2D light homogenization

Abstract

An optical reflective device is disclosed, the device comprising a waveguide and a longitudinal light homogenizing structure mounted to a surface of the waveguide. The light homogenizing structure can receive input light and generate longitudinal homogenized light by homogenizing the input light along the longitudinal dimension of the waveguide. A cross-coupler in the waveguide can receive the longitudinal homogenized light from the light homogenizing structure and can generate two-dimensional homogenized light by redirecting the longitudinal homogenized light along the lateral dimension of the waveguide. The light homogenizing structure can include a partially reflective layer, a stacked substrate layer with a refractive index mismatch, and / or a combination of a partially reflective layer and a completely reflective layer. The cross-coupler and / or the partially reflective layer can be formed using a hologram group. A prism or an inclined substrate surface can couple the input light into the substrate.

Description

[0001] This application is a divisional application of the invention patent application with application number 201880066065.2, priority date October 16, 2017, application date September 27, 2018, and invention name “Two-dimensional Light Homogenization”.

[0002] This patent application claims priority to U.S. Provisional Patent Application No. 62 / 572,779, filed on October 16, 2017, which is hereby incorporated by reference in its entirety. Background Art

[0003] The present invention generally relates to optical reflective devices, including light homogenization in optical reflective devices.

[0004] An optical waveguide has a physical structure that guides electromagnetic waves in a spectrum (e.g., light). In some cases, the optical waveguide can be an optical fiber or a planar waveguide structure. The optical waveguide can use total internal reflection (TIR) ​​to guide light to an output end. Depending on the angle of incidence of the light relative to the surface normal of the TIR surface, the intensity or power distribution of the light can vary. Therefore, due to the angle of incidence of the light, an area of ​​the optical waveguide can receive more or less energy from the guided light. In some cases, this spatially uneven power distribution is irrelevant to the operation of the optical waveguide. However, in other cases, the inhomogeneity of light propagating through the waveguide can lead to performance deficiencies in certain TIR device applications. Therefore, it may be beneficial to homogenize the power distribution of light for a specific area of ​​the optical waveguide or TIR device.

[0005] TIR imaging devices may suffer from performance deficiencies due to pattern non-uniformity of light propagating through a waveguide. For example, light may enter a waveguide, propagate through the waveguide, and reflect or diffract toward an exit pupil to form a projected image. The waveguide may include optical coupling devices (e.g., an input coupler, a cross coupler, and / or an output coupler). However, the light propagating through the waveguide may exhibit a spatially non-uniform power distribution within the coupled mode. Such spatially non-uniform power distribution may also result in a non-uniform intensity distribution at the optical coupling device. Therefore, such a non-uniform intensity distribution may enlarge the point spread function associated with the output light beam reflected toward the exit pupil, thereby reducing the resolution of the projected image and resulting in non-uniform brightness of the projected image. Summary of the invention

[0006] The features generally relate to one or more improved methods, systems or devices for homogenizing light in one or more dimensions. The methods, systems or devices can employ one or more light homogenizers or homogenization techniques to change the power distribution of light in a waveguide or TIR device.

[0007] In some examples, an optical device may include a waveguide having a waveguide surface. The waveguide surface may have a longitudinal dimension and a perpendicular transverse dimension. A light homogenizing structure may be formed on the waveguide. The light homogenizing structure may receive input light and generate longitudinal homogenized light by homogenizing the input light along the longitudinal dimension. The optical device may include a cross-coupler that receives the longitudinal homogenized light from the light homogenizing structure and generates two-dimensional homogenized light by redirecting the longitudinal homogenized light along the transverse dimension. The cross-coupler may include a hologram that diffracts light on both the upward path and the downward path.

[0008] In some examples, the light homogenizing structure may include a substrate and a partial reflector located on or in the substrate. The partial reflector may include a dielectric coating, a metal coating, a polymer film, a set of holograms, or other structures. In some examples, the light homogenizing structure may include a substrate having a first surface and a second surface relative to each other, a first total reflection layer located on the first surface, a second total reflection layer located on the second surface, and a partial reflection layer located on the substrate and a partial reflection layer interposed between the first total reflection layer and the second total reflection layer. In some examples, the light homogenizing structure may include: a first substrate located on a waveguide surface and having a first refractive index, a second substrate located on the first substrate and having a second refractive index different from the first refractive index, and a third substrate located on the first substrate and having a third refractive index different from the second refractive index.

[0009] In some examples, the light homogenizing structure may include a prism mounted to a substrate surface opposite to a waveguide surface. The prism may direct input light into the substrate and toward a partial reflector. In some examples, the light homogenizing structure includes a substrate having an inclined side surface that couples the input light into the substrate. The partial reflector may have a spatially varying reflectivity and may produce homogenized light at the waveguide opposite the prism and within approximately one pupil period of the waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A further understanding of the nature and advantages of specific implementations of the present disclosure may be achieved by reference to the following drawings. In the drawings, similar components or features may have the same reference label. In addition, various components of the same type may be distinguished by a reference label followed by a hyphen and a second label that distinguishes the similar components. If only the first reference label is used in the specification, the description applies to any one of the similar components having the same first reference label regardless of the second reference label.

[0011] Figure 1 is an illustration of an illustrative head mounted display (HMD) according to some embodiments, in which the principles embodied herein may be implemented.

[0012] Figure 2Ais a diagram illustrating the reflective properties of an exemplary skew mirror in real space according to some embodiments.

[0013] Figure 2B An exemplary skew mirror in k-space is shown according to some embodiments.

[0014] Figure 3A is a diagram illustrating the reflective properties of an exemplary skew mirror in real space according to some embodiments.

[0015] Figure 3B An exemplary skew mirror in k-space is shown according to some embodiments.

[0016] Figure 4A-4B An example of an illustrative optical system supporting light homogenization according to some embodiments is shown.

[0017] Figure 5A-5E An example of an illustrative optical system supporting light homogenization according to some embodiments is shown.

[0018] Fig. 6A An example of an illustrative system supporting light homogenization according to some embodiments is shown.

[0019] Figure 6B An example of an illustrative graph supporting light homogenization according to some embodiments is shown.

[0020] Figure 7A-7C An example of an illustrative optical lens supporting light homogenization according to some embodiments is shown.

[0021] Figure 8 An exemplary method for light homogenization according to some embodiments is shown.

[0022] Fig. 9 is a side view of an example optical system including a longitudinal light homogenizing structure formed above a waveguide according to some embodiments.

[0023] Fig. 10A and Fig. 10B is a top view of an exemplary optical system including a longitudinal light homogenizing structure formed above a waveguide to perform two-dimensional light homogenization across the waveguide, according to some embodiments.

[0024] Fig.11 is a side view of an illustrative longitudinal light homogenizing structure including a partially reflective layer mounted to a bottom surface of a substrate of an underlying waveguide according to some embodiments.

[0025] Fig.12is a top perspective view of an illustrative longitudinal light homogenizing structure including a vertical reflective surface mounted to a substrate of an underlying waveguide according to some embodiments.

[0026] Fig.13A and Fig. 13B An exemplary light homogenizing structure having spatially varying partial reflectors formed within a waveguide is shown according to some embodiments.

[0027] Fig.14 is a side view of an illustrative light homogenizing structure having spatially varying partial reflectors formed outside of a waveguide according to some embodiments.

[0028] Fig.15 is a top view of an illustrative spatially varying partial reflector according to some embodiments. DETAILED DESCRIPTION

[0029] An optical head-mounted display (HMD) is a wearable device that has the ability to reflect projected images and allow users to experience augmented reality. Head-mounted displays typically involve near-eye optics to produce "virtual" images. In the past, HMDs have addressed various technical limitations that reduce image quality and increase weight and size. Past implementations have included traditional optics for reflecting, refracting, or diffracting light, however, the designs are often bulky. In addition, traditional reflectors and grating structures have inherent limitations. For example, a traditional reflector may have a certain reflection axis that coincides with the surface normal. The reflection axis of a traditional reflector may result in suboptimal orientation or performance of the reflector. In addition, a traditional grating structure may include multiple reflection axes that do not accept covariance with the incident angle and / or wavelength.

[0030] Thus, an apparatus for reflecting light may include the following features: light is reflected about a reflection axis that is not constrained by a surface normal, and the reflection angle for a given incident angle is constant at multiple wavelengths. For incident light of a given wavelength, embodiments of the apparatus may have a substantially constant reflection axis (i.e., a reflection axis with a variation in the reflection axis angle of less than 1.0 degrees) over a range of incident angles, and this phenomenon may be observed using incident light of various wavelengths.

[0031] In some examples, the waveguide may have one or more light homogenizing elements to minimize the point spread function of the output beam and improve the projected image quality such as resolution and brightness. The light homogenizing element may be formed in or on a substrate that overlaps the waveguide (e.g., on the surface of the waveguide) (e.g., the light homogenizing element may be outside the waveguide). For example, the light homogenizing element may be a partially reflective element positioned parallel to the waveguide surface. In some cases, the partially reflective element may be disposed at the substrate interface of the waveguide or disposed to pass through a substrate (e.g., a plate or pipe structure) mounted to the waveguide. In some examples, the partially reflective element may be an optical element (e.g., an oblique mirror) having a reflection axis perpendicular to the waveguide surface. The light homogenizing element may split the energy of the propagating light. For example, a first portion of the energy of light incident on the surface of the light homogenizing element and propagating in a first mode direction (e.g., a downward light direction) may be reflected in a second mode direction (e.g., an upward light direction) different from the first mode direction. A second portion of the energy of light incident on the surface of the light homogenizing element and propagating in the first mode direction may be refracted or continue to propagate in the first mode direction.

[0032] Various aspects of the disclosure are initially described in the context of an apparatus for reflecting light into an eye box that is located a fixed distance away from a skew mirror. Specific examples for apparatuses including grating media are described. The grating media may include one or more grating structures. The grating structure may be configured to reflect light of a particular wavelength about a reflection axis at a particular plurality of incident angles, wherein the reflection axis is offset from a surface normal of the grating structure. Various aspects of the disclosure are further illustrated and described with reference to apparatus diagrams, system diagrams, and flow charts related to light homogenization.

[0033] This description provides examples and is not intended to limit the scope, applicability or configuration of specific implementations of the principles described herein. Rather, the following description will provide those skilled in the art with a useful description of specific implementations for implementing the principles described herein. Various changes may be made to the function and arrangement of elements.

[0034] Therefore, various specific implementations may appropriately omit, replace or add various programs or components. For example, it should be understood that the method can be performed in an order different from the order described, and various steps can be added, omitted or combined. In addition, the aspects and elements described in certain specific implementations can be combined in various other specific implementations. It should also be understood that the following systems, methods, devices and software can be individually or collectively components of a larger system, in which other programs can take precedence over or otherwise modify their application programs.

[0035] Figure 11 is an illustration of a head mounted display (HMD) 100 in which the principles included herein may be implemented. The HMD 100 may include glasses or headgear in which a near eye display (NED) 105 may be attached in front of the user's eyes. The NED 105 may include a diffractive element portion disposed within or in conjunction with a lens assembly of the HMD 100. In some examples, the diffractive element portion is a holographic optical element (HOE), which may include an angled mirror 110. Coordinates (x-axis, y-axis, and z-axis) are provided with reference to the angled mirror 110. The HMD 100 may include a light source or light projector 115 operatively coupled to the lens assembly. In some examples, the light source or light projector 115 may be operatively coupled to the lens assembly in a waveguide configuration. In some examples, the light source or light projector 115 may be operatively coupled to the lens assembly in a free space configuration.

[0036] The skew mirror 110 is a reflective device that may include a grating medium in which a volume hologram or other grating structure resides. The skew mirror 110 may sometimes be referred to herein as a volume holographic grating structure 110. The skew mirror 110 may include additional layers, such as a glass cover or a glass substrate. The additional layers may be used to protect the grating medium from contamination, moisture, oxygen, reactive chemicals, damage, etc. The additional layers may also have a refractive index that matches the grating medium. The grating medium has physical properties that allow it to diffract light around an axis called a reflection axis by virtue of the grating structure residing therein, wherein the diffraction angle (hereinafter referred to as the reflection angle) varies by less than 1° for multiple wavelengths of light incident on the grating medium at a given angle of incidence. In some cases, the reflection axis is also constant for multiple wavelengths and / or angles of incidence. In some cases, the grating structure may be formed by one or more holograms. In some implementations, one or more holograms may be volume phase holograms. Other types of holograms may also be used in various implementations of the grating structure.

[0037] Similarly, for incident light of a given wavelength, implementations typically have a substantially constant reflection axis (i.e., a reflection axis with a reflection axis angle that varies by less than 1°) over a range of incident angles, and this phenomenon can be observed using incident light of various wavelengths. In some implementations, the reflection axis remains substantially constant for each combination of a set of multiple incident angles and a set of multiple wavelengths.

[0038] A hologram can be a record of an interference pattern and can include both intensity and phase information from the light used to record it. This information can be recorded in a photosensitive medium that converts the interference pattern into an optical element that modifies the amplitude or phase of a subsequent incident light beam based on the intensity of the initial interference pattern. Grating media can include photosensitive polymers, photorefractive crystals, dichromated gelatin, photothermorefractive glass, films containing dispersed silver halide particles, or other materials that have the ability to react and record incident interference patterns. In some cases, coherent lasers can be used to record and / or read the recorded holograms.

[0039] In some cases, two or more laser beams, referred to as recording beams, may be used to record a hologram. In some cases, the recording beams may be monochromatic collimated plane beams that are similar to each other except for the angle of incidence on the grating medium. In some specific implementations, the recording beams may have different amplitudes or phase distributions from each other. The recording beams may be guided so that they intersect within the recording medium. In the case where the recording beams intersect, they interact with the recording medium in a manner that varies according to the intensity of each point of the interference pattern. This produces patterns of different optical properties in the recording medium. For example, in some embodiments, the refractive index may vary within the recording medium. In some cases, the resulting interference pattern may be spatially distributed (e.g., with a mask, etc.) in a manner that is uniform for all such grating structures recorded on the grating medium. In some cases, multiple grating structures may be superimposed in a single recording medium by changing the wavelength or angle of incidence to produce different interference patterns in the recording medium. In some cases, after recording one or more holograms in the medium, the medium may be treated with light in a post-recording light process. Post-recording photoprocessing can be performed with highly incoherent light to substantially consume remaining reactive medium components, such as photoinitiators or photosensitive monomers, so that the photosensitivity of the recording medium is greatly reduced or eliminated. After recording of a hologram or other grating structure in a recording medium is complete, the medium is often referred to as a grating medium. The grating medium is typically rendered non-photosensitive.

