Diffractive grating coherence control

Abstract

By slightly varying a pitch and / or direction of a grating structure continuously and non-uniformly across a waveguide diffractive grating, the distance traveled by a portion of the display light along a first path through the grating differs from the distance traveled by the remainder of the display light along a second path through the grating such that the amount of phase accumulated by the respective portions of display light differs. The grating tunes the phase difference between the two paths traveled by the portions of display light by spatially modulating the grating structure. The grating thus reliably controls constructive and destructive interference for the light scattering pathways and improves the overall grating efficiency.

Description

DIFFRACTIVE GRATING COHERENCE CONTROL BACKGROUND

[0001] The present disclosure relates generally to augmented reality (AR) eyewear, which fuses a view of the real world with a heads-up display overlay. Wearable display devices, which include wearable heads-up displays (WHUDs) and head- mounted display (HMD) devices (all of which may be used interchangeably herein), are wearable electronic devices that combine real world and virtual images via one or more optical combiners, such as one or more integrated combiner lenses, to provide a virtual display that is viewable by a user when the wearable display device is worn on the head of the user. One class of optical combiner uses a waveguide (also termed a lightguide) to transfer light. In general, light from a projector of the wearable display device enters the waveguide of the optical combiner through an incoupler, propagates along the waveguide, and exits the waveguide through an outcoupler. If the pupil of the eye is aligned with one or more exit pupils provided by the outcoupler, at least a portion of the light exiting through the outcoupler will enter the pupil of the eye, thereby enabling the user to see a virtual image. Since the combiner lens is transparent, the user will also be able to see the real world. SUMMARY OF EMBODIMENTS

[0002] In accordance with some embodiments, a method includes varying a first total phase accumulated by a first portion of display light along a first path through a diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide. The method further includes modulating constructive and destructive interference of the first portion of display light and the second portion of display light based on a difference between the first total phase and the second total phase.

[0003] In some embodiments, varying the first total phase from the second total phase includes continuously and non-uniformly varying a grating pitch across a grating of the diffractive waveguide such that a second order derivative of thevariance is non-zero. In some embodiments, varying includes continuously and non- uniformly varying a grating direction across a grating of the diffractive waveguide.

[0004] The grating is a one-dimensional exit pupil expander in some embodiments. In some embodiments in which the grating is a one-dimensional exit pupil expander, the difference between the first total phase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

[0005] In some embodiments, the grating is a two-dimensional exit pupil expander and outcoupler and the method also includes coupling the first portion and the second portion of display light out of the diffractive waveguide, such that the difference between the first total phase and the second total phase results in a phase difference range of approximately pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

[0006] A net pointing error between the first path and the second path is approximately zero in some embodiments.

[0007] In accordance with another embodiment, a diffractive waveguide includes a grating configured to vary a first total phase accumulated by a first portion of display light along a first path through the diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide. The grating is further configured to modulate constructive and destructive interference of the first portion of display light and the second portion of display light based on a difference between the first total phase and the second total phase.

[0008] In some embodiments, the grating includes a continuously and non-uniformly varying pitch across the grating, wherein a second order derivative of the variance is non-zero. In some embodiments, the grating includes a continuously and non- uniformly varying grating direction across the grating.

[0009] The grating may be a one-dimensional exit pupil expander. In some embodiments, the grating is configured to couple the first portion and the second portion of display light out of the diffractive waveguide, wherein the difference between the first total phase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

[0010] The grating may be a two-dimensional exit pupil expander and outcoupler. In such embodiments, the grating may be configured to couple the first portion and the second portion of display light out of the diffractive waveguide, wherein the difference between the first total phase and the second total phase results in a phase difference of pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

[0011] In some embodiments, a net pointing error between the first path and the second path is approximately zero.

[0012] In another embodiment, a diffractive waveguide includes a grating having a continuous and non-uniformly varying pitch across the grating to vary a first total phase accumulated by a first portion of display light along a first path through the diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide, wherein a second order derivative of the variance is non-zero.

[0013] In some embodiments, the grating has a continuous and non-uniformly varying grating direction across the grating.

[0014] The grating may be a one-dimensional exit pupil expander. In some embodiments in which the grating is a one-dimensional exit pupil expander, the grating is configured to couple the first portion and the second portion of display light out of the diffractive waveguide, wherein a difference between the first total phase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

[0015] In some embodiments, the grating is a two-dimensional exit pupil expander and outcoupler. In some such embodiments, the grating is configured to couple the first portion and the second portion of display light out of the diffractive waveguide, wherein a difference between the first total phase and the second total phase results in a phase difference of pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

[0016] In some embodiments, a net pointing error between the first path and the second path is approximately zero. In some embodiments, the continuous and non- uniformly varying pitch compensates for a total thickness variation profile across the diffractive waveguide. In some embodiments, a wafer containing diffractive waveguides comprises a plurality of dies, wherein each die of the plurality of dies comprises a grating having a continuous and non-uniformly varying pitch across the grating, wherein a second order derivative of the variance is non-zero, such that the pitch variation is different for different dies to compensate for an expected thickness profile within each die location. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items.

