Facebook Patent | Switchable diffractive optical element and waveguide containing the same

Patent: Switchable diffractive optical element and waveguide containing the same

Drawings: Click to check drawins

Publication Number: 20210191122

Publication Date: 20210624

Applicant: Facebook

Abstract

An optical device includes a light source assembly configured to generate an image light; and at least one waveguide including an in-coupling element and an out-coupling element configured to transmit, via the at least one waveguide, a plurality of light fields of the image light to an eye-box of the optical device, in a time-multiplexing manner. At least one of the in-coupling element or the out-coupling element includes at least one switchable diffractive optical grating, which includes a surface relief grating (SRG) filled with an optically anisotropic material having a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction. One of the first and second refractive principal refractive indices substantially matches a refractive index of the SRG, and the other mismatches.

Claims

1. An optical device, comprising: a light source assembly configured to generate an image light; and at least one waveguide including an in-coupling element and an out-coupling element configured to transmit, via the at least one waveguide, a plurality of light fields of the image light to an eye-box of the optical device, in a time-multiplexing manner, wherein at least one of the in-coupling element or the out-coupling element includes at least one switchable diffractive optical element, comprising: a surface relief grating (SRG) filled with a switchable optically anisotropic material having a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction, one of the first and second refractive principal refractive indices substantially matching a refractive index of the SRG, and the other mismatching the refractive index of the SRG.

2. The optical device of claim 1, wherein a light field corresponds to a predetermined portion of a field of view (FOV) of a single-color image.

3. The optical device of claim 1, wherein a light field corresponds to a predetermined portion of a field of view (FOV) of a full-color image.

4. The optical device of claim 1, wherein a light field corresponds to a single-color image of a predetermined color.

5. The optical device of claim 1, wherein the switchable optically anisotropic material includes active liquid crystals (LCs).

6. The optical device of claim 1, wherein the at least one switchable diffractive optical element is switchable between a diffraction state and a non-diffraction state via an external electric field applied to the at least one switchable diffractive optical element.

7. The optical device of claim 1, wherein the SRG is one of a slanted grating and a non-slanted grating.

8. The optical device of claim 1, wherein the at least one switchable diffractive optical element is a one-dimensional diffraction grating.

9. The optical device of claim 1, wherein: the at least one switchable diffractive optical element includes N number of switchable diffraction gratings, N being a positive integer and N.gtoreq.2, during respective time periods, the N number of switchable diffraction gratings sequentially configured to be in a diffraction state to transmit respective light fields of the plurality of light fields, and during one time period, one of the N number of switchable diffraction gratings configured to be in the diffraction state to transmit a light field of the plurality of light fields and the remaining switchable diffraction gratings configured to be in a non-diffraction state.

10. The optical device of claim 9, wherein: the N number of switchable diffraction gratings are arranged to be partially overlapped with each other.

11. The optical device of claim 9, wherein: the N number of switchable diffraction gratings are arranged to be stacked.

12. The optical device of claim 1, wherein: the at least one waveguide includes M number of waveguides arranged to be stacked, M being a positive integer and M.gtoreq.2, each of the M number of waveguides includes the at least one switchable diffractive optical element, during respective time periods, the at least one switchable diffractive optical elements included in the respective waveguides sequentially configured to be in a diffraction state to transmit respective light fields of the plurality of light fields, and during one time period, the at least one switchable diffractive optical element included in one of the M number of waveguides configured to be in the diffraction state to transmit a light field of the plurality of light fields, and the at least one switchable diffractive optical elements included in the remaining waveguides configured to be in a non-diffraction state.

13. The optical device of claim 12, wherein: the at least one switchable diffractive optical element included in the respective waveguides includes N number of switchable diffraction gratings, N being a positive integer and N.gtoreq.2.

14. A method of an optical device, comprising: during a first time period, in-coupling, by a first in-coupling grating, a first plurality of image lights corresponding to a first light field of a plurality of light fields into a first waveguide via diffraction, and decoupling, by a first out-coupling grating, the first plurality of image lights out of the first waveguide towards an eye-box of the optical device via the diffraction; during a second time period, in-coupling, by a second in-coupling grating, a second plurality of image lights corresponding to a second light field of the plurality of light fields into a second waveguide via diffraction, and decoupling, by a second out-coupling grating, the second plurality of image lights out of the second waveguide towards an eye-box of the optical device via the diffraction.

15. The method of claim 14, wherein the first light field corresponds to a first portion of a field of view (FOV) of a single-color image, and the second light field corresponds to a second portion of the FOV of the single-color image.

16. The method of claim 14, wherein the first light field corresponds to a first portion of a field of view (FOV) of a full-color image, and the second light field corresponds to a second portion of the FOV of the full-color image.

17. The method of claim 14, wherein the first light field corresponds to a single-color image of a first color, and the second light field corresponds to the single-color image of a second color.

18. The method of claim 14, wherein the first and second waveguides are a same common waveguide, the first and second in-coupling gratings are partially overlapped and disposed at a first surface or a second surface of the common waveguide, and the first and second out-coupling gratings are partially overlapped and disposed at the first surface or the second surface of the common waveguide.

