Optical system with compact coupling from projector into waveguide

Abstract

To provide an optical system with compact coupling of an image from a projector into a waveguide.SOLUTION: An optical system includes a light-guide optical element (LOE) (10) having mutually parallel first and second major external surfaces (11, 12) for guiding light by internal reflection, and a projector (100) that projects illumination corresponding to a collimated image from an aperture (101). The projector injects light into the LOE via a coupling prism (30) attached to the first major external surface (11) that projects an image injection surface. A reflective polarizing beam splitter (51) is deployed at an interface between the major external surface (11) and the coupling prism (30) parallel to the major external surfaces, to selectively transmit illumination from the coupling prism into the LOE while trapping light already within the LOE so as to propagate within the LOE by internal reflection.SELECTED DRAWING: Figure 6B

Description

[Technical Field]

[0001] The present invention relates to optical systems, and in particular to optical systems with compact coupling of an image from a projector to a waveguide. [Background technology]

[0002] Many virtual reality and augmented reality displays employ a light-guiding optical element (LOE) with two major parallel planes through which an image propagates by internal reflection. Illumination corresponding to a collimated image is generated by a projector and introduced into the LOE at a coupling input region. The illumination propagates within the LOE by internal reflection until it reaches a coupling output region where it is coupled from the LOE toward the viewer's eye. Coupling the illumination toward the eye can be by using a set of obliquely angled, partially reflective internal surfaces or by using one or more diffractive optical elements, as is well known in the art. Coupling of image illumination from the projector into the LOE can be accomplished via a coupling prism. Summary of the Invention

[0003] The present invention relates to an optical system with compact coupling of an image from a projector into a waveguide.

[0004] In accordance with the teachings of one embodiment of the present invention, an optical system includes: (a) a light-directing optical element (LOE) formed from a transparent material and having first and second outer major surfaces that are parallel to each other for directing light by internal reflection; (b) a projector configured to project illumination corresponding to a collimated image from an aperture, the illumination having a chief ray that defines an optical axis of the projector and exiting the aperture with an angular field about the chief ray; and (c) a combining prism attached to the first outer major surface of the LOE, the combining prism providing at least a portion of an image injection surface that is obliquely angled relative to the outer major surface, the projector configured to project an image corresponding to the image injection surface and a light-directing optical element (LOE) formed from a transparent material and having first and second outer major surfaces that are parallel to each other for directing light by internal reflection at the outer major surface. and (d) a reflective polarizing beam splitter positioned at an interface between the outer major surface and the combining prism, parallel to the outer major surface, where at least a portion of the illumination is incident on the beam splitter with a first polarization and is transmitted by the beam splitter from the combining prism into the LOE, and where light corresponding to a conjugate image of the collimated image, having a second polarization that enters the beam splitter from within the LOE, is reflected from the beam splitter to propagate within the LOE by internal reflection.

[0005] According to a further feature of an embodiment of the invention, there is also provided a wave plate disposed in the path of at least a portion of the illumination to convert the illumination between the first polarization and the second polarization.

[0006] According to a further feature of an embodiment of the invention, the wave plate is a quarter wave plate associated with at least a portion of the second outer major surface of the LOE.

[0007] According to a further feature of an embodiment of the invention, the wave plate is a half-wave plate positioned in overlapping relationship with the first portion of the opening without overlapping with the second portion of the opening.

[0008] According to a further feature of an embodiment of the present invention, a first portion of the aperture projects illumination through a portion of the image injection surface where the light enters the LOE without traversing a beam splitter.

[0009] According to a further feature of an embodiment of the invention, the projector is configured to project illumination of a second polarization, a first portion of the aperture projects the illumination through a portion of the image injection surface where the light passes through the beam splitter, and a half-wave plate converts the illumination of the second polarization to illumination of the first polarization.

[0010] According to a further feature of an embodiment of the present invention, the image injection surface is provided in part by the coupling prism and in part by a surface of the LOE.

