement in the performance of a previously designed monitor system is presented. This improvement is accomplished by the employment of a dual spatial-spectral technique, namely imaging spectroscopy. This technique allows the simultaneous determination of the optical spectrum components and the spatial location of an object in a surface. In this way, the spatially characterization of the plasma emitted during a tungsten inert gas (TIG) welding is performed. The main advantage of this technique is that the spectra of all the points in the line of vision are measured at the same time. Not only are all the spectra captured simultaneously, but they are also processed as a batch, allowing the investigation of the welding quality. Moreover, imaging spectroscopy provides the desired real-time operation. To simultaneously acquire the information of both domains, spectral and spatial, a passive Prism-Grating-Prism (PGP) device can be used. In this paper the plasma spectra is captured during the welding test by means of a near infrared imaging spectroscopic system which consists of input optics, an imaging spectrograph and a monochrome camera. Technique features regarding on-line welding quality monitoring are discussed by means of several experimental welding tests.

A multispectral system based on a monochrome camera and an adaptive illumination source is presented in this paper. Its preliminary application is focused on material discrimination for food and beverage industries, where monochrome, color and infrared imaging have been successfully applied for this task. This work proposes a different approach, in which the relevant wavelengths for the required discrimination task are selected in advance using a Sequential Forward Floating Selection (SFFS) Algorithm. A light source, based on Light Emitting Diodes (LEDs) at these wavelengths is then used to sequentially illuminate the material under analysis, and the resulting images are captured by a CCD camera with spectral response in the entire range of the selected wavelengths. Finally, the several multispectral planes obtained are processed using a Spectral Angle Mapping (SAM) algorithm, whose output is the desired material classification. Among other advantages, this approach of controlled and specific illumination produces multispectral imaging with a simple monochrome camera, and cold illumination restricted to specific relevant wavelengths, which is desirable for the food and beverage industry. The proposed system has been tested with success for the automatic detection of foreign object in the tobacco processing industry.

In real sensors, the crosstalk or undesirable crossed sensitivities must be minimized. Distributed Brillouin sensing is a very useful technique to measure fluctuations of temperature along an optical fiber. However, the later measurement can be influenced by the humidity on the fiber; therefore its effect must be minimized. Because the aforementioned, the Brillouin frequency changes with the humidity. Thus, for a given temperature on a distributed fiber sensor such variations have been investigated. The experimental results obtained using three different types of single mode fibers with 1000m length, at 25°C are reported in this paper.

The effective index, fundamental mode width, numerical aperture are some important parameters of a fibre. The behaviour of these parameters for a Modified Total Internal Reflection (MTIR) Photonic Crystal Fibre (PCF) is studied when strain is applied. We chose a range of normalized frequency (L/l) and a range of d/L and we analyzed the behaviour for different values of strain. The sensibilities of the studied parameter are obtained in the chosen range. Finally, the region of maximum influence of the strain is observed.

A level and flexible quasi-distributed liquid sensor based on the changes in the light transmittance in a plastic optical fiber cable is proposed. The measurement points are constituted by small areas created by side-polishing on a curved fiber and the removal of a portion of the core. These points are distributed and adapted on each full-turn of a spire of fiber built on a cylindrical tube vertically positioned in a tank. The changes between the refractive indexes of air and liquid generate a signal power proportional to the position and level of the liquid. The sensor system was successfully demonstrated in the laboratory and experimental results of three prototypes with 10, 8, and 5 measurement points and with bend radius of 5mm are presented in this paper.

A new spectral processing technique designed for its application in the on-line detection and classification of arc-welding defects is presented in this paper. A non-invasive fiber sensor embedded within a TIG torch collects the plasma radiation originated during the welding process. The spectral information is then processed by means of two consecutive stages. A compression algorithm is first applied to the data allowing real-time analysis. The selected spectral bands are then used to feed a classification algorithm, which will be demonstrated to provide an efficient weld defect detection and classification. The results obtained with the proposed technique are compared to a similar processing scheme presented in a previous paper, giving rise to an improvement in the performance of the monitoring system.

