WO2025128849A1

WO2025128849A1 - Hybrid waveguide for low image leakage, low cosmetic defects and high efficiency ar displays

Info

Publication number: WO2025128849A1
Application number: PCT/US2024/059817
Authority: WO
Inventors: Jinxin FU; Yangyang Sun; Sihui He; Ludovic Godet
Original assignee: Applied Materials Inc
Current assignee: Applied Materials Inc
Priority date: 2023-12-13
Filing date: 2024-12-12
Publication date: 2025-06-19
Anticipated expiration: 2026-06-13

Abstract

Embodiments described relate to hybrid waveguides with a diffractive relay grating, a diffractive fold grating, and a reflective output mirror array. The waveguide includes a substrate, a prism coupler to couple light disposed over the substrate, a relay grating, a fold grating, and an output coupling mirror array. The relay grating is disposed over the substrate adjacent to the prism coupler. The relay grating includes relay structures to diffract light to a fold grating. The fold grating is disposed over the substrate and is adjacent to the relay grating. The fold grating includes folding structures operable to split beams in a first direction to the output coupling mirror array and a second direction which are diffracted in the first direction. The output coupling mirror array is disposed in the substrate and adjacent to the fold grating. The output coupling mirror array has a plurality of mirrors to out-couple the beams.

Description

HYBRID WAVEGUIDE FOR LOW IMAGE LEAKAGE, LOW COSMETIC DEFECTS AND HIGH EFFICIENCY AR DISPLAYS

BACKGROUND

Field

[0001] Embodiments of the present disclosure generally relate to waveguides for augmented, mixed, and/or virtual reality displays. More specifically, embodiments described herein relate to hybrid waveguides with a diffractive relay grating, a diffractive fold grating, and a reflective output mirror array.

Description of the Related Art

[0002] Virtual reality is generally considered to be a computer generated simulated environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed with a headmounted display (HMD), such as glasses or other wearable display devices that have near-eye display panels as lenses to display a virtual reality environment that replaces an actual environment.

[0003] Augmented reality (AR), however, enables an experience in which a user can still see through the display lenses of the glasses or other HMD device to view the surrounding environment, yet also see images of virtual objects that are generated to appear as part of the environment. Augmented reality can include any type of input, such as audio and haptic inputs, as well as virtual images, graphics, and video that enhance or augment the environment that the user experiences. As an emerging technology, there are many challenges and design constraints with augmented reality. Therefore, what is needed in the art are improved waveguides.

SUMMARY

[0004] In one embodiment, a waveguide is provided. The waveguide includes a substrate, a prism coupler disposed over the substrate, a relay grating, a fold grating, and an output coupling mirror array. The prism coupler is operable to couple light into the substrate. The relay grating is disposed over the substrate adjacent to the prism coupler. The relay grating includes relay structures operable to diffract light to a fold grating. The fold grating is disposed over the substrate and is adjacent to the relay grating. The fold grating includes folding structures operable to split beams in a first direction to the output coupling mirror array and a second direction in the fold grating where beams in the second direction are diffracted in the first direction. The output coupling mirror array is disposed in the substrate and adjacent to the fold grating. The output coupling mirror array has a plurality of mirrors disposed in the substrate. The mirrors are operable to receive the beams in the first direction from the fold grating and out-couple the beams.

[0005] In another embodiment, a waveguide is provided. The waveguide includes a substrate, a prism coupler disposed over the substrate, a relay grating, a fold grating, and an output coupling mirror array. The prism coupler is operable to couple light into the substrate. The relay grating disposed over the substrate adjacent to the prism coupler. The relay grating includes relay structures operable to diffract light to a fold grating. The fold grating is disposed over the substrate adjacent to the relay grating. The fold grating includes folding structures operable to split beams in a first direction to an output coupling mirror array and a second direction in the fold grating where beams in the second direction are diffracted in the first direction. The relay structures and the folding structures have the same pitch and orientation, and the relay grating has a greater diffraction efficiency than the fold grating. The output coupling mirror array is disposed in the substrate and adjacent to the fold grating. The output coupling mirror array includes a plurality of mirrors disposed in the substrate. The mirrors are operable to receive the beams in the first direction from the fold grating and out-couple the beams.

