Astigmatism-based display and aberration compensation

Abstract

A display for providing an image to an observer's eye is disclosed. An image generator provides image illumination corresponding to an image to a substrate having mutually parallel major exterior surfaces for propagation within the substrate. An intermediate optical device including at least one astigmatic optical element directs the image illumination from the image generator for propagation within the substrate in a first in-plane direction by internal reflection. A reflective optical device having cylindrical optical power reflects the image illumination for propagation in a second in-plane direction by internal reflection. A coupling output device couples the collimated image illumination toward the observer's eye. The intermediate optical device includes an element configured to generate corrective optical aberrations that at least partially offset characteristic aberrations introduced by the reflective optical device.

Description

[Technical field]

[0001] The present invention relates to a myopic display system, and more particularly to a myopic display system including a waveguide system for two-dimensional image expansion.

[0002] To be commercially viable, compact waveguide-based optical systems, such as myopic displays, typically require two-dimensional pupil expansion to achieve a wide field of view (FOV) without increasing the size of the image projection optics. Some display systems use waveguide systems for image expansion. In these systems, light corresponding to an image is coupled into a waveguide. The light propagates along the waveguide via total internal reflection (TIR) ​​and is gradually coupled out through one or more light extraction elements, such as a series of partially reflective angular facets or diffractive elements, thereby increasing the expansion of the image in one dimension.

[0003] To expand in two dimensions using a waveguide system, various solutions exist. For example, US Patent Publication No. 2014 / 0036361 A1 describes a waveguide incorporating optical elements integrated into the waveguide plane at the waveguide end. As shown in FIG. 1 (corresponding to FIG. 46 of the above referenced publication), light from a 2D image source 515 is collimated in the plane of the waveguide 503 by optical element 501 and injected into the waveguide. In the waveguide, the light expands towards a reflective waveguide end 500 (usually a cylindrical mirror) with a positive optical power in the transverse direction, where it is collimated after reflection. The reflected light is then extracted from the waveguide via a series of facets 512, thus expanding in the other dimension.

[0004] Another solution is described in US Patent Publication No. 2019 / 0033598 A1. This system employs pupil 2D imaging with a cylindrical mirror at the end of the waveguide to achieve pupil expansion in the plane of the waveguide. As shown in FIG. 2 (corresponding to FIGS. 4(a)-4(c) of US2019 / 033598 A1), a light source 24 is projected by an imaging system 23 onto a first guide element 26. For each FOV point, the imaging system 23 provides a light beam that is collimated in a plane perpendicular to the waveguide surface and converges to a focus in the waveguide plane. Upon reflection from a cylindrical mirror 30 at the end of the waveguide, the beam becomes parallel in all directions. A second waveguide 27 with an extraction zone 31 is used for light extraction (e.g., using a mirror or a diffractive element) and image expansion in the other dimension.

[0005] However, the above existing solutions suffer from various drawbacks, including image aberration problems introduced by cylindrical mirrors and other drawbacks, as will be explained in more detail below. Summary of the Invention

[0006] The present invention is a display for providing an image to an observer's eye. The display includes a substrate ("waveguide") having first and second major external surfaces parallel to each other. The waveguide includes a reflective optical device with cylindrical optical power ("cylindrical mirror") at the waveguide end and one or more additional components configured to compensate for image aberrations caused by the cylindrical mirror. The system disclosed herein aims to achieve high image resolution with a compact size projection optics that could potentially fit into the frame of a conventional goggle. Some of the waveguides disclosed herein include an etendue expander inside the waveguide to increase the numerical aperture of the system. Some of the waveguides include a projection optics module configured to compensate for aberrations caused by the cylindrical mirror.

[0007] Thus, in accordance with the teachings of an embodiment of the present invention, there is provided a display for providing an image to an eye of an observer, the display comprising: (a) an image generator providing image illumination corresponding to the image; (b) a substrate having first and second major outer surfaces parallel to each other; and (c) an intermediate optical arrangement configured to direct the image illumination from the image generator to propagate in a first in-plane direction within the substrate by internal reflection from the first and second major outer surfaces, the intermediate optical arrangement including at least one astigmatic optical element such that the image illumination propagating within the substrate is collimated in a plane perpendicular to the first major outer surface and is uncollimated in a plane parallel to the first major outer surface. (d) a reflective optical device having cylindrical optical power, the reflective optical device being integrated with the substrate for reflecting at least a portion of the image illumination to propagate in a second in-plane direction by internal reflection from the first and second major exterior surfaces, the image illumination after reflection from the reflective optical device being collimated image illumination that is collimated in both planes perpendicular and parallel to the first major exterior surface; and (e) a coupling output device for coupling at least a portion of the collimated image illumination toward an eye of an observer, the reflective optical device having characteristic aberrations, the intermediate optical device generating corrective optical aberrations that at least partially offset the characteristic aberrations of the reflective optical device.

[0008] According to some embodiments, the reflective optical device includes a cylindrical mirror integrated with an edge of the substrate, the cylindrical mirror having a cylindrical axis perpendicular to the first major exterior surface.

[0009] According to some aspects, the intermediate optical device is configured to converge to a plane parallel to the first major exterior surface such that image illumination propagating within the substrate converges toward a focal locus within the substrate and then diverges before reaching the reflecting optical device.

[0010] According to some aspects, the display further includes an etendue optical expander integrated with the substrate at the focal locus. Optionally, the etendue expander can be polarization selective.

[0011] According to some aspects, the etendue optical expander is a one-dimensional etendue optical expander configured to expand image illumination in a plane parallel to the first major outer surface without widening the image illumination in a plane perpendicular to the first major outer surface.

[0012] According to some aspects, the etendue optical expander includes an array of cylindrical microlenses.

[0013] According to some embodiments, the intermediate optical device includes at least one astigmatic optical element having an effective axis of cylindrical optical power, the effective axis of the cylindrical optical power being optically parallel to the cylindrical axis of the reflective optical device.

[0014] According to some aspects, the astigmatic optical element is integrated with the substrate.

[0015] According to some aspects, the astigmatic optical element includes a cylindrical refractive lens integrated with the substrate.

[0016] According to some aspects, the astigmatic optical element includes a cylindrical reflective lens integrated with the substrate.

[0017] According to some aspects, the astigmatic optical element includes a diffractive optical element having cylindrical optical power integrated with the substrate.

[0018] According to some aspects, the reflective optical device includes a diffractive optical element having cylindrical optical power integrated with a substrate.

[0019] According to some aspects, the astigmatic optical element is external to the substrate.

[0020] According to some aspects, the astigmatic optical element includes a refractive lens having a cylindrical optical power.

[0021] According to some aspects, the astigmatic optical element includes a reflective lens having cylindrical optical power.

[0022] According to some aspects, the astigmatic optical element includes a diffractive optical element having cylindrical optical power.

[0023] According to some aspects, the intermediate optical device is configured such that image illumination propagating within the substrate towards the reflecting optical device diverges in a plane parallel to the first major exterior surface.

[0024] According to some embodiments, the image generator includes an elongated spatial light modulator having a width and a length, the length being at least five times the width, and the spatial light modulator is disposed with the length aligned parallel to the first major exterior surface.

[0025] According to some aspects, the intermediate optical device includes a polarizing beam splitter (PBS) prism optically coupled to the substrate, and the spatial light modulator is optically coupled to a face of the PBS prism.

[0026] According to some aspects, the spatial light modulator has an elliptical active pixel area and has a first pixel pitch parallel to the length and a second pixel pitch parallel to the width that is smaller than the first pixel pitch.

[0027] According to some aspects, each physical pixel element of the spatial light modulator is smaller than a first pixel pitch and larger than a second pixel pitch, and a plurality of physical pixel elements are staggered within each first pixel pitch.

[0028] According to some aspects, the reflective optical device is configured to have a first reflectivity associated with a first subset of angles of incidence and a second reflectivity, lower than the first reflectivity, associated with a second subset of angles of incidence, the first subset including angles within a range corresponding to image illumination and the second subset including angles not within the first subset.

[0029] According to some aspects, the combining output device is polarization selective to at least partially combine image illumination of the first polarization while substantially transmitting image illumination of the second polarization, the intermediate optical device is configured to direct image illumination from the image generator to propagate within the substrate in a direction in the first plane with the second polarization, and a quarter-wave plate is associated with the reflecting optical device such that collimated image illumination propagating in the second in-plane direction has the first polarization and is combined out by the combining output device.

