PATENT DOCUMENT

Publication Number: US-12105287-B1
Application Number: US-202117373190-A
Country: US
Kind Code: B1

Title: Optical systems for leveraging the non-zero transition time of display panel mirrors

Abstract:
A display having a reflective display panel may provide image light to an eye box. The panel may include mirrors rotatable between first and second angles. The mirrors may take a non-zero time period to transition between the first and second angles. During the non-zero time period, the light source may emit pulses of illumination. The mirrors may reflect the pulses of illumination as offset pupils of image light. The mirrors may be at respective intermediate angles while reflecting each of the pulses of illumination. The mirrors may toggle between the first and second angles at a sufficiently fast rate such that the offset pupils form an effective pupil that is expanded in at least one dimension. If desired, the offset pupils may be used to display virtual objects in different focal planes at the eye box.

Claims:
What is claimed is: 
     
       1. A display system comprising:
 a light source configured to emit illumination light; 
 a reflective display panel configured to reflect the illumination light as image light, wherein the reflective display panel comprises a plurality of mirrors, a mirror from the plurality of mirrors is adjustable between an off state and an on state, the mirror is oriented at first tilt angle in the off state, and the mirror is oriented at a second tilt angle that is different from the first tilt angle in the on state; 
 a waveguide having an input coupler configured to couple the image light into the waveguide and having an output coupler configured to couple the image light out of the waveguide and towards an eye box; and 
 control circuitry coupled to the reflective display panel, wherein the control circuitry is configured to:
 rotate the mirror from the first tilt angle to the second tilt angle, and 
 pulse the light source while the mirror is rotating from the first tilt angle to the second tilt angle. 
 
 
     
     
       2. The display system of  claim 1 , wherein the reflective display panel comprises a digital-micromirror device (DMD) and wherein the mirror comprises a micromirror. 
     
     
       3. The display system of  claim 1 , further comprising:
 at least one lens element, wherein the at least one lens element is configured to provide the illumination light to each mirror of the plurality of mirrors at a common angle of incidence. 
 
     
     
       4. The display system of  claim 1 , wherein the control circuitry is configured to pulse the light source while the mirror is at an intermediate tilt angle that is between the first tilt angle and the second tilt angle. 
     
     
       5. The display system of  claim 4 , wherein the control circuitry is configured to pulse the light source while the mirror is at the second tilt angle. 
     
     
       6. The display system of  claim 5 , wherein the mirror is configured to provide the image light to the input coupler within a first pupil while the mirror is at the intermediate tilt angle, wherein the mirror is configured to provide the image light to the input coupler within a second pupil while the mirror is at the second tilt angle, and wherein the second pupil is at least partially non-overlapping with respect to the first tilt angle. 
     
     
       7. The display system of  claim 6 , wherein the first and second pupils collectively form an expanded pupil that is larger than the first pupil and that is larger than the second pupil. 
     
     
       8. The display system of  claim 6 , wherein the waveguide comprises first and second layers of grating medium, wherein the input coupler comprises a first input coupling structure in the first layer of grating medium and a second input coupling structure in the second layer of grating medium, and wherein the display system further comprises:
 at least one lens element that is configured to focus the first pupil on the first input coupling structure and that is configured to focus the second pupil on the second input coupling structure. 
 
     
     
       9. The display system of  claim 8 , wherein the output coupler comprises a first output coupling structure in the first layer of grating medium and a second output coupling structure in the second layer of grating medium, wherein the first output coupling structure is configured to couple the image light out of the waveguide and towards the eye box with a first optical power, and wherein the second output coupling structure is configured to couple the image light out of the waveguide and towards the eye box with a second optical power that is different from the first optical power. 
     
     
       10. The display system of  claim 4 , wherein the control circuitry is configured to pulse the light source while the mirror is at an additional intermediate tilt angle that is between the intermediate tilt angle and the second tilt angle. 
     
     
       11. The display system of  claim 10 , wherein the mirror is configured to provide the image light to the input coupler within a first pupil while the mirror is at the intermediate tilt angle, wherein the mirror is configured to provide the image light to the input coupler within a second pupil while the mirror is at the additional intermediate tilt angle, and wherein the second pupil is at least partially non-overlapping with respect to the first tilt angle. 
     
     
       12. The display system of  claim 11 , wherein the first and second pupils collectively form an expanded pupil that is larger than the first pupil and that is larger than the second pupil. 
     
     
       13. The display system of  claim 11 , wherein the waveguide comprises first and second layers of grating medium, wherein the input coupler comprises a first diffractive grating structure in the first layer of grating medium and a second diffractive grating structure in the second layer of grating medium, wherein the first diffractive grating structure is Bragg-matched to the first pupil, and wherein the second diffractive grating structure is Bragg-matched to the second pupil. 
     
     
       14. The display system of  claim 13 , wherein the output coupler comprises a first output coupling structure in the first layer of grating medium and a second output coupling structure in the second layer of grating medium, wherein the first output coupling structure is configured to couple a first portion of the image light having a first virtual object in a first focal plane out of the waveguide and towards the eye box, and wherein the second output coupling structure is configured to couple a second portion of the image light having a second virtual object in a second focal plane that is different from the first focal plane out of the waveguide. 
     
     
       15. The display system of  claim 1 , wherein the control circuitry is further configured to:
 rotate the mirror from the second tilt angle to the first tilt angle, and 
 pulse the light source while the mirror is rotating from the second tilt angle to the first tilt angle. 
 
     
     
       16. The display system of  claim 15 , wherein the control circuitry is further configured to:
 toggle the mirror between the first and second tilt angles at a rate greater than or equal to 24 Hz. 
 
     
     
       17. A method of operating a display system to provide image light to an eye box, wherein the display system comprises a light source, a digital-micromirror device (DMD) panel, control circuitry, and a waveguide having an input coupler and an output coupler, wherein the DMD panel comprises a micromirror that is adjustable between a first tilt angle and a second tilt angle, and wherein the method comprises:
 with the control circuitry, adjusting the micromirror from the first tilt angle to the second tilt angle, wherein the micromirror takes a non-zero time period to transition from the first tilt angle to the second tilt angle; 
 with the control circuitry, during the non-zero time period, controlling the light source to emit at least one pulse of illumination light; 
 with the micromirror, during the non-zero time period, reflecting the at least one pulse of illumination light as image light; 
 with the input coupler, coupling the image light into the waveguide; and 
 with the output coupler, coupling the image light out of the waveguide and towards the eye box. 
 