[0011] According to a further feature of an embodiment of the present invention, the image injection surface is provided entirely by a coupling prism. [Brief explanation of the drawings]

[0012] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1] 1 is a schematic side view of a projector injecting an image into a waveguide through an edge surface, showing incomplete filling of the image into the waveguide. FIG. [Figure 2A] FIG. 10 is a schematic side view of a projector injecting an image into a waveguide through a coupling prism, showing the shallow end of the projected angular field, so as to fill the thickness of the waveguide with the image. [Figure 2B] FIG. 10 is a schematic side view of a projector injecting an image into a waveguide through a coupling prism, showing the steepest edge of the projected angular field, so as to fill the thickness of the waveguide with the image. [Figure 3A] FIG. 1 is a schematic side view of an optical system according to one embodiment of the present invention, including a projector that uses a reflective polarizing beam splitter to inject an image into a waveguide through a coupling prism, showing the shallow end of the projected angular field. [Figure 3B]FIG. 1 is a schematic side view of an optical system according to one embodiment of the present invention, including a projector that uses a reflective polarizing beam splitter to inject an image into a waveguide through a coupling prism, showing the middle portion of the projected angular field. [Figure 3C] FIG. 1 is a schematic side view of an optical system according to one embodiment of the present invention, including a projector that uses a reflective polarizing beam splitter to inject an image into a waveguide through a coupling prism, showing the steepest edge of the projected angular field. [Figure 4] 1 is a graph showing the transmission for p-polarized light and the reflectance for s-polarized light as a function of angle of incidence for a polarizing beam splitter suitable for use in one embodiment of the present invention. [Figure 5A] FIG. 10 is a schematic side view of an optical system according to an alternative embodiment of the present invention, showing the shallow end of the projected angular field. [Figure 5B] FIG. 10 is a schematic side view of an optical system according to an alternative embodiment of the present invention, showing the steepest edge of the projected angular field. [Figure 6A] FIG. 10 is a schematic side view of an optical system according to a further alternative embodiment of the present invention, showing the shallow end of the projected angular field. [Figure 6B] 10 is a schematic side view of an optical system according to a further alternative embodiment of the present invention, showing the steepest edge of the projected angular field; [Figure 7A] 10A-10C are schematic side views of optical systems according to any of the above embodiments, illustrating possible placement of retarder elements to reduce polarization-related banding effects. [Figure 7B] 10A-10C are schematic side views of optical systems according to any of the above embodiments, illustrating possible placement of retarder elements to reduce polarization-related banding effects. [Figure 7C] 10A-10C are schematic side views of optical systems according to any of the above embodiments, illustrating possible placement of retarder elements to reduce polarization-related banding effects. [Figure 8A] 10 is a schematic side view of an optical system according to any of the above embodiments, showing a possible deployment of one internal partial reflector to achieve mixing to reduce polarization-related banding effects. [Figure 8B] 10 is a schematic side view of an optical system according to any of the above embodiments, showing a possible deployment of two internal partial reflectors to achieve mixing to reduce polarization-related banding effects. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention relates to an optical system with compact coupling of an image from a projector into a waveguide.

[0014] The principles and operation of an optical system according to the present invention may be better understood with reference to the drawings and accompanying description.

[0015] 1 shows light rays propagating within a light-directing optical element (LOE) 10 (interchangeably referred to herein as a "waveguide") by internal reflections at first and second outer major surfaces 11 and 12 that are parallel to each other. In this example, the light rays are coupled out toward a viewer's eye 40 by an obliquely angled partially reflecting mirror 20 embedded in the outer major surface of the LOE. The present invention is equally applicable to displays that use diffractive optical elements to couple out image illumination toward a viewer's eye, as is well known in the art.

[0016] Illumination from projector 100 corresponding to a collimated image is shown here injected into the waveguide at surface 13 in a simple manner that does not replicate the injected image, and therefore no conjugate image is produced. As a result, the light rays propagating through the waveguide contain "holes," i.e., areas where the image illumination does not reach, and the light rays reaching the observer's eye 40 are not uniform. Therefore, the intensity distribution detected by the observer is not uniform, but varies at different positions of the eye within the "oculomotor box" (the eye's allowed viewing positions), and depends on the particular field being observed.

