Patent Description:
In some implementations an optical waveguide can be used to spatially translate a generated image from one position to another position in an optical system. For example, in a near-eye display (NED) device, an optical waveguide can spatially translate propagating light rays representing imagery generated by a microdisplay and convey them toward an eye of a user. Such technology may be incorporated into an NED device in the form of eyeglasses, goggles, a helmet, a visor, or some other type of eyewear.

Document <CIT> teaches a projection display for projecting a colour image to a viewer overlaid on a real world scene viewed through the display. The display comprises an image generator for generating image bearing chromatic light for injection into a waveguide assembly. The waveguide assembly comprises a first waveguide having a first input diffraction region arranged to couple image bearing chromatic light in a first range of field angles into the first waveguide to propagate by total internal reflection. A second waveguide having a second input diffraction region is arranged to couple image bearing chromatic light in a second range of field angles into the second waveguide to propagate by total internal reflection. The first and second waveguides have first and second output diffraction regions arranged to output image bearing chromatic light from the respective waveguides for projecting a colour image in the first and second range of field angles to a viewer overlaid on a real world scene viewed through the waveguide assembly.

Document <CIT> teaches the grating having a flat metal substrate coated with a metal layer, and a thin dielectric layer made of high index material having thickness and a refractive index difference with respect to a refractive index of an incident medium i.e. planar waveguide. The thickness and the refractive index difference are preset such that diffraction efficiency along a magnetic transverse linear polarization state is higher than diffraction efficiency along an electric transverse linear polarization state for an incidence angle ranging between zero and plus or minus <NUM>. Independent claims are also included for the following: a grating coupler comprising a reflection diffraction grating a planar imaging or optical beam transporting system comprising a set of reflection diffraction gratings.

Document <CIT> teaches an optical waveguide comprising a body of material configured for the contained propagation of light therethrough, a surface relief grating configured to receive the propagating light and at least partially to diffract or reflect it out of the waveguide, and at least one layer of dielectric material of varying thickness having a first surface and a second surface which conforms to a profiled surface of the grating so that the grating exhibits a spatial variation in efficiency dependent on the varying thickness of the dielectric material.

Document <CIT> teaches an optical waveguide having wavelength specific incoupling and outcoupling gratings on each side of the waveguide substrate.

According to aspects of the present invention there is provided an optical waveguide, a near-eye display system and a method of manufacturing a waveguide display method as defined in the accompanying claims. The technique introduced here includes an optical waveguide and a method of manufacturing such a waveguide and an optical waveguide display. In various embodiments the optical waveguide includes a light-transmissive substrate configured for use in a near-eye display (NED) device. The substrate includes a plurality of internally reflective surfaces, where the substrate is made of a first material having a first refractive index. The optical waveguide further includes a diffractive optical element (DOE) formed on a first surface of the plurality of surfaces of the substrate, where the DOE is configured to input light rays to the substrate or output light rays from the substrate. The DOE in various embodiments includes a diffraction grating made of a second material having a second refractive index; and a coating over the diffraction grating made of a third material having a third refractive index, wherein the second refractive index is not equal to the third refractive index. Other aspects of the technique will be apparent from the accompanying figures and detailed description.

One or more embodiments of the present disclosure are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements.

In this description, references to "an embodiment", "one embodiment" or the like, mean that the particular feature, function, structure or characteristic being described is included in at least one embodiment of the technique introduced here. Occurrences of such phrases in this specification do not necessarily all refer to the same embodiment. On the other hand, the embodiments referred to also are not necessarily mutually exclusive.

The following description generally assumes that a "user" of a display device is a human. Note, however, that a display device according to the embodiments disclosed herein can potentially be used by a user that is not human, such as a machine or an animal. Hence, the term "user" can refer to any of those possibilities, except as may be otherwise stated or evident from the context. Further, except where stated otherwise, the term "eye" is used herein as a general term to refer to an optical receptor of any type of user of a display device, and therefore can refer to a human eye, an animal eye, or a machine-implemented optical sensor designed to detect an image in a manner analogous to a human eye.

Some NED devices include optical systems for spatially translating a pupil from one position to another position, for example from a microdisplay imager to an eye of a user. This optical system is sometimes referred to as a pupil relay system. An NED device can include one or more transparent waveguides arranged so that they are located directly in front of each eye of the user when the NED device is worn by the user, to project light representing generated images into the eye of the user. With such a configuration, images generated by the NED device can be overlaid on the user's view of the surrounding physical environment. Waveguides configured for use in NED devices include reflective surfaces configured to propagate light rays through total internal reflection (TIR). One aspect of translating a pupil from one position to another via a waveguide involves receiving the light rays into the waveguide ("in-coupling") at a first location and outputting the light rays from the waveguide ("out-coupling") at a second location.

Light rays can in-coupled to and out-coupled from a waveguide via a DOE that functions as an input port or an output port for the light rays. A DOE can include a diffraction grating structure, for example a surface relief diffraction grating (SRG). However, the diffraction efficiency of a conventional grating-based DOE is highly sensitive to both the wavelength and angle of incidence of the diffracted light rays. This results in waveguide displays that are only able to effectively in-couple and out-couple a narrow field of view (FOV) (e.g., less than <NUM> degrees) of a generated image. This high sensitivity to wavelength and angle of incidence restricts the use of such components when large FOVs (e.g., over <NUM> degrees) are considered, or when the range of wavelength of light is relatively broad (e.g., full color displays). If a large FOV is desired, the diffraction efficiency of the grating should be sufficiently high over the whole FOV to avoid greatly compromising the uniformity of the image across the FOV.

