Development of fiber-optic sensors for structural health monitoring applications

Increasing applications of composite materials in aerospace structures require developing new sensors and structural health monitoring (SHM) strategies. The sensors investigated in the present study are intended for applications in such area. The first sensor type, whispering gallery mode (WGM) sensor, is known to have ultra high sensitivity. These sensors are constructed by optically coupling a microscale resonator with an optical fiber. SHM applications require physically bonding the fiber and the particle by encapsulating the sensor system inside a solid material to maintain optimum conditions and to obtain a robust sensor, which is a significant challenge. Finding appropriate materials for encapsulation because of complex mechanical and optical requirements and developing processing methods due to the microscale structure and fragile nature of the fiber are important research tasks, which are addressed in this work. A mechanics based approach is adopted in this work to model the response of WGM sensors and also the encapsulated sensor system. A second type of sensor, called fiber-loop sensor, is developed in this study, which relies on the same optical phenomena of WGM resonances. This sensor is robust, inexpensive, and has simple instrumentation compared to the WGM sensors. This intensity modulation type sensor combines the advantage of high sensitivity of frequency-modulation based sensors and simplicity of intensity-modulation based sensors. This sensor is shown to have sensitivity of better than 10-4 N for force and 10-5 m for displacement. Fiber-loop sensors are also shown to survive at least 104 cycles of loading and unloading at high displacement. Further, they are embedded in fiber reinforced composites, which are tested under flexural loading conditions. The sensors have shown very stable and repeatable response. SHM applications are expected to benefit from the advancements in the area of fiber-optic sensors and availability of new types of sensors. It is also expected that fiber-loop sensors will find applications in other areas such as infrastructure, biomedical, and materials research.