Method and apparatus for generating input images for holographic waveguide displays

[0001] This application is the U.S. national phase of PCT Application No. PCT/GB2015/000203 filed on Jun. 29, 2015, which claims the benefit of U.S. Provisional Patent Application No. 62/071,277 filed on Sep. 19, 2014, the disclosures of which are incorporated herein by reference in their entireties.

[0002] This invention relates to an apparatus for generating an image, and more particularly to an apparatus for generating an input image for a holographic waveguide display.

[0003] The invention addresses the problem of providing uniform output illumination in holographic waveguide displays. U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, PCT Application No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY all of which are incorporated herein by reference in their entireties.

[0004] The invention addresses a particular category of holographic waveguides based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results. SBGs may be used to provide transmission or reflection gratings for free space applications. In waveguide applications the parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have lower diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence.

[0005] Waveguides offer many features that are attractive in HMDs and HUDs. They are thin and transparent. Wide fields of views can be obtained by recording multiple holographs and tiling the field of view regions formed by each hologram. A key feature of these waveguides is that they provide pupil expansion in two orthogonal directions. The pupil expansion in a given direction is achieved by diffracting equal amounts of light out of output grating toward the eye box at each beam grating interaction. Uniformity of output is achieved by designing the output grating to have diffraction efficiency varying from a low value near the input end of the waveguide to a high value at the furthest extremity of the output grating. The inventors refer to grating having such properties as lossy gratings. The diffraction efficiency profile along the waveguide may be controlled by varying one or both of the grating refractive index modulation and the grating thickness. According to the theory of Bragg gratings higher index modulations give higher peak efficiency and narrow diffraction efficiency angular bandwidths. Reducing the thickness of the grating leads to a decrease in the diffraction efficiency and a broadening of the diffraction efficiency angular bandwidth. The input image data is provided by a microdisplay external to the waveguide. The microdisplay which is usually a reflective array must be illuminated via a beam splitter. The reflected image light is collimated such that each pixel of the image provides a parallel beam in a unique direction. Finally, the image light must be coupled efficiently into the waveguide so that the image content can by transferred to the waveguide components used for orthogonal pupil expansion. The image light from the microdisplay is normally coupled into the waveguide via an input grating. Alternatively a prism may be used.

[0006] A major design challenge is coupling the image content into the waveguide efficiently and in such a way the waveguide image is free from chromatic dispersion and brightness non uniformity. To overcome chromatic dispersion and to achieve the best possible collimation it is desirable to use lasers. However, lasers and other narrow band sources such as LEDs suffer from the problem of pupil banding artifacts which manifest themselves as output illumination non uniformity. Banding artifacts are formed when the collimated pupil is replicated (expanded) in a TIR waveguide. In very basic terms the light beams diffracted out of the waveguide each time the beam interacts with the grating have gaps or overlaps. This leads to an illumination ripple. The degree of ripple is a function of field angle, waveguide thickness, and aperture thickness. The inventors have found by experiment and simulation that the effect of banding can be smoothed by dispersion with broadband sources such as LEDs. The effects are therefore most noticed in narrowband (e.g. laser) illumination sources.

[0007] There is a requirement for an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity.

[0008] There is a further requirement for a waveguide display comprising an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity, and holographic waveguides for providing pupil expansion in orthogonal directions.

[0009] It is a first object of the invention to provide an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity.

[0010] It is a second object of the invention to provide a waveguide display comprising an image generator for illuminating a microdisplay, collimating the reflected image light from the microdisplay and efficiently coupling it into a thin holographic waveguide with high efficiency, with low chromatic dispersion and with high illumination uniformity, and holographic waveguides for providing pupil expansion in orthogonal directions.

