Optimization of an optical device for low frequency electric field measurement

The conventional measurement systems for low frequency (50-60 Hz) high electric fields measurement currently lay on active metallic probes which can disturb the measured electric field. The present study concerns an optical device using a LiNbO3 crystal, which is known to be electrooptic, as sensing medium. The experimental set-up uses two electrodes to create the electric field to be measured. The principle of measurement implements the electrooptical effect in the crystal without any contacting electrode and is based on the classical static method in which the polarization of light is analyzed after going through the crystal. In order to minimize thermo-optical effects, the light beam propagates along the optical axis and the electric field is applied perpendicularly to the beam. Thanks to an excellent agreement between simulation with Finite Element method and experiment, the shape of the crystal and Its protection ring are optimized. The crystal used has a regular octagonal section (radius # 1.5 mm) and a length of 8 mm. The output signal of the device is proportional to the applied electric field in the range 0-200 kV.cm(-1). A good insulation is obtained. Besides, good linearity and reproducibility are observed, the sensibility of the sensor being about 4 muA/(kV.cm(-1)).