Patent Description:
Optical imaging systems, such as wearable display systems (e.g., wearable display headsets) can include one or more eyepieces that present projected images to a user. Eyepieces can be constructed using thin layers of one or more highly refractive materials. As examples, eyepieces can be constructed from one or more layers of highly refractive glass, silicon, metal, or polymer substrates.

In some cases, an eyepiece can be patterned (e.g., with one or more light diffractive nanostructures) such that it projects an image according to a particular focal depth. For an example, to a user viewing a patterned eyepiece, the projected image can appear to be a particular distance away from the user.

Further, multiple eyepieces can be used in conjunction to project a simulated three-dimensional image. For example, multiple eyepieces-each having a different pattern-can be layered one atop another, and each eyepiece can project a different depth layer of a volumetric image. Thus, the eyepieces can collectively present the volumetric image to the user across three-dimensions. This can be useful, for example, in presenting the user with a "virtual reality" environment.

<CIT> discloses a head-mounted display device comprising a plurality of optical elements in optical communication, the plurality of optical elements being configured, during operation of the head-mounted display device, to project an image in a field of view of a user wearing the head-mounted display device, wherein a first optical element of the plurality of optical elements is configured to receive light from a second optical element of the plurality of optical elements; wherein the first optical element defines a grating at along a periphery of the first optical element, the grating being a metasurface comprising: a plurality of protrusions extending from a base portion of the first optical element, the protrusions comprising a first material having a first optical dispersion profile for visible wavelengths of light, and a second material disposed between at least some of the plurality of protrusions along the base portion of the first optical element, the second material having a second optical dispersion profile for visible wavelengths of light. The metasurface is however highly selective.

The invention is directed to a head-mounted display device includes a plurality of optical elements in optical communication, according to claim <NUM>. The plurality of optical elements is configured, during operation of the head-mounted display device, to project an image in a field of view of a user wearing the head-mounted display device. A first optical element of the plurality of optical elements is configured to receive light from a second optical element of the plurality of optical elements. The first optical element defines a grating at along a periphery of the first optical element. The grating includes a plurality of protrusions extending from a base portion of the first optical element. The protrusions include a first material having a first optical dispersion profile for visible wavelengths of light. The grating also includes a second material disposed between at least some of the plurality of protrusions along the base portion of the first optical element. The second material has a second optical dispersion profile for visible wavelengths of light.

The invention is also directed to a method of constructing a head-mounted display device, according to claim <NUM>, including providing a first optical element including a grating formed along a first surface of the first optical element. The grating includes a plurality of protrusions including a first material having a first optical dispersion profile for visible wavelengths of light, and a second material deposited between at least some of the plurality of protrusions along the first surface of the first optical element. The second material has a second optical dispersion profile for visible wavelengths of light, The method also includes positioning the first optical element in optical communication with a second optical element in the head-mounted display device.

<FIG> illustrates an example wearable display system <NUM> that incorporates a high index material grating. The display system <NUM> includes a display or eyepiece <NUM>, and various mechanical and electronic modules and systems to support the functioning of that display <NUM>. The display <NUM> may be coupled to a frame <NUM>, which is wearable by a display system user <NUM> and which is configured to position the display <NUM> in front of the eyes of the user <NUM>. The display <NUM> may be considered eyewear in some embodiments. In some embodiments, a speaker <NUM> is coupled to the frame <NUM> and is positioned adjacent the ear canal of the user <NUM>. The display system may also include one or more microphones <NUM> to detect sound. The microphone <NUM> can allow the user to provide inputs or commands to the system <NUM> (e.g., the selection of voice menu commands, natural language questions, etc.), and/or can allow audio communication with other persons (e.g., with other users of similar display systems). The microphone <NUM> can also collect audio data from the user's surroundings (e.g., sounds from the user and/or environment). In some embodiments, the display system may also include a peripheral sensor <NUM> a, which may be separate from the frame <NUM> and attached to the body of the user <NUM> (e.g., on the head, torso, an extremity, etc.). The peripheral sensor <NUM> a may acquire data characterizing the physiological state of the user <NUM> in some embodiments.

The display <NUM> is operatively coupled by a communications link <NUM>, such as by a wired lead or wireless connectivity, to a local data processing module <NUM> which may be mounted in a variety of configurations, such as fixedly attached to the frame <NUM>, fixedly attached to a helmet or hat worn by the user, embedded in headphones, or removably attached to the user <NUM> (e.g., in a backpack-style configuration or in a belt-coupling style configuration). Similarly, the sensor <NUM> a may be operatively coupled by communications link <NUM> b (e.g., a wired lead or wireless connectivity) to the local processor and data module <NUM>. The local processing and data module <NUM> may include a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory or a hard disk drive), both of which may be utilized to assist in the processing, caching, and storage of data. The data may include data <NUM>) captured from sensors (which may be, e.g., operatively coupled to the frame <NUM> or otherwise attached to the user <NUM>), such as image capture devices (e.g., cameras), microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, gyros, and/or other sensors disclosed herein; and/or <NUM>) acquired and/or processed using a remote processing module <NUM> and/or a remote data repository <NUM> (including data relating to virtual content), possibly for passage to the display <NUM> after such processing or retrieval. The local processing and data module <NUM> may be operatively coupled by communication links <NUM>, <NUM>, such as via a wired or wireless communication links, to the remote processing module <NUM> and the remote data repository <NUM> such that these remote modules <NUM>, <NUM> are operatively coupled to each other and available as resources to the local processing and data module <NUM>. In some embodiments, the local processing and data module <NUM> may include one or more of the image capture devices, microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. In some other embodiments, one or more of these sensors may be attached to the frame <NUM>, or may be standalone devices that communicate with the local processing and data module <NUM> by wired or wireless communication pathways.

