PATENT DOCUMENT

Publication Number: US-11994681-B2
Application Number: US-202117477321-A
Country: US
Kind Code: B2

Title: Optical systems with reflective prism input couplers

Abstract:
An electronic device may include a display module that generates light and an optical system that redirects the light towards an eye box. The system may include an input coupler on a waveguide and a lens that directs the light towards the input coupler. The input coupler may include a prism having a reflective surface that reflects the light into the waveguide. The reflective surface may be curved to provide the light with an optical power. The prism may be configured to expand a field of view of the light. A birefringent beam displacer may expand the effective pupil size of the light. The lens may include lens elements that converge the light at a location between the lens elements and the waveguide. A switchable panel may be placed at the location and toggled between first and second orientations to increase the effective resolution of the light.

Claims:
What is claimed is: 
     
       1. A display system comprising:
 a display module that produces image light; 
 a waveguide; 
 a reflective input coupling prism mounted to the waveguide, wherein the reflective input coupling prism is configured to receive the image light through the waveguide, the reflective input coupling prism is configured to reflect the image light into the waveguide, and the waveguide is configured to propagate the reflected image light via total internal reflection; and 
 an output coupler on the waveguide and configured to couple the reflected image light out of the waveguide, wherein the output coupler comprises diffractive grating structures in a grating medium embedded within the waveguide. 
 
     
     
       2. The display system defined in  claim 1 , wherein the diffractive grating structures comprise volume holograms, and the display system further comprises a dispersion compensation wedge optically interposed between the display module and the waveguide. 
     
     
       3. The display system defined in  claim 1 , wherein the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the display system further comprising a reflective layer on the reflective surface. 
     
     
       4. The display system defined in  claim 3 , wherein the reflective layer comprises a switchable reflective layer, the switchable reflective layer being adjustable between a first state at which the switchable reflective layer reflects the image light into the waveguide in a first direction and a second state at which the switchable reflective layer reflects the image light into the waveguide in a second direction that is different from the first direction, and wherein the switchable reflective layer comprises a structure selected from the group consisting of: a switchable liquid crystal grating and a digital micromirror device. 
     
     
       5. The display system defined in  claim 3 , wherein the reflective layer comprises a layer selected from the group consisting of: a dielectric layer, a metallic layer, a layer of three-dimensional metal structures, and a layer of diffractive gratings. 
     
     
       6. The display system defined in  claim 3 , wherein the reflective surface comprises a curved surface that is configured to provide the reflected light with an optical power. 
     
     
       7. The display system defined in  claim 3 , wherein the reflective layer covers a portion of the reflective surface that overlaps a pupil of the image light that has been focused onto the reflective surface and wherein the display system further comprises an optical absorber that covers a remainder of the reflective surface. 
     
     
       8. The display system defined in  claim 3 , further comprising:
 a lens configured to direct the image light from the display module towards the waveguide, wherein the lens is configured to converge the image light at a location interposed between the waveguide and the lens; and 
 a transparent plate at the location, the transparent plate being adjustable between a first orientation at which the transparent plate directs the image light towards the waveguide with a first alignment and a second orientation at which the transparent plate directs the image light towards the waveguide with a second alignment that is parallel to and offset from the first alignment. 
 
     
     
       9. The display system defined in  claim 3 , wherein the waveguide has opposing first and second surfaces, the reflective input coupling prism comprises a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the second wedge includes the reflective surface, the surface of the first wedge is configured to reflect the image light into the waveguide in a first direction, and the reflective surface is configured to reflect the image light into the waveguide in a second direction that is different from the first direction. 
     
     
       10. The display system defined in  claim 9 , wherein the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the display further comprising:
 a polarization-sensitive film interposed between the first and second wedges, wherein the polarization-sensitive film is configured to reflect a first polarization of the image light in the first direction and to transmit a second polarization of the image light, and wherein the reflective surface is configured to reflect, in the second direction, the second polarization of the image light transmitted by the polarization-sensitive film. 
 
     
     
       11. The display system defined in  claim 10 , further comprising:
 a switchable polarizer, wherein the switchable polarizer is switchable between first and second states, wherein the switchable polarizer is configured to transmit, in the first state, the first polarization of the image light through the second surface of the waveguide, and wherein the switchable polarizer is configured to transmit, in the second state, the second polarization of the image light through the second surface of the waveguide. 
 
     
     
       12. The display system defined in  claim 10 , wherein the display module comprises a first display panel configured to transmit the first polarization of the image light towards the reflective input coupling prism and wherein the display module comprises a second display panel configured to concurrently transmit the second polarization of the image light towards the reflective input coupling prism. 
     
     
       13. The display system defined in  claim 1 , wherein the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the waveguide has opposing first and second surfaces, the reflective input coupling prism comprises a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the second wedge includes the reflective surface, the surface of the first wedge is configured to reflect the image light into the waveguide in a first direction, and the reflective surface is configured to reflect the image light into the waveguide in a second direction that is different from the first direction, the display further comprising:
 a switchable reflective layer interposed between the first and second wedges, wherein the switchable reflective layer is switchable between first and second states, wherein the switchable reflective layer is configured to reflect, in the first state, the image light in the first direction, wherein the switchable reflective layer is configured to transmit, in the second state, the image light towards the reflective surface, and wherein the switchable reflective layer comprises a cholesteric liquid crystal reflector. 
 
     
     
       14. The display system defined in  claim 1 , further comprising:
 a birefringent beam displacer configured to receive a pupil of image light, wherein the birefringent beam displacer is configured to transmit a first polarization of the pupil towards the reflective input coupling prism within a first beam, and wherein the birefringent beam displacer is configured to transmit a second polarization of the pupil towards the reflective input coupling prism in a second beam that is offset from the first beam. 
 
     
     
       15. The display system defined in  claim 14 , wherein the waveguide has opposing first and second surfaces, the reflective input coupling prism comprises a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the birefringent beam displacer is mounted to the second surface, the first wedge has a first refractive index, and the second wedge has a second refractive index that is different from the first refractive index. 
     
     
       16. The display system defined in  claim 1 , wherein the reflective input coupling prism comprises a birefringent beam displacer. 
     
     
       17. The display system defined in  claim 1 , further comprising:
 a lens configured to direct the image light from the display module towards the reflective input coupling prism, wherein the lens comprises:
 a first set of lens elements, and 
 a second set of lens elements separated from the first set of lens elements by a gap, wherein the second set of lens elements is interposed between the first set of lens elements and the waveguide, and wherein the first set of lens elements is configured to converge the image light at a location within the gap. 
 
 
     
     
       18. The display system defined in  claim 17 , further comprising:
 a switchable element at the location within the gap, wherein the switchable element is switchable between a first state at which the switchable element passes the image light to the second set of lens elements with a first alignment and a second state at which the switchable element passes the image light to the second set of lens elements with a second alignment that is displaced with respect to the first alignment, and wherein the switchable element comprises a transparent panel and a piezoelectric component configured to rotate the transparent panel between a first orientation in the first state and a second orientation in the second state. 
 
     
     
       19. An electronic device comprising:
 a display module that produces image light; 
 a waveguide having opposing first and second surfaces; 
 collimating optics that direct the image light towards the second surface of the waveguide; and 
 an input coupling prism configured to receive the image light from the collimating optics through the waveguide, wherein the input coupling prism comprises:
 a first wedge mounted to the first surface of the waveguide and having a first reflective surface oriented at a first angle with respect to the first surface of the waveguide, wherein the first wedge is configured to reflect the image light into the waveguide in a first direction, and 
 a second wedge mounted to the first reflective surface and having a second reflective surface oriented at a second angle with respect to the first surface of the waveguide, wherein the second angle is greater than the first angle, wherein the second wedge is configured to receive the image light through the first wedge, and wherein the second wedge is configured to reflect the image light into the waveguide in a second direction that is different from the first direction. 
 
 
     
     
       20. The electronic device defined in  claim 19 , further comprising:
 a polarizing beam splitter layered on the first reflective surface. 
 
     
     
       21. The electronic device defined in  claim 19 , further comprising:
 a switchable reflective layer on the first reflective surface. 
 
     
     
       22. The electronic device defined in  claim 19 , wherein the first wedge has a first refractive index and the second wedge has a second refractive index that is different from the first refractive index. 
     
     
       23. The electronic device defined in  claim 19 , wherein the second reflective surface is curved and wherein the first reflective surface is curved. 
     
