PATENT DOCUMENT

Publication Number: US-11467407-B2
Application Number: US-201816632830-A
Country: US
Kind Code: B2

Title: Displays with volume phase gratings

Abstract:
An electronic device may have a display system that produces images. The display system may have one or more pixel arrays such as liquid-crystal-on-silicon pixel arrays. Images from the display system may be coupled into a waveguide by an input coupler and may be coupled out of the waveguide using an output coupler. The input and output couplers may be formed from volume phase holographic gratings. An additional grating may be used to shift light that would otherwise pass above or below the user&#39;s field of view towards the viewer. Holographic gratings in the waveguide may have fringes with constant pitch and variable period. The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given incident angle.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 a display system configured to produce images; and 
 an optical system having an input portion and an output portion, wherein the optical system comprises:
 a waveguide that extends between the input portion and the output portion; 
 an input coupler in the input portion, wherein the input coupler is configured to couple the images from the display system into the waveguide; 
 an output coupler in the output portion, wherein the output coupler is configured to couple the images out of the waveguide; and 
 a volume phase holographic grating with a constant pitch and a variable period, wherein the volume phase holographic grating comprises fringes in a holographic medium, wherein the fringes comprise a first set of fringes oriented at a first fringe angle and a second set of fringes oriented at a second fringe angle, wherein the first set of fringes has a first period that is Bragg-matched to incident light associated with an upper field of view, and wherein the second set of fringes has a second period that is Bragg-matched to incident light associated with a lower field of view. 
 
 
     
     
       2. The electronic device defined in  claim 1  wherein at least one of the input coupler and the output coupler comprises an additional volume phase holographic grating with a constant pitch and a variable period, wherein the additional volume phase holographic grating comprises fringes in a holographic medium, wherein each fringe is oriented at a fringe angle relative to a surface normal of the holographic medium, and wherein the fringe angle varies across the holographic medium. 
     
     
       3. The electronic device defined in  claim 2  wherein the fringes of the additional volume phase holographic grating comprise a first set of fringes oriented at a first fringe angle, a second set of fringes oriented at a second fringe angle, and a third set of fringes oriented at a third fringe angle. 
     
     
       4. The electronic device defined in  claim 3  wherein the first set of fringes has a first period that is Bragg-matched to incident light associated with a left field of view, the second set of fringes has a second period that is Bragg-matched to incident light associated with a center field of view, and the third set of fringes has a third period that is Bragg-matched to incident light associated with a right field of view. 
     
     
       5. The electronic device defined in  claim 2  wherein the period of the additional volume phase holographic grating varies continuously across the additional volume phase holographic grating. 
     
     
       6. The electronic device defined in  claim 2  wherein the additional volume phase holographic grating comprises first, second, and third gratings arranged in a stack, wherein the first grating has fringes with a first pitch and a first period, the second grating has fringes with the first pitch and a second period, and the third grating has fringes with the first pitch and a third period. 
     
     
       7. The electronic device defined in  claim 1  wherein each fringe is oriented at a fringe angle relative to a surface normal of the holographic medium, and wherein the fringe angle varies across the holographic medium. 
     
     
       8. A volume phase holographic grating, comprising:
 a medium having first, second, and third portions; 
 fringes in the medium, wherein the fringes have a uniform pitch across the first, second, and third portions of the medium, wherein the fringes in the first portion have a first period, the fringes in the second portion have a second period, and the fringes in the third portion have a third period, wherein the first, second, and third periods are different, and wherein the first period maximizes diffraction efficiency for incident light of a given wavelength and a first incident angle, the second period maximizes diffraction efficiency for incident light of the given wavelength and a second incident angle, and the third period maximizes diffraction efficiency for incident light of the given wavelength and a third incident angle; and 
 additional fringes in the medium, wherein the additional fringes comprise a first set of fringes oriented to maximize diffraction efficiency for incident angles associated with an upper field of view and a second set of fringes oriented to maximize diffraction efficiency for incident angles associated with a lower field of view. 
 
     
     
       9. The volume phase holographic grating defined in  claim 8  wherein the period of the fringes in the holographic medium varies continuously across the medium. 
     
     
       10. The volume phase holographic grating defined in  claim 8  wherein the medium comprises first, second, and third holographic mediums, wherein the fringes with the first period are formed in the first holographic medium, the fringes with the second period are formed in the second holographic medium, and the fringes with the third period are formed in the third holographic medium. 
     
     
       11. The volume phase holographic grating defined in  claim 8  wherein the medium has a surface normal, wherein the fringes are oriented at a fringe angle relative to the surface normal, and wherein the fringe angle varies across the medium. 
     
     
       12. A display system, comprising:
 a waveguide; 
 a first holographic grating that couples light into the waveguide; 
 a second holographic grating that couples light out of the waveguide, wherein at least one of the first and second holographic gratings has fringes with a constant pitch and a variable period; and 
 a third holographic grating that redirects light within the waveguide, wherein the third holographic grating has fringes with a constant pitch and a variable period, and wherein the fringes of the third holographic grating comprise a first set of fringes oriented to maximize diffraction efficiency for incident angles associated with an upper field of view and a second set of fringes oriented to maximize diffraction efficiency for incident angles associated with a lower field of view. 
 
     
     
       13. The display system defined in  claim 12  wherein the first and second holographic gratings comprise volume phase holographic gratings. 
     
     
       14. The display system defined in  claim 12  wherein the fringes of at least one of the first and second holographic gratings comprise a first set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a left field of view, a second set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a center field of view, and a third set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a right field of view. 
     
     
       15. The display system defined in  claim 14  wherein the first, second, and third sets of fringes are multiplexed within a holographic medium.

