Optical systems including light-guide optical element for two-dimensional expansion with retarder element

Abstract

To solve one of known problems with a type of an optical system, in which a very small amount of light is lost due to polarization mismatch between partial reflections from non-parallel facets.SOLUTION: An optical system for directing image illumination incident in a combined input region to an eye-motion box for viewing by an eye of a user, comprises a light-guide optical element (LOE) formed from a transparent material. The LOE includes: a first region containing a first set of planar, mutually-parallel, partially-reflecting surfaces having a first orientation; a second region containing a second set of planar, mutually-parallel, partially-reflecting surfaces having a second orientation non-parallel to the first orientation; a set of mutually-parallel major external surfaces extending across the first and second regions, and an optical retarder deployed between the first region and the second region so as to rotate a polarization of light deflected by the first set of partially-reflecting surfaces prior to reaching the second set of partially-reflecting surfaces.SELECTED DRAWING: Figure 2B

Description

[Technical Field]

[0001] The presently disclosed subject matter relates to optical systems, and more particularly to optical systems including light-directing optical elements (LOEs) configured for two-dimensional image expansion. [Background technology]

[0002] In recent years, consumer demand for "smart" eyewear, such as head-mounted displays (HMDs) and augmented reality (AR) glasses, collectively known as near-eye display systems, has increased. This rapidly evolving technology field therefore requires optical systems that are smaller and lighter, yet offer a relatively large field of view (FOV), and produce bright, high-quality images.

[0003] Some known optical systems use a waveguide (also referred to herein as a "light guide," "light guide optical element," or "LOE") to magnify an input image by propagating the image along a substrate embedded with one or more sets of partially reflective internal surfaces ("facets"). One known problem with this type of optical system is that a small amount of light is lost due to polarization mismatch between partial reflections from non-parallel facets. Summary of the Invention [Means for solving the problem]

[0004] In accordance with one aspect of the presently disclosed subject matter, there is provided an optical system for directing image illumination incident at a coupling input region to an eye movement box for viewing by a user's eyes, the optical system including a light-directing optical element (LOE) formed from a transparent material, the LOE including a first region including a first set of planar, mutually parallel partially reflective surfaces having a first orientation, a second region including a second set of planar, mutually parallel partially reflective surfaces having a second orientation non-parallel to the first orientation, and a set of mutually parallel outer major surfaces extending across the first and second regions such that both the first set of partially reflective surfaces and the second set of partially reflective surfaces are located between the major outer surfaces. and a set of primary outer surfaces, wherein the second set of partially reflective surfaces are at an oblique angle to the primary outer surfaces such that a portion of image illumination propagating within the LOE from a first region to a second region due to internal reflection at the primary outer surfaces is coupled out from the LOE toward the eye movement box, and the first set of partially reflective surfaces are oriented such that a portion of image illumination propagating within the LOE from the coupling input region is deflected toward the second region due to internal reflection at the primary outer surfaces, and the LOE further includes an optical retarder disposed between the first and second regions to rotate the polarization of light deflected by the partially reflective surfaces of the first set before reaching the partially reflective surfaces of the second set.

[0005] According to some embodiments, the optical system includes a miniature image projector (POD) optically coupled to the LOE to direct image illumination into a coupling input region of the LOE such that the image illumination is confined to one dimension by internal reflection at one set of major exterior surfaces.

[0006] According to some embodiments, the POD is configured to generate a collimated image that is collimated to infinity such that the image illumination spans an angular range corresponding to a two-dimensional angular field of view.

[0007] According to some embodiments, the first set of partially reflective surfaces are oriented orthogonal to the major outer surface of the LOE.

[0008] According to some embodiments, both the image illumination and a conjugate of the image illumination are deflected to the second region.

[0009] According to some embodiments, the first set of partially reflective surfaces are oriented obliquely relative to the outer major surface of the LOE.

[0010] According to some embodiments, either the image illumination or a conjugate of the image illumination is deflected to the second region.

[0011] According to some aspects, the first set of partially reflective surfaces sequentially reflects a percentage of the image illumination propagating within the first region such that the image illumination undergoes expansion in a first dimension.

[0012] According to some aspects, the second set of partially reflective surfaces continuously reflects a proportion of the image illumination propagating within the second region such that the image illumination undergoes expansion in the second dimension.

[0013] According to some embodiments, the first region is configured to achieve aperture enlargement in one of the X-axis direction or the Y-axis direction, and the second region is configured to achieve aperture enlargement in the other of the X-axis direction or the Y-axis direction.

