The invention relates to a technique for detecting electric fields by modulating the phase of an optical beam. A length of optical fiber is jacketed with or attached to piezoelectric material that is poled perpendicular to the length of the fiber. An electric field is applied across the piezoelectric element, i.e. in the direction of poling, resulting in a change in the element thickness and a change in the axial dimension, which, in turn, changes the length of the optical fiber. The change in fiber length is accompanied by a smaller change in the refractive index of the fiber. The result is a shift in the optical phase.