Laser-frequency locking techniques for high-sensitivity strain measurements by high-birefringence fiber Bragg gratings and resonators

A new approach to simultaneously interrogate orthogonal axes of single Fiber-Bragg-Gratings (FBGs) and FBG-Fabry Perot resonator sensors fabricated in linearly highly birefringent (HiBi) fibre is presented. Novel interrogation techniques of single Fiber-Bragg-Gratings (FBGs) and FBG-resonator sensors are presented. For a single FBG, we combined a laser-modulation technique to an electronic feedback loop that keeps the source always frequency locked to one peak of the sensor’s reflected spectrum. Two different lasers, with orthogonally-polarized states, were adopted to monitor simultaneously both the “fast” and “slow” FBG peaks. The corresponding correction signals from the servo-loop outputs can be interpreted as strain or temperature induced on the FBG. Detection limits ranging from 1 n / Hz to 100 n / Hz, for axial dynamic and static deformations, respectively, and of 0.025 °C/ Hz for temperature variations, are expected. A similar approach was developed for sub-på resolution interrogation of an optical resonator made of a high-reflectivity FBG-pair, using the Pound-Drever-Hall (PDH) stabilization method.