APPARENT SPECKLE REDUCTION APPARATUS AND METHOD FOR MEMS LASER PROJECTION SYSTEM

• US 20080297731A1
• Filed: 06/01/2007
• Published: 12/04/2008
• Est. Priority Date: 06/01/2007
• Status: Abandoned Application

• Current Assignee: Microvision, Inc.
• Original Assignee: Microvision, Inc.
• Inventors: Montague, Thomas W., Powell, Karlton D., Xue, Bin, Brown, Margaret K.
• Application Number:
  US11/757,226
• Publication Number:
  US 20080297731A1
• Time in Patent Office: Days
• US Class Current: 353/37
• CPC Class Codes: G03B 21/26 Projecting separately subsidiary matter simultaneously with main image
• CPC Class Codes: G03B 21/28 Reflectors in projection beam in illumination beam G03B21/2066

Abstract

A laser projection system is disclosed having reduced apparent speckle. The system includes a laser emitting a first beam on an optical element. The optical element emits a second beam incident on a scanner that scans the beam onto a projection screen. The optical element may be an exit pupil expander, delay plate, or have a locally electrically modulated index of refraction. In other embodiments, the laser has a tunable wavelength distribution that is changed for each frame displayed by the projection system to reduce apparent speckle. In still other embodiments, the angular content of a beam incident on a scanner is modulated to produce a time varying speckle pattern.

Claims

1. An imaging system comprising:

a coherent light source emitting a first beam;

a scanner comprising a mirror positioned an optical distance from the coherent light source, the mirror having a width greater than an expected width of the first beam projected the optical distance from the coherent light source; and

an optical element interposed between the scanner and coherent light source, the optical element receiving the first beam and emitting a second beam having a numerical aperture substantially larger than the first beam, the second beam being projected onto the mirror.

2. The imaging system of claim 1, wherein the second beam comprises multiple beams.

3. The imaging system of claim 2, wherein the multiple beams overlap.

4. The imaging system of claim 3, wherein the multiple beams are arranged in an ordered array.

5. The imaging system of claim 1, wherein the optical element is an exit pupil expander (EPE).

6. The imaging system of claim 5, wherein the EPE is positioned in a focal plane of the first beam.

7. The imaging system of claim 6, further comprising an image screen, the mirror projecting the second beam onto the image screen.

8. The imaging system of claim 7, wherein the EPE is a two-dimensional array of optical components.

9. The imaging system of claim 1, wherein the optical element comprises multiple light paths each having a distinct optical path length.

10. The imaging system of claim 9, wherein the multiple light paths are arranged in an ordered array.

11. The imaging system of claim 10, wherein the multiple light paths have distinct optical path lengths differing from one another by more than a coherence length of light emitted by the coherent light source.

12. The imaging system of claim 8, further comprising a multi-mode element positioned between the multiple light paths and the scanner.

13. The imaging system of claim 12, wherein the multi-mode element is a delay block.

14. The imaging system of claim 12, wherein the multi-mode element comprises at least two delay blocks.

15. An imaging system comprising:

a coherent light source emitting a first beam;

a scanner comprising a mirror positioned an optical distance from the coherent light source;

an optical element interposed between the scanner and coherent light source, the optical element receiving the first beam and emitting a second beam, the second beam being projected onto the mirror; and

wherein the optical element has a locally electrically modulated index of refraction and wherein the optical element is coupled to one or more drive circuits programmed to exert one or more time-varying voltage signals on the optical element.

16. The imaging system of claim 15, wherein the optical element comprises a lithium niobium oxide (LiNbO₃) wafer.

17. The imaging system of claim 16, wherein the optical element comprises:

inversed and non inversed portions adjoining one another along a domain boundary;

first and second faces parallel to one another and positioned proximate opposite ends of the optical element the first and second parallel faces at a non-perpendicular angle relative to the domain boundary, the first beam being incident on the first face and the second beam emitting from the second face.

18. The imaging system of claim 17, wherein normal vectors of the first and second faces are at an angle between about 4 and about 6 degrees relative to the domain boundary.

19. The imaging system of claim 18, wherein the normal vectors of the first and second faces are at an angle of about 5 degrees relative to the domain boundary.

20. The imaging system of claim 17, further comprising a plurality of electrodes secured to the optical element, each of the electrodes spanning the domain boundary, and wherein the drive circuits are coupled to the electrodes.

21. The imaging system of claim 20, wherein the drive circuits are programmed to exert oscillating signals on the electrodes.

22. The imaging system of claim 21, wherein the scanner has a scan rate and wherein the oscillating signals have a frequency larger than the scan rate.

23. The imaging system of claim 21, wherein the scanner has a scan rate and wherein the oscillating signals are effective to modulate an optical path of the optical element at a frequency substantially larger than the scan rate.

24. The imaging system of claim 23, wherein the scanner comprises horizontal and vertical actuators operable to direct the second beam to form a two dimensional array of pixels at a pixel scan rate, and wherein the oscillating signals are effective to modulate the optical path of the optical element at a frequency larger than the pixel scan rate.

25. A method for improving an image projected from a coherent light source comprising:

emitting a first beam onto a scanner;

actuating the scanner to direct the first beam onto an exit pupil expander (EPE); and

emitting a second beam from the EPE onto an imaging screen, the imaging screen transmitting the second beam to a user'"'"'s eye, the second beam being substantially more angularly diverse than the first beam.

26. The method of claim 25, wherein the EPE comprises a two dimensional array of optical elements operable to emit the second beam that is substantially more angularly diverse than the first beam.

