GB1123840A - Electro-optic apparatus - Google Patents

Abstract

1,123,840. Light modulators. INTERNATIONAL BUSINESS MACHINES CORP. 7 Dec., 1966 [3 Jan., 1966], No. 54705/66. Heading H4F. [Also in Division H1] Relates to electro-optic apparatus for controlling the direction of emission of laser light comprising a body of material exhibiting electro-optic and photoconductive effects on which a charge pattern is deposited to produce the electro-optic effect which alters the polarization of an incident polarized light beam, the charge pattern being erased by means of a source of light having a frequency within the range of frequencies which cause the body of material to become conductive. The frequency range of the incident polarized light is selected to be outside the range of frequencies capable of producing a photoconductive effect. The direction of emission of laser light from a ruby 10 is controlled by a cathode-ray tube 12, the face of which comprises a crystal 18 of material selected to exhibit the electro-optic effect at the location where the electron beam 14 strikes and thus cause laser emission along a particular mode of oscillation 16. To discharge the field set up across the crystal the material of the crystal is selected to exhibit a photoconductive effect in response to the light from lamp 20. The laser cavity is formed by a mirror 28 and a dielectric mirror coating 26 applied to the inner surface of crystal 18. A transparent coating 24 of stannous oxide provides a ground reference for the electron beam. In operation light in a plurality of angularly oriented modes of oscillation from ruby 10 is plane polarized 32 and passed via phase plate 34 to the crystal 18. Plate 34 introduces a relative phase delay between the two othogonal components of light and is oriented at 45 degrees with respect to polarizer 32. The phase delay introduced by plate 34 is selected so that the intensity of the light returning to laser 10 is insufficient to cause the laser to oscillate. To obtain laser emission, the phase delay introduced by plate 34 must be compensated for by an opposite phase delay introduced by crystal 18 due to the deposition of charge by electron beam 14 producing a field across the crystal. Laser emission along other modes of oscillation may be achieved by moving the position of electron beam 14. Crystal 18 may comprise ZnS, GaAs, GaP, SiC or ZnTe. When illuminated by lamp 20 the charge is conducted through to coating 24 or a battery 40 connected by an electrode 41 to the upper portion of the crystal and another electrode 43 connected to ground may be employed to drain off the charge when the crystal becomes conductive. The charge pattern may be erased by a pulse of illumination from lamp 20 and in addition the lamp may provide a constant illumination, battery 44 and resistor 46, to increase the inherent conductivity built into the crystal during manufacture.