Laser despeckler based on angular diversity

Abstract

A device for reducing laser speckle using a micro scanner and a holographic diffuser. The micro scanner includes a first transparent optical substrate with an input surface and an output surface and a second transparent optical substrate with an input surface and an output surface and a variable refractive index medium sandwiched between the output surface of the first substrate and the input surface of the second substrate. Transparent electrodes are applied to the output surface of the first substrate and the input surface of the second substrate. The electrodes are coupled to a voltage generator. The input surface of the first substrate is optically coupled to a laser source. The input surface of the second substrate is configured as an array of prismatic elements. At least one of the input surface of the first substrate or the output surfaces of the second substrate is planar.

Claims

1. An illumination device comprising: a source of light; a microdisplay; a beam coupling optics comprising: a total internal reflection (TIR) lightguide; a coupling grating for admitting collimated light from said source into said lightguide; and a second coupling grating for extracting light out of said light guide; and disposed between said beam coupling optics and said microdisplay: a first transparent substrate having an output surface; a variable refractive index layer having an input surface and an output surface, wherein said input surface of said refractive index layer overlays said output surface of said first transparent substrate; a second transparent substrate having an input surface, wherein said input surface of said second transparent layer overlays said output layer of said variable refractive index layer; and a holographic diffuser, wherein said output surface of said variable refractive index layer and said input layer of said second transparent substrate are each configured as a multiplicity of ray deflecting features to deflect incident light into a temporally varying random distribution of ray directions in a first predefined range of angles having a plurality of angles of deflection such that the plurality of angles of deflection are varied as a voltage applied across the variable refractive index is iteratively varied, and wherein each point in said holographic diffuser diffracts incident light rays of said first predefined range of angles into output light rays having a second predefined range of angles forming a diffuse illumination patch.

2. The illumination device of claim 1, wherein said diffuse illumination patch is disposed in proximity to the surface of said microdisplay or in proximity to the surface of a projection screen.

3. The illumination device of claim 1, holographic diffuser comprises a hologram of a scattering surface.

4. The illumination device of claim 1, wherein said holographic diffuser comprises multiplicity of holograms of scattering surfaces.

5. The illumination device of claim 1, wherein said holographic diffuser comprises one or more of a group consisting of a Bragg hologram, a switchable Bragg grating, a computer-generated hologram, a transmission hologram and a reflection hologram.

6. The illumination device of claim 1, wherein said holographic diffuser encodes properties for performing one or both of varying the intensity distribution of said incident light and shaping the cross-sectional geometry of incident light.

7. The illumination device of claim 1, wherein said holographic diffuser is an array of selectively switchable holographic elements.

8. The illumination device of claim 1, further comprising one or more of a group consisting of a beam steering means, a beam splitter, a beam expander, a polarizer, a light integrator, a light guide, and a projection lens.

9. The illumination device of claim 1, wherein said light source comprises emitters of at least two different colors.

10. The illumination device of claim 1, wherein said ray deflecting features have three or more facets for deflecting light.

11. The illumination device of claim 1, wherein said ray deflecting features have surface angles defined by a random distribution.

12. The illumination device of claim 1, wherein said ray deflecting features are characterized by at least two different geometrical prescriptions.

13. The illumination device of claim 12, wherein said geometrical prescriptions are distributed uniformly across said input surface of said second transparent substrate.

14. The illumination device of claim 1, wherein said ray deflecting features have at least one light diffusing surface.

15. The illumination device of claim 1, wherein electrodes are applied to each the output surface of the first substrate and the input surface of the second substrate, with at least one said electrode patterned into electrode elements for providing addressable regions of said variable refractive index layer, wherein the average refractive index of any of said addressable regions is proportional to a voltage applied across electrodes overlapping said regions, wherein said first range of angles is determined by refractive index of said substrates that the average refractive index of said regions of said variable refractive index layer, wherein the voltage applied across each of said electrode elements is varied temporally.

