Staircase in-coupling for waveguide display

A waveguide display includes a waveguide and a staircase structure coupled to the waveguide. The waveguide includes a first substrate, a second substrate, and a holographic material layer between the first substrate and the second substrate. The holographic material layer includes a first grating and a second grating. The staircase structure is positioned on top of at least a portion of the first grating but not on top of the second grating. The staircase structure includes an input grating that is on top of the first grating and is configured to couple display light into the waveguide. The first grating is configured to redirect the display light coupled into the waveguide by the input grating towards the second grating.

CROSS-REFERENCES TO RELATED APPLICATIONS

The following two U.S. patent applications (including this one) are being filed concurrently, and the entire disclosure of the other applications are incorporated by reference into this application for all purposes:Application Ser. No. 17/184,312, filed Feb. 24, 2021, entitled “STAIRCASE IN-COUPLING FOR WAVEGUIDE DISPLAY”; andApplication Ser. No. 17/184,316, filed Feb. 24, 2021, entitled “WAVEGUIDE DISPLAY WITH MULTIPLE MONOCHROMATIC PROJECTORS”.

BACKGROUND

An artificial reality system, such as a head-mounted display (HMD) or heads-up display (HUD) system, generally includes a near-eye display (e.g., in the form of a headset or a pair of glasses) configured to present content to a user via an electronic or optic display within, for example, about 10-20 mm in front of the user's eyes. The near-eye display may display virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view both images of virtual objects (e.g., computer-generated images (CGIs)) and the surrounding environment by, for example, seeing through transparent display glasses or lenses (often referred to as optical see-through).

One example of an optical see-through AR system may use a waveguide-based optical display, where light of projected images may be coupled into a waveguide (e.g., a transparent substrate), propagate within the waveguide, and be coupled out of the waveguide at different locations. In some implementations, the light of the projected images may be coupled into or out of the waveguide using diffractive optical elements, such as volume holographic gratings and/or surface-relief gratings. Light from the surrounding environment may pass through a see-through region of the waveguide and reach the user's eyes as well.

SUMMARY

This disclosure relates generally to grating-based waveguide displays for near-eye display. More specifically, disclosed herein are techniques for improving the coupling efficiencies of grating-based near-eye display systems. Various inventive embodiments are described herein, including devices, systems, methods, and the like.

According to some embodiments, a waveguide display may include a waveguide that includes a first substrate, a second substrate, and a holographic material layer between the first substrate and the second substrate. The holographic material layer may include a first grating and a second grating. The waveguide display may also include a staircase structure coupled to the waveguide and positioned on top of at least a portion of the first grating but not on top of the second grating. The staircase structure may include an input grating that is on top of the first grating and is configured to couple display light into the waveguide. The first grating is configured to redirect the display light coupled into the waveguide by the input grating towards the second grating.

In some embodiments of the waveguide display, the staircase structure is characterized by a total thickness less than about 100 μm. In some embodiments, the staircase structure may include two or more holographic material layers. In some embodiments, the staircase structure may include a staircase substrate, and the input grating is on a top or bottom surface of the staircase substrate. In some embodiments, a shape and a thickness of the staircase structure may be selected to avoid clipping of a field of view of the waveguide display by the staircase structure.

In some embodiments, the waveguide display may also include an output grating. The second grating may be configured to diffract, at two or more regions of the second grating, the display light from the first grating towards the output grating. The output grating may be configured to couple the display light from each of the two or more regions of the second grating out of the waveguide at two or more regions of the output grating. In some embodiments, the waveguide display may also include a phase structure on the first substrate or the second substrate. The phase structure may be configured to change a polarization state of the display light incident on the phase structure before or after the display light is redirected by the first grating. The phase structure may include a waveplate, a layer of a birefringent material, or a subwavelength structure and an overcoat layer. The phase structure may be in selected regions of the waveguide or may be characterized by a spatially varying phase retardation across different regions of the phase structure.

In some embodiments, the input grating, the first grating, and the second grating may include transmissive volume Bragg gratings or reflective volume Bragg gratings. In some embodiments, the waveguide display may include two or more holographic material layers between the first substrate and the second substrate. The first grating and the second grating may be formed in the two or more holographic material layers.

According to some embodiments, a waveguide display may include a projector configured to transmit display light, a waveguide, a first grating and a second grating in a first region and a second region, respectively, of the waveguide, and a staircase structure on the first region of the waveguide. The staircase structure may include an input grating on top of the first grating and configured to couple the display light from the projector into the waveguide. The first grating is configured to redirect the display light coupled into the waveguide by the input grating towards the second grating.

In some embodiments of the waveguide display, the waveguide may include a first substrate, a second substrate, and one or more holographic material layers between the first substrate and the second substrate. The first grating and the second grating may be formed in the one or more holographic material layers. In some embodiments, the staircase structure may be characterized by a total thickness less than about 100 μm. In some embodiments, the staircase structure may include two or more holographic material layers, and the input grating may be formed in the two or more holographic material layers. In some embodiments, the staircase structure may include a staircase substrate, and the input grating is on a top or bottom surface of the staircase substrate.

In some embodiments, the waveguide display may include an output grating in the waveguide. The second grating may be configured to diffract, at two or more regions of the second grating, the display light from the first grating towards the output grating. The output grating may be configured to couple the display light from each of the two or more regions of the second grating out of the waveguide at two or more regions of the output grating. In some embodiments, the waveguide display may include a phase structure on the waveguide, where the phase structure may be configured to change a polarization state of the display light incident on the phase structure before or after the display light is redirected by the first grating. The phase structure may include, for example, a waveplate, a layer of a birefringent material, or a subwavelength structure and an overcoat layer. In some embodiments, the input grating, the first grating, and the second grating may include transmissive volume Bragg gratings or reflective volume Bragg gratings. A shape and a thickness of the staircase structure may be selected to avoid clipping of a field of view of the waveguide display by the staircase structure.

According to some embodiments, a waveguide display may include a waveguide, three input gratings configured to couple display light in different respective colors into the waveguide, one or more first middle gratings configured to receive and redirect the display light from the three input gratings, a second middle grating configured to diffract, at two or more regions of the second middle grating, the display light from the one or more first middle gratings, and an output grating configured to couple the display light from each of the two or more regions of the second middle grating out of the waveguide at two or more regions of the output grating.

In some embodiments of the waveguide display, the one or more first middle gratings may include three first middle gratings, each first middle grating of the three first middle gratings corresponding to a respective input grating of the three input gratings and configured to receive and redirect the display light of the respective color from the corresponding respective input grating. In some embodiments, the waveguide may include a first substrate, a second substrate, and one or more holographic material layers between the first substrate and the second substrate. The one or more first middle gratings and the second middle grating may be formed in the one or more holographic material layers.

In some embodiments, the waveguide display may include three projectors. Each projector of the three projectors may be configured to generate a monochromatic image, and each input grating of the three input gratings may be configured to couple the monochromatic image from a corresponding projector of the three projectors into the waveguide. In some embodiments, each projector of the three projectors may include a two-dimensional array of micro-LEDs. In some embodiments, the waveguide display may include a phase structure on the waveguide. The phase structure may be configured to change a polarization state of the display light incident on the phase structure before or after the display light is redirected by the one or more first middle gratings. In some embodiments, the phase structure may include a waveplate, a layer of a birefringent material, or a subwavelength structure and an overcoat layer. In some embodiments, the phase structure may be in selected regions of the waveguide or may be characterized by a spatially varying phase retardation across different regions of the phase structure.

In some embodiments of the waveguide display, the three input gratings, the one or more first middle gratings, and the second middle grating may include multiplexed transmissive volume Bragg gratings or multiplexed reflective volume Bragg gratings. In some embodiments, each input grating of the three input gratings may be on a respective staircase structure bonded to the waveguide, and each respective staircase structure may be on top of the one or more first middle gratings. In some embodiments, each respective staircase structure may include a staircase substrate, and each input grating of the three input gratings may be on a top or a bottom of the staircase substrate of the respective staircase structure. In some embodiments, the respective staircase structure may be characterized by a total thickness less than about 100 μm. In some embodiments, the respective staircase structure may include two or more holographic material layers, where the input grating may be formed in the two or more holographic material layers. In some embodiments, a shape and a thickness of the respective staircase structure may be selected to avoid clipping of a field of view of the waveguide display by the respective staircase structure.

According to some embodiments, a waveguide display may include a waveguide, three projectors configured to generate display light of different respective colors, three input gratings configured to couple the display light in the different respective colors into the waveguide, three first middle gratings configured to receive and redirect the display light from the respective input gratings of the three input gratings, a second middle grating configured to receive and redirect the display light from the three first middle gratings, and an output grating configured to couple the display light from the second middle grating out of the waveguide.

In some embodiments, the waveguide may include a first substrate, a second substrate, and one or more holographic material layers between the first substrate and the second substrate. The three first middle gratings and the second middle grating may be formed in the one or more holographic material layers. In some embodiments, the waveguide display may include a phase structure on the waveguide, where the phase structure may be configured to change a polarization state of the display light incident on the phase structure after or before the display light is diffracted by the three first middle gratings. In some embodiments, the phase structure may include a waveplate, a layer of a birefringent material, or a subwavelength structure and an overcoat layer. In some embodiments, each input grating of the three input gratings may be on a respective staircase structure bonded to the waveguide, and each respective staircase structure may be on top a first middle grating of the three first middle gratings. In some embodiments, each respective staircase structure may include a staircase substrate, each input grating of the three input gratings may be on a top or a bottom of the staircase substrate of the respective staircase structure, and each respective staircase structure may be characterized by a total thickness less than about 100 μm.

DETAILED DESCRIPTION

This disclosure relates generally to grating-based waveguide displays for near-eye display. More specifically, disclosed herein are techniques for improving the coupling efficiencies of grating-based near-eye display systems. Various inventive embodiments are described herein, including devices, systems, methods, and the like.

In a near-eye display system, it is generally desirable to expand the eyebox, improve image quality (e.g., resolution and contrast), reduce physical size, increase power efficiency, and increase the field of view (FOV). In a waveguide-based near-eye display system, light of projected images may be coupled into a waveguide (e.g., a substrate), propagate within the waveguide, and be coupled out of the waveguide at different locations to replicate exit pupils and expand the eyebox. Two or more gratings may be used to expand the eyebox in two dimensions. In a waveguide-based near-eye display system for augmented reality applications, light from the surrounding environment may pass through at least a see-through region of the waveguide display (e.g., the substrate) and reach the user's eyes. In some implementations, the light of the projected images may be coupled into or out of the waveguide using diffractive optical elements, such as gratings, which may also allow light from the surrounding environment to pass through.

Couplers implemented using diffractive optical elements may have limited coupling efficiencies due to, for example, less than 100% diffraction efficiency to the desired diffraction order, leakage, crosstalk, polarization dependence, angular dependence, wavelength dependence, and the like. Grating couplers may be optimized to maximize the power of the display light in the desired path. For example, the grating shape, the slant angle, the grating period, the duty cycle, the grating height or depth, the refractive index, the refractive index modulation, the overcoating material, and the spatial variations of these grating parameters across the grating may be adjusted to improve the efficiencies of directing display light to the desired directions towards the eyebox. Varying these parameters may provide some but limited improvements to the efficiencies of the waveguide display due to the intrinsic characteristics of the SRGs and VBGs, such as a limited maximum achievable refractive index modulation in a holographic recording material. In addition, in waveguide displays using surface-relief grating (SRG) couplers or volume Bragg grating (VBG) couplers, due to the size of the input pupil and thus the size of the input grating coupler, display light coupled into the waveguide by the input grating coupler may be reflected back to the input grating coupler and may be diffracted again by the input grating coupler to undesired directions and thus may not reach the eyebox of the waveguide display.

According to certain embodiments, a second grating (e.g., a grating for pupil expansion) may be place underneath the input grating coupler, such that the in-coupled light by the input grating coupler may be diffracted by the second grating before the in-coupled light would reach the input grating coupler again due to total internal reflection at the surfaces of the waveguide. The in-coupled light that is diffracted by the second grating may change the propagation direction within the waveguide and thus may not meet the Bragg condition of the input grating coupler when it reaches the input grating coupler again. Therefore, the in-coupled light that is diffracted by the second grating may not be diffracted by the input grating coupler even if it reaches the input grating coupler again.

In some embodiments, the second grating may be fabricated in a holographic material layer within the waveguide, the input grating coupler may be fabricated in a different holographic material layer on a staircase structure that has a small area to avoid changing the thickness in other regions of the waveguide. Separately recording the input grating coupler and the second grating in different holographic material layers may make the manufacturing process easier, and may also help to improve the diffraction efficiency of the input grating coupler and the second grating due to the higher overall achievable refractive index modulation in more holographic material layers. The staircase structure including the input grating coupler may then be attached to the waveguide and aligned with the second grating. To avoid FOV clipping, the total thickness and the overall shape of the staircase structure that includes the holographic material layer in which the input grating coupler is recorded and the staircase substrate (if needed) on which the holographic material layer is attached may be optimized. In some embodiments, the thickness of the holographic material layer may be appropriate and thus the staircase substrate may not be used. In some embodiments, a staircase substrate (e.g., a thin glass plate) with a certain shape and thickness may be used such that the total thickness of the holographic material layer and the staircase substrate may be equal to the desired thickness.

According to certain embodiments, three color projectors may be used to generate three monochromatic images that can be combined to form a color image, and three input grating couplers may be used to couple the three monochromatic images respectively into the waveguide. Each color projector may include, for example, a micro-LED array that emits display light in one color. The three color projectors may include, for example, a red micro-LED array, a green micro-LED array, and a blue micro-LED array. Because of the separate input gratings for different colors, each input grating may use the total achievable refractive index modulation of the holographic material layer to achieve a higher diffraction efficiency for display light of the respective color. For example, the overall in-coupling efficiency may be about five to ten times of the overall in-coupling efficiency of a waveguide display without separate projectors and input gratings for three different colors.

According to certain embodiments, the efficiency of a waveguide display may further be improved by controlling the polarization state of the display light along its propagation path. For example, a phase structure may be coupled to a surface of the waveguide and used to change the polarization state of the light reflected at the surface of the waveguide, such that the reflected light, when reaching a grating coupler in its propagation path, may be more preferentially diffracted or reflected to the desired directions to improve the overall efficiency of the waveguide display. The phase structure may include any birefringent materials (e.g., birefringent crystals, liquid crystals, or polymers) or structures (e.g., gratings or other subwavelength structures) that can cause a desired phase delay between two orthogonal linear polarization components (e.g., s-polarized light and p-polarized light), such that the incident light beam may be changed to, for example, an s-polarized, p-polarized, circularly polarized, or elliptically polarized beam. The phase structure may be placed at various locations in a waveguide display, such as at the input coupler region, between the input coupler and the output coupler, at the output coupler region, or any combinations. Adding phase structures to waveguide displays can add more degrees of design freedom for optimizing the efficiencies of the waveguide display. For example, the location, the phase delay, the orientation, and other parameters of the phase structure may be selected to change the polarization state of the display light such that the display light may be more preferentially diffracted by the polarization-dependent gratings to desired diffraction orders and directions to reach user's eye eventually.