[0014] According to some embodiments, the first and second sets of partially reflective surfaces are implemented as interior surfaces coated with a dielectric thin film coating configured to reflect light impinging on the interior surfaces over a predetermined range of angles.

[0015] According to some embodiments, the retarder is disposed within the LOE so as to extend between the outer major surfaces substantially perpendicular to the outer major surfaces.

[0016] According to some embodiments, the retarder is disposed within the LOE such that it extends between the outer major surfaces at an oblique angle relative to the outer major surfaces.

[0017] According to some embodiments, the retarder is disposed within the LOE so as to be oriented substantially parallel to the outer major surface.

[0018] According to some embodiments, the retarder is oriented substantially adjacent to one of the major exterior surfaces. [Brief explanation of the drawings]

[0019] In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which: [Figure 1A] 1A-1D show an example of a near-eye display system using an LOE for two-dimensional image magnification according to the prior art; [Figure 1B] 1A-1D show an example of a near-eye display system using an LOE for two-dimensional image magnification according to the prior art; [Figure 1C] 1A-1D show an example of a near-eye display system using an LOE for two-dimensional image magnification according to the prior art; [Figure 1D] 1A-1D show an example of a near-eye display system using an LOE for two-dimensional image magnification according to the prior art; [Figure 2A] 2A-2B show an enlarged view of the LOE of FIGS. 1A-1B according to the prior art; [Figure 2B] 2A-2B show an enlarged view of the LOE of FIGS. 1A-1B according to the prior art; [Figure 3] Figure 3 shows the reflectance characteristics of light rays at a certain angle of incidence in the S-polarized state and the P-polarized state. [Figure 4] FIG. 4 shows an LOE with an embedded retarder, according to one embodiment of the disclosed subject matter; [Figure 5A] 5A-5E illustrate various configurations of retarder 40 with LOE according to embodiments of the disclosed subject matter; [Figure 5B]5A-5E illustrate various configurations of retarder 40 with LOE according to embodiments of the disclosed subject matter; [Figure 5C] 5A-5E illustrate various configurations of retarder 40 with LOE according to embodiments of the disclosed subject matter; [Figure 5D] 5A-5E illustrate various configurations of retarder 40 with LOE according to embodiments of the disclosed subject matter; [Figure 5E] 5A-5E illustrate various configurations of retarder 40 with LOE according to embodiments of the disclosed subject matter; [Figure 6A] 6A-6C illustrate a known method for fabricating an optical polarization retarder; [Figure 6B] 6A-6C illustrate a known method for fabricating an optical polarization retarder; [Figure 6C] 6A-6C illustrate a known method for fabricating an optical polarization retarder; [Figure 7A] 7A-7D show an example of a method for fabricating an LOE with an embedded retarder according to an embodiment of the disclosed subject matter; [Figure 7B] 7A-7D show an example of a method for fabricating an LOE with an embedded retarder according to an embodiment of the disclosed subject matter; [Figure 7C] 7A-7D show an example of a method for fabricating an LOE with an embedded retarder according to an embodiment of the disclosed subject matter; [Figure 7D] 7A-7D show an example of a method for fabricating an LOE with an embedded retarder according to an embodiment of the disclosed subject matter; [Figure 8A] 8A-8E show an example of a method for manufacturing an LOE with an embedded retarder according to another embodiment of the disclosed subject matter; and [Figure 8B] 8A-8E show an example of a method for manufacturing an LOE with an embedded retarder according to another embodiment of the disclosed subject matter; and [Figure 8C]8A-8E show an example of a method for manufacturing an LOE with an embedded retarder according to another embodiment of the disclosed subject matter; and [Figure 8D] 8A-8E show an example of a method for manufacturing an LOE with an embedded retarder according to another embodiment of the disclosed subject matter; and [Figure 8E] 8A-8E show an example of a method for manufacturing an LOE with an embedded retarder according to another embodiment of the disclosed subject matter; and [Figure 9A] 9A-9B show an example of a diffractive LOE with an embedded retarder. [Figure 9B] 9A-9B show an example of a diffractive LOE with an embedded retarder. DETAILED DESCRIPTION OF THE INVENTION

[0020] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be understood by those skilled in the art that the presently disclosed subject matter may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the disclosed subject matter.

[0021] By way of background, near-eye displays using waveguides for image magnification typically include or are coupled to a projector that projects an image into a waveguide consisting of a transparent substrate, which propagates the image by total internal reflection (TIR) ​​between the parallel outer surfaces of the waveguide. Optical elements, such as partially reflective inner surfaces embedded within the waveguide, redirect the image toward the viewer in the case of a one-dimensional waveguide, or toward a second waveguide in the case of a two-dimensional waveguide. In the latter case, the second waveguide propagates the image again via TIR along an axis perpendicular to the first waveguide, magnifying the image in two dimensions. Facets embedded in the second waveguide couple the magnified image out toward the viewer.

