Hybrid reflective and diffractive waveguide architecture

Abstract

A hybrid waveguide system employs reflective grating structures to couple display light into and out of the hybrid waveguide system, and a diffractive grating structure to expand the display light in an orthogonal direction. In some embodiments, the hybrid waveguide system includes a reflective waveguide with segmented mirrors that is bonded to a diffractive waveguide including a one-dimensional (1D) diffractive grating. To further minimize any image gaps at high frequencies, some embodiments of the hybrid waveguide system include a partially reflective thin film coating between the a high-refractive index substrate of a diffractive waveguide including the diffractive grating and a lower refractive index substrate of a reflective waveguide including the reflective grating structures.

Description

HYBRID REFLECTIVE AND DIFFRACTIVE WAVEGUIDE ARCHITECTUREBACKGROUND

[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 eyewear display 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] Embodiments described herein leverage the advantages of reflective waveguides while avoiding the challenges of image gap artifacts and visibility of mirror structures. In an embodiment, a waveguide system includes a reflective incoupler to couple display light into the waveguide system, a diffractive exit pupil expander to expand an exit pupil of the display light, and a reflective outcoupler to couple the expanded exit pupil out of the waveguide system. In some embodiments, the waveguide system includes a reflective waveguide comprising the reflective incoupler and the reflective outcoupler and a diffractive waveguide comprising the diffractive exit pupil expander.

[0003] In some embodiments, the reflective waveguide comprises a polymer having a first refractive index and is bonded to the diffractive waveguide with an adhesive having a refractive index matched to the first refractive index. The diffractive exit pupil expander comprises a diffractive grating having a grating vector that is approximately perpendicular to a propagation direction of the display light in some embodiments. The diffractive exit pupilexpander is configured to diffract the display light an even number of times to an eyebox of the waveguide system in some embodiments.

[0004] Some embodiments further include a thin film coating between the adhesive and the diffractive waveguide configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system. The diffractive grating is placed on an outer surface of the diffractive waveguide in some embodiments. In some embodiments, the diffractive exit pupil expander comprises a two-dimensional grating. Further, in some embodiments, the waveguide system further includes a light engine to generate display light having an input polarization of one of s or p polarization and a coating on mirrors of the reflective outcoupler configured to attenuate the input polarization from non-diffracted display light.

[0005] In another embodiment, a waveguide system includes a reflective waveguide comprising a reflective incoupler and a reflective outcoupler to couple display light into and out of the waveguide system and a diffractive waveguide comprising a diffractive exit pupil expander to expand an exit pupil of the display light. In some embodiments, the reflective waveguide comprises a polymer having a first refractive index and is bonded to the diffractive waveguide with an adhesive having a refractive index matched to the first refractive index. Further, in some embodiments, the diffractive exit pupil expander comprises a diffractive grating having a grating vector that is approximately perpendicular to a propagation direction of the display light. The diffractive exit pupil expander is configured to diffract the display light an even number of times to an eyebox of the waveguide system in some embodiments.

[0006] In some embodiments, the waveguide system further includes a thin film coating between the adhesive and the diffractive waveguide configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system. The diffractive grating may be placed on an outer surface of the diffractive waveguide. The diffractive exit pupil expander may comprise a two-dimensional grating. In some embodiments, the waveguide system further includes a light engine to generate display light having an input polarization of one of s or p polarization and a coating on mirrors of the reflective outcoupler configured to attenuate the input polarization from non-diffracted display light.

[0007] In another embodiment, a method includes directing display light into a waveguide system at a reflective incoupler, expanding an exit pupil of the display light at a diffractive exitpupil expander, and directing the expanded exit pupil out of the waveguide system at a reflective outcoupler. In some embodiments, the method further includes partially reflecting display light impinging at angles guided within the waveguide system at a partially reflective thin film coating between a diffractive waveguide of the waveguide system comprising the diffractive exit pupil expander and a reflective waveguide of the waveguide system comprising the reflective incoupler and the reflective outcoupler.

