Abstract: A device for measuring electric fields, including electric fields having frequencies extending into the GHz range, including an optically active crystal sensor having an optical property which changes depending upon the strength of an applied electric field; a body contacting the optically active crystal sensor at a common surface portion, the body having a dielectric constant which is different from that of the optically active crystal sensor; a first optical conductor connected to the optically active crystal sensor for supplying light thereto; a device for polarizing the light supplied to the optically active crystal sensor; an analyzer device for converting shift in phase of the light transmitted through the optically active crystal sensor into a change in intensity of the transmitted light; a second optical conductor connected to the analyzer device for conducting the transmitted light from the analyzer device; and a detector for detecting change in the intensity of the transmitted light connected to the