[0018] FIG.1 is a diagram illustrating a rear perspective view of an augmented reality display device implementing diffractive grating coherence control in accordance with some embodiments.

[0019] FIG.2 is a k-space diagram and a corresponding x-space diagram for components of a waveguide implementing diffractive grating coherence control in accordance with some embodiments.

[0020] FIG.3 is a diagram illustrating respective paths of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in accordance with some embodiments.

[0021] FIG.4 is a side view of a grating structure in accordance with some embodiments.

[0022] FIG.5 illustrates a conventional grating structure and a grating structure having a continuously and non-uniformly varying pitch and direction in accordance with some embodiments.

[0023] FIG.6 is a k-space diagram and a corresponding x-space diagram for components of a two-dimensional grating implementing diffractive grating coherence control in accordance with some embodiments.

[0024] FIG.7 is a diagram illustrating respective paths of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in accordance with some embodiments.

[0025] FIG.8 is a diagram illustrating respective paths of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in accordance with some embodiments.

[0026] FIG.9 is a diagram illustrating respective paths of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in accordance with some embodiments.

[0027] FIG.10 is a flow diagram illustrating a method for controlling coherence of light traveling along different paths through a diffractive grating in accordance with some embodiments. DETAILED DESCRIPTION

[0028] A diffractive waveguide typically includes an incoupler (IC), an exit pupil expander (EPE), and an outcoupler (OC). In some cases, the waveguide includes anIC and a two-dimensional combined EPE / OC. The optical beam cross section at a light source (i.e., display engine) of an AR eyewear display device is typically smaller than the cross section of display light that is guided to an eyebox for viewing by a user at the user’s pupil position. The EPE expands the optical beam cross section by diffracting a portion of the display light each time the light hits a grating structure of the EPE, thus replicating the exit pupil multiple times to achieve a larger eyebox. The EPE allows the remainder of the display light pass through or go through total internal reflection (TIR).

[0029] In a conventional EPE, light entering the waveguide from the display engine travels along two intersecting paths having identical lengths. The identical lengths of the intersecting paths cause light traveling along each path to accumulate the same amount of phase, resulting in constructive interference along a first direction at a first intersection and destructive interference along a second direction at the first intersection and reducing the overall efficiency of the EPE. Further, because the two paths have identical lengths, the interference occurs for all wavelengths of display light.

[0030] FIGs.1-10 illustrate techniques for controlling coherence of display light propagating through a diffractive waveguide to modulate constructive and destructive interference of portions of the display light. By slightly varying the waveguide thickness continuously and non-uniformly across the EPE, the distance traveled by a portion of the display light along a first path differs from the distance traveled by the remainder of the display light along a second path such that the amount of phase accumulated by the respective portions of display light differs. The EPE tunes the phase difference between the two paths traveled by the portions of display light by spatially modulating the grating structure. The EPE thus reliably controls constructive and destructive interference for the light scattering pathways and improves the overall grating efficiency.

[0031] In some embodiments, the grating pitch of the EPE varies continuously and non-uniformly across the entire EPE. Thus, the distances between adjacent grating structures of the EPE are spatially shifted by small amounts that vary continuouslyacross the EPE such that portions of display light that traverse different regions of the EPE accumulate different phases. The varying phases that accumulate for each path traveled by the portions of display light are referred to as the “detour phase”. In some embodiments, the detour phase is achieved by continuously and non-uniformly modulating the pitch of the grating, the direction of the grating, or both the pitch and direction of the grating.

[0032] In some embodiments, the detour phase is applied to a one-dimensional EPE and results in an additional phase difference of π between the portions of display light traveling along the different paths such that constructive interference occurs for light directed sideways toward the OC and destructive interference occurs for light directed downward. In other embodiments, the detour phase is applied to a two-dimensional combined EPE and OC and results in an additional phase difference of π between the portions of display light traveling along the different paths. Such a phase difference is used to cause destructive interference for light traveling toward a world-side of an AR display device.

[0033] Conventional fabrication methods for diffractive waveguides include making a master with high precision lithography tools and then replicating the pattern. Fabrication of a diffractive waveguide with such a detour phase does not add additional cost, as the change of the grating pitch and direction can be readily incorporated into making the master and does not require any additional processing steps.

