Image light guide with composite diffractive optical element and head mounted display using same

Abstract

An image light guide for transmitting image-bearing light includes a substrate operable to transmit an image-bearing light beam along its length. An incoupling diffractive optical element is formed along the substrate and operable to diffract a portion of the image-bearing light beam from an image source into the substrate in an angularly encoded manner. An outcoupling diffractive optical element is formed along the substrate at least partially in a plane having an x-axis and a y-axis and operable to diffract a portion of the image-bearing light beam from the substrate in an angularly decoded manner. The outcoupling diffractive optical element includes a first plurality of periodic structures and a second plurality of periodic structures operable to diffract a portion of the image-bearing light beam into diffraction orders. The first and second plurality of periodic structures include a plurality of vertices, with adjacent vertices along the x-axis being offset in the y-axis direction.

Description

[Technical field]

[0001] The present invention relates to electronic displays, and in particular to displays that use image light guides having diffractive optical elements to deliver image-bearing light to a viewer. [Background technology]

[0002] Head-mounted displays (HMDs) are being developed for a variety of applications, including military, commercial, industrial, firefighting, and entertainment applications. In many of these applications, it is valuable to create a virtual image that can be visually superimposed on a real-world image in the HMD user's field of view. An optical image light guide delivers image-bearing light to the viewer within a small space and directs the virtual image to the viewer's pupil, making this superimposition possible.

[0003] While conventional image light guide configurations have significantly reduced the bulk, weight, and cost of near-eye display optics, further improvements are desired.

[0004] In some cases, the size of the eyebox is limited, which limits the allowable range of motion and device placement in the HMD design. Light can often be unevenly distributed in the field of view, which can result in hot spots, such as high light levels in the center of the field of view and low light levels at the periphery. Beam management functions within the waveguide, including beam expansion and light distribution functions, can increase the size, manufacturing cost, and complexity of the waveguide. Summary of the Invention

[0005] In a first exemplary embodiment, an image light guide for transmitting image-bearing light comprises a substrate (602, 702, 802, 902) operable to transmit an image-bearing light beam along a longitudinal direction. The substrate has a first surface and a second surface parallel to the first surface. The image light guide for transmitting image-bearing light comprises an in-coupling diffractive optical element (604, 704, 804, 904) formed along the substrate. The in-coupling diffractive optical element is operable to diffract a portion of the image-bearing light beam from an image source (16) into the substrate in an angularly encoded manner. The image light guide for transmitting image-bearing light comprises an out-coupling diffractive optical element (500, 706, 806, 906) formed along the substrate. The out-coupling diffractive optical element is at least partially disposed in a plane having an x-axis and a y-axis and is operable to diffract a portion of the image-bearing light beam from the substrate in an angularly decoded manner. Further, the outcoupling diffractive optical element includes a first plurality of periodic structures (412, 424, 414) and a second plurality of periodic structures (416, 426, 418), the first and second plurality of periodic structures operable to diffract a portion of the image-bearing light beam in a diffraction order, and the first and second plurality of periodic structures include a plurality of vertices (352, 354, 452, 454, 2154), each vertex adjacent along the x-axis being offset along the y-axis.

[0006] In a second exemplary embodiment, a method of manufacturing an image light guide for transmitting image-bearing light includes providing a substrate (602, 702, 802, 902) having a planar surface and a coating bonded to the surface, providing a beam writing system operable to write in a first direction and a second direction orthogonal to the first direction, and providing a diffraction grating layout pattern including a plurality of unit cells (310, 410A, 410B), each unit cell including a first plurality of linear diffraction features (412, 424, 414) and a second plurality of linear diffraction features (416, 426, 418), one or more intersections of the first and second plurality of linear diffraction features defining corresponding one or more vertices (352, 354, 452, 454, 2154), adjacent vertices along the first direction including an offset along the second direction. The method of manufacturing an image light guide for transmitting image-bearing light further includes placing the substrate in the beam writing system such that the beam writing system is operable to write to the coating. The method also includes aligning one of the first and second pluralities of linear diffractive features parallel to a first direction of the beam writing system and writing the diffraction grating layout pattern into the coating via the beam writing system. [Brief description of the drawings]

[0007] The accompanying drawings are incorporated herein as part of the detailed description. The disclosed drawings illustrate embodiments of the presently disclosed subject matter and illustrate selected principles and teachings of the present disclosure. However, the drawings do not illustrate every possible embodiment of the presently disclosed subject matter and are not intended to limit the scope of the present disclosure.

[0008] [Figure 1] FIG. 2 is a simplified cross-sectional view of an image light guide illustrating the expansion of an image-bearing beam along the propagation direction to expand one dimension of the eyebox.

[0009] [Diagram 2] FIG. 13 is a perspective view of an image light guide with a rotating grating, illustrating the expansion of the image-bearing beam perpendicular to the propagation direction to expand the second dimension of the eyebox.

[0010] [Diagram 3] 1 is a plan view of an image light guide comprising an outcoupling diffractive optical element having a pattern of alternating grating vectors according to an exemplary embodiment of the disclosed subject matter.

[0011] [Figure 4] 1A-1C are diagrams illustrating the behavior of light within individual diffraction patterns according to exemplary embodiments of the presently disclosed subject matter.

[0012] [Diagram 5] 1A-1C show schematics of composite diffraction patterns according to exemplary embodiments of the presently disclosed subject matter.

[0013] [Figure 6] 1 is a schematic plan view of a composite diffraction pattern according to an exemplary embodiment of the presently disclosed subject matter.

[0014] [Figure 7] FIG. 1 illustrates a configuration of a unit cell of a diffraction pattern according to an exemplary embodiment of the presently disclosed subject matter.

[0015] [Figure 8A] FIG. 13 illustrates another configuration of a unit cell of a diffraction pattern according to an exemplary embodiment of the presently disclosed subject matter. [Figure 8B] FIG. 1 illustrates a unit cell of a diffraction pattern according to an exemplary embodiment of the presently disclosed subject matter.

[0016] [Figure 9A] FIG. 2 is a plan view of a portion of a composite grating having three overlapping grating patterns according to one embodiment of the present disclosure.

[0017] [Figure 9B]FIG. 13 is a vector diagram of the sum lattice vectors that form a closed triangle.

[0018] [Figure 10A] 1 is a schematic plan view of an image light guide having a complex grating pattern operable to expand and output an image-bearing beam in accordance with an exemplary embodiment of the disclosed subject matter;

[0019] [Figure 10B] 1 is a schematic plan view of an image light guide according to an exemplary embodiment of the disclosed subject matter, the image light guide having a complex diffraction grating pattern operable to expand and output an image-bearing beam;

[0020] [Figure 10C] The arrangement of the unit cell of the diffraction pattern according to Figure 8A is shown, which is rotated around the z-axis.

[0021] [Figure 10D] 1 is a schematic plan view of an image light guide having a complex grating pattern operable to expand and output an image-bearing beam, according to an exemplary embodiment of the disclosed subject matter;

[0022] [Figure 10E] FIG. 13 is a vector diagram of the sum lattice vectors that form a closed triangle.