[0040] In some implementations, the grating structure includes a hologram generated by interference between multiple light beams, referred to as recording beams. Typically, but not necessarily, the grating structure includes multiple holograms. The multiple holograms can be recorded using a recording beam that is incident on the grating medium at different angles (i.e., multiple angles) between the multiple holograms and / or a recording beam of different wavelengths (i.e., multiple wavelengths) between the multiple holograms. In some implementations, the grating structure includes a hologram recorded using two recording beams, the two recording beams having angles of incidence on the grating medium that change when recording the holograms, and / or wavelengths that change when recording the holograms. Implementations also include an apparatus wherein the reflection axis differs from the surface normal of the grating medium by at least 1.0 degrees; or at least 2.0 degrees; or at least 4.0 degrees; or at least 9.0 degrees.

[0041] The light projector 115 may provide image-bearing light to the lens assembly. In some examples, the lens assembly and the skew mirror 110 may be substantially flat relative to the xy plane; however, in certain implementations, the lens assembly may include some curvature relative to the xy plane. Reflected light 120 from the skew mirror 110 may be reflected to an eye box located at a fixed distance away from the skew mirror 110 along the z-axis. In some examples, the skew mirror 110 may be at least partially contained in a waveguide. The waveguide may propagate incident light 130 toward the skew mirror 110 by total internal reflection. In some examples, the incident light 130 may propagate toward the skew mirror 110 through free space. The skew mirror 110 may include a grating medium made of a photosensitive polymer. The skew mirror 110 may also include one or more grating structures within the grating medium. Each grating structure may include one or more holograms or sinusoidal volume gratings that may overlap each other. In some embodiments, a hologram or a non-holographic sinusoidal volume grating is used in the grating medium. In other embodiments, both holograms and non-holographic sinusoidal volume gratings may be used in the same grating medium. The grating structures may be configured to reflect light of a specific wavelength at a specific plurality of incident angles about a reflection axis that is offset from the surface normal of the grating medium. Each grating structure within the grating medium may be configured to reflect a portion of the light toward an exit pupil in the eyebox at a fixed distance from the waveguide.

[0042] Each grating structure (e.g., each volume hologram) can reflect light differently than another grating structure. For example, a first grating structure can reflect incident light of a first wavelength at a first incident angle, while a second grating structure can reflect incident light of a second wavelength at the first incident angle (e.g., different grating structures can be configured to reflect light of different wavelengths for incident light at the same incident angle). Additionally, a first grating structure can reflect incident light of a first wavelength at a first incident angle, while a second grating structure can reflect incident light of the first wavelength at a second incident angle (e.g., different grating structures can be configured to reflect light of the same wavelength for incident light at different incident angles). Furthermore, a grating structure can reflect first incident light of a first wavelength and a first incident angle, and a grating structure can reflect second incident light at a second wavelength and a second incident angle about the same reflection axis. In this way, different grating structures can be used to selectively reflect light of a particular wavelength of incident light at a given incident angle. These different grating structures can be superimposed in the grating medium of the skew mirror 110. The skew mirror 110 may have a substantially constant (uniform) axis of reflection (eg, each grating structure of the skew mirror 110 has the same substantially constant axis of reflection).

[0043] In some examples, the head mounted display device may include a light source or light projector 115 for providing image-bearing light and a lens assembly. The lens assembly may include a skew mirror 110. The lens assembly may include a light input portion for receiving image-bearing light from the light source or light projector 115. A waveguide may be disposed within the lens assembly and operatively coupled to the light input portion. The waveguide may include at least two substrates (not shown), a grating medium disposed between the at least two substrates, a first grating structure within the grating medium, and a second grating structure within the grating medium. In some examples, the waveguide may be omitted and the light source or light projector 115 may be operatively coupled to the lens assembly in a free space configuration. The first grating structure may be configured to reflect light of a wavelength about a first reflection axis of the first grating structure, the first reflection axis being offset from a surface normal of the grating medium. The first grating structure may be configured to reflect light at a first angle of incidence. The second grating structure may be configured to at least partially not overlap with the first grating structure. The second grating structure may be configured to reflect light of the same wavelength as the light reflected by the first grating structure. The second grating structure may be configured to reflect light of a wavelength about a second reflection axis of the second grating structure that is offset from a surface normal of the grating medium. The second grating structure may be configured to reflect light at a second angle of incidence that is different from the first angle of incidence.

[0044] Figure 1 This is illustrative only and not limiting. For example, an imaging component such as a light source (e.g., a light source or light projector 115) may provide image-bearing light. A waveguide component such as an optical lens may include a light input portion. The light input portion of the optical lens may receive the image-bearing light; the waveguide may be disposed within the optical lens and operatively coupled to the light input portion. In some cases, the waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. An equalizing component may be included within the waveguide or mounted to the waveguide (and / or overlap with the waveguide). For example, a partially reflective element may be positioned between a substrate (pipe structure) mounted to the waveguide and the first waveguide surface, or may be embedded within the substrate. The partially reflective element may have a reflection axis parallel to the normal of the waveguide surface. A coupling component such as an optical coupling device may be operatively coupled to the waveguide medium.

[0045] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0046] Figure 2A is a cross-sectional view 200 illustrating the reflective properties of a skew mirror 210 in real space according to one example. The cross-sectional view 200 may include a grating structure, such as a hologram 230 in a grating medium. Figure 2ASkew mirror components other than the grating medium are omitted, such as additional layers that can serve as substrates or protective layers for the grating medium. The substrate or protective layer can be used to protect the grating medium from contamination, moisture, oxygen, reactive chemicals, damage, etc. Specific implementations of the skew mirrors used for optical coupling and / or pupil equalization can be partially reflective. For example, the skew mirrors used for pupil equalization can be configured to selectively reflect light when the light is needed to form an exit pupil toward the peep box. The skew mirrors used for pupil equalization can be configured to avoid reflecting light for certain incident angles, where such reflections reflect the light to areas that are not toward the desired exit pupil. Specific implementations of some skew mirror embodiments may require a relatively high dynamic range recording medium to achieve high reflectivity of the resulting grating medium over a relatively wide wavelength bandwidth and angle range. In contrast, the skew mirrors used for pupil equalization may require a smaller dynamic range, allowing each hologram to be stronger (e.g., recorded with greater intensity and / or longer exposure time). A skew mirror containing a stronger hologram can provide a brighter image, or allow a dimmer light projector to provide an image of the same brightness. In some cases, pupil equalization techniques are not used as skew mirrors for optical coupling devices. The skew mirror 205 is characterized by the angle of the reflection axis 225 measured relative to the z-axis. The z-axis is perpendicular to the skew mirror axis 210. The skew mirror 205 is illuminated by the incident light 215 at an internal angle of incidence measured relative to the z-axis. The main reflected light 220 can be reflected at an internal reflection angle of 180° measured relative to the z-axis. The main reflected light 220 can correspond to wavelengths of light residing in the red, green, and blue regions of the visible spectrum.

[0047] The bevel mirror 210 is characterized by an angle measured by a reflection axis 225 relative to the z-axis. The z-axis is perpendicular to the bevel mirror axis 205. The bevel mirror 210 is illuminated by an incident light 215 at an internal angle of incidence measured relative to the z-axis. The main reflected light 220 can be reflected with an internal reflection angle axis substantially perpendicular to the surface of the bevel mirror 210. In some examples, the main reflected light 220 may correspond to wavelengths of light residing in the red, green, and blue regions of the visible spectrum. For example, the red, green, and blue regions of the visible spectrum may include a red wavelength (e.g., 610 to 780 nm) band, a green wavelength (e.g., 493 to 577 nm) band, and a blue wavelength (e.g., 405 to 492 nm) band. In other examples, the main reflected light 220 may correspond to wavelengths of light residing outside the visible spectrum (e.g., infrared wavelengths and ultraviolet wavelengths).

[0048] The skew mirror 210 may have multiple hologram regions, all of which share substantially the same reflection axis 225. However, these multiple regions may each reflect light at different incident angle ranges. For example, the bottom third of the HOE containing the skew mirror 210 may contain only a subset of grating structures that reflect light upward toward the corresponding eye box. The middle third may then reflect light directly toward the corresponding eye box. The top third may then need to contain only a subset of grating structures that reflect light downward toward the corresponding eye box.

[0049] Figure 2B Shows Figure 2A The k-space distribution of the spatially varying refractive index component is usually expressed as Spatial distribution 260 passes through the origin and has an angle measured relative to the z-axis that is equal to the angle of reflection axis 225. Recording k-sphere 255 is a k-sphere corresponding to a particular write wavelength. K-space 250 may include various k-spheres corresponding to wavelengths of light residing in the red, green, and blue regions of the visible spectrum.

[0050] The k-space formalism is a method for analyzing holographic recordings and diffraction. In k-space, propagating light waves and holograms are represented by the three-dimensional Fourier transform of their distribution in real space. For example, an infinite collimated monochromatic reference beam can be represented in real space and k-space by equation (1):

[0051]

[0052] in Yes all The optical scalar field distribution under the 3D space vector position and its transformation Yes all The optical scalar field distribution under the 3D spatial frequency vector. r is the scalar complex amplitude of the field; and is a wave vector whose length represents the spatial frequency of the light wave and whose direction represents the propagation direction. In some embodiments, all light beams are composed of light of the same wavelength, so all light wave vectors must have the same length, i.e. Therefore, all optical propagation vectors must lie within the radius k. n =2πn0 / λ on a sphere, where n0 is the average refractive index (“volume index”) of the hologram, and λ is the vacuum wavelength of light. This structure is called a k-sphere. In other embodiments, multiple wavelengths of light can be decomposed into a superposition of wave vectors of different lengths located on different k-spheres.

[0053] Another important k-space distribution is the hologram itself. A volume hologram usually consists of a spatial variation of the refractive index within the grating medium. The spatial variation of the refractive index is usually expressed as It can be called a refractive index modulation pattern, and its k-space distribution is usually expressed as The refractive index modulation pattern generated by the interference between the first recording beam and the second recording beam is generally proportional to the spatial intensity of the recorded interference pattern, as shown in equation (2):

[0054]

[0055] in is the spatial distribution of the first recording beam field, and is the spatial distribution of the second recording beam field. The unary operator “*” represents the complex conjugate. The last term in equation (2) The incident second recording beam is mapped into the diffracted first recording beam. Therefore, the following equation may be obtained:

[0056]

[0057] in is a 3D cross-correlation operator. That is, the product of one light field and the complex conjugate of another light field in the spatial domain has a cross-correlation with their corresponding Fourier transforms in the frequency domain.

[0058] Typically, the hologram 230 constitutes a real-valued refractive index distribution in real space. The spatial distribution positions can be calculated by cross-correlation and Determined mathematically, or by vector difference and It is determined geometrically that and is the corresponding hologram The grating vectors spatially distributed to the origin (not shown separately). Note that by convention, wave vectors are represented by lowercase "k" and grating vectors are represented by uppercase "K".

[0059] Once recorded, the hologram 230 can be illuminated by a probe beam to produce a diffracted beam. For the purposes of this disclosure, the diffracted beam can be considered to be a reflection of the probe beam, which can be referred to as the incident beam (e.g., image-bearing light). The probe beam and its reflected beam are angularly divided by the reflection axis 225 (i.e., the angle of incidence of the probe beam relative to the reflection axis is the same as the angle of reflection of the reflected beam relative to the reflection axis). The diffraction process can be represented by a set of mathematical and geometric operations in k-space similar to those in the recording process. In the weak diffraction limit, the diffracted light distribution of the diffracted beam is given by equation (4),

[0060]

[0061] in and are the k-space distributions of the diffracted beam and the probe beam, respectively; and “*” is the 3D convolution operator. Indicates that the preceding expression is valid only when Convolution is valid when the result lies on the k-sphere. represents the polarization density distribution and is related to the inhomogeneous electric dipole moment of the grating medium induced by the probe beam. is proportional to the macro-sum.

[0062] In general, when the probe beam is similar to one of the recording beams used for recording, the effect of convolution is to reverse the cross-correlation during recording, and the diffracted beam will be substantially similar to the other recording beams used to record the hologram. When the probe beam has a different k-space distribution than the recording beam used for recording, the hologram can produce a diffracted beam that is completely different from the beam used to record the hologram. It should also be noted that although the recording beams are generally coherent, the probe beam (and the diffracted beam) are not so limited. The multi-wavelength probe beam can be analyzed as a superposition of single-wavelength beams, each beam following equation (4) with a different k-sphere radius.

[0063] The term "probe beam", which is sometimes used when describing the properties of a skew mirror in k-space, may be similar to the term "incident light", which is sometimes used when describing the reflection properties of a skew mirror in real space. Similarly, the term "diffraction beam", which is sometimes used herein when describing the properties of a skew mirror in k-space, may be similar to the term "primary reflected light", which is sometimes used herein when describing the properties of a skew mirror in real space. Therefore, when describing the reflection properties of a skew mirror in real space, sometimes it is said that the incident light is reflected by a hologram (or other grating structure) as the primary reflected light, but the statement that the probe beam is diffracted by the hologram to produce a diffraction beam essentially expresses the same meaning. Similarly, when describing the reflection properties of a skew mirror in k-space, sometimes it is said that the probe beam is diffracted by a hologram (or other grating structure) to produce a diffraction beam, but the statement that the incident light is reflected by the grating structure to produce the primary reflected light has the same meaning in the context of a specific implementation of the present disclosure.

[0064] Figure 3A is a cross-sectional view 300 illustrating the reflective properties of a skew mirror 310 in real space according to one example. The cross-sectional view 300 may include a grating structure, such as a hologram 330 in a grating medium. Figure 3AThe components of the skew mirror other than the grating medium are omitted, such as additional layers that can be used as substrates or protective layers for the grating medium. The substrate or protective layer can be used to protect the grating medium from contamination, moisture, oxygen, reactive chemicals, damage, etc. The specific implementation of the skew mirror for light homogenization can be partially reflective. In this way, the skew mirror for light homogenization can be configured to selectively reflect light in cases where the propagating light may need to be homogenized. The skew mirror for light homogenization can be configured to avoid such reflections in cases where reflecting light at certain incident angles may not be beneficial. The specific implementation of some skew mirror embodiments may require a recording medium with a relatively high dynamic range to achieve high reflectivity of the resulting grating medium over a relatively wide wavelength bandwidth and angle range. In some cases, the skew mirror for light homogenization may require a smaller dynamic range, thereby allowing each hologram to be stronger (e.g., recorded with greater intensity and / or longer exposure time). The skew mirror containing a stronger hologram can provide a brighter image, or allow a dimmer light projector to provide an image of the same brightness. The skew mirror 305 is characterized by the angle of the reflection axis 325 measured relative to the z-axis. The z-axis is perpendicular to the bevel mirror axis 310. The reflection axis 325 may coincide with the surface normal. The bevel mirror 305 is illuminated by the incident light 315 at an internal angle of incidence measured relative to the z-axis. The primary reflected light 320 may be reflected at an internal reflection angle equal to the internal angle of incidence measured relative to the z-axis. The primary reflected light 320 may correspond to wavelengths of light residing in the red, green, and blue regions of the visible spectrum.