Foreign object detection processes are improving thanks to imaging spectroscopy techniques through the employment of hyperspectral systems such as prism-grating-prism spectrographs. These devices offer a valuable but sometimes huge and redundant amount of spectral and spatial information that facilitates and speed up the classification and sorting procedures of materials in industrial production chains. In this work, different algorithms of supervised and non-supervised Principal Components Analysis (PCA) are thoroughly applied on the experimentally acquired hyperspectral images. The evaluated PCA versions implement different statistical mechanisms to maximize the class separability. PCA alternatives (traditional "m-method", "J-measure", SEPCOR and "Supervised PCA") are compared taking into account how the achieved spectral compression affects the classification performance in terms of accuracy and execution time. During the whole process, the classification stage is fixed and performed by an Artificial Neural Network (ANN). The developed techniques have been probed and successfully checked in tobacco industry where detection of plastics, cords, cardboards, papers, textile threads, etc. must be done in order to enter only tobacco leaves in the industrial chain.

Distributed fiber sensing based on Brillouin gain scattering (BGS) principle is a useful way to develop devices capable to measure temperature or/and strain in optical fibers. New effects or technologies that could achieve a larger distance and/or a better spatial resolution are a topic of special interest in this fiber sensing area. The influence of the probe-pulse shape in the interaction between the pulsed light and the continuous wave laser in a pump-probe system is presented. The purpose of this study is to improve the spatial resolution of the measurement without losing stability in the BGS. Also it is showed how the backscattering Brillouin gain is affected by inducing variations on the final value of the BGS intensity; this effect is illustrated by using an experimental set up based on the Brillouin optical time-domain analysis (BOTDA). Theoretical analysis of the probe pulse in the Brillouin shift and intensity value using triangular, sinc and saw tooth shapes around the medium phonon life time (~10ns) are presented; as well as the experimental results and possible applications are explained.

The introduction of arc and laser welding in the aerospace, automotive and nuclear sectors, among others, has led to a great effort in research concerning quality assurance of these processes. Hence, an on-line, real-time welding monitor system able to detect instabilities affecting the welding quality would be of great interest, as it would allow to reduce the use of off-line inspection techniques, some of them by means of destructive-testing evaluation, improving process productivity. Among several different approaches, plasma optical spectroscopy has proved to be a feasible solution for the on-line detection of weld defects. However, the direct interpretation of the results offered by this technique can be difficult. Therefore, Artificial Neural Networks (ANN), due to their ability to handle non-linearity, is a highly suitable solution to identify and detect disturbances along the seam. In this paper plasma spectra captured during welding tests are compressed by means of Principal Component Analysis (PCA) and, then, processed in a back propagation ANN. Experimental tests performed on stainless steel plates show the feasibility of the proposed solution to be implemented as an on-line arc-welding quality monitor system.

The gases accumulated inside the landfill as result of the fermentation of Municipal Solid Waste (MSW) known as biogas, are taking into consideration all possible uses as direct transformation into electricity. The system for collecting, regulating and controlling the biogas must include all the necessary safety features where the biogas leakage presents a high impact. Infrared thermography can be use to detect gas leakages due to the differences in temperature between the gas and the immediate surroundings. This method is able to monitor a wide area of landfill sites, quickly. This technology will not be effective if the differences in temperature are not better than five degrees. This paper describes a field test conducted to study the limitations of the infrared thermography caused by weather conditions and the moment of day or/and season when the thermal images was captured. Pipelines, borders, cells, covers, slopes and leakage (hot spots) are studied and optimum conditions are defined.

A new plasma spectroscopy analysis technique based on the generation of synthetic spectra by means of optimization processes is presented in this paper. The technique has been developed for its application in arc-welding quality assurance. The new approach has been checked through several experimental tests, yielding results in reasonably good agreement with the ones offered by the traditional spectroscopic analysis technique.

