LIGHTGUIDES WITH TUNABLE GRATINGS FOR DYNAMICALLY VARIABLE FIELD-OF-VIEW

A display apparatus includes a lightguide for conveying images to a user in a target field-of-view (FOV). The lightguide includes a tunable output diffraction grating for displaying different portions of the target field-of-view at different time instances. The tunable output diffraction grating may include grating segments that are selectively switchable between a diffracting state and a non-diffracting state in dependence on a content of an image being displayed, providing content-dependent FOV switching.

TECHNICAL FIELD

The present disclosure relates visual display devices and related components, modules, and methods.

BACKGROUND

Visual displays provide information to viewer(s) including still images, video, data, etc. Visual displays have applications in diverse fields including entertainment, education, engineering, science, professional training, advertising, to name just a few examples. Some visual displays such as TV sets display images to several users, and some visual display systems such s near-eye displays (NEDs) are intended for individual users.

An artificial reality system generally includes an NED (e.g., a headset or a pair of glasses) configured to present content to a user. 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 images of virtual objects (e.g., computer-generated images (CGIs)) superimposed with the surrounding environment by seeing through a “combiner” component. The combiner of a wearable display is typically transparent to external light but includes some light routing optic to direct the display light into the user's field of view.

Because a display of HMD or NED is usually worn on the head of a user, a large, bulky, unbalanced, and/or heavy display device with a heavy battery would be cumbersome and uncomfortable for the user to wear. Consequently, head-mounted display devices can benefit from a compact and efficient configuration, including efficient light sources and illuminators providing illumination of a display panel, high-throughput combiner components, ocular lenses, and other optical elements in the image forming train.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated.

AR and VR displays may use pupil-replicating lightguides to carry images to an eyebox and/or to illuminate display panels that generate images to be displayed. Herein the term “eyebox” means a geometrical area for the user's eye where a good-quality image may be observed by a user of the NED. A pupil-replicating lightguide may include grating structures for in-coupling a light beam into the lightguide, and/or for out-coupling portions of the light beam along the waveguide surface. In accordance with this disclosure, a grating structure of a pupil-replicating lightguide may include a tunable/switchable grating with switchable or tunable diffraction efficiency, grating pitch or grating period, blazing angle, etc. The terms “grating pitch” and “grating period” are used herein interchangeably. The term “tunable” encompasses both continuously tunable and switchable between two or more states. The term “diffraction efficiency” refers to aspects of the performance of the diffraction grating in terms of power throughput of the diffraction grating. In particular, the diffraction efficiency can be a measure of the optical power diffracted into a given direction compared to the power incident onto the diffractive element. In examples described herein, the diffraction efficiency is typically a measure of the optical power diffracted by the grating or a segment thereof in the first order of diffraction relative to the power incident onto the grating or the segment thereof. The term “output efficiency” as used herein refers to a fraction of the optical power of a light source of a display apparatus that is available to the user for viewing images.

An aspect of the present disclosure relates to a display system comprising a lightguide and an image light source coupled to the lightguide. The lightguide is configured to receive image light emitted by the image light source and to convey the image light received in a target field of view (FOV) of the display to an eyebox for presenting to a user. The term “field of view” (FOV), when used in relation to a display system, may refer to an angular range of light propagation supported by the system or visible to the user. A two-dimensional (2D) FOV may be defined by angular ranges in two orthogonal planes. For example, a 2D FOV of a NED device may be defined by two one-dimensional (1D) FOVs, which may be a vertical FOV, for example +\−20° relative to a horizontal plane, and a horizontal FOV, for example +\−30° relative to the vertical plane. With respect to a FOV of a NED, the “vertical” and “horizontal” planes or directions may be defined relative to the head of a standing person wearing the NED. Otherwise the terms “vertical” and “horizontal” may be used in the present disclosure with reference to two orthogonal planes of an optical system or device being described, without implying any particular relationship to the environment in which the optical system or device is used, or any particular orientation thereof to the environment.

Embodiments described herein relate to a pupil replicating lightguide operable to convey to a user different FOV portions at different time instances. Such lightguides include active, i.e. dynamically tunable, diffraction gratings configured to support a variable or switchable FOV and to enable adjusting one or more of the grating's properties to the particular FOV portion being displayed for providing an enhanced viewer experience. In some embodiments, an out-coupling (“output”) diffraction grating of a pupil-replicating lightguide may be segmented, with the segments individually switchable between a diffracting and a non-diffracting state depending on a FOV portion being displayed. By switching to a non-diffracting state a portion of an output grating that doesn't contribute to a FOV portion being currently displayed, grating-related artifacts, e.g. the “rainbow” in AR displays, can be reduced and the image brightness improved. Furthermore, the diffraction efficiency of a currently “FOV-contributing” subs-set of the grating segments may be adjusted to the FOV portion being conveyed, e.g. to provide enhanced image uniformity. In some embodiments, the FOV portion being displayed depends on a content of the image.

In some embodiments, an in-coupling (“input”) grating and an out-coupling (“output”) grating of a lightguide may be operable, i.e. their grating pitch simultaneously tuned, to quickly scan through a sequence of FOV portions of an image. In such embodiments, the image is presented to the viewer in a time-multiplexed manner, with the visual cortex of a viewer integrating the different FOV portions into a single image FOV; this approach may enable a greater overall FOV than can be instantaneously supported by the lightguide. By segmenting the output grating and selectively adjusting the diffraction efficiency of different segments during the FOV scan, the image brightness and/or uniformity of the lightguide may be further improved.

Accordingly, an aspect of the present disclosure provides a display apparatus for displaying images within a target field-of-view (FOV), the display apparatus comprising a lightguide for relaying image light carrying the images to an eyebox. The lightguide comprises a substrate of optically transparent material, the substrate comprising two opposing surfaces for guiding the image light in the substrate by reflections therefrom. The lightguide further comprises an output diffraction grating disposed in or upon the substrate and configured to diffract the image light out of the lightguide toward the eyebox, wherein the output diffraction grating has one or more electrically tunable characteristics and is operable to convey, to the eyebox, different FOV portions of the target FOV at different time instances. The display apparatus further comprises a controller configured to selectively tune the one or more electrically tunable characteristics in dependence on a FOV portion being conveyed.

In some implementations, the one or more electrically tunable characteristics may comprise a diffraction efficiency, and the output diffraction grating may comprise a plurality of grating segments disposed along the surfaces; the controller may be configured to selectively reduce the diffraction efficiency for one or more of the grating segments depending on the FOV portion being conveyed.

In some implementations, the one or more electrically tunable characteristics may comprise an output grating pitch, and the controller may be configured to selectively tune the output grating pitch in at least a segment of the output diffraction grating depending on the portion of the target FOV being displayed. The display apparatus may further comprise an input diffraction grating having an electrically tunable input grating pitch, with the controller configured to tune the electrically tunable input grating pitch in coordination with tuning the output grating pitch. In some of such implementations, the controller is configured to tune the input grating pitch, so as to direct beams of the image light from non-overlapping portions of the target FOV to propagate within the substrate at a same angle of incidence at the surfaces. In these or other implementations, the controller may be configured to tune the input grating pitch so that the image light propagates within the substrate at angles of incidence upon the opposing surfaces thereof smaller than 70 degrees for any FOV portion of the target FOV being conveyed.