[0008] In another embodiment, a waveguide system includes a first substrate comprising a first material having a first refractive index, a second substrate comprising a second material having a second refractive index higher than the first refractive index, and a partially reflective thin film coating between the first substrate and the second substrate configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 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.

[0010] FIG. 1 is a diagram illustrating a rear perspective view of an augmented reality display device implementing a waveguide system including reflective waveguide to couple display light into and out of the waveguide system and a diffractive waveguide to expand an exit pupil of the display light in accordance with some embodiments.

[0011] FIG. 2 illustrates a cross-sectional view of a facet structure formed in the surface of a reflective waveguide in accordance with some embodiments.

[0012] FIG. 3 is an x-space diagram and a cross-sectional view of components of a hybrid waveguide system implementing refractive incoupler and outcoupler gratings and a diffractive exit pupil expander grating in accordance with some embodiments.

[0013] FIG. 4 is a cross-sectional view of a partially reflective coating layer sandwiched between a diffractive waveguide and a reflective waveguide in accordance with some embodiments.

[0014] FIG. 5 illustrates alternative configurations of a diffractive waveguide bonded to a reflective waveguide in accordance with some embodiments.

[0015] FIG. 6 is an x-space diagram illustrating components of a hybrid waveguide including a two-dimensional diffractive exit pupil expander in accordance with some embodiments.

[0016] FIG. 7 is a flow diagram illustrating a method of coupling light into and out of a hybrid waveguide system using a reflective incoupler and a reflective outcoupler and expanding an exit pupil using a diffractive exit pupil expander in accordance with some embodiments.DETAILED DESCRIPTION

[0017] Reflective waveguides can enable a small form factor augmented reality display with high efficiency and color uniformity due to a lack of spectral dispersion while having limited artifacts (low eye glow, low rainbow, etc.) due to the highly directional nature of the mirror reflection. Current reflective waveguides use facets to expand light in two directions and outcouple the expanded light to a user’s eye, producing a large two-dimensional (2D) eyebox. However, current reflective waveguides encounter challenges in achieving a sufficient image sharpness, as indicated by a modulation transfer function (MTF), in reducing the image gap artifacts associated with pupil replication, and in achieving low visibility of the mirror structures.

[0018] To leverage the advantages of reflective waveguides while avoiding the challenges of image gap artifacts and visibility of mirror structures, FIGs. 1-7 illustrate a hybrid waveguide system that employs reflective grating structures to couple display light into and out of the hybrid waveguide system, and a diffractive grating structure to expand the display light in an orthogonal direction. In some embodiments, the hybrid waveguide system includes a reflective waveguide with segmented mirrors that is bonded to a diffractive waveguide including a one-dimensional (1D) diffractive grating. In some embodiments, the reflective waveguide and the diffractive waveguide are bonded together with an adhesive having a refractive index that matches a refractive index of a polymer substrate of the reflective waveguide.

[0019] The diffractive exit pupil expander includes a diffractive grating that has a grating vector that is approximately perpendicular to a propagation direction of the display light in some embodiments. Incoming light diffracts from the diffractive grating toward the top or bottom of the waveguide, thereby expanding the pupil, and diffracts an even number of times (e.g., twice) on the diffractive grating to reach the eyebox. After the even number of diffractions, the light propagates in the original direction and any spectral dispersion is canceled out in angular space. Furthermore, adjacent pupils expanded by the diffractive grating partially overlap each other, due to the spectrally dispersive nature of grating diffraction, thereby reducing or eliminating image gap artifacts at high frequencies.

[0020] In some embodiments, the diffractive waveguide includes a high refractive index (HRI) substrate (e.g., having a higher refractive index than the refractive index of the substrate of the reflective waveguide and the index-matched adhesive) on which the diffractive grating is formed. The diffractive grating of the diffractive waveguide faces toward the reflective waveguide in some embodiments, and in other embodiments, the diffractive grating of the diffractive waveguide is on the surface of the HRI substrate that faces opposite the reflective waveguide. In some embodiments, the diffractive waveguide includes a 2D diffractive grating, which is formed in some embodiments by superposing two 1 D diffractive gratings.

