Optical Systems with Gratings and Anti-reflective Layers

A display system may include a waveguide and at least one surface relief grating (SRG) structure. The SRG structure may include a plurality of ridges separated by a plurality of troughs. The SRG structure may include an anti-reflective layer to mitigate specular reflections off of the ridges. The anti-reflective layer may be formed above the ridges such that the ridges are interposed between the waveguide and the anti-reflective layer. In this type of arrangement, the anti-reflective layer may fill the troughs between the ridges or may be patterned to overlap the ridges without filing the troughs. The anti-reflective layer may be formed below the ridges such that the anti-reflective layer is interposed between the ridges and the waveguide. The SRG structure may include multiple anti-reflective layers. When multiple anti-reflective layers are included, the anti-reflective layers may have at least one differing property (e.g., material, number of layers, dimension).

BACKGROUND

This disclosure relates generally to optical systems and, more particularly, to optical systems for electronic devices with displays.

Electronic devices often include displays that present images close to a user's eyes. For example, virtual and augmented reality headsets may include displays with optical elements that allow users to view the displays.

Devices such as these can be challenging to design. If care is not taken, the components used to display images in these devices can be unsightly and bulky and may not exhibit a desired optical performance.

SUMMARY

An electronic device may have a display system. The display system may include a waveguide and at least one surface relief grating (SRG) structure. The SRG structure may include a plurality of ridges separated by a plurality of troughs.

The SRG structure may include an anti-reflective layer to mitigate specular reflections off of the SRG surface due to its high refractive indices. The anti-reflective layer may be formed above the ridges such that the ridges are interposed between the waveguide and the anti-reflective layer. In this type of arrangement, the anti-reflective layer may fill the troughs between the ridges or may be patterned to overlap the ridges without filing the troughs. The anti-reflective layer may be formed below the ridges such that the anti-reflective layer is interposed between the ridges and the waveguide.

The SRG structure may include multiple anti-reflective layers. One anti-reflective layer may be above the ridges while another anti-reflective layer may be below the ridges. Alternatively, one anti-reflective layer may be aligned with a first subset of the ridges while another anti-reflective layer may be aligned with a second subset of the ridges. When multiple anti-reflective layers are included, the anti-reflective layers may have at least one differing property (e.g., material, number of layers, dimension).

DETAILED DESCRIPTION

System10ofFIG.1may be a head-mounted device having one or more displays. The displays in system10may include near-eye displays20mounted within support structure (housing)8. Support structure8may have the shape of a pair of eyeglasses or goggles (e.g., supporting frames), may form a housing having a helmet shape, or may have other configurations to help in mounting and securing the components of near-eye displays20on the head or near the eye of a user. Near-eye displays20may include one or more display modules such as display modules20A and one or more optical systems such as optical systems20B. Display modules20A may be mounted in a support structure such as support structure8. Each display module20A may emit light38(image light) that is redirected towards a user's eyes at eye box24using an associated one of optical systems20B.

The operation of system10may be controlled using control circuitry16. Control circuitry16may include storage and processing circuitry for controlling the operation of system10. Circuitry16may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry16may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry16and run on processing circuitry in circuitry16to implement operations for system10(e.g., data gathering operations, operations involving the adjustment of components using control signals, image rendering operations to produce image content to be displayed for a user, etc.).

System10may include input-output circuitry such as input-output devices12. Input-output devices12may be used to allow data to be received by system10from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, or other electrical equipment) and to allow a user to provide head-mounted device10with user input. Input-output devices12may also be used to gather information on the environment in which system10(e.g., head-mounted device10) is operating. Output components in devices12may allow system10to provide a user with output and may be used to communicate with external electrical equipment. Input-output devices12may include sensors and other components18(e.g., image sensors for gathering images of real-world object that are digitally merged with virtual objects on a display in system10, accelerometers, depth sensors, light sensors, haptic output devices, speakers, batteries, wireless communications circuits for communicating between system10and external electronic equipment, etc.).

