GB1112019A - Magneto-optical device - Google Patents

Abstract

1,112,019. Light deflectors. MULLARD Ltd. 14 April, 1967 [22 April, 1986], No. 17757/86. Heading H4F. Apparatus for deflecting a beam of plane polarized infra-red radiation into one of two directions comprises a Faraday-rotation device in the form of a single-crystal block of yttriumiron-garnet (YIG) and a birefringent prism, a magnetic field being applied to the YIG block in a direction parallel to the beam path to cause the plane of polarization of the beam to be rotated through + 45 or - 45 degrees depending on the direction of the magnetic field and according to the direction of rotation the beam is deflected into one of two directions by means of the birefringent prism. As shown, Fig. 4, an infra-red beam from a laser X is plane polarized by polarizer R and passed through a YIG block G of such length as to give a 45- degree rotation when magnetized by a coil E, the magnetic circuit being completed by two yoke members Y of ferrite material, e g polycrystalline YIG material. Prism K made from a birefringent crystal of rutile (TiO 2 ) with its refracting edge parallel to the optic axis is disposed with its optic axis perpendicular to the beam path so as to deflect the beam into one of two positions on screen S depending on the direction of polarization of the light beam, i.e. the direction of the magnetic field. The YIG block G (a polarization switch) and prism K constitute a binary deflection unit and a succession of N such units form a one-dimension digital deflection bank with 2<SP>N</SP> displacement positions if the prism angles are so arranged that each successive prism gives twice the deviation. With a second bank, the birefringent prisms being orthogonal to the prism of the first bank a two dimensional array is formed. Since the length of the beam path in the YIG is only approximately 1À8 mm. for 45 degrees rotation with an infra-red beam of wavelength 1À15 microns the design of the magnetic yoke is difficult and thus is not used. Instead the disc D, Fig 8, of YIG is held coaxially within a coil E which produces the magnetic field. The disc D is magnetically saturated by means not shown, but to reduce the high value of saturation magnetization of YIG a gallium-YIG crystal (Y 3 Fe 3.85 Ga 1.15 O 12 ) is used. The disc is cut with its easy direction [111] normal to the plane of the disc and is coated with an anti-reflection coating F and an aluminized mirror coating C. In order to produce a rectangular array of positions to which the beam is deflected 45 degrees polarization rotators are provided between the polarization switches so that the net effect of beam rotation is from zero to 90 degrees rather than from + 45 to - 45 degrees. The rotators may comprise a disc D or YIG, Fig 9, coated with an aluminized mirror coating C and an antireflecting coating F, fixed to a permanent magnet M magnetized in the direction H. A two-dimensional rectangular deflector utilizing the switches J and rotators L, Figs. 8 and 9, is described with reference to Fig. 10, not shown.