SYSTEM AND METHOD OF ORBITAL ANGULAR MOMENTUM (OAM) DIVERSE SIGNAL PROCESSING USING CLASSICAL BEAMS

• US 20100013696A1
• Filed: 08/26/2008
• Published: 01/21/2010
• Est. Priority Date: 07/18/2008
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation), University of Melbourne
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Shah, Nitesh N., Waagen, Donald E., Greentree, Andrew D., SCHMITT, HARRY A., Barker, Delmar L.
• Granted Patent: US 7,701,381 B2
• Time in Patent Office: Days
• US Class Current: 342/54
• CPC Class Codes: G01S 13/751 wherein the responder or reflector radiates a coded signal
• CPC Class Codes: G01S 13/78 discriminating between different kinds of targets, e.g. IFF-radar, i.e. identification of friend or foe G01S13/75, G01S13/79 take precedence
• CPC Class Codes: G01S 17/74 Systems using reradiation of electromagnetic waves other than radio waves, e.g. IFF, i.e. identification of friend or foe
• CPC Class Codes: G01S 7/024 using polarisation effects in waveguides H01P1/165; for aerials H01Q, e.g. H01Q15/22, H01Q15/24, H01Q19/195
• CPC Class Codes: G01S 7/499 using polarisation effects

Abstract

The present invention describes a system and method of OAM diverse signal processing using classical beams for applications in which OAM signal character is controlled such as optical tagging and applications in which OAM signal character is not controlled such as clutter mitigation and interference cancellation for target detection, identification etc. This is accomplished by transmitting a source beam having a prescribed state with one or more non-zero OAM components, reflecting the beam off a ‘tagged’ or ‘untagged’ target and receiving the return beam in the direct return path to measure the one or more OAM components to identify the target. OAM processing provides additional degrees of processing freedom to greatly enhance the processing capabilities to detect and identify both ‘tagged’ and ‘untagged’ targets.

Claims

1. An apparatus, comprising:

a source that generates an electromagnetic beam in an initial state having zero orbital angular momentum (OAM);

a first diffractive element that converts the beam into a source beam having a prescribed source state with one or more non-zero OAM components;

a tag that reflects the source beam to control the non-zero OAM components in a return beam; and

a receiver that measures the OAM components of the return beam and compares the measured OAM components to the nonzero OAM components of the prescribed source state to identify the tag.

2. The apparatus of claim 1, wherein said receiver measures a non-OAM property of the return beam.

3. The apparatus of claim 2, wherein said electromagnetic beam has a prescribed polarization, said tag reflecting the source beam to control the polarization of the return beam, said receiver measuring the polarization of the return beam.

4. The apparatus of claim 2, wherein said electromagnetic beam is a laser beam, said receiver computing the range to the tag from the measured non-OAM property.

5. The apparatus of claim 2, wherein said electromagnetic beam is an RF beam, said receiver performing target engagement functions from the measured non-OAM property.

6. The apparatus of claim 1, wherein said first diffractive element produces multiple non-zero OAM components.

7. The apparatus of claim 1, wherein said tag comprises a reflective surface that preserves the one or more OAM components of the prescribed source state.

8. The apparatus of claim 1, wherein said tag comprises a second diffractive element that alters the one or more non-zero OAM components of the prescribed source state in a known manner.

9. The apparatus of claim 8, wherein the electromagnetic beam is an electro-optic (EO) beam, said first diffractive element comprising a computer generated hologram that is regenerated to change the non-zero OAM components of the source beam in a time sequence, said tag'"'"'s second diffractive element comprising a computer generated hologram that is regenerated synchronously with the source beam.

10. The apparatus of claim 8, wherein the electromagnetic beam is a radio-frequency (RF) beam, said first diffractive element comprising multiple different prefabricated gratings that are selected in sequence to change the non-zero OAM components of the source beam in a time sequence, said tag'"'"'s second diffractive element comprising a like sequence of gratings that are selected synchronously with the source beam.

11. The apparatus of claim 8, wherein the electromagnetic beam is an EO beam, said first and second diffractive elements comprising holograms.

12. The apparatus of claim 8, wherein the electromagnetic beam is an RF beam, said first and second diffractive elements comprising gratings.

13. The apparatus of claim 8, wherein said second diffractive element is configured as a retro-reflector to reflect the return beam back along a direct return path to the receiver.