[0030] In accordance with other teachings of the present invention, a display for providing an image to an eye of an observer is provided, the display including: (a) an image generator providing image illumination corresponding to the image; (b) a substrate having first and second outer major surfaces parallel to each other; (c) an intermediate optical arrangement configured to direct the image illumination from the image generator to propagate in a first in-plane direction within the substrate by internal reflection from the first and second outer major surfaces, the intermediate optical arrangement including at least one astigmatic optical element such that the image illumination propagating within the substrate is collimated in a plane perpendicular to the first outer major surface and is uncollimated in a plane parallel to the first outer major surface; and (d) a reflective optical arrangement having cylindrical optical power, the intermediate optical arrangement configured to direct the image illumination from the image generator to propagate in a second in-plane direction by internal reflection from the first and second outer major surfaces. (e) a coupling output device for coupling at least a portion of the collimated image illumination toward an eye of an observer, the coupling output device having a polarization selectivity that at least partially couples out the image illumination of a first polarization while substantially transmitting the image illumination of a second polarization, the intermediate optical device configured to couple the image illumination from the image generator in the second polarization through the substrate in a first in-plane direction, and a quarter-wave plate is associated with the reflective optical device such that the collimated image illumination propagating in the second in-plane direction has the first polarization to be coupled out by the coupling output device. [Brief description of the drawings]

[0031] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1] 1 shows an embodiment of a known display. [Diagram 2] 1 shows an embodiment of a known display. [Figure 3A] 1 shows a schematic diagram illustrating the working principle of a known system; [Figure 3B] 1 shows a schematic diagram illustrating the working principle of a known system; [Figure 3C] 1 shows a schematic diagram illustrating the working principle of a known system; [Figure 4A] 1 shows a schematic diagram of certain image aberrations present in known systems; [Figure 4B] 1 shows a schematic diagram of certain image aberrations present in known systems; [Figure 5A] 2 shows a schematic diagram of another type of image aberration present in known systems; [Figure 5B] 2 shows a schematic diagram of another type of image aberration present in known systems; [Figure 6A] 2 shows a schematic diagram of another type of image aberration present in known systems; [Figure 6B] 2 shows a schematic diagram of another type of image aberration present in known systems; [Figure 6C] 2 shows a schematic diagram of another type of image aberration present in known systems; [Figure 7A] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 7B] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 7C] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 8] 1 shows a schematic diagram of a solution to the problem of image aberrations. [Figure 9A] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 9B] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 9C] A schematic diagram of light reflected by a cylindrical mirror is shown. [Figure 10A] 1 shows a schematic diagram of a system with two mirrors. [Figure 10B] 1 shows a schematic diagram of a system with two mirrors. [Figure 10C] 1 shows a schematic diagram of a system with two mirrors. [Figure 11A] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system. [Figure 11B] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system. [Figure 11C] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system. [Figure 12A] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a polarization control element. [Figure 12B] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a polarization control element. [Figure 13A] 1 illustrates ghost ray blocking according to one embodiment of a waveguide system. [Figure 13B] 1 illustrates ghost ray blocking according to one embodiment of a waveguide system. [Figure 14A] 1 shows a schematic diagram of one embodiment of a waveguide system including a mixer. [Figure 14B] 1 shows a schematic diagram of one embodiment of a waveguide system including a mixer. [Figure 15A] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a beam splitter mirror. [Figure 15B] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a beam splitter mirror. [Figure 15C] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a beam splitter mirror. [Figure 16] 15A-15C depict a variation of the system shown in FIGS. 15A-15C that includes a polarization control element. [Figure 17] 15A-15C depict a further variation of the system shown in FIGS. 15A-15C that includes a refractive cylindrical element. [Figure 18]1 shows a schematic diagram of one embodiment of a waveguide system including a diffractive element. [Figure 19] FIG. 2 shows a schematic diagram of an embodiment of a waveguide system including a second waveguide. [Figure 20A] FIG. 1 shows a schematic diagram of an embodiment of a waveguide system including an etendue expander. [Figure 20B] FIG. 1 shows a schematic diagram of an embodiment of a waveguide system including an etendue expander. [Figure 20C] FIG. 1 shows a schematic diagram of an embodiment of a waveguide system including an etendue expander. [Figure 21A] Shown is a converging light beam falling on a microlens array with a small numerical aperture. [Figure 22A] 1 illustrates various embodiments of a polarization selective etendue expander. [Figure 22B] 1 illustrates various embodiments of a polarization selective etendue expander. [Figure 23A] 1 shows a schematic diagram of an embodiment of a waveguide system having a focusing element within a POD. [Figure 23B] 1 shows a schematic diagram of an embodiment of a waveguide system having a focusing element within a POD. [Figure 23C] 1 shows a schematic diagram of an embodiment of a waveguide system having a focusing element within a POD. [Figure 24A] 1 illustrates the location of stops according to various embodiments. [Figure 24B] 1 illustrates the location of stops according to various embodiments. [Figure 25A] 1 shows a schematic diagram of an embodiment of a waveguide system including a biconic mirror in a POD. [Figure 25B] 1 shows a schematic diagram of an embodiment of a waveguide system including a biconic mirror in a POD. [Figure 26A] 25A-25B show schematic diagrams of one embodiment of a system including an etendue expander. [Figure 26B] 25A-25B show schematic diagrams of one embodiment of a system including an etendue expander. [Figure 27]1 shows a schematic diagram of an embodiment of a waveguide system with a large POD aperture. [Figure 28] 1 shows a schematic diagram of an embodiment of a waveguide system including a cylindrical optical surface within a POD. [Figure 29A] 29 illustrates a mixer and cover plate according to one embodiment of the system shown in FIG. 28. [Figure 29B] 29 illustrates a mixer and cover plate according to one embodiment of the system shown in FIG. 28. [Figure 30A] 1 illustrates an alternative arrangement of components within a POD. [Figure 30B] 1 illustrates an alternative arrangement of components within a POD. [Diagram 31] FIG. 29 shows a schematic diagram of an embodiment of the system of FIG. 28 including an additional cylindrical lens. [Diagram 32] Indicates the existing pixel layout of the image source. [Diagram 33] 4 shows the new pixel layout of the image source. [Diagram 34] 1 shows a series of graphs illustrating the qualitative behavior of image aberrations as the angle of incidence increases. [Figure 35A] 1 shows a series of ray diagrams illustrating a diverging beam passing through a planar refractive interface. [Figure 35B] 1 shows a series of ray diagrams illustrating a diverging beam passing through a planar refractive interface. [Figure 36A] 1 shows a series of ray diagrams illustrating a diverging beam passing through a wedge prism. [Figure 36B] 1 shows a series of ray diagrams illustrating a diverging beam passing through a wedge prism. [Figure 37A] 1 shows a graph illustrating the maximum y-aberration introduced by a wedge prism relative to a cylindrical mirror. [Figure 37B] 1 shows a series of graphs illustrating maximum aberration for different FOVs. [Figure 38A] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a wedge prism. [Figure 38B] FIG. 1 shows a schematic diagram of one embodiment of a waveguide system including a wedge prism. [Figure 39A]1 shows a schematic diagram of an embodiment of a waveguide system including additional optical elements in the POD. [Figure 39B] 1 shows a schematic diagram of one embodiment of a waveguide system including a refractive component in a POD. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The term "display" is used herein to describe any type of display, including but not limited to head-up displays and near-sight displays, whether for virtual or augmented reality applications. "Display" is used interchangeably with "waveguide system" herein.

[0033] The term "image generator" is used herein to refer to any element or device used to generate an image, including, but not limited to, spatial light modulators (SLMs) such as LCOS (liquid crystal on silicon) chips, micro LED displays, and various scanning arrangements in which a scanning beam is modulated in synchronism with its scanning motion to generate an image.

[0034] The term "substrate" is used interchangeably herein with "waveguide" to refer to any light transmitting structure having mutually parallel first and second major external surfaces along which light propagates by internal reflection at the major surfaces. Light is said to propagate in an "in-plane direction", which is the direction that light propagates when viewed perpendicular to the waveguide, i.e., ignoring internal reflections above and below the major surfaces. The phrase "in-plane" is used to refer to a direction parallel to the major external surfaces of the substrate.

[0035] The phrase "having cylindrical optical power" is used in the description herein and is claimed to refer to any optical element or arrangement that has unequal optical power in two axes, such elements are also referred to as "astigmatism". In such cases, the axis with the higher optical power is referred to as the "effective cylindrical axis". An element or arrangement is referred to as having "only cylindrical optical power" if it has optical power along one axis with no (or negligible) optical power along the perpendicular axis. Such elements are also referred to as "cylindrical lenses", although the shape of the lens does not necessarily have to be a portion of a geometric cylinder, and instead may use a surface formed by sweeping a circle, a conic section, or a more complex curve along a direction perpendicular to the waveguide surface.