Some implementations may comprise a source of the image light and an image processor for controlling the source depending on a content of the image, the image processor being operatively coupled to the controller, wherein the FOV portion being conveyed depends on the content of the image.

In any of the above implementations, the one or more electrically tunable characteristics of the output diffraction grating comprise a grating efficiency, and the controller may be configured to tune the grating efficiency depending on the FOV portion being displayed.

In some implementations, the display apparatus is configured for conveying, to the eyebox, an image in the target FOV sequentially portion by portion, and the controller is configured to at least once tune the one or more electrically tunable characteristics while the image is being displayed. In some of such implementations, the one or more electrically tunable grating characteristics being tuned comprise at least one of an output grating pitch in at least a segment of the output diffraction grating or diffraction efficiency in at least a segment of the output diffraction grating. In some of the above implementations, the output diffraction grating comprises a plurality of individually tunable grating segments, and the controller is configured to selectively tune at least one of the grating pitch or the diffraction efficiency for a subset of the individually tunable grating segments while the image is being displayed, the subset being dependent on the FOV portion being conveyed to the eyebox.

An aspect of the present disclosure provides a display apparatus for displaying an augmented reality (AR) image, the display apparatus comprising: an image projector for providing image light carrying the AR image, a lightguide, and a controller. The lightguide comprises a substrate of optically transparent material, the substrate comprising two opposing surfaces for guiding the image light in the substrate by reflections from the surfaces, and an output diffraction grating configured to diffract the image light out of the substrate for combining with ambient light carrying real scenery and for presenting the AR image to a user within a target field-of-view (FOV), wherein the output diffraction grating comprises a plurality of grating segments, each having an electrically variable diffraction efficiency. The controller is configured to selectively reduce the diffraction efficiency for one or more of the grating segments in dependence on a content of the AR image.

In some implementations of this aspect, the display apparatus is configured to present the AR image in a FOV portion of the target FOV dependent on the content of the AR image, and the controller is configured to switch the one or more grating segments to a substantially non-diffracting state when the one or more grating segments are disposed outside of the FOV portion presenting the AR image. In some implementations, the controller is configured to switch the one or more grating segments from the substantially non-diffracting state to a diffracting state when the content of the AR image changes. In some implementations, the controller may be configured to tune the diffraction efficiency of the one or more grating segments in the diffracting state.

An aspect of the present disclosure provides a method for displaying images to a user within a target field-of-view (FOV), comprising: a) coupling image light into a lightguide having an output region adjacent an eyebox; b) using an output diffraction grating located in the output region of the lightguide to convey to the eyebox different FOV portions of the image light at different time instances, the different FOV portions of the image light being conveyed within different portions of the target FOV; and c) tuning at least one of a grating pitch or a grating efficiency of the output diffraction grating in dependence on a FOV portion being conveyed to the eyebox.

In some implementations, the method may comprise selecting the FOV portion being conveyed depending on a content of the image. In some implementations the method may comprise selecting the FOV portion being conveyed depending on a location of the image content within the target FOV.

In some implementations, the method may comprise using the output diffraction grating to convey, to the eyebox over a frame time interval, the target FOV sequentially portion by portion, at least once during the frame time interval tuning the at least one of the grating pitch or the diffraction efficiency for the FOV portion being displayed.

In some implementations, the method may comprise i) using a scanning image projector to provide image light carrying the image to an input diffraction grating of the lightguide by sequentially scanning a beam of the image light through different portions of the target FOV over a frame time interval, the input diffraction grating tunable to couple the beam into the lightguide; and ii) tuning a pitch of each of the input diffraction grating and the at least a segment of the output diffraction grating in coordination with the scanning.

FIG.1illustrates an example display apparatus100for presenting images to a user, e.g. in the form of image frames. The display apparatus100may include an image projector103configured to provide image light101carrying the images in angular domain within a target field-of-view (FOV)110supported by the display. The target FOV110may also be referred to herein as the supported FOV or the frame FOV, and may represent a cross-section of a 2D FOV of the display apparatus100. A lightguide120relays the image light101to an eyebox150of the display apparatus100. The lightguide120includes a substrate125, which may be e.g. a slab of a material that is transparent to visible light. The substrate125has two opposing surfaces121and122, e.g. the main outer surfaces thereof, and is configured for guiding the image light in the substrate in a zig-zag fashion by reflections from the surfaces121and122. An output diffraction grating (ODG)140is disposed in or upon the substrate125in an output region thereof facing the eyebox150, and is configured to diffract the image light out of the substrate125toward the eyebox150for viewing the images within a FOV110A matching the image FOV110. The ODG140has one or more electrically tunable characteristics or parameters, such as a grating pitch and/or diffraction efficiency, which may be tuned or switched by a controller160. In some embodiments, the ODG140may include a plurality of individually tunable segments tiled side by side along one of the surfaces121,122, e.g. as described below with reference toFIG.4A. The display apparatus100may be operated so that different FOV portions of the target FOV110or110A are conveyed to the eyebox150at different time instances, with the controller160tuning the one or more electrically tunable characteristics of the ODG140in dependence on a FOV portion which is currently being conveyed to the eyebox150. As a non-limiting example, FOV portions111,112,113of the image FOV110may be conveyed to the eyebox150at different time instances, for viewing within corresponding FOV portions111A,112A, and113A of the user's FOV110A, respectively, and the controller160may be operated to tune at least one of the grating pitch and the diffraction efficiency in at least a segment of the ODG140in dependence on the FOV portion being conveyed.

The image projector103may be embodied, for example, using a pixelated display panel, e.g. an LC micro display, optionally having suitable optics at its output. It may also be embodied using a light source, such as e.g. one or more light-emitting diodes (LED), superluminescent light-emitting diodes (SLED), side-emitting laser diodes, vertical-cavity surface-emitting laser diodes (VCSEL), etc., followed by an image beam scanner. The image light101provided by the projector103within the target FOV110is coupled into the substrate125by an input optical coupler, such as e.g. an input diffraction grating (IDG)130disposed in an input region of the lightguide as illustrated inFIG.1, or a coupling prism or other suitable coupling means in other embodiments. Is some embodiments, the IDG130may also be electrically tunable, e.g. have an electrically tunable pitch, and may be operable to couple different portions of the FOV110at different time instances. In some embodiments, both the IDG130and the ODG140may be segmented, with the segments individually tunable by the controller. In some embodiments, only the ODG140may be segmented.

In some embodiments, the image projector103may project an image spanning up to the target FOV110sequentially portion by portion, each image portion carried by the image light in a corresponding FOV portion, e.g.111,112, or113, with the controller160adjusting tuning the grating pitch of the IDG130and the ODG140, or at least a segment thereof, for one or more of the FOV portions. In some embodiments, the controller160may tune the ODG140in dependence on image content being displayed in a current image frame; in some embodiments this may include e.g. deactivating a segment of the ODG140, i.e. switching off, or at least substantially reducing the diffraction efficiency, of the segment, in dependence on an image content of the image being displayed. In some embodiments, e.g. when the image content is present only in a portion of the target FOV, the controller160may switch off, or at least substantially reduce the diffraction efficiency, of a segment of the ODG140that is outside of a FOV portion wherein the image content is present. Here “substantially” refers to a reduction by at least a factor of 5.

