Facebook Patent | Display Waveguide Assembly With Color Cross-Coupling

Patent: Display Waveguide Assembly With Color Cross-Coupling

Publication Number: 20200116996

Publication Date: 20200416

Applicants: Facebook

Abstract

A waveguide display includes a display projector for emitting polychromatic image light, and a waveguide assembly for transmitting image light to an exit pupil. The waveguide assembly includes two or more waveguides disposed in a stack, each having an in-coupler aligned with the other in-couplers and an offset out-coupler aligned with the other out-couplers. The assembly is configured so that at least one color channel of the image light propagates to the exit pupil along at least two waveguides. A method for selecting the waveguides of the stack to suppress color channel splitting at the exit pupil is provided.

TECHNICAL FIELD

[0001] The present disclosure generally relates to optical display systems and devices, and in particular to waveguide displays and components therefor.

BACKGROUND

[0002] In certain types of display systems, such as wearable displays for augmented reality (AR) applications, heads-up displays, heads-down displays, and the like, an electronic display may be positioned away from the direct line of sight of the user. One approach that can be used in such systems to bring images from a display projector to the user of the system is by means of an optical waveguide. Optical waveguides also enable expanding an image beam from a micro-display within a small device volume, which is advantageous for wearable displays that have to be compact and lightweight. However, optical waveguides typically provide a limited field of view, in particular when the image light is polychromatic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Embodiments disclosed herein will be described in greater detail with reference to the accompanying drawings which represent example embodiments thereof, in which like elements are indicated with like reference numerals, and wherein:

[0004] FIG. 1A is a schematic isometric view of a waveguide display system using a waveguide assembly with color cross-coupling for transmitting images to a user;

[0005] FIG. 1B is a schematic block diagram of a display projector of the waveguide display of FIG. 1A;

[0006] FIG. 2A is a schematic diagram illustrating the coupling of a first color channel into a waveguide and an input FOV for the first color channel;

[0007] FIG. 2B is a schematic diagram illustrating the coupling of a second color channel into the display waveguide of FIG. 2A and an input FOV of the second color channel;

[0008] FIG. 3A is a schematic diagram illustrating input and output FOVs of a display waveguide for a selected color channel;

[0009] FIG. 3B is a schematic diagram illustrating the transmission of light by a display waveguide with two output gratings at opposing faces of the waveguide;

[0010] FIG. 4 is a graph schematically illustrating the input FOV of a display waveguide as an area in a plane with coordinates “wavelength” (.lamda.) and “angle of incidence” (.alpha.);

[0011] FIG. 5 is a schematic cross-section of a three-waveguide stack configured for separately transmitting three color channels within different waveguides without color cross-coupling;

[0012] FIG. 6 is a graph schematically illustrating the input FOVs of the three waveguides of the three-waveguide stack of FIG. 5 in the (.lamda., .alpha.) plane;

[0013] FIG. 7 is a schematic cross-section of a two-waveguide stack configured for transmitting three color channels with cross-coupling in a second color channel;

[0014] FIG. 8 is a graph schematically illustrating the input FOVs of the two waveguides of the two-waveguide stack of FIG. 7 in the (.lamda., .alpha.) plane;

[0015] FIG. 9 is a schematic cross-section of a three-waveguide stack configured for transmitting three color channels with cross-coupling in each channel to broaden the FOV of the stack for polychromatic light;

[0016] FIG. 10 is a graph schematically illustrating the input FOVs of the three waveguides of the three-waveguide stack of FIG. 9 in the (.lamda., .alpha.) plane configured to support a broader FOV;

[0017] FIG. 11 is a graph illustrating the input FOVs of the three waveguides of the three-waveguide stack of FIG. 9 in the (.lamda., .alpha.) plane computed for the waveguides with the refractive index n.apprxeq.1.8;

[0018] FIG. 12 is a schematic diagram of an example layout of a waveguide with a 2D FOV and a vertically aligned in-coupler;

[0019] FIG. 13 is a schematic diagram illustrating the formation of a 2D FOV in the waveguide of FIG. 12 in k-space;

[0020] FIG. 14 is a graph illustrating the 2D FOV of the waveguide of FIG. 12 in the plane of incidence angles .theta.x, .theta.y according to an embodiment;