[0034] Variation of the grating pitch can lead to a change in the direction of the light beam, referred to as a pointing error, which can degrade the image quality if the pointing error exceeds a threshold amount. To reduce or eliminate the pointing error, the continuous and non-uniform spatial shift of regions of the grating are kept small (on the order of a fraction of a wavelength to a few multiples of a wavelength) in comparison to the size of the grating, which is on the order of millimeters. Further in some embodiments, a pointing error introduced by a detour phase of an IC is canceled or compensated by a pointing error in the opposite direction introduced bythe detour phase of other gratings of the waveguide, such as an EPE and OC or a two-dimensional combined EPE / OC.

[0035] FIG.1 illustrates an example AR eyewear display system 100 implementing diffractive grating coherence control in accordance with some embodiments. The AR eyewear display system 100 includes a support structure 102 (e.g., a support frame) to mount to a head of a user and that includes an arm 104 that houses a laser projection system, micro-display (e.g., micro-light emitting diode (LED) display), or other light engine configured to project display light representative of images toward the eye of a user, such that the user perceives the projected display light as a sequence of images displayed in a field of view (FOV) area 106 at one or both of lens elements 108, 110 supported by the support structure 102. In some embodiments, the support structure 102 further includes various sensors, such as one or more front- facing cameras, rear-facing cameras, other light sensors, motion sensors, accelerometers, and the like. The support structure 102 further can include one or more radio frequency (RF) interfaces or other wireless interfaces, such as a Bluetooth(TM) interface, a WiFi interface, and the like.

[0036] The support structure 102 further can include one or more batteries or other portable power sources for supplying power to the electrical components of the AR eyewear display system 100. In some embodiments, some or all of these components of the AR eyewear display system 100 are fully or partially contained within an inner volume of support structure 102, such as within the arm 104 in region 112 of the support structure 102. In the illustrated implementation, the AR eyewear display system 100 utilizes an eyeglasses form factor. However, the AR eyewear display system 100 is not limited to this form factor and thus may have a different shape and appearance from the eyeglasses frame depicted in FIG.1.

[0037] One or both of the lens elements 108, 110 are used by the AR eyewear display system 100 to provide an AR display in which rendered graphical content can be superimposed over or otherwise provided in conjunction with a real-world view as perceived by the user through the lens elements 108, 110. For example, laser light or other display light is used to form a perceptible image or series of images that areprojected onto the eye of the user via one or more optical elements, including a waveguide, formed at least partially in the corresponding lens element. One or both of the lens elements 108, 110 thus includes at least a portion of a waveguide that routes display light received by an incoupler (IC) (not shown in FIG.1) of the waveguide to an outcoupler (OC) (not shown in FIG.1) of the waveguide, which outputs the display light toward an eye of a user of the AR eyewear display system 100. Additionally, the waveguide employs an exit pupil expander (EPE) (not shown in FIG.1) in the light path between the IC and OC, or in combination with the OC, in order to increase the dimensions of the display exit pupil. Each of the lens elements 108, 110 is sufficiently transparent to allow a user to see through the lens elements to provide a field of view of the user’s real-world environment such that the image appears superimposed over at least a portion of the real-world environment.

[0038] In some embodiments, one or more of the IC, OC, and / or EPE use diffractive waveguide facets either to diffract light from one surface of the waveguide back to the same surface or to allow light to travel through the facets from one surface of the waveguide to a different, opposing surface of the waveguide. In order to improve efficiency of the EPE, the pitch and / or grating direction of the grating structure of the EPE varies continuously and non-uniformly from one region of the grating structure to the next to introduce a “detour phase” by which light traveling along different paths through the EPE accumulates different phases such that constructive and destructive interference of the light at intersections of the paths is controlled.

[0039] FIG.2 illustrates a normalized k-space representation 200 of the display light propagating through a waveguide and an x-space diagram 220 for a waveguide including an IC 222, an EPE 224, and an OC 226 that experience constructive and destructive interference at intersections of portions of light traveling along different paths through the EPE 224. The k-space diagram is a tool used in optical design to represent directions of light rays that propagate within a waveguide. In the k-space representation 200, an inner refractive boundary 202 is depicted as a circle with radius of n=1, the refractive index associated with the external transmission medium(air). An outer refractive boundary 204 corresponds to an effective refractive index of the medium of the waveguide.

[0040] In the context of the k-space representation 200, for RGB display light to be successfully and accurately directed to an eye of a user via a waveguide with the indicated refractive index, each red, green, and blue component of that display light enters the waveguide system from an external position 206, which is included in the space depicted within inner refractive boundary 202. The color components are directed along one or more paths within the waveguide via total internal reflection (TIR) (light that undergoes TIR within the waveguide resides in the space depicted between inner refractive boundary 202 and outer refractive boundary 204) and are then redirected to exit the waveguide (and thereby return to the external space within inner refractive boundary 202 within which light does not undergo TIR). Display light components represented between the inner refractive boundary 202 and outer refractive boundary 304 are propagated to the user via the waveguide. Any display light components represented outside the outer refractive boundary 204 (of which there are none in the k-space representation 200) are non-propagating and cannot exist.