16. The illumination device of claim 1, wherein each ray deflecting feature is configured to output a bundle of light rays that overlaps with at least one other bundle of light rays outputted by another ray deflecting feature.

17. The illumination device of claim 1, wherein said light source is LED or laser.

18. The illumination device of claim 1, wherein said microdisplay comprises a reflective microdisplay and said light source is configured to illuminate said reflective microdisplay.

19. The illumination device of claim 18, wherein light reflected from said reflective microdisplay propagates through said diffuser and through said variable refractive index layer.

20. The illumination device of claim 18, wherein a beam splitter cube is disposed along the light path from said source to said reflective microdisplay, wherein light reflected from said reflective microdisplay is reflected by said beam splitter cube into said illumination patch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic side elevation view of a micro scanner.

(2) FIG. 2 is a schematic side elevation view of one aspect of a micro scanner.

(3) FIG. 3A is a schematic side elevation view of a first aspect of a micro scanner.

(4) FIG. 3B is a schematic side elevation view of a second aspect of a micro scanner.

(5) FIG. 3C is a schematic side elevation view of a third aspect of a micro scanner.

(6) FIG. 4A is a schematic view illustrating a method of recording a holographic diffuser.

(7) FIG. 4B is a schematic view of a first aspect of the operation of a holographic diffuser.

(8) FIG. 4C is a schematic view of a second aspect of the operation of a holographic diffuser.

(9) FIG. 5 is schematic side elevation view of a despeckler according to one embodiment of the invention.

(10) FIG. 6A is detail of the operation of a despeckler according to one embodiment of the invention.

(11) FIG. 6B is detail of the operation of a despeckler according to one embodiment of the invention.

(12) FIG. 7 is a schematic side elevation view of despeckler according to one embodiment of the invention.

(13) FIG. 8 is a schematic side elevation view of a projection display incorporating a despeckler according to one embodiment of the invention.

(14) FIG. 9 is a schematic side elevation view of a projection display incorporating a despeckler according to one embodiment of the invention.

(15) FIG. 10 is a schematic side elevation view of a projection display incorporating a despeckler according to one embodiment of the invention.

(16) FIG. 11 is a schematic side elevation view of a projection display incorporating a despeckler according to one embodiment of the invention.

(17) FIG. 12 is a schematic side elevation view of despeckler for use with a reflective microdisplay according to one embodiment of the invention.

(18) FIG. 13 is a schematic side elevation view of despeckler for use with a reflective microdisplay according to one embodiment of the invention.

DETAILED DESCRIPTION

(19) It an object of the present invention to provide a despeckler with improved speckle contrast reduction.

(20) It will be apparent to those skilled in the art that the present invention may be practiced with only some or all aspects of the present invention as disclosed in the following description. For the purposes of explaining the invention well-known features of laser technology and laser displays have been omitted or simplified in order not to obscure the basic principles of the invention.

(21) Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optics and laser displays in particular.

(22) In the following description the terms light, ray, beam and direction will used interchangeably and in association with each other to indicate the propagation of light along rectilinear trajectories.

(23) Unless otherwise stated the term optical axis in relation to a ray or beam direction refers to propagation parallel to an axis normal to the surfaces of the optical components described in relation to the embodiments of the invention.

(24) It should also be noted that in the following description of the invention repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment.

(25) The despeckler embodiments disclosed herein are directed at overcoming both objective and subjective speckle. The devices generate set of unique speckle patterns within an eye resolution cell by operating on the angular characteristic of rays propagating through the despeckler according to the angular diversity principle.

(26) Specifically the invention provides a despeckler that combines a means for generating illumination light with a temporally varying random distribution of ray directions to be referred to as a micro scanner with a holographic means for generating a diffuse illumination patch at a specified location. Said holographic means will be referred to as a holographic diffuser in the following description of the invention. The location of the diffuse illumination patch typically coincides with the surface of a microdisplay but in some display applications it may be advantageous for it located at or near to an intermediated image plane.