19. The method of claim 14, wherein the first and second waveguides are a same common waveguide, the first and second in-coupling gratings stacked and each disposed at a first surface or a second surface of the common waveguide, and the first and second out-coupling gratings stacked and each disposed at the first surface or the second surface of the common waveguide.

20. The method of claim 14, wherein the first and second waveguides are individual waveguides, the first in-coupling and first out-coupling gratings each disposed at a first surface or a second surface of the first waveguide, and the second in-coupling and second out-coupling gratings each disposed at a first surface or a second surface of the second waveguide.

21. The method of claim 14, further comprising: during a third time period, in-coupling, by a third in-coupling grating, a third plurality of image lights corresponding to a third light field of the plurality of light fields into a third waveguide via diffraction, and decoupling, by a third out-coupling grating, the third plurality of image lights out of the third waveguide towards an eye-box of the optical device via the diffraction.

Description

BACKGROUND

[0001] Near-eye displays (NEDs) have been widely used in a variety of applications, such as video playback, gaming, and sports. NEDs have been used to realize virtual reality (VR), augmented reality (AR) or mixed reality (MR). AR or MR headsets display a virtual image overlapping with real-world images or see-through images. Pupil-expansion waveguide display systems with diffractive coupling structures are one of the most promising designs for AR/MR displays, potentially offering sun/eye-glasses form factors, a moderately large field of view (FOV), high transmittance and a large eye-box. A waveguide display system often includes a micro-display, collimator, and waveguide optics such as a waveguide combiner. The waveguide combiner integrates in-coupling and out-coupling elements that are often diffraction gratings, and a corresponding waveguide is referred to as a diffractive waveguide. Various diffraction gratings have been integrated into the waveguide, such as surface relief gratings obtained by nanofabrication or holographic gratings of various types.

BRIEF SUMMARY OF THE DISCLOSURE

[0002] One aspect of the present disclosure provides an optical device. The optical device includes a light source assembly configured to generate an image light; and at least one waveguide configured to guide the image light towards an eye-box of the optical device. The at least one waveguide includes an in-coupling element and an out-coupling element configured to transmit, via the at least one waveguide, a plurality of light fields of the image light to an eye-box of the optical device, in a time-multiplexing manner. At least one of the in-coupling element or the out-coupling element includes at least one switchable diffractive optical element, which includes a surface relief grating (SRG) filled with an optically anisotropic material. The optically anisotropic material has a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction. One of the first and second refractive principal refractive indices substantially matches a refractive index of the SRG, and the other mismatches the refractive index of the SRG.

[0003] Another aspect of the present disclosure provides a method for an optical device. The method includes during a first time period, in-coupling, by a first in-coupling grating, a first plurality of image lights corresponding to a first light field of a plurality of light fields into a first waveguide via diffraction, and decoupling, by a first out-coupling grating, the first plurality of image lights out of the first waveguide towards an eye-box of the optical device via the diffraction. The method further includes during a second time period, in-coupling, by a second in-coupling grating, a second plurality of image lights corresponding to a second light field of the plurality of light fields into a second waveguide via diffraction, and decoupling, by a second out-coupling grating, the second plurality of image lights out of the second waveguide towards an eye-box of the optical device via the diffraction.

[0004] Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The following drawings are provided for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure.

[0006] FIG. 1A illustrates a schematic diagram of a near-eye display (NED), according to an embodiment of the disclosure;

[0007] FIG. 1B illustrates a cross-section of the NED in FIG. 1A, according to an embodiment of the disclosure;

[0008] FIG. 2A illustrates a schematic diagram of a waveguide display assembly, according to an embodiment of the disclosure;

[0009] FIG. 2B illustrates a schematic diagram of a waveguide display assembly, according to another embodiment of the disclosure;

[0010] FIG. 3A illustrates a schematic diagram of a grating in a non-diffraction state, according to an embodiment of the disclosure;

[0011] FIG. 3B illustrates a schematic diagram of a grating in a diffraction state, according to an embodiment of the disclosure;

[0012] FIG. 4A illustrates a schematic diagram of a grating in a diffraction state, according to another embodiment of the disclosure;

[0013] FIG. 4B illustrates a schematic diagram of a grating in a non-diffraction state, according to another embodiment of the disclosure;

[0014] FIG. 5A illustrates a schematic diagram of a grating in a non-diffraction state, according to another embodiment of the disclosure;

[0015] FIG. 5B illustrates a schematic diagram of a grating in a diffraction state, according to another embodiment of the disclosure;

[0016] FIG. 6A illustrates a schematic diagram of a grating in a diffraction state, according to another embodiment of the disclosure;

[0017] FIG. 6B illustrates a schematic diagram of a grating in a non-diffraction state, according to another embodiment of the disclosure;

[0018] FIG. 7 illustrates a schematic diagram of a switchable diffractive optical element, according to another embodiment of the disclosure;

[0019] FIGS. 8A-8C illustrate a schematic diagram of switching an active grating, according to an embodiment of the disclosure;

[0020] FIGS. 9A-9B illustrate a schematic diagram of switching an active grating, according to another embodiment of the disclosure;

[0021] FIG. 10A illustrates a schematic diagram of a waveguide, according to an embodiment of the disclosure;