Tests have demonstrated that the diffraction efficiency of a grating-based DOE at a given wavelength or angle of incidence of light can depend on the refractive index of the material used to form the grating structure. In some configurations, increasing the refractive index of the material used to form the diffraction grating structure increases and provides for a more uniform diffraction efficiency over a range of angles of incidence and wavelength of light rays. Glass materials with sufficiently high refractive indices (e.g., n=<NUM>) can be used to form the substrate of a waveguide. However, using glass to form diffraction grating structures typically involves a process of etching the structure into the glass. This process is expensive, time consuming and generally impractical, particularly for mass produced waveguide displays for consumer NED devices.

A cheaper and quicker process for forming the grating structure of a DOE involves using standard UV-curable resin, however standard UV-curable resins are polymer-based and typically have a lower refractive index (e.g., n=<NUM>-<NUM>) than glass. A waveguide display with a glass substrate and grating structures made of polymer-based resin will, as a result, be limited to displaying a relatively narrow FOV.

Accordingly, introduced here are solutions to this problem, which include coating and/or burying a grating structure made of a material with a first refractive index material in another with a second refractive index that is not equal to the first refractive index. It has been found that increasing the contrast between the refractive index of the grating structure and the refractive index of the coating can increase the diffraction efficiency of a grating-based DOE over a range of wavelengths and angles of incidence of diffracted light rays.

<FIG> shows an example of a near-eye display (NED) device in which the technique introduced here can be incorporated. The NED device <NUM> may provide virtual reality (VR) and/or augmented reality (AR) display modes for a user, i.e., the wearer of the device. To facilitate description, it is henceforth assumed that the NED device <NUM> is designed for AR visualization.

In the illustrated embodiment, the NED device <NUM> includes a chassis <NUM>, a transparent protective visor <NUM> mounted to the chassis <NUM>, and left and right side arms <NUM> mounted to the chassis <NUM>. The visor <NUM> forms a protective enclosure for various display elements (not shown) that are discussed below.

The chassis <NUM> is the mounting structure for the visor <NUM> and side arms <NUM>, as well as for various sensors and other components (not shown) that are not germane to this description. A display assembly (not shown in <FIG>) that can generate images for AR visualization is also mounted to the chassis <NUM> and enclosed within the protective visor <NUM>. The visor assembly <NUM> and/or chassis <NUM> may also house electronics (not shown) to control the functionality of the display assembly and other functions of the NED device <NUM>. The NED device <NUM> shown in <FIG> is a head-mounted display HMD device and so further includes an adjustable headband <NUM> attached to the chassis <NUM>, by which the NED device <NUM> can be worn on a user's head.

<FIG> show, in accordance with certain embodiments, right side and front orthogonal views, respectively, of display components that may be contained within the visor <NUM> of the NED device <NUM>. During operation of the NED device <NUM>, the display components are positioned relative to the user's left eye <NUM>L and right eye <NUM>R as shown. The display components are mounted to an interior surface of the chassis <NUM>. The chassis <NUM> is shown in cross-section in <FIG>.

The display components are designed to overlay three-dimensional images on the user's view of his real-world environment, e.g., by projecting light into the user's eyes. Accordingly, the display components include a display module <NUM> that houses a light engine including components such as: one or more light sources (e.g., one or more light emitting diodes (LEDs)); one or more microdisplay imagers, such as liquid crystal on silicon (LCOS), liquid crystal display (LCD), digital micromirror device (DMD); and one or more lenses, beam splitters and/or waveguides. The microdisplay imager(s) (not shown) within the display module <NUM> may be connected via a flexible circuit connector <NUM> to a printed circuit board <NUM> that has image generation/control electronics (not shown) mounted on it.

The display components further include a transparent waveguide carrier <NUM> to which the display module <NUM> is mounted, and one or more transparent waveguides <NUM> stacked on the user's side of the waveguide carrier <NUM>, for each of the left eye and right eye of the user. The waveguide carrier <NUM> has a central nose bridge portion <NUM>, from which its left and right waveguide mounting surfaces extend. One or more waveguides <NUM> are stacked on each of the left and right waveguide mounting surfaces of the waveguide carrier <NUM>, to project light emitted from the display module and representing images into the left eye <NUM>L and right eye <NUM>R, respectively, of the user. The display assembly <NUM> can be mounted to the chassis <NUM> through a center tab <NUM> located at the top of the waveguide carrier <NUM> over the central nose bridge section <NUM>.

<FIG> shows a single input pupil design for a waveguide 252a that can be mounted on the waveguide carrier <NUM> to convey light to a particular eye of the user, in this example, the right eye of user. A similar waveguide can be designed for the left eye, for example, as a (horizontal) mirror image of the waveguide shown in <FIG>. The waveguide 252a is transparent and, as can be seen from <FIG>, would normally be disposed directly in front of the right eye of the user during operation of the NED device, e.g., as one of the waveguides <NUM> in <FIG>. The waveguide 252a is, therefore, shown from the user's perspective during operation of the NED device <NUM>.

