Hybrid angular/spatial holographic multiplexer

• US 5,793,504 A
• Filed: 08/07/1996
• Issued: 08/11/1998
• Est. Priority Date: 08/07/1996
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Stoll, Harold McDowell
• Primary Examiner(s): Nguyen, Thong
• Assistant Examiner(s): CHANG, AUDREY Y
• Application Number:
  US08/695,245
• Time in Patent Office: 734 Days
• Field of Search: 359/3, 359/7, 359/10, 359/11, 359/21, 359/28, 359/30, 359/32, 359/24, 365/125, 365/124, 365/216, 369/103
• US Class Current: 359/11
• CPC Class Codes: G03H 1/26 Processes or apparatus specially adapted to produce multiple sub- holograms or to obtain images from them, e.g. multicolour technique
• CPC Class Codes: G03H 1/265 Angle multiplexing; Multichannel holograms G03H1/268 takes precedence
• CPC Class Codes: G11B 7/0065 Recording, reproducing or erasing by using optical interference patterns, e.g. holograms
• CPC Class Codes: G11B 7/08564 using galvanomirrors

Abstract

A plurality of holograms are simultaneously angularly and spatially multiplexed within a storage medium by writing the holograms at different locations within the storage medium using reference and object beams directed thereto via cooperating reference beam and object beam scanners, respectively. The simultaneously angular and spatially multiplexed holograms are subsequently read-out by directing a reference beam through a desired location within the storage medium using the reference beam scanner. The reference beam is directed from the storage medium to a phase conjugator which conjugates the phase of the reference beam and directs the phase conjugated reference beam back to the same desired location within the storage medium so as to excite a corresponding conjugate object beam which is then directed onto a detector array.

Claims

1. A hybrid angular/spatial holographic multiplexer comprising:

a) a storage medium;

b) a reference beam scanner for scanning a reference beam spatially through said storage medium;

c) an object beam scanner for scanning an object beam spatially through said storage medium;

d) a spatial light modulator for modulating a laser beam to form the object beam;

e) a phase conjugator for conjugating the reference beam during read-out so as to excite a corresponding conjugate object beam;

f) a detector array for converting the conjugate object beam into an electrical signal representative thereof; and

g) wherein said reference beam scanner and said object beam scanner cooperate to form a plurality of angular/spatial multiplexed holograms within said storage medium.

2. The hybrid angular/spatial holographic multiplexer as recited in claim 1 wherein said storage medium comprises Fe:

LiNbO₃.

3. The hybrid angular/spatial holographic multiplexer as recited in claim 1 further comprising a circuit for electrically shorting at least two surfaces of said storage medium to effect discharge of photo-generated charges generated therein.

4. The hybrid angular/spatial holographic multiplexer as recited in claim 1 wherein said reference beam scanner and said object beam scanner are configured to spatially overlap adjacent angular/spatial multiplexed holograms so as to equalize exposure of the storage medium to the reference and object beams and, thereby, mitigate undesirable optical damage to the storage medium.

5. The hybrid angular/spatial holographic multiplexer as recited in claim 1 wherein said reference beam scanner and said object beam scanner are configured to uniformly distribute holograms through substantially the entire storage medium so as to enhance storage density.

6. The hybrid angular/spatial holographic multiplexer as recited in claim 1 wherein said reference beam scanner comprises an articulated mirror for directing the reference beam to different portions of said storage medium.

7. The hybrid angular/spatial holographic multiplexer as recited in claim 1 wherein said object beam scanner comprises an articulated Mangin mirror for directing the object beam to different portions of said storage medium.

8. The hybrid angular/spatial holographic multiplexer as recited in claim 1 further comprising an optical switch for preventing the reference beam from being incident upon the phase conjugator during write operations and for allowing the reference beam to be incident upon the phase conjugation during read-out operations.

9. The hybrid angular/spatial holographic multiplexer as recited in claim 1 further comprising:

a) a wedge for directing the reference beam from the storage medium onto the phase conjugator; and

b) a lens for focusing the reference beam directed by the wedge onto the phase conjugator.

10. The hybrid angular/spatial holographic multiplexer as recited in claim 1 further comprising:

a) a beam splitter for transmitting the object beam to the storage medium during write operations and for reflecting the conjugate object beam to the detector array during read-out operations; and

b) a polarization rotator for transmitting the object beam to the storage medium during write operations and for rotating the phase of the excited corresponding conjugate object beam during read-out operations so as to cause the beam splitter to reflect the excited corresponding conjugate object beam to said detector array.

