Superimposed Bragg gratings written into polarization maintaining fiber for monitoring micro-strains

Fiber Bragg gratings constitute excellent sensor elements, able to measure static and dynamic fields, such as temperature and pressure. We demonstrate that superimposed Bragg gratings written into hydrogen-loaded polarization maintaining fiber can be used to quantitatively monitor micro-strains or characterize the shrinkage of different cements. In fact, since the fiber Bragg gratings are written with a sufficient wavelength spacing between their Bragg wavelength, they exhibit different sensitivities and consequently, they allow the simultaneous measurement of temperature, strain, pressure and transversal force, which is sufficient to completely characterize shrinkage. We discuss the calibration of these superimposed gratings making use of a completely automated system allowing to load the fiber both in the axial and transversal direction and to apply controlled temperature changes.