Broadband frequency response of the optic fiber phase modulator with piezoelectric ZnO coating

Abstract

An exact theory for an optical acousto-optic phase modulator with a ZnO coating (OF-AOPM) has been developed, which is made by considering only the radial strain induced by acoustic wave in the resonator. It has been shown that radial strain alone gives strong modulation in the vicinity of a modulating frequency close to half-wave resonance of the Piezoelectric ZnO Coating. At frequency far from this region, not only radial strain but also axial strain may be counted on for optical modulation. In this paper, phase modulation in OFPM for the entire frequency region is presented by considering both axial and radial strains. The theoretical calculation shows good agreement with the experimental results. The frequency response of the OF-AOPM for optical phase modulation has been investigated using this theory. For broadband frequency responses, both radial and axial strains play important roles in optical phase modulation. At low frequencies below the vicinity of the half-wave resonant frequency of ZnO film, the axial strain is dominant. For the other frequency regions, strong phase modulation arises from the strain and the simplified theory presented in the previous paper [1] can be applied. This theoretical model is expected to be useful in device design as well as for many device applications.