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

Abstract

A waveguide display includes: a waveguide that supports a single grating layer; a data modulation light source; a first input coupler for guiding light of a first spectral band from the source into a first waveguide pupil; a second input coupler for guiding light of a second spectral band from the source into a second waveguide pupil; an output coupler including a multiplexed first grating and a second grating; at least one folding grating for guiding the first spectral band along a first path from the first pupil to the output coupler and providing a first beam expansion; at least one folding grating for guiding the second spectral band along a second path from the second pupil to the output coupler and providing a first beam expansion. The first multiplexed grating guides the first spectral band out of the waveguide in a first direction in which the beam expansion is orthogonal to the first beam expansion. The second multiplexed grating guides the second spectral band out of the waveguide in a first direction in which the beam expansion is orthogonal to the first beam expansion.

Description

Technical Field

[0001] The present invention generally relates to waveguide devices, and more particularly, to color holographic waveguide displays. Background Art

[0002] A waveguide can be referred to as a structure that has the ability to confine and guide waves (i.e., a space region in which a confined wave 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 use many different mechanisms to control the propagation path of waves. For example, a planar waveguide can be designed to utilize diffraction gratings to diffract incident light and couple the incident light into the waveguide structure such that the incident coupled light can continue to propagate within the planar structure via total internal reflection (“TIR”).

[0003] Fabricating waveguides can include the use of material systems that allow for the recording of holographic optical elements within the waveguide. One class of such materials includes polymer dispersed liquid crystal (“PDLC”) mixtures, which are mixtures containing a photopolymerizable monomer and a liquid crystal. 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 photo-polymerization-induced phase separation, creating regions densely filled with liquid crystal microdroplets, interspersed with regions of transparent polymer. The alternating liquid crystal-rich regions and liquid crystal-deficient regions form the grating’s fringe planes.

[0004] Waveguide optical devices such as those described above can be considered 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 (“HUDs”) for aviation and road transportation, and sensor fields for biometric and lidar (“LIDAR”) applications. Summary of the Invention

[0005] 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 waveguide displays having various components, including: a waveguide that supports a single grating layer; a data modulation light source optically coupled to the waveguide; a first input coupler for guiding light of a first spectral band from the source into a first waveguide pupil; a second input coupler for guiding light of a second spectral band from the source into a second waveguide pupil; and an output coupler including multiplexed first and second gratings. Additionally, many embodiments include at least one folded grating for guiding the first spectral band along a first path from the first pupil to the output coupler that provides a first beam expansion. At least one folded grating can be used to guide the second spectral band along a second path from the second pupil to the output coupler and provide a first beam expansion. A first multiplexed grating can direct the first spectral band out of the waveguide in a first direction orthogonal to the first beam expansion. A second multiplexed grating can direct the second spectral band out of the waveguide in the first direction orthogonal to the first beam expansion.

[0006] In other embodiments, each of the first and second input couplers includes at least one of a prism and a grating.

[0007] In still other embodiments, the first input coupler includes a first prism and the second input coupler includes a second prism, where the first and second prisms are arranged along the general light propagation direction of the waveguide.

[0008] In yet other embodiments, the first input coupler includes a first prism and the second light input coupler includes a second prism, where the first prism and the second prism are arranged along a direction orthogonal to the general light propagation direction of the waveguide.

[0009] In still further other embodiments, the first input coupler includes a first grating and the second input coupler includes a second grating, where the first and second gratings are arranged along the general light propagation direction of the waveguide.

[0010] In other embodiments, the first input coupler includes a first grating and the second input coupler includes a second grating, where the first and second gratings are arranged along a direction orthogonal to the general light propagation direction of the waveguide.

[0011] 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, where the first and second gratings are arranged along the general light propagation direction of the waveguide.

[0012] 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 arranged along a direction orthogonal to the general light propagation direction of the waveguide.

[0013] In yet 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.

[0014] In other embodiments, the folded grating is multiplexed and has a prescription for performing two-dimensional beam expansion and extracting light from the waveguide.

[0015] In still other embodiments, the folded 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.

[0016] In still other embodiments, the source includes at least one LED.

[0017] In yet still other embodiments, the source includes at least one LED having a spectral output with a peak wavelength biased towards the first spectral band and at least one LED having a spectral output with a peak wavelength biased towards the second spectral band.

[0018] In other embodiments, at least one of the gratings is a rolling k-vector grating.

[0019] In still other embodiments, light undergoes double interaction within at least one of the folded gratings.

[0020] In still other embodiments, the data modulation light source has a microdisplay for displaying image pixels and collimating optics for projecting the image displayed on the microdisplay panel such that each image pixel on the microdisplay is converted into a unique angular direction within the first waveguide.

[0021] In yet still other embodiments, at least one of the gratings has a spatially varying pitch.

[0022] 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, an HPDLC material, or a homogeneous modulated holographic liquid crystal polymer material.

[0023] In still other embodiments, each of the first and second input couplers includes at least one grating, and the at least one grating of each of the first and the input couplers, the folded grating, and the first and second multiplexers is disposed in a single grating layer.

[0024] Other embodiments include a method of displaying a color image, comprising the steps of:

[0025] a) providing a waveguide supporting a single grating layer; a light source; a first input coupler; a second input coupler; an output coupler including multiplexed first and second gratings; a first folded grating; and a second folded grating;

[0026] b) guiding a first spectral band from the source into a first waveguide pupil via the first input coupler;

[0027] c) guiding a second spectral band from the source into a second waveguide pupil via the second input coupler;

[0028] d) beam expanding the first spectral band light by means of the first folded grating and redirecting it onto the output coupler;

[0029] e) beam expanding the second spectral band light by means of the second folded grating and redirecting it onto the output coupler;

[0030] f) beam expanding the first spectral band light by means of the first multiplexing grating and extracting the first spectral band light from the waveguide;

[0031] 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.

[0032] Other embodiments include a waveguide display, wherein the waveguide supports a single grating layer. Additionally, the waveguide display may include an image modulating light source that is optically coupled to the waveguide through a first input coupler for guiding light of a first spectral band from a source into a first waveguide pupil. The waveguide display may also have a second input coupler for guiding light of a second spectral band from the source into a second waveguide pupil. Additionally, first and second folded gratings for diffracting the first and second spectral bands, respectively, may be used in conjunction with an output coupler that includes multiplexed first and second gratings for diffracting the first and second bands out of the waveguide, respectively.

[0033] Other embodiments include a light field display having a first waveguide display and a second waveguide display as in many embodiments. The input couplers and output couplers of the first and second waveguides overlap, wherein at least one grating in the first waveguide display has a focal power for focusing light extracted from the first waveguide onto a first focal plane, and wherein at least one grating display in the second waveguide has a focal power for focusing light extracted from the first waveguide onto a second focal plane, and wherein the input couplers of the first waveguide display and the second waveguide display each have a grating that can be switched between a diffractive and a non-diffractive state.

[0034] In still other embodiments, when the grating of the second waveguide display is in its non-diffractive state, the grating of the first waveguide display is in its diffractive state for incident-coupling image modulation light for viewing at the first focal plane, and when the grating of the first waveguide display is in its non-diffractive state, the grating of the second waveguide display is in its diffractive state for incident-coupling second image modulation light for viewing at the second focal plane. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] This description will be more fully understood with reference to the following drawings and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention.

