FBG-based sensors

Figure 4.15 Concept of a polymer-based single-yam temperature sensor (a) and FBG-based sensor (b) (Li et al., 2012 Sibmski et al., 2010).

Several directional sensors types have been proposed, one based on fiber optics [40], the other based on piezo-wafers [41]. Betz et al. [40] reports a rosette constmcted from three FBG optical sensors arranged in a triangular pattern (Figure 16.18(b)). Strain gauge rosette principles are used to resolve the principal directions and obtain the sensing ray direction. Matt and Lanza di Scalea [41] and Salamone et al. [42] report a piezo rosette constructed from three MFC sensors arranged in a star pattern... 

FBG sensors provide a potential solution for strain based SHM techniques especially for composites and bonded structures. In composite materials, FBGs can be embedded inside the composite texture with a negligible effect on the mechanical properties [30, 31, 32, 33]. FBG strain sensors have been used in order to detect microscopic damages in composite laminates [34] and also for the investigation of delamination detection in CFRP laminates [35]. 

The aim of this chapter is to give an introduction to the polyimide as an excellent moisture sensitive coating material used to form the fiber-optic humidity sensors. The remainder of this chapter will be arranged as follows a brief review on the structure, optical properties and sensing principles of polyimide-coated FBG humidity sensor as well as preliminary experimental results for investigating the sensor properties are given in Section 2. In Section 3, we will demonstrate a prototype of semiconductor optical amplifier (SOA) based fiber laser sensor system used for multiple physical parameter measurements and present... 

Fig ure 11. (a) Structure of FBG-based vibration/temperature sensor and (b) operation prindple for... 

It should be noted that since the FBG-based humidity sensor has a humidity-temperature cross sensitivity, for a pure humidity measurement, however the temperature effects on the sensor should be taken into account and the temperature compensation should be taken with suitable techniques. Our system was intended to be used in a relatively closed environment for multiple parameters monitoring, for example, in a cabinet of high-voltage equipment, in which the temperature distribution is regarded to be uniform. Therefore the temperature... 

Liyin, H., Lutang, W., Nian, R, Huishan, C. Huang, Z. [2011]. Fbg moisture sensor system using soa-based fiber laser with temperature compensation. Proceedings of SPIE in Optical Sensors and Biophotonics HI, Vol. 8311, SPIE, Shanghai, China, pp. 1-6. 

Temperature assessment performed by fibre-based sensors can be implemented also by optical technology. One of the promising approaches is coating of FBG fibres with thermo-sensitive substances such as a mixture of methyl ethyl ketone peroxide and cobalt naphthenate. Optical estimation of temperature measurements ensures high accuracy of the results, but the functional coating enhances the performance of the sensor (Li et al., 2012). 

Sensitivities to external fields. Sensor applications of FBGs are dependent on the sensitivities to external physical fields. The relative center wavelength shift of silica-fiber-based Bragg gratings can be written as ... 

The spectrum width at the half-maximum was proposed as a quantitative indicator of transverse crack density. However, it was noticed in practice that the residual thermal stresses resulting from the initial CFRP processing may pose a problem because the spectrum had already split into two peaks and became much broader before the tensile load was even apphed. Hence, this proposed baseline-free method could be used only if the FBG sensor could be embedded without the non-axisymmetric residual thermal stresses being present. Nonetheless, the direct measurement of actual strain with the FBG sensor can be used as abasehne-based method of inferring the crack density from the strain values, although this would have to rely on an initial baseline... 

As BFS technique is based on sensing the pattern of strain of the baekface, strain sensors are needed for the sensing. Both conventional electrical resistance strain gages as well as FBG sensors can be used for BFS teehniques. 

Based on the phenomenon of the Bragg wavelength of FBG changing with many physical parameters, FBG sensors are mainly used for measurements of temperature, strain, pressure, and vibration [3, 39]. On the other hand, incorporated with recently developed fiber laser technologies, fiber laser sensors as active sensor systems with many unique advantages, such... 

Figure 10. Schematic of proposed SOA-based fiber laser sensor system for multi-parameter sensing, in which FBGi and FBG2 are taken as vibration/temperature and humidity sensors, respectively. FBG3 and FBG4 are two wavelength-matched reference FBGs. 1-km long fiber used in sensor system is to verify a remote sensing ability of system...

The coupled mode theory is often used to describe the polarization characteristics, or modes coupling in optical fibre, and the reflection spectra of FBG sensors. When the optical response of an optical fibre is analysed based on the coupled mode theory, the electric field is normalized as j j E ( E (dxdy = 1. Based on the perturbation approach, the slowly varying amplitudes c are determined by the following coupled mode equations ... 

Based on the preceding theoretical analysis, the FBG sensors can be treated as the wave-plates. Apparently, when an FBG sensor is under torsion, these... 

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