Description:
This application claims the benefit of provisional patent application No. 62/563,422, filed Sep. 26, 2017, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to devices with displays, and, more particularly, to head-mounted displays. 
     Head-mounted displays may be used to display virtual reality and augmented reality content. A head-mounted display that is displaying augmented reality content may overlay computer-generated images on real-world objects. Displays and optical systems may be used to create images and to present those images to a user. 
     If care is not taken, however, the components used in displaying content for a user in a head-mounted display may not exhibit desired levels of optical performance. 
     SUMMARY 
     An electronic device may have a display system that produces images. An optical system with one or more waveguides and input and output coupler systems may be used to distribute the images to a user. 
     The display system may have one or more pixel arrays such as liquid-crystal-on-silicon pixel arrays. Images from the display system may be coupled into one or more waveguides by an input coupler system and may be coupled out of the waveguide in multiple image planes using an output coupler system. The input and output coupler systems may include single couplers, stacks of couplers, and tiled arrays of couplers. The couplers may be volume phase holographic gratings or other optical couplers for coupling light into and out of the upper and lower surfaces of elongated strip-shaped waveguides. 
     Holographic gratings in the waveguide may have fringes with constant pitch and variable period. The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given wavelength and incident angle. For example, a first set of fringes may have a first period that maximizes diffraction efficiency for incident light associated with a left field of view, a second set of fringes may have a second period that maximizes diffraction efficiency for incident light associated with a center field of view, and a third set of fringes may have a third period that maximizes diffraction efficiency for a right field of view. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative head-mounted display in accordance with an embodiment. 
         FIG. 2  is a top view of an illustrative head-mounted display in accordance with an embodiment. 
         FIG. 3  is a diagram of an illustrative optical system and associated display system for a head-mounted display in accordance with an embodiment. 
         FIG. 4  is a top view of an illustrative volume phase holographic grating with constant pitch and constant period in accordance with an embodiment. 
         FIG. 5  is a graph showing how different angles of incidence can result in different diffraction efficiencies for a volume phase holographic grating with constant period in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative volume phase holographic grating with constant pitch and variable period in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative optical system having one or more volume phase holographic gratings of the type shown in  FIG. 6  in accordance with an embodiment. 
         FIG. 8  is a side view of an illustrative optical system having a volume phase holographic grating with constant pitch and variable period that redirects upper field of view light and lower field of view light so that it exits the optical system towards the eyebox in accordance with an embodiment. 
         FIG. 9  is a top view of an illustrative stack of volume phase holographic gratings with the same pitch and different periods in accordance with an embodiment. 
         FIG. 10  is a top view of an illustrative volume phase holographic grating having patches of gratings with periods that vary from patch to patch in accordance with an embodiment. 
         FIG. 11  is a top view of an illustrative stack of volume phase gratings with overlapping patches of gratings with periods that vary from patch to patch in accordance with an embodiment. 
         FIG. 12  is a diagram of an illustrative recording setup for patch-writing a constant-pitch, variable-period volume phase holographic grating in accordance with an embodiment. 
         FIG. 13  is a diagram of an illustrative recording setup for recording a constant-pitch, variable-period volume phase holographic grating using a complex wavefront reference recording beam in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Head-mounted displays and other devices may be used for virtual reality and augmented reality systems. These devices may include portable consumer electronics (e.g., portable electronic devices such as cellular telephones, tablet computers, glasses, other wearable equipment), head-up displays in cockpits, vehicles, etc., display-based equipment (projectors, televisions, etc.). Devices such as these may include displays and other optical components. Device configurations in which virtual reality and/or augmented reality content is provided to a user with a head-mounted display are described herein as an example. This is, however, merely illustrative. Any suitable equipment may be used in providing a user with virtual reality and/or augmented reality content. 
     A head-mounted display such as a pair of augmented reality glasses that is worn on the head of a user may be used to provide a user with computer-generated content that is overlaid on top of real-world content. The real-world content may be viewed directly by a user through a transparent portion of an optical system. The optical system may be used to route images from one or more pixel arrays in a display system to the eyes of a user. One or more waveguides may be included in the optical system. Input optical couplers may be used to couple images into the waveguides from one or more pixel arrays. Output optical couplers may be used to couple images out of the waveguides for viewing by the user. One or more additional optical couplers in the waveguides may be used to shift the vertical component of light that is out of the field of view towards the user&#39;s eyes. 
     The input couplers, output couplers, and other optical couplers for the optical system may form structures such as Bragg gratings that couple light into the waveguides from the displays and that couple light out of the waveguides for viewing by the user. Optical couplers may be formed from volume phase holographic gratings or other holographic coupling elements. The optical couplers may, for example, be formed from thin layers of polymers, dichromated gelatin, and/or other optical coupler structures in which holographic patterns are recorded using lasers. For example, the interference of two collimated laser beams may produce modulations in the refractive index in the dichromated gelatin, thereby forming a holographic grating. In some configurations, optical couplers may be formed from dynamically adjustable devices such as adjustable gratings formed from microelectromechanical systems (MEMs) components, liquid crystal components (e.g., tunable liquid crystal gratings, polymer dispersed liquid crystal devices), or other adjustable optical couplers. Arrangements in which optical couplers are formed from volume phase holographic gratings are sometimes described herein as an example. 
     One or more of the volume phase holographic gratings in the optical system may have fringes with a constant pitch, a variable period, and/or a variable fringe angle. Constant-pitch, variable-period gratings may help maintain high diffraction efficiency across a range of angles of incidence for a given wavelength. This type of volume phase holographic grating may in turn help avoid undesirable color shifts, efficiency losses, and brightness variations in the optical system. 
