Fiber Bragg gratings as ultrasonic waves sensors

In this work, the response of fiber Bragg gratings (FBG) subjected to longitudinal ultrasonic (US) field has been theoretically and numerically investigated. US field interaction has been modeled taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to elasto-optic effect. Numerical results show that both broadband and narrowband FBG interrogation schemes can be used to detect US presence. In both schemes, sensitivity of the FBG response decreases when reducing the ultrasound wavelength below the grating length. US excitation has been also found to induce a FBG optical spectrum shape distortion for high power ultrasounds and US wavelengths comparable to the grating length. The results of this analysis provide useful tools for the design of grating based ultrasound sensors meeting specific requirements in terms of field intensity and frequency.