[0006] In another embodiment, a method of forming a waveguide is provided. The method includes bonding mirrors of varied reflectivity to a bonding substrate to form a mirror stack, cutting the mirror stack at an angle to form an output coupling mirror array with varied reflectivity, and embedding the output coupling mirror array in a waveguide substrate. The method further includes disposing a relay grating and a fold grating over a first surface of the waveguide substrate forming a waveguide, and disposing a prism coupler on the waveguide substrate. The prism coupler is operable to couple light to the relay grating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of scope, as the disclosure may admit to other equally effective embodiments.

[0008] Figure 1 A is a perspective, top view of a hybrid waveguide, according to embodiments.

[0009] Figure 1 B is a perspective, cross sectional view of a relay grating and a prism coupler of the waveguide, according to embodiments.

[0010] Figure 1 C is a perspective, cross sectional view of a fold grating of the waveguide, according to embodiments.

[0011] Figure 1 D is a perspective, cross sectional view of an output coupling mirror array of the waveguide, according to embodiments.

[0012] Figure 2 is a flow diagram of a method of forming a waveguide, according to embodiments.

[0013] Figure 3A-3D are schematic, cross-sectional views of a substrate during a method of forming a waveguide, according to embodiments.

[0014] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

[0015] The present disclosure generally relate to optical devices. More specifically, embodiments described herein relate to waveguide combiners with hybrid gratings including both diffractive gratings and mirror arrays. Diffractive waveguides may have high eye glow, low efficiency, and cosmetic defects. Reflective waveguides may have cosmetic defects, modulation transfer function (MTF) degradation, and process difficulty causing low yield. Hybrid waveguides with a diffractive relay grating, a diffractive fold grating, and a reflective output mirror array reduce the negative effects of waveguides with either all diffractive gratings or all reflective gratings.

[0016] Eye glow (i.e. image leakage) is the amount of light traveling in the wrong direction away from an eye of a user. Besides being a loss of light, eye glow is visible on the side opposite of the user of an AR device making it hard to see the eyes of the user through the AR device. The eye glow leads to low efficiency as the light leakage takes away from the light out-coupled to the eyes of the user. Reducing the eye glow will increase the efficiency of the waveguide. Cosmetic defects of diffractive waveguides include rainbow effects caused by ambient light diffracted in the gratings of the waveguide. Diffractive gratings lead to light being split by color in total internal reflection (TIR). This light out-coupled by a diffractive output coupling grating has lower field of view (FOV).

[0017] Cosmetic defects of reflective waveguides include light reflections and scattering from folding mirror arrays. The MTF degradation is caused by denser mirror arrays needed to reduce the color/brightness uniformity. The MTF degradation causes pupil splits leading to the user seeing blurry images. The process difficulty is caused by bonding the mirrors leading to low image yield. [0018] In some embodiments, which may be combined with other embodiments, the hybrid waveguide contains a diffractive fold grating. The diffractive fold grating in the hybrid waveguide does not have the cosmetic defects of light reflection and scattering present in the folding mirror arrays in reflective waveguides. The diffractive fold grating also has less MTF degradation because the diffractive fold grating does not have dense mirror arrays present in the folding mirror arrays. Diffractive fold gratings do not have the process difficulty of bonding the mirror arrays. This increases the image yield across the fold grating. In some embodiments, which may be combined with other embodiments, the hybrid waveguide contains a reflective output mirror array. The reflective output mirror array may reduce the eye glow. For example, the eye glow may be reduced due to the reflective output mirror array having more light coupling to human eyes than coupling towards the world side compared to a diffractive output grating. Advantages of the present disclosure include the hybrid waveguide design supporting a larger FOV compared to the all diffractive gratings design for the same substrate with a given refractive index. The hybrid waveguide design also has less rainbow artifact due to the use of the reflective output mirror array compared to diffractive output grating.