The remote processing module <NUM> may include one or more processors to analyze and process data, such as image and audio information. In some embodiments, the remote data repository <NUM> may be a digital data storage facility, which may be available through the internet or other networking configuration in a "cloud" resource configuration. In some embodiments, the remote data repository <NUM> may include one or more remote servers, which provide information (e.g., information for generating augmented reality content) to the local processing and data module <NUM> and/or the remote processing module <NUM>. In other embodiments, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module.

The perception of an image as being "three-dimensional" or "<NUM>-D" may be achieved by providing slightly different presentations of the image to each eye of the user. <FIG> illustrates a conventional display system for simulating three-dimensional image data for a user. Two distinct images <NUM>, <NUM>-one for each eye <NUM>, <NUM>-are output to the user. The images <NUM>, <NUM> are spaced from the eyes <NUM>, <NUM> by a distance <NUM> along an optical or z-axis that is parallel to the line of sight of the user. The images <NUM>, <NUM> are flat and the eyes <NUM>, <NUM> may focus on the images by assuming a single accommodated state. Such <NUM>-D display systems rely on the human visual system to combine the images <NUM>, <NUM> to provide a perception of depth and/or scale for the combined image.

However, the human visual system is complicated and providing a realistic perception of depth is challenging. For example, many users of conventional "<NUM>-D" display systems find such systems to be uncomfortable or may not perceive a sense of depth at all. Objects may be perceived as being "three-dimensional" due to a combination of vergence and accommodation. Vergence movements (e.g., rotation of the eyes so that the pupils move toward or away from each other to converge the respective lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with focusing (or "accommodation") of the lenses of the eyes. Under normal conditions, changing the focus of the lenses of the eyes, or accommodating the eyes, to change focus from one object to another object at a different distance will automatically cause a matching change in vergence to the same distance, under a relationship known as the "accommodation-vergence reflex," as well as pupil dilation or constriction. Likewise, under normal conditions, a change in vergence will trigger a matching change in accommodation of lens shape and pupil size. As noted herein, many stereoscopic or "<NUM>-D" display systems display a scene using slightly different presentations (and, so, slightly different images) to each eye such that a three-dimensional perspective is perceived by the human visual system. Such systems can be uncomfortable for some users, however, since they simply provide image information at a single accommodated state and work against the "accommodation-vergence reflex. " Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional image data.

<FIG> illustrates aspects of an approach for simulating three-dimensional image data using multiple depth planes. With reference to <FIG>, the eyes <NUM>, <NUM> assume different accommodated states to focus on objects at various distances on the z-axis. Consequently, a particular accommodated state may be said to be associated with a particular one of the illustrated depth planes <NUM>, which has an associated focal distance, such that objects or parts of objects in a particular depth plane are in focus when the eye is in the accommodated state for that depth plane. In some embodiments, three-dimensional image data may be simulated by providing different presentations of an image for each of the eyes <NUM>, <NUM>, and also by providing different presentations of the image corresponding to multiple depth planes. While the respective fields of view of the eyes <NUM>, <NUM> are shown as being separate for clarity of illustration, they may overlap, for example, as distance along the z-axis increases. In addition, while the depth planes are shown as being flat for ease of illustration, it will be appreciated that the contours of a depth plane may be curved in physical space, such that all features in a depth plane are in focus with the eye in a particular accommodated state.

The distance between an object and an eye <NUM> or <NUM> may also change the amount of divergence of light from that object, as viewed by that eye. <FIG> illustrate relationships between distance and the divergence of light rays. The distance between the object and the eye <NUM> is represented by, in order of decreasing distance, R1, R2, and R3. As shown in <FIG>, the light rays become more divergent as distance to the object decreases. As distance increases, the light rays become more collimated. Stated another way, it may be said that the light field produced by a point (the object or a part of the object) has a spherical wavefront curvature, which is a function of how far away the point is from the eye of the user. The curvature increases with decreasing distance between the object and the eye <NUM>. Consequently, at different depth planes, the degree of divergence of light rays is also different, with the degree of divergence increasing with decreasing distance between depth planes and the user's eye <NUM>. While only a single eye <NUM> is illustrated for clarity of illustration in <FIG> and other figures herein, it will be appreciated that the discussions regarding the eye <NUM> may be applied to both eyes <NUM> and <NUM> of a user.

