Optical system having a compact coupling from a projector to a waveguide

Abstract

The optical system includes a light directing optical element (LOE) (10) having first and second outer major surfaces (11, 12) parallel to each other for directing light by internal reflection, and a projector (100) for projecting illumination corresponding to a collimated image from an aperture (101). The projector injects light into the LOE through a coupling prism (30) attached to the first outer major surface (11) that projects an image injection surface. A reflective polarizing beam splitter (51) is disposed at an interface between the outer major surface (11) and the coupling prism (30) parallel to the outer major surfaces, and selectively transmits illumination from the coupling prism into the LOE for propagation within the LOE by internal reflection while capturing light already existing within the LOE.

Description

Technical Field

[0001] The present invention relates to an optical system, and more particularly to an optical system having a compact coupling of an image from a projector to a waveguide.

Background Art

[0002] Many virtual reality displays and augmented reality displays employ a light guiding optical element (LOE) having two main 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 towards the viewer's eye. Coupling from the illumination towards the eye may be by using a set of obliquely angled partially reflecting inner surfaces, or by using one or more diffractive optical elements as is well known in the art. Coupling of the image illumination from the projector to the LOE can be achieved via a coupling prism.

Summary of the Invention

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

[0004] According to the teachings of one embodiment of the present invention, an optical system includes: (a) a light guiding optical element (LOE) formed of a transparent material and having first and second major outer surfaces that are mutually parallel for guiding 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 having an angular field around the chief ray and exiting the aperture; (c) a coupling prism attached to the first major outer surface of the LOE, the coupling prism providing at least a portion of an image injection surface that is angled obliquely with respect to the major outer surface, the projector being associated with the image injection surface and being 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 with respect to the major outer surface that is greater than a critical angle for internal reflection at the major outer surface; and (d) a reflective polarizing beam splitter disposed at an interface parallel to the major outer surface between the major outer surface and the coupling prism, at least a portion of the illumination being incident on the beam splitter with a first polarization, being transmitted by the beam splitter from the coupling prism into the LOE, and light having a second polarization that is incident on the beam splitter from within the LOE and corresponds to a conjugate image of the collimated image being reflected from the beam splitter so as to propagate within the LOE by internal reflection.

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

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

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

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

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

[0010] According to a further feature of an embodiment of the present invention, the image injection surface is provided partly by a coupling prism and partly by the 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 Description of the Drawings

[0012] The invention will be described herein only by way of example with reference to the accompanying drawings.

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

[0014] The principle and operation of the optical system according to the present invention can be better understood by referring to the drawings and the accompanying description.

[0015] As an introduction, FIG. 1 shows light rays propagating within a light guiding optical element (LOE) 10 (alternatively referred to herein as a "waveguide") by internal reflection at first and second major outer surfaces 11 and 12 that are parallel to each other. In this example, the light rays are coupled out towards an observer's eye 40 by an embedded partial mirror 20 angled obliquely to the major outer surface of the LOE. The present invention is similarly applicable to displays that use diffractive optical elements to couple out image illumination towards an observer's eye, as is well known in the art.

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

[0017] To achieve uniform illumination of the combined output light, more advanced combined input configurations, such as those presented in FIGS. 2A and 2B, are often used. Here, the projector 100 provides a larger aperture and is coupled to the LOE via the prism 30, such that the light rays injected into the waveguide and reflected by the lower surface of the waveguide 12 overlap with the light rays directly injected from the projector. As a result, both the image and its conjugate are fully present within the waveguide, ensuring that the waveguide is "filled" with image illumination. Of course, this needs to apply to all fields supported by the waveguide. FIGS. 2A and 2B show two extreme examples of a typical field of view of about 20 degrees width in the medium, corresponding to a width of about 30 degrees in air. Note that a significant proportion of the illumination for each field is lost (represented by the direction of the light rays that end at the back surface of the coupling prism rather than entering the LOE because they are outside the LOE aperture).

[0018] In principle, advanced projectors can be designed such that each field is composed only of the light rays that are ultimately coupled to the waveguide. However, these are difficult to design and imply many technical complexities (for example, the apertures of such systems are at an oblique angle to the chief ray, far from the projector, and typically require large projectors). Further, this type of projector has to be designed for a specific waveguide and a "one-size-fits-all" general-purpose projector is not possible.

[0019] Here, referring generally to a particular and particularly preferred implementation of the present invention, an optical system is provided that includes a light guiding optical element (LOE) 10 formed from a transparent material and having first and second major outer surfaces 11, 12 that are parallel to each other for guiding light by internal reflection. The projector 100 is configured to project illumination corresponding to a collimated image from the aperture 101, the illumination having a chief ray that defines the optical axis 102 of the projector and having an angular field around the chief ray and exiting the aperture. FIG. 3B shows a set of light rays parallel to the chief ray, and FIGS. 3A and 3C show the shallowest and steepest angular light rays of the angular field, respectively.

