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-FabryPerot resonator sensors fabricated in linearly highly birefringent (HiBi) fibre is presented. Novel interrogation techniquesof single Fiber-Bragg-Gratings (FBGs) and FBG-resonator sensors are presented. For a single FBG, we combined alaser-modulation technique to an electronic feedback loop that keeps the source always frequency locked to one peak ofthe sensor's reflected spectrum. Two different lasers, with orthogonally- polarized states, were adopted to monitorsimultaneously both the "fast" and "slow" FBG peaks. The corresponding correction signals from the servo-loop outputscan be interpreted as strain or temperature induced on the FBG. Detection limits ranging from 1 nε/√Hz to 100 nε/√Hz,for axial dynamic and static deformations, respectively, and of 0.025 °C/√Hz for temperature variations, are expected. Asimilar approach was developed for sub-pϵ resolution interrogation of an optical resonator made of a high-reflectivityFBG-pair, using the Pound- Drever-Hall (PDH) stabilizati