(27) The invention provides a solid state analogue of the classical moving-screen speckle reduction technique. The holographic means for creating a diffuse illumination patch also provides a means for shaping the beam cross section and controlling the spatial homogeneity of the illumination all of said features being encoded within a single hologram.

(28) We start by considering the micro scanner. The basic principles of a micro scanner for use with the invention are illustrated in the schematic side elevation view of FIG. 1. The apparatus comprises: a first transparent optical substrate 91 with an input surface 91A and an output surface 91B and a second transparent optical substrate 93 with an input surface 93A and an output surface 93B. The input surface of the second substrate 93 is configured as an array of prismatic elements each prismatic element containing surfaces such as 93A. Advantageously, at least one of the input surface of the first substrate or the output surfaces of the second substrate is planar. Transparent electrodes 98A,98B are applied to the output surface 91B of said first substrate and the input surface 93A of said second substrate. A variable refractive index layer 92 having input surfaces 92A, 92B is sandwiched between the output surface of the first substrate and the output surface of the second substrate providing an array of variable refractive index prismatic elements. Advantageously the variable refractive index layer is a HPDLC material. In one embodiment of the invention the variable refractive index layer is a SBG. In one embodiment of the invention the variable refractive index layer is a sub wavelength grating. The insets marked by dashed lies show portions of the first substrate-electrode-HPDLC layer interface and the second substrate-electrode-HPDLC layer interface in more detail. The electrodes are coupled to a voltage generator 90 by means of an electrical circuit 99. The input surface 91A of the first substrate 91 is optically coupled to a laser source which is not illustrated. The substrates are fabricated from an optical glass such as BK7. Alternatively, optical plastics may be used.

(29) We consider the propagation of light through one of the prismatic elements. Input laser light indicated by the rays 440A,440B is transmitted through substrate 91 into the HPDLC. Refracted rays from a first prism surface 93B are indicated by 441A and refracted rays from a second prism surface are indicated by 441B. Each of the refracted rays in the groups indicated by 441A,441B corresponds to a unique average refractive index resulting from a unique applied voltage. The rays 441A,441B are refracted at the output surface of the second substrate 93 to provide the output rays 442A,442B. As indicated in the drawing each prism will provide overlapping rays indicated by the divergent ray bundles 440,450,460,470.

(30) The ray geometry is illustrated in more detail in FIG. 2 which provides a schematic illustration of the ray propagation around one prism face. The angle of deflection in the prism is given by α.sub.2=arcsin ((n.sub.h/n.sub.g) sin (α.sub.1), which is approximately equal to (n.sub.h/n.sub.g) α.sub.1. The prism angle α.sub.1 is given by α.sub.1=arctan (h/D), where D is the length of the prism (or period) and h is its height. It can be shown that the resulting angle of prism deflection δ is given by δ=arcsin (n.sub.g sin(α.sub.2−α.sub.1). Making the approximation that δ=n.sub.g (α.sub.2−α.sub.1), we obtain: δ=n.sub.g as (n.sub.h/n.sub.g−1). Combining both previous equations, the deflection angle may be expressed as a function of the prism characteristics and index. Based on the above equations the ray deflection is given by δ=n.sub.g ((h/D) (n.sub.h/n.sub.g−1). The directions of the output rays are swept by increasing the effective refractive index in the HPDLC between the substrate-HPDLC index match condition and the full effective index shift. Typically, the index of glass is n.sub.g=1.55. The index of the HPDLC n.sub.h in its non diffracting state is matched to the index of the substrate glass which is typically 1.55. The inventors have found that the maximum refractive variation of the HPDLC is typically +0.065. The HPDLC material has a sinusoidal sub-wavelength grating with a duty cycle of 50% of the index swing regions (bright fringes). Therefore the maximum effective refractive index change extends from 1.55 to 1.55+0.065/2=1.5825. Assuming a prism height of 1 micron, a prism length of 30 microns, and n.sub.g=1.55 and n.sub.h=1.5825, we obtain a deflection angle of 0.062 degrees.