[0021] FIG. 15A is a graph illustrating the 2D FOV of a first waveguide (WG1) of an example two-waveguide stack at a first (blue) color channel in the plane of incidence angles according to an embodiment;

[0022] FIG. 15B is a graph illustrating the 2D FOV of the first waveguide (WG1) of the example two-waveguide stack at a second (green) color channel in the plane of incidence angles according to the embodiment;

[0023] FIG. 15C is a graph illustrating the 2D FOV of the first waveguide (WG1) of the example two-waveguide stack at a third (red) color channel in the plane of incidence angles according to the embodiment;

[0024] FIG. 15D is a graph illustrating the 2D FOV of a second waveguide (WG2) of the example two-waveguide stack at the first (blue) color channel in the plane of incidence angles according to the embodiment;

[0025] FIG. 15E is a graph illustrating the 2D FOV of the second waveguide (WG2) of the example two-waveguide stack at the second (green) color channel in the plane of incidence angles according to the embodiment;

[0026] FIG. 15F is a graph illustrating the 2D FOV of the second waveguide (WG2) of the example two-waveguide stack at the third (red) color channel in the plane of incidence angles according to the embodiment;

[0027] FIG. 16 is a schematic front view of a binocular NED using a waveguide assembly with the layout of FIG. 12;

[0028] FIG. 17A is a schematic diagram illustrating an example layout of a waveguide assembly with the in-coupler and out-coupler side by side;

[0029] FIG. 17B is a schematic diagram illustrating a vector diagram of grating vectors in the example layout of FIG. 17A;

[0030] FIG. 17C is a schematic plan view of a NED using two waveguide assemblies with the layout of FIG. 17A and the in-couplers at the middle;

[0031] FIG. 18A is a schematic diagram illustrating an example layout for a 2D waveguide assembly with an in-coupler diagonally offset from an exit pupil of an out-coupler;

[0032] FIG. 18B is a schematic diagram illustrating a vector diagram of grating vectors in the example layout of FIG. 17A;

[0033] FIG. 18C is a schematic plan view of a NED using two waveguide assemblies with diagonally offset in-couplers side by side;

[0034] FIG. 19 is a schematic cross-sectional diagram of a two-waveguide stack illustrating the divergence of light beams of a same color (“color splitting”) after propagating in waveguides with differing wedge angles;

[0035] FIG. 20 is a flowchart illustrating general steps of a method for fabricating a waveguide stack with color cross-coupling to reduce the color splitting in non-ideal waveguides;

[0036] FIG. 21 is a schematic diagram illustrating a setup for measuring exit angles of a reference beam for display waveguides in accordance with the method of FIG. 20;

[0037] FIG. 22 is a flowchart of an embodiment of the method of FIG. 20 using waveguide binning based on measured exit angles;

[0038] FIG. 23 is a flowchart of a waveguide selection method for fabricating a waveguide stack with color cross-coupling between three waveguides according to an embodiment.

DETAILED DESCRIPTION

[0039] In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular optical and electronic circuits, optical and electronic components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the example embodiments. All statements herein reciting principles, aspects, and embodiments, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

[0040] Note that as used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method or process steps does not imply a sequential order of their execution, unless explicitly stated.

[0041] Furthermore, the following abbreviations and acronyms may be used in the present document: HMD (Head Mounted Display); NED (Near Eye Display); VR (Virtual Reality); AR (Augmented Reality); MR (Mixed Reality); LED (Light Emitting Diode); FOV (Field of View); TIR (Total Internal Reflection).

[0042] Example embodiments may be described hereinbelow with reference to polychromatic light that is comprised of three distinct color channels, generally referred to as the first color channel having a first center wavelength .lamda..sub.1, the second color channel having a second center wavelength .lamda..sub.2, and the third color channel having a third center wavelength .lamda..sub.3. For certainty it will be assumed that the second color channel is positioned spectrally between the first and second color channels, although this is a matter of convention and is not meant to be limiting. In at least some embodiments it may be assumed that .lamda..sub.1<.lamda..sub.2.lamda..sub.3 for further certainty, which is also not limiting. The first color channel may be referred to as the blue (B) channel or color and may represent the blue channel of an RGB color scheme, the second color channel may be referred to as the green (G) channel or color and may represent the green channel of the RBG color scheme, and the third color channel may be referred to as the red (R) channel or color and may represent the red channel of the RGB color scheme. It will be appreciated however that the embodiments described herein may be adapted for use with polychromatic light comprised of any combination of two or more, or preferably three or more color channels, which may represent non-overlapping portions of a relevant optical spectrum.