     
       24. A display comprising:
 a display module that generates image light; 
 a waveguide having opposing first and second surfaces; 
 a lens configured to direct the image light towards the second surface of the waveguide, wherein the lens comprises a set of lens elements configured to converge the image light at a location interposed between the set of lens elements and the second surface of the waveguide; and 
 a reflective input coupling prism mounted to the first surface of the waveguide and configured to receive the image light from the lens through the waveguide, wherein the reflective input coupling prism is configured to reflect the image light into the waveguide. 
 
     
     
       25. The display defined in  claim 24 , wherein the lens further comprises an additional set of lens elements interposed between the location and the second surface of the waveguide, further comprising:
 a transparent panel at the location, wherein the transparent panel is switchable between first and second orientations, wherein the transparent panel is configured to pass, in the first orientation, the image light to the second set of lens elements with a first alignment, and wherein the transparent panel is configured to pass, in the second orientation, the image light to the second set of lens elements with a second alignment that is offset from the first alignment.

Description:
This application is a continuation of international patent application No. PCT/US2020/050709, filed Sep. 14, 2020, which claims the benefit of U.S. provisional patent application No. 62/902,645, filed Sep. 19, 2019, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This relates generally to optical systems and, more particularly, to optical systems for displays. 
     Electronic devices may include displays that present images to a user&#39;s eyes. For example, devices such as virtual reality and augmented reality headsets may include displays with optical elements that allow users to view the displays. 
     It can be challenging to design devices such as these. If care is not taken, the components used in displaying content may be unsightly and bulky and may not exhibit desired levels of optical performance. 
     SUMMARY 
     An electronic device such as a head-mounted device may have one or more near-eye displays that produce images for a user. The head-mounted device may be a pair of virtual reality glasses or may be an augmented reality headset that allows a viewer to view both computer-generated images and real-world objects in the viewer&#39;s surrounding environment. 
     The near-eye display may include a display module that generates light and an optical system that redirects the light from the display module towards an eye box. The optical system may include an input coupler and an output coupler formed on a waveguide. The input coupler may redirect light from the display module so that the light propagates in the waveguide towards the output coupler. The output coupler may couple the light out of the waveguide and towards the eye box. The output coupler may include diffractive grating structures such as volume holograms in the waveguide. 
     The input coupler may be a reflective input coupling prism mounted to the waveguide. A lens may direct the light from the display module to the input coupler through the waveguide. The prism may receive the light from the lens through the waveguide and may have a reflective surface that reflects the light into the waveguide. The reflective surface may be curved to provide the reflected light with an optical power. This may allow the lens to have fewer lens elements than in scenarios where the prism does not impart optical power on the light. 
     The prism may be configured to expand a field of view of the light. For example, a switchable reflective layer may be provided on the reflective surface for expanding a field of view of the light. If desired, the prism may be a split prism that includes a first wedge having a first reflective surface and a second wedge mounted to the first reflective surface. The first reflective surface may reflect the image light in a first direction whereas the second reflective surface reflects the image light in a second direction. A polarization-sensitive film may be provided on the first reflective surface and may reflect a first polarization of light while transmitting a second polarization of light. A switchable polarizer may be used to sequentially provide the prism with light of the first and second polarizations. In another suitable arrangement, the display panel may concurrently provide the prism with light of the first and second polarizations. If desired, a switchable reflective layer may be provided on the first reflective surface and may selectively reflect or pass the light from the display module. If desired, a birefringent beam displacer may be interposed on the optical path of the display for expanding the effective pupil size of the light. 
     The lens may include a first set of lens elements that converge the light from the display module at a location between the first set of lens elements and the waveguide. A transparent panel that is switchable between first and second orientations may be placed at the location. The transparent panel may be toggled between the first and second orientations to increase the effective resolution of the light. If desired, a second set of lens elements may be interposed between the location and the waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an illustrative system having a display in accordance with some embodiments. 
         FIG.  2    is a top view of an illustrative optical system for a display having a waveguide with an input coupler in accordance with some embodiments. 
         FIG.  3    is a top view of an illustrative input coupler formed from a reflective prism in accordance with some embodiments. 
         FIG.  4    is a top view of an illustrative input coupler formed from a reflective prism having a curved reflective surface in accordance with some embodiments. 
         FIG.  5    is a top view of an illustrative input coupler formed from a reflective prism having a switchable reflective component for expanding the field of view for a display in accordance with some embodiments. 
         FIG.  6    is a top view of an illustrative input coupler formed from a split reflective prism having a polarization-sensitive reflective film for expanding a field of view in accordance with some embodiments. 
         FIG.  7    is a top view of an illustrative input coupler formed from a split reflective prism having a switchable reflective component for expanding a field of view in accordance with some embodiments. 
         FIG.  8    is a top view of an illustrative display system in which light of different polarizations is concurrently provided to a split reflective prism having a polarization-sensitive reflective film for expanding a field of view in accordance with some embodiments. 
         FIG.  9    is a perspective view of an illustrative birefringent beam displacer that may be provided on a waveguide for expanding a pupil of light within the waveguide in accordance with some embodiments. 
         FIG.  10    is a top view showing how an illustrative birefringent beam displacer of the type shown in  FIG.  9    may pass light to an input coupler having a split reflective prism in accordance with some embodiments. 
         FIG.  11    is a top view of an illustrative split multi-element lens that may provide image light to an input coupler of the types shown in  FIGS.  2 - 10    in accordance with some embodiments. 
         FIG.  12    is a top view showing how an illustrative switchable light displacing element may be mounted within a split multi-element lens of the type shown in  FIG.  11    in accordance with some embodiments. 
         FIG.  13    is a top view showing how a switchable light displacing element of the type shown in  FIG.  12    may be adjusted between first and second states for displacing image light in accordance with some embodiments. 
         FIG.  14    is a front view of pixels of image light that illustrates how an illustrative switchable light displacing element of the type shown in  FIGS.  12  and  13    may increase the effective resolution of the image light in accordance with some embodiments. 
         FIG.  15    is a top view showing how an input coupler formed from a reflective prism with a curved reflective surface may form part of a split multi-element lens in accordance with some embodiments. 
         FIG.  16    is a top view showing how an input coupler formed from a reflective prism may receive image light through a dispersion compensation wedge and/or may include an optical absorber in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative system having a device with one or more near-eye display systems is shown in  FIG.  1   . System  10  may be a head-mounted device having one or more displays such as near-eye displays  14  mounted within support structure (housing)  20 . Support structure  20  may have the shape of a pair of eyeglasses (e.g., supporting frames), may form a housing having a helmet shape, or may have other configurations to help in mounting and securing the components of near-eye displays  14  on the head or near the eye of a user. Near-eye displays  14  may include one or more display modules such as display modules  14 A and one or more optical systems such as optical systems  14 B. Display modules  14 A may be mounted in a support structure such as support structure  20 . Each display module  14 A may emit light  22  (image light) that is redirected towards a user&#39;s eyes at eye box  24  using an associated one of optical systems  14 B. 
     The operation of system  10  may be controlled using control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for controlling the operation of system  10 . Circuitry  16  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code (instructions) may be stored on storage in circuitry  16  and run on processing circuitry in circuitry  16  to implement operations for system  10  (e.g., data gathering operations, operations involving the adjustment of components using control signals, image rendering operations to produce image content to be displayed for a user, etc.). 