     
     
       18. The method of  claim 17 , further comprising:
 with the control circuitry, adjusting the micromirror from the second tilt angle to the first tilt angle, wherein the micromirror takes an additional non-zero time period to transition from the second tilt angle to the first tilt angle; and 
 with the control circuitry, during the additional non-zero time period, controlling the light source to emit at least one additional pulse of illumination light. 
 
     
     
       19. The method of  claim 18 , further comprising:
 with the control circuitry, toggling the micromirror between the first and second tilt angles at a rate greater than or equal to 30 Hz. 
 
     
     
       20. The method of  claim 17 , wherein the at least one pulse of illumination light comprises a plurality of pulses of the illumination light and wherein reflecting the at least one pulse of illumination light comprises reflecting, at different respective intermediate tilt angles between the first and second tilt angles, each of the pulses from the plurality of pulses of the illumination light as the image light. 
     
     
       21. The method of  claim 20 , wherein the plurality of pulses comprises first and second pulses and wherein the different respective intermediate tilt angles comprise a first intermediate tilt angle at which the micromirror reflects the first pulse within a first pupil and a second intermediate tilt angle at which the micromirror reflects the second pulse within a second pupil that is at least partially non-overlapping with respect to the first pupil. 
     
     
       22. The method of  claim 21 , wherein coupling the image light out of the waveguide and towards the eye box comprises coupling the first pupil out of the waveguide with a first optical power and coupling the second pupil out of the waveguide with a second optical power that is different from the first optical power. 
     
     
       23. A display system comprising:
 a light source; 
 a digital-micromirror device (DMD) panel that comprises a micromirror rotatable between a first tilt angle and a second tilt angle; 
 control circuitry coupled to the DMD panel, wherein the control circuitry is configured to:
 control the micromirror to rotate from the first tilt angle to the second tilt angle, and 
 control the light source to emit first and second pulses of illumination light while the micromirror is rotating from the first tilt angle to the second tilt angle, wherein the micromirror is configured to reflect the first pulse of illumination light at a first output angle as first image light, wherein the micromirror is configured to reflect the second pulse of illumination light at a second output angle as second image light, and wherein the second output angle is different from the first output angle; and 
 
 a waveguide, wherein the waveguide has an input coupler configured to couple the first and second image light into the waveguide and wherein the waveguide has an output coupler configured to couple the first and second image light out of the waveguide and towards an eye box. 
 
     
     
       24. The display system of  claim 23 , wherein the DMD panel comprises an additional micromirror rotatable between the first tilt angle and the second tilt angle, wherein the control circuitry is further configured to:
 control the additional micromirror to rotate from the first tilt angle to the second tilt angle, wherein the additional micromirror is configured to reflect the first pulse of illumination light at the first output angle as a part of the first image light, and wherein the additional micromirror is configured to reflect the second pulse of illumination light at the second output angle as part of the second image light. 
 
     
     
       25. The display system of  claim 24 , further comprising:
 a lens configured to focus the first image light onto the input coupler within a first pupil and configured to focus the second image light onto the input coupler within a second pupil that is at least partially offset with respect to the first pupil. 
 
     
     
       26. The display system of  claim 25 , wherein the control circuitry is configured to toggle the micromirror and the additional micromirror between the first and second tilt angles at a rate greater than or equal to 60 Hz. 
     
     
       27. The display system of  claim 25 , wherein the waveguide comprises:
 a first waveguide portion, wherein the input coupler comprises a first input coupling structure in the first waveguide portion, the first input coupling structure being configured to redirect the first image light towards the output coupler via the first waveguide portion; and 
 a second waveguide portion stacked onto the first waveguide portion, wherein the input coupler comprises a second input coupling structure in the second waveguide portion, the second input coupling structure being configured to redirect the second image light towards the output coupler via the second waveguide portion. 
 
     
     
       28. The display system of  claim 27 , wherein the output coupler comprises;
 a first diffractive grating structure in the first waveguide portion, wherein the first diffractive grating structure is configured to couple the first image light out of the waveguide and towards the eye box; and 
 a second diffractive grating structure in the second waveguide portion, wherein the second diffractive grating structure is configured to couple the second image light out of the waveguide and towards the eye box.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 63/051,325, filed Jul. 13, 2020, which is hereby incorporated by reference herein in its entirety. 
    
    
     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, can consume excessive power, 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 display may include a display module and a waveguide. The display module may include a light source and a reflective display panel. The reflective display panel may be a digital-micromirror device (DMD) panel. The DMD panel may include micromirrors rotatable between an “OFF” state in which the micromirrors are oriented at a first tilt angle and an “ON” state in which the micromirrors are oriented at a second tilt angle. The control circuitry may control at least one of the micromirrors to rotate between the first and second tilt angles. The micromirror may take a non-zero time period to transition between the first and second tilt angles. During the non-zero time period, the control circuitry may control the light source to emit pulses of illumination light. The mirror may reflect the pulses of illumination light as offset pupils of image light. The mirror may be at a respective intermediate tilt angle while reflecting each of the pulses of illumination light. The control circuitry may toggle the mirror between the first and second tilt angles at a sufficiently fast rate such that the offset pupils form an effective pupil that is expanded in at least one dimension. 
     An input coupler may couple the image light into the waveguide. An output coupler may couple the image light out of the waveguide and towards an eye box. If desired, the waveguide may include stacked layers. The input coupler may include different input coupling structures in each of the layers. Each input coupling structure may couple a respective one of the pupils of image light into the waveguide. The output coupler may include different output coupling structures in each of the layers. Each output coupling structure may couple the image light from a respective one of the input coupling structures out of the waveguide. If desired, each output coupling structure may impart a different respective optical power to the image light. In this way, by leveraging the non-zero time period required by the mirrors to rotate between the first and second tilt angles, the display may provide effectively expanded pupils of image light and/or virtual objects in different focal planes at the eye box. Performing pupil expansion using the intermediate tilt angles may allow other pupil expanding components such as a cross-coupler to be omitted from the waveguide. Omitting the cross-coupler may minimize the number of diffractions performed on the image light, thereby maximizing throughput and brightness of the image light at the eye box. 
    