The waveguide 252a includes a single input port 310a (also called an incoupling element, and corresponding to the single input pupil) located in the region of the waveguide 252a that is closest to the user's nose bridge when the NED device <NUM> is worn by the user. In certain embodiments the input port 310a is or includes a DOE which can include, for example a surface diffraction grating, volume diffraction grating, and/or a Switchable Bragg Grating (SBG). The waveguide 252a further includes an output port 330a (also called out-coupling element) and a transmission channel 320a. As with the input port 310a, in certain embodiments, the output port 330a is or includes a DOE which can include, for example a surface diffraction grating, volume diffraction grating, and/or an SBG. A right-eye output port of the display module (not shown) is optically coupled (but not necessarily physically coupled) to the input port 310a of the waveguide <NUM>. During operation, the display module <NUM> (not shown in <FIG>) outputs light representing an image for the right eye from its right-eye output port (not shown) into the input port 310a of the waveguide 252a.

The transmission channel 320a conveys light from the input port <NUM> to the output port <NUM> and may include, for example, a surface diffraction grating, volume diffraction grating, or a reflective component such as a substrate with multiple internally reflective surfaces. The transmission channel 320a may be designed to accomplish this by use of total internal reflection (TIR). Light representing the image for the right eye is then projected from the output port 330a to the user's eye.

As shown in <FIG>, in some embodiments a waveguide may include multiple input ports 310b and 312b, for example to provide a greater overall FOV through multiplexing different FOVs of the projected image. Note that while the present disclosure describes waveguides with one or two input ports/pupils and a single output port/pupil, a display device incorporating the technique introduced here may have a waveguide with more than two input ports/pupils and/or more than one output port/pupil for a given eye. Further, while the example of <FIG> is for the right eye, a similar waveguide can be designed for the left eye, for example, as a (horizontal) mirror image of the waveguide in <FIG>.

As shown in <FIG>, the waveguide 252b includes two separate input ports 310b and 312b, two transmission channels 320b and 322b, and an output port 330b. During operation, each of the input ports 310b, 312b receives light (from the display module <NUM>) representing a different portion of the image for the right eye of the user. Each of the transmission channels 320b, 322b is optically coupled to a separate one of the input ports 310b or 312b and conveys light from only the corresponding input port 310b or 312b to the output port 330b. Each of the transmission channels 320b, 322b may be, for example, an internal or surface diffraction grating design to channel light by TIR. Light from the two different portions of the image is combined at the output port 330b and projected into the eye of the user as a single integrated image.

In some embodiments, the left input port 310b receives the left portion (e.g., half) of the image for one eye of the user (e.g., the right eye) while the right input port 312b receives the right portion (e.g., half) of the image for that same eye. Each portion of the image can include all of the color components that are present in the complete image, e.g., red, green and blue color components. The portions of the image may be generated in a tiled manner, i.e., where they are spatially contiguous and nonoverlapping, or they may at least partially overlap spatially. Further, in other embodiments, rather than generating left and right portions of the image, the separate portions of the image could be upper and lower portions of the image, or the image could be spatially divided in some other manner. Additionally, the waveguide 252b could have more than two input ports, in which case the image could be provided to the waveguide 252b in the form of three or more separate image portions, which are reintegrated in the waveguide 252b.

Hence, in at least some embodiments, different portions of an image for a given eye of the user are generated and input simultaneously into separate input ports of a waveguide, then reintegrated within the waveguide and projected into the eye of the user as a single integrated image, to produce a larger FOV. In other embodiments, the separate portions of the image could be input to the waveguide in a time division multiplexed manner, rather than simultaneously. Further, in some embodiments, the physical placement of the input ports on the waveguide may be different from that shown in <FIG>. For example, the input ports could be spaced apart vertically on the waveguide rather than, or in addition to, horizontally. Other input port configurations are also possible.

As shown in <FIG>, in some embodiments a waveguide may include multiple output ports 330c and 332c, for example to provide a greater overall field of view (FOV) through multiplexing different fields of view of the projected image. Note that while the present disclosure describes a waveguides with two output ports/pupils and a single input port/pupil, a display device incorporating the technique introduced here may have a waveguide with more than two input ports/pupils and/or more than two output ports/pupils for a given eye. Further, while the example of <FIG> is for the right eye, a similar waveguide can be designed for the left eye, for example, as a (horizontal) mirror image of the waveguide in <FIG>.

As shown in <FIG>, the waveguide 252c includes two separate output ports 330c and 332c, a transmission channel 320c, and an input port 310c. During operation, the input ports 310c receives light (from the display module <NUM>) representing a different portion of the image for the right eye of the user. The transmission channel 320c is optically coupled to the input ports 310c and conveys light from the input port 310c or the output ports 330c and 332c. The transmission channels 320c may be, for example, an internal or surface diffraction grating design to channel light by TIR. Each of the two output ports 330c and 332c output light corresponding to one of two different portions of an image and project the respective protons of the image into the eye of the user as a single integrated image.

In some embodiments, the left output port 330c projects the left portion (e.g., half) of the image for one eye of the user (e.g., the right eye) while the right output port 332c projects the right portion (e.g., half) of the image for that same eye. Each portion of the image can include all of the color components that are present in the complete image, e.g., red, green and blue color components. The portions of the image may be generated in a tiled manner, i.e., where they are spatially contiguous and nonoverlapping, or they may at least partially overlap spatially. Further, in other embodiments, rather than generating left and right portions of the image, the separate portions of the image could be upper and lower portions of the image, or the image could be spatially divided in some other manner. Additionally, the waveguide 252c could have more than two output ports, in which case the image can be projected to the eye of the user in the form of three or more separate image portions
Hence, in at least some embodiments, different portions of an image for a given eye of the user are generated and input simultaneously into separate input ports of a waveguide, then reintegrated within the waveguide and projected into the eye of the user as a single integrated image, to produce a larger FOV. In other embodiments, the separate portions of the image could be input to the waveguide in a time division multiplexed manner, rather than simultaneously. Further, in some embodiments, the physical placement of the input ports on the waveguide may be different from that shown in <FIG>. For example, the input ports could be spaced apart vertically on the waveguide rather than, or in addition to, horizontally. Alternatively, in some embodiments, the multiple output ports of a waveguide may be overlaid over one another instead of oriented side by side as shown in <FIG>. Other input port configurations are also possible.