     A schematic diagram of an illustrative head-mounted display is shown in  FIG. 1 . As shown in  FIG. 1 , head-mounted display  10  may have control circuitry  50 . Control circuitry  50  may include storage and processing circuitry for controlling the operation of head-mounted display  10 . Circuitry  50  may 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 circuitry  50  may 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 circuitry  50  and run on processing circuitry in circuitry  50  to implement operations for head-mounted display  10  (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.). 
     Head-mounted display  10  may include input-output circuitry  52 . Input-output circuitry  52  may be used to allow data to be received by head-mounted display  10  from 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 display  10  with user input. Input-output circuitry  52  may also be used to gather information on the environment in which head-mounted display  10  is operating. Output components in circuitry  52  may allow head-mounted display  10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIG. 1 , input-output circuitry  52  may include one or more displays such as display(s)  26 . Display(s)  26  may be used to display images for a user of head-mounted display. Display(s)  26  have pixel array(s) to generate images that are presented to a user through an optical system. The optical system may, if desired, have a transparent portion through which the user (viewer) can observe real-world objects while computer-generated content is overlaid on top of the real-world objects by producing computer-generated images on the display(s)  26 . 
     Optical components  54  may be used in forming the optical system that presents images to the user. Components  54  may include static components such as waveguides, static optical couplers, and fixed lenses. If desired, components  54  may also include adjustable optical components such as an adjustable polarizer, tunable lenses (e.g., liquid crystal tunable lenses, tunable lenses based on electrooptic materials, tunable liquid lenses, microelectromechanical systems (MEMS) tunable lenses, or other tunable lenses), a dynamically adjustable coupler (e.g., an adjustable MEMs grating or other coupler, an adjustable liquid crystal holographic coupler such as an adjustable liquid crystal Bragg grating coupler, adjustable holographic couplers (e.g., electro-optical devices such as tunable Bragg grating couplers, polymer dispersed liquid crystal devices), couplers, lenses, and other optical devices formed from electro-optical materials (e.g., lithium niobate or other materials exhibiting the electro-optic effect), or other static and/or tunable optical components. Components  54  may be used in receiving and modifying light (images) from display  26  and in providing images to a user for viewing. In some configurations, one or more of components  54  may be stacked, so that light passes through multiple components in series (e.g., optical couplers may be stacked or may partially overlap one another). In other configurations, components may be spread out laterally (e.g., optical couplers may be tiled side-by-side). Configurations may also be used in which both tiling and stacking are present. 
     Input-output circuitry  52  may include components such as input-output devices  60  for gathering data and user input and for supplying a user with output. Devices  60  may include sensors  70 , audio components  72 , and other components for gathering input from a user or the environment surrounding device  10  and for providing output to a user. Devices  60  may, for example, include keyboards, buttons, joysticks, touch sensors for trackpads and other touch sensitive input devices, cameras, light-emitting diodes, and/or other input-output components. 
     Cameras or other devices in input-output circuitry  52  may face a user&#39;s eyes and may track a user&#39;s gaze. Sensors  70  may include position and motion sensors (e.g., compasses, gyroscopes, accelerometers, and/or other devices for monitoring the location, orientation, and movement of head-mounted display  10 , satellite navigation system circuitry such as Global Positioning System circuitry for monitoring user location, etc.). Using sensors  70 , for example, control circuitry  50  can monitor the current direction in which a user&#39;s head is oriented relative to the surrounding environment. Movements of the user&#39;s head (e.g., motion to the left and/or right to track on-screen objects and/or to view additional real-world objects) may also be monitored using sensors  70 . 
     If desired, sensors  70  may include ambient light sensors that measure ambient light intensity and/or ambient light color, force sensors, temperature sensors, touch sensors, capacitive proximity sensors, light-based proximity sensors, other proximity sensors, strain gauges, gas sensors, pressure sensors, moisture sensors, magnetic sensors, etc. Audio components  72  may include microphones for gathering voice commands and other audio input and speakers for providing audio output (e.g., ear buds, bone conduction speakers, or other speakers for providing sound to the left and right ears of a user). If desired, input-output devices  60  may include haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, and other output components. Circuitry  52  may include wired and wireless communications circuitry  74  that allows head-mounted display  10  (e.g., control circuitry  50 ) to communicate with external equipment (e.g., remote controls, joysticks and other input controllers, portable electronic devices, computers, displays, etc.) and that allows signals to be conveyed between components (circuitry) at different locations in head-mounted display  10 . 
     The components of head-mounted display  10  may be supported by a head-mountable support structure such as illustrative support structure  16  of  FIG. 2 . Support structure  16  may be configured to form a frame of a pair of glasses (e.g., left and right temples and other frame members), may be configured to form a helmet, may be configured to form a pair of goggles, or may have other head-mountable configurations. 
     Optical system  84  may be supported within support structure  16  and may be used to provide images from displays  26  to a user (see, e.g., the eyes of user  90  of  FIG. 2 ). With one illustrative configuration, displays  26  may be located in outer (edge) portions  88  of optical system  84  and may have one or more pixel arrays that produce images. Light associated with the images may be coupled into waveguides in outer portions  88  using input coupler systems. The waveguides may traverse intermediate regions  82 . In central portion(s)  86  of system  84  (at the opposing ends of the waveguides from the input coupler systems and displays  26 ), output coupler systems formed from one or more output couplers may couple the light out of the waveguides. This light may pass through optional lenses  80  in direction  92  for viewing by user  90 . Portion  86  of optical system  84  may be transparent, so that user  90  may view external objects such as object  30  through this region of system  84  while system  84  overlays computer-generated content (image content generated by control circuitry  50 ) with objects such as object  30 V. 
     A portion of an illustrative head-mounted device is shown in  FIG. 3 . Device  10  may include one or more pixel arrays such as pixel array  26 . Pixel array  26  is formed from pixels  26 P. There may be any suitable number of pixels  26 P in display  26  (e.g., 0-1000, 10-10,000, 1000-1,000,000, 1,000,000 to 10,000,000, more than 1,000,000, fewer than 1,000,000, fewer than 10,000, fewer than 100, etc.). Pixel array  26  may have any suitable type of display pixels (e.g., pixel array  26  may form a display such as an organic light-emitting diode display, a display having a pixel array formed from an array of light-emitting diodes each of which is formed from a respective crystalline semiconductor die, a liquid crystal display, a liquid-crystal-on-silicon display, a microelectromechanical systems display, or any other suitable display). In the illustrative configuration of  FIG. 3 , pixel array  26  forms part of display system  100  in which pixel array  26  is illuminated by light from an illumination system. The illumination system includes light source  104  and optical coupler  102 . Light source  104  may include one or more light-emitting components  106 . Components  106  may be, for example, light-emitting diodes such as red, green, and blue light emitting diodes, white light emitting diodes and/or light-emitting diodes, lamps, lasers, or other light sources of one or more other colors. Optical coupler  102  may be a beam splitter or other optical component(s) that helps direct light  108  from light source  104  toward pixel array  26 . 
