Abstract
In this work, a fibre loop mirror for the simultaneous measurement of strain and temperature is presented. The loop mirror contains a section of a small core microstructured fibre characterized for strain and temperature sensing. Due to the small core geometry and using a small section length, the structure presents high birefringence and also intermodal interference. The spectral response of this configuration shows the presence of three interferometers. One of them corresponds to the interference of light that propagates in the fast and slow axes (group birefringence) and the others are associated with the interference of light in the two lowest order spatial modes in each of the fibre eigenaxis. These interferometers present distinct sensitivities to strain and temperature for different wavelengths.

Abstract
Composite structures integrity is sensible to service life. Their application in the aeronautical and space engineering implies the necessity to insure their integrity through non-destructive evaluations. On-line health monitoring procedure capable to detect, acquire, and identify damage in fibre reinforced plastic composite materials are necessary. Among the different non-destructive techniques, acoustic emission was chosen for its ability to detect evolutive defects during in-service life of structures. Traditionally, the AE waves are detected at the surface of the structure by piezoelectric transducers. Such transducers have some limitations (e.g. they can't be used at low/high temperature, and are sensible to electromagnetic interferences). Optic fibre sensors have revealed to be a good alternative. Due to their low dimensions they can be easily embedded in fibre reinforced composite at manufacturing. In this chapter is discussed the use of an optic fibre system developed for damage monitoring in composite materials from the rapid release of elastic strain energy they generate, detected in the form of elastic waves. Among the different optic fibre sensors, the Fabry-Pérot interferometer is chosen for its high sensitivity to transient phenomena. The propagating acoustic emission waves induce variations of the light in the interferometer. The difficulty when using such sensor remains the phase recovery. In this study an original set-up is proposed for phase recovery based on the generation of two quadrature-shifted phase interferometric signals from two fibre Bragg gratings. The optic fibre sensor is embedded in a cross-ply carbon fibre/epoxy laminate. The optic fibre sensor system successfully detects periodic ultrasonic waves propagating into the material as well as simulated acoustic emission waves. These tests demonstrate that the optic fibre system is suitable for damage detection from acoustic emission waves. Such in-service health monitoring methodology can be used to locate damage and to determine its severity.

Abstract
In this Letter, we present a fiber loop mirror configuration based on a suspended twin-core fiber for sensing applications. Using the suspended twin-core fiber, the fringe pattern is due to the differential optical patch of the light in the two cores associated with a refractive index difference of similar to 10(-3), which indicates an advantage of this approach compared with those based on high-birefringent fibers, namely, the possibility of using a small length of fiber. The sensing configuration was characterized for torsion, temperature, and strain. Using the fast Fourier transform technique, it is possible to obtain measurand-induced amplitude variations of the fringe pattern. The results obtained indicate the viability of a temperature- and strain-independent torsion sensor. (C) 2010 Optical Society of America

Abstract
In this research programme, methodologies for structural health monitoring (SHM) of composite overwrapped pressure vessels (COPV) were addressed. This works fit in the development of a COPV laboratorial prototype incorporating non-destructive sensing technologies, in order to detect and identify critical aspects that can occur during operation, aiming to reduce possible unwanted and unpredicted failures. Fibre Bragg grating (FBG) optical sensors and polyvinylidene Fluoride (PVDF) polymeric piezoelectric were the selected sensing technologies. These sensors were embedded in the liner-composite interface during its manufacturing and monitored the prototype while being tested under cyclic internal pressure loading. The data collected from the different sensors type were compared. The Analysis of the data, together with the assessment to the suitability of the two sensing technologies in SHM applications are discussed.

Abstract
In this work, a fiber loop mirror for the simultaneous measurement of strain and temperature is presented. The loop mirror contains a section of a small core microstructured fiber characterized for strain and temperature sensing. Due to the small core geometry and using a small section length, the structure presents high birefringence and also intermodal interference. The spectral response of this configuration shows the presence of three interferometers. One of them corresponds to the interference of light that propagates in the fast and slow axes (group birefringence) and the others are associated with the interference of light in the two lowest order spatial modes in each of the fiber eigenaxis. These interferometers present distinct sensitivities to strain and temperature for different wavelengths.

Abstract
In this research programme, methodologies for structural health monitoring (SHM) of composite over-wrapped pressure vessels (COPV) were addressed. So, this work is part of the development of a COPV laboratorial prototype incorporated with non-destructive sensing technologies. The aim is to detect and identify critical aspects that can happen during operation, in order to reduce possible safety problems. Fibre Bragg grating (FBG) optical sensors and polyvinylidene fluoride (PVDF) polymeric piezoelectric were the selected sensing technologies. These sensors were embedded in the liner-composite interface during its manufacturing and monitored the prototype while tested under cyclic internal pressure loading. The measurements collected from the sensors were compiled with the analysis of test data and are presented in this paper. Also, the suitability of the two sensing technologies, issues related to sensor embedding and the monitoring strategy are discussed.