PATENT DOCUMENT

Publication Number: US-11822079-B2
Application Number: US-201916520718-A
Country: US
Kind Code: B2

Title: Waveguided display system with adjustable lenses

Abstract:
An electronic device may have a display that provides image light to a waveguide. First and second liquid crystal lenses may be mounted to opposing surfaces of the waveguide. An coupler may couple the image light out of the waveguide through the first lens. The second lens may convey world light to the first lens. Control circuitry may control the first lens to apply a first optical power to the image light and the world light and may control the second lens to apply a second optical power to the world light that cancels out the first optical power. Each lens may include two layers of liquid crystal molecules having antiparallel pretilt angles. The pretilt angles and rubbing directions of the first lens may be antiparallel to corresponding pretilt angles and rubbing directions of the second lens about the waveguide.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 a waveguide having opposing first and second surfaces; 
 a display configured to emit first light into the waveguide; 
 a first liquid crystal lens at the first surface and comprising a first liquid crystal layer having first liquid crystal molecules with a first pretilt angle across a thickness of the first liquid crystal layer; 
 a second liquid crystal lens at the second surface and comprising a second liquid crystal layer having second liquid crystal molecules with a second pretilt angle across a thickness of the second liquid crystal layer, the second pretilt angle being different from the first pretilt angle; and 
 an output coupler configured to couple the first light out of the waveguide through the first liquid crystal lens. 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the first liquid crystal lens comprises a third liquid crystal layer having third liquid crystal molecules with a third pretilt angle that is opposite to the first pretilt angle and wherein the second liquid crystal lens comprises a fourth liquid crystal layer having fourth liquid crystal molecules with a fourth pretilt angle that is opposite to the second pretilt angle. 
     
     
       3. The electronic device defined in  claim 2 , wherein the second liquid crystal layer is interposed between the fourth liquid crystal layer and the waveguide and wherein the first liquid crystal layer is interposed between the third liquid crystal layer and the waveguide. 
     
     
       4. The electronic device defined in  claim 1 , wherein the first liquid crystal lens comprises a first transparent substrate layer mounted at the waveguide, a first liquid crystal layer on the first transparent substrate layer, and a second transparent substrate layer on the first liquid crystal layer, wherein the second liquid crystal lens comprises a third transparent substrate layer mounted at the waveguide, a second liquid crystal layer on the third transparent substrate layer, and a fourth transparent substrate layer on the second liquid crystal layer, wherein the first and third transparent substrate layers have a first rubbing direction, and wherein the second and fourth transparent substrate layers have a second rubbing direction antiparallel to the first rubbing direction. 
     
     
       5. The electronic device defined in  claim 4 , wherein the first liquid crystal lens comprises a fifth transparent substrate layer on the second transparent substrate layer, a third liquid crystal layer on the fifth transparent substrate layer, and a sixth transparent substrate layer on the third liquid crystal layer, wherein the second liquid crystal lens comprises a seventh transparent substrate layer on the fourth transparent substrate layer, a fourth liquid crystal layer on the seventh transparent substrate layer, and an eighth transparent substrate layer on the fourth liquid crystal layer, wherein the seventh and fifth transparent substrate layers have the second rubbing direction, and wherein the eighth and sixth transparent substrate layers have the first rubbing direction. 
     
     
       6. The electronic device defined in  claim 4 , wherein the first liquid crystal lens comprises a first electrode layer on the first transparent substrate layer and a second electrode layer on the second transparent substrate layer, wherein the second liquid crystal lens comprises a third electrode layer on the third transparent substrate layer and a fourth electrode layer on the fourth transparent substrate layer, wherein first voltages are provided between the first and second electrode layers that configure the first liquid crystal layer to exhibit a first index of refraction profile, and wherein the second voltages are provided between the third and fourth electrode layers that configure the second liquid crystal layer to exhibit a second index of refraction profile that is an inverse of the first index of refraction profile. 
     
     
       7. The electronic device defined in  claim 1 , further comprising:
 a linear polarizer interposed between the first liquid crystal lens and the first surface of the waveguide. 
 
     
     
       8. The electronic device defined in  claim 7 , wherein the linear polarizer has a transmission axis, wherein the first liquid crystal lens comprises a first plurality of stacked transparent substrates having first rubbing directions, wherein the second liquid crystal lens comprises a second plurality of stacked transparent substrates having second rubbing directions, and wherein the first and second rubbing directions are aligned with the transmission axis. 
     
     
       9. The electronic device defined in  claim 1 , further comprising:
 a third liquid crystal lens mounted to the first liquid crystal lens and aligned with the first and second liquid crystal lenses; and 
 a fourth liquid crystal lens mounted to the second liquid crystal lens and aligned with the first, second, and third liquid crystal lenses, wherein the first and second liquid crystal lenses have first rubbing directions, and the third and fourth liquid crystal lenses have second rubbing directions orthogonal to the first rubbing directions. 
 
     
     
       10. The electronic device defined in  claim 1 , wherein the output coupler comprises a diffractive grating embedded within the waveguide and wherein the second pretilt angle is opposite the first pretilt angle. 
     
     
       11. An optical system configured to convey first light from a display and second light, the optical system comprising:
 a waveguide having opposing first and second surfaces; 
 a first transparent layer at the first surface; 
 a first liquid crystal layer mounted to the first transparent layer; 
 a second transparent layer mounted to the first liquid crystal layer, wherein the first and second transparent layers and the first liquid crystal layer are configured to pass the first light and the second light; 
 a third transparent layer at the second surface; 
 a second liquid crystal layer mounted to the third transparent layer; and 
 a fourth transparent layer mounted to the second liquid crystal layer, wherein the third and fourth transparent layers and the second liquid crystal layer are configured to pass the second light, wherein the first and third transparent layers have a first rubbing direction, and wherein the second and fourth transparent layers have a second rubbing direction different from the first rubbing direction. 
 
     
     
       12. The optical system defined in  claim 11 , further comprising:
 a fifth transparent layer mounted to the second transparent layer; 
 a third liquid crystal layer mounted to the fifth transparent layer; and 
 a sixth transparent layer mounted to the third liquid crystal layer, wherein the fifth transparent layer has the second rubbing direction and the sixth transparent layer has the first rubbing direction, and wherein the fifth and sixth transparent layers and the third liquid crystal layer are configured to pass the first light and the second light. 
 
     
     
       13. The optical system defined in  claim 12 , further comprising:
 a seventh transparent layer mounted to the fourth transparent layer; 
 a fourth liquid crystal layer mounted to the seventh transparent layer; and 
 an eighth transparent layer mounted to the fourth liquid crystal layer, wherein the seventh transparent layer has the second rubbing direction and the eight transparent layer has the first rubbing direction, and wherein the seventh and eighth transparent layers and the fourth liquid crystal layer are configured to pass the second light. 
 
     
     
       14. The optical system defined in  claim 13 , further comprising:
 first elongated electrodes and a first common electrode coupled to opposing sides of the first liquid crystal layer; 
 second elongated electrodes and a second common electrode coupled to opposing sides of the second liquid crystal layer, wherein the first elongated electrodes extend parallel to the second elongated electrodes; 
 third elongated electrodes and a third common electrode coupled to opposing sides of the third liquid crystal layer; and 
 fourth elongated electrodes and a fourth common electrode coupled to opposing sides of the fourth liquid crystal layer, wherein the third elongated electrodes extend parallel to the fourth elongated electrodes and orthogonal to the first and second elongated electrodes. 
 
     
     
       15. The optical system defined in  claim 13 , further comprising:
 a linear polarizer interposed between the third transparent layer and the waveguide, wherein the linear polarizer has a transmission axis aligned with the first and second rubbing directions. 
 