[0023] [Figure 11] 1 is a schematic plan view of an image light guide having a pupil expansion diffractive optical element and an exit pupil diffractive optical element, according to an exemplary embodiment of the disclosed subject matter. [Figure 12] 1 is a schematic plan view of an image light guide having a pupil expansion diffractive optical element and an exit pupil diffractive optical element, according to an exemplary embodiment of the disclosed subject matter.

[0024] [Figure 13]In an exemplary embodiment of the disclosed subject matter, an input central ray is shown incident on the waveguide surface at an angle other than normal. [Figure 14] FIG. 14 is a simplified cross-sectional view of the image light guide of FIG. 13, showing light rays propagating within the waveguide.

[0025] [Figure 15] 1A-1C are simplified diagrams illustrating a portion of a composite diffractive optical element according to an exemplary embodiment of the presently disclosed subject matter.

[0026] [Figure 16] 1 is a schematic diagram of a portion of a composite grating pattern operable to expand and output an image-bearing beam, according to an exemplary embodiment of the disclosed subject matter.

[0027] [Figure 17] 1 illustrates a portion of a composite grating pattern, according to an exemplary embodiment of the disclosed subject matter, that is operable to expand and output an image-bearing beam.

[0028] [Figure 18] 1 is a perspective view of a binocular display system for augmented reality viewing that uses at least one image light guide according to an exemplary embodiment of the subject matter of this disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] It should be understood that the present invention may assume various alternative orientations and step sequences, unless expressly specified. It should also be understood that the specific assemblies and systems illustrated in the attached drawings and described in the following specification are merely exemplary embodiments of the inventive concepts defined herein. Thus, specific dimensions, orientations, or other physical characteristics relating to the disclosed embodiments are not to be considered as limiting, unless specifically stated otherwise. Also, where not otherwise stated, similar elements in the various embodiments described herein are commonly referred to with like reference numerals.

[0030] As used herein, the terms "first," "second," and the like do not necessarily denote an order, sequential, or priority relationship, but simply refer to an element or collection of elements, unless otherwise specified. Used to clearly distinguish one from the others.

[0031] As used herein, the terms "viewer," "operator," "observer," and "user" are considered equivalent and refer to a person or machine that views an image using or wears a device having an image light guide.

[0032] As used herein, the term "set" refers to a nonempty set, as commonly understood in elementary mathematics to refer to a collection of elements or members. As used herein, the term "subset," unless otherwise specified, refers to a nonempty proper subset, i.e., a subset of a larger set that has one or more elements. A subset may constitute the entire set S. However, a "proper subset" of a set S refers to a set that is entirely contained in set S but excludes at least one member of set S.

[0033] As used herein, the terms "coupled," "coupler," and "coupling" in the optical arts refer to coupling that allows light to be transmitted from one optical medium or device to another optical medium or device.

[0034] As used herein, the term "vertex" or "vertices" refers to repeating intersection features within a composite diffraction pattern. For example, vertices include areas where two or more lines meet, diffraction features that include posts, or areas where two or more unit cells meet.

[0035] Optical systems such as HMDs can display virtual images. In contrast to real image projection, the virtual image is not projected onto the display surface, i.e., if the display surface were in a position where the virtual image could be perceived, no image would be projected onto it. Virtual image projection offers a number of inherent advantages in augmented reality displays. For example, the apparent size of the virtual image is not limited by the dimensions or position of the display surface. Furthermore, the object from which the virtual image is sourced can be small. For example, a magnifying glass provides a virtual image of an object. By projecting a virtual image that appears to be at some distance, a more realistic visual experience can be provided compared to systems that project real images. Also, providing a virtual image avoids the need to correct screen artifacts, which is necessary in the case of real image projection.

[0036] The image light guide can display a virtual image using image-bearing light from a light source such as a projector. For example, a collimated and relatively angularly encoded light beam from a projector is coupled into the planar waveguide by input coupling, such as an in-coupling diffractive optical element. This in-coupling diffractive optical element is attached to or formed on the surface of the planar waveguide or embedded within the waveguide. Such a diffractive optical element can be formed as a diffraction grating, a holographic optical element (HOE), or in other known manner. For example, a diffraction grating can be formed by a surface relief. After the diffracted light propagates along the waveguide, it can be returned out of the waveguide by a similar output coupling, such as an out-coupling diffractive optical element. This out-coupling diffractive optical element can be positioned to provide pupil expansion along one dimension of the virtual image. Additionally, a rotating grating can be placed on or within the waveguide to provide pupil expansion in the orthogonal dimension of the virtual image. The image-bearing light output from the waveguide provides the viewer with an expanded eyebox.

[0037] As shown in FIG. 1, the image light guide 10 comprises a planar waveguide 22 having flat and parallel surfaces. The planar waveguide 22 comprises a transparent substrate S having an outer surface 12 and an inner surface 14 disposed opposite the outer surface 12. In this embodiment, an incoupling diffractive optical element IDO and an output coupling diffractive optical element ODO are disposed on the inner surface 14. The incoupling diffractive optical element IDO is a reflective diffraction grating through which the image-bearing light WI is coupled into the planar waveguide 22. However, the incoupling diffractive optical element IDO may be a transmission grating, a volume hologram or other holographic diffractive element, or any other type of optical element that diffracts the incoming image-bearing light WI. The incoupling diffractive optical element IDO may be disposed on the outer surface 12 or the inner surface 14 of the planar waveguide 22 and may be transmission or reflection type in combination with the direction in which the image-bearing light WI approaches the planar waveguide 22.

[0038] When used as part of a virtual display system, the incoupling diffractive optical element IDO couples image-bearing light WI from a real, virtual, or hybrid image source into the substrate S of the planar waveguide 22. The real or image dimension is first transformed into an array of overlapping angularly related beams that encode different locations within the virtual image presented to the incoupling diffractive optical element IDO. The image-bearing light WI is diffracted (approximately by first order diffraction) and thereby redirected by the incoupling diffractive optical element IDO into the planar waveguide 22 as image-bearing light WG, which is further propagated along the planar waveguide 22 by Total Internal Reflection (TIR). The image-bearing light WG is diffracted into a more condensed range of approximately angularly related beams to meet the boundaries defined by the TIR, but retains the image information in an encoded form. The outcoupling diffractive optical element ODO receives the encoded image-bearing light WG and diffracts the image-bearing light WG as image-bearing light WO out of the planar waveguide 22 towards the intended location of the observer's eye (again, approximately by first order diffraction). Typically, the outcoupling diffractive optical element ODO is designed symmetrically with respect to the incoupling diffractive optical element IDO to restore the original angular relationship of the image-bearing light WI in the output angularly associated beams of image-bearing light WO. However, in order to increase one dimension of the overlap of the angularly associated beams in the so-called eyebox E, where the virtual image can be seen, the outcoupling diffractive optical element ODO is configured to encounter the image-bearing light WG multiple times and diffract only a portion of the image-bearing light WG at each encounter. The multiple encounters along the length of the outcoupling diffractive optical element ODO have the effect of expanding each angularly associated beam of image-bearing light WO in one dimension, thereby expanding one dimension of the eyebox E where the beams overlap. The enlarged eyebox E reduces the sensitivity to the viewer's eye position for viewing the virtual image.

