Patent GB-1313057-A (GB)
Title: Magneto-optical probes
IPC: G02F1/09
Filed: 1971-04-19
Abstract: 1313057 Magneto-optical apparatus BROWN BOVERI & CO Ltd 19 April 1971 [4 Feb 1970] 20855/71 Heading G1A [Also in Division H4] In the measurement of H.T. electric currents by the use of an optically active medium which under the influence of the magnetic field produced by the current causes a Faraday rotation of the plane of polarization of polarized light passing through the medium, errors due to the residual birefringence of the medium are compensated for in a number of ways. In one arrangement, the beam is elliptically polarized before its entrance into the medium using e.g. a plane polarized beam direct, or via a polarizer, from a laser, and passing this through a rotatable half-wave and a rotatable quarter wave plate, by which the inclination and eccentricity, respectively, of the ellipse may be adjusted so as to provide the required compensation. The eccentricity may alternatively be adjusted by variation of thickness instead of the rotation of the quarter wave plate. The inclination of the ellipse may alternatively be adjusted by the use of an optically active material e.g. a solution, whose thickness or concentration is varied, or the magnetic field applied to which, is varied. As an alternative to or in addition to the above means, the magneto-optical medium whose Faraday rotation is under test may be of a particular construction i.e. comprised of at least two component media, connected by cement or liquid of approximately the same refractive index. The device is provided, for example, by parts cut from the same flint glass body and having the birefringence principal axes alternately rotated <SP>#</SP>/ 2  with respect to one another. The essential condition is that the refractive indices and/or thicknesses are selected so that the value of the expression # f  {n x <SP>(i)</SP>-ny<SP>(i)</SP>} l i  is less than 0.1/2## o  where n x <SP>(i)</SP>and n y  <SP>(i)</SP> represent the refractive indices of the i<SP>th</SP> portion of the medium for two perpendicular directions x and y of vibration, l i  is the length of the light path in the i<SP>th</SP> portion an # o  is the wavelength of light in vacuum. Fig. 1 shows a suitable system including laser 1, polarizer 2, half and quarter wave plates 4, 5 together with the complex medium 6. In this case, the thickness of the sections 6a, 6c of the flint glass block add up to the thickness of section 6b, and 6b is at 90 with respect to the other two. Multiple paths through the block are provided by use of mirror end surfaces to give maximum Faraday rotation, while the construction gives substantially equal optical path lengths for the different refractive indices to provide the necessary avoidance of residual birefringence. The issuing substantially plane polarized beam 2 is beamed by a dual analyser photo-detector arrangement to determine the Faraday rotation. In constructing the magneto-optical medium, the component media are made thin so as to avoid birefringence error building up in each component. A method of checking when the birefringence error is minimal is described. This involves applying the detector outputs to a C.R.T. and examining the deviation from a perfect circle, (when no Faraday effect is present).