Method and apparatus for providing single grating layer color holographic waveguide display

Abstract

Methods and apparatus for providing a single grating layer color holographic waveguide display are disclosed. The waveguide display includes: a waveguide supporting a single grating layer; a data modulation light source; a first input coupler that directs light of a first spectral band from the source into a first waveguide pupil; a second input coupler that directs light of a second spectral band from the source into a second waveguide pupil; the output coupler comprises a first optical grating and a second optical grating which are multiplexed; at least one folding grating that directs the first spectral band along a first path from the first pupil to the output coupler and provides a first beam expansion; and at least one folding grating that directs the second spectral band along a second path from the second pupil to the output coupler and provides a first beam expansion. The first multiplexing grating directs the first spectral band out of the waveguide in a first direction in which the beam spread is orthogonal to the first beam spread. The second multiplexing grating directs the second spectral band out of the waveguide in a first direction in which the beam spread is orthogonal to the first beam spread.

Description

[0001] This application is a divisional application of the Chinese invention patent application with application number 201980089763.9, application date December 10, 2019, and titled “Method and device for providing a single grating layer color holographic waveguide display”. Technical Field

[0002] The present invention relates generally to waveguide devices and, more particularly, to color holographic waveguide displays. Background Art

[0003] A waveguide can be described as a structure that has the ability to confine and guide waves (i.e., limit the region of space in which waves can propagate). One class of waveguides includes optical waveguides, which are structures capable of guiding electromagnetic waves, typically those in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using many different mechanisms. For example, a planar waveguide can be designed to utilize a diffraction grating to diffract incident light and couple the incident light into the waveguide structure so that the incident coupled light can continue to propagate within the planar structure via total internal reflection ("TIR").

[0004] Fabrication of waveguides can include the use of material systems that allow for recording of holographic optical elements within the waveguide. One class of such materials includes polymer-dispersed liquid crystal ("PDLC") mixtures, which are mixtures comprising photopolymerizable monomers and liquid crystals. Another subclass of such mixtures includes holographic polymer-dispersed liquid crystal ("HPDLC") mixtures. Holographic optical elements, such as volume phase gratings, can be recorded in such liquid mixtures by illuminating the material with two mutually coherent laser beams. During the recording process, the monomers polymerize and the mixture undergoes photopolymerization-induced phase separation, thereby creating regions densely packed with liquid crystal droplets interspersed with regions of transparent polymer. Alternating regions rich in liquid crystal and regions poor in liquid crystal form the fringe plane of the grating.

[0005] Waveguide optics such as those described above are contemplated for use in a range of display and sensor applications. In many applications, waveguides containing one or more grating layers encoding multiple optical functions can be implemented using a variety of waveguide architectures and material systems, enabling new innovations in near-eye displays for augmented reality ("AR") and virtual reality ("VR"), compact head-up displays ("HUD") for aviation and road transportation, and sensors for biometric and laser radar ("LIDAR") applications. Summary of the Invention

[0006] Many embodiments relate to waveguide displays configured to implement a full-color display capable of providing two-dimensional beam expansion and light extraction. For example, many embodiments relate to a waveguide display having various components, including: a waveguide supporting a single grating layer; a data-modulated light source optically coupled to the waveguide; a first input coupler for directing a first spectral band of light from the source into a first waveguide pupil; a second input coupler for directing a second spectral band of light from the source into a second waveguide pupil; and an output coupler comprising a multiplexed first and second gratings. Furthermore, many embodiments include at least one folding grating for directing the first spectral band along a first path from the first pupil to an output coupler that provides a first beam expansion. At least one folding grating can be used to direct the second spectral band along a second path from the second pupil to the output coupler that provides a first beam expansion. The first multiplexing grating can direct the first spectral band out of the waveguide in a first direction orthogonal to the beam expansion of the first beam. The second multiplexing grating can direct the second spectral band out of the waveguide in a first direction orthogonal to the beam expansion of the first beam.

[0007] In other embodiments, the first and second input couplers each include at least one of a prism and a grating.

[0008] In still other embodiments, the first input coupler comprises a first prism and the second input coupler comprises a second prism, wherein the first and second prisms are disposed along a general direction of light propagation of the waveguide.

[0009] In yet other embodiments, the first light input coupler comprises a first prism and the second light input coupler comprises a second prism, wherein the first and second prisms are arranged along a direction orthogonal to a general light propagation direction of the waveguide.

[0010] In still other embodiments, the first input coupler comprises a first grating and the second input coupler comprises a second grating, wherein the first and second gratings are arranged along a general light propagation direction of the waveguide.

[0011] In other embodiments, the first input coupler comprises a first grating and the second input coupler comprises a second grating, wherein the first and second gratings are arranged along a direction orthogonal to a general light propagation direction of the waveguide.

[0012] In still other embodiments, the first input coupler comprises a prism and a first grating, and the second input coupler comprises the prism and a second grating, wherein the first and second gratings are disposed along a general direction of light propagation of the waveguide.

[0013] In yet other embodiments, the first input coupler comprises a prism and a first grating, and the second input coupler comprises the prism and a second grating, wherein the first and second gratings are arranged along a direction orthogonal to a general light propagation direction of the waveguide.

[0014] In still other embodiments, the first input coupler includes a prism and a first grating, and the second input coupler includes the prism and a second grating, wherein the first and second gratings are multiplexed.

[0015] In other embodiments, folded gratings are multiplexed and have a prescription for performing two-dimensional beam expansion and extracting light from the waveguide.

[0016] In still other embodiments, the fold grating is configured to provide pupil expansion in a first direction, wherein the output grating is configured to provide pupil expansion in a second direction different from the first direction.

[0017] In yet other embodiments, the source comprises at least one LED.

[0018] In still other embodiments, the source includes at least one LED having a spectral output biased toward a peak wavelength of the first spectral band and at least one LED having a spectral output biased toward a peak wavelength of the second spectral band.

[0019] In other embodiments, at least one of the gratings is a scrolled k-vector grating.

[0020] In still other embodiments, light undergoes a double interaction within at least one of the folded gratings.

[0021] In yet other embodiments, the data modulated light source has a microdisplay for displaying image pixels and collimating optics for projecting an image displayed on the microdisplay panel such that each image pixel on the microdisplay is translated into a unique angular direction within the first waveguide.

[0022] In still other embodiments, at least one grating has a spatially varying pitch.

[0023] In other embodiments, at least one of the input coupler, the folded grating and the output grating is one of a switchable Bragg grating or a surface relief grating recorded in a holographic photopolymer, a HPDLC material or a uniformly modulated holographic liquid crystal polymer material.

[0024] In still other embodiments, the first and second input couplers each comprise at least one grating, wherein the at least one grating of each of the first and input couplers, the folding grating and the first and second multiplexers is arranged in a single grating layer.

[0025] Other embodiments include a method of displaying a color image, comprising the steps of:

[0026] a) providing a waveguide supporting a single grating layer; a light source; a first input coupler; a second input coupler; an output coupler comprising a multiplexed first and second gratings; a first folded grating; and a second folded grating;

[0027] b) directing a first spectral band from the source into a first waveguide pupil via the first input coupler;

[0028] c) directing a second spectral band from the source into a second waveguide pupil via the second input coupler;

[0029] d) expanding the first spectral band light by means of the first folding grating and redirecting the light to the output coupler;

[0030] e) expanding the second spectral band light by means of the second folding grating and redirecting the light to the output coupler;

[0031] f) performing beam expansion on the light in the first spectral band by means of the first multiplexing grating and extracting the light in the first spectral band from the waveguide;

[0032] g) beam-expanding the second spectral band light by means of the second multiplexing grating and extracting the second spectral band light from the waveguide.

[0033] Other embodiments include waveguide displays in which the waveguide supports a single grating layer. Furthermore, the waveguide display can include an image-modulated light source optically coupled to the waveguide via a first input coupler for directing light from the source in a first spectral band into a first waveguide pupil. The waveguide display can also have a second input coupler for directing light from the source in a second spectral band into a second waveguide pupil. Furthermore, first and second folded gratings for diffracting the first and second spectral bands, respectively, can be used with an output coupler comprising multiplexed first and second gratings for diffracting the first and second bands out of the waveguide, respectively.

[0034] Other embodiments include a light field display having a first waveguide display and a second waveguide display, as in many embodiments, wherein input couplers and output couplers of the first and second waveguides overlap, wherein at least one grating in the first waveguide display has optical power for focusing light extracted from the first waveguide to a first focal plane, wherein at least one grating in the second waveguide display has optical power for focusing light extracted from the first waveguide to a second focal plane, wherein the input couplers of the first waveguide display and the second waveguide display each have a grating switchable between a diffractive and a non-diffractive state.

[0035] In yet other embodiments, the grating of the first waveguide display is in its diffraction state for incoupling image modulated light for viewing at a first focal plane when the grating of the second waveguide display is in its non-diffraction state, wherein the grating of the second waveguide display is in its diffraction state for incoupling second image modulated light for viewing at a second focal plane when the grating of the first waveguide display is in its non-diffraction state. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete description of the scope of the invention.