[0039] In this embodiment, the outcoupling diffractive optical element ODO is a transmissive diffraction grating arranged on the inner surface 14 of the planar waveguide 22. However, like the incoupling diffractive optical element IDO, the outcoupling diffractive optical element ODO may be arranged on the outer surface 12 or on the inner surface 14 of the planar waveguide 22 and may be transmissive or reflective in combination with the direction in which the image-bearing light WG is intended to exit the planar waveguide 22.

[0040] As shown in FIG. 2, the image light guide 20 can be configured to expand the eyebox 74 in two dimensions, i.e., along the x-axis and y-axis of the intended image. To achieve beam expansion in the second dimension, the incoupling diffractive optical element IDO is oriented to diffract the image-bearing light WG with a grating vector k0 toward the intermediate rotating grating TG, whose grating vector k1 is oriented to diffract the image-bearing light WG in a reflection mode toward the outcoupling diffractive optical element ODO. Each of the many encounters with the intermediate rotating grating TG diffracts only a portion of the image-bearing light WG, thereby laterally expanding each angularly associated beam of the image-bearing light WG approaching the outcoupling diffractive optical element ODO. The intermediate rotating grating TG redirects the image-bearing light WG to a direction that is at least approximately aligned with the grating vector k2 of the outcoupling diffractive optical element ODO, in order to vertically expand the angularly associated beam of the image-bearing light WG in the second dimension before it exits the planar waveguide 22 as the image-bearing light WO. The depicted grating vectors, such as grating vectors k0, k1, k2, extend in a direction perpendicular to the diffractive features (e.g., grooves, lines, or rulings) of the diffractive optical element and have a magnitude that is inversely proportional to the period or pitch d (i.e., the center-to-center distance between grooves) of the diffractive optical elements IDO, TG, ODO.

[0041] As shown in FIG. 2, the incoupling diffractive optical element IDO receives an incoming image-bearing light WI. This image-bearing light W1 includes a set of angularly related beams that correspond to pixels or equivalent locations in an image generated by the image source 16. The image source 16 is operable to generate a range of angularly encoded beams to create a virtual image. The image source 16 can be, but is not limited to, a real image display used with focusing optics, a beam scanner to set the beam angle more directly, or a one-dimensional real image display combined with a scanner. The image light guide 20 outputs a set of angularly related beams expanded in two dimensions of the image by providing multiple encounters of the image-bearing light WG with the intermediate rotating grating TG and the outcoupling diffractive optical element ODO in different orientations. In the original orientation of the planar waveguide 22, the intermediate grating TG provides beam expansion in the y-axis direction, and the outcoupling diffractive optical element ODO provides a similar beam expansion in the x-axis direction. The reflectivity characteristics and respective periods d of the diffractive optical elements IDO, ODO, TG, and the orientation of their respective grating vectors expand the beam in two dimensions while preserving the intended relationships between the angularly related beams in the image-bearing light WI, and output it from the image light guide 20 as image-bearing light WO.

[0042] The image-bearing light WI input to the image light guide 20 is encoded by the incoupling diffractive optical element IDO into a different set of angularly related beams, but the information necessary to reconstruct the image is preserved by the systematic effect of the incoupling diffractive optical element IDO. The rotating grating TG is located at an intermediate position between the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO and is typically configured so as not to cause a significant change in the encoding of the image-bearing light WG. The outcoupling diffractive optical element ODO is typically configured symmetrically with respect to the incoupling diffractive optical element IDO, e.g., including diffractive features that share the same period. Similarly, the period of the rotating grating TG also typically matches the common period of the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO. As shown in FIG. 2, the grating vector k1 of the rotating grating is illustrated as being oriented at 45° with respect to the other grating vectors (all as unoriented line segments). However, in one embodiment, the grating vector k1 of the rotating grating TG is oriented at 60° relative to the grating vectors k0, k2 of the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO, and the image-bearing light WG is rotated by 120°. By orienting the grating vector k1 of the intermediate rotating grating TG at 60° relative to the grating vectors k0, k2 of the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO, the grating vectors k0, k2 of the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO are also oriented at 60° relative to each other (again, the grating vectors are considered as unoriented line segments). Since the magnitude of the grating vector is based on the common pitch of the rotating grating TG, the incoupling diffractive optical element IDO, and the outcoupling diffractive optical element ODO, the three grating vectors k0, k1, k2 as oriented line segments form an equilateral triangle, and the sum of the magnitudes is zero. This avoids asymmetric effects that may cause undesirable aberrations, including chromatic dispersion.

[0043] The image-bearing light WI diffracted into the planar waveguide 22 is effectively encoded by the incoupling diffractive optical element, whether the incoupling diffractive optical element is a grating, a hologram, a prism, a mirror, or other mechanism. The reflection, refraction, and / or diffraction of light occurring at the incoupling diffractive optical element IDO must be decoded by the outcoupling diffractive optical element ODO to recreate the virtual image presented to the viewer. The rotating grating TG, preferably located at an intermediate position between the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO, is typically designed and oriented so as not to cause any modification to the encoded light. The outcoupling diffractive optical element ODO decodes the image-bearing light WG into an angularly related beam of the original or desired shape. This beam is expanded to fill the eyebox 74.

[0044] Regardless of whether symmetry is maintained between the rotating grating TG and the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO, and regardless of whether any changes occur along the planar waveguide 22 in the encoding of the angularly associated beams of the image-bearing light WI, the rotating grating TG, the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO are associated such that the image-bearing light WO output from the planar waveguide 22 maintains or retains the original shape of the image-bearing light WI or a desired shape for generating the intended virtual image.

[0045] The letter "R" represents the orientation of the virtual image as seen by a viewer with eyes in the eyebox 74. As shown, the orientation of the letter "R" in the represented virtual image corresponds to the orientation of the letter "R" encoded in the image-bearing light WI. A change in the rotation or angular orientation of the incident image-bearing light WI about the z-axis in the xy plane causes a corresponding symmetric change in the rotation or angular orientation of the light exiting the outcoupling diffractive optical element ODO. In terms of the orientation of the image, the rotating grating TG simply acts as a kind of optical relay, expanding the angularly encoded beam of the image-bearing light WG along one axis of the image (e.g., along the y-axis). The outcoupling diffractive optical element ODO expands the angularly encoded beam of the image-bearing light WG along another axis of the image (e.g., along the x-axis) while maintaining the original orientation of the virtual image encoded by the image-bearing light WI. As shown in FIG. 2, the rotating grating TG is a tilted or square diffraction grating and is placed on the front or rear surface of the planar waveguide 22. Alternatively, the rotating grating TG may be a blazed diffraction grating.

[0046] The present disclosure provides an improved image light guide that eliminates the need for a separate rotating grating TG in the light path. More specifically, the present disclosure provides, among other things, a waveguide having a diffractive array operable to expand an image-bearing light beam in two dimensions and output the expanded image-bearing light beam toward the eyebox.