(31) FIG. 3 illustrates the sweeping of output rays as the voltage applied across the HPDLC via the electrodes 98A,98B is varied. At the maximum voltage condition illustrated in FIG. 3A there is nor deflection in the incoming rays 430 which propagate into the from the HPDLC region 92 into region 93 as the rays 431 and subsequently into air as rays 432 without deviation. FIGS. 3B-3C show how the ray deviation increases as the voltage is reduced. In FIG. 3B input collimated light 433 is deflected into the ray directions 434 in the HPDLC medium and into ray direction 435 in air. In FIG. 3C input collimated light 436 is deflected into the ray directions 437 in the HPDLC medium and into ray direction 438 in air.

(32) In one embodiment of the invention both of the transparent electrodes are continuous. The HPDLC is selectively switched in discrete steps from a fully diffracting to a non diffracting state by an electric field applied across the transparent electrodes.

(33) At least one of said transparent electrodes is patterned to provide independently switchable electrode elements such that portions of the HPDLC may be selectively switched in discrete steps from a fully diffracting to a non diffracting state by an electric field applied across the transparent electrodes. Desirably, the electrodes are fabricated from ITO.

(34) We next consider the principles of and method of recording of the holographic diffuser referring to FIGS. 4A-4C. The beam shaping device is essentially a hologram of a diffuser or scatter plate. Using a traditional holographic recording procedure, which is illustrated schematically in FIG. 4A, the holographic diffuser is recorded by illuminating a holographic recording medium 2A by light scattered from a real diffuser 5 and a second collimated reference beam indicated by the rays 201,202. Advantageously, the rays 201,202 are parallel. Groups of rays from two points on the diffuser are generally indicated by 211 and 212. Each group of rays from a point on the diffuser surface fills the aperture of the hologram. Under playback the processed hologram indicated by 2B is illuminated by a beam parallel to the reference beam such that the hologram forms a static image of the diffuser. Every point of the diffuse illumination patch reproduces each divergent point of the diffuser. Two equivalent interpretations may be used to characterize the formation of a diffuse illumination patch by the hologram. Referring to FIG. 4A it will be seen that each point on the hologram diffracts incident light into the entire area of the diffuse illumination patch. On the other hand, referring to FIG. 4B, it will be seen that in this case each point in the diffuse illumination patch receives light from the entire area of the hologram. It will be clear to those skilled in the art of holographic optics that other optical configurations may be used to form a diffuse illumination patch that exists at any location in front of or behind the holographic diffuser. It should also be apparent that the same principles may be used to provide a holographic diffuser based on reflection holograms. Whichever interpretation is used, the effect is to provide random spatio temporal averaging of the speckle pattern, multiple speckle patters are superimposed and the speckle contrast is decreased. If the hologram is now illuminated by light from the micro scanner each point on the hologram is illuminated by rays having random incident angles covering a small angular sweep. This is equivalent to the classical displaced diffuser approach.

(35) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 5 a despeckler comprises a micro scanner 1 based on the principles described above, a holographic diffuser 2 based on the principles described above. The micro scanner is illuminated by light from a laser 21 which is expanded and collimated by the beam coupling optics 22. The beam coupling optics typically includes a beam expander and collimator together with means such as a grating or prism for optically coupling the laser beam to the micro scanner. Specifically, the light from the laser indicated by 101 is expanded and collimated to provide the beam indicated by 102. The invention does not assume any particular optical design for the beam coupling optics. The micro scanner deflects the beam 102 in small angular sweeps of random amplitude indicated by 111, 112 according to the principles discussed above. The holographic diffuser then diffracts light to form a diffuse illumination patch as discussed above.