[0041] Initially, display light entering the waveguide at the incoupler forms an image that is centered at or around the origin of the k-space representation 200. The image is initially disposed at a first position 206 with respect to k-space. Upon redirection of the display light by the incoupler, the image is shifted in k-space to a second position 208, corresponding to a shift in the negative kyand kxdimensions. Upon redirection of the display light by the exit pupil expander, the image is shifted in k-space to a third position 210, corresponding to a shift in the positive kydimension and the negative kx dimension. Upon redirection of the display light by the outcoupler, the image is shifted in k-space back to the first position 206, corresponding to a shift in the positive kx dimension. In the present example, it is assumed that the angle at which the display light enters the waveguide system via the incoupler is the same as or substantially the same as (e.g., within 5% of) the angle at which the display light exits the waveguide via the outcoupler.

[0042] The k-space diagram 200 shows the angles at which light is coupled into the waveguide and the x-space diagram 220 illustrates the sizes and locations of the IC 222, the EPE 224 and the OC 226 with respect to a lens 230. The arrows within the EPE 224 indicate propagation directions of portions of light along various paths through the EPE 224.

[0043] FIG.3 is a diagram 300 corresponding to the arrows within the EPE 224 of FIG.2 illustrating respective paths of a first portion of display light and a second portion of display light through an exit pupil expander grating of a conventional diffractive waveguide. As display light is guided through the EPE 224, portions of the display light traverse different paths, such as path 302 and path 304. At each node at which the paths intersect (e.g., nodes 306, 308, 310, 312), the portions of display light intersect and experience constructive interference if the portions of display light have the same phase. The spatial displacement between each of the nodes 306, 308, 310, 312 is denoted r1320 and r2322.

[0044] In a conventional diffractive grating having a uniform grating structure across the area of the EPE 224, the same phase will accumulate for light traveling along path 302 and path 304, resulting in constructive interference for all wavelengths at the intersections of path 302 and path 304. Thus, for example at node 312, the display light will tend to go in a downward direction rather than sideways, toward the OC 226. The purpose of the EPE 224 is to redirect the light sideways into the OC 226, and the constructive interference causing the display light to deflect downward reduces the efficiency of the EPE 224.

[0045] FIG.4 is a side view of a one-dimensional (1D) grating structure 400 in accordance with some embodiments. In some embodiments, the grating structure 400 is included in an EPE such as EPE 224. The grating structure 400 includes a substrate 402 and a grating 404 having diffractive facets that diffract incident display light. In the illustrated example, a beam of incident display light is represented by a vector kin408, which hits the grating 404 and is scattered into a new direction with a vector kout 410. The scattering is characterized by an amplitude (which corresponds to the efficiency of the scattering) and a phase. If the light beam reflects at the sameangle as the angle of incidence (e.g., if kin 408 = kout 410 in a lateral dimension), then the phase value does not depend on the lateral displacement of the grating Δs.

[0046] However, if the beam of incident display light is deflected by (i.e., has its direction changed by) the grating 404, then kin 408 ് kout 410, and an additional phase value can be introduced. By laterally shifting portions of the grating 404 with respect to each other along a plane of the grating 404 by a displacement denoted as lateral shift ∆s 406, the amplitude of the scattering does not change, but the net response of light will have a non-zero phase shift. Thus, shifting the grating 404 shifts the interference condition for the paths 302, 304 for deflected light beams as follows: Δφ = (kin - kout)^Δs

[0047] If the magnitude of the lateral shift ∆s 406 is small but spatially non-uniform, the grating 404 maintains the same structure locally, but different regions of the grating 404 will produce different phases of scattered light, referred to herein as a detour phase. Although in the illustrated example the grating structure 400 includes a blazed reflective grating 404, in other embodiments the detour phase is applied to any type of diffraction grating.

[0048] In some embodiments, the grating 404 is shifted by a fraction of a wavelength to a few multiples of a wavelength (e.g., 15 nm – 1 micron), which is relatively small compared to the overall size of the grating 404, which is on the order of millimeters. The relatively small lateral shift ∆s 406 of portions of the grating with respect to each other changes the phase conditions of the two paths 302, 304. In particular, the shift ∆s 406 introduces a phase difference that affects the coherence of light that diffracts off different regions of the grating structure 400. In some embodiments, the lateral shift ∆s 406 is non-uniform but continuous across the grating structure 400 such that different regions of the grating structure 400 have different lateral shifts without any abrupt changes from one region to the next. In some embodiments, the grating structure 400 is shifted laterally only at the nodes (e.g., nodes 306, 308, 310, 312).