Referring toFIG.2for a non-limiting illustrative example, a pupil-replicating lightguide220, shown in a top view, includes a slab225of transparent material for guiding image light in the slab by a series of internal reflections from outer surfaces of the slab225. The pupil-replicating lightguide220includes a switchable grating240adjacent to an eyebox255, both shown inFIG.2in a top view in projection on an (x,y) plane parallel to the outer surfaces of the slab. The pupil-replicating lightguide220, the switchable grating240, and the eyebox255may be embodiments of the lightguide120, the ODG140, and the eyebox155described above with reference toFIG.1. An active area, efficiency, and/or pitch of the switchable grating240may be adjusted based on the image content being displayed and the position of the image content in the field of view; for example, when displaying an image with the image content confined to a portion of a target image FOV of the display, only a portion240A of the switchable grating240may be configured to diffract image light propagating in the slab225toward the eyebox250. Such an approach may enable higher output efficiency and uniformity when the displayed image content, e.g. colored or b/w text, indicators, auxiliary information, etc., is not covering all of the FOV or does not include one of the R (red), G (green), and B (blue) color channels that is being conveyed by the lightguide. Furthermore, the output diffraction efficiency can be reduced substantially to zero, i.e. turned off (“de-activated”), in a portion of the output grating that is outside of a FOV portion where the image content is present. In displays configured for AR applications, i.e. wherein the region of the lightguide220including the output grating240is see-through and combines image light carrying AR images with ambient light carrying real life scenery on the other side of the slab, switching at least a portion of the output grating to a substantially non-diffracting state may have an advantage of eliminating see-through artifacts such as the rainbow artifact, and to improve throughput for the ambient light being transmitted through the slab.

Referring now toFIG.3, an AR display300is an example embodiment of the display apparatus100ofFIG.1. In the AR display300ofFIG.3, the substrate125is substantially transparent to ambient visible light171, and the ODG140is configured to combine the image light carrying AR images with the ambient light171carrying real-life scenery, with the combined light illuminating the eyebox155. The image projector103of the AR display apparatus300may project an AR image105A, which content is carried by image light101A within only the FOV portion111of the target FOV110supported by the display. From the eyebox155, the AR image is visible within a corresponding FOV portion111A, replicated by the ODG140to a plurality of locations in the eyebox. A portion140A of the ODG140is outside of the FOV portion111A carrying the AR image105A for any location in the eyebox155. Accordingly, the controller160may be configured to de-activate the ODG portion140A while the image105A being displayed, i.e. to switch the ODG portion140A from a diffracting state, in which it may diffract a portion of image light incident thereon from within the substrate125toward the eyebox155, to a substantially non-diffracting state, or to at least reduce the diffraction efficiency thereof. Here, “substantially non-diffracting” means having a diffraction efficiency that is at least 5 time lower than the diffraction efficiency of the same grating portion in the diffracting state. Switching the ODG portion140A to a non-diffracting state may reduce undesired grating-related artifacts such as image light leakage and the rainbow artifact, which may be caused by undesired diffractions of the image and ambient light, respectively, upon the grating structure of the ODG140. When the image provided by the image projector103changes so that it is carried at least in part by image light within a FOV portion of the target image FOV110that is complementary to FOV111A, the controller160may tune the ODG portion140back to a substantially diffracting state. The ODG portion140A may include one or more segments of the ODG140, which diffraction efficiency at least is individually electrically tunable, including individually switchable between a diffracting and a non-diffracting states in some embodiments. Similarly, the remaining portion140B of the ODG140may also include one or more individually tunable grating segments.

Referring now toFIG.4Afor a non-limiting illustrative example, a pupil-replicating lightguide320, shown in a top view, includes a slab substrate325of transparent material for guiding light therein by a series of internal reflections from outer surfaces of the substrate325. An electrically tunable/switchable grating340disposed in or upon the substrate325in an output region thereof includes a plurality of grating segments341ii,34112, . . . ,34145, which may be generally referred to as grating segments341, and which characteristics are individually tunable, e.g. switchable from a diffracting state to a substantially non-diffracting state, by a controller360. The pupil-replicating lightguide320, the substrate325, and the tunable/switchable grating340may be embodiments of the lightguide120, substrate125, and ODG140described above with reference toFIG.3. Although a 2D array of 20 segments is shown, the number of segments may be different, generally at least two, which may be disposed in a 2D array or along a single direction. By way of example, to display an image in a top right corner of the display FOV, e.g. as illustrated inFIG.2, the controller360may de-activate the grating segments341in the first and second columns and the fourth row of the segment array of the tunable/switchable grating340, as illustrated inFIG.4Bby the non-patterned segments. To display an image in a lower left corner of the FOV, the controller360may de-activate the grating segments341in the fourth and fifths columns and the first row of the segment array of the tunable/switchable grating340, as illustrated inFIG.4Cby the non-patterned segments.

Accordingly, in some embodiments a pupil-replicating lightguide includes a slab substrate for guiding light therein and an output grating supported by the substrate, wherein the output grating has a spatially variant tunable efficiency. The output grating may be controlled to out-couple light by a portion of the output grating to form an image only in a portion of a field of view. Such a pupil-replicating lightguide may be used in an AR display.

In some embodiments, a display apparatus such as those described above may have an output diffraction grating having at least a segment with an electrically tunable grating pitch. In some embodiments, the grating pitch of both the input and output diffraction gratings may be synchronously electrically tunable, e.g. in dependence on a FOV portion being displayed, so that rays of the image light are coupled in and out of the substrate at a same angle, thereby preserving the correspondence between the image FOV at the input coupler and the display FOV at seen from the eyebox, while potentially enhancing at least one of the output uniformity of the display or the FOV supported by the display.

FIGS.5A-5Cschematically illustrate, in a side cross-sectional view, a lightguide520including a substrate525, e.g. a slab of an optically transparent material, having two outer surfaces521,522for guiding image light within the substrate by TIR upon the surfaces. The lightguide520further includes an input diffraction grating (IDG)530and an output diffraction grating (ODG)540disposed in laterally separated input and output regions of the lightguide, respectively, each of IDG530and ODG540having a grating pitch that is electrically tunable by a controller560. The IDG530may be configured to couple, into the substrate525, image light501incident thereon in a target FOV505(FIG.5A). The target FOV505depends on the image light wavelength λ, the grating pitch pinof the IDG530, and the refractive index n of the substrate525. The target FOV234may be e.g. symmetrical to a normal207to the substrate525. In the illustrated example the target FOV234includes all rays of the image light that in the angular domain are between ray511a(dotted line) and ray511c(solid line), which correspond to propagation angles θ within the substrate525between a minimum angle β1, that may be equal or somewhat exceed the critical angle of TIR βc=a sin(1/n), and some maximum angle β3=βmax≤90°. Here β is the angle of incidence of an in-coupled ray of the image light501upon one of the surfaces521or522from within the substrate. For a given grating pitch pin=p1, the IDG530conveys image light within non-overlapping portions of the FOV505into non-overlapping angular ranges of the in-coupled light within the substrate. For example, image light within a first FOV portion10bound by rays511aand511bis coupled into the substrate in angular range between β1and β2. A second FOV portion11bound by rays511band511cis coupled into the substrate in angular range between β2and β3. The in-coupled rays of image light at opposing edges of the FOV505, e.g. adjacent rays511aand511c, propagate toward the ODG540along different zig-zag paths, with the more oblique ray511a, which propagates in the substrate525at the greatest angle βmax, impinging the ODG540fewer times and experiencing fewer diffractions out of the substrate than the ray511cat the opposite edge of the FOV505. Therefore, it may be difficult to provide a good output uniformity across a wide FOV.