[0036] The phrase "optically parallel" is used to refer to axes of an optical system that are physically parallel or optically equivalent to parallel axes. In certain implementations, for reasons of compactness or other design considerations, the optical path may be folded by using one or more reflective elements. In such cases, "optically parallel" refers to axes that would be parallel if the optical path were unfolded.

[0037] All "lenses" referred to herein, unless otherwise specified, may be refractive lenses, reflective lenses, diffractive optical elements having optical power (such as those based on Bragg gratings), or any combination of the above. The singular term "lens" should be understood to include multiple lenses arranged in a lens group.

[0038] Various embodiments of the present invention refer to image illumination that is collimated in a plane perpendicular to the (first major external surface of) the substrate and not collimated in a plane parallel to the substrate. If the in-plane image illumination is converging, one can refer to a "focal locus" where the in-plane rays focus. This is sometimes referred to as a "focal plane" or "focal plane". However, it does not have to be a plane and is not a "focal plane" in the conventional sense, since the illumination is not focused in a perpendicular plane so that no actual image is produced. However, the focal locus indicates the location (surface or plane) where the in-plane rays of a particular pixel converge.

[0039] The phrases "étendue optical expander" and "étendue expander" are used herein to refer to any structure that increases the etendue (spatial spread) of a beam. Particularly preferred examples of the invention use orientation-specific expanders that have minimal effect on the pattern of image illumination in the plane perpendicular to the substrate.

[0040] As used herein, the term "intermediate optical device" refers to an astigmatic optical element configured to generate corrective optical aberrations, or any combination of optical elements including at least one astigmatic optical element and collectively configured to generate corrective optical aberrations, optionally combined with additional optical properties required for a particular implementation. It is described as "intermediate" in the sense of being located in the optical path between the image generator and the reflective optical device, but may be anywhere in that optical path, whether integrated with the substrate or as part of the image projector as exemplified below.

[0041] As explained above in the Technical Field section, existing solutions for two-dimensional image enhancement using waveguide systems suffer from various types of image aberrations, leading to poor quality images. These aberrations are explained in more detail with reference to Figures 3A-7C, which show various problems associated with prior art systems.

[0042] 3A-3C show a schematic of the general operation of a waveguide such as US2019 / 033598 A1. As shown in FIG. 3A, light is focused in the plane of the waveguide 700 by a lens 702 integrated into the projection optics 704. A cylindrical mirror 706 collimates the light and completes the pupil imaging in an eye motion box (EMB) (not shown). The image is extracted using an optical light extraction element such as a facet (or other optical element) located in the light extraction region 708 of the waveguide. As shown in FIG. 3B, the pupil image magnification and FOV are defined by the focal lengths f1 and f2 of the lenses 702 and 706, respectively. Note that in this system, the lens 702 is oriented at an angle to the plane of the waveguide, as shown in FIG. 3C, which shows a side view of the system.

[0043] Problem #1: Aberrations caused by optical path differences in the FOV due to reflections from cylindrical mirrors With reference to Figures 4A-4B, which show schematic diagrams of the YZ and ZX planes of the light paths in the waveguide system, consider two light rays 800, 802, each associated with a different respective point in the FOV. Due to the difference in the respective incidence angles a1 and a2 of the light rays, the paths of the light rays in the waveguide plane are different. As shown in Figure 4B, the light rays 800 and 802 have the same convergence before being injected into the waveguide, but inside the waveguide, the light rays converge to different foci. As a result, the beam in FOV1 can be collimated after reflection from the cylindrical mirror, while the beam in FOV2 diverges after reflection from the cylindrical mirror, resulting in aberrations in the perceived image.

[0044] Problem #2: Aberrations caused by optical path differences introduced by the injection of light into the waveguide. With reference to Figures 5A-5B, which show a schematic diagram of the light paths in a waveguide system, consider a light beam that is injected into a waveguide 900 using a wedge prism 902. The beam is collimated in the YZ plane and converges in a direction orthogonal to the YZ plane. As shown in Figure 5A, the light beam will have an optical path difference 904 after being injected into the waveguide. The optical path difference 904 results in image aberrations, as shown in Figure 5B, since light rays injected through different points along the wedge prism will converge to different focal points in the waveguide plane.

[0045] Problem #3: Aberrations caused by non-parallel light rays after reflection from a cylindrical mirror Other image aberrations are caused by the cylindrical mirror at the end of the waveguide, where light rays that were parallel in the direction perpendicular to the waveguide plane become non-parallel after reflection from the cylindrical mirror.

[0046] By way of background, Figures 6A-6C show a schematic diagram of light reflecting off a cylindrical mirror 1001. A light beam collimated in the YZ plane and converging in an orthogonal direction is directed to the cylindrical mirror 1001 oriented parallel to the y-axis. Note that the XZ plane is orthogonal to the surface of the mirror. After reflection from the mirror, the beam is no longer parallel in the YZ plane. This is evident from Figures 6B and 6C, which show a fan of rays in the focal plane of an ideal lens placed after the mirror. The fan of rays exhibits a shape typical of coma aberration. It should be noted that this type of aberration cannot be corrected by optimizing the shape of the cylindrical mirror, such as by giving it a conical or more complex shape.

[0047] The aberrations of a cylindrical mirror can be further understood with reference to Figures 7A-7C, which show schematic diagrams of light rays reflecting off a cylindrical mirror. As shown in Figures 7A-7B, parallel light rays R1 and R2 are incident on the cylindrical mirror at an angle α with respect to the plane to which the mirror surface is orthogonal (i.e., the XZ plane). At point O where R1 intersects with the mirror surface, the mirror surface is perpendicular to the z-axis. As a result, the angle of reflection of light ray R1 is the same as its angle of incidence. Light ray R2 rotates by an angle θ around the y-axis after reflection. As a result, after reflection, R1 and R2 are not parallel in the YZ plane, as shown in Figure 7C.

[0048] In addition to the problems mentioned above, some prior art systems, including US2014 / 0036361 and / or US2019 / 0033598, suffer from various other shortcomings. For example, some of these systems require a relatively small EMB in the z-direction. Increasing the EMB requires decreasing the F# ("f-number") of the projection optics, which complicates the design of the projection optics.

[0049] Another drawback of the arrangement described in US2019 / 0033598 A1 is that coupling of light between the two waveguides can cause the apertures of the waveguides to be incompletely filled, resulting in dark stripes in the observed image, a phenomenon which is explained in more detail below with reference to Figures 7-11.

[0050] The waveguide-based near-vision display systems described herein (also referred to herein as "waveguide systems") improve upon existing systems by providing enhanced image quality (e.g., higher achievable resolution, absence of black lines, etc.) while maintaining a compact design. In some embodiments, an etendue expander is used to achieve an even more compact size and simpler projection optics design while allowing for a larger EMB size, as described in more detail below. In some embodiments, additional components are used to manage the polarization state of the light within the waveguide, such that 2D imaging is achieved with a single waveguide.

[0051] General embodiment of the proposed waveguide system As an introduction to the concepts detailed herein, FIG. 8 shows a schematic diagram of a solution to the problem posed above. In general, the inventors have found it beneficial to introduce an astigmatic focusing element 4 ("Lens 4") into the waveguide 2 at the end of the waveguide opposite a cylindrical mirror ("Lens 5"). Lens 4 can be implemented as a reflective or refractive cylindrical optical element. As shown in FIG. 8, Lens 4 bends the light rays (shown as arrows) exiting the projection optics unit 1 ("POD1") in the plane of the waveguide. The bending of the light rays is the same for all fields, so that after reflection from Lens 5, no light path difference occurs in the waveguide plane. The waveguide 2 further includes a light extraction region 3 with one or more optical elements (e.g., shown here as facets) for extracting light from the waveguide towards the EMB (not shown).

[0052] The integrated lens 4 also solves the second problem identified above with reference to Figures 5A-5B by allowing the light rays to converge to the same focal point, since focusing of the light rays at the waveguide plane is achieved after the light rays are coupled inside the waveguide.

[0053] Furthermore, lens 4 solves the third problem identified above with reference to Figures 7A-7C because lens 4 acts as a second mirror element in the waveguide, thereby introducing image aberrations similar to but opposite in sign to those caused by lens 5, thereby correcting the aberrations caused by lens 5.

[0054] To further demonstrate this principle, consider a collimated beam propagating to a cylindrical mirror as shown in Figures 9A-9B. The aberration of the beam after reflection from the mirror is shown in Figure 9C. This aberration is similar to the aberration shown in Figure 6C above, but with the opposite sign. As shown in Figures 10A-10B, in a system with two mirrors, it is possible to achieve a collimated beam after reflection from the second mirror. As a result, such a system of two cylindrical mirrors in a waveguide can transform an input collimated light beam propagating in the waveguide into a different collimated light beam. As Figure 10C shows, the resulting aberration remaining after reflection from both mirrors is very small and can be neglected in most cases. It should be noted that at least one of the two cylindrical mirrors can be replaced by a cylindrical lens, which will mutually compensate the aberrations.

