Trial manufacture of electric field measuring system based on Kerr effect in gases

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A non-contact measurement of electric field is proposed based on Kerr effect in gas. With the Kerr sensing technique, the electric field can be measured without causing any field disturbance, Kerr constants of gases are, however, very small compared with those of liquids and this ideal technique has not been applied to the electric field measurement in gas.In this study, a system is developed for high sensitive measurement of the small phase retardation caused by the Kerr electro-optic effect of gas. An optical phase modulation technique is effectively adopted to improve the sensitivity of the system.The light beam emitting from a stabilized He-Ne laser (632.8 nm in wavelength, 3 mW in power) passes through a photoelastic modulator (PEM) and is put into a test vessel or open air depending on the measured gaseous material. The test vessel is made of stainless steel with glass (BK7) windows and filled with material gas. A pair of aluminum plane electrodes with length 1.3 m, width 160 mm and 20 mm gap is placed in parallel with each other in the vessel. A pair of reflection mirrors is set outside of the glass windows. The laser light goes back and forth 5 times in the middle space between the electrodes. The cross section of light bundle is approximately 50 x 4 mm rectangular.When the laser light passes through the PEM, the phase shift between two orthogonal components of the light is modulated with an external high frequency (50 kHz) driver. The phase offset based on the additional birefringence, which generates a noise, is compensated by the Soleil-Babinet compensator.With this optical system, (I) dc stressed uniform electric fields in sulphur hexafluoride, nitrogen, and carbon dioxide gases at the pressure of 0.2 to 0.5 Mpa and (ii) ac stressed uniform electric fields in atmospheric air are measured. The optical system is successfully used to obtain the Kerr constant of atmospheric air. 1.3xlO^<-15>[m/V^2].