In one embodiment, the lightguide520may represent the lightguide120of the display apparatus100ofFIG.1or the AR display apparatus300ofFIG.3, with the ODG540being an embodiment of ODG140including a plurality of grating segments, e.g. as illustrated inFIG.4A, with the segments having individually tunable diffraction efficiency and grating pitch. The controller560may selectively tune the diffraction efficiency and/or the grating pitch of different grating segments in dependence on an image content being displayed. In some embodiments, the controller560may de-activate, i.e. switch off, one or more first grating segments while tuning the grating pitch of one or more second grating segments.

By way of example, the display apparatus including the lightguide520may be configured so that when at a first time instance an image projector thereof, e.g. the image projector103shown inFIG.3, generates image light501athat is confined to the first FOV portion11of the FOV505, as illustrated inFIG.5B. Responsive to the image content, the controller560operates the IDG530so that its grating pitch pin=p1, wherein p1is such that the image light501apropagates within the substrate in the first angular range between β1≃βc and β2. Simultaneously, the controller506may tune the grating pitch poutin at least a portion542of the ODG540so as to out-couple the image rays out of the substrate525at the angles of their incidence upon the substrate, a, to convey the first FOV portion11to an eye555of the viewer substantially without distortion. In some embodiments, the controller560may de-activate, i.e. switch off, another portion541of the ODG540which is outside of the first FOV portion11visible to the eye555.

At a second time instance, the image projector may generate image light501bconfined to the second FOV portion10of the FOV505, as illustrated inFIG.5C. Were the grating pitch of the IDG530stay equal to p1, the image light501bwould be coupled into the substrate to propagate at more oblique angles to the ODG140, β2and β3, which may result in a lower output efficiency and/or output uniformity. Instead, in some embodiments, the controller560may tune the IDG530so that its grating pitch pinis reduced, thereby also reducing the maximum angle of propagation within the substrate to β4<β3. In some embodiments, the controller560may reduce the grating pitch pinto a value p2<p1so that the image light501bis coupled into the substrate525in the third angular range (β1, β4), which at least partially overlaps with the first angular range (β1, β2). Simultaneously, the controller560may correspondingly tune the grating pitch poutin a third portion543of the ODG540, so as to convey the second FOV portion10to the eye555substantially without distortion. In some embodiments, the controller560may be configured to tune the input grating pitch so that the image light propagates within the substrate525at angles of incidence β upon the opposing surfaces thereof521,522smaller than 70 degrees for any FOV portion of the target FOV being conveyed. In some embodiments, the controller560may de-activate, i.e. switch off, a fourth portion544of the ODG540which is outside of the second FOV portion10being conveyed to the eye555.

In other embodiments, the lightguide520having in-coupling and out-coupling diffraction gratings with a tunable pitch may be operated to display an image sequentially portion by portion, so that different portions of the image corresponding to different partial FOVs are being displayed at different time instances, each time adjusting the grating pitch synchronously in the IDG530and the ODG540, or at least in some portions thereof, depending on the partial FOV being displayed. When the overall target FOV of the image is being scanned over a sufficiently short time, the sequentially displayed FOV portions of the image are integrated by a visual cortex of the viewer into a single image. Using this method, the overall FOV perceived by the viewer may be enhanced.

Referring toFIG.6, a display apparatus600is configured to present an image to a user in a time-sequential manner, portion by portion. The display apparatus600includes an image projector603disposed to provide image light601to an input region633of a pupil replicating lightguide620shown in a side cross-sectional view. The input region633includes an IDG630having an electrically tunable grating pitch. The lightguide620, which may be an embodiment of the lightguide520described above, includes a substrate625, e.g. a slab of a material that is transparent to visible light, with two opposing outer surfaces621and622for guiding the image light in the substrate in a zig-zag fashion by TIR at the surfaces. An ODG640is disposed in or upon the substrate625in an output region643thereof facing an eyebox650where the eye555of a user is to be located. The output region643includes an ODG640that has an electrically tunable grating pitch and is configured to out-couple, i.e. diffract, a fraction of in-coupled image light out of the substrate625at consecutive incidences, and to direct the out-coupled image light toward the eyebox650. A controller660is electrically connected to each of the IDG630and ODG640for tuning a grating pitch therein. The lightguide620may also incorporate other optical elements, such as e.g. a folding grating, a beam splitter, polarization converter, etc. which are not shown in the figure to avoid clutter.

The image projector603may be e.g. an LC display panel or another pixelated display configured to project images in the form of two-dimensional (2D) image frames. Each image frame is carried by the image light601in an angular domain, spanning a frame FOV605, which is indicated inFIG.6in a cross-section by the plane of the figure, corresponding to the (y, z) plane of a Cartesian coordinate system (x,y,z) indicated inFIG.6.

FIG.7illustrates a 2D view of the FOV605, in projection on the plane of the substrate625, corresponding to the (x,y) plane of a Cartesian coordinate system (x,y,z) indicated inFIG.6. The y-axis direction may correspond e.g. to a “horizontal” direction of the 2D FOV of the display apparatus. In some embodiments, at least one of the input and output regions633,643of lightguide620(FIG.6) may include a second diffraction grating, to support a 2D FOV of the display. In some embodiments, the second diffraction grating may also be tunable in pitch. In some embodiments, at least one of the IDG630and the ODG640may have a 2D grating structure.

The image projector603may project an image frame upon the IDG630in a time multiplexed manner, i.e. portion by portion sequentially in time. Each portion may be projected in a corresponding FOV portion of the frame FOV605for a fraction of a frame duration T, e.g. for a time interval Δt=T/N, to be perceived by the user as a single image spanning the frame FOV605; here N is the number of FOV portions being sequentially transmitted per frame. An image processor670provides image information for each FOV portion to the projector603, e.g. in a digital form, sequentially in time so that the whole frame is displayed over the frame duration T One of the image processor670and the image projector603may also be operatively connected to the controller660to provide information indicative of the FOV portion being displayed, with the controller660being configured to adjust the grating pitch of the IDG630and the ODG640as the FOV portion being displayed changes. Since at each moment in time the lightguide620conveys at most a portion of the full frame FOV605, the frame FOV605may include an angular range that is broader than the angular range of light of the same color band that could be coupled into the substrate632by a diffraction grating with a fixed grating pitch based on the substrate's refractive index.