In some embodiments, a waveguide display may include any combination of the input grating in a staircase structure, the phase structure for changing the polarization state of the display light, and a respective set of a projector and an input coupler for each color of the display light. For example, a waveguide display may include an input grating in a staircase structure, a phase structure for changing the polarization state of the display light, and a set of a projector and an input coupler for all colors. In another example, a waveguide display may include an input grating in a staircase structure, and a respective set of a projector and an input coupler for each color of the display light. In another example, a waveguide display may include an input grating on a waveguide, a phase structure for changing the polarization state of the display light, and a respective set of a projector and an input coupler for each color of the display light. In yet another example, a waveguide display may include an input grating in a staircase structure, a phase structure for changing the polarization state of the display light, and a respective set of a projector and an input coupler for each color of the display light.

In the following description, various inventive embodiments are described, including devices, systems, methods, and the like. For the purposes of explanation, specific details are set forth in order to provide a thorough understanding of examples of the disclosure. However, it will be apparent that various examples may be practiced without these specific details. For example, devices, systems, structures, assemblies, methods, and other components may be shown as components in block diagram form in order not to obscure the examples in unnecessary detail. In other instances, well-known devices, processes, systems, structures, and techniques may be shown without necessary detail in order to avoid obscuring the examples. The figures and description are not intended to be restrictive. The terms and expressions that have been employed in this disclosure are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. The word “example” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “example” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

FIG.1is a simplified block diagram of an example of an artificial reality system environment100including a near-eye display120in accordance with certain embodiments. Artificial reality system environment100shown inFIG.1may include near-eye display120, an optional external imaging device150, and an optional input/output interface140, each of which may be coupled to an optional console110. WhileFIG.1shows an example of artificial reality system environment100including one near-eye display120, one external imaging device150, and one input/output interface140, any number of these components may be included in artificial reality system environment100, or any of the components may be omitted. For example, there may be multiple near-eye displays120monitored by one or more external imaging devices150in communication with console110. In some configurations, artificial reality system environment100may not include external imaging device150, optional input/output interface140, and optional console110. In alternative configurations, different or additional components may be included in artificial reality system environment100.

Near-eye display120may be a head-mounted display that presents content to a user. Examples of content presented by near-eye display120include one or more of images, videos, audio, or any combination thereof. In some embodiments, audio may be presented via an external device (e.g., speakers and/or headphones) that receives audio information from near-eye display120, console110, or both, and presents audio data based on the audio information. Near-eye display120may include one or more rigid bodies, which may be rigidly or non-rigidly coupled to each other. A rigid coupling between rigid bodies may cause the coupled rigid bodies to act as a single rigid entity. A non-rigid coupling between rigid bodies may allow the rigid bodies to move relative to each other. In various embodiments, near-eye display120may be implemented in any suitable form-factor, including a pair of glasses. Some embodiments of near-eye display120are further described below with respect toFIGS.2and3. Additionally, in various embodiments, the functionality described herein may be used in a headset that combines images of an environment external to near-eye display120and artificial reality content (e.g., computer-generated images). Therefore, near-eye display120may augment images of a physical, real-world environment external to near-eye display120with generated content (e.g., images, video, sound, etc.) to present an augmented reality to a user.

In various embodiments, near-eye display120may include one or more of display electronics122, display optics124, and an eye-tracking unit130. In some embodiments, near-eye display120may also include one or more locators126, one or more position sensors128, and an inertial measurement unit (IMU)132. Near-eye display120may omit any of eye-tracking unit130, locators126, position sensors128, and IMU132, or include additional elements in various embodiments. Additionally, in some embodiments, near-eye display120may include elements combining the function of various elements described in conjunction withFIG.1.

Display electronics122may display or facilitate the display of images to the user according to data received from, for example, console110. In various embodiments, display electronics122may include one or more display panels, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, an inorganic light emitting diode (ILED) display, a micro light emitting diode (μLED) display, an active-matrix OLED display (AMOLED), a transparent OLED display (TOLED), or some other display. For example, in one implementation of near-eye display120, display electronics122may include a front TOLED panel, a rear display panel, and an optical component (e.g., an attenuator, polarizer, or diffractive or spectral film) between the front and rear display panels. Display electronics122may include pixels to emit light of a predominant color such as red, green, blue, white, or yellow. In some implementations, display electronics122may display a three-dimensional (3D) image through stereoscopic effects produced by two-dimensional panels to create a subjective perception of image depth. For example, display electronics122may include a left display and a right display positioned in front of a user's left eye and right eye, respectively. The left and right displays may present copies of an image shifted horizontally relative to each other to create a stereoscopic effect (e.g., a perception of image depth by a user viewing the image).

In certain embodiments, display optics124may display image content optically (e.g., using optical waveguides and couplers) or magnify image light received from display electronics122, correct optical errors associated with the image light, and present the corrected image light to a user of near-eye display120. In various embodiments, display optics124may include one or more optical elements, such as, for example, a substrate, optical waveguides, an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, input/output couplers, or any other suitable optical elements that may affect image light emitted from display electronics122. Display optics124may include a combination of different optical elements as well as mechanical couplings to maintain relative spacing and orientation of the optical elements in the combination. One or more optical elements in display optics124may have an optical coating, such as an anti-reflective coating, a reflective coating, a filtering coating, or a combination of different optical coatings.

Magnification of the image light by display optics124may allow display electronics122to be physically smaller, weigh less, and consume less power than larger displays. Additionally, magnification may increase a field of view of the displayed content. The amount of magnification of image light by display optics124may be changed by adjusting, adding, or removing optical elements from display optics124. In some embodiments, display optics124may project displayed images to one or more image planes that may be further away from the user's eyes than near-eye display120.

Display optics124may also be designed to correct one or more types of optical errors, such as two-dimensional optical errors, three-dimensional optical errors, or any combination thereof. Two-dimensional errors may include optical aberrations that occur in two dimensions. Example types of two-dimensional errors may include barrel distortion, pincushion distortion, longitudinal chromatic aberration, and transverse chromatic aberration. Three-dimensional errors may include optical errors that occur in three dimensions. Example types of three-dimensional errors may include spherical aberration, comatic aberration, field curvature, and astigmatism.

Locators126may be objects located in specific positions on near-eye display120relative to one another and relative to a reference point on near-eye display120. In some implementations, console110may identify locators126in images captured by external imaging device150to determine the artificial reality headset's position, orientation, or both. A locator126may be an LED, a corner cube reflector, a reflective marker, a type of light source that contrasts with an environment in which near-eye display120operates, or any combination thereof. In embodiments where locators126are active components (e.g., LEDs or other types of light emitting devices), locators126may emit light in the visible band (e.g., about 380 nm to 750 nm), in the infrared (IR) band (e.g., about 750 nm to 1 mm), in the ultraviolet band (e.g., about 10 nm to about 380 nm), in another portion of the electromagnetic spectrum, or in any combination of portions of the electromagnetic spectrum.

External imaging device150may include one or more cameras, one or more video cameras, any other device capable of capturing images including one or more of locators126, or any combination thereof. Additionally, external imaging device150may include one or more filters (e.g., to increase signal to noise ratio). External imaging device150may be configured to detect light emitted or reflected from locators126in a field of view of external imaging device150. In embodiments where locators126include passive elements (e.g., retroreflectors), external imaging device150may include a light source that illuminates some or all of locators126, which may retro-reflect the light to the light source in external imaging device150. Slow calibration data may be communicated from external imaging device150to console110, and external imaging device150may receive one or more calibration parameters from console110to adjust one or more imaging parameters (e.g., focal length, focus, frame rate, sensor temperature, shutter speed, aperture, etc.).

Position sensors128may generate one or more measurement signals in response to motion of near-eye display120. Examples of position sensors128may include accelerometers, gyroscopes, magnetometers, other motion-detecting or error-correcting sensors, or any combination thereof. For example, in some embodiments, position sensors128may include multiple accelerometers to measure translational motion (e.g., forward/back, up/down, or left/right) and multiple gyroscopes to measure rotational motion (e.g., pitch, yaw, or roll). In some embodiments, various position sensors may be oriented orthogonally to each other.

IMU132may be an electronic device that generates fast calibration data based on measurement signals received from one or more of position sensors128. Position sensors128may be located external to IMU132, internal to IMU132, or any combination thereof. Based on the one or more measurement signals from one or more position sensors128, IMU132may generate fast calibration data indicating an estimated position of near-eye display120relative to an initial position of near-eye display120. For example, IMU132may integrate measurement signals received from accelerometers over time to estimate a velocity vector and integrate the velocity vector over time to determine an estimated position of a reference point on near-eye display120. Alternatively, IMU132may provide the sampled measurement signals to console110, which may determine the fast calibration data. While the reference point may generally be defined as a point in space, in various embodiments, the reference point may also be defined as a point within near-eye display120(e.g., a center of IMU132).

Eye-tracking unit130may include one or more eye-tracking systems. Eye tracking may refer to determining an eye's position, including orientation and location of the eye, relative to near-eye display120. An eye-tracking system may include an imaging system to image one or more eyes and may optionally include a light emitter, which may generate light that is directed to an eye such that light reflected by the eye may be captured by the imaging system. For example, eye-tracking unit130may include a non-coherent or coherent light source (e.g., a laser diode) emitting light in the visible spectrum or infrared spectrum, and a camera capturing the light reflected by the user's eye. As another example, eye-tracking unit130may capture reflected radio waves emitted by a miniature radar unit. Eye-tracking unit130may use low-power light emitters that emit light at frequencies and intensities that would not injure the eye or cause physical discomfort. Eye-tracking unit130may be arranged to increase contrast in images of an eye captured by eye-tracking unit130while reducing the overall power consumed by eye-tracking unit130(e.g., reducing power consumed by a light emitter and an imaging system included in eye-tracking unit130). For example, in some implementations, eye-tracking unit130may consume less than 100 milliwatts of power.

Near-eye display120may use the orientation of the eye to, e.g., determine an inter-pupillary distance (IPD) of the user, determine gaze direction, introduce depth cues (e.g., blur image outside of the user's main line of sight), collect heuristics on the user interaction in the VR media (e.g., time spent on any particular subject, object, or frame as a function of exposed stimuli), some other functions that are based in part on the orientation of at least one of the user's eyes, or any combination thereof. Because the orientation may be determined for both eyes of the user, eye-tracking unit130may be able to determine where the user is looking. For example, determining a direction of a user's gaze may include determining a point of convergence based on the determined orientations of the user's left and right eyes. A point of convergence may be the point where the two foveal axes of the user's eyes intersect. The direction of the user's gaze may be the direction of a line passing through the point of convergence and the mid-point between the pupils of the user's eyes.

Input/output interface140may be a device that allows a user to send action requests to console110. An action request may be a request to perform a particular action. For example, an action request may be to start or to end an application or to perform a particular action within the application. Input/output interface140may include one or more input devices. Example input devices may include a keyboard, a mouse, a game controller, a glove, a button, a touch screen, or any other suitable device for receiving action requests and communicating the received action requests to console110. An action request received by the input/output interface140may be communicated to console110, which may perform an action corresponding to the requested action. In some embodiments, input/output interface140may provide haptic feedback to the user in accordance with instructions received from console110. For example, input/output interface140may provide haptic feedback when an action request is received, or when console110has performed a requested action and communicates instructions to input/output interface140. In some embodiments, external imaging device150may be used to track input/output interface140, such as tracking the location or position of a controller (which may include, for example, an IR light source) or a hand of the user to determine the motion of the user. In some embodiments, near-eye display120may include one or more imaging devices to track input/output interface140, such as tracking the location or position of a controller or a hand of the user to determine the motion of the user.

Console110may provide content to near-eye display120for presentation to the user in accordance with information received from one or more of external imaging device150, near-eye display120, and input/output interface140. In the example shown inFIG.1, console110may include an application store112, a headset tracking module114, an artificial reality engine116, and an eye-tracking module118. Some embodiments of console110may include different or additional modules than those described in conjunction withFIG.1. Functions further described below may be distributed among components of console110in a different manner than is described here.

In some embodiments, console110may include a processor and a non-transitory computer-readable storage medium storing instructions executable by the processor. The processor may include multiple processing units executing instructions in parallel. The non-transitory computer-readable storage medium may be any memory, such as a hard disk drive, a removable memory, or a solid-state drive (e.g., flash memory or dynamic random access memory (DRAM)). In various embodiments, the modules of console110described in conjunction withFIG.1may be encoded as instructions in the non-transitory computer-readable storage medium that, when executed by the processor, cause the processor to perform the functions further described below.

Application store112may store one or more applications for execution by console110. An application may include a group of instructions that, when executed by a processor, generates content for presentation to the user. Content generated by an application may be in response to inputs received from the user via movement of the user's eyes or inputs received from the input/output interface140. Examples of the applications may include gaming applications, conferencing applications, video playback application, or other suitable applications.

Headset tracking module114may track movements of near-eye display120using slow calibration information from external imaging device150. For example, headset tracking module114may determine positions of a reference point of near-eye display120using observed locators from the slow calibration information and a model of near-eye display120. Headset tracking module114may also determine positions of a reference point of near-eye display120using position information from the fast calibration information. Additionally, in some embodiments, headset tracking module114may use portions of the fast calibration information, the slow calibration information, or any combination thereof, to predict a future location of near-eye display120. Headset tracking module114may provide the estimated or predicted future position of near-eye display120to artificial reality engine116.

Artificial reality engine116may execute applications within artificial reality system environment100and receive position information of near-eye display120, acceleration information of near-eye display120, velocity information of near-eye display120, predicted future positions of near-eye display120, or any combination thereof from headset tracking module114. Artificial reality engine116may also receive estimated eye position and orientation information from eye-tracking module118. Based on the received information, artificial reality engine116may determine content to provide to near-eye display120for presentation to the user. For example, if the received information indicates that the user has looked to the left, artificial reality engine116may generate content for near-eye display120that mirrors the user's eye movement in a virtual environment. Additionally, artificial reality engine116may perform an action within an application executing on console110in response to an action request received from input/output interface140, and provide feedback to the user indicating that the action has been performed. The feedback may be visual or audible feedback via near-eye display120or haptic feedback via input/output interface140.

Eye-tracking module118may receive eye-tracking data from eye-tracking unit130and determine the position of the user's eye based on the eye tracking data. The position of the eye may include an eye's orientation, location, or both relative to near-eye display120or any element thereof. Because the eye's axes of rotation change as a function of the eye's location in its socket, determining the eye's location in its socket may allow eye-tracking module118to more accurately determine the eye's orientation.

FIG.2is a perspective view of an example of a near-eye display in the form of an HMD device200for implementing some of the examples disclosed herein. HMD device200may be a part of, e.g., a VR system, an AR system, an MR system, or any combination thereof. HMD device200may include a body220and a head strap230.FIG.2shows a bottom side223, a front side225, and a left side227of body220in the perspective view. Head strap230may have an adjustable or extendible length. There may be a sufficient space between body220and head strap230of HMD device200for allowing a user to mount HMD device200onto the user's head. In various embodiments, HMD device200may include additional, fewer, or different components. For example, in some embodiments, HMD device200may include eyeglass temples and temple tips as shown in, for example,FIG.3below, rather than head strap230.

