Spatially weighted fiber-optic sensors for vibration modal filters

This dissertation attempts to develop fiber-optic vibration modal filters, which can retain specific vibration mode(s) information for the applications involving structural vibration control. We began the research with a fundamental analysis to carefully examine the theoretical basis of these kinds of fiber-optic sensors. During this examination, we found that except for the clamped-clamped beam case, it is very difficult to retain a specific vibration mode information and filter all unwanted vibration mode information for beam-like structures by using the classic approach of designing a dual-mode fiber-optic sensor such that the second derivative of the propagation constant difference (Deltabeta) is proportional to the specific mode shape. A deeper examination of fiber-optic sensors applied to cantilever beam structures resulted in three discoveries. The first discovery is that in addition to modifying the core geometry, a variable propagation constant profile along the fiber length can be created by modifying the refractive index of the core using the photorefractive effect. The second discovery is that vibration modal filters can be designed using approaches different from the commonly used approach of modifying the second derivative of the propagation constant difference (Deltabeta) to be proportional to the specific mode shape. The third discovery is that, except for the clamped-clamped beam case, a pure fiber-optic vibration modal filter can only be obtained by using two colocated optical fiber sensors.;After a careful analysis based on the perturbation theory, the polarimetric and dual-mode sensor configurations using elliptical core fibers were deemed most promising for vibration modal filters. It was found in this analysis that by changing the E-core geometry, the dual-mode sensor gives a larger range of variable propagation constant difference than the polarimetric sensor does. Alternatively, the analysis showed that by changing the refractive index of the core, the polarimetric sensor gives a larger range of variable propagation constant difference than the dual-mode sensor does. (Abstract shortened by UMI.).