Simultaneous sensing of displacement and temperature with a single FBG

Abstract

A novel fiber Bragg grating (FBG) sensor with simultaneous sensing of displacement and temperature is presented. The FBG is affixed on the cantilever inclinedly. The midpoint of FBG exactly coincides with the zero strain layer of a rectangular beam. The vertical displacement can be measured by the broadened bandwidth of FBG as the bandwidth is insensitive to temperature, while the temperature can be measured by the center wavelength shift as the wavelength shift is insensitive to vertical displacement. With 0.1 nm spectral resolution of the analyzer, sensitivities of bandwidth-displacement and center wavelength-temperature are 0.48 nm/mm and 0.05 nm/ o C, resolutions are 0.2 mm and 2.0 o C, and sensing ranges of displacement and temperature are up to 8.5 mm and 45 o C respectively. Experimental results match theoretical analyses very well.The fiber Bragg grating (FBG) is a new type of passive optical devices [1] , by which physical quantities, such as strain, stress, displacement and temperature can be precisely measured. However, Bragg wavelength is sensitive to strain and temperature simultaneously, which limits its applications. In order to overcome the cross-sensitivity, some solutions [2][3][4][5] have been proposed, most of which used several FBGs and some required two demodulation systems. These bring application inconvenience and considerable cost.Based on the FBG chirped effect and cantilever beam scheme [6,7] , a novel simultaneous sensor is proposed: the vertical displacement can be measured by the broadened bandwidth of FBG as bandwidth is insensitive to temperature, while the temperature can be measured by the center wavelength shift of FBG as wavelength shift is insensitive to vertical displacement. The simultaneous sensing is analyzed theoretically, and a highly linear response is obtained in experiment, which is expected for practical applications in optical fiber sensing field.The reflected bandwidth of FBG varies with the displacement of cantilever and is insensitive to temperature. The basic principle is shown in Fig.1.The research shows that when the temperature remains a constant, the relationship between center wavelength variation of FBG c ' c and the axial strain ax is given by [8] ,(1) c ax c ' c K where ax is axial strain on FBG, ' c is the center wavelength after exerting strain ax on FBG, c is the initial center wavelength, and 78 . 0 K [8] is the strain sensitivity coefficient. We can effectively modulate the reflected chirped bandwidth by exerting a gradient strain on axes of FBG, which will produce FBG's grids gradient variation. By choosing the suitable material, which possesses greater thermal expansion coefficient and better flexibility, a cantilever beam is designed as shown in Fig.1. The cantilever is a rectangular beam, one end of which is fixed and the other end is free. The FBG is inclined to be sticked on one side of the cantilever beam, and the angle between FBG's axes and cantilever's zero strain layer is = 45 o . To ensure that the reflected spectrum is broadened towards...

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