[0036] Figure 1 Conceptual diagram schematically illustrates a plan view of a waveguide display having a single-layer waveguide according to an embodiment of the present invention, the single-layer waveguide supporting an input coupler including a prism and a spatially separated input grating.

[0037] Figure 2 Conceptual diagram schematically illustrates a plan view of a waveguide display having a single-layer waveguide according to an embodiment of the present invention, the single-layer waveguide supporting an input coupler including a prism and a multiplexed input grating.

[0038] Figure 3 Conceptual diagram schematically illustrates a plan view of a waveguide display having a single-layer waveguide according to an embodiment of the present invention, the single-layer waveguide supporting an input coupler including a spatially separated input grating.

[0039] Figure 4 Conceptual diagram schematically illustrates a plan view of a waveguide display having a single-layer waveguide according to an embodiment of the present invention, the single-layer waveguide supporting an input coupler including a multiplexed input grating.

[0040] Figure 5 and Figure 6The conceptual diagram schematically shows a plan view of a waveguide display with a single-layer waveguide according to various embodiments of the present invention, the single-layer waveguide supporting first and second spatially separated input prisms.

[0041] Figure 7 The conceptual diagram schematically shows a plan view of a waveguide display according to an embodiment of the present invention, the waveguide display having spatially separated input gratings and multiplexed grating pairs that combine the dual functions of two-dimensional beam expansion and beam extraction in the waveguide.

[0042] Figure 8 The conceptual diagram schematically shows a flowchart according to an embodiment of the present invention, the flowchart illustrating a method for providing a color waveguide display with two-dimensional beam expansion using a single grating layer.

[0043] Figure 9 The conceptual diagram schematically shows a cross-sectional view of a light field display with a single-layer color waveguide stack according to an embodiment of the present invention.

[0044] Figure 10A The conceptual diagram schematically shows a cross-sectional view according to an embodiment of the present invention, which shows a first operating state of a light field display corresponding to the formation of a visible image in a first range.

[0045] Figure 10B The conceptual diagram schematically shows a cross-sectional view according to an embodiment of the present invention, which shows a second operating state of a light field display corresponding to the formation of a visible image in a second range.

[0046] Figure 11A and Figure 11B The conceptual diagram schematically shows the grating geometry of a set of exemplary gratings according to an embodiment of the present invention.

[0047] Figure 12 and Figure 13 The conceptual diagram schematically shows a plan view of a waveguide for providing a color image using a single grating layer having an input grating, a folded grating, and an output grating according to an embodiment of the present invention.

[0048] Figure 14 The conceptual diagram schematically shows a cross-sectional view of a dichroic prism system according to an embodiment of the present invention for coupling illumination from red, green, and blue sources into the waveguide such that the red-green and green-blue bands of the illumination are spatially sheared when entering the waveguide.

[0049] Figure 15 is a graph showing the spectra of two LEDs having similar peak wavelengths combined for use to provide a primary illumination color according to an embodiment of the present invention.

[0050] Figure 16A conceptual diagram illustrates a schematic cross-sectional view of a rolling K-vector input grating configured to receive spatially-sheared illumination providing red-green and blue bands, in accordance with an embodiment of the present invention. DETAILED DESCRIPTION

[0051] To describe the embodiments, some well-known features of optical techniques in the fields of optical design and visual display that are known to those skilled in the art 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 a light beam refers to propagation parallel to an axis perpendicular to the surface of an optical component described in the present invention. In the following description, the terms light, ray, light beam, and direction may be used interchangeably and are related to each other to indicate the propagation direction of light energy along a straight-line trajectory. The following description will use terms commonly employed by those skilled in the field 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.

[0052] Turning now to the 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 head-mounted displays (HMDs) for AR and VR, helmet-mounted displays, projection displays, heads-up displays (HUDs), heads-down displays (HDDs), autostereoscopic displays, and other 3D displays. Additionally, similar techniques can be applied to waveguide sensors, such as, for example, eye trackers, fingerprint scanners, and LIDAR systems. Due to several factors, waveguide manufacturing, especially color waveguide manufacturing, can be expensive and tends to be low-volume. One such contributing effect is the difficulty in aligning the separate red, green, and blue waveguide layers required in a full-color display. This situation can be alleviated to a large extent by reducing the number of waveguide layers used to achieve full color. For example, a full-color waveguide display can be implemented using two waveguide layers, one transmitting blue-green light and the other transmitting green-red light. Ideally, the display should have as few waveguide layers as possible. However, a single configuration of a Bragg grating typically does not operate efficiently over the entire visual spectral bandwidth. Thus, implementing a full-color display using a single grating layer can be challenging. Accordingly, many embodiments of the present invention are directed to implementing a full-color waveguide capable of providing two-dimensional beam expansion and light extraction using gratings with different configurations within a single grating layer.

[0053] In many embodiments, a waveguide display is implemented as a waveguide including a single grating layer. The waveguide display may also include a data modulation 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 folded grating for guiding the first spectral band along a first path from the first pupil to the output coupler, and at least one folded grating for guiding the second spectral band along a second path from the second pupil to the output coupler. These folded gratings may be configured to provide a first beam expansion for their respective spectral bands. With respect to the output coupler, the first multiplexed grating may be configured to direct the first spectral band out of the waveguide in a first direction, where the beam expansion is orthogonal to the first beam expansion, and the second multiplexed grating may be configured to direct the second spectral band out of the waveguide in the first direction, where the beam expansion is orthogonal to the first beam expansion.

[0054] Waveguide displays according to various embodiments of the present invention may be implemented and configured in many different ways. In some embodiments, the waveguide display is implemented as a curved biaxial beam expansion waveguide.

[0055] Single layer waveguide displays, color waveguide displays, materials, and related manufacturing methods are discussed in more detail below.

[0056] Waveguide Display

[0057] Waveguide displays according to various embodiments of the present invention may be implemented and configured in many different ways. For purposes of illustration and simplification, the general propagation direction discussed throughout this disclosure is from left to right. As can be readily understood, the waveguide configuration and light propagation direction may be configured accordingly depending on the specific application. The single layer color waveguide architecture described in this disclosure has several major advantages compared to multi-layer architectures. The first is that the assembly and alignment of multiple layers are not required, thus increasing yield and reducing manufacturing costs. The second advantage is reduced manufacturing complexity because only a single layer is required during the manufacturing process using a single exposure process. This results in a reduction in exposure throughput time and thus a cost reduction. The principles of the present invention may be applied to various waveguide display and sensor applications, including but not limited to HUDs and HMDs. While the present invention is directed to single layer color waveguides, many of the embodiments and teachings disclosed herein may also be applied to monochromatic waveguides.

[0058] 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 still 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 direct light to the output grating via total internal reflection, where the output grating is configured to provide pupil expansion in a second direction different from the first direction, according to the embodiments and teachings disclosed in the cited references. By using a folded grating, according to some embodiments, the waveguide device advantageously requires fewer layers than previous systems and methods for displaying information. Additionally, by using a folded grating, light can travel via 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.