HMD device200may present to a user media including virtual and/or augmented views of a physical, real-world environment with computer-generated elements. Examples of the media presented by HMD device200may include images (e.g., two-dimensional (2D) or three-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio, or any combination thereof.

The images and videos may be presented to each eye of the user by one or more display assemblies (not shown inFIG.2) enclosed in body220of HMD device200. In various embodiments, the one or more display assemblies may include a single electronic display panel or multiple electronic display panels (e.g., one display panel for each eye of the user). Examples of the electronic display panel(s) may include, for example, an LCD, an OLED display, an ILED display, a μLED display, an AMOLED, a TOLED, some other display, or any combination thereof. HMD device200may include two eyebox regions.

In some implementations, HMD device200may include various sensors (not shown), such as depth sensors, motion sensors, position sensors, and eye tracking sensors. Some of these sensors may use a structured light pattern for sensing. In some implementations, HMD device200may include an input/output interface for communicating with a console. In some implementations, HMD device200may include a virtual reality engine (not shown) that can execute applications within HMD device200and receive depth information, position information, acceleration information, velocity information, predicted future positions, or any combination thereof of HMD device200from the various sensors. In some implementations, the information received by the virtual reality engine may be used for producing a signal (e.g., display instructions) to the one or more display assemblies. In some implementations, HMD device200may include locators (not shown, such as locators126) located in fixed positions on body220relative to one another and relative to a reference point. Each of the locators may emit light that is detectable by an external imaging device.

FIG.3is a perspective view of an example of a near-eye display300in the form of a pair of glasses for implementing some of the examples disclosed herein. Near-eye display300may be a specific implementation of near-eye display120ofFIG.1, and may be configured to operate as a virtual reality display, an augmented reality display, and/or a mixed reality display. Near-eye display300may include a frame305and a display310. Display310may be configured to present content to a user. In some embodiments, display310may include display electronics and/or display optics. For example, as described above with respect to near-eye display120ofFIG.1, display310may include an LCD display panel, an LED display panel, or an optical display panel (e.g., a waveguide display assembly).

Near-eye display300may further include various sensors350a,350b,350c,350d, and350eon or within frame305. In some embodiments, sensors350a-350emay include one or more depth sensors, motion sensors, position sensors, inertial sensors, or ambient light sensors. In some embodiments, sensors350a-350emay include one or more image sensors configured to generate image data representing different regions in a field of views in different directions. In some embodiments, sensors350a-350emay be used as input devices to control or influence the displayed content of near-eye display300, and/or to provide an interactive VR/AR/MR experience to a user of near-eye display300. In some embodiments, sensors350a-350emay also be used for stereoscopic imaging.

In some embodiments, near-eye display300may further include one or more illuminators330to project light into the physical environment. The projected light may be associated with different frequency bands (e.g., visible light, infra-red light, ultra-violet light, etc.), and may serve various purposes. For example, illuminator(s)330may project light in a dark environment (or in an environment with low intensity of infra-red light, ultra-violet light, etc.) to assist sensors350a-350ein capturing images of different objects within the dark environment. In some embodiments, illuminator(s)330may be used to project certain light pattern onto the objects within the environment. In some embodiments, illuminator(s)330may be used as locators, such as locators126described above with respect toFIG.1.

In some embodiments, near-eye display300may also include a high-resolution camera340. Camera340may capture images of the physical environment in the field of view. The captured images may be processed, for example, by a virtual reality engine (e.g., artificial reality engine116ofFIG.1) to add virtual objects to the captured images or modify physical objects in the captured images, and the processed images may be displayed to the user by display310for AR or MR applications.

FIG.4is a simplified diagram illustrating an example of an optical system400in a near-eye display system. Optical system400may include an image source410and projector optics420. In the example shown inFIG.4, image source410is in front of projector optics420. In various embodiments, image source410may be located outside of the field of view of user's eye490. For example, one or more reflectors or directional couplers may be used to deflect light from an image source that is outside of the field of view of user's eye490to make the image source appear to be at the location of image source410shown inFIG.4. Light from an area (e.g., a pixel or a light emitting device) on image source410may be collimated and directed to an exit pupil430by projector optics420. Thus, objects at different spatial locations on image source410may appear to be objects far away from user's eye490in different viewing angles (FOVs). The collimated light from different viewing angles may then be focused by the lens of user's eye490onto different locations on retina492of user's eye490. For example, at least some portions of the light may be focused on a fovea494on retina492. Collimated light rays from an area on image source410and incident on user's eye490from a same direction may be focused onto a same location on retina492. As such, a single image of image source410may be formed on retina492.

The user experience of using an artificial reality system may depend on several characteristics of the optical system, including field of view (FOV), image quality (e.g., angular resolution), size of the eyebox (to accommodate for eye and head movements), and brightness of the light (or contrast) within the eyebox. Field of view describes the angular range of the image as seen by the user, usually measured in degrees as observed by one eye (for a monocular HMD) or both eyes (for either biocular or binocular HMDs). The human visual system may have a total binocular FOV of about 200° (horizontal) by 130° (vertical). To create a fully immersive visual environment, a large FOV is desirable because a large FOV (e.g., greater than about 60°) may provide a sense of “being in” an image, rather than merely viewing the image. Smaller fields of view may also preclude some important visual information. For example, an HMD system with a small FOV may use a gesture interface, but the users may not see their hands in the small FOV to be sure that they are using the correct motions. On the other hand, wider fields of view may require larger displays or optical systems, which may influence the size, weight, cost, and comfort of using the HMD.

Resolution may refer to the angular size of a displayed pixel or image element appearing to a user, or the ability for the user to view and correctly interpret an object as imaged by a pixel and/or other pixels. The resolution of an HMD may be specified as the number of pixels on the image source for a given FOV value, from which an angular resolution may be determined by dividing the FOV in one direction by the number of pixels in the same direction on the image source. For example, for a horizontal FOV of 40° and 1080 pixels in the horizontal direction on the image source, the corresponding angular resolution may be about 2.2 arc-minutes, compared with the one-arc-minute resolution associated with Snellen 20/20 human visual acuity.

In some cases, the eyebox may be a two-dimensional box in front of the user's eye, from which the displayed image from the image source may be viewed. If the pupil of the user moves outside of the eyebox, the displayed image may not be seen by the user. For example, in a non-pupil-forming configuration, there exists a viewing eyebox within which there will be unvignetted viewing of the HMD image source, and the displayed image may vignette or may be clipped but may still be viewable when the pupil of user's eye is outside of the viewing eyebox. In a pupil-forming configuration, the image may not be viewable outside the exit pupil.

The fovea of a human eye, where the highest resolution may be achieved on the retina, may correspond to an FOV of about 2° to about 3°. This may require that the eye rotates in order to view off-axis objects with a highest resolution. The rotation of the eye to view the off-axis objects may introduce a translation of the pupil because the eye rotates around a point that is about 10 mm behind the pupil. In addition, a user may not always be able to accurately position the pupil (e.g., having a radius of about 2.5 mm) of the user's eye at an ideal location in the eyebox. Furthermore, the environment where the HMD is used may require the eyebox to be larger to allow for movement of the user's eye and/or head relative the HMD, for example, when the HMD is used in a moving vehicle or designed to be used while the user is moving on foot. The amount of movement in these situations may depend on how well the HMD is coupled to the user's head.

Thus, the optical system of the HMD may need to provide a sufficiently large exit pupil or viewing eyebox for viewing the full FOV with full resolution, in order to accommodate the movements of the user's pupil relative to the HMD. For example, in a pupil-forming configuration, a minimum size of 12 mm to 15 mm may be desired for the exit pupil. If the eyebox is too small, minor misalignments between the eye and the HMD may result in at least partial loss of the image, and the user experience may be substantially impaired. In general, the lateral extent of the eyebox is more critical than the vertical extent of the eyebox. This may be in part due to the significant variances in eye separation distance between users, and the fact that misalignments to eyewear tend to more frequently occur in the lateral dimension and users tend to more frequently adjust their gaze left and right, and with greater amplitude, than adjusting the gaze up and down. Thus, techniques that can increase the lateral dimension of the eyebox may substantially improve a user's experience with an HMD. On the other hand, the larger the eyebox, the larger the optics and the heavier and bulkier the near-eye display device may be.

In order to view the displayed image against a bright background, the image source of an AR HMD may need to be sufficiently bright, and the optical system may need to be efficient to provide a bright image to the user's eye such that the displayed image may be visible in a background including strong ambient light, such as sunlight. The optical system of an HMD may be designed to concentrate light in the eyebox. When the eyebox is large, an image source with high power may be used to provide a bright image viewable within the large eyebox. Thus, there may be trade-offs among the size of the eyebox, cost, brightness, optical complexity, image quality, and size and weight of the optical system.

FIG.5illustrates an example of an optical see-through augmented reality system500including a waveguide display for exit pupil expansion according to certain embodiments. Augmented reality system500may include a projector510and a combiner515. Projector510may include a light source or image source512and projector optics514. In some embodiments, light source or image source512may include one or more micro-LED devices. In some embodiments, image source512may include a plurality of pixels that displays virtual objects, such as an LCD display panel or an LED display panel. In some embodiments, image source512may include a light source that generates coherent or partially coherent light. For example, image source512may include a laser diode, a vertical cavity surface emitting laser, an LED, a superluminescent LED (sLED), and/or a micro-LED described above. In some embodiments, image source512may include a plurality of light sources (e.g., an array of micro-LEDs described above) each emitting a monochromatic image light corresponding to a primary color (e.g., red, green, or blue). In some embodiments, image source512may include three two-dimensional arrays of micro-LEDs, where each two-dimensional array of micro-LEDs may include micro-LEDs configured to emit light of a primary color (e.g., red, green, or blue). In some embodiments, image source512may include an optical pattern generator, such as a spatial light modulator. Projector optics514may include one or more optical components that can condition the light from image source512, such as expanding, collimating, scanning, or projecting light from image source512to combiner515. The one or more optical components may include, for example, one or more lenses, liquid lenses, mirrors, free-form optics, apertures, and/or gratings. For example, in some embodiments, image source512may include one or more one-dimensional arrays or elongated two-dimensional arrays of micro-LEDs, and projector optics514may include one or more one-dimensional scanners (e.g., micro-mirrors or prisms) configured to scan the one-dimensional arrays or elongated two-dimensional arrays of micro-LEDs to generate image frames. In some embodiments, projector optics514may include a liquid lens (e.g., a liquid crystal lens) with a plurality of electrodes that allows scanning of the light from image source512.

Combiner515may include an input coupler530for coupling light from projector510into a substrate520of combiner515. Input coupler530may include a volume holographic grating or another diffractive optical element (DOE) (e.g., a surface-relief grating (SRG)), a slanted reflective surface of substrate520, or a refractive coupler (e.g., a wedge or a prism). For example, input coupler530may include a reflective volume Bragg grating or a transmissive volume Bragg grating. Input coupler530may have a coupling efficiency of greater than 30%, 50%, 75%, 90%, or higher for visible light. Light coupled into substrate520may propagate within substrate520through, for example, total internal reflection (TIR). Substrate520may be in the form of a lens of a pair of eyeglasses. Substrate520may have a flat or a curved surface, and may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, ceramic, or the like. A thickness of the substrate may range from, for example, less than about 1 mm to about 10 mm or more. Substrate520may be transparent to visible light.

Substrate520may include or may be coupled to a plurality of output couplers540each configured to extract at least a portion of the light guided by and propagating within substrate520from substrate520, and direct extracted light560to an eyebox595where an eye590of the user of augmented reality system500may be located when augmented reality system500is in use. The plurality of output couplers540may replicate the exit pupil to increase the size of eyebox595, such that the displayed image may be visible in a larger area. As input coupler530, output couplers540may include grating couplers (e.g., volume holographic gratings or surface-relief gratings), other diffraction optical elements (DOEs), prisms, etc. For example, output couplers540may include reflective volume Bragg gratings or transmissive volume Bragg gratings. Output couplers540may have different coupling (e.g., diffraction) efficiencies at different locations. Substrate520may also allow light550from the environment in front of combiner515to pass through with little or no loss. Output couplers540may also allow light550to pass through with little loss. For example, in some implementations, output couplers540may have a very low diffraction efficiency for light550such that light550may be refracted or otherwise pass through output couplers540with little loss, and thus may have a higher intensity than extracted light560. In some implementations, output couplers540may have a high diffraction efficiency for light550and may diffract light550in certain desired directions (i.e., diffraction angles) with little loss. As a result, the user may be able to view combined images of the environment in front of combiner515and images of virtual objects projected by projector510. In some implementations, output couplers540may have a high diffraction efficiency for light550and may diffract light550to certain desired directions (e.g., diffraction angles) with little loss.

In some embodiments, projector510, input coupler530, and output coupler540may be on any side of substrate520. Input coupler530and output coupler540may be reflective gratings (also referred to as reflective gratings) or transmissive gratings (also referred to as transmissive gratings) to couple display light into or out of substrate520.

FIG.6illustrates an example of an optical see-through augmented reality system600including a waveguide display for exit pupil expansion according to certain embodiments. Augmented reality system600may be similar to augmented reality system500, and may include the waveguide display and a projector that may include a light source or image source612and projector optics614. The waveguide display may include a substrate630, an input coupler640, and a plurality of output couplers650as described above with respect to augmented reality system500. WhileFIG.5only shows the propagation of light from a single field of view,FIG.6shows the propagation of light from multiple fields of view.

FIG.6shows that the exit pupil is replicated by output couplers650to form an aggregated exit pupil or eyebox, where different regions in a field of view (e.g., different pixels on image source612) may be associated with different respective propagation directions towards the eyebox, and light from a same field of view (e.g., a same pixel on image source612) may have a same propagation direction for the different individual exit pupils. Thus, a single image of image source612may be formed by the user's eye located anywhere in the eyebox, where light from different individual exit pupils and propagating in the same direction may be from a same pixel on image source612and may be focused onto a same location on the retina of the user's eye.FIG.6shows that the image of the image source is visible by the user's eye even if the user's eye moves to different locations in the eyebox.

In many waveguide-based near-eye display systems, in order to expand the eyebox of the waveguide-based near-eye display in two dimensions, two or more output gratings may be used to expand the display light in two dimensions or along two axes (which may be referred to as dual-axis pupil expansion). The two gratings may have different grating parameters, such that one grating may be used to replicate the exit pupil in one direction and the other grating may be used to replicate the exit pupil in another direction.