[0049] The phase change between path 302 and path 304 (Δφpath1 - Δφpath2) for a spatially varying displacement s(r) is calculated as follows: Δφpath1 = φds (r1) + φsd (r2) + φdd(r4) = (k1 - k2) ^ s(r1) + (k2 - k1) ^ s(r2) Δφpath2 = φdd (r1) + φds(r3) + φsd(r4) = (k1 - k2) ^ s(r3) + (k2 - k1) ^ s(r4) Δφpath1 - Δφpath2 = (k1 - k2) ^(s(r1) - s(r2) - s(r3) + s(r4)) = (k1 - k2) ^Δr1 ^ Δr2 ^ ∇∇s

[0050] where Δr1 and Δr2 denote the spatial displacement from r1320 and r2322 and S is the detour profile. S is a vector field defined at every point on the two- dimensional space. φds(r1) is the additional detour phase r1 from "downward" to "sideways", φsd(r2) is the additional detour phase at r2 from "sideways" to "downward", φds(r3) is the additional detour phase r3 from "downward" to "sideways", and φdd(r4) is the additional detour phase at r4 from "downward" to "downward", which is always 0. The direction is important because the actual phase value depends on kin and kout, which is also represented as (k1-k2) and (k2-k1). r1, r2, r3, and r4are the physical locations of the four nodes during propagation of light. r1, r2, r3, and r4 differ from each other by one bounce spacing in some embodiments. s(r1), s(r2), s(r3), and s(r4) are the local displacements of the gratings at the four locations r1, r2, r3, r4. In some embodiments, each of s(r1), s(r2), s(r3), and s(r4) are different from each other and can be considered a displacement field. If ∆ ^^ is on the order of 1, the phase difference between path 302 and path 304 changes from a default of zero for a nominally flat waveguide to ^^ / 2. This can be achieved if ∇∇s is non zero, that is, if s is designed to be a at least a second-order functionThus, every loop in the 2D space will have the same non-zero phase difference. A number of solutions satisfy this requirement, with smaller shifts resulting in smaller pointing errors and larger shifts resulting in larger pointing errors, or changes of chief ray angle, which can degrade image quality.

[0051] If the lateral shift ∆s 406 is in the direction of ^ ^^^െ ^^ଶ^ ൌ ∆ ^^, which is also the grating direction, and has a non-zero second order derivative in the Δr1 ^ Δr2 direction, then the phase change between path 302 and path 304 Δφpath1 - Δφpath2 will have a non-zero value across the grating structure 400. An example of spatial displacement that satisfies this requirement is^^ ൌ ∆ ^^ / |∆ ^^| ∙ ^^ ∙ ^^ ∙ ^^ where C is a constant and ^^ ∙ ^^ is a spatial profile indicating the amount by which each point of the grating 404 is shifted in the x and y directions. In this case, Δφpath1 - Δφpath2 = ^^ |∆ ^^| ൫∆ ^^^௫∆ ^^ଶ௬^ ∆ ^^^௬∆ ^^ଶ௫൯ is also a constant,

[0052] FIG.5 illustrates a conventional grating structure 500 and a grating structure 510 having a continuously and non-uniformly varying pitch and direction in accordance with some embodiments. The conventional grating structure 500 exhibits a uniform pitch and direction of the grating across the length and width of the grating structure 500.

[0053] In contrast, the grating structure 510 has a continuous and non-uniform lateral shift of the pitch and direction of the grating across the length and width of the grating structure 510. The amount of the shift is exaggerated in the grating structure 510 to illustrate the resulting grating shift, which appears to curve the gratings. In the illustrated example, a second order spatial variation of the shift results in small grating pitch variation and grating direction variation across the entire grating.

[0054] In some embodiments, a relatively large s(r) is applied to the grating structure such that a light source with moderate bandwidth (e.g., approximately 30 nm full width half max (FWHM)), will experience phase differences for a unit interference loop (of which there may be on the order of hundreds or thousands across a grating structure) for different wavelengths that span the entire range of [0,2π]. Thus, maxλ[φpath1- φpath2] - minλ[φpath1- φpath2] ~ 2π In such embodiments, different wavelengths undergo constructive or destructive interference such that the net average result is similar to incoherent propagation. In this case, C ≈ 10-3mm-1.

[0055] In other embodiments, a smaller s(r) is applied to the grating structure, such that avgλ[φpath1- φpath2] ~ π In this case, the average constructive interference for downward scattering is transformed into destructive interference, and C ≈ 10-5to 10-4mm-1.

[0056] In practice, not all waveguides are uniformly flat such that φpath1- φpath2= 0. Rather, a waveguide may have non-uniformities such as total thickness variations (TTVs) that result in φpath1- φpath2≠ 0. If the non-uniformities are relatively large but reproducible from sample to sample and measurable, a map of s(r) can be designed in some embodiments with the waveguide non-uniformities included as the baseline. Thus, in some embodiments, the detour phase is designed to compensate for the expected TTV profile. Further, in some embodiments, a different detour phase is designed for different dies across a wafer to compensate for different expected TTVs across the different dies.

