Photorefractive systems and methods - Patent Review 5440669

Claims

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;

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