In an example embodiment illustrated inFIGS.6and7, for each frame the image projector603may project a first FOV portion605aof the frame FOV605during a first half of the frame duration, and may project a second FOV portion605bof the frame FOV605during a second half of the frame duration, with the controller660adjusting the grating pitch of the IDG630and the ODG640accordingly each time the FOV portion changes, to convey the respective first or second FOV portion605to the eyebox650, e.g. as described above with reference toFIG.5. The FOV portions605a,605bpropagate in the substrate625in a partially overlapping angular ranges, also when the FOV portions605a,605bdo not angularly overlap outside of the substrate. In some embodiments, the ODG640may be non-segmented, with the grating pitch being tuned simultaneously along the full length of the grating in the direction of the y-axis.FIG.7illustrates the 2D frame FOV605as seen from the eyebox650. In the illustrated embodiment, the FOV portions605aand605boverlap in a center portion of the frame FOV605; in other embodiments, the FOV portions605aand605bdo not overlap. In some embodiments, the ODG640may be segmented, e.g. as illustrated inFIG.4a. In some embodiment, each segment of the ODG640may be independently tunable in both the grating pitch and the diffraction efficiency. In some embodiments, the diffraction efficiency of a portion of the ODG640that is outside of a FOV portion being transmitted, may be de-activated.

FIG.8illustrates a display apparatus800which may be an embodiment of the display apparatus600in which the image projector603is replaced with a scanning projector803. InFIG.8, elements having the same or similar functions as corresponding elements shown inFIG.6are indicated by same reference numerals and may not be described again. The scanning projector803generates each image frame over the frame duration T by angularly scanning a beam801of image light in 2D, e.g. using one or more scanning reflectors808, across the frame FOV605, responsive to signals from the image processor670. The beam801may be provided by one or more point light sources802, such as laser diodes (LDs) or light emitting diodes (LEDs) (not shown), which may also be controlled by the image processor670in dependence on the image content of the frame being displayed. The controller660may be configured to adjust the grating pitch of the IDG630and the ODG640in synchronization with the scanning projector803, e.g. synchronously with the tilting of the reflector(s)808, in dependence on the tilt angle of the reflector(s)808and a corresponding angular position of the scanned beam801in the frame FOV605. In some embodiments, the controller660may be configured to adjust the grating pitch of the IDG630and the ODG640two or more times during the frame duration in dependence on a FOV portion currently being scanned. In the example embodiment illustrated inFIG.8, the controller660is configured to synchronously adjust the grating pitch of the IDG630and the ODG640three times during the frame duration, when the projector803scans through three FOV portions605a,605b, and605c, respectively. Similarly to the display apparatus600, in some embodiments of the display apparatus800the ODG640may be segmented, e.g. as illustrated inFIG.4A. In some embodiment, each segment may be independently tunable in both the grating pitch and the diffraction efficiency. In some embodiments, the diffraction efficiency may be de-activated in a portion of the ODG640that is outside of a FOV portion being transmitted.

FIG.9provides a non-limiting illustrative example of an operation of a pupil-replicating lightguide having a 2D segmented ODG with switchable grating segments for displaying an image frame FOV portion by FOB portion. The pupil replicating lightguide900includes a slab substrate902of transparent material for guiding light therein by a series of internal reflections from outer surfaces of the slab substrate902. The pupil-replicating lightguide900includes a tunable grating904, e.g. as illustrated inFIG.4A, configured to provide, at any given time, only a portion911,912, . . . ,919of an overall frame FOV910. In some embodiments the tunable grating904may be substantially continuous and have an electrically tunable grating pitch. In some embodiments the tunable grating904may be segmented, with each grating segment having an independently tunable pitch. In some embodiments the tunable grating904may be segmented, with each grating segment having an independently tunable grating efficiency. An image projector including a display panel or a beam scanner, e.g. as described above with reference toFIGS.6and8, may generate the portions911,912, . . . ,919of the frame FOV910in a time-sequential manner, and selected segmented of the switchable segmented grating904may be switched, and/or the grating pitch in at least a segment of the tunable grating904adjusted, in sync with the projector to “place” the portions911,912, . . . ,919at their respective locations in the overall frame FOV910one by one. The visual cortex of the viewer will time-integrate the FOV portions911,912, . . . ,919perceiving the frame FOV910as a single image.

The approach described above enables providing a large field of view display with a relatively low-index lightguide, which can potentially reduce the weight and/or cost of the display combiner. Furthermore for scanning displays, the output grating(s) may be synced with the instant scanning angle to improve the efficiency of light utilization.

Example embodiments described above include pupil-replicating or illuminating lightguides incorporating one or more grating structures that have variable, i.e. switchable or continuously tunable, grating pitch, also referred to as grating period, and/or variable diffraction efficiency. Some of such grating structures may have other tunable parameters, e.g. the blazing angle that defines the orientation of the grating grooves, or fringes, relative to the input/output surfaces of the lightguide. Some of such switchable or tunable gratings include a material with electrically tunable refractive index, such as but not exclusively a liquid crystal (LC) medium.

FIG.10illustrates an example of an electrically tunable diffraction grating1040, as may be used in embodiments described above. In the illustrated example, the electrically tunable diffraction grating1040is in contact with an outer surface of a substrate1025. In some embodiments the electrically tunable diffraction grating1040may be incorporate within the substrate1025. The electrically tunable diffraction grating1040includes a layer1043sandwiched between electrodes1041formed with an optically transparent material, e.g. a glass or plastic substrate coated with indium tin oxide (ITO), the layer having an electrically variable periodic refractive index pattern. A controller1060is configured to vary at least one of the period, the amplitude, and the blazing angle of the refractive index refractive index pattern, e.g. by varying a voltage applied to the electrodes1061.

In some embodiments, the material of layer1043may contain LC medium, in which case the tunable diffraction grating1040may be referred to as an LC grating. The LC medium may include e.g. nematic-type liquid crystals. Nematic liquid crystals may be composed of rod-like molecules that may have non-zero dipole moments and can be approximately aligned by an electrical filed. In another example, the liquid-crystal medium may include cholesteric liquid crystals, in which a molecular stack has a twisted, helical or heliconical structure. The liquid-crystal medium may also include any suitable mixture of nematic liquid crystals, which may have larger, better defined dipole moments and relatively high birefringence, and cholesteric-type liquid crystals, which may have smaller dipole moments but may have the advantage of responding more quickly to changing electric fields. For example, a layer of nematic liquid crystals may be doped with chiral dopants, which may increase the response time of the nematic liquid crystals. The application of an electric field, e.g., by applying a suitable voltage between the electrodes1041, may orient the dipole moments of LC molecules. For example, the application of an electric field to an LC layer, e.g. layer1043, may cause the formation of a molecular orientation pattern of the LC molecules, e.g., for nematic liquid crystals, or may modify an existing orientation pattern of the LC molecules, e.g., for cholesteric liquid crystals.