     System  10  may include input-output circuitry such as input-output devices  12 . Input-output devices  12  may be used to allow data to be received by system  10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, or other electrical equipment) and to allow a user to provide head-mounted device  10  with user input. Input-output devices  12  may also be used to gather information on the environment in which system  10  (e.g., head-mounted device  10 ) is operating. Output components in devices  12  may allow system  10  to provide a user with output and may be used to communicate with external electrical equipment. Input-output devices  12  may include sensors and other components  18  (e.g., image sensors for gathering images of real-world object that are digitally merged with virtual objects on a display in system  10 , accelerometers, depth sensors, light sensors, haptic output devices, speakers, batteries, wireless communications circuits for communicating between system  10  and external electronic equipment, etc.). 
     Display modules  14 A may include reflective displays (e.g., liquid crystal on silicon (LCOS) displays, digital-micromirror device (DMD) displays, or other spatial light modulators), emissive displays (e.g., micro-light-emitting diode (uLED) displays, organic light-emitting diode (OLED) displays, laser-based displays, etc.), or displays of other types. Light sources in display modules  14 A may include uLEDs, OLEDs, LEDs, lasers, combinations of these, or any other desired light-emitting components. 
     Optical systems  14 B may form lenses that allow a viewer (see, e.g., a viewer&#39;s eyes at eye box  24 ) to view images on display(s)  14 . There may be two optical systems  14 B (e.g., for forming left and right lenses) associated with respective left and right eyes of the user. A single display  14  may produce images for both eyes or a pair of displays  14  may be used to display images. In configurations with multiple displays (e.g., left and right eye displays), the focal length and positions of the lenses formed by components in optical system  14 B may be selected so that any gap present between the displays will not be visible to a user (e.g., so that the images of the left and right displays overlap or merge seamlessly). 
     If desired, optical system  14 B may contain components (e.g., an optical combiner, etc.) to allow real-world image light from real-world images or objects  25  to be combined optically with virtual (computer-generated) images such as virtual images in image light  22 . In this type of system, which is sometimes referred to as an augmented reality system, a user of system  10  may view both real-world content and computer-generated content that is overlaid on top of the real-world content. Camera-based augmented reality systems may also be used in device  10  (e.g., in an arrangement which a camera captures real-world images of object  25  and this content is digitally merged with virtual content at optical system  14 B). 
     System  10  may, if desired, include wireless circuitry and/or other circuitry to support communications with a computer or other external equipment (e.g., a computer that supplies display  14  with image content). During operation, control circuitry  16  may supply image content to display  14 . The content may be remotely received (e.g., from a computer or other content source coupled to system  10 ) and/or may be generated by control circuitry  16  (e.g., text, other computer-generated content, etc.). The content that is supplied to display  14  by control circuitry  16  may be viewed by a viewer at eye box  24 . 
       FIG.  2    is a top view of an illustrative display  14  that may be used in system  10  of  FIG.  1   . As shown in  FIG.  2   , near-eye display  14  may include one or more display modules such as display module  14 A and an optical system such as optical system  14 B. Optical system  14 B may include optical elements such as one or more waveguides  26 . Waveguide  26  may include one or more stacked substrates (e.g., stacked planar and/or curved layers sometimes referred to herein as waveguide substrates) of optically transparent material such as plastic, polymer, glass, etc. 
     If desired, waveguide  26  may also include one or more layers of holographic recording media (sometimes referred to herein as holographic media, grating media, or diffraction grating media) on which one or more diffractive gratings are recorded (e.g., holographic phase gratings, sometimes referred to herein as holograms). A holographic recording may be stored as an optical interference pattern (e.g., alternating regions of different indices of refraction) within a photosensitive optical material such as the holographic media. The optical interference pattern may create a holographic phase grating that, when illuminated with a given light source, diffracts light to create a three-dimensional reconstruction of the holographic recording. The holographic phase grating may be a non-switchable diffractive grating that is encoded with a permanent interference pattern or may be a switchable diffractive grating in which the diffracted light can be modulated by controlling an electric field applied to the holographic recording medium. Multiple holographic phase gratings (holograms) may be recorded within (e.g., superimposed within) the same volume of holographic medium if desired. The holographic phase gratings may be, for example, volume holograms or thin-film holograms in the grating medium. The grating media may include photopolymers, gelatin such as dichromated gelatin, silver halides, holographic polymer dispersed liquid crystal, or other suitable holographic media. 
     Diffractive gratings on waveguide  26  may include holographic phase gratings such as volume holograms or thin-film holograms, meta-gratings, or any other desired diffractive grating structures. The diffractive gratings on waveguide  26  may also include surface relief gratings formed on one or more surfaces of the substrates in waveguides  26 , gratings formed from patterns of metal structures, etc. The diffractive gratings may, for example, include multiple multiplexed gratings (e.g., holograms) that at least partially overlap within the same volume of grating medium (e.g., for diffracting different colors of light and/or light from a range of different input angles at one or more corresponding output angles). 
     Optical system  14 B may include collimating optics such as collimating lens  34 . Collimating lens  34  may include one or more lens elements that help direct image light  22  towards waveguide  26 . If desired, display module  14 A may be mounted within support structure  20  of  FIG.  1    while optical system  14 B may be mounted between portions of support structure  20  (e.g., to form a lens that aligns with eye box  24 ). Other mounting arrangements may be used, if desired. 
     As shown in  FIG.  2   , display module  14 A may generate light  22  associated with image content to be displayed to eye box  24 . Light  22  may be collimated using a lens such as collimating lens  34 . Optical system  14 B may be used to present light  22  output from display module  14 A to eye box  24 . 
     Optical system  14 B may include one or more optical couplers such as input coupler  28 , cross-coupler  32 , and output coupler  30 . In the example of  FIG.  2   , input coupler  28 , cross-coupler  32 , and output coupler  30  are formed at or on waveguide  26 . Input coupler  28 , cross-coupler  32 , and/or output coupler  30  may be completely embedded within the substrate layers of waveguide  26 , may be partially embedded within the substrate layers of waveguide  26 , may be mounted to waveguide  26  (e.g., mounted to an exterior surface of waveguide  26 ), etc. 
     The example of  FIG.  2    is merely illustrative. One or more of these couplers (e.g., cross-coupler  32 ) may be omitted. Optical system  14 B may include multiple waveguides that are laterally and/or vertically stacked with respect to each other. Each waveguide may include one, two, all, or none of couplers  28 ,  32 , and  30 . Waveguide  26  may be at least partially curved or bent if desired. 
     Waveguide  26  may guide light  22  down its length via total internal reflection. Input coupler  28  may be configured to couple light  22  from display module  14 A (lens  34 ) into waveguide  26 , whereas output coupler  30  may be configured to couple light  22  from within waveguide  26  to the exterior of waveguide  26  and towards eye box  24 . For example, display module  14 A may emit light  22  in direction +Y towards optical system  14 B. When light  22  strikes input coupler  28 , input coupler  28  may redirect light  22  so that the light propagates within waveguide  26  via total internal reflection towards output coupler  30  (e.g., in direction X). When light  22  strikes output coupler  30 , output coupler  30  may redirect light  22  out of waveguide  26  towards eye box  24  (e.g., back along the Y-axis). In scenarios where cross-coupler  32  is formed at waveguide  26 , cross-coupler  32  may redirect light  22  in one or more directions as it propagates down the length of waveguide  26 , for example. 
     Input coupler  28 , cross-coupler  32 , and/or output coupler  30  may be based on reflective and refractive optics or may be based on holographic (e.g., diffractive) optics. In arrangements where couplers  28 ,  30 , and  32  are formed from reflective and refractive optics, couplers  28 ,  30 , and  32  may include one or more reflectors (e.g., an array of micromirrors, partial mirrors, or other reflectors). In arrangements where couplers  28 ,  30 , and  32  are based on holographic optics, couplers  28 ,  30 , and  32  may include diffractive gratings (e.g., volume holograms, surface relief gratings, etc.). 
     In one suitable arrangement that is sometimes described herein as an example, output coupler  30  is formed from diffractive gratings or micromirrors embedded within waveguide  26  (e.g., volume holograms recorded on a grating medium stacked between transparent polymer waveguide substrates, an array of micromirrors embedded in a polymer layer interposed between transparent polymer waveguide substrates, etc.), whereas input coupler  28  is formed from a reflective prism mounted to an exterior surface of waveguide  26  (e.g., an exterior surface defined by a waveguide substrate that contacts the grating medium or the polymer layer used to form output coupler  30 ). 