[0059] In many embodiments, at least one of the input grating, the folded grating, or the output grating may combine two or more angular diffraction specifications to expand the angular bandwidth. Similarly, in some embodiments, at least one of the input grating, the folded grating, or the output grating may combine two or more spectral diffraction specifications to expand the spectral bandwidth. For example, a color multiplexing grating may be used to diffract two or more primary colors.

[0060] In several embodiments, the grating layer includes a plurality of components, including the input coupler, the folded grating, and the output grating (or portions thereof), which are laminated together to form a single substrate waveguide. These components may be separated by an optical adhesive or other transparent material having a refractive index that matches these components. In some embodiments, the grating layer may be formed via a cell manufacturing process by creating cells having a desired grating thickness for each of the input coupler, the folded grating, and the output grating and vacuum filling each cell with an SBG material. In many embodiments, the cells are formed by positioning a plurality of glass plates with gaps therebetween that define the desired grating thickness for the input coupler, the folded grating, and the output grating. In several embodiments, a single cell may be made to have a plurality of holes such that separate holes are filled with different bags of SBG material. Separate regions may then be separated by a separating material (e.g., glue, oil, etc.) to separate any intermediate spaces. In some embodiments, the SBG material may be spin-coated onto a substrate and then covered by a second substrate after the material has cured.

[0061] In many embodiments directed to display applications, the folded grating can be oriented (clocked) such that its grating vector is in the diagonal direction within the waveguide plane. This ensures that the folded light has sufficient angular bandwidth. However, some embodiments of the present invention can utilize other clocking angles to meet the spatial constraints on grating positioning that may arise in the ergonomic design of the display. The grating vector azimuth angle can be referred to as the "clocking angle". In some embodiments, the longitudinal edges of each folded grating are tilted with respect to the alignment axis of the input coupler such that each folded grating is disposed on the diagonal with respect to the propagation direction of the display light. The angle of the folded grating is such that light from the input coupler is redirected to the output grating. In one example, the folded grating is disposed at a forty-five degree angle with respect to the direction in which the display image is released from the input coupler. This feature can cause the display image propagating down the folded grating to be adjusted into the output grating. For example, in several embodiments, the folded grating rotates the image ninety degrees into the output grating. In this way, a single waveguide can provide biaxial pupil expansion in the horizontal and vertical directions. In multiple embodiments, each folded grating can have a partially diffractive structure. The output grating receives the image light from the folded 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.

[0062] In many embodiments, the folded grating angular bandwidth can be enhanced by designing the grating specifications to facilitate a dual interaction of the guided light with the grating. Exemplary embodiments of the dual interaction folded grating are disclosed in U.S. Patent Application No.: 14 / 620,969, entitled "WAVEGUIDE GRATING DEVICE", the disclosure of which is incorporated herein by reference. In some embodiments, the waveguide operates in the infrared band based on the above principles. 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.

[0063] As discussed above, waveguide displays in accordance with various embodiments of the present invention may include a light source. In some embodiments, the data modulation light source used with the waveguide embodiments above includes an input image node (IIN) in combination with a microdisplay. The input grating may be configured to receive collimated light from the IIN and cause the light to travel within the waveguide to the folding grating via total internal reflection between the first and second surfaces. Generally, 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 into the desired FOV. Each image pixel on the microdisplay may be converted into a unique angular direction within the first waveguide. Any of a variety of microdisplay technologies may be used. In some embodiments, the microdisplay panel may be a liquid crystal device or a microelectromechanical systems (MEMS) device. In several embodiments, the microdisplay may be based on organic light emitting diode (OLED) technology. Such light emitting devices generally do not require a separate light source and thus have the benefit of a smaller form factor. In multiple embodiments, the IIN may be based on a scanned modulation laser. According to some embodiments, the IIN projects an image onto the microdisplay panel such that each display pixel is converted into a unique angular direction 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 multiple embodiments, the beam splitter is a grating recorded in HPDLC and uses the inherent polarization selectivity of such a grating to separate the light illuminating the display and the image modulation light reflected from the display. In some embodiments, the beam splitter is a polarization beam splitter cube.

[0064] In many embodiments, the IIN includes a speckle remover. Advantageously, the speckle remover is a holographic waveguide device based on the embodiments and teachings of U.S. Patent No. US 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 speckle remover is particularly important when the source is a laser and the image source is a laser-illuminated microdisplay or a laser-based emissive display. The LED will provide better uniformity than the laser. If laser illumination is used, there is a risk of illumination banding at the waveguide output. In some embodiments, 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" can be used to overcome laser illumination banding in the waveguide, the disclosure of which is incorporated herein by reference. In several embodiments, the light from the light source is polarized. In multiple embodiments, the image source is a liquid crystal display (LCD) microdisplay or a liquid crystal on silicon (LCoS) microdisplay.

[0065] In many embodiments, the 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, the first and second input gratings being disposed along the general light propagation direction of the waveguide. In several embodiments, the first and second gratings are disposed along a direction orthogonal to the general light propagation direction of the waveguide. The first and second input gratings can be implemented in the waveguide and configured in many different ways. In multiple embodiments, the input gratings are spatially separated. In other embodiments, the input gratings are implemented as multiplexed gratings. The cross configuration of the multiplexed gratings can be advantageous for gratings recorded in HPDLC materials as it can enable efficient phase separation of the liquid crystal and monomer components during grating recording. Figure 1 and Figure 2 The conceptual map illustrates these differences.

[0066] Figure 1Conceptually illustrates a schematic plan view of a waveguide display with a single-layer waveguide according to an embodiment of the present invention. The single-layer waveguide supports an input coupler that includes a prism and spatially separated input gratings. In an illustrative embodiment, waveguide display 100 includes waveguide 101 that supports 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 disposed 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 path from input to extraction in the waveguide.

[0067] Figure 2 Conceptually illustrates a schematic plan view of a waveguide display with a single-layer waveguide according to an embodiment of the present invention. The single-layer waveguide supports an input coupler that includes a prism and multiplexed input gratings. As shown, waveguide display 120 includes waveguide 121 that supports input prism 122. Waveguide 121 also includes multiplexed input gratings 123, 124, folding gratings 125, 126, and multiplexed output gratings 127, 128 disposed 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 path from input to extraction in the waveguide.

[0068] Although Figure 1 and Figure 2 illustrate specific waveguide configurations, 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 each include a first and second input grating, and a waveguide display can be implemented without a prism. In still other embodiments, the first and second input gratings are disposed along a direction orthogonal to the general light propagation direction of the waveguide. In other embodiments, the first and second input gratings are disposed along the general light propagation direction of the waveguide. Figure 3 and Figure 4 Conceptually illustrates a schematic plan view of a waveguide display implemented with spatially separated input gratings and a prims-less input coupler according to various embodiments of the present invention. As shown, Figure 3Illustrated is a waveguide display 140, which includes a waveguide 141 that supports input gratings 142, 143 and layers, folding gratings 144, 145 and multiplexed output gratings 146, 147, all of the gratings being disposed in a single layer. The beam path from input to extraction in the waveguide is 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 Illustrated is a waveguide display 160, which has a waveguide 161 that supports input gratings 162, 163, folding gratings 164, 165 and multiplexed output gratings 166, 167, all of the gratings being disposed in a single layer. The beam path from input to extraction in the waveguide is 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 illustrated embodiments lies in the arrangement of the input gratings - that is, Figure 4 Illustrated is an embodiment in which the first and second gratings are disposed along the general light propagation direction of the waveguide. In embodiments such as Figure 3 and Figure 4 and other embodiments to be described below, two spatially separated input couplers can provide two separate input pupils.