<FIG> schematically shows an example display module <NUM> configured for use with a NED device such as NED device <NUM> in <FIG>. Note, that example display module <NUM> shown in <FIG> is configured for use with a dual input port waveguide as shown at example waveguide 252b in <FIG>. However, similar principles can be applied for a display module configured for use with fewer or more input ports, for example as shown at example waveguides 252a and 252c in <FIG> (respectively).

As shown in <FIG>, example display module <NUM> includes a light engine <NUM>, an optical switch <NUM> and a pupil relay <NUM>. Though not shown, the display module <NUM> may also include similar or identical components for the other eye of the user. In some embodiments, the light engine <NUM> includes one or more light sources (not shown), such as one or more colored LEDs. For example, the light engine <NUM> can include red, green and blue LEDs to produce the red, green and blue color components, respectively, of the image. Additionally, the light engine <NUM> includes at least one microdisplay imager (not shown), such as an LCOS imager, LCD or DMD; and may further include one or more lenses, beam splitters, waveguides, and/or other optical components (not shown).

The optical switch <NUM> controls the propagation direction of the light output by the light engine <NUM>, representing each particular portion of the image, to one of two different optical paths. In the illustrated embodiment, the first path is for the left half of the image and leads to an output port <NUM> of the display module <NUM> that is coupled to one corresponding input port <NUM> of the waveguide <NUM>. The other optical path is for the right portion of the image and includes a pupil relay <NUM>, which propagates that portion of the image to a second output port <NUM> of the display module <NUM>, which is optically coupled to a second corresponding input port <NUM> of the waveguide <NUM>.

The optical switch <NUM> selectively controls the propagation direction of light from the light engine <NUM> based on a switching criterion, such as polarization. For example, one half of the image may have s-polarization while the other half of image has p-polarization, where the optical switch <NUM> conveys s-polarized light along one optical path and conveys p-polarized light along the other optical path. The switch <NUM> can be, for example, an LCD mirror that either transmits light or acts as a perfect mirror, depending on the applied voltage. Note, however, that a switching criterion (or criteria) other than polarization could be used. For example, time division multiplexing could be used to switch between the optical paths. The optical switch <NUM>, therefore, enables a single light engine <NUM> to provide two pupils for an image to two separate in-coupling elements <NUM>, <NUM> on a waveguide <NUM>. In an example waveguide with only one input port, optical switch <NUM> may direct light emitted from light engine <NUM> to the single input port.

The pupil relay <NUM> is optional but enables larger distances between the input ports <NUM>, <NUM> on the waveguide <NUM>. The pupil relay <NUM> may be constructed using any known or convenient method and materials for transferring an image pupil from one location to another. For example, the pupil relay <NUM> may be constructed from a sequence of paraxial lenses that focus the pupil to an intermediate image and then collimate it, followed by a mirror to redirect the light into the corresponding input port of the waveguide.

<FIG> schematically illustrates an example light engine <NUM> including certain relevant components. The view in <FIG> is from the right side of the display module <NUM>. Note that some embodiments may include other active and/or passive components, not shown. The light engine <NUM> in the illustrated embodiment includes at least one light source <NUM>, such as a color LED. Although only one light source <NUM> is shown in <FIG>, in practice there may be multiple light sources provided for each eye of the user, e.g., one for each color component of whatever color model is being employed (e.g., red, green and blue). The same or a similar configuration as shown in <FIG> can be used to combine light from such multiple light sources.

The light engine <NUM> further includes one or more imagers (e.g., LCOS microdisplays) 572a and 572b that generate an image intended for display to a particular eye of the user. Note that the example light engine <NUM> shown in <FIG> includes two imagers 572a and 572b, however another light engine may include one or more than two imagers. In the case of multiple imagers 572a and 572b, each imager may generate a portion of the image to be displayed to the user. A retarder (e.g., quarter-wave plate) can be placed before the waveguide at one of the waveguide inputs to have optimum polarization entering the waveguide.

Additionally, the light engine <NUM> can include a combination of polarizing beam splitters (PBSs) <NUM>, <NUM>, one or more reflective lenses <NUM> and one or more quarter-wave plates <NUM>, that generate and propagate the image(s) through the output port <NUM> of the light engine <NUM>. In the example shown in <FIG>, a first PBS <NUM> reflects s-polarized light from the light source <NUM> upward to a first microdisplay imager 572a, which generates one portion of the image. The PBS <NUM> also causes p-polarized light from the light source <NUM> to be propagated straight through to the other microdisplay imager 572b, which produces a second portion of the image. Both portions of the image (separately constituting s- polarized and p-polarized light) then propagate downward through the PBS <NUM> to a second PBS <NUM>, which directs them to birdbath-shaped reflective lenses <NUM> via quarter-wave plates (retarders) <NUM>. The image portions are then reflected back by the reflective lenses <NUM> through the quarter-wave plates <NUM> and then through the PBS <NUM>. From there, the image portions are output through the output port <NUM> of the light engine <NUM> and provided to additional optics in the display module <NUM>, as shown by the example in <FIG>.