[0011] The objects of the invention are met in a first embodiment in which there is provided an image generation device comprising: a spatial light modulator (SLM); a source emitting first wavelength light; a beam deflector for forming the light into a scanned beam; an illumination waveguide containing at least one switchable grating disposed in at least one layer; an image transport waveguide containing at least one switchable grating disposed in at least one layer; and a coupler for directing the scanned beam into a first set of TIR paths in the illumination waveguide. The at least one switchable grating in the illumination waveguide diffracts light out of the first set of TIR paths onto the SLM. The at least one switchable grating in the image transport waveguide diffracts image-modulated from the SLM into a second set of TIR paths in the image transport waveguide

[0012] In one embodiment the extent of the at least one grating along the image transport waveguide defines a coupling aperture, wherein the coupling aperture defines a numerical aperture for each pixel of the SLM.

[0013] In one embodiment at least one grating in the illumination waveguide and at least one grating in the image transport waveguide are switched into their diffracting states simultaneously.

[0014] In one embodiment the switchable grating in the illumination waveguide comprises an array of elements each having a unique angular diffraction characteristic.

[0015] In one embodiment the switchable grating in the image transport waveguide comprises an array of elements each having a unique angular diffraction characteristic.

[0016] In one embodiment regions of the SLM are illuminated cyclically the regions being updated with new image information in phase with the illumination, wherein the at least one grating in the illumination waveguide and the at least one grating in the image transport waveguide are switched synchronously with the SLM image formation updates.

[0017] In one embodiment a collimating lens is disposed between the illumination waveguide and the image transport waveguide.

[0018] In one embodiment the apparatus further comprises a source emitting second wavelength light wherein the illumination grating waveguide and the image transport waveguide each contain at least one grating for diffracting the first wavelength light and at least one grating for diffraction second wavelength light, wherein the SLM displays first wavelength image information when the first wavelength diffracting gratings are in their diffracting states, wherein the SLM displays second wavelength image information when the second wavelength diffracting gratings are in their diffracting states.

[0019] In one embodiment the image transfer waveguide provides an optical path to a waveguide display device.

[0020] In one embodiment further comprises a lossy grating operative to provide a spatial variation of intensity across the beam.

[0021] In one embodiment the coupler comprises a prism or grating.

[0022] In one embodiment the SLM, the illumination waveguide and the image transport waveguide are configured in a stack.

[0023] In one embodiment the apparatus further comprises despeckler.

[0024] In one embodiment the switchable grating layers are recorded in one of a HPDLC reverse mode HPDLC, uniform modulation.

[0025] In one embodiment the source is a laser or LED.

[0026] In one embodiment the SLM is a liquid crystal device.

[0027] In one embodiment of the invention there is provided a method of generating a wave guided image comprising the steps of:

• [0028] a) providing a SLM, a illumination waveguide containing at least one switchable grating layer, an image transport waveguide containing at least one switchable grating layer, a laser module, a micro mirror and a coupler;
• [0029] b) the micro-mirror sweeping light from the laser module, through a defined angular range the light being injected into the illumination waveguide;
• [0030] c) switching a SBG layer in the illumination waveguide to diffract light onto the SLM;
• [0031] d) switching a SBG layer in the image transport waveguide to diffract light reflected from the SLM into a waveguide path in the image transport waveguide.

[0032] In one embodiment of the invention there is provide a method of generating a wave guided image wherein at least one grating in the illumination waveguide and at least one grating in the image transport waveguide are switched into their diffracting states simultaneously.

[0033] In one embodiment there is provided a method of generating a wave guided image comprising the steps of:

• [0034] a) providing a SLM, a illumination waveguide containing at least one switchable grating layer, an image transport waveguide containing at least one switchable grating layer, a laser module, a micro mirror and a coupler;
• [0035] b) the micro-mirror sweeping light from the laser module through a defined angular range the light being injected into the illumination waveguide;
• [0036] c) sequentially update regions of the SLM with image information;
• [0037] d) switching a SBG layer in the illumination waveguide to diffract light onto a region of the SLM.
• [0038] e) switching a SBG layer in the image transport waveguide to diffract light reflected from the SLM into a waveguide path in the image transport waveguide.

[0039] In one embodiment there is provide a method of generating a wave guided image wherein simultaneously at least one grating in the illumination waveguide is switched into a first diffracting state and least one grating in the image transport waveguide is switched into a second diffracting state and a region of the SLM is updated with image information.