A highly believable simulation of perceived depth may be achieved by providing, to the eye, different presentations of an image corresponding to each of a limited number of depth planes. The different presentations may be separately focused by the user's eye, thereby helping to provide the user with depth cues based on the amount of accommodation of the eye required to bring into focus different image features for the scene located on different depth planes and/or based on observing different image features on different depth planes being out of focus.

<FIG> illustrates an example of a waveguide stack for outputting image information to a user in an AR eyepiece. A display system <NUM> includes a stack of waveguides, or stacked waveguide assembly, <NUM> that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some embodiments, the display system <NUM> is the system <NUM> of <FIG>, with <FIG> schematically showing some parts of that system <NUM> in greater detail. For example, the waveguide assembly <NUM> may be part of the display <NUM> of <FIG>. It will be appreciated that the display system <NUM> may be considered a light field display in some embodiments.

The waveguide assembly <NUM> may also include a plurality of features <NUM>, <NUM>, <NUM>, <NUM> between the waveguides. In some embodiments, the features <NUM>, <NUM>, <NUM>, <NUM> may be one or more lenses. The waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or the plurality of lenses <NUM>, <NUM>, <NUM>, <NUM> may be configured to send image information to the eye with various levels of wavefront curvature or light ray divergence. Each waveguide level may be associated with a particular depth plane and may be configured to output image information corresponding to that depth plane. Image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may function as a source of light for the waveguides and may be utilized to inject image information into the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, each of which may be configured, as described herein, to distribute incoming light across each respective waveguide, for output toward the eye <NUM>. Light exits an output surface <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of each respective image injection device <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and is injected into a corresponding input surface <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the respective waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some embodiments, the each of the input surfaces <NUM>, <NUM><NUM>, <NUM>, <NUM>, <NUM> may be an edge of a corresponding waveguide, or may be part of a major surface of the corresponding waveguide (that is, one of the waveguide surfaces directly facing the world <NUM> or the user's eye <NUM>). In some embodiments, a beam of light (e.g., a collimated beam) may be injected into each waveguide and may be replicated, such as by sampling into beamlets by diffraction, in the waveguide and then directed toward the eye <NUM> with an amount of optical power corresponding to the depth plane associated with that particular waveguide. In some embodiments, a single one of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be associated with, and inject light into, a plurality (e.g., three) of the waveguides <NUM>, <NUM>, <NUM>, <NUM>,<NUM>.

In some embodiments, the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are discrete displays that each produce image information for injection into a corresponding waveguide <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, respectively. In some other embodiments, the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are the output ends of a single multiplexed display which may transmit image information via one or more optical conduits (such as fiber optic cables) to each of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. It will be appreciated that the image information provided by the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may include light of different wavelengths, or colors.

In some embodiments, the light injected into the waveguides <NUM>, <NUM>,
<NUM>, <NUM>, <NUM> is provided by a light projector system <NUM>, which includes a light module <NUM>, which may include a light source or light emitter, such as a light emitting diode (LED). The light from the light module <NUM> may be directed to, and modulated by, a light modulator <NUM> (e.g., a spatial light modulator), via a beamsplitter (BS) <NUM>. The light modulator <NUM> may spatially and/or temporally change the perceived intensity of the light injected into the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. Examples of spatial light modulators include liquid crystal displays (LCD), including a liquid crystal on silicon (LCOS) displays, and digital light processing (DLP) displays.

In some embodiments, the light projector system <NUM>, or one or more components thereof, may be attached to the frame <NUM> (<FIG>). For example, the light projector system <NUM> may be part of a temporal portion (e.g., ear stem <NUM>) of the frame <NUM> or disposed at an edge of the display <NUM>. In some embodiments, the light module <NUM> may be separate from the BS <NUM> and/or light modulator <NUM>.

In some embodiments, the display system <NUM> may be a scanning fiber display comprising one or more scanning fibers to project light in various patterns (e.g., raster scan, spiral scan, Lissajous patterns, etc.) into one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and ultimately into the eye <NUM> of the user. In some embodiments, the illustrated image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may schematically represent a single scanning fiber or a bundle of scanning fibers configured to inject light into one or a plurality of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some other embodiments, the illustrated image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may schematically represent a plurality of scanning fibers or a plurality of bundles of scanning fibers, each of which are configured to inject light into an associated one of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. One or more optical fibers may transmit light from the light module <NUM> to the one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. In addition, one or more intervening optical structures may be provided between the scanning fiber, or fibers, and the one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to, for example, redirect light exiting the scanning fiber into the one or more waveguides <NUM>,<NUM>,<NUM>,<NUM>,<NUM>.

A controller <NUM> controls the operation of the stacked waveguide assembly <NUM>, including operation of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, the light source <NUM>, and the light modulator <NUM>. In some embodiments, the controller <NUM> is part of the local data processing module <NUM>. The controller <NUM> includes programing (e.g., instructions in a non-transitory medium) that regulates the timing and provision of image information to the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some embodiments, the controller may be a single integral device, or a distributed system connected by wired or wireless communication channels. The controller <NUM> may be part of the processing modules <NUM> or <NUM> (<FIG>) in some embodiments.

The waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to propagate light within each respective waveguide by total internal reflection (TIR). The waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may each be planar or have another shape (e.g., curved), with major top and bottom surfaces and edges extending between those major top and bottom surfaces. In the illustrated configuration, the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may each include out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> that are configured to extract light out of a waveguide by redirecting the light, propagating within each respective waveguide, out of the waveguide to output image information to the eye <NUM>. Extracted light may also be referred to as out-coupled light and the out-coupling optical elements light may also be referred to light extracting optical elements. An extracted beam of light may be output by the waveguide at locations at which the light propagating in the waveguide strikes a light extracting optical element. The out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be, for example, diffractive optical features, including diffractive gratings, as discussed further herein. While the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are illustrated as being disposed at the bottom major surfaces of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, in some embodiments they may be disposed at the top and/or bottom major surfaces, and/or may be disposed directly in the volume of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, as discussed further herein. In some embodiments, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be formed in a layer of material that is attached to a transparent substrate to form the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some other embodiments, the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be a monolithic piece of material and the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be formed on a surface and/or in the interior of that piece of material.

Each waveguide <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may output light to form an image corresponding to a particular depth plane. For example, the waveguide <NUM> nearest the eye may deliver collimated beams of light to the eye <NUM>. The collimated beams of light may be representative of the optical infinity focal plane. The next waveguide up <NUM> may output collimated beams of light which pass through the first lens <NUM> (e.g., a negative lens) before reaching the eye <NUM>. The first lens <NUM> may add a slight convex wavefront curvature to the collimated beams so that the eye/brain interprets light coming from that waveguide <NUM> as originating from a first focal plane closer inward toward the eye <NUM> from optical infinity. Similarly, the third waveguide <NUM> passes its output light through both the first lens <NUM> and the second lens <NUM> before reaching the eye <NUM>. The combined optical power of the first lens <NUM> and the second lens <NUM> may add another incremental amount of wavefront curvature so that the eye/brain interprets light coming from the third waveguide <NUM> as originating from a second focal plane that is even closer inward from optical infinity than was light from the second waveguide <NUM>.

The other waveguide layers <NUM>, <NUM> and lenses <NUM>, <NUM> are similarly configured, with the highest waveguide <NUM> in the stack sending its output through all of the lenses between it and the eye for an aggregate focal power representative of the closest focal plane to the person. To compensate for the stack of lenses <NUM>, <NUM>, <NUM>, <NUM> when viewing/interpreting light coming from the world <NUM> on the other side of the stacked waveguide assembly <NUM>, a compensating lens layer <NUM> may be disposed at the top of the stack to compensate for the aggregate optical power of the lens stack <NUM>, <NUM>, <NUM>, <NUM> below. Such a configuration provides as many perceived focal planes as there are available waveguide/lens pairings. Both the out-coupling optical elements of the waveguides and the focusing aspects of the lenses may be static (i.e., not dynamic or electro-active). In some alternative embodiments, either or both may be dynamic using electro-active features.

In some embodiments, two or more of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may have the same associated depth plane. For example, multiple waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may output images set to the same depth plane, or multiple subsets of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may output images set to the same plurality of depth planes, with one set for each depth plane. This can provide advantages for forming a tiled image to provide an expanded field of view at those depth planes.

The out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to both redirect light out of their respective waveguides and to output this light with the appropriate amount of divergence or collimation for a particular depth plane associated with the waveguide. As a result, waveguides having different associated depth planes may have different configurations of out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, which output light with a different amount of divergence depending on the associated depth plane. In some embodiments, the light extracting optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be volumetric or surface features, which may be configured to output light at specific angles. For example, the light extracting optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be volume holograms, surface holograms, and/or diffraction gratings. In some embodiments, the features <NUM>, <NUM>, <NUM>, <NUM> may not be lenses; rather, they may simply be spacers (e.g., cladding layers and/or structures for forming air gaps).

In some embodiments, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are diffractive features with a diffractive efficiency sufficiently low such that only a portion of the power of the light in a beam is re-directed toward the eye <NUM> with each interaction, while the rest continues to move through a waveguide via TIR. Accordingly, the exit pupil of the light module <NUM> is replicated across the waveguide to create a plurality of output beams carrying the image information from light source <NUM>, effectively expanding the number of locations where the eye <NUM> may intercept the replicated light source exit pupil. These diffractive features may also have a variable diffractive efficiency across their geometry to improve uniformity of light output by the waveguide.

In some embodiments, one or more diffractive features may be switchable between "on" states in which they actively diffract, and "off states in which they do not significantly diffract. For instance, a switchable diffractive element may include a layer of polymer dispersed liquid crystal in which microdroplets form a diffraction pattern in a host medium, and the refractive index of the microdroplets may be switched to substantially match the refractive index of the host material (in which case the pattern does not appreciably diffract incident light) or the microdroplet may be switched to an index that does not match that of the host medium (in which case the pattern actively diffracts incident light).