[0065] In some examples, primary reflected light 320 may correspond to wavelengths of light residing in the red, green, and blue regions of the visible spectrum. For example, the red, green, and blue regions of the visible spectrum may include a red wavelength (e.g., 610 to 780 nm) band, a green wavelength (e.g., 493 to 577 nm) band, and a blue wavelength (e.g., 405 to 492 nm) band. In other examples, primary reflected light 320 may correspond to wavelengths of light residing outside the visible spectrum (e.g., infrared and ultraviolet wavelengths).

[0066] The skew mirror 310 may have multiple hologram regions that share substantially the same reflection axis 325. However, each of these multiple regions may reflect light at different ranges of incident angles. For example, the bottom third of the HOE containing the skew mirror 310 may contain only a subset of grating structures that reflect light upward toward the corresponding eye box. The middle third may then reflect light directly toward the corresponding eye box. The top third may then need to contain only a subset of grating structures that reflect light downward toward the corresponding eye box.

[0067] Figure 3B Shows Figure 3A The k-space distribution of the spatially varying refractive index component is usually expressed as Spatial distribution 360 passes through the origin and coincides with the z-axis, which is equal to the angle of reflection axis 325. Reflection axis 325 is substantially parallel to the surface normal. Recording k-sphere 355 is a k-sphere corresponding to a particular write wavelength. K-space 350 may include various k-spheres corresponding to wavelengths of light residing in the red, green, and blue regions of the visible spectrum.

[0068] The k-space formalism method for analyzing the holographic recording and diffraction associated with the k-space representation 350 may be described above with respect to Figure 2A The above is executed.

[0069] Figure 4A An example of an optical system 400-a supporting light homogenization according to various aspects of the present disclosure is shown. The optical system 400-a can be used in applications such as, but not limited to, head-mounted displays. The optical system 400-a can employ selective coupling to allow the optical coupling device 405-a to reflect the light 410-a toward a specific location and project a replica pupil 415-a and a replica pupil 415-b. The angles represented are internal angles relative to the surface normal of the waveguide 420-a, and refraction at the substrate interface and at the substrate-air interface is ignored for illustration purposes. The replica pupil 415-a and the replica pupil 415-b can project images corresponding to the reflection axis 425-a and the reflection axis 425-b, respectively. The optical coupling device 405-a can include a grating medium and a grating structure within the grating medium. The grating structure can have multiple holograms or sinusoidal volume gratings.

[0070] Optical system 400-a shows light source 430-a, where light 410-a enters waveguide 420-a. In order for reflected light 410-a to be reflected toward optical coupling device 405-a and project replica pupil 415-a and replica pupil 415-b, light 410-a can propagate through waveguide 420-a by total internal reflection. For example, light 410-a can propagate through waveguide 420-a and reflect between a downward mode direction and an upward mode direction. Light 410-a can be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, light 410-a can be an example of light from multiple pixels.

[0071] In some cases, reflected light 410-a in a TIR device may experience mode inhomogeneities. For example, a gap 435 caused by a TIR mode of light (e.g., a spatial mode along the propagation direction) may exist between the continuous interaction of the light and the waveguide surface. In some cases, light 410-a propagating with a large angle of incidence relative to the surface normal of the waveguide 420-a may increase the size of the gap 435 between the interaction of the light 410-a with the waveguide 420-a. Depending on the size of the gap 435 caused by the coupling mode of the light 410-a, the light 410-a reflected within the waveguide 420-a may only partially interact with the optical coupling device 405-a. Figure 4A Such partial interaction in the example of non-overlapping pattern non-uniformity can result in a non-uniform intensity distribution at the optical coupling device 405-a. The non-uniform intensity distribution can result in a spatially non-uniform power distribution 440-a. The spatially non-uniform power distribution 440-a can result in a point spread function (PSF) of the output beam associated with the replica pupil of the optical coupling device 405-a expanding, thereby reducing the resolution of the projected image.

[0072] Figure 4B An illustration of an optical system 400-b incorporating light homogenization is shown. The optical system 400-b may be used in applications such as head mounted displays. The optical system 400-b may employ selective coupling to allow the optical coupling device 405-b to reflect the light 410-b toward a specific location and project a replica pupil 415-c and a replica pupil 415-d. The angles represented are internal angles relative to the surface normal of the waveguide 420-b, and refraction at the substrate interface and at the substrate air interface is ignored for illustration purposes. The replica pupil 415-c and the replica pupil 415-d may project images corresponding to the reflection axis 425-c and the reflection axis 425-d, respectively. The optical coupling device 405-b may include a grating medium and a grating structure within the grating medium. The grating structure may have multiple holograms or sinusoidal volume gratings.

[0073] Optical system 400-b shows light source 430-b, where light 410-b enters waveguide 420-b. In order for reflected light 410-b to be reflected toward optical coupling device 405-b and project replica pupil 415-c and replica pupil 415-d, light 410-b can propagate through waveguide 420-b by total internal reflection. For example, light 410-b can propagate through waveguide 420-b and reflect between a downward mode direction and an upward mode direction. Light 410-b can be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, light 410-b can be an example of light from multiple pixels. In some cases, light 410-b can propagate downward through waveguide 420-b in one mode. In other examples, light 410-b can propagate downward through waveguide 420-b in multiple modes. That is, multiple reflections of light 410-b may occur along the surface of waveguide 420-b.

[0074] In some implementations, the light homogenizing element 445 can be positioned adjacent to where the light 410-b enters the waveguide 420-b. The light 410-b can propagate through the light homogenizing element 445, pass downward through the waveguide medium 420-b, reflect off the optical coupling device 405-b, and project substantially uniform replica pupils 415-c and 415-d. The light homogenizing element 445 can split the energy of the propagating light 410-b. In some cases, each portion of the light 410-b can also propagate through the waveguide 420-b in a first mode direction (e.g., a downward ray direction) and in a second mode direction different from the first mode direction (e.g., an upward ray direction). For example, each portion of the light 410-b can reflect off the optical coupling device 405-b and project the replica pupils 415-c and 415-d with a uniform intensity distribution. In some examples, the uniform intensity distribution can represent partial homogenization. The uniform intensity distribution at the optical coupling device 405-b can result in a partially spatially uniform power distribution 440-b. In some substantially homogenous and completely homogenous replica pupil examples, the intensity distribution at the optical coupling device 405-b can result in a step function power distribution, where each portion of the light can overlap and reflect off the optical coupling device 405-b. The partially spatially uniform power distribution 440-b can reduce the PSF of the output beams associated with the replica pupils 415-c and 415-d when compared to the spatially non-uniform power distribution 440-a.

[0075] Figure 4BThis is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 445) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0076] In some cases, the partially reflective element is configured to reflect light incident on the partially reflective element at a first set of incident angles with a first reflectivity, and to reflect light incident on the partially reflective element at a second set of incident angles with a second reflectivity. The first reflectivity may be different from the second reflectivity. In some cases, the partially reflective element is configured to reflect 40% to 60% of the light incident on the partially reflective element.

[0077] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0078] Figure 5A An example of an optical system 500-a supporting light homogenization according to aspects of the present disclosure is shown. The optical system 500-a can be used in applications such as, but not limited to, head-mounted displays. The optical system 500-a can employ selective coupling to allow a partially reflective element of the light homogenization element 505-a to reflect light 510-a toward a specific location. The angles represented are internal angles relative to the surface normal of the grating medium 515-a, and refraction at the grating medium 515-a and / or substrate interface and at the substrate air interface is ignored for illustration purposes. The replica pupil 520-a can project an image corresponding to the reflection axis 525-a. The grating medium 515-a can include a grating structure. The grating structure can have multiple holograms or a sinusoidal volume grating.

[0079] Optical system 500-a shows light source 530-a, where light 510-a enters waveguide medium 535-a. In order for light 510-a to be reflected and project a replica pupil 520-a, light 510-a can propagate through waveguide medium 535-a by total internal reflection. For example, light 510-a can propagate through waveguide medium 535-a and reflect between a downward mode direction and an upward mode direction. Light 510-a can be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, light 510-a can be an example of light from multiple pixels.

[0080] The optical system 500-a may include a grating medium 515-a at least partially located within a waveguide medium 535-a. The grating medium 515-a may be implemented in an optical coupling device (e.g., an input coupler, a cross coupler, or an output coupler) located within the waveguide medium 535-a. In some cases, the light homogenizing element 505-a may include a partially reflective element (e.g., having a reflectivity of 50% in some cases, but generally having a reflectivity between 10% and 90%) and may be positioned parallel to a first substrate 540-a (e.g., a first layer) and a second substrate 545-a (e.g., a second layer) of the waveguide medium 535-a. In other cases, the light homogenizing element 505-a may include a partially reflective element (e.g., having a reflectivity between 33% and 67% in some cases, and having a reflectivity between 10% and 90% in other cases). In some examples, the reflectivity of the partially reflective element may be based on the thickness of the waveguide and waveguide components (e.g., substrates, grating media, etc.). For example, a waveguide having a total thickness of 200 microns may be optimized using a partially reflective element having a reflectivity of approximately 33%, while a waveguide having a total thickness of 2 mm may be optimized using a partially reflective element having a reflectivity of approximately 50%. In some embodiments, the partially reflective element may have a reflectivity between 10% and 90%.

[0081] The partially reflective element of the light homogenizing element 505-a can be a metal material or an insulating material. A portion (e.g., half) of the incident light 510-a can be refracted or propagated through the partially reflective element, and another portion of the light 510-a can be reflected off the partially reflective element. In some cases, the first partially reflective plate and the second partially reflective plate of the light homogenizing element 505-a can form a baffle. That is, the first partially reflective plate can be parallel and at least partially overlap with the second partially reflective plate. In some examples, the first partially reflective plate can reflect light 510-a (e.g., at a first set of incident angles), and the second partially reflective plate can reflect light 510-a (e.g., at a second set of incident angles).

[0082] The reflectivity of the light homogenizing element 505-a may vary spatially. For example, the reflectivity of the light homogenizing element 505-a may vary throughout the length of the light homogenizing element 505-a. In some examples, the reflectivity of the spatially varying light homogenizing element 505-a may vary from 50% to 0% over a longitudinal distance of 2 mm. In other examples, the reflectivity of the spatially varying light homogenizing element 505-a may vary from 0% to 50% to 0% over a longitudinal distance of 2 mm. In some cases, the light homogenizing element 505-a may have a central portion having a constant reflectivity of 50%. The spatially varying light homogenizing element 505-a may also selectively reflect propagating light 510-a that forms an incident angle within a specific range relative to the surface normal of the waveguide.

[0083] Figure 5A This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 505-a) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0084] In some cases, the partially reflective element includes a partially reflective plate disposed within the waveguide. The partially reflective plate may include a plate surface parallel to the first waveguide surface. In some cases, the partially reflective element includes a first partially reflective plate and a second partially reflective plate disposed within the waveguide. Each of the first partially reflective plate and the second partially reflective plate may include a plate surface parallel to the first waveguide surface. Each of the first partially reflective plate and the second partially reflective plate may be configured to reflect 10% to 90% of light incident on a respective one of the first partially reflective plate and the second partially reflective plate.

[0085] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0086] Figure 5B An illustration of an optical system 500-b incorporating light homogenization is shown. Optical system 500-b may be used in applications such as, but not limited to, head mounted displays. Optical system 500-b may employ selective coupling to allow a partially reflective element of light homogenizing element 505-b to reflect light 510-b toward a specific location. Figure 5B In the figure, refraction at the grating medium 515-b, at the substrate interface, and at the substrate air interface is ignored for illustration purposes. The replica pupil 520-b can project an image corresponding to the reflection axis 525-b. The grating medium 515-b can include a grating structure. The grating structure can have multiple holograms or a sinusoidal volume grating.

[0087] Optical system 500-b shows light source 530-b, where light 510-b and light 510-c enter waveguide medium 535-b. In order for light 510-b and light 510-c to be reflected and project a replica pupil 520-b, light 510-b and light 510-c can propagate through waveguide medium 535-b by total internal reflection. For example, light 510-b and light 510-c can propagate through waveguide medium 535-b and reflect between a downward mode direction and an upward mode direction. Based on the angle of incidence, light 510-b can be reflected by light homogenizing element 505-b, while light 510-c is not reflected by light homogenizing element 505-b. Light 510-b and light 510-c can be examples of light from one pixel (e.g., an image point) and have corresponding widths (e.g., pupil widths). In some examples, light 510-b and light 510-c can be examples of light from multiple pixels.

[0088] Figure 5B This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 505-b) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0089] Alternatively or in addition, the waveguide may include a first layer (e.g., first substrate 540-b) having parallel planar surfaces and a second layer (e.g., waveguide medium 535-b) having parallel planar surfaces. The inner planar surfaces of the parallel planar surfaces of the first layer may abut the inner planar surfaces of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer has a different refractive index than the second layer. In these cases, a partially reflective element may be a boundary condition associated with the first layer and the second layer.

[0090] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0091] Figure 5CAn illustration of an optical system 500-c incorporating light homogenization is shown. The optical system 500-c may be used in applications such as, but not limited to, head mounted displays. The optical system 500-c may employ selective coupling to allow a partially reflective element of the light homogenizing element 505-c to reflect light 510-f toward a specific location. The angles represented are internal angles relative to the surface normal of the grating medium 515-c, and refraction at the grating medium 515-c and / or substrate interface and at the substrate air interface is ignored for illustration purposes. The replica pupil 520-c may project an image corresponding to the reflection axis 525-c. The grating medium 515-c may include a grating structure. The grating structure may have multiple holograms or a sinusoidal volume grating.