[0040] A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like index numerals indicate like parts. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

[0048] The invention will now be further described by way of example only with reference to the accompanying drawings. It will apparent to those skilled in the art that the present invention may be practiced with some or all of the present invention as disclosed in the following description. For the purposes of explaining the invention well-known features of optical technology known to those skilled in the art of optical design and visual displays have been omitted or simplified in order not to obscure the basic principles of the invention. Unless otherwise stated the term “on-axis” in relation to a ray or a beam direction refers to propagation parallel to an axis normal to the surfaces of the optical components described in relation to the invention. In the following description the terms light, ray, beam and direction may be used interchangeably and in association with each other to indicate the direction of propagation of light energy along rectilinear trajectories. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optical design. 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.

[0049] In one embodiment illustrated in FIG. 1 the apparatus comprises a microdisplay 1, illumination waveguide layer 2, an image transport waveguide layer which is divided into image transport waveguide aperture 3A and an image transport waveguide layer 3B. The illumination waveguide layer contains a threes layer SBG 4 with the separate layers indicated by 4A,4B,4C The image transport grating aperture contains a SBG layer 5 with the separate layers indicated by 5A,5B,5C. The apparatus further comprises laser module 6 micro mirror 7, prism 8, a SBG module comprising lossy SBG layer 9 sandwiched by the substrates 10,11 and a coupling prism 12 abutting the illumination waveguide layer 2 for coupling scanned light into the illumination waveguide layer, and a collimation lens 13.

[0050] Note that although three SBG layers are used in the illumination waveguide layer, the invention may be applied with more or fewer SBG layers according to the angular range requirement. The number layers will be roughly equal to the angular range to be covered divided by the diffraction efficiency angular bandwidth of the SBGs. The micro mirror will have a rotation range equal to cover the required angular range to cover the angular acceptance range of the microdisplay. The collimation lens 13 has a relative aperture of typically F/2. However other relative aperture may be used as dictated by the system requirements. The image transport waveguide is typically less than 1 mm in thickness. The coupling prism 12 is not an essential feature of the invention and could be replaced by a grating. The purpose of the lossy SBG 9 is to expand the input laser beam across the height of the microdisplay pixel column. Typically this distance would be a few millimeters.

[0051] The invention provides a method and apparatus for time-sequential aperture switching using SBGs to create a multiplicity of time distributed spatially discrete pupil apertures in a waveguide optic that are temporally integrated to construct a spatially homogenized expanded pupil. The temporal integration of the multiple pupils serves to reduce the contrast of banding artifacts that arise from spatially discrete pupil replication across the waveguide. Perfect homogenization will normally only be achieved for a singe field angle. Typically, this will be at the centre of the field of view. In other regions of the field of view bright banding or dark banding may occur depending on the field angle range. Pupil artifacts manifest themselves in two different ways according to whether the display is beyond the limits of eye accommodation or within the eye accommodation range. Beyond the limits of eye accommodation: that is when the display is close to the eye, as in the case of Head Mounted Displays (HMDs) and smart eyewear (eye relief around 30 mm or less), uniformity variations are seen as a function of field angle. The contrast of these artifacts can vary as a function of pupil diameter (influenced display brightness and/or see-thru brightness). Within the eye accommodation distance pupil artifacts can be directly imaged when the eye focuses on the pupil and not at infinity; this can occurs in Head Up Displays (HUDs) and Head Down Displays (HDDs) where the display is several inches from the eye. This is distracting to display users, impairing the functionality of the display; a user focusing on a pupil artifact will then not be focusing on the far field image projected by the display.