Display modules20A may be liquid crystal displays, organic light-emitting diode displays, laser-based displays, or displays of other types. Optical systems20B may form lenses that allow a viewer (see, e.g., a viewer's eyes at eye box24) to view images on display(s)20. There may be two optical systems20B (e.g., for forming left and right lenses) associated with respective left and right eyes of the user. A single display20may produce images for both eyes or a pair of displays20may be used to display images. In configurations with multiple displays (e.g., left and right eye displays), the focal length and positions of the lenses formed by system20B may be selected so that any gap present between the displays will not be visible to a user (e.g., so that the images of the left and right displays overlap or merge seamlessly).

If desired, optical system20B may contain components (e.g., an optical combiner, etc.) to allow real-world image light from real-world images or objects28to be combined optically with virtual (computer-generated) images such as virtual images in image light38. In this type of system, which is sometimes referred to as an augmented reality system, a user of system10may view both real-world content and computer-generated content that is overlaid on top of the real-world content. Camera-based augmented reality systems may also be used in device10(e.g., in an arrangement in which a camera captures real-world images of object28and this content is digitally merged with virtual content at optical system20B).

System10may, if desired, include wireless circuitry and/or other circuitry to support communications with a computer or other external equipment (e.g., a computer that supplies display20with image content). During operation, control circuitry16may supply image content to display20. The content may be remotely received (e.g., from a computer or other content source coupled to system10) and/or may be generated by control circuitry16(e.g., text, other computer-generated content, etc.). The content that is supplied to display20by control circuitry16may be viewed by a viewer at eye box24.

FIG.2is a top view of an illustrative display20that may be used in system10ofFIG.1. As shown inFIG.2, near-eye display20may include one or more display modules such as display module(s)20A and an optical system such as optical system20B. Optical system20B may include optical elements such as one or more waveguides50. Waveguide50may include one or more stacked substrates (e.g., stacked planar and/or curved layers sometimes referred to herein as waveguide substrates) of optically transparent material such as plastic, polymer, glass, etc.

If desired, waveguide50may also include one or more layers of holographic recording media (sometimes referred to herein as holographic media, grating media, or diffraction grating media) on which one or more diffractive gratings are recorded (e.g., holographic phase gratings, sometimes referred to herein as holograms). A holographic recording may be stored as an optical interference pattern (e.g., alternating regions of different indices of refraction) within a photosensitive optical material such as the holographic media. The optical interference pattern may create a holographic phase grating that, when illuminated with a given light source, diffracts light to the targeted direction with the designed phase modulation. The holographic phase grating may be a non-switchable diffractive grating that is encoded with a permanent interference pattern or may be a switchable diffractive grating in which the diffracted light can be modulated by controlling an electric field applied to the holographic recording medium. Multiple holographic phase gratings (holograms) may be recorded within (e.g., superimposed within) the same volume of holographic medium if desired. The holographic phase gratings may be, for example, volume holograms or thin-film holograms in the grating medium. The grating media may include photopolymers, gelatin such as dichromated gelatin, silver halides, holographic polymer dispersed liquid crystal, or other suitable holographic media.

Diffractive gratings on waveguide50may include holographic phase gratings such as volume holograms or thin-film holograms, meta-gratings, or any other desired diffractive grating structures. The diffractive gratings on waveguide50may also include surface relief gratings formed on one or more surfaces of the substrates in waveguides50, gratings formed from patterns of metal or dielectric structures, etc. The diffractive gratings may, for example, include multiple multiplexed gratings (e.g., holograms) that at least partially overlap within the same volume of grating medium (e.g., for diffracting different colors of light and/or light from a range of different input angles at one or more corresponding output angles). Other light redirecting elements such as louvered mirrors may be used in place of diffractive gratings in waveguide50if desired.

As shown inFIG.2, display module20A may generate image light38associated with image content to be displayed to eye box24. Image light38may be collimated using a collimating lens if desired. Optical system20B may be used to present image light38output from display module20A to eye box24. If desired, display module20A may be mounted within support structure8ofFIG.1while optical system20B may be mounted between portions of support structure8(e.g., to form a lens that aligns with eye box24). Other mounting arrangements may be used, if desired.

Optical system20B may include one or more optical couplers (e.g., light redirecting elements) such as input coupler52, cross-coupler54, and output coupler56. In the example ofFIG.2, input coupler52, cross-coupler54, and output coupler56are formed at or on waveguide50. Input coupler52, cross-coupler54, and/or output coupler56may be completely embedded within the substrate layers of waveguide50, may be partially embedded within the substrate layers of waveguide50, may be mounted to waveguide50(e.g., mounted to an exterior surface of waveguide50), etc.