[0092] Optical system 500-c shows light source 530-c, where light 510-f enters waveguide medium 535-c. In order for reflected light 510-f to be reflected and project a replica pupil 520-c, light 510-f can propagate through waveguide medium 535-c by total internal reflection. For example, light 510-f can propagate through waveguide medium 535-c and reflect between a downward mode direction and an upward mode direction. Light 510-f can be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, light 510-c can be an example of light from multiple pixels.

[0093] The optical system 500-c may include a grating medium 515-c at least partially located within a waveguide medium 535-c. The grating medium 515-c may be implemented in an optical coupling device (e.g., an input coupler, a cross coupler, or an output coupler) located within the waveguide medium 535-c. In some specific implementations, the light homogenizing element 505-c may be positioned at the bottom interface of the first substrate 540-c, and the light homogenizing element 505-d may be positioned at the top interface of the second substrate 545-c. In some cases, the light homogenizing element 505-c and the light homogenizing element 505-d may include a partially reflective element (e.g., having a reflectivity of 50%). For example, multiple partially reflective elements of the light homogenizing element 505-c and the light homogenizing element 505-d may be positioned in parallel at the bottom interface of the first substrate 540-c and the top interface of the second substrate 545-c, respectively.

[0094] Figure 5C This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., one or both of light homogenizing element 505-c and light homogenizing element 505-d) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0095] Alternatively or in addition, the waveguide may include a first layer having parallel planar surfaces (e.g., first substrate 540-c) and a second layer having parallel planar surfaces (e.g., waveguide medium 535-c). The inner planar surface of the parallel planar surfaces of the first layer may abut the inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer and the second layer have matching refractive indices. In these cases, the partially reflective element may be a partially reflective coating (e.g., light homogenizing element 505-c) disposed on one or both of the inner planar surface of the parallel planar surfaces of the first layer or the inner planar surface of the parallel planar surfaces of the second layer.

[0096] Alternatively or in addition, the waveguide may include a first layer (e.g., second substrate 545-c) having parallel planar surfaces and a second layer (e.g., waveguide medium 535-c) having parallel planar surfaces. The inner planar surface of the parallel planar surfaces of the first layer may abut the inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer and the second layer have matching refractive indices. In these cases, the partially reflective element may be a partially reflective coating (e.g., light homogenizing element 505-d) disposed on one or both of the inner planar surface of the parallel planar surfaces of the first layer or the inner planar surface of the parallel planar surfaces of the second layer. The partially reflective coating may include a metallic material or an insulating material.

[0097] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0098] Figure 5D An illustration of an optical system 500-d incorporating light homogenization is shown. Optical system 500-d may be used in applications such as, but not limited to, head mounted displays. Optical system 500-d may employ selective coupling to allow a partially reflective element of light homogenizing element 505-e to reflect light toward a specific location. Figure 5D In the figure, refraction at the grating medium, at the substrate interface, and at the substrate air interface is neglected for illustration purposes. The replica pupil 520-d may project an image corresponding to the reflection axis 525-d. The grating medium 515-d may include a grating structure. The grating structure may have multiple holograms or a sinusoidal volume grating.

[0099] Optical system 500-d shows light source 530-d, where light 510-g enters waveguide medium 535-d. In order for light 510-g to be reflected and project a replica pupil 520-d, light 510-g can propagate through waveguide medium 535-d by total internal reflection. For example, light 510-g can propagate through waveguide medium 535-c and reflect between a downward mode direction and an upward mode direction. Light 510-g can be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, light 510-g can be an example of light from multiple pixels.

[0100] The optical system 500-d may include a grating medium 515-d at least partially located within a waveguide medium 535-d. The grating medium 515-d may be implemented in an optical coupling device (e.g., an input coupler, a cross coupler, or an output coupler) located within the waveguide medium 535-d. In some cases, the light homogenizing element 505-e may include a skew mirror having a light homogenizing reflection axis 550 parallel to a surface of the waveguide medium 535-d. In some cases, the skew mirror may selectively reflect based at least in part on an incident angle associated with light 510-g incident to or propagating through the skew mirror. Light 510-g including a first set of incident angles (e.g., having an absolute angle of 65°-77° relative to the surface normal) may have a higher reflectivity (e.g., a reflectivity of 50%), while light 510-g including a second set of incident angles (e.g., having an absolute angle of 49°-57° relative to the surface normal) may have a lower reflectivity (e.g., a reflectivity of 25%). For example, a hologram configured to reflect light having an incident angle within a first range (e.g., an absolute angle of 65°-77° relative to the surface normal) can be recorded more strongly. In some cases, the skew mirror can be configured to selectively reflect light 510-g that is incident on or propagates through the skew mirror in two dimensions.

[0101] The light homogenizing element 505-e may also include a grating medium and a plurality of grating structures within the grating medium. The grating structure may include a plurality of holograms or sinusoidal volume gratings. The grating structure may reflect light 510-g having a first wavelength around a reflection axis at a first set of incident angles, wherein the reflection axis deviates from a surface normal of the grating structure, and may reflect light 510-g having a second wavelength around a reflection axis at a second set of incident angles, wherein the reflection axis deviates from a surface normal of the grating structure. Each grating structure may include a different reflection axis that deviates from the normal of the corresponding grating structure.

[0102] Figure 5DThis is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 505-e) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0103] Alternatively or in addition, the waveguide can include a first layer (e.g., first substrate 540-d) having parallel planar surfaces and a second layer (e.g., waveguide medium 535-d) having parallel planar surfaces. The inner planar surfaces of the parallel planar surfaces of the first layer can abut the inner planar surfaces of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface can be a planar surface of the first layer opposite the inner planar surfaces of the parallel planar surfaces of the first layer, and the second waveguide surface can be a planar surface of the second layer opposite the inner planar surfaces of the parallel planar surfaces of the second layer.

[0104] In some examples, the optical coupling device may be disposed within at least one of the first layer or the second layer of the waveguide. The optical coupling device may include a grating medium (e.g., grating medium 515-d), a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure may be configured to reflect light of a certain wavelength around a first reflection axis (e.g., reflection axis 525-d) at a first incident angle, wherein the first reflection axis deviates from the waveguide surface normal. The second grating structure may be configured to reflect light of a certain wavelength around a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis deviates from the waveguide surface normal. In some cases, the first reflection axis and the second reflection axis are substantially parallel.

[0105] In some cases, at least one of the first grating structure or the second grating structure comprises a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure comprises a non-holographic diffractive optical element. The non-holographic diffractive optical element may include, for example, a louvered mirror or a liquid crystal grating.

[0106] Alternatively or in addition, the partially reflective element can include a grating medium (e.g., light homogenizing element 505-e), a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure can be configured to reflect light of a certain wavelength about a first reflection axis (e.g., light homogenizing reflection axis 550) at a first incident angle, wherein the first reflection axis is parallel to the waveguide surface normal. The second grating structure can be configured to reflect light of a certain wavelength about a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis is parallel to the waveguide surface normal.

[0107] In some cases, at least one of the first grating structure or the second grating structure of the partially reflective element comprises a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure of the partially reflective element comprises a non-holographic diffractive optical element. The non-holographic diffractive optical element may include, for example, a louvered mirror or a liquid crystal grating.

[0108] In some cases, the grating medium of the partially reflective element is configured to reflect 10% to 90% of light having the wavelength incident on the grating medium. In some cases, each of the first angle of incidence and the second angle of incidence has a value between 65° and 77° relative to the waveguide surface normal. In some cases, the grating medium of the partially reflective element is configured to allow a majority of the light having the wavelength to pass through the grating medium at a third angle of incidence that is different from the first angle of incidence and the second angle of incidence. That is, the light incident on the grating medium at the third angle of incidence may pass through the grating medium at a greater percentage than the light incident on the grating medium at the first angle of incidence or the second angle of incidence. In some cases, the third angle of incidence may be closer to the critical angle of the waveguide (e.g., approximately 41° in some implementations) than either of the first angle of incidence or the second angle of incidence. For example, the third angle of incidence may have a value between 49° and 57° relative to the waveguide surface normal.

[0109] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0110] Figure 5E An illustration of an optical system 500-e incorporating light homogenization is shown. The optical system 500-e may be used in applications such as, but not limited to, a head mounted display. The optical system 500-e may employ selective coupling to allow a partially reflective element of the light homogenizing element 505-f to reflect light toward a specific location. The optical system 500-e illustrates a light source 530-e, wherein light 510-h enters a waveguide 535-e. For example, the light 510-h may propagate through the waveguide 535-e and reflect between a downward mode direction and an upward mode direction. The light 510-h may be an example of light from one pixel (e.g., an image point) and have a corresponding width (e.g., a pupil width). In some examples, the light 510-h may be an example of light from multiple pixels.

[0111] In some examples, the light homogenizing element 505-f may include a partially reflective element (e.g., having a reflectivity of 50%) in a waveguide 535-e without a recording medium layer. The waveguide 535-e may be formed by bonding two glass plates having a thickness of half the total thickness of the waveguide 535-e, such that the partially reflective element is disposed between two adjacent surfaces, a top plate 555-a and a bottom plate 555-b. In some cases, the partially reflective element may be a partially reflective coating applied to the surface of at least one of the two plates. In some examples, the light homogenizing element 505-f may be used with or disposed within a waveguide system including one or more couplers consisting of a volume hologram. In other examples, the light homogenizing element 505-f may be used with or disposed within a waveguide system including one or more couplers consisting of a diffractive optical element (DOE), a louvered mirror, and / or a liquid crystal grating.

[0112] Figure 5E This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 505-f) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0113] Alternatively or in addition, the waveguide may include a first layer (e.g., first plate 555-a) having parallel planar surfaces and a second layer (e.g., second plate 555-b) having parallel planar surfaces. The inner planar surface of the parallel planar surfaces of the first layer may abut the inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer and the second layer have matching refractive indices. In these cases, the partially reflective element may be a partially reflective coating (e.g., light homogenizing element 505-f) disposed on one or both of the inner planar surface of the parallel planar surfaces of the first layer or the inner planar surface of the parallel planar surfaces of the second layer.

[0114] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0115] Fig. 6AAn exemplary system 600-a supporting light homogenization according to aspects of the present disclosure is shown. System 600-a may include a first substrate 605, a second substrate 610, and a waveguide medium 615. Light 620-a may enter the first substrate 605 at an incident angle 622 relative to the surface normal. Light 620-b may reflect off the bottom surface of the first substrate 605. Refracted light 625 may enter the waveguide medium 615 at a refraction angle 627 relative to the surface normal.

[0116] In some examples, a refractive index mismatch may be purposefully applied to the waveguide 615 in order to homogenize the light. In some cases, a refractive index mismatch may be configured between the first substrate 605 and the waveguide medium 615 and / or between the second substrate 610 and the waveguide medium 615. For example, the first substrate 605 and the second substrate 610 may have a refractive index of 1.83, and the waveguide medium 615 may have a refractive index of 1.53. Light incident on the boundary of the first substrate 605 and the waveguide medium 615 at an incident angle of 54.5° relative to the surface normal will produce refracted light with a second incident angle of 77° and a reflectivity of 0.26 associated with the reflected wave. When the waveguide medium 615, the top substrate 605, and the bottom substrate 610 are each 0.5 mm thick, the resulting replicated pupil period is 6.6 mm. In contrast, when the top substrate 605 and the bottom substrate 610 have a refractive index of 1.53, the incident angle of light remains at 77° but the pupil replication period becomes 13.0 mm.

[0117] In some cases, increasing the refractive index of the first substrate 605 and the second substrate 610 may result in a shortened pupil replication period. For example, the TIR geometry and properties of the optical elements in the waveguide may be configured to homogenize the light. In other cases, increasing the refractive index of the first substrate 605 and the second substrate 610 may not affect the pupil size of the waveguide. For example, reducing the pupil replication period and maintaining the same pupil size improves homogenization of propagating light, especially light with an incident angle close to the critical angle.

[0118] In some cases, the system 600-a may include a grating medium (not shown) at least partially located within the waveguide medium 615. The grating medium may be implemented in an optical coupling device (e.g., an input coupler, a cross coupler, or an output coupler) located within the waveguide medium 615. In some examples, a refractive index mismatch may be imposed on a portion of the first substrate 605 or the second substrate 610 of the waveguide medium 615. That is, the first substrate 605 and the waveguide medium 615 may have matching refractive indices, and the second substrate 610 and the waveguide medium 615 may have different refractive indices. For example, the refractive index mismatch between the second substrate 610 and the waveguide medium 615 may result in Fresnel reflections off the top surface of the second substrate 610. In some cases, the TIR pupil geometry may improve homogenization by reducing the pupil replication period and keeping the pupil size constant. In other examples, the reflectivity mismatch may produce reflections that affect the hologram recording during hologram recording.

[0119] Fig. 6A This is exemplary only and not limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. The waveguide may include a first layer (e.g., first substrate 605) having parallel planar surfaces and a second layer (e.g., waveguide medium 615) having parallel planar surfaces. The inner planar surface of the parallel planar surfaces of the first layer may abut the inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer has a different refractive index than the second layer. For example, the first layer may have a higher refractive index than the second layer. In these cases, a partially reflective element may be a boundary condition associated with the first layer and the second layer. The partially reflective element may have a reflection axis parallel to the normal of the waveguide surface.

[0120] Alternatively or in addition, the waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. The waveguide may include a first layer (e.g., bottom substrate 610) having parallel planar surfaces and a second layer (e.g., waveguide medium 615) having parallel planar surfaces. The inner planar surface of the parallel planar surfaces of the first layer may abut the inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer. In some cases, the first layer has a different refractive index than the second layer. For example, the first layer may have a higher refractive index than the second layer. In these cases, a partially reflective element may be a boundary condition associated with the first layer and the second layer. The partially reflective element may have a reflection axis parallel to the normal of the waveguide surface.

[0121] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0122] Figure 6B An exemplary graph 600 - b supporting light homogenization according to aspects of the present disclosure is shown. The graph 600 - b includes an axis 625 - a that can represent reflectivity and an axis 625 - b that can represent incident angle. The graph 600 - b may also include a first graph line 630 , a second graph line 635 , and a difference segment 645 .

[0123] The first curve graph 630 may represent the reflectivity of a transverse electric (TE) polarized incident light wave for matched boundary conditions (i.e., equal refractive index) over a range of incidence angles. In some examples, the first curve graph 630 may represent the reflectivity of a transverse electric (TE) polarized incident light wave for light traveling from the top substrate to the waveguide medium (i.e., from a medium with a higher refractive index to a medium with a lower refractive index) over a range of incidence angles.

