A single piece of optical fiber can be utilized to sense both strain and temperature simultaneously. To develop such a sensor, we sandwich a section of two-mode elliptical core (e-core) fiber between two partially reflecting mirrors. This configuration can be considered as an intrinsic Fabry-Perot interferometer, in which the two-mode, e-core fiber serves as the resonant cavity. Two different types of phase modulation can be extracted under perturbations of strain and temperature on the fiber. These phase changes are due to the two-mode interference and intrinsic Fabry-Perot interference, respectively. The relationship between the phase information and the two physical measurands, i.e. strain and temperature, can be established using two coupled equations, in which the strain and temperature are considered as two unknowns. By solving these two coupled equations, we can simultaneously determine the strain and temperature. The waveguide theory and the Cross sensitivity analysis of this sensor are presented. The descriptions of four independent experiments that have been used to determine the coefficients of the two coupled equations are given. The resolutions of the strain and temperature measurements have been obtained to be 31 μm/m and 4.5 °C, respectively. / Master of Science