Patent GB-1328282-A (GB)
Title: Electronic-optical memory system
IPC: G11C11/411
Filed: 1970-10-08
Abstract: 1328282 Magnets-optic storage arrangements RCA CORPORATION 8 Oct 1970 [15 Oct 1969] 48021/70 Heading H3B [Also in Divisions G4 and H1] An electrically and optically accessible memory comprises a light source, e.g. laser 10, a light sensitive storage medium 26, and a matrix array of [light valve] units 30 each of which consists of a flip-flop, a light valve, and a photosensor, the light valve being operated in response to the state of the flip-flop during a write operation and the flip-flop being set or reset by the output of the photosensor during a read operation. Herein the terms "read" and "write" are used in respect of the light sensitive storage medium 26 rather than the flip-flop. The laser beam is passed through a polarization rotator 11 which is responsive to one of two electrical inputs R and W to rotate the polarization of the beam according to whether a read or write operation is required. A prism 17 which may be formed from a pair of birefringent crystals reflects the beam via lenses 18 and 35 when it is read polarized and transmits the beam to a beam splitter 20 when it is write polarized. An X-Y beam deflector 12 deflects the beam along one of the parts 14, 14<SP>1</SP> or 14<SP>11</SP> or any intermediate path in order to selectively address the storage medium 26. A light valve unit from the matrix array 30 is shown in Fig. 5, the flip-flops in the array are electrically addressable via word Wi and bit Di lines in the usual way, the bit lines being divided into two, one, w, for writing and the other, r, for reading. Each flip-flop is connected to its associated light valve which is opened or closed depending on the state of the flip-flop when a write enabling signal Ew occurs. A photosensor is also connected to the flip-flop which is set when light passes through the sensor during reading and a read enabling signal Er is present. In a write operation the beam passes through prism 17 and a portion of it is passed through the beam splitter 20 and focussed on the array 30. A hologram mask 27 is provided to further split the beam such that only the light valves in the array 30 which is formed as an integrated circuit are illuminated. The beam is then focussed on to a small portion of the storage medium 26 in accordance with the X-Y deflection imparted by the deflector 12. The data to be stored is electrically written into the flipflops in the array 30 in the usual way. When the write enable signal Ew occurs, which in practice is when the laser 10 is pulsed, the light valves are selectively actuated in accordance with the data stored in the flip-flops. The beam is then passed or not passed by each valve and impinges on the storage medium 26 together with the unmodulated part of the beam from the beam splitter 20 to form a hologram. On read out the polarization of the beam is reversed, the beam is transmitted via prism 17, lenses 18 and 35 and mirror 34, and impinges on a small portion of the storage medium 26 in accordance with the X-Y deflection imparted by deflector 12. The array 30 is illuminated by the reflected light and the photosensors in the array cause the corresponding flip-flops to be set. The data may be electrically read from the flip-flop array in the usual way. The storage medium may consist of a thin layer of manganese bismuth deposited on mica or saphire which is heated to form the manganese bismuth into a single crystal and subjected to a strong magnetic field to align the magnetic atoms. The direction of magnetization of the surface is changed by incident optical energy having an intensity above a threshold value. The read beam is reflected with a polarization dependent on the stored image due to the magneto-Kerr effect. Alternatively read out is accomplished by Faraday magneto-optic rotation of a reference beam transmitted through the manganese bismuth film. Read out may be destructive or non-destructive depending on the intensity of the read beam. The light valves may be a liquid crystal device or an area of natural or intrinsic silicon deposited on a saphire substrate and the photosensors may be PN-diodes.