[0055] Generally speaking, if a 2D imaging system in a waveguide transforms a collimated beam propagating in the waveguide into another beam collimated in the plane of the waveguide, this new beam will also be collimated in a plane orthogonal to the plane of the waveguide. Thus, such a 2D imaging system will correct the aberrations orthogonal to the plane of the waveguide caused by cylindrical optics.

[0056] Thus, the presently disclosed subject matter improves upon the prior art in several respects. i) The image completely fills the waveguide aperture in the direction perpendicular to the waveguide plane, thereby improving the overall image in general, and in particular preventing dark lines from being seen in the observed image. ii) The aberrations caused by the cylindrical mirror at the end of the waveguide are compensated, thereby enabling higher resolution images of sufficient quality. iii) As described in more detail below, embodiments include systems with enhanced pixel layouts at the image source that enable achieving higher image resolution and smaller image source and projection optics dimensions compared to the prior art. iv) The intensity of ghost images caused by reflection of external objects by the cylindrical mirror is reduced, as explained below.

[0057] Having explained the general concept, Figures 11A-11C show schematic diagrams of embodiments of a waveguide system in more detail. Referring now to Figure 11A, the illustrated waveguide system includes an image generator contained within a projection optics module 1 (hereinafter "POD1"). The image generator provides image illumination corresponding to an image. POD1 is optically coupled to a waveguide 2 and provides image illumination corresponding to an image for injection into the waveguide.

[0058] The waveguide 2 further includes an intermediate optical device configured to direct image illumination from the image generator to propagate within the waveguide in a first in-plane direction by internal reflection from a major external surface of the waveguide. The intermediate optical device provides collimated image illumination in a plane perpendicular to the major external surface of the waveguide and non-collimated image illumination in a plane parallel to the major external surface. In Figures 11A-11C, the major parallel external surfaces of the waveguide are parallel to the plane of the paper. The intermediate optical device is implemented in Figures 11A-11B as an astigmatic optical element 4 (referred to herein as "lens 4").

[0059] The waveguide 2 further includes an integrated reflective optics device 5 having a cylindrical optical power (hereinafter referred to as "lens 5") for reflecting at least a portion of the image illumination to propagate in an in-plane direction by internal reflection from a major external surface. The image illumination after reflection from lens 5 is collimated image illumination that is collimated in both a plane parallel to the plane perpendicular to the major external surface of the waveguide.

[0060] Lens 5 introduces characteristic aberrations and lens 4 introduces corrective aberrations to at least partially offset the characteristic aberrations produced by lens 5 .

[0061] In some embodiments, lens 5 can be integrated with the edge of the waveguide and can have a cylindrical axis perpendicular to the major external surface of the waveguide, as shown in Figures 11A-11C. In some embodiments, lens 5 can be or include a diffractive optical element with cylindrical power integrated with the waveguide.

[0062] In some embodiments, the lens 5 may have a high reflectivity in a range of angles corresponding to the image illumination 1, and a low reflectivity (either transmissive or absorptive) at angles outside this range. In this way, the lens 5 reflects only the light emitted by the POD that propagates in the waveguide by internal reflection. Light from external objects (e.g., the sun) hits the lens 5 at angles in the range of low reflectivity. Thus, the intensity of any possible ghost images caused by external objects is reduced. Such reflectivity depending on the angle of incidence can be implemented, for example, using multi-layer coating techniques. Another potentially suitable technique is volume Bragg gratings, which have high diffraction efficiency only in a relatively narrow range of angles.

[0063] 11A-11C, lens 4 is configured to converge to a plane parallel to the major exterior surface of the waveguide such that image illumination propagating within the waveguide converges toward a focal locus within the waveguide before diverging before reaching lens 5. In some embodiments, lens 4 has an effective axis of cylindrical optical power that is optically parallel to the cylindrical axis of lens 5.

[0064] The waveguide 2 further includes a coupling-out device 3 for coupling at least a portion of the collimated image illumination towards the eye of an observer. The coupling-out device is shown herein, by way of example, as a set of mutually parallel partially reflective facets that are non-parallel (i.e., angled) with respect to the main parallel exterior surfaces of the waveguide. For simplicity, the coupling-out device is alternatively referred to herein as a light extraction region 3 or facet 3, although other types of coupling-out devices (e.g., diffractive elements) are possible and within the scope of the present disclosure.

[0065] 11B shows an alternative embodiment in which lens 4 is integrated into POD 1. In this case, to achieve good image quality, the plane of orthogonal optical surfaces of lens 4 must be parallel to the plane of orthogonal optical surfaces of lens 5 (taking into account the possible use of folding mirrors in the POD).

[0066] FIG. 11C shows another alternative embodiment including a specially configured POD1 including an elongated SLM that provides a beam diverging in the waveguide plane from each pixel of the display, as will be described in more detail below with reference to FIGS. 28-33. After reflection from lens 5, the beam is collimated. However, the beam provided by POD1 in this embodiment is not collimated in the plane orthogonal to the waveguide (as in the prior art). The beam is shaped to have aberrations that are equal in magnitude but opposite in sign to those of lens 5. In this way, the beam reflected from lens 5 is collimated in both the waveguide plane and the orthogonal plane.

[0067] It should be noted that waveguide 2 may be a single waveguide, or a system of two or more optically coupled waveguides.

[0068] Having described the general embodiments and operating principles of the disclosed waveguide system, a number of alternative configurations will now be described in detail.

[0069] Variation #1: Waveguide system with polarization control elements A polarized output of the projection optics can offer several advantages, such as the elimination of some types of ghosting and higher system efficiency. In this case, polarization control elements such as polarizers and quarter wave plates can be integrated into the system to provide polarization management.

[0070] 12A-12B show schematic diagrams of the top and side views, respectively, of another embodiment of a waveguide system. In this embodiment, the waveguide 2 includes an integrated quarter-wave plate 6 and polarizer 7. The quarter-wave plate 6 can be implemented, for example, using a transparent polymer film attached to the surface of the lens 5.

[0071] Assume that the light at the output of POD1 is p-polarized relative to the plane of the facets. In this case, polarizer 7 should be configured to transmit p-polarized light, while the light extraction element (e.g., facet 3) should be configured to reflect only s-polarized light. The light passes through waveplate 6, reflects off lens 5, passes through waveplate 6 a second time, and becomes s-polarized. Facet 3, optimized for s-polarization reflectivity, reflects the light out of the waveguide toward the EMB (not shown). The reflective properties of facet 3, or other light extraction elements, can be optimized for a particular polarization state of the light using techniques well known in the art, such as polarization splitting coatings.

[0072] In some embodiments, the quarter-wave plate can be optimized to provide an equal phase shift to rays within a given range of incidence angles α1-α2, 0<α1<α2, which corresponds to the range of incidence angles α1-α2 of the rays in the waveguide.

[0073] In some embodiments, as shown in Figure 12B, a polarizer 8 can be bonded to the main external surface of the waveguide. In this case, the polarizer should preferably completely cover at least the light extraction region of the waveguide (represented as facet 3 in Figure 12B). The polarizer blocks light of the same polarization state as the light output from POD 1. Thus, if the POD output is p-polarized, polarizer 8 should be configured to block p-polarized light and transmit s-polarized light.

[0074] Note that a polarizer 7 (transmitting light of the same polarization as that emitted by the POD) should preferably be placed between the light extraction region of the waveguide (in this case facet 3) and lens 4. Such a polarizer can be effective in blocking image "ghosts" created by multiple reflections between lens 5 and lens 4.

[0075] 13A-13B, an embodiment including a polarizer 8 blocks ghost rays 10 generated by forward-propagating p-polarized light rays 9 while still allowing backward-propagating s-polarized light rays 12 to be extracted towards an EMB (not shown).

[0076] The polarizers described herein with reference to Figures 12A-13B can also be used in the other waveguide systems described herein.

[0077] Variation #2: Waveguide system including mixers In some embodiments, it may be desirable to reduce the aperture of the POD in a direction perpendicular to the waveguide plane, for example, to reduce optical aberrations and / or to make the POD module more compact. However, such reduction in aperture can result in dark lines in the observed image because the waveguide aperture is not completely filled.

[0078] 14A-14B show schematic diagrams of the top and side views, respectively, of another embodiment of a waveguide system. The waveguide 2 includes an optical mixer 14, in this case implementing a semi-reflective surface parallel to the waveguide's major external surface. The mixer splits the light beam propagating within the waveguide and directs the split beams into the waveguide aperture area that is not filled with POD.