     
     
       16. A display comprising:
 pixels; 
 a waveguide configured to convey first light emitted by the pixels; 
 a first liquid crystal lens at a first side of the waveguide; 
 a second liquid crystal lens on the first liquid crystal lens; 
 a third liquid crystal lens at a second side of the waveguide; and 
 a fourth liquid crystal lens on the third liquid crystal lens, wherein the third and fourth liquid crystal lenses are configured to pass the first light and second light from the first and second liquid crystal lenses, the first liquid crystal lens has first antiparallel rubbing directions, the second liquid crystal lens has second antiparallel rubbing directions different from the first antiparallel rubbing directions, the third liquid crystal lens has third antiparallel rubbing directions aligned with the first antiparallel rubbing directions, and the fourth liquid crystal lens has fourth antiparallel rubbing directions aligned with the second antiparallel rubbing directions. 
 
     
     
       17. The display defined in  claim 16 , wherein each of the first, second, third, and fourth liquid crystal lenses comprises:
 a first layer of liquid crystal molecules having a first pretilt angle; and 
 a second layer of liquid crystal molecules having a second pretilt angle different from the first pretilt angle. 
 
     
     
       18. A display comprising:
 a waveguide having opposing first and second sides, the waveguide being configured to propagate light via total internal reflection and comprising an output coupler; 
 a first liquid crystal layer at the first side; and 
 a second liquid crystal layer at the second side, wherein the first liquid crystal layer comprises first liquid crystal molecules having a first pretilt angle across a thickness of the first liquid crystal layer and the second liquid crystal layer comprises second liquid crystal molecules having a second pretilt angle, and wherein the output coupler is configured to couple the light propagated via total internal reflection out of the waveguide through the first liquid crystal layer. 
 
     
     
       19. The display of  claim 18 , wherein the second liquid crystal molecules have the second pretilt angle across a thickness of the second liquid crystal layer and the second pretilt angle is opposite the first pretilt angle. 
     
     
       20. The display of  claim 18 , wherein the first liquid crystal layer has a lateral area and the thickness is uniform across the lateral area.