In some embodiments grating1040may be an LC grating in which the period of the grating pattern is defined e.g. by pattering a photo-sensitive LC alignment layer, or by a surface relief pattern at an interface between the layer1043and a substrate. In the case of tunable surface relief gratings (SRG), LC molecules between the surface relief groves have a different refractive index than the material of the groves. In the absence of the electric field the LC molecules are aligned horizontally, i.e. parallel to the substrate and along the groves, diffracting light polarized along the groves. A voltage applied across the LC layer may align the LC molecules along the electric filed direction, i.e. normally to the layer, thereby substantially eliminating the diffraction.

FIG.11illustrates a tunable LC SRG1100that may be an embodiment of the tunable grating1040. The tunable LC SRG1100includes a first substrate1101supporting a first conductive layer1111and a surface-relief grating structure1104having a plurality of ridges1106extending from the first substrate1101and/or the first conductive layer1111. A second substrate1102is spaced apart from the first substrate1101. The second substrate1102supports a second conductive layer1112. A cell is formed by the first1111and second1112conductive layers. The cell is filled with a LC fluid, forming an LC layer1108. The LC layer1108includes nematic LC molecules1110, which may be oriented by an electric field across the LC layer1108. The electric field may be provided by applying a voltage V to the first1111and second1112conductive layers.

The surface-relief grating structure1104may be polymer-based, e.g. it may be formed from a polymer having an isotropic refractive index npof about 1.5, for example. The LC fluid has an anisotropic refractive index. For light polarization parallel to a director of the LC fluid, i.e. to the direction of orientation of the nematic LC molecules1110, the LC fluid has an extraordinary refractive index ne, which may be higher than an ordinary refractive index noof the LC fluid for light polarization perpendicular to the director. For example, the extraordinary refractive index nemay be about 1.7, and the ordinary refractive index nomay be about 1.5, i.e. matched to the refractive index npof the surface-relief grating structure1104.

When the voltage Vis not applied (left side ofFIG.11), the LC molecules1110are aligned approximately parallel to the grooves of the surface-relief grating structure1104. At this configuration, a linearly polarized light beam1121with e-vector oriented along the grooves of the surface-relief grating structure1104will undergo diffraction, since the surface-relief grating structure1104will have a non-zero refractive index contrast. When the voltage V is applied (right side ofFIG.11), the LC molecules1110are aligned approximately perpendicular to the grooves of the surface-relief grating structure1104. At this configuration, a linearly polarized light beam1121with e-vector oriented along the grooves of the surface-relief grating structure1104will not undergo diffraction because the surface-relief grating structure1104will appear to be index-matched and, accordingly, will have a substantially zero refractive index contrast. For the linearly polarized light beam1121with e-vector oriented perpendicular to the grooves of the surface-relief grating structure1104, no diffraction will occur in either case (i.e. when the voltage is applied and when it is not) because at this polarization of the linearly polarized light beam1121, the surface-relief grating structure1104are index-matched. Thus, the tunable LC surface-relief grating1100can be switched on and off (for polarized light) by controlling the voltage across the LC layer1108. Several such gratings with differing pitch/slant angle/refractive index contrast may be used to switch between several grating configurations.

In some embodiments of the LC surface-relief grating1100, the surface-relief grating structure1104may be formed from an anisotropic polymer with substantially the same or similar ordinary noand extraordinary nerefractive indices as the LC fluid. When the LC director aligns with the optic axis of the birefringent polymer, the refractive index contrast is close to zero at any polarization of impinging light, and there is no diffraction. When the LC director is misaligned with the optic axis of the birefringent polymer e.g. due to application of an external electric field, the refractive index contrast is non-zero for any or most polarizations of the impinging light, and accordingly there is diffraction and beam deflection.

In some embodiments, the grating1040may be a holographic polymer-dispersed liquid crystal (H-PDLC) grating that may be manufactured by causing interference between two coherent laser beams in layer1143, containing a photosensitive monomer/liquid crystal (LC) mixture, between the two electrodes1041having a conductive coating. Upon irradiation, a photoinitiator contained within the mixture initiates a free-radical reaction, causing the monomer to polymerize. As the polymer network grows, the mixture phase separates into polymer-rich and liquid-crystal rich regions. The refractive index modulation between the two phases causes light passing through the layer1143to be scattered in the case of traditional PDLC2 or diffracted in the case of H-PDLC. When an electric field is applied across the cell, the index modulation is removed and light passing through the cell is unaffected. A description of such tunable diffraction gratings, which may be switched on and off, is provided in an article entitled “Electrically Switchable Bragg Gratings from Liquid Crystal/Polymer Composites” by Pogue et al., Applied Spectroscopy, v. 54 No. 1, 2000, which is incorporated herein by reference in its entirety.

In some embodiments, the grating1040may be a polarization volume hologram (PVH) and/or a Pancharatnam—Berry phase (PBP) liquid crystal (LC) grating. Such gratings may be controlled either directly by applying an electric field to the LC layer, or indirectly by providing a serially coupled half-wave plate (HWP). When the electric field is applied to the LC layer, LC molecules are aligned in the electric field, changing effective refractive index, depending on polarization state of the impinging light.

In some embodiments, layer1043of the electrically tunable grating1040may include a flexoelectric LC. LC molecules typically are electrical dipoles having a non-zero dipole moment, which usually do not exhibit spontaneous polarization because of equal probability for the dipoles to point to two opposite directions respectively, but become polarized in an external electric field. However LC molecules that do not have a perfect rod-shaped structure, but have e.g. a bend-shaped or a pear-shaped structure, may exhibit spontaneous polarization, termed flexoelectric polarization or flexoelectric effect. In materials with a low dielectric anisotropy and a non-zero flexoelectric coefficient difference (e1-e3), where e1 and e3 are the splay and bent flexoelectric coefficients, respectively, electric fields exceeding certain threshold values may result in a transition from the homogeneous planar state to a spatially periodic one, producing a diffraction grating in layer1043. The field-induced grating is characterized by rotation of the LC director about the alignment axis in the alignment layer(s) adjacent layer1043, with the wavevector of the grating oriented perpendicular to the initial alignment direction. The rotation sign is defined by both the electric field vector and the sign of the (e1-e3) difference. The wavenumber characterizing the field-induced periodicity is increased linearly with the applied voltage starting from a threshold value of about it/d, where d is the thickness of the layer. Examples of suitable flexoelectric LC materials, and of LC gratings incorporating such materials that may be used in embodiments of the present disclosure, are described in e.g. in an article entitled “Dynamic and Photonic Properties of Field-Induced Gratings in Flexoelectric LC Layers” by Palto in Crystals 2021, 11, 894, and a U.S. Pat. No. 10,890,823, both of which being incorporated herein by reference in their entireties.

