2.1.2. Strain Measurement with Temperature Compensation

The literature refers to many techniques for performing temperature compensation for strain measurements [9,21,23]. In the present work, a method was used that utilizes two identical FBGs, inscribed in the same type of optical fiber and with the same grating period (Λ). One of the FBGs acts as a strain sensor; this sensor must be temperature-compensated. The strain sensor thus plays the role of the strain-measuring sensor. The other FBG sensor, the compensation sensor, must be located close to the first one. Therefore, it can be assumed that both sensors are subjected to the same temperature variation. The temperature compensation sensor must be isolated from any external mechanical load. If we consider Equation (7) to represent the strain measuring sensor and Equation (8) for the temperature compensation sensor, then by combining both equations, the mathematical expression of temperature-compensated strain(εm) can be obtained:

where ∆λm and ∆λc are the wavelength shifts of measuring and compensation FBGs, respectively; λ0m and λ0c are the base or reference wavelengths for measuring and compensation sensors and k is the calibration factor that is characteristic of the optical bench structure. The factor k is also strain and temperature dependent [22,24].