[0022] FIGS. 10B-10D illustrates an operation scheme of the waveguide in FIG. 10A to deliver different portions of field of view (FOV) in a time-multiplexing manner, according to an embodiment of the disclosure;

[0023] FIG. 10E illustrates a schematic diagram of an overlapping configuration of subgratings included in the waveguide in FIG. 10A, according to an embodiment of the disclosure;

[0024] FIG. 10F illustrates a schematic diagram of an overlapping configuration of subgratings included in the waveguide in FIG. 10A, according to another embodiment of the disclosure;

[0025] FIG. 11A illustrates a schematic diagram of a stack of waveguides, according to an embodiment of the disclosure;

[0026] FIGS. 11B-11D illustrate an operation scheme of the stack of waveguides in FIG. 11A to deliver different portions of FOV in a time-multiplexing manner, according to an embodiment of the disclosure;

[0027] FIG. 12A illustrates a schematic diagram of a stack of waveguides, according to another embodiment of the disclosure;

[0028] FIGS. 12B-12D illustrate an operation scheme of the stack of waveguides in FIG. 12A to deliver single-color images of different colors in a time-multiplexing manner, according to an embodiment of the disclosure;

[0029] FIG. 13 illustrates a flow chart of a method of an optical device for delivering different portions of FOV in a time-multiplexing manner, according to an embodiment of the disclosure;

[0030] FIG. 14 illustrates a flow chart of a method of an optical device for delivering single-color images of different colors in a time-multiplexing manner, according to another embodiment of the disclosure; and

[0031] FIG. 15 illustrate a schematic diagram of a waveguide of delivering different portions of FOV in a time-multiplexing manner, according to another embodiment of the disclosure.

DETAILED DESCRIPTION

[0032] Hereinafter, embodiments consistent with the disclosure will be described with reference to drawings, which are merely examples for illustrative purposes and are not intended to limit the scope of the disclosure. In the drawings, the shape and size may be exaggerated, distorted, or simplified for clarity. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts, and a detailed description thereof may be omitted. Further, in the present disclosure, the disclosed embodiments and the features of the disclosed embodiments may be combined under conditions without conflicts. The described embodiments are some but not all of the embodiments of the present disclosure. Based on the disclosed embodiments, persons of ordinary skill in the art may derive other embodiments consistent with the present disclosure, all of which are within the scope of the present disclosure.

[0033] The present disclosure provides an optical device. The optical device may include a light source assembly configured to generate an image light; and at least one waveguide configured to guide the image light to an eye-box of the optical device. The waveguide may include an in-coupling element and an out-coupling element, which are configured to transmit, via the waveguide, a plurality of light fields corresponding to the image light to the eye-box in a time-multiplexing manner. A light field may correspond to a predetermined portion of a field of view (FOV) of a single-color image, a predetermined portion of the FOV of a full-color image, or a single-color image of a predetermined color. At least one of the in-coupling element or the out-coupling element may include at least one switchable diffractive optical element. The switchable diffractive optical element may include a surface relief grating (SRG) filled with an active optically anisotropic material having a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction. One of the first and second principal refractive indices may substantially match a refractive index of the SRG, and the other may mismatch the refractive index of the SRG. The optically anisotropic material may include active or reorientable liquid crystals (LCs). The switchable diffractive optical element may be switchable between a diffraction state and a non-diffraction state due to reorientation of LCs in an external field, e.g., an electric field, a magnetic field, or a light, etc. The optical device may be a component of a near-eye display (NED).

[0034] The optically anisotropic material may be a uniaxial anisotropic material, whose refractive index ellipsoid has an axial symmetry with regard to its optic axis. n.sup.o.sub.AN and n.sup.e.sub.AN are principal refractive indices of the uniaxial anisotropic material. Nematic liquid crystals (LC) (except some exotic types like bend-core shaped) belong to the category of uniaxial anisotropic materials. Refractive index experienced by a light propagating in the nematic LC layer may be variable in a range between ordinary refractive index n.sup.o.sub.AN and extraordinary refractive index n.sup.e.sub.AN, depending on the angle .alpha. between the light polarization and optical axis of the optically anisotropic material. For example, the refractive index experienced by a light propagating in the nematic LC layer may be varied from n.sup.o.sub.AN to n.sup.e.sub.AN when the angle .alpha. changes from 90.degree. to 0.degree..

[0035] The switchable diffractive optical element may be polarization selective, for example, the diffractive optical element may selectively diffract a linearly polarized light having a first polarization, and transmit a linearly polarized light having a second polarization with negligible diffraction. The diffraction efficiency of the linearly polarized light having the first polarization may be controllable by the external field. The diffraction efficiency of the linearly polarized light having the first polarization may be lower than a predetermined threshold, for example, about 10%, 5%, 1%, 0.5%, 0.1% or 0.05%. In some embodiments, one of the first and second principal refractive indices may be the same as the refractive index of the SRG and, thus, the diffractive optical element may transmit the linearly polarized light having the second polarization without any diffraction.