    
     
       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 that receives light from a display module in accordance with some embodiments. 
         FIG.  3    is a top view of an illustrative display module having a reflective display panel in accordance with some embodiments. 
         FIG.  4    is a schematic diagram of an illustrative reflective display panel mirror that transitions between ON and OFF states in accordance with some embodiments. 
         FIG.  5    includes graphs that show how an illustrative light source may be synchronized with a reflective display panel mirror that transitions between ON and OFF states in accordance with some embodiments. 
         FIG.  6    is a schematic diagram of an illustrative reflective display panel mirror that shows how the reflective display panel mirror may have intermediate orientations that are used to reflect illumination light as the mirror transitions between ON and OFF states in accordance with some embodiments. 
         FIG.  7    includes graphs that show how an illustrative light source may be pulsed to provide image light to an input coupler while a reflective display panel mirror is at intermediate orientations as the mirror transitions between ON and OFF states in accordance with some embodiments. 
         FIG.  8    is a flow chart of illustrative steps involved in providing image light to an input coupler with a reflective display panel mirror at intermediate orientations as the mirror transitions between ON and OFF states in accordance with some embodiments. 
         FIG.  9    is a top view showing how multiple reflective display panel mirrors may provide image light within different pupils at an input coupler as the mirrors transition between ON and OFF states in accordance with some embodiments. 
         FIG.  10    is a top view of an illustrative waveguide having multiple input coupling structures for providing images at an eye box with virtual objects in multiple focal planes 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  (sometimes referred to herein as image light  22 ) 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.). In one suitable arrangement that is sometimes described herein as an example, components  18  may include gaze tracking sensors that gather gaze image data from a user&#39;s eye at eye box  24  to track the direction of the user&#39;s gaze in real time. 
     Display modules  14 A (sometimes referred to herein as display engines  14 A, light engines  14 A, or projectors  14 A) may include reflective displays (e.g., displays with a light source that produces illumination light that reflects off of a reflective display panel to produce image light such as 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 in 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(s)  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 . Collimating lens  34  may be omitted if desired. If desired, display module(s)  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(s)  14 A may generate image light  22  associated with image content to be displayed to eye box  24 . Image light  22  may be collimated using a lens such as collimating lens  34 . Optical system  14 B may be used to present image light  22  output from display module(s)  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 image light  22  down its length via total internal reflection. Input coupler  28  may be configured to couple image light  22  from display module(s)  14 A into waveguide  26 , whereas output coupler  30  may be configured to couple image light  22  from within waveguide  26  to the exterior of waveguide  26  and towards eye box  24 . Input coupler  28  may include an input coupling prism if desired. As an example, display module(s)  14 A may emit image light  22  in the +Y direction towards optical system  14 B. When image light  22  strikes input coupler  28 , input coupler  28  may redirect image light  22  so that the light propagates within waveguide  26  via total internal reflection towards output coupler  30  (e.g., in the +X direction). When image light  22  strikes output coupler  30 , output coupler  30  may redirect image light  22  out of waveguide  26  towards eye box  24  (e.g., back in the −Y direction). In scenarios where cross-coupler  32  is formed at waveguide  26 , cross-coupler  32  may redirect image 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, louvered 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  includes a 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 ) or one or more layers of diffractive grating structures. 
     In one suitable arrangement that is described herein as an example, display module  14 A may include a reflective display panel for providing image light  22  to waveguide  26 .  FIG.  3    is a top view showing how display module  14 A may include a reflective display panel for providing image light  22  to waveguide  26 . As shown in  FIG.  3   , display module  14 A may include a light source (LS) such as light source  44 . Light source  44  may be an LED light source, OLED light source, uLED light source, laser light source, or any other desired light source. 
     Display module  14 A may include a reflective display panel such as reflective display panel  46  (e.g., a reflective spatial light modulator). Reflective display panel  46  may be an LCOS display panel, a DMD display panel, or other types of reflective display panel. Reflective display panel  46  may have an array of individually adjustable pixels. Each pixel may be formed by a respective reflective element  48  in reflective display panel  46 . In one suitable arrangement that is described herein as an example, reflective display panel  46  is a DMD display panel. Reflective display panel  46  may therefore sometimes be referred to herein as DMD panel  46  or 
     DMD  46 . In this example, reflective elements  48  may be mirrors such as micromirrors (e.g., micro-electromechanical-systems (MEMS)-based micromirrors). Reflective elements  48  may therefore sometimes be referred to herein as micromirrors  48  or mirrors  48 . Each mirror  48  may form a respective pixel for reflective display panel  46 . 
     Light source  44  may emit illumination light  40 . Illumination light  40  may include light in one or more wavelength bands (e.g., red, green, and/or blue wavelength bands). One or more illumination light lens elements  42  in display module  14 A may direct illumination light  40  onto reflective display panel  46 . Lens elements  42  may include one or more prisms, partial reflectors, polarizers, and/or other optical components if desired. Lens elements  42  may, for example, provide illumination light  40  to DMD panel  46  at an incident angle B (relative to the X-axis) across each of the mirrors  48  in DMD panel  46 . During operation, control circuitry  16  ( FIG.  1   ) may control DMD panel  46  to selectively reflect illumination light  40  at each pixel (mirror) location to produce image light  22  (e.g., image light having an image as modulated onto the illumination light by the mirrors  48  in DMD panel  46 ). 