[0021] To further minimize any image gaps at high frequencies, some embodiments of the hybrid waveguide system include a partially reflective thin film coating between the HRI substrate of the diffractive waveguide and the reflective waveguide. The thin film coating is anti-reflective for light impinging at near-normal incidence to improve see-through efficiency and partially reflective for angles of display light that are guided inside the hybrid waveguide system via total internal reflection (TIR). The thin film coating effectively homogenizes the propagating pupil to reduce the occurrence of image gaps.

[0022] Because light that was not diffracted by the diffractive grating can still outcouple to the eyebox (as zero diffractions is an even number), a bright stripe could appear in the center of the eyebox. To mitigate the appearance of the stripe, some embodiments employ polarization. A light engine that generates display light for the hybrid waveguide system produces pure s or p polarized light in some embodiments. The polarization state of the display light is not affected by TIR interactions within the reflective waveguide, so the polarization state remains as the display light propagates through the reflective waveguide.However, when the display light diffracts from the diffraction grating, the polarization state changes at each TIR surface interaction such that the polarization state becomes mixed. Thus, if s-polarized light is coupled into the hybrid waveguide system, non-diffracted light will remain s-polarized, while diffracted and expanded light will have largely mixed and random polarization. Some embodiments include a coating on the mirrors of the reflective outcoupler that is more efficient for p-polarization to attenuate the s-polarized bright stripe from the nondiffracted display light. In other embodiments, the display light is p-polarized and the coating on the mirrors of the reflective outcoupler is more efficient for s-polarization to attenuate the p-polarized bright stripe from the non-diffracted display light.

[0023] FIG. 1 illustrates an example AR eyewear display system 100 implementing a hybrid waveguide system including reflective waveguide to couple display light into and out of the waveguide system and a diffractive waveguide to expand an exit pupil of the display light 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.

[0024] 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.

[0025] 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 are projected 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.

[0026] T o allow for high efficiency and color uniformity, lack of eye glow, low visibility of anEPE, and improved image uniformity, the hybrid waveguide system includes a reflective IC and OC and a diffractive EPE. The reflective IC and OC are incorporated into a reflective waveguide in some embodiments while the diffractive EPE is incorporated into in a diffractive waveguide that is bonded to the reflective waveguide. In some embodiments, a partially reflective thin film coated is sandwiched between the reflective waveguide and the diffractive waveguide to provide additional homogenization of the propagating pupil.

[0027] FIG. 2 illustrates a cross-sectional view of a facet structure 200 formed in the surface of a reflective waveguide in accordance with some embodiments. To reduce the thickness of the reflective waveguide, current reflective waveguides employ segmented mirrors, such that the mirror does not span the entire thickness of the waveguide. The reflective facets include at least a primary reflective surface (also referred to herein as a “first face”) and a secondary surface (also referred to herein as a “second face”). The primary surface is typically used to partially relay light and is coated with a partially reflective coating. The secondary surface is typically left uncoated to avoid production of ghost or image artifacts.

[0028] The facet structure 200 is formed on a light-transmitting substrate, constructed from a transparent material such as plastic, that has a pair of parallel faces 210, 212 (also referred to as major surfaces) and a series of planar partially reflective surfaces deployed within the substrate at an oblique angle to the parallel faces 210, 212. The partially reflective surfaces (referred to herein as facets) are coated with partially reflective coatings that are applied to at least part of the sides or surfaces of the facet structure 200. The coatings are designed with reflective characteristics such that the coatings are at least partially reflective to incident light having particularly corresponding characteristics in order to generate a desired reflectivity characteristic.