[0036] FIG. 1A illustrates a schematic diagram of a near-eye display (NED) 100 according to an embodiment of the disclosure. In some embodiments, the NED 100 may be referred to as a head-mounted display (HMD). The NED 100 may present media to a user. Examples of media presented by the NED 100 include one or more images, video, audio, or some combination thereof. In some embodiments, audio is presented via an external device (e.g., speakers and/or headphones) that receives audio information from the NED 100, a console (not shown), or both, and presents audio data based on the audio information. The NED 100 acts as a VR device, an AR device and/or a MR device, or some combination thereof. In some embodiments, when the NED 100 acts as an AR and/or MR device, portions of the NED 100 and its internal components may be at least partially transparent.

[0037] As shown in FIG. 1A, the NED 100 may include a frame 105 and a display 110. Certain device(s) may be omitted, and other devices or components may also be included. The frame 110 may include any appropriate type of mounting structure to ensure the display 110 to be viewed as a NED by a user. The frame 105 may be coupled to one or more optical elements which together display media to users. In some embodiments, the frame 105 may represent a frame of eye-wear glasses. The display 110 is configured for users to see the content presented by the NED 100. As discussed below in conjunction with FIG. 1B, the display 110 may include at least one display assembly (not shown) for directing image light to an eye of the user. In some embodiments, the at least one display assembly may be a projection system. For illustrative purposes, FIG. 1A shows the projection system may include a projector 135 that is coupled to the frame 105.

[0038] FIG. 1B is a cross-section 150 of the NED 100 shown in FIG. 1A according to an embodiment of the disclosure. The display 110 may include at least one waveguide display assembly 115. The waveguide display assembly 115 may include a waveguide or a stack of waveguides. An exit pupil 125 may be a location where an eye 120 is positioned in an eye-box region when the user wears the NED 100. For purposes of illustration, FIG. 1B shows the cross section 150 associated with a single eye 120 and a single waveguide display assembly 115, but in alternative embodiments not shown, another display assembly which is separate from the waveguide display assembly 115 shown in FIG. 1B, may provide image light to an eye-box located at an exit pupil of another eye of the user.

[0039] The waveguide display assembly 115, as illustrated below in FIG. 1B, may be configured to direct the image light to an eye-box located at the exit pupil 125 of the eye 120. The waveguide display assembly 115 may be composed of one or more materials (e.g., plastic, glass, etc.) with one or more refractive indices that effectively minimize the weight and widen the FOV of the NED 100. In some embodiments, the waveguide display assembly 115 may be a component (e.g., the display 110) of the NED 100. In some embodiments, the waveguide display assembly 115 may be part of some other NED, or other system that directs display image light to a particular location. As shown in FIG. 1B, the waveguide display assembly 115 may be for one eye 120 of the user. The waveguide display assembly 115 for one eye may be separated or partially separated from the waveguide display assembly 115 for the other eye. In certain embodiments, a single waveguide display assembly 115 may be used for both eyes 120 of the user.

[0040] In some embodiments, the NED 100 may include one or more optical elements between the waveguide display assembly 115 and the eye 120. The optical elements may act to, e.g., correct aberrations in image light emitted from the waveguide display assembly 115, magnify image light emitted from the waveguide display assembly 115, some other optical adjustment of image light emitted from the waveguide display assembly 115, or some combination thereof. The example for optical elements may include an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, or any other suitable optical element that affects image light. In some embodiments, the NED 100 may include an adaptive dimming element 130, which may dynamically adjust the transmittance of the real-world objects viewed through the NED 100, thereby switching the NED 100 between a VR device and an AR device or between a VR device and a MR device. In some embodiments, along with switching between the AR/MR device and the VR device, the adaptive dimming element 130 may be used in the AR and/MR device to mitigate difference in brightness of real and virtual objects.

[0041] FIG. 2A illustrates a schematic diagram of a waveguide display assembly 200 according to an embodiment of the disclosure. The waveguide display assembly 200 may be implemented into NEDs for VR, AR or MR applications. As shown in FIG. 2A, the waveguide display assembly 200 may include a light source assembly 205, a waveguide 210, and a controller 215. The light source assembly 205 may include a light source 220 and an optics system 225. In some embodiments, the light source 220 may be a light source that generates coherent or partially coherent light. The light source 220 may include, e.g., a laser diode, a vertical cavity surface emitting laser, and/or a light emitting diode. In some embodiments, the light source 220 may be a display panel, such as a liquid crystal display (LCD) panel, an liquid-crystal-on-silicon (LCoS) display panel, an organic light-emitting diode (OLED) display panel, a micro light-emitting diode (micro-LED) display panel, a digital light processing (DLP) display panel, or some combination thereof. In some embodiments, the light source 220 may be a self-emissive panel, such as an OLED display panel or a micro-LED display panel. In some embodiments, the light source 220 may be a display panel that is illuminated by an external source, such as an LCD panel, an LCoS display panel, or a DLP display panel. Examples of an external sources may include a laser, an LED, an OLED, or some combination thereof. The optics system 225 may include one or more optical components that condition the light from the light source 220. Conditioning light from the light source 220 may include, e.g., transmitting, attenuating, expanding, collimating, and/or adjusting orientation in accordance with instructions from the controller 215.