[0019] Figure 1A is a perspective, top view of a hybrid waveguide 100. The hybrid waveguide 100 includes a plurality of structures 102. The structures 102 may be disposed over, under, or on a first surface 103 of a substrate 101 , or disposed in the substrate 101 . In some embodiments, which may be combined with other embodiments, the substrate 101 comprises a material with a reflective index between 1.5 and 2.3. The material includes glass containing materials or plastic containing materials with optical transparency. For example, the substrate 101 may include polycarbonate. In some embodiments, which may be combined with other embodiments, the substrate 101 may have a thickness greater than 200 pm. Other embodiments of the substrate 101 having thinner or greater thicknesses are also contemplated.

[0020] The structures 102 are nanostructures having a sub-micron critical dimension, e.g., a width less than 1 micrometer. Regions of the structures 102 may correspond to one or more gratings 104. In some embodiments, which may be combined with other embodiments, the one or more gratings 104 of the hybrid waveguide 100 includes at least a relay grating 104A, a fold grating 104B, and an output coupling mirror array 104C (i.e. reflective grating). The output coupling mirror array 104C may be a diffractive grating or a reflective mirror array. For example, in an embodiment, the output coupling mirror array 104C is a reflective mirror array. In some embodiments, which may be combined with other embodiments, the relay grating 104A is a diffractive grating. The fold grating 104B is a diffractive grating and may correspond to a pupil expansion grating. A prism coupler 105 is positioned above the first surface 103 of the substrate 101. In some embodiments as shown in Figure 1A and 1 B, the prism coupler 105 is disposed on the first surface 103 of the substrate 101 adjacent to the relay grating 104A. In other embodiments, the prism coupler 105 is disposed above the relay grating 104A.

[0021] Light beams are shown traveling through the hybrid waveguide 100 on interference paths 106. Light is projected to the prism coupler 105. The prism coupler 105 couples the light beam to the relay grating 104A. The light is diffracted by the relay grating 104A to the fold grating 104B. In the fold grating 104B, the light is split into multiple light beams on different interference paths 106. The split beams are split in a first direction (x-direction) to an output coupling mirror array 104C and a second direction (y-direction) in the fold grating. The beams in the second direction are diffracted in the first direction. The light beams then travel to the output coupling mirror array 104C. The light is then out-coupled from the hybrid waveguide 100 to the eyes of the user.

[0022] Figure 1 B is a perspective, cross sectional view of the relay grating 104A and the prism coupler 105 of the hybrid waveguide 100. Figure 1 B is taken along section line 1 B-1 B of Figure 1 . The prism coupler 105 couples light into the substrate 101 . Light in-coupled at an angle larger than a critical angle $ undergoes total internal reflection (TIR) in the substrate 101. The critical angle $ is determined according to the refractive index of the hybrid waveguide 100. In some embodiments, which may be combined with other embodiments, the refractive index of the hybrid waveguide 100 is 2 and the critical angle & is about 30°. The light in TIR travels to the relay grating 104A. In an embodiment, the relay grating 104A is a surface relief grating.