[0079] The mixer 14 is preferably implemented as a non-absorbing surface that has approximately 50% reflectivity for light propagating inside the waveguide and relatively low reflectivity for light entering the waveguide from outside the system, so that the transparency of the display is not reduced. The mixer is preferably located between the lens 4 and the light extraction region 3.

[0080] It should be noted that the use of the mixer is not limited to the particular system described herein, and indeed may be incorporated into other embodiments described in this document.

[0081] Variation #3: Double Mirror System 15A-15C show schematic diagrams of a top view, a side view, and a top perspective view, respectively, of another embodiment of a waveguide system. In this embodiment, the waveguide 2 includes a beam splitter mirror 17. The beam splitter mirror 17 can be a polarizing beam splitter (PBS), which can optimize the light coupling efficiency when the output of POD1 is polarized. A light ray 15 is emitted by an image source 16 and propagates through POD1, which outputs a collimated light ray for a given point in the FOV. The light ray 15 is coupled into the waveguide by the beam splitter mirror 17. The light ray 15 propagates through the waveguide towards lens 4 (now acting as a focusing element). After reflecting off lens 4, the light ray propagates through facet 3 towards lens 5 (acting as a collimating mirror). Upon reflecting off lens 5, the light ray propagates in the opposite direction through the waveguide. Facet 3 emits a portion of the energy carried by ray 15 towards the EMB 18 .

[0082] FIG. 16 shows an alternative configuration of a waveguide 2 including a PBS 17 in combination with quarter wave plates 6, 9 and a polarizer 7, as discussed above with reference to FIGS. 12A-14B.

[0083] FIG. 17 shows a further variation of the general system of FIG. 15, including one or more refractive cylindrical elements 20. The refractive cylindrical elements ("lenses") 20 can be used to correct the aberrations of the double mirror system in the waveguide plane. The lenses 20 are preferably made of a material with a different refractive index compared to the refractive index of the waveguide substrate and can have a convex or concave shape. The lenses 20 can also be a combination of two or more lenses made of different materials. In a similar manner, the lenses 4 can also be used in combination with one or more refractive elements.

[0084] Lenses 4 and 5, as well as any of the refractive elements, may have a cylindrical, swept conical (a shape obtained by sweeping a 2D conic section along a direction perpendicular to the waveguide plane), or freeform 2D shape to achieve better image quality in the system. Fresnel lenses may also be used in combination with any of the above elements (i.e., lenses 4, 5, or 20).

[0085] Variation #4: Diffractive Optical Elements 18 shows a schematic diagram of another embodiment of a waveguide system. In this embodiment, 2D imaging in the plane of the waveguide is implemented using diffractive optical elements 21 and 22. In this case, the function of the diffractive elements is similar to that of optical elements such as lenses (including mirrors). In some cases, more than two diffractive elements can be used, depending on the particular design of the imaging system.

[0086] It will be appreciated that diffractive elements such as 21 or 22 shown cannot be infinitesimally small in the z-direction and therefore introduce aberrations in the plane of the waveguide. To compensate for such aberrations, the optical power of the diffractive element must preferably be non-uniform along the z-axis. Such a diffractive element can be implemented, for example, as a surface relief grating with curved grooves in the surface of the waveguide. Since the waveguide in principle does not need to be transparent at the ends, the grating can be covered with a reflective coating and the grating pitch can be made such that only the zeroth and first diffracted orders are present, thus maximizing the efficiency of the first order.

[0087] Alternatively, Bragg gratings can be used as diffractive elements 21 and 22 to obtain high diffraction efficiency. Because Bragg gratings exhibit high angular selectivity, two or more Bragg gratings can be used to implement each of elements 21 and 22. Alternatively, polarization Bragg gratings can be implemented as elements 21 and 22, such that only selected polarizations can be used for imaging.

[0088] Variation #5: Expanding the projection optics aperture with an additional waveguide Figure 19 shows a schematic diagram of another embodiment of a waveguide system. In this embodiment, the POD 1 can be combined with a waveguide 33 of the type disclosed in US Patent No. 7,643,214. As shown in Figure 19, this arrangement can be used to expand the optical aperture in the dimension parallel to the plane of the waveguide 2. This arrangement may be useful to increase the size of the EMB (not shown) in the x dimension.

[0089] Transformation #6: Etendue Expander 20A-20C show schematic diagrams of another embodiment of a waveguide system. In this embodiment, the waveguide system includes an optical etendue expander. The etendue expander is preferably integrated with the waveguide at the focal locus of the waveguide. The etendue expander is preferably configured to expand the image in a plane parallel to the major external surface of the waveguide without widening the image in a plane perpendicular to the major external surface.

[0090] 11A-11B, note that the width of the beam D2 at lens 5 must be greater than or equal to the width of the EMB. The relationship between D2 and the POD aperture width D1 is given by Equation 1.

number

[0091] At the same time, the relationship between the FOV of the system (FOV) and the FOV of the projection optical system (FOVo) is given by Equation 2.

number

[0092] The width of the EMB and FOV are fixed parameters. Therefore, D2 is the width of the EMB ("W EMB "), the condition that must be satisfied for the FOV and aperture width D1 of the projection optical system is expressed by Equation 3. D1 ×FOVo=W EMB ×FOVs (3)

[0093] Therefore, the optical aperture width can be smaller than the width of the EMB only at the expense of increasing the projection optics FOV, which limits the miniaturization possibilities of the projection optics and also makes the optical system design difficult, since a large aperture means a low F# of the projection optics.

[0094] Having described the problem, we propose the following solution. As seen in Figures 11A-11B, lens 4 forms an intermediate image in the waveguide along imaginary curve 25. This is further illustrated in Figure 20A, which shows light 34 focused to a point in the waveguide in the plane of the waveguide. Beams corresponding to different FOVs are focused to points along imaginary curve 25. Assume that an optical element 36 (herein referred to as an "étendue expander") that transforms the light in one dimension parallel to the waveguide plane is placed at the location of imaginary curve 25 as shown in Figure 20B. The etendue expander 36 can be a one-dimensional diffuser, a diffraction grating, or an array of cylindrical microlenses. The etendue expander 36 increases the angle of the cone of light rays that strike the etendue expander in the waveguide plane. In other words, the etendue expander increases the numerical aperture (NA) of the beam or decreases the F# ("f-number") of the system in the waveguide plane. This optical function is accomplished either by scattering in the case of a diffuser, diffraction in the case of a grating, or optical power in the case of a microlens array (MLA), as described below with reference to Figures 21A-21B.

[0095] FIG. 21A shows a converging beam with a small numerical aperture (NA) hitting the MLA from the left. The NA of the beam that passes through the MLA and propagates further to the right becomes significantly larger compared to the NA of the beam to the left of the MLA. FIG. 21B shows a close-up of the beam passing through one of the microlenses of the MLA. As can be seen, to increase the NA of the beam, the MLA needs to be positioned slightly out of focus of the converging beam. If such an MLA is used as an etendue expander 36, each microlens becomes a pixel in the image projected to the user's eye. Therefore, the microlenses are preferably small enough so that they are not resolved in the resulting image.

[0096] For simplicity, we assume that a diffraction grating is used as the etendue expander. In this case, each ray in the beam 34 in Fig. 20A generates several diffracted rays. All rays in the original beam 34 and the respective diffracted rays form a beam 37 with a larger angular divergence. Thus, the width D3 of the beam after reflection from the lens 5 can be significantly larger compared to the width D2 of a system without a diffuser, a diffraction grating, or an MLA. It is important that the angular distribution of energy in the diffracted beam 37 is uniform, as shown in Fig. 20C. A non-uniform energy distribution can result in a non-uniform image observed by the system user and variations in image brightness across the EMB. Furthermore, the divergence of the beam should not be too large to avoid the generation of stray light.

[0097] Note that the described approach of etendue extension does not sacrifice the FOV of the system, potentially allowing the aperture of the POD to be as small as allowed by the diffraction limit.

[0098] It is further noted that the etendue expander 36, if implemented using a diffuser or grating, may in some cases be visible to a viewer of the display system, in which case it may be desirable to use a polarization sensitive grating or a diffuser that can be made invisible to the viewer.