[0042] The light source assembly 205 may generate an image light 230 and output the image light 230 to an in-coupling element 235 located at the waveguide 210. The waveguide 210 may expanded image light 230 to an eye 265 of the user. The waveguide 210 may receive the image light 230 at one or more in-coupling elements 235 located at the waveguide 210, and guide the received image light 230 to an out-coupling element 245 located at the waveguide 210, such that the received input image light 230 is decoupled out of the waveguide 210 towards the eye 265 via the out-coupling element 245. In some embodiments, the in-coupling element 235 may couple the image light 230 from the light source assembly 205 into the waveguide 210. The waveguide 210 may include a first surface 210-1 facing the real-world and an opposing second surface 210-2 facing the eye 265. In some embodiments, as shown in FIG. 2A, the in-coupling element 235 may be part of, or affixed to, the first surface 210-1 of the waveguide 210. In some embodiments, the in-coupling element 235 may be part of, or affixed to, the second surface 210-2 of the waveguide 210. In some embodiments, the in-coupling element 235 may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface grating, or other types of diffractive elements, or some combination thereof. In some embodiments, the in-coupling element 235 may include a diffraction grating, and a pitch of the diffraction grating may be chosen, such that the total internal reflection (TIR) occurs in the waveguide 210, and the image light 230 may propagate internally in the waveguide 210 (e.g., by total internal reflection). The in-coupling element 235 is also referred to as an in-coupling grating.

[0043] The out-coupling element 245 may be part of, or affixed to, the first surface 210-1 or the second surface 210-2 of the waveguide 210. In some embodiments, as shown in FIG. 2A, the out-coupling element 245 may be part of, or affixed to, the second surface 210-2 of the waveguide 210. In some embodiments, the out-coupling element 245 may be part of, or affixed to, the first surface 210-1 of the waveguide 210. In some embodiments, the out-coupling element 245 may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface grating, or other types of diffractive elements, or some combination thereof. In some embodiments, the out-coupling element 245 may include a diffraction grating, and the pitch of the diffraction grating may be configured to cause the incident image light 230 to exit the waveguide 210, i.e., redirecting image light 230 so that the TIR no longer occurs. Such a grating is also referred to as an out-coupling grating.

[0044] The waveguide 210 may be composed of one or more materials that facilitate total internal reflection of the image light 230. The waveguide 210 may be composed of, for example, plastic, glass, and/or polymers. The waveguide 210 may have a relatively small form factor. For example, the waveguide 210 may be approximately 50 mm wide along the x-dimension, 30 mm long along the y-dimension and 0.5-1 mm thick along the z-dimension. The controller 215 may control the operation scheme of the light source assembly 205. In some embodiments, the waveguide 210 may output the expanded image light 230 to the eye 265 with a large field of view (FOV). For example, the expanded image light 230 may be provided to the eye 265 with a diagonal FOV (in x and y) of 60 degrees and or greater and/or 150 degrees and/or less. The waveguide 210 may be configured to provide an eye-box with a width of 8 mm or greater and/or equal to or less than 50 mm, and/or a height of 6 mm or greater and/or equal to or less than 20 mm. Using the waveguide display assembly 200, the physical display and electronics may be moved to the side of the front rigid body and a fully unobstructed view of the real world may be achieved, therefore opening up the possibilities to true AR.

[0045] In some embodiments, the waveguide 210 may include additional gratings which redirect/fold and/or expand the pupil of the light source assembly 205. For example, as shown in FIG. 2B, in a waveguide display assembly 250, the waveguide 210 may further include a directing element 240 that redirects the received input image light 230 to the out-coupling element 245, such that the received input image light 230 is decoupled out of the waveguide 210 via the out-coupling element 245. The directing element 240 may be part of, or affixed to, the first side 210-1 of the waveguide 210, and the out-coupling element 245 may be part of, or affixed to, the second side 210-2 of the waveguide 210, such that the directing element 240 is arranged opposed to the out-coupling element 245. In some embodiments, the directing element 240 and the out-coupling element 245 may be structurally similar. In some embodiments, the directing element 240 may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface grating, or other types of diffractive elements, or some combination thereof. In some embodiments, the directing element 240 may include a diffraction grating, and in this case the directing element 240 is also referred to as a folding grating. In some embodiments, multiple functions, e.g., redirecting/folding and/or expanding the pupil of the light source assembly 205 may be combined into a single grating, e.g. an out-coupling grating.

[0046] Referring to FIGS. 2A-2B, in some embodiments, the waveguide display assembly 200/250 may include a plurality of waveguides stacked together, where each waveguide 210 is designed to handle, e.g., some portion of the FOV and/or some portion of the color spectrum of the virtual image. In some embodiments, the waveguide display assembly 200/250 may include a plurality of source assemblies 205 and/or a plurality of waveguides 210. Each of the source assemblies 205 may emit a monochromatic image light of a specific wavelength band corresponding to a primary color (e.g., red, green, or blue). The plurality of waveguides 210 may be stacked together to output an expanded image light 230 that is multi-colored, i.e., image light 230 of full colors. In some embodiments, each of the source assemblies 205 may emit full-color image lights corresponding to different portions of the FOV provided by the waveguide display assembly 200/250. In some embodiments, the source assembly 205 may include a plurality of light sources 220. Each of the light sources 220 may emit image lights of full colors. The full-color image lights emitted by different light sources 220 may correspond to different portions of the FOV provided by the waveguide display assembly 200/250. For example, the source assembly 205 may include three light sources 220 emitting full-color image lights corresponding to a left portion, a center portion and a right portion of the FOV, respectively.