[0037] Figure 1 A schematic plan view of a waveguide display having a single layer waveguide supporting an input coupler including a prism and spatially separated input gratings in accordance with an embodiment of the present invention is conceptually illustrated.

[0038] Figure 2 A schematic plan view of a waveguide display having a single layer waveguide supporting an input coupler including a prism and a multiplexed input grating in accordance with an embodiment of the present invention is conceptually illustrated.

[0039] Figure 3 A schematic plan view of a waveguide display having a single layer waveguide supporting input couplers including spatially separated input gratings in accordance with an embodiment of the present invention is conceptually illustrated.

[0040] Figure 4 A schematic plan view of a waveguide display having a single layer waveguide supporting an input coupler including a multiplexed input grating is conceptually illustrated in accordance with an embodiment of the present invention.

[0041] Figure 5 and Figure 6Schematic plan views of waveguide displays having a single layer of waveguide supporting first and second spatially separated input prisms are conceptually illustrated according to various embodiments of the present invention.

[0042] Figure 7 A schematic plan view of a waveguide display having spatially separated input gratings and multiplexed grating pairs that combines the dual functions of two-dimensional beam expansion and beam extraction in a waveguide is conceptually illustrated according to an embodiment of the present invention.

[0043] Figure 8 A flow chart illustrating a method of providing a color waveguide display with two-dimensional beam expansion using a single grating layer is conceptually illustrated according to an embodiment of the present invention.

[0044] Figure 9 A schematic cross-sectional view of a lightfield display with a single-layer color waveguide stack is conceptually illustrated in accordance with an embodiment of the present invention.

[0045] Figure 10A A schematic cross-sectional view conceptually illustrates a first operational state of a light field display corresponding to forming a visible image at a first range, in accordance with an embodiment of the present invention.

[0046] Figure 10B A schematic cross-sectional view showing a second operational state of a light field display corresponding to forming a visible image at a second range is conceptually illustrated in accordance with an embodiment of the present invention.

[0047] Figure 11A and Figure 11B The grating geometries of a set of exemplary gratings according to embodiments of the present invention are conceptually illustrated.

[0048] Figure 12 and Figure 13 A plan view of a waveguide for providing a color image using a single grating layer having an input grating, a fold grating, and an output grating is conceptually illustrated in accordance with an embodiment of the present invention.

[0049] Figure 14 A cross-sectional view of a dichroic prism system for coupling illumination from red, green, and blue sources into a waveguide such that the red-green and green-blue bands of the illumination are spatially sheared upon entering the waveguide is conceptually illustrated in accordance with an embodiment of the present invention.

[0050] Figure 15 is a graph illustrating the spectra of two LEDs having similar peak wavelengths used in combination to provide a primary lighting color according to an embodiment of the present invention.

[0051] Figure 16A schematic cross-sectional view conceptually illustrates a scrolling K-vector input grating configured to receive spatially sheared illumination to provide red-green and blue bands, in accordance with an embodiment of the present invention. DETAILED DESCRIPTION

[0052] For the purpose of describing the embodiments, some well-known features of optical technology known to those skilled in the art of optical design and visual display have been omitted or simplified to avoid obscuring the basic principles of the present invention. Unless otherwise specified, the term "coaxial" with respect to the direction of a ray or beam refers to propagation parallel to an axis perpendicular to the surface of the optical component described in the present invention. In the following description, the terms light, ray, beam and direction can be used interchangeably and are associated with each other to indicate the direction of propagation of light energy along a straight line trajectory. The parts described below will be represented using terms commonly used by those skilled in the art of optical design. For illustrative purposes, it should be understood that the drawings are not drawn to scale unless otherwise specified. For example, the dimensions in some of the drawings have been exaggerated.

[0053] Turning now to the accompanying drawings, a color holographic waveguide display and related manufacturing methods are illustrated. Waveguide displays can be used in many different applications, including but not limited to HMDs for AR and VR, helmet-mounted displays, projection displays, head-up displays (HUDs), head-down displays (HDDs), autostereoscopic displays, and other 3D displays. Furthermore, similar technology can be applied to waveguide sensors such as, for example, eye trackers, fingerprint scanners, and LIDAR systems. Waveguide manufacturing, especially color waveguide manufacturing, can be expensive and prone to low yields due to several factors. One such contributing factor is the difficulty in aligning the separate red, green, and blue waveguide layers required for a full-color display. This can be largely mitigated by reducing the number of waveguide layers used to achieve full color. For example, a full-color waveguide display can be achieved using two waveguide layers, one transmitting blue-green light and the other transmitting green-red light. Ideally, a display should have as few waveguide layers as possible. However, a single configuration of Bragg gratings typically cannot operate efficiently across the entire visual spectral bandwidth. Therefore, achieving a full-color display using a single grating layer can be challenging. Thus, many embodiments of the present invention are directed to realizing a full-color waveguide capable of providing two-dimensional beam expansion and light extraction using gratings of different configurations within a single grating layer.

[0054] In many embodiments, a waveguide display is implemented as a waveguide having a single grating layer. The waveguide display may also include a data-modulated light source optically coupled to the waveguide, a first input coupler for directing light of a first spectral band from the source into a first waveguide pupil, and a second input coupler for directing light of a second spectral band from the source into a second waveguide pupil. The light source may include at least one of an LED or a laser. In some embodiments, the source includes separate red, green, and blue emitters. In several embodiments, the waveguide display includes an output coupler having multiplexed first and second gratings, at least one folding grating for directing the first spectral band along a first path from the first pupil to the output coupler, and at least one folding grating for directing the second spectral band along a second path from the second pupil to the output coupler. The folding gratings may be configured to provide a first beam expansion for their respective spectral bands. With respect to the output coupler, the first multiplexing grating can be configured to direct a first spectral band out of the waveguide in a first direction, wherein the beam expansion is orthogonal to the first beam expansion, and the second multiplexing grating can be configured to direct a second spectral band out of the waveguide in the first direction, wherein the beam expansion is orthogonal to the first beam expansion.

[0055] Waveguide displays according to various embodiments of the present invention can be implemented and configured in many different ways.In some embodiments, the waveguide display is implemented as a curved, dual-axis beam-expanding waveguide.

[0056] Single layer waveguide displays, color waveguide displays, materials, and related manufacturing methods are discussed in more detail below.

[0057] Waveguide Display

[0058] Waveguide displays according to various embodiments of the present invention can be implemented and configured in many different ways. For purposes of illustration and simplicity, the general propagation direction discussed throughout this disclosure is from left to right. As can be readily appreciated, the waveguide configuration and light propagation direction can be configured accordingly depending on the specific application. The single-layer color waveguide architecture described in this disclosure has several major advantages over multi-layer architectures. The first is that assembly and alignment of multiple layers is not required, thereby increasing yield and reducing manufacturing costs. The second advantage is reduced manufacturing complexity because only a single layer is required during manufacturing using a single exposure process. This results in a reduction in exposure throughput time and therefore reduced costs. The principles of the present invention can be applied to a variety of waveguide display and sensor applications, including but not limited to HUDs and HMDs. While the present invention is dedicated to single-layer color waveguides, many of the embodiments and teachings disclosed herein can also be applied to monochrome waveguides.

[0059] In many embodiments, a waveguide display may include a light source, an input coupler, and an output coupler. The input coupler may include at least one of a prism and an input grating. In several embodiments, the output coupler is implemented using an output grating. In yet other embodiments, the waveguide display may include a folded grating. In several embodiments, each folded grating is configured to provide pupil expansion in a first direction and to guide light to an output grating via total internal reflection, wherein the output grating is configured to provide pupil expansion in a second direction different from the first direction, in accordance with the embodiments and teachings disclosed in the cited references. By using folded gratings, according to some embodiments, the waveguide device advantageously requires fewer layers than previous systems and methods for displaying information. In addition, by using folded gratings, light can travel by total internal reflection within the waveguide in a single right-angle prism defined by the outer surface of the waveguide, while achieving dual pupil expansion.

[0060] In many embodiments, at least one of the input grating, the fold grating, or the output grating can combine two or more angular diffraction specifications to extend the angular bandwidth. Similarly, in some embodiments, at least one of the input grating, the fold grating, or the output grating can combine two or more spectral diffraction specifications to extend the spectral bandwidth. For example, a color multiplexing grating can be used to diffract two or more primary colors.

[0061] In several embodiments, the grating layer comprises multiple components, including an input coupler, a folded grating, and an output grating (or portions thereof), laminated together to form a single substrate waveguide. These components can be separated by optical glue or other transparent material with a refractive index matching that of the components. In some embodiments, the grating layer can be formed via a cell fabrication process by creating a cell with the desired grating thickness for each of the input coupler, folded grating, and output grating and vacuum-filling each cell with SBG material. In many embodiments, the cell is formed by positioning multiple glass plates with gaps between them that define the desired grating thickness for the input coupler, folded grating, and output grating. In several embodiments, a cell can be fabricated with multiple holes, such that individual holes are filled with different pockets of SBG material. Separate regions can then be separated by separating any intervening spaces with a separator material (e.g., glue, oil, etc.). In some embodiments, the SBG material can be spin-coated onto a substrate and then covered with a second substrate after the material cures.