[0124] In some examples, the second graph 635 may represent the reflectivity of a TE polarized incident light wave for a mismatched boundary condition (i.e., the refractive index is different on opposite sides of the boundary) over a range of incident angles. In other examples, the second graph 635 may represent the reflectivity of a transverse electric (TE) polarized incident light wave for light traveling from the waveguide medium to the second substrate 610 (i.e., from a medium with a lower refractive index to a medium with a higher refractive index) over a range of incident angles.

[0125] The difference segment 645 can show the difference between n s The incident angle of light is 54.5° and n m The difference associated with light with an incident angle of 77° (e.g., n m =1.53 and n s=1.83). The reflectivity of the boundary in both directions is 0.26. A second curve graph 635 may show the reflectivity for the boundary condition associated with a refractive index difference of 0.3. Light with an incident angle of 77° may result in a reflectivity of 0.26. Therefore, Fresnel reflection may be highest for light with a large incident angle relative to the surface normal of the waveguide (e.g., propagating light with an incident angle close to 90°). Fresnel reflection caused by light with an incident angle close to the critical angle will have little effect on improving light homogenization in the waveguide.

[0126] Fig. 7A An example of an optical lens 700-a supporting light homogenization according to aspects of the present disclosure is shown. The optical lens 700-a may include a waveguide 705-a, a light input portion 710-a, a first optical coupling device 715-a (e.g., a cross coupler), a second optical coupling device 720-a (e.g., an output coupler), and light homogenizing elements 725-a and 725-b. The waveguide 705-a may also include another optical coupling device (e.g., an input coupler); however, for illustration purposes, the input coupler is omitted. The waveguide 705-a may include a first surface and a second surface parallel to each other, as well as a light receiving end adjacent to the light input portion 710-a and a light output end located far from the light input portion 710-a.

[0127] In some examples, the light homogenizing element 725-a may be located between the light input portion 710-a and the first optical coupling device 715-a. In other examples, the light homogenizing element 720-b may be located between the first optical coupling device 715-a and the second optical coupling device 720-a. In some cases, the light homogenizing element 725-a may be located between the input coupler (not shown) and the first optical coupling device 715-a. The light homogenizing element 725-a or the light homogenizing element 725-b may be located throughout the waveguide 705-a. However, if the light homogenizing element 725-a and the light homogenizing element 725-b may be located throughout the waveguide 705-a, there may be problems in recording the first optical coupling device 715-a and the second optical coupling device 720-a. In some cases, the light homogenizing element 725-a and the light homogenizing element 725-b may or may not overlap with the first optical coupling device 715-a and the second optical coupling device 720-a in space. In some examples, the light homogenizing element 725 - a may or may not spatially overlap with the light input portion 710 - a .

[0128] In some cases, light homogenizing element 725-a may be configured to reflect and refract light in a first direction (e.g., propagate light in the y-axis direction), and light homogenizing element 725-b may be configured to reflect and refract light in a second direction (e.g., propagate light in the x-axis direction). In other examples, light homogenizing element 725-a and light homogenizing element 725-b may be two-dimensional such that the homogenizing elements are configured to simultaneously direct light in a first direction (e.g., propagate light in the x-axis direction) and a second direction (e.g., propagate light in the y-axis direction or the z-axis direction).

[0129] Fig. 7A This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., one of light homogenizing element 725-a and light homogenizing element 725-b) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0130] Alternatively or additionally, the waveguide may include a first layer having parallel planar surfaces and a second layer having parallel planar surfaces. An inner planar surface of the parallel planar surfaces of the first layer may abut an inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer.

[0131] In some examples, the partially reflective element can be a partially reflective coating disposed on one or both of the inner planar surface of the planar-parallel surfaces of the first layer or the inner planar surface of the planar-parallel surfaces of the second layer.

[0132] In some examples, an optical coupling device (e.g., one or both of optical coupling device 715-a and optical coupling device 720-a) may be disposed within at least one of the first layer or the second layer of the waveguide. The optical coupling device may include a grating medium, a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure may be configured to reflect light of a certain wavelength around a first reflection axis at a first incident angle, wherein the first reflection axis deviates from the normal to the waveguide surface. The second grating structure may be configured to reflect light of a certain wavelength around a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis deviates from the normal to the waveguide surface. In some cases, the first reflection axis and the second reflection axis are substantially parallel.

[0133] In some cases, at least one of the first grating structure or the second grating structure comprises a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure comprises a non-holographic diffractive optical element. The non-holographic diffractive optical element may include, for example, a louvered mirror or a liquid crystal grating.

[0134] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0135] Figure 7B An example of an optical lens 700-b supporting light homogenization according to aspects of the present disclosure is shown. The optical lens 700-b may include a waveguide 705-b, a light input portion 710-b, a first light coupling device 715-b (e.g., a cross coupler), a second light coupling device 720-b (e.g., an output coupler), and a light homogenizing element 725-c. The pupil expander may also include another light coupling device (e.g., an input coupler); however, for illustration purposes, the input coupler is ignored. The waveguide 705-b may include a first surface and a second surface parallel to each other, and a light receiving end adjacent to the light input portion 710-b and a light output end located far from the light input portion 710-b. In some cases, the light homogenizing element 725-c may spatially overlap with the first light coupling device 715-b. In some examples, the light homogenizing element 725-c may or may not spatially overlap with the light input portion 710-b.

[0136] Figure 7B This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 725-c) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0137] Alternatively or additionally, the waveguide may include a first layer having parallel planar surfaces and a second layer having parallel planar surfaces. An inner planar surface of the parallel planar surfaces of the first layer may abut an inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer.

[0138] Alternatively or in addition, the partially reflective element can include a grating medium (e.g., light homogenizing element 725-c), a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure can be configured to reflect light of a certain wavelength about a first reflection axis at a first incident angle, wherein the first reflection axis is parallel to the waveguide surface normal. The second grating structure can be configured to reflect light of a certain wavelength about a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis is parallel to the waveguide surface normal.

[0139] In some cases, at least one of the first grating structure or the second grating structure of the partially reflective element includes a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure of the partially reflective element includes a non-holographic diffractive optical element.

[0140] In some examples, an optical coupling device (e.g., one or both of optical coupling device 715-b and optical coupling device 720-b) may be disposed within at least one of the first layer or the second layer of the waveguide. The optical coupling device may include a grating medium, a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure may be configured to reflect light of a certain wavelength around a first reflection axis at a first incident angle, wherein the first reflection axis deviates from the normal to the waveguide surface. The second grating structure may be configured to reflect light of a certain wavelength around a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis deviates from the normal to the waveguide surface. In some cases, the first reflection axis and the second reflection axis are substantially parallel.

[0141] In some cases, at least one of the first grating structure or the second grating structure comprises a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure comprises a non-holographic diffractive optical element. The non-holographic diffractive optical element may include, for example, a louvered mirror or a liquid crystal grating.

[0142] In some cases, the partially reflective element spatially overlaps the grating medium. For example, at least one hologram of the plurality of holograms in the grating structure of the partially reflective element (e.g., light homogenizing element 725-c) may at least partially spatially overlap at least one hologram of the plurality of holograms in the grating structure of the optical coupling device (e.g., optical coupling device 715-b). For example, the spatially overlapping holograms overlap relative to the space occupied or shared volume by the two holograms in a continuous grating medium (e.g., the two spatially overlapping holograms share or coexist in at least a portion of the same space or volume within one or more grating media). In this way, at least some of the varying refractive index properties and associated fringe patterns of the first hologram will occupy the same space or volume as (and be superimposed or mixed with) at least some of the varying refractive index properties and associated fringe patterns of the second hologram within the one or more grating media.

[0143] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0144] Figure 7C An example of an optical lens 700-c supporting light homogenization according to aspects of the present disclosure is shown. The optical lens 700-c may include a waveguide 705-c, a light input portion 710-c, a first light coupling device 715-c (e.g., a cross coupler), a second light coupling device 720-c (e.g., an output coupler), and a light homogenizing element 725-d. The pupil expander may also include another light coupling device (e.g., an input coupler); however, for illustration purposes, the input coupler is ignored. The waveguide 705-c may include a first surface and a second surface parallel to each other, and a light receiving end adjacent to the light input portion 710-c and a light output end located distal to the light input portion 710-c. In some cases, the light homogenizing element 725-d may include a radial component and spatially overlap a portion of the first light coupling device 715-c. In some examples, the light homogenizing element 725-d may or may not spatially overlap the light input portion 710-c.

[0145] Figure 7C This is for illustrative purposes only and is not intended to be limiting. For example, a waveguide may have a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. A partially reflective element (e.g., light homogenizing element 725-d) may be positioned between the first waveguide surface and the second waveguide surface. The partially reflective element may have a reflection axis parallel to the normal to the waveguide surface.

[0146] Alternatively or additionally, the waveguide may include a first layer having parallel planar surfaces and a second layer having parallel planar surfaces. An inner planar surface of the parallel planar surfaces of the first layer may abut an inner planar surface of the parallel planar surfaces of the second layer of waveguide. In some examples, the first waveguide surface may be a planar surface of the first layer opposite to the inner planar surface of the parallel planar surfaces of the first layer, and the second waveguide surface may be a planar surface of the second layer opposite to the inner planar surface of the parallel planar surfaces of the second layer.

[0147] In some examples, the partially reflective element can be a partially reflective coating disposed on one or both of the inner planar surface of the planar-parallel surfaces of the first layer or the inner planar surface of the planar-parallel surfaces of the second layer.

[0148] In some examples, an optical coupling device (e.g., one or both of optical coupling device 715-c and optical coupling device 720-c) may be disposed within at least one of the first layer or the second layer of the waveguide. The optical coupling device may include a grating medium, a first grating structure within the grating medium, and a second grating structure within the grating medium. The first grating structure may be configured to reflect light of a certain wavelength around a first reflection axis at a first incident angle, wherein the first reflection axis deviates from the normal to the waveguide surface. The second grating structure may be configured to reflect light of a certain wavelength around a second reflection axis at a second incident angle different from the first incident angle, wherein the second reflection axis deviates from the normal to the waveguide surface. In some cases, the first reflection axis and the second reflection axis are substantially parallel.

[0149] In some cases, at least one of the first grating structure or the second grating structure comprises a hologram (e.g., a volume phase hologram). In some cases, at least one of the first grating structure or the second grating structure comprises a non-holographic diffractive optical element. The non-holographic diffractive optical element may include, for example, a louvered mirror or a liquid crystal grating.

[0150] In some cases, the partially reflective element may at least partially overlap the grating medium of an optical coupling device (e.g., optical coupling device 715-c) in a direction normal to the plane of the first waveguide surface.

[0151] Additional examples and various implementations are contemplated using the light homogenization techniques described herein.

[0152] Figure 8 A flow chart illustrating a method 800 for light homogenization according to aspects of the present disclosure is shown. The operations of the method 800 may be implemented by a waveguide or components thereof as described herein. For example, the operations of the method 800 may be performed by the waveguide described with reference to FIGS. 4-7 .

[0153] At block 805, the waveguide may propagate light in a TIR waveguide having a first waveguide surface and a second waveguide surface parallel to the first waveguide surface. The operations of block 805 may be performed according to the methods described herein. In some examples, aspects of the operations of block 805 may be performed by the waveguide described with reference to FIGS. 4-7.

[0154] At block 810, the waveguide may reflect a portion of the light via a partially reflective element located between a first waveguide surface and a second waveguide surface, the partially reflective element having a reflection axis parallel to a normal to the waveguide surface. The operations of block 810 may be performed according to the methods described herein. In some examples, aspects of the operations of block 810 may be performed by the partially reflective element described with reference to FIGS. 4-7.

[0155] In the above text, combined with Figure 4- Figure 8 In the examples described, light homogenization is performed within the waveguide. Sometimes, it may be desirable to be able to perform light homogenization at a location outside the waveguide and outside the grating medium. If desired, the above description in conjunction with FIG. Figure 8 The described structures and methods may be performed using a light homogenizing structure located on or at a surface (e.g., outer surface) of a waveguide (e.g., external to the waveguide and separate from a grating medium used to perform redirecting operations such as input coupling, output coupling, or cross coupling).

[0156] Fig. 9 4 is a side view showing how the light homogenizing structure can achieve light homogenization in a waveguide (eg, the waveguide 420 of FIG. 4 , the substrate and grating medium shown in FIG. 5-FIG . 7 , etc.). Fig. 9 As shown, the system 900 may include a light homogenizing structure 904 mounted to a surface 910 (e.g., an exterior of the waveguide 902) of the waveguide 902. The surface 910 may be, for example, a surface of a top waveguide substrate of the waveguide 902 (e.g., a grating medium may be sandwiched between a top waveguide substrate and a bottom waveguide substrate of the waveguide 902). The example of using a waveguide to form the structure 902 is described herein only as an example, and the structure 902 may be any desired substrate with a grating medium having a holographic element (e.g., for performing input coupling, output coupling, and / or cross coupling) if desired.

[0157] The light homogenizing structure 904 may be implemented using any of the systems described above in conjunction with FIGS. 4-7 (e.g., the system 400 of FIG. 4 , the system 500 of FIG. 5 , the system 600 of FIG. 6 , and / or the system 700 of FIG. 7 may be implemented in any of the systems described above in conjunction with FIGS. 4-7 ). Fig. 9 The waveguide 902 may have a lateral surface extending in a two-dimensional XY plane. The waveguide 902 may have a longitudinal dimension (parallel to the X-axis) and a perpendicular lateral dimension (parallel to the Y-axis). Fig. 9906. As shown, the light homogenizing structure 904 can homogenize light along the longitudinal dimension as indicated by arrow 906. The light homogenizing structure 904 can also couple light downward into a cross-coupler 909 within the waveguide 902, and / or couple to a cross-coupler outside the waveguide 902 (e.g., a cross-coupler formed using an angled mirror or other optical element). The cross-coupler 909 can be formed using gratings in a grating medium sandwiched between a top waveguide substrate and a bottom waveguide substrate of the waveguide 902, can be formed at the surface of the waveguide, or can be formed outside the waveguide and inserted between the structure 904 and the waveguide 902. The cross-coupler 909 can be used to homogenize light along the transverse dimension that is homogenized by the structure 904 along the longitudinal dimension as indicated by arrow 908 (out of the page). In general, the light homogenizing structure 904 and the cross-coupler can be used to homogenize the input light 908 across the entire two-dimensional transverse area of ​​the waveguide (e.g., homogenize in two dimensions along the longitudinal axis and the transverse axis).