[0047] Referring to FIGS. 2A-2B, in the waveguide display assembly 200/250, at least one of the in-coupling grating 235, the out-coupling grating 245 or the directing grating 235 may include at least one switchable diffractive optical element in accordance with an embodiment of the present disclosure. In some embodiments, the switchable diffractive optical element may be switchable between a diffraction state (or an On-state) and a non-diffraction state (or an Off-state) by an external field.

[0048] FIG. 3A-3B illustrate schematic diagrams of a grating 300 in a non-diffraction state and a diffraction state, respectively, according to an embodiment of the disclosure. As shown in FIGS. 3A-3B, the grating 300 may include a surface relief grating (SRG) 305 filled with an optically anisotropic material 315 consisting of elongated molecules. The SRG 305 may be a binary non-slanted grating. Molecules 310 of the optically anisotropic material 315 may be homogeneously aligned within the groove in the groove direction, for example, in the y-direction in FIGS. 3A-3B. The optically anisotropic material 315 may be uniaxial and have a first principal refractive index (e.g., n.sup.e.sub.AN) in the groove direction (e.g., y-direction) of the SRG 305 and a second principal refractive index (e.g., n.sup.o.sub.AN) along an in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 305. The second principal refractive index (e.g., n.sup.o.sub.AN) may substantially match a refractive index n.sub.g of the SRG 305, and the first principal refractive index (e.g., n.sup.e.sub.AN) may mismatch the refractive index n.sub.g of the SRG 305.

[0049] The SRG 305 may be fabricated from an organic material, such as amorphous or liquid crystalline polymers, crosslinkable monomers including those having LC properties (reactive mesogens (RM)), or fabricated from an inorganic material, such as metals or oxides used for manufacturing of metasurfaces. The materials of the SRG 305 may be isotropic or anisotropic. In some embodiments, the SRG 305 may be nanofabricated from a resist material that is transparent or nearly transparent to a range of EM frequencies, such as the visible band. The resist material may be a form of thermoplastic, polymer, optically transparent photoresist, and so on. After set or cured, the resist material may provide an alignment of the optically anisotropic material 315 filled into the SRG 305. That is, the SRG 305 may function as an alignment layer for the optically anisotropic material 315. Various alignment patterns and features (e.g., sub 10 nm) may be formed using the nanofabrication techniques of the SRG 305, which allows the creation of an alignment pattern of the optically anisotropic material 315 with high customizability. For example, the molecules of the optically anisotropic material 315 may be homeotropically or homogeneously or hybrid aligned within the grooves of the SRG 305. In some embodiments, the molecules 310 of the optically anisotropic material 315 may be homeotropically or homogeneously aligned within the grooves of the SRG 305 by a stretch, a light (e.g., photoalignment), an electric field, a magnetic field, or any appropriate aligning methods.

[0050] The optically anisotropic material 315 may include active materials that are switchable by an external field. The active materials may include active or reorientable liquid crystals (LCs), or polymerizable liquid crystal (LC) precursors, or some combinations thereof. In some embodiments, the polymerizable LC precursors may include reactive mesogens (RMs) that are polymerizable LC materials. In some embodiments, the grating 300 may further include two opposite substrates that form a container of the SRG 305 and the optically anisotropic material 315. In some embodiments, to enable an electrical switching of the grating 300, each substrate may be provided with a transparent electrode, such as an indium tin oxide (ITO) electrode. In some embodiments, the alignment of the optically anisotropic material 315 may be provided by one or more alignment layers other than the SRG 305, where the alignment layer may be disposed at the substrate. In some embodiments, the thickness of the optically anisotropic material 315 may be the same as a depth d of the SRG 305. In some embodiments, the thickness of the optically anisotropic material 315 may be different from the depth of the SRG 305, where the optically anisotropic material 315 disposed above the SRG 105 may be uniform and may not contribute to the diffraction.

[0051] The grating 300 may be sensitive to a linearly polarized incident light. As shown in FIG. 3A, for an incident light 320 polarized in the in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 305, due to the substantial refractive index match between n.sup.o.sub.AN and n.sub.g, the grating 300 may appear to be a substantially optically uniform plate for the incident light 320 with negligible diffraction. That is, the grating 300 may be in a non-diffraction state for the incident light 320 polarized in the in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 305. In some embodiments, the second principal refractive index (e.g., n.sup.o.sub.AN) may exactly match (or may be the same as) the refractive index n.sub.g of the SRG 305 and, thus, the incident light 320 may be transmitted through without any diffraction. That is, the diffraction effect of the grating 300 may be completely turned off.