Waveguide50may guide image light38down its length via total internal reflection. Input coupler52may be configured to couple image light38from display module20A into waveguide50, whereas output coupler56may be configured to couple image light38from within waveguide50to the exterior of waveguide50and towards eye box24. Input coupler52may include an input coupling prism, an edge or face of waveguide50, a lens, a steering mirror or liquid crystal steering element, or any other desired input coupling elements. As an example, display module20A may emit image light38in the +Y direction towards optical system20B. When image light38strikes input coupler52, input coupler52may redirect image light38so that the light propagates within waveguide50via total internal reflection towards output coupler56(e.g., in the +X direction within the total internal reflection (TIR) range of waveguide50). When image light38strikes output coupler56, output coupler56may redirect image light38out of waveguide50towards eye box24(e.g., back along the Y-axis). A lens such as lens60may help to direct or focus image light38onto eye box24. Lens60may be omitted if desired. In scenarios where cross-coupler54is formed on waveguide50, cross-coupler54may redirect image light38in one or more directions as it propagates down the length of waveguide50, for example. In redirecting image light38, cross-coupler54may also perform pupil expansion on image light38.

Input coupler52, cross-coupler54, and/or output coupler56may be based on reflective and refractive optics or may be based on diffractive (e.g., holographic) optics. In arrangements where couplers52,54, and56are formed from reflective and refractive optics, couplers52,54, and56may include one or more reflectors (e.g., an array of micromirrors, partial mirrors, louvered mirrors, or other reflectors). In arrangements where couplers52,54, and56are based on diffractive optics, couplers52,54, and56may include diffractive gratings (e.g., volume holograms, surface relief gratings, etc.).

The example ofFIG.2is merely illustrative. Optical system20B may include multiple waveguides that are laterally and/or vertically stacked with respect to each other. Each waveguide may include one, two, all, or none of couplers52,54, and56. Waveguide50may be at least partially curved or bent if desired. One or more of couplers52,54, and56may be omitted. If desired, optical system20B may include an optical coupler such as a surface relief grating structure that performs the operations of both cross-coupler54and output coupler56. For example, the surface relief grating structure may redirect image light38as the image propagates down waveguide50(e.g., while expanding the image light) and the surface relief grating structure may also couple image light38out of waveguide50and towards eye box24.

FIG.3Ais a top view showing one example of how a surface relief grating structure may be formed on waveguide50. As shown inFIG.3A, waveguide50may have a first lateral (e.g., exterior) surface70and a second lateral surface72opposite lateral surface70. Waveguide50may include any desired number of one or more stacked waveguide substrates. If desired, waveguide50may also include a layer of grating medium sandwiched (interposed) between first and second waveguide substrates (e.g., where the first waveguide substrate includes lateral surface70and the second waveguide substrate includes lateral surface72).

Waveguide50may be provided with a surface relief grating structure such as surface relief grating structure74. Surface relief grating (SRG) structure74may be formed within a substrate such as a layer of SRG substrate (medium)76. In the example ofFIG.3A, SRG substrate76is layered onto lateral surface70of waveguide50. This is merely illustrative and, if desired, SRG substrate76may be layered onto lateral surface72(e.g., the surface of waveguide50that faces the eye box).

SRG structure74may include at least two partially-overlapping surface relief gratings. Each surface relief grating in SRG structure74may be defined by corresponding ridges (peaks)78and troughs (minima)80in the thickness of SRG substrate76. In the example ofFIG.3A, SRG structure74is illustrated for the sake of clarity as a binary structure in which the surface relief gratings in SRG structure74are defined either by a first thickness associated with peaks78or a second thickness associated with troughs80. This is merely illustrative. If desired, SRG structure74may be non-binary (e.g., may include any desired number of thicknesses following any desired profile, may include peaks78that are angled at non-parallel fringe angles with respect to the Y axis, etc.). If desired, SRG substrate76may be adhered to lateral surface70of waveguide50using a layer of adhesive (not shown). SRG structure74may be fabricated separately from waveguide50and may be adhered to waveguide50after fabrication, for example. As another example, ridges78may be patterned to substrate50without an intervening SRG substrate76.