<FIG> is a block diagram of example hardware components including a computer system within control circuitry <NUM> of a NED device <NUM>. Control circuitry <NUM> provides various electronics that support the other components of NED device <NUM>. In this example, the control circuitry <NUM> for includes a processing unit <NUM>, a memory <NUM> accessible to the processing unit <NUM> for storing processor readable instructions and data, a communication module <NUM> communicatively coupled to the processing unit <NUM> which can act as a network interface for connecting the NED device to another computer system. A power supply <NUM> provides power for the components of the control circuitry <NUM> and the other components of the NED device <NUM> like sensor units <NUM> which may include, but are not limited to, image capture devices (e.g. cameras), audio capture devices (e.g. microphones), and location/ motion capture devices (e.g. accelerometers).

The processing unit <NUM> may include one or more processors including a central processing unit (CPU) and/or a graphics processing unit (GPU). Memory <NUM> is representative of the various types of memory which may be used by the system such as random access memory (RAM) for application use during execution, buffers for sensor data including captured image data and display data, read only memory (ROM) or Flash for instructions and system data, and other types of nonvolatile memory for storing other items, some examples of which are applications for which image light representing image data is generated. In this example, an electrical connection of a data bus <NUM> connects the sensor units <NUM>, a display driver <NUM>, processing unit <NUM>, memory <NUM>, and the communication module <NUM>. The data bus <NUM> also derives power from the power supply <NUM> through a power bus <NUM> to which all the illustrated elements of the control circuitry are connected for drawing power.

The control circuitry <NUM> further includes the display driver <NUM> for selecting digital control data, e.g. control bits, to represent image data which digital control data may be decoded by microdisplay circuitry <NUM> and different active component drivers. An example of an active component driver is a display illumination driver <NUM> which converts digital control data to analog signals for driving an illumination unit <NUM> which includes one or more light sources (e.g. similar to light source <NUM> in <FIG>) like one or more light emitting diodes (LEDs). A microdisplay <NUM> may be an active transmissive, emissive, or reflective device. For example, microdisplay <NUM> may be similar to the one or more imagers 572a-b described with reference to <FIG>. Microdisplay <NUM> may be a liquid crystal on silicon (LCoS) device requiring power or a micromechanical machine (MEMs) based device requiring power to move individual mirrors. In some embodiments, a waveguide display may include one or more active gratings <NUM> such as a Switchable Bragg Grating (SBG). An active grating(s) controller <NUM> converts digital control data into signals for changing the properties of one or more active gratings <NUM>.

In some embodiments discussed below, the control circuitry <NUM> may include other control units not illustrated here but related to other functions of a NED device <NUM> device such as, for example, polarization control, providing audio output, identifying head orientation and location information. In other embodiments, some of the processing and memory resources identified in <FIG> can be shared between the control circuitry <NUM> and a companion processing module embodied in, for example, a mobile device (e.g. a smart phone) communicatively coupled to the NED device <NUM>.

<FIG> illustrate the propagation of light rays in a pupil relay using diffractive optical elements DOEs on a waveguide substrate. The term "pupil relay" describes the system of components used to spatially transfer a pupil from on location to another, for example from entry pupil 710a-b to exit pupil 712a-b. In some embodiments, the pupil relay includes a waveguide with optical properties such that the entry pupil and exit pupil of the waveguide have substantially identical size and shape, and such that polychromatic light rays input to the pupil relay propagate collinearly through the pupil relay by total internal reflection (TIR), so that the corresponding output light rays have substantially identical chromatic properties to those of the input light rays; that is, the pupil relay is achromatic. In this context, "substantially identical" means that there is no perceivable difference in these properties to a human user. In other embodiments, the optical properties of the entry pupil may differ from the optical properties of the exit pupil, for example, for pupil expansion.

As shown in <FIG>, in some embodiments, a pupil relay is a waveguide 700a that includes a light-transmissive substrate 750a with at least two surfaces 752a and 754a that are substantially parallel to each other and that are internally reflective so as to provide TIR of light rays propagating within the substrate <NUM>. Waveguide 700a also includes two DOEs 760a and 762a that facilitate light entry and exit from the substrate(one on the input end and one on the output end). In some embodiments, DOEs 760a and 762a are surface relief diffraction gratings formed as part of or proximate to a given surface (i.e., a surface parallel to the direction of propagation of the light rays within the substrate) of the substrate 750a of the waveguide 700a. For example, as shown in <FIG>, DOEs 760a and 762a may be formed on or proximate to surface 752a of substrate 750a. In this description, "proximate to" means no deeper than one micrometer from the surface. It may be desirable to make the depth of each DOE relatively large compared to its period.

The DOEs 760a and 762a are designed to cause light rays of different colors to propagate collinearly through the substrate 752a and to continue to propagate collinearly upon exiting the waveguide 700a, respectively. For example, <FIG> shows collinear light rays 714a and 716a of two different colors entering substrate 750a via DOE 760a (the in-coupling element), propagating through substrate 750a through TIR, and exiting substrate 750a at DOE 762a (the out-coupling element).

<FIG> shows another way of combining multiple colors into a single waveguide by using DOEs. In <FIG>, the waveguide 700b includes at least four diffraction DOEs 760b, 762b, 764b, and 766b according to the claimed invention formed on or proximate to two opposite surfaces 752b and 754b of the substrate 750b that are parallel to the direction of propagation of the light rays within the substrate. The DOEs 760b and 762b on one surface 752b (e.g., top surface) of the waveguide 700b couple a first color (represented by light ray 714b), and the DOEs 764b and 766b on the opposite surface 754b (e.g., bottom surface) of the substrate 750b couple a second color (represented by light ray 716b). This is done using, for example, DOEs that work only on one polarization (colors have orthogonal polarizations) or using switchable diffraction gratings to enable selection of the coupled color for each diffraction gating. As mentioned above, the same principle can be applied to allow collinear propagation of three or more colors through the pupil relay in the embodiments of <FIG>.

