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
Fibre optic Bragg gratings (FBGs) written in normal and reduced diameter high birefringence (HiBi) fibres are studied. Chemical etching is used to reduce the diameter of fibres while the optical properties of the FBG spectrum are measured. The results obtained agree qualitatively with the stress enhanced chemical etching. The birefringence of the fibre is determined as a function of the diameter. Optical characterization of the FBG under transverse strain and temperature is also performed. The results obtained show the feasibility of the simultaneous measurement of those parameters with a HiBi FBG sensor.

Abstract
In this letter, a temperature sensor using a fiber loop mirror containing a piece of highly birefringent erbium doped fiber is presented. A Hi-Bi PANDA erbium-doped silica fiber was used and compared with the conventional Hi-Bi PANDA fiber. Different results for strain and temperature sensitivity were obtained. The temperature coefficient sensitivity was -2.22 nm/degrees C and significantly higher when compared with others conventional Hi-Bi fibers. Strain experiments were also performed. (C) 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3152-3154 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23907

Abstract
The use of fiber Bragg grating sensors embedded in hybrid composite laminates for simultaneous measurement of strain and temperature is proposed. The hybrid structure, formed by a pre-impregnated thermoset and thermoplastic composites, contains one single fiber Bragg grating embedded in each material, connected in series with each other. A different response is observed when the smart composite laminate is subjected to strain and to temperature. This is expected due to the distinct properties presented by each material. The rms deviation obtained for a temperature range between 20 and 60 degrees C is +/- 0.97 degrees C and for a strain range from 0 to 1100 mu epsilon is +/- 13.04 mu epsilon.

Abstract
A suspended multicore fiber sensor for simultaneous measurement of curvature and strain is proposed. The spectral response shows evidences of several interferences arising from the seven cores of the fiber. Once the sensing head presents different sensitivities for curvature and strain measurements, these physical parameters can be discriminated by using the matrix method. The rms deviations are +/- 19m(-1) and +/- 12:90 mu epsilon for curvature and strain measurements, respectively. (C) 2011 Optical Society of America