11. The hybrid angular/spatial holographic multiplexer as recited in claim 1, wherein the spatial light modulator and detector array are formed to have the same pixel pitch.

12. The hybrid angular/spatial holographic multiplexer as recited in claim 1, wherein the spatial light modulator is formed to have a pixel pitch that is an integral multiple of the detector array pixel pitch.

13. A method for angularly and spatially multiplexing a plurality of holograms within a storage medium, said method comprising the steps of:

a) writing a plurality of angularly/spatially multiplexed holograms at different locations within the storage medium using reference and object beams directed thereto via cooperating reference beam and object beam scanners, respectively; and

b) reading out selected ones of said holograms via a reference beam directed to a desired location within the storage medium using the reference beam scanner, the reference beam being directed from the storage medium to a phase conjugator which conjugates the phase of the reference beam and directs the phase conjugated reference beam back to the same desired location within the storage medium to excite a corresponding conjugate object beam which is directed onto a detector array.

14. A method for angularly and spatially multiplexing a plurality of holograms within a storage medium, said method comprising the steps of:

a) performing a write operation comprising the steps of;

(i) directing a reference beam to a desired location within the storage medium via a reference beam scanner;

(ii) directing an object beam to the same desired location within the storage medium via an object beam scanner;

(iii) forming a hologram within the storage medium via interference of the reference beam and the object beam;

(iv) repeating steps (i) through (iii) for a plurality of different desired locations within the storage medium for forming a plurality of angularly/spatially multiplexed holograms;

b) performing a read-out operation comprising the steps of;

(i) directing a reference beam to a desired location within the storage medium via a reference beam scanner;

(ii) directing the reference beam from the storage medium to a phase conjugator;

(iii) conjugating the phase of the reference beam; and

(iv) directing the excited corresponding conjugate object beam onto a detector array via the object beam scanner.

15. The method as recited in claim 14 wherein the step of directing a reference beam to a desired location within the storage medium comprises directing a reference beam to a desired location within a storage medium comprised of Fe:

LiNbO₃.

16. The method as recited in claim 14 wherein the steps of both performing a write operation and performing a read-out operation include electrically shorting surfaces of the storage medium to effect discharge of photo-generated charge therein.

17. The method as recited in claim 14 wherein the step of directing a reference beam to a desired location within the storage medium via a reference beam scanner and the step of directing an object beam to the same location within the storage medium via an object beam scanner, both during the write operation, comprise directing the reference beam and the object beam so as to spatially overlap adjacent angle/spatial multiplexed holograms in order to equalize exposure of the storage medium to the reference and object beams and, thereby, to mitigate undesirable optical damage to the storage medium.

18. The method as recited in claim 14 wherein the step of directing a reference beam to a desired location within the storage medium via a reference beam scanner and directing an object beam to the same desired location within the storage medium via an object beam scanner, both comprise forming holograms such that the holograms are uniformly distributed throughout substantially the entire storage medium so as to enhance storage density.

19. The method as recited in claim 14 wherein the step of directing a reference beam to a desired location within the storage medium comprises directing the reference beam via an articulated mirror.

20. The method as recited in claim 14 wherein the step of directing an object beam to a desired location within the storage medium comprises directing the object beam via an articulated Mangin mirror.

21. The method as recited in claim 14 further comprising the step of configuring an optical switch during write operations such that the object and reference beams are not transmitted therethrough to the phase conjugator and configuring the optical switch during read-out operations such that the reference beam is transmitted therethrough to the phase conjugator.

22. The method as recited in claim 14 further comprising the steps of:

a) transmitting the object beam through a beam splitter and polarization rotator during write operations such that the object beam is substantially unaffected thereby; and

b) transmitting the excited corresponding conjugate object beam through the same phase rotator during read-out operations such that the phase of the exited corresponding conjugate object beam is rotated 90 degrees thereby such that the excited corresponding conjugate object beam is reflected by the beam splitter to the detector array.

23. The method as recited in claim 14 wherein the step of performing a write operation comprises performing a write operation according to an exposure schedule so as to mitigate diffraction efficiency degradation of stored holograms.

24. The method as recited in claim 14 wherein the step of performing a write operation comprises spatially overlapping adjacent holograms so as to average the exposure of the storage medium to the reference and object beams and thus mitigate optical damage to the storage medium.

25. The method as recited in claim 14 wherein the step of performing a write operation comprises performing a plurality of write operations so as to uniformly distribute holograms throughout the entire storage medium and thus enhance the storage density thereof.