Description:
This application claims the benefit of provisional patent application No. 62/717,628, filed Aug. 10, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with displays. 
     Electronic devices with displays may be used to display content for a user. If care is not taken, the components used in displaying content for a user in an electronic device may be unsightly and bulky and may not exhibit desired levels of optical performance. 
     SUMMARY 
     An electronic device may have a display that provides image light to a waveguide via an input coupler. The image light may propagate through the waveguide under the principle of total internal reflection. A first liquid crystal lens may be mounted to a first surface of the waveguide and a second liquid crystal lens may be mounted to a second surface of the waveguide. An output coupler may couple the image light out of the waveguide through the first liquid crystal lens. The second liquid crystal lens may convey world light from objects external to the electronic device to the first liquid crystal lens through the waveguide. 
     Control circuitry may control the first liquid crystal lens to exhibit a first index-of-refraction profile that applies a first optical power to the image light and the world light. The control circuitry may control the second liquid crystal lens to exhibit a second index-of-refraction profile that applies a second optical power to the world light that at least partially cancels out the first optical power. A linear polarizer may be interposed between the waveguide and the first liquid crystal lens to ensure the linear polarization aligns with the rubbing directions of the liquid crystal lenses. If desired, a third liquid crystal lens may be mounted to the first liquid crystal lens and a fourth liquid crystal lens may be mounted to the second liquid crystal lens. If the third and fourth liquid crystal lenses are employed, the linear polarizer may be omitted and both polarization states from the world light and the image light may be used. 
     The first and second liquid crystal lenses may include transparent substrates with aligned rubbing directions. Each liquid crystal lens may include two layers of liquid crystal molecules having antiparallel pretilt angles. The pretilt angles and rubbing directions of the first liquid crystal lens may be antiparallel to corresponding pretilt angles and rubbing directions of the second liquid crystal lens about the waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  2    is a diagram of an illustrative head-mounted device for a single eye in accordance with an embodiment. 
         FIG.  3    is a top view of an illustrative display system for a head-mounted device having a waveguide and liquid crystal lenses in accordance with an embodiment. 
         FIG.  4    is a top view of illustrative liquid crystal lenses of the type shown in  FIG.  3    in accordance with an embodiment. 
         FIGS.  5  and  6    are graphs showing how illustrative liquid crystal lenses may be adjusted so that their refractive indices vary as a function of position to produce desired lens profiles in accordance with an embodiment. 
         FIG.  7    is a top view of an illustrative display system for a head-mounted device having a waveguide and multiple pairs of liquid crystal lenses for covering multiple polarizations in accordance with an embodiment. 
         FIGS.  8  and  9    are top views of illustrative liquid crystal lenses of the type shown in  FIG.  7    in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as head-mounted devices and other devices may be used for augmented reality and virtual reality systems. These devices may include portable consumer electronics (e.g., portable electronic devices such as tablet computers, cellular telephones, glasses, other wearable equipment, etc.), head-up displays in cockpits, vehicles, etc., and display-based equipment (televisions, projectors, 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 device 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 device 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 or a scanning device in a display system to the eyes of a viewer. A waveguide such as a thin planar waveguide formed from one or more sheets of transparent material such as glass or plastic or other light guides may be included in the optical system to convey image light from the pixel arrays to the viewer. 
     The illumination system may include a light source that supplies illumination for the display. The illuminated display produces image light. An input optical coupler may be used to couple light from the light source into a waveguide in the illumination system. An output optical coupler may be used to couple display illumination out of the waveguide. Input and output couplers may also be used to couple image light from the display into a waveguide in the optical system and to couple the image light out of the waveguide for viewing by the viewer. 
     The input and output couplers for the head-mounted device may form structures such as Bragg gratings, prisms, angled transparent structures, and/or lenses that couple light into the waveguide and that couple light out of the waveguide. Input and output optical couplers may be formed from diffractive couplers such as volume holograms, other holographic coupling elements, or other diffractive coupling structures. The input and output couplers may, for example, be formed from thin or thick layers of photopolymers and/or other optical coupler structures in which holographic patterns are recorded using lasers. In some configurations, optical couplers may be formed from dynamically adjustable devices such as liquid crystal components (e.g., tunable liquid crystal gratings, polymer dispersed liquid crystal devices), or other adjustable optical couplers. 
     A schematic diagram of an illustrative head-mounted device is shown in  FIG.  1   . As shown in  FIG.  1   , head-mounted device  10  may have control circuitry  12 . Control circuitry  12  may include storage and processing circuitry for controlling the operation of head-mounted display  10 . Circuitry  12  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  12  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  12  and run on processing circuitry in circuitry  12  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 device  10  may include input-output circuitry  14 . Input-output circuitry  14  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 device  10  with user input. Input-output circuitry  14  may also be used to gather information on the environment in which head-mounted device  10  is operating. Output components in circuitry  14  may allow head-mounted device  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  14  may include one or more displays such as display(s)  18 . Display(s)  18  may be used to display images for a user of head-mounted device  10 . Display(s)  18  have pixel array(s) or laser scanning patterns 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)  18 . 
     Optical components  16  may be used in forming the optical system that presents images to the user. Components  16  may include static components such as waveguides, static optical couplers, and fixed lenses. Components  16  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  16  may be used in providing light to display(s)  18  to illuminate display(s)  18  and may be used in providing images from display(s)  18  to a user for viewing. In some configurations, one or more of components  16  may be stacked, so that light passes through multiple components in series. In other configurations, components may be spread out laterally (e.g., multiple displays may be arranged on a waveguide or set of waveguides using a tiled set of laterally adjacent couplers). Configurations may also be used in which both tiling and stacking are present. 
     Input-output circuitry  14  may include components such as input-output devices  22  for gathering data and user input and for supplying a user with output. Devices  22  may include sensors  26 , audio components  24 , and other components for gathering input from a user or the environment surrounding device  10  and for providing output to a user. Devices  22  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 in input-output devices  22  may gather images of the user&#39;s eyes and/or the environment surrounding the user. As an example, eye-facing cameras may be used by control circuitry  12  to gather images of the pupils and other portions of the eyes of the viewer. The locations of the viewer&#39;s pupils and the locations of the viewer&#39;s pupils relative to the rest of the viewer&#39;s eyes may be used to determine the locations of the centers of the viewer&#39;s eyes (i.e., the centers of the user&#39;s pupils) and the direction of view (gaze direction) of the viewer&#39;s eyes. 
     Sensors  26  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  26 , for example, control circuitry  12  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  26 . 
     If desired, sensors  26  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  24  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  22  may include haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, and other output components. Circuitry  14  may include wired and wireless communications circuitry  20  that allows head-mounted display  10  (e.g., control circuitry  12 ) 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  28  of  FIG.  2   . Support structure  28 , which may sometimes be referred to as a housing, 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  33  may be supported within support structure  28  and may be used to provide images from display(s)  18  to a user (see, e.g., the eyes of user  42  of  FIG.  2   ). With one illustrative configuration, display(s)  18  may be located in outer (edge) portions  38  of optical system  33  and may have one or more pixel arrays that produce images. Light associated with the images may be coupled into waveguides in outer portions  38  using input coupler systems. The light within the waveguide may traverse intermediate regions  32 . In central portion(s)  34  of system  33  (at the opposing ends of the waveguides from the input coupler systems and display(s)  18 ), 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  30  in direction  40  for viewing by user  42 . Portion(s)  34  of optical system  33  may be transparent, so that user  42  may view external objects such as object  36  through this region of system  33  while system  33  overlays computer-generated content (image content generated by control circuitry  12  of  FIG.  1   ) with objects such as object  36 . 