[0069] In addition to prism-less input couplers, waveguide displays can implement input couplers that include only prisms. Figure 5 and Figure 6 Conceptual diagrams illustrate schematic plan views of waveguide displays implementing input couplers without input gratings according to various embodiments of the present invention. As shown, the first input coupler includes a first prism and the second light input coupler includes a second prism. In Figure 5 , the first and second prisms are disposed along a direction orthogonal to the general light propagation direction of the waveguide. In Figure 6 , the first and second prisms are disposed along the general light propagation direction of the waveguide.

[0070] Referring to Figure 5 , waveguide display 210 includes a waveguide 211 that supports input prisms 212, 213. Waveguide 211 also includes folding gratings 214, 215 and multiplexed output gratings 216, 217 disposed in a single grating layer. The beam path from input to extraction in the waveguide is 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 6Illustrated is a waveguide display 220 including a waveguide 231 that supports input prisms 232, 233. The waveguide 231 also includes folded gratings 234, 235 and multiplexed output gratings 236, 237 disposed in a single grating layer. The beam path from input to extraction in the waveguide is 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 the waveguide displays shown therein, the pitch angles and clock angles of the folded and output gratings can be used to satisfy the conditions of grating reciprocity.

[0071] As described in the previous section, the input coupler can be configured in a number of different ways. Additionally, the folded gratings and output couplers of the waveguide display can also be configured in many different ways. Figure 7 The conceptual diagram illustrates a schematic plan view of a waveguide display with a waveguide according to an embodiment of the present invention. The waveguide display has spatially separated input gratings and multiplexed grating pairs that combine the dual functions of two-dimensional beam expansion and beam extraction in the waveguide. As shown, the waveguide display 190 includes a waveguide 191 that supports input coupling prisms 192, 193. The waveguide 191 also includes combined folded and multiplexed output gratings 194 - 197. In an illustrative embodiment, gratings 194, 195 diffract and expand the light entering the waveguide 191 two-dimensionally via prism 192. Similarly, gratings 196, 197 diffract and expand the light entering the waveguide 191 two-dimensionally via prisms 192, 193. The beam path from input to extraction in the waveguide is illustrated by ray paths 198 - 200 in the case of prism 192 and ray paths 201 - 203 in the case of prism 193. Although four gratings are multiplexed, the pairs of gratings corresponding to each of the two paths have crossed Bragg stripes. In some embodiments, the input coupling prisms 192, 193 can be replaced by gratings.

[0072] In some embodiments directed to displays using non-polarized light sources, the input gratings used can be a combination of gratings oriented such that each grating diffracts a specific polarization of the incident non-polarized light into the waveguide path. Such embodiments can incorporate some of the embodiments and teachings disclosed by Waldern et al. in PCT application PCT / GB2017 / 000040 “METHOD AND APPARATUS FOR PROVIDING A POLARIZATION SELECTIVE HOLOGRAPHIC WAVGUIDE DEVICE”, the disclosure of which is incorporated herein by reference in its entirety. The output grating can be configured in a similar manner such that light from the waveguide path is combined and coupled out of the waveguide as non-polarized light. For example, in some embodiments, the input grating and the output grating each comprise a crossed grating having peak diffraction efficiencies for orthogonal polarization states. In several embodiments, the polarization states are S polarization and P polarization. In multiple embodiments, the polarization states are opposite circular polarization senses. The advantages of recording gratings in liquid crystal polymer systems, such as but not limited to SBG, are that, in this regard, due to their inherent birefringence, they can exhibit strong polarization selectivity. However, other grating technologies that can be configured to provide unique polarization states can also be used.

[0073] In embodiments utilizing gratings recorded in liquid crystal polymer material systems, at least one polarization control layer can be provided that overlaps at least one of the folded grating, the input grating, or the output grating for the purpose of compensating for polarization rotation in any of the gratings, particularly the folded grating. In many embodiments, all of the 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 grating. The polarization control layer can include an optical retardation film. In several embodiments based on HPDLC materials, the birefringence of the grating can be used to control the polarization properties of the waveguide device. Using the birefringence tensor, K-vectors, and grating footprints of the HPDLC grating as design variables opens up a design space for optimizing the angular capabilities and optical efficiency of the waveguide device. In some embodiments, a quarter-wave plate disposed on the glass-air interface of the waveguide rotates the polarization of the 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 substrate of the waveguide can help the light maintain alignment with the expected line of sight by compensating for skew waves in the waveguide. In multiple embodiments, the quarter-wave plate can be provided as a multi-layer coating.

[0074] Figure 8The conceptual diagram illustrates a flowchart according to an embodiment of the present invention, which illustrates a method of providing a color waveguide display with two-dimensional beam expansion using a single grating layer. As shown, a method 240 of coupling light of more than one polarization component into a waveguide is provided. Referring to the flowchart, method 240 includes providing (241) a waveguide that supports a single grating layer; a light source; a first input coupler; a second input coupler; an output coupler having multiplexed first and second gratings; a first folded grating; and a second folded grating. A first spectral band can be directed (242) from the source through the first input coupler into a first waveguide pupil, and a second spectral band can be directed (243) from the source through the second input coupler into a second waveguide pupil. The first spectral band light can be beam-expanded and redirected (244) to the output coupler by means of the first folded grating. The second spectral band light can be beam-expanded and redirected (245) to the output coupler by means of the second folded grating. The first spectral band light can be beam-expanded and extracted (246) from the waveguide by means of the first multiplexed grating. The second spectral band light can be beam-expanded and extracted (247) from the waveguide by means of the second multiplexed grating.

[0075] The above discussion and Figures 1-8 The embodiments illustrated therein are based on the principle of input pupil splitting using split pupil input coupling or multiplexed input coupling to provide upward and downward waveguide paths to an output grating using two spatially separated folded gratings. One challenge in implementing this method is that having two folded gratings results in an increase in waveguide size, especially in the vertical direction above the eye center point. Another challenge is manufacturing an efficient multiplexed output grating. Accordingly, various embodiments of the present invention are directed to a color waveguide architecture based on a single waveguide layer that supports a single grating layer and that does not use the beam splitting principle.

[0076] In many embodiments, a waveguide display is implemented to provide an image at infinity. 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.

[0077] In some embodiments, the waveguide provides one layer of a multi-layer waveguide architecture that includes a single layer grating waveguide, as described above with respect to Figure 3 、 Figure 4 and Figure 7As described in the embodiments shown, where each waveguide provides a full-color image within a specified viewing range measured from the eye frame. 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 only be encoded into the multiplexed output grating to produce a minimum amount of 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) can generally be referred to as light field displays. In many embodiments, the input gratings will be switched to their diffractive states such that only one input grating is in its diffractive state at any given time (such that the image content is projected only onto one range). The projection range can be determined using an eye tracker that tracks both eyes to triangulate the desired viewing range from the measured left and right eye gaze vectors. The image data typically provided by the microdisplay can be updated for each viewing range.