In the embodiments of <FIG>, the substrates 750a-b are formed of material(s) with appropriate optical properties to facilitate light propagation through TIR. In some embodiments, substrates 750a-b are made of glass, for example, formed through an injection molding process. As mentioned, each of the DOEs 760a-b, 762a-b, 764b, and 766b may include surface relief diffraction gratings (SRG) which can be part of a surface of the substrate 750a-b (e.g. formed through etching into a surface of substrate 750a-b or formed during an injection molding process) not according to the claimed invention, which are formed on a surface of substrate 750a-b (e.g. through application and curing of material on the surface) according to the claimed invention, or which can be buried within the substrate 750a-b not according to the claimed invention. Hence, the light input surface and light output surface of the waveguide 700a-b, respectively, are each a DOE, or a portion of the substrate surface directly over a DOE if the DOE is buried below the surface. It can be assumed that each DOE in the pupil relay is substantially coplanar with at least one of the surfaces of the substrate that are parallel to the long axis of the pupil relay (i.e., each DOE is parallel to such surface and within one micrometer of depth of such surface).

<FIG> show cross-sectional views of example waveguides 800a-b at interface that include a substrate 850a-b (respectively) and DOEs 860a-b (respectively) that include a surface relief grating (SRG) structure. Waveguides 800a-b can be considered analogous to waveguides 700a-b described with respect to <FIG>. Similarly, substrates 850a-b can be considered analogous to substrates 750a-b and DOEs 860a-b can be considered analogous to any of DOEs 760a-b, 762a-b, 764b, and 766b as described with respect to <FIG>.

<FIG> illustrate alternative methods for forming the grating structure of a DOE. For example <FIG> shows an example waveguide 800a formed of a single material, such as glass. As previously explained glass has sufficient reflective properties to facilitate light propagation along the substrate 850a through TIR and has a sufficiently high refractive index (~<NUM>-<NUM>) to facilitate a high diffraction efficiency for in-coupling/out-coupling of light via the DOE 860a. In some embodiments not according to the claimed invention, substrate 850a and DOE 860a are formed as a single element out of glass through an injection molding process. More likely, however, substrate 850a is formed out of glass through injection molding and the SRG structure of DOE 860a is formed by a precise etching process into the surface of substrate 850a.

As previously mentioned, a waveguide made of a single high index material such as waveguide 800a has performance advantages, but is impractical to manufacture particularly in the context of mass produced consumer-level NED devices. Waveguide 800b in <FIG> is an alternative embodiment in which DOE 860b is formed on at least one surface of substrate 850b through a replication process. As with waveguide 800a, the substrate 850b of waveguide 800b is formed of a material with appropriate optical properties to facilitate light propagation through TIR, for example glass. Substrate 850b can be manufactured out of glass using any standard manufacturing process. The SRG of DOE 860b is then formed on a surface of substrate 850b. In some embodiments, the SRG of DOE 860b is formed through a curing process of standard UV-curable polymer-based resin. While much cheaper and quicker than forming the grating structure out of a material such as glass, standard UV-curable resin has a relatively low refractive index (e.g. ~<NUM> - <NUM>). This results in relatively low diffraction efficiency at certain wavelengths and angles of incidence of light rays, thereby resulting in limited FOV for displayed images.

Note that that substrates 850a-b and DOEs 860a-b are illustrated conceptually in <FIG> and are not intended to show limiting structural configurations or true dimensions. For example the dimensions of the diffraction grating structures of DOEs 860a-b are greatly exaggerated relative to the dimensions of substrate 850a-b for clarity purposes. Similarly, the number, shape, and orientation of diffraction grating structures of DOEs 860a-b are intended to be illustrative and not limiting. In some embodiments, the diffraction grating structures may vary in one or more parameters across the area of the DOE. <FIG> shows a cross-sectional view of another example waveguide <NUM> including a DOE <NUM> and a substrate <NUM>. The cross section of example waveguide <NUM> shown in <FIG> shows that the grating structure of DOE <NUM> can vary across an area of the DOE <NUM> in one or more parameters, such as grating period P, grating line width W, grating fill factor F, grating depth D, slant angle Φ, line shape, surface pattern (not shown) and modulation direction. The grating fill factor F is the fraction of the grating period that is filled with grating material. In other words, fill factor F=W/P.

Again, the structural elements of waveguide <NUM> are illustrated conceptually and not intended to show limiting structural configurations or dimensions. Configuration of the diffraction grating structure (i.e. setting the aforementioned parameters) to achieve a desired in-coupling or out-coupling effect is well understood in the art.

<FIG> shows the in-coupling of light rays <NUM> into substrate 850b via a DOE 860b of waveguide 800b. As shown in <FIG>, light rays <NUM> are diffracted and refracted at DOE 860b and propagate along substrate 850b through TIR. As previously mentioned with respect to <FIG>, the substrate 850b of waveguide 800b can be made of a high index material such as glass and the diffraction grating structure of DOE 860b can be made of a low index (relative to the substrate) material such as UV-curable polymer-based resin. The out-coupling of light rays <NUM> from the substrate 850b via a second DOE would follow the same principles as illustrated in <FIG>.