     As shown in  FIG. 3 , light  108  from light source  104  may be directed towards the surface of pixel array  26  (e.g., a liquid-crystal-on-silicon pixel array) by coupler  102 . Light  108  is reflected by pixels  26 P, which create an image for viewing by user  90 . Reflected light  108 R (e.g., image light corresponding to an image formed from the array of pixels  26 P) passes through coupler  102  and optional lens  110 . This reflected image light (image  112 ) is received by an input coupler system in input portion  88  of optical system  84 . 
     As shown in  FIG. 3 , optical system  84  may include one or more waveguides such as waveguide  116 . Waveguide  116  may be formed from a transparent material such as clear glass or plastic. Waveguide  116  may be a film or rigid plate that carries holographic media (e.g., photosensitive material such as dichromate gelatin, photopolymer, or other optical structures in which holographic patterns are recorded using lasers). A left-hand waveguide for providing images to a user&#39;s left eye  90  is shown in  FIG. 3 , but system  84  may, in general, include waveguide structures for providing image light to both of a user&#39;s eyes. With one illustrative configuration, each waveguide  116  has an elongated strip shape that extends along axis X between opposing first and second ends. Waveguide  116  may, for example, have a height (e.g., a length in dimension Y) of about 1 mm to 100 mm, at least 2 mm, at least 5 mm, less than 50 mm, or other suitable size. Waveguide  116  may have a thickness (e.g., a length in dimension Z) of about 3 mm, 1-5 mm, at least 0.1 mm, at least 0.5 mm, at least 1.5 mm, at least 3 mm, less than 4 mm, less than 5 mm, or other suitable thickness. In dimension X, a left-hand waveguide  116  may extend across about half of a user&#39;s face and a right-hand waveguide  116  may extend across the other half of the user&#39;s face. Accordingly, waveguides  116  may have lengths in dimension X of about 10 mm to 300 mm, at least 5 mm, at least 20 mm, at least 40 mm, at least 80 mm, at least 100 mm, at least 130 mm, less than 200 mm, less than 150 mm, less than 100 mm, less than 90 mm, etc. Waveguides  116  may be straight (as shown in  FIG. 3 ) or may have a curved shape that wraps around a user&#39;s head. 
     System  84  may have an input coupler system in portion  88 . The input coupler system may include one or more input couplers such as input coupler  114 . Image light  112  from display  26  may be coupled into waveguide  116  using input coupler  114 . Input coupler  114  of  FIG. 3  is a reflective coupler (light reflects from coupler  114  into waveguide  116 ). This is merely illustrative, however. If desired, input couplers such as input coupler  114  may be transmissive couplers (light is coupled into waveguide  116  upon passing through coupler  114 ). 
     Within waveguide  116 , the light that has been coupled into waveguide  116  may propagate along dimension X in accordance with the principal of total internal reflection. Light  118  may then be coupled out of waveguide  116  by an output coupler system in output portion  86 . The output coupler system may include one or more output couplers such as output coupler  120 , which couples light  118  out of waveguide  116 , as illustrated by light  122 . Light  122  may then pass through lenses such as lens  80  in direction  92  for viewing by user  90 . 
     If desired, there may be additional optical couplers in waveguide  116  such as optical coupler  124 . Optical coupler  124  may, for example, be used to shift the vertical component of light that would otherwise be outside of the user&#39;s field of view (e.g., field of view  140  of  FIG. 3 ) towards the user&#39;s eyes  90 . Because this type of optical coupler enlarges the user&#39;s field of view  140  along dimension Y of  FIG. 3 , optical coupler  124  may sometimes be referred to as a Y-pupil expansion coupler or vertical field of view expansion grating. Vertical field of view expansion gratings such as grating  124  may be located between input coupler  114  and output coupler  120  (as shown in the example of  FIG. 3 ), maybe located in front of input coupler  114 , may be integrated with input coupler  114  and/or output coupler  120 , or may be located in other suitable positions. 
     The optical couplers in system  84  may be holographic couplers (e.g., volume phase holographic gratings). The couplers may be plane-to-plane couplers (infinite focal length) or may have an associated finite focal length f (e.g., these couplers may have an associated positive or negative lens power). 
     The example of  FIG. 3  in which couplers  114 ,  120 , and  124  are located on a rear surface of waveguide  116  is merely illustrative. If desired, couplers  114 ,  120 , and  124  may be located on a front surface of waveguide  116  (e.g., opposite the surface shown in  FIG. 3 ), may be embedded within waveguide  116 , or may be partially embedded in waveguide  116 . 
     Optical couplers in waveguide  116  such as input coupler  114 , output coupler  120 , and vertical field of view expansion grating  124  may be formed from volume phase holographic gratings or other holographic coupling elements. The optical couplers may, for example, be formed from thin layers of polymers, dichromated gelatin, and/or other optical coupler structures in which holographic patterns are recorded using lasers. For example, the interference of two collimated laser beams may produce periodic modulations in the refractive index in the dichromated gelatin, thereby forming a holographic grating. 
       FIG. 4  is a top view of an illustrative volume phase holographic grating that may be used to form optical couplers in a head-mounted display. As shown in  FIG. 4 , grating  126  may include a medium such as holographic medium  128 . Holographic medium  128  may be dichromated gelatin, polymer, or other suitable material. Holographic medium  128  may have a periodic modulation of refractive index. The modulation of refractive index in grating  126  occurs in periodic fringes such as fringes  130 . The portions of holographic medium  128  that form fringes  130  may have one refractive index, whereas the portions of holographic medium  128  between fringes  130  may have a different refractive index. Grating  126  may form an optical interference pattern that stores a holographic recording within holographic medium  128 . 
     In the example of  FIG. 4 , fringes  130  are parallel to and equidistant from one another. Gratings of this type may sometimes be referred to as linear gratings. In other words, the period A is constant across grating  126 , the pitch ρ is constant across grating  126 , and the fringe angle φ is constant across grating  126 . The period Λ refers to the spacing between fringes  130  as measured along the grating vector K (i.e., vector  134  orthogonal to fringes  130 ). The pitch p refers to the distance between fringes  130  as measured along the grating surface  128 S. The fringe angle φ refers to the angle between the grating surface normal (i.e., vector  132 ) and the grating vector K (i.e., vector  134  orthogonal to fringes  130 ). 
     When light is incident on the surface of grating  128 , the pitch ρ determines the diffraction angle according to the following grating equation: 
     
