| An optical fiber bending sensor and a quasi-distributed optical fiber sensor, where isolated sensing regions can be designed to sense strain, bending, etc., are developed as sensor candidates for smart civil engineering materials and structures. Both sensors use two-mode fibers. With the same two-mode fibers, acousto-optic frequency shifters are developed as the main components to accomplish these sensors. Elliptical-core two-mode fibers are chosen to improve the stability and sensitivity of the sensors. A perturbation theory approach is used to investigate the sensing mechanism of the bending sensor. A system to simultaneously measure all the sensing regions of the quasi-distributed sensor is also demonstrated.; Because conventional polymer-based fiber coatings often do not adequately protect the fibers themselves nor the fiber sensors during their installations into the concrete or metallic structures, two different processing methods are introduced. They provide superior protection to the fibers and sensors for the installation. The first method is to coat the fiber with a thick metallic layer (aluminum or tin) using a continuous casting process. Coating thicknesses of 0.5 to 1.5 mm and length up to 3 m are presented. Using a birefrigent fiber coated with such a thick metallic layer, a sensor for dynamic load or pressure was also implemented. The second method uses the thermal spray technology for directly embedding optical fibers and sensors into or surface attaching them onto concrete and metallic plates.; Finally, a sol-gel based process is used for coating optical fibers with piezoelectric lead zirconate titanate films. Coating thicknesses up to 120 μm have been achieved. Piezoelectric-coated fibers can be used as in-line optical fiber signal processing components for various fiber sensors relevant for smart structure applications. As an example, an in-line fiber phase modulator is also developed. Theoretical analysis is carried out based on an axial-symmetrical multi-layer structure operated in a radially resonant configuration. The phase modulation induced by radial acoustic modes excited on optical fibers, with a coating thickness of 120 μm and length of 5.8 cm, ranges from 100 kHz to 25 MHz. |