Laser welding is being introducing for the manufacture of critical parts of engines in the aeronautical industry - among others - due to its many advantages over other welding techniques. However, process reliability must be improved; particularly, precise control of the focal point of the laser is required to ensure full penetration over the entire seam. We present an optoelectronic sensor that is able to optimize the focal position of the welding laser by monitoring the light emitted by the process in two different spectral bands and picked up by an optical fibre head. The system has been optimized for use in a real manufacturing environment. It is robust, compact, easy to operate, and able to adjust itself to different welding conditions and laser setups.

A new real-time implementation of the Boltzmann-plot technique for optical spectroscopic analysis of thermal plasmas is presented. Applications are found in the monitoring of industrial processes in which plasma emission naturally occurs, such as arc and laser welding. Experimental results show the feasibility of using the Boltzmann-plot technique for real-time calculation of the electronic temperature of the plasma, a parameter that has been correlated with common defects found in the weld seam. The foundations of the proposed technique, its implementation, and its capability for the on-line detection of defects in tungsten inert gas TIG welding of steel parts are discussed.

A new kind of fiber-optic sensor, to our knowledge, is proposed: a chaotic sensor. The sensor is based on a fiberring resonator that is optically driven by a train of pulses whose period equals the round-trip delay of the resonator. This sensor exploits the extremely high dependence of chaotic systems on initial conditions to sense several physical parameters (elongation, attenuation, index of refraction). The measured information is encoded in different geometrical characteristics of the chaotic attractor. The proposed sensing scheme has been modeled and simulated. The simulations reveal high potential for this structure as a multiparameter sensor

The spectroscopic analysis of the light emitted by thermal plasmas has found many applications, from chemical analysis to monitoring and control of industrial processes. Particularly, it has been demonstrated that the analysis of the thermal plasma generated during arc or laser welding can supply information about the process and, thus, about the quality of the weld. In some critical applications (e.g. the aerospace sector), an early, real-time detection of defects in the weld seam (oxidation, porosity, lack of penetration, ...) is highly desirable as it can reduce expensive non-destructive testing (NDT). Among others techniques, full spectroscopic analysis of the plasma emission is known to offer rich information about the process itself, but it is also very demanding in terms of real-time implementations. In this paper, we proposed a technique for the analysis of the plasma emission spectrum that is able to detect, in real-time, changes in the process parameters that could lead to the formation of defects in the weld seam. It is based on the estimation of the electronic temperature of the plasma through the analysis of the emission peaks from multiple atomic species. Unlike traditional techniques, which usually involve peak fitting to Voigt functions using the Levenberg-Marquardt recursive method, we employ the LPO (Linear Phase Operator) sub-pixel algorithm to accurately estimate the central wavelength of the peaks (allowing an automatic identification of each atomic species) and cubic-spline interpolation of the noisy data to obtain the intensity and width of the peaks. Experimental tests on TIG-welding using fiber-optic capture of light and a low-cost CCD-based spectrometer, show that some typical defects can be easily detected and identified with this technique, whose typical processing time for multiple peak analysis is less than 20msec. running in a conventional PC.

In this paper the application of the Inverse Least Squares algorithm (ILS) to the detection of methane using its behaviour in the near-infrared band is presented. In order to test the effectiveness of this method, different methane concentrations were measured. Wavelength Modulation Spectroscopy (WMS) was employed to obtain the first and second harmonics of the modulation signal. The use of both harmonics in spectroscopy eliminates the dependence of the measured absorbance on parameters such as: fiber misalignments, optical power fluctuations, etc. This property greatly increases the accuracy of the concentration readings. The benefits of analysing multiple lines in gas detection are discussed together with the capabilities of the ILS algorithm. The ILS algorithm is based on the Beer-Lambert law. This law is extended to include multiple wavelengths and rearranged in such a way that the concentration of the chemical species depends on the measured absorbances. In order to apply the previous algorithm, three absorption lines centered at 1665.961 nm, 1666.201 nm and 1666.483 nm were used. The obtained results are compared with the most usual single-line calibration method based on linear regression. This comparison shows that ILS gives a superior performance. Specifically, results indicate that the ILS multiline algorithm is less noise dependent and has a higher reliability than single-line calibration methods.