     For example, mirrors  48  may be individually (e.g., independently) rotatable between two predetermined orientations (states) such as an “ON” state and an “OFF” state. Control circuitry  16  of  FIG.  1    may individually adjust the state of each pixel based on the images to be displayed using display  14 . The example of  FIG.  3    illustrates the operation of a single pixel P* on illumination light  40  for the sake of clarity. However, in general, similar operations are performed at each pixel (mirror) across the lateral area of DMD panel  46 . DMD panel  46  may include any desired number of pixels arranged into rows and columns or in any other desired pattern (e.g., tens of pixels, hundreds of pixels, thousands of pixels, tens of thousands of pixels, hundreds of thousands of pixels, etc.). 
     When pixel P* is in the “ON” state, the mirror  48  used to form that pixel P* may be at a first orientation (e.g., an “ON” orientation or state). In this orientation, mirror  48  may reflect illumination light  40  (as image light  22 ) at an output angle A towards lens  34 . Lens  34  may direct image light  22  towards input coupler  28  of optical system  14 B. When pixel P* is in the “OFF” state, pixel P* may direct illumination light  40  away from optical system  14 B, as shown by arrow  50  (e.g., towards a light sink  52  such as a baffle that includes light absorbing materials and/or textured structures that effectively extinguish the reflected light to prevent the reflected light from being received at the eye box). By adjusting pixel P* between the “ON” and “OFF” states in this way, pixel P* may either direct illumination light  40  towards input coupler  28  and the eye box (as image light  22 ) or may direct illumination light  40  outside of the projection optics (i.e., towards light sink  52 ) so that the beam is not received at the eye box. 
       FIG.  4    is a schematic diagram of a given mirror  48  in DMD panel  46  (e.g., the mirror  48  that is used to form pixel P* of  FIG.  3   ). As shown in  FIG.  4   , mirror  48  may be adjustable (rotatable) about a pivot point  66 . Pivot point  66  may be located at the center of mirror  48 , at an end (edge) of mirror  48 , elsewhere on mirror  48 , or mirror  48  may be rotatable about multiple pivot points. 
     The orientation (position) of mirror  48  may be defined by a tilt angle C (e.g., with respect to horizontal axis  65 ). Control circuitry  16  ( FIG.  1   ) may adjust mirror  48  between the “ON” state and the “OFF” state, as shown by arrows  60 . In the “ON” state, the (mirror) plane of mirror  48  may be at orientation  64  (e.g., an orientation in which the mirror plane is oriented at tilt angle C ON ). When in this state, mirror  48  may reflect illumination light  40 , as image light  22 , onto an output (reflected) angle A such as output angle A ON  (e.g., as measured with respect to horizontal axis  65 ). Image light  22  output from mirror  48  at output angle A ON  may be directed towards (e.g., focused onto) input coupler  28  (e.g., by lens  34  of  FIG.  3   ). For example, the image light  22  reflected by mirror  48  may be focused onto input coupler  28  within a corresponding pupil  68 . Pupil  68  may extend across a finite non-zero area. 
     In the “OFF” state, mirror  48  may be at orientation  62  (e.g., an orientation in which the mirror plane is oriented at tilt angle C OFF ). Tilt angle C OFF  may be less than zero whereas tilt angle C ON  is greater than zero. This is merely illustrative. In another suitable arrangement, tilt angle C OFF  may be equal to zero or greater than zero and less than tilt angle C ON . Tilt angles C ON  and C OFF  may be the limits of the range of rotation achievable by mirror  48  (e.g., tilt angle C ON  may be the maximum tilt angle of mirror  48  and tilt angle C OFF  may be the minimum tilt angle of mirror  48 ). When in the “OFF” state, mirror  48  may reflect illumination light  40  onto an output (reflected) angle A such as output angle A OFF , as shown by arrow  50 . The light reflected at output angle A OFF  may be directed towards light sink  52  of  FIG.  3   , for example. 
     Over time, control circuitry  16  ( FIG.  1   ) may toggle mirror  48  between orientations  62  and  64 , as shown by arrows  60  (e.g., as the pixel P* formed by mirror  48  is toggled on or off to produce images such as images from a stream of image frames at the eye box). As an example, tilt angle C ON  may be 12 degrees, 15 degrees, between 10 and 15 degrees, between 11 and 13 degrees, 5 degrees, between 5 and 15 degrees, greater than 10 degrees, greater than 5 degrees, etc. Tilt angle C OFF  may be −12 degrees, −15 degrees, between −10 and −15 degrees, between −11 and −13 degrees, −5 degrees, between −5 and −15 degrees, less than −10 degrees, less than −5 degrees, 0 degrees, 2 degrees, less than 5 degrees, between −12 and 0 degrees, or any other desired angle less than tilt angle C ON . 
     In practice, it takes a non-zero amount of time to rotate mirror  48  between tilt angles C OFF  and C ON  (e.g., between orientations  64  and  62 ). Graph  70  of  FIG.  5    plots the tilt angle C of mirror  48  as the mirror transitions from orientation  62  (tilt angle C OFF ) to orientation  64  (tilt angle C ON ) of  FIG.  4    over time (e.g., on a microsecond (μs) scale). As shown by curve  74  of graph  70 , mirror  48  may be at tilt angle C OFF  between time T0 and time T OFF . At time T OFF , control circuitry  16  ( FIG.  1   ) may control mirror  48  to rotate to tilt angle C ON . At and after time T ON , mirror  48  may be at tilt angle C ON . Between times T OFF  and T ON , mirror  48  may rotate through intermediate tilt angles as mirror  48  transitions from tilt angle C OFF  to tilt angle C ON . It may take mirror  48  a non-zero transition time (period) TX (e.g., where TX=T ON −T OFF ) to rotate from tilt angle C OFF  to tilt angle C ON  (e.g., curve  70  may have a non-vertical slope between times T OFF  and T ON ). 
     In order to mitigate stray light in the system, light source  44  ( FIG.  3   ) may be in an “OFF” state until mirror  48  has arrived at tilt angle C ON . Graph  72  of  FIG.  5    plots the state of light source  44  as mirror  48  transitions between tilt angle C OFF  and tilt angle C ON  over time. As shown by curve  76  of graph  72 , light source  44  may be turned off until time T ON  (e.g., light source  44  may not emit illumination light  40  between times TO and T ON ). At time T ON , light source  44  may be turned on (e.g., placed in an “ON” state) to emit illumination light  40 . The illumination light  40  may then reflect off of mirror  48  (which is at tilt angle C ON  at time T ON ) as image light  22  provided to input coupler  28 . 
     While graph  70  and graph  72  of  FIG.  5    plot the transition of mirror  48  from the “OFF” state to the “ON” state, similar graphs may also plot the transition of mirror  48  from the “ON” state to the “OFF” state (e.g., by reversing the time axes of graphs  70  and  72 ). Control circuitry  16  may toggle in both directions between the “ON” and “OFF” states over time (e.g., as shown by arrows  60  of  FIG.  4   ). If desired, the flexibility and performance of the display may be enhanced by leveraging the fact that mirror  48  has a non-zero transition time TX as the mirror transitions between “ON” and “OFF” states. For example, mirror  48  may be used to reflect light towards input coupler  28  while mirror  48  is oriented at one or more intermediate tilt angles between tilt angles C OFF  and C ON  (e.g., as the mirror transitions between tilt angles C OFF  and C ON ). 