[0057] In some embodiments, rather than employing a separate 1D EPE and 1D OC, a diffractive waveguide includes a 2D EPE that also serves as an OC by scattering the light outside the waveguide back into the user’s eye. Some light, however, will also be scattered out of the waveguide through a world-facing side. Leakage of light through the world side of the waveguide degrades the overall display efficiency and also poses privacy concerns, as observers can see displayed content intended for a user of the AR eyewear display system. The ratio of the amount of light scattered through the world side of the waveguide to the amount of light directed to the eye of the user is referred to as the worldside-to-eyeside ratio. Use of the detour phase can improve the worldside-to-eyeside ratio.

[0058] FIG.6 is a k-space diagram 600 and a corresponding x-space diagram 620 for components of display light traveling through a waveguide that includes a two- dimensional grating 624 providing combined EPE / OC functionality and implementing diffractive grating coherence control in accordance with some embodiments. As shown in the k-space diagram 600, display light enters the waveguide from anexternal position 606 through an IC 622, which is included in the space depicted within the inner refractive boundary 602. The IC 622 produces one k-vector 608 and directs the light inside the waveguide. A 2D combined EPE / OC 624 produces two k- vectors 610, 612, changing the direction of light within the waveguide and diffracting the light into air to produce an outcoupling function, such that the color components are directed along two or more paths within a volume the space depicted between inner refractive boundary 602 and outer refractive boundary 604 and are then redirected to exit the waveguide and return to the external space within inner refractive boundary 602.

[0059] Thus, the k-space diagram 600 illustrates the angles at which light is coupled into the waveguide and the x-space diagram 620 illustrates the sizes and locations of an IC 622, and the combined EPE / OC 624 with respect to a lens 630. The arrows within the combined EPE / OC 624 indicate propagation directions of portions of light along various paths through the combined EPE / OC 624.

[0060] FIG.7 is a diagram 700 corresponding to the arrows within the combined EPE / OC 624 of FIG.6 illustrating respective paths of a first portion of display light and a second portion of display light through a 2D exit pupil expander / outcoupler grating of a conventional diffractive waveguide. As display light is guided through the combined EPE / OC 624, portions of the display light traverse different paths, such as path 702 and path 704. At each node at which the paths intersect (e.g., nodes 706, 708, 710, 712), the portions of display light intersect and experience constructive interference if the portions of display light have the same phase. The spatial displacement between each of the nodes 706, 708, 710, 712 is denoted r1720 and r2 722.

[0061] In a conventional diffractive grating having a uniform grating structure across the area of a 2D combined EPE / OC, a fixed phase difference will accumulate for light traveling along path 702 and path 704, resulting in some interference at the intersections of path 702 and path 704 (i.e., at node 712). The exact value of the phase difference depends on the grating design. By spatially shifting the nodes 706, 708, 710, 712 to introduce a phase difference between path 702 and path 704, the2D combined EPE / OC can cause destructive interference of display light going toward the worldside of the AR display device and / or constructive interference of display light going toward the user’s eye. Because the display light has multiple paths along which it can travel through the 2D combined EPE / OC 624, destructive interference toward the worldside does not necessarily coincide with constructive interference toward the eyeside.

[0062] Light traveling along path 702 and path 704 starts at the same point (node 704) and ends at the same spatial point (node 712) in the same direction (going to the worldside of the AR display device). The exact phase difference between path 702 and path 704 depends on the design of the grating for the 2D combined EPE / OC 624, and the wavelength, polarization, and incident angle of the display light. Using the detour phase, the grating of the 2D combined EPE / OC 624 can reliably change the phase difference between light traveling along the two paths 702, 704 as follows: Δφpath1 = φ12(r1)+φ21(r2 )+φ13(r4)=(k1 - k2) ^s(r1)+(k2 - k1) ^s(r2)+(k1 - k3) ^s(r4) Δφpath2 = φ11(r1)+φ12(r3)+φ23(r4)=(k1 - k1) ^s(r1)+(k1 - k2) ^s(r3)+(k2 - k3) ^s(r4) Δφpath1 - Δφpath2 = (k1 - k2) ^(s(r1)-s(r2)-s(r3)+s(r4))=(k1 - k2) ^Δr1 ^ Δr2 ^∇∇s In some embodiments, the phase difference between the two paths 702, 704 is designed to be π. If the phase difference between the two paths 702, 704 is π, light traveling along the two paths will destructively interfere at the node 712, and thus significantly decrease scattering to the world-side.

[0063] The spatially non-uniform shift of the gratings for the detour phase introduces a pointing error, which refers to grating vector change that results in a difference between the actual beam direction and the intended beam direction. If the change in the chief ray angle is different for different beam paths, the pointing error results in a degradation of image sharpness, negatively impacting the user experience.