[0062] In many embodiments for display applications, the folded gratings can be oriented (clocked) with their grating vectors oriented diagonally within the waveguide plane. This ensures sufficient angular bandwidth for the folded light. However, some embodiments of the present invention may utilize other clocking angles to accommodate spatial constraints on grating positioning that may arise in the ergonomic design of the display. The grating vector orientation angle may be referred to as the "clocking angle." In some embodiments, the longitudinal edges of each folded grating are tilted relative to the alignment axis of the input coupler, such that each folded grating is positioned diagonally relative to the propagation direction of the display light. The angle of the folded gratings is such that light from the input coupler is redirected to the output grating. In one example, the folded gratings are positioned at a forty-five-degree angle relative to the direction in which the display image is released from the input coupler. This feature allows the display image propagating down the folded grating to be aligned with the output grating. For example, in several embodiments, the folded gratings rotate the image 90 degrees into the output grating. In this way, a single waveguide can provide biaxial pupil expansion in both the horizontal and vertical directions. In various embodiments, each folded grating can have a partially diffractive structure. The output grating receives image light from the fold grating via total internal reflection and provides pupil expansion in a second direction. The output grating can be configured to provide pupil expansion in a second direction different from the first direction and cause light to exit the waveguide from the first surface or the second surface.

[0063] In many embodiments, the angular bandwidth of a folded grating can be enhanced by designing the grating specifications to promote dual interaction between the guided light and the grating. Exemplary embodiments of dual-interaction folded gratings are disclosed in U.S. Patent Application No. 14 / 620,969, entitled "WAVEGUIDEGRATING DEVICE," the disclosure of which is incorporated herein by reference. In some embodiments, waveguides based on the above principles operate in the infrared. In some embodiments, at least one of the input grating, the folded grating, or the output grating can be based on a surface relief structure.

[0064] As discussed above, waveguide displays according to various embodiments of the present invention may include a light source. In some embodiments, a data-modulated light source used with the above-described waveguide embodiments includes an input image node (IIN) in conjunction with a microdisplay. The input grating may be configured to receive collimated light from the IIN and direct the light within the waveguide to a folding grating via total internal reflection between a first surface and a second surface. Typically, in addition to the microdisplay panel, the IIN also integrates the light source and optical components required to illuminate the display panel, separate the reflected light, and collimate it to a desired field of view (FOV). Each image pixel on the microdisplay can be converted to a unique angular orientation within the first waveguide. Any of a variety of microdisplay technologies can be used. In some embodiments, the microdisplay panel can be a liquid crystal device or a microelectromechanical system (MEMS) device. In several embodiments, the microdisplay can be based on organic light-emitting diode (OLED) technology. Such light-emitting devices generally do not require a separate light source and therefore have the benefit of a smaller form factor. In various embodiments, the IIN can be based on a scanned modulated laser. According to some embodiments, the IIN projects the image displayed on the microdisplay panel such that each display pixel is converted to a unique angular orientation within the substrate waveguide. The collimating optics included in the IIN may include lenses and mirrors, which may be diffractive lenses and mirrors. In some embodiments, the IIN may be based on the embodiments and teachings disclosed in U.S. Patent Application No. 13 / 869,866, entitled “HOLOGRAPHIC WIDE ANGLE DISPLAY,” and U.S. Patent Application No. 13 / 844,456, entitled “TRANSPARENT WAVEGUIDE DISPLAY,” the disclosures of which are incorporated herein by reference. In several embodiments, the IIN includes a beam splitter for directing light onto the microdisplay and transmitting the reflected light to the waveguide. In many embodiments, the beam splitter is a grating recorded in the HPDLC and uses the inherent polarization selectivity of the grating to separate the light illuminating the display and the image-modulated light reflected from the display. In some embodiments, the beam splitter is a polarizing beam splitter cube.

[0065] In many embodiments, the IIN includes a despeckler. Advantageously, the despeckler is a holographic waveguide device based on the embodiments and teachings of U.S. Patent No. 8,565,560, entitled "LASER ILLUMINATION DEVICE," the disclosure of which is incorporated herein by reference. The light source can be a laser or an LED and can include one or more lenses for modifying the angular characteristics of the illumination beam. The use of a despeckler is particularly important when the source is a laser and the image source is a laser-lit microdisplay or a laser-based emissive display. LEDs provide better uniformity than lasers. If laser illumination is used, there is a risk of illumination banding at the waveguide output. In some embodiments, laser illumination banding in the waveguide can be overcome using the techniques and teachings disclosed in U.S. Provisional Patent Application No. 62 / 071,277, entitled "METHOD AND APPARATUS FOR GENERATING INPUT IMAGES FOR HOLOGRAPHIC WAVEGUIDE DISPLAYS," the disclosure of which is incorporated herein by reference. In several embodiments, the light from the light source is polarized. In various embodiments, the image source is a liquid crystal display (LCD) microdisplay or a liquid crystal on silicon (LCoS) microdisplay.

[0066] In many embodiments, a waveguide display includes first and second input couplers. The first and second input couplers can each include at least one of a prism and a grating. In some embodiments, the coupler utilizes a single prism and is associated with a pair of first and second input gratings, respectively, with the first and second input gratings disposed along the general direction of light propagation of the waveguide. In several embodiments, the first and second gratings are disposed along a direction orthogonal to the general direction of light propagation of the waveguide. The first and second input gratings can be implemented in the waveguide and configured in many different ways. In many embodiments, the input gratings are spatially separated. In other embodiments, the input gratings are implemented as multiplexed gratings. A crossed configuration of multiplexed gratings can be advantageous for gratings recorded in HPDLC materials because it can achieve efficient phase separation of the liquid crystal and monomer components during grating recording. Figure 1 and Figure 2 These differences are conceptually illustrated.

[0067] Figure 1A schematic plan view of a waveguide display having a single-layer waveguide supporting an input coupler comprising a prism and spatially separated input gratings, in accordance with an embodiment of the present invention, is conceptually illustrated. In the illustrative embodiment, waveguide display 100 includes a waveguide 101 supporting an input prism 102. Waveguide 101 also includes input gratings 103, 104, folding gratings 105, 106, and multiplexed output gratings 107, 108. As shown, the gratings are arranged in a single grating layer. Ray paths 109-112 of rays diffracted by input grating 103 and ray paths 113-116 of rays diffracted by input grating 104 illustrate the beam paths in the waveguide from input to extraction.

[0068] Figure 2 A schematic plan view of a waveguide display having a single-layer waveguide supporting an input coupler including a prism and a multiplexed input grating, in accordance with an embodiment of the present invention, is conceptually illustrated. As shown, waveguide display 120 includes a waveguide 121 supporting an input prism 122. Waveguide 121 also includes multiplexed input gratings 123, 124, folding gratings 125, 126, and multiplexed output gratings 127, 128 arranged in a single grating layer. Ray paths 129-132 of rays diffracted by grating 123 and ray paths 133-136 of rays diffracted by grating 124 illustrate the beam paths from input to extraction in the waveguide.

[0069] Although Figure 1 and Figure 2 While a specific waveguide configuration is illustrated, waveguide displays according to various embodiments of the present invention can be implemented in many different ways, depending on the specific requirements of a given application. For example, in many embodiments, the first and second input couplers comprise first and second input gratings, respectively, and the waveguide display can be implemented without prisms. In other embodiments, the first and second input gratings are positioned orthogonal to the general direction of light propagation of the waveguide. In other embodiments, the first and second input gratings are positioned along the general direction of light propagation of the waveguide. Figure 3 and Figure 4 A schematic plan view of a waveguide display implemented with spatially separated input gratings and prims-less input couplers according to various embodiments of the present invention is conceptually illustrated. Figure 3A waveguide display 140 is shown comprising a waveguide 141 supporting input gratings 142, 143 and layers, folded gratings 144, 145 and multiplexed output gratings 146, 147, all arranged in a single layer. The beam paths in the waveguide from input to extraction are illustrated by ray paths 148-151 in the case of input grating 142 and by ray paths 152-155 in the case of input grating 143. Similarly, Figure 4 A waveguide display 160 is shown having a waveguide 161 supporting input gratings 162, 163 and folding gratings 164, 165 and multiplexed output gratings 166, 167, all arranged in a single layer. The beam paths in the waveguide from input to extraction are illustrated by ray paths 168-171 in the case of input grating 163 and by ray paths 172-175 in the case of input grating 162. Waveguide display 160 and Figure 3 The main difference between the embodiments shown in is the arrangement of the input grating - i.e. Figure 4 An embodiment is shown in which the first and second gratings are arranged along the general direction of light propagation in the waveguide. Figure 3 and Figure 4 In the embodiments described above and in other embodiments described below, two spatially separated input couplers can provide two separate input pupils.