In some embodiments, layer1043of the electrically tunable grating1040having a variable grating period or a slant angle may include helical and helicoidal LC. Cholesteric LCs (CLC), which have intrinsic periodicity in the form of the helical supramolecular structure, may be obtained e.g. by doping the nematic LC matrix with chiral components. The LC molecules in the mixture may self-organize into a periodic helically twisted configuration including helical structures extending between the top and bottom surfaces of the LC layer1043. Depending on a type of alignment conditions at the layer surfaces, the helical twist axes of the helical structures may be normal to the surfaces or tilt. The helical structures may form a volume grating that acts as a Bragg grating with the Bragg period equal to one half of the distance P, termed cholesteric pitch, along the helical axis where the LC director and the optic axis rotate by 360°. By varying the applied electrical field, the cholesteric pitch P and thus the Bragg period P/2 of the LC grating may be varied. In some embodiments of LC grating, e.g. some of those including a planar-aligned CLC layer, a diffractive pattern may appear when the applied electric field exceeds a threshold, and may vary in amplitude with the applied field, thereby enabling tuning the diffraction efficiency. In some embodiments a tunable LC grating may include oblique helicoidal LCs, in which the LC director is tilted at an oblique angle to the helical axis. Such LC gratings may have superior tunability because the applied electric field may tune the oblique angles and the pitch lengths in a relatively wide range without disturbing the helical axis orientation. Tunable gratings with oblique helicoidal LCs have been described e.g. in an article entitled “Electrooptic Response of Chiral Nematic Liquid Crystals with Oblique Helicoidal Director” by Xiang et al. Phys. Rev. Lett. 112, 217801, 2014, which is incorporated herein by reference in its entirety.

In at least some embodiments, an LC-based grating1040such as those described above may be polarization selective. Such gratings may selectively diffract a light beam having a first polarization, e.g. linear or circular, but transmit a light beam having a second, typically orthogonal, polarization with negligible diffraction. In such embodiments, the display apparatuses described above may operate with polarized image light, e.g. to enhance the display's efficiency, and/or may include various polarizers and polarization converters, such as e.g. quarter-wave plates (QWP), half-wave plates (HWP), a-plates, and lightguides incorporating the same.

Tunable diffraction gratings other than LC grating may also be used in the example embodiments described above. In some embodiments, switchable/tunable gratings may be formed on a surface of the lightguide by providing a surface acoustic wave as disclosed e.g. in an article entitled “Status of Leaky Mode Holography” by Smalley et al., Photonics 2021, 8, 29, 2 which is incorporated herein by reference in its entirety. Such diffraction gratings may be tunable in both the grating pitch, by tuning the frequency of the acoustic wave, and the diffraction efficiency, by tuning its amplitude.

Diffraction gratings with a tunable/switchable diffraction efficiency may also be implemented as fluidic gratings. A fluidic grating may include two immiscible fluid layers, like water and oil, whose interface deforms when a spatially inhomogeneous electric field is applied. The spatially inhomogeneous electric field may be provided e.g. by using spatially inhomogeneous and/or discrete electrodes.

Referring toFIGS.12A and12B, a fluidic grating1200includes first1201and second1202immiscible fluids separated by an inter-fluid boundary1203. One of the fluids may be a hydrophobic fluid such as oil, e.g. silicone oil, while the other fluid may be water-based. One of the first1201and second1202fluids may be a gas in some embodiments. The first1201and second1202fluids may be contained in a cell formed by first1211and second1212substrates supporting first1221and second1222electrode structures. The first1221and/or second1222electrode structures may be at least partially transparent, absorptive, and/or reflective.

At least one of the first1221and second1222electrode structures may be patterned for imposing a spatially variant electric field onto the1201and second1202fluids. For example, in12A and12B, the first electrode1221is patterned, and the second electrodes1222is not patterned, i.e. the second electrodes1222is a backplane electrode. In the embodiment shown, both the first1221and second1222electrodes are substantially transparent. For example, the first1221and second1222electrodes may be indium tin oxide (ITO) electrodes.

FIG.12Ashows the fluidic grating1200in a non-driven state when no electric field is applied across the inter-fluid boundary1203. When no electric field is present, the inter-fluid boundary1203is straight and smooth; accordingly, a light beam1205impinging onto the fluidic grating1200does not diffract, propagating right through as illustrated.FIG.12Bshows the fluidic grating1200in a driven state when a voltage V is applied between the first1221and second1222electrodes, producing a spatially variant electric field across the first1201and second1202fluids separated by the inter-fluid boundary1203.

The application of the spatially variant electric field causes the inter-fluid boundary1203to distort as illustrated inFIG.12B, forming a periodic variation of effective refractive index, i.e. a surface-relief diffraction grating. The light beam1205impinging onto the fluidic grating1200will diffract, forming first1231and second1232diffracted sub-beams. By varying the amplitude of the applied voltage V, the strength of the fluidic grating1200may be varied. By applying different patterns of the electric field e.g. with individually addressable sub-electrodes or pixels of the first electrode1221, the grating period and, accordingly, the diffraction angle, may be varied. More generally, varying the effective voltage between separate sub-electrodes or pixels of the first electrode1221may result in a three-dimensional conformal change of the fluidic interface i.e. the inter-fluid boundary1203inside the fluidic volume to impart a desired optical response to the fluidic grating1200. The applied voltage pattern may be pre-biased to compensate or offset gravity effects, i.e. gravity-caused distortions of the inter-fluid boundary1203.

Portions of a patterned electrode may be individually addressable. In some embodiments, the first electrode1221may be a continuous, non-patterned electrode coupled to a patterned dielectric layer for creating a spatially non-uniform electric field across the first1201and second1202fluids. Also in some embodiments, the backplane electrode is omitted, and the voltage is applied between the segmented electrodes themselves.

The thickness of the first1221and second1222electrodes may be e.g. between 10 nm and 50 nm. The materials of the first1221and second1222electrodes besides ITO may be e.g. indium zinc oxide (IZO), zinc oxide (ZO), indium oxide (TO), tin oxide (TO), indium gallium zinc oxide (IGZO), etc. The first1201and second1202fluids may have a refractive index difference of at least 0.1, and may be as high as 0.2 and higher. One of the first1201or second1202fluids may include polyphenylether, 1,3-bis(phenylthio)benzene, etc. The first1211and/or second1212substrates may include e.g. fused silica, quartz, sapphire, etc. The first1211and/or second1212substrates may be straight or curved, and may include vias and other electrical interconnects. The applied voltage may be varied in amplitude and/or duty cycle when applied at a frequency of between 100 Hz and 100 kHz. The applied voltage can change polarity and/or be bipolar. Individual first1201and/r second1202fluid layers may have a thickness of between 0.5-5 micrometers, more preferably between 0.5-2 micrometer.

To separate the first1201and second1202fluids, surfactants containing one hydrophilic end functional group and one hydrophobic end functional group may be used. The examples of a hydrophilic end functional group are hydroxyl, carboxyl, carbonyl, amino, phosphate, sulfhydryl. The hydrophilic functional groups may also be anionic groups such as sulfate, sulfonate, carboxylates, phosphates, for example. Non-limiting examples of a hydrophobic end functional group are aliphatic groups, aromatic groups, fluorinated groups. For example, when polyphenyl thioether and fluorinated fluid may be selected as a fluid pair, a surfactant containing aromatic end group and fluronirated end group may be used. When phenyl silicone oil and water are selected as the fluid pair, a surfactant containing aromatic end group and hydroxyl (or amino, or ionic) end group may be used. These are only non-limiting examples.