[0029] As shown in the depicted embodiment, the facet structure 200 includes a series of facets. Each facet has a height on the order of 100’s of microns and width in the range of millimeters, and the series of facets collectively has a width in the range of centimeters. Each facet has top surface 202 that slopes at an angle 204 with respect to a lateral dimension of the series of facets and is separated by a distance referred to as a pitch 206 from the next facet. The top surface 202 of each facet is formed to have a desired shape and smoothness. For example, in the illustrated embodiment, each facet has a flat top surface 202. In some embodiments, the top surface 202 of each facet is curved.

[0030] The facet structure 200 is configured to form at least one of an incoupler, and an outcoupler. In some embodiments, characteristics of the facet structure 200 varies between portions used as the incoupler and the outcoupler. For example, in some embodiments, the pitch 206 for the incoupler varies from the pitch 206 for the outcoupler. In some embodiments, the angle 204 of the top surface 202 of the facet structure 200 varies between the incoupler and the outcoupler. Thus, in some embodiments, the gratings of each of the incoupler and the outcoupler are not parallel to each other.

[0031] FIG. 3 is an x-space diagram 300 and a cross-sectional view 320 of components of a hybrid waveguide system 330 implementing reflective incoupler 302 and outcoupler 306 mirrorsand a diffractive exit pupil expander grating 304 in accordance with some embodiments. As illustrated in the x-space diagram 300, the gratings 302, 304, 306 are shown within a lens 312 such as lens elements 108, 110. The reflective incoupler 302 couples display light from a light source such as a light engine or microdisplay (not shown) into the hybrid waveguide system 330 and directs the display light toward the diffractive exit pupil expander grating 304. The exit pupil expander grating 304 includes a series of facetshaving a grating vector 308 that is oriented approximately orthogonal to the direction of propagation of the display light (e.g., within approximately 15° of perpendicular). The exit pupil expander grating 304 splits rays of light that impinge on the exit pupil expander grating 304 and diffracts the light to either the top or bottom halves of the hybrid waveguide system 330 (i.e., in either a positive or negative y-axis direction), thus expanding the pupil.

[0032] In some embodiments, the display light undergoes an even number of diffractions on the exit pupil expander grating 304 (e.g., two diffractions). After the even number of diffractions, the display light propagates in the same direction as the display light that was input to the exit pupil expander grating 304 and any spectral dispersion effect is canceled out in angular space. The exit pupil expander grating 304 thus laterally expands the pupil of display light output from the light engine. The magnitude of the lateral shift that a pupil experiences after diffraction by the exit pupil expander grating 304 is based on the wavelength of the display light due to the spectral dispersion of the exit pupil expander grating 304. Thus, for a broadband light source such as an LED or micro-LED (pLED), the lateral pupil expansion results in elongation of the pupil along the expansion direction, which fills in gaps between pupil replicas.

[0033] The exit pupil expander grating 304 thus improves image uniformity and removes image gaps while directing the display light toward the reflective outcoupler 306. The reflective outcoupler 306 includes a series of reflective facets (i.e., mirrors) that redirects the light out of the hybrid waveguide system 330 toward an eye of a user.

[0034] However, because zero diffractions is an even number of diffractions, light that was not diffracted by the diffractive grating can still outcouple to the eyebox and result in a bright stripe in the center of the eyebox. In some embodiments, a light engine that generates display light for the hybrid waveguide system 330 produces display light having pure s or p polarization. The polarization state remains unchanged as the display light propagates through the reflective waveguide. However, when the display light diffracts from the diffraction grating, the polarization state changes at each TIR surface interaction. Thus, if light having an input polarization that is s-polarized is coupled into the hybrid waveguide system, the non-diffracted light will remain s-polarized, but the diffracted and expanded light will have largely mixed and random polarization.

[0035] In some embodiments, the mirrors of the reflective outcoupler are coated with a coating that is more efficient for p-polarization to attenuate the s-polarized bright stripe fromthe non-diffracted display light. In other embodiments, the display light is p-polarized and the coating on the mirrors of the reflective outcoupler is more efficient for s-polarization to attenuate the p-polarized bright stripe from the non-diffracted display light.