The example ofFIG.3Ais merely illustrative. In another implementation, SRG structure74may be placed at a location within the interior of waveguide50, as shown in the example ofFIG.3B. As shown inFIG.3B, waveguide50may include a first waveguide substrate84, a second waveguide substrate86, and a media layer82interposed between waveguide substrate84and waveguide substrate86. Media layer82may be a grating or holographic recording medium, a layer of adhesive, a polymer layer, a layer of waveguide substrate, or any other desired layer within waveguide50. SRG substrate76may be layered onto the surface of waveguide substrate84that faces waveguide substrate86. Alternatively, SRG substrate76may be layered onto the surface of waveguide substrate86that faces waveguide substrate84.

If desired, SRG structure74may be distributed across multiple layers of SRG substrate, as shown in the example ofFIG.3C. As shown inFIG.3C, the optical system may include multiple stacked waveguides such as at least a first waveguide50and a second waveguide50′. A first SRG substrate76may be layered onto one of the lateral surfaces of waveguide50whereas a second SRG substrate76′ is layered onto one of the lateral surfaces of waveguide50′. First SRG substrate76may include one or more of the surface relief gratings in SRG structure74. Second SRG substrate76′ may include one or more of the surface relief gratings in SRG structure74. This example is merely illustrative. If desired, the optical system may include more than two stacked waveguides. In examples where the optical system includes more than two waveguides, each waveguide that is provided with an SRG substrate may include one or more of the surface relief gratings in SRG structure74. While described herein as separate waveguides, waveguides50and50′ ofFIG.3Cmay also be formed from respective waveguide substrates of the same waveguide, if desired. The arrangements inFIGS.3A,3B, and/or3C may be combined if desired.

If desired, multiple surface relief gratings may be co-located for redirecting (expanding) image light38in different directions (e.g., in an overlapping or interleaved arrangement in or on waveguide50). The surface relief gratings in SRG structure74may overlap in physical space (e.g., when viewed in the −Y direction ofFIGS.3A-3C) and, in implementations where only a single SRG substrate76is used, may each at least partially overlap within the same volume of SRG substrate76. Despite overlapping on waveguide50, the surface relief gratings in SRG structure74may diffract incoming light from and/or onto different respective directions.

The material used to form SRG substrate76and/or ridges78may be a high refractive index material (e.g., silicon nitride, titanium dioxide, etc.). The high refractive index material may be organic or inorganic. The refractive index of SRG substrate76and/or ridges78may be greater than 1.5, greater than 1.7, greater than 1.9, greater than 2.0, greater than 2.2, etc. Using a high refractive index material for ridges78achieves a strong diffraction effect when immersed in air or another low-index material. However, if care is not taken, specular reflections may be greater than desired. Specular reflections off of the SRG structures may cause glare both on the eye box side of the waveguide (e.g., light travelling in the positive Y-direction may reflect in the negative Y-direction towards the eye box on the negative Y-side of the structure ofFIG.3A) and the non-eye-box side of the waveguide (e.g., light travelling in the negative Y-direction may reflect in the positive Y-direction towards a third person observer of device10on the positive Y-side of the structure ofFIG.3A).

To mitigate specular reflections caused by the SRG structure(s)74, one or more anti-reflective layers may be included in the SRG structures. There are numerous ways for an anti-reflective layer to be incorporated into SRG structure74. The anti-reflective layer may be incorporated above ridges78(such that the ridges are interposed between the anti-reflective layer and the waveguide), below ridges78(such that the anti-reflective layer is interposed between the waveguide and the ridges), or both above and below the ridges.

FIG.4is a cross-sectional side view of an illustrative SRG structure74. As shown inFIG.4, the SRG structure includes a high-index layer90that defines ridges78on waveguide50. Additionally, high-index layer90forms a non-SRG portion92that does not define any ridges. As previously discussed, high-index layer90may be formed from an organic or inorganic high refractive index material (e.g., silicon nitride, titanium dioxide, etc.) with a refractive index that is greater than 1.5, greater than 1.7, greater than 1.9, greater than 2.0, greater than 2.2, etc. Troughs80are formed between the ridges78in the high-index layer90. In the example ofFIG.4, high-index layer90has a thickness of 0 in troughs80(e.g., the high-index layer is totally omitted). Alternatively, the high-index layer may have a non-zero thickness in troughs80(e.g., as inFIG.3A).