       FIG.  6    is a diagram showing how mirror  48  may be used to reflect light towards input coupler  28  while mirror  48  is oriented at one or more intermediate tilt angles between tilt angles C OFF  and C ON  (e.g., as the mirror transitions between tilt angles C OFF  and C ON ). As shown in  FIG.  6   , as mirror  48  rotates from tilt angle C OFF  to tilt angle C ON  (or vice versa), mirror  48  may have at least one intermediate tilt angle at which mirror  48  reflects illumination light  40  towards output coupler  28  as image light  22 . 
     In the example of  FIG.  6   , mirror  48  may reflect illumination light  40  as image light  22  at output angle A 1  while oriented at tilt angle C 2  (e.g., where output angle A 1  is less than the output angle A OFF  associated with tilt angle C OFF  and greater than the output angle A ON  associated with tilt angle C ON ). The image light  22  reflected by mirror  48  at tilt angle C 2  may be focused onto input coupler  28  (e.g., by lens  34  of  FIG.  3   ) within a corresponding pupil  80 - 1 . Pupil  80 - 1  may extend across a finite, non-zero area, and may be spatially offset from the pupil  68  associated with output angle A ON . For example, pupil  80 - 1  may not overlap any of pupil  68  or may partially overlap pupil  68  (e.g., pupil  80 - 1  may be at least partially non-overlapping with respect to pupil  68 ). 
     Similarly, mirror  48  may reflect illumination light  40  as image light  22  at output angle A 2  while oriented at tilt angle C 1  (e.g., where tilt angle C 1  is less than tilt angle C OFF  and greater than tilt angle C 1  and where output angle A 2  is greater than output angle A 1  and less than output angle A OFF . The image light  22  reflected by mirror  48  at tilt angle C 1  may be focused onto input coupler  28  (e.g., by lens  34  of  FIG.  3   ) within a corresponding pupil  80 - 2 . Pupil  80 - 2  may extend across a finite, non-zero area, and may be spatially offset from pupil  80 - 1 . For example, pupil  80 - 2  may not overlap any of pupil  80 - 1  or may partially overlap pupil  80 - 1  (e.g., pupil  80 - 2  may be at least partially non-overlapping with respect to pupil  80 - 1 ). 
     Collectively, pupils  68 ,  80 - 1 , and  80 - 2  may form an effective pupil  82  that extends across a larger area than each individual pupil on its own. By rapidly toggling between tilt angles C OFF  and C ON  (e.g., faster than the response rate of the human eye such as at 24 Hz or greater, 30 Hz or greater, 60 Hz or greater, 120 Hz or greater, 240 Hz or greater, etc.), as shown by arrows  60 , and also reflecting light while at intermediate tilt angles C 1  and C 2  during each transition between tilt angles C OFF  and C ON , mirror  48  may “paint” effective pupil  82  at input coupler  28  over time. Effective pupil  82  may then be used to display frames of image data at the eye box. This may serve to effectively expand the size of the pupil of light coupled into waveguide  26  by input coupler  28  (e.g., in one dimension such as the dimension parallel to the X-axis), thereby ensuring that as much of the eye box is filled with uniformly-distributed image light  22 . By performing pupil expansion in this way using the mirrors  48  in DMD panel  46  (e.g., by leveraging non-zero transition time TX and reflecting light at intermediate tilt angles), other pupil-expanding components in display  14  such as cross-coupler  32  may be omitted. Omitting cross-coupler  32  may, for example, minimize the number of diffractions performed on image light  22  in directing the image light towards the eye box, thereby maximizing light throughput and the peak brightness of the images provided at the eye box. 
     The example of  FIG.  6    is merely illustrative. Mirror  48  may reflect illumination light  40  at any desired number N of intermediate tilt angles during each transition between tilt angles C OFF  and C ON  (e.g., at one intermediate tilt angle, at two intermediate tilt angles as shown in  FIG.  6   , at three intermediate tilt angles, at four intermediate tilt angles, at more than four intermediate tilt angles, etc.). Each intermediate tilt angle that is used to reflect light may produce a corresponding pupil  80  that is used to pain the larger effective pupil  82  (e.g., mirror  48  may produce N+1 pupils at input coupler  28  over time, where N pupils  80  are produced at intermediate tilt angles and one pupil  68  is produced at tilt angle C ON ). 
     Graph  83  of  FIG.  7    plots the tilt angle C of mirror  48  as the mirror transitions from tilt angle C OFF  to tilt angle C ON  while also reflecting illumination light  40  at one or more intermediate tilt angles. As shown by curve  81  of graph  83 , mirror  48  may be at tilt angle C OFF  between time T0 and time T OFF . At time T OFF , control circuitry  16  ( FIG.  1   ) may control mirror  48  to rotate to tilt angle C ON . At and after time T ON , mirror  48  may be at tilt angle C ON . Between times T OFF  and T ON , mirror  48  may rotate through intermediate tilt angles that include intermediate tilt angle C 1  and intermediate tilt angle C 2 , as mirror  48  rotates from tilt angle C OFF  to tilt angle C ON . For example, mirror  48  may be oriented at tilt angle C 1  at time T 1  and may be oriented at tilt angle C 2  at time T 2  (e.g., where times T 1  and T 2  are during transition time TX). 
     Graph  85  of  FIG.  7    plots the state of light source  44  as mirror  48  transitions from tilt angle C OFF  to tilt angle C ON  while also reflecting illumination light  40  at one or more intermediate tilt angles. As shown by curve  87  of graph  85 , light source  44  may be turned off until time T 1 . At time T 1  (e.g., when mirror  48  is at intermediate tilt angle C 1 ), control circuitry  16  ( FIG.  1   ) may control light source  44  ( FIG.  3   ) to pulse “ON” (e.g., for pulse duration DT). Pulse duration DT may be, for example, 1 μs, 2 μs, 3 μs, 4 μs, 1-5 μs, 2-4 μs, less than 5 μs, less than 4 μs, less than 3 μs, less than 2 μs, less than 1 μs, or other durations. Pulsing light source  44  in this way may produce a pulse of illumination light  40  ( FIG.  3   ) that is reflected off of mirror  48  while the mirror is at intermediate tilt angle C 1 , providing image light  22  to input coupler  28  within pupil  80 - 2  ( FIG.  6   ). 
     Similarly, at time T 2  (e.g., when mirror  48  is at intermediate tilt angle C 2 ), control circuitry  16  may control light source  44  to pulse “ON” (e.g., for pulse duration DT). Pulsing light source  44  at this time may produce a pulse of illumination light  40  ( FIG.  3   ) that is reflected off of mirror  48  while the mirror is at intermediate tilt angle C 2 , providing image light  22  to input coupler  28  within pupil  80 - 1  ( FIG.  6   ). Then, at time T ON  (e.g., when mirror  48  is at tilt angle C ON ), control circuitry  16  may control light source  44  to pulse “ON” (e.g., for pulse duration DT). Pulsing light source  44  at this time may produce a pulse of illumination light  40  ( FIG.  3   ) that is reflected off of mirror  48  while the mirror is at tilt angle C ON , providing image light  22  to input coupler  28  within pupil  68  ( FIG.  6   ). Each pulse of light source  44  may be separated from the subsequent or previous pulse by pulse gap WT. Pulse gap WT may, for example, be longer than pulse duration DT. 