[0020] The coupling prism 30 is attached to the first major outer surface 11 of the LOE and provides at least a portion of an image injection surface 32 that is angled obliquely with respect to the major outer surfaces 11 and 12. In the non-limiting examples 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, and both are polished to form a continuous surface. The projector 100 is oriented such that the principal ray and the angular field around the principal ray are injected through the image injection surface at an incident angle with respect to the major outer surface that is greater than the critical angle for internal reflection at the major outer surface and is associated with the image injection surface 32. In other words, the orientation of the projector and the coupling prism is such that image illumination can propagate within the LOE by internal reflection at the projected angle.

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

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

[0023] The operation of this implementation is as follows. Light is projected from the projector 100 onto the waveguide 10 and p-polarized. (The option of using p-polarized projection illumination is arbitrarily selected in this example, but this example could be presented equivalently with s-polarized illumination projection, and it should be understood that the p / s polarization designations are interchangeable throughout.) In this example, a reflective polarizing beam splitter 51 that transmits p-polarized light and reflects s-polarized light is disposed between the coupling prism 30 and the upper surface 11. An optical retarder (quarter-wave plate) 52 is disposed on at least a portion of the lower surface 12 and acts to change the polarization of the incident light beam.

[0024] FIG. 4 presents plots of the reflectivity and transmittance of s-polarized and p-polarized light as a function of the angle of incidence (respectively), and illustrates a typical coating layer on the surface 51, as is known in the field of polarizing beam splitters. Alternatively, the appropriate effect can be achieved using a wire grid polarizer. Preferably, the retarder 52 is a quarter-wave plate, and the polarization of the light transmitted back and forth by the retarder is rotated, converting from p-polarized light to s-polarized light (or vice versa). As a result of this structure, the light beam projected by the projector 100 is transmitted by the beam splitter 51 and passes through the waveguide. As shown in FIG. 3C, the light beam that impinges on 52 changes its polarization, and when the light beam impinges on the beam splitter a second time, it is reflected by the beam splitter 51. The selective properties of the beam splitter enable a very large percentage of the injected image illumination to be coupled into the waveguide with a substantial reduction in energy loss. In addition, the required size of the projector aperture is significantly smaller than FIGS. 2A and 2B.

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

[0026] Optimally, the system is implemented such that the light rays of all fields are reflected only once from surface 51 before reaching the end of the combined prism. Otherwise, typically some light losses occur.

[0027] In this embodiment, steep propagation light rays (Figure 3C) can, in some cases, be adversely affected by a non-uniform intensity profile. This can be mitigated in different ways, for example, by using an embedded mixer element (i.e., a partially reflective surface parallel to the main axis of the waveguide described below with reference to Figures 8A and 8B), or by placing closely spaced coupling output facets within the waveguide. The non-uniformity can be reduced or eliminated by careful design of the geometry of the projector aperture and the coupling configuration.

[0028] The retarder 52 may be placed only within the coupling input region, or may extend over part or all of the waveguide. The retarder can also serve to rotate and mix the polarization along the waveguide, for example, to mitigate any polarization artifacts that may arise due to the polarization-dependent coupling input configuration of this embodiment. The retarder can be placed on the outer surface of the waveguide, or between the waveguide 10 and an outer thin cover plate (not shown), which can be used to enhance the uniformity of the coupled output illumination.

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

[0030] FIGS. 5A and 5B show an alternative implementation of an embodiment of the present invention in which the image injection surface 32 is provided entirely by the coupling prism 30 and the projector 100 is disposed above 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 are similar to the structure and operation of FIGS. 3A-3C.

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

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

[0033] Obviously, by using a projector that generates polarized light transmitted by the beam splitter and disposing the half-wave plate 52 on a portion of the image injection surface 32 where light is directly coupled into the LOE without crossing the beam splitter (lower portion, the orientation shown here), an equivalent effect can be achieved.

[0034] In all of the embodiments described herein, the beam splitter is described as being at an interface parallel to the major outer surface between the first major outer surface 11 and the coupling prism 30. The "interface" for this purpose is functionally defined as the region through which light passes from the coupling prism 30 into the LOE 10. Most preferably, the beam splitter is typically disposed in the same plane as the first major outer surface 11 as a coating applied to one or the other of the opposing surfaces of the coupling prism 30 into the LOE 10 prior to adhesion, or as a film or other layer sandwiched between the coupling prism 30 and the LOE 10. However, an arrangement of the beam splitter embedded within the coupling prism 30 or within the LOE 10 will also be considered to be "at the interface" as long as it is close enough to the interface to provide the above functionality. In all of the cases shown, the parallelism of the beam splitter to the major surface of the LOE is essential to avoid generating ghost images when image illumination propagates along the LOE.

[0035] The various coupling input configurations described above essentially couple light in a mixed polarization state, i.e., p-polarized light and s-polarized light are superimposed, into the waveguide, such 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. Since the embedded (refractive or diffractive) components that couple light out of the waveguide are typically polarization sensitive, this can result in a streaky (non-uniform intensity) image at the output.

[0036] In principle, the embedded elements can be designed and optimized to maximize uniformity by matching the conditions for both polarization states, but this is usually very difficult to achieve and strong trade-offs in terms of efficiency, color uniformity, etc. are imposed. Therefore, several alternative approaches for improving the effects of mixed polarization illumination coupled into the waveguide are proposed below.

