Heterodyne readout holographic memory

Abstract

A holographic optical memory utilizes an optical heterodyne technique to significantly increase the signal-to-noise ratio during the readout stage of operation. A light source provides a coherent light beam which is split into a readout beam and a local oscillator beam. The readout beam is directed to one of the holograms stored in the memory medium and a portion of the readout beam is diffracted by the hologram to form a reconstructed image of the bit pattern stored in the hologram at the reconstructed image plane. The local oscillator beam is superimposed with the diffracted portion of the readout beam. An optical frequency translator is positioned in either the readout beam or the local oscillator beam to cause the beams to have different optical frequencies. Therefore, when the two beams are superimposed, a beat frequency signal is produced. An array of detectors is positioned at the reconstructed image plane to receive the superimposed beams. Each detector of the array is positioned to receive the light representing one bit of the bit pattern and to provide an output signal indicative of the intensity of the beat frequency signal received.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 
     
       1. In a holographic optical memory having a memory medium upon which a plurality of holograms are stored, a system for reading out a bit pattern stored in one of the holograms, comprising: light source means for providing a coherent light beam,   beam splitter means for splitting the coherent beam into a first and a second beam,   light beam directing means for directing the first beam to one of the plurality of holograms, a portion of the first beam being diffracted by the hologram to form at a reconstructed image plane a reconstructed image of the bit pattern stored in the hologram,   light beam superimposing means for superimposing the second beam with the diffracted portion of the first beam,   optical frequency translator means positioned in the path of one of the first and second beams to cause the one beam to have a different frequency from that of the other beam during readout, such that a beat frequency signal is produced when the first and second beams are superimposed, and   an array of detectors positioned at the reconstructed image plane, each detector positioned to receive light representing one bit of the bit pattern and to provide an output signal indicative of the intensity of the beat frequency signal received.   
     
     
       2. The invention as described in claim 1 wherein the memory medium is a magnetic film. 
     
     
       3. The invention as described in claim 2 wherein the magnetic film is manganese bismuth. 
     
     
       4. A holographic optical memory comprising: light source means for providing a coherent light beam,   beam splitter means for splitting the coherent light beam into a first and a second beam,   a memory medium for the storage of a plurality of holograms,   beam directing means for simultaneously directing the first and second beams to coincide at a selected region of the memory medium,   page composer means positioned in the path of the second beam between the beam splitter means and the memory medium for creating a bit pattern in the second beam during the writing stage,   optical frequency translator means positioned in the path of one of the first and second beams to cause the one beam to have a different frequency from that of the other beam, such that a beat frequency signal is produced when the first and second beams are superimposed,   beam intensity control means for causing the combined intensity of the first and second beams to be sufficient to store the bit pattern as a hologram during the writing stage, and insufficient to alter the hologram during the reading stage,   pivoting means positioned proximate the memory medium for pivoting, during the reading stage, superimposed beams comprising a diffracted portion of the first beam and an undiffracted portion of the second beam into a common reconstructed image plane, and   an array of detectors positioned at the common reconstructed image plane, each detector positioned to receive the light representing one bit of a reconstructed bit pattern formed by the diffracted portion of the first beam and to provide an output signal indicative of the intensity of the beat frequency signal received.   
     
     
       5. The holographic optical memory of claim 4 wherein the beam directing means comprises: light beam deflector means positioned between the light source means and the beam splitter means for deflecting the first and second beams to a plurality of resolvable spots,   mirror means positioned in the path of one of the first and second beams for changing the direction of propagation of the beam,   inverting means positioned in the path of one of the first and second beams for inverting the angular direction of the beam,   an array of individual lenses positioned in the path of the second beam, each lens being positioned at one of the plurality of resolvable spots, for reducing the beam diameter of the resolvable spots, and   field lens means positioned in the path of the second beam between the array of individual lenses and the page composer means for pivoting the second beam at a first pivot plane.   
     
     
       6. The holographic optical memory of claim 5 wherein beam inverting means comprises first and second lenses. 
     
     
       7. The holographic optical memory of claim 5 wherein the beam inverting means is positioned in the path of the second beam. 
     
     
       8. The holographic optical memory of claim 7 wherein the beam inverting means is positioned essentially at the first pivot plane and wherein the beam inverting means further pivots the second beam at a second pivot plane. 
     
     
       9. The holographic optical memory of claim 8 wherein the page composer means is positioned proximate the second pivot plane. 
     
     
       10. The holographic optical memory of claim 5 wherein the page composer means is positioned essentially at the first pivot plane. 
     
     
       11. The holographic optical memory of claim 4 and further comprising Fourier transform lens means positioned in the path of the second beam proximate the page composer means for performing a Fourier transform of the bit pattern produced by the page composer means. 
     