The magnitude of the height of each ridge may be greater than 50 nanometers, greater than 100 nanometers, greater than 200 nanometers, greater than 300 nanometers, greater than 500 nanometers, greater than 750 nanometers, greater than 1000 nanometers, less than 50 nanometers, less than 100 nanometers, less than 200 nanometers, less than 300 nanometers, less than 500 nanometers, less than 750 nanometers, less than 1000 nanometers, between 200 nanometers and 400 nanometers, between 100 nanometers and 750 nanometers, between 50 nanometers and 1000 nanometers, etc. Each ridge may have a width that is greater than 50 nanometers, greater than 100 nanometers, greater than 200 nanometers, greater than 300 nanometers, greater than 500 nanometers, less than 50 nanometers, less than 100 nanometers, less than 200 nanometers, less than 300 nanometers, less than 500 nanometers, between 50 nanometers and 300 nanometers, between 300 nanometers and 400 nanometers, etc. The center-to-center spacing between the ridges (pitch) may be any desired magnitude (e.g., greater than 50 nanometers, greater than 100 nanometers, greater than 200 nanometers, greater than 300 nanometers, greater than 500 nanometers, greater than 750 nanometers, greater than 1000 nanometers, less than 50 nanometers, less than 100 nanometers, less than 200 nanometers, less than 300 nanometers, less than 500 nanometers, less than 750 nanometers, less than 1000 nanometers, between 200 nanometers and 400 nanometers, between 300 nanometers and 400 nanometers, between 100 nanometers and 750 nanometers, etc.). The duty cycle of the ridges (defined as ridge width divided by ridge pitch) may be greater than 60%, greater than 70%, greater than 80%, greater than 90%, greater than 95%, less than 99%, less than 70%, less than 80%, less than 90%, less than 95%, between 60% and 99%, etc. The ridges may have a uniform spacing or may have a varying spacing. Similarly, the dimensions of each ridge may be uniform or may vary.

As shown inFIG.4, an anti-reflective layer94may be formed over ridges78and non-SRG portion92of the high-index layer90. Anti-reflective layer94may be formed from a single layer of optical coating (e.g., a single layer of silicon dioxide). In this type of example, the anti-reflective layer94has a lower refractive index than the material of layer90(and therefore the material of ridges78). The refractive index of anti-reflective layer94may be lower than the refractive index of ridges78by at least 0.1, at least 0.3, at least 0.5, at least 0.7, at least 1.0, etc. The refractive index of anti-reflective layer94may be less than 2.0, less than 1.8, less than 1.6, less than 1.4, etc.

In another possible arrangement, anti-reflective layer94(sometimes referred to as anti-reflective coating94) may include multiple layers of material (e.g., organic or inorganic dielectric material) with varying refractive indices. For example, the anti-reflective layer may include alternating layers of high refractive index material and low refractive index material (e.g., with a refractive index that is at least 0.1 lower than the high refractive index material, at least 0.3 lower than the high refractive index material, at least 0.5 lower than the high refractive index material, at least 0.7 lower than the high refractive index material, etc.). In general, a multi-layer anti-reflective layer includes at least two layers with different refractive indices. The refractive index of a layer within multi-layer anti-reflective layer94may be lower than the refractive index of ridges78by at least 0.1, at least 0.3, at least 0.5, at least 0.7, at least 1.0, etc.

The example inFIG.4of anti-reflective layer94filling the troughs80between adjacent ridges is merely illustrative. In another possible arrangement, shown inFIG.5, anti-reflective layer94is patterned to overlap ridges78(and non-SRG portion92) without overlapping (filling) troughs80. This type of arrangement may have improved diffraction efficiency compared withFIG.4(due to the troughs being filled with air inFIG.5instead of layer94as inFIG.4). However, the arrangement ofFIG.4may have reduced manufacturing cost and complexity compared to the arrangement ofFIG.5. In yet another example, shown inFIG.6, ridges78are formed by the anti-reflective layer94itself.