[0070] In addition to prism-less input couplers, waveguide displays can implement input couplers that include only prisms. Figure 5 and Figure 6 A schematic plan view of a waveguide display implementing an input coupler without an input grating according to various embodiments of the present invention is conceptually illustrated. As shown, the first input coupler includes a first prism and the second light input coupler includes a second prism. Figure 5 In the embodiment, the first and second prisms are arranged along a direction orthogonal to the general light propagation direction of the waveguide. Figure 6 In the embodiment, the first and second prisms are arranged along the general light propagation direction of the waveguide.

[0071] refer to Figure 5 , a waveguide display 210 includes a waveguide 211 supporting input prisms 212, 213. The waveguide 211 also includes folded gratings 214, 215 and multiplexed output gratings 216, 217 arranged in a single grating layer. The beam paths in the waveguide from input to extraction are illustrated by ray paths 219A-219D for rays coupled into the waveguide by prism 213 and ray paths 218A-218D for rays coupled into the waveguide by prism 212. Similarly, Figure 6A waveguide display 220 is shown that includes a waveguide 231 supporting input prisms 232, 233. The waveguide 231 also includes folded gratings 234, 235 and multiplexed output gratings 236, 237 arranged in a single grating layer. The beam paths from input to extraction in the waveguide are illustrated by ray paths 238-241 for rays coupled into the waveguide by prism 233 and ray paths 242-245 for rays coupled into the waveguide by prism 222. In embodiments using only prism-based input couplers, such as Figure 5 and Figure 6 For the waveguide display shown in , the grating reciprocity condition can be solved using the pitch angle and clock angle of the fold and output gratings.

[0072] As described in the previous section, the input coupler can be configured in many different ways. Furthermore, the folded grating and output coupler of the waveguide display can also be configured in many different ways. Figure 7 A schematic plan view of a waveguide display having a waveguide with spatially separated input gratings and a multiplexed grating pair, combining the dual functions of two-dimensional beam expansion and beam extraction in the waveguide, according to an embodiment of the present invention, is conceptually illustrated. As shown, waveguide display 190 includes a waveguide 191 supporting input coupling prisms 192 and 193. Waveguide 191 also includes combined folding and multiplexing output gratings 194-197. In the illustrative embodiment, gratings 194 and 195 diffract and expand light entering waveguide 191 in two dimensions via prism 192. Similarly, gratings 196 and 197 diffract and expand light entering waveguide 191 in two dimensions via prisms 192 and 193. The beam paths in the waveguide from input to extraction are illustrated by ray paths 198-200 in the case of prism 192 and by ray paths 201-203 in the case of prism 193. Although the four gratings are multiplexed, the pairs of gratings corresponding to each of the two paths have crossed Bragg fringes.In some embodiments, the input coupling prisms 192, 193 may be replaced by gratings.

[0073] In some embodiments for displays using unpolarized light sources, the input gratings used can be combined with gratings oriented such that each grating diffracts a specific polarization of incident unpolarized light into the waveguide path. Such embodiments can incorporate some of the embodiments and teachings disclosed in PCT application PCT / GB2017 / 000040, "METHOD AND APPARATUS FOR PROVIDING A POLARIZATION SELECTIVE HOLOGRAPHIC WAVGUIDE DEVICE," by Waldern et al., the disclosure of which is incorporated herein by reference in its entirety. Output gratings can be configured in a similar manner so that light from the waveguide paths is combined and coupled out of the waveguide as unpolarized light. For example, in some embodiments, the input and output gratings each combine crossed gratings having peak diffraction efficiencies for orthogonal polarization states. In several embodiments, the polarization states are S and P polarizations. In multiple embodiments, the polarization states are opposite circular polarizations. The advantages of gratings, such as, but not limited to, SBGs, have been documented in liquid crystal polymer systems, where they can exhibit strong polarization selectivity due to their inherent birefringence. However, other grating technologies that can be configured to provide unique polarization states may also be used.

[0074] In embodiments utilizing gratings recorded in a liquid crystal polymer material system, at least one polarization control layer can be provided overlapping at least one of the folded gratings, input grating, or output grating for the purpose of compensating for polarization rotation in any grating, particularly the folded gratings. In many embodiments, all gratings are covered by the polarization control layer. In some embodiments, the polarization control layer is applied only to a subset of the gratings, such as only to the folded gratings. The polarization control layer can include an optical retardation film. In several embodiments based on HPDLC materials, the birefringence of the gratings can be used to control the polarization properties of the waveguide device. Using the birefringence tensor, K-vector, and grating footprint of the HPDLC grating as design variables opens up a design space for optimizing the angular capability and optical efficiency of the waveguide device. In some embodiments, a quarter-wave plate disposed at the glass-air interface of the waveguide rotates the polarization of light to maintain efficient coupling with the grating. For example, in one embodiment, the quarter-wave plate is a coating applied to the waveguide substrate. In some waveguide display embodiments, applying a quarter-wave coating to the waveguide substrate can help maintain alignment of light with the intended viewing axis by compensating for skew waves in the waveguide. In various embodiments, the quarter wave plate may be provided as a multilayer coating.

[0075] Figure 8A flowchart is conceptually illustrated according to an embodiment of the present invention, which illustrates a method for providing a color waveguide display with two-dimensional beam expansion using a single grating layer. As shown, a method 240 for coupling light of more than one polarization component into a waveguide is provided. Referring to the flowchart, the method 240 includes providing (241) a waveguide supporting a single grating layer; a light source; a first input coupler; a second input coupler; an output coupler having a multiplexed first and second gratings; a first folding grating; and a second folding grating. A first spectral band can be directed (242) from the source into the first waveguide pupil via the first input coupler, and a second spectral band can be directed (243) from the source into the second waveguide pupil via the second input coupler. The first spectral band light can be beam expanded and redirected (244) onto the output coupler by means of the first folding grating. The second spectral band light can be beam expanded and redirected (245) onto the output coupler by means of the second folding grating. The first spectral band light can be beam expanded and extracted (246) from the waveguide by means of the first multiplexing grating. Light of the second spectral band may be beam expanded and extracted from the waveguide by means of a second multiplexing grating (247).

[0076] The above discussion and Figure 1-8 The embodiments illustrated in FIG are based on the principle of input pupil bifurcation using split-pupil input coupling or multiplexed input coupling to provide upward and downward waveguide paths to the output grating using two spatially separated folded gratings. One challenge in implementing this approach is that having two folded gratings results in an increase in waveguide size, particularly in the vertical direction above the eye center. Another challenge is fabricating efficient multiplexing of the output gratings. Thus, various embodiments according to the present invention are directed to color waveguide architectures based on a single waveguide layer supporting a single grating layer, which do not utilize the beam bifurcation principle.

[0077] In many embodiments, waveguide displays are implemented to provide an image at infinite distance. In some embodiments, the image can be at some intermediate distance. In several embodiments, the image can be at a distance compatible with the relaxed viewing range of the human eye. For example, many waveguides according to various embodiments of the present invention can cover a viewing range from about 2 meters to about 10 meters.

[0078] In some embodiments, the waveguide provides a layer of a multilayer waveguide architecture comprising a single layer grating waveguide, as described above with respect to Figure 3 、 Figure 4 and Figure 7The embodiments shown in are described in detail in which each waveguide provides a full color image within a specified viewing range measured from the eye box. The viewing range can be determined by the optical power encoded into one or more gratings in the waveguide. In several embodiments, the optical power will be encoded only into the multiplexed output gratings to produce minimal de-collimation of the guided light. Techniques for encoding optical power into gratings are known to those skilled in the art. Displays that provide multiple viewing ranges (or focal planes) may generally be referred to as light field displays. In many embodiments, the input gratings will be switched to their diffraction states so that only one input grating is in its diffraction state at any time (so that the image content is projected into only one range). The projection range can be determined using an eye tracker that tracks both eyes to determine the desired viewing range by triangulating the measured left and right eye gaze vectors. The image data typically provided by the microdisplay can be updated for each viewing range.

[0079] Figure 9 A schematic cross-sectional view of a lightfield display 310 comprising a stack of single-layer color waveguides 301A-301C is conceptually illustrated, in accordance with an embodiment of the present invention. In the illustrative embodiment, each waveguide comprises an input grating, a folding grating, and a multiplexed output grating, labeled by the numerals 312, 313, and 314 and the characters A, B, and C, respectively, according to the waveguide layer. The input grating of each waveguide can be a switchable grating. In many embodiments, the switchable grating is an SBG. Figure 9 The input grating shown in corresponds to Figure 3-4 and Figure 7 One of the two input gratings shown in any one of the waveguides, in each case both input gratings are switched on simultaneously. At least one grating in the grating layer has an optical power for forming a visible image within a predefined range, so that each waveguide provides a unique visible range.

