We describe a fiber optic voltage sensor with optically controlled sensitivity. The sensor operates at two widely separated wavelengths (633 and 976 nm), one of which is a control signal (976 nm). We show that at a properly chosen wavelength of the control signal and of the phase-retarding element, variations of the power of the control signal allow increases or decreases in the sensitivity of the sensor. A theoretical analysis of sensitivity as a function of the optical power of a control signal is presented. We have demonstrated experimentally variation of the sensor's sensitivity from 0.01% to 0 per 1 V/sub rms/ of control power changes in the range of 0-7 /spl mu/W.