       FIG.  3    is a diagram of illustrative components that may be used in forming optical system  33  of device  10 . The diagram of  FIG.  3    includes components for one of the user&#39;s eyes. Device  10  may contain two sets of such components to present images to both of a user&#39;s eyes. 
     As shown in  FIG.  3   , device  10  may include one or more displays such as display(s)  18  for producing image light  44 . Image light  44  may be generated by illuminating a reflective display containing an array of pixels or using any other desired display components. The images presented on the array of pixels may be conveyed through lens  46  to input coupler  50  (e.g., at end  56  of waveguide  48 ), which couples image light  44  into waveguide  48  (e.g., a planar waveguide). The image light coupled into waveguide  48  is confined within waveguide  48  in accordance with the principle of total internal reflection and travels towards output coupler  52 , as indicated by light  54 . 
     Output coupler  52  (e.g., at end  58  of waveguide  48 ) couples light  54  (image light) out of waveguide  48  and towards viewer  42  (an eye of a user), as output light (output image light)  76 . Input coupler  50  and output coupler  52  may, for example, include structures such as Bragg gratings or other diffraction gratings that couple light into waveguides and that couple light out of the waveguides. Couplers  50  and  52  may be formed from volume holograms or other holographic coupling elements (e.g., thin layers of polymers and/or other optical coupler structures in which holographic patterns are recorded using lasers), prisms, angled transparent structures, lenses, or any other desired light coupling elements. Couplers  50  and  52  may have infinite focal lengths (e.g., couplers  50  and  52  may be plane-to-plane couplers) or may have associated finite focal lengths. Couplers  50  and/or  52  may be embedded within waveguide  48 , formed on surface  62  of waveguide  48 , and/or formed on surface  64  of waveguide  48 . 
     Light  80  from external objects such as object  36  may pass through waveguide  48  (e.g., at end  58  of waveguide  48 , within central region  34  of  FIG.  2   ). Light  80  may sometimes be referred to herein as scene light, real world light, or world light. Output image light  76  and world light  80  may both be provided to eye box  78  (e.g., an area at which user  42  of  FIG.  3    places their eye for viewing light from display(s)  18 ). Output image light  76  may be overlaid with world light  80  when viewed by the user at eye box  78 . 
     One or more lenses may be used to direct output image light  76  towards eye box  78  with desired optical characteristics (e.g., with a desired optical power, focal length, image depth, etc.). It may be desirable to be able to adjust these optical characteristics over time (e.g., based on changes in sensor data gathered using sensors  26  of  FIG.  1   ). Device  10  may include adjustable lens components such as adjustable lens components  66  and  68  of  FIG.  3    that can be adjusted in real time by control circuitry  12 . 
     Adjustable lens components  66  and  68 , which may sometimes be referred to as adjustable lenses, adjustable lens systems, adjustable optical systems, adjustable lens devices, tunable lenses, etc., may contain electrically adjustable material such as liquid crystal material, volume Bragg gratings, or other electrically modulated material that may be adjusted to produce customized lenses. Each of components  66  and  68  may contain an array of electrodes that apply electric fields to portions of a layer of liquid crystal material or other voltage-modulated optical material with an electrically adjustable index of refraction (sometimes referred to as an adjustable lens power or adjustable phase profile). By adjusting the voltages of signals applied to the electrodes, the index of refraction profile of components  66  and  68  may be dynamically adjusted. This allows the size, shape, and location of the lenses formed within components  66  and  68  to be adjusted. An example in which adjustable lens components  66  and  68  are formed from layers of liquid crystal material is described herein as an example. Adjustable lens components  66  and  68  may therefore be referred to herein as liquid crystal lenses  66  and  68 . 
     Control circuitry  12  may adjust liquid crystal lens  68  in real time to modulate output image light  76  with different desired optical characteristics over time (e.g., optical power, image depth, etc.). In general, it may be desirable to modulate output image light  76  without also modulating world light  80  (e.g., to adjust the optical characteristics of output image light  76  without also adjusting those optical characteristics of world light  80 ). 
     World light  80  passes through liquid crystal lens  66  prior to passing through waveguide  48 . Control circuitry  12  may control liquid crystal lens  66  to exhibit an index of refraction profile that is the opposite (reverse) of the index of refraction profile provided by liquid crystal lens  66  (e.g., so that the optical power provided by liquid crystal lens  66  to world light  80  cancels out the optical power provided by liquid crystal lens  68  to world light  80 ). In this way, output image light  76  may be modulated by liquid crystal lens  68  whereas any modulation by liquid crystal lens  68  on world light  80  is canceled out by modulation from liquid crystal lens  66  (e.g., control circuitry  12  may modulate output image light  76  from display(s)  18  without modulating world light  80  from external object  36 ). 
     If desired, device  10  may include additional lenses such as lenses  74  and  72 . As shown in  FIG.  3   , lens  74  may be mounted to liquid crystal lens  66  and lens  72  may be mounted to liquid crystal lens  68 . Lens  72  may serve to bias the optical power of liquid crystal lens  68  and may sometimes be referred to herein as bias lens  72 . For example, liquid crystal lens  68  may have an adjustable optical power of −1.5 D to 1.5 D and bias lens  72  may have an optical power of −1.5 D, which generates a combined optical power of −3 D to 0 D (e.g., because negative power is needed to bring an object at infinity closer to the eye). Lens  74  may be used to cancel the power of lens  72 . Lenses  74  and  72  may include fixed and/or adjustable lenses. Lens  74  may be a convex lens whereas lens  72  is a concave lens in one suitable arrangement. This is merely illustrative. In general, lenses  74  and  72  may include any desired lens structures (e.g., concave lenses, convex lenses, Fresnel lenses, etc.). Lens  74  and/or lens  72  may be omitted if desired. 
     In some scenarios, it may be desirable to provide output image light  76  with a particular linear polarization. In one arrangement, a first linear polarizer is interposed between display(s)  18  and input coupler  50  for providing image light  44  with a first linear polarization and a second linear polarizer is interposed between lens  74  and waveguide  48  for providing world light  80  with a second linear polarization. However, in practice, linearly-polarized light does not maintain its linear polarization state as it propagates down the length of waveguide  48  from end  56  to output coupler  52  (e.g., due to phase shifts generated at each reflection off at surfaces  62  and  64  of waveguide  48 ). This may lead to undesirable artifacts such as double images from display(s)  18 . 
     In order to maintain the linear polarization state of output image light  76  by the time the light reaches liquid crystal lens  68 , a linear polarizer such as linear polarizer  70  may be interposed between surface  64  of waveguide  48  and liquid crystal lens  68 . This allows light  54  to retain its polarization before passing through liquid crystal lens  68 . Linear polarizer  70  also serves to linearly polarize world light  80 . In the example of  FIG.  3   , linear polarizer  70  has an absorption axis that extends parallel to the X-axis (as shown by arrows  86 ) and a transmission axis that extends parallel to the Y-axis (e.g., as shown by arrow  88 ). Adhesive such as optically-transparent adhesive films or curable liquids may be used to attach lens  74  and liquid crystal lens  66 , and likewise to attach waveguide  48  and linear polarizer  70 , and/or to attach liquid crystal lens  68  to lens  72 , etc. A small air gap may be maintained between surface  62  of waveguide  48  and liquid crystal lens  66  and surface  64  of waveguide  48  and linear polarizer  70  to generate an index of refraction variation and to allow total internal reflection of light  54  prior to interacting with out coupler  52  if desired. Other means may be used to generate a reflection interface at the waveguide surface as well (e.g. dielectric coatings, metallic coatings, material selection, etc.). 
