Patent GB-1221355-A (GB)
Title: Arrangement for controlling electromagnetic wave energy
Inventor: BALLMAN ALBERT ANTHONY
IPC: G11C13/04
Filed: 1968-06-14
Abstract: 1,221,355. Light modulators. WESTERN ELECTRIC CO. Inc. 14 June, 1968 [16 June, 1967 (2)], No. 28349/68. Heading H4F. [Also in Division G4] In a light modulating arrangement, Fig. 1, the optical activity of a crystal 13 of bismuth germanium oxide is controlled by the combination of an electric bias filed 18 applied across the crystal and a light beam 26 directed at the crystal at least part of the spectrum of which lies inside the absorption band of the crystal, the light beam 11 to be modulated (the major part of the spectrum of which lies outside the absorption band of the crystal) being preferably polarized 12 and detected 17 via an analyser 16 or a doubly refracting member (36, Fig. 3, not shown) in which the modulated light beam is deflected accordnig to polarization. Crystal 13 comprises photo-active material which exhibits photoconductivity and natural optical activity which rotates the plane of polarization of beam 11, a further rotation resulting from the stimulus of both bias field 18 and side light 26 which together generate a significant number of charge carriers in the crystal. Variations in the intensity of beam 26 and/or, in the magnitude of the bias field supplied by source 18 change the rotational effect of crystal 13 and consequently the intensity of illumination at detector 17. Variations in the intensity of light beam 11 do not affect the rotational effect of crystal 13. The presence of the electric field in crystal 13 causes some linear birefringence that results in some elliptical polarization in beam 11 emerging from crystal 13, but this may be compensated. To reduce the bias field required ferroelectric enhancement may be utilized by positioning on either side of the crystal, barium titanate crystals (21, Fig. 1A, not shown), each poled to one of its two relatively stable charge states by a voltage appropriate to its thickness, or by poling a single ferroelectric crystal in a separate circuit and placing that crystal in contact with one edge of crystal 13 parallel to the electric field. A plurality of crystals 13 may be arranged in tandem, Fig. 2 (not shown) each subjected to variable electric bias fields so that in operation when light from light source 23 or from a plurality of such light sources is directed on to one or more of the crystals the change in polarization rotation of the light beam 11 is dependent upon the electric field or fields established in the crystal or crystals. A scratch-pad memory application of the above arrangement, Fig. 5, comprises a bismuth germanium oxide crystal 110 which stores domains of altered optical activity for periods approximating one minute. As shown light source 111 emits a broad monochromatic light beam 112 via polarizer 113, half-silvered 116, analyser 122 and crystal 110 to a detector array 117. The crystal 110 is located in an electric field produced by an adjustable bias source 118. The magnitude of the bias source affects the contrast of domains of altered optical activity in the crystal, the domains being stored in the crystal by means of a light source 129 the modulated light beam 128 from which is deflected to activate selected areas (domains) of the crystal 110 by means of deflection control 131 controlled by address data signals from a central control 127. The outputs of the detector array 117 are fed to a buffer register 123 from which they may be utilized 126 or rewritten into the crystal if gate 139 is opened. Although read-out from crystal 110 is continuously available (light source 111 is continuously operative), writing beam 128 is gated on and off to avoid shifting the position of the nucleated domain of enhanced optical activity. To accomplish single bit (domain) erasure control system 127 operates an adjustable resistor 121 to reduce the voltage on bias source 118 when the beam 128 is incident on the domain. To erase a word the relevant adjacent bit locations are scanned by beam 128 while the bias is at reduced level. The entire memory can be erased in a single operation by flooding the crystal with intense light 140 in its absorption band, e.g. white light. Instead of storing binary information the intensity of beam 128 may be varied to any one of a plurality of different intensity levels so that the resulting signals applied to buffer 123 have correspondingly different magnitude levels. Ferroelectric enhancement may be utilized, Fig. 6 (not shown) to improve the contrast of nucleated storage domains. To increase the time during which domains of enhanced optical activity are retained the crystal is positioned in a magnetic field which is perpendicular to the axis of the electric field.