[0158] Fig. 10A yes Fig. 9 A top view of the structure shown (e.g., along Fig. 9 912). Fig. 10A As shown, system 1000 (eg, Fig. 9 The system 900 may include a surface 1010 (e.g., Fig. 9 The surface 910 of the waveguide 1002 (e.g., Fig. 9 waveguide 902). Light homogenizing structure 1004 (e.g., Fig. 9 The light homogenizing structure 904 of the waveguide 1002 may extend along the longitudinal dimension of the waveguide 1002 (e.g., along the X-axis) and the transverse dimension of the waveguide 1002 (e.g., along the Y-axis). An input coupler 1012 (e.g., a prism, an angled surface of a substrate in the light homogenizing structure 1004, other collimating optical devices, etc.) may couple light into the light homogenizing structure 1004. The light may propagate in the longitudinal dimension through the homogenizing structure 1004, as indicated by arrows 1006. During the propagation of the light, a replicated pupil 1014 may be generated at locations where the light reflects off the surface of the waveguide 1002. The light homogenizing structure 1004 may be used to homogenize the light in the longitudinal dimension 1006 to generate longitudinally homogenized light 1016 in the longitudinal dimension (e.g., the light may be dispersed throughout the longitudinal dimension by the light homogenizing structure, rather than generating a single pupil 1014, thereby providing a more continuous, more uniform light in the longitudinal dimension). Fig. 10A The pupils 1014 shown illustrate a pre-homogenized condition, where the gaps between the pre-homogenized pupils have not yet been filled by the homogenization structure. Fig. 10A As shown, the homogenizer may need to run a distance (e.g. Fig. 10A The three pupil periods shown in Figure 2) can achieve complete averaging in any dimension. Therefore, Fig. 10AThe top row shown in FIG. 1004 is non-homogeneous before the third pupil (e.g., the beginning of frame 1016). Then, when light from the top row is coupled downward, the entire pattern from the top row is non-homogeneous in the downward direction before the third row (e.g., the beginning of frame 1020). Thus, the homogenizing structure 1004 may extend over at least the first three columns and rows of the pupil 1014 in the waveguide 1002. This is merely illustrative, and the homogenizing structure 1004 may extend over a larger area (e.g., over the entire waveguide) if desired. In some embodiments, longitudinal homogenization may be achieved in a single worst pupil replication cycle.

[0159] The light homogenizing structure 1004 can also couple the light homogenized in the longitudinal direction to a cross-coupler located on the surface 1010 (e.g., adjacent to the light homogenizing structure 1004 or below the light homogenizing structure 1004) or located within the waveguide 1010 (e.g., the cross-coupler can be embedded in the waveguide 1002 such as Fig. 9 gratings within the cross-coupler 909 shown). The light homogenizing structure 1004 may overlap with, for example, the cross-coupler. The cross-coupler itself may homogenize the light in the lateral dimension, as shown by arrows 1008 and homogenized light 1018. Since the cross-coupler receives light from the light homogenizing structure 1004 that has been homogenized in the longitudinal dimension, the cross-coupler may homogenize the longitudinal homogenized light in the lateral direction to produce a two-dimensional homogenized light 1020 region (e.g., the longitudinal light homogenizing structure 1004 and the cross-coupler may effectively produce the two-dimensional homogenized light 1020). The two-dimensional homogenized light may be an output that is coupled to an eyebox or redirected in any desired direction (e.g., using a diffraction grating in the waveguide 1002).

[0160] exist Fig. 10A In the example of FIG. 1 , the cross-coupler laterally homogenizes the light by diffracting only light passing upward (or downward) as the light propagates through the waveguide 1012 (e.g., where "upward" and "downward" are described with reference to the Z-axis). This is merely illustrative. In another suitable arrangement, a cross-coupler located near the center thickness of the waveguide may diffract light both upward through the dielectric layers in the waveguide 1002 and downward through the dielectric layers in the waveguide 1002. Fig. 10B is a diagram showing how the cross-coupler can homogenize the light in this example. Fig. 10B As shown, in this scenario, additional interactions of the cross-couplers can produce additional rows of homogenized light 1016' that are offset (e.g., by half a pupil period) relative to row 1016. This produces a denser pupil hexagonal pattern (see, e.g., Fig. 10B 1020'), so that relative to Fig. 10A The rectangular pattern shown further improves homogenization.

[0161] Fig.11FIG. 4 is a side view of an exemplary longitudinal light homogenizing structure that can be used to homogenize input light in the longitudinal dimension. Fig.11 As shown, system 1100 may include a waveguide 1101 (e.g., waveguide 1002 and Fig. 9 10 and 1110 of the waveguide 902) (e.g., the surface 1010 and Fig. 9 The waveguide 1102 may include a longitudinal light homogenizing structure 1104 (e.g., the light homogenizing structure 1004 of FIG. 10 and the light homogenizing structure 904 of FIG. 4 ) on a surface 910 of the waveguide. The waveguide 1102 may include, for example, a grating medium 1114 sandwiched between two waveguide substrates. The grating medium 1114 may include a grating structure (e.g., a hologram).

[0162] The light homogenizing structure 1104 may include a substrate 1116 (sometimes referred to herein as a duct 1116 or duct structure 1116). The substrate 1116 may extend along the longitudinal dimension of the waveguide 1102 (e.g., parallel to the X-axis). In addition, the optical system 1100 may include a light input portion such as a prism 1112. The prism 1112 may be mounted to a surface 1118 of the substrate 1116 or to a separate prism mount coupled to the left edge of the substrate 1116. The light 1108 may be incident at the prism 1112 and coupled to the substrate 1116 via the prism 1112 (or via the prism mount).

[0163] The bottom surface of the substrate 1116 may be coated with a partially reflective layer such as partially reflective layer 1120. The partially reflective layer 1120 may be a dielectric coating, a metal coating, a polymer film, a polarizing layer, a refractive index mismatch layer, a grating structure (e.g., one or more holograms), or any other desired structure. The partially reflective layer 1120 may be formed on the bottom surface of the substrate 1116, on the surface 1110 of the waveguide 1102, formed by a separate layer interposed between the substrate 1116 and the surface 1110, or may be embedded within the substrate 1116.

[0164] One or more light beams (e.g., modes) 1108 of the input-coupled pupil region 1122 may be input-coupled via the prism 1112 and directed through the substrate 1116. The modes 1108 may be collimated and propagated through the light homogenizing element 1104 (e.g., in the longitudinal dimension as indicated by arrow 1106) and uniformly transmitted into the waveguide 1120 through the partially reflective layer 1102. As an example, each collimated mode 1108 may correspond to a display pixel for image projection. The properties of the light homogenizing structure 1104 (including adhesion to the waveguide 1102 and reflectivity of the partially reflective layer 1120) may allow for a larger replica pupil copy across the region 1124 at the waveguide 1102. The region 1124 of the replica pupil copy may correspond to a length W associated with a ray path of the one or more input-coupled light modes 1108 through the substrate 1104. For example, the region 1124 may correspond to the light 1016 of FIG. 10. Thus, the coupling structure can act as a pupil expander, thereby increasing the replica pupil copy of the optical system 1100. The replica pupil copy can be completely homogenized in the longitudinal dimension throughout the optical system 1100 (see, for example, light 1016 of FIG. 10) until a maximum mode angle θ is reached. max , the mode angle is given by the trigonometric formula definition.

[0165] When the pupil length is substantially equal to the pupil replication period, each mode 1108 can be characterized as being completely homogenized. Thus, homogenization can correspond to the absence of overlap or the presence of gaps between each of the one or more replicated pupil regions 1126. The maximum input pupil input coupled at the prism 1112 can correspond to the transverse prism length p. In some cases, the prism length p can be equivalent to the input coupling pupil region 1122. The large mode angle can contribute to the ability of the optical system 1100 to achieve a large FOV in the longitudinal dimension according to the prism length p and the thickness D1 of the substrate 1116. In the case of complete homogenization, θ max The trigonometric formula can be used is defined based on the prism length p and the thickness D1 of the substrate 1116. For example, for a non-uniform waveguide 1102 with a thickness of 1.5 mm interconnected with a universal waveguide system including the substrate 1116 and the prism 1112, full homogenization of the reflected modes 1108 can be achieved within each replicated pupil copy 1126 over a range of mode angles. Specifically, for a thickness D1 = 1.5 mm, and a 12 mm prism 1112, an external FOV of 53.4 degrees can be achieved in the waveguide dimensions (e.g., in the case where the prism is directly attached to a 1.5 mm thick waveguide). For example, each mode 1108 contained in the emission output pupil can be fully homogenized across the achieved external FOV until a maximum mode angle θ of approximately 76 degrees is reached. max .

[0166] In some cases, partially reflective layer 1120 can help isolate bright, unattenuated light within substrate 1116 from alternative light from waveguide 1102. Alternatively, it may be beneficial to reduce the thickness D1 of substrate 1116 to accommodate smaller prisms 1112 and compact external projection optics while achieving substantially equal maximum mode angles θ required for complete homogenization. max In some cases, the foregoing embodiments may reduce the input coupling pupil of the optical system 1100 and reduce the required working distance for the propagating mode 1108, thereby allowing for more compact external projection optics. For example, a substrate 1116 having a thickness D1 = 0.5 mm may be attached to a waveguide 1102 having a thickness D2 = 1.5 mm and, via input coupling at a 4 mm prism 1112, may allow for the same 76 degree maximum mode angle θ max , as just one example (eg, using a 0.5 mm tubing may allow for the use of a smaller prism 1112 relative to a scenario where the prism is attached directly to the waveguide). Generally speaking, dimension D1 of substrate 1116 may be smaller than dimension D2 of waveguide 1102 .

[0167] If desired, absorbers such as absorber 1128 may be formed on one or more surfaces of prism 1112, and / or absorbers such as absorber 1130 may be formed at the ends of substrate 1116 to minimize stray light. The reflectivity of layer 1120 may vary in the longitudinal dimension of system 1100. For example, layer 1120 may have a first region 1132 closest to prism 1112 having a first reflectivity (e.g., 80%, 70%, 90%, between 70% and 90%, etc.), a second region 1134 having a second reflectivity less than the first reflectivity (e.g., 60%, 70%, 50%, between 50% and 70%, etc.), and a third region 1136 having a third reflectivity less than the second reflectivity (40%, 50%, 30%, between 30% and 50%, etc.). This is merely exemplary, and in general, layer 1120 may have any desired number of regions with different reflectivities. If desired, the reflectivity of layer 1120 can vary continuously. This can be used, for example, to optimize the uniformity of light across the longitudinal dimension of system 1100. If desired, layer 1120 can have a reflectivity that is dependent on angle, wavelength, or polarization (e.g., for incident light, layer 1120 is more reflective at low angles of incidence than at high angles of incidence).

[0168] Fig.11The examples are merely illustrative. The longitudinal light homogenizing structure 1104 may include other structures, such as multiple partially reflective layers or fully reflective layers, and / or reflective layers oriented perpendicular to the surface of the underlying waveguide, phase retarder, polarizer, or other optical element. In another suitable arrangement, the substrate 1116 may have a relatively high refractive index (e.g., 1.83 or higher) to achieve a shortened pupil period. This can also be used to reduce the required size (e.g., size p) of the prism 1112. Fig.12 is a top down perspective view showing one example of how a longitudinal light homogenizing structure may include multiple reflective layers oriented perpendicular to the underlying waveguide.

[0169] like Fig.12 As shown, the system 1200 may include a waveguide 1202 (e.g., the waveguide 1002 of FIG. 10 and Fig. 9 10 and 1210 of the waveguide 902) (e.g., the surface 1010 and Fig. 9 910) on the lateral light homogenizing structure 1204. Light may be coupled into the homogenizing structure 1204 as shown by the input pupil 1208 (e.g., using a Fig.11 10). The light can propagate along the longitudinal dimension (eg, parallel to the X-axis) of the waveguide 1202. The homogenizing structure 1204 can homogenize the light 1208 in the transverse direction.

[0170] For example, homogenizing structure 1204 may include a plurality of reflective layers (sometimes referred to herein as ducts 1206 or duct structures 1206) formed on or within substrate 1206. Substrate 1206 may be mounted to surface 1210. The reflective layers may be oriented perpendicular to surface 1210. Homogenizing structure 1204 may emit a laterally expanded (homogenized) output pupil 1212 (e.g., light 1016 of FIG. 10). Substrate 1206 may be comprised of a single substrate (e.g., a piece of glass) and may include at least bottom surface 1214 (e.g., a surface on surface 1210 of waveguide 1202) and one or more parallel planar layers 1216, 1218, and 1220. Layers 1216, 1218, and 1220 may lie in planes perpendicular to surfaces 1214 and 1210. Planar layer 1216 , planar layer 1218 , and planar layer 1220 may extend across some or all of the height of substrate 1206 (eg, parallel to the Z-axis) and may extend along a longitudinal axis of system 1200 .

[0171] For example, planarization layer 1216 and planarization layer 1218 can be fully reflective layers (e.g., coatings with a reflectivity of approximately 100%), such as fully reflective or nearly fully reflective dielectric coatings, metal coatings, grating structures, etc. Planarization layer 1220 can be a partially reflective coating. Planarization layer 1220 can be a dielectric coating, a metal coating, a grating structure, etc. If desired, planarization layer 1220 can have a variable reflectivity along the longitudinal dimension (e.g., as combined with Figure 1 1206). Layer 1218 and layer 1216 may be formed on an outer surface of substrate 1206, with layer 1220 embedded within substrate 1206. In another suitable arrangement, substrate 1206 may be formed of two different substrate layers, with layer 1220 sandwiched between the two substrate layers.