[0047] As shown in FIG. 3, the image light guide 100 includes an incoupling diffractive optical element IDO and an outcoupling diffractive optical element ODO. The diffractive optical elements are formed on a first surface 102 of the image light guide. Alternatively, one or both of the incoupling diffractive optical element IDO and the outcoupling diffractive optical element ODO are formed on a second surface of the image light guide 100 opposite the first surface 102. The incoupling diffractive optical element IDO has a grating vector k1 extending in the x-axis direction. The outcoupling diffractive optical element ODO includes a diffractive array 104. The diffractive array 104 includes a number of components, the diffractive optical elements 106. In the row of the diffractive array 104, the series of diffractive optical elements 106 have alternating grating vectors k2, k3. The arrangement of the alternating grating vectors k2, k3 is shown diagrammatically in an enlarged view of a portion of the array 100. The grating vector k2 is offset from the incoupling grating vector k1 and the x-axis by +60° (in other words, offset from the y-axis by −30°). The lattice vector k3 is offset from the incoupling lattice vector k1 and the x-axis by −60°.

[0048] The diffractive array 104 can be considered structurally as a union of disjoint, non-overlapping subsets of diffractive or diffractive optical elements formed on a single surface. In terms of set theory, the union of the subsets forms a "partition". Each subset of the partition has its own lattice vector, and the subsets are distinguished from each other by the direction of the lattice vector. That is, all diffractive elements of each subset have a common lattice vector. The spatial arrangement of the diffractive optical elements 106 has elements of at least two subsets interleaved, such that each diffractive optical element 106 of a subset with lattice vector k2 is adjacent to one or more diffractive optical elements 106 of the other subset with lattice vector k3. Two or more subsets of adjacent diffractive optical elements 106 can constitute a partition of the diffractive array 104. The lattice vector of each subset extends in a different direction than the lattice vectors of the other subsets.

[0049] 4, when the light WG incident from the incoupling diffractive optical element IDO interacts with the diffractive array 104, a portion of the incident light WG is diffracted and directed to another portion of the diffractive array 104 at an angle to the original path of the light. As shown in FIG. 4, in one embodiment, when the incident light WG interacts with the diffractive optical element 106 having the grating vector k2, a portion of the incident light WG is diffracted and thereby deflected 120° to the original path of the light from the incoupling diffractive optical element IDO. Similarly, when the incident light WG interacts with the diffractive optical element 106 having the grating vector k3, a portion of the incident light WG is diffracted and thereby deflected −120° to the original path of the light from the incoupling diffractive optical element IDO. Another portion of the incident light WG travels through the diffractive optical element 106 to an adjacent diffractive optical element 106 having a different grating vector. When the diffracted and deflected light WG is incident on the diffractive optical element 106 at an angle substantially parallel to its grating vectors k2, k3, a portion of the light WO is output (outcoupled) from the image light guide 100.

[0050] As shown in FIG. 5, in one embodiment, the outcoupling diffractive optical ODO comprises a first diffraction grating having a grating vector k2 and a second diffraction grating having a grating vector k3. The first and second diffraction gratings overlap to create an angular relationship of the grating vectors k2, k3 between 0° and 180°. In one embodiment, the angular relationship between the grating vectors k2, k3 is about 60°. When the light diffracted and directed by the first diffraction grating is incident on the second diffraction grating at an angle substantially perpendicular to the features (e.g., lines) of the second diffraction grating, a portion of the light is outcoupled from the image light guide.

[0051] 5, in one embodiment, the period d1 of the first diffraction grating is equal to the period d2 of the second diffraction grating. In another embodiment, the period d1 is greater than the period d2. In another embodiment, the period d1 is less than the period d2. In yet another embodiment, at least one of the first and second diffraction patterns includes a chirp period d1, d2 that varies in the direction of the grating vector k2, k3.

[0052] As shown in FIG. 6, in one embodiment, the outcoupling diffractive optical ODO has a composite diffraction pattern including a first diffraction grating 206A with a grating vector k1, a second diffraction grating 206B with a grating vector k2, and a third diffraction grating 206C with a grating vector k3. The first, second, and third diffraction gratings 206A, 206B, 206C are coplanar (i.e., on the same surface of the image light guide) overlapping each other. In one embodiment, the period d of each of the diffraction gratings 206A, 206B, 206C is equal. In another embodiment, the period d of one or more of the diffraction gratings 206A, 206B, 206C is different. In one embodiment, the period d of one or more of the diffraction gratings 206A, 206B, 206C is chirped. As shown, all three grating vectors k1, k2, and k3 are related by an angle of 60° (when considered as undirected line segments). Because the magnitudes of the lattice vectors are based on a common pitch, the three lattice vectors k1, k2, k3 (as directed line segments) can be organized into a vector diagram that forms an equilateral triangle and has a sum of zero magnitude. In other configurations, the lattice vectors k1, k2, k3 can be oriented relative to one another at different angular amounts.

[0053] With continued reference to Figure 6, when a composite diffractive pattern is produced by subtractive manufacturing techniques, the remaining material forms diffractive features 208. As shown in Figure 6, in one embodiment, diffractive features 208 are triangular. However, in other embodiments, diffractive features 208 may be any shape, such as, but not limited to, hexagonal, as determined by the distribution of the removed material.

[0054] In one embodiment, the diffraction gratings 206A, 206B, 206C are formed by an array of replicas of the unit cells 210 arranged in a two-dimensional lattice. As shown in FIG. 6, the entire composite diffraction pattern of the outcoupling diffractive optical system ODO is formed by the replica and consecutive arrangement of the hexagonal unit cells 210. Adjacent unit cells 210 share vertices in the two-dimensional lattice. The unit cells 210 allow for the formation and orientation of the diffractive features 208 in the composite diffraction pattern, although the lattice vectors k1, k2, k3 are oriented at 60° relative to each other. For example, the relative orientation and period of the lattice vectors k1, k2, k3 remain unchanged even if the diffractive features 208 are formed as squares, rectangles, circles, or ellipses. The diffractive features 208 can be defined by material remaining after machining or other subtractive manufacturing processes, or by material removed by machining or other subtractive manufacturing processes. The diffractive features 208 can also be defined by optical properties that distinguish the diffractive features 208 from their surroundings, such as differences in refractive index.

[0055] As shown in FIG. 7, in one embodiment, the unit cell 310 defines a non-regular hexagon operable to form a composite diffraction pattern. The unit cell 310 includes a first pair of diffractive features 312, 314 having approximately the same length, a second pair of diffractive features 316, 318 having approximately the same length, and a third pair of diffractive features 320, 322 having approximately the same length. The first, second, and third pairs of diffractive features are not the same length. For example, the diffractive features 312, 320, 316 have different lengths. The first, second, and third pairs of diffractive features 312, 320, 318, 314, 322, 316 define the non-regular hexagon of the unit cell 310.

[0056] The unit cell 310 also includes a fourth diffractive feature 324, a fifth diffractive feature 326, and a sixth diffractive feature 328. The fourth, fifth, and sixth diffractive features 324, 326, and 328 intersect within the non-regular hexagon of the unit cell 310. The first, second, and third pairs of diffractive features 312, 320, 318, 314, 322, and 316 and the fourth, fifth, and sixth diffractive features 324, 326, and 328 define six area domains 330, 332, 334, 336, 338, and 340.