[0017] To achieve uniform illumination of the coupled-out light, more advanced input coupling configurations, such as those presented in Figures 2A and 2B, are often used. Here, projector 100 provides a larger aperture and is coupled into the LOE via prism 30, so that light rays injected into the waveguide and reflected by the bottom surface of waveguide 12 overlap with light rays injected directly from the projector. This ensures that both the image and its conjugate are fully present within the waveguide, "filling" it with image illumination. Of course, this must be true for the entire field supported by the waveguide. Figures 2A and 2B show two extreme examples of a typical field of view approximately 20 degrees wide in a medium, corresponding to approximately 30 degrees wide in air. Note that a significant fraction of the illumination for each field is lost (represented by ray directions that miss the LOE aperture and therefore end up on the back surface of the coupling prism rather than entering the LOE).

[0018] In principle, advanced projectors could be designed so that each field consists only of rays that are ultimately coupled into a waveguide. However, these are difficult to design and imply many technical complexities (e.g., the aperture of such a system would be at an oblique angle to the chief ray and far away from the projector, typically requiring a large projector). Furthermore, this type of projector must be designed for a specific waveguide, making a "one-fits-all" general-purpose projector impossible.

[0019] Referring now generally to certain particularly preferred implementations of the present invention, there is provided an optical system including a light-directing optical element (LOE) 10 formed from a transparent material and having first and second mutually parallel exterior major surfaces 11, 12 for directing light by internal reflection. A projector 100 is configured to project illumination corresponding to a collimated image from an aperture 101, the illumination having a chief ray defining an optical axis 102 of the projector and exiting the aperture with an angular field about the chief ray. Figure 3B shows a set of rays parallel to the chief ray, while Figures 3A and 3C show the shallowest and steepest angular rays of the angular field, respectively.

[0020] A coupling prism 30 is attached to the first outer major surface 11 of the LOE and provides at least a portion of an image injection surface 32 that is angled obliquely relative to the outer major surfaces 11 and 12. In the non-limiting example of FIGS. 3A-3C , the image injection surface 32 is provided in part by the coupling prism 30 and in part by the edge of the LOE 10, which are polished together to form a continuous surface. A projector 100 is associated with the image injection surface 32 and oriented such that the chief ray and the angular field around the chief ray are injected through the image injection surface at an angle of incidence relative to the outer major surface that is greater than the critical angle for internal reflection at the outer major surface. In other words, the orientation of the projector and coupling prism is such that the image illumination can propagate within the LOE by internal reflection at the projected angle.

[0021] A particular feature of certain preferred implementations of the present invention is that a reflective polarizing beam splitter 51 is disposed at the interface between first outer major surface 11 and combining prism 30, parallel to the outer major surface. At least a portion of illumination from projector 100 enters beam splitter 51 with a first polarization that is transmitted by the beam splitter from combining prism 30 into LOE 10, while light having a second polarization, corresponding to a conjugate image of the collimated image, enters the beam splitter from within the LOE and is reflected from the beam splitter to propagate within the LOE by internal reflection. Thus, the beam splitter distinguishes between image illumination from projector 100 that is permitted to enter the LOE and image illumination already within the LOE that is prevented from exiting, and begins its propagation along the LOE via internal reflection.

[0022] Various configurations can be used to achieve polarization conditioning that achieves the above functions. In a particularly preferred subset of embodiments, a wave plate is placed in the path of at least a portion of the image illumination to convert the illumination between a first polarization and a second polarization. Figures 3A-3C show one example of this, where the wave plate is implemented as a quarter-wave plate 52 associated with at least a portion of the second outer major surface 12 of the LOE.

[0023] The operation of this implementation is as follows: Light is projected from projector 100 into waveguide 10 as p-polarized. (The option of using p-polarized projection illumination is chosen arbitrarily in this example, but it should be understood that this example could equally well be presented as projecting s-polarized illumination, and p / s polarization designations are interchanged throughout.) A reflective polarizing beam splitter 51, which (in this example) transmits p-polarized light and reflects s-polarized light, is positioned between combining prism 30 and top surface 11. An optical retarder (quarter-wave plate) 52 is positioned on at least a portion of bottom surface 12 and acts to change the polarization of the incident light.