As described above, the input and output grating couplers described above can be volume holographic gratings or surface-relief gratings, which may have very different Klein-Cook parameter Q:

Q=2⁢πλ⁢dn⁢Λ2,
where d is the thickness of the grating, λ is the wavelength of the incident light in free space, Λ is the grating period, and n is the refractive index of the recording medium. The Klein-Cook parameter Q may divide light diffraction by gratings into three regimes. When a grating is characterized by Q<<1, light diffraction by the grating may be referred to as Raman-Nath diffraction, where multiple diffraction orders may occur for normal and/or oblique incident light. When a grating is characterized by Q>>1 (e.g., Q≥10), light diffraction by the grating may be referred to as Bragg diffraction, where generally only the zeroth and the ±1 diffraction orders may occur for light incident on the grating at an angle satisfying the Bragg condition. When a grating is characterized by Q≈1, the diffraction by the grating may be between the Raman-Nath diffraction and the Bragg diffraction. To meet Bragg conditions, the thickness d of the grating may be higher than certain values to occupy a volume (rather than at a surface) of a medium, and thus may be referred to as a volume Bragg grating. VBGs may generally have relatively small refractive index modulations (e.g., Δn≤0.05) and high spectral and angular selectivity, while surface-relief gratings may generally have large refractive index modulations (e.g., Δn≥0.5) and wide spectral and angular bandwidths.

FIG.7Aillustrates the spectral bandwidth of an example of a volume Bragg grating (e.g., a reflective VBG) and the spectral bandwidth of an example of a surface-relief grating (e.g., a transmissive SRG). The horizontal axis represents the wavelength of the incident visible light and the vertical axis corresponds to the diffraction efficiency. As shown by a curve710, the diffraction efficiency of the reflective VBG is high in a narrow wavelength range, such as green light. In contrast, the diffraction efficiency of the transmissive SRG may be high in a very wide wavelength range, such as from blue to red light, as shown by a curve720.

FIG.7Billustrates the angular bandwidth of an example of a volume Bragg grating (e.g., a reflective VBG) and the angular bandwidth of an example of a surface-relief grating (e.g., a transmissive SRG). The horizontal axis represents the incident angle of the visible light incident on the grating, and the vertical axis corresponds to the diffraction efficiency. As shown by a curve715, the diffraction efficiency of the reflective VBG is high for light incident on the grating from a narrow angular range, such as about ±2.5° from the perfect Bragg condition. In contrast, the diffraction efficiency of the transmissive SRG is high in a very wide angular range, such as greater than about ±10° or wider, as shown by a curve725.

FIG.8Aillustrates an example of an optical see-through augmented reality system including a waveguide display800and surface-relief gratings for exit pupil expansion according to certain embodiments. Waveguide display800may include a substrate810(e.g., a waveguide), which may be similar to substrate520. Substrate810may be transparent to visible light and may include, for example, a glass, quartz, plastic, polymer, PMMA, ceramic, Si3N4, or crystal substrate. Substrate810may be a flat substrate or a curved substrate. Substrate810may include a first surface812and a second surface814. Display light may be coupled into substrate810by an input coupler820, and may be reflected by first surface812and second surface814through total internal reflection, such that the display light may propagate within substrate810. Input coupler820may include a grating, a refractive coupler (e.g., a wedge or a prism), or a reflective coupler (e.g., a reflective surface having a slant angle with respect to substrate810). For example, in one embodiment, input coupler820may include a prism that may couple display light of different colors into substrate810at a same refraction angle. In another example, input coupler820may include a grating coupler that may diffract light of different colors into substrate810at different directions. Input coupler820may have a coupling efficiency of greater than 10%, 20%, 30%, 50%, 75%, 90%, or higher for visible light.

Waveguide display800may also include a first output grating830and a second output grating840positioned on one or two surfaces (e.g., first surface812and second surface814) of substrate810for expanding incident display light beam in two dimensions in order to fill an eyebox with the display light. First output grating830may be configured to expand at least a portion of the display light beam along one direction, such as approximately in the x direction. Display light coupled into substrate810may propagate in a direction shown by a line832. While the display light propagates within substrate810along a direction shown by line832, a portion of the display light may be diffracted by a region of first output grating830towards second output grating840as shown by a line834each time the display light propagating within substrate810reaches first output grating830. Second output grating840may then expand the display light from first output grating830in a different direction (e.g., approximately in the y direction) by diffracting a portion of the display light from an exit region850to the eyebox each time the display light propagating within substrate810reaches second output grating840.

FIG.8Billustrates an example of an eye box including two-dimensional replicated exit pupils.FIG.8Bshows that a single input pupil805may be replicated by first output grating830and second output grating840to form an aggregated exit pupil860that includes a two-dimensional array of individual exit pupils862. For example, the exit pupil may be replicated in approximately the x direction by first output grating830and in approximately the y direction by second output grating840. As described above, output light from individual exit pupils862and propagating in a same direction may be focused onto a same location in the retina of the user's eye. Thus, a single image may be formed by the user's eye from the output light in the two-dimensional array of individual exit pupils862.

FIG.9illustrates an example of a volume Bragg grating-based waveguide display900with exit pupil expansion and dispersion reduction according to certain embodiments. Waveguide display900may be an example of waveguide display800and may include a waveguide905, and an input grating910, a first middle grating920, a second middle grating930, and an output grating940formed on or in waveguide905. Each of input grating910, first middle grating920, second middle grating930, and output grating940may be a transmissive grating or a reflective grating. Display light from a light source (e.g., one or more micro-LED arrays) may be coupled into waveguide905by input grating910. The in-coupled display light may be reflected by surfaces of waveguide905through total internal reflection, such that the display light may propagate within waveguide905. Input grating910may include multiplexed VBGs and may couple display light of different colors and from different fields of view into waveguide905at corresponding diffraction angles.

First middle grating920and second middle grating930may be in different regions of a same holographic material layer or may be on different holographic material layers. In some embodiments, first middle grating920may be spatially separate from second middle grating930. First middle grating920and second middle grating930may each include multiplexed VBGs. In some embodiments, first middle grating920and second middle grating930may be recorded in a same number of exposures and under similar recording conditions, such that each VBG in first middle grating920may match a respective VBG in second middle grating930(e.g., having the same grating vector in the x-y plane and having the same and/or opposite grating vectors in the z direction). For example, in some embodiments, a VBG in first middle grating920and a corresponding VBG in second middle grating930may have the same grating period and the same grating slant angle (and thus the same grating vector), and the same thickness. In one example, first middle grating920and second middle grating930may have a thickness about 20 μm and may each include about 20 or more VBGs recorded through about 20 or more exposures.

Output grating940may be formed in the see-through region of waveguide display900and may include an exit region950that overlaps with the eyebox of waveguide display900when viewed in the z direction (e.g., at a distance about 18 mm from output grating940in +z or −z direction). Output grating940may include multiplexed VBG gratings that include many VBGs. In some embodiments, output grating940and second middle grating930may at least partially overlap in the x-y plane, thereby reducing the form factor of waveguide display900. Output grating940, in combination with first middle grating920and second middle grating930, may perform the dual-axis pupil expansion described above to expand the incident display light beam in two dimensions to fill the eyebox with the display light.

Input grating910may couple the display light from the light source into waveguide905. The display light may reach first middle grating920directly or may be reflected by surfaces of waveguide905to first middle grating920, where the size of the display light beam may be slightly larger than the size of the display light beam at input grating910. Each VBG in first middle grating920may diffract a portion of the display light within a FOV range and a wavelength range that approximately satisfies the Bragg condition of the VBG to second middle grating930. While the display light diffracted by a VBG in first middle grating920propagates within waveguide905(e.g., along a direction shown by a line922) through total internal reflection, a portion of the display light may be diffracted by the corresponding VBG in second middle grating930towards output grating940each time the display light propagating within waveguide905reaches second middle grating930. Output grating940may then expand the display light from second middle grating930in a different direction by diffracting a portion of the display light to the eyebox each time the display light propagating within waveguide905reaches exit region950of output grating940.

As described above, each VBG in first middle grating920may match a respective VBG in second middle grating930(e.g., having the same grating vector in the x-y plane and having the same and/or opposite grating vector in the z direction). The two matching VBGs may work under opposite Bragg conditions (e.g., +1 order diffraction versus −1 order diffraction) due to the opposite propagation directions of the display light at the two matching VBGs. For example, as shown inFIG.9, the VBG in first middle grating920may change the propagation direction of the display light from a downward direction to a rightward direction, while the matching VBG in second middle grating930may change the propagation direction of the display light from a rightward direction to a downward direction. Thus, the dispersion caused by second middle grating930may be opposite to the dispersion caused by first middle grating920, thereby reducing or minimizing the overall dispersion.

Similarly, each VBG in input grating910may match a respective VBG in output grating940(e.g., having the same grating vector in the x-y plane and having the same and/or opposite grating vector in the z direction). The two matching VBGs may also work under opposite Bragg conditions (e.g., +1 order diffraction versus −1 order diffraction) due to the opposite propagation directions of the display light (e.g., into and out of waveguide905) at the two matching VBGs. Therefore, the dispersion caused by input grating910may be opposite to the dispersion caused by output grating940, thereby reducing or minimizing the overall dispersion.

In the examples of waveguide displays described above, couplers implemented using diffractive optical elements may have limited coupling efficiencies due to, for example, less than 100% diffraction efficiency to the desired diffraction order, leakage, polarization dependence, angular dependence, wavelength dependence, and the like. In addition, due to the size of the input pupil and thus the size of the input coupler, the display light coupled into the waveguide by an input coupler may be reflected back to the input coupler and may be at least partially diffracted by the input coupler again to undesired directions and become leakage light.

FIG.10Aillustrates an example of a grating coupler1020for coupling display light into a waveguide1010of a waveguide display1000. Grating coupler1020may have a finite area to receive an incident light beam1005having a finite beam width from a projector.FIG.10Ashows the desired optical path of an incident light beam1030. Grating coupler1020on a top surface1012of waveguide1010may diffract incident light beam1030into a first diffraction order1032having a certain diffraction angle. First diffraction order1032may propagate in waveguide1010and reach a bottom surface1014of waveguide1010. Bottom surface1014of waveguide1010may reflect all first diffraction order1032back towards grating coupler1020as shown by a light beam1034due to total internal reflection. It may be desirable that light beam1034is fully reflected at top surface1012of waveguide1010as shown by a light beam1036, such that all first diffraction order1032coupled into waveguide1010by grating coupler1020may propagate within waveguide1010to reach an output coupler.

FIG.10Billustrates examples of undesired light diffraction by grating coupler1020that may reduce the efficiency of waveguide display1000. As illustrated, when incident light beam1030reaches grating coupler1020, it may be diffracted by grating coupler1020into multiple diffraction orders including first diffraction order1032and other diffraction orders1040(e.g., zeroth order, —1storder, and higher orders). When the reflected light beam1034from the bottom surface1014reaches top surface1012of waveguide1010, it may be at least partially diffracted by grating coupler1020into higher diffraction orders (e.g., ±1, ±2, and the like) as shown by light beams1042and1044. Therefore, the power of the reflected portion (shown by light beam1036) may be much lower than the power of incident light beam1030or first diffraction order1032. For example, about 20% or more of the in-coupled light may be diffracted out of waveguide1010by grating coupler1020, thereby reducing the overall in-coupling efficiency of grating coupler1020.

FIG.11illustrates an example of a grating coupler1120for coupling display light into a substrate1110of a waveguide display1100. In the illustrated example, grating coupler1120may be formed on a top surface1112of substrate1110. A light ray1130may be coupled by a first region1122of grating coupler1120into substrate1110at a certain diffraction angle. The in-coupled display light of light ray1130may be reflected by a bottom surface1114of substrate1110and reach grating coupler1120at the top surface1112of substrate1110again. A portion of the in-coupled display light of light ray1130may be reflected at the top surface1112to continue to propagate within substrate1110. However, a portion1132of the in-coupled display light of light ray1130may be diffracted by a second region1124of grating coupler1120out of substrate1110to cause input coupling leakage. The reflected portion may propagate within substrate1110and reach a third region1126of grating coupler1120, and may be at least partially diffracted out of substrate1110by third region1126of grating coupler1120as shown by a light ray1134to cause further input coupling leakage.

Similarly, a light ray1140(e.g., for a different field of view angle or color) may be coupled by grating coupler1120into substrate1110at a certain diffraction angle, and at least a portion1142of the in-coupled light of light ray1140may be diffracted out of substrate1110by grating coupler1120when the portion1142of the in-coupled light of light ray1140reaches grating coupler1120again due to the total internal reflection at the bottom surface1114of substrate1110. An input light ray1150(e.g., for another field of view angle or color) may be coupled by grating coupler1120into substrate1110at a certain diffraction angle, and at least a portion1152of the in-coupled light of light ray1150may be diffracted out of substrate1110by grating coupler1120when the portion1152of the in-coupled light of light ray1150reaches grating coupler1120again due to the total internal reflection at the bottom surface1114of substrate1110.

Therefore, the overall input coupling efficiency of grating coupler1120may be reduced due to the input coupling leakage. In some embodiments, the input coupling leakage may be reduced by, for example, increasing the thickness of substrate1110such that the in-coupled light, when reflected back to the top surface1112, may be outside of the region of grating coupler1120. In another example, the pupil size may be reduced such that grating coupler1120can have a smaller area and thus the in-coupled light, when reflected back to the top surface1112, may be outside of the region of grating coupler1120. However, increasing the thickness of the substrate and/or reducing the pupil size may decrease the pupil density and/or reduce the display resolution.

According to certain embodiments, the first middle grating (e.g., first middle grating920) may be place underneath the input grating (e.g., input grating910or grating coupler1120), such that the in-coupled display light may be diffracted by the first middle grating before the in-coupled display light would reach the input grating again due to the total internal reflection at the surfaces of the waveguide. The in-coupled display light that is diffracted by the first middle grating may change the propagation direction within the waveguide and thus may not meet the Bragg condition of the input grating when it reaches the input grating again. Therefore, the in-coupled display light that is diffracted by the first middle grating may not be diffracted by the input grating even if it reaches the input grating again. In certain embodiments, a phase structure (e.g., a phase retarder) may alternatively or additionally be used to change the polarization state of the in-coupled display light, such that the out-coupling of the in-coupled display light by the input grating may be reduced due to the polarization dependence of the input grating.

FIG.12Aillustrates an example of a method of improving the in-coupling efficiency of a waveguide display according to certain embodiments.FIG.12Ashows a waveguide1210, and an input grating1220and a first middle grating1230in waveguide1210. Waveguide1210may include one or more substrates. Input grating1220and first middle grating1230may be formed on the one or more substrates. Input grating1220in the example shown inFIG.12Amay be a reflective VBG. In some embodiments, input grating1220may be a transmissive grating. As illustrated inFIG.12A, input grating1220may overlap with first middle grating1230when viewed in the z direction. In-coupled display light from input grating1220may be diffracted by first middle grating1230, and thus may change the propagation direction within the waveguide. As a result, the in-coupled display light diffracted by first middle grating1230may not meet the Bragg condition of input grating1220when it reaches input grating1220again. Therefore, the in-coupled display light that is diffracted by first middle grating1230may not be diffracted by input grating1220out of waveguide1210to cause input coupling leakage. The overlapping of input grating1220and first middle grating1230may also help to reduce the size of the waveguide display. In one example, input grating1220and first middle grating1230may be recorded in a same holographic material layer.