       FIG.  3    is a top view showing how input coupler  28  at waveguide  26  may be formed from a reflective prism. As shown in  FIG.  3   , input coupler  28  may include a reflective prism (e.g., a reflective input coupling prism) such as prism  36 . Prism  36  may have a bottom surface  38  mounted to exterior surface  40  of waveguide  26  (e.g., using an optically clear adhesive not shown in  FIG.  3    for the sake of clarity). 
     As shown in  FIG.  3   , lens  34  may receive light  22  (e.g., from display module  14 A of  FIG.  2   ). Prism  36  may be mounted to the side of waveguide  26  opposite to lens  34  and display module  14 A. For example, waveguide  26  may have an exterior surface  42  that opposes exterior surface  40 . Exterior surface  42  may be interposed between prism  36  and lens  34  (e.g., waveguide  26  may be interposed between prism  36  and display module  14 A). 
     Lens  34  may direct light  22  towards waveguide  26 . Light  22  may enter waveguide  26  through surface  42  (e.g., at a sufficiently low angle with respect to the normal surface of surface  42  such that no total internal reflection occurs). Light  22  may pass through surface  40  of waveguide  26  into prism  36 . Light  22  may reflect off of surface  44  of prism  36  (sometimes referred to herein as reflective surface  44  or reflection surface  44 ) and back into waveguide  26  through surfaces  38  and  40 . Surface  44  may be tilted in one or more directions (e.g., including out of the plane of the page, where the normal axis of surface  44  is oriented at a non-zero angle with respect to the +Y axis within the Z-Y plane in addition to a non-zero angle with respect to the +Y axis within the X-Y plane). Light  22  may then propagate down the length of waveguide  26  via total internal reflection. In this way, prism  36  may couple light  22  into waveguide  26  despite being located on the side of waveguide  26  opposite to lens  34  and display module  14 A ( FIG.  2   ). 
     The angular spread of light  22  may be confined to pupil  46  within waveguide  26  after being coupled into waveguide  26  by prism  36  (e.g., the rays of light reflected by prism  36  may converge at pupil  46  within waveguide  26 ). The optical path length from surface  42  to surface  38 , from surface  38  to surface  44 , and to pupil  46  from surface  44  may, for example, be substantially greater (e.g., two times greater or more than two times greater) than the optical path length from lens  34  to the point within the waveguide where the rays of light from the prism converge in scenarios where input coupler  28  is formed from a transmissive input coupling prism mounted to surface  42  of waveguide  26 . This may serve to optimize the optical performance of optical system  14 B while minimizing the distance  53  from the input of lens  34  to surface  42  (e.g., thereby allowing optical system  14 B to occupy less space and to better fit within the form factor of a head mounted support structure such as support structure  20  of  FIG.  1   ) relative to scenarios where a transmissive input coupling prism is used. 
     Lens  34  may be formed from one or more lens elements  48  (e.g., a first lens element  48 - 1 , an Nth lens element  48 -N, etc.). Lens elements  48  may include any desired lens elements (e.g., lenses with concave surfaces, convex surfaces, planar surfaces, spherical surfaces, aspherical surfaces, and/or free-form curved surfaces, lenses with microstructures, Fresnel lenses, combinations of these and/or other types of lens elements, etc.). Lens  34  may have a width  52  and a length  50 . 
     If desired, an optional reflective layer such as reflective layer  45  may be layered onto surface  44  of prism  36 . Reflective layer  45  may include a reflective dielectric coating, a reflective metallic coating, patterns of three-dimensional metal structures, and/or a layer of (reflective) diffraction gratings such as one or more thin-film or volume holograms in a grating medium or surface relief gratings layered onto surface  44 . Reflective layer  45  may, for example, help to increase the amount of light  22  that is reflected off of surface  44 . If desired, reflective layer  45  may also be configured to provide light  22  with an optical power upon reflecting off of reflective layer  45  at surface  44  of prism  36 . For example, diffractive gratings in reflective layer  45  may diffract light  22  back towards waveguide  26  while also imparting optical power onto the diffracted light. Reflective layer  45  may be omitted if desired. 
     In the example of  FIG.  3   , surface  44  of prism  36  is planar. This example is merely illustrative. If desired, surface  44  may be curved.  FIG.  4    is a top view showing how surface  44  of prism  36  may be curved. As shown in  FIG.  4   , the curved shape of surface  44  may impart light  22  with optical power upon reflection off of surface  44 . Surface  44  may have a spherically-curved shape, an aspherically-curved shape, or a free-form curved shape following any desired curved three-dimensional surface. Reflective layer  45  on the curved surface  44  of  FIG.  4    may help to increase the amount of light  22  that is reflected off of surface  44  and/or may help contribute to the optical power of the light  22  reflected into waveguide  26 . Reflective layer  45  may be omitted if desired. 
     In the example of  FIG.  4   , lens  34  includes M lens elements  48  (e.g., a first lens element  48 - 1 , an Mth lens element  48 -M, etc.). The number M of lens elements  48  in lens  34  in the example of  FIG.  4    may be less than the number N of lens elements  48  in lens  34  in the example of  FIG.  3   . The optical power imparted by curved surface  44  and/or reflective layer  45  may, for example, contribute the optical power to light  22  that would otherwise have been provided by the N-M extra lenses in lens  34  of  FIG.  3   . This may allow the input of lens  34  to be separated from surface  42  of waveguide  26  by a distance  54  that is shorter than distance  53  of  FIG.  3   . This may serve to further reduce the amount of space occupied by optical system  14 B in system  10  ( FIG.  1   ), for example. 
     In the example of  FIGS.  3  and  4   , light  22  may be confined to a relatively small pupil within waveguide  26  (e.g., pupil  46  of  FIG.  3   ). In practice, it may be desirable for optical system  14 B to fill as large of an eye box  24  with as uniform an intensity of light  22  as possible. If desired, light redirecting elements such as input coupler  28  may be configured to expand light  22  in one or more dimensions while also coupling light  22  into waveguide  26 , to fill as large of an eye box  24  with as uniform an intensity of light  22  as possible. 
       FIG.  5    shows an example in which prism  36  includes a switchable reflective surface for expanding the field of view of light  22 . As shown in  FIG.  5   , a switchable reflective layer such as switchable reflective layer  56  may be layered onto surface  44  of prism  36 . Switchable reflective layer  56  may include, for example, a liquid crystal (LC) grating, a digital micromirror device (DMD), or other microelectromechanical (MEMs) structures that are switchable between at least first and second states. Switchable reflective layer  56  may receive control signals over control path  58  (e.g., from control circuitry  16  of  FIG.  1   ) that place switchable reflective layer  56  into a selected one of the first and second states at any given time. 
     In the first state, image light  22  may reflect off of surface  44  and switchable reflective layer  56  (sometimes referred to herein collectively as a switchable reflective surface) in a first direction into waveguide  26 , as shown by arrow (ray)  60 . This reflected light may exhibit a relatively-small field of view  62 . In the second state, image light  22  may reflect off of surface  44  and switchable reflective layer  56  in a second direction into waveguide  26 , as shown by arrow  64 . This reflected light may exhibit a relatively-small field of view  66 . 
     The control circuitry may rapidly toggle switchable reflective layer  56  between the first and second states, as shown by arrow  68 , so that the light  22  coupled into waveguide  26  exhibits a relatively wide effective field of view  70 . Switchable reflective layer  56  may be switched between the first and second states at a speed greater than the response speed of the human eye (e.g., greater than 60 Hz, greater than 120 Hz, greater than 240 Hz, greater than 1 kHz, greater than 10 kHz, etc.) so that a user at eye box  24  ( FIG.  2   ) is unable to separately perceive each state and instead perceives a single effective field of view  70 . In this way, image light  22  may be coupled into waveguide  26  and provided to the eye box with a wider effective field of view than would otherwise be provided to the eye box. As an example, fields of view  62  and  66  may each be 30 degrees, 25 degrees, between 25 and 35 degrees, less than 45 degrees, etc., whereas field of view  70  is 60 degrees, between 55 and 65 degrees, greater than 45 degrees, or any other desired angle greater than field of view  62  or field of view  66 . 
       FIG.  6    shows another example in which prism  36  expands the field of view of light  22 . As shown in  FIG.  6   , prism  36  may be a split prism having a first portion  76  and a second portion  78  stacked on upper surface  72  of first portion  76 . Upper surface  72  may be planar, may be tilted in one or more directions (e.g., including out of the plane of the page), or may be curved (e.g., free-form curved). Portions  78  and  76  may be formed from the same material (e.g., may have the same index of refraction) or may be formed from different materials (e.g., materials having different indices of refraction). 
     Portion  76  may have a wedge shape and may therefore sometimes be referred to herein as wedge  76 . Portion  78  may have a wedge shape and may therefore sometimes be referred to herein as wedge  78 . This is merely illustrative and, in general, portions  76  and  78  may have any desired shapes. Wedge  76  may include bottom surface  38  of prism  36  and may be mounted to surface  40  of waveguide  26 . Upper surface  72  of wedge  76  may be oriented at an acute angle such as angle  80  with respect to surface  38  (e.g., surfaces  38 ,  40 , and/or  42  may be planar). Wedge  78  may include the reflective surface  44  of prism  36 . Surface  44  may be oriented at an acute angle such as angle  82  with respect to surface  80  (e.g., angle  82  may be greater than angle  80 ). 