[0080] The operation of light field display is Figure 10A and Figure 10B Conceptual illustration. Figure 10A 3 is a schematic cross-sectional view illustrating a first operational state 320 of the waveguide corresponding to the formation of a visible image 322 at a first range labeled R1. The black-shaded input grating 312A is in its diffractive state 321, and the input gratings 312B and 312C are in their non-diffractive states. Thus, in the first operational state, light propagates only in the waveguide 301A. Figure 10B 3 is a schematic cross-sectional view illustrating a second operating state 330 of the waveguide corresponding to the formation of a visible image 332 at a second range labeled R2. The black-shaded input grating 312C is in its diffractive state 331, and the input gratings 312A and 312B are in their non-diffracting states. Thus, in the second operating state, light propagates only in waveguide 301C.

[0081] Switchable Bragg grating

[0082] The optical structure recorded in the waveguide can include many different types of optical elements, such as, but not limited to, diffraction gratings. In many embodiments, the grating implemented is a Bragg grating (also known as a volume grating). Bragg gratings can have high efficiency, with almost no light being diffracted into higher orders. The relative amounts of light in the diffracted and zeroth orders can be varied by controlling the refractive index modulation of the grating, a property that can be used to create lossy waveguide gratings to extract light over a larger pupil. One type of grating used in holographic waveguide devices is the switchable Bragg grating ("SBG"). SBGs can be fabricated by first placing a thin film of a mixture of a photopolymerizable monomer and a liquid crystal material between glass plates or substrates. In many cases, the glass plates are in a parallel configuration. One or both glass plates can support electrodes, typically transparent tin oxide films, for applying an electric field across the film. The grating structure in the SBG can be recorded in a liquid material (often called a slurry) by photopolymerization-induced phase separation using interferometric exposure with spatially periodic intensity modulation. Controlling factors such as, but not limited to, radiation intensity, the volume fractions of the components of the materials in the mixture, and the exposure temperature can determine the resulting grating morphology and performance. It will be readily appreciated that a variety of materials and mixtures can be used, depending on the specific requirements of a given application. In many embodiments, HPDLC materials are used. During the recording process, the monomers polymerize and the mixture undergoes phase separation. The LC molecules aggregate to form discrete or coalesced droplets that are periodically distributed in the polymer network on an optical wavelength scale. Alternating liquid crystal-rich and liquid crystal-poor regions form the fringe planes of the grating, which can produce Bragg diffraction with strong optical polarization caused by the orientation order of the LC molecules in the droplets. In some embodiments, the grating in a given layer is recorded in a step-by-step manner by scanning or stepping a recording laser beam across the grating area. In several embodiments, the grating is recorded using mastering and contact replication processes currently used in the holographic printing industry.

[0083] The resulting volume phase grating can exhibit very high diffraction efficiency, which can be controlled by the strength of the electric field applied to the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets can change, resulting in a decrease in the refractive index modulation of the fringes and a reduction in the hologram diffraction efficiency to very low levels. Typically, the electrodes are configured so that the applied electric field is perpendicular to the substrate. In many embodiments, the electrodes are made of indium tin oxide ("ITO"). In the OFF state, with no applied electric field, the extraordinary axis of the liquid crystal is typically aligned perpendicular to the fringes. Therefore, the grating exhibits high refractive index modulation and high diffraction efficiency for P-polarized light. When an electric field is applied to the HPDLC, the grating switches to the ON state, where the extraordinary axes of the liquid crystal molecules align parallel to the applied electric field and, therefore, perpendicular to the substrate. In the ON state, the grating exhibits low refractive index modulation and low diffraction efficiency for both S- and P-polarized light. Consequently, the grating region no longer diffracts light. Depending on the function of the HPDLC device, each grating region can be divided into multiple grating elements, such as, for example, a matrix of pixels. Typically, the electrodes on one substrate surface are uniform and continuous, while the electrodes on the opposing substrate surface are patterned according to a plurality of selectively switchable grating elements.

[0084] Typically, an SBG element clears to zero within 30 μs and switches on with a longer relaxation time. It is important to note that the device's diffraction efficiency can be tuned over a continuous range via the applied voltage. In many cases, the device exhibits near-100% efficiency without an applied voltage, while exhibiting essentially zero efficiency with a sufficiently high applied voltage. In certain types of HPDLC devices, a magnetic field can be used to control the LC orientation. In some HPDLC applications, the LC material and polymer phase separation can reach a point where no discernible droplet structure is produced. SBGs can also be used as passive gratings. In this mode, their primary advantage is the unique high refractive index modulation. SBGs can be used to provide transmissive or reflective gratings for free-space applications. SBGs can be implemented as waveguide devices, with the HPDLC forming the waveguide core or evanescent coupling layer near the waveguide. The glass plates used to form the HPDLC unit cell provide a total internal reflection (TIR) ​​lightguide structure. When the switchable grating diffracts light at angles exceeding the TIR condition, light can be coupled out of the SBG.

[0085] In many embodiments, the SBGs are recorded in a uniformly modulated material, such as POLICRYPS or POLIPHEM, which have a solid liquid crystal matrix dispersed in a liquid polymer. Exemplary uniformly modulated liquid crystal-polymer material systems are disclosed in U.S. Patent Application Publication No. US2007 / 0019152 to Caputo et al. and PCT Application No. PCT / EP2005 / 006950 to Stumpe et al., both of which are incorporated herein by reference in their entireties. Uniformly modulated gratings are characterized by high refractive index modulation (and therefore high diffraction efficiency) and low scattering. In some embodiments, at least one of the gratings is recorded in a reverse-mode HPDLC material. Reverse-mode HPDLC differs from conventional HPDLC in that the grating is passive when no electric field is applied, but becomes diffracting in the presence of an electric field. Reverse mode HPDLC may be based on any of the formulations and processes disclosed in PCT Application No.: PCT / GB2012 / 000680 entitled IMPROVEMENTS TOHOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, the disclosure of which is incorporated herein by reference.The optical recording material system is discussed in more detail below.

[0086] Grating structure and configuration

[0087] Each grating within a waveguide can be characterized in 3D space by a grating vector (or K-vector), which, in the case of a Bragg grating, is defined as a vector perpendicular to the Bragg fringes. The grating vector can determine the optical efficiency for a given range of input and diffraction angles. The gratings described throughout this disclosure can be implemented in any of a variety of different grating configurations. For example, the input and output gratings of some embodiments can be designed to have a common surface grating pitch.

[0088] Figure 11A and Figure 11B The grating geometry of a set of exemplary gratings according to an embodiment of the present invention is conceptually illustrated. Vector N is the unit vector normal to the grating surface; r1-r3 are the incident and diffracted unit ray vectors; K1, K2 are the grating K-vectors (not necessarily in the drawing plane); q1, q2 are unit vectors parallel to the holographic fringes (defining the grating clock angle); d1, d2 are the grating pitches; and λ a ,λ b is the wavelength. The reciprocity condition for the ray path defined by rays r1-r3 can be obtained by first applying the grating equation to the folded grating: r1 x N - r2 x N = λ a (q1 / d1) is then applied to the output grating: r2 x N - r3 x N = λ bThe q-vector is obtained by taking the dot product of the vectors q1 and z, which yields the relationship q1.z / d1 = q2.z / d2, where z is a unit vector along the main waveguide dimension, typically parallel to the mean beam propagation direction in the waveguide. The q-vector is perpendicular to the drawing plane.

[0089] In many embodiments, the fold grating and output grating functions are combined into two overlapping, multiplexed fold gratings with opposite clock angles. In some embodiments, the opposite clock angles have different magnitudes. The crossed fold gratings can be configured to perform two-dimensional beam expansion and extract light from the waveguide. A separate grating pair can be provided for each of the first and second paths. Thus, many embodiments include a total of four fold gratings multiplexed into a single waveguide layer. By combining the fold gratings and the output grating, a significant reduction in grating real estate can be achieved.

[0090] In many embodiments, a waveguide includes at least one grating with a spatially varying pitch. In some embodiments, each grating has a fixed K-vector. In several embodiments, at least one of the gratings is a scrolled K-vector grating. Scrolling the K-vector can extend the angular bandwidth of the grating without increasing the waveguide thickness. In several embodiments, the scrolled K-vector grating includes a waveguide portion containing discrete grating elements with differently arranged K-vectors. In some embodiments, the scrolled K-vector grating includes a waveguide portion containing a single grating element within which the K-vector undergoes a smooth, monotonic variation in direction. Light can be input into the waveguide using various configurations of scrolled K-vector gratings, such as but not limited to those described above. Using a prism to couple light into the waveguide has the advantage of avoiding the significant light loss and limited angular bandwidth associated with scrolled K-vector gratings. Practical scrolled K-vector input gratings are generally unable to match the much larger angular bandwidth of folded gratings, which can be 40 degrees or greater.