[0036] Because the outcoupling is performed by mirrors, which are highly directional, the hybrid waveguide system 330 experiences very little or no light leakage to a world-facing side of the hybrid waveguide system 330. In addition, by employing a reflective (mirror) incoupler 302 and outcoupler 306, coupling of display light into and out of the hybrid waveguide system 330 is more efficient and less color dependent compared to diffractive incouplers and outcouplers. The hybrid waveguide system 330 thus has increased overall efficiency and uniformity than fully diffractive waveguide systems.

[0037] Compared to a fully reflective waveguide, the hybrid waveguide system 330 achieves reduced pupil replication artifacts, improved see-through aesthetics, and improved MTF, or image sharpness. The spectral dispersion of the diffractive exit pupil expander grating 304 helps to fill gaps between pupil replicas, thus reducing or eliminating pupil replication artifacts. Further, whereas a reflective pupil expander is visible to the user at an oblique angle, complicating design of a mirror coating that appears transparent to the user, the diffractive exit pupil expander grating 304 is generally much less visible than a semitransparent mirror used for a reflective exit pupil expander. In addition, the discrete nature of the mirrors in reflective waveguides results in pupil clipping which, in turn, causes diffraction and MTF degradation. By replacing some of the mirrors with the continuous diffraction grating of the diffractive exit pupil expander grating 304 improves the overall resolution (MTF) of the hybrid waveguide system 330.

[0038] As illustrated in the cross-sectional view 320, the hybrid waveguide system 330 includes a reflective waveguide 322 and a diffractive waveguide 332. The reflective waveguide 322 includes a reflective incoupler 302 and a reflective outcoupler 306 that are each formed with segmented mirrors. The diffractive waveguide 332 includes a substrate of HRI material 324 and a diffractive grating 304.

[0039] In the illustrated example, the reflective waveguide 322 is bonded to the diffractive waveguide 332 with an index-matched adhesive 326 that has a refractive index approximately equal to the refractive index of a polymer substrate of the reflective waveguide 322. For example, in some embodiments the polymer material of the reflective waveguide is approximately 1 .45 < nrefiective 1 .7. The refractive index of the diffractive waveguide substrateis higher than the refractive index of the reflective waveguide substrate in some embodiments (e.g., 1.7 < ndiffractive 2.1), such that any angle of display light propagating inside the reflective waveguide 322 can enter the higher refractive index diffractive waveguide 332 and interact with the diffractive grating 304. For example, in some embodiments,0.1.

[0040] In some embodiments, the reflective waveguide 322 is thicker than the diffractive waveguide 332. For example, in some embodiments, the reflective waveguide 322 is approximately 1-3 mm thick and the diffractive waveguide 332 is approximately 0.4-1 mm thick. The index-matched adhesive 326 layer is approximately 0.1-50 pm thick in some embodiments, and in some embodiments is 0.5-20 pm thick.

[0041] A partially reflective thin film coating 328 is sandwiched between the index-matched adhesive 326 layer and the HRI material 324 of the diffractive waveguide 332 in the illustrated example. The partially reflective thin film coating 328 is anti-reflective for light impinging at near-normal incidence to improve see-through efficiency and is partially reflective for light impinging at angles that are guided through the hybrid waveguide system 330 via TIR. The partially reflective thin film coating 328 provides additional homogenization of the propagating pupil, as explained further below with reference to FIG. 4.

[0042] FIG. 4 is a cross-sectional view 400 of a partially reflective coating layer 328 sandwiched between an HRI material substrate 424 (e.g., the diffractive waveguide 332) and a lower refractive index material substrate 422 (e.g., the reflective waveguide 322) to form a homogenizer 410 in accordance with some embodiments. Pupil d1 402 is the width of a pupil traveling through the lower refractive index material substrate 422 and encountering the partially reflective coating 328. A first portion of the light reflects off the partially reflective coating 328, while a second portion of the incident light travels through the partially reflective coating 328 and the HRI material substrate 424. The second portion of the incident light reflects off an outer surface 426 of the HRI material substrate 424 (e.g., through TIR at the interface of the HRI material substrate 424 and air) and propagates through the partial reflective coating 328 and the lower refractive index material substrate 422, such that pupil d1 402 is expanded to fill pupil d2 404.

