Simultaneous measurements of strain and temperature with dual fiber Bragg gratings for pervasive computing

Smart sensors are the key components of pervasive computing networks. The fiber Bragg grating (FBG) can be composed of the distributed sensing networks for the high sensitivity. The cross-sensitivity is the intrinsic problem of FBG sensors. We analyzed the influence of the measurand-strain (or temperature) effect on temperature (or strain) in fiber-optic sensors that employ FBGs. Based on the mode-coupling theory, equation of fiber Bragg grating and the formula of the cross-sensitivity of strain and temperature were obtained. The physical mechanics of the cross-sensitivity in fiber gratings sensors is described. We show that a sensor scheme consisting of two FBGs gives large discrimination against temperature and strain. The characteristics of the solution with dual-wavelength matrix measuring strain and temperature simultaneously is discussed. The matrix of strain and temperature with wavelength change of dual FBG is explained. The relationship of strain (temperature) with two wavelengths shifts was obtained. By enhancing the FBG's temperature sensitivity or using polymer grating instead of one FBGs the condition number of the matrix of sensitivities can be dropped, so the accuracy of the sensor can be improved. The feasibility of using this type of embedded sensors configuration for simultaneous strain and temperature measurements was demonstrated.