FIG.12Bis a top view of an example of a waveguide display1200including grating couplers arranged to improve the in-coupling efficiency according to certain embodiments. Waveguide display1200may include waveguide1210(e.g., a substrate), input grating1220, and first middle grating1230as described above with respect toFIG.12A. As illustrated, input grating1220may overlap with a portion of first middle grating1230in the top view. As described above with respect to, for example,FIG.9, waveguide display1200may also include a second middle grating1240and an output grating1250that may expand the input pupil in two directions.

Input grating1220may couple the display light from a light source (e.g., a projector) into waveguide1210. The display light may reach first middle grating1230directly or may be reflected by surfaces of waveguide1210to first middle grating1230. First middle grating1230may change the propagation direction of the in-coupled display light by diffracting the in-coupled display light towards second middle grating1240. The display light diffracted by first middle grating1230, even if reaching input grating1220again, would not be diffracted by input grating1220out of waveguide1210to cause leakage.

As described above with respect toFIGS.8A and9, while the display light diffracted by first middle grating1230propagates within waveguide1210through total internal reflection, a portion of the display light may be diffracted by second middle grating1240towards output grating1250each time the display light propagating within waveguide1210reaches second middle grating1240. Output grating1250may then expand the display light from second middle grating1240in a different direction by diffracting a portion of the display light to an eyebox of waveguide display1200each time the display light propagating within waveguide1210reaches an exit region1260of output grating1250that may overlap with the eyebox when viewed in the z direction.

FIG.12Cis a zoom-in top view of a portion of waveguide display1200shown inFIG.12Baccording to certain embodiments.FIG.12Dis a zoom-in side view of the portion of waveguide display1200shown inFIG.12Baccording to certain embodiments.FIGS.12C and12Dshows input grating1220and first middle grating1230in waveguide1210.FIG.12Cshows that input grating1220overlaps with a portion of first middle grating1230when reviewed in the z direction. First middle grating1230may include multiple VBGs1232that may be used to diffract light from different fields of view and/or in different colors.FIG.12Dshows that input grating1220and first middle grating1230may be recorded in a same holographic material layer and may be multiplexed in a same region of the holographic material layer.

As described above, overlapping input grating1220and first middle grating1230may help to reduce the physical dimension of waveguide display1200. However, due to the limited maximum achievable refractive index modulation in a holographic material layer, multiplexing input grating1220and first middle grating1230in a same holographic material layer as shown inFIG.12Dmay not achieve a high diffraction efficiency for input grating1220and first middle grating1230. In addition, it can be challenging to fabricate input grating1220and first middle grating1230in a same holographic material layer.

According to certain embodiments, the first middle grating (e.g., first middle grating1230) may be fabricated in a holographic material layer within a waveguide as shown inFIG.12A, the input grating (e.g., input grating1220) may be fabricated in a different holographic material layer in a staircase structure that has a small size to avoid changing the thickness in other regions of the waveguide. Separately recording the input grating and the first middle grating in different holographic material layers may make the manufacturing process easier, and may help to improve the diffraction efficiency of the input grating and the first middle grating due to the higher overall achievable refractive index modulation in multiple holographic material layers. The staircase structure including the input grating may then be attached to the waveguide and aligned with the first middle grating. To avoid FOV clipping, the total thickness and the overall shape of the staircase structure that includes the holographic material layer in which the input grating is recorded and the staircase substrate (if needed) on which the holographic material layer is attached may be optimized. In some embodiments, the thickness of the holographic material layer may be sufficiently high and thus the staircase substrate may not be used. In some embodiments, a staircase substrate (e.g., a thin glass plate) with a certain shape and thickness may be used such that the total thickness of the holographic material layer and the staircase substrate may be equal to the desired thickness of the staircase structure.

FIG.13Ais a top view of an input section of an example of a waveguide display1300including an input grating1322in a staircase structure1330for improving the in-coupling efficiency according to certain embodiments.FIG.13Bis a side view of the example of waveguide display1300shown inFIG.13Aaccording to certain embodiments. In the illustrated example, the input section of waveguide display1300may include a waveguide1310, which may include two or more substrates. First middle grating1340may be formed on a surface of one of the two or more substrates and may be sandwiched by two substrates. As described above, first middle grating1340may include multiple VBGs for different fields of views.

Staircase structure1330may be attached to a top surface of waveguide1310. As illustrated byFIG.13B, input grating1322may be formed in a holographic material layer1320on the top or bottom surface of a staircase substrate1332of staircase structure1330. Input grating1322may generally have a circular shape or rectangular shape that matches the shape of the output image of the light source (e.g., a micro-LED array) or a projector. In some embodiments, input grating1322may be recorded in holographic material layer1320formed (e.g., coated or laminated) on another substrate, and then transferred to staircase substrate1332to form staircase structure1330, which may then be aligned and bonded to waveguide1310. In some embodiments, holographic material layer1320may be coated or laminated on staircase substrate1332, and may then then be recorded before or after bonding staircase structure1330to waveguide1310.

As shown inFIGS.13A and13B, staircase structure1330may be flat and may have a certain shape in the x-y plane. Staircase structure1330(including input grating1322and staircase substrate1332, if used) may have a certain total thickness. Because the non-zero thickness of staircase structure1330, the shape of staircase structure1330may need to be optimized to avoid pupil clipping and the resultant efficiency reduction.

FIG.13Cillustrates an example of pupil clipping in a waveguide display having a prism1304as the input coupler. As shown inFIG.13C, prism1304may couple display light from a projector1302(or another light source, such as a micro-LED array) into a waveguide1306. Due to the physical size of prism1304, light from certain fields of view may be clipped as shown by the dashed lines and thus may not be coupled into waveguide1306. In waveguide display1300shown inFIGS.13A and13B, light from certain fields of view that is diffracted by input grating1322may also be clipped if the thickness and the shape of staircase structure1330are not appropriately selected.

FIG.13Dincludes a perspective view of an example of staircase structure1330according to certain embodiments. Holographic material layer1320may include input grating1322recorded therein. Holographic material layer1320may be cut according to the desired shape of staircase structure1330. Staircase substrate1332may also be cut from a substrate (e.g., a glass substrate with a thickness about 100 μm or thicker) according to the desired shape of staircase structure1330. In the illustrated example, holographic material layer1320may be attached to the top surface of staircase substrate1332. In another example, holographic material layer1320may be attached to the bottom surface of staircase substrate1332. Staircase structure1330may then be attached (e.g., bonded) to waveguide1310. If input grating1322has not been recorded in holographic material layer1320before holographic material layer1320is attached to staircase substrate1332to form staircase structure1330, a recording process may be performed before or after staircase structure1330is attached to waveguide1310to form input grating1322in holographic material layer1320.

To optimize the input coupling efficiency and minimize pupil clipping for all FOVs of waveguide display1300, the thickness and/or shape of staircase structure1330may be tuned, which may depend on whether input grating1322is on the top of staircase structure1330or the bottom of staircase structure1330. As described above, thickness of staircase substrate1332may depend on the thickness of holography material layer1320in which input grating1322is recorded. In some embodiments, one or more holographic material layers1320may be used to achieve the desired thickness of staircase structure1330, and thus staircase substrate1332may not be used. For example, in some implementations, if the desired thickness of staircase structure1330is 80 μm, four holographic material layers1320each having a thickness about 20 μm may be used (if the overall haziness is sufficiently low) and no additional staircase substrate1332may be needed. The spatial multiplexing of VBGs in first middle grating1340, such as the number of VBGs, the FOV covered by each VBG, and the refractive index modulation and the physical dimension of each VBG, may also be optimized.

FIG.14illustrates an example of a VBG-based waveguide display1400used as a baseline for comparing with waveguide displays having the staircase structures disclosed herein. As described above, VBG-based waveguide display1400may include one or more substrates1410. VBG-based waveguide display1400may be an example of a waveguide display that includes a single image projector and two or more sets of gratings for two or more FOVs. Only the first set of gratings (e.g., for the left-half FOV) is shown inFIG.14. The first set of gratings may include an input grating1420, a first middle grating1430, a second middle grating1440, and an output grating1450in one or more holographic material layers. Input grating1420may be on top of first middle grating1430and overlap with first middle grating1430as shown in, for example,FIGS.12A-12D. VBG-based waveguide display1400may not include a staircase structure. The first set of gratings may include transmissive VBGs or reflective VBGs. In some embodiments, input grating1420and output grating1450may have matching grating vectors (e.g., having the same grating vector in the x-y plane) and thus may compensate the dispersion of display light caused by each other.

As described above, the display light from a projector may be coupled into a substrate1410by input grating1420, and may propagate within substrate1410and reach first middle grating1430. First middle grating1430may diffract the display light towards second middle grating1440. Second middle grating1440may diffract the display light towards output grating1450at multiple locations along substantially the x direction to replicate the exit pupil in substantially the x direction. Output grating1450may diffract the display light from each of the multiple locations of second middle grating1440out of substrate1410at multiple locations along substantially the y direction, such that the exit pupil may be replicated in substantially the y direction. On output grating1450, an exit region1460corresponds to the region where display light for the left-half FOV at one pupil location in the eyebox (e.g., at the center the eyebox) may be coupled out of output grating1450. A region1442in second middle grating1440represents the region of second middle grating1440that maps to exit region1460.

The performance of VBG-based waveguide display1400, such as the minimum in-coupling efficiency for the entire field of view and the average in-coupling efficiency for the entire field of view, is compared with waveguide displays having the staircase structures disclosed herein to show the efficacy of the input grating on the staircase structure disclosed herein. The waveguide displays having the staircase structures may include an input grating on the top or bottom of the staircase structure as shown in, for example,FIG.13B.

FIG.15includes a chart1500illustrating optimization results for examples of waveguide displays including an input grating in a staircase structure according to certain embodiments. The horizontal axis of chart1500corresponds to the average in-coupling efficiency for an entire FOV, while the vertical axis corresponds to the minimum in-coupling efficiency for the entire FOV. Each individual data point in chart1500corresponds to a waveguide display with a unique combination of structure (e.g., without or with a staircase structure or with a staircase structure at a different respective location), staircase thickness, and staircase shape. Data points1530inFIG.15show the simulation results for VBG-based waveguide display1400, where it is assumed that the input grating and the first middle grating may each use the total achievable refractive index modulation (Δn) of a holographic material layer (which may not be achieved in reality). Data points1510inFIG.15show the simulation results for VBG-based waveguide displays including an input grating on the top surface of a staircase substrate and having different staircase shapes and/or staircase thicknesses. Data points1520inFIG.15show the simulation results for VBG-based waveguide displays including an input grating on the bottom surface of a staircase substrate and having different staircase shapes and/or staircase thicknesses.

Data points1530inFIG.15show that the average in-coupling efficiency of VBG-based waveguide display1400may be slightly higher, but the minimum in-coupling efficiency in the entire FOV is much lower. Data points1510show that, with the input grating on the top of a staircase structure as shown inFIGS.13A-13B and13D, the average in-coupling efficiency can be fairly high, such as close to the average in-coupling efficiency of VBG-based waveguide display1400, and the minimum in-coupling efficiency can be much higher than the minimum in-coupling efficiency of VBG-based waveguide display1400. Data points1520show that, with the input grating on the bottom surface of a staircase structure as shown inFIGS.13A-13B, the average in-coupling efficiency can be fairly high too, and the minimum in-coupling efficiency can be higher than the minimum in-coupling efficiency of VBG-based waveguide display1400.

FIG.16Aillustrates staircase thickness optimization results for examples of waveguide displays including a grating coupler on top of a staircase structure according to certain embodiments. In the optimization, the thickness of the staircase structure (including the holographic material layer and the staircase substrate) is varied between 50 μm and 500 μm. A curve1610includes data points each showing the average in-coupling efficiency for an entire field of view and the minimum in-coupling efficiency for the field of view for a staircase structure having a different respective thickness. As shown by the optimization results, for staircase structures with the input grating at the top, better in-coupling efficiencies can be achieved when the staircase structures are thinner, such as about 50 μm, where both the average in-coupling efficiency and the minimum in-coupling efficiency can be higher.

FIG.16Billustrates staircase thickness optimization results for examples of waveguide displays including a grating coupler on the bottom of a staircase structure according to certain embodiments. In the optimization, the thickness of the staircase structure (including the holographic material layer and the staircase substrate) is varied between 50 μm and 500 μm. A curve1620includes data points each showing the average in-coupling efficiency for an entire field of view and the minimum in-coupling efficiency for the field of view for a staircase structure having a different respective thickness. As shown by the optimization results, for staircase structures with the input grating at the bottom, better average in-coupling efficiencies can be achieved when the staircase structures are thinner, such as close to 50 μm, while better minimum in-coupling efficiencies can be achieved when the staircase structures are relatively thicker, such as about 80 μm.

FIGS.17A-17Cillustrate staircase shape optimization results for examples of waveguide displays including a grating coupler on the top of a staircase structure according to certain embodiments. Each example of waveguide display shown inFIGS.17A-17Ccorresponds to a data point1510inFIG.15and has a unique combination of staircase shape and stair thickness and a unique combination of average in-coupling efficiency and minimum in-coupling efficiency.FIG.17Ashows a portion of a waveguide display that includes a substrate1710, a staircase structure1730that includes an input grating1720on a staircase substrate, and a first middle grating1740as described above. The thickness of staircase structure1730including input grating1720and the staircase substrate is about 50 μm.FIG.17Ashows the optimized shape of staircase structure1730, which has a small size that may be slightly larger than the (circular) input grating1720.

FIG.17Bshows a portion of a waveguide display that includes a substrate1712, a staircase structure1732that includes an input grating1722on a staircase substrate, and a first middle grating1742as described above. The thickness of staircase structure1732including input grating1722and the staircase substrate is about 50 μm.FIG.17Bshows the optimized shape of staircase structure1732, which has a small size that may be slightly larger than the (circular) input grating1722. Staircase structure1732and staircase structure1730may have different shapes and/or thicknesses.

FIG.17Cshows a portion of a waveguide display that includes a substrate1714, a staircase structure1734that includes an input grating1724on a staircase substrate, and a first middle grating1744as described above. The thickness of staircase structure1734including input grating1724and the staircase substrate is about 50 μm.FIG.17Cshows the optimized shape of staircase structure1734, which has a small size that may be slightly larger than the (circular) input grating1724. Staircase structure1734may have a shape (and/or thickness) different from the shapes (and/or thicknesses) of staircase structure1730and staircase structure1732.

FIGS.17D-17Fillustrate staircase thickness optimization results for examples of waveguide displays including a grating coupler on the bottom of a staircase structure according to certain embodiments. Each example of waveguide display shown inFIGS.17D-17Fcorresponds to a data point1520inFIG.15and has a unique combination of staircase shape and stair thickness and a unique combination of average in-coupling efficiency and minimum in-coupling efficiency.FIG.17Dshows a portion of a waveguide display that includes a substrate1715, a staircase structure1735that includes an input grating1725under a staircase substrate, and a first middle grating1745as described above. The thickness of staircase structure1735including input grating1725and the staircase substrate is about 63 μm.FIG.17Dshows the optimized shape of staircase structure1735.