     Liquid crystal lens  66  and liquid crystal lens  68  may each include a respective pair of liquid crystal cells. Each liquid crystal cell may include a corresponding layer of liquid crystal molecules. The pretilt angle of the liquid crystal molecules in the first liquid crystal cell of liquid crystal lens  66  may be opposite to the pretilt angle of the liquid crystal molecules in the second liquid crystal cell of liquid crystal lens  66 . Similarly, the pretilt angle of the liquid crystal molecules in the first liquid crystal cell of lens  68  may be opposite to the pretilt angle of the liquid crystal molecules in the second liquid crystal cell of lens  68 . 
     The pretilt angles of the liquid crystal molecules can be set by providing substrates within liquid crystal lenses  66  and  68  with particular rubbing directions (e.g., the liquid crystal molecules may contact the substrates and align along the corresponding rubbing directions). In the example of  FIG.  3   , the rubbing directions of liquid crystal lens  66  may extend parallel to the Y-axis (e.g., as shown by arrows  82 ). Similarly, the rubbing directions of liquid crystal lens  68  may also extend parallel to the Y-axis (e.g., as shown by arrows  84 ). In this way, the rubbing directions of liquid crystal lenses  66  and  68  may both extend parallel to the transmission axis of linear polarizer  88 . 
     The example of  FIG.  3    is merely illustrative. In general, the transmission axis of linear polarizer  70  and the rubbing directions of liquid crystal lenses  66  and  68  may extend parallel to any desired axes (e.g., such that the rubbing directions of liquid crystal lens  66  extend parallel/antiparallel to the rubbing directions of liquid crystal lens  68  and parallel to the transmission axis of linear polarizer  70 ). However, if care is not taken, off-axis light artifacts from liquid crystal lens  68  may exacerbate off-axis light artifacts from liquid crystal lens  66  to create undesirable image artifacts in world light  80  at eye box  78 . 
     In order to mitigate these effects, the pretilt angles of the liquid crystal molecules in liquid crystal lens  66  may be opposite to the corresponding pretilt angles in liquid crystal lens  68  (e.g., with respect to waveguide  48 ).  FIG.  4    is a cross-sectional diagram showing how the structures of liquid crystal lens  66  may be arranged antiparallel to the structures of liquid crystal lens  68 . 
     As shown in  FIG.  4   , liquid crystal lens  66  may include a pair of liquid crystal cells such as liquid crystal cell  90  and liquid crystal cell  92 . Liquid crystal cell  90  may have a layer of voltage-modulated optical material such as liquid crystal layer  110 . Liquid crystal layer  110  may be interposed between transparent substrates such as upper substrate  106  and lower substrate  114 . Substrates  106  and  114  may be formed from clear glass, sapphire or other transparent crystalline material, transparent plastic, thin coatings or films, other transparent layers, and/or combinations of these. 
     Liquid crystal cell  90  may have patterns of electrodes such as electrode layers  108  and  112  that can be supplied with signals from control circuitry  12  to produce desired voltages on liquid crystal cell  90 . Electrode layer  108  may be patterned on substrate  106  whereas electrode layer  112  may be patterned on substrate  114 . In one suitable arrangement, electrode layer  108  may include a common electrode (e.g., a blanket or plane of conductive material on substrate  106 ) whereas electrode layer  112  includes elongated electrodes (e.g., strip-shaped electrodes or finger electrodes) on substrate  114 . The strip-shaped electrodes may extend parallel to the X-axis of  FIG.  4   , for example, and may sometimes be referred to herein as vertical electrodes  112 . 
     At each location of the vertical electrodes  112 , a desired voltage may be applied across liquid crystal layer  110  by supplying a first voltage to electrodes  112  and a second voltage (e.g., a ground voltage) to common electrode  108 . The liquid crystal between the two electrode layers will receive an applied electric field with a magnitude that is proportional to the difference between the first and second voltages on the electrode layers. By controlling the voltages on electrodes  112  and  108 , the index of refraction of liquid crystal layer  110  of liquid crystal cell  90  can be dynamically adjusted to produce customized lenses. 
     When an electric field is applied to the liquid crystals of layer  110 , the liquid crystals change orientation. The speed at which a given liquid crystal material can be reoriented is limited by factors such as the thickness of layer  110 . To increase the tuning speed of liquid crystal layer  110  while still achieving a suitable tuning range, liquid crystal lens  66  may include two or more liquid crystal cells stacked on top of one another. This type of arrangement is illustrated in  FIG.  4   . 
     As shown in  FIG.  4   , liquid crystal cell  92  of liquid crystal lens  66  may be stacked under liquid crystal cell  90 . Liquid crystal cell  92  may include liquid crystal layer  120 . Liquid crystal layer  120  may be interposed between transparent substrates such as upper substrate  116  and lower substrate  124 . Upper substrate  116  of cell  92  may be mounted (attached) to lower substrate  114  of cell  90  (e.g., using optically transparent adhesive or other structures). 
     Substrates  116  and  124  may be formed from clear glass, sapphire or other transparent crystalline material, transparent plastic, thin coatings or films, other transparent layers, and/or combinations of these. Liquid crystal cell  92  may have patterns of electrodes such as electrode layers  118  and  122  that can be supplied with signals from control circuitry  12  to produce desired voltages on liquid crystal cell  92 . Electrode layer  118  may be patterned on substrate  116  whereas electrode layer  122  may be patterned on substrate  124 . In one suitable arrangement, electrode layer  122  may include a common electrode (e.g., a blanket or plane of conductive material on substrate  124 ) whereas electrode layer  118  includes elongated electrodes (e.g., strip-shaped electrodes or finger electrodes) on substrate  116 . The strip-shaped electrodes may extend parallel to the Y-axis of  FIG.  4   , for example, and may sometimes be referred to herein as horizontal electrodes  118 . 
     At each location of the horizontal electrodes  118 , a desired voltage may be applied across liquid crystal layer  120  by supplying a first voltage to electrodes  118  and a second voltage (e.g., a ground voltage) to common electrode  122 . The liquid crystal between the two electrode layers will receive an applied electric field with a magnitude that is proportional to the difference between the first and second voltages on the electrodes. By controlling the voltages on electrodes  118  and  124 , the index of refraction of liquid crystal layer  120  of liquid crystal cell  92  can be dynamically adjusted to produce customized lenses. 
     If desired, the rubbing direction and pretilt angles of liquid crystal cells  90  and  92  in lens  66  may be antiparallel with respect to each other. In particular, liquid crystal molecules  126 A of upper liquid crystal cell  90  may have a first pretilt angle and liquid crystal molecules  126 B of lower liquid crystal cell  92  may have a second pretilt angle that is opposite to the first pretilt angle (e.g., the optical axis of liquid crystal molecules  126 B may be oriented at a positive angle with respect to the Z-axis of  FIG.  4    whereas the optical axis of liquid crystal molecules  126 A are oriented at an equal but negative angle with respect to the Z-axis when no voltage is applied to the electrodes). In other words, liquid crystal molecules  126 B may have a positive pretilt angle with respect to the substrate plane whereas liquid crystal molecules  126 A are in the same plane formed by the optical axis of liquid crystal molecules  126 B and surface normal of the substrate, and have an equal but negative pretilt angle with respect to the substrate plane when no voltage is applied to the electrodes (e.g., liquid crystal molecules  126 A and liquid crystal molecules  126 B may have opposite pretilt angles). 
     In order to obtain these opposing pretilt angles, the upper substrate of each liquid crystal cell may be formed using opposite (antiparallel) rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite (antiparallel) rubbing directions. As shown in  FIG.  4   , substrates  106  and  124  of lens  66  may be have a first rubbing direction, as shown by arrows  150  and  156 , whereas substrates  114  and  116  have a second rubbing direction, as shown by arrows  152  and  154  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  152  and  154 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  82  of  FIG.  3   . 
     Overlapping portions of liquid crystal layers  110  and  120  in lens  66  may be controlled using the same or different voltages to achieve the desired index of refraction at that portion of lens  66 . For example, a first voltage V 1  may be applied across a given portion of upper liquid crystal layer  110  whereas a second voltage V 2  may be applied across that portion of lower liquid crystal layer  120 . Voltages V 1  and V 2  may be different or may be the same. Control circuitry  12  may determine the magnitudes and ratio of V 1  to V 2  based on the desired index of refraction at that portion of the liquid crystal lens  66  (e.g., to provide lens  66  with an opposite optical power to lens  68 ). 
