| Intracore fiber Bragg grating sensors capable of truly measuring distributed strain within the grating length, are reported for the first time. Three methods are developed: the phase-based, intensity-based and Fourier transform techniques. All are used to decipher the optical response of the sensor and convert it into a strain profile measurement. Intra-grating strain measurements are demonstrated by determining the strain field caused by several devised mechanical structures: in the through-the-thickness direction of a L-shaped graphite/epoxy specimen, in close proximity to a hole in an aluminum plate, and along a cantilevered beam with slots milled through the cross-section. Issues relating to the accuracy of the Fourier transform relationship in fiber Bragg gratings are discussed. Numerical simulations using a transfer matrix solution to the coupled mode equations, relating to the accuracy of the strain measurement are presented. A tunable laser diode system to interrogate the fiber Bragg sensor is developed; different methods of sensing are identified in this scenario. Finally, a novel implementation of Bragg intra-grating control using a segmented piezoelectric stack for strain-tuning was demonstrated; the fiber Bragg grating was surface bonded to the piezoelectric stack. Different optical functionality and filter shaping was demonstrated with fiber Bragg gratings. |