[0172] Each of the planar layers 1218, 1216, and 1220 can be maintained parallel throughout the respective substrates 1206 as a means of maintaining image resolution. In addition, the planar layers 1218, 1216, and 1220 can be maintained perpendicular to the surfaces 1214 and 1210, thereby presenting right angles within the substrate 1206 and maintaining image resolution. The reflective layers 1218, 1216, and partially reflective layers 1220 can be used to provide uniform output intensity at the output pupil 1212 (e.g., having a constant pupil brightness in the output light including the output pupil 1212) according to uniform or varying incident angles, wavelengths, polarization, etc. In some embodiments, the reflective layers 1218, 1216, and 1220 can reflect the input light at an angle below the critical angle of the substrate (e.g., approximately 42 degrees for a glass to air interface). The reflectivity can result in a reduction in the pupil replica gap, thereby achieving pupil expansion. That is, adjacent beams of output pupil 1212 may be directly adjacent to each other (e.g., some beams from output pupil 1212 may be generated by reflection from layer 1216 and transmission through layer 1220, while other beams from output pupil 1212 may be generated by reflection from layer 1220). In some cases, each end may be coated with an absorptive or reflective coating element as a method of at least mitigating stray light emission from substrate 1206. Output pupil 1212 may be provided downwardly into waveguide 1202 and / or a cross-coupler (e.g., light may pass through surface 1214 at the bottom of substrate 1206 because layer 1218 is a fully reflective layer).

[0173] One or more of the planar layers 1216, 1218, and 1220 and / or one end of the substrate 1206 (eg, Fig.121206) may include an area or aperture for input pupil 1208. The area or aperture may be uncoated or have a special coating to facilitate input. Light from input pupil 1208 may enter the aperture of substrate 1206. In some cases, the light pattern contained in input pupil 1208 may be reflected by at least planar layer 1216, planar layer 1218, and planar layer 1220 within substrate 1206. The reflected light from input pupil 1208 may have a first reflection angle within the total internal reflection (TIR) ​​range relative to a common axis of the propagation direction of duct structure 1206. At least planar layer 1216 and planar layer 1218 may direct the reflected light from input pupil 1208 to the propagation direction (e.g., a direction along the longitudinal dimension) at a second reflection angle relative to a second transverse axis. Substrate 1206 may then emit the reflected light pattern as output pupil 1212 for guiding image projection. In the case of one or more partially reflective coating elements, the configuration of the coating elements can provide a uniform output pupil intensity (eg, constant pupil brightness) of the reflected light pattern based on uniform or varying incident angle, wavelength, and polarization.

[0174] Partially reflective layer 1220 may remain parallel to planar layer 1216 and planar layer 1218 and remain perpendicular to surface 1214. Layer 1220 may include a partially reflective coating element to homogenize the TIR mode of input pupil 1208, thereby improving lateral homogenization. For example, layer 1220 may maintain a reflectivity of approximately 50% and may be oriented at an offset from planar layer 1216 (e.g., offset at 1 / 3 of the width of substrate 1206) and extend the length of substrate 1206 parallel to planar layer 1218. Layer 1220 may guide light to propagate in the lateral dimension of substrate 1206 according to the partially reflective properties of layer 1220, thereby improving lateral homogenization of light reflected back to planar layer 1216 and homogenization of light allowed to pass through to planar layer 1218. In some embodiments, layer 1220 may implement one or more skew mirrors or alternative partially reflective surfaces.

[0175] An input light pattern including an input pupil 1208 may enter substrate 1206 via an aperture of layer 1208. In alternative embodiments, the aperture may be located at an alternative planar region, surface, or end of substrate 1206. The input light may propagate into substrate 1206 and reflect at a first angle of incidence within a TIR range common to a first axis. In some cases, the propagation direction of the first axis to substrate 1206 may be common. Planar layer 1216 and planar layer 1218 may guide the light pattern of input pupil 1208 in a lateral (e.g., non-TIR) dimension of a second alternative axis. The propagated light may be directed to partially reflective layer 1220. Light transmitted through partially reflective layer 1220 may constitute an extended output pupil 1212 for projection. Output pupil 1212 may be composed of one or more copies of input pupil 1208, wherein there are no gaps between copies. The intensity of output pupil 1212 may be substantially uniform with continuous image fidelity.

[0176] If desired, in some embodiments, spatially varying reflectivity (e.g., as described above in connection with FIG. 10- Fig.12 The described arrangement uses a spatially varying reflective layer to homogenize the light over approximately one pupil period. Fig.13A , Fig. 13B and Fig.14 is a diagram showing how spatially varying reflectivity can be used to homogenize light over approximately one pupil period. Fig.13A and Fig. 13B In the example of FIG. 4-1 , the light homogenizing structure is formed inside the waveguide (eg, similar to the above description in conjunction with FIG. 4-1 ). Figure 8 The arrangement described above). Fig.14 In an example, the light homogenizing structure is formed outside the waveguide (e.g., similar to the above combination Fig.11 described arrangement).

[0177] like Fig.13A As shown, optical system 1300 may include waveguide 1302 having grating medium 1306 stacked between waveguide substrate 1304 and waveguide substrate 1308 (e.g., similar to the arrangement shown in FIG. 5 , but with spatially varying reflectivity). Positions along the x-axis on the waveguide are shown in FIG. Fig.13A, the position starts at x=0 on the left side of the waveguide and is in meters. The prism 1310 can be mounted on the waveguide 1302 and can receive input light 1314 at the input face 1312. The prism 1310 can couple the light 1314 into the waveguide 1302. A partially reflective structure (sometimes referred to herein as a partial reflector or a partially reflective layer) can be formed in the region 1316 (e.g., embedded in the grating medium 1306, on the surface of the grating medium 1306, on the surface of the waveguide substrate 1304, and / or on the surface of the waveguide substrate 1308, etc.). The partial reflector can include one or more partially reflective coatings, dielectric layers, metal layers, grating structures, etc. In the example of FIG. 3, the waveguide 1302 can have a thickness approximately equal to 1.3 mm (e.g., the waveguide substrate 1304 and the waveguide substrate 1308 can each be 0.3 mm thick and the grating medium 1306 can be 0.7 mm thick). This is merely illustrative, and in general, other thicknesses may be used. In this arrangement, the prisms 1310 may have a relatively low length P (eg, approximately 3 mm).

[0178] Region 1316 may have any desired width. In one suitable arrangement, the width of region 1316 may be approximately equal to one pupil period (e.g., approximately 13 mm). The partial reflector may exhibit a spatially varying reflectivity along the x-axis within region 1316. For example, the partial reflector may have a reflectivity given by a graph (reflectivity profile) 1318 that plots the reflectivity R as a function of distance x across the entire length of the partial reflector in region 1316. As shown in graph 1318, Fig.13A The partial reflector in the example of 1316 has four discrete reflectivities in different regions of the partial reflector (e.g., starting at x=0, R=4 / 5, then R=2 / 3, then R=1 / 2, then R=0 to the right of region 1316). In this example, the reflectivity of the partial reflector can be calculated by the formula R i ~2–1 / (R i-1 ) is obtained, where R iis the reflectivity of the i-th region of the partial reflector along the x-axis. This is merely illustrative, and in general, any desired reflectivity may be used. The partial reflector may have any desired number of discrete reflectivities, or a continuously variable reflectivity curve (e.g., in the case where a metal coating is used to form the partial reflector). As just one example, the partial reflector may obtain such a reflectivity profile by providing four stacked partially reflective layers (e.g., where R=4 / 5) in the leftmost portion of region 1316, three stacked partially reflective layers (e.g., where R=3 / 4) in the second left portion of region 1316, two stacked partially reflective layers (e.g., where R=2 / 3) in the second right portion of region 1316, and one partial reflective layer (e.g., where R=1 / 2) in the rightmost portion of region 1316.

[0179] The light 1314 that has been coupled into the waveguide may propagate down the length of the waveguide (e.g., where Fig.13A , the light intensity is shown by lighter shading). The light can be substantially homogenized in regions 1318 at opposite ends of the waveguide. For example, region 1318 can have a width approximately equal to one pupil period (e.g., 12 mm). In this way, homogenization can be performed over approximately one pupil period. Fig.13A In the example of , a worst-case pupil period is assumed (eg, where light is incident at a relatively high angle, such as 78 degrees). If incident at a lower angle, the light will be homogenized more quickly within the waveguide.

[0180] Fig. 13B An exemplary averaging curve 1330 for optical system 1300 within region 1318 is shown. Fig. 13B Curve 1334 plots the raw intensity of light within region 1318 (e.g., as a function of x position across the entire region). Curve 1332 plots the filtered intensity of light within region 1318 (e.g., when having a 4 mm iris filter). Fig. 13B The associated metric is 1 - std / mean. As shown by curves 1332 and 1334, light is provided with a relatively uniform intensity across the entire region 1318. Thus, the spatially varying partial reflectors in the waveguide 1302 can homogenize the light within approximately one pupil period at the region 1318.

[0181] Fig.14 shows how a light homogenizing structure with spatially varying reflectivity can be formed outside a waveguide (e.g., similar to the above in conjunction with Fig.11 , but with spatially varying reflectivity). Fig.14As shown, optical system 1400 may include waveguide 1402 having grating medium 1406 stacked between waveguide substrate 1404 and waveguide substrate 1408. The position along the x-axis on the waveguide is shown in FIG. Fig.14 , the position is from the left side of the waveguide at x=0 and is in meters. Substrate 1420 can be mounted to waveguide substrate 1404. Prism 1410 can be mounted to waveguide substrate 1404 at the end of substrate 1420 and can receive input light 1414 at input face 1412. In another suitable arrangement, prism 1410 (e.g., similar to Fig.11 1410) is mounted to substrate 1420. Prism 1410 can couple light 1414 into waveguide 1302. Partially reflective structures such as partial reflectors 1418 can be formed in region 1416 (e.g., on the upper surface of waveguide substrate 1404, on the lower surface of substrate 1420 and prism 1410, embedded within substrate 1420 and prism 1410, etc.). Partial reflectors 1418 may include partially reflective coatings, dielectric layers, metal layers, grating structures, etc. Fig.14 In the example of , waveguide 1402 may have a thickness approximately equal to 1.3 mm (e.g., waveguide substrate 1404 and waveguide substrate 1408 may each be 0.3 mm thick, and grating medium 1406 may be 0.7 mm thick), while substrate 1420 has a thickness approximately equal to 0.65 mm. This is merely exemplary, and in general, other thicknesses may be used. In this arrangement, prism 1410 may have a relatively low width P that is approximately equal to half the period of the tube pupil (e.g., approximately 3 mm). If desired, optional light absorbing layers, such as absorbing layer 1424 on prism 1410 and absorbing layer 1426 at the end of substrate 1420, may be provided to mitigate stray light.

[0182] The partial reflector 1418 may have a spatially varying reflectivity (e.g., the partial reflector 1418 may have different regions with discrete reflectivities, such as Fig.13A 1318 or other reflectivity). Fig.14 As shown, similar to the above combination Fig.13A In the arrangement described above, partial reflectors may be provided at opposite ends of the waveguide (e.g. Fig.14 The light is homogenized within approximately one pupil period at the end 1422 shown. For example, Fig.14 The arrangement can allow a thicker tube to match the projector pupil size without requiring a thicker waveguide, can allow a projector aperture stop (e.g., where the prisms are formed directly on the waveguide substrate), and can alleviate the need for writing through the coating.

[0183] exist Fig.13A , Fig. 13Band Fig.14 In the examples of FIG. 5 , the light homogenizing structure is shown as having a partial reflector and, for clarity, its reflectivity varies only along a single dimension. Fig.15 A top view of a light homogenizing structure having a spatially varying reflectivity that can be used to form Fig.13A and Fig.14 homogenized structure or form other homogenized structures. Fig.15 In the example, the waveguide is omitted for clarity.

[0184] like Fig.15 As shown, the optical system 1500 may include a prism 1506 formed above a partial reflector 1508 (e.g., Fig.14 Partial reflector 1418 or Fig.13A 1316). The partial reflector 1508 can be used to homogenize light input coupled by the prism 1506 (e.g., along the x-axis). The homogenized light can be redirected by the cross coupler 1518 to the output coupler 1520, as just one example. The partial reflector 1508 can have different regions with different discrete reflectivities. The reflectivity of the partial reflector 1508 can be a function of the radial distance (radius) r from the center of the input coupling face (edge) of the prism 1506. The above examples are merely illustrative, where the prism (e.g., Fig.14 Prism 1410, Fig.15 13 prism 1506, prism 1310 and Fig.11 A prism 1112) is used as an input coupling device for the waveguide. In general, any desired input coupling device can be used.

[0185] Graph 1502 plots the reflectivity R of the partial reflector 1508 as a function of radius r. As shown in curve 1504 of graph 1502, the partial reflector 1508 may have a first reflectivity R1 (e.g., 0.73, between 0.7 and 0.8, etc.) from r=0 to r=r1 (e.g., in region 1510 of the partial reflector 1508), a second reflectivity R2 (e.g., 0.61, between 0.6 and 0.7, etc.) from r=r1 to r=r2 (e.g., in region 1512 of the partial reflector 1508), a third reflectivity R3 (e.g., 0.48, between 0.4 and 0.6, etc.) from r=r2 to r=r3 (e.g., in region 1514 of the partial reflector 1508), and a fourth reflectivity R4 (e.g., 0.361, between 0.3 and 0.4, etc.) from r=r3 to r=r4 (e.g., in region 1516 of the partial reflector 1508). In one suitable arrangement, four layers of coating are used to form partial reflector 1508. For example, a first partially reflective layer may overlap regions 1510, 1512, 1514, and 1516, a second partially reflective layer may overlap regions 1510, 1512, and 1514, a third partially reflective layer may overlap regions 1510 and 1512, and a fourth partially reflective layer may overlap region 1510 to produce a reflectivity profile associated with curve 1504. These coatings may be deposited using a coating mask. Radius r1 may be approximately 5.9 mm, radius r2 may be approximately 8.8 mm, radius r3 may be approximately 11.6 mm, and radius r4 may be approximately 14.3 mm, as just one example. Other radii may be used if desired. Partial reflector 1508 may have any desired number of regions with different reflectivities, and the regions may have any desired shape. If desired, a number of discrete regions may be used to approximate a continuous reflectivity curve (e.g., to optimize light homogenization).

[0186] Although various embodiments have been described and illustrated herein, other means and / or structures may be used to perform the corresponding functions and / or obtain one or more of the corresponding results and / or benefits described herein, and each of these variations and / or modifications is considered to be within the scope of the embodiments described herein. More generally, all parameters, dimensions, materials, and configurations described herein are merely illustrative, and actual parameters, dimensions, materials, and / or configurations may depend on the application for which a specific application or embodiment is used. These embodiments may be practiced in any desired combination. In addition, various concepts may be embodied as one or more methods, devices, or systems for which examples have been provided. The actions performed as part of a method or operation may be sorted in any suitable manner. Therefore, embodiments in which actions are performed in an order different from the order shown may be constructed, which may include performing some actions simultaneously, even if these actions are shown as sequential actions in the embodiments. As used herein, with reference to one or more element lists, the phrase "at least one" should be understood to mean at least one element selected from any one or more elements in the element list, but not necessarily including at least one of each element specifically listed in the element list, and not excluding any combination of elements in the element list. Transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "retaining," "consisting of," etc. should be construed as open ended, i.e., meaning including, but not limited to, the term "about" means plus or minus 10% of a given value.

