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MEASUREMENT OF SURFACE CHARGE USING THE POCKELS EFFECT P L Lewin' The electro-optic effect is well documented and electro-optical sensors have been used for measuring high voltages and space charge fields in electric discharges'. Generally these sensors have been designed for point measurement and in order to create a two dimensional measurement mechanical scanning has been incorporated. In order to apply the electro-optic effect to the measurement of the dynamic surface charge distribution on an insulating material there are several significant design considerations. 0 The sample material should ideally be bonded to the electro-optic crystal. Unlike the field probe which may be positioned above the material surface. The measurement is dependent on reflection of polarised light from the interface between the crystal and the sample. With a field probe the light passes through the crystal. Noise and interference generated by the light source, external light sources and mechanical shock must be minimised to ensure a reasonable accuracy of measurement. The system requires careful calibration. 0 Several materials exhibit the electro-optic effect and research has been completed in Japan using Bismuth Silicate (Bil$i020). Recent work at Southampton has concentrated on the use of Lithium Niobate (LiNb03) wafers to monitor surface charge. The Electro-optic Effect Consider linearly polarised light propagating along the z optical axis of a crystal (fig I) exhibiting a birefringence along the x and y optical axes (where x is the slowest axis). When light propagates through the crystal the velocities v, and vy of the electric field components (e,, and eyl) of the light become different due to the birefringence. Thus an optical phase retardation, 6, is produced between the x and y electric field components (ex2 and ey2) of the light transmitted from the crystal. I
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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