     The example of  FIG.  7    is merely illustrative. Pulse gaps WT may be uniform across transition time TX or may be non-uniform across transition time TX (e.g., pulse gaps WT may be selected so that light source  44  is pulsed on while mirror  48  is at any desired intermediate tilt angle(s) between tilt angle C OFF  and tilt angle C ON ). Each pulse may have the same pulse duration DT or different pulses may have different durations. The pulse of light source  44  at time T ON  may be omitted if desired. Light source  44  may be pulsed “ON” any desired number of times during transition time TX (e.g., N times to produce N pupils  80  ( FIG.  6   ) at N different intermediate tilt angles of mirror  48 ). While graph  81  and graph  85  of  FIG.  7    plot the transition of mirror  48  from the “OFF” state to the “ON” state, similar graphs may also plot the transition of mirror  48  from the “ON” state to the “OFF” state (e.g., by reversing the time axes of graphs  70  and  72 ). Control circuitry  16  may toggle in both directions between the “ON” and “OFF” states over time (e.g., as shown by arrows  60  of  FIG.  6   ). This may, for example, serve to paint an effective pupil  82  of image light  22  provided to input coupler  28  ( FIG.  6   ). 
       FIG.  8    is a flow chart of illustrative steps that may be performed by control circuitry  16  ( FIG.  1   ) in controlling mirror  48  to reflect light using one or more intermediate tilt angles. At step  100 , control circuitry  16  may place mirror  48  in the “OFF” state (e.g., where mirror  48  is oriented at tilt angle C OFF ). Control circuitry  16  may also place light source  44  in the “OFF” state (e.g., so light source  44  does not emit illumination light  40 ). 
     At step  102 , control circuitry  16  may control mirror  48  to begin rotating from tilt angle C OFF  to tilt angle C ON . Mirror  48  may be oriented at intermediate tilt angles as the mirror rotates from tilt angle C OFF  to tilt angle C ON . Mirror  48  may take transition time TX to rotate from tilt angle C OFF  to tilt angle C ON . 
     At step  104 , while mirror  48  is rotating from tilt angle C OFF  to tilt angle C ON  (e.g., without stopping the rotation of mirror  48 ), control circuitry  16  may perform N pulses of light source  44  for duration DT (e.g., where the pulses are separated in time by pulse gaps WT). The pulse timing may be selected so that offset pupils  80  of image light  22  ( FIG.  6   ) are provided to input coupler  28  while mirror  48  is at a desired set of different intermediate tilt angles in its transition from tilt angle C OFF  to tilt angle C ON . For example, control circuitry  16  may pulse light source  44  “ON” at time T 1  (e.g., while mirror  48  is at tilt angle C 1 ) and again at time T 2  (e.g., at pulse gap WT after the previous pulse, while mirror  48  is at tilt angle C 2 ). This may serve to provide image light  22  to input coupler  28  within pupil  80 - 2  and then within pupil  80 - 1  of  FIG.  6   . 
     At step  106 , after transition time TX has elapsed since beginning the transition of mirror  48  to tilt angle C ON , control circuitry  16  may pulse light source  44  “ON” (e.g., at time T ON ). 
     At step  108 , control circuitry  16  may then control mirror  48  to begin rotating back from tilt angle C ON  to tilt angle C OFF . 
     At step  110 , while mirror  48  is rotating from tilt angle C ON  to tilt angle C OFF , control circuitry  16  may perform N pulses of light source  44  for duration DT (e.g., where the pulses are separated in time by pulse gaps WT). The pulse timing may be selected so that offset pupils  80  of image light  22  ( FIG.  6   ) are provided to input coupler  28  while mirror  48  is at a desired set of different intermediate tilt angles in its transition from tilt angle C ON  to tilt angle C OFF . Processing may subsequently loop back to step  102  via path  112 . Control circuitry  16  may continue to repeat these steps (e.g., where each transition between tilt angles C ON  and C OFF  occurs faster than the response time of the human eye) to provide an effective pupil  82  of light at input coupler  28  that is much larger than the individual pupil size that would otherwise be provided to the input coupler (e.g., as shown by pupil  68  of  FIG.  4   ). 
     The example of  FIG.  8    is merely illustrative. If desired, light source  44  may be pulsed ON only during the transition of mirror  48  from tilt angle C OFF  to tilt angle C ON  (e.g., steps  108  and  110  may be omitted and processing may loop back to step  102  after step  106 ) or light source  44  may be pulsed ON only during the transition of mirror  48  from tilt angle C ON  to tilt angle C OFF  (e.g., mirror  48  may be placed in the “ON” state at step  100 , steps  102 - 106  may be omitted, and processing may loop back to step  100  from step  110 ). If desired, step  106  may be omitted (e.g., mirror  48  may provide image light to input coupler  28  only while oriented at intermediate tilt angles if desired). Two or more of the steps of  FIG.  8    may be performed concurrently and/or combinations of these arrangements may be used if desired. 
     In the example of  FIGS.  3 - 7   , the operation of a single mirror  48  is illustrated for the sake of clarity. In general, these operations may be performed for all of the mirrors across DMD panel  46  ( FIG.  3   ).  FIG.  9    is a diagram showing how three mirrors  48  in DMD panel  46  may be provide image light  22  to input coupler  28  while the mirrors transition between tilt angles C OFF  and C ON . 
     As shown in  FIG.  9   , three mirrors  48  (e.g., three adjacent mirrors in display panel  46 ) may receive illumination light  40  (e.g., at the same incident angle B and from the same light source  44  of  FIG.  3   ). Control circuitry  16  ( FIG.  1   ) may control these mirrors to transition between tilt angles C OFF  and C ON . At a first tilt angle (e.g., at intermediate tilt angle C 1  and time T 1  of  FIG.  7   ), mirrors  48  may produce image light  22  by reflecting illumination light  40  onto output angle A 2 , as shown by rays  126 . Lens  34  (e.g., one or more lens elements in lens  34 ) may focus the image light  22  incident at angle A 2  to within pupil  80 - 2  on input coupler  28  of optical system  14 B. 
     At a second tilt angle (e.g., at intermediate tilt angle C 2  and time T 2  of  FIG.  7   ), mirrors  48  may produce image light  22  by reflecting illumination light  40  onto output angle A 1 , as shown by rays  124 . Lens  34  (e.g., one or more lens elements in lens  34 ) may focus the image light  22  incident at angle A 1  to within pupil  80 - 1  on input coupler  28  of optical system  14 B. 
     At a third tilt angle (e.g., at tilt angle C ON  and time T ON  of  FIG.  7   ), mirrors  48  may produce image light  22  by reflecting illumination light  40  onto output angle A ON , as shown by rays  122 . Lens  34  (e.g., one or more lens elements in lens  34 ) may focus the image light  22  incident at angle A ON  to within pupil  68  on input coupler  28  of optical system  14 B. Pupil  80 - 2  may be at least partially non-overlapping with respect to pupil  80 - 1 . Pupil  80 - 1  may be at least partially non-overlapping with respect to pupil  68 . By rapidly toggling the tilt angle of mirrors  48 , pupils  80 - 2 ,  80 - 1 , and  68  may collectively form the one-dimensionally expanded effective pupil  82  at input coupler  28 . The same light source  44  and the same illumination light  40  may be used to produce image light  22  for each of the mirrors  48  in DMD panel  46  regardless of which tilt angle is being used by the mirrors to reflect illumination light  40  (e.g., the N pupils  80  and pupil  68  of image light  22  may be produced by the same light source  44 , the same illumination light  44 , and the same mirrors  48  of the same DMD panel  46 ). 