[0022] While this disclosure is primarily concerned with partially reflective surfaces as a coupling-out method, it should be noted that, with appropriate modifications, the techniques described herein may be similarly applied to waveguides employing other light coupling-out elements (e.g., diffractive elements or combinations of reflective and diffractive elements, etc.), as described in more detail below with reference to Figures 9A-9B. Similarly, while this disclosure is primarily concerned with waveguides configured for two-dimensional image expansion, the techniques disclosed herein may also be applied to one-dimensional waveguides with appropriate modifications that would be known to one of ordinary skill in the art.

[0023] 1A and 1B schematically illustrate an exemplary implementation of a known device in the form of a near-eye display, generally designated 10, within which LOE 12 may be deployed. Near-eye display 10 uses a miniature image projector (or “POD”) 14 optically coupled to LOE 12 to project an image (also referred to herein as “image illumination”) onto LOE 12, confining the image light in one dimension by internal reflection at a series of parallel, planar exterior surfaces of LOE 12. The light impinges on a set of parallel, obliquely tilted facets relative to the propagation direction of the image light, with each successive facet deflecting a percentage of the image light, also trapped / guided by internal reflection within the substrate, in a deflected direction. This first set of facets, not individually shown in FIGS. 1A and 1B, is located in a first region of the LOE, designated region 16. Partial reflection at these successive facets achieves optical aperture expansion in the first dimension.

[0024] In some embodiments, the aforementioned set of facets is orthogonal to the outer major surface of the substrate. In this case, both the incident image and its conjugate, which undergoes internal reflection as it propagates within region 16, are polarized to become conjugate images propagating in the polarized direction. In other embodiments, the first set of facets is angled obliquely relative to the outer major surface of the LOE. In the latter case, either the incident image or its conjugate forms the desired polarized image that propagates within the LOE, while other reflections can be minimized, for example, by using angle-selective coatings on the facets that make them relatively transparent for the range of incident angles represented by the image for which reflection is not desired.

[0025] The first set of facets deflects the image illumination from a first propagation direction, which is confined within the substrate by total internal reflection (TIR), to a second propagation direction, which is also confined within the substrate by TIR. The deflected image illumination then enters a second substrate region 18, which may be implemented as an adjacent, separate substrate or as an extension of a single substrate, within which a coupling-out arrangement (a further set of partially reflective facets or a diffractive optical element) progressively couples out a proportion of the image illumination toward the eye of an observer located within an area defined as the oculomotor box, thereby achieving a second dimension of optical aperture expansion.

[0026] The entire device 10 may be implemented separately for each eye, and is preferably supported against the user's head, with each LOE 12 facing the user's corresponding eye. In one particularly preferred option as shown here, the support structure is implemented as an eyeglass frame having sides 20 for supporting the device against the user's ears. Other forms of support structure may also be used, including, but not limited to, a headband, a sun visor, or a device suspended from a helmet.

[0027] In the drawings and claims herein, reference is made to an X-axis extending horizontally (FIG. 1A) or vertically (FIG. 1B) in the general direction of extension of the first region of the LOE, and a Y-axis extending perpendicular thereto, i.e., vertically in FIG. 1A and horizontally in FIG. 1B.

[0028] Very broadly speaking, first region 16 of LOE 12 can be considered to achieve aperture expansion in the X direction, while second region 18 of LOE 12 achieves aperture expansion in the Y direction. Note that an orientation such as that shown in FIG. 1A can be considered a “top-down” implementation, in which image illumination entering the main portion (second region) of the LOE enters from the top edge, and the orientation shown in FIG. 1B can be considered a “side-entry” implementation, in which an axis, referred to herein as the Y axis, is deployed horizontally. In the remaining figures, various features of particular embodiments of the present invention are illustrated in the context of either a top-down orientation or side-entry, but it should be understood that all of those features are equally applicable to both embodiments.

[0029] The PODs used in the devices of the present invention are preferably configured to produce collimated images, i.e., images in which the light for each image pixel is a parallel beam collimated to infinity with an angular orientation corresponding to the pixel location, and the image illumination therefore spans an angular range corresponding to the two-dimensional field of view angle.