[0099] For example, assume that the system shown in Figures 20A-20C includes a polarization control element similar to that shown in Figures 12A-13B. Further assume that the POD output is p-polarized relative to the plane of the facets, and the polarizer 8 (Figures 12A-13B) transmits s-polarized light. Thus, if the diffuser or grating etendue expander 36 only diffuses or diffracts p-polarized light, the viewer would not be able to see the diffuser or grating. Such a polarization sensitive grating (or diffuser) can be made, for example, using a polarizing Bragg grating (or a combination thereof) or the structure shown in Figure 22A and consisting of two matching parts 28 and 29. Part 29 is an optically transparent homogeneous material, and part 28 is made of a uniaxially birefringent material, e.g., liquid crystal. The refractive indices of parts 28 and 29 are matched for s-polarized light and different (relative to the plane of the facets) for p-polarized light. Implemented in this way, the grating diffracts p-polarized light and does not affect s-polarized light. The same mechanism of polarization sensitivity can also be applied to polarization sensitive diffusers. Alternatively, an array of cylindrical microlenses can be used to refract p-polarized light while leaving s-polarized light unaffected, as shown in Figure 22B.

[0100] Variant #7: A POD with an integrated focusing element with an optional biconic mirror. 23A-31 show schematic diagrams of another embodiment of the waveguide system. In this embodiment, the lens 4 is integrated into the POD so that the beam injected into the waveguide is already focused at the waveguide plane, which may reduce the manufacturing cost of the waveguide.

[0101] For context, Figure 23A shows an isometric view of the waveguide with lenses 4 and 5 integrated into the waveguide (similar to Figure 15C). Figures 23B and 23C show the unfolded optical paths in the system for four different FOVs. Figure 23B shows a stop aperture 40 ("stop"), which may coincide with the output aperture of the POD or with a virtual or real plane in the waveguide or the plane of the optical input wedge. Also, the stops in the x and y directions do not necessarily coincide.

[0102] FIG. 24A shows a system in which stop 40 is placed between lenses 4 and 5. The part of the system including stop and lens 5 can be folded and integrated into the waveguide so that the plane of stop 40 is located at the end of the waveguide. The part of the system including lens 4 can be integrated into POD 1 so that the exit aperture of the POD coincides with stop 40. The optical path can be further folded within the POD. However, because of this folding, the angle of incidence of the light ray on lens 4 should not change. Otherwise, aberrations caused by the cylindrical optical power of lens 5 will occur, as shown in FIGS. 6, 7, and 11.

[0103] FIG. 24B shows an alternative implementation of the system shown in FIG. 24A, in which the POD 1 includes an optical module 41 that outputs a collimated beam towards a lens 4.

[0104] Note that stop 40 acts as a stop along the y-axis because it coincides with the entrance to the waveguide. The aperture that limits the light beam in the direction of the x-axis can be placed at stop 40 or elsewhere.

[0105] In some embodiments, the POD may include additional optical elements to minimize optical aberrations, such as cylindrical, biconic, spherical, aspheric or non-spheric or freeform lenses or mirrors made of the same or different optical materials.

[0106] 25A-25B show top and side views, respectively, of a system in which lens 4 is implemented as a biconic mirror in a POD 1. The system of Figs. 25A-25B includes a display 32 with a polarizer, each pixel of which emits polarized light towards lens 4 via a PBS 33. Lens 4 is combined with a quarter wave plate such that the polarization of the reflected light is rotated 90 degrees. Light reflected by lens 4 is subsequently reflected by the PBS 33 and injected into the waveguide through a stop 40 at the waveguide edge.

[0107] 25A-25B does not fill the waveguide aperture (in the plane perpendicular to the waveguide). Therefore, mixer 14 is used to fill the waveguide aperture.

[0108] 26A-26B show top and side views, respectively, of a system similar to that of FIGS. 25A-25B, which includes an etendue expander 36 that allows for reduction of the projection optics aperture in the plane of the waveguide (along the x-axis).

[0109] In some embodiments, the systems of Figures 25A-26B can further include polarization control elements, such as polarizers and quarter wave plates, as described above with reference to Figures 12A-13B.

[0110] Note also that the POD aperture can be larger than the thickness of the waveguide along the y-axis, as shown in Figure 27. In this case, the waveguide is thicker at the coupling end, providing a larger entrance aperture 44. In some cases, a larger aperture can be used instead of or in addition to a mixer.

[0111] Variant #8: Optical system with cylindrical optical surface in the projection module 28-33 show schematic diagrams of another embodiment of a waveguide system in which an intermediate optical device is integrated into the POD. In this embodiment, the waveguide system includes a cylindrical optical surface within the POD. With reference to FIG. 28, which shows a schematic diagram of a top view of a system according to this embodiment, the POD 1 is optically coupled to a waveguide 2. The waveguide 2 includes one or more light extraction elements 3 (shown as multiple partially reflective facets). The waveguide further includes a lens 5 having a focal length f2. With reference to FIG. 29A, which shows a schematic diagram of a side view of a system according to this embodiment, the POD 1 includes an image source 104 located at the focal plane of the lens 5. The light beams 35 emitted by the pixels of the image source are collimated in a plane perpendicular to the waveguide by a collimating optic (not shown in FIG. 28). The light beams propagate through the waveguide 2 towards the lens 5. Upon reflection from lens 5, the light is collimated in the plane of the waveguide and propagates through the waveguide and is directed by a facet towards the EMB (not shown).

[0112] Note that upon exiting POD 1, the divergence of beam 35 in the plane of the waveguide is defined by the focal length of lens 5 and the desired width D of the EMB.

number

[0113] The larger the angle φ, the greater the optical aberration of the light rays at the edge of the beam aperture caused by the cylindrical mirror 5. Therefore, the larger the EMB, the worse the image quality seen by the user when his / her pupil is at the edge of the EMB. The angular aperture of the image source should not significantly exceed the value given by equation (4). Otherwise, part of the light will be directed outside the EMB, reducing the energy efficiency of the system.

[0114] The image source 104 may be, for example, a micro LED display, an SLM, etc. For the purposes of FIG. 28, the image source 104 is assumed to be an SLM. The POD 1 includes an illumination system 200 and prisms 102, 105 with a PBS 103 sandwiched between them. The POD 1 further includes an SLM (e.g., LCOS, etc.) 104, a quarter wave plate 106, and a cylindrical mirror 107. Polarized light from the illumination system 200 enters the prism 102 and is reflected by the PBS 103 towards the SLM 104. Since the modulated light reflected by the SLM 104 has a polarization state rotated by 90 degrees, it is directed through the PBS 103 to the prism 105 towards the cylindrical mirror 107. The quarter wave plate 106 between the prism 105 and the mirror 107 rotates the polarization of the light reflected by the cylindrical mirror 107 by 90 degrees. The reflected light re-enters the prism 105 and is reflected by the PBS 103 towards the waveguide 2 .

[0115] The image source 104 is preferably an elongated SLM having a length at least five times greater than its width and is arranged with its length aligned parallel to a major external surface of the waveguide. Preferably, the SLM is elliptical and has an effective pixel area smaller than a first pixel pitch, with a first pixel pitch parallel to the length of the SLM and a second pixel pitch parallel to the width of the SLM, as described in more detail below. Preferably, each physical pixel element of the SLM is smaller than the first pixel pitch and larger than the second pixel pitch, with a plurality of physical pixel elements staggered within each first pixel pitch, as described in more detail below.

[0116] In some embodiments, the cylindrical mirror 107 and the lens 5 can have a cylindrical shape, an aspheric shape, or a freeform shape, or any combination of the above.

[0117] In some embodiments, the aperture of POD 1 in the y-direction can be smaller than the aperture size required to fill the entrance aperture of waveguide 2. In this case, mixer 14 can be used as described above to completely fill the waveguide aperture.

[0118] 29A-29B, the mixer may be disposed between the POD and the light extraction element 3 (shown here as a facet), and the facet is disposed between the mixer and the lens 5. In another embodiment, the mixer can be disposed between the lens 5 and the facet, and the facet can be disposed between the POD and the mixer.

[0119] In yet another embodiment, the system may include two mixers (not shown), one positioned between the POD and the facet and the other positioned between the lens 5 and the facet, with the facet being positioned between the two mixers.

[0120] In some embodiments, to prevent image ghosting due to back-reflected light within the waveguide, a polarizer 108 can be used between the prism 105 and the waveguide at the waveguide opening, and a quarter wave plate 106 is placed in front of the lens 5. Thus, the polarization of the back-reflected light is rotated by 90 degrees and blocked by the polarizer 108. Such polarization rotation is also beneficial to optimize the reflectivity of the facet for a selected polarization of light, as discussed above.

[0121] In some embodiments, as shown in Figure 29A, the waveguide may include a cover plate 46 bonded to a parallel external surface. In this case, a polarizer may be implemented in front of the cover plate 46 (in a manner similar to that shown in Figures 12A and 13B). The polarizer in front of the cover plate 46 furthest from the user of the display system (to the left of the waveguide 2 in Figure 29A) may block light emitted by facets 3 oriented away from the user. The polarizer in front of the cover plate 46 closest to the user (to the right of the waveguide 2 in Figure 29A) may block ghost light reflected towards the user by the surface of the opposite cover plate 46 (i.e., the cover plate to the left of the waveguide in Figure 29A).