     The example of  FIG.  9    is merely illustrative. Mirrors  48  may reflect illumination light  40  at any desired number N of intermediate tilt angles during each transition between tilt angles C OFF  and C ON  (e.g., at one intermediate tilt angle, at two intermediate tilt angles as shown in  FIG.  6   , at three intermediate tilt angles, at four intermediate tilt angles, at more than four intermediate tilt angles, etc.). Each intermediate tilt angle that is used to reflect light may produce a corresponding pupil  80  that is used to paint the larger effective pupil  82  (e.g., mirrors  48  may produce N+1 pupils at input coupler  28  over time, where N pupils  80  are produced at intermediate tilt angles and one pupil  68  is produced at tilt angle C ON ). 
     The examples described above in which pupils  80 - 2 ,  80 - 1 , and  68  are used to perform one-dimensional pupil expansion are merely illustrative. In another suitable arrangement, pupils  80 - 2 ,  80 - 1 , and  68  may be used to provide image light  22  with different optical powers and/or to produce virtual objects in different focal planes at eye box  24  ( FIG.  2   ). In these arrangements, pupil  80 - 2  may be separated from pupil  80 - 1  by a non-zero offset  120  and pupil  80 - 1  may be separated from pupil  68  by a non-zero offset  120 . The same offset  120  or different offsets  120  may be used between each of the pupils. Each pupil may then be received by different input coupling structures on waveguide  26 . 
       FIG.  10    is a top view showing how mirrors  48  may be used to provide image light with different optical powers and/or in different focal planes to eye box  24  ( FIG.  2   ). As shown in  FIG.  10   , waveguide  26  may include a first waveguide portion (layer)  150 , a second waveguide portion (layer)  148  stacked onto first waveguide portion  150 , and a third waveguide portion (layer)  146  stacked onto second waveguide portion  148 . Each of waveguide portions  150 ,  148 , and  146  may include one, two, or more than two substantially planar waveguide substrates and a layer of grating medium (e.g., a layer of holographic recording medium layered onto one waveguide substrate or sandwiched (interposed) between two waveguide substrates). 
     Input coupler  28  may include a first input coupling structure  144  in waveguide portion  150 , a second input coupling structure  142  in waveguide portion  148 , and a third input coupling structure  140  in waveguide portion  146 . Input coupling structure  144  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  150 . Input coupling structure  142  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  148 . Input coupling structure  140  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  146 . 
     Output coupler  30  may include a first output coupling structure  156  in waveguide portion  150 , a second output coupling structure  154  in waveguide portion  148 , and a third output coupling structure  152  in waveguide portion  146 . Output coupling structure  156  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  150 . Output coupling structure  154  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  148 . Output coupling structure  152  may include a diffractive grating structure (e.g., a set of volume holograms) or a louvered mirror formed in the layer of grating medium in waveguide portion  146 . 
     Input coupling structure  144  may be completely or partially non-overlapping with respect to input coupling structures  142  and  140 . Input coupling structure  142  may be completely or partially non-overlapping with respect to input coupling structures  140  and  144 . Lens  34  ( FIG.  9   ) may focus the pupil  80 - 2  of image light  22  onto input coupling structure  140  of waveguide layer  146 . Lens  34  may focus the pupil  80 - 1  of image light  22  onto input coupling structure  142  of waveguide layer  148 . Lens  34  may focus the pupil  68  of image light  22  onto input coupling structure  142  of waveguide layer  148 . 
     Input coupling structure  140  may redirect the image light  22  incident within pupil  80 - 2  towards output coupling structure  152  and the redirected image light may propagate down waveguide portion  146  via total internal reflection, as shown by arrow  141 . Input coupling structure  142  may redirect the image light  22  incident within pupil  80 - 1  towards output coupling structure  154  and the redirected image light may propagate down waveguide portion  148  via total internal reflection, as shown by arrow  143 . Input coupling structure  144  may redirect the image light  22  incident within pupil  68  towards output coupling structure  156  and the redirected image light may propagate down waveguide portion  150  via total internal reflection, as shown by arrow  145 . In scenarios where input coupling structures  140 ,  142 , and  144  are formed using diffractive grating structures, the diffractive grating structure in input coupling structure  140  may be Bragg-matched with the wavelengths and incident angles associated with pupil  80 - 2 , the diffractive grating structure in input coupling structure  142  may be Bragg-matched with the wavelengths and incident angles associated with pupil  80 - 1 , and the diffractive grating structure in input coupling structure  144  may be Bragg-matched with the wavelengths and incident angles associated with pupil  68 . 
     Output coupler  30  may couple the image light  22  received from input coupler  28  out of waveguide  26  and towards eye box  24 . Output coupling structure  152  may couple the image light associated with arrow  141  out of waveguide  26  and towards eye box  24 . Output coupling structure  154  may couple the image light associated with arrow  143  out of waveguide  26  and towards eye box  24 . Output coupling structure  156  may couple the image light associated with arrow  145  out of waveguide  26  and towards eye box  24 . 
     In coupling the image light associated with arrow  141  out of waveguide  26 , output coupling structure  152  may impart the image light with a first optical power (e.g., a positive or negative lens power) and/or may provide the image light to eye box  24  such that objects in the image light are provided in a first image (focal) plane  160 - 1  (e.g., the holograms used to form output coupling structure  152  may have a finite focal length rather than being plane-to-plane holograms). Additionally or alternatively, if desired, waveguide portion  146  may be curved and/or a lens may be interposed between waveguide portions  146  and  148  to impart the image light associated with arrow  141  with the first optical power and/or to provide the image light to eye box  24  such that objects in the image light are provided in first image plane  160 - 1 . If desired, input coupling structure  140  may impart optical power to the image light associated with arrow  141 . 