(36) FIGS. 6A-6B illustrates the operation of the diffuse illumination patch in more detail. In FIG. 6A illumination from a collimated beam in the ray direction 111 resulting in the diffracted ray group 121A-121D which is uniformly distributed around the average ray direction 121. FIG. 6B illustrates the ray paths follow when the incidence beam being swept to the ray direction 112 resulting in the diffracted ray group 122A-122D which is uniformly distributed around the average ray direction 122. Each position on the incident rays sweep gives rise to a unique diffuse ray distribution across the diffuse illumination patch. Superposing and temporally integrating the set of such patterns generated by the full incident ray sweep over the eye integration time results in a reduction of the speckle contrast.

(37) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 7 the despeckler of FIG. 5 further comprises a beam steering means for directing the output ray angles 111, 112 from the micro-scanner into the input angles 131, 132 required by the holographic diffuser. Typically, the holographic diffuser will require off-axis incidence angles. Advantageously, the beam steering means is a diffractive device based on Bragg gratings. However, other means for steering the beams into the required angles will be apparent to those skilled in the art.

(38) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 8 a projector incorporating the despeckler of FIG. 5 further comprises a microdisplay 3 a projection lens 41 and a screen 42 which is observed from an eye position indicated by the symbol 43.

(39) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 9 a projector incorporating the despeckler of FIG. 7 further comprises a microdisplay 3 a projection lens 41 and a screen 42 which is observed from an eye position indicated by the symbol 43.

(40) Advantageously, each point in the holographic diffuser diffracts light into the maximum available area of the diffuse illumination patch, as defined by the active area of a microdisplay, for example. In some embodiments of the invention it may be sufficient for light diffracted from any point to fill only a portion of the maximum available area. Easing the diffraction angle range will generally make the hologram prescription less demanding.

(41) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 10 two micro scanners 1A, 1B of identical prescriptions are provided. The micro scanners are stacked and operated independently. The number of micro scanners that can be stacked in this way is limited only by the transmission losses incurred by the substrates, HPDLC and ITO. Speckle reduction increases with the number of layers.

(42) In one embodiment of the invention illustrated in the schematic side elevation view of FIG. 11 the beam coupling optics comprises a TIR lightguide 24. A coupling grating 23 admits collimated light from the laser 21 into a TIR path indicated by 103. A second coupling grating 25 directs light into the micro scanner. A TIR lightguide may be used to couple in light from RGB laser sources or multiple monochromatic sources depending on the application.

(43) In one embodiment of the invention the holographic diffuser is a CGH.

(44) In one embodiment of the invention the holographic diffuser is recorded into a HPDLC using the same procedure as described above. In this case the holographic diffuser can be switched on and off.

(45) In one embodiment of the invention in which the holographic diffuser is recorded into a HPDLC the holographic diffuser may be configured as an array of selectively switchable diffuser elements recorded each operating according to the above principles.

(46) In one embodiment of the invention the holographic diffuser also encodes the properties of beam shaping and homogenization. The principles are known to those skilled in the art of diffractive optical element design. The holographic diffuser is made by fabricating a CGH with the required optical properties and recording the CGH into the holographic diffuser. (essentially forming a hologram of the CGH).

(47) In one embodiment of the invention illustrated in FIG. 12 there is provided a despeckler for use with a reflective display. The despeckler is similar to the embodiment of FIG. 7 but further comprise a polarizing beamsplitter 44 and a quarter wave plate 45. Linearly polarized light from the laser is transmitted through the micro scanner, beam coupling optics and holographic diffuser as described above to provide illumination light 131,132. The illumination light is transmitted through the polarizing beamsplitter transmitted through the quarter wave plate reflected at the microdisplay and transmitted once again through the quartet wave plate emerging with polarization orthogonal to that of the incident light and is then reflected at the polarizing beam splitter towards projection lens which is not illustrated. The entire reflected light path is indicated by 150. In the case of a liquid crystal display panel the quarter wave plate will not be required.

(48) The inventors have found that micro scanner according to the principles described above is polarization sensitive.