       
         
           
             
               
                 
                   
                     
                       λ 
                       0 
                     
                     
                       ρ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       n 
                     
                   
                   = 
                   
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         1 
                       
                     
                     + 
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where λ 0  is the wavelength of incident light  136  in air, ρ is the pitch of grating  126 , n is the refractive index of medium  128 , θ1 is the angle of incident light  136  (as measured from grating surface normal  132 ), and θ2 is the angle of diffracted light  138  (as measured from grating surface normal  132 ). Thus, in order to achieve the same diffraction angle θ2 across grating  126 , grating  126  has the same pitch ρ across grating  126 . 
     In a volume phase holographic grating, maximum diffraction efficiency occurs when the Bragg condition is satisfied, which occurs when the following is true: 
     
       
         
           
             
               
                 
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         ϕ 
                         - 
                         
                           θ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       λ 
                       0 
                     
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       n 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Λ 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     As shown in Equation 2 above, different incident angles may result in a change in wavelength at which maximum diffraction efficiency occurs.  FIG. 5  illustrates how diffraction efficiency may be dependent upon incident angle and wavelength.  FIG. 5  is a graph showing diffraction efficiency for a grating having parallel fringes with constant pitch and constant period (e.g., a grating of the type shown in  FIG. 4 ). Curve  142  represents the diffraction efficiency for light of a first wavelength λ1, curve  144  represents the diffraction efficiency for light of a second wavelength λ2, and curve  146  represents the diffraction efficiency for light of a third wavelength λ3. When illuminated with a given color of light (e.g., having a peak wavelength λ3), the wavelength at which maximum diffraction efficiency occurs will change as the angle of incidence changes (according to Equation 2). At angle of incidence A3, maximum diffraction efficiency occurs at the desired wavelength λ3, as illustrated by curve  146 . At angle of incidence A2, however, maximum diffraction efficiency occurs at wavelength λ2, as illustrated by curve  144 . And at angle of incidence A1, maximum diffraction efficiency occurs at wavelength  1   l , as illustrated by curve  142 . Wavelengths λ1 and λ2 may be slightly different than the desired wavelength λ3. 
     If care is not taken, the dependence of diffraction efficiency on incident angle can present obstacles. For example, lasers may not be suitable for the illumination source because the narrow spectrum would result in a small field of view. Users may perceive visible color shifts and/or brightness variations because the wavelength with highest diffraction efficiency can vary across the field of view. Additionally, at any given incident angle, a subset of the spectra of incident light may be diffracted into the waveguide, resulting in efficiency loss. 
     To achieve high diffraction efficiency for the same wavelength at different incident angles, the period and/or fringe angle of the grating may be varied across the grating. In other words, the period Λ may be adjusted to satisfy Equation 2, even as incident angle changes. To ensure that the diffraction angle remains constant across the grating, the pitch ρ should remain constant across the grating. To vary the period without changing the pitch, the fringe angle φ may be adjusted accordingly. Period, pitch, and fringe angle are related by the following equation: 
     
       
         
           
             
               
                 