[0091] Although the figures indicate a high degree of symmetry in the grating geometry and grating layout in the different wavelength channels, in reality, the grating specifications and footprints may be asymmetric due to different spectral bandwidths. Although the gratings in the upper and lower parts of the waveguide are illustrated with similar areas, the two spectral bands may require adjustment of the grating specifications (including pitch, tilt angle and clock angle) to balance the two optical paths. A symmetrical prism arrangement (i.e., the prisms are arranged along a direction orthogonal to the general beam propagation direction) may be easier to design than an in-line arrangement (i.e., the prisms are arranged along the general beam propagation direction). The optimal solution may require consideration of optical efficiency, form factor and cost. The shape of the input grating, folded grating or output grating may depend on the waveguide application and can be any polygonal geometry influenced by factors such as, but not limited to, the required beam expansion, output beam geometry, beam uniformity and ergonomic factors.

[0092] Figure 12 A schematic plan view of a waveguide 250 supporting a single grating layer 251 is conceptually illustrated, the grating layer 251 having an input grating 252 with a rolled K-vector, a folded grating 253, and an output grating 254. In some embodiments, one or both of the folded grating and the output grating may have a rolled K-vector. Figure 13 , which shows a cross-section 260 of a waveguide, with the grating layer 251 shown sandwiched between substrates 261, 262 having different refractive indices n1, n2. Operation in the visible band can be achieved by selecting appropriate refractive indices n1, n2 and optimizing the scrolling K-vector specification of the input grating to provide high diffraction efficiency in the visible band. In several embodiments, the scrolling K-vector specification of the output grating can also be adjusted as part of the optimization in the visible band. The following paragraphs and figures provide a basis for the optimization of the visible band. Figure 12 and Figure 13 It should be noted that many features of this approach may also be relevant to single-layer color waveguides based on the beam bifurcation principle.

[0093] In many embodiments, the substrate refractive index is approximately n1 = 1.5 and n2 = 1.7. The substrates can be glass or plastic. For higher angles in TIR, having different refractive indices can promote more bounce in the waveguide (less interaction compared to lower angles closer to TIR). Using substrates with different refractive indices can also promote uniformity of illumination output from the waveguide. In some embodiments, using a high refractive index material (typically with a refractive index of 1.7 or higher) for one of the substrates supports higher waveguide angular carrying capacity. In several embodiments where the higher glass refractive index has a refractive index greater than the average refractive index of the grating formed by the HPDLC, the grating material can set the limit of the angular carrying capacity of the waveguide. In several embodiments, the upper refractive index is set to be slightly higher than the average level of the grating material. It should be noted that in such embodiments, the purpose of achieving high waveguide angular carrying capacity is not to expand the field of view, but to expand the spectral range that a single waveguide can carry. This is because the dispersion of a wider spectral band from red to blue produces a wider angular range in the waveguide.

[0094] In many embodiments, the rolling K-vector specifications required to achieve a color single-layer grating can be achieved by optimizing the spatial position of the rolling K-vector input grating to match the red-green and green-blue bands of the input illumination by shearing the input pupil through a dichroic prism step. Figure 14 One such arrangement 270 is shown for shearing illumination from an RGB source into relatively displaced red-green and green-blue bands using a prismatic element comprising a reflective surface for reflecting long wavelengths and a dichroic coating for partially reflecting short wavelengths and transmitting long wavelengths. Figure 14As shown in FIG, device 270 includes an illumination module 271 containing red, green, and blue light sources 272-274 that emit light in the general direction indicated by block arrow 275. In the illustrative embodiment, illumination module 271 is optically coupled to a prism system that includes a prism 276 having an inner surface 277 to which a dichroic coating is applied to reflect short-wavelength light and transmit long-wavelength light. A prism face 278 adjacent to and parallel to the inner surface can reflect long-wavelength light into the prism. An opposing prism surface 287 can reflect both short-wavelength and long-wavelength light out of the prism via face 288 to provide output beams indicated by block arrows 285 and 286. The ray paths of light reflected from the dichroic coating are represented by rays 280, 281, and 282. The ray paths of rays reflected from surface 278 are represented by rays 279, 283, and 284. In some embodiments, the source includes at least one LED having a spectral output with a peak wavelength biased toward a first shorter wavelength band and at least one LED having a spectral output with a peak wavelength biased toward a longer wavelength band. In many embodiments, the long wavelength band corresponds to light extending over the green to red region of the visible spectrum, while the short wavelengths correspond to the blue to green region. In other embodiments, the long wavelength band corresponds to red light, while the short wavelength band corresponds to light extending over the blue to green region. Figure 14 It will be apparent from the consideration of that other prism configurations can be used to achieve the separation of light into two sheared spectral bands or arbitrarily defined spectral bandwidths. In some embodiments, Figure 14 The device may also employ reflector coatings, polarizers, and / or spectral filter coatings to provide greater differentiation of the output spectral bands, for example, to reduce crosstalk between the spectral bands. In some embodiments, the color reproduction of the waveguide may be improved by using two or more LEDs that are spectrally shifted by a small amount to provide the desired primary colors. Figure 15 Graph 290 is conceptually illustrated showing the LED output spectra of two such LEDs, where the vertical axis labeled 291 corresponds to output intensity and the horizontal axis 292 represents wavelength. In this case, the LEDs have peak output in the green (G) band, with the spectrum 293 of one LED shifting toward blue (B) and the spectrum 294 of the other LED shifting toward red (R).

[0095] Figure 16A schematic cross-sectional diagram 300 is conceptually illustrated showing a portion of a scrolled K-vector input grating illuminated by spectrally sheared illumination across the visible band. The grating comprises Bragg fringes 302A-302F having tilt angles that continuously decrease from left to right. The incident light is represented by effective red, green and blue light sources labeled R, G and B, which emit rays labeled numerals 301-307. Typical diffracted rays that will undergo TIR in the waveguide are indicated by 308. Due to spectral shearing, the Bragg fringes on the left side of the grating, such as 302A, diffract red rays 301 and green rays 303. On the other hand, the Bragg fringes on the right side of the grating, such as 302F, diffract green rays 305 and blue rays 307. Using a dichroic prism arrangement, such as but not limited to Figure 14 Those described in, can produce a step function shift of two spectral bands. Other techniques can be used to provide spectral shearing. In some embodiments, spectral shearing uses the dispersion properties of a prism as a function of wavelength, for example, performed continuously using a pair of color correction prisms. The benefits of the spectral shearing technique are not limited to the color waveguides disclosed herein. The technique can also be used to enhance the performance of color waveguides or monochromatic waveguides using scrolled K-vector gratings that are illuminated using green LED emitters whose spectral bandwidth can be 80nm or higher. In several embodiments, continuous spectral shearing can be provided with the aid of gratings.

[0096] Based on Figure 14 In many embodiments of the system principles shown in , more dichroic layers can be used for fine tuning. However, this can complicate prism fabrication, and in most cases one dichroic layer may be sufficient. In some embodiments, the dichroic prism can be designed to reflect incident light to an angle suitable for waveguide propagation. In several embodiments, the dichroic prism can have high transmittance in the visible band for high angles of incidence (in air) to support see-through viewing of the peripheral field of view. In multiple embodiments, the dichroic prism can also be configured to achieve angular alignment of the input image projector with the input grating. This feature is particularly important for tilted waveguides, which are waveguides with a surface normal that is at an angle to the principal axis of the field of view.

[0097] In many embodiments, according to Figure 12 and Figure 13The waveguide of the present invention can operate in the spectral range of about 460nm to 640nm. In some embodiments, the source is an LED. In other embodiments, a laser is used. In several embodiments, the light from the source is modulated using a DLP micro-projector with a pupil size of about 4mm. In multiple embodiments, an LCoS or other micro-projector can be used. In some embodiments, the waveguide is designed to have a tilt angle of 30 degrees. In several embodiments, a prism is used to couple the input light into the waveguide. In multiple embodiments, the waveguide provides a brightness greater than 1,500 nits at the aiming eye from a 30 lumen DLP projector. In some embodiments, spatially varying grating refractive index modulation is used to control the diffraction efficiency of the waveguide, thereby achieving greater uniformity of the waveguide output. Methods and systems for spatially varying grating refractive index modulation are further discussed in detail in U.S. Patent Application No.: 16 / 203,071, entitled "Systems and Methods for Manufacturing Waveguide Cells," the disclosure of which is incorporated herein by reference in its entirety. Alternatively, the same or similar effect can be achieved by spatially varying the thickness of the grating layer comprising the input grating, fold grating, and output grating. Spatially varying the refractive index modulation has the benefit of enabling a single-thickness grating layer. In some embodiments, an LCP layer positioned after the input grating can be used to rotate the polarization to minimize input grating re-interaction outcoupling losses. Compared to multilayer waveguide architectures, this type of waveguide typically has a relatively small field of view. In several embodiments, the waveguide supports a resolution of at least nHD (640x360) with a 15-degree horizontal x 15-degree vertical FOV. In several embodiments, the field of view can be improved by tilting the fold grating. In some embodiments, the above-mentioned field of view is provided with an eyebox of 18 mm horizontal x 14 mm vertical. Advantageously, the grating can be exposed through low-index (or more transparent) glass to minimize holographic recording haze. The waveguide refractive index arrangement (ocular / non-ocular) can be determined by the RKV exposure design.

