A new optical field sensor using a Pockels device for measuring the electric field distribution in the discharge space has been developed. This system consists of both azimuth angle modulation and PS-polarized light division methods. By using a ferroelectric liquid crystal phase modulator (FLCM) instead of an azimuth angle modulator，we succeeded to theoretically eliminate all of noises existing in the optical field measurement system,especially，the birefringence eftects in a longer optical fiber cable. This fact has been confirmed experimentally under the following circumstance； (1) the size of Pockels crystal is 1mm φ in diameter and 5mm in length，(2 ) the gap　length between needle and plane electrodes is 20mm，(3 ) A.C applied voltage is 1 to 15 kV，etc. From the theoretical and the experimental works，it is possible to say that the present system is a kind of noiseless sensors and furthermore is one of the most useful sensors for holding the safety in the field of electrical insulation facilities.

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