[0024] FIG. 4 presents plots of reflectance and transmittance for s-polarized and p-polarized light (respectively) as a function of incidence angle and illustrates typical coating layers on surface 51, as known in the field of polarizing beam splitters. Alternatively, a suitable effect can be achieved using a wire grid polarizer. Preferably, retarder 52 is a quarter-wave plate that rotates the polarization of light transmitted back and forth by the retarder, converting p-polarized light to s-polarized light (or vice versa). As a result of this structure, light rays projected by projector 100 are transmitted by beam splitter 51 and pass through the waveguide. As shown in FIG. 3C, light rays impinging on beam splitter 52 change their polarization and are reflected by beam splitter 51 when they strike the beam splitter a second time. The selective properties of the beam splitter allow a significantly larger percentage of the injected image illumination to be coupled into the waveguide, significantly reducing energy loss. Additionally, the required size of the projector aperture is significantly smaller than in FIGS. 2A and 2B.

[0025] The retarder 52 can be implemented in many ways, including but not limited to, a crystalline zero-order crystalline retarder, a thin-film polycrystalline true zero-order retarder, a sub-wavelength structure, and an advanced dielectric layer coated directly onto the waveguide.

[0026] Optimally, the system is implemented so that all field rays are reflected only once from surface 51 before reaching the end of the combining prism, otherwise some light loss will typically occur.

[0027] In this embodiment, the steep propagation beam (FIG. 3C) may in some cases suffer from a non-uniform intensity profile. This can be mitigated in different ways, for example, by using embedded mixer elements (i.e., partially reflecting surfaces parallel to the major axis of the waveguide, as described below with reference to FIGS. 8A and 8B) or by placing closely spaced coupling-out facets within the waveguide. Non-uniformity can be reduced or eliminated by careful design of the projector aperture and the geometry of the coupling configuration.

[0028] The retarder 52 may be located only in the coupling input region, or may extend across some or all of the waveguide. The retarder may also rotate and mix the polarization along the waveguide, helping to mitigate any polarization artifacts that may result from, for example, the polarization-dependent coupling input configuration of this embodiment. The retarder may be located on the outer surface of the waveguide or between the waveguide 10 and an outer thin cover plate (not shown), which may be used to increase the uniformity of the coupled-out illumination.

[0029] In this and other embodiments described herein, capturing light within the LOE by beamsplitter 51 relatively close to the image injection plane provides advantages for the design of image projector 100. Specifically, for optical efficiency, the entrance aperture of the waveguide is preferably imaged by the projector optics (illumination optics + collimating optics not shown) to the projector's illumination stop. In the designs of FIGS. 2A and 2B, the effective aperture to the waveguide is at the end of the coupling prism, far from the image injection plane. In contrast, the designs of FIGS. 3A-3C and subsequent examples herein provide the effective waveguide aperture very close to image injection plane 32, allowing the use of common projector designs in which the illumination stop is imaged to the projector exit aperture, typically facilitating the use of projectors with smaller overall sizes.

[0030] 5A and 5B show an alternative implementation of an embodiment of the present invention in which image injection surface 32 is provided entirely by coupling prism 30 and projector 100 is placed on top of the waveguide. Such a configuration is much easier to manufacture, but results in a slightly larger aperture. In all other respects, the structure and operation of the implementation of FIGS. 5A and 5B is similar to that of FIGS. 3A-3C.