[0064] The pointing error can be kept relatively small (i.e., below a threshold level that is visually detectable) in some embodiments by keeping s(r) small. In otherembodiments, the detour phase is implemented in each of the grating components of a waveguide (i.e., the IC, the EPE, and the OC, or the IC and 2D combined EPE / OC) such that a pointing error introduced at the IC is corrected or compensated by a pointing error introduced at the EPE and OC or the 2D combined EPE / OC. For example, after applying the detour phase, the unit lattice ^^^^^→ ^^^^^^ ^^ ^^^^^^ ^^^, ^^ ^^^^^^ ^^^ ൌ ^^ ∙ ^^ ^^^ ^^^, and the reciprocal lattice changes by ^^^^^→ ^^^^^^ ^^ ^^^^^^ ^^^, ^^ ^^^^^^ ^^^ ൌ െ ^^ ^^^ ^^^ ∙ ^^, where a(i)is the ith unit vector; δa(i)is the difference of the unit vector due to applying the grating perturbation; b(i)is the ith reciprocal vector (in k space). δb(i)is the difference of the reciprocal vector due to applying the grating perturbation.

[0065] For example, referring back to FIG.3, the portions of light traveling along the respective paths 302, 304 through the grating have a same starting point (at node 306) and a same endpoint (at node 312) in accordance with some embodiments. The pointing error difference between path 302 and path 304 is ൫^^ ^^^ோ^1^െ ^^ ^^^ோ^2^൯ െ ^ ^^ ^^^ோ^3^െ ^^ ^^^ோ^4^^~ ^^ ^^ ^^ ^^^ோ~ ^^ଷ^^^ ^^, ^^^Thus, if sthen its third order derivative is net zero, and there is no point error between light paths that start and end at the same points.

[0066] FIG.8 is a diagram 800 illustrating respective paths 802, 804 of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in which the paths have a same starting point but different endpoints within a pupil 806 in accordance with some embodiments. In the illustrated example, replications of the same input light are directed simultaneously to a single exit pupil. Due to the change of the grating vector across the pupil 806, the pointing error will be non-zero. However, the pointing error is proportional to the size of the pupil (i.e., on the order of several mm) rather than the size of the OC or EPE, which are on the order of 10s of mm. Thus, the pointing error will be sufficiently small to not significantly degrade the image quality.

[0067] FIG.9 is a diagram 900 illustrating respective paths 902, 904 of a first portion of display light and a second portion of display light through a grating of a diffractive waveguide in which the paths 902, 904 have different starting and end points in accordance with some embodiments. For two adjacent paths 902, 904, ^ ^^ ^^ ∙ ^^^ ^^^^^ൌ െ ^^ ^^ ∙ ^^ ^^ ^^^ ^^^ ∙ ^^^^^.

[0068] If two paths aredistance is the same at every scattering node. The total pointing error between the two paths 902, 904 is therefore proportional to ^^ ^^ ∙ ^^ ^^ ^^^ ^^^ ∙ ^∑ ∆ ^^^. If the detour phase covers the three waveguide components of the IC 222, EPE 224 and OC 226, then∑∆ ^^^ ൌ 0. Thus, for such embodiments, there is no net pointing error.

[0069] FIG.10 is a flow diagram illustrating a method 1000 for controlling coherence of light traveling along different paths through a diffractive grating in accordance with some embodiments. In some embodiments, the method is practiced at a waveguide employing either an IC, EPE, and OC, such as the IC 222, EPE 224, and OC 226 of FIG.3, or an IC and a 2D combined EPE / OC, such as the IC 622 and the 2D combined EPE / OC 624 of FIG.6.

[0070] At block 1002, at least one of a grating pitch and a grating direction is continuously and non-uniformly varied across a grating such as the EPE 224 or the 2D combined EPE / OC 624 of the waveguide. Thus, different regions of the grating exhibit different pitches and grating directions without any abrupt changes in pitch or grating direction from one region to the next. As a result, a first distance traveled by a first portion of display light along a first path through the grating differs from a second distance traveled by a second portion of display light along a second path through the grating.

[0071] At block 1004, the grating modulates constructive and destructive interference of the first portion of display light and the second portion of display light as a function of the difference between the first distance and the second distance. In some embodiments, a lateral shift of the grating structure introduces a phasedifference of π, resulting in destructive interference for light directed toward a world- side of an AR eyewear display device. In other embodiments, the lateral shift of the grating structure introduces a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light, resulting in constructive interference for light directed sideways from the EPE towards the OC and destructive interference for light direct downward from the EPE. In some embodiments the phase difference range introduced by the lateral shift of the grating structure is between 2π and 10-20π between a shortest wavelength and a longest wavelength of a color of the display light.