FIG.17Eshows a portion of a waveguide display that includes a substrate1716, a staircase structure1736that includes an input grating1726under a staircase substrate, and a first middle grating1746as described above. The thickness of staircase structure1736including input grating1726and the staircase substrate is about 56 μm.FIG.17Eshows the optimized shape of staircase structure1736, which is larger than the (circular) input grating1726and has an edge that extends towards the right. Staircase structure1736and staircase structure1735may have different shapes and/or thicknesses.

FIG.17Fshows a portion of a waveguide display that includes a substrate1718, a staircase structure1738that includes an input grating1728under a staircase substrate, and a first middle grating1748as described above. The thickness of staircase structure1738including input grating1728and the staircase substrate is about 81 μm.FIG.17Fshows that the optimized shape of staircase structure1738, which is larger than the (circular) input grating1728and has an edge that extends towards the right.

FIG.18illustrates an example of a waveguide display1800including volume Bragg grating couplers. In the illustrated example, waveguide display1800may include a first assembly1810and a second assembly1820that are separated by a spacer1830. First assembly1810may include a first substrate1812, a second substrate1816, and one or more holographic grating layers1814between first substrate1812and second substrate1816. First substrate1812and second substrate1816may each be a thin transparent substrate, such as a glass substrate having a thickness about 100 μm or few hundred micrometers. Holographic grating layers1814may include multiplexed reflective VBGs, transmissive VBGs, or both. Holographic grating layers1814may have a total thickness less than about 100 μm, such as between about 20 μm and about 80 μm. Similarly, second assembly1820may include a first substrate1822, a second substrate1826, and one or more holographic grating layers1824between first substrate1822and second substrate1826. Holographic grating layers1824may include multiplexed reflective VBGs, transmissive VBGs, or both. First assembly1810may be used to couple display light in red, green, and blue colors from certain fields of view to user's eyes, and second assembly1820may be used to couple display light in red, green, and blue colors from other fields of view to user's eyes.

FIG.19Ais a front view of an example of a volume Bragg grating-based waveguide display1900according to certain embodiments.FIG.19Bis a side view of the example of volume Bragg grating-based waveguide display1900shown inFIG.19A. Waveguide display1900may be an example of first assembly1810or second assembly1820, or a part of first assembly1810or second assembly1820. In the illustrated example, waveguide display1900may include a waveguide1910, an input coupler, and a middle grating1930and an output grating1940in waveguide1910. The input coupler may include projector optics1920(e.g., a lens) and an input grating1922. Display light may be collimated by projector optics1920and projected onto input grating1922, which may couple the display light into waveguide1910by diffraction as described above. The display light may reach a first portion1932of middle grating1930and may be diffracted by first portion1932of middle grating1930to change the propagation direction and reach a second portion1934of middle grating1930, which may then diffract the display light towards output grating1940. Output grating1940may diffract the display light out of waveguide1910at different locations to form multiple exit pupils as described above.

First portion1932and second portion1934of middle grating1930may be on a same holographic material layer and may have matching grating vectors (e.g., having a same grating vector in the x-y plane and a same grating vector and/or opposite grating vectors in the z direction). Therefore, they may compensate for the dispersion of display light caused by each other to reduce the overall dispersion, due to the opposite Bragg conditions (e.g., +1 order and −1 order diffractions) for the diffractions at first portion1932and second portion1934of middle grating1930. In addition, input grating1922and output grating1940may have matching grating vectors (e.g., having the same grating vector in the x-y plane and having the same or opposite grating vectors in the z direction), where input grating1922may couple the display light into waveguide1910, while output grating1940may couple the display light out of the waveguide. Therefore, input grating1922and output grating1940may compensate for the dispersion of display light caused by each other to reduce the overall dispersion, due to the opposite diffraction directions and opposite Bragg conditions (e.g., +1 order and −1 order diffractions) for the diffractions at input grating1922and output grating1940. In this way, the dispersion by first portion1932and second portion1934of middle grating1930may be canceled out, and the dispersion by input grating1922and output grating1940may also be canceled out. Therefore, the overall dispersion of the display light by waveguide display1900can be minimized. As such, a higher resolution of the displayed image may be achieved.

Each of input grating1922, first portion1932and second portion1934of middle grating1930, and output grating1940may include multiplexed volume Bragg gratings configured to diffract display light of different colors and/or from different fields of view. Due to the limited wavelength range and/or angular range of each VBG grating as described above with respect toFIGS.7A and7B, different VBGs may need to be used to diffract different color components of the display light and/or display light from different fields of view. Thus, to cover a large field of view for all colors, many VBGs may be needed. However, the achievable total refractive index modulations of a holographic material layer may be limited. Therefore, limited number of VBGs may be recorded in the holographic material layer, and the overall diffraction efficiency of VBG-based waveguide display1900may be low and/or the field of view of VBG-based waveguide display1900may be small. As such, multiple holographic material layers may be needed to cover the entire fields of view for all colors.

FIG.20Ais a front view of an example of a volume Bragg grating-based waveguide display2000according to certain embodiments.FIG.20Bis a side view of the example of volume Bragg grating-based waveguide display2000shown inFIG.20A. Waveguide display2000may be an example of first assembly1810, second assembly1820, or waveguide display1800. Waveguide display2000may include a waveguide2010, an input coupler, and a middle grating2030and an output grating2040formed on or in waveguide2010. The input coupler may include projector optics2020(e.g., a lens) and an input grating2022. Display light may be collimated by projector optics2020and projected onto input grating2022, which may couple the display light into waveguide2010by diffraction as described above. The display light may reach a first portion2032of middle grating2030and may be diffracted by first portion2032of middle grating2030to change the propagation direction and reach a second portion2034of middle grating2030, which may diffract the display light towards output grating2040. Output grating2040may diffract the display light out of waveguide2010at different locations to form multiple exit pupils as described above.

As described above, first portion2032and second portion2034of middle grating2030may have matching grating vectors (e.g., having a same grating vector in the x-y plane and a same grating vector and/or opposite grating vectors in the z direction). Input grating2022and output grating2040may have matching grating vectors (e.g., having the same grating vector in the x-y plane and having the same and/or opposite grating vectors in the z direction). Therefore, due to the opposite diffraction directions and opposite Bragg conditions (e.g., +1 order and −1 order diffractions), the overall dispersion by first portion2032and second portion2034of middle grating2030may be reduced or canceled out, and the overall dispersion by input grating2022and output grating2040may also be reduced or canceled out. Therefore, the overall dispersion of the display light by waveguide display2000can be minimized. As such, a higher resolution of the displayed image may be achieved.

As illustrated inFIG.20B, waveguide display2000may include multiple polymer layers on one or more waveguide plates, where input grating2022, middle grating2030, and output grating2040may each be split into multiple gratings recorded in the multiple polymer layers. The gratings on each polymer layer may cover different respective FOVs and light spectra, and the combination of the multiple polymer layers may provide the full FOV and spectral coverage. In this way, each polymer layer can be thin (e.g., about 20 μm to about 100 μm), and can be exposed for fewer times (e.g., less than about 100) to record fewer gratings to reduce haziness and increase the refractive index modulation for each VBG grating. Therefore, the diffraction efficiency of each VBG grating can be high for the covered FOV and spectrum, and the overall diffraction efficiency of waveguide display2000may be high for the entire FOV and spectrum due to the multiple polymer layers used. In the example shown inFIGS.20A and20B, waveguide display2000may include a first assembly2012that includes multiple polymer layers on one or more substrates, and a second assembly2014that includes multiple polymer layers on one or more substrates. Each polymer layer in first assembly2012and second assembly2014may include part of input grating2022, middle grating2030, and/or output grating2040for certain fields of view.

FIG.21Aillustrates an example of a volume Bragg grating-based waveguide display2100including multiple grating layers for different fields of view according to certain embodiments. VBG-based waveguide display2100may be an example of waveguide display1800or VBG-based waveguide display2000described above. In waveguide display2100, gratings may be spatially multiplexed along the z direction. For example, waveguide display2100may include multiple substrates, such as substrates2110,2112,2114, and the like. The substrates may include a same material or materials having similar refractive indexes. One or more VBGs (e.g., VBGs2120,2122,2124, etc.) may be made on each substrate, such as recorded in a holographic material layer formed on the substrate. The VBGs may be reflective gratings or transmissive gratings. The substrates with the VBGs may be arranged in a substrate stack along the z direction for spatial multiplexing. Each VBG may be a multiplexed VBG that includes multiple gratings designed for different Bragg conditions to couple display light in different wavelength ranges and/or different FOVs into or out of the waveguide.

In the example shown inFIG.21A, VBG2120may couple light2134from a positive field of view into the waveguide as shown by a light ray2144within the waveguide. VBG2122may couple light2130from around 0° field of view into the waveguide as shown by a light ray2140within the waveguide. VBG2124may couple light2132from a negative field of view into the waveguide as shown by a light ray2142within the waveguide. As described above, each of VBGs2120,2122, and2124may be a multiplexed VBG with many exposures, and thus may couple light from different FOV ranges into or out of the waveguide.

FIG.21Billustrates the fields of view of multiple gratings in an example of a volume Bragg grating-based waveguide display (e.g., waveguide display2100) according to certain embodiments. In some embodiments, each of the gratings may be in a respective grating layer and/or on a respective waveguide plate. Each of the gratings may be a multiplexed grating including many exposures, and may be used to couple display light from multiple FOV ranges into or out of the waveguide at high efficiencies. For example, a curve2150shows the diffraction efficiency of a first VBG (e.g., VBG2122ofFIG.21A) for light from different fields of view. A curve2160shows the diffraction efficiency of a second VBG (e.g., VBG2120ofFIG.21A) for light from different fields of view. A curve2170shows the diffraction efficiency of a third VBG (e.g., VBG2124ofFIG.21A) for light from different fields of view. The first, second, and third VBGs arranged in a stack may more uniformly diffract light in the full field of view (e.g., from about −20° to about 20°) at high efficiencies. The first VBG, the second VBG, and the third VBG may be used to couple display light of the same color or different colors. Different sets of VBGS may be used to cover the full field of view for display light of different colors.

FIG.22Aillustrates an example of a near-eye display (NED) device2200including a waveguide display2230according to certain embodiments. NED device2200may be an example of near-eye display120, augmented reality system500, or another type of waveguide displays disclosed herein. NED device2200may include a light source2210, projection optics2220, and waveguide display2230. Light source2210may include multiple panels of light emitters for different colors, such as a panel of red light emitters2212, a panel of green light emitters2214, and a panel of blue light emitters2216. The red light emitters2212are organized into an array; the green light emitters2214are organized into an array; and the blue light emitters2216are organized into an array. The dimensions and pitches of light emitters in light source2210may be small. For example, each light emitter may have a diameter less than 2 μm (e.g., about 1.2 μm) and the pitch may be less than 3 μm (e.g., about 2 μm). As such, the number of light emitters in each of red light emitters2212, green light emitters2214, and blue light emitters2216can be equal to or greater than the number of pixels in a display image, such as 960×720, 1280×720, 1440×1080, 1920×1080, 2160×1080, or 2560×1080 pixels. Thus, a display image may be generated simultaneously by light source2210. A scanning element may not be needed in NED device2200.

Before reaching waveguide display2230, the light emitted by light source2210may be conditioned by projection optics2220, which may include a lens array. Projection optics2220may collimate or focus the light emitted by light source2210to waveguide display2230. Waveguide display2230may include three input couplers2232,2234, and2236for coupling the light emitted by red light emitters2212, green light emitters2214, and blue light emitters2216, respectively, into waveguide display2230. The light coupled into waveguide display2230may propagate within waveguide display2230through, for example, total internal reflection as described above. Gratings2238may expand the display light in two directions and couple portions of the light propagating within waveguide display2230out of waveguide display2230and towards user's eye2290as described above.

FIG.22Billustrates another example of a near-eye display (NED) device2250including a waveguide display2280according to certain embodiments. In some embodiments, NED device2250may use a scanning mirror2270to project light from a light source2240to an image field where a user's eye2290may be located. NED device2250may be an example of near-eye display120, augmented reality system500, or another type of display devices. Light source2240may include one or more rows or one or more columns of light emitters of different colors, such as multiple rows of red light emitters2242, multiple rows of green light emitters2244, and multiple rows of blue light emitters2246. For example, red light emitters2242, green light emitters2244, and blue light emitters2246may each include N rows, each row including, for example,2560light emitters (pixels). The red light emitters2242are organized into an array; the green light emitters2244are organized into an array; and the blue light emitters2246are organized into an array. In some embodiments, light source2240may include a single line of light emitters for each color. In some embodiments, light source2240may include multiple columns of light emitters for each of red, green, and blue colors, where each column may include, for example, 1080 light emitters. In some embodiments, the dimensions and/or pitches of the light emitters in light source2240may be relatively large (e.g., about 3-5 μm) and thus light source2240may not include sufficient light emitters for simultaneously generating a full display image. For example, the number of light emitters for a single color may be fewer than the number of pixels (e.g., 2560×1080 pixels) in a display image. The light emitted by light source2240may be a set of collimated or diverging beams of light.

Before reaching scanning mirror2270, the light emitted by light source2240may be conditioned by various optical devices, such as collimating lenses or a freeform optical element2260. Freeform optical element2260may include, for example, a multi-facet prism or another light folding element that may direct the light emitted by light source2240towards scanning mirror2270, such as changing the propagation direction of the light emitted by light source2240by, for example, less than 90°, about 90°, or greater than 90°. In some embodiments, freeform optical element2260may be rotatable to scan the light. Scanning mirror2270and/or freeform optical element2260may reflect and project the light emitted by light source2240to waveguide display2280, which may include gratings2282for coupling the light emitted by light source2240into waveguide display2280. For example, gratings2282may include a respective input coupler for each color. The light coupled into waveguide display2280may propagate within waveguide display2280through, for example, total internal reflection as described above. Gratings2282may also expand the display light in two directions and couple portions of the light propagating within waveguide display2280out of waveguide display2280and towards user's eye2290.

Scanning mirror2270may include a microelectromechanical system (MEMS) mirror or any other suitable mirrors. Scanning mirror2270may rotate to scan in one or two dimensions. As scanning mirror2270rotates, the light emitted by light source2240may be directed to a different area of waveguide display2280at a different angle such that a full display image may be projected onto waveguide display2280and directed to user's eye2290by waveguide display2280in each scanning cycle. For example, in embodiments where light source2240includes light emitters for all pixels in one or more rows or columns, scanning mirror2270may be rotated in the column or row direction (e.g., x or y direction) to scan an image. In embodiments where light source2240includes light emitters for some but not all pixels in one or more rows or columns, scanning mirror2270may be rotated in both the row and column directions (e.g., both x and y directions) to project a display image (e.g., using a raster-type scanning pattern).

