<title>Optical dc current and voltage measurement by superposing ac magnetic or electric field</title>

Abstract

Optical DC current and voltage measurement using Faraday or Pockels effect necessitates the compensation of light intensity variation with some methods, because it is possible the intensity variation may give rise to a measuring error. We have proposed a method which suggests superposing AC magnetic field on linearly polarized light or AC electric field on elliptically polarized light and compensating light intensity variation by detecting AC components caused by constant AC magnetic or electric field. Experiments of long term stability for DC current and voltage measurement were carried out by using flint glass for DC current measurement and B<SUB>12</SUB>SiO<SUB>20</SUB> (BSO) crystals for DC voltage measurement. An experimental result of DC current measurement where DC current of 1.0 A was passed under AC current of 1 kHz 1.5 A<SUB>rms</SUB> showed that the modulation depth without compensation through AC magnetic field varied largely with time because of the temperature rise of the flint glass, but that with compensation through AC magnetic field hardly varied with time and the effectiveness of AC magnetic field superposition was confirmed. The same compensation method was applied to optical DC voltage measurement using Pockels effect. An experimental result of DC voltage measurement where DC voltage of 500 V was applied under AC voltage of 5 kHz 50 V<SUB>rms</SUB> clarified that the time variation of the modulation depth with compensation through AC electric field was small compared with that without compensation through AC electric field.