[0057] The widths of the diffractive features 312, 324, 314 are approximately the same and are greater than 50 nm. In one embodiment, the widths of the diffractive features 312, 324, 314 are in a range between 200 nm and 600 nm. The widths of the diffractive features 316, 326, 318 are approximately the same and are greater than 50 nm. In one embodiment, the widths of the diffractive features 316, 326, 318 are in a range between 200 nm and 600 nm. The widths of the diffractive features 320, 328, 322 are approximately the same and are greater than 50 nm. In one embodiment, the widths of the diffractive features 320, 328, 322 are in a range between 200 nm and 600 nm. The widths of the diffractive features 312, 324, 314, the widths of the diffractive features 316, 326, 318, and the widths of the diffractive features 320, 328, 322 do not have to be the same. Furthermore, in one embodiment, the depths of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318 are the same or similar. In another embodiment, the depths of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318 are not the same or similar.

[0058] In one embodiment, the refractive indices of the six regions 330, 332, 334, 336, 338, 340 are the same or nearly the same. For example, the refractive indices of the six regions 330, 332, 334, 336, 338, 340 may be equal to or nearly equal to the refractive index of air. The refractive indices of the six regions 330, 332, 334, 336, 338, 340 are not the same as the refractive indices of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318. The refractive indices of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318 may be the same or nearly the same as one another. The refractive index of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318 may be approximately the same as the refractive index of air. In one embodiment, the refractive index of the diffractive features 312, 324, 314, 320, 328, 322, 316, 326, 318 is in the range of 1.25 to 3.5.

[0059] Continuing to refer to FIG. 7, in one embodiment, the unit cell 310 includes a vertical offset 350 in the y-axis direction (i.e., the vertical direction shown in FIG. 7) between vertex 352 and vertex 354. In one embodiment, the vertical offset 350 has a distance between 10 nm and 100 nm. In one embodiment, the vertical offset 350 is about 65 nm. The vertex 352 is defined, at least in part, by the intersection of the diffractive features 318, 320, 324. The vertex 354 is defined, at least in part, by the intersection of the diffractive features 314, 322, 326. As shown in FIG. 9A, the vertical offset 350 is a step change from one unit cell 310 to an adjacent unit cell 310. In one embodiment, the vertical offset 350 defines the shape of the six regions 330, 332, 334, 336, 338, 340 as a scalene triangle (rather than an equilateral triangle). The shape of the six area domains 330, 332, 334, 336, 338, 340 as scalene triangles allows for the diffraction of off axis inputs symmetrically positioned through the center of the composite diffraction pattern.

[0060] The vertical offset 350 is generally consistent within the composite diffraction pattern. As described in more detail below, in one embodiment, the consistency of the vertical offset 350 facilitates the production of the composite diffraction pattern by a digital writing process.

[0061] Continuing to refer to FIG. 7, in one embodiment, diffractive features 316, 326 have a pitch 360, diffractive features 312, 324 have a pitch 362, and diffractive features 322, 328 have a pitch 364. In one embodiment, diffractive feature pitches 360, 362, 364 are different from one another. Diffractive feature pitches 360, 362, 364 can range from 300 nm to 500 nm. In one embodiment, pitch 360 is about 356 nm and pitch 362 is in the range of 300 nm to 500 nm. In one embodiment, pitch 362 is about 323 nm. In one embodiment, pitch 364 is in the range of 300 nm to 500 nm. In another embodiment, pitch 364 is about 305 nm.

[0062] As shown in FIG. 8A, in one embodiment, the unit cell 410A defines an offset diffraction pattern operable to form a composite diffraction pattern. In the unit cell 410A, the diffraction features of the unit cell 310 that are parallel to the diffraction features of the associated incoupling diffractive optical element IDO (e.g., the diffraction features 320, 328, 322 shown in FIG. 7) are not created. The unit cell 410A includes a first pair of diffraction features 412, 414 having approximately the same length and a second pair of diffraction features 416, 418 having approximately the same length. The first and second pairs of diffraction features are not the same length. The different lengths of the first and second pairs of diffraction features 412, 414, 416, 418 at least partially define an offset 450 of the unit cell 310. In one embodiment, the vertical offset 450 is between the apex 452 and the apex 454 in the y-axis direction. Vertex 452 is defined by at least a partial intersection of diffractive feature 424 and diffractive feature 418. Vertex 454 is defined by at least a partial intersection of diffractive feature 426 and diffractive feature 414.

[0063] The unit cell 410A also includes intersecting diffractive features 424 and 426. The first and second pairs of diffractive features 412, 414, 416, 418, and diffractive features 424, 426 define six regions 430, 432, 434, 436, 438, 440. In one embodiment, the diffractive features 416, 426 have a pitch 460 and the diffractive features 412, 424 have a pitch 462.

[0064] Diffractive features are also referred to herein as periodic structures. In an embodiment, the periodic structures can be, but are not limited to, linear diffractive features, cylinders, or elliptical cylinders. For example, FIG. 16 shows a composite diffraction pattern 2000 having a periodic structure including a cylinder 2002. The periodic structure 2002 includes an array of unit cells 410 in a periodic grid to form a two-dimensional periodic lattice structure. The unit cells 410 represent an offset 450 such that adjacent periodic structures 2002 along a line 472 are offset in the y-axis direction.

[0065] In one embodiment, the regular variations that define the periodicity of the composite diffraction pattern are patterns of sinusoidal structures 2100, as shown in FIG. 17. The composite diffraction pattern can have more than two vector components. Thus, the generated diffraction orders of the composite diffraction pattern are optimized to promote the desired performance. The periodicity of the composite diffraction pattern in the y-axis direction is generated by the regular or average spacing between adjacent rows of grating features 2100 in the y-axis direction. In one embodiment, the rows of sinusoidal patterns 2100 may be out of phase with adjacent rows in the y-axis direction. The periodic structure 2100 includes an arrangement of unit cells 410A and / or unit cells 410B in a periodic grid to form a two-dimensional periodic grating structure. The unit cells 410B define a parallelogram and the unit cells 410A define an irregular hexagon. The unit cells 410A, 410B represent an offset 450 such that each adjacent peak 2102 of the periodic structure 2100 along the x-axis direction is offset in the y-axis direction. In other words, the unit cells 410A define vertices 2154 at the intersection of two or more unit cells 410A, with adjacent vertices along the x-axis direction including an offset 450 in the y-axis direction.

[0066] As shown in FIG. 10B, the vertical offset 450 is a gradual change from one unit cell 410A to an adjacent unit cell 410A. The unit cells 410A create a lattice and / or regular tiling (i.e., a tessellation) where three unit cells 410A meet at each interior vertex (i.e., a vertex that contains two or more linear diffractive features). The unit cells 410A form a lattice with diagonal rows, each having a centerline 470 disposed at a non-zero angle with the lattice vector k0 of the incoupling diffractive optical element 704.

[0067] In one embodiment, as shown in FIG. 10C, the widths and refractive indices of the first and second pairs of diffractive features 412, 414, 416, 418, and diffractive features 424, 426 are similar to unit cell 310. The diffractive features do not separate regions 430, 440 and 434, 436, respectively, but six separate regions are implicitly defined by unit cell 410A. The design of unit cell 410A and the composite diffractive pattern formed therewith is utilized to prevent undesired diffraction of certain diffractive orders of image-bearing light WG without the use of diffractive features parallel to those of the associated incoupling diffractive optical element IDO.