[0122] 30A-30B show various alternative arrangements of the above-mentioned components within the POD 1. The light paths are indicated by solid arrows.

[0123] FIG. 31 shows a further embodiment of POD 1 in which an additional cylindrical lens 109 (or a combination of cylindrical lenses such as an achromatic cylindrical doublet) and a cylindrical lens (or lenses) 110 are used to improve image quality.

[0124] 28-31, note that the focal length f2 of lens 5 is much larger than the focal length f1 of the collimating optics of POD1 (i.e., cylindrical mirror 107 in FIGS. 29A, 30A, and 30B, or mirror 107 in combination with cylindrical elements 109 and 110 in FIG. 31).

[0125] The angular resolution of the eye in the x and y dimensions (dx, dy), respectively, translates to the dimensions of a resolvable pixel in the image source plane as follows:

number

[0126] In equation (5), n1 refers to the refractive index of the material of POD1, and n2 refers to the refractive index of the material of waveguide 2. From equation (5), the resolvable spot at the image source plane is a rectangle with dx>>dy (i.e., dx is significantly larger than dy), as follows: Figure 32 shows a pixel arrangement for an image source such as an SLM or micro LED display that uses rectangular pixels 111 for different spatial resolutions of dx and dy. Although rectangular pixels have been described in the prior art, the link between pixel dimensions and eye resolution has not been established.

[0127] As can be seen from Figures 28 and 31, the focal length f1 defines the smallest possible size of the POD. A more compact POD therefore requires a smaller f1 according to equation (5) and, consequently, a smaller pixel pitch. However, the minimum pixel pitch is limited by current display manufacturing technology and is currently limited to about 3 microns. From equation (5), it can be estimated that the smallest possible focal length f1 is about 15 mm (assuming the eye's resolution is 1 arc minute).

[0128] FIG. 33 shows a new pixel arrangement that can be used in an SLM 104 (or in some embodiments a micro LED display) that takes advantage of different spatial resolutions in dx and dy by using shorter pixels 112 that are staggered in a staircase pattern. The y distance between adjacent pixel centers (y pitch) can be much smaller than the pixel size py. The emissive area 1112 of a pixel in the y direction can also be smaller than py. Thus, the resolution in the y direction is higher than py. Meanwhile, some adjacent pixels in the x direction will not be resolved by the eye as long as they occupy a distance equal to or less than dx.

[0129] Assuming that the pixel arrangement shown in FIG. 33 can support y-pitch as small as 1 micron, the minimum f1 can now be as small as about 5 mm, allowing for a very compact POD.

[0130] It should be noted that the above systems based on cylindrical optics can have strong distortions. However, this can be corrected by image processing algorithms in the display (SLM or LCOS) control chain. Image processing algorithms can also be used to compensate for chromatic aberrations.

[0131] Variation #9: Optical system with corrected cylindrical mirror aberrations 34-39B show schematic diagrams of another embodiment of a waveguide system, in which the waveguide system includes one or more additional components to compensate for aberrations caused by the cylindrical mirror.

[0132] By way of introduction, the systems shown above in Figures 28-31 may suffer from poor image quality for large FOVs and EMBs. Considering a light incidence angle α as shown in Figures 7A-7C, the aberration caused by lens 5 increases as the angle α increases or the beam becomes wider in the XZ plane. Figure 34 shows a series of graphs illustrating the qualitative behavior of this aberration, represented by "ey", as a function of position along the x-axis ("px" or x-coordinate of the pupil) as the angle α increases.

[0133] Now, with reference to Figs. 35A-35B, which illustrate the above problem, consider a beam that is parallel in the YZ plane and diverging in the XZ plane, incident on a planar refractive interface at an angle β, as shown in Fig. 39B. The beam is represented by two rays, R1 and R2. Figs. 35A-35B assume an interface between media with refractive indices n1 and n2, where n2>n1. After refraction, the beam is no longer parallel in the YZ plane. The larger the angle β, the more aberration is introduced into the beam in the y direction after refraction. Also, the larger the difference between n1 and n2, the greater the aberration.

[0134] Figures 36A-36B show the above behavior when a wedge prism is introduced between the media of Figures 35A and 35B. In this case, a beam parallel in the YZ plane and diverging in the XZ plane propagates through three media including a center wedge prism with refractive indices n1, n2, and n3, respectively. n2 is smaller than both n1 and n3. The wedge-shaped medium with refractive index n2 shown in Figure 36B introduces aberrations similar to those of a refractive interface. The larger the wedge angle γ, or the greater the difference between n1 and n2 and between n3 and n2, the larger the aberrations introduced by the wedge in the y direction.

[0135] However, it should be noted that the maximum refractive index difference of available optical glasses suitable for the visible range is about 0.5, which is too small for the refractive interface shown in Figures 35A-35B to introduce aberrations large enough to counterbalance those introduced by a cylindrical mirror such as lens 5 in the waveguide system disclosed herein. One possible solution is to increase the angle of incidence β, thereby increasing the aberrations in the y direction. However, a large angle may add undesirable aberrations to the beam.

[0136] On the other hand, when air is used as a medium instead of a wedge prism, air allows for a large refractive index difference and therefore a large y-aberration. The magnitude of this aberration depends weakly on the angle of incidence β. The maximum y-aberration introduced by a wedge is qualitatively shown in FIG. 37A by the dashed line. Conversely, the cylindrical mirror aberration depends strongly on the angle of incidence, as shown in FIG. 37A by the solid line. As a result, as shown in FIG. 37B for α=A2, when a wedge is used to compensate for the aberrations in the central FOV, the aberrations in the edge FOVs A1 and A3 are nearly equal and opposite in sign.

[0137] Thus, a wedge element as shown in Figures 36A-36B can reduce the aberration of a cylindrical mirror by about a factor of two. Figures 38A-38B show schematic side and top views of an embodiment of a waveguide system including a wedge 113 disposed between the POD 1 and the waveguide 2. The wedge 113 couples light from the POD to the waveguide. In this case, the waveguide aperture stop should be disposed behind the wedge 113. The system can further include a polarizer and a quarter wave plate similar to the system described above with reference to Figures 12A-13B and 29A-29B. The surfaces 113a and 113b of the wedge 113 can optionally have one- or two-dimensional optical power to compensate for the optical aberrations of the POD 1 and achieve better image quality.

[0138] In a system where beams belonging to different FOVs are separated and each such beam passes through a wedge with a different angle γ, it is possible to introduce different y-aberrations in each beam, thus perfectly compensating the aberrations of a cylindrical mirror over the full FOV. The separation of beams belonging to different FOVs occurs within the POD. Thus, aberration compensation can be achieved using refractive optical elements within the POD.

[0139] Thus, FIG. 39A shows a system in which POD1 includes a polarizing beam splitter 103, a quarter wave plate (QWP) 106, and prisms 102 and 105 separated by cylindrical photorefractive elements 109, 110, 114, and 116. The QWP 106 can be located between the prism 105 and element 109, or can be located inside element 114. Element 114 has a reflective surface 107. The gap between elements 109 and 114 is filled with air or a material with a refractive index different from the refractive index of elements 109 and 114. In the latter case, elements 109 and 114 form a group of cemented lenses. The photorefractive elements 109, 110, 114, and 116 can include cemented glass elements or can include elements separated by air.

[0140] In one embodiment, some or all of the optical elements 109, 110, 114, and 116 can be implemented using volume Bragg diffractive lenses. Volume Bragg gratings allow all diffracted energy to be directed into a single diffraction order, and the diffraction efficiency of such gratings is highly sensitive to wavelength. Therefore, conventional reflective or refractive interfaces can be replaced by a combination of three volume Bragg gratings, each optimized for the blue, red, and white components of the spectrum.

[0141] In the system shown in Figure 39A, the refractive interfaces effectively act as wedges of different angles γ for different FOVs, such as rays belonging to FOV A and FOB B emanating from image source 104. The refractive interfaces of elements 109, 110, 114, and 116 may have cylindrical, aspheric or freeform shapes, or any combination of shape types, in some embodiments. The number and shapes of refractive components shown in Figure 43A are merely non-limiting examples of the systems described herein.

[0142] In some embodiments, POD 1 provides a beam that diverges in the x-direction and includes an introduced aberration that is opposite to that of lens 5 (FIG. 28) in the y-direction. Alternatively, the POD can include only a refractive component 115 to produce the same desired aberration, as shown in FIG. 39B.

[0143] It should be noted that the components in the POD that have optical power (i.e., components 107, 114, 109, 110, and 115) are essentially cylindrical, which means that they have no optical power in the x-direction, however, in principle these components could in fact have some optical power in the x-direction, especially if it would improve the overall image quality.