[0098] In conjunction with the single-layer color waveguide embodiments disclosed herein, a rolling K-vector exposure method is provided for recording a rolling K-vector input grating with high angular bandwidth. This exposure method can incorporate many of the embodiments and teachings disclosed in U.S. Provisional Application No. 62 / 614,932, filed January 8, 2018, by Waldern et al., entitled “METHODS FOR FABRICATING OPTICAL WAVEGUIDES,” the disclosure of which is incorporated herein by reference.

[0099] In many embodiments, the master grating used in fabrication is an amplitude grating. Rolling K-vector recording typically employs a cylindrical lens positioned along the exposure beam path. By timing the cylindrical exposure lens relative to the input grating on the master, a wider angular bandwidth increase can be achieved. In some embodiments, the input grating on the master can be a chirped grating as disclosed in U.S. Provisional Application No. 62 / 614,932, the disclosure of which is incorporated herein by reference. A chirped grating may be required to overcome the effects of non-parallel recording beams and the finite thickness between the master and replica gratings. In other words, to ensure a constant surface period in the replica, which may be required to meet grating reciprocity in the final waveguide, the master period should vary spatially. In many embodiments, using this mastering technique, a single plane wavefront input beam interacts with a cylindrical lens to provide one-dimensional focusing, and a portion of the light then either generates a diffracted beam from the chirped master or passes as a zero-order beam (with attenuation) and retains the original one-dimensional focusing function of the cylindrical lens. In some embodiments, the local rolling K-vector grating angular bandwidth is maximized as a function of position (e.g., height on the input grating structure if the input grating is clocked relative to the orthogonal field). This will cause the input grating chirp specification to vary in 2D relative to the input wavefront from the cylindrical lens.

[0100] Advantageously, to improve color uniformity, the grating can be designed using reverse ray tracing from the eyepiece, through the output grating and fold grating, to the input grating. This process allows identification of the required physical extent of the grating, particularly the fold grating. Unnecessary grating space that contributes to haze can be reduced or eliminated. Ray paths are optimized for red, green, and blue, each following a slightly different path due to dispersion effects between the input and output gratings via the fold grating. The design should allow sufficient clearance between the input and fold, and between the fold and output, to allow the use of exposure lenses in a rolling K-vector grating exposure setup. This is primarily to prevent clipping of the desired fold grating aperture size, thereby avoiding support for direct path ray coupling required for optimized uniformity.

[0101] As used with respect to any of the embodiments described herein, the term grating may encompass a grating comprising a group of gratings. For example, in many embodiments, the input grating and the output grating each comprise two or more gratings multiplexed into a single layer. Recording more than one holographic format into a single holographic layer is well established in the holographic literature. Methods for recording such multiplexed holograms are well known to those skilled in the art. In some embodiments, the input grating and the output grating may each comprise two overlapping grating layers contacted or vertically separated by one or more thin optical substrates. In several embodiments, the grating layers are sandwiched between glass or plastic substrates. In several embodiments, two or more such grating layers may form a stack, wherein total internal reflection occurs at the outer substrate and air interface. In some embodiments, the waveguide may comprise only one grating layer. In several embodiments, electrodes may be applied to the facets of the substrates to switch the grating between diffractive and transparent states. The stack may also include additional layers, such as beam-splitting coatings and environmental protection layers.

[0102] In many embodiments of the present invention directed to displays, a waveguide display may be combined with an eye tracker. In a preferred embodiment, the eye tracker is a waveguide device that overlays a display waveguide and is based on the embodiments and teachings of PCT Application No. GB2014 / 000197 entitled “HOLOGRAPHIC WAVEGUIDE EYE TRACKER,” PCT Application No. GB2015 / 000274 entitled “HOLOGRAPHIC WAVEGUIDE OPTICAL TRACKER,” and PCT Application No. GB2013 / 000210 entitled “APPARATUS FOR EYE TRACKING,” the disclosures of which are incorporated herein by reference. Many embodiments of the present invention are directed to waveguide displays that may also include a dynamic focusing element. The dynamic focusing element may be based on the embodiments and teachings of U.S. Provisional Patent Application No. 62 / 176,572 entitled “ELECTRICALLY FOCUS TUNABLE LENS,” the disclosure of which is incorporated herein by reference. In some embodiments, a waveguide display according to the principles of the present invention also includes a dynamic focusing element and an eye tracker to provide a dynamic focusing element based on the disclosure of U.S. Provisional Patent Application No. 62 / 176,572 entitled “ELECTRICALLY FOCUS TUNABLE LENS.” WAVEGUIDE LIGHTFIELDDISPLAYS”, the disclosure of which is incorporated herein by reference. Some embodiments of the present invention may be directed to waveguide displays based on some embodiments of U.S. Patent Application No. 13 / 869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY and U.S. Patent Application No. 13 / 844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, the disclosures of which are incorporated herein by reference. In some embodiments, a waveguide device according to the principles of the present invention may be integrated into a window, such as a windshield-integrated HUD for road vehicle applications. In some embodiments, a window-integrated display may be based on some embodiments of U.S. Patent Application No. 13 / 869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY and U.S. Patent Application No. 13 / 844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, the disclosures of which are incorporated herein by reference. The embodiments and teachings disclosed in U.S. Provisional Patent Application No.: PCT Application No.: PCT / GB2016 / 000005 for ENVIRONMENTALLY ISOLATED WAVEGUIDE DISPLAY, the disclosure of which is incorporated herein by reference. In some embodiments, the waveguide device may include a gradient index (GRIN) waveguide assembly for relaying image content between the IIN and the waveguide. Exemplary embodiments are disclosed in PCT Application No.: PCT / GB2016 / 000005, entitled ENVIRONMENTALLY ISOLATED WAVEGUIDE DISPLAY, the disclosure of which is incorporated herein by reference.In some embodiments, the waveguide device may include an optical pipe for providing beam expansion in one direction based on the embodiments disclosed in U.S. Provisional Patent Application No. 62 / 177,494, entitled WAVEGUIDE DEVICE INCORPORATING ALIGHT PIPE, the disclosure of which is incorporated herein by reference. An optical device based on any of the above embodiments may be implemented using a plastic substrate using the materials and processes disclosed in PCT Application No. PCT / GB2012 / 000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUIDCRYSTAL MATERIALS AND DEVICES, which is incorporated herein by reference.

[0103] HPDLC Material System

[0104] HPDLC mixtures according to various embodiments of the present invention typically include LC, monomers, a photoinitiator dye, and a coinitiator. The mixture (commonly referred to as a slurry) typically also includes a surfactant. For the purposes of the present invention, a surfactant is defined as any chemical agent that reduces the surface tension of the overall liquid mixture. The use of surfactants in HPDLC mixtures is known and dates back to the earliest research on HPDLC. For example, RL Sutherland et al. described a PDLC mixture in a paper published in SPIE, Vol. 2689, pp. 158-169, 1996, which includes monomers, a photoinitiator, a coinitiator, a chain extender, and an LC to which a surfactant may be added, the disclosure of which is incorporated herein by reference. Natarajan et al. also mentioned surfactants in a paper published in Journal of Nonlinear Optical Physics and Materials, Vol. 5, No. 1, pp. 89-98, 1996, the disclosure of which is incorporated herein by reference. In addition, U.S. Patent No. 7,018,563 to Sutherland et al. discusses a polymer-dispersed liquid crystal material for forming a polymer-dispersed liquid crystal optical element, the material comprising: at least one acrylic monomer; at least one type of liquid crystal material; a photoinitiator dye; a co-initiator; and a surfactant. The disclosure of U.S. Patent No. 7,018,563 is incorporated herein by reference in its entirety.

[0105] The patent and scientific literature contains many examples of material systems and processes that can be used to make SBGs, including research into formulating such material systems to achieve high diffraction efficiency, fast response time, low drive voltage, etc. U.S. Patent No. 5,942,157 to Sutherland and U.S. Patent No. 5,751,452 to Tanaka et al. both describe combinations of monomers and liquid crystal materials suitable for making SBG devices. Examples of recipes can also be found in papers from the early 1990s. Many of these materials use acrylate monomers, including:

[0106] RL Sutherland et al., Chem. Mater., Vol. 5, p. 1533 (1993), describes the use of acrylate polymers and surfactants, the disclosure of which is incorporated herein by reference. Specifically, the formulation includes a crosslinking multifunctional acrylate monomer; a chain extender, N-vinyl pyrrolidone, LC E7, a photoinitiator, Red Bengal, and a coinitiator, N-phenylglycine. In some variations, the surfactant octanoic acid is added.

[0107] Fontecchio et al., SID 00 Digest, pp. 774-776, 2000, describes UV-curable HPDLCs for reflective display applications comprising a multifunctional acrylate monomer, LC, a photoinitiator, a co-initiator, and a chain terminator, the disclosure of which is incorporated herein by reference.