     Surface  100  of substrate  124  may be mounted to surface  62  of waveguide  48  of  FIG.  3   . If desired, a small air gap may be maintained between substrate  124  and surface  62  (e.g., to allow for and index of refraction difference with waveguide  48  that enables total internal reflection within waveguide  48 ). Substrate  124  may be considered to be “mounted” to (at) waveguide  48  even when this small air gap is present (e.g., no adhesive may be present between the waveguide and the liquid crystal lenses closest to the waveguide). Lens  74  of  FIG.  3    may be mounted to surface  98  of substrate  106 . Surface  102  of liquid crystal lens  68  may be mounted to surface  64  of waveguide  48 . Similarly, surface  102  may be considered to be “mounted” to waveguide  48  even when a small air gap is present between liquid crystal lens  68  and waveguide  48  to allow for total internal reflection. Waveguide  48  has been omitted from  FIG.  4    for the sake of clarity. The pretilt angles of the liquid crystal molecules in lens  68  may be opposite to the corresponding pretilt angles of the liquid crystal molecules in lens  66  with respect to waveguide  48  (e.g., the liquid crystal molecules in the liquid crystal layer with a certain single electrode direction in lens  66  may have pretilt angles opposite to the liquid crystal layer with the same electrode direction in lens  68 , etc.). 
     As shown in  FIG.  4   , liquid crystal lens  68  may include a pair of liquid crystal cells such as liquid crystal cell  94  and liquid crystal cell  96 . Liquid crystal cell  94  may have liquid crystal layer  132 . Liquid crystal layer  132  may be interposed between transparent substrates such as upper substrate  128  and lower substrate  136 . Substrates  128  and  136  may be formed from clear glass, sapphire or other transparent crystalline material, transparent plastic, thin coatings or films, other transparent layers, and/or combinations of these. 
     Liquid crystal cell  94  may have patterns of electrodes such as electrode layers  130  and  134  that can be supplied with signals from control circuitry  12  to produce desired voltages on liquid crystal cell  94 . Electrode layer  130  may be patterned on substrate  128  whereas electrode layer  134  may be patterned on substrate  136 . In one suitable arrangement, electrode layer  130  may include a common electrode (e.g., a blanket or plane of conductive material on substrate  128 ) whereas electrode layer  134  includes elongated electrodes (e.g., strip-shaped electrodes or finger electrodes) on substrate  136 . The strip-shaped electrodes may extend parallel to the Y-axis of  FIG.  4   , for example, and may sometimes be referred to herein as horizontal electrodes  134 . 
     At each location of the horizontal electrodes  134 , a desired voltage may be applied across liquid crystal layer  132  by supplying a first voltage to electrodes  134  and a second voltage (e.g., a ground voltage) to common electrode  130 . The liquid crystal between the two electrode layers will receive an applied electric field with a magnitude that is proportional to the difference between the first and second voltages on the electrodes. By controlling the voltages on electrodes  130  and  134 , the index of refraction of liquid crystal layer  132  of liquid crystal cell  94  can be dynamically adjusted to produce customized lenses. 
     As shown in  FIG.  4   , liquid crystal cell  96  of liquid crystal lens  68  may be stacked under liquid crystal cell  94 . Liquid crystal cell  96  may include liquid crystal layer  142 . Liquid crystal layer  142  may be interposed between transparent substrates such as upper substrate  138  and lower substrate  146 . Upper substrate  138  of cell  96  may be mounted (attached) to lower substrate  136  of cell  94  (e.g., using optically transparent adhesive or other structures). 
     Substrates  138  and  146  may be formed from clear glass, sapphire or other transparent crystalline material, transparent plastic, thin coatings or films, other transparent layers, and/or combinations of these. Liquid crystal cell  96  may have patterns of electrodes such as electrode layers  140  and  144  that can be supplied with signals from control circuitry  12  to produce desired voltages on liquid crystal cell  96 . Electrode layer  140  may be patterned on substrate  138  whereas electrode layer  144  may be patterned on substrate  146 . In one suitable arrangement, electrode layer  144  may include a common electrode (e.g., a blanket or plane of conductive material on substrate  146 ) whereas electrode layer  140  includes elongated electrodes (e.g., strip-shaped electrodes or finger electrodes) on substrate  138 . The strip-shaped electrodes may extend parallel to the X-axis of  FIG.  4   , for example, and may sometimes be referred to herein as vertical electrodes  140 . 
     At each location of the vertical electrodes  140 , a desired voltage may be applied across liquid crystal layer  142  by supplying a first voltage to electrodes  140  and a second voltage (e.g., a ground voltage) to common electrode  144 . The liquid crystal between the two electrode layers will receive an applied electric field with a magnitude that is proportional to the difference between the first and second voltages on the electrodes. By controlling the voltages on electrodes  140  and  144 , the index of refraction of liquid crystal layer  142  of liquid crystal cell  96  can be dynamically adjusted to produce customized lenses. 
     If desired, the rubbing direction and pretilt angles of liquid crystal cells  94  and  96  in lens  68  may be antiparallel or opposite with respect to each other. In particular, liquid crystal molecules  148 A of upper liquid crystal cell  94  may have a first pretilt angle and liquid crystal molecules  148 B of lower liquid crystal cell  96  may have a second pretilt angle that is opposite to the first pretilt angle (e.g., the optical axis of liquid crystal molecules  148 B may be oriented at a positive angle with respect to the Z-axis of  FIG.  4    whereas the optical axis of liquid crystal molecules  148 A are oriented at an equal but negative angle with respect to the Z-axis when no voltage is applied to the electrodes). 
     In order to obtain these opposing pretilt angles, the upper substrate of each liquid crystal cell may be formed using opposite rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite rubbing directions. As shown in  FIG.  4   , substrates  128  and  146  of lens  68  may be have a first rubbing direction, as shown by arrows  158  and  164 , whereas substrates  136  and  138  have a second rubbing direction, as shown by arrows  160  and  162  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  158  and  164 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  84  of  FIG.  3   . 
     In this way, the rubbing direction of substrate  106  of lens  66  may be parallel to the rubbing direction of substrate  124  of lens  66  and substrates  128  and  146  of lens  68 . Similarly, the rubbing direction of substrate  106  of lens  66  may be antiparallel to the rubbing direction of substrates  114  and  116  of lens  66  and substrates  136  and  138  of lens  68 . In addition, the pretilt angle of liquid crystal molecules  126 B in lens  66  may be opposite to the pretilt angle of liquid crystal molecules  148 A. Similarly, the pretilt angle of liquid crystal molecules  126 A in lens  66  may be opposite to the pretilt angle of liquid crystal molecules  148 B. In other words, the pretilt angles of liquid crystal lens  66  may be opposite to the corresponding pretilt angles of liquid crystal lens  68  about waveguide  48 . 
     Overlapping portions of liquid crystal layers  132  and  142  in lens  68  may be controlled using the same or different voltages to achieve the desired index of refraction at that portion of lens  68 . For example, a first voltage V 3  may be applied across a given portion of upper liquid crystal layer  132  whereas a second voltage V 4  may be applied across that portion of lower liquid crystal layer  142 . Voltages V 3  and V 4  may be different or may be the same. Control circuitry  12  may determine the ratio of V 3  to V 4  based on the desired index of refraction at that portion of the liquid crystal lens  66  (e.g., based on the disposition of the user&#39;s eyes  16 ). This may serve to configure lens  68  to provide output image light  76  ( FIG.  3   ) with desired optical characteristics (e.g., optical power, image depth, etc.). Control circuitry  12  may determine voltages V 1  and V 2  of lens  66  to control lens  66  to reverse the optical characteristics imparted on world light  80  by lens  68  (e.g., control circuitry  12  may provide voltages V 1  and V 2  to counteract the optical characteristics of lens  68 ). 
     The example of  FIG.  4    is merely illustrative. If desired, electrode layers  108  and  122  of lens  66  may be formed using strip-shaped electrodes (e.g., where one electrode layer is formed using horizontal strips and the other is formed using vertical strips) whereas electrode layers  112  and  118  are formed using common (planar) electrodes. Similarly, if desired, electrode layers  130  and  144  of lens  68  may be formed using strip-shaped electrodes (e.g., where one electrode layer is formed using horizontal strips and the other is formed using vertical strips) whereas electrode layers  134  and  140  are formed using common (planar) electrodes. Rubbing directions  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 , and  164  may extend parallel to any desired axis (e.g., as long as rubbing directions  150 ,  156 ,  158 , and  164  are parallel to each other and antiparallel to rubbing directions  152 ,  154 ,  160 , and  162 ). When arranged in this way, lens  68  may modulate output image light  76  provided to eye box  78  ( FIG.  3   ) while preventing light artifacts from lens  66  from adding with light artifacts from lens  68 . 
       FIGS.  5  and  6    show examples of illustrative index-of-refraction profiles that may be generated by liquid crystal lenses  66  and/or  68  of  FIG.  4   . In the example of  FIG.  5   , curve  166  plots an exemplary refractive index n generated for liquid crystal lens  66 . As shown by curve  166 , refractive index n has been varied continuously between peripheral lens edges Y 1  and Y 2 . This may provide liquid crystal lens  66  with a desired optical power and/or any other desired optical characteristics. 