Referring toFIG.13, example display apparatuses described above may implement a method1300for displaying images to a user within a target field-of-view (FOV), which in one embodiment includes the following steps or operations: (1310) using a lightguide with a tunable diffraction grating associated therewith to convey different portions of the target FOV at different time instances to an eyebox of a display apparatus; and, (1320) tuning at least one of a grating pitch or a diffraction efficiency in the output diffraction grating in dependence on a FOV portion being conveyed to the eyebox.

In some embodiments, the method includes coupling, into the lightguide, image light carrying the images in an angular domain, and using an output diffraction grating to couple different FOV portions of the image light out of the lightguide at different time instances. In some embodiments, the method includes selecting the FOV portion being conveyed depending on an image content. In some embodiments, the method includes selecting the FOV portion being conveyed depending on a location of the image content within the target FOV. In some embodiments, the method includes using only a portion of an output area of the output grating to diffract the image light out of the lightguide depending on the FOV portion being conveyed.

In some embodiments, the method includes operating at least a segment of the output diffraction grating to convey, to the eyebox over a frame time interval, the target FOV sequentially portion by portion, at least once during the frame time interval tuning the at least one of the grating pitch or the diffraction efficiency for the FOV portion being displayed.

In some embodiments, the method includes switching the FOV portions being conveyed in synchronization with FOV portions being displayed by an image projector providing the images to the lightguide. In some embodiments, the method includes using a scanning image projector to provide image light carrying the image to an input diffraction grating of the lightguide by sequentially scanning a beam of the image light through different portions of the target FOV over a frame time interval, the input diffraction grating tunable to couple the beam into the lightguide, and tuning a pitch of each of the input diffraction grating and the at least a segment of the output diffraction grating in synchronization with the scanning.

Example embodiments described above have been described by way of example to assist in better understanding of salient features of their operation, and are capable of many variations and modifications. For example in some cases the example embodiments described above may be effective for monochromatic image light. For color images, the image light of different colors may be spatially and/or temporally multiplexed. In some embodiments, two or more stacked lightguides may be used to guide different color channels. In some embodiments, the grating pitch of the input and output couplers of the same lightguide may be tuned to accommodate different color channels in a time multiplexed manner. In at least some embodiments, more than one diffraction grating may be used to couple light out of the lightguide. Some embodiments may utilize more than one input diffraction grating and/or more than one output diffraction gratings, e.g. to support a 2D FOV. When two or more diffraction gratings are used for the in-coupling or the out-coupling, the gratings may be superimposed to form a 2D grating structure, e.g. at a same outer surface of the lightguide's substrate. In other embodiments, different in-coupling gratings or different out-coupling gratings may be disposed at the opposite outer surfaces of the substrate. Embodiments in which one or more input gratings or one or more output gratings are disposed in the bulk of the substrate are also within the scope of the present disclosure. In embodiments where the image light may get diffracted by N≥2 diffraction gratings with grating vectors ki, i=1, . . . , Nin succession, the grating periods of two or more of the diffraction gratings are adjusted so that the grating vectors ki, i=1, . . . , N, sum to zero, i.e. Σ1Nki=0, for every FOV portion being transmitted.

Embodiments of the present disclosure may include, or be implemented in conjunction with, an artificial reality system. An artificial reality system adjusts sensory information about outside world obtained through the senses such as visual information, audio, touch (somatosensation) information, acceleration, balance, etc., in some manner before presentation to a user. By way of non-limiting examples, artificial reality may include virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include entirely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, somatic or haptic feedback, or some combination thereof. Any of this content may be presented in a single channel or in multiple channels, such as in a stereo video that produces a three-dimensional effect to the viewer. Furthermore, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in artificial reality and/or are otherwise used in (e.g., perform activities in) artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a wearable display such as an HMD connected to a host computer system, a standalone HMD, a near-eye display having a form factor of eyeglasses, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers.

Referring toFIG.14, a virtual reality (VR) near-eye display1400includes a frame1401supporting, for each eye: an image projector1430, e.g. an LC display panel or a scanning projector; a lightguide1410including one or more tunable gratings, e.g. as described above, for relaying image light generated by the image projector1430to an eyebox1412in a dynamically variable FOV. A plurality of eyebox illuminators1406, shown as black dots, may be placed around the lightguide1410on a surface that faces the eyebox1412. An eye-tracking camera1404may be provided for each eyebox1412.

The purpose of the eye-tracking cameras1404is to determine position and/or orientation of both eyes of the user. The eyebox illuminators1406illuminate the eyes at the corresponding eyeboxes1412, allowing the eye-tracking cameras1404to obtain the images of the eyes, as well as to provide reference reflections i.e. glints. The glints may function as reference points in the captured eye image, facilitating the eye gazing direction determination by determining position of the eye pupil images relative to the glints images. To avoid distracting the user with the light of the eyebox illuminators1406, the latter may be made to emit light invisible to the user. For example, infrared light may be used to illuminate the eyeboxes1412.

Turning toFIG.15, an HMD1500is an example of an AR/VR wearable display system which encloses the user's face, for a greater degree of immersion into the AR/VR environment. The HMD1500may generate the entirely virtual 3D imagery. The HMD1500may include a front body1502and a band1504that can be secured around the user's head. The front body1502is configured for placement in front of eyes of a user in a reliable and comfortable manner. A display system1580may be disposed in the front body1502for presenting AR/VR imagery to the user. The display system1580may include any of the display devices, lightguides, and tunable diffraction gratings disclosed herein. Sides1506of the front body1502may be opaque or transparent.

In some embodiments, the front body1502includes locators1508and an inertial measurement unit (IMU)1510for tracking acceleration of the HMD1500, and position sensors1512for tracking position of the HMD1500. The IMU1510is an electronic device that generates data indicating a position of the HMD1500based on measurement signals received from one or more of position sensors1512, which generate one or more measurement signals in response to motion of the HMD1500. Examples of position sensors1512include: 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 IMU1510, or some combination thereof. The position sensors1512may be located external to the IMU1510, internal to the IMU1510, or some combination thereof.

The locators1508are traced by an external imaging device of a virtual reality system, such that the virtual reality system can track the location and orientation of the entire HMD1500. Information generated by the IMU1510and the position sensors1512may be compared with the position and orientation obtained by tracking the locators1508, for improved tracking accuracy of position and orientation of the HMD1500. Accurate position and orientation is important for presenting appropriate virtual scenery to the user as the latter moves and turns in 3D space.

The HMD1500may further include a depth camera assembly (DCA)1511, which captures data describing depth information of a local area surrounding some or all of the HMD1500. The depth information may be compared with the information from the IMU1510, for better accuracy of determination of position and orientation of the HMD1500in 3D space.

The HMD1500may further include an eye tracking system1514for determining orientation and position of user's eyes in real time. The obtained position and orientation of the eyes also allows the HMD1500to determine the gaze direction of the user and to adjust the image generated by the display system1580accordingly. The determined gaze direction and vergence angle may be used to adjust the display system1580to reduce the vergence-accommodation conflict. The direction and vergence may also be used for displays' exit pupil steering as disclosed herein. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided including e.g. a set of small speakers built into the front body1502.