[0030] The image projector 14 typically includes at least one light source arranged to illuminate a spatial light modulator, such as an LCOS chip. The spatial light modulator modulates the projection intensity of each pixel of the image, thereby generating the image. Alternatively, the image projector can include an array of LEDs (typically implemented using a microLED or OLED array) or a scanning arrangement (typically implemented using a high-speed scanning mirror) that scans illumination from a laser light source across the projector's image plane, varying the beam intensity pixel-by-pixel in synchronization with the movement, thereby projecting the desired intensity at each pixel. In either case, collimating optics are provided to generate an output projected image that is collimated to infinity. Some or all of the above components are typically disposed on the surface of one or more polarizing beam splitter (PBS) cubes or other prism configurations, as is well known in the art.

[0031] Optical coupling of image projector 14 to LOE 12 may be achieved by any suitable optical coupling, such as via a coupling prism with an obliquely angled input face or via a reflective coupling arrangement, via a side edge and / or one of the major outer surfaces of the LOE. The details of the coupling input arrangement are not important to the present invention and are shown here schematically as a non-limiting example wedge prism 15 applied to one of the major outer surfaces of the LOE.

[0032] It will be understood that near-eye display 10 includes various additional components, including a controller 22 for operating image projector 14, typically employing power from a small on-board battery (not shown) or some other suitable power source. It will be understood that controller 22 includes all necessary electronic components, such as at least one processor or processing circuitry, for driving the image projector, all as known in the art.

[0033] 1C-1D are schematic diagrams illustrating another example of an existing near-eye display system 10 with three types of integrated elements. Similar to FIGS. 1A-1B, a projector (POD) 14 projects an image onto the LOE 12, where the light is one-dimensionally confined by TIR to the region between the LOE's two parallel outer major surfaces. The image light rays propagate through the waveguide at a certain angular orientation until they are reflected by one of two sets of parallel facets in regions 16a and 16b. These two parallel facets redirect the light rays to a different angular orientation, where they are one-dimensionally confined by TIR to the region between the LOE's two parallel major surfaces. The light rays are then reflected a second time by the facets in regions 16a and 16b, respectively, to return to their original orientation when they entered the LOE. Finally, the light rays are reflected by a third set of mutually parallel facets of region 18 which redirect the light rays so that the incident image is coupled out of the LOE and propagated towards the oculomotor box where the observer's eyes are located.

[0034] 2A, the optical characteristics of an embodiment of a near-eye display are illustrated in more detail. Specifically, a more detailed view of a light-directing optical element (LOE) 12 formed from a transparent material is shown, including a first region 16 including a first set of planar, mutually parallel partially reflective surfaces 17 having a first orientation, and a second region 18 including a second set of planar, mutually parallel partially reflective surfaces 19 having a second orientation non-parallel to the first orientation. A set of mutually parallel outer major surfaces 24 extends across first and second regions 16 and 18 such that both first set of partially reflective surfaces 17 and second set of partially reflective surfaces 19 are located between outer major surfaces 24. Most preferably, sets of outer major surfaces 24 are each a pair of continuous surfaces throughout first and second regions 16 and 18, although the option of having a setdown or increase in thickness between regions 16 and 18 is also within the scope of the present invention. Regions 16 and 18 may be directly juxtaposed so that they meet at a boundary, which may be a straight boundary or some other shape, or there may be one or more additional LOE regions interposed between them to provide various additional optical or mechanical functions, depending on the particular application. While the present invention is not limited to any particular manufacturing technique, in certain particularly preferred implementations, a particularly high quality primary outer surface is achieved by employing a continuous outer plate between separately formed regions 16 and 18 to form a composite LOE structure.

[0035] The optical properties of the LOE can be understood by tracing the image illumination path backward. The second set of partially reflective surfaces 19 are at an oblique angle to the outer major surface 24 so that a portion of the image illumination propagating within LOE 12 from first region 16 to second region 18 due to internal reflections at the outer major surface is coupled out of the LOE toward eye movement box 26. The first set of partially reflective surfaces 17 are oriented so that a portion of the image illumination propagating within LOE 12 from the coupling input region (combining prism 15) is deflected toward second region 18 due to internal reflections at the outer major surface.

[0036] One dimension of the angular spread of the projected image from image projector 14 is represented in FIG. 2A by a cone of illumination (projected onto the plane of major exterior surface 24) extending from the POD opening on the right side of the LOE toward the left side of the LOE. In the non-limiting example shown here, the central optical axis of the POD defines a propagation direction within the LOE aligned with the X-axis, and the angular spread (within the LOE) is approximately ±16°. (Note that the FOV angle is larger in air due to changes in refractive index.) A first set of partially reflective surfaces 17 is shown in first region 16, and a second set of partially reflective surfaces 19 is shown in second region 18.