     In coupling the image light associated with arrow  143  out of waveguide  26 , output coupling structure  154  may impart the image light with a second optical power (e.g., an optical power that is different from the first optical power) and/or may provide the image light to eye box  24  such that objects in the image light are provided in a second image (focal) plane  160 - 2  (e.g., the holograms used to form output coupling structure  154  may have a finite focal length rather than being plane-to-plane holograms). Additionally or alternatively, if desired, waveguide portion  148  may be curved and/or a lens may be interposed between waveguide portions  148  and  150  to impart the image light associated with arrow  143  with the second optical power and/or to provide the image light to eye box  24  such that objects in the image light are provided in second image plane  160 - 2 . If desired, input coupling structure  142  may impart optical power to the image light associated with arrow  143 . 
     In coupling the image light associated with arrow  145  out of waveguide  26 , output coupling structure  156  may impart the image light with a third optical power (e.g., an optical power that is different from the first and second optical powers) and/or may provide the image light to eye box  24  such that objects in the image light are provided in a third image (focal) plane  160 - 3  (e.g., the holograms used to form output coupling structure  154  may have a finite focal length rather than being plane-to-plane holograms). Additionally or alternatively, if desired, waveguide portion  150  may be curved and/or a lens may be interposed between waveguide portion  150  and eye box  24  to impart the image light associated with arrow  145  with the third optical power and/or to provide the image light to eye box  24  such that objects in the image light are provided in second image plane  160 - 3 . If desired, input coupling structure  144  may impart optical power to the image light associated with arrow  145 . 
     When configured in this way, waveguide  26  may be configured to display different images from display module  14 A in different image (focal) planes  160  (e.g., virtual image locations at different respective distances from eye box  24 ). This allows distant objects (e.g., mountain peaks in a landscape) to be presented in a distant image plane  160  (see, e.g., far-field image plane  160 - 3 ) and allows close objects (e.g., the face of a person in the user&#39;s field of view) to be presented in a close image plane (see, e.g., near-field image plane  160 - 1 ), providing eye box  24  with three-dimensional image content. Other objects may be presented in intermediate-distance image planes (e.g., intermediate image plane  160 - 2 ). By displaying far objects in distant image planes and close objects in nearby image planes, three-dimensional imagery may be displayed naturally for the user with minimal eye fatigue and discomfort. 
     When configured in this way, display module  14 A may provide waveguide  26  with multiple images per unit time (e.g., per “frame” of image data). These images may be presented in multiple different focal planes  160 . Display module  14 A may present multiple images to a user through waveguide  26  using a multiplexing scheme (e.g., in which the image light associated with arrows  141 ,  143 , and  145  are provided to eye box  24  at different times as mirrors  48  transition between “ON” and “OFF” states). 
     The example of  FIG.  10    is merely illustrative. In general, there may be N input coupling structures in input coupler  28  and N output coupling structures in output coupler  30 , where each of the N input coupling structures couples the image light  22  from a respective pupil into a respective waveguide portion (e.g., there may be N stacked waveguide portions in waveguide  26 , each with a respective input coupling structure and output coupling structure). Similarly, image light  22  may be provided to eye box  24  with N optical powers and/or with virtual objects in N different focal planes  160 . Waveguide portions  146 ,  148 , and  150  may sometimes be referred to as separate waveguides rather than as portions of the same waveguide  26 . 
     Input coupling structures  140 ,  142 , and/or  144  be overlapping (e.g., completely overlapping) if desired. In this arrangement, because the diffractive grating structure in input coupling structure  144  is Bragg-matched to the image light  22  incident within pupil  68 , the image light  22  incident within pupils  80 - 2  and  80 - 1  may pass through input coupling structure  144  to input coupling structures  142  and  144  without being diffracted in waveguide portion  150 . Similarly, because the diffractive grating structure in input coupling structure  142  is Bragg-matched to the image light  22  incident within pupil  80 - 1 , the image light  22  incident within pupil  80 - 2  may pass through input coupling structure  144  to input coupling structure  140  without being diffracted in waveguide portion  148 . 
     If desired, two or more of the input coupling structures (e.g., each of input coupling structures  140 ,  142 , and  144 ) may be formed in the same waveguide portion (e.g., from different sets of overlapping holograms recorded in the same volume of the same layer of grating medium). In this example, the set of holograms in input coupling structure  140  may only diffract the image light  22  incident within pupil  80 - 2 , the set of holograms in input coupling structure  142  may only diffract the image light  22  incident within pupil  80 - 1 , and the set of holograms in input coupling structure  144  may only diffract the image light  22  incident within pupil  68 . By toggling mirrors  48  between tilt angles C OFF  and C ON  and pulsing light source  44  while mirror  48  is at one or more intermediate tilt angles, display  14  may provide image light  22  to eye box  24  with different optical powers and/or with virtual objects in different focal planes over time. By toggling the tilt angles faster than the response time of the human eye, the image light  22  received at eye box  24  may appear to the human eye to have been provided concurrently with each of the optical powers and with virtual objects in each of the different focal places at once. 
     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: 20210712
Publication Date: 20241001
Grant Date: 20241001
Priority Date: 20200713
Inventors: BHAKTA, Vikrant
PENG, GUOLIN
CHOI, Hyungryul
DELAPP, SCOTT M.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/0076", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0125", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/014", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B26/0833", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0081", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B26/0833", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0081", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/014", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B2027/014", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0081", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B26/0833", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 92899573