[0068] In one embodiment, unit cell 410B can be defined as a parallelogram, as shown in FIG. 8B. In this embodiment, unit cell 410B includes diffractive features 424, 426. Vertex 452 is defined, at least in part, by an end of diffractive feature 424. Vertex 454 is defined, at least in part, by an end of diffractive feature 426.

[0069] 9A shows a schematic plan view of a portion of a compound grating 500 according to the present disclosure. The compound grating 500 includes a repeat of the unit cell 310 of FIG. 7, thereby forming three overlapping gratings. A grating vector k1 extends in a direction perpendicular to the diffractive features 316, 326, and 318. A grating vector k2 extends in a direction perpendicular to the diffractive features 320, 328, and 322, and a grating vector k3 extends in a direction perpendicular to the diffractive features 312, 324, and 314.

[0070] As shown in FIG. 9B, in one embodiment, the combination of the grating vector k1, the grating vector k3, and the vector k0 of the incoupling diffractive optical grating (see FIG. 10A) defines a closed triangle and forms a vector diagram with a substantially zero magnitude. In other words, the combination of the grating vectors k0, k1, and k3 forms a vector diagram with a substantially zero magnitude. Similarly, the combination of the grating vectors k1, k2, and k3 defines a closed triangle and forms a vector diagram with a substantially zero magnitude. Thus, all the grating vectors in the waveguide (e.g., the parallel plate waveguide system 600, 700) sum to a substantially zero magnitude. In one embodiment, the grating vectors k0, k1, and k3 form a closed scalene triangle. In another embodiment, the grating vectors k0, k1, and k3 form a closed equilateral triangle. In another embodiment, the grating vectors k0, k1, and k3 form a closed isosceles triangle.

[0071] It is understood that due to manufacturing variability, the dimensions specified may vary depending on the manufacturing method. Although the figures show sharp edges and sharp apexes, in practice, the edges and apexes are rounded when manufactured as known to those skilled in the art. The degree of sharpness or roundness of the sharp features described in this disclosure depends, at least in part, on the manufacturing process. Similarly, where the figures show sharp edges and sharp apexes, the features may be designed to have rounded edges and / or rounded apexes.

[0072] 10A is a schematic diagram of a parallel plate waveguide system 600 with a waveguide 602 (i.e., a substrate). The waveguide 602 has an input diffractive optical element 604 and a composite pupil expansion grating 500. The input diffractive optical element 604 has a grating vector k0. The composite pupil expansion grating 500 functions as an output grating in addition to performing pupil expansion. As described with respect to FIG. 9B, in one embodiment, the grating vectors k0, k1, and k3 form a closed triangle. Similarly, the grating vectors k0, k1, and k2 and the grating vectors k0, k3, and k2 form a vector diagram of a closed triangle.

[0073] FIG. 10B is a schematic diagram of a parallel plate waveguide system 700 comprising a waveguide 702 (i.e., a substrate). The waveguide 702 comprises an input diffractive optical element 704 and a pupil expansion compound grating 706. The input diffractive optical element 704 has a grating vector k0. The pupil expansion compound grating 706 comprises a repeat of the unit cell 410 shown in FIG. 8A and FIG. 8B. The pupil expansion compound grating 706 serves as an exit grating and a pupil expander. The grating vector k1 extends in a direction perpendicular to the diffractive features 416, 426, 418, and the grating vector k3 extends in a direction perpendicular to the diffractive features 412, 424, 414. In one embodiment, the grating vectors k0, k1, k3 combine to form a vector diagram that defines a closed triangle having a substantially zero magnitude.

[0074] As shown in FIG. 10C, FIG. 10D, in one embodiment, the compound grating 706 is rotated in the xy plane of the waveguide 702, so that the first plurality of periodic structures are oriented parallel to the x-axis and the edge of the waveguide 702. The vector diagram formed by the grating vectors k0, k1, k3 defines a scalene triangle. If the input central ray WI is positioned at a complex compound angle with respect to the grating vector k0 of the incoupling diffractive optical element 704, as shown in FIG. 10D, the vector diagram formed by the vectors grating vectors k0, k1, k2 / k3 defines a scalene triangle. In FIG. 10D, the grating vector k0 of the incoupling diffractive optical element is tilted downward in the xy plane. If the grating vector k0 of the incoupling diffractive optical element is wobbled to the left in the xy plane, the central image path WI will not be aligned with the grating vector k0 of the incoupling diffractive optical element, and a non-regular hexagonal (or parallelogram) unit cell 410 will be required. When the optical system is rotating or when the input central ray WI is at a complex compound angle with respect to the input grating vector k0, there should be a scalene triangle relationship between the grating vectors k0, k1, k2 / k3. In one embodiment, the grating vector k0 of the incoupling diffractive optical element is aligned parallel to the grating vector k1 of the outcoupling diffractive optical element. In another embodiment, the grating vector k0 of the incoupling diffractive optical element is aligned parallel to the grating vector k3 of the outcoupling diffractive optical element. In yet another embodiment, the grating vector k0 of the incoupling diffractive optical element is aligned parallel to a line that bisects (halves) the grating vectors k1, k3 of the outcoupling diffractive optical element. As shown in FIG. 13, when the input central ray 1006 is positioned at a complex compound angle with respect to the grating vector k0 of the incoupling diffractive optical element 1002, the grating vectors k0, k1, k3 of the incoupling diffractive optical element and the outcoupling diffractive optical element need to be considered independently to form a vector diagram of a closed scalene triangle.

[0075] As shown in FIG. 11, in one embodiment, the parallel plate waveguide system 800 includes a waveguide 802. The waveguide 802 includes an in-coupling diffractive optical element 804, a pupil widening composite diffractive optical element 806, and an out-coupling diffractive optical element 808. The in-coupling diffractive optical element 804 has a grating vector k0, and the pupil widening composite diffractive optical element 806 has grating vectors k1, k2, and k3. In one embodiment, the pupil widening composite diffractive optical element 806 is defined by a unit cell 310. The out-coupling diffractive optical element 808 has a grating vector k4. In one embodiment, the grating vector k0 is equal in magnitude and direction to the grating vector k4. In one embodiment, the grating vectors k0, k1, and k3 form a vector diagram representing a closed triangle, as described with reference to FIG. 9B. With continued reference to FIG. 11, in one embodiment, the grating vectors k4, k1, and k3 form a vector diagram of a closed triangle. In one embodiment, the grating vectors k4, k1, and k3 form an equilateral triangle. In another embodiment, the lattice vectors k4, k1, and k3 form a scalene triangle. In yet another embodiment, the lattice vectors k4, k1, and k3 form an isosceles triangle.

[0076] As shown in FIG. 12, in one embodiment, the parallel plate waveguide system 900 includes a waveguide 902. The waveguide 902 includes an in-coupling diffractive optical element 904, a pupil widening composite diffractive optical element 906, and an out-coupling diffractive optical element 908. The in-coupling diffractive optical element 904 has a grating vector k0, and the pupil widening composite diffractive optical element 906 has grating vectors k1, k3. In one embodiment, the pupil widening composite diffractive optical element 906 is defined by a unit cell 410. The out-coupling diffractive optical element 908 has a grating vector k4. In one embodiment, the grating vector k0 is equal in magnitude and direction to the grating vector k4. In one embodiment, the grating vectors k0, k1, k3 form a vector diagram representing a closed triangle, as described with reference to FIG. 9B. With continued reference to FIG. 12, in one embodiment, the grating vectors k4, k1, k3 form a vector diagram of a closed triangle. In one embodiment, the grating vectors k4, k1, k3 form an equilateral triangle. In another embodiment, the lattice vectors k4, k1, and k3 form a scalene triangle. In yet another embodiment, the lattice vectors k4, k1, and k3 form an isosceles triangle.