In some embodiments, a camera assembly <NUM> (e.g., a digital camera, including visible light and IR light cameras) may be provided to capture images of the eye <NUM>, parts of the eye <NUM>, or at least a portion of the tissue surrounding the eye <NUM> to, for example, detect user inputs, extract biometric information from the eye, estimate and track the gaze direction of the eye, to monitor the physiological state of the user, etc. In some embodiments, the camera assembly <NUM> may include an image capture device and a light source to project light (e.g., IR or near-IR light) to the eye, which may then be reflected by the eye and detected by the image capture device. In some embodiments, the light source includes light emitting diodes ("LEDs"), emitting in IR or near-IR. In some embodiments, the camera assembly <NUM> may be attached to the frame <NUM> (<FIG>) and may be in electrical communication with the processing modules <NUM> or <NUM>, which may process image information from the camera assembly <NUM> to make various determinations regarding, for example, the physiological state of the user, the gaze direction of the wearer, iris identification, etc. In some embodiments, one camera assembly <NUM> may be utilized for each eye, to separately monitor each eye.

<FIG> illustrates an example of exit beams output by a waveguide. One waveguide is illustrated (with a perspective view), but other waveguides in the waveguide assembly <NUM> (<FIG>) may function similarly. Light <NUM> is injected into the waveguide <NUM> at the input surface <NUM> of the waveguide <NUM> and propagates within the waveguide <NUM> by TIR. Through interaction with diffractive features, light exits the waveguide as exit beams <NUM>. The exit beams <NUM> replicate the exit pupil from a projector device which projects images into the waveguide. Any one of the exit beams <NUM> includes a sub-portion of the total energy of the input light <NUM>. And in a perfectly efficient system, the summation of the energy in all the exit beams <NUM> would equal the energy of the input light <NUM>. The exit beams <NUM> are illustrated as being substantially parallel in <FIG> but, as discussed herein, some amount of optical power may be imparted depending on the depth plane associated with the waveguide <NUM>. Parallel exit beams may be indicative of a waveguide with out-coupling optical elements that out-couple light to form images that appear to be set on a depth plane at a large distance (e.g., optical infinity) from the eye <NUM>. Other waveguides or other sets of out-coupling optical elements may output an exit beam pattern that is more divergent, as shown in <FIG>, which would require the eye <NUM> to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a distance closer to the eye <NUM> than optical infinity.

Additional information regarding wearable display systems (e.g., including optical elements used in wearable display systems) can be found in <CIT>, and entitled "EYEPIECES FOR AUGMENTED REALITY DISPLAY SYSTEM,".

As noted above, wearable display system <NUM> includes one or more optical elements having one or more grating structures that enhance an optical performance of the wearable display system. As an example, one or more optical elements forming an eyepiece of the wearable display system <NUM>, such as the waveguide stack shown in <FIG>, can include gratings structures defined along their peripheries (e.g., along an interface between an optical element and another optical element, or along an interface between an optical element and air, such as out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), and formed from one or more high index materials, such as such as titanium dioxide (TiO<NUM>), silicon carbide (SiC), and/or lithium niobate (LiNbO<NUM>). In particular, the differential dispersion of these materials can be used to achieve a uniform diffraction efficiency across the visible spectrum. This can be beneficial, for example, in fabricating of single layer eyepieces (e.g., for use in the wearable display system) that can display a high-quality multi-color image (e.g., a red-green-blue image) having high color uniformity over a wide field of view. For instance, referring to <FIG>, each of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> can be configured to send image information to the eye according to multiple wavelengths of light (e.g., corresponding to a red-green-blue image).

In general, high refractive index substrates (such as LiNbO<NUM> or SiC) offer the possibility of multiplexing the full red-green-blue (RGB) spectrum onto a single layer substrate. However, the use of certain types of grating structures, such as binary or blazed structures, can result poor eyepiece performance, as such grating structures may impart color-selective properties onto the eyepiece. For instance, the grating structures may cause shorter wavelengths of light to be diffracted more efficiently compared to longer wavelengths of light. This effect can be undesirable in some circumstances. For example, this effect may make it more difficult to obtain a good color balance or uniformity in single layer RGB eyepiece while maintaining an acceptable eyepiece efficiency.

However, as described herein, the differential dispersion of certain high index materials can be used to achieve uniform diffraction efficiency across the visible spectrum. This enables the fabrication of efficient single layer RGB eyepieces exhibiting good color balance and/or uniformity that may not be achievable through other techniques. For example, in some cases, an eyepiece can be formed from wavelength selective volume holographic materials (e.g., instead of using the techniques described herein), but they may have field of view limitations stemming from their lower refractive indices (e.g., around <NUM>).

In general, some materials can have similar refractive indices at a specific wavelength λ<NUM>, while exhibiting different dispersions (e.g., refractive index with respect to wavelength). As a result, light crossing the interface between these materials will not be affected at that specific wavelength λ<NUM>, while being affected at other wavelengths. If the interface includes a grating structure, the diffraction efficiency of that grating structure will be close to zero around wavelength λ<NUM>, because there would be no phase modulation. However, light would be diffracted at other wavelengths, with a diffraction efficiency that is proportional to the refractive index difference between the materials.

