Method and apparatus for phase-encoded homogenized fourier transform holographic data storage and recovery

Abstract

An apparatus for writing and reading holograms, comprising a spatial light modulator (SLM) operable in phase mode, having a plurality of pixels for generating an object beam that overlaps with a reference beam; a holographic recording medium (HRM) in the path of the object beam; and a first lens element disposed in the path of the object beam between the SLM and the HRM; wherein the HRM is disposed at or near the Fourier transform plane of the first lens element.

1 . A method of reading a phase-encoded hologram, comprising:
 directing a reference beam at a location in a holographic recording medium where a phase-encoded Fourier transform hologram or fractional Fourier transform hologram was recorded, thereby reconstructing said phase-encoded hologram, said reconstructed hologram comprising images of edges of pixels, said images corresponding to transitions between pixels recorded by light wavefronts having different phases;   detecting the reconstructed hologram with a detector having resolution sufficient to detect the edges of pixels; and   assigning a value of “0” or “1” to each pixel based on the image of the edges of pixels, said image corresponding to transitions between pixels recorded by light wavefronts having different phases.   
   
   
       2 . The method of  claim 1  wherein assigning a value of “0” or “1” to each pixel includes oversampling, whereby each pixel of the SLM is detected by more than one pixel of the detector. 
   
   
       3 . The method of  claim 1  wherein assigning a value of “0” or “1” to each pixel includes image processing. 
   
   
       4 . The method of  claim 1  wherein assigning a value of “0” or “1” to each pixel includes detecting the gradient of intensity of the reconstructed hologram. 
   
   
       5 . The method of  claim 1  further including translating the detector in the plane of the detector. 
   
   
       6 . The method of  claim 1  wherein assigning a value of “0” or “1” to each pixel includes detecting at least one fiducial mark corresponding to a pixel of a known state and assigning a value of “0” or “1” to each pixel relative to the detected fiducial mark. 
   
   
       7 . A method of reading holograms, comprising:
 directing a reference beam at a selected location in a holographic recording medium where a phase-encoded Fourier transform hologram or fractional Fourier transform hologram was recorded, reconstructing a first object beam and directing the first object beam to a detector;   illuminating a phase spatial light modulator (SLM) displaying a uniform data page, thereby forming a second object beam and directing the second object beam to said detector, thereby producing an interference pattern between the first object beam and the second object beam at the detector that reproduces an amplitude-modulated data page; and   detecting the amplitude-modulated data page.   
   
   
       8 . The method of  claim 7  wherein the light wavefront reflected from or transmitted through any pixel of the SLM has a phase of φ=0 or π. 
   
   
       9 . The method of  claim 7  wherein the intensity of the second object beam is substantially equal to the intensity of the first object beam. 
   
   
       10 . A method of recording a homogenized hologram, comprising:
 recording a uniform data page hologram; and   recording a Fourier transform hologram or a fractional Fourier transform hologram at the same storage location.   
   
   
       11 . The method of  claim 10  wherein the Fourier transform hologram or a fractional Fourier transform hologram is a phase-encoded homogenized hologram. 
   
   
       12 . The method of  claim 10  wherein the light wavefront reflected from or transmitted though any pixel of the SLM has a phase of φ=0 or π. 
   
   
       13 . The method of  claim 11  wherein the uniform data page hologram and the homogenized hologram are recorded using the same reference beam. 
   
   
       14 . The method of  claim 11  wherein the uniform data page hologram and the homogenized hologram have substantially the same diffraction efficiency. 
   
   
       15 . The method of  claim 13  wherein a first uniform data page hologram and a first homogenized hologram are recorded using a first reference beam, and further wherein a second uniform data page hologram and second homogenized hologram are recorded at the same storage location, or a substantially overlapped storage location using a second reference beam. 
   
   
       16 . The method of  claim 11  wherein the uniform data page hologram and the phase-encoded homogenized hologram are recorded with substantially the same diffraction efficiency. 
   
   
       17 . The method of  claim 11  wherein a uniform data page hologram and more than one phase-encoded homogenized holograms are recorded at the same storage location. 
   
   
       18 . The method of  claim 11  wherein the uniform data page hologram and the homogenized hologram are recorded using different reference beams. 
   
   
       19 . The method of  claim 18  wherein the uniform data page hologram and the phase-encoded homogenized hologram are recorded with substantially the same diffraction efficiency. 
   
   
       20 . The method of  claim 18  wherein two or more homogenized holograms are recorded, each homogenized hologram being recorded with a different reference beam. 
   
   
       21 . A method of reading a homogenized hologram, comprising:
 directing a reference beam at a selected location in a holographic recording medium where a phase modulated Fourier transform hologram or fractional Fourier transform hologram and a uniform phase-encoded data page hologram were recorded using the same reference beam, thereby reconstructing a first object beam, used to record a first homogenized hologram, and a second object beam, used to record the uniform data page hologram, thereby producing an interference pattern between the first and second object beams that reproduces an amplitude-modulated data page; and   detecting the amplitude-modulated data page with a light detector.   
   
   
       22 . A method of reading a homogenized hologram, comprising:
 directing a first reference beam at a selected location in a holographic recording medium where a phase-encoded Fourier transform hologram or fractional Fourier transform hologram and a uniform phase-encoded data page hologram were recorded using the first and a second reference beams, thereby reconstructing a first object beam used to record a first phase-encoded Fourier transform hologram or fractional Fourier transform hologram;   directing the second reference beam at the selected storage location in the holographic recording medium, thereby reconstructing a second object beam used to record the uniform data page, thereby producing an interference pattern between the first and second object beams that reproduces an amplitude-modulated data page; and   detecting the said amplitude-modulated data page with a light detector.   
   
   
       23 . The method of  22  wherein the intensities of the first and second reference beams are adjusted so that the intensities of the first and second reconstructed object beams are substantially the same. 
   
   
       24 . The method of  22  wherein the phases of the first and second reference beams are substantially the same. 
   
   
       25 . A method of searching a holographic recording medium for a specified content, comprising:
 illuminating a spatial light modulator (SLM) displaying at least one search pattern corresponding to a selected content, thereby forming a search beam;   directing the search beam to one or more storage locations on a holographic recording medium where at least one phase-encoded Fourier transform hologram or fractional Fourier transform hologram is recorded, thereby producing at least one reconstructed reference beam when the one or more storage locations contain at least one phase-encoded Fourier transform hologram or fractional Fourier transform hologram that contains selected content of the search pattern; and   detecting the at least one said reconstructed reference beam with one or more light detectors.   
   
   
       26 . The method of  claim 25  wherein the SLM is operable in a phase mode. 
   
   
       27 . A method of searching a holographic recording medium for a specified content, comprising:
 illuminating a spatial light modulator (SLM) operable in phase mode and displaying at least one search pattern corresponding to a selected content, thereby forming a search beam;   directing the said search beam to one or more storage locations on a holographic recording medium where at least one Fourier transform hologram or fractional Fourier transform hologram is recorded, thereby producing at least one reconstructed reference beam when the one or more storage locations contain at least one Fourier transform hologram or fractional Fourier transform hologram that contains selected content of the search pattern; and   detecting the at least one reconstructed reference beam with one or more light detectors.   
   
   
       28 . A method of recording a homogenized hologram, comprising:
 illuminating a spatial light modulator (SLM) operable in phase mode, said SLM having a plurality of pixels, each pixel of the SLM that operates in phase mode being in at least either a first state or a second state, the number of pixels of the SLM in the first state is about equal to the number of pixels in the second state;   controllably changing the state of selected pixels of the SLM, thereby changing the polarization of a light wavefront reflected from or transmitted through each pixel of the SLM, thereby forming an output beam;   directing the output beam at a polarization filter, thereby changing the phase of a light wavefront reflected from or transmitted through each pixel of the SLM by φ, depending on the state of the pixel, and thereby producing a phase encoded object beam;   directing the phase encoded object beam through a first lens element disposed in the path of the object beam between the SLM and a holographic recording medium (HRM), wherein the HRM is so disposed at or near the Fourier transform plane of the first lens element; and   directing a reference beam at the HRM so as to overlap the phase encoded object beam at a selected storage location in the HRM, thereby producing an interference pattern at the HRM and recording a hologram;   wherein the reference beam overlaps the object beam at or near the Fourier transform plane of the first lens element, and   further wherein one or more holograms are recorded at the storage location by one or more multiplexing methods, and wherein the multiplexed holograms are fully or partially overlapped at the storage location.