Patent GB-1170939-A (GB)
Title: Optical Memory Systems.
Inventor: KURTZ STEWART KENDALL
IPC: G11C13/04
Filed: 1966-12-22
Abstract: 1,170,939. Optical memory system. WESTERN ELECTRIC CO. Inc. 22 Dec., 1966 [23 Dec., 1965; 24 March, 1966], No. 57427/66. Heading H4F. [Also in Division G4] An optical storage device 114 comprises a polarizing layer P having an electro-optic layer EO on one side thereof and a photo-conducting layer PC on the other side, the three layers being electrically in series and having a voltage connected across them arranged such that when the electro-optic layer is illuminated with light polarized in a direction that renders the polarizing layer opaque to the light and the photoconductor layer is illuminated by another beam of light, the voltage across the electrooptic layer at a position corresponding to the illumination of the photo-conducting layer becomes such that the polarization is rotated to a direction rendering the polarizing layer transparent so that light is passed by the polarizing layer to represent information. In one embodiment light from a light source III such as a laser passes through a digital light deflector to illuminate one of a matrix of predetermined areas on the storage device. The storage device has on each face a transparent electrode 31, 32 (Fig. 5), to which is connected a voltage source. For writing, the illumination intensity of the light from source III is such as to lower the impedance of the photoconducting layer, which otherwise is very much greater than the resistance of the polarizing and electro-optic layers. Thus the voltage applied now appears mainly across the electrooptic layer and is sufficient to cause rotation of the plane of polarization of light applied to it. Light is applied from a source 115 polarized such as to render layer P opaque. The EO layer rotates the plane of polarization and the light now is able to pass through the P layer and maintains the low resistance of the PC layer when light source 111 is extinguished. On reading, light of a lower intensity is emitted from source 111, passes through any selected transparent location representing a " 1 " and is detected by unit 120. Alternatively light from source 115 passing through appropriate transparent locations in the storage unit passes back through the digital light deflector, retracing the steps shown and is detected by a unit, not shown, situated between source 111 and deflector 112. When zeros are to be written light from source 111 is extinguished and the storage unit remains opaque. In an alternative storage unit (Fig. 5) the electro-optic layer comprises electro-optic spots 35 corresponding to the output positions of the deflector 112 but surrounded by photo-conductive rings 37, each ring and spots being in contact with a transparent electrode e. The unit works as previously described except that when the rings 37 are illuminated by a suitable light beam their resistance is lowered to effectively short-circuit the voltage across the electro-optic layer and so erase any " 1 " that is written. The light effecting each one of the photo-conducting layers should be such as to have no effect on the other photo-conducting layers. Alternatively (Fig. 7), the electrodes are formed of vertical Y-strips and horizontal X- strips, memory locations being formed at their cross-over points which are transparent or apertured to transmit light. At one end of each rod, outside the area of the memory matrix is situated a photo-conducting layer Pcy, Per. Above and below this layer are resistors connected to ground. In the unilluminated state the layers Pcy, Pcx are effectively open circuit and the voltage across the storage unit is determined by voltage source 130 and the value of resistors R, Rx1, Ry1. When a layer Pcy and a layer Pcx are illuminated the resistance of Pcx, Pcy are lowered and resistors Ry2, Rx2 partly short out Ry1, Rx1 to lower the voltage across the selected junction to a value less than that required to rotate the polarizing direction, thus extinguishing any data recorded.