As an example, <FIG> shows, in cross-section, an example grating structure <NUM> defined at an interface between a first optical element 702a (having a refractive index of n<NUM>) and a second optical element 702b (having a different refractive index n<NUM>). As the diffraction efficiency of that grating structure <NUM> is close to zero at or around wavelength λ<NUM>, light 704a having a wavelength λ<NUM> passes through the grating structure <NUM> with little or no diffraction. However, light 704a having a different wavelength λ<NUM> is diffracted as it passes through the grating structure <NUM>, with a diffraction efficiency that is proportional to the difference in refractive index between the two materials (e.g., the difference between n<NUM> and n<NUM>).

The spectral response of the grating structure is dictated, at least in part, by the refractive indices and the dispersion properties of the materials used to form the grating structure. Accordingly, a particular spectral response of the grating structure can be achieved by selecting certain materials (e.g., having certain refractive indices and the dispersion properties) to form the grating structure.

Further, spectral response of the grating structure is dictated, at least in part, by the physical dimensions of the gratings (e.g., their height, width, periodicity, duty cycle, etc.). Accordingly, a particular spectral response of the grating structure can be achieved by further forming gratings having certain dimensions using the selected materials.

As an embodiment of the invention, <FIG> shows, in cross-section, a single unit <NUM> of a grating structure <NUM>. The unit <NUM> can repeat one or more times periodically along a periphery of an optical element (e.g., at an interface between the optical element and another optical element, or along an interface between the optical element and air). The unit <NUM> includes a base portion <NUM> composed of a first material, and protrusion <NUM> composed of the first material and extending from the base portion <NUM>. The unit <NUM> also includes filling portions <NUM> composed of a second material different from the first material, and disposed on the base portion <NUM> along opposing sides of the protrusion <NUM>. As shown in <FIG> and <FIG>, the unit <NUM> repeats periodically along the periphery of an optical element <NUM>, forming the grating structure <NUM> (e.g., a "binary" grating in filling portions disposed between each protrusion). In the example shown in <FIG>, the optical element <NUM> receives light through the grating structure <NUM> (e.g., from a light source <NUM>), and light <NUM> incident upon the grating structure <NUM> is diffracted as it enters the optical element <NUM>. In the example shown in <FIG>, the optical element <NUM> emits light <NUM> through the grating structure <NUM> (e.g., towards another optical element, into the air, and/or towards a user's eye), and light incident upon the grating structure <NUM> is diffracted as it exits the optical element <NUM>. In some implementations, one or more of the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (e.g., as shown in <FIG>) can include a respective grating struvtuce <NUM>. In some other instantiations such as those used in waveguide displays, the light to be coupled out propagates through the substrate by total internal reflection (TIR) and is extracted from the waveguide by the grating structure.

Referring back to <FIG>, the unit <NUM> has a cross-sectional width wt (e.g., corresponding to the periodicity of the unit <NUM>), and a cross-sectional height ht (e.g., corresponding to the maximal height of the unit <NUM>). Further, the protrusion <NUM> has a cross-sectional width w<NUM> and a cross-sectional height h<NUM> (e.g., the difference in height between the top surface of the protrusion <NUM> and the top surface of the base portion <NUM>). The base portion <NUM> has a cross-sectional width wt, and a cross-sectional height h<NUM>. Each filler portion <NUM> has a cross-sectional width w<NUM>, and a cross-sectional height h<NUM>.

Each of the parameters wt, w<NUM>. w<NUM>, ht, h<NUM>, h<NUM>, h<NUM>, and h<NUM>, can be selected to impart certain optical properties with respect the grating structure. Further, the materials of the base portion <NUM>, protrusion <NUM>, and filling portions <NUM> also can be selected (e.g., based at least in part on their respective refractive indices and the dispersion properties) to impart certain optical properties with respect the grating structure. In particular, if the repeating period of the grating structures is sufficiently small (e.g., between <NUM> and <NUM>), the extraction efficiency of light propagating by total interface reflection (TIR) within the substrate can be controlled with respect to wavelength.

In some implementations, these parameters can be selected such that the grating structure exhibits a diffraction efficiency that is uniform or more uniform over a particular range of incident angles of light and with respect to particular wavelengths of light (e.g., compared to grating structures designed using techniques different from those described herein). As an example, the base portion <NUM> and the protrusion <NUM> can be formed from SiC (e.g., through a deposition and etching process), and the filling portions <NUM> can be formed from TiO<NUM> (e.g., through a deposition process, such as sputtering). In some cases, the refractive index of the SiC portions can be between <NUM> and <NUM>, and the refractive index of the TiO2 portions can be between <NUM> and <NUM>. Further, the grating structure can be formed such that w<NUM> is equal to or approximately equal to <NUM> (e.g., between <NUM> and <NUM>), w<NUM> is equal to or approximately equal to <NUM> (e.g., between <NUM> and <NUM>), h<NUM> is equal to or approximately equal to <NUM> (e.g., between <NUM> and <NUM>), h<NUM> is equal to or approximately equal to <NUM> (e.g., between <NUM> and <NUM>), wt is equal to or approximately equal to <NUM> (e.g., between <NUM> and <NUM>. It should be noted that the numbers given in the example above correspond to the out-coupling of TIR-guided light within the substrate.