[0023] In an embodiment, the relay grating 104A is disposed above the substrate 101. The relay grating 104A may include relay structures 102A. In some embodiments, the relay structures 102A are formed out of the substrate 101. In other embodiments, the relay structures 102A are formed from a different material. The relay structures 102A may include silicon nitrate (SiN), titanium oxide (TiOx), niobium oxide (NbOx), zirconium oxide (ZrOx), hafnium oxide (HFOx), or combinations thereof. The relay structures 102A have a refractive index between 1 .7 and 2.7. In some embodiments, each of the relay structures 102A are defined by a depth 107, critical dimension 109, a pitch 111 , and a duty cycle. The depth 107 of the relay structure 102A is defined by the distance between a top surface 108 of the relay structure 102A and the first surface 103 of the substrate 101 . The critical dimension 109 is the width of the relay structure 102A. A space width 110 defines a distance between two relay structures 102A. The pitch 111 is defined by the distance between the trailing edges of adjacent relay structure 102A. The pitch 111 is the summation of the critical dimension 109 and the space width 110 of a particular relay structure 102A. The duty cycle is defined is defined as the ratio of the space width 110 to the pitch 111. In some embodiments, which may be combined with other embodiments, the duty cycle of the relay grating 104A ranges between about 0.2 and about 0.8. In some embodiments, which may be combined with other embodiments, the depth 107 of the relay grating 104A ranges between 20 nm and 400 nm. In some embodiments, which may be combined with other embodiments, the critical dimension 109 of the relay grating 104A ranges between 60 nm and 300 nm. In some embodiments, which may be combined with other embodiments, the pitch 111 of the relay grating 104A ranges between 200 nm and 300 nm. As shown in Figure 1 B, the depth 107 and the duty cycle can vary across the relay grating 104A. The pitch 111 can vary across the relay grating 104A. A variable depth, a variable duty cycle, and/or a varying pitch modulates a diffraction efficiency and interference path of the relay structures 102A. The diffraction efficiency is defined as the ratio between light diffracted by the relay grating 104A and the light entering the relay grating 104A. The relay grating 104A is operable to diffract light to the fold grating 104B.

[0024] In some embodiments as shown in Figure 1 B, the relay structures 102A are angled at structure angles $ relative to the substrate 101. The structure angle $ is the angle between the first surface 103 of the substrate 101 and the sidewalls 112 of the relay structures 102A. The structure angles $ range from 0° to 70°. In one embodiment, which can be combined with other embodiments described herein, each respective structure angle for each relay structures 102A is substantially equal throughout the substrate 101. In another embodiment, which can be combined with other embodiments described herein, at least one respective structure angle $ of the plurality of relay structures 102A is different from another device angle $ of the plurality of the relay structures 102A. In other embodiments, the relay structures 102A may be substantially vertical, (i.e. binary). Sidewalls 112 of the relay structures 102A are parallel to each other and perpendicular with the first surface 103 of the substrate 101 .

[0025] Figure 1 C is a perspective, cross sectional view of the fold grating 104B of the hybrid waveguide 100. Figure 1 C is taken along section line 1 C- 1 C of Figure 1A. The fold grating 104B is defined by a plurality of folding structures 102B. In some embodiments, the folding structures 102B are formed out of the substrate 101. In other embodiments, the folding structures 102B are formed from a different material. The folding structures 102B may include silicon nitrate (SiN), titanium oxide (TiOx), niobium oxide (NbOx), zirconium oxide (ZrOx), hafnium oxide (HfOx), or combinations thereof. The folding structures 102B have a refractive index between 1.7 and 2.7. Each of the folding structures 102B have the depth 107, the critical dimension 109, the space width 110, the pitch 111 , and the duty cycle defined above in Figure 1 B for the relay structures 102A. In some embodiments, which may be combined with other embodiments, the duty cycle of the fold grating 104B ranges between 0.2 and 0.8. In some embodiments, which may be combined with other embodiments, the depth 107 of the fold grating 104B ranges between 20 nm and 400 nm. In some embodiments, which may be combined with other embodiments, the critical dimension 109 of the fold grating 104B ranges between 60 nm and 300 nm. In some embodiments, which may be combined with other embodiments, the pitch 111 of the fold grating 104B ranges between 200 nm and 300 nm. As shown in Figure 1 C, the depth 107 and the duty cycle can vary across the fold grating 104B. In some embodiments, which may be combined with other embodiments, the pitch 111 can vary across the fold grating 104B. In some embodiments, which may be combined with other embodiments, the depth 107, the duty cycle, or the pitch 111 varies in the folding grating 104B to modulate a diffraction efficiency and interference paths 106 of the fold grating 104B. The diffraction efficiency is defined as the ratio between light diffracted by the fold grating 104B and the light entering the fold grating 104B. The fold grating 104B is a surface relief grating.

