Patent GB-1424958-A (GB)
Title: Optical memory apparatus
IPC: G11C11/42
Filed: 1974-04-24
Abstract: 1424958 Data store INTERNATIONAL BUSINESS MACHINES CORP 24 April 1974 [18 June 1973] 17960/74 Heading G4C [Also in Divisions H3-H5] A single number, e.g. 0 to 15, is stored along one of a plurality of discrete spatial paths extending through a stack 60 of ferroelectric layers, Fig. 1, as a characteristic series of localized remanent potentials obtained by application to each layer of a selected potential from the group W1<W2<W3<W4, these localized remanent potentials through the layers determining the birefringence effect on a broad spectrum light beam 20L projected along a selected discrete path and therefore identifying the number stored in terms of the colour over a limited spectral range passed by the layers. This colour is identified and passed to an encoder 52 by a group of photo-detectors each responsive to a different colour. To successively read out numbers stored in different discrete paths, or for erasing or writing, the light beam 20L from a tungsten or xenon lamp 11 is stepped by a scanner 20 through an X-Y raster scan. The scanner, Fig. 2, comprises a polarizer 24P and a series of ferroelectric analyser plates 21, the localized optical properties of the plates being changeable to provide a selected spatial path for the light beam 20L. Such a transmission path is determined by applying a potential 31 to a selected single electrode in each of respective groups 22, 23 of parallel transparent electrodes interleaved between the plates, adjacent electrodes being electrically separated by transparent insulators 24. The selected application of potential 31 is determined by a scanner energizer and control circuit, Fig. 1. The combination of the scanner and the ferroelectric storage stack is shown in Fig. 3, each ferroelectric layer 63 forming part of a section 60A- 60E which includes a transparent electrode 64 and the combination 62 of a photo-conductive layer 62A isolating a transparent electrode 62B from its ferroelectric layer. The stack is terminated by an analyser 65<SP>1</SP>. Fig. 7 shows the remanent birefringent properties of a ferroelectric layer 63, the degree of birefringence being determined by a write potential W1-W4, while birefringence is removed by an appropriate erase potential E1-E4. To write into the store a potential Vx is applied over leads 66, 67 to section 60A, and selected write potentials of amplitude Va-Vd are applied over leads 68-75 to sections 60B-60E. Thus to write the number 6, for example, section 60B is placed at a potential Vd, section 60C is placed at potential Vc and sections 60D, 50E are at zero potential. These potentials are applied from a storage energizer and control circuit 40, Fig. 1. The discrete region in each ferroelectric layer where a birefringence state is to be established is determined by the path taken by the light beam, this beam causing a localized conduction of each photo-conductive layer 62A which effectively couples the correspondingly localized regions of the electrodes 62B with the ferroelectric layers. A similar combination of appropriate potentials E1-E4 and a light beam is used for selective erasure. Each writing or erasure operation is followed by a verification test in which the photo-detector output to the encoder is compared by a comparator with the write or erase instruction. Control circuit detals are disclosed, Figs. 4, 5, 6 and 8 (not shown).