Phase-based Bragg intra-grating sensing of strain gradients

Light reflected from chirped fiber Bragg gratings possess both phase and amplitude information which is unique. In this paper we report for the first time the use of the relative phase to determine the strain-induced linear chirp. By measuring this intramodal (chromatic) dispersion induced on the incident wave, the wavelength dependent penetration depth and therefore the strain gradient can be found by using a closed-form approximation. Moreover, the error can be reduced by selecting the appropriate grating properties like the writing induced chirp and the index variation parameters.