[0077] 13, a waveguide assembly 1000 includes an incoupling diffractive optical element 1002 operable to couple an image-bearing light beam into a waveguide 1004. A central ray 1006 of the image-bearing light beam representing an input image can impinge on the incoupling diffractive optical element 1002 at an angle other than perpendicular to a first surface 1008 of the waveguide 1004. As shown in FIG. 13, in one embodiment, the central ray 1006 makes an angle θ with respect to the z-axis. The z-axis is perpendicular to the first surface 1008 of the waveguide. As shown by a line segment 1010 projected onto the xy plane, the central ray 1006 incident on the incoupling diffractive optical element 1002 makes an angle φ with respect to the y-axis.

[0078] FIG. 14 is a side view of the waveguide assembly 1000 shown in FIG. 13. A central ray 1006 is incident on the incoupling diffractive optical element 1002. The resulting diffracted ray 1012 travels through the waveguide 1004 to the second surface 1014 of the waveguide 1004 where the diffracted ray 1012 undergoes total internal reflection (TIR) ​​as ray 1016. The ray 1016 continues to propagate through the waveguide 1004 by TIR. The grating vector k0 (see FIG. 13) of the incoupling diffractive optical element 1002 is designed such that the central ray 1006 is diffracted to produce an angled diffracted ray 1012 halfway between the TIR minimum boundary 1018 and the TIR maximum boundary 1020. The TIR minimum boundary 1018 is the line defined by the angle ξ measured from the z-axis. The angle ξ is the minimum angle at which TIR begins. That is, any ray with an angle with respect to the z-axis greater than angle ξ will undergo TIR when incident on the second surface 1014 and the first surface 1008 of the waveguide. In one embodiment, angle α is approximately the same as angle β. The TIR maximum boundary 1020 is the optical path with the maximum angle with respect to the z-axis at which the ray will undergo TIR. In other words, only diffracted rays with angles between the TIR minimum boundary 1018 and maximum boundary 1020 will be those rays that will be incident on the compound diffractive optical element 500, 706 at least once, via multiple TIR reflections at the first surface 1018 and the second surface 1014. The z-axis, TIR minimum boundary 1018, TIR maximum boundary 1020, and diffracted rays 1012 are all shown in the same plane. As shown in Figures 13 and 14, when the input central ray 1006 is positioned at a complex compound angle with respect to the grating vector k0 of the incoupling diffractive optical element 1002, the vector diagram formed by the grating vectors k0, k1, and k2 / k3 defines a scalene triangle.

[0079] In one embodiment, the layout pattern of diffractive optical elements can be formed or written directly on and / or into the surface of a mold substrate using, but not limited to, electron beam (e-beam) lithography, ion beam lithography, laser lithography, and / or other digital beam writing methods. In digital beam writing to the mold substrate, linear diffractive features that are not parallel to the x-axis and / or y-axis of the beam writer are generated in a zigzag or step pattern. As previously described with respect to FIG. 7, the unit cell 310 includes a vertical offset 350 and the unit cells 410A, 410B include a vertical offset 450. The vertical offset 350, 450 is a gradual change from one unit cell 310, 410A, 410B to an adjacent unit cell 310, 410A, 410B.

[0080] For digital beam writing, the vertical offsets 350, 450 should be evenly divided into multiples of the height h of the unit cells 310, 410A, 410B by discrete values ​​to ensure that the non-regular hexagonal unit cells 310, 410A, 410B are repeated to form the composite diffractive optical pattern. FIG. 15 is a simplified diagram of a portion of an expanded pupil composite diffraction grating 706 that includes repeating unit cells 410B. FIG. 15 illustrates the principle of creating a composite diffractive optical pattern 706 that utilizes repeating unit cells 310, 410A, 410B. Here, the unit cells 410B are parallelograms, and the vertical offsets 350, 450 are selected to be evenly divided into multiples of the height of the unit cells 310, 410A, 410B by discrete values. For example, the height h of the unit cells can be four times the vertical offset 450, as shown in FIG. 15. The unit cells are the smallest area that is repeated within the composite diffractive optical pattern. When unit cell 410B describes a parallelogram with a perpendicular offset, the composite diffraction pattern forms a scalene triangle of lattice vectors.

[0081] Digitally writing the unit cells 310, 410A, 410B provides greater reproducibility than traditional methods of creating diffractive elements. Additionally, digital beam writing facilitates optimization of the diffraction orders. The diffraction orders can be optimized by varying the duty cycle, shape, and depth of the diffractive features that can be produced symmetrically by digital writing.

[0082] For example, the mold substrate is oriented such that diffractive features 320, 328, 322 are substantially parallel to the preferred writing direction of the beam writer (parallel to the x-axis or y-axis of the beam writer), and the angles of diffractive features 312, 324, 314 and the angles of diffractive features 316, 326, 318 are oriented such that any errors in the writing process are respectively reflected in each unit cell 310. In other words, the lattice vectors of unit cells 310, 410A, 410B are aligned parallel to the preferred writing direction of the beam writer (parallel to the x-axis or y-axis of the beam writer).

[0083] The perspective view of FIG. 18 shows a display system 60 for three-dimensional (3D) augmented reality display using a pair of image light guides of the present disclosure. The display system 60 is shown as an HMD with a left-eye optical system 64L with an image light guide 140L for the left eye and a right-eye optical system 64R with an image light guide 140R for the right eye. An image source 52, such as a picoprojector or similar device, is provided and operated to generate separate images for each eye. These images are formed as virtual images with the image orientation required for an upright image display. The images generated can be paired into stereoscopic images for a 3D display. The virtual images formed by the optical system can be viewed as overlaid on the real-world view seen by the viewer through the image light guide. Additional components well known to those skilled in the art of augmented reality visualization can also be provided, such as one or more cameras attached to the frame of the HMD to view the scene content or the viewer's eye tracking. Alternative arrangements are possible, including a display device (e.g., a monocular display) for providing an image to one eye.

[0084] One or more features of the embodiments described herein may be combined to create additional embodiments not shown. Although various embodiments have been described in detail, it should be understood that they are presented by way of example and not limitation. It will be apparent to those skilled in the art that the disclosed subject matter may be embodied in other specific forms, variations, and modifications without departing from the scope, spirit, or essential characteristics thereof. The above-described embodiments should therefore be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced.