     Prism  36  may include a passive partially reflective layer such as polarization-sensitive reflective film  74 . Polarization-sensitive reflective film  74  (sometimes referred to herein as polarizing beam splitter  74  or reflective film  74 ) may be layered onto upper surface  72  of wedge  76  (e.g., wedge  78  may be mounted to reflective film  74 ). Reflective film  74  may reflect light of a first polarization and may pass (transmit) light of a second polarization without reflecting the light of the second polarization. The first polarization may be a p-polarization whereas the second polarization is an s-polarization or the first polarization may be an s-polarization whereas the second polarization is a p-polarization, as an example. If desired, optically clear adhesive (not shown in  FIG.  6    for the sake of clarity) may be used to help adhere wedge  78  to wedge  76 . Reflective film  74  may be formed from a wire grid polarizer, if desired. 
     In this example, optical system  14 B may also include a switchable polarizer such as switchable polarizer  84 . Waveguide  26  may be interposed between switchable polarizer  84  and prism  36 . Switchable polarizer  84  may be mounted to surface  42  of waveguide  26  (e.g., using an optically clear adhesive not shown in  FIG.  6    for the sake of clarity) or may be spaced apart from surface  42 . One or more lens elements (e.g., from lens  34  of  FIG.  2   ) may be interposed between switchable polarizer  84  and waveguide  26 . In another suitable arrangement, display module  14 A may include a spatial light modulator such as a reflective display panel (e.g., a DMD or LCOS display panel) or a transmissive display panel that receives illumination light from one or more light sources. In this example, switchable polarizer  84  may, if desired, be optically coupled between the light sources and the spatial light modulator. 
     Switchable polarizer  84  may receive light  22 . Switchable polarizer  84  may have a first state at which switchable polarizer  84  only passes (transmits) light  22 A of the first polarization and a second state at which switchable polarizer  84  only passes (transmits) light  22 B of the second polarization. When switchable polarizer  84  is in the first state, light  22 A of the first polarization passes through wedge  76  and reflects off of reflective film  72  to produce field of view  62  within waveguide  26 . When switchable polarizer  84  is in the second state, light  22 B of the second polarization passes through wedge  76  and reflective film  74  (e.g., without being reflected by reflective film  74 ). Light  22 B then passes through wedge  78  and is reflected off of surface  44  of wedge  78  to produce field of view  66  within waveguide  26 . If desired, reflective layer  45  may be formed on surface  44  to help increase the reflected intensity and/or to provide optical power to light  22 B. Surface  44  and/or surface  72  (e.g., reflective film  74 ) may be curved if desired (e.g., to provide light  22 A and  22 B with optical power). Reflective layer  45  may be omitted if desired. 
     Switchable polarizer  84  may receive control signals over control path  86  (e.g., from control circuitry  16  of  FIG.  1   ) that place switchable polarizer  84  into a selected one of the first and second states at any given time. The control circuitry may rapidly toggle switchable polarizer  84  between the first and second states, as shown by arrow  68 , so that light  22  (e.g., light  22 A and  22 B) is coupled into waveguide  26  and exhibits the relatively wide effective field of view  70 . Switchable polarizer  84  may be switched between the first and second states at a speed greater than the response speed of the human eye (e.g., greater than 60 Hz, greater than 120 Hz, greater than 240 Hz, greater than 1 kHz, greater than 10 kHz, etc.) so that a user at eye box  24  ( FIG.  1   ) is unable to perceive each state and instead perceives a single effective field of view  70 . In this way, image light  22  may be coupled into waveguide  26  and provided to the eye box with a wider effective field of view than would otherwise be provided to the eye box. In another suitable arrangement, rapidly toggling between light  22 A and  22 B may be used to increase the effective resolution of the images provided at the eye box (e.g., where the light reflected by surfaces  72  and  44  is angularly separated by about one-half of the pixel pitch of the image data to be displayed, thereby doubling the effective resolution of the images provided to the eye box). The methods and systems for increasing field of view as described in connection with any of the embodiments herein may additionally or alternatively be used to increase the effective resolution of the images provided at the eye box in this way (e.g., the arrangement of  FIG.  5    may also be used to produce an image having an increased effective resolution at the eye box rather than to expand the field of view of the light at the eye box). 
       FIG.  7    shows another example in which prism  36  expands the field of view of light  22 . As shown in  FIG.  7   , prism  36  may include a switchable reflective layer such as switchable reflective layer  88  interposed between wedges  76  and  78 . Switchable reflective layer  88  may include a cholesteric liquid crystal reflector or any other desired switchable reflective (mirror) layered onto surface  72  of wedge  76  (e.g., wedge  78  may be mounted to switchable reflective layer  88 ). 
     Switchable reflective layer  88  may receive light  22  through waveguide  26  and wedge  76 . Switchable reflective layer  88  may have a first state at which switchable reflective layer  88  reflects light  22  and a second state at which switchable reflective layer  88  only passes (transmits) light  22  without reflecting light  22 . When switchable reflective layer  88  is in the first state, light  22  passes through wedge  76  and reflects off of switchable reflective layer  88  to produce field of view  62  within waveguide  26 . When switchable reflective layer  88  is in the second state, light  22  passes through wedge  76  and switchable reflective layer  88  (e.g., without being reflected by switchable reflective layer  88 ). Light  22  then passes through wedge  78  and is reflected off of surface  44  of wedge  78  to produce field of view  66  within waveguide  26 . If desired, reflective layer  45  may be formed on surface  44  to help increase the reflected intensity and/or to provide optical power to light  22 . Surface  44  and/or surface  72  (e.g., switchable reflective layer  88 ) may be curved if desired (e.g., to provide light  22  with optical power). Reflective layer  45  may be omitted if desired. 
     Switchable reflective layer  88  may receive control signals over control path  90  (e.g., from control circuitry  16  of  FIG.  1   ) that place switchable reflective layer  88  into a selected one of the first and second states at any given time. The control circuitry may rapidly toggle switchable reflective layer  88  between the first and second states, as shown by arrow  68 , so that light  22  is coupled into waveguide  26  and exhibits the relatively wide effective field of view  70 . Switchable reflective layer  88  may be switched between the first and second states at a speed greater than the response speed of the human eye (e.g., greater than 60 Hz, greater than 120 Hz, greater than 240 Hz, greater than 1 kHz, greater than 10 kHz, etc.) so that a user at eye box  24  ( FIG.  1   ) is unable to perceive each state and instead perceives a single effective field of view  70 . In this way, image light  22  may be coupled into waveguide  26  and provided to the eye box with a wider effective field of view than would otherwise be provided to the eye box. Because switchable reflective layer  88  is insensitive to the polarization of light  22 , prism  36  of  FIG.  7    may expand the field of view of light  22  without requiring other components that polarize light  22 . 
     In the example of  FIGS.  6  and  7   , light  22  is expanded by toggling a switchable element faster than the response speed of the human eye (e.g., because image light is only provided within one of fields of view  62  or  66  at any given time). To accommodate this switching, display module  14 A needs to produce frames of image data in light  22  at the same relatively high speed, so that a full stream of image data is provided to a user at the eye box. In practice, this can consume an undesirably high amount of processing resources and power within system  10 . 
     In another suitable arrangement, light  22  may be concurrently provided within both fields of view  62  and  66  in waveguide  26 . This may allow the field of view of light  22  to be expanded without rapidly toggling a switchable element and without requiring the display module to display image data at a corresponding high speed. This may serve to minimize the amount of processing resources and power consumed by system  10  while still expanding the field of view of light  22  provided to the eye box. In another suitable arrangement, the structures of  FIG.  7    may be used to produce an image having an increased effective resolution at the eye box rather than to expand the field of view of the light at the eye box). 
       FIG.  8    is a diagram showing how light  22  may be concurrently provided within both fields of view  62  and  66  in waveguide  26 . As shown in  FIG.  8   , display module  14 A may include at least a first display panel  93  and a second display panel  95 . Display module  14 A may also include a prism such as prism  94 . Display panels  93  and  95  may be, for example, uLED panels or other emissive displays. Collimating optics, polarizers, and/or any other desired optical components may be interposed between display panel  93  and prism  94  and between display panel  95  and prism  94 . Display panels  93  and  95  may include reflective spatial light modulators (e.g., DMD panels, LCOS panels, etc.) in another suitable arrangement. 