<FIG> shows the angular response of a binary grating structure etched in SiC (e.g., without any filling portions of TiO<NUM> deposited between the protrusions of the grating structure) with respect to three different wavelengths of light (red, green and blue). <FIG> shows the angular response of a similar binary grating structure etched in SiC—but also having filling portions of TiO<NUM> deposited between the protrusions of the grating structure (e.g., as shown in FIGS. 5A-5C)-with respect to the same three wavelengths of light (red, green and blue). As shown in <FIG> and <FIG>, the addition of the filling portions of TiO<NUM> increases a diffraction efficiency of the grating structure across a range of incident angles of light (e.g., from -<NUM>° to <NUM>°) with respect to each of the different wavelengths of light. Accordingly, light incident on the grating structures described herein exhibit are less likely to exhibit color-dependent or incident angle-dependent diffraction characteristics.

As described herein, optical elements having the grating structures described herein may be particularly suitable for use as eyepieces in a wearable display headset. For example, a wearable display headset may be configured to display multicolored images (e.g., RGB images). Accordingly, one or more optical elements of the wearable display headset (e.g., the eyepiece and/or any other optical elements) can be formed having the grating structures described herein, such that they are less likely to exhibit color-dependent or incident angle-dependent diffraction characteristics. This can facilitate the display of multi-color images with improved uniformity (e.g., with respect to light intensity) over a wide field of view. For instance, referring to <FIG>, one of the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be diffractive optical features, including the diffractive gratings described herein.

As an example, <FIG> shows an intensity map of light emitted by an eyepiece including a binary grating structure etched in SiC (e.g., without any filling portions of TiO<NUM> deposited between the protrusions of the grating structure, as described with respect to <FIG>). <FIG> shows an intensity map of light emitted by a similar binary grating structure etched in SiC-but also having filling portions of TiO<NUM> deposited between the protrusions of the grating structure (e.g., as described with respect to <FIG> and <FIG>). As shown in <FIG>, the addition of the filling portions of TiO<NUM> increases the uniformity of projected light. For example, the eyepiece of <FIG> exhibits localized bands of high-intensity light (e.g., a C-shaped artifact of higher-intensity light surrounded by regions of lower-intensity light). In contrast, the eyepiece of <FIG> exhibits a more uniform light intensity pattern.

Although example parameters and materials are described here, these are merely illustrative examples. In practice, one or more parameters may differ, depending on the implementation.

Further, different materials can be used other than those described above with respect to <FIG>, <FIG>, <FIG>, <FIG>, and 1000B. As an example, in some implementations, the base portion and the protrusions of a grating structure can be composed of SiC, LiNbO<NUM>, or a combination thereof. Further, the filling portions can be composed of TiO<NUM>. In some implementations, the principles described herein can be applied to other combination of materials in which the waveguide substrate exhibits a higher refractive index and a lower dispersion than the coating material. Examples include diamond/LiNbO<NUM> and diamond/SrTiO<NUM> systems.

In some implementations, the refractive index of a material may vary, depending on the manner in which the material is deposited (e.g., on an underlying substrate). As an example, the refractive index of a sputtered layer of titanium dioxide can be varied between <NUM> and <NUM> by changing the deposition parameters, such as the temperature and/or the pressure at which the materials were sputtered onto the underlying material. Accordingly, the spectral response of a grating structure can be "tuned" by changing the deposition conditions of one or more materials used to define the grating structure.

As an example, <FIG> shows the refractive index curves of crystalline SiC and TiO<NUM> deposited according to the atomic deposition (ALD) technique. Each of these materials exhibits a refractive index that varies with respect to the incident wavelength of light (e.g., defining particular refractive index curves). These refractive index curves can be modified, at least in part, by varying the deposition parameters of each of the materials (e.g., the temperature and/or pressure when the materials are sputtered onto a substrate or other structure).

In the embodiments shown in <FIG>, each repeating unit of the grating includes a protrusion that is rectangular in cross-section (e.g., forming a binary grating). However, this need not be the case. For instance, in some implementations, each unit of the grating can include differently shaped protrusions. As an example, as shown in <FIG>, each unit of a grating can include a protrusion that is rectangular in cross-section (e.g., a isosceles triangle as shown in <FIG>, a right triangle as shown in <FIG>, etc.). In practice, any other grating configurations also can be used, depending on the implementation. It should be noted that the technique is also applicable to two-dimensional diffractive lattices such as two-dimensional arrays of rods, squares, or pyramids.

<FIG> shows an embodiment of method <NUM> for constructing a head-mounted display device using the optical elements and grating structures described herein.

According to the process <NUM>, a first optical element is provided (step <NUM>). The first optical element includes a grating formed along a first surface of the first optical element. The grating includes plurality of protrusions including a first material having a first optical dispersion profile for visible wavelengths of light, and a second material deposited between at least some of the plurality of protrusions along the first surface of the first optical element. The second material has a second optical dispersion profile for visible wavelengths of light. Example first optical elements are shown and described with respect to <FIG>.

In some implementations, the second material can be titanium dioxide (TiO<NUM>). In some implementation, the first material can be silicon carbide (SiC) or lithium niobate (LiNbO<NUM>).