     Curve  168  plots an exemplary refractive index n generated for liquid crystal lens  68 . As shown by curve  168 , refractive index n has been varied inversely to the refractive index associated with curve  166  (e.g., refractive index n for lens  66  may be relatively low at locations where refractive index n for lens  68  is relatively high and refractive index n for lens  66  may be relatively high at locations where refractive index n for lens  68  is relatively low). This may, for example, provide liquid crystal lens  68  with an optical power opposite to the optical power of liquid lens  66 . In this way, the optical power of lens  68  may cancel out the optical power applied to world light  80  ( FIG.  3   ) by lens  66  so that the world light is not modulated by the time it reaches eye box  78 . Control circuitry  12  may adjust curve  166  over time (e.g., by adjusting voltages V 1  and V 2  of  FIG.  4   ) and may adjust curve  168  to counteract these changes (e.g., by adjusting voltages V 3  and V 4  of  FIG.  4   ). 
     In the example of  FIG.  6   , refractive index n has been varied discontinuously to produce an index-of-refraction profile appropriate for forming a Fresnel lens. When the index-of-refraction profile of  FIG.  6    is applied to lens  66 , an inverse profile may be applied to lens  68 , for example. These examples are merely illustrative. If desired, other suitable index-of-refraction profiles may be used using adjustable lens components of the type shown in  FIGS.  3  and  4   . 
     In the example of  FIG.  4   , liquid crystal lenses  66  and  68  include electrodes that extend in multiple directions (e.g., horizontal electrodes  118  and  134  and vertical electrodes  112  and  140 ).  FIGS.  5  and  6    illustrate phase profile modulation for lenses  66  and  68  along only a single direction for the sake of simplicity (e.g., the Y-dimension of  FIGS.  3 - 6   ). In general, curves  166  and  168  of  FIG.  5    and the curve of  FIG.  6    may lie on three-dimensional index-of-refraction surfaces that are controlled/adjusted using control circuitry  12  (e.g., the index of refraction of lenses  66  and  68  may be actively varied in both the X and Y dimensions of  FIGS.  3 - 6   ). In another suitable arrangement, liquid crystal lenses  66  and  68  of  FIG.  4    may include electrodes that extend only in one direction, allowing lenses  66  and  68  to modulate their phase profiles only along one direction. 
     In the example of  FIGS.  3  and  4   , light of a single linear polarization is provided to eye box  78 . It may sometimes be desirable to be able to provide light of additional polarizations to eye box  78 .  FIG.  7    is a diagram showing how waveguide  48  may be used to provide unpolarized light to eye box  78 . 
     As shown in  FIG.  7   , four liquid crystal lenses such as liquid crystal lenses  170 ,  172 ,  174 , and  176  may be mounted adjacent to waveguide  48  at end  58 . Liquid crystal lens  172  may be mounted to of waveguide  48 . Liquid crystal lens  170  may be mounted to liquid crystal lens  172 . Optional lens  74  may be mounted to liquid crystal lens  170 . Liquid crystal lens  174  may be mounted to waveguide  48 . Liquid crystal lenses  172  and  174  may be referred to as being “mounted” to waveguide  48  even though small air gaps may be present between the liquid crystal lenses and the waveguide to maintain total internal reflection within the waveguide. Liquid crystal lens  176  may be mounted to liquid crystal lens  174 . Optional lens  72  may be mounted to liquid crystal lens  176 . Linear polarizer  70  of  FIG.  3    may be omitted in this example, allowing unpolarized output image light  76  and world light  80  to pass to eye box  78 . 
     Liquid crystal lenses  174  and  176  may be used to provide output image light  76  with desired optical power. Liquid crystal lenses  170  and  172  may be used to cancel out the optical power provided to world light  80  by lenses  174  and  176  (e.g., so that world light  80  is not modulated by the time it reaches eye box  78 ). For example, control circuitry  12  may control liquid crystal lens  172  to cancel out the optical power provided by liquid crystal lens  174  (e.g., by providing lens  172  with an inverse index-of-refraction profile relative to that provided to lens  174 ). Similarly, control circuitry  12  may control liquid crystal lens  170  to cancel out the optical power provided by liquid crystal lens  176  (e.g., by providing lens  170  with an inverse index-of-refraction profile relative to that provided to lens  176 ). 
     The pretilt angles of the liquid crystal molecules and the rubbing directions of the substrates in lenses  170  and  176  may extend along orthogonal directions relative to the pretilt angles of the liquid crystal molecules and the rubbing directions of the substrates in lenses  172  and  174 . For example, the rubbing directions in lens  172  may extend parallel to the Y-axis of  FIG.  7   , as shown by arrows  180 . Similarly, the rubbing directions in lens  174  may also extend parallel to the Y-axis, as shown by arrows  182 . At the same time, the rubbing directions in lens  170  may extend parallel to the X-axis, as shown by arrows  178 . Similarly, the rubbing directions in lens  176  may also extend parallel to the X-axis, as shown by arrows  184 . This may allow the lenses to modulate output image light  76  with multiple polarizations and to pass un-modulated world light  80  to eye box  78  with multiple polarizations. 
     The example of  FIG.  7    is merely illustrative. In general, the rubbing directions of lenses  170 ,  172 ,  174 , and  176  may extend parallel to any desired axes (e.g., such that the rubbing directions of liquid crystal lens  172  extend along an axis parallel/antiparallel to the rubbing directions of liquid crystal lens  174  and orthogonal to the rubbing directions of liquid crystal lenses  176  and  170 ). 
       FIG.  8    is a cross-sectional diagram showing how the structures of liquid crystal lens  172  may be arranged antiparallel to the structures of liquid crystal lens  174 . As shown in  FIG.  8   , liquid crystal lens  172  may include a pair of liquid crystal cells such as liquid crystal cell  186  and liquid crystal cell  188 . Liquid crystal cell  186  may include liquid crystal layer  204 . Liquid crystal layer  204  may be interposed between transparent substrates such as upper substrate  202  and lower substrate  206 . 
     Liquid crystal cell  188  of liquid crystal lens  172  may be stacked under liquid crystal cell  186 . Liquid crystal cell  188  may include liquid crystal layer  210 . Liquid crystal layer  210  may be interposed between transparent substrates such as upper substrate  208  and lower substrate  212 . The electrode layers of lenses  172  and  174  have been omitted from  FIG.  8    for the sake of clarity. In general, lenses  172  and  174  may be provided with electrode layers similar to electrode layers  108 ,  112 ,  118 , and  122  of  FIG.  4   . 
     The rubbing directions and pretilt angle of liquid crystal cell  186  may be antiparallel to the rubbing directions and pretilt angle of liquid crystal cell  188 . In particular, liquid crystal molecules  214 A of upper liquid crystal cell  186  may have a first pretilt angle and liquid crystal molecules  214 B of lower liquid crystal cell  188  may have a second pretilt angle that is opposite to the first pretilt angle. Similarly, the upper substrate of each liquid crystal cell in lens  172  may be formed using opposite rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite rubbing directions. 
     As shown in  FIG.  8   , substrates  202  and  212  of lens  172  may be have a first rubbing direction, as shown by arrows  216  and  222 , whereas substrates  206  and  208  have a second rubbing direction, as shown by arrows  218  and  220  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  216  and  222 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  180  of  FIG.  7   . 
     Surface  196  of substrate  212  may be mounted to surface  62  of waveguide  48  of  FIG.  7    (e.g., allowing for a small air gap between substrate  212  and waveguide  48 ). Lens  170  of  FIG.  7    may be mounted to surface  194  of substrate  202 . Surface  198  of lens  174  may be mounted to surface  64  of waveguide  48 . Waveguide  48  has been omitted from  FIG.  8    for the sake of clarity. The pretilt angles of the liquid crystal molecules in lens  174  may be opposite to the corresponding pretilt angles of the liquid crystal molecules in lens  172  about waveguide  48  (e.g., the liquid crystal molecules in the liquid crystal layer with a certain single electrode direction in lens  174  may have pretilt angles opposite to the liquid crystal layer with the same electrode direction in lens  172 , etc.). 
     As shown in  FIG.  8   , lens  174  may include a pair of liquid crystal cells such as liquid crystal cell  190  and liquid crystal cell  192 . Liquid crystal cell  190  may include liquid crystal layer  226 . Liquid crystal layer  226  may be interposed between transparent substrates such as upper substrate  224  and lower substrate  228 . Liquid crystal cell  192  of liquid crystal lens  174  may be stacked under liquid crystal cell  190 . Liquid crystal cell  192  may include liquid crystal layer  232 . Liquid crystal layer  232  may be interposed between transparent substrates such as upper substrate  230  and lower substrate  234 . 
     The rubbing directions and pretilt angle of liquid crystal cell  190  may be antiparallel or opposite to the rubbing directions and pretilt angle of liquid crystal cell  192 . In particular, liquid crystal molecules  236 A of upper liquid crystal cell  190  may have a first pretilt angle and liquid crystal molecules  236 B of lower liquid crystal cell  192  may have a second pretilt angle that is opposite to the first pretilt angle. Similarly, the upper substrate of each liquid crystal cell in lens  174  may be formed using opposite rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite rubbing directions. 