The use of digital adaptative filters for the processing of signals coming from fiber optic sensors is discussed. An insight into the theoretical operating principle and the experimental parameters that affect their performance is given.

In interferometric fibre optic sensor constructed using conventional low-bi fibres, random fluctuations in the state of the polarization (SOP) of the interfering beams give rise to variations in the output fringe visibility, and consequently fading of the interferometric signal. Kersey et al.1 have reported the first demonstration of a polarisationinsensitive fibre-optic Michelson interferometer (FOMI) based on a birefringence compensation technique using a Faraday rotator mirror (FRM) in the final end of each arm of the interferometer.

A technique for writing of chirped fiber Bragg gratings with an arbitrary chirp profile (periodic or non periodic) is presented. The technique uses a second exposure with an UV beam to obtain a non-uniformly increase the refraction index of the fiber along the fiber Bragg grating.

In this paper a theoretical prediction of a new kind of in-fiber devices is presented. They behave essentially like long-period Bragg gratings except for the fact that they are not permanent but volatile. This is because these devices are not formed by the UV-induced increment of the refractive index but by the twisting of a pristine HiBi fiber. This way, a long period fiber grating whose characteristics can be modified by changing the polarization of the incoming light is obtained.

Laser welding is being introduced in the aerospace industry due to its many advantages over traditional techniques. However, welding of parts with complex shapes requires precise control of the focal point of the laser in order to achieve full penetration over the entire seam. In this paper, we present an improved control system for real-time adjustment of the correct focal position, which is based on the monitoring of the light emitted by the process in two different spectral bands. The reported system has been optimized for use in a real environment: it is robust, compact, easy to operate, able to adjust itself to different welding conditions, materials and laser setups, and includes a direct connection to an external PC. Results from recent field trials on complex aerospace structures are provided.

A novel method for interrogating interferometric sensors, based on analyzing the characteristics of the light expelled from a tilted fiber Bragg grating and captured by a photodetector linear array, is presented in the paper. Also a novel algorithm for the recovery of the measured information is proposed and its performance explored. It belongs to the parametric frequency estimation methods and is an adaptation of the MUltiple SIgnal Classification (MUSIC) algorithm. Finally an insight to the possibility of interrogating multiplexed interferometric sensors is given.

A new interrogation scheme for civil engineering transducers is presented. Although it was developed in the first place for Fiber Bragg Grating based transducers, it has also demonstrated a good performance when interrogating other type of sensors such as those based on Low Finesse Fabry-Perot Cavities. The proposed unit is able to extract the required information from the transducers by measuring the characteristics of the near field radiation created in the surroundings of a tilted fiber Bragg grating. This working principle makes it very attractive and suitable for civil engineering structural monitoring. Among the technical characteristics of the scheme, the following should be highlighted: inherent capability for extracting information of wavelength multiplexed FBG-based transducers, very wide wavelength operation range, and good wavelength resolution.

A novel software tool has been developed to improve and optimize the design of optical multipass cells in long-path absorption spectroscopy. One of the main application fields of these cells is the monitoring of hazardous gases, where they become the transducers of a global fiber optic sensor. New techniques, based on solids modeling, have been applied for the description of the mirrors that constitute the multipass cell. These mirrors have been represented in terms of parametric surfaces known as NURBS (Non-Uniform Rational B-Spline). Under this approach, the design can reach a high degree of flexibility and versatility. The behavior of the multipass cell has been evaluated by means of different specific ray-tracing techniques that have been implemented taking into account the parametric nature of NURBS surfaces.