(49) In one embodiment of the invention illustrated in FIG. 13 there is provided a further despeckler for use with a reflective microdisplay. The despeckler is similar to the embodiment of FIG. 5 but further comprise a polarizing beamsplitter 44 and a quarter wave plate 45. Linearly polarized light from the laser is transmitted through the polarization beam splitter, micro scanner, and holographic diffuser as described above to provide illumination light 131, 132. The illumination light is transmitted through the quarter wave plate reflected at the microdisplay and transmitted once again through the quartet wave plate, holographic diffuser and micro scanner and is then reflected at the polarizing beam splitter towards projection lens which is not illustrated. The entire reflected light path is indicated by 151. In this embodiment of the invention the holographic diffuser will ideally be sensitive to the same polarization as the micro scanner.

(50) In one embodiment of the invention the electrode elements of the micro scanner have substantially the same cross sectional area as a prismatic element.

(51) In one embodiment of the invention the center of said electrode element of the micro scanner overlaps the vertex of a prismatic element.

(52) In one embodiment of the invention the center of an electrode element of the micro scanner is offset from the vertex of a prismatic element.

(53) In one embodiment of the invention the prism array of the micro scanner is a linear array of elements of triangular cross section as illustrated in FIG. 1.

(54) In one embodiment of the invention the prism array of the micro scanner is a two-dimensional array comprising pyramidal elements of cross section similar to the one illustrated in FIG. 15. In such an embodiment ray deflections occur in two directions.

(55) In one embodiment of the invention the prismatic elements of the micro scanner are identical. Such an embodiment of the invention is also illustrated by FIG. 1.

(56) In one embodiment of the invention the surface angles of the prismatic elements of the micro scanner have a random distribution. Such an embodiment of the invention is also illustrated by FIG. 1.

(57) In one embodiment of the invention the prismatic elements of the micro scanner are each characterized by one of at least two different surface geometries. Such an embodiment of the invention is also illustrated by FIG. 1.

(58) In one embodiment of the invention the prismatic elements of the micro scanner are each characterized by one of at least two different surface geometries with the prismatic elements of each surface geometry being distributed uniformly across the prism array.

(59) In one embodiment of the invention the prismatic elements of the micro scanner have diffusing surfaces.

(60) In one embodiment of the invention the laser source comprises red green and blue emitters.

(61) The invention is not restricted to any particular laser source configuration. The HPDLC drive electronics are not illustrated. The apparatus may further comprise relay optics, beam folding mirrors, light integrators, filters, prisms, polarizers and other optical elements commonly used in displays

(62) The present invention does not assume any particular process for fabricating a despeckler devices. The fabrication steps may be carried out used standard etching and masking processes. The number of steps may be further increased depending on the requirements of the fabrication plant used. For example, further steps may be required for surface preparation, cleaning, monitoring, mask alignment and other process operations that are well known to those skilled in the art but which do not form part of the present invention

(63) It will be clear from the above description of the invention that the despeckler embodiments disclose here may be applied to the reduction of speckle in a wide range of laser displays including front and rear projection displays, wearable displays, scanned laser beam displays and transparent displays for use in viewfinders and HUDs.

(64) The invention is not limited to any particular type of HPDLC or recipe for fabricating HPDLC. The HPDLC material currently used by the inventors typically switches at 170 us and restores at 320 us. The inventors believe that with further optimization the switching times may be reduced to 140 microseconds.

(65) It should be emphasized that the Figures are exemplary and that the dimensions have been exaggerated. For example thicknesses of the HPDLC layers have been greatly exaggerated.

(66) The HPDLC may be based on any crystal material including nematic and chiral types.

(67) In particular embodiments of the invention any of the HPDLC devices discussed above may be implemented using super twisted nematic (STN) liquid crystal materials. STN offers the benefits of pattern diversity and adoption of simpler process technology by eliminating the need for the dual ITO patterning process described earlier.

(68) The invention may also be used in other applications such as optical telecommunications

(69) Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.