Patent GB-1098932-A (GB)
Title: Improvements in and relating to optical data storage system
IPC: G09G3/00
Filed: 1965-04-01
Abstract: 1,098,932. Lasers. INTERNATIONAL BUSINESS MACHINES CORPORATION. April 1, 1965 [April 1, 1964], No. 13793/65. Heading H1C. [Also in Divisions G4 and H3. An optical data storage system comprises a laser, having a partially transparent cavity reflector, a storage element for causing predetermined filaments of the active medium to lase in accordance with stored information, and means including a photo-responsive device arranged in the path of the laser output beam for providing an indication of the information stored. In the arrangement of Fig. 1 a ruby 1 laser has inserted in its resonant cavity 6, 8 a storage element 2 in the form of a plate of magnetic material having a highly reflective surface which surface is divided into areas magnetized in different directions. Due to the magneto-optic effect the light in the laser becomes fully polarised in one direction or another at right angles thereto depending on the direction of magnetization of the areas and hence is representative of the bit stored in the area. Information is stored by applying coincident currents to selected wires of two orthogonal sets of conductors, Fig. 2, extending over the magnetic storage plate 2. Analyser 12, orientated to pass light polarized in the direction indicated by arrows 9 and to block light polarized in the direction indicated by arrows 10, is positioned as shown with crossed analyser 22 between mirror 8 and photo-detector 25. By directing an electron beam 16 against one face of a crystal of ammonium di-hydrogen phosphate 14 situated between the two analysers the plane of polarization of the light from the analyser 12 is rotated so that it then passes through the crossed analyser 22. If the electron beam is directed to a spot which receives no light from the analyser then no light is passed to the second analyser. All the information stored in the plate 2 is read out by directing the electron beam 16 to the areas of mirror 8 corresponding to the storage areas in turn, an output from photodetector 25 representing a " 1 " and no output a " 0." In the arrangement of Fig. 3 (not shown) a light deflection system comprising a plurality of birefringent crystals and electrooptic elements are employed to deflect light from any selected point on the mirror 8 to the photo-detector. The system of Fig. 5 employs an electro-optic device 70 and electron beam 73 as a storage element. Brewster windows 65, 66 at the end of the negative temperature medium 64 allow light polarised linearly in the vertical direction to pass through the medium without loss. A quarter wave plate 68 positioned between device 70 and mirror 6 when the electro-optic device 70 is not actuated results in the linearly polarized light being firstly circularly polarized and then after reflection by mirror 6 horizontally polarized so that insufficient light passes through the medium 64 to effect a lasing action. If the electron beam 73 is directed to a point on the surface of the device 70 light from the medium 64 becomes circularly polarized at that point, is linearly polarized by quarter-wave plate 68 and after reflection by mirror 6 reaches window 65 polarized in a vertical direction and effects a lasing action within this portion of the beam. The electron beam is directed to different portions of the device 70 where it is required to establish light rays, the electric charge at a point remaining for a sufficient period of time until the beam is again directed there. The arrows marked on partially reflecting mirror 8 represent one value of a binary bit and the absence thereof the other. This information is read out using the arrangement of Fig. 1 less analyser 12 or of Fig. 3 suitably modified. In the arrangement of Fig. 7 (not shown), one of the cavity mirrors of a laser comprises a plurality of small mirrors mounted at points where it is desired to store information representative of "1" bits. Thus lasing occurs only in those filaments which are provided with a small mirror.