27. The method of claim 25, wherein the EPE comprises a two dimensional array of diffracting elements and wherein the second beam comprises multiple angularly diverse beamlets.

28. A method for improving an image projected from a coherent light source comprising:

emitting a first beam having a first wavelength distribution from a coherent light source onto a scanner;

actuating the scanner to direct the first beam onto an imaging screen to produce a first image on the imaging screen, the imaging screen reflecting the second beam to a user'"'"'s eye;

modulating the coherent light source of the coherent light source to emit a second wave length distribution substantially different from the first wavelength distribution; and

emitting a second beam having the second wavelength distribution from the coherent light source onto the imaging screen to produce a second image on the imaging screen, the imaging screen reflecting the second beam to a user'"'"'s eye.

29. The method of claim 28, wherein the first beam reflects from the imaging screen producing a first speckle pattern and wherein the second beam reflects from the imaging screen producing a second speckle pattern substantially different from the first speckle pattern.

30. The method of claim 29, further comprising modulating an intensity of the second beam substantially effective to compensate for a human perceptible difference between the first wavelength distribution and the second wavelength distribution.

31. The method of claim 29, wherein the coherent light source is a distributed Bragg reflector (DBR) laser.

32. The method of claim 31, wherein modulating the coherent light source to emit a second wavelength distribution comprises tuning a temperature of the DBR laser.

33. The method of claim 32, wherein the step of modulating the coherent light source to emit the second wavelength distribution occurs during a scan fly-back period of the scanner.

34. A method for improving an image projected from a coherent light source comprising:

emitting a beam from a coherent light source onto a scanner;

actuating the scanner to scan the beam across a screen to produce a series of images at a frame rate; and

wherein emitting a beam onto the scanner comprises modulating a wavelength distribution of the beam at a rate equal or greater than the frame rate.

35. The method of claim 34, further comprising modulating an intensity of the second beam substantially effective to compensate for a human perception of modulation of the wavelength distribution.

36. The method of claim 34, wherein the coherent light source is a distributed Bragg reflector (DBR) laser.

37. The method of claim 36, wherein modulating the wave length distribution of the beam comprises a temperature of the DBR laser.

38. The method of claim 34, wherein the step of modulating the wavelength distribution of the beam occurs during a scan fly-back period of the scanner.

39. An imaging system comprising:

a coherent light source emitting a first beam;

a scanner comprising a mirror positioned an optical distance from the coherent light source, the mirror having a width greater than an expected width of the first beam projected the optical distance from the coherent light source; and

an optical element receiving the first beam , the optical element emitting a second beam onto the scanner and modulating angular content of the second beam at a frequency effective to reduce speckle as apparent to a human viewer.

40. The imaging system of claim 39, wherein the optical element is an angular deflector.

41. The imaging system of claim 40, wherein a drive circuit is coupled to the angular deflector, the drive circuit programmed to cause the angular deflector to modulate an angle of the second beam at a frequency equal or greater than a frame rate of the scanner.

42. The imaging system of claim 41, wherein the drive circuit is programmed to cause the angular deflector to modulate the angle of the second beam at a frequency equal or greater than a pixel scan rate of the scanner.

43. The imaging system of claim 41, wherein the angular deflector is a first angular deflector oriented to modulate the angle of the second beam in a first plane, the imaging system further comprising a second angular deflector oriented to modulate the angle of the second beam in a second plane orthogonal to the first plane.

44. The imaging system of claim 39, wherein the optical element comprises a liquid crystal lens coupled to a driver, the driver programmed to modulate the numerical aperture of the liquid crystal lens at a frequency effective to reduce apparent speckle of an image produced by the second beam.

45. The imaging system of claim 39, wherein the optical element comprises an optical fiber having a first end receiving the first beam and a second end emitting the second beam;

an actuator coupled to the optical fiber proximate the first end and operable to change an angle of the fiber proximate the first end; and

a drive circuit coupled to the actuator, the drive circuit operable to cause the actuator to modulate the angle of the fiber proximate the first end at a frequency effective to reduce apparent speckle of an image produced by the second beam.

46. The imaging system of claim 39, wherein the optical element comprises a multimode optical fiber having a first end receiving the first beam and a second end emitting the second beam;

an actuator engaging the optical fiber at a middle portion between the first and second ends and operable to change a shape of the optical fiber between the first and second ends; and

a drive circuit coupled to the actuator, the drive circuit operable to cause the actuator to modulate the shape of the optical fiber to an extent and at a frequency effective to reduce apparent speckle of an image produced by the second beam.

47. The imaging system of claim 39, wherein the optical element is a variable aperture modulated as to at least one of size and position at a frequency effective to reduce apparent speckle of an image produced by the second beam.

48. The imaging system of claim 47, wherein the variable aperture is a liquid crystal aperture coupled to a drive circuit operable to modulate the size and position of a transmissive portion of the liquid crystal aperture.

49. A user device, comprising:

a coherent light source emitting a first beam;

a scanner comprising a mirror positioned an optical distance from the coherent light source, the mirror having a width greater than an expected width of the first beam projected the optical distance from the coherent light source; and

an optical element interposed between the scanner and coherent light source, the optical element receiving the first beam and emitting a second beam having a numerical aperture substantially larger than the first beam, the second beam being projected onto the mirror.

50. The user device of claim 49, wherein the user device is a small form-factor device selected from the group consisting of a computing device, a portable device, a wireless device, a cell phone, a portable DVD player, a portable television device, a laptop, a portable e-mail device, a portable music player, and a personal digital assistant.