     Display panel  93  may emit light  22 A of the first polarization whereas display panel  95  emits light  22 B of the second polarization. Prism  94  may include a polarizing beam splitter that reflects light  22 A of the first polarization towards lens  34  and input coupler  28  and that transmits light  22 B of the second polarization towards lens  34  and input coupler  28 . Reflective film  74  on wedge  76  may reflect light  22 A to produce field of view  62  within waveguide  26 . At the same time, reflective film  74  may transmit light  22 B, which reflects off of surface  44  to produce field of view  66  within waveguide  26 . Display panels  93  and  95  may be active at the same time to concurrently provide light  22 A and  22 B to input coupler  28 . This may allow prism  36  to concurrently produce fields of view  62  and  66 , so that the in-coupled light collectively exhibits the wide effective field of view  70  within waveguide  26 . 
     The pupils of light  22  within waveguide  26  (e.g., pupil  46  of  FIG.  3   ) may be replicated during propagation down the length of waveguide  26  to fill the eye box with light. In practice, there may be gaps between the replicated pupils that produce uneven brightness uniformity across the area of the eye box. In some scenarios, increasing the size of display module  14 A may help to fill the gaps between the replicated pupils so that the eye box is filled with light of uniform intensity. However, larger display modules  14 A may occupy an excessive amount of space within system  10 . If desired, the optical path of optical system  14 B may include a birefringent beam displacer that helps to fill the gaps between the replicated pupils so that the eye box is filled with light of uniform intensity (e.g., without increasing the size of display module  14 A). In another suitable arrangement, the structures of  FIG.  8    may be used to produce an image having an increased effective resolution at the eye box. 
       FIG.  9    is a perspective view of an illustrative birefringent beam displacer  102  that may be provided in optical system  14 B. As shown in  FIG.  9   , birefringent beam displacer  102  may be formed from a birefringent material such as calcite and may have a length  100  (e.g., in the direction of the optical path). Birefringent beam displacer  102  may receive light  22 . Light  22  may include light of a first polarization  96  in alignment with light of a second polarization  98  (e.g., within a corresponding pupil  104  at the input face of birefringent beam displacer  102 ). 
     Birefringent beam displacer  102  may separate light  22  into a first beam  108  that includes the light of the first polarization  96  and a second beam  109  that includes the light of the second polarization  98 . Upon exiting birefringent beam displacer  102 , beam  109  may be separated from beam  108  by displacement  107 . The magnitude of displacement  107  may be directly proportional to the length  100  of birefringent beam displacer  102 , for example. Beams  108  and  109  may each individually exhibit relatively small pupils  104 . However, displacement  107  may separate the pupils  104  of beams  108  and  109  so that beams  108  and  109  collectively exhibit an expanded pupil  106 . Expanded pupil  106  may be replicated by waveguide  26 . Because expanded pupil  106  is larger than pupil  104 , expanded pupil  106  may fill the eye box with light of more uniform intensity relative to scenarios where the birefringent beam displacer is omitted, for example. 
       FIG.  10    shows one example of how birefringent beam displacer  102  may be mounted within optical system  14 B. As shown in  FIG.  10   , waveguide  26  may be interposed between prism  36  and birefringent beam displacer  102 . Birefringent beam displacer  102  may be mounted to surface  42  (e.g., using optically clear adhesive not shown in  FIG.  10    for the sake of clarity) or may be separated from waveguide  26 . 
     Wedge  76  of prism  36  may, for example, be formed from a material having a first index of refraction n 1  whereas wedge  78  is formed from a material having a second index of refraction n 2 . The difference in indices n 1  and n 2  may cause the light (e.g., after being split into displaced beams  108  and  109  of  FIG.  9   ) to reflect off of surface  72  so that the light (e.g., expanded pupil  106  of  FIG.  9   ) is coupled into waveguide  26 . Partially or completely reflective layers may, if desired, be formed on any of the surfaces of prism  36  shown in  FIG.  10   , which may be planar and/or curved. In another suitable arrangement, birefringent beam displacer  102  may be used to form some or all of prism  36  (e.g., wedges  76  and/or  78 ). In another suitable arrangement, the structures of  FIG.  10    may be used to produce an image having an increased effective resolution at the eye box. 
     In one suitable arrangement, lens  34  of  FIGS.  2 - 4    may include a single set or grouping of lens elements (e.g., lens elements  48  of  FIGS.  3  and  4   ) that direct light  22  towards waveguide  26  without converging the light at a point between two or more of the lens elements. This is merely illustrative. If desired, lens  34  may be provided with a relay optics arrangement in which lens  34  is formed from multiple groups or sets of lens elements that are separated by a gap, where the lens elements converge light  22  at a point between two or more of the lens elements. 
       FIG.  11    is a diagram showing how lens  34  may include multiple groups or sets of lens elements that are separated by a gap, where the lens elements converge light  22  at a point between two or more of the lens elements. As shown in  FIG.  11   , lens  34  may include a first set (group) of lens elements  34 A and a second set (group) of lens elements  34 B (e.g., sets of lens elements such as lens elements  48  of  FIGS.  3  and  4   ). Lens elements  34 A and lens elements  34 B may each include any desired number of lens elements (e.g., one lens element, two lens elements, three lens elements, four lens elements, more than four lens elements, etc.). Lens  34  may include the same number of lens elements  34 A as lens elements  34 B or may include a different number of lens elements  34 A and  34 B. Lens elements  34 A and  34 B may include any desired types of lens elements. If desired, physical light stops such as optical absorbers  114  may be placed at one or more locations in lens  34  (e.g., in gap  110 , around lens elements  34 A, etc.) to help mitigate stray light in the system. 
     Lens elements  34 A may be separated from lens elements  34 B in lens  34  by gap  110 . Gap  110  may be greater than the distance between adjacent lens elements  34 B and greater than the distance between adjacent lens elements  34 A, for example. Lens elements  34 A may receive light  22  from display module  14 A. Lens elements  34 A may focus (converge) light  22  at point (pupil)  112  between lens elements  34 A and lens elements  34 B. The rays of light  22  may then reach lens elements  34 B, which redirect the light towards waveguide  26  (e.g., surface  42  of waveguide  26  as shown in  FIGS.  3 - 8  and  10   ). Arranging the lens elements in lens  34  in this way may allow lens  34  to exhibit a length  116  and a width  118 . Length  116  of lens  34  in  FIG.  11    may, for example, be greater than the length  50  of lens  34  in scenarios where only a single group of lens elements is used (e.g., as shown in  FIG.  3   ). However, width  118  may be less than the width  52  of lens  34  in scenarios where only a single group of lens elements is used (e.g., as shown in  FIG.  3   ). This may, for example, reduce the amount of space required within device  10  of  FIG.  1    without sacrificing image quality. The example of  FIG.  11    is merely illustrative and, if desired, other lens element arrangements may be used. 
     If desired, a switchable element may be mounted within gap  110  of lens  34  to help increase the effective resolution of display  14 .  FIG.  12    is a diagram showing how a switchable element may be mounted within gap  110  of lens  34  to help increase the effective resolution of display  14 . As shown in  FIG.  12   , a switchable element such as switchable light displacing element  120  may be mounted at point  112  within gap  110 . Switchable light displacing element  120  may be a flat transparent plate (e.g., a glass plate, plastic plate, etc.) that is switchable between first and second states. 
     In the first state, switchable light displacing element  120  may have a first orientation. In the second state, switchable light displacing element  120  may have a second orientation. Switchable light displacing element  120  may include MEMs structures, piezoelectric structures, or other structures for adjusting the orientation of the switchable element so that the switchable element is placed in a selected one of the first and second states. Switchable light displacing element  120  may receive control signals over control path  122  (e.g., from control circuitry  16  of  FIG.  1   ) that place switchable light displacing element  120  into a selected one of the first and second states at any given time. 
       FIG.  13    is a diagram showing how switchable light displacing element  120  may be placed into the first and second states. As shown in  FIG.  13   , switchable light displacing element  120  may have a first orientation such as first orientation  124  in the first state. Switchable light displacing element  120  may have a second orientation such as second orientation  126  in the second state. Light  22  from a single pixel in display module  14 A is shown in the example of  FIG.  13    for the sake of clarity. In general, light  22  is received at switchable light displacing element  120  for each pixel in the display module. 
     In the first state, light  22  may pass directly through switchable light displacing element  120 , as shown by ray  128 . In the first state, light  22  may be incident upon switchable light displacing element  120  parallel to a normal axis of the lateral area of switchable light displacing element  120 . This may allow light  22  to pass through switchable light displacing element  120  without refracting. In the second state, light  22  may be incident upon switchable light displacing element  120  at a non-zero angle with respect to the normal axis of switchable light displacing element  120 . This may cause light  22  to refract prior to being output by switchable light displacing element  120 , as shown by ray  130 . This may cause light  22  (ray  130 ) to be displaced in the second state by an offset such as offset  132  relative to the first state (ray  128 ). 