[0026] In some embodiments, the folding structures 102B may be substantially vertical. In other embodiments as shown in Figure 1 C, the folding structures 102B are angled at the structure angle $ (i.e. critical angle) relative to the substrate 101 as described above. In some embodiments, which may be combined with other embodiments, the structure angles $ range from 0° to about 70°. In one embodiment, which can be combined with other embodiments described herein, each respective structure angle $ for each folding structures 102B is substantially equal throughout the substrate 101 . In another embodiment, which can be combined with other embodiments described herein, at least one respective structure angle $ of the plurality of the folding structures 102B is different from another device angle $ of the plurality of the folding structures 102B.

[0027] In some embodiments, which may be combined with other embodiments, the relay grating 104A and the fold grating 104B have the same pitch 111 and structure angle $ (i.e. orientation.) In some embodiments, which may be combined with other embodiments, the relay grating 104A has a greater diffraction efficiency than the fold grating 104B. The difference in diffraction efficiency may be caused by a difference in the depth 107 of the relay structures 102A and the folding structures 102B. In some embodiments, the relay structures 102A and the folding structures 102B are made of different materials and have different refractive indices leading to the difference in diffraction efficiency.

[0028] In some embodiments, which may be combined with other embodiments, the folding structures 102B are disposed on a second surface 113 of the substrate 101 . The second surface 113 of the substrate is opposite the first surface 103. The folding structures 102B on the second surface 113 increase the pupil density. The pupil density is defined as number of light beam spots per unit area for a given light propagation angle inside the substrate 101 . Increasing the pupil density improves the color uniformity and reduces image defects.

[0029] Figure 1 D is a perspective, cross sectional view of the output coupling mirror array 104C of the hybrid waveguide 100. Figure 1 D is taken along section line 1 D-1 D of Figure 1A. The output coupling mirror array 104C is embedded in the substrate 101. The output coupling mirror array 104C is operable to receive light beams from the fold grating 104B. The output coupling mirror array 104C includes a mirror array 102C. The output coupling mirror array 104C is configured to out-couple light from the hybrid waveguide 100. The mirror array 102C includes a plurality of mirrors 115 bonded together. The mirror 115 may include glass or a dielectric material. The dielectric material may include titanium oxide (TiOx), silicon oxide (SiOx), niobium oxide (NiOx), zirconium oxide (ZrOx), and hafnium oxide (HfOx), or combinations thereof. The mirror 115 are coated with a dialectical material. The dialectical material includes titanium oxide (TiOx), silicon oxide (SiOx), niobium oxide (NiOx), zirconium oxide (ZrOx), and hafnium oxide (HfOx), or a combination thereof. The dialectal materials used to coat and the thickness of the coating vary across the mirror 115. The difference in coating allows the mirrors 115 to have different reflectivity. The reflectivity of the mirrors 115 ranges between 2% and 50%. In one embodiment, at least one of the mirrors have a different reflectivity compared to a different mirror. For example, the reflectivity can increase from a first side 117 of the output coupling mirror array 104C to the second side 119 of the output coupling mirror array 104C. The light will be reflected weakly out of the first side 117 gradually increasing to the second side 119. The change in reflectivity value allows for uniform out-coupling of light across the FOV. In some embodiments, a multilayer coating is used on the mirror 115. The output coupling mirror array 104C has a larger FOV and significantly less light leakage than a diffractive output coupling grating. The lower light leakage leads to less eye glow in the hybrid waveguide 100.