NED device2250may operate in predefined display periods. A display period (e.g., display cycle) may refer to a duration of time in which a full image is scanned or projected. For example, a display period may be a reciprocal of the desired frame rate. In NED device2250that includes scanning mirror2270, the display period may also be referred to as a scanning period or scanning cycle. The light generation by light source2240may be synchronized with the rotation of scanning mirror2270. For example, each scanning cycle may include multiple scanning steps, where light source2240may generate a different light pattern in each respective scanning step. In each scanning cycle, as scanning mirror2270rotates, a display image may be projected onto waveguide display2280and user's eye2290. The actual color value and light intensity (e.g., brightness) of a given pixel location of the display image may be an average of the light beams of the three colors (e.g., red, green, and blue) illuminating the pixel location during the scanning period. After completing a scanning period, scanning mirror2270may revert back to the initial position to project light for the first few rows of the next display image or may rotate in a reverse direction or scan pattern to project light for the next display image, where a new set of driving signals may be fed to light source2240. The same process may be repeated as scanning mirror2270rotates in each scanning cycle. As such, different images may be projected to user's eye2290in different scanning cycles.

FIG.23Aillustrates the layout of a first set of gratings in an example of waveguide display2300including three projectors for three different colors according to certain embodiments.FIG.23Billustrates the layout of a second set of gratings in the example of waveguide display2300including three projectors for three different colors according to certain embodiments. Waveguide display2300may be an example of waveguide display1800,2000, or2200, and may include two assemblies as described above, whereFIG.23Amay show the first assembly andFIG.23Bmay show the second assembly. The first assembly may be used to couple display light for some fields of view from three color projectors to user's eyes, while the second assembly may be used to couple display light for some other fields of view from the three color projectors to user's eyes. Each color projector may include, for example, a micro-LED array that emits display light in one color as described above with respect toFIGS.22A and22B. The three color projectors may include, for example, a red micro-LED array, a green micro-LED array, and a blue micro-LED array. Each micro-LED array may generate a monochromatic image of a corresponding color, and thus the three micro-LED arrays may generate a color image.

The first assembly of waveguide display2300shown inFIG.23Amay include a waveguide2310(e.g., a substrate), three input gratings2320, three first middle gratings2330, a second middle grating2340, and an output grating2350. Each of the three input gratings2320may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2310. Each of the three first middle grating2330may be used to direct display light from a corresponding input grating2320towards second middle grating2340as described above. Second middle grating2340and output grating2350may expand the input pupil in two directions and deliver the display light to user's eyes.

The second assembly of waveguide display2300shown inFIG.23Bmay include a waveguide2312(e.g., a substrate), three input gratings2322, three first middle gratings2332, a second middle grating2342, and an output grating2352. Each of the three input gratings2322may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2312. Each of the three first middle grating2332may be used to direct display light from a corresponding input grating2323towards second middle grating2342as described above. Second middle grating2342and output grating2352may expand the input pupil in two directions and deliver the display light to user's eyes.

Because of the separate input gratings and/or first middle gratings for display light of different colors, each input grating and/or first middle grating may use the total achievable refractive index modulation of a holographic material layer to achieve a higher diffraction efficiency for display light of the respective color. In various embodiments, waveguide display2300may have an improved overall in-coupling efficiency that is about five to ten times of the overall in-coupling efficiency of a waveguide display without separate projectors and input gratings for three different colors.

V. Phase Structures

Gratings described above may be optimized to maximize the power of the display light in the desired path. For example, the grating shape, the slant angle, the grating period, the duty cycle, the grating height or depth, the refractive index, the refractive index modulation, the overcoating material, and the spatial variations of these grating parameters across the grating may be adjusted to improve the efficiencies of directing display light to the desired directions. In addition, as described with respect toFIGS.10A-11, display light coupled into a waveguide by an input grating coupler may reach the input grating coupler again and may be partially coupled out the waveguide by the input grating coupler. Thus, the overall input coupling efficiency of the input grating coupler may be low. As described above, in some embodiments, staircase structures and/or separate grating couplers for different colors may help to improve the overall in-coupling efficiencies. Furthermore, grating couplers may have different diffraction efficiencies for s-polarized light and p-polarized light. For example, a grating coupler may have a higher in-coupling efficiency for s-polarized input light than for p-polarized input light, and may also have a higher out-coupling efficiency for s-polarized light than for p-polarized light.

According to certain embodiments, the efficiency of a waveguide display may be further improved by controlling the polarization state of the display light beam along its propagation path. For example, a phase structure may be coupled to the waveguide and used to change the polarization state of the light reflected at the surface of the waveguide, such that the reflected light, when reaching a polarization-dependent grating coupler, may be preferentially diffracted or reflected to the desired directions towards the eyebox to improve the overall efficiency of the waveguide display.

FIG.24Aillustrates an example of a waveguide display2400including volume Bragg grating couplers. Waveguide display2400may include a VBG layer2420within a substrate2410or between two substrates. VBG layer2420may include an input VBG2422and an output VBG2424. In the illustrated example, input VBG2422may reflectively diffract incident light, and thus may function as a reflective VBG. Output VBG2424may partially reflectively diffract the light from input VBG2422out of substrate2410towards an eyebox of waveguide display2400.

FIG.24Billustrates an example of an input coupler2430including a volume Bragg grating2436in a substrate2432. VBG2436may be an example of input VBG2422or output VBG2424. As illustrated, VBG2436may function as multiple reflectors that strongly reflect light of a specific wavelength and from a specific angle that satisfies the Bragg condition. Both transmissive VBGs and reflective VBGs may function as multilayer reflectors. Depending on the slant angle of the multiple reflectors in VBG2436, the reflected light may or may not pass through VBG2436such that VBG2436may transmissively or reflectively diffract incident light2438as shown inFIG.24B. In the illustrated example, the reflectively diffracted light may be reflected at a top surface2434of substrate2432and may reach VBG2436again. VBG2436may at least partially diffract the reflected light out of substrate2432and thus may decrease the input coupling efficiency of input coupler2430. The reflectivity of each of the multiple reflectors may depend on the polarization state and the incident angle of the incident light, and the base refractive index and the refractive index modulation (Δn) of the VBG.

FIG.24Cillustrates examples of reflection coefficients of s-polarized and p-polarized light with different incident angles at an interface between a low refractive index material and a high refractive index material. In the illustrated example, the refractive index of the first medium is 1.0, the refractive index of the second medium is 1.5, and the s-polarized or p-polarized light reaches the interface between the two media from the first medium. A curve2440inFIG.24Cshows the reflection coefficients for s-polarized light with different incident angles. A curve2442shows the reflection coefficients for p-polarized light with different incident angles. Curve2442shows that, when the incident angle is equal to or close to the Brewster's angle, the reflection coefficient for p-polarized light is about or close to zero. Thus, the reflectivity at the interface between the two media can be very low for p-polarized light from certain incident angles.

FIG.24Dillustrates examples of reflection coefficients of s-polarization and p-polarization light with different incident angles at an interface between a high refractive index material and a low refractive index material. In the illustrated example, the refractive index of the first medium is 1.5, the refractive index of the second medium is 1.0, and the s-polarized or p-polarized light reaches the interface between the two media from the first medium. A curve2444inFIG.24Dshows the reflection coefficients for s-polarized light with different incident angles. A curve2446shows the reflection coefficients for p-polarized light with different incident angles. As shown by curves2444and2446, the incident light may be totally reflected when the incident angle is greater than the critical angle. When the incident angle is less than the critical angle, the reflection coefficients for p-polarized light with incident angles at or near the Brewster's angle may be close to zero. Thus, the reflectivity at the interface between the two media can be very low for p-polarized light from certain incident angles. Thus, in a VBG-based waveguide display, it may be desirable to alter the polarization state of the incident light to preferentially diffract or transmit the incident light in order to achieve a high efficiency of the VBG-based waveguide display

FIG.24Eillustrates a cross-sectional view of an example of a waveguide display2402including VBG couplers and a phase structure2456according to certain embodiments. Waveguide display2402may be similar to waveguide display2400and may additionally include phase structure2456. As illustrated, waveguide display2402may include VBGs2460and2462in a substrate2450or between two substrates. VBG2460may reflectively diffract incident display light (e.g., s-polarized light) towards a top surface2452of substrate2450. Top surface2452may reflect the display light towards a bottom surface2454of substrate2450. Phase structure2456at bottom surface2454of substrate2450may receive the reflected display light and change the polarization state of the display light, for example, to p-polarized light. The display light may be reflected at bottom surface2454of substrate2450or a bottom surface of phase structure2456. The reflected display light may incident on VBG2462as s-polarized light due to the different orientation and different grating vector of VBG2462compared to VBG2460, and may be diffracted out of substrate2450towards an eyebox at a higher diffraction efficiency by VBG2462. Simulation results show that, by using phase structure2456, the maximum coupling efficiency may be improved by about 42% from the baseline efficiency (without using phase structure2456).

FIG.24Fillustrates a cross-sectional view of an example of a waveguide display2404including volume Bragg gratings2480and2482and phase structures2490and2492according to certain embodiments. As illustrated, waveguide display2404may include VBGs2480and2482in a substrate2470or between two substrates. VBG2480may reflectively diffract incident display light (e.g., s-polarized light) towards a top surface of substrate2470. Phase structure2492may be coupled to the top surface of substrate2470, and may change the polarization state of the incident display light. The top surface of substrate2470or phase structure2492may reflect the display light towards the bottom surface of substrate2470. Phase structure2490at the bottom surface of substrate2470may change the polarization state of the incident display light. The display light may be reflected at the bottom surface of substrate2470or phase structure2490. The reflected display light may be incident on VBG2482, and may be diffracted by VBG2482out of substrate2470towards an eyebox at a high diffraction efficiency.

In some embodiments, phase structures2490and2492may be only at selected locations on the top and bottom surfaces of substrate2470. In some embodiments, either phase structure2490or phase structure2492may be used in a waveguide display. In some embodiments, both phase structure2490and phase structure2492may be used in a waveguide display, where the desired phase change or retardation may be achieved by the combination of the two phase structures. For example, to convert s-polarized light to p-polarized light, a first phase structure may convert the s-polarized light to circularly polarized light, and a second phase structure may convert the circularly polarized light to p-polarized light. In some embodiments, the polarization alteration characteristics of phase structure2490or phase structure2492may vary at different locations.

Phase structures2456,2490, and2492described above may include any birefringent materials (e.g., birefringent crystals, liquid crystals, or polymers) or structures (e.g., gratings, meta-gratings, micro-structures, nano-structures, or other subwavelength structures) that can cause a desired phase delay between two orthogonal linear polarization components (e.g., s-polarized component and p-polarized component) of a light beam, such that the incident light beam may be changed to an s-polarized, p-polarized, circularly polarized, or elliptically polarized beam. In one example, phase structure2456,2490, or2492may include a waveplate having a desired phase delay, such as a quarter-wave plate (QWP) or a waveplate have another phase delay. The phase structure may be placed at various locations in a waveguide display, such as at the input coupler region, between the input coupler and the output coupler, at the output coupler region, or any combinations.

FIG.25Aillustrates a cross-sectional view of an input portion of an example of a waveguide display2500including VBG couplers and a phase structure2540according to certain embodiments. As illustrated, waveguide display2500may include an input grating2520and a first middle grating2530in a waveguide2510or between two substrates. Input grating2520may be an example of the input grating described above, and first middle grating2530may be an example of the first middle grating described above. Input grating2520may reflectively diffract incident display light towards the top surface of waveguide2510. The top surface of waveguide2510may reflect the display light towards a bottom surface of waveguide2510. Phase structure2540at bottom surface2514of waveguide2510may receive the reflected display light and change the polarization state of the display light, for example, from s-polarized light to p-polarized light or from p-polarized light to s-polarized light. The display light may be reflected at the bottom surface of waveguide2510or a bottom surface of phase structure2540. The reflected p-polarized display light may be incident on input grating2520again, but may be minimally diffracted by input grating2520. The p-polarized display light may be reflected at the top surface of waveguide2510and reach first middle grating2530as s-polarized light due to the different orientation and different grating vector of first middle grating2530compared to input grating2520, and may be diffracted by first middle grating2530to a second middle grating at a higher diffraction efficiency.

FIG.25Billustrates a top view of the example of waveguide display2500including VBG couplers and phase structure2540according to certain embodiments. As illustrated, in addition to input grating2520, phase structure2540, and first middle grating2530, waveguide display2500may also include a second middle grating2550and an output grating2560. Each of gratings2550and2560may be a reflective VBG or a transmissive VBG. As described above with respect toFIG.9, second middle grating2550may receive the display light diffracted by first middle grating2530and replicate the input pupil in one direction (e.g., approximately the x direction) and direct the display light towards output grating2560. Output grating2560may replicate the input pupil in a second direction (e.g., approximately the y direction) and direct the display light towards an eyebox2570.

In the illustrated example, phase structure2540is shown to be at a region where input grating2520and/or first middle grating2530are located, to change the polarization state of the display light coupled into waveguide2510, for example, from p-polarized to s-polarized or from s-polarized to p-polarized. In some other embodiments, phase structure2540may also be at regions where second middle grating2550and/or output grating2560are located, to change the polarization state of the display light during its propagation within waveguide2510.

FIG.26Aillustrates a simulation result for an example of a volume Bragg grating-based waveguide display2600according to certain embodiments. Waveguide display2600may be an example of waveguide display900.FIG.26Ashows a display light beam coupled into a waveguide by an input grating (e.g., input grating910) and then directed by a first middle grating (e.g., first middle grating920) to a second middle grating (e.g., second middle grating930). The in-coupling efficiency of waveguide display2600may be measured after the display light is diffracted by the first middle grating and before the display light reaches the second middle grating.

FIG.26Billustrates a simulation result of an example of a waveguide display2605including volume Bragg gratings and a phase structure according to certain embodiments. Waveguide display2605may be an example of waveguide display2500, where a phase structure (e.g., phase structure2540) may be located at a region where an input grating (e.g., input grating2520) and a first middle grating (e.g., first middle grating2530) are located.FIG.26Bshows a display light beam coupled into a waveguide by the input grating and then directed by the first middle grating to a second middle grating (e.g., second middle grating2550). The in-coupling efficiency of waveguide display2605may be measured after the display light is diffracted by the first middle grating and before the display light reaches the second middle grating.FIG.26Bshows that the intensity of the display light beam after the first middle grating may be much higher than that shown inFIG.26A.

FIG.27illustrates a portion of an example of a waveguide display2700including a staircase structure2720and a phase structure2750according to certain embodiments. As waveguide display1300, waveguide display2700may include a waveguide2710and a first middle grating2740formed in waveguide2710. Staircase structure2720may be bonded to waveguide2710. Staircase structure2720may include an input grating2730on the top or the bottom of a staircase substrate as described above with respect to, for example,FIGS.13A,13B,13D, and17A-17F, to reduce the undesired coupling of the display light out of waveguide2710by input grating2730and to reduce FOV clipping. In addition, phase structure2750as described above with respect toFIGS.24C-25Bmay be formed at the bottom or top surface of waveguide2710, to further improve the in-coupling efficiency of the waveguide display2700. In the example shown inFIG.27, input grating2730may be formed on the top of staircase structure2720, and the staircase substrate may have a size that is close to the size of input grating2730(e.g., have a circular shape).