[0052] As shown in FIG. 3B, for an incident light 330 polarized in the groove direction (e.g., y-direction) of the SRG 305, due to the refractive index difference between n.sup.e.sub.AN and n.sub.g, the light 330 may experience a periodic modulation of the refractive index in the grating 300 and become diffracted. That is, the grating 300 may be in a diffraction state for the incident light 330 polarized in the groove direction (e.g., y-direction) of the SRG 305. The diffraction efficiency of the light 330 may be determined by the modulation of refractive index nm (i.e., the difference between the n.sup.e.sub.AN and n.sub.g) provided by the grating 300. The diffraction efficiency may be controllable by an externa field, e.g. an electric field, a magnetic field, or a light, etc.

[0053] FIGS. 4A-4B illustrate a schematic diagram of a grating 400 in a diffraction state and a non-diffraction state, respectively. The similarities between FIGS. 4A-4B and FIGS. 3A-3B are not repeated, while certain differences may be explained. As shown in FIGS. 4A-4B, molecules 410 of an optically anisotropic material 415 may be homogeneously aligned within the groove in the groove direction, for example, in the y-direction in FIGS. 4A-4B. The optically anisotropic material 415 may have a first principal refractive index (e.g., n.sup.eAN) in a groove direction (e.g., y-direction) of the SRG 405 and a second principal refractive index (e.g., n.sup.oAN) along an in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 405. The second principal refractive index (e.g., n.sup.oAN) may mismatch a refractive index n.sub.g of the SRG 405, and the first principal refractive index (e.g., n.sup.e.sub.AN) may substantially match the refractive index n.sub.g of the SRG 405.

[0054] The grating 400 may be sensitive to a linearly polarized incident light. As shown in FIG. 4A, for an incident light 420 polarized in the in-plane direction (e.g., x-direction) perpendicular to the groove direction of an SRG 405, due to the refractive index difference between n.sup.o.sub.AN and n.sub.g, the light 420 may experience a periodic modulation of the refractive index in the grating 400 and, thus, get diffracted. That is, the grating 400 may be in a diffraction state for the incident light 430 polarized in the in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 405. The diffraction efficiency of the light 420 may be determined by the modulation of refractive index nm (i.e., the difference between the n.sup.o.sub.AN and n.sub.g) provided by the grating 400.

[0055] As shown in FIG. 4B, for an incident light 430 polarized in the groove direction (e.g., y-direction) of the SRG 405, due to the substantial refractive index match between n.sup.e.sub.AN and n.sub.g, the grating 400 may appear to be a substantially optically uniform plate for the incident light 420 with negligible diffraction. That is, the grating 400 may be in a non-diffraction state for the incident light 430 polarized in the groove direction (e.g., y-direction) of the SRG 405. In some embodiments, the first principal refractive index (e.g., n.sup.eAN) may exactly match (or may be the same as) the refractive index n.sub.g of the SRG 405 and, thus, the incident light 430 may be transmitted through without any diffraction. That is, the diffraction effect of the grating 400 may be completely turned off.

[0056] FIGS. 5A-5B illustrate a schematic diagram of a grating 500 in a non-diffraction state and a diffraction state, respectively. The similarities between FIGS. 3A-3B and FIGS. 5A-5B are not repeated, while certain differences may be explained. Similar to the grating 300 in FIGS. 3A-3B, the grating 500 shown in FIGS. 5A-5B may include an SRG 505 filled with an optically anisotropic material 515. Different from the binary non-slanted SRG 305 in FIGS. 3A-3B, the SRG 505 in FIGS. 5A-5B may be a binary slanted grating. The diffraction state and the non-diffraction state of the grating 500 in FIGS. 5A-5B may be referred to that of the grating 300 in FIGS. 3A-3B, and the details are not repeated here.

[0057] FIGS. 6A-6B illustrate a schematic diagram of a grating 600 in a non-diffraction state and a diffraction state, respectively. The similarities between FIGS. 4A-4B and FIGS. 6A-6B are not repeated, while certain differences may be explained. Similar to the grating 400 in FIGS. 4A-4B, the grating 600 shown in FIGS. 6A-6B may include an SRG 605 filled with an optically anisotropic material 615. Different from the binary non-slanted SRG 405 in FIGS. 4A-4B, the SRG 605 in FIGS. 6A-6B may be a binary slanted grating. The diffraction state and the non-diffraction state of the grating 600 in FIGS. 6A-6B may be referred to that of the grating 400 in FIGS. 4A-4B, and the details are not repeated here.

[0058] FIGS. 3A-6B show the diffractive optical element is an active grating that includes an SRG having a periodic rectangular profile, i.e., the cross-sectional profile of the grooves of the SRG has a periodic rectangular shape, which is for illustrative purposes and not intended to limit the scope of the present disclosure. In some embodiments, the fringes of the grating may be linear, i.e. the grating may be a one-dimensional grating. In some embodiments, the diffractive optical element may include a plurality of SRGs that are patterned and/or stacked. In some embodiments, the cross-sectional profile of the grooves of the SRG may be non-rectangular, for example, sinusoidal, triangular or saw-tooth, depending on the application scenarios. In some embodiments, the cross-sectional profile of the grooves of the SRG may be non-periodic, an exemplary diffractive optical element will be described in FIG. 7. In some embodiments, the diffractive optical element may be configured with or without optical power. The disclosed diffractive optical elements may also realize almost the same optical functions as conventional refractive optics, such as lenses, prisms or aspheres, but may be much smaller and lighter.