[0037] The near-eye display is designed to provide the full FOV of the projected image to a user's eye, positioned within a range of permitted positions specified by an eye movement box (EMB) 26 (i.e., a shape, typically represented as a rectangle, away from the plane of the LOE where the eye's pupil will see the projected image). To reach the eye movement box, light must be coupled out from the second region 18 toward the EMB 26 by a second set of partially reflective surfaces 19. To provide the full image field, each point within the EMB must receive a full angular range of images from the LOE. Tracing back the field of view from the EMB suggests a larger rectangle 28 into which the relevant illumination is coupled out from the LOE toward the EMB.

[0038] FIG. 2A shows a first end of the field of view, corresponding to the bottom-left pixel of the projected image. A beam of width corresponding to the projector's optical aperture when coupled into the LOE is shown propagating upward and left from the POD and being partially reflected from a series of partially reflective surfaces 17. As shown here, only a subset of the facets produce reflections useful for providing the corresponding pixel in the image viewed by the user, and only a subregion of those facets contributes to the observed image of this pixel. The relevant region is indicated by a thick black line, showing the light rays corresponding to this pixel in the redirected image reflected from facet 17 and then coupled out by facet 19 to reach the four corners of the EMB 26.

[0039] Here, and throughout the discussion, only the in-plane propagation direction of the rays is shown, here as they propagate within the LOE; however, it should be noted that the rays actually follow a zigzag path of repeated internal reflections from the two major exterior surfaces, and the overall image field in one dimension is encoded by the angle of inclination of the rays relative to the major exterior surfaces, which corresponds to the pixel location in the Y dimension. As an additional example, the deflected and combined out rays, corresponding to the upper left extremity of the image, as seen in the upper left corner of the EMB, are shown in dashed lines. Figure 2B schematically illustrates the LOE of Figure 2A rotated 90 degrees and with the rays and eye movement box removed from Figure 2A to aid in visualization of the LOE.

[0040] Facets 17 and 19 are implemented as interior surfaces coated with a partially reflective coating, preferably a dielectric thin-film coating, specifically designed to partially reflect light impinging on the surface over a predetermined angular range, each angle associated with a predetermined field of view, which in turn is associated with the full FOV of the projected image. Note that the light impinging on the facets contains light of different wavelengths across a relatively broad wavelength spectrum determined by the illumination source. Furthermore, note that, generally speaking, the incident image may be polarized or unpolarized, and in each case the facet coating must be designed accordingly. For example, if the incident image is unpolarized (i.e., contains both p- and s-polarized light), the coating must be designed to account for the effects of reflection of both p- and s-polarized light.

[0041] By definition, the polarization state of light is defined according to the angular orientation of a particular ray (i.e., the orientation of the field k-vector, or plane wave) relative to the normal of the surface on which that ray impinges. Thus, a polarized incident ray of light may have one polarization state relative to one surface and another polarization state relative to a different surface. Thus, when light propagates through an optical system with many surfaces, the polarization state of an impinging ray is defined according to the direction of the incident ray and the angular orientation of the surface on which it impinges. As evident from Figure 2A, as light propagates through a near-eye display, it impinges on parallel outer surfaces (also referred to herein as "faces") 24, a first set of facets, and a second set of facets, each of which has a different angular orientation relative to one another. Thus, a polarized ray associated with a given field can be described as having a first polarization state relative to LOE surface 24, a second polarization state relative to facet 17, and a third polarization state relative to facet 19. Because the polarization state of a ray relative to a surface affects the reflectivity of the ray from that surface, ideally, the partially reflective coating on facet 17 should be designed differently from the partially reflective coating on facet 19 to achieve sufficiently high reflectivity at each set of facets. Furthermore, it is often extremely difficult to design an optical coating with the required optical properties for a certain polarization or angular range. For example, designing a coating with high reflectivity for p-polarized light near Brewster's angle is extremely difficult, if possible at all. Therefore, polarization mismatch of the illumination light between the first and second sets of facets 17 and 19 can limit the feasibility of a particular coating requirement and force a compromise on the initial polarization state. Furthermore, if the polarization state relative to the outer major surface 24 at any point along the LOE 12 is a combination of s- and p-polarized light, the polarization will rotate during TIR, resulting in significant differences in the reflectivity of light for different fields from different facets, often resulting in black lines in the output image.

[0042] As mentioned above, a drawback of existing LOEs is that the polarization of incoming light can vary across different regions of the LOE, across different sets of facets, and across the LOE's major exterior surfaces. This often leads to "impure" polarization of the incoming image relative to the major surfaces. Because TIR induces different phases in s- and p-polarized light, the polarization of the incoming image can rotate and change as the light propagates through the LOE. This significantly complicates the design of thin optical coatings on the facets, potentially reducing output efficiency and introducing local or global nonuniformities in the projected output image.