[0031] 6A and 6B show an alternative implementation in which, instead of employing a retarder on the second major surface of the waveguide, a retarder 52 in the form of a half-wave plate is positioned in an overlapping relationship with respect to a first portion of the aperture 101 without overlapping the second portion of the aperture. In the case shown, the "first" portion of the aperture projects illumination through a portion of the image injection surface 32 through which light passes through the beamsplitter 51. This is appropriate when the projector projects the polarized light reflected by the beamsplitter. As shown, the polarized light reflected by the beamsplitter is introduced directly into the LOE below the coupling input surface 32 and is thus captured by the beamsplitter and propagates by internal reflection along the LOE, while the half-wave plate 52 converts illumination of the second polarization to illumination of the first polarization at the top of the aperture, as shown, allowing that portion of the image illumination to be transmitted by the beamsplitter and enter the LOE.

[0032] Thus, as a specific example, in the case of Figures 6A and 6B where the beam splitter passes p-polarized light and reflects s-polarized light, light rays below the image injection surface that are injected directly into the waveguide and do not propagate through retarder 51 will be s-polarized, while light rays above the image injection surface will propagate through retarder 51 (here preferably acting as a half-wave plate) and be injected into the waveguide as p-polarized.

[0033] Clearly, an equivalent effect can be achieved by using a projector that produces polarized light that is transmitted by the beamsplitter and placing a half-wave plate 52 in a portion of the image injection surface 32 where the light is coupled directly into the LOE without traversing the beamsplitter (bottom, orientation shown here).

[0034] In all of the embodiments described herein, the beamsplitter is described as being at the interface between first outer major surface 11 and coupling prism 30, parallel to the outer major surface. For this purpose, the “interface” is functionally defined as the area where light passes from coupling prism 30 into LOE 10. Most preferably, the beamsplitter is located flush with first outer major surface 11, typically as a coating applied to one or other of the opposing surfaces of coupling prism 30 prior to bonding into LOE 10, or as a film or other layer sandwiched between coupling prism 30 and LOE 10. However, placement of a beamsplitter embedded within coupling prism 30 or LOE 10 would also be considered “at the interface,” as long as it is close enough to the interface to provide the above functionality. In all cases shown, parallelism of the beamsplitter to the LOE major surface is essential to avoid creating ghost images as the image illumination propagates along the LOE.

[0035] The various coupling input configurations described above essentially couple light into the waveguide in mixed polarization states, i.e., superimposed p- and s-polarized light, so that for a particular field, some regions of the input aperture are composed of p-polarized light and other regions of the input aperture are composed of s-polarized light. Because the embedded (refractive or diffractive) components that couple light out of the waveguide are typically polarization sensitive, this can result in streaked (non-uniform intensity) images at the output.

[0036] In principle, the embedded elements could be designed and optimized to maximize uniformity by matching the conditions for both polarization states, but this is usually very difficult to achieve and comes at the expense of efficiency, color uniformity, etc. Therefore, several alternative approaches are proposed below to improve the effect of mixed polarized illumination coupled into a waveguide.

[0037] As shown in Figures 7A-7C, a polarization retarder 201 can be placed within the waveguide to control the polarization state of light within the waveguide. The retarder can be made of a birefringent crystal, a thin layer of polymer, or a structurally or spatially varying coating or a spatially varying diffraction grating. Such an element can be embedded within the waveguide (as described in PCT Patent Application No. PCT / IL2021 / 051143), or, if fabricated separately, can be separately glued between the waveguide and the coupling input wedge. The thickness of the retarder can be set to a desired thickness. For example, it can be thinned so that it acts as a true quarter-wave plate for the relevant wavelength, taking into account the incidence angles of all fields within the field of view (FOV), and s-polarized and p-polarized light transmitted by the retarder is converted to (nearly) circularly polarized light (but with opposite polarization rotations).

[0038] In an alternative, but conceptually related, implementation, projector 100 may be configured to generate circularly polarized image illumination, and polarizing beam splitter 51 may correspondingly be implemented as a circularly polarizing beam splitter. In this way, light coupled into the waveguide is either right-handed or left-handed circularly polarized, greatly improving the uniformity of the output light.

[0039] Alternatively, the retarder can be "thickened"

[0040]

number

[0041] A thick retarder can cause unwanted artifacts in the configurations of Figures 7B and 7C due to different light paths through the retarder, which would create ghost images. This can be resolved if the retarder is positioned perpendicular to the waveguide with sufficient precision, as in Figure 7A. In this case, the angular orientation of all light rays propagating through the retarder is maintained, and no ghost images are expected.