[0030] Figure 2 is a flow diagram of a method 200 of forming the hybrid waveguide 100. Figure 3A-3D are schematic, cross-sectional views of the substrate 101 during the method 200 of forming the hybrid waveguide 100. Prior to operation 201 , the plurality of mirrors 115 are formed. The mirrors 115 are cut from a partially reflective material using wire saw cutting. The partially reflective material includes include glass or a dielectric material such as titanium oxide, silicon oxide, and niobium oxide. The mirrors 115 are then polished and coated. The mirrors 115 are coated with a dialectical material such as titanium oxide, silicon oxide, niobium oxide, or combination thereof. In some embodiments, a multilayer coating is used on the mirrors 115. The dialectal materials used to coat may vary for the different mirrors 115. The thickness of the coatings may vary for the different mirrors 115. The difference in coating allows the mirrors 115 to have different reflectivity.

[0031] At operation 201 , the mirrors 115 are bonded together. Figure 3A is a cross-sectional view of a mirror stack 300 bonded to a bonding substrate 302. The mirror stack 300 includes a first mirror 300A bonded to a first surface 303 of the bonding substrate 302. In some embodiments, the first mirror 300A has the highest reflectivity in the mirror stack 300. The mirror stack 300 includes a second mirror 300B bonded to a top surface of the first mirror 300A with the second mirror 300B offset from the first mirror 300A by a distance. Subsequent mirrors are bonded to a previous adjacent mirror in the same manner and offset the distance. Finally, a final mirror 300C is bonded to a previous mirror. In some embodiments, the final mirror 300C has the lowest reflectivity in the mirror stack 300. In one example, seven mirrors are used in the mirror stack 300 though embodiments with other amounts of mirrors are contemplated.

[0032] At operation 203, the output coupling mirror array 104C is cut and disposed in the substrate 101 . Multiple output coupling mirror arrays 104C are cut from the mirror stack 300 along the cut lines 305. The mirrors 115 are cut into the shape shown in Figure 1 D. The mirrors 115, bonded together, form the output coupling mirror arrays 104C. The multiple output coupling mirror arrays 104C fabricated from each mirror stack 300 may be used on the same waveguide 100 or multiple waveguides 100 depending on the size of the output coupling mirror arrays 104C and waveguides 100. The output coupling mirror array 104C is embedded in the substrate 101 as shown in Figure 3B. In some embodiments, the output coupling mirror array 104C is oriented least reflective to most reflective in the X direction. At each operation prior to operation 205, optional cutting, grinding, and polishing of the output coupling mirror arrays 104C or the mirrors 115 may be performed.

[0033] At operation 205, the relay grating 104A and the fold grating 104B are disposed on the substrate 101. Figure 3C is a cross-sectional view of the hybrid waveguide 100 with the relay grating 104A and the fold grating 104B. In some embodiments, the relay structures 102A and the folding structures 102B are etched into the substrate 101. In other embodiments, the relay structures 102A and the folding structures 102B are formed from a different material than the substrate 101 and disposed thereon. The relay structures 102A and the folding structures 102B are disposed onto the first surface 103 of the substrate 101. Optionally, the folding structures 102B may be disposed on the second surface 113 of the substrate 101 .

[0034] At operation 207, the prism coupler 105 is disposed on the substrate 101 . Figure 3D is a cross-sectional view of the hybrid waveguide 100 with the prism coupler 105. The prism coupler 105 is operable to in-couple light to the relay grating 104A. In some embodiments, the prism coupler 105 is disposed on the first surface 103 of the substrate 101 adjacent to the relay grating 104A. In other embodiments, the prism coupler 105 is disposed on top of the relay grating 104A.

[0035] In summation, embodiments of the present disclosure generally relate to waveguides for augmented, mixed, and/or virtual reality displays. More specifically, embodiments described herein relate to hybrid waveguides with a diffractive relay grating, a diffractive fold grating, and a reflective output mirror array. Benefits of the hybrid waveguides described herein includes reduced eye glow, reduced cosmetic defects, and increased efficiency from diffractive waveguides. Benefits also include reduced cosmetic defects, reduced modulation transfer function (MTF) degradation, and elimination of process difficulty causing low yield from reflective waveguides.

