2.1. FBG Sensing Principle

In-fiber Bragg gratings exhibit dual sensitivity since their output relative wavelength shift (Δλ_B/λ_B) depends on either the applied strain variation or the temperature difference, as shown by the sensing equation [8]:

where λ_B is the Bragg wavelength, p_e the photo-elastic constant of the optical fiber, ξ the thermo-optic constant, α_f the thermal expansion coefficient, ΔT the temperature interval (defined as the difference between the actual temperature T and the reference one T_ref at which λ_B has been measured) and Δε the total strain variation along the grating due to the combined action of external forces and apparent strain induced by differential thermal expansion of the optical fiber relative to the host structure. These constants can be condensed into two coefficients, K_ε and K_T, which represent the strain sensitivity and the temperature sensitivity, respectively. If the two wavelength contributions of Equation (1) can be discriminated, it is then possible to measure simultaneously both temperature and strain.