<FIG> is a graph that charts first-order diffraction efficiency of an in-coupler DOE with a diffraction grating formed of low index material (or that does not include an applied coating) over a range of angles of incidence θ for various wavelengths λ of light. The example in-coupler (e.g. similar to in-coupler DOE 860b of waveguide 800b) is optimized for a maximal in-coupling diffraction efficiency over the FOV for light in the <NUM>±<NUM> wavelength range. As show in <FIG> the minimum efficiency over the FOV drops well below <NUM>%, and at certain angles the wavelength sensitivity is very high (<NUM>% vs. <NUM>% for instance). In practical terms, <FIG> demonstrates that a diffraction grating structure with a relatively low refractive index will result in poor image uniformity across the <NUM>° field of view. For example, a user may experience a rainbow effect or other image artifacts at the peripheral edge of the a displayed FOV.

<FIG> are graphs that chart first-order diffraction efficiency over a range of angles of incidence θ for two different in-coupler DOEs with diffraction grating structures formed of materials with different refractive indices (<NUM> and <NUM>, respectively). In the cases of both <FIG>, the waveguide includes a substrate made of glass with a refractive index of <NUM> and a grating structure optimized for maximal in-coupling over a FOV for monochromatic green light at ~<NUM> wavelength. The diffraction gratings have a period P of <NUM> and are otherwise constrained in other structural parameters as follows: D < <NUM>,. <NUM> < F <. <NUM>, -<NUM>°< Φ < +<NUM>°. As shown in <FIG>, changing the refractive index of the diffraction grating material from <NUM> to <NUM> changes the angle of incidence at which diffraction efficiency is maximized, but still results in high variation across a FOV.

<FIG> show cross-sectional views of example waveguides 1300a-b at interface that include a coating of high index material intended to improve diffraction efficiency over a wider FOV and range of light wavelengths. As shown in <FIG> waveguides 130a-b include a substrate 13550a-b (respectively) and DOEs 1360a-b (respectively) that include an SRG structure 1370a-b (respectively). Waveguides 1300a-b can be considered analogous to waveguides 800a-b described with respect to <FIG>. Similarly, substrates 1350a-b can be considered analogous to substrates 850a-b and DOEs 1360a-b can be considered analogous to DOEs 850a-b as described with respect to <FIG>. As mentioned, waveguides 1300a-b additionally include a coating of high index material 1380a-1380b over the grating structure 1370a-b (respectively). <FIG> shows an embodiment of waveguide 1300a in which the coating material 1380a is applied to the interface surfaces of grating structure 1370a of DOE 1360a. <FIG> shows an alternative embodiment in which the grating structure 1370b of DOE 1360b is embedded (at least substantially) in the coating material 1380b. As will be demonstrated, in either embodiment, the application of a coating with a high refractive index to the diffraction grating structure decreases the sensitivity of the component to the angle of incidence and wavelength of light, and hence improves the overall efficiency of the in-coupler/out-coupler.

In some embodiments, coatings 1380a and 1380b are an anti-reflection type coating made of a material such as aluminum dioxide, titanium dioxide, or some combination thereof and have refractive indices up to <NUM>. Such coatings can be applied using a number of different industry standard processes such as evaporation, atomic layer deposition (ALD), chemical vapor deposition (CVD), spin coating, and dip coating. Such coating processes are relatively cheap when performed in batch and are suitable for mass manufacturing. Although the selected thickness of coating will depend on the material used and the overall optical configuration of the waveguide, in some embodiments coatings 1380a-b are generally on the order of <NUM>-<NUM> thick.

<FIG> shows the in-coupling of light rays <NUM> into substrate 1350b via a DOE 1360b of waveguide 1300b. As shown in <FIG>, light rays <NUM> are diffracted and refracted at DOE 1360b and propagate through substrate 1350b by TIR. The out-coupling of light rays <NUM> from the substrate 1350b via a second DOE would follow the same principles as illustrated in <FIG>. The in-coupling of light rays <NUM> are shown via waveguide 1300b as described with respect to <FIG>, however in-coupling via waveguide 1350a (i.e. with a coating 1380a applied to the interface surface of diffraction grating structure 1360a) would follow the same principles as illustrated in <FIG>
<FIG> is a graph that charts first-order diffraction efficiency over a range of angles of incidence θ for various wavelengths λ of light for an in-coupler DOE with a high index material applied as a coating to a diffraction grating. The example in-coupler (e.g. similar to in-coupler DOE 1360a-bof waveguide 1300a-b) is optimized for a maximal in-coupling over the FOV for light in the <NUM>±<NUM> wavelength range. As show in <FIG>, first order diffraction efficiency remains at between <NUM>% and <NUM>% over a <NUM>° range of angle of incidence θ and <NUM> range of wavelength for diffracted light rays. In practical terms, <FIG> demonstrates that a diffraction grating structure with an applied coating of high index material will result in improved image uniformity across a wide FOV.

<FIG> are graphs that chart first-order diffraction efficiency over a range of angles of incidence θ for two different in-coupler DOEs that include a high index coating (n=<NUM>) applied to diffraction grating structures formed of materials with different refractive indices (n=<NUM> and <NUM>, respectively). The waveguide tested in both <FIG> include a substrate made of glass with a refractive index of <NUM> and a grating structure optimized for maximal in-coupling over the FOV for monochromatic green light at ~<NUM> wavelength. The diffraction gratings have a period P of <NUM> and are otherwise constrained in other structural parameters as follows: D < <NUM>,. <NUM> < F <. <NUM>, -<NUM>°< Φ < +<NUM>°. The applied coating has a thickness of ~<NUM> to <NUM>. As shown in <FIG>, a high refractive index coating (n=<NUM>) applied to the diffraction grating structure results in relatively high and uniform diffraction efficiency (~ <NUM>% to <NUM>%) across a <NUM>° range of angle of incidence θ for diffracted light rays. Notably, although the refractive index of the coating material is the same in both <FIG>, use of the lower index (n=<NUM>) grating material (as seen in <FIG>) yields improved results.