FIG.28Aillustrates an example of a waveguide display2800including separate projectors and input grating couplers for different colors according to certain embodiments. Waveguide display2800may be similar to waveguide display2300and may include one or more assemblies.FIG.28Ashows one of the one or more assemblies. The projectors may include three projectors. Each projector may include, for example, a micro-LED array that emits display light in one color as described above with respect toFIGS.22A and22B. The three projectors may include, for example, a red micro-LED array, a green micro-LED array, and a blue micro-LED array. Each micro-LED array may generate a monochromatic image of a corresponding color, where the combination of the three monochromatic images may form a color image. Waveguide display2800may include a waveguide2810(e.g., a substrate), multiple (e.g., three) input gratings2820, multiple first middle gratings2830(which may be separate from each other or may be in contiguous regions), a second middle grating2840, and an output grating2850. Each of the multiple input gratings2820may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2810. Each of the multiple first middle grating2830may be used to direct display light from an input grating2820towards second middle grating2840as described above. Second middle grating2840and output grating2850may expand the input pupil in two directions and couple the display light to user's eyes.

FIG.28Billustrates an example of a waveguide display2802including separate projectors and input gratings for different colors and a phase structure2860according to certain embodiments. Waveguide display2802may be similar to waveguide display2800, and may include one or more assemblies. One of the one or more assemblies may include a waveguide2812(e.g., a substrate), multiple (e.g., three) input gratings2822, multiple first middle gratings2832(which may be separate from each other or may be in contiguous regions), a second middle grating2842, and an output grating2852. The projectors may include three projectors. Each projector may include, for example, a micro-LED array that emits display light in one color. The three projectors may include, for example, a red micro-LED array, a green micro-LED array, and a blue micro-LED array. Each micro-LED array may generate a monochromatic image of a corresponding color, where the combination of the three monochromatic images may form a color image. Each of the multiple input gratings2822may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2812. Each of the multiple first middle grating2832may be used to direct display light from an input grating2822towards second middle grating2842as described above. Second middle grating2842and output grating2852may expand the input pupil in two directions and couple the display light to user's eyes.

Waveguide display2802may also include an additional phase structure2860as described above. Phase structure2540may be at a region where input gratings2822and/or first middle grating2832are located, to change the polarization state of the display light coupled into waveguide2812, for example, from p-polarized to s-polarized or from s-polarized to p-polarized. In some other embodiments, phase structure2860may also be at regions where second middle grating2842and/or output grating2552are located, to change the polarization state of the display light during its propagation within waveguide2812.

FIG.29Aillustrates an input portion of an example of a waveguide display2900including multiple projectors and multiples input gratings2930on multiple staircase structures2920according to certain embodiments. Each projector may include, for example, a micro-LED array that emits display light in one color as described above with respect toFIGS.22A and22B. The three color projectors may include, for example, a red micro-LED array, a green micro-LED array, and a blue micro-LED array. Each micro-LED array may generate a monochromatic image of a corresponding color, and thus the three micro-LED arrays in combination may generate a color image. Waveguide display2900may include a waveguide2910(e.g., a substrate), three staircase structures2920each including an input grating2930, and three first middle gratings2940, which may be separate from each other or may be in contiguous regions. The shape and the thickness of staircase structures2920may be selected to optimize the input efficiency and reduce the FOV clipping as described above. As also described above, the staircase structures2920may or may not include a staircase substrate, the thickness of which may be the difference between the total desired thickness of staircase structure2920and the thickness of the holographic material layer for input grating2930. Each input grating2930may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2910. Each first middle grating2940may be used to direct display light from an input grating2930towards a second middle grating (e.g., second middle grating2340,2840, or2842, not shown inFIG.29A) as described above. The second middle grating and an output grating (e.g., output grating2350,2850, or2852, not shown inFIG.29A) may expand the input pupil in two directions and couple the display light to user's eyes.

FIG.29Billustrates an input portion of an example of a waveguide display2902including multiple projectors, multiples input gratings2932on multiple staircase structures2922, and a phase structure2950according to certain embodiments. Waveguide display2902may be similar to waveguide display2900, and may include a waveguide2912(e.g., a substrate), three staircase structures2922each including an input grating2932, and three first middle gratings2942, which may be separate from each other or may be in contiguous regions. The shape and the thickness of staircase structures2922may be selected to optimize the input efficiency and reduce the FOV clipping as described above. As also described above, the staircase structures2922may or may not include a staircase substrate, the thickness of which may be the difference between the total desired thickness of staircase structure2922and the thickness of the holographic material layer for input grating2932. Each input grating2932may be used to couple display light of one color from a light source (e.g., a micro-LED array) into waveguide2912. Each first middle grating2942may be used to receive and redirect display light from an input grating2932towards a second middle grating (e.g., second middle grating2340,2840, or2842, not shown inFIG.29B) as described above. The second middle grating and an output grating (e.g., output grating2350,2850, or2852, not shown inFIG.29B) may expand the input pupil in two directions and couple the display light to user's eyes.

Waveguide display2902may also include an additional phase structure2950as described above. Phase structure2950may be at a region where input gratings2932and/or first middle gratings2942are located, to change the polarization state of the display light coupled into waveguide2912, for example, from p-polarized to s-polarized or from s-polarized to p-polarized. In some other embodiments, phase structure2950may also be at regions where the second middle grating and/or the output grating are located, to change the polarization state of the display light during its propagation within waveguide2912.

The phase structures described above (e.g., phase structure2456,2490,2492,2540,2750,2860, or2950) may include any birefringent materials (e.g., birefringent crystals, liquid crystals, or polymers) or structures (e.g., gratings, meta-gratings, nano-structures, or other subwavelength structures) that can cause a desired phase delay between two orthogonal linear polarization components (e.g., s-polarized light and p-polarized light), such that the incident light beam may be changed to an s-polarized, p-polarized, circularly polarized, or elliptically polarized beam.

In some embodiments, in order to reduce the loss (e.g., due to undesired Fresnel reflection) at the interfaces between the phase structures and the adjacent components of the waveguide display, such as the substrate, it may be desirable to use a phase structure that has an effective refractive index close to the refractive index of the adjacent component. In some embodiments where the substrate has a high refractive index (e.g., >2.0, such as 2.5), it may be difficult to find a birefringent material that has a matching refractive index. In such cases, gratings or other subwavelength structures may be used to achieve the phase delay, polarization conversion, and refractive index matching, such that a difference between the refractive index of the substrate and the effective refractive index of the phase structure may be less than about 0.35, less than about 0.2, less than about 0.1, or less than about 0.05.

FIG.30is a simplified block diagram of an example of an electronic system3000of an example near-eye display (e.g., HMD device) for implementing some of the examples disclosed herein. Electronic system3000may be used as the electronic system of an HMD device or other near-eye displays described above. In this example, electronic system3000may include one or more processor(s)3010and a memory3020. Processor(s)3010may be configured to execute instructions for performing operations at a number of components, and can be, for example, a general-purpose processor or microprocessor suitable for implementation within a portable electronic device. Processor(s)3010may be communicatively coupled with a plurality of components within electronic system3000. To realize this communicative coupling, processor(s)3010may communicate with the other illustrated components across a bus3040. Bus3040may be any subsystem adapted to transfer data within electronic system3000. Bus3040may include a plurality of computer buses and additional circuitry to transfer data.

Memory3020may be coupled to processor(s)3010. In some embodiments, memory3020may offer both short-term and long-term storage and may be divided into several units. Memory3020may be volatile, such as static random access memory (SRAM) and/or dynamic random access memory (DRAM) and/or non-volatile, such as read-only memory (ROM), flash memory, and the like. Furthermore, memory3020may include removable storage devices, such as secure digital (SD) cards. Memory3020may provide storage of computer-readable instructions, data structures, program modules, and other data for electronic system3000. In some embodiments, memory3020may be distributed into different hardware modules. A set of instructions and/or code might be stored on memory3020. The instructions might take the form of executable code that may be executable by electronic system3000, and/or might take the form of source and/or installable code, which, upon compilation and/or installation on electronic system3000(e.g., using any of a variety of generally available compilers, installation programs, compression/decompression utilities, etc.), may take the form of executable code.

In some embodiments, memory3020may store a plurality of application modules3022through3024, which may include any number of applications. Examples of applications may include gaming applications, conferencing applications, video playback applications, or other suitable applications. The applications may include a depth sensing function or eye tracking function. Application modules3022-3024may include particular instructions to be executed by processor(s)3010. In some embodiments, certain applications or parts of application modules3022-3024may be executable by other hardware modules3080. In certain embodiments, memory3020may additionally include secure memory, which may include additional security controls to prevent copying or other unauthorized access to secure information.

In some embodiments, memory3020may include an operating system3025loaded therein. Operating system3025may be operable to initiate the execution of the instructions provided by application modules3022-3024and/or manage other hardware modules3080as well as interfaces with a wireless communication subsystem3030which may include one or more wireless transceivers. Operating system3025may be adapted to perform other operations across the components of electronic system3000including threading, resource management, data storage control and other similar functionality.

Wireless communication subsystem3030may include, for example, an infrared communication device, a wireless communication device and/or chipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fi device, a WiMax device, cellular communication facilities, etc.), and/or similar communication interfaces. Electronic system3000may include one or more antennas3034for wireless communication as part of wireless communication subsystem3030or as a separate component coupled to any portion of the system. Depending on desired functionality, wireless communication subsystem3030may include separate transceivers to communicate with base transceiver stations and other wireless devices and access points, which may include communicating with different data networks and/or network types, such as wireless wide-area networks (WWANs), wireless local area networks (WLANs), or wireless personal area networks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16) network. A WLAN may be, for example, an IEEE 802.11x network. A WPAN may be, for example, a Bluetooth network, an IEEE 802.15x, or some other types of network. The techniques described herein may also be used for any combination of WWAN, WLAN, and/or WPAN. Wireless communications subsystem3030may permit data to be exchanged with a network, other computer systems, and/or any other devices described herein. Wireless communication subsystem3030may include a means for transmitting or receiving data, such as identifiers of HMD devices, position data, a geographic map, a heat map, photos, or videos, using antenna(s)3034and wireless link(s)3032. Wireless communication subsystem3030, processor(s)3010, and memory3020may together comprise at least a part of one or more of a means for performing some functions disclosed herein.

Embodiments of electronic system3000may also include one or more sensors3090. Sensor(s)3090may include, for example, an image sensor, an accelerometer, a pressure sensor, a temperature sensor, a proximity sensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a module that combines an accelerometer and a gyroscope), an ambient light sensor, or any other similar module operable to provide sensory output and/or receive sensory input, such as a depth sensor or a position sensor. For example, in some implementations, sensor(s)3090may include one or more inertial measurement units (IMUs) and/or one or more position sensors. An IMU may generate calibration data indicating an estimated position of the HMD device relative to an initial position of the HMD device, based on measurement signals received from one or more of the position sensors. A position sensor may generate one or more measurement signals in response to motion of the HMD device. Examples of the position sensors may include, but are not limited to, one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU, or some combination thereof. The position sensors may be located external to the IMU, internal to the IMU, or some combination thereof. At least some sensors may use a structured light pattern for sensing.

Electronic system3000may include a display module3060. Display module3060may be a near-eye display, and may graphically present information, such as images, videos, and various instructions, from electronic system3000to a user. Such information may be derived from one or more application modules3022-3024, virtual reality engine3026, one or more other hardware modules3080, a combination thereof, or any other suitable means for resolving graphical content for the user (e.g., by operating system3025). Display module3060may use liquid crystal display (LCD) technology, light-emitting diode (LED) technology (including, for example, OLED, ILED, μLED, AMOLED, TOLED, etc.), light emitting polymer display (LPD) technology, or some other display technology.

Electronic system3000may include a user input/output module3070. User input/output module3070may allow a user to send action requests to electronic system3000. An action request may be a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application. User input/output module3070may include one or more input devices. Example input devices may include a touchscreen, a touch pad, microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, a game controller, or any other suitable device for receiving action requests and communicating the received action requests to electronic system3000. In some embodiments, user input/output module3070may provide haptic feedback to the user in accordance with instructions received from electronic system3000. For example, the haptic feedback may be provided when an action request is received or has been performed.

Electronic system3000may include a camera3050that may be used to take photos or videos of a user, for example, for tracking the user's eye position. Camera3050may also be used to take photos or videos of the environment, for example, for VR, AR, or MR applications. Camera3050may include, for example, a complementary metal-oxide-semiconductor (CMOS) image sensor with a few millions or tens of millions of pixels. In some implementations, camera3050may include two or more cameras that may be used to capture 3-D images.

In some embodiments, electronic system3000may include a plurality of other hardware modules3080. Each of other hardware modules3080may be a physical module within electronic system3000. While each of other hardware modules3080may be permanently configured as a structure, some of other hardware modules3080may be temporarily configured to perform specific functions or temporarily activated. Examples of other hardware modules3080may include, for example, an audio output and/or input module (e.g., a microphone or speaker), a near field communication (NFC) module, a rechargeable battery, a battery management system, a wired/wireless battery charging system, etc. In some embodiments, one or more functions of other hardware modules3080may be implemented in software.

In some embodiments, memory3020of electronic system3000may also store a virtual reality engine3026. Virtual reality engine3026may execute applications within electronic system3000and receive position information, acceleration information, velocity information, predicted future positions, or some combination thereof of the HMD device from the various sensors. In some embodiments, the information received by virtual reality engine3026may be used for producing a signal (e.g., display instructions) to display module3060. For example, if the received information indicates that the user has looked to the left, virtual reality engine3026may generate content for the HMD device that mirrors the user's movement in a virtual environment. Additionally, virtual reality engine3026may perform an action within an application in response to an action request received from user input/output module3070and provide feedback to the user. The provided feedback may be visual, audible, or haptic feedback. In some implementations, processor(s)3010may include one or more GPUs that may execute virtual reality engine3026.

In various implementations, the above-described hardware and modules may be implemented on a single device or on multiple devices that can communicate with one another using wired or wireless connections. For example, in some implementations, some components or modules, such as GPUs, virtual reality engine3026, and applications (e.g., tracking application), may be implemented on a console separate from the head-mounted display device. In some implementations, one console may be connected to or support more than one HMD.

In alternative configurations, different and/or additional components may be included in electronic system3000. Similarly, functionality of one or more of the components can be distributed among the components in a manner different from the manner described above. For example, in some embodiments, electronic system3000may be modified to include other system environments, such as an AR system environment and/or an MR environment.

It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized or special-purpose hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets, etc.), or both. Further, connection to other computing devices such as network input/output devices may be employed.

Further, while certain embodiments have been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are also possible. Certain embodiments may be implemented only in hardware, or only in software, or using combinations thereof. In one example, software may be implemented with a computer program product containing computer program code or instructions executable by one or more processors for performing any or all of the steps, operations, or processes described in this disclosure, where the computer program may be stored on a non-transitory computer readable medium. The various processes described herein can be implemented on the same processor or different processors in any combination.