     The control circuitry may rapidly toggle (e.g., rotate) switchable light displacing element  120  between the first and second states, as shown by arrow  134 , so that the light  22  is provided to the waveguide with a first alignment, as shown by ray  128 , and a second alignment (e.g., a second alignment that is parallel to and offset from the first alignment), as shown by ray  130 . Similar rays are produced for each pixel in the display, such that the first and second alignments collectively cause the light to exhibit a greater resolution than would otherwise be available in the absence of switchable light displacing element  120 . Switchable light displacing element  120  may be switched between the first and second states at a speed greater than the response speed of the human eye (e.g., greater than 60 Hz, greater than 120 Hz, greater than 240 Hz, greater than 1 kHz, greater than 10 kHz, etc.) so that a user at eye box  24  ( FIG.  2   ) is unable to perceive each state. In this way, light  22  may be coupled into waveguide  26  and provided to the eye box with a greater effective resolution than would otherwise be provided to the eye box in the absence of switchable light displacing element  120 . 
       FIG.  14    is a front view showing how switchable light displacing element  120  may provide light  22  with an increased effective resolution to the waveguide (e.g., as taken in the direction of arrow  131  of  FIG.  14   ). In the example of  FIG.  14   , four pixels of light  22  are shown for the sake of clarity. In general, light  22  and the display module may include any desired number of pixels. 
     As shown in  FIG.  14   , light  22  may include pixels P 1 , P 2 , P 3 , and P 4  when switchable light displacing element  120  is in the first state (e.g., orientation  124  of  FIG.  13   ). When switchable light displacing element  120  is in the second state (e.g., orientation  126  of  FIG.  13   ), pixels P 1 , P 2 , P 3 , and P 4  may be displaced by displacement  138 , as shown by respective pixels P 1 ′, P 2 ′, P 3 ′, and P 4 ′. Displacement  138  may, for example, be a two-dimensional displacement that includes offset  136  parallel to the Z-axis and/or offset  132  parallel to the X-axis. 
     Pixels P 1 , P 2 , P 3 , and P 4  may exhibit a first pixel pitch and pixels P 1 ′, P 2 ′, P 3 ′, and P 4 ′ may exhibit the first pixel pitch. However, the combination of pixels P 1 , P 2 , P 3 , and P 4  with pixels P 1 ′, P 2 ′, P 3 ′, and P 4 ′ may exhibit a second pixel pitch that is less than (e.g., half) the first pixel pitch. By rapidly toggling between the first and second states, light  22  may effectively include each of pixels P 1 , P 2 , P 3 , P 4 , P 1 ′, P 2 ′, P 3 ′, and P 4 ′ (e.g., as perceived by a user at the eye box) and thus the second pixel pitch, rather than only pixels P 1 , P 2 , P 3 , and P 4  and the first pixel pitch (e.g., in scenarios where switchable light displacing element  120  is omitted). This may serve to increase the effective resolution of light  22  relative to scenarios where switchable light displacing element  120  is omitted (e.g., to twice the resolution that light  22  would otherwise have in the absence of switchable light displacing element  120 ), without requiring an increase in size or processing resources for display module  14 A. 
     In scenarios where prism  36  includes a curved reflective surface (e.g., surface  44  of  FIG.  4   ), the optical power introduced by the curved reflective surface may allow lens elements  34 B of lens  34  ( FIGS.  11  and  12   ) to be omitted.  FIG.  15    is a diagram showing how the curved reflective surface of prism  36  may replace lens elements  34 B of  FIGS.  11  and  12   . 
     As shown in  FIG.  15   , lens elements  34 A of lens  34  may converge light  22  at point  112 . Light  22  may subsequently pass through waveguide  26  to prism  36  and may be reflected off of the curved surface  44  of prism  36 . The curve of surface  44  may introduce optical power to the light that would otherwise have been introduced by lens elements  34 B of  FIGS.  11  and  12   . This may allow lens elements  34 B to be omitted, allowing lens elements  34 A to be placed closer to waveguide  26  than in scenarios where lens elements  34 B are included in lens  34 . For example, there may be a distance  140  between the input to lens  34  and surface  42  of waveguide  26 , where distance  140  is less than length  116  of  FIG.  11    and distance  53  of  FIG.  3   . Switchable light displacing element  120  may be mounted at point  112  for increasing the effective resolution of light  22  if desired. 
     The example of  FIG.  15    is merely illustrative. If desired, reflective layer  45  may be layered onto surface  44  or may be omitted. Switchable light displacing element  120  may be omitted if desired. Prism  36  of  FIG.  15    may include one or more wedges such as wedges  76  and  78  of  FIGS.  6 - 8  and  10   ). Surface  44  may have any desired free-form curved shape. Any desired combination of the arrangements of  FIGS.  3 - 15    may be used. 
     In practice, display module  14 A and lens  34  may focus image light  22  onto the reflective surface(s) of prism  36  within a corresponding pupil. If desired, the structures layered onto the reflective surface(s) of prism  36  may extend over (overlap) only a portion of the reflective surface(s), where the portion overlaps the pupil of the image light  22  focused on the reflective surface(s) (e.g., the portion may have the same size and shape as the pupil or may be slightly larger than the pupil). The remainder of the reflective surface(s) may be covered (overlapped) by an optical absorber that serves as a physical stop for the image light. For example, as shown in  FIG.  16   , prism  36  may include reflective layer  150  on reflective surface  44  that reflects image light  22  into waveguide  26 . The size of reflective layer  150  may match the pupil size of image light  22 . The remainder of reflective surface  44  may be covered by optical absorber  152 . This example is merely illustrative and, in general, reflective layer  45  of  FIGS.  3 ,  4 ,  6 - 8 , and  15   , switchable reflective layer  56  of  FIG.  5   , polarization-sensitive reflective film  74  of  FIGS.  6  and  8   , and switchable reflective layer  88  of  FIG.  7    may overlap only a portion of the corresponding reflective surface of prism  36  that overlaps (e.g., matches) the area spanned by the pupil of image light  22  focused on the reflective surface (e.g., as shown by reflective layer  150  of  FIG.  16   ), whereas the remainder of the reflective surface is covered with an optical absorber (e.g., as shown by optical absorber  152  of  FIG.  16   ). This may serve to minimize stray light in optical system  14 B, thereby maximizing the contrast of images provided at eye box  24 . 
     If desired, an optional optical wedge such may be interposed on the optical path between waveguide  26  and display modules  14 A, as shown by optical wedge  158  of  FIG.  16   . Optical wedge  158  may have a first surface  154  facing and parallel to the lateral surface of waveguide  26 . Optical wedge  158  may have an opposing second surface  156  that is oriented at a non-parallel angle (e.g., tilted) with respect to first surface  154 . Image light  22  may pass through optical wedge  158  before passing through waveguide  26 . Optical wedge  158  may be a dispersion compensation wedge that compensates for dispersion of image light  22  by prism  36  and/or the other optical components of the system (e.g., in scenarios where prism  36  has a refractive index as a function of wavelength that is different from the bulk refractive index as a function of wavelength of the grating medium in waveguide  26 ). If desired, optical wedge  158  may additionally or alternatively help redirect image light  22  incident at other angles (e.g., angles non-parallel with respect to the Y-axis) towards prism  36 . This may allow display module  14 A to be mounted at different locations or orientations with respect to waveguide  26  than would otherwise be possible in the absence of the optical wedge. For example, the optical wedge may allow display module  14 A to be located within a main frame for waveguide  26  (e.g., within support structures  20  of  FIG.  1   ) without needing to be located in the temple or other portions of the support structures (e.g., thereby optimizing space consumption within system  10 ). Optical wedge  158  of  FIG.  16    may be interposed on the optical path for image light  22  between waveguide  26  and display module  14 A in any of the arrangements of  FIGS.  3 - 8 ,  10 , and  15    if desired. 
     In accordance with an embodiment, a display system is provided that includes a display module that produces image light, a waveguide, a reflective input coupling prism mounted to the waveguide, the reflective input coupling prism is configured to receive the image light through the waveguide, the reflective input coupling prism is configured to reflect the image light into the waveguide, and the waveguide is configured to propagate the reflected image light via total internal reflection, and an output coupler on the waveguide and configured to couple the reflected image light out of the waveguide. 
     In accordance with another embodiment, the output coupler includes diffractive grating structures in a grating medium embedded within the waveguide, the diffractive grating structures include volume holograms, and the display system includes a dispersion compensation wedge optically interposed between the display module and the waveguide. 
     In accordance with another embodiment, the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the display system includes a reflective layer on the reflective surface. 
     In accordance with another embodiment, the reflective layer includes a switchable reflective layer, the switchable reflective layer being adjustable between a first state at which the switchable reflective layer reflects the image light into the waveguide in a first direction and a second state at which the switchable reflective layer reflects the image light into the waveguide in a second direction that is different from the first direction, and the switchable reflective layer includes a structure selected from the group consisting of a switchable liquid crystal grating and a digital micromirror device. 