                   ρ 
                   = 
                   
                     Λ 
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       φ 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
       FIG. 6  is a top view of an illustrative volume phase grating having constant pitch and variable period that may be used to form one or more optical couplers in a head-mounted display (e.g., a head-mounted display of the type shown in  FIG. 3 ). As shown in  FIG. 6 , grating  148  may include a medium such as holographic medium  150 . Holographic medium  150  may be dichromated gelatin, polymer, or other suitable material. Holographic medium  150  may have a periodic modulation of refractive index. The modulation of refractive index in grating  148  occurs in periodic fringes such as fringes  152 . The portions of holographic medium  150  that form fringes  152  may have one refractive index, whereas the portions of holographic medium  150  between fringes  152  may have a different refractive index. Grating  148  may form an optical interference pattern that stores a holographic recording within holographic medium  150 . 
     In the example of  FIG. 6 , fringes  152  have constant pitch and variable period. In other words, the period Λ(x) varies across grating  148  (e.g., varies as a function of position along the X-axis of  FIG. 6 ), the pitch ρ is constant across grating  148 , and the fringe angle φ(x) varies across grating  148  (e.g., varies as a function of position along the X-axis of  FIG. 6 ). As in the example of  FIG. 4 , the period Λ refers to the spacing between fringes  152  as measured along the grating vector K (i.e., vector  156  orthogonal to fringes  152 ); the pitch ρ refers to the distance between fringes  152  as measured along the grating surface  150 S; the fringe angle φ refers to the angle between the grating surface normal (i.e., vector  154 ) and the grating vector K (i.e., vector  156  orthogonal to fringe  152 ). 
     Because the pitch of grating  148  remains constant while the period changes, diffraction angle θ2 may remain the same across grating  148 , while the maximum diffraction efficiency for a given wavelength may remain high even at different field angles (i.e., different angles of incidence). This is achieved by ensuring that the Bragg condition (Equation 2) is satisfied at each location on grating  148 . In other words, the diffraction efficiency at each portion of grating  148  may be tailored (e.g., Bragg-matched) to the field angles that are incident on that portion of grating  148 . For example, for a given wavelength, fringes  152  in portion  164  of grating  148  may have a period that maximizes diffraction efficiency at incident angles associated with a left field of view; fringes  152  in portion  166  of grating  148  may have a period that maximizes diffraction efficiency at incident angles associated with a center field of view; and fringes  152  in portion  168  of grating  148  may have a period that maximizes diffraction efficiency at incident angles associated with a right field of view. 
       FIG. 7  illustrates how gratings with constant pitch and variable period (e.g., gratings of the type shown in  FIG. 6 ) may be incorporated into optical system  84  of head-mounted display  10  (e.g., a head-mounted device of the type shown in  FIG. 3 ). In this example, optical system  84  may be used to deliver light  112  from display system  100  to a user&#39;s right eye  90 , as illustrated by light  122 . 
     As discussed in connection with  FIG. 3 , input coupler  114  may be used to couple light  112  from display system  100  into waveguide  116 . Within waveguide  116 , the light that has been coupled into waveguide  116  may propagate along dimension X in accordance with the principal of total internal reflection. Light  118  may then be coupled out of waveguide  116  towards viewer  90  by output coupler  120 , as illustrated by light  122 . 
     Incident light  112  may have different field angles such as left field of view light  158 , center field of view light  160 , and right field of view light  158 . Left field of view light  158  reaches viewer  90  as light  158 ′ to form a left portion of an image, center field of view light  160  reaches viewer  90  as light  160 ′ to form a center portion of an image, and right field of view light  162  reaches viewer  90  as light  162 ′ to form a right portion of an image. 
     Input coupler  114  and output coupler  120  may be formed from gratings having constant pitch and variable period, as discussed in connection with  FIG. 6 . For example, input coupler  114  may be formed from grating  148 A having fringes  152 A, and output coupler  120  may be formed from grating  148 B having fringes  152 B. In each grating, the fringes may be tailored to maximize diffraction efficiency for a given range of field angles. Fringes in portion  164  of each grating may have a period that maximizes diffraction efficiency for left field of view light  158 ; fringes in portion  166  of each grating may have a period that maximizes diffraction efficiency for center field of view light  160 ; and fringes in portion  168  of each grating may have a period that maximizes diffraction efficiency right field of view light  162 . To maximize diffraction efficiency at a given field angle for a given wavelength at a given location of the grating, the period of the grating at that location may be determined according to Equation 2. This is sometimes referred to as Bragg-matching the grating to the incident angle and wavelength of incident light. This ensures that, for a given color of incident light, left field of view light  158 ′ of that color, center field of view light  160 ′ of that color, and right field of view light  162 ′ of that color all reach viewer  90  with uniform diffraction efficiency. This helps avoid color shifts, brightness variations, and efficiency losses, while also allowing for both broadband illumination sources and narrow spectrum illumination sources (e.g., lasers). 
     If desired, volume phase holographic gratings with constant pitch and variable period may also be used to form vertical field of view expansion gratings. This type of arrangement is illustrated in  FIG. 8 . As shown in  FIG. 8 , optical system  84  may include vertical field of view expansion grating  124 . In this example, grating  124  is located between input coupler  114  (not shown in  FIG. 8  because it is behind grating  124 ) and output coupler  120 . Grating  124  may be used to minimize the amount of light that is above or below the user&#39;s field view  140 . For example, lower field of view light  170 , which might otherwise pass under field of view  140 , is redirected by grating  124  to reach viewer  90 . 
     Grating  124  may be formed from a volume phase holographic grating having constant pitch and variable period, as discussed in connection with  FIG. 6 . For example, vertical field of view expansion grating  124  may be formed from grating  148 C having fringes  152 C. In grating  148 C, the fringes may be tailored to maximize diffraction efficiency for a given range of field angles. Fringes  152 C in portion  176  of grating  148 C may have a period that maximizes diffraction efficiency for lower field of view light  170 ; fringes  152 C in portion  178  of grating  148 C may have a period that maximizes diffraction efficiency center field of view light  172 ; and the lower portion of grating  148 C (not shown because it is behind output coupler  120  of  FIG. 8 ) may have fringes  152 C with a period that maximizes diffraction efficiency for upper field of view light (not shown in  FIG. 8 ). To maximize diffraction efficiency at a given field angle for a given wavelength at a given location of the grating, the period of the grating at that location may be determined according to Equation 2 (e.g., the grating may be Bragg-matched to the incident angle and wavelength of incident light). This ensures that, for a given color of incident light, upper field of view light, center field of view light  172 , and lower field of view light  170  all reach viewer  90  with uniform diffraction efficiency. 