InFIGS.4and5, anti-reflective layer94is formed over ridges78in SRG structure74. In this arrangement, ridges78are interposed between waveguide50and anti-reflective layer94. This example is merely illustrative. In another possible arrangement, shown inFIG.7, the anti-reflective layer94is formed below ridges78in SRG structure74. In this arrangement, anti-reflective layer94is interposed between waveguide50and ridges78. InFIG.7, anti-reflective layer94has a uniform thickness and covers waveguide50. Ridges78are then formed directly on the upper surface of anti-reflective layer94.

Anti-reflective layers may be formed both above and below ridges78if desired. As shown inFIG.8, a first anti-reflective layer94-1is formed below ridges78(such that the anti-reflective layer94-1is interposed between waveguide50and ridges78). A second anti-reflective layer94-2is formed above ridges78(such that the ridges78are interposed between waveguide50and anti-reflective layer94-2).

When multiple anti-reflective layers are included in the SRG structure (as inFIGS.8and9, for example), each anti-reflective layer may either be a single-layer anti-reflection layer (e.g., a single layer of optical coating with a refractive index lower than the refractive index of the high-index layer90) or a multi-layer anti-reflection layer (as described above). Anti-reflective layer94-1may be a single-layer anti-reflective layer while anti-reflective layer94-2may be a multi-layer anti-reflective layer, or vice versa.

In another possible arrangement, shown inFIG.10, different ridges may be covered by different anti-reflective layers. For example, a first subset of ridges78inFIG.10are covered by anti-reflective layer94-1while a second subset of ridges78inFIG.10are covered by anti-reflective layer94-2. Anti-reflective layers94-1and94-2may have at least one differing property (e.g., material, dimension, number of layers, etc.). Anti-reflective layer94-1may be a single-layer anti-reflective layer while anti-reflective layer94-2may be a multi-layer anti-reflective layer, or vice versa. Anti-reflective layers94-1and94-2may be multi-layer anti-reflective layers with differing layer stack-ups.

Including different anti-reflective layers over different subsets of ridges as inFIG.10is merely illustrative. If desired, an anti-reflective layer that is formed below ridges (e.g., as inFIGS.7-9) may be split into two anti-reflective layers with at least one differing property (with each anti-reflective layer below a respective subset of the ridges). Additionally, different subsets of ridges may be aligned with anti-reflective coatings in different locations if desired. For example, a first subset of ridges may be positioned below a first anti-reflective coating (as inFIG.4orFIG.5) while a second subset of ridges may be positioned above a second anti-reflective coating (a inFIG.7)

FIG.11shows another possible arrangement for an anti-reflective layer in surface relief grating structure74. InFIG.11, an anti-reflective layer94is formed as a conformal coating over ridges78. In other words, the thickness of anti-reflective layer94inFIG.11is the same above ridges78as between ridges78in troughs80(e.g., anti-reflective layer94has a uniform thickness). Across the surface relief grating structure, the thickness of anti-reflective layer94may be uniform within 10%, within 5%, etc.

FIG.11shows how an encapsulation layer98may be formed over ridges78with conformal anti-reflective layer94. Encapsulation layer98may be formed from the same material as ridges78or from a different material than ridges78. Encapsulation layer98has a planar upper surface. Encapsulation layer98has a greater thickness over troughs80than over ridges78.

In the example ofFIG.11, the anti-reflective layer94has a lower refractive index than the material of ridges78. The refractive index of anti-reflective layer94may be lower than the refractive index of ridges78by at least 0.1, at least 0.3, at least 0.5, at least 0.7, at least 1.0, etc. The refractive index of anti-reflective layer94may be less than 2.0, less than 1.8, less than 1.6, less than 1.4, etc.

In another possible arrangement, anti-reflective layer94(sometimes referred to as anti-reflective coating94) may include multiple layers of material (e.g., organic or inorganic dielectric material). Multiple coatings may collectively be referred to as a single anti-reflective layer94. As one example, anti-reflective layer94may include multiple layers of material with varying refractive indices. For example, the anti-reflective layer may include alternating layers of high refractive index material and low refractive index material. The refractive index of a layer within multi-layer anti-reflective layer94may be lower than the refractive index of ridges78by at least 0.1, at least 0.3, at least 0.5, at least 0.7, at least 1.0, etc.