     As shown in  FIG.  8   , substrates  224  and  234  of lens  174  may be have a first rubbing direction, as shown by arrows  238  and  244 , whereas substrates  228  and  230  have a second rubbing direction, as shown by arrows  240  and  242  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  238  and  244 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  182  of  FIG.  7   . 
     In this way, the rubbing direction of substrate  202  of lens  172  may be parallel to the rubbing direction of substrate  212  of lens  172  and substrates  224  and  234  of lens  174 . Similarly, the rubbing direction of substrate  202  of lens  172  may be antiparallel to the rubbing direction of substrates  206  and  208  of lens  172  and substrates  228  and  230  of lens  174 . In addition, the pretilt angle of liquid crystal molecules  214 B in lens  172  may be opposite to the pretilt angle of liquid crystal molecules  236 A. Similarly, the pretilt angle of liquid crystal molecules  214 A in lens  66  may be opposite to the pretilt angle of liquid crystal molecules  236 B. In other words, the pretilt angles of liquid crystal lens  172  may be opposite to the corresponding pretilt angles of liquid crystal lens  174  about waveguide  48  (e.g., as long as the liquid crystal cells have the same electrode directions). 
     The example of  FIG.  8    is merely illustrative. Rubbing directions  216 ,  218 ,  220 ,  222 ,  238 ,  240 ,  242 , and  244  may extend parallel to any desired axis (e.g., so long as rubbing directions  216 ,  222 ,  238 , and  244  are parallel to each other, antiparallel to rubbing directions  218 ,  220 ,  240 , and  242 , and orthogonal to the rubbing directions of lenses  170  and  176  of  FIG.  7   ). When arranged in this way, lens  174  may modulate output image light  76  provided to eye box  78  ( FIG.  7   ) while preventing light artifacts from lens  174  from adding with light artifacts from lens  172 . 
       FIG.  9    is a cross-sectional diagram showing how the structures of liquid crystal lens  170  may be arranged antiparallel to the structures of liquid crystal lens  176  (e.g., the plane of the page of  FIG.  9    my lie in the X-Z plane of  FIGS.  3 - 8   ). As shown in  FIG.  9   , liquid crystal lens  170  may include a pair of liquid crystal cells such as liquid crystal cell  246  and liquid crystal cell  248 . Liquid crystal cell  246  may include liquid crystal layer  264 . Liquid crystal layer  264  may be interposed between transparent substrates such as upper substrate  262  and lower substrate  266 . 
     Liquid crystal cell  248  of liquid crystal lens  170  may be stacked under liquid crystal cell  246 . Liquid crystal cell  248  may include liquid crystal layer  270 . Liquid crystal layer  270  may be interposed between transparent substrates such as upper substrate  268  and lower substrate  272 . The electrode layers of lenses  170  and  176  have been omitted from  FIG.  9    for the sake of clarity. If desired, lenses  170  and  176  may be provided with electrode layers similar to electrode layers  108 ,  112 ,  118 , and  122  of  FIG.  4   . 
     The rubbing directions and pretilt angle of liquid crystal cell  246  may be antiparallel to the rubbing directions and pretilt angle of liquid crystal cell  248 . In particular, liquid crystal molecules  274 A of upper liquid crystal cell  246  may have a first pretilt angle and liquid crystal molecules  274 B of lower liquid crystal cell  248  may have a second pretilt angle that is opposite to the first pretilt angle (e.g., orthogonal to the pretilt angles of liquid crystal molecules  214 A,  214 B,  236 A, and  236 B of  FIG.  8   ). Similarly, the upper substrate of each liquid crystal cell in lens  170  may be formed using opposite rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite rubbing directions. 
     As shown in  FIG.  9   , substrates  262  and  272  of lens  170  may be have a first rubbing direction, as shown by arrows  276  and  282 , whereas substrates  266  and  268  have a second rubbing direction, as shown by arrows  278  and  280  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  276  and  282 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  178  of  FIG.  7    and orthogonal to the rubbing directions of lenses  172  and  174  of  FIG.  8   . 
     Surface  256  of substrate  272  may be mounted to surface  194  of lens  172  ( FIG.  8   ). Lens  74  of  FIG.  7    may be mounted to surface  254  of substrate  262 . Surface  258  of lens  176  may be mounted to surface  200  of lens  174  ( FIG.  8   ). Lens  72  of  FIG.  7    may be mounted to surface  260  of substrate  294 . Waveguide  48 , lens  172 , and lens  174  have been omitted from  FIG.  9    for the sake of clarity. The pretilt angles of the liquid crystal molecules in lens  170  may be opposite to the corresponding pretilt angles of the liquid crystal molecules in lens  176  about waveguide  48  (e.g., the liquid crystal molecules with a certain single electrode direction in the liquid crystal layer may have pretilt angles opposite to the liquid crystal layer with the same electrode direction in lens  170 , etc.). 
     As shown in  FIG.  9   , lens  176  may include a pair of liquid crystal cells such as liquid crystal cell  250  and liquid crystal cell  252 . Liquid crystal cell  250  may include liquid crystal layer  286 . Liquid crystal layer  286  may be interposed between transparent substrates such as upper substrate  284  and lower substrate  288 . Liquid crystal cell  252  of liquid crystal lens  176  may be stacked under liquid crystal cell  250 . Liquid crystal cell  252  may include liquid crystal layer  292 . Liquid crystal layer  292  may be interposed between transparent substrates such as upper substrate  290  and lower substrate  294 . 
     The rubbing directions and pretilt angle of liquid crystal cell  250  may be antiparallel or opposite to the rubbing directions and pretilt angle of liquid crystal cell  252 . In particular, liquid crystal molecules  296 A of upper liquid crystal cell  250  may have a first pretilt angle and liquid crystal molecules  296 B of lower liquid crystal cell  252  may have a second pretilt angle that is opposite to the first pretilt angle. Similarly, the upper substrate of each liquid crystal cell in lens  176  may be formed using opposite rubbing directions and the lower substrate of each liquid crystal cell may be formed using opposite rubbing directions. 
     As shown in  FIG.  9   , substrates  284  and  294  of lens  176  may be have a first rubbing direction, as shown by arrows  298  and  304 , whereas substrates  288  and  290  have a second rubbing direction, as shown by arrows  300  and  302  (e.g., a rubbing direction antiparallel to the rubbing direction associated with arrows  298  and  304 ). These rubbing directions may, for example, extend parallel/antiparallel to arrows  184  of  FIG.  7    and orthogonal to the rubbing directions of lenses  172  and  174  of  FIG.  8   . 
     In this way, the rubbing direction of substrate  262  of lens  170  may be parallel to the rubbing direction of substrate  272  of lens  170  and substrates  284  and  294  of lens  176 . Similarly, the rubbing direction of substrate  262  of lens  170  may be antiparallel to the rubbing direction of substrates  266  and  268  of lens  170  and substrates  288  and  290  of lens  176 . In addition, the pretilt angle of liquid crystal molecules  274 B in lens  170  may be opposite to the pretilt angle of liquid crystal molecules  296 A. Similarly, the pretilt angle of liquid crystal molecules  274 A in lens  170  may be opposite to the pretilt angle of liquid crystal molecules  296 B. In other words, the pretilt angles of liquid crystal lens  170  may be opposite to the corresponding pretilt angles of liquid crystal lens  176  about waveguide  48 . 
     The example of  FIG.  9    is merely illustrative. Rubbing directions  276 ,  278 ,  280 ,  282 ,  298 ,  300 ,  302 , and  304  may extend parallel to any desired axis (e.g., so long as rubbing directions  276 ,  282 ,  298 , and  304  are parallel to each other, antiparallel to rubbing directions  278 ,  280 ,  300 , and  302 , and orthogonal to the rubbing directions of lenses  172  and  174  of  FIGS.  7  and  8   ). When arranged in this way, lens  176  may modulate output image light  76  provided to eye box  78  ( FIG.  7   ) while preventing light artifacts from lens  170  from adding with light artifacts from lens  176 . This may allow also multiple polarizations of light to pass to eye box  78  of  FIG.  7   . 
     In general, lenses  170 ,  172 ,  174 , and  176  of  FIG.  7    may be stacked in any desired order. More than four liquid crystal lenses may be stacked at end  58  of waveguide  48  for modulating output image light  76  without modulating world light  80  if desired. By configuring device  10  in this way (e.g., using the arrangements of  FIGS.  1 - 9   ), image light from display(s)  18  may be modulated with desired optical characteristics (e.g., optical power) without modulating world light  80 , while also minimizing light artifacts associated with the liquid crystal lenses and while eliminating polarization loss associated with light propagating down the length of waveguide  48 . 
     Substrates  202 ,  206 ,  208 ,  212 ,  224 ,  228 ,  230 , and  234  of  FIG.  8   , substrates  262 ,  266 ,  268 ,  272 ,  284 ,  288 ,  290 , and  294  of  FIG.  9   , and substrates  106 ,  114 ,  116 ,  124 ,  128 ,  136 ,  138 , and  146  of  FIG.  4    may each be referred to as “substrates,” “substrate layers,” or “transparent layers” herein (e.g., substrate  202 , transparent layer  206 , substrate layer  294 , etc.) but each include a layer of a given transparent material such as glass or transparent plastic that is coated with a polyimide layer/coating (e.g., the layