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Photorefractive systems and methods    
United States Patent5440669   
Link to this pagehttp://www.wikipatents.com/5440669.html
Inventor(s)Rakuljic; George A. (Santa Monica, CA); Yariv; Amnon (San Marino, CA)
AbstractA new type of holographic recording in photorefractive crystals is disclosed, termed orthogonal data storage, in which counterpropagating, reflection mode holograms are wavelength multiplexed to form gratings lying along a common axis. It is shown that this configuration provides substantial improvements in data storage capacity in comparison to prior art systems and that higher coupling coefficients are achieved. In consequence, not only storage of data but a wide range of other devices including wavelength selective filters, lenses and optical correlation systems are disclosed. Further, methods of writing and processing metastable images are disclosed which substantially improve the strength of the gratings that are permanently developed in the medium. These methods include advantageous use of electrical potentials and photovoltaic properties, together with different interrelationships during processing.
   














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Drawing from US Patent 5440669
Photorefractive systems and methods - US Patent 5440669 Drawing
Photorefractive systems and methods
Inventor     Rakuljic; George A. (Santa Monica, CA); Yariv; Amnon (San Marino, CA)
Owner/Assignee     Accuwave Corporation (Santa Monica, CA)
Patent assignment
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Publication Date     August 8, 1995
Application Number     07/908,298
PAIR File History     Application Data   Transaction History
Image File Wrapper   Patent Term   Fees
Litigation
Filing Date     July 2, 1992
US Classification    
Int'l Classification    
Examiner     Sikes; William L.
Assistant Examiner     Parsons; David R.
Attorney/Law Firm     Merchant, Gould, Smith, Edell, Welter & Schmidt
Address
Parent Case     CROSS-REFERENCE TO RELATED APPLICATIONS This application is a Continuation-In-Part of application Ser. No. 07/736,736, filed Jul. 26, 1991, now abandoned by George A. Rakuljic, et al., entitled PHOTOREFRACTIVE SYSTEMS AND METHODS, which application is incorporated by reference herein.
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Patent Tags     photorefractive methods
   
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What is claimed is:

1. The method of recording holograms in a photorefractive medium comprising the steps of:

generating a plurality of different distributed electron charge reflection holograms at different non-harmonically related wavelengths in the medium; and

converting the holograms into a plurality of spatially varying index of refraction patterns in the medium that cannot be erased by illumination.

2. The method as set forth in claim 1 above, wherein the holograms have a half-life at ambient temperature in excess of 50 years and a coupling coefficient k.sub.FIXED of greater than 1 cm.sup.-1.

3. The method as set forth in claim 2 above, wherein the medium has a thickness dimension inversely proportioned to the wavelength separation and the holograms are generated by counterpropagating object and reference beams into the medium substantially normal to the thickness dimension.

4. The method as set forth in claim 3 above, wherein the holograms are further generated by modulating the object beam impinging on the medium with a selected image for each wavelength, and wherein the medium has a C-axis and the beams are counterpropagated parallel to the C-axis.

5. The method as set forth in claim 4 above, wherein the distributed electron charge holograms are generated with the medium being electrically shorted, and the holograms are then stabilized by first converting the distributed charges to ionic charge patterns while the medium is electrically shorted, and then developing the ionic charge patterns while the medium is electrically open.

6. The method of recording holograms in a photorefractive medium comprising the steps of:

illuminating the medium at a given angle with a reference beam and an interactive subject beam to effect migration of charges in the medium into an interference grating, the reference beam and the subject beam being at a same first frequency; and

repeating the illuminating step successively at the given angle with beams at other frequencies different from the first frequency to form other interference gratings, wherein the diffusion field E.sub.d is approximately equal to the limiting space charge field E.sub.q.

7. The method as set forth in claim 6 above, wherein the reference beam and subject beam are directed into the medium in opposite directions, and wherein the method includes the step of converting the migrated charges to ionic displacements by heating the medium and neutralizing the migrated charges.

8. The method of recording data in a thick photorefractive crystal comprising the steps of:

generating a plurality of holographic interference pattern images within the crystal, each of the interference patterns having a different wavelength, but the fringes defined by the interference patterns lying substantially parallel to a common axis, the amount of illumination energy varying in decreasing fashion as successive images are generated;

fixing all the interference pattern images concurrently and

developing the interference pattern images concurrently, such that all images provide substantially equal diffraction efficiency on readout.

9. The method as set forth in claim 8 above, wherein the illumination energy is varied by decreasing the illumination duration.

10. The method of generating an improved, stable, high diffraction efficiency volumetric hologram in a photorefractive crystal comprising the steps of:

recording at least one electronic grating in the crystal while maintaining the crystal shorted;

compensating the charge distribution of the electronic grating with an ionic grating while maintaining the field in the crystal shorted, and

erasing the electronic grating while maintaining a large dc field in the crystal.

11. The method of claim 10 above, wherein the crystal is heated to a level at which ions are thermally mobile during the recording step such that the compensating step is effected simultaneously with recording.

12. The method of creating substantially permanent holograms in a photorefractive crystal comprising the steps of:

electrically shorting the crystal;

neutralizing space charge fields by heating the electrically shorted crystal;

generating a metastable electronic space charge hologram by exposing the crystal to light in a selected pattern while maintaining the crystal electrically shorted;

re