[0052] The invention provides two primary modes of operation based on the switching of the micro mirror and the SBG elements in the two waveguides. The first mode, which is illustrated in FIGS. 1-2, is pupil switching that is aperture switching at the input to the image transport waveguide aperture. The second mode, which is illustrated in FIGS. 3-4, is field switching, that is switching at the LCoS to control the portion of the LCoS that is illuminated at any given time. We next consider the propagation of light through the embodiment of FIG. 1. Collimated light 1000 from the laser is swept by the microscanner into a range of ray directions such as 1001. The light is then expanded by the lossy SBG to provide the light 1002 incident on the coupling prism which enters the TIR path indicated by 1003. The function of the SBGs in the illumination waveguide is to illuminate the desired microdisplay column, the SBGs being switch in synchronism with the rotation of the micro mirror. Each SBG will have a unique diffracting characteristic design to diffract a set of angularly separated ray directions from the microscanner into a range of output angularly separated ray directions incident on the image surface of the microdisplay. The incident and diffracted angles from a given SBG element are given by the Bragg equation. In FIG. 1 an active SBG 4D element deflects the TIR beam towards a column 1A of the microdisplay in the direction 1004. From consideration of FIG. 1 it should be apparent that by sweeping the micro mirror through a given angular range and selectively switching SBG elements into their diffracting state during the sweep time any given column of pixels will be illuminated by a range of incidence angles. For a reflective microdisplay as shown in FIG. 1 these rays are in turn reflected into opposing angles towards the collimation lens. The extreme rays reflected from the microdisplay column 1A are indicated by 1005 and 1007 which give rise to collimated beam represented by the extreme rays 1008,1009 which enter the image transport waveguide aperture to illuminate the SBG array 5. In FIG. 1 the SBG element 5B is in its diffracting state and all others are in their non-diffracting states. The block arrow 1012 indicates the direction of scrolling of the SBG elements in synchronism with the scanning of the beam. The ray 1008 incident on element 5B is diffracted in to the TIR path indicated by 1010 within the image transport waveguide aperture and proceeds into the image transport waveguide as the TIR light 1011. In the envisaged applications of the invention the light 1011 would be wave guided to separate waveguide grating elements for pupil expansion in orthogonal directions. FIG. 2 illustrates the pupil switching process used in the embodiment of FIG. 1 in more detail with the filed regions diffracted by the elements of transport waveguide being indicated by the boundary rays originating at the microdisplay column 1A indicated 1006A-1006F ad th boundary collimated rays by 1013A-1013B.

[0053] We next consider the aperture switching mode referring to FIG. 3. The apparatus is similar to that of FIG. 1 except that the image transport waveguide aperture 20A now contains three SBG array layers, 21,22,23. Each stack of elements such as 21A,22A,23A receives light from three separate portions of the microdisplay. Only one stack diffracts at any time. All elements of all other SBGs are in their non diffracting states. This process is shown in more detail in FIG. 4 which shows the microdisplay divided into the three regions 1B,1C,1D. Ray paths from the centre column of each region to the SBG stack elements 21A,22A,23A by the rays paths 1021-1026, which include the paths through the collimator lens 13 and the SBG stack, are illustrated. In one embodiment all three SBGs in the stack are in their active state simultaneously. In one embodiment the SBGs are switched simultaneously. In one embodiment the SBGs are switched synchronously with the switching of the microdisplay columns such that for example: when the microdisplay columns 1B are displayed the element 21A is in its active state, when the microdisplay columns 1C are displayed the element 22A is in its active state, and so on. The captured light proceeds via TIR into the image transport waveguide 20A.

[0054] It should be apparent from consideration of the drawings and the above description that pupil switching and aperture switching can be combined in a single image generation process. This embodiment would use the same optical components as the embodiments discussed above with an appropriate switching procedure for the illumination waveguide and image transport waveguide SBGs. The chief limitation on the switching scheme would be the switching on and off times for a SBG element.