<FIG> are graphs that chart first-order diffraction efficiency over a range of angles of incidence θ for in-coupler DOEs that are otherwise the same as those tested in <FIG> (respectively) except that the refractive index of the coating material is lowered from <NUM> to <NUM>. A shown in <FIG>, the diffraction efficiency is still relatively uniform across a <NUM>° range of angle of incidence θ, but is significantly lower than with the higher index coating (~<NUM>% to <NUM>% where the diffraction grating material has a refractive index of <NUM> and ~<NUM>% to <NUM>% where the diffraction grating material has a refractive index of <NUM>.

<FIG> demonstrate that a key factor in determining the optimal material to apply as a coating is the refractive index of the diffraction grating structure itself. While in general, coatings with higher refractive indices yield increased diffraction efficiency over a FOV, optimal results are achieved where the contrast in refractive index between the diffraction grating material and coating is highest. For example, a relatively small difference (n=<NUM>) in refractive index between the diffraction grating material (n=<NUM>) and coating (n=<NUM>) appeared to yield the worst results (See <FIG>) while a relatively large difference (n=<NUM>) in refractive index between the diffraction grating material (n=<NUM>) and coating (n=<NUM>) appeared to yield the best results (See <FIG>). In some embodiments, the difference in refractive index between the coating an diffraction grating material in a DOE configured for use with a NED device is at least <NUM>.

<FIG>, are graphs that chart first-order diffraction efficiency over a range of angles of incidence θ for two different in-coupler DOEs that both include diffraction grating structures made of relatively low refractive index (n=<NUM>) that are formed on a glass substrate with a relatively high refractive index (n=<NUM>). Different coatings are applied to each waveguide, one with a refractive index of <NUM> as shown in <FIG> and one with a refractive index of <NUM>. as shown in <FIG>. As shown in <FIG>, even though the difference in refractive index between the diffraction grating material and coating is relatively high (. <NUM>), the increased refractive index of the substrate (<NUM> as opposed to <NUM>) negatively impacts overall diffraction efficiency and/or uniformity across the <NUM>° range of angle of incidence θ. Accordingly, another key factor in determining the optimal material to apply as a coating is the refractive index of the substrate material. Based on the results shown in <FIG>, increasing the refractive index of the substrate material requires further increasing the refractive index of the coating to optimize diffraction efficiency over a wide field of view.

<FIG> is a flow chart describing an example process <NUM> for manufacturing a waveguide configured for use with an NED device that is consistent with the above teachings. As shown in <FIG>, process <NUM> begins at step <NUM> with forming a layer of a curable first material (e.g. UV-curable polymer-based resin) on at a surface of a light-transmissive substrate made of a second material (e.g. glass). The process continues at step <NUM> with curing the first material to form a diffraction grating on or proximate to the surface of the substrate. In some embodiments, this diffraction grating is configure to input and/or output light rays into and/or out of the substrate. The required curing process will depend on the type of material used. Curing can be performed using number of processes such as application of heat, light, or particular chemicals. The process continues at step <NUM> with coating the diffraction grating with a third material (e.g. titanium dioxide and/or aluminum dioxide) that has a higher refractive index than the first material (i.e. applying the high index coating). In some embodiments this coating process is performed using atomic layer deposition and/or chemical vapor deposition.

Any or all of the features and functions described above can be combined with each other, except to the extent it may be otherwise stated above or to the extent that any such embodiments may be incompatible by virtue of their function or structure, as will be apparent to persons of ordinary skill in the art. Unless contrary to physical possibility, it is envisioned that (i) the methods/steps described herein may be performed in any sequence and/or in any combination, and that (ii) the components of respective embodiments may be combined in any manner.

Claim 1:
An optical waveguide (1300a, 1300b) comprising:
a light-transmissive substrate (1350a, 1350b) configured for use in a near-eye display, NED, device (<NUM>), the substrate (1350a, 1350b) including a plurality of internally reflective surfaces, the substrate made of a first material having a first refractive index; and
a diffractive optical element, DOE, (1360a, 1360b) formed on a first surface of the plurality of surfaces of the substrate (1350a, 1350b), the DOE (1360a, 1360b) configured to input light rays to the substrate (1350a, 1350b) or output light rays from the substrate, the DOE including:
a diffraction grating (1370a, 1370b) made of a second material having a second refractive index; and
a coating (1380a, 1380b) over the diffraction grating (1370a, 1370b) made of a third material having a third refractive index;
wherein the second refractive index is not equal to the third refractive index; and
wherein the optical waveguide includes at least three further DOEs (762b, 764b, 766b), each including the diffraction grating (1370a, 1370b) and the coating (1380a, 1380b), the at least four DOEs being formed on two opposite surfaces (752b, 754b) of the substrate that are parallel to the direction of propagation of light rays within the substrate, wherein the two opposite surfaces (752b, 754b) comprise the first surface, wherein the DOEs on one surface of the waveguide couple a first color and the DOEs on the opposite surface of the substrate couple a second color by the DOEs working only on one polarization or using switchable DOEs to enable selection of the coupled color for each DOE.