     In some arrangements, the fringe period of grating  148  (e.g., grating  148 A,  148 B, and/or grating  148 C) may vary continuously across grating  148 , such that the spacing between each pair of fringes is different from the spacing between the next pair of fringes (e.g., as shown in the example of  FIG. 6 ). In other arrangements, a constant-pitch, variable-period grating may be achieved using gratings that are entirely linear (e.g., using gratings of the type shown in  FIG. 4 ) or that have patches of linear fringes.  FIGS. 9, 10, and 11  illustrate various examples of grating arrangements that may be used to achieve a similar result as a single grating with a constant pitch and continuously varying period. 
     In the example of  FIG. 9 , grating  148  is formed from multiple stacked gratings, with each grating tailored to a specific subset of field angles. In particular, for a given color of light, grating  148 - 1  may have fringes  152 - 1  with a fringe period that maximizes diffraction efficiency for left field of view angles; grating  148 - 2  may have fringes  152 - 2  with a fringe period that maximizes diffraction efficiency for center field of view angles, and grating  148 - 3  may have fringes  152 - 3  with a fringe period that maximizes diffraction efficiency for right field of view angles. All three gratings may have the same pitch, and each grating may also have a constant period across that grating. However, the period of each grating may be different from the period of the other gratings in the stack (e.g., the period of fringes  152 - 1 , the period of fringes  152 - 2 , and the period of fringes  152 - 3  may be different from one another), yielding a similar effect as that of a single grating with constant pitch and variable period. 
     It should be understood that the example of  FIG. 9  in which grating  148  is formed from a stack of three gratings is merely illustrative. If desired, grating  148  may be made up of four, five, six, more than six, or fewer than six gratings. 
     In the example of  FIG. 10 , grating  148  is made up of patches of constant-pitch, constant-period fringes. Each patch may be tailored to a specific subset of field angles. In particular, for a given color of light, portion  164  of grating  148  may have fringes  152  with a period that maximizes diffraction efficiency for left field of view angles; portion  166  of grating  148  may have fringes  152  with a period that maximizes diffraction efficiency for center field of view angles, and portion  168  of grating  148  may have fringes  152  with a period that maximizes diffraction efficiency for right field of view angles. All three patches in grating  148  may have the same pitch, and each patch may also have a constant period across that patch. However, the period of each patch may be different from the period of the other patches in the grating, yielding a similar effect as that of a grating with a constant pitch and continuously varied period. 
     In the example of  FIG. 11 , grating  148  is formed from multiple stacked gratings, with each grating tailored to a specific subset of field angles. In particular, for a given color of light, grating  148 - 1  may have a patch of fringes  152 - 1  with a period that maximizes diffraction efficiency for left field of view angles; grating  148 - 2  may have a patch of fringes  152 - 2  with a period that maximizes diffraction efficiency for center field of view angles, and grating  148 - 3  may have a patch of fringes  152 - 3  with a period that maximizes diffraction efficiency for right field of view angles. All three patches of fringes may have the same pitch, and each patch may also have a constant period across that patch. However, the period of each patch may be different from the period of the other patches in the stack, yielding a similar effect as that of a single grating with constant pitch and variable period. 
     The examples of  FIGS. 6-11  in which the different grating regions are spatially separated are merely illustrative. If desired, a multiplexing arrangement in which multiple gratings are superimposed in a common holographic medium may be used to achieve a constant-pitch, variable-period grating. For example, fringes of a first pitch and a first period, fringes of the first pitch and a second period, and fringes of the first pitch and a third period may be multiplexed within a common holographic medium. Multiplexing constant pitch gratings may be used, for example, to address multiple colors (e.g., multiple wavelengths) in a single layer of holographic media. 
     Illustrative examples of holographic recordings systems that may be used to record a constant-pitch, variable-period volume phase holographic grating of the type described in connection with  FIGS. 6-11  are shown in  FIGS. 12 and 13 . 
     In the example of  FIG. 12 , a constant-pitch, variable period volume phase holographic grating is recorded in patches using a sequential recording setup. As shown in  FIG. 12 , system  180  may include recording beams such as signal laser beams  182  and reference laser beams  184 . Signal beams  182  and reference beams  184  can be positioned at various angles with respect to index-matching material  186 . Index-matching material  186  may have a cavity that receives holographic structure  148 . Holographic structure  148  initially includes an unexposed recording medium such as layer  150  of  FIG. 6 . Following exposure to laser light from the recording beams of system  180 , a grating is recorded in the recording medium such as grating  148  of  FIG. 7 . 
     A laser system may produce laser light for use in recording grating  148  in structure  150 . During operation, signal and reference laser beams pass through index-matching material  186  to reach holographic structure  150 . To form fringes  150  in grating  148  with constant pitch and variable period, patches such as patches  188  of fringes in grating  148  may be recorded sequentially, using plane-wave signal and reference beams to record each patch. With each subsequent patch of fringes, the recording beams may be reoriented to maintain the same pitch while varying the period to achieve the desired diffraction efficiency for a given input angle. For example, a first patch  188  of fringes with a first pitch and a first period may be recorded using plane-wave signal beams  182  and  184 . A second patch  188  of fringes with the same pitch as the first patch but with a different period may then be recorded by changing the orientation of plane-wave signal beams  182  and  184  (e.g., by changing the separation between the recording beams and changing the angle of the recording beams relative to medium  150 ). Subsequent patches  188  may be recorded, reorienting the recording beams for each patch to achieve the desired diffraction efficiency at that location of grating  148 . 