[0055] The above described pupil switching and aperture switching schemes are characterised by five synchronized switching events which are described below with reference to FIG. 2 and FIG. 4. In a first event the micro-mirror sweeps a laser beam, through a defined angular range the light being injected into the illumination waveguide. The following two events (second and third) are concerned with pupil switching. In the second event the diffracting SBG grating layer in the illumination waveguide is switched (between the three layers). This SBG switching covers the angular bandwidth required to fill the full acceptance angle of the microdisplay. The angular range covered by each SBG layer and microscanner sweep corresponds with the image transport waveguide aperture element selected. In the third event the SBG element under the illuminated portion of the image transport waveguide aperture is switched to a diffracting state. The following two events (fourth and fifth) are concerned with field switching. In the fourth event three defined portions of the microdisplay are time sequentially illuminated by SBG elements of the illumination waveguide layer. Note that only a small field angle is illuminated at a given time. In the fifth event SBG elements in stack of SBG elements in the image transfer waveguide aperture are switching into their diffracting states in synchronism with the switching of the SBG elements of the illumination waveguide layer.

[0056] In the embodiment of FIG. 5, which is identical to that of FIG. 1, the illumination waveguide also includes a wave guide despeckler 30. The despeckler is desirable a waveguide despeckler based on principles disclosed in PCT Application No.: PCT/GB2013/000500 entitled WAVEGUIDE FOR HOMOGENIZING ILLUMINATION, and U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATION DEVICE both of which are incorporated herein by reference in their entireties.

[0057] In one embodiment illustrated in the flow diagram of FIG. 6 a method of generating a image using the pupil switching scheme of the embodiment of FIG. 1 is provided. Referring to the flow diagram, we see that the said method comprises the following steps:

[0058] At step 2000 provide a SLM, a illumination waveguide layer containing at least one switchable grating layer, an image transport waveguide layer containing at least one switchable grating layer, an image transport waveguide containing at least one switchable grating layer, a laser module, a micro mirror and a coupler;

[0059] At step 2001 the micro-mirror sweeps light from said laser module, through a defined angular range, the light being injected into the illumination waveguide

[0060] At step 2002 switch a grating layer in the illumination waveguide to diffract light onto the SLM.

[0061] At step 2003 switch a grating layer in the image transport waveguide to diffract light reflected from the SLM into a waveguide path in the image transport waveguide.

[0062] In one embodiment illustrated in the flow diagram of FIG. 7 a method of generating a image using the pupil switching scheme of the embodiment of FIG. 3 is provided. Referring to the flow diagram, we see that the said method comprises the following steps:

[0063] At step 2010 provide a SLM, a illumination waveguide layer containing at least one switchable grating layer, an image transport waveguide layer containing at least one switchable grating layer, an image transport waveguide containing at least one switchable grating layer, a laser module, a micro mirror and a coupler;

[0064] At step 2011 the micro-mirror sweeps light from said laser module, through a defined angular range the light being injected into the illumination waveguide

[0065] At step 2012 sequentially update regions of the SLM with image information.

[0066] At step 2013 switch a grating layer in the illumination waveguide to diffract light onto a region of the SLM.

[0067] At step 2014 switch a grating layer in the image transport waveguide to diffract light reflected from the SLM into a waveguide path in the image transport waveguide.

[0068] It should be emphasized that the drawings are exemplary and that the dimensions have been exaggerated. For example thicknesses of the SBG layers have been greatly exaggerated.

[0069] In any of the above embodiments the waveguides may be curved or formed from a mosaic of planar or curved facets.

[0070] An image generator based on any of the above-described embodiments may be implemented using plastic substrates using the materials and processes disclosed in PCT Application No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. Advantageously, the SBGs are recorded in a reverse mode HPDLC material in which the diffracting state of SBG occurs when an electric field is applied across the electrodes. An eye tracker based on any of the above-described embodiments may be implemented using reverse mode materials and processes disclosed in the above PCT application.

[0071] While the invention may be applied with gratings of any type including switching or non-switching gratings based on Bragg (volume) holograms, or surface-relief gratings the preferred grating technology is a SBG, which offers the advantages of fast switching, high optical efficiency and transparency and high index modulation.

[0072] The method of fabricating the SBG pixel elements and the ITO electrodes used in any of the above-described embodiments of the invention may be based on the process disclosed in the PCT Application No. US2006/043938, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY.

[0073] It should be understood by those skilled in the art that while the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. Various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.