[0043] Also, as mentioned above, the reflectivity of a ray of light striking a surface varies depending on the polarization state of the ray relative to the surface. Figure 3 shows the reflectivity as a function of angle of incidence for exemplary coating designs for the first and second facets. As can be seen, the reflectivity of p-polarized light is zero near the Brewster angle. For this reason, it is generally preferred that the light propagating through the display be s-polarized, or at least predominantly s-polarized, relative to facets 17 and 19 for maximum efficiency and simplified coating design.

[0044] The inventors have now discovered that the efficiency and simplicity of a near-eye display system using an LOE configured for two-dimensional expansion can be improved by rotating the polarization of light propagating between a first set of facets and a second set of facets so that it is always s-polarized (or at least predominantly s-polarized) for both sets of facets, where "efficiency" means that more of the projection light initially coupled into the near-eye display system is reflected back towards the viewer, resulting in a brighter and / or more uniform output image.

[0045] FIG. 4 schematically illustrates one embodiment of an LOE similar to that shown in FIG. 2B, but now including an optical retarder 40 positioned along the optical path between facet 17 (which, in the illustrated embodiment, is orthogonal to the outer surface) and facet 19 and configured to rotate the polarization of the light after reflection from facet 17 and before reflection from facet 19. Thus, assuming light input to the LOE is s-polarized relative to facet 17, after reflection from facet 17 the light will be predominantly p-polarized relative to facet 19. Retarder 40 then rotates the polarization so that the light is s-polarized (or at least predominantly s-polarized) relative to facet 19. Because facets 17 and 19 now both reflect s-polarized light off their respective surfaces in each set of facets, no additional coating design considerations are necessary. Furthermore, the retarder ensures that light propagating inside the waveguide is nearly purely polarized relative to the major outer surfaces, and therefore does not experience polarization rotation due to TIR. As will be described in more detail below, the retarder 40 can be implemented in a variety of ways, including but not limited to as a half-wave plate.

[0046] As shown in Figures 5A-5E, the retarder 40 can be physically positioned in a variety of possible locations within the LOE and oriented at a variety of different angles. For example, as shown in Figure 5A, the retarder can extend across the thickness of the LOE (the z-axis in the drawings) between parallel planes and be oriented approximately perpendicular to the planes of the LOE. Figure 5B shows another configuration in which the retarder 40 is oriented obliquely to the planes of the LOE and non-parallel to the facets 19. Figure 5C shows a further configuration in which the retarder 40 is oriented parallel to the facets 19.

[0047] Figure 5D shows yet another configuration in which the retarder 40 is oriented parallel to the planes of the LOE and is physically located at a point between the planes, which may be midway between the planes but is not necessarily limited to such. Finally, Figure 5E shows yet another configuration in which the retarder 40 is oriented parallel to the planes of the display and is physically located adjacent to one of the planes.

[0048] It should be noted that in all cases the size, position and / or angle of the retarder should be determined so that all, or substantially all, of the light reflected from facet 17 passes through the retarder before being reflected by facet 19.

[0049] Furthermore, while the retarder has been described above as simply rotating the polarization of light, it should be noted that in some cases it may be desirable for the retarder to perform additional functions. For example, referring to the configuration shown in FIG. 5D, the retarder 40 may include a coating with a 50% reflectivity. This allows the retarder 40 to additionally function as a "mixer," mixing the propagating light rays to improve the intensity uniformity of the output image. Near-eye displays with embedded mixer elements have previously been described in PCT Publication WO2021001841A1.

[0050] The retarder 40 can be implemented in a variety of ways, including, but not limited to, as a half-wave plate or a coated inner surface. Suitable coatings include, for example, dielectric, birefringent, thin film polymer, crystalline retarders, geometric phase grating retarders, etc. In some embodiments, such as the configuration shown in Figure 5C, the retarder can be implemented as a coating applied to a first facet in the second set of facets.

[0051] An exemplary method for fabricating an LOE having a retarder element will now be described with reference to FIGS. 6A-8E.

[0052] 6A-6C show schematic diagrams of known methods for fabricating optical retarders suitable for deployment in LOEs. In FIG. 6A, the retarder is fabricated from a crystalline material, such as quartz. A first transparent crystal plate 42 made of a birefringent material is bonded to a transparent substrate 41. The substrate 41 is preferably made of the same material as the LOE. The bonded structure is then thinned, for example, by double-side polishing, until the required thickness of the birefringent material is achieved. FIGS. 6B-6C show alternative methods for fabricating the retarder: either by coating a dielectric coating (homogeneous or non-homogeneous) onto the substrate 41 (FIG. 6B) or by bonding a polycrystalline thin film to the substrate 41 (FIG. 6C).