[0036] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

What is claimed is:

1. A waveguide, comprising: a substrate; a prism coupler disposed over the substrate, the prism coupler operable to couple light into the substrate; a relay grating disposed over the substrate adjacent to the prism coupler, the relay grating comprising relay structures operable to diffract light to a fold grating; the fold grating disposed over the substrate and adjacent to the relay grating, the fold grating comprising folding structures operable to split beams in a first direction to an output coupling mirror array and a second direction in the fold grating where beams in the second direction are diffracted in the first direction; and the output coupling mirror array disposed in the substrate and adjacent to the fold grating having a plurality of mirrors disposed in the substrate, the mirrors are operable to receive the beams in the first direction from the fold grating and out-couple the beams.

2. The waveguide of claim 1 , wherein at least one of a duty cycle, a depth, or a pitch varies in the fold grating.

3. The waveguide of claim 2, wherein the relay grating and the fold grating have the same pitch and orientation.

4. The waveguide of claim 1 , wherein the substrate comprises a material with a refractive index between 1.5 and 2.3.

5. The waveguide of claim 1 , wherein the relay grating, the fold grating, and the prism coupler are disposed on a first surface of the substrate.

6. The waveguide of claim 5, wherein the fold grating includes folding structures disposed on a second surface opposite the first surface of the substrate.

7. The waveguide of claim 1 , wherein the relay grating and the fold grating are surface relief gratings.

8. The waveguide of claim 1 , wherein the plurality of mirrors has at least one of the mirrors having a different reflectivity to a different mirror.

9. A waveguide, comprising: a substrate; a prism coupler disposed over the substrate, the prism coupler operable to couple light into the substrate; a relay grating disposed over the substrate adjacent to the prism coupler, the relay grating comprising relay structures operable to diffract light to a fold grating; the fold grating disposed over the substrate adjacent to the relay grating, the fold grating comprising folding structures operable to split beams in a first direction to an output coupling mirror array and a second direction in the fold grating where beams in the second direction are diffracted in the first direction, wherein the relay structures and the folding structures have the same pitch and orientation and the relay grating has a greater diffraction efficiency than the fold grating; and the output coupling mirror array disposed in the substrate and adjacent to the fold grating having a plurality of mirrors disposed in the substrate, the mirrors are operable to receive the beams in the first direction from the fold grating and out-couple the beams.

10. The waveguide of claim 9, wherein the substrate comprises with a refractive index between 1 .5 and 2.3.

11 . The waveguide of claim 9, wherein the relay grating, the fold grating, and the prism coupler are disposed on a first surface of the substrate.

12. The waveguide of claim 11 , wherein the fold grating includes folding structures disposed on a second surface opposite the first surface of the substrate.

13. The waveguide of claim 9, wherein the plurality of mirrors has at least one of the mirrors having a different reflectivity to a different mirror.

14. The waveguide of claim 9, wherein at least one of a duty cycle, a depth, or a pitch varies in the fold grating.

15. A method of forming a waveguide, comprising: bonding mirrors of varied reflectivity to a bonding substrate to form a mirror stack; cutting the mirror stack at an angle to form an output coupling mirror array with varied reflectivity; embedding the output coupling mirror array in a waveguide substrate; disposing a relay grating and a fold grating over a first surface of the waveguide substrate forming a waveguide; and disposing a prism coupler on the waveguide substrate, the prism coupler operable to couple light to the relay grating.

16. The method of claim 15, wherein forming the mirrors, comprises: cutting the mirrors from a partially reflective material using wire saw cutting; polishing the mirrors; and coating the mirrors with a dielectric material to vary a reflectivity value.

17. The method of claim 15, wherein the prism coupler is positioned adjacent to the relay grating.

18. The method of claim 15, wherein the prism coupler is positioned above the relay grating.

19. The method of claim 16, wherein the dielectric material includes titanium oxide, silicon oxide, niobium oxide, or combinations thereof.

20. The method of claim 15, wherein the relay grating and the fold grating are surface relief gratings.