[0043] An aspect of the present disclosure relates to a display system comprising a waveguide stack and an image light source coupled thereto, wherein the waveguide stack is configured to receive polychromatic image light emitted by the image light source and to convey the image light received in the polychromatic FOV of the waveguide stack to an eyebox for presenting to a user, wherein at least one of color channels of the polychromatic image light may be conveyed to the eyebox over two or more waveguides of the waveguide stack.

[0044] An aspect of the present disclosure relates to a waveguide stack for conveying image light comprising a plurality of color channels from an image light source to an exit pupil or an eyebox of a waveguide display. The waveguide stack may comprise a plurality of waveguides stacked one over another, each waveguide comprising an input coupler and an output coupler. The input coupler and the output coupler of each waveguide define a field of view (FOV) of the waveguide at each of the plurality of color channels, the FOVs of the plurality of waveguides in combination defining a polychromatic FOV of the waveguide stack. The plurality of color channels of the image light may comprise a first, second, and third color channels, with the second color channels located spectrally between the first and third color channels. The waveguide stack may be configured for transmitting at least one of the color channels of the image light to the eyebox within different waveguides of the waveguide stack. Each input coupler may comprise an input diffraction grating configured to couple a portion of the image light into a corresponding waveguide thereby obtaining in-coupled light propagating in the waveguide toward the output coupler thereof. Each output coupler may comprise one or more output diffraction gratings configured to extract the in-coupled light out of the waveguide toward the eyebox. In some implementations the output coupler of at least one waveguide may comprise two diffraction gratings, which may be configured to expand the in-coupled light in two dimensions and to extract expanded light out of the waveguide.

[0045] In some implementations the plurality of waveguides of the waveguide stack may comprise a first waveguide and a second waveguide, each of which configured to transmit the second color channel of the image light to the eyebox. In some implementations the input couplers of the first and second waveguides are configured so that a beam of the image light of the second color channel received from a first portion of the polychromatic FOV of the waveguide stack is transmitted to the eyebox over the first waveguide, and a beam of the image light of the second color channel received from a second portion of the polychromatic FOV of the waveguide stack is transmitted to the eyebox over the second waveguide.

[0046] In some implementations the first and second waveguides may be configured so that the FOV of the first waveguide at the first color channel and the FOV of the second waveguide at the third color channel share a common FOV portion comprising the polychromatic FOV of the waveguide stack. In some implementations the first and second waveguides may be configured so that the FOV of the first waveguide at the first color channel is aligned with the FOV of the second waveguide at the third color channel. In some implementations the input coupler of the first waveguide may comprise a diffraction grating having a first pitch p.sub.1, the input coupler of the second waveguide may comprises a diffraction grating having a second pitch p.sub.2>p.sub.1. In some implementations .lamda..sub.1/p.sub.1 may be generally equal to .lamda..sub.3/p.sub.2, where .lamda..sub.1 and .lamda..sub.3 are central wavelengths of the first color channels, respectively.

[0047] In some implementations the plurality of waveguides of the waveguide stack may further comprise a third waveguide, wherein the input coupler of the third waveguide comprises a diffraction grating having a third pitch p.sub.3>p.sub.2. In some implementations each of the first, second, and third waveguides is configured to transmit at least two color channels of the image light to the eyebox for broadening the polychromatic FOV of the waveguide stack. The FOVs of the first and second waveguides may partially overlap at each color channel to define a first shared FOV, the FOVs of the second and third waveguides may partially overlap at each color channel to define a second shared FOV. In some implementations the input couplers of the first, second, and third waveguides may be configured so that the polychromatic FOV of the waveguide stack exceeds, in at least one dimension, the FOV of each one of the first, second, and third waveguides at each of the first, second, and third color channels. In some implementations each of the first and second shared FOVs does not exceed 20 degrees in at least one of the first, second, and third color channels in at least one dimension.