Each coating in anti-reflective layer94may be conformally coated or directionally coated on the underlying ridges78.FIG.12is a cross-sectional sideview of an illustrative surface relief grating structure with ridges78having angled surfaces.FIG.12shows an example where the anti-reflective layer94is formed using first and second coatings94-1and94-2that are deposited using a directional coating technique.

When an encapsulation layer is formed over ridges78, the encapsulation layer may be formed using two deposition steps to improve the flatness of the upper surface of the encapsulation layer.FIG.13shows a two-step method of forming an encapsulation layer over ridges in a surface relief grating structure.

At step102, a first layer94-1is formed in troughs80between ridges78. In a subsequent deposition step104, a second layer94-2is formed over the ridges78and first layer94-1. The upper surface of the second layer94-2is planar. Each one of layers94-1and94-2may be spin coating resins.

InFIG.13, an example is shown where the two-part deposition process is applied to anti-reflective layer94. This example is merely illustrative. In general, the two-part deposition process ofFIG.13may be applied to any desired layer with a planar upper surface (e.g., encapsulation layer98inFIGS.11and12, layer94in inFIG.4, layer94-2inFIG.9, layers94-1and94-2inFIG.10, etc.).

If desired, a surface relief grating may be encapsulated by a metal material. In other words, encapsulation layer98inFIG.11and/orFIG.12may be formed from metal.

All of the layers described herein (e.g., the substrate, ridges, anti-reflective layer(s), etc.) can form complex near-field effects with incident light propagating in total internal reflection (TIR). The layer thicknesses must be co-designed for a given grating. Including extra layers allows for more free parameters. The overall device end-to-end image color uniformity can be improved by optimizing these thickness combinations.

Optical system20B may include one or more optical couplers (e.g., an input coupler, a cross-coupler, and an output coupler) formed at or on a waveguide. As examples, the optical system may have a sequential architecture or a combined architecture.

In a sequential architecture, image light may be directed to an input coupler, a cross coupler, and an output coupler in that order. As a specific example, a cross coupler may be at least partially laterally interposed between an input coupler (e.g., an input prism) and an output coupler. The input coupler may be laterally interposed between the cross coupler and an edge of the waveguide. The input prism may couple light into the waveguide. A cross coupler may expand the in-coupled light in a first direction and may provide the light to the output coupler. The output coupler may expand the light in a second direction that is different than the first direction.

In a combined architecture, image light may be directed from an input coupler to a combined optical coupler that performs the function of both a cross coupler and an output coupler. It may be desirable for the output coupler on the waveguide to fill as large of an eye box with as uniform-intensity image light as possible. The combined optical coupler may perform the functionality of both a cross-coupler and an output coupler for the waveguide. The combined optical coupler may therefore be configured to expand image light in one or more dimensions while also coupling the image light out of the waveguide. By using a combined optical coupler in this manner, space may be conserved within the display.

Any of the SRG structures described herein may be used to form any optical coupler (e.g., an input coupler, a cross coupler, an output coupler, a combined optical coupler that performs the function of both a cross coupler and an output coupler, etc.) in optical systems with either a sequential architecture or a combined architecture. When SRG structures form multiple optical couplers, a single SRG structure may be described has having ridges, a first subset of which form a first optical coupler and a second subset of which form a second optical coupler. Alternatively, a first SRG structure may be described as defining a first optical coupler while a second SRG structure may be described as defining a second optical coupler.

Different optical couplers may have different anti-reflective layer arrangements. For example, a first optical coupler (e.g., an input coupler) may have ridges and an anti-reflective layer formed above the ridges while a second optical coupler (e.g., a combined optical coupler that performs the function of both a cross coupler and an output coupler) may have ridges and an anti-reflective layer formed below the ridges. As another example, a first optical coupler (e.g., an input coupler) may have ridges and an anti-reflective layer formed above and between the ridges (as inFIG.4) while a second optical coupler (e.g., a combined optical coupler that performs the function of both a cross coupler and an output coupler) may have ridges and an anti-reflective layer patterned to overlap ridges but not fill troughs between the ridges (as inFIG.5). As yet another example, a first optical coupler (e.g., an input coupler) may have ridges and a single-layer anti-reflective layer that overlaps the ridges while a second optical coupler (e.g., a combined optical coupler that performs the function of both a cross coupler and an output coupler) may have ridges and a multi-layer anti-reflective layer that overlaps the ridges.