[0078] Figure 9 The conceptual diagram shows a schematic cross-sectional view of a light field display 310 including a stack of single-layer color waveguides 301A - 301C according to an embodiment of the present invention. In the illustrative embodiment, each waveguide includes an input grating, a folded grating, and a multiplexed output grating labeled 312, 313, 314 and characters A, B, 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 gratings shown in Figures 3-4 and Figure 7 correspond to one of the two input gratings shown in either, in each case, the two input gratings being turned on simultaneously. At least one grating in the grating layer has an optical power for forming a visible image within a predefined range such that each waveguide provides a unique visible range.

[0079] The operation of the light field display is conceptually illustrated in Figure 10A and Figure 10B is a schematic cross-sectional view that shows a first operating state 320 of the waveguide corresponding to the formation of a visible image 322 at a first range labeled R1. The input grating 312A with black shading is in its diffractive state 321, and the input gratings 312B, 312C are in their non-diffractive states. Thus, in the first operating state, light only propagates in waveguide 301A. Figure 10A is a schematic cross-sectional view that shows a second operating state 330 of the waveguide corresponding to the formation of a visible image 332 at a second range labeled R2. The input grating 312C with black shading is in its diffractive state 331, and the input gratings 312A, 312B are in their non-diffractive states. Thus, in the second operating state, light only propagates in waveguide 301C. Figure 10B is a schematic cross-sectional view that shows a second operating state 330 of the waveguide corresponding to the formation of a visible image 332 at a second range labeled R2. The input grating 312C with black shading is in its diffractive state 331, and the input gratings 312A, 312B are in their non-diffractive states. Thus, in the second operating state, light only propagates in waveguide 301C.

[0080] Switchable Bragg grating

[0081] The optical structures recorded in waveguides can include many different types of optical elements, such as, but not limited to, diffraction gratings. In many embodiments, the implemented grating is a Bragg grating (also known as a volume grating). Bragg gratings can have high efficiency, with little light diffracted into higher orders. The relative amounts of light in diffraction and zero order can be changed by controlling the refractive index modulation of the grating, and this property can be used to fabricate lossy waveguide gratings for extracting light over a large 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 photo-polymerizable 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 of the glass plates can support electrodes, typically a transparent tin oxide film, for applying an electric field across the film. The grating structure in the SBG can be recorded in a liquid material (commonly referred to as a slurry) by photo-polymerization-induced phase separation with interference exposure having a spatially periodic intensity modulation. Factors such as, but not limited to, controlling the radiation intensity, the component volume fractions of the materials in the mixture, and the exposure temperature can determine the resulting grating morphology and performance. It can be readily understood that depending on the specific requirements of a given application, a variety of materials and mixtures can be used. In many embodiments, HPDLC materials are used. During the recording process, the monomer polymerizes and the mixture undergoes phase separation. The LC molecules aggregate to form discrete or coalesced droplets that are periodically distributed within a polymer network on the optical wavelength scale. Alternating liquid crystal-rich regions and liquid crystal-deficient regions form the grating fringe planes, which can produce Bragg diffraction with strong optical polarization caused by the director order of the LC molecules in the droplets. In some embodiments, the grating in a given layer is recorded in a stepwise manner by scanning or stepping a recording laser beam across the grating region. In several embodiments, the grating is recorded using a master and contact replication process currently used in the holographic printing industry.

[0082] The resulting volume phase grating can exhibit very high diffraction efficiency, which can be controlled by the intensity of the electric field applied to the film. In the case of applying an electric field to the grating via a transparent electrode, the natural orientation of the LC droplets may change, resulting in a reduced refractive index modulation of the fringes and a reduction of the hologram diffraction efficiency to a very low level. Typically, the electrodes are configured such 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 without an applied electric field, the extraordinary axis of the liquid crystal is typically aligned perpendicular to the fringes. Thus, the grating exhibits a high refractive index modulation and a high diffraction efficiency for P-polarized light. In the case of applying an electric field to the HPDLC, the grating switches to the ON state, in which the extraordinary axis of the liquid crystal molecules is aligned parallel to the applied electric field and thus perpendicular to the substrate. In the ON state, the grating exhibits a lower refractive index modulation and a lower diffraction efficiency for both S-polarized light and P-polarized light. Thus, the grating region no longer diffracts light. Depending on the function of the HPDLC device, each grating region can be divided into a plurality of grating elements, such as, for example, a pixel matrix. Typically, the electrodes on one substrate surface are uniform and continuous, while the electrodes on the opposite substrate surface are patterned according to a plurality of selectively switchable grating elements.

[0083] Typically, the SBG element is cleared within 30 μs and turned on with a longer relaxation time. It should be noted that the diffraction efficiency of the device can be adjusted in a continuous range by means of the applied voltage. In many cases, the device exhibits an efficiency close to 100% without an applied voltage, while exhibiting substantially zero efficiency when a sufficiently high voltage is applied. In some types of HPDLC devices, a magnetic field can be used to control the LC orientation. In some HPDLC applications, the phase separation of the LC material from the polymer can be such that no distinguishable droplet structure is produced. The SBG can also be used as a passive grating. In this mode, its main advantage is the unique high refractive index modulation. The SBG can be used to provide a transmission or reflection grating for free space applications. The SBG can be implemented as a waveguide device, where the HPDLC forms a waveguide core or an evanescent coupling layer near the waveguide. The glass plates used to form the HPDLC cell provide a total internal reflection (“TIR”) light guiding structure. When the switchable grating diffracts light at an angle exceeding the TIR condition, the light can be coupled out of the SBG.

[0084] In many embodiments, the SBG is recorded in a homogeneous modulation material, such as POLICRYPS or POLIPHEM having a solid liquid crystal matrix dispersed in a liquid polymer. Exemplary homogeneous modulation 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 entirety. Homogeneous modulation gratings are characterized by high refractive index modulation (and thus 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 and becomes diffractive in the presence of an electric field. Reverse mode HPDLC can be based on any of the formulations and processes disclosed in PCT Application No.: PCT / GB2012 / 000680 entitled IMPROVEMENTS TO HOLOGRAPHIC 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.

[0085] Grating Structures and Configurations

[0086] Each grating within the 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 the 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.

[0087] Figure 11A and Figure 11B Conceptual diagrams illustrate the grating geometry of a set of exemplary gratings in accordance with embodiments of the present invention. Vector N is the unit vector normal to the grating surface; r1-r3 are the unit ray vectors for incidence and diffraction; K1, K2 are the grating K-vectors (not necessarily in the plane of the drawing); q1, q2 are the 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 paths 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) and then applying it to the output grating: r2 x N - r3 xN = λ b(q2 / d2) is obtained by taking the vector dot product of vectors q1 and z to yield the relationship q1.z / d1 = q2.z / d2, where z is the unit vector along the main waveguide dimension, typically parallel to the average beam propagation direction in the waveguide. The q vectors are perpendicular to the drawing plane.

[0088] In many embodiments, the functions of the folded grating and the output grating are combined in two overlapping multiplexed folded gratings with opposite clock angles. In some embodiments, the opposite clock angles have different magnitudes. The crossed folded grating can be configured to perform two-dimensional beam expansion and extract light from the waveguide. Separate grating pairs can be provided for each of the first and second paths. Thus, many embodiments include a total of four folded gratings multiplexed into a single waveguide layer. By combining the folded grating and the output grating, a significant reduction in grating substrate real estate can be achieved.