14. The apparatus of claim 8, wherein said second diffractive element is a static physical element.

15. The apparatus of claim 14, wherein said tag comprises a plurality of second diffractive elements that are associated with different source beams having different prescribed source states.

16. The apparatus of claim 8, wherein said tag'"'"'s second diffractive element comprises a computer generated hologram (CGH) that is temporally agile.

17. The apparatus of claim 16, wherein a spatial light modulator (SLM) is configured to provide the temporally agile CGH.

18. The apparatus of claim 16, wherein said CGH changes to present a sequence of different holograms associated with different source beams having different known prescribed source states.

19. The apparatus of claim 16, wherein said CGH changes to present a sequence of different holograms synchronized to changes in the one or more OAM components of the prescribed source state of the source beam.

20. The apparatus of claim 16, wherein said CGH changes to present different holograms to impart information in addition to the tag identification onto the OAM components of the return beam.

21. The apparatus of claim 8, wherein the tag is matched to the prescribed source state of the source beam such that for any deviations from the prescribed source state the OAM components in the return beam are unintelligible to the receiver.

22. The apparatus of claim 1, wherein the receiver comprises:

an interferometric device that separates the non-zero OAM components, and

a processor that processes the components to determine the corresponding weights of the OAM components and to identify the optical tag.

23. The apparatus of claim 1, wherein said tag comprises a second diffractive element that transforms the one or more non-zero OAM components of the prescribed source state to a different prescribed return state in the return beam, said receiver knowing the prescribed source state of the source beam and the transformation implemented by the OAM tag compares the measured OAM to the non-zero OAM components of the prescribed source state to identify the tag.

24. The apparatus of claim 23, wherein the receiver lies in a direct return path of the return beam.

25. The apparatus of claim 24, wherein said tag is configured as a retro-reflector to reflect the return beam back along the direct return path.

26. The apparatus of claim 24, wherein the tag is positionally constrained approximately normal to the incident source beam to reflect the return beam along the direct return path to the receiver.

27. An apparatus, comprising:

a source that generates an electromagnetic beam in an initial state having zero orbital angular momentum (OAM);

a first diffractive element that converts the beam into a source beam that is transmitted along a source path, said source beam having a prescribed source state with one or more non-zero OAM components;

a tag having a second diffractive element that reflects the source beam to transform the non-zero OAM components to a different prescribed return state in a return beam; and

a receiver in a direct return path that approximately retraces the source path, said receiver measures the OAM components of the return beam and with knowledge of the prescribed source state of the source beam and the transformation implemented by the OAM tag compares the measured OAM to the non-zero OAM components of the prescribed source state to identify the tag.

28. The apparatus of claim 27, wherein said second diffractive element is configured as a retro-reflector to reflect the return beam back along the direct return path.

29. The apparatus of claim 27, wherein the tag is positionally constrained approximately normal to the incident source beam so that the second diffractive element reflects the return beam along the direct return path to the receiver.

30. An apparatus, comprising:

a source that generates an electromagnetic beam in an initial state having zero orbital angular momentum (OAM);

a diffractive element that converts the beam into a source beam having a prescribed source state with one or more non-zero OAM components, said source beam reflected off a target in a background to produce a return beam in which the OAM components are altered differently by the target and background; and

a receiver in the directed reflected path of the return beam that measures the one or more OAM components of the return beam and processes the alterations of the OAM components from the prescribed source state to discriminate the target from the background.

31. The apparatus of claim 30, wherein the electromagnetic beam is a radio frequency (RF) beam.

32. The apparatus of claim 31, further comprising:

a plurality of sources that generate RF beams in an initial state having zero OAM;

a plurality of different diffractive elements that convert respective beams passing there through into source beams each having a different prescribed source state with one or more non-zero OAM components;

a multiplexer that multiplexes the source beams into a time sequence; and

an antenna that transmits the multiplexed source beams.

33. The apparatus of claim 32, wherein said sources generate the RF beams with polarization diversity said receiver measuring and processing the altered polarization in the return beam.

34. The apparatus of claim 31, further comprising a plurality of different diffractive elements that are selected in sequence to change the one or more OAM components of the beam in a time sequence.

35. The apparatus of claim 31, wherein the first diffractive element converts the beam into a source beam having multiple non-zero OAM components.