[0108] YH Cho et al., Polymer International, Vol. 48, pp. 1085-1090, 1999, discloses HPDLC formulations including acrylates, the disclosure of which is incorporated herein by reference.

[0109] Karasawa et al., Japanese Journal of Applied Physics, Vol. 36, pp. 6388-6392, 1997, describe acrylates of various functional sequences, the disclosure of which is incorporated herein by reference.

[0110] • TJ Bunning et al., Polymer Science: Part B: Polymer Physics, Vol. 35, pp. 2825-2833, 1997, also describes multifunctional acrylate monomers, the disclosure of which is incorporated herein by reference.

[0111] G. Slannacchione et al., Europhysics Letters, Vol. 36(6), pp. 425-430, 1996, describe PDLC mixtures comprising a pentaacrylate monomer, LC, a chain extender, a co-initiator, and a photoinitiator, the disclosure of which is incorporated herein by reference.

[0112] Acrylates offer the advantages of fast kinetics, good mixing with other materials, and good compatibility with film-forming processes. Because acrylates are cross-linked, they tend to be mechanically robust and flexible. For example, urethane acrylates with functionalities of 2 (di) and 3 (tri) have been widely used in HPDLC technology. Higher-functionality materials, such as pentagonal and hexagonal functional rods, have also been used.

[0113] One of the known properties of transmissive SBGs is that the LC molecules tend to align with an average direction perpendicular to the plane of the grating stripes (i.e., parallel to the grating or K-vector). The effect of this LC molecular alignment is that transmissive SBGs diffract P-polarized light (i.e., light with a polarization vector in the plane of incidence) efficiently, but have almost zero diffraction efficiency for S-polarized light (i.e., light with a polarization vector perpendicular to the plane of incidence).

[0114] Principle of Equivalence

[0115] Although the above description includes many specific embodiments of the present invention, these should not be interpreted as limiting the scope of the present invention, but as an example of one embodiment thereof. Although only several embodiments are described in detail in this disclosure, many modifications are possible (for example, the size, dimensions, structure, shape and proportion of various elements, parameter values, installation arrangements, use of materials, colors, orientations, etc.). For example, the position of the elements can be reversed or otherwise changed, and the nature or quantity of the discrete elements or positions can be changed or changed. Therefore, all such modifications are intended to be included within the scope of this disclosure. According to alternative embodiments, the order or sequence of any process or method steps can be changed or reordered. Without departing from the scope of this disclosure, other replacements, modifications, changes and omissions can be made in the design, operating conditions and arrangement of the exemplary embodiments. Therefore, it should be understood that, without departing from the scope and spirit of the present invention, the present invention can be practiced in a manner different from that specifically described. Therefore, the embodiments of the present invention should be considered illustrative and not restrictive in all aspects. Therefore, the scope of the present invention should not be determined by the embodiments shown, but by the appended claims and their equivalents.

Claims

1. A waveguide display comprising: a waveguide supporting a single grating layer with a general light propagation direction; a source of data-modulated light optically coupled to the waveguide; a first input coupler for directing light of a first spectral band from the source into a first waveguide pupil; a second input coupler for directing light of a second spectral band from the source into a second waveguide pupil; an output coupler comprising a first multiplexing grating and a second multiplexing grating; a first folding grating for directing the first spectral band along a first path from the first waveguide pupil to the output coupler and providing a first beam expansion; at least a second folding grating for directing the second spectral band along a second path from the second waveguide pupil to the output coupler and providing a first beam expansion; the first multiplexing grating directing the first spectral band out of the waveguide in a first direction having a beam expansion orthogonal to the first beam expansion, the second multiplexing grating directing the second spectral band out of the waveguide in the first direction with a beam expansion orthogonal to the first beam expansion, Wherein the first input coupler and the second input coupler are spatially separated in the plane of the waveguide.

2. The waveguide display of claim 1, wherein the first input coupler and the second input coupler each comprise at least one of a prism and a grating.

3. A waveguide display as described in claim 1, wherein the first input coupler comprises a first prism and the second input coupler comprises a second prism, wherein the first prism and the second prism are arranged along the general light propagation direction of the waveguide.

4. A waveguide display as described in claim 1, wherein the first input coupler comprises a first prism and the second input coupler comprises a second prism, wherein the first prism and the second prism are arranged along a direction orthogonal to the general light propagation direction of the waveguide.

5. A waveguide display as claimed in claim 1, wherein the first input coupler comprises a first grating and the second input coupler comprises a second grating, wherein the first and second gratings are arranged along a general light propagation direction of the waveguide.

6. A waveguide display as claimed in claim 1, wherein the first input coupler comprises a first grating and the second input coupler comprises a second grating, wherein the first and second gratings are arranged along a direction orthogonal to a general light propagation direction of the waveguide.

7. A waveguide display as claimed in claim 1, wherein the first input coupler comprises a prism and a first grating, and the second input coupler comprises the prism and a second grating, wherein the first grating and the second grating are arranged along a general light propagation direction of the waveguide.

8. A waveguide display as described in claim 1, wherein the first input coupler includes a prism and a first grating, and the second input coupler includes the prism and a second grating, wherein the first grating and the second grating are arranged along a direction orthogonal to the general light propagation direction of the waveguide.

9. The waveguide display of claim 1, wherein the first input coupler comprises a first prism and a first grating, and the second input coupler comprises a second prism and a second grating, wherein the first grating and the second grating are multiplexed.

10. The waveguide display of claim 1, wherein folded gratings are multiplexed and have specifications for performing two-dimensional beam expansion and extracting light from the waveguide.

11. The waveguide display of claim 1 , wherein each of the first and second fold gratings is configured to provide pupil expansion in a first direction, and wherein the output grating is configured to provide pupil expansion in a second direction different from the first direction.

12. The waveguide display of claim 1, wherein the source comprises at least one LED.

13. The waveguide display of claim 1, wherein the source comprises at least one LED having a spectral output with a peak wavelength biased toward a first spectral band and at least one LED having a spectral output with a peak wavelength biased toward a second spectral band.

14. A waveguide display as claimed in claim 1, wherein at least one of the gratings is a scrolled k-vector grating.

15. The waveguide display of claim 1, wherein light undergoes a double interaction within at least one of the fold gratings.

16. The waveguide display of claim 1 , wherein the source of data modulated light comprises: a microdisplay panel, wherein the microdisplay is configured to display image pixels; as well as An input image node having collimating optics, wherein the input image node projects an image displayed on the microdisplay panel such that each image pixel on the microdisplay panel is translated into a unique angular orientation within the first waveguide.

17. A waveguide display as claimed in claim 1 comprising at least one grating having a spatially varying pitch.

18. A waveguide display as claimed in claim 1, wherein at least one of the input coupler, the folded grating and the output grating is one of a switchable Bragg grating or a surface relief grating recorded in a holographic photopolymer, an HPDLC material or a uniformly modulated holographic liquid crystal polymer material.

19. A waveguide display as described in claim 1, wherein the first input coupler and the second input coupler each include at least one grating, wherein the at least one grating of each of the first input coupler and the second input coupler, the folding grating, and the first multiplexing grating and the second multiplexing grating are arranged in a single grating layer.

20. A light field display comprising: a first waveguide display and a second waveguide display, each of the first waveguide display and the second waveguide display comprising: a waveguide supporting a single grating layer with a general light propagation direction; a source of data-modulated light optically coupled to the waveguide; a first input coupler for directing light of a first spectral band from the source into a first waveguide pupil; a second input coupler for directing light of a second spectral band from the source into a second waveguide pupil; an output coupler comprising a first multiplexing grating and a second multiplexing grating; a first folding grating for directing the first spectral band along a first path from the first waveguide pupil to the output coupler and providing a first beam expansion; at least a second folding grating for directing the second spectral band along a second path from the second waveguide pupil to the output coupler and providing a first beam expansion; the first multiplexing grating directing the first spectral band out of the waveguide in a first direction having a beam expansion orthogonal to the first beam expansion, the second multiplexing grating directing the second spectral band out of the waveguide in the first direction with a beam expansion orthogonal to the first beam expansion, wherein the first input coupler and the second input coupler are spatially separated in the plane of the waveguide, wherein input couplers and output couplers of the first waveguide display and the second waveguide display overlap, wherein at least one grating in the first waveguide display has an optical power for focusing light extracted from the first waveguide to a first focal plane, wherein at least one grating in the second waveguide display has an optical power for focusing light extracted from the first waveguide to a second focal plane, wherein the input couplers of the first waveguide display and the second waveguide display each comprise a grating capable of switching between diffractive and non-diffractive states.

21. The light field display of claim 20, wherein the grating of the first waveguide display is in its diffraction state for incoupling the first image modulated light for viewing at the first focal plane when the grating of the second waveguide display is in its non-diffraction state, wherein the grating of the second waveguide display is in its diffraction state for incoupling the second image modulated light for viewing at the second focal plane when the grating of the first waveguide display is in its non-diffraction state.

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