[0089] In many embodiments, the waveguide includes at least one grating having 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 rolling k-vector grating. The rolling K-vector can expand the angular bandwidth of the grating without increasing the waveguide thickness. In multiple embodiments, the rolling K-vector grating includes a waveguide section that contains discrete grating elements having different arrangements of K-vectors. In some embodiments, the rolling K-vector grating includes a waveguide section that contains a single grating element within which the K-vector undergoes a smooth monotonic change in direction. Various configurations of the rolling K-vector grating, such as but not limited to the configurations described above, can be used to input light into the waveguide. The advantage of using a prism to couple light into the waveguide is that it avoids significant optical losses and limited angular bandwidths that can result from using a rolling K-vector grating. Practical rolling K-vector input gratings typically cannot match the much larger angular bandwidth of the folded grating, which can be 40 degrees or greater.

[0090] Although the drawings indicate a high degree of symmetry in the grating geometry and grating layout in different wavelength channels, in reality, due to different spectral bandwidths, the grating specifications and coverage areas may be asymmetric. Although the gratings on the upper and lower parts of the waveguide are illustrated with similar areas, the grating specifications (including pitch, tilt angle, and clock angle) may need to be adjusted for the two spectral bands to balance the two optical paths. A symmetric 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 need to consider optical efficiency, form factor, and cost. The shape of the input grating, folded grating, or output grating can depend on the waveguide application and can be any polygonal geometry affected by factors such as but not limited to the required beam expansion, output beam geometry, beam uniformity, and ergonomic factors.

[0091] Figure 12 The conceptual map shows a schematic plan view of a waveguide 250 supporting a single grating layer 251, which has an input grating 252 with a rolling 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 rolling K-vector. Refer Figure 13 , which shows a cross-section 260 of the waveguide, and the grating layer 251 is shown to be 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 rolling K-vector specifications of the input grating to provide high diffraction efficiency in the visible band. In several embodiments, the rolling K-vector specifications of the output grating can also be adjusted as part of the optimization in the visible band. Further details of the embodiments based on Figure 12 and Figure 13 are provided in the following paragraphs and drawings. It should be noted that many features of this method may also be relevant to single-layer color waveguides based on the principle of beam splitting.

[0092] In many embodiments, the substrate refractive indices are approximately n1 = 1.5 and n2 = 1.7. The substrate can be glass or plastic. For higher angles in TIR, having different refractive indices can promote more bounces in the waveguide (less interaction compared to lower angles closer to TIR). Using substrates with different refractive indices can also promote the uniformity of the illumination output from the waveguide. In some embodiments, using a high refractive index material (usually with a refractive index of 1.7 or higher) for one of the substrates supports a higher waveguide angle-carrying capacity. In several embodiments where the higher glass refractive index is greater than the average refractive index of the grating formed by HPDLC, the grating material can set the limit of the waveguide's angle-carrying capacity limit. In multiple embodiments, the upper refractive index is set slightly higher than the average level of the grating material. It should be noted that in such embodiments, the purpose of achieving a high waveguide angle-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.

[0093] 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 via a dichroic prism step. Figure 14 An arrangement 270 is shown for shearing the illumination from an RGB source into relatively displaced red-green and green-blue bands using a prism element that includes a reflective surface for reflecting long wavelengths and a dichroic coating for partially reflecting short wavelengths and transmitting long wavelengths. As Figure 14As shown in, device 270 includes a lighting module 271 that includes red, green, and blue light sources 272 - 274 that emit light along a generally direction indicated by the box arrow 275. In an illustrative embodiment, the lighting module 271 is optically coupled to a prism system that includes a prism 276 that has 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 that is adjacent and parallel to the inner surface can reflect the long wavelength light into the prism. The opposing prism surface 287 can reflect the short and long wavelength light out of the prism via face 288 to provide output beams indicated by box arrows 285, 286. The ray paths of the light reflected from the dichroic coating are represented by rays 280, 281, 282. The ray paths of the rays reflected by surface 278 are represented by rays 279, 283, 284. In some embodiments, the sources include at least one LED having a spectral output with a peak wavelength biased towards a first shorter wavelength band and at least one LED having a spectral output with a peak wavelength biased towards a longer wavelength band. In many embodiments, the longer wavelength band corresponds to light extending over the green to red regions of the visible spectrum, while the shorter wavelength corresponds to the blue to green regions. In other embodiments, the longer wavelength band corresponds to red light, while the shorter wavelength band corresponds to light extending over the blue to green regions. From Figure 14 the considerations, it is apparent that other prism configurations can be used to achieve splitting of light into two sheared spectral bands or arbitrarily defined spectral bandwidths. In some embodiments, Figure 14 the device can also employ mirror coatings, polarizers, and / or spectral filtering coatings to provide greater discrimination of the output spectral bands, e.g., to reduce crosstalk between the spectral bands. In some embodiments, color reproduction of the waveguide can be improved by using two or more LEDs with slightly different spectral relative displacements to provide the desired primary colors. Figure 15 The conceptual diagram illustrates a graph 290 that shows the LED output spectra of two such LEDs, where the vertical axis labeled 291 corresponds to the output intensity and the horizontal axis 292 represents wavelength. In this case, the LEDs have peak outputs in the green (G) band, where the spectrum 293 of one LED is biased towards blue (B), while the spectrum 294 of the other LED is biased towards red (R).

[0094] Figure 16The conceptual diagram schematically shows a cross-sectional view 300, which shows a part of a rolling K-vector input grating illuminated by spectral shear illumination across the visible band. The grating includes Bragg stripes 302A - 302F with continuously decreasing tilt angles from left to right. The incident light is represented by effective red, green, and blue light sources labeled R, G, and B, and the rays they emit are labeled with numbers 301 - 307. A typical diffracted ray that will undergo TIR in the waveguide is indicated by 308. Due to spectral shear, the Bragg stripes on the left side of the grating, such as 302A, diffract the red ray 301 and the green ray 303. On the other hand, the Bragg stripes on the right side of the grating, such as 302F, diffract the green ray 305 and the blue ray 307. Using a dichroic prism arrangement, such as but not limited to Figure 14 those described in

[0095] can produce a step function shift of two spectral bands. Other techniques can be used to provide spectral shear. In some embodiments, spectral shear uses the dispersion property of a prism as a function of wavelength, for example, using a pair of color-corrected prisms to be performed continuously. The benefits of spectral shear technology are not limited to the color waveguides disclosed herein. This technology can also be used to enhance the performance of color waveguides or monochromatic waveguides using rolling K-vector gratings, which are illuminated by green LED emitters with a spectral bandwidth that can be 80 nm or higher. In several embodiments, continuous spectral shear can be provided by means of the grating. Figure 14 In many embodiments based on the system principle shown in

