Interaction of ultrasound with a polarization preserving optic fiber

The objective of this study is to develop a model for the interaction of ultrasound with a polarization preserving optic fiber. Polarization preserving fibers have proven valuable for sensing acoustic pressure. Here, the incident ultrasound will produce cross-sectional strains beyond the existing static strains. The work presented in this thesis combines theoretical, numerical and experimental analysis of fiber response to ultrasonic pressure.;A numerical model uses the finite element method for predicting fiber sensitivity to incident ultrasound. This method allows for constructing models with any fiber geometry or material properties. Here, any cross-sectional parameters are easily modified for examining changes in fiber response. It can be used to determine the thermal residual stress field as well as solve coupled structural-acoustic equations to predict ultrasonically induced strains in the fiber core. In this study, the model calculates acoustically induced birefringence in bare and plastic coated Bow-tie and Panda fibers. Also, the model shows a fiber sensitivity dependence upon cross-sectional orientation with respect to the propagation direction of the ultrasound. Given any fiber orientation, the model calculates absolute levels of birefringence for a frequency range of 20.0 kHz. to 6.0 MHz. Over this frequency range, the induced strains are radial and effects from hydrostatic acoustic pressure can be examined.;The theoretical nature of this study entails deriving equations which describe the pressure dependence of material properties. These equations use third-order constants to determine changes in these properties in regions of large residual stresses. They are incorporated into the finite element program to modify the material property matrix given by Lame constants. This program evaluates fiber response using these third-order constants and allows comparisons with predictions using stiffness coefficients from linear elasticity.;Finally, the predictions of the finite element model are verified with experimental results. The ultrasound induces strains in the fiber cross-section which modify the indices of refraction in the core and thus, alter the optical polarization state. This effect is used by polarimetric sensors or detecting ultrasonic pressure. Applications of fiber optic sensors include calibration of medical ultrasound, as well as uses in nondestructive testing.