[0043] Multiple pupils incident on the homogenizer 410 are expanded in this manner such that they overlap upon expansion. The homogenizer 410 thus further fills any gaps between pupils, resulting in a more uniform image. In some embodiments, the homogenizer 410 has a reflectance in a range of 15% to 85% (or 30-70%, or 50%). In some embodiments, the HRI material substrate 424 has a thickness between 0.4-1.0 mm and the low refractive index material substrate 422 has a thickness between 1-3 mm.

[0044] FIG. 5 illustrates alternative configurations of the hybrid waveguide system in accordance with some embodiments. In various embodiments, a diffractive grating 502 of the diffractive waveguide 332 can be placed on either side of the HRI substrate 424. For example, in a first configuration 500, the diffractive grating 502 is placed on an outer surface 504 of the HRI substrate 424 and is left uncoated, such that there is a high refractive index contrast between the diffractive grating 502 material and air.

[0045] In a second configuration 510, the diffractive grating 502 is placed on an inner surface 506 of the HRI substrate 424 and is encapsulated in an encapsulation material 512. In the second configuration 510, two diffraction orders exist — a transmitted diffraction order 516 and a reflected diffraction order 518. In addition, incoming light 514 interacts with the diffractive grating 502 twice as often (twice per bounce as opposed to once per bounce). Therefore, the second configuration 510 may have higher overall diffraction efficiency.

[0046] FIG. 6 is an x-space diagram illustrating components of a hybrid waveguide system 600 including a two-dimensional (2D) diffractive exit pupil expander 604 in accordance with some embodiments. In contrast to the hybrid waveguide system 330 illustrated in FIG. 3 that includes a 1 D diffractive exit pupil expander 304 having a single grating vector 308, the 2D diffractive exit pupil expander 604 combines two grating vectors 608 (i.e., k-vectors) in the same expansion region. The 2D diffractive exit pupil expander 604 can diffract the light to larger angles, as indicated by the light propagation direction 610, so that the beam can expand faster and requires a shorter propagation distance. The 2D diffractive exit pupil expander 604 therefore reduces the overall footprint of the hybrid waveguide system 330.

[0047] In some embodiments, the 2D diffractive exit pupil expander 604 is produced by superposition of two 1 D gratings. In other embodiments, the HRI substrate 324 of the diffractive waveguide 332 incorporates two 1 D gratings fabricated on opposite surfaces of the HRI substrate 324.

[0048] FIG. 7 is a flow diagram illustrating a method 700 of coupling light into and out of a hybrid waveguide system using a reflective incoupler and a reflective outcoupler and expanding an exit pupil using a diffractive exit pupil expander in accordance with some embodiments. In some embodiments, the method 700 is practiced by the hybrid waveguide system 330 or the hybrid waveguide system 600.

[0049] At block 702, display light is coupled into the hybrid waveguide system 330 via a reflective incoupler such as reflective incoupler 302. In some embodiments, the reflective incoupler 302 is included within a reflective waveguide 322 of the hybrid waveguide system 330. At block 704, an exit pupil of the display light is expanded at a diffractive exit pupil expander. In some embodiments, the diffractive exit pupil expander is included within a diffractive waveguide 332 of the hybrid waveguide system 330. In some embodiments, the diffractive exit pupil expander has a grating vector that is aligned perpendicular to the direction of light propagation.

[0050] At block 706, the expanded exit pupil of the display light is coupled out of the hybrid waveguide system 330 by a reflective outcoupler such as reflective outcoupler 306. In some embodiments, the reflective outcoupler is included within the reflective waveguide 322.