Claims

1. a substrate operable to transmit an image-bearing light beam along a longitudinal direction; an incoupling diffractive optical element formed along the substrate and operable to diffract at least a portion of the image-bearing light beam from an image source into the substrate in an angularly encoded manner; an outcoupling diffractive optical element formed along the substrate and operable to diffract a portion of the image-bearing light beam from the substrate in an angularly decoded manner; Equipped with the outcoupling diffractive optical element includes a plurality of unit cells arranged in a two-dimensional periodic lattice, each of which is repeated within the outcoupling diffractive optical element to form a plurality of periodic structures, the plurality of periodic structures operable to diffract at least a portion of the image-bearing light beam in diffraction orders; An image light guide for transmitting image-bearing light, wherein the outcoupling diffractive optical element includes a plurality of vertices, each vertex including an overlapping region of two or more unit cells, and adjacent vertices along a first direction are offset in a second direction, the first direction being a direction of transmission of an image-bearing light beam within the substrate, and the second direction being different from the first direction.

2. 2. An image light guide for transmitting image-bearing light as claimed in claim 1, wherein the incoupling diffractive optical element defines a first grating vector and the outcoupling diffractive optical element defines a second grating vector and a third grating vector.

3. 3. An image light guide for transmitting image-bearing light as described in claim 2, wherein the second lattice vector is disposed at a first angle relative to the first lattice vector and the third lattice vector is disposed at a second angle relative to the first lattice vector.

4. 3. The image light guide for transmitting image-bearing light of claim 2, wherein the first, second and third lattice vectors form a scalene triangle.

5. 2. An image light guide for transmitting image-bearing light as described in claim 1, wherein a central ray of the image-bearing light beam from the image source is disposed at a first angle relative to an x-y plane and at a second angle within the x-y plane relative to the incoupling diffractive optical element.

6. the plurality of periodic structures includes a first plurality of periodic structures and a second plurality of periodic structures; the outcoupling diffractive optical element is operable to diffract a portion of a first portion of each of the image-bearing light beams in a first direction via incidence on the first plurality of periodic structures, thereby expanding the first portion of each of the image-bearing light beams in a first dimension; 2. An image light guide for transmitting image-bearing light as described in claim 1, wherein the outcoupling diffractive optical element is operable to diffract a portion of each of the second portions of the image-bearing light beams in a second direction via incidence on one or more of the second plurality of periodic structures, thereby expanding the second portions of each of the image-bearing light beams in a second dimension.

7. the outcoupling diffractive optical element is operable to diffract a portion of the first portion of each of the image-bearing light beams expanded in the first dimension out of the substrate via incidence on the second plurality of periodic structures; 7. An image light guide for transmitting image-bearing light as described in claim 6, wherein the outcoupling diffractive optical element is operable to diffract a portion of each of the second portions of the image-bearing light beams expanded in the second dimension out of the substrate via incidence on the first plurality of periodic structures.

8. the plurality of periodic structures includes a first plurality of periodic structures and a second plurality of periodic structures; The image light guide for transmitting image-bearing light of claim 1 , wherein the first and second pluralities of periodic structures comprise overlapping linear diffractive features.

9. the incoupling diffractive optical element comprises a plurality of linear diffractive features; the plurality of periodic structures includes a first plurality of periodic structures and a second plurality of periodic structures; the first plurality of periodic structures are oriented at a first angle with respect to the plurality of linear diffractive features of the incoupling diffractive optical element; the second plurality of periodic structures are oriented at a second angle with respect to the plurality of linear diffractive features of the incoupling diffractive optical element; 9. The image light guide for transmitting image-bearing light of claim 8, wherein said first angle is greater than said second angle.

10. The image light guide for transmitting image-bearing light of claim 1 , wherein the incoupling diffractive optical element and the outcoupling diffractive optical element are formed on a first surface of the substrate.

11. 2. The image light guide for transmitting image-bearing light as recited in claim 1, wherein each of said unit cells defines a non-regular hexagon.

12. 2. The image light guide for transmitting image-bearing light of claim 1, wherein the height of each unit cell is a multiple of said offset of said vertices.

13. An image light guide for transmitting image-bearing light as described in claim 1, wherein the plurality of periodic structures comprises a first plurality of periodic structures and a second plurality of periodic structures, and the first and second plurality of periodic structures define a triangular region.

14. 14. An image light guide for transmitting image-bearing light as described in claim 13, wherein the region comprises a scalene triangle, thereby diffracting the image-bearing light beam incident at a non-perpendicular angle from an image source onto the incoupling diffractive optical element.

15. providing a substrate having a first surface and a coating bonded to the first surface; providing a beam writing system operable to write in a first direction and a second direction orthogonal to the first direction; providing a grating layout pattern comprising a plurality of unit cells, each unit cell comprising a first plurality of linear diffractive features and a second plurality of linear diffractive features, the grating layout pattern comprising a plurality of vertices, each vertex comprising an area where two or more unit cells meet, adjacent vertices along the first direction comprising an offset along the second direction; placing the substrate in the beam writing system, whereby the beam writing system is operable to write to the coating; aligning one of the first and second pluralities of linear diffractive features parallel to a first direction of the beam writing system; writing the diffraction grating layout pattern into the coating via the beam writing system; 16. A method of manufacturing an image light guide for transmitting image-bearing light, comprising:

16. 16. The method of manufacturing an image light guide for transmitting image-bearing light of claim 15, wherein the height of each unit cell is a multiple of the offset of the vertices.

17. a substrate operable to transmit an image-bearing light beam along a longitudinal direction; an incoupling diffractive optical element formed along the substrate and operable to diffract a portion of the image-bearing light beam from an image source into the substrate in an angularly encoded manner; an outcoupling diffractive optical element formed along the substrate and disposed at least partially in a plane having a first direction and a second direction, the first direction being a direction of propagation of an image-bearing light beam within the substrate and the second direction being different from the first direction, the outcoupling diffractive optical element operable to diffract a portion of the image-bearing light beam from the substrate in an angularly decoded manner; Equipped with the outcoupling diffractive optical element includes a plurality of successive periodic structures defining a series of sinusoidal waves operable to diffract a portion of the image-bearing light beam in a diffraction order; An image light guide for transmitting image-bearing light, wherein the plurality of consecutive periodic structures includes a plurality of peaks, each adjacent peak along the first direction being offset in the second direction.

18. 20. The image light guide for transmitting image-bearing light of claim 17, wherein a first row of sine waves of the outcoupling diffractive optical element are out of phase with a second row of sine waves.

19. 3. The image light guide for transmitting image-bearing light of claim 2 , wherein the outcoupling diffractive optical element further defines a fourth grating vector, the fourth grating vector being defined by a third plurality of periodic structures.

20. a substrate operable to transmit an image-bearing light beam along a longitudinal direction; an incoupling diffractive optical element formed along the substrate and operable to diffract at least a portion of the image-bearing light beam from an image source into the substrate in an angularly encoded manner; an outcoupling diffractive optical element formed along the substrate and operable to diffract a portion of the image-bearing light beam from the substrate in an angularly decoded manner; Equipped with the outcoupling diffractive optical element includes a plurality of unit cells forming a plurality of periodic structures, the plurality of periodic structures operable to diffract at least a portion of the image-bearing light beam in a diffraction order; the plurality of unit cells includes a first unit cell and a second unit cell adjacent to the first unit cell in a first direction, the second unit cell being offset from the first unit cell in a second direction perpendicular to the first direction, and a height of each unit cell being an integer multiple of the offset; An image light guide for transmitting image-bearing light, said first direction being a direction of transmission of an image-bearing light beam within said substrate.

Images