[0144] It should be noted that the aberrations caused by the cylindrical mirror are similar to the coma of a conventional optical system. In fact, it becomes clear when considering rays passing through the exit pupil line perpendicular to the tangent plane of the system with coma. In theory, the POD module can balance the aberrations of the cylindrical mirror if it has a coma of a different sign than the aberrations of the cylindrical mirror. However, in such a POD system with coma, additional aberrations such as spherical aberrations may also occur. The POD design must balance all aberrations except coma.

[0145] For example, referring back to FIG. 39A, note that there is a group of optical elements 116 with optical power located between the POD and the waveguide. This group of elements can include one or more elements that help improve image quality, for example, by compensating for the spherical aberration of the POD. The group of elements 116 can include cylindrical lenses or have lenses with more complex shapes for more efficient compensation of aberrations. Gaps between elements in a group can be filled with air or optical materials of different refractive index. In the latter case, the group 116 is composed of several cemented lenses.

[0146] In one particularly preferred embodiment, elements 109 and 107 operate in an off-axis mode and introduce aberrations that depend on the FOV, and element 116 operates in an on-axis mode and compensates for aberrations that are independent of the FOV or that depend weakly on the FOV compared to the aberrations introduced by lens 5.

[0147] In other embodiments, it is also possible for at least some of the elements in group 116 to operate in an off-axis mode.

[0148] In yet another embodiment, at least some of elements 107, 109, 110, and 114 may operate in an on-axis mode, while some of the elements in group 116 may operate in an off-axis mode.

[0149] Thus, the above embodiments can be used to achieve 2D FOV expansion of an image injected into a waveguide while improving the quality of the image presented to a viewer compared to image quality of prior art optical systems.

[0150] Furthermore, the above-described embodiments can accommodate a more compact projection optics module as compared to prior art systems.

[0151] It will be appreciated that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. 1. A display for providing an image to an observer's eye, comprising: (a) an image generator providing image illumination corresponding to the image; (b) a substrate having first and second major exterior surfaces parallel to each other; (c) an intermediate optical arrangement configured to direct the image illumination from the image generator to propagate within the substrate in a first in-plane direction by internal reflection from the first and second outer major surfaces, the intermediate optical arrangement including at least one astigmatic optical element such that the image illumination propagating within the substrate is collimated in a plane perpendicular to the first outer major surface and uncollimated in a plane parallel to the first outer major surface, the first in-plane direction being parallel to the first and second outer major surfaces; and (d) a reflective optical device having cylindrical optical power, the reflective optical device being integrated with the substrate to reflect at least a portion of the image illumination to propagate in a second in-plane direction by internal reflection from the first and second outer major surfaces, the image illumination after reflection from the reflective optical device being collimated in both a plane perpendicular to and parallel to the first outer major surface, the second in-plane direction being parallel to the first and second outer major surfaces; and (e) a coupling device for coupling at least a portion of the collimated image illumination toward an eye of an observer; the reflective optical device has a characteristic aberration, and the intermediate optical device includes at least one element arranged to generate a corrective optical aberration that at least partially offsets the characteristic aberration of the reflective optical device; the intermediate optical device is configured such that the image illumination propagating within the substrate converges in a plane parallel to the first major exterior surface, converges towards a focal locus within the substrate, and then diverges before reaching the reflecting optical device; The display further comprises an etendue optical expander integrated with the substrate at the focal locus.

2. 10. The display of claim 1, wherein said reflective optical device comprises a cylindrical mirror integrated with an edge of said substrate, said cylindrical mirror having a cylindrical axis perpendicular to said first major exterior surface.

3. The display of claim 1 , wherein the etendue optical expander is polarization selective.

4. 2. The display of claim 1, wherein the etendue optical expander is a one-dimensional etendue optical expander configured to expand the image illumination in a plane parallel to the first major exterior surface without widening the image illumination in a plane perpendicular to the first major exterior surface.

5. The display of claim 1 , wherein the etendue optical expander comprises an array of cylindrical microlenses.

6. the intermediate optical arrangement includes at least one astigmatic optical element having a cylindrical optical power; 2. The display of claim 1, wherein a cylindrical axis of said cylindrical optical power of said at least one astigmatic optical element is optically parallel to a cylindrical axis of said reflecting optical device.

7. The display of claim 6 , wherein the astigmatic optical element is integrated with the substrate.

8. The display of claim 7 , wherein the astigmatic optical element comprises a cylindrical refractive lens integrated with the substrate.

9. The display of claim 7 , wherein the astigmatic optical element comprises a cylindrical reflective lens integrated with the substrate.

10. The display of claim 7 , wherein the astigmatic optical element comprises a diffractive optical element having cylindrical optical power integrated with the substrate.

11. The display of claim 10 , wherein the reflective optical device comprises a diffractive optical element having cylindrical optical power integrated with the substrate.

12. The display of claim 6 , wherein the astigmatic optical element is external to the substrate.

13. 13. The display of claim 12, wherein the astigmatic optical element comprises a refractive lens having cylindrical optical power.

14. 13. The display of claim 12, wherein the astigmatic optical element comprises a reflective lens having cylindrical optical power.

15. 13. The display of claim 12, wherein the astigmatic optical element comprises a diffractive optical element having cylindrical optical power.

16. The display of claim 1 , wherein the intermediate optical device is configured such that image illumination propagating within the substrate towards the reflective optical device diverges in a plane parallel to the first major exterior surface.

17. 17. The display of claim 16, wherein the image generator comprises an elongated spatial light modulator having a width and a length, the length being at least five times the width, the spatial light modulator being disposed with the length aligned parallel to the first major exterior surface.

18. 20. The display of claim 17, wherein the intermediate optical device comprises a polarizing beam splitter (PBS) prism optically coupled to the substrate, the spatial light modulator being optically coupled to a face of the PBS prism.

19. 20. The display of claim 17, wherein the spatial light modulator has an elliptical effective pixel area with a first pixel pitch parallel to the length and a second pixel pitch parallel to the width that is smaller than the first pixel pitch.

20. 20. The display of claim 19, wherein each physical pixel element of the spatial light modulator is smaller than the first pixel pitch and larger than the second pixel pitch, a plurality of said physical pixel elements being staggered along said length.

21. 2. The display of claim 1, wherein the reflective optical device is configured to have a first reflectance associated with a first subset of angles of incidence and a second reflectance, lower than the first reflectance, associated with a second subset of angles of incidence, the first subset including angles within a range corresponding to the image illumination, and the second subset including angles not within the first subset.

22. 2. The display of claim 1 , wherein the combining output device is polarization selective to at least partially combine image illumination of a first polarization while substantially transmitting image illumination of a second polarization, the intermediate optical device is configured to direct the image illumination from the image generator to propagate within the substrate in the first in-plane direction in the second polarization, and a quarter-wave plate is associated with the reflecting optical device such that the collimated image illumination propagating in the second in-plane direction has the first polarization and is combined out by the combining output device.

23. 1. A display for providing an image to an observer's eye, comprising: (a) an image generator providing image illumination corresponding to the image; (b) a substrate having first and second major exterior surfaces parallel to each other; (c) an intermediate optical arrangement configured to direct the image illumination from the image generator to propagate within the substrate in a first in-plane direction by internal reflection from the first and second outer major surfaces, the intermediate optical arrangement including at least one astigmatic optical element such that the image illumination propagating within the substrate is collimated in a plane perpendicular to the first outer major surface and uncollimated in a plane parallel to the first outer major surface, the first in-plane direction being parallel to the first and second outer major surfaces; and (d) a reflective optical device having cylindrical optical power, the reflective optical device being integrated with the substrate to reflect at least a portion of the image illumination to propagate in a second in-plane direction by internal reflection from the first and second outer major surfaces, the image illumination after reflection from the reflective optical device being collimated in both a plane perpendicular to and parallel to the first outer major surface, the second in-plane direction being parallel to the first and second outer major surfaces; and (e) a coupling device for coupling at least a portion of the collimated image illumination toward an eye of an observer; the combining output device is polarization selective to at least partially combine image illumination of a first polarization while substantially transmitting image illumination of a second polarization, the intermediate optical device is configured to direct the image illumination from the image generator to propagate within the substrate in the first in-plane direction with the second polarization, a quarter wave plate is associated with the reflecting optical device such that the collimated image illumination propagating in the second in-plane direction has the first polarization and is combined out by the combining output device; the intermediate optical device is configured such that the image illumination propagating within the substrate converges in a plane parallel to the first major exterior surface, converges towards a focal locus within the substrate, and then diverges before reaching the reflecting optical device; The display further comprises an etendue optical expander integrated with the substrate at the focal locus.

Images