[0051] 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.

[0052] 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 butequivalent 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 waveguide system comprising: a reflective incoupler to couple display light into the waveguide system; a diffractive exit pupil expander to expand an exit pupil of the display light; and a reflective outcoupler to couple the expanded exit pupil out of the waveguide system.

2. The waveguide system of claim 1 , further comprising: a reflective waveguide comprising the reflective incoupler and the reflective outcoupler; and a diffractive waveguide comprising the diffractive exit pupil expander.

3. The waveguide system of claim 2, wherein the reflective waveguide comprises a polymer having a first refractive index and is bonded to the diffractive waveguide with an adhesive having a refractive index matched to the first refractive index.

4. The waveguide system of claim 2 or claim 3, wherein the diffractive exit pupil expander comprises a diffractive grating having a grating vector that is approximately perpendicular to a propagation direction of the display light.

5. The waveguide system of claim 4, wherein the diffractive exit pupil expander is configured to diffract the display light an even number of times to an eyebox of the waveguide system.

6. The waveguide system of any of claims 3 to 5, further comprising: a thin film coating between the adhesive and the diffractive waveguide configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system.

7. The waveguide system of any of claims 4 to 6, wherein the diffractive grating is placed on an outer surface of the diffractive waveguide.

8. The waveguide system of any of the preceding claims, wherein the diffractive exit pupil expander comprises a two-dimensional grating.

9. The waveguide system of any of the preceding claims, further comprising: a light engine to generate display light having an input polarization of one of s or p polarization; and a coating on mirrors of the reflective outcoupler configured to attenuate the input polarization from non-diffracted display light.

10. A waveguide system, comprising: a reflective waveguide comprising a reflective incoupler and a reflective outcoupler to couple display light into and out of the waveguide system; and a diffractive waveguide comprising a diffractive exit pupil expander to expand an exit pupil of the display light.11 . The waveguide system of claim 10, wherein the reflective waveguide comprises a polymer having a first refractive index and is bonded to the diffractive waveguide with an adhesive having a refractive index matched to the first refractive index.

12. The waveguide system of claim 10 or claim 11 , wherein the diffractive exit pupil expander comprises a diffractive grating having a grating vector that is approximately perpendicular to a propagation direction of the display light.

13. The waveguide system of any of claims 10 to 12, wherein the diffractive exit pupil expander is configured to diffract the display light an even number of times to an eyebox of the waveguide system.

14. The waveguide system of any of claims 11 to 13, further comprising: a thin film coating between the adhesive and the diffractive waveguide configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system.

15. The waveguide system of any of claims 12 to 14, wherein the diffractive grating is placed on an outer surface of the diffractive waveguide.

16. The waveguide system of any of claims 10 to 15, wherein the diffractive exit pupil expander comprises a two-dimensional grating.

17. The waveguide system of any of claims 10 to 16, further comprising: a light engine to generate display light having an input polarization of one of s or p polarization; and a coating on mirrors of the reflective outcoupler configured to attenuate the input polarization from non-diffracted display light.

18. A method comprising: directing display light into a waveguide system at a reflective incoupler; expanding an exit pupil of the display light at a diffractive exit pupil expander; and directing the expanded exit pupil out of the waveguide system at a reflective outcoupler.

19. The method of claim 18, further comprising: partially reflecting display light impinging at angles guided within the waveguide system at a partially reflective thin film coating between a diffractive waveguide of the waveguide system comprising the diffractive exit pupil expander and a reflective waveguide of the waveguide system comprising the reflective incoupler and the reflective outcoupler.

20. A waveguide system, comprising: a first substrate comprising a first material having a first refractive index; a second substrate comprising a second material having a second refractive index higher than the first refractive index; and a partially reflective thin film coating between the first substrate and the second substrate configured to be anti-reflective for light impinging at near-normal incidence and partially reflective for display light impinging at angles guided within the waveguide system.

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