[0042] If the projector outputs image illumination with polarization states that are not orthogonal to the waveguide, i.e., not pure s-polarized or p-polarized light along the waveguide axis, but rather a linear superposition of the two, the polarization of each wavelength will be rotated with each reflection of the TIR on the major surfaces of the waveguide. This will effectively produce an effect similar to the thick retarder of Figure 7. This coating intermixing can be further enhanced by coating the major surfaces of the waveguide with a specialized coating, as in Patent No. WO2021 / 105978A1.

[0043] Another approach may be to place a light-mixing partially reflective layer 202 in the center of the waveguide, parallel to the outer major surfaces (as disclosed in PCT Patent Application Publication No. WO2021 / 079372). An example of such a structure is illustrated in Figures 8A and 8B. With this option, the light in each field is uniform throughout the waveguide, but the proportion of p-polarized and s-polarized light can still vary from field to field. This effect must be taken into account when designing the properties of any diffractive or refractive elements embedded within the waveguide.

[0044] The various implementations of the invention described herein are applicable to a wide range of contexts and use any type of waveguide and any type of projector. For example, projector 100 can employ any suitable image generation technology, including, but not limited to, a liquid crystal transmissive or reflective (LCOS) projector, a scanned laser projector, or a DLP projector, all of which employ any suitable collimating optics.

[0045] It will be understood 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. An optical system comprising: (a) a light-directing optical element (LOE) formed from a transparent material and having first and second outer major surfaces parallel to each other for directing light by internal reflection; (b) a projector configured to project light corresponding to a collimated image from an aperture, the light having a chief ray that defines an optical axis of the projector and exiting the aperture with a set of rays about the chief ray; (c) a coupling-in arrangement associated with the projector and the LOE for coupling the light from the projector into the LOE at a coupling-in region such that the light undergoes internal reflection at the major outer surface, the coupling-in arrangement including a reflective polarizing beam splitter and a wave plate positioned such that the light propagating within the LOE has a spatial non-uniformity of polarization for at least a portion of the set of light rays; (d) a coupling-out arrangement associated with the LOE for redirecting the light propagating within the LOE towards a viewer, the coupling-out arrangement being polarization sensitive; and (e) a non-uniformity mitigating element selected from the group consisting of a retarder, a birefringent depolarizing element, and a partially reflective mixing element, associated with the LOE and positioned to interact with the light propagating within the LOE between the coupling-in and coupling-out configurations to reduce non-uniformity in the light redirected towards the viewer caused by spatial non-uniformity in the polarization.

2. The optical system of claim 1 , wherein the non-uniformity mitigating element comprises a retarder associated with the LOE and positioned to convert linear polarization of the light propagating within the LOE to circular polarization.

3. 2. The optical system of claim 1, wherein the nonuniformity mitigation element comprises a block of birefringent material having a thickness d greater than Δλ / Δn to achieve depolarization, where Δλ is the spectral bandwidth of the light and Δn= the difference between the extraordinary and ordinary refractive indices of the birefringent material.

4. The optical system of claim 1 , wherein the non-uniformity mitigating element is a flat element disposed substantially perpendicular to the outer major surface of the LOE.

5. The optical system of claim 1 , wherein the non-uniformity mitigating element is a flat element located on the outer major surface of the LOE.

6. The optical system of claim 1 , wherein the non-uniformity mitigating element is a flat element disposed within the LOE in a plane oblique to the outer major surface of the LOE.

7. The optical system of claim 1 , wherein the non-uniformity mitigating element is at least one partially reflective surface disposed within the LOE parallel to the first outer major surface and the second outer major surface.

8. The optical system of claim 7 , wherein the partially reflective surface is located at the center of the thickness of the LOE.

9. The optical system of claim 7 , wherein the at least one partially reflective surface is implemented as two partially reflective surfaces arranged to subdivide the thickness of the LOE into three portions.

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