[0096] more dichroic layers can be used for fine-tuning. However, this may complicate the prism manufacturing, and in most cases one dichroic layer may be sufficient. In some embodiments, the dichroic prism can be designed to reflect the incident light at an angle suitable for waveguide propagation. In several embodiments, the dichroic prism can have a high transmittance for high incident angles (in air) in the visible band to support perspective viewing of the peripheral field of view. In multiple embodiments, the dichroic prism can also be configured to achieve the 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 at an angle to the main axis of the field of view. Figure 12 and Figure 13Waveguides based on the principle can operate in the spectral range of approximately 460 nm to 640 nm. 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 approximately 4 mm. In multiple embodiments, an LCoS or other micro-projector can be used. In some embodiments, the waveguide is designed to have an inclination 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. The methods and systems for spatially-varying grating refractive index modulation are further discussed in detail in U.S. Patent Application No.: 16 / 203,071, titled "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 including the input grating, the folded grating, and the output grating. Spatially varying the refractive index modulation has the benefit of enabling a single-thickness grating layer. In some embodiments, an LCP layer disposed after the input grating can be used to rotate the polarization to minimize the input grating re-interaction output coupling loss. Compared with the multi-layer waveguide architecture, this type of waveguide generally has a relatively small field of view. In several embodiments, the waveguide supports a resolution of at least the nHD (640x360) standard with a 15-degree horizontal x 15-degree vertical FOV. In multiple embodiments, the field of view can be improved by tilting the folded grating. In some embodiments, the above field of view is provided with an eye box of 18 mm horizontally x 14 mm vertically. Advantageously, the grating can be exposed with a low refractive index (or more transparent glass) to minimize the holographic recording haze. The waveguide refractive index arrangement on the eye side / non-eye side can depend on the RKV exposure design.

[0097] Associated 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 a high angular bandwidth. This exposure method can incorporate many of the embodiments and teachings disclosed in U.S. Provisional Application No. 62 / 614,932, titled "METHODS FOR FABRICATING OPTICAL WAVEGUIDES", filed by Waldern et al. on January 8, 2018, the disclosure of which is incorporated herein by reference.

[0098] In many embodiments, the master grating used in fabrication is an amplitude grating. Rolled K-vector recording typically employs a cylindrical lens disposed 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 limited thickness between the master grating and the replicated grating. In other words, in order to ensure a constant surface period in the replica that may be required to satisfy grating reciprocity in the final waveguide, the master period should vary spatially. In many embodiments, using this master fabrication technique, a single plane wavefront input beam interacts with the cylindrical lens to provide one-dimensional focusing, and then a portion of the light either generates a diffracted beam from the chirped master or passes through as the zero order (with attenuation) and retains the original one-dimensional focusing function of the cylindrical lens. In some embodiments, the local rolled K-vector grating angular bandwidth is maximized as a function of position (e.g., if the input grating is timed relative to the orthogonal field, then the height on the input grating structure. This will result in a variation of the input grating chirp specification in 2D relative to the input wavefront from the cylindrical lens.

[0099] Advantageously, to improve color uniformity, reverse ray tracing from the eye socket via the output grating and the folding grating to the input grating can be used to design the gratings. This process can allow identification of the physical extent required for the gratings, particularly the folding grating. Unnecessary grating spaces that cause haze can be reduced or eliminated. The ray paths are optimized for red, green, and blue, and each path follows a slightly different path due to the dispersion effect generated between the input and output gratings via the folding grating. The design should allow sufficient clearance between the input and the folding and between the folding and the output to allow the use of an exposure lens in the rolled K-vector grating exposure apparatus. This is mainly to prevent the ideal folding grating aperture size from being clipped, thus avoiding support for the direct path ray coupling required for optimizing uniformity.

[0100] As used with respect to any of the embodiments described herein, the term grating may encompass a grating that includes a set of gratings. For example, in many embodiments, each of the input grating and the output grating includes two or more gratings multiplexed into a single layer. It has been well established in the holography literature that more than one holographic specification can be recorded into a single holographic layer. Methods for recording such multiplexed holograms are well known to those skilled in the art. In some embodiments, each of the input grating and the output grating may each include two overlapping grating layers that are either in contact or vertically separated by one or more thin optical substrates. In several embodiments, the grating layers are sandwiched between glass or plastic substrates. In many embodiments, two or more such grating layers may form a stack in which total internal reflection occurs at the outer substrate and air interface. In some embodiments, the waveguide may include only one grating layer. In several embodiments, electrodes may be applied to the faces of the substrate 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.

[0101] In many embodiments of the present invention directed to a display, a waveguide display can be combined with an eye tracker. In a preferred embodiment, the eye tracker is a waveguide device that covers the 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 a waveguide display that may also include a dynamic focusing element. The dynamic focusing element can 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 further includes a dynamic focusing element and an eye tracker to provide a light field display based on the embodiments and teachings disclosed in U.S. Provisional Patent Application No.: 62 / 125,089 entitled "HOLOGRAPHIC WAVEGUIDE LIGHT FIELD DISPLAYS", the disclosure of which is incorporated herein by reference. Some embodiments of the present invention can be directed to a waveguide display 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 can be integrated within a window, such as a HUD for an integrated windshield for road vehicle applications. In some embodiments, a window-integrated display can be based on the embodiments and teachings disclosed in U.S. Provisional Patent Application No.: 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 can 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, based on the embodiments disclosed in U.S. Provisional Patent Application No.: 62 / 177,494, entitled WAVEGUIDE DEVICE INCORPORATING A LIGHT PIPE, the waveguide device may include a light pipe for providing beam expansion in one direction, and the disclosure of this application is incorporated herein by reference. The optical device based on any of the above embodiments may be implemented using a plastic substrate, which uses the materials and processes disclosed in PCT Application No.: PCT / GB2012 / 000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, and this application is incorporated herein by reference.

[0102] HPDLC material system

[0103] HPDLC mixtures according to various embodiments of the present invention generally include LC, monomers, photoinitiator dyes, and coinitiators. The mixture (commonly referred to as a slurry) generally also contains surfactants. For the purposes of describing the present invention, a surfactant is defined as any chemical reagent that reduces the surface tension of the total liquid mixture. The use of surfactants in HPDLC mixtures is known and dates back to the earliest studies of HPDLC. For example, R.L. Sutherland et al. described in a paper in SPIE, Volume 2689, pages 158 - 169, 1996, a PDLC mixture that includes monomers, photoinitiators, coinitiators, chain extenders, and LC to which surfactants can be added, and the disclosure of this paper is incorporated herein by reference. Natarajan et al. also mentioned surfactants in a paper in Journal of Nonlinear Optical Physics and Materials, Volume 5, Issue 1, pages 89 - 98, 1996, and the disclosure of this paper 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 polymer - dispersed liquid crystal optical elements, which includes: at least one acrylic monomer; at least one type of liquid crystal material; photoinitiator dye; coinitiator; and surfactant. The disclosure of U.S. Patent No. 7,018,563 is incorporated herein by reference in its entirety.

