A Highly-Sensitive Fiber Bragg Grating Transverse Strain Sensor Using Micro-Structure Fibers

摘要

In-fiber Bragg gratings are key components for optical sensing. As embedded sensors, in-fiber gratings find important applications in mechanical structural health monitoring, vibration detection, pressure monitoring and acoustic sensing. However, the current state-of-the-art grating sensors inscribed in standard fiber only provide for the detection of stress and vibration in one dimension, along the axial dimension (length) of the fiber.In this thesis, we developed fiber Bragg grating sensors in specialty micro-structural fibers that can not only detect structural stress along the fiber, but also can detect stress along the transverse directions with high sensitivity. Finite element analysis (ANSYS) was used to design and study both longitudinal and transverse strain induced in microstructural fibers by external loads. Air holes were strategically placed in the fiber cladding to focus the external stress into the fiber cores to enhance the response of the fiber Bragg grating sensors. Stress-induced birefringence in the fiber core was studied in both standard and two-hole microstructural fibers with various air hole and fiber core configurations. Based on simulation studies, the optimal position to place the fiber core was determined to maximize the influence of transverse stress on the fiber cores. To validate the simulation results, Bragg grating sensors was inscribed in two-hole microstructure fibers for transverse stress sensing. Transverse stress, as measured by resonance peak splitting of the grating sensor, was used to compare with both predictions from simulation results. Both the simulation and measurement results indicate that the sensitivity of grating sensors to the transverse stress can be enhanced eight times more in a two-hole fiber than in a standard fiber.