       FIG. 13  illustrates an example in which a constant-pitch, variable-period grating is recorded using recording beams with a non-planar wavefronts. As shown in  FIG. 13 , recording beams may include signal beam  184  and reference beam  182 . Recording signal beam  184  may be generated by a diverging point source  190  positioned at a nominal location of the eye during playback. Reference recording beam  182  may contain a complex wavefront. The wavefront may, for example, be identical to the wavefront generated by a diverging point source (e.g., points source  190 ) diffracting off of medium  150 . Rather than using multiple exposures as in the recording example of  FIG. 12 , the recording beams of  FIG. 13  are oriented relative to medium  150  so as to produce constant-pitch, variable-period fringes  152  in medium  150  in a single exposure (although multiple exposures may be used, if desired). 
     In accordance with an embodiment, an electronic device is provided that includes, a display system configured to produce images, and an optical system having an input portion and an output portion, the optical system includes, a waveguide that extends between the input portion and the output portion, an input coupler in the input portion, the input coupler is configured to couple the images from the display system into the waveguide, and an output coupler in the output portion, the output coupler is configured to couple the images out of the waveguide, at least one of the input coupler and the output coupler includes: a volume phase holographic grating with a constant pitch and a variable period. 
     In accordance with another embodiment, the volume phase holographic grating includes fringes in a holographic medium, each fringe is oriented at a fringe angle relative to a surface normal of the holographic medium, and the fringe angle varies across the holographic medium. 
     In accordance with another embodiment, the fringes include a first set of fringes oriented at a first fringe angle, a second set of fringes oriented at a second fringe angle, and a third set of fringes oriented at a third fringe angle. 
     In accordance with another embodiment, the first set of fringes has a first period that is Bragg-matched to incident light associated with a left field of view, the second set of fringes has a second period that is Bragg-matched to incident light associated with a center field of view, and the third set of fringes has a third period that is Bragg-matched to incident light associated with a right field of view. 
     In accordance with another embodiment, the period of the volume phase holographic grating varies continuously across the volume phase holographic grating. 
     In accordance with another embodiment, the volume phase holographic grating includes first, second, and third gratings arranged in a stack, the first grating has fringes with a first pitch and a first period, the second grating has fringes with the first pitch and a second period, and the third grating has fringes with the first pitch and a third period. 
     In accordance with another embodiment, including an additional volume phase holographic grating in the waveguide between the input coupler and the output coupler. 
     In accordance with another embodiment, the additional volume phase holographic grating has a constant pitch and a variable period. 
     In accordance with another embodiment, the additional volume phase holographic grating includes fringes in a holographic medium, each fringe is oriented at a fringe angle relative to a surface normal of the holographic medium, and the fringe angle varies across the holographic medium. 
     In accordance with another embodiment, the fringes include a first set of fringes oriented at a first fringe angle and a second set of fringes oriented at a second fringe angle. 
     In accordance with another embodiment, the first set of fringes has a first period that is Bragg-matched to incident light associated with an upper field of view, and the second set of fringes has a second period that is Bragg-matched to incident light associated with a lower field of view. 
     In accordance with an embodiment, a volume phase holographic grating is provided that includes, a medium having first, second, and third portions, and fringes in the medium, the fringes have a uniform pitch across the first, second, and third portions of the medium, the fringes in the first portion have a first period, the fringes in the second portion have a second period, and the fringes in the third portion have a third period, the first, second, and third periods are different, and the first period maximizes diffraction efficiency for incident light of a given wavelength and a first incident angle, the second period maximizes diffraction efficiency for incident light of the given wavelength and a second incident angle, and the third period maximizes diffraction efficiency for incident light of the given wavelength and a third incident angle. 
     In accordance with another embodiment, the period of the fringes in the holographic medium varies continuously across the medium. 
     In accordance with another embodiment, the medium includes first, second, and third holographic mediums, the fringes with the first period are formed in the first holographic medium, the fringes with the second period are formed in the second holographic medium, and the fringes with the third period are formed in the third holographic medium. 
     In accordance with another embodiment, the medium has a surface normal, the fringes are oriented at a fringe angle relative to the surface normal, and the fringe angle varies across the medium. 
     In accordance with an embodiment, a display system is provided that includes, a waveguide, a first holographic grating that couples light into the waveguide, and a second holographic grating that couples light out of the waveguide, at least one of the first and second holographic gratings has fringes with a constant pitch and a variable period. 
     In accordance with another embodiment, the first and second holographic gratings include volume phase holographic gratings. 
     In accordance with another embodiment, the fringes include a first set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a left field of view, a second set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a center field of view, and a third set of fringes that are oriented to maximize diffraction efficiency for incident angles associated with a right field of view. 
     In accordance with another embodiment, the first, second, and third sets of fringes are multiplexed within a holographic medium. 
     In accordance with another embodiment, a third holographic grating that redirects light within the waveguide, the third holographic grating has fringes with a constant pitch and a variable period. 
     In accordance with another embodiment, the fringes of the third holographic grating include a first set of fringes oriented to maximize diffraction efficiency for incident angles associated with an upper field of view and a second set of fringes oriented to maximize diffraction efficiency for incident angles associated with a lower field of view. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20180814
Publication Date: 20221011
Grant Date: 20221011
Priority Date: 20170926
Inventors: DELAPP, SCOTT M.
COCILOVO, BYRON R.
OH, SE BAEK
STEELE, BRADLEY C.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G03H1/0248", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0178", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B5/32", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B2027/0105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "G03H1/0248", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0174", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0116", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0174", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G03H1/0248", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2027/0178", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 63524371