[0037] As shown in FIGS. 7A - 7C, a polarization retarder 201 can be disposed within the waveguide so as to control the polarization state of light within the waveguide. The retarder can be made from a birefringent crystal, a thin layer of polymer, or a structurally or spatially varying coating or spatially varying diffraction grating. Such elements can be embedded within the waveguide (as described in PCT patent application No. PCT / IL2021 / 051143), or, if they are manufactured separately, can be adhesively bonded separately between the waveguide and the coupling input wedge. The thickness of the retarder can be set to a preferred thickness. For example, it can act as a true quarter - wave plate for the relevant wavelength, taking into account the angle of incidence of all fields within the field of view (FOV), and can be made thin such that the s - polarized light and p - polarized light transmitted by the retarder are converted to (substantially) circularly polarized light (but with opposite polarization rotation).

[0038] In an alternative but conceptually related implementation, the projector 100 can be configured to generate circularly polarized image illumination, and correspondingly, the polarization beam splitter 51 can be implemented as a circularly polarized beam splitter. In this way, the light coupled to the waveguide is circularly polarized either clockwise or counter - clockwise, and the uniformity of the output light is significantly improved.

[0039] Alternatively, the retarder can be made “thick”.

[0040]

Number

[0041] A thick retarder can cause unwanted artifacts in the configurations of FIGS. 7B and 7C due to different optical paths passing through the retarder that would generate ghost images. This can be resolved when the retarder is placed perpendicular to the waveguide with sufficient accuracy, as in FIG. 7A. In this case, the angular orientation of all light rays propagating through the retarder is maintained, and ghost images are not expected.

[0042] When the projector outputs image illumination in a polarization state that is not perpendicular to the waveguide, i.e., not pure s-polarized light or p-polarized light along the waveguide axis, but rather in a linear superposition state of the two, the polarization of each wavelength rotates at each reflection of TIR on the major surface of the waveguide. This will effectively produce an effect similar to that of the thick retarder in FIG. 7. The mixing of this coating can be further enhanced by coating the major surface of the waveguide with a dedicated coating, as in Patent No. WO2021 / 105978A1.

[0043] Another approach may be to place the partially reflective layer 202 that mixes light at the center of the waveguide, parallel to the major outer surface (as disclosed in PCT Patent Application Publication No. WO2021 / 079372). Examples of such structures are illustrated in FIGS. 8A and 8B. According to this option, the light in each field is uniform throughout the waveguide, but the ratio of p-polarized light to s-polarized light may still vary from field to field. This effect needs to be considered when designing the characteristics of diffractive or refractive elements embedded in the waveguide.

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

[0045] It should be understood that the above description is intended to serve only as an example, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

Claim 1 An optical system comprising: (a) A light guiding optical element (LOE) formed from a transparent material and having first and second major outer surfaces that are mutually parallel for guiding light by internal reflection, the LOE further having partial reflectors angled obliquely to the first and second major outer surfaces, the light being coupled and output towards a user's eye by the partial reflectors; (b) A projector configured to project illumination corresponding to a collimated image from an aperture, the illumination having a chief ray defining an optical axis of the projector and a set of rays around the chief ray, exiting the aperture; (c) A coupling prism attached to the first major outer surface of the LOE, the coupling prism providing at least a portion of an image injection surface angled obliquely to the major outer surface, the projector being associated with the image injection surface and oriented such that the chief ray and the set of rays around the chief ray are injected through the image injection surface at an angle of incidence to the major outer surface that is greater than a critical angle for internal reflection at the major outer surface; (d) A reflective polarizing beam splitter disposed at an interface parallel to the major outer surface between the major outer surface and the coupling prism, at least a portion of the illumination being incident on the beam splitter with a first polarization, being transmitted by the beam splitter from the coupling prism into the LOE, and light having a second polarization that is incident on the beam splitter from within the LOE and corresponding to a conjugate image of the collimated image being reflected from the beam splitter to propagate within the LOE by internal reflection. Claim 2 The optical system according to claim 1, further comprising a waveplate disposed in at least a portion of the path of at least a portion of the illumination for converting the illumination between the first polarization and the second polarization. Claim 3 The optical system according to claim 2, wherein the waveplate is a quarter-wave plate associated with at least a portion of the second major outer surface of the LOE. Claim 4 The optical system according to claim 2, wherein the waveplate is a half-wave plate disposed in an overlapping relationship with only a portion of the aperture. Claim 5 The optical system according to claim 4, wherein a part of the opening projects illumination through a part of the image injection surface, and light from a part of the image injection surface enters the LOE without crossing the beam splitter.

6. The optical system according to claim 4, wherein the projector is configured to project illumination of the second polarization, a part of the opening projects illumination through a part of the image injection surface, light from a part of the image injection surface passes through the beam splitter, and the half-wave plate converts the illumination of the second polarization into illumination of the first polarization.

7. The optical system according to claim 1, wherein the image injection surface is provided partly by the coupling prism and partly by the surface of the LOE.

8. The optical system according to claim 1, wherein the image injection surface is entirely provided by the coupling prism.

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