Jose Miguel Lopez-Higuera

Fiber Optic Sensors in Structural Health Monitoring

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Brillouin Distributed Fiber Sensors: An Overview and Applications

A review focused on real world applications of Brillouin distributed fiber sensors is presented in this paper. After a brief overview of the theoretical principles, some works to face the two main technical challenges (large dynamic range and higher spatial resolution) are commented. Then an overview of some real and on-field applications is done.

Methane detection at 1670-nm band using a hollow-core photonic bandgap fiber and a multiline algorithm

The long interaction pathlengths provided by hollow-core photonic bandgap fibers (HC-PBFs) are especially advantageous for the detection of weakly absorbing gases such as methane (CH(4)). In this paper, we demonstrate methane sensing with a 1670-nm band HC-PBF. A multiline algorithm is used to fit the R(6) manifold (near 1645 nm) and, in this way, to measure the gas concentration. With this method, a minimum detectivity of 10 ppmv for the system configuration was estimated.

Laser-Induced Breakdown Spectroscopy: Fundamentals, Applications, and Challenges

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.

Optical techniques for real-time penetration monitoring for laser welding

Optical techniques for real-time full-penetration monitoring for Nd:YAG laser welding have been investigated. Coaxial light emission from the keyhole is imaged onto three photodiodes and a camera. We describe the spectral and statistical analyses from photodiode signals, which indicate the presence of a full penetration. Two image processing techniques based on the keyhole shape recognition and the keyhole image intensity profile along the welding path are presented. An intensity ratio parameter is used to determine the extent of opening at the rear of a fully opened keyhole. We show that this parameter clearly interprets a hole in formation or a lack of penetration when welding is performed on workpieces with variable thicknesses at constant laser power.

Fiber Bragg grating as an optical filter tuned by a magnetic field

We describe a novel tunable optical filter for use in optical-frequency-domain multiplexed communication systems. The shift in the Bragg condition of a fiber Bragg grating as a result of magnetically induced circular birefringence is calculated with coupled-mode theory on the basis of circular states of polarization, and the values obtained for silica and terbium-doped optical fibers are compared.

A quasi-distributed level sensor based on a bent side-polished plastic optical fibre cable

A flexible quasi-distributed liquid level sensor based on the changes in the light transmittance in a plastic optical fibre (POF) cable is proposed. The measurement points are constituted by small areas created by side-polishing on a curved fibre and the removal of a portion of the core. These points are distributed on each full turn of a coil of fibre built on a cylindrical tube vertically positioned in a tank. The changes between the refractive indices of air and liquid generate a signal power proportional to the position and level of the liquid. The sensor system has been successfully demonstrated in the laboratory, and experimental results of two prototypes with 15 and 18 measurement points in a range of 33 mm and 39 mm respectively and a resolution of 0.08 mm with bend radii of 5 mm and 8 mm are presented.

Fast algorithm for spectral processing with application to on-line welding quality assurance

A new technique is presented in this paper for the analysis of welding process emission spectra to accurately estimate in real-time the plasma electronic temperature. The estimation of the electronic temperature of the plasma, through the analysis of the emission lines from multiple atomic species, may be used to monitor possible perturbations during the welding process. Unlike traditional techniques, which usually involve peak fitting to Voigt functions using the Levenberg-Marquardt recursive method, sub-pixel algorithms are used to more accurately estimate the central wavelength of the peaks. Three different sub-pixel algorithms will be analysed and compared, and it will be shown that the LPO (linear phase operator) sub-pixel algorithm is a better solution within the proposed system. Experimental tests during TIG-welding using a fibre optic to capture the arc light, together with a low cost CCD-based spectrometer, show that some typical defects associated with perturbations in the electron temperature can be easily detected and identified with this technique. A typical processing time for multiple peak analysis is less than 20 ms running on a conventional PC.

Uniform fiber Bragg grating first- and second-order diffraction wavelength experimental characterization for strain-temperature discrimination

A full experimental characterization of the first- and second-order diffraction wavelengths of fiber Bragg gratings (FBGs) fabricated in a single optimized UV-writing process is presented in this letter. These devices are useful for the simultaneous measurement of strain and temperature.

Stabilization of Dual-Wavelength Erbium-Doped Fiber Ring Lasers by Single-Mode Operation

In this work, a novel single-longitudinal-mode (SLM) dual-wavelength laser configuration is proposed and demonstrated. This laser is based on ring resonators, and employs fiber Bragg gratings to select the operation wavelengths. It includes a short piece of highly doped Er-fiber that acts as the active medium. The stable SLM operation is guaranteed when the two lasing channels present similar output powers. This behavior is shown for different pump powers.