Use of weakly tilted fiber Bragg gratings for sensing purposes

Weakly tilted fiber Bragg gratings with gratings planes tilted at small angles with respect to the fiber axis couple light to backward going core mode and cladding modes. Their transmitted spectrum is characterized by narrow resonance dips below the Bragg wavelength corresponding to the core mode coupling. The amplitude spectral evolutions of weakly tilted fiber Bragg gratings in response to diverse physical perturbations such as temperature, mechanical strains, bending and surrounding refractive index changes are presented. Different techniques allowing to efficiently correlate the spectral evolution with the information to be measured are reported. We demonstrate that a selective monitoring of one cladding mode shift with respect to the Bragg wavelength gives temperature-insensitive strain measurements whereas a global monitoring of the cladding modes spectrum offers temperature-insensitive surrounding refractive index measurements. We also point out the possibility of using this global monitoring for bending and transverse strain sensing purposes. Finally, we present the effect of coating (the grating is covered by a polymer) on the sensitivity of weakly tilted fiber Bragg grating to surrounding refractive index changes. For every application, the performances of weakly tilted fiber Bragg gratings sensors are discussed.