In general, any of the anti-reflective layers described herein in connection withFIGS.4-13may be a single-layer anti-reflective layer or a multi-layer anti-reflective layer.

In accordance with an embodiment, a display system is provided that includes a waveguide configured to propagate image light and a surface relief grating structure at the waveguide, the surface relief grating structure includes a plurality of ridges separated by a plurality of troughs and an anti-reflective layer formed over the plurality of ridges, the anti-reflective layer fills the plurality of troughs and the plurality of ridges is interposed between the waveguide and the anti-reflective layer.

In accordance with another embodiment, the anti-reflective layer includes a plurality of layers with different refractive indices.

In accordance with another embodiment, the surface relief grating structure further includes an additional anti-reflective layer that is interposed between the plurality of ridges and the waveguide.

In accordance with another embodiment, the anti-reflective layer is a single layer of optical coating.

In accordance with another embodiment, the plurality of ridges is formed from a material with a first refractive index, the anti-reflective layer includes a material with a second refractive index, and the second refractive index is lower than the first refractive index by at least 0.3.

In accordance with another embodiment, the anti-reflective layer is a first anti-reflective layer, the plurality of ridges is a first plurality of ridges, the plurality of troughs is a first plurality of troughs, and the surface relief grating structure further includes a second plurality of ridges separated by a second plurality of troughs and a second anti-reflective layer formed over the second plurality of ridges, the second plurality of ridges is interposed between the waveguide and the second anti-reflective layer.

In accordance with another embodiment, the first anti-reflective layer has at least one property that is different than in the second anti-reflective layer.

In accordance with another embodiment, the first plurality of ridges defines an input coupler and the second plurality of ridges defines a combined optical coupler that performs the function of both a cross coupler and an output coupler.

In accordance with another embodiment, the first anti-reflective layer is a single-layer anti-reflective layer and the second anti-reflective layer is a multi-layer anti-reflective layer.

In accordance with another embodiment, the anti-reflective layer has a uniform thickness over both the plurality of ridges and the plurality of troughs.

In accordance with another embodiment, the surface relief grating structure further includes an encapsulation layer that is formed over the anti-reflective layer, the encapsulation layer has a planar surface, a first thickness over the plurality of ridges, and a second thickness that is greater than the first thickness over the plurality of troughs.

In accordance with another embodiment, the encapsulation layer is formed from a same material as the plurality of ridges.

In accordance with an embodiment, a display system is provided that includes a waveguide configured to propagate image light and a surface relief grating structure at the waveguide, the surface relief grating structure includes a plurality of ridges separated by a plurality of troughs and an anti-reflective layer formed between the waveguide and the plurality of ridges.

In accordance with another embodiment, the anti-reflective layer includes a plurality of layers with different refractive indices.

In accordance with another embodiment, the surface relief grating structure further includes an additional anti-reflective layer that is formed over the plurality of ridges and the plurality of ridges is interposed between the additional anti-reflective layer and the anti-reflective layer.

In accordance with another embodiment, the additional anti-reflective layer is patterned to overlap the plurality of ridges without filling the plurality of troughs.

In accordance with another embodiment, the additional anti-reflective layer overlaps the plurality of ridges and fills the plurality of troughs.

In accordance with another embodiment, the anti-reflective layer is a single layer of optical coating.

In accordance with another embodiment, the plurality of ridges is formed from a material with a first refractive index, the anti-reflective layer includes a material with a second refractive index, and the second refractive index is lower than the first refractive index by at least 0.3.

In accordance with an embodiment, a display system is provided that includes a waveguide configured to propagate image light, the waveguide has first and second opposing sides and at least one surface relief grating structure at the waveguide, the at least one surface relief grating structure includes a plurality of ridges separated by a plurality of troughs, the plurality of ridges is formed on the first side of the waveguide, a first anti-reflective layer formed on the first side of the waveguide and aligned with a first subset of the plurality of ridges and a second anti-reflective layer formed on the first side of the waveguide and aligned with a second subset of the plurality of ridges, the first anti-reflective layer has at least one property that is different than in the second anti-reflective layer.

In accordance with another embodiment, the at least one property is a dimension.

In accordance with another embodiment, the at least one property is a material.

In accordance with another embodiment, the at least one property is a number of layers.