[0059] FIG. 7 illustrates a schematic diagram of a switchable diffractive optical element 700, according to another embodiment of the disclosure. As shown in FIG. 7, the diffractive optical element 700 may include an SRG 705 filled with an optically anisotropic material 715. Molecules 710 of the optically anisotropic material 715 may be homogeneously or homeotropically aligned within the groove, for example, homogeneously aligned in a groove direction (e.g., y-direction) of the SRG 705. The optically anisotropic material 715 may have a first principal refractive index (e.g., an extraordinary refractive index n.sup.o.sub.AN) in the groove direction (e.g., y-direction) of the SRG 705 and a second principal refractive index (e.g., an ordinary refractive index n.sup.o.sub.AN) along an in-plane direction (e.g., x-direction) perpendicular to the groove direction. One of the first principal refractive index and the second principal refractive index may substantially match a refractive index n.sub.g of the SRG 705, and the other may mismatch the refractive index n.sub.g of the SRG 705. For discussion purposes, in the diffractive optical element 700, the second principal refractive index (e.g., n.sup.o.sub.AN) of the optically anisotropic material 715 may substantially match the refractive index n.sub.g of the SRG 705, and the first principal refractive index (e.g., n.sup.e.sub.AN) in the groove direction (e.g., y-direction) of the SRG 705 may mismatch the refractive index n.sub.g of the SRG 705.

[0060] The cross-sectional profile of the grooves of the SRG 705 may have a non-periodic rectangular profile. In the in-plane direction (e.g., x-direction) perpendicular to the groove direction (e.g., y-direction) of the SRG 705, the periodicity ((w.sub.groove+w.sub.hill) of the SRG 705 may monotonically decrease from a center (c) to a periphery of the SRG 705, through which a light focusing effect is achieved. For an incident light 730 polarized in the groove direction (e.g., y-direction) of the SRG 705, due to the refractive index difference between n.sup.e.sub.AN and n.sub.g, the light 730 may experience a periodic modulation of the refractive index in the diffractive optical element 700 and become diffracted. Through configuring the cross-sectional profile of the grooves of the SRG 705, as well as, the refractive indices of the optically anisotropic material 715 and the SRG 705, diffracted light beams 740 may be further focused. That is, the diffractive optical element 700 may function a cylindrical diffractive lens. The diffractive optical element 700 may also include other elements, such as substrates, electrodes for electrically switching, alignment layers, etc., and FIG. 7 merely shows a partial structure of the diffractive optical element 700.

[0061] A switching between a diffraction state and a non-diffraction state of an active diffractive optical element (e.g., an active grating) in accordance with an embodiment of the present disclosure will be explained in the following with the accompanying FIGS. 8A-8C and FIGS. 9A-9B. In some embodiments, an active diffractive optical element in accordance with an embodiment of the present disclosure may be switchable between a non-diffraction state and a diffraction state by applying an electric field to the active LC materials, due to an electric-field-induced reorientation of the LCs filled into the SRG. In some embodiments, the active diffractive optical element may be switchable between a non-diffraction state and a diffraction state by applying a light to the active LC materials, due to a photo-induced reorientation of the LCs filled into the SRG. In some embodiments, the active diffractive optical element may be switchable between a non-diffraction state and a diffraction state by applying a magnetic field to the active LC materials, due to a magnetic-field-induced reorientation of the LCs filled into the SRG. Further, in the diffraction state, the diffraction efficiency of a light incident onto the active diffractive optical element may be continuously changeable via continuously varying the applied electric field or light or magnetic field. That is, the active diffractive optical element may be configured to provide different diffraction efficiency to an incident light, thereby satisfying different application scenarios. In some embodiments, at least one of the electrodes of the active diffractive optical element may include pixelated electrodes. A light incident onto the active diffractive optical element may irradiate one or more pixelated electrodes, and the diffraction efficiency of the light may spatially vary by applying different voltages to the different pixelated electrodes. For discussion purposes, a grating filled with active LCs is used as example to explain the switching of the disclosed diffractive optical elements in FIGS. 8A-8C and FIGS. 9A-9B.

[0062] FIGS. 8A-8C illustrate a schematic diagram of switching a diffractive optical element 800, according to an embodiment of the disclosure. For discussion purposes, the diffractive optical element 800 may be a grating 800. As shown in FIGS. 8A-8C, the grating 800 may include upper and lower substrates 810 arranged opposite to each other. Each substrate 810 may be provided with a transparent electrode at an inner surface of the substrate 810 for applying an electric field to the grating 800, such as an ITO electrode (not drawn). The grating 800 may include an SRG 805 bonded to or formed on the lower substrate 810 and an optical anisotropic material 815 filled into grooves of the SRG 805. The optical anisotropic material 815 may include active anisotropic materials, such as active LCs having positive or negative dielectric anisotropy. The optically anisotropic material 815 may have a first principal refractive index (e.g., n.sup.eAN) in the groove direction (e.g., y-direction) of the SRG 805 and a second principal refractive index (e.g., n.sup.o.sub.AN) along an in-plane direction (e.g., x-direction) perpendicular to the groove direction of the SRG 805.

……
……
……