[0104] Patents and scientific literature contain many examples of material systems and processes that can be used to fabricate SBGs, including research on formulating such material systems to achieve high diffraction efficiency, fast response times, low drive voltages, etc. Both U.S. Patent No. 5,942,157 to Sutherland and U.S. Patent No. 5,751,452 to Tanaka et al. describe combinations of monomers and liquid crystal materials suitable for fabricating SBG devices. Examples of recipes can also be found in papers from the early 1990s. Many of these materials use acrylate monomers, including:

[0105] · Chem. Mater., Vol. 5, p. 1533 (1993) by R.L. Sutherland et al. describes the use of acrylate polymers and surfactants, the disclosure of which is incorporated herein by reference. Specifically, the recipe includes crosslinking multifunctional acrylate monomers; chain extender N-vinylpyrrolidone, LC E7, photoinitiator Rose Bengal, and co-initiator N-phenylglycine. Surfactant octanoic acid was added in certain variants.

[0106] · SID 00 Digest, pp. 774 - 776, 2000 by Fontecchio et al. describes UV-curable HPDLCs for reflective display applications, which include multifunctional acrylate monomers, LC, photoinitiator, co-initiator, and chain terminator, the disclosure of which is incorporated herein by reference.

[0107] · Polymer International, Vol. 48, pp. 1085 - 1090, 1999 by Y.H. Cho et al. discloses HPDLC recipes including acrylates, the disclosure of which is incorporated herein by reference.

[0108] · Japanese Journal of Applied Physics, Vol. 36, pp. 6388 - 6392, 1997 by Karasawa et al. describes acrylates with various functional sequences, the disclosure of which is incorporated herein by reference.

[0109] · Polymer Science: Part B: Polymer Physics, Vol. 35, pp. 2825 - 2833, 1997 by T.J. Bunning et al. also describes multifunctional acrylate monomers, the disclosure of which is incorporated herein by reference.

[0110] ·Europhysics Letters, Volume 36(6), pages 425 - 430, 1996, by G.S. lannacchione et al., describes a PDLC mixture including a pentaacrylate monomer, LC, a chain extender, a co - initiator, and a photo - initiator, the disclosure of which is incorporated herein by reference.

[0111] Acrylates have the advantages of fast kinetics, good mixing with other materials, and good compatibility with film - forming processes. Since acrylates are cross - linked, they tend to be mechanically robust and flexible. For example, urethane acrylates with functionality 2 (di) and 3 (tri) have been widely used in HPDLC technology. Higher - functionality materials such as penta - and hexa - functional rods have also been used.

[0112] One of the known properties of transmissive SBGs is that 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 the LC molecular alignment is that the transmissive SBG efficiently diffracts P - polarized light (i.e., light with a polarization vector in the plane of incidence), but has an almost zero diffraction efficiency for S - polarized light (i.e., light with a polarization vector perpendicular to the plane of incidence).

[0113] Doctrine of equivalents

[0114] Although the above description contains many specific embodiments of the present invention, these should not be construed as limiting the scope of the present invention, but rather as examples of one of its embodiments. Although only several embodiments are described in detail in this disclosure, many modifications are possible (e.g., the dimensions, sizes, structures, shapes and proportions of various elements, parameter values, installation arrangements, use of materials, colors, orientations, etc.). For example, the positions of elements can be reversed or otherwise changed, and the nature or quantity of discrete elements or positions can be altered or changed. Accordingly, 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 re - ordered. Other substitutions, modifications, changes and omissions can be made in the design, operating conditions and arrangements of the exemplary embodiments without departing from the scope of this disclosure. It should thus be understood that the present invention can be practiced in a manner different from that specifically described without departing from the scope and spirit of the present invention. Accordingly, the embodiments of the present invention should be considered illustrative in all respects and not restrictive. Thus, 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 that supports a single grating layer with a general light propagation direction; A source for data-modulated light, optically coupled to the waveguide; A first input coupler for guiding light of a first spectral band from the source into a first waveguide pupil; A second input coupler for guiding light of a second spectral band from the source into a second waveguide pupil; An output coupler including a first multiplexing grating and a second multiplexing grating; A first folding grating for guiding 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 guiding 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 guides the first spectral band out of the waveguide in a first direction where the beam expansion is orthogonal to the first beam expansion, The second multiplexing grating guides the second spectral band out of the waveguide in the first direction where the beam expansion is 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 according to claim 1, wherein each of the first input coupler and the second input coupler comprises at least one of a prism and a grating.

3. The waveguide display according to 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. The waveguide display according to 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. The waveguide display according to claim 1, wherein the first input coupler comprises a first grating, and the second input coupler comprises a second grating, wherein the first grating and the second grating are arranged along the general light propagation direction of the waveguide.

6. The waveguide display according to claim 1, wherein the first input coupler comprises a first grating, and the second input coupler comprises 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.

7. The waveguide display according to 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 the general light propagation direction of the waveguide.

8. The waveguide display according to 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 direction orthogonal to the general light propagation direction of the waveguide.

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

10. The waveguide display according to claim 1, wherein the folded grating is multiplexed and has specifications for performing two-dimensional beam expansion and extracting light from the waveguide.

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

12. The waveguide display according to claim 1, wherein the source includes at least one LED.

13. The waveguide display according to claim 1, wherein the source includes at least one LED having a spectral output with a peak wavelength biased towards a first spectral band and at least one LED having a spectral output with a peak wavelength biased towards a second spectral band.

14. The waveguide display according to claim 1, wherein at least one of the gratings is a rolling k-vector grating.

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

16. The waveguide display according to claim 1, wherein the source for modulating data light includes: A microdisplay panel, wherein the microdisplay is configured to display image pixels; And An input image node having collimating optics, wherein the input image node projects an image to be displayed on the microdisplay panel such that each image pixel on the microdisplay panel is converted into a unique angular direction within the first waveguide.

17. The waveguide display according to claim 1, including at least one grating having a spatially varying pitch.

18. The waveguide display according to 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 homogeneous modulation holographic liquid crystal polymer material.

19. The waveguide display according to claim 1, wherein each of the first input coupler and the second input coupler includes at least one grating, and at least one grating of each of the first input coupler and the second input coupler, the folded grating, and the first multiplexed grating and the second multiplexed grating is disposed 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 that supports a single grating layer with a general light propagation direction; A source for data-modulated light, optically coupled to the waveguide; A first input coupler for guiding light of a first spectral band from the source into a first waveguide pupil; A second input coupler for guiding light of a second spectral band from the source into a second waveguide pupil; An output coupler including a first multiplexing grating and a second multiplexing grating; A first folding grating for guiding 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 guiding 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 guides the first spectral band out of the waveguide in a first direction where the beam expansion is orthogonal to the first beam expansion, The second multiplexing grating guides the second spectral band out of the waveguide in the first direction where the beam expansion is 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, The input couplers and output couplers of the first waveguide display and the second waveguide display overlap. At least one grating in the first waveguide display has a optical power for focusing the light extracted from the first waveguide onto a first focal plane. At least one grating in the second waveguide display has a optical power for focusing the light extracted from the first waveguide onto a second focal plane. The input couplers of the first waveguide display and the second waveguide display each include a grating capable of switching between a diffractive state and a non-diffractive state.

21. The light field display according to claim 20, wherein when the grating of the second waveguide display is in its non-diffractive state, the grating of the first waveguide display is in its diffractive state for incidentally coupling the first image modulation light for viewing at the first focal plane, and when the grating of the first waveguide display is in its non-diffractive state, the grating of the second waveguide display is in its diffractive state for incidentally coupling the second image modulation light for viewing at the second focal plane.

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