     In accordance with another embodiment, the reflective layer includes a layer selected from the group consisting of a dielectric layer, a metallic layer, a layer of three-dimensional metal structures, and a layer of diffractive gratings. 
     In accordance with another embodiment, the reflective surface includes a curved surface that is configured to provide the reflected light with an optical power. 
     In accordance with another embodiment, the reflective layer covers a portion of the reflective surface that overlaps a pupil of the image light that has been focused onto the reflective surface and the display system includes an optical absorber that covers a remainder of the reflective surface. 
     In accordance with another embodiment, the display includes a lens configured to direct the image light from the display module towards the waveguide, the lens is configured to converge the image light at a location interposed between the waveguide and the lens, and a transparent plate at the location, the transparent plate being adjustable between a first orientation at which the transparent plate directs the image light towards the waveguide with a first alignment and a second orientation at which the transparent plate directs the image light towards the waveguide with a second alignment that is parallel to and offset from the first alignment. 
     In accordance with another embodiment, the waveguide has opposing first and second surfaces, the reflective input coupling prism includes a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the second wedge includes the reflective surface, the surface of the first wedge is configured to reflect the image light into the waveguide in a first direction, and the reflective surface is configured to reflect the image light into the waveguide in a second direction that is different from the first direction. 
     In accordance with another embodiment, the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the display includes a polarization-sensitive film interposed between the first and second wedges, the polarization-sensitive film is configured to reflect a first polarization of the image light in the first direction and to transmit a second polarization of the image light, and the reflective surface is configured to reflect, in the second direction, the second polarization of the image light transmitted by the polarization-sensitive film. 
     In accordance with another embodiment, the display includes a switchable polarizer, the switchable polarizer is switchable between first and second states, the switchable polarizer is configured to transmit, in the first state, the first polarization of the image light through the second surface of the waveguide, and the switchable polarizer is configured to transmit, in the second state, the second polarization of the image light through the second surface of the waveguide. 
     In accordance with another embodiment, the display module includes a first display panel configured to transmit the first polarization of the image light towards the reflective input coupling prism and the display module includes a second display panel configured to concurrently transmit the second polarization of the image light towards the reflective input coupling prism. 
     In accordance with another embodiment, the reflective input coupling prism has a reflective surface configured to reflect the image light into the waveguide, the waveguide has opposing first and second surfaces, the reflective input coupling prism includes a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the second wedge includes the reflective surface, the surface of the first wedge is configured to reflect the image light into the waveguide in a first direction, and the reflective surface is configured to reflect the image light into the waveguide in a second direction that is different from the first direction, the display includes a switchable reflective layer interposed between the first and second wedges, the switchable reflective layer is switchable between first and second states, the switchable reflective layer is configured to reflect, in the first state, the image light in the first direction, the switchable reflective layer is configured to transmit, in the second state, the image light towards the reflective surface, and the switchable reflective layer includes a cholesteric liquid crystal reflector. 
     In accordance with another embodiment, the display includes a birefringent beam displacer configured to receive a pupil of image light, the birefringent beam displacer is configured to transmit a first polarization of the pupil towards the reflective input coupling prism within a first beam, and the birefringent beam displacer is configured to transmit a second polarization of the pupil towards the reflective input coupling prism in a second beam that is offset from the first beam. 
     In accordance with another embodiment, the waveguide has opposing first and second surfaces, the reflective input coupling prism includes a first wedge mounted to the first surface and a second wedge mounted to a surface of the first wedge, the birefringent beam displacer is mounted to the second surface, the first wedge has a first refractive index, and the second wedge has a second refractive index that is different from the first refractive index. 
     In accordance with another embodiment, the reflective input coupling prism includes a birefringent beam displacer. 
     In accordance with another embodiment, the display includes a lens configured to direct the image light from the display module towards the reflective input coupling prism, the lens includes a first set of lens elements, and a second set of lens elements separated from the first set of lens elements by a gap, the second set of lens elements is interposed between the first set of lens elements and the waveguide, and the first set of lens elements is configured to converge the image light at a location within the gap. 
     In accordance with another embodiment, the display includes a switchable element at the location within the gap, the switchable element is switchable between a first state at which the switchable element passes the image light to the second set of lens elements with a first alignment and a second state at which the switchable element passes the image light to the second set of lens elements with a second alignment that is displaced with respect to the first alignment, and the switchable element includes a transparent panel and a piezoelectric component configured to rotate the transparent panel between a first orientation in the first state and a second orientation in the second state. 
     In accordance with an embodiment, an electronic device is provided that includes a display module that produces image light, a waveguide having opposing first and second surfaces, collimating optics that direct the image light towards the second surface of the waveguide, and an input coupling prism configured to receive the image light from the collimating optics through the waveguide, the input coupling prism includes a first wedge mounted to the first surface of the waveguide and having a first reflective surface oriented at a first angle with respect to the first surface of the waveguide, the first wedge is configured to reflect the image light into the waveguide in a first direction, and a second wedge mounted to the first reflective surface and having a second reflective surface oriented at a second angle with respect to the first surface of the waveguide, the second angle is greater than the first angle, the second wedge is configured to receive the image light through the first wedge, and the second wedge is configured to reflect the image light into the waveguide in a second direction that is different from the first direction. 
     In accordance with another embodiment, the electronic device includes a polarizing beam splitter layered on the first reflective surface. 
     In accordance with another embodiment, the electronic device includes a switchable reflective layer on the first reflective surface. 
     In accordance with another embodiment, the first wedge has a first refractive index and the second wedge has a second refractive index that is different from the first refractive index. 
     In accordance with another embodiment, the second reflective surface is curved and the first reflective surface is curved. 
     In accordance with an embodiment, a display is provided that includes a display module that generates image light, a waveguide having opposing first and second surfaces, a lens configured to direct the image light towards the second surface of the waveguide, the lens includes a set of lens elements configured to converge the image light at a location interposed between the set of lens elements and the second surface of the waveguide, and a reflective input coupling prism mounted to the first surface of the waveguide and configured to receive the image light from the lens through the waveguide, the reflective input coupling prism is configured to reflect the image light into the waveguide. 
     In accordance with another embodiment, the lens includes an additional set of lens elements interposed between the location and the second surface of the waveguide, including a transparent panel at the location, the transparent panel is switchable between first and second orientations, the transparent panel is configured to pass, in the first orientation, the image light to the second set of lens elements with a first alignment, and the transparent panel is configured to pass, in the second orientation, the image light to the second set of lens elements with a second alignment that is offset from the first alignment. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20210916
Publication Date: 20240528
Grant Date: 20240528
Priority Date: 20190919
Inventors: BHAKTA, Vikrant
PENG, GUOLIN
CHOI, Hyungryul
KEILBACH, KEVIN A.
DELAPP, SCOTT M.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0123", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0147", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0163", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0025", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0031", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0023", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0123", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0147", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0163", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 72659366