     
       12. The holographic optical memory of claim 4 and wherein the pivoting means comprises pivoting lens means. 
     
     
       13. The holographic optical memory of claim 12 wherein the pivoting lens means comprises a lens having a substantially flat surface and a curved surface. 
     
     
       14. The holographic optical memory of claim 13 wherein the memory medium comprises a deposited layer on the substantially flat surface. 
     
     
       15. The holographic optical memory of claim 4 wherein the memory medium is a magnetic film. 
     
     
       16. The holographic optical memory of claim 15 wherein the diffracted portion of the first beam and the undiffracted portion of the second beam are transmitted through the magnetic film. 
     
     
       17. The holographic optical memory of claim 15 wherein the diffracted portion of the first beam and the undiffracted portion of the second beam are reflected by the magnetic film. 
     
     
       18. The holographic optical memory of claim 15 wherein the magnetic film is manganese bismuth. .Iadd. 19. A system for reading out a bit pattern stored in a hologram, the system comprising: means for directing a first beam to the hologram, a portion of the first beam being diffracted by the hologram to form a reconstructed image of the bit pattern;   means for superimposing a second beam with the diffracted portion of the first beam, the second beam differing from the first beam such that a beat frequency is produced when the first and second beams are superimposed; and   means for detecting a beat frequency signal produced by the superimposed first and second beams for each bit of the bit pattern. .Iaddend..Iadd. 20. A system for reading out a bit pattern stored in a hologram formed by a reference beam and an information beam, the system comprising:   means for directing first and second beams having different frequencies onto the hologram, the first beam following the path of the reference beam, the second beam following the path of the information beam;   means for pivoting superimposed beams comprising a diffracted portion of the first beam and a portion of the second beam into a common reconstructed image plane; and   means for detecting a beat frequency signal produced by the superimposed first and second beams, the beat frequency signal representing a bit of the bit pattern. .Iaddend..Iadd. 21. A heterodyne system for reading out a bit pattern stored in a hologram formed by a reference beam and an information beam, the heterodyne system comprising:   light beam directing means for directing a readout beam to the hologram, a portion of the readout beam being diffracted by the hologram to form a reconstructed image;   light beam superimposing means for superimposing a local oscillator beam with the diffracted portion of the readout beam; and   detector means for detecting a beat frequency signal generated by the superimposed beams, the beat frequency signal representing a bit of the   
     
     
        bit pattern. .Iaddend..Iadd. 22.  The invention of claim 21 wherein the readout beam follows a reference beam path and the local oscillator beam follows an information beam path. .Iaddend..Iadd. 23. In a holographic optical memory having a memory medium upon which a plurality of holograms are stored, a system for reading out a bit pattern stored in one of the holograms, the system comprising: light beam directing means for directing a first beam to one of the plurality holograms, a portion of the first beam being diffracted by the hologram to form, at a reconstructed image plane, a reconstructed image of the bit pattern stored in the hologram;   light beam superimposing means for superimposing a second beam with the diffracted portion of the first beam, the first and second beams having different frequencies; and   an array of detectors positioned at the reconstructed image plane, each detector positioned to receive light representing one bit of the bit pattern and to provide an output signal indicative of the intensity of a beat frequency signal produced by the superimposed beams. .Iaddend. .Iadd. 24. The invention of claim 23 wherein the first beam follows a reference beam path and the second beam follows an information beam path. .Iaddend..Iadd. 25. An arrangement for reading out a hologram formed by the interference of a reference beam and an object beam comprising in combination:   means for directing two spatially-unmodulated beams onto the hologram, one following the path of the reference beam and the other following the path of the object beam, the two beams differing from one another such that a beat frequency is produced when the first and second beams are superimposed; and   means for detecting the beat frequency component of the image reconstructed by the two beams. .Iaddend. .Iadd. 26. A method of reading out a hologram formed by the interference of a reference beam and an object beam, comprising the steps of:   directing two spatially unmodulated beams onto the hologram, one following the path of the reference beam and the other following the path of the object beam;   frequency translating one of the beams such that a beat frequency is produced when the two beams are superimposed; and   detecting at least the beat frequency component of the image reconstructed by the two beams. .Iaddend..Iadd. 27. An arrangement for reading out a hologram formed by the interference of a reference beam and an object beam comprising, in combination:   means for directing two spatially unmodulated beams onto the hologram, one following the path of the reference beam and the other following the path of the object beam;   means for frequency translating one of said beams such that a beat frequency is produced when the two beams are superimposed; and   means for detecting at least the beat frequency component of the image reconstructed by the two beams. .Iaddend.