[0048] In some implementations the one or more output diffraction gratings of the output coupler of at least one waveguide may be configured to define an eyebox projection area of the waveguide from which the image light is projected onto the eyebox, the eyebox projection area having a horizontal axis defined relative to the eyebox. In some implementations the one or more output diffraction gratings of the output coupler of at least one waveguide may comprise at least one of: a two-dimensional diffraction grating, or two linear diffraction gratings disposed at an angle to one another and to the input diffraction grating. In some implementations the input diffraction grating may have a grating vector oriented at an angle to the horizontal axis of the eyebox projection area that is less than 40 degrees.

[0049] An aspect of the present disclosure relates to a near-eye display system comprising: at least one light projector configured to emit image light comprising a plurality of color channels; and, two waveguide assemblies, each configured to convey image light from the at least one light projector to a different eye of a user, wherein each of the two waveguide assemblies comprises an in-coupler for receiving the image light from the at least one light projector and an out-coupler for conveying the image light from the waveguide assembly to an eye of the user, and wherein the in-couplers are disposed at least partially between the out-couplers, or the out-couplers are disposed at least partially between the in-couplers. In some implementations of the near-eye display system each out-coupler of the two waveguide assemblies comprises an eyebox projection area from which the image light is projected to an eye of the user, wherein the eyebox projection areas are disposed on a horizontal axis, and wherein the in-couplers of the two waveguide assemblies are offset from the horizontal axis.

[0050] An aspect of the present disclosure provides a method for fabricating a waveguide stack with color cross-coupling wherein a same color channel of the polychromatic image light may be conveyed to the exit pupil over two different waveguides of the waveguide stack. The method may comprise: a) determining an exit angle of a first reference light beam for each waveguide from a plurality of first waveguides and a plurality of second waveguides, and b) selecting, for the waveguide stack, a first waveguide from the plurality of first waveguides and a second waveguide from the plurality of second waveguide based on the exit angles of the first reference beam. The selecting in b) may comprise selecting the first waveguide and second waveguide for which the exit angles match with a pre-defined accuracy.

[0051] Step a) of the method in some embodiments thereof may comprise directing the first reference light beam to impinge upon the in-coupler of each waveguide at a first angle of incidence, and measuring the exit angle at which the first reference light beam exits the out-coupler of the corresponding waveguide.

[0052] In at least some implementations, each waveguide from the plurality of first waveguides may be configured for conveying at least a first color channel of the polychromatic image light to the exit pupil, each waveguide from the plurality of second waveguides may be configured for conveying at least one of a second color channel or a third color channel of the polychromatic image light to the exit pupil, wherein the second color channel may be located spectrally between the first and third color channels.

[0053] Each first waveguide may have a first FOV defining a range of incidence angles of the polychromatic image light upon the first waveguide that can be conveyed to the exit pupil, and each second waveguide may have a second FOV defining a range of incidence angles of the polychromatic image light upon the second waveguide that can be conveyed to the exit pupil. In some implementations the first reference light beam may comprise a first reference wavelength, and the first FOV and second FOV may partially overlap at the first reference wavelength to define a first shared FOV. The first angle of incidence may be selected within the first shared FOV. In some implementations the first reference wavelength may be a wavelength of the second color channel.

[0054]* In some implementations the method may comprise combining the selected first and second waveguides to form the waveguide stack so as to allow the second color channel to be partially coupled into both the first and second waveguides by the in-couplers thereof*

[0055] In some implementations the method may comprise binning the first and second waveguides based on the exit angles measured therefor. The binning may comprise: assigning at least some of the first waveguides to one of a plurality of first bins based on the exit angle measured therefor, so that the exit angles measured for the first waveguides assigned to a same first bin differ by no more than a first threshold value; and, assigning at least some of the second waveguides to one of a plurality of second bins based on the exit angle measured therefor, so that the exit angles measured for the second waveguides assigned to a same second bin differ by no more than a second threshold value. The method may further comprise selecting the first and second waveguides from matching first and second bins, respectively.