[0187] As used herein, the term "approximately" refers to a given value plus or minus 10%. The term "about" refers to a given value plus or minus 20%. The term "substantially" with respect to reflected light refers to light reflected by the grating structure. Light reflected substantially at the angle includes more light than light reflected at any other angle (excluding surface reflections). Light reflected substantially about the reflection axis includes more reflected light than light reflected about any other reflection axis (excluding surface reflections). When considering the main reflected light, light reflected by the surface of the device is not included. The term "reflection axis" refers to the axis that bisects the angle at which the incident light is reflected. The absolute value of the angle of incidence of the incident light relative to the reflection axis is equal to the absolute value of the angle of reflection of the incident light relative to the reflection axis. For conventional reflectors, the reflection axis coincides with the surface normal (i.e., the reflection axis is perpendicular to the reflector surface). In contrast, a specific implementation of the skew mirror according to the present disclosure may have a reflection axis that is different from the surface normal, or in some cases may have a reflection axis that coincides with the surface normal. The reflection axis angle may be determined by adding the angle of incidence and its corresponding angle of reflection and dividing the sum by two. The angles of incidence and reflection may be determined empirically, taking multiple measurements (typically three or more) to produce an average.

[0188] The term "reflection" and similar terms are used in the present disclosure in some cases where "diffraction" is generally considered to be an appropriate term. The use of "reflection" is consistent with the mirror properties exhibited by skew mirrors and helps to avoid potentially confusing terms. For example, in the case of saying that a grating structure is configured to "reflect" incident light, a conventional technician may prefer to say that the grating structure is configured to "diffract" the incident light because it is generally believed that the grating structure acts on the light by diffraction. However, such use of the term "diffraction" will lead to statements such as "the incident light is diffracted about a generally stable reflection axis", which may cause confusion. Therefore, in the case of stating that the incident light is "reflected" by the grating structure, based on the present disclosure, a person of ordinary skill in the art will realize that the grating structure actually "reflects" the light through the mechanism of diffraction. Such use of "reflection" is not without precedent in optics, and conventional reflectors are generally referred to as "reflecting" light, even though diffraction plays a major role in such reflection. Those skilled in the art therefore recognize that most "reflections" include the characteristics of diffraction, and that "reflection" performed by a skew mirror or its components also includes diffraction.

[0189] The term "light" refers to electromagnetic radiation. Electromagnetic radiation may have any wavelength unless reference is made to a particular wavelength or range of wavelengths, such as "visible light" which refers to a portion of the electromagnetic spectrum visible to the human eye. The terms "hologram" and "holographic grating" refer to the recording of an interference pattern produced by interference between multiple crossed light beams. In some examples, a hologram or holographic grating may be produced by interference between multiple crossed light beams, wherein each of the multiple crossed light beams remains unchanged during the exposure time. In other examples, a hologram or holographic grating may be produced by interference between multiple crossed light beams, wherein the angle of incidence of at least one of the multiple crossed light beams on the grating medium is changed when recording the hologram, and / or the wavelength is changed when recording the hologram (e.g., a complex hologram or a complex holographic grating).

[0190] The term "sinusoidal volume grating" refers to an optical component having optical properties, such as a refractive index, modulated with a generally sinusoidal profile throughout a volumetric region. Each (simple / sinusoidal) grating corresponds to a single conjugate vector pair in k-space (or a generally point-like distribution of conjugate pairs in k-space). The term "diffraction efficiency" refers to the ratio of the power of the light reflected on the grating medium to the power of the incident light. The term "entrance pupil" refers to the real or virtual aperture through which a beam of light entering an imaging optical device passes with its minimum size. The term "eyebox" refers to a two-dimensional area that outlines an area in which a human pupil can be placed for viewing the full field of view at a fixed distance from a grating structure. The term "eye relief" refers to the fixed distance between a grating structure and a corresponding eyebox. The term "exit pupil" refers to the real or virtual aperture through which a beam of light emerging from an imaging optical device passes with its minimum size. In use, the imaging optical system is typically configured to direct the beam of light toward an image capture device. Examples of image capture devices include, but are not limited to, a user's eye, a camera, or other photodetector. In some cases, the exit pupil may include a subset of the light beams that emerge from the imaging optics.

[0191] The term "grating medium" refers to a physical medium configured to have a grating structure for reflecting light. The grating medium may include a plurality of grating structures. The term "grating structure" refers to one or more gratings configured to reflect light. In some examples, the grating structure may include a group of gratings that share at least one common property or characteristic (e.g., each group of gratings responds to light of the same wavelength). In some implementations, the grating structure may include one or more holograms. In other implementations, the grating structure may include one or more sinusoidal volume gratings. In some examples, for each of the one or more gratings (e.g., a hologram or a sinusoidal grating), the grating structure may be uniform relative to the reflection axis. Alternatively or in addition, for each of the one or more gratings in the grating medium (e.g., a hologram or a sinusoidal volume grating), the grating structure may be uniform relative to the length or volume. The skew mirrors described herein are sometimes also referred to herein as grating structures, holographic grating structures, or volume holographic grating structures.

[0192] The above embodiments can be implemented in any of a variety of ways. For example, hardware, software, or a combination thereof can be used to implement the design and manufacture of embodiments of the technology disclosed herein. When implemented in software, whether provided in a single computer or distributed in multiple computers, the software code can be executed on any suitable processor or collection of processors.

[0193] According to one embodiment, an optical device is provided, comprising: a waveguide having a waveguide surface, wherein the waveguide surface has a longitudinal dimension and a perpendicular lateral dimension; and a light homogenizing structure located on the waveguide, wherein the light homogenizing structure is configured to receive input light and generate longitudinally homogenized light by homogenizing the input light along the longitudinal dimension.

[0194] According to any combination of the above embodiments, the optical device further includes a cross-coupler configured to receive the longitudinally homogenized light from the light homogenizing structure and generate two-dimensionally homogenized light by redirecting the longitudinally homogenized light along a transverse dimension.

[0195] According to any combination of the above embodiments, the cross-coupler is configured to diffract the longitudinally homogenized light both upwardly through the cross-coupler and downwardly through the cross-coupler.

[0196] According to any combination of the above embodiments, the light homogenizing structure includes a substrate and a partially reflective layer on the substrate, wherein the partially reflective layer is configured to transmit and reflect input light at least twice.

[0197] According to any combination of the above embodiments, the partially reflective layer has a spatially varying reflectivity.

[0198] According to any combination of the above embodiments, the substrate is mounted to the waveguide surface.

[0199] According to any combination of the above embodiments, the partially reflective layer is interposed between the substrate and the waveguide surface.

[0200] According to any combination of the above embodiments, the partially reflective layer is embedded in the substrate.

[0201] According to any combination of the above embodiments, the substrate is a waveguide substrate and includes a waveguide surface.

[0202] According to any combination of the above embodiments, the partially reflective layer includes a structure selected from the group consisting of: a dielectric coating, a metal coating, a polymer film, and a set of holograms.

[0203] According to any combination of the above embodiments, the light homogenizing structure includes: a substrate having opposite first and second surfaces, a first reflective layer on the first surface, a second reflective layer on the second surface, and a partially reflective layer on the substrate and interposed between the first and second reflective layers.

[0204] According to any combination of the above embodiments, the first and second reflective layers and the partially reflective layer extend perpendicular to the waveguide surface.

[0205] According to any combination of the above embodiments, the first reflective layer includes an aperture for input light, and the partially reflective layer is closer to the first reflective layer than the second reflective layer.

[0206] According to any combination of the above embodiments, the light homogenizing structure includes a substrate mounted on the waveguide surface and having a refractive index greater than or equal to 1.83 and a prism mounted on the substrate.

[0207] According to another embodiment, an optical system is provided, comprising: a waveguide having a first waveguide substrate and a second waveguide substrate and a grating medium interposed between the first waveguide substrate and the second waveguide substrate; an input coupling device mounted to the waveguide at a first end of the waveguide and configured to couple input light into the waveguide; and a partial reflector on the waveguide, wherein the partial reflector has a spatially varying reflectivity and is configured to homogenize the input light within a pupil period at a second end of the waveguide.

[0208] According to any combination of the above embodiments, the partial reflector includes a first region having a first reflectivity at a first end of the waveguide and a second region having a second reflectivity less than the first reflectivity, wherein the second region is interposed between the first region and the second end of the waveguide.

[0209] According to any combination of the above embodiments, the partial reflector has a continuously variable reflectivity profile throughout its length.

[0210] According to any combination of the above embodiments, the partial reflector is embedded in the waveguide.

[0211] According to any combination of the above embodiments, the optical system further comprises a substrate mounted to the waveguide and having an end at the input coupling device, wherein the partial reflector is formed on the input coupling device and a bottom surface of the substrate.

[0212] According to another embodiment, a head-mounted display device is provided, the device including: a light source; a waveguide having a planar outer surface; a longitudinal light homogenizing structure, which includes a substrate mounted to the planar outer surface and configured to homogenize light from the light source along the longitudinal dimension of the waveguide; and a cross-coupler embedded in the waveguide, wherein the cross-coupler is configured to redirect the homogenized light along the lateral dimension of the waveguide.

[0213] The foregoing is merely illustrative and various modifications may be made to the embodiments described. The foregoing embodiments may be implemented independently or in any combination.

Claims

1. An optical system, comprising: a waveguide having a first waveguide substrate and a second waveguide substrate and a grating medium interposed between the first waveguide substrate and the second waveguide substrate; an input coupling prism mounted to the waveguide at a first end of the waveguide and configured to couple input light into the waveguide; as well as A partial reflector sandwiched between the input coupling prism and the waveguide, wherein the partial reflector has a spatially varying reflectivity and is configured to homogenize the input light within a pupil period at the second end of the waveguide, wherein the partial reflector includes a first region having a first reflectivity at the first end of the waveguide and a second region having a second reflectivity less than the first reflectivity, wherein the second region is interposed between the second end of the waveguide and the first region.

2. The optical system according to claim 1, wherein: The partial reflector has a continuously variable reflectivity profile over its entire length.

3. The optical system of claim 1 further comprising a substrate mounted to the waveguide and having an end at the input coupling prism, wherein at least a portion of the partial reflector is sandwiched between the substrate and the waveguide.

4. The optical system according to claim 1, wherein: The input coupling prism is mounted to the first waveguide substrate.

5. The optical system according to claim 4, wherein: The input coupling prism has a first surface for receiving the input light and has a second surface, and the optical system further comprises: An optically absorbing layer is disposed on the second surface.

6. The optical system according to claim 1, wherein: The partial reflector includes a third region between the first region and the second region, the third region having a third reflectivity greater than the second reflectivity and less than the first reflectivity.

7. A head mounted display device, comprising: light source; a waveguide having a planar outer surface; a longitudinal light homogenizing structure comprising a substrate mounted to the planar outer surface and configured to homogenize light from the light source along a longitudinal dimension of the waveguide; a cross-coupler embedded in the waveguide, wherein the cross-coupler is configured to redirect homogenized light along a lateral dimension of the waveguide, the substrate having a first surface and a second surface opposite the first surface, the waveguide comprising a first waveguide substrate having the planar out-of-plane surface, a second waveguide substrate, and a grating medium sandwiched between the first waveguide substrate and the second waveguide substrate, the cross-coupler being embedded in the grating medium, the longitudinal light homogenizing structure comprising a partially reflective layer interposed between the substrate and the planar out-of-plane surface, and the partially reflective layer contacts both the second surface of the substrate and the planar out-of-plane surface of the first waveguide substrate; as well as An input coupling prism is mounted to the first surface of the substrate.

8. The head mounted display device according to claim 7, wherein: The partially reflective layer has a reflectivity that decreases along a propagation direction of the light within the substrate.

9. The head mounted display device according to claim 8, wherein: The reflectivity decreases in a set of discrete steps.

10. The head mounted display device according to claim 7, wherein: The input coupling prism has a first face and a second face, and the head mounted display device further comprises an absorber stacked on the second face.

11. The head mounted display device according to claim 7, further comprising: An output coupler is configured to couple the homogenized light redirected by the cross-coupler out of the waveguide.

12. The head mounted display device according to claim 11, wherein: The output coupler comprises a volume hologram.

13. The head mounted display device according to claim 12, wherein: The cross-coupler includes a louvered mirror.

14. The head mounted display device according to claim 12, wherein: The cross-coupler comprises an additional volume hologram.

15. The head mounted display device according to claim 7, wherein: The cross-coupler comprises a volume hologram.

16. The head mounted display device according to claim 7, wherein: The cross-coupler includes a louvered mirror.

17. A head mounted display device, comprising: light source; a waveguide having a planar outer surface; a longitudinal light homogenizing structure comprising a substrate mounted to the planar outer surface and configured to homogenize light from the light source along a longitudinal dimension of the waveguide; a cross-coupler embedded in the waveguide, wherein the cross-coupler is configured to redirect homogenized light along a lateral dimension of the waveguide, the substrate having a first surface and a second surface opposite the first surface, the waveguide comprising a first waveguide substrate having the planar out-of-plane surface, a second waveguide substrate, and a grating medium sandwiched between the first waveguide substrate and the second waveguide substrate, the cross-coupler being embedded in the grating medium, the longitudinal light homogenizing structure comprising a partially reflective layer interposed between the substrate and the planar out-of-plane surface, and the partially reflective layer contacts both the second surface of the substrate and the planar out-of-plane surface of the first waveguide substrate; and an input coupling prism mounted to the first surface of the substrate at the first end of the substrate, wherein the partially reflective layer has a spatially varying reflectivity and is configured to homogenize input light within one pupil period proximate a second end of the substrate opposite the first end of the substrate, wherein the partially reflective layer includes a first region having a first reflectivity at the first end of the substrate and a second region having a second reflectivity less than the first reflectivity, and wherein the second region is interposed between the second end of the substrate and the first region.

18. The head mounted display device according to claim 17, further comprising: An output coupler is configured to couple the homogenized light redirected by the cross-coupler out of the waveguide.

19. The head mounted display device according to claim 18, wherein: The output coupler and the cross coupler include volume holograms.

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