[0053] Next, LOE regions incorporating the second set of facets are formed according to known methods. Figure 7A schematically illustrates forming LOE regions 18 by stacking and bonding a series of flat, transparent, coated plates 38 and slicing the stack along oblique planes parallel to the flat surfaces of the plates (Figure 7A). The slices are then polished to form multiple LOE regions 18 (Figure 7B). Regions 16 can be formed similarly.

[0054] Two alternative methods are proposed for forming the final LOE structure including the retarder: Figures 7C-7D show a first method in which a single retarder element is bonded to a single LOE region 16 on one side and a single LOE region 18 on the other side (Figure 7C) to form the final LOE (Figure 7D).

[0055] The second method, as shown in Figures 8A-8B, is to stack and glue multiple LOE regions 18 (Figure 8A), and then glue a retarder element spanning the thickness of the stack 18' to the edge of the stack (Figures 8B-8C). Next, as shown in Figures 8D-8E, a block of material 16' representing multiple formed but unsliced ​​LOE regions 16 (i.e., produced at an intermediate stage in the fabrication of the LOE regions 16) is glued to the opposite side of the retarder element, and the combined block is sliced ​​into multiple LOEs (Figure 8E), each containing a retarder embedded between two sets of facets.

[0056] In each of the above alternatives, the final LOE may require shaping and polishing on both sides for precise parallelism between the faces. In some embodiments, a transparent cover plate may be glued to the face, as is known in the art.

[0057] While the present invention has thus far been described herein primarily in the context of LOEs based on partially reflective internal surfaces, it will be appreciated that the principles of the present invention may also be advantageously implemented in light-guiding optical elements that use diffractive optical elements (DOEs) to achieve one or both of an enlarged optical aperture and / or a larger dimension of the coupled output of image illumination from the waveguide toward the observer.

[0058] 9A-9B illustrate an example of an embodiment of an optical system implemented using a diffractive waveguide for directing image illumination incident at a coupling input region to an eye movement box for viewing by a user's eyes. The optical system includes an LOE 12 formed from a transparent material and including a first region including a first DOE 27, a second region including a second DOE 29, and a set of parallel outer major surfaces 24. The outer major surfaces extend across the first and second regions such that both the first DOE 27 and the second DOE 29 are located between the outer major surfaces 24. In the example shown in FIG. 9A, image illumination enters one end of the first region and propagates unidirectionally along the length of the first region until it is deflected by one or more DOEs 27 into the second region. In the example of Figure 9B, image illumination is incident on the center of a first region and propagates in the opposite direction until it is deflected by two or more DOEs 27a, 27b into a second region. In either case, the image illumination is coupled out of the second region by one or more DOEs 29 into an eye movement box (not shown).

[0059] As shown in Figures 9A-9B, the LOE further includes an optical retarder 40 disposed between the first region and the second region to rotate the polarization of light deflected by the first DOE (i.e., DOE 27 in the case of Figure 9A, and DOEs 27a, 27b in the case of Figure 9B) before reaching the second DOE 29.

[0060] It should be noted that in some embodiments, each of the first and second DOEs may actually be implemented as a set of DOEs, in which case a "first DOE" should be understood to include the first set of DOEs, and a "second DOE" should be understood to include the second set of DOEs.

[0061] Non-limiting examples of DOEs include, for example, surface gratings and / or volume gratings (e.g., holographic gratings). In some embodiments (not shown), the LOE may include a DOE in one of the first and second regions and a facet in the other of the first and second regions. For example, the first region may include a DOE while the second region includes a facet, or the first region may include a facet while the second region includes a DOE. It should be understood that a diffractive LOE can be made by first fabricating a waveguide without an embedded retarder using any of the known methods described above, and then "writing" a holographic grating structure into the waveguide.

[0062] It should be understood that the embedded retarders described above with reference to FIG. 4 in the context of LOEs with facets can also be used with other forms of LOEs, such as LOEs with “partial” facets in regions 16 or 18 (e.g., as described in further detail in WO2020 / 049542A1), with appropriate modifications as required, as known to those skilled in the art.

[0063] It is understood that the present invention is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Accordingly, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. Thus, those skilled in the art will appreciate that the conception on which the present disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out some of the purposes of the presently disclosed subject matter.