In some implementations, the grating can be formed by etching a plurality of channels onto the first optical element along the first surface. Each channel can have a first depth. Further, the second material can be depositied between at least some of the plurality of protrusions along the first surface. An example of this configuration is shown, for example, in <FIG>.

In some implementations, each channel can have a substantially rectangular cross-section. In some implementations, each channel can have a substantially equal width (e.g., approximately <NUM>).

In some implementations, depositing the second material can include depositing the second material into at least some of the channels.

In some implementations, depositing the second material can include sputtering the second material into at least some of the channels. The second material can be sputtered at different temperatures and/or pressures to varying the optical properties (e.g., the refractive index) of the material.

In some implementations, the second material can be deposited such that it extends a first height within the channel.

In some implementations, the first depth can be greater than the first height. As an example, the first depth can be approximately <NUM>, and the first height can be approximately <NUM>.

In some implementations, the grating can be formed according to a period along a length of the first surface. As an example, the period can correspond to a length of approximately <NUM>.

Further, the first optical element is positioned in optical communication with a second optical element in the head-mounted display device (step <NUM>). Example configurations of a first optical element and a second optical element in a head-mounted display device are shown and described with respect to <FIG> and <FIG>.

Some implementations of subject matter and operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. For example, in some implementations, the local processing and data module <NUM>, the remote processing module <NUM>, and/or the remote data repository <NUM> can be implemented using digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations of one or more of them. In another example, the process <NUM> shown in FIG. <NUM> can be implemented, at least in part, using digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations of one or more of them (e.g., as a part of an automated or computer-assisted manufacturing process).

Some implementations described in this specification can be implemented as one or more groups or modules of digital electronic circuitry, computer software, firmware, or hardware, or in combinations of one or more of them. Although different modules can be used, each module need not be distinct, and multiple modules can be implemented on the same digital electronic circuitry, computer software, firmware, or hardware, or combination thereof.

Some implementations described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. A computer storage medium can be, or can be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially generated propagated signal.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages.

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and processors of any kind of digital computer. A computer includes a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. A computer may also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices (e.g., EPROM, EEPROM, flash memory devices, and others), magnetic disks (e.g., internal hard disks, removable disks, and others), magneto optical disks, and CD ROM and DVD-ROM disks.

A computer system may include a single computing device, or multiple computers that operate in proximity or generally remote from each other and typically interact through a communication network. Examples of communication networks include a local area network ("LAN") and a wide area network ("WAN"), an inter-network (e.g., the Internet), a network comprising a satellite link, and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). A relationship of client and server may arise by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

<FIG> shows an example computer system <NUM> that includes a processor <NUM>, a memory <NUM>, a storage device <NUM> and an input/output device <NUM>. Each of the components <NUM>, <NUM>, <NUM> and <NUM> can be interconnected, for example, by a system bus <NUM>. The processor <NUM> is capable of processing instructions for execution within the system <NUM>. In some implementations, the processor <NUM> is a single-threaded processor, a multi-threaded processor, or another type of processor. The processor <NUM> is capable of processing instructions stored in the memory <NUM> or on the storage device <NUM>. The memory <NUM> and the storage device <NUM> can store information within the system <NUM>.

In some implementations, the input/output device <NUM> can include one or more of a network interface device, e.g., an Ethernet card, a serial communication device, e.g., an RS-<NUM> port, and/or a wireless interface device, e.g., an <NUM> card, a <NUM> wireless modem, a <NUM> wireless modem, etc. In some implementations, the input/output device can include driver devices configured to receive input data and send output data to other input/output devices, e.g., keyboard, printer and display devices <NUM>. In some implementations, mobile computing devices, mobile communication devices, and other devices can be used.

While this specification contains many details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features that are described in this specification in the context of separate implementations can also be combined. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple embodiments separately or in any suitable subcombination.

Claim 1:
A head-mounted display device (<NUM>) comprising:
a plurality of optical elements (<NUM>) in optical communication, the plurality of optical elements being configured, during operation of the head-mounted display device, to project an image in a field of view of a user (<NUM>) wearing the head-mounted display device (<NUM>),
wherein a first optical element (<NUM>) of the plurality of optical elements is configured to receive light from a second optical element of the plurality of optical elements;
wherein the first optical element (<NUM>) defines a grating (<NUM>) at along a periphery of the first optical element (<NUM>), the grating (<NUM>) comprising:
a plurality of protrusions (<NUM>) extending from a base portion (<NUM>) of the first optical element (<NUM>), the protrusions (<NUM>) comprising a first material having a first optical dispersion profile for visible wavelengths of light, and each of the protrusions (<NUM>) comprising a first surface opposite the base portion (<NUM>) of the first optical element (<NUM>), and
a second material (<NUM>) disposed between at least some of the plurality of protrusions (<NUM>) along the base portion (<NUM>) of the first optical element (<NUM>), the second material (<NUM>) having a second optical dispersion profile for visible wavelengths of light,
wherein there is an absence of any additional material on the first surfaces of the protrusions (<NUM>).