[0072] At block 1006, a pointing error resulting from the different paths through the grating is minimized. In some embodiments, the pointing error is kept below a threshold that would degrade image quality by limiting the amount by which the grating structure is spatially shifted. In some embodiments, a pointing error introduced by one grating of the waveguide (e.g., an incoupler grating) is compensated for or canceled by a pointing error in the opposite direction introduced by another grating of the waveguide (e.g., at least one of an EPE grating and an OC grating, or a 2D combined EPE / OC grating).

[0073] In some embodiments, certain aspects of the techniques described above may be implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, orother instruction format that is interpreted or otherwise executable by one or more processors.

[0074] A computer readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and / or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disk, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)).

[0075] Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure.

[0076] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical,required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

WHAT IS CLAIMED IS:

1. A method comprising: varying a first total phase accumulated by a first portion of display light along a first path through a diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide; and modulating constructive and destructive interference of the first portion of display light and the second portion of display light based on a difference between the first total phase and the second total phase.

2. The method of claim 1, wherein varying comprises: continuously and non-uniformly varying a grating pitch across a grating of the diffractive waveguide, wherein a second order derivative of the variance is non-zero.

3. The method of claim 1 or claim 2, wherein varying comprises: continuously and non-uniformly varying a grating direction across a grating of the diffractive waveguide.

4. The method of claim 2 or claim 3, wherein the grating comprises a one- dimensional exit pupil expander.

5. The method of claim 2 or claim 3, wherein the difference between the first total phase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

6. The method of claim 2 or claim 3, further comprising: coupling the first portion and the second portion of display light out of the diffractive waveguide, wherein the grating comprises a two-dimensional exit pupil expander and outcoupler and the difference between the firsttotal phase and the second total phase results in a phase difference range of approximately pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

7. The method of any of claims 1 to 6, wherein a net pointing error between the first path and the second path is approximately zero.

8. A diffractive waveguide, comprising: a grating configured to: vary a first total phase accumulated by a first portion of display light along a first path through the diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide; and modulate constructive and destructive interference of the first portion of display light and the second portion of display light based on a difference between the first total phase and the second total phase.

9. The diffractive waveguide of claim 8, wherein the grating comprises: a continuously and non-uniformly varying pitch across the grating, wherein a second order derivative of the variance is non-zero.

10. The diffractive waveguide of claim 8 or 9, wherein the grating comprises: a continuously and non-uniformly varying grating direction across the grating.

11. The diffractive waveguide of claim 9 or 10, wherein the grating comprises a one- dimensional exit pupil expander.

12. The diffractive waveguide of claim 9 or 10, wherein the grating is configured to: couple the first portion and the second portion of display light out of the diffractive waveguide, wherein the difference between the first totalphase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

13. The diffractive waveguide of claim 9 or 10, wherein the grating is configured to: couple the first portion and the second portion of display light out of the diffractive waveguide, wherein the grating comprises a two-dimensional exit pupil expander and outcoupler and the difference between the first total phase and the second total phase results in a phase difference of pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

14. The diffractive waveguide of any of claims 8 to 13, wherein a net pointing error between the first path and the second path is approximately zero.

15. A diffractive waveguide, comprising: a grating having a continuous and non-uniformly varying pitch across the grating to vary a first total phase accumulated by a first portion of display light along a first path through the diffractive waveguide from a second total phase accumulated by a second portion of display light along a second path through the diffractive waveguide, wherein a second order derivative of the variance is non-zero.

16. The diffractive waveguide of claim 15, wherein the grating has a continuous and non-uniformly varying grating direction across the grating.

17. The diffractive waveguide of claim 15 or 16, wherein the grating comprises a one-dimensional exit pupil expander.

18. The diffractive waveguide of any of claims 15 to 17, wherein the grating is configured to:couple the first portion and the second portion of display light out of the diffractive waveguide, wherein a difference between the first total phase and the second total phase results in a phase difference of at least 2π, but no more than 20π, between a shortest wavelength and a longest wavelength of a color of the display light.

19. The diffractive waveguide of claim 15 or 16, wherein the grating is configured to: couple the first portion and the second portion of display light out of the diffractive waveguide, wherein the grating comprises a two-dimensional exit pupil expander and outcoupler and a difference between the first total phase and the second total phase results in a phase difference of pi between the first portion of display light and the second portion of display light for an average wavelength of the display light.

20. The diffractive waveguide of any of claims 15 to 19, wherein a net pointing error between the first path and the second path is approximately zero.

21. The diffractive waveguide of any of claims 15 to 20, wherein the continuous and non-uniformly varying pitch compensates for a total thickness variation profile across the diffractive waveguide.

22. The diffractive waveguide of any of claims 15 to 21, wherein a wafer containing diffractive waveguides comprises a plurality of dies, wherein each die of the plurality of dies comprises a grating having a continuous and non-uniformly varying pitch across the grating, wherein a second order derivative of the variance is non-zero, such that a pitch variation is different for different dies to compensate for an expected thickness profile within each die location.

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