Claims

1. 1. An optical system for directing image illumination incident on a combined input region to an eye movement box for viewing by a user's eye, the optical system comprising: a light-directing optical element (LOE) formed from a transparent material, the LOE comprising: a first region including a first set of planar, mutually parallel partially reflective surfaces having a first orientation; a second region including a second set of planar, mutually parallel partially reflective surfaces having a second orientation non-parallel to the first orientation; a set of mutually parallel outer major surfaces that extend across the first and second regions such that both the first set of partially reflective surfaces and the second set of partially reflective surfaces are located between the outer major surfaces without an intervening air gap; the second set of partially reflective surfaces are at an oblique angle to the outer major surfaces such that a portion of image illumination propagating within the LOE due to internal reflection at the outer major surfaces from the first region to the second region is coupled out of the LOE toward the eye movement box; the first set of partially reflective surfaces are oriented such that internal reflection at the major outer surfaces deflects a portion of image illumination propagating within the LOE from the coupling input region toward the second region; the LOE further includes an optical retarder disposed between the first region and the second region to rotate the polarization of light deflected by the first set of partially reflective surfaces before it reaches the second set of partially reflective surfaces.

2. 2. The optical system of claim 1, further comprising a miniature image projector (POD) optically coupled to the LOE to direct the image illumination into the coupling input region of the LOE such that the image illumination is confined to one dimension by internal reflection at the one set of major outer surfaces.

3. The optical system of claim 2 , wherein the POD is configured to produce a collimated image that is collimated to infinity such that the image illumination spans an angular range corresponding to a two-dimensional angular field of view.

4. The optical system of claim 1 , wherein the first set of partially reflective surfaces are oriented orthogonal to the outer major surfaces of the LOE.

5. The optical system of claim 4 , wherein both the image illumination and a conjugate of the image illumination are deflected to the second region.

6. The optical system of claim 1 , wherein the first set of partially reflective surfaces are oriented obliquely relative to the outer major surfaces of the LOE.

7. The optical system of claim 6 , wherein either the image illumination or a conjugate of the image illumination is deflected to the second region.

8. The optical system of claim 1 , wherein the first set of partially reflective surfaces successively reflects a percentage of the image illumination propagating within the first region such that the image illumination undergoes expansion in a first dimension.

9. The optical system of claim 1 , wherein the second set of partially reflective surfaces continuously reflects a percentage of the image illumination propagating within the second region such that the image illumination undergoes expansion in a second dimension.

10. 2. The optical system of claim 1, wherein the first region is configured to realize an aperture expansion in one of an X-axis direction or a Y-axis direction, and the second region is configured to realize an aperture expansion in the other of the X-axis direction or the Y-axis direction.

11. 10. The optical system of claim 1, wherein the first and second sets of partially reflective surfaces are implemented as interior surfaces coated with a dielectric thin film coating configured to reflect light impinging on the interior surfaces over a predetermined range of angles.

12. The optical system of claim 1 , wherein the retarder is disposed within the LOE so as to extend between the outer major surfaces substantially perpendicular to the outer major surfaces.

13. The optical system of claim 1 , wherein the retarder is disposed within the LOE so as to extend between the outer major surfaces at an oblique angle relative to the outer major surfaces.

14. The optical system of claim 1 , wherein the retarder is disposed within the LOE so as to be oriented substantially parallel to the outer major surface.

15. The optical system of claim 14 , wherein the retarder is oriented substantially adjacent one of the outer major surfaces.

16. 1. An optical system for directing image illumination incident on a combined input region to an eye movement box for viewing by a user's eye, the optical system comprising: a light-directing optical element (LOE) formed from a transparent material, the LOE comprising: a first region including a first set of planar, mutually parallel partially reflective surfaces having a first orientation; a second region including a second set of planar, mutually parallel partially reflective surfaces having a second orientation non-parallel to the first orientation; a set of mutually parallel outer major surfaces that extend across the first and second regions such that both the first set of partially reflective surfaces and the second set of partially reflective surfaces are located between the outer major surfaces without an intervening air gap; the second set of partially reflective surfaces are at an oblique angle to the outer major surfaces such that a portion of image illumination propagating within the LOE due to internal reflection at the outer major surfaces from the first region to the second region is coupled out of the LOE toward the eye movement box; the first set of partially reflective surfaces are oriented such that internal reflections at the major outer surfaces deflect a portion of image illumination propagating within the LOE from a coupling input region toward the second region, the deflection being the only in-plane element redirection of the propagation direction of the image illumination occurring between the coupling input region and the second region of the LOE; the LOE further includes an optical retarder disposed between the first region and the second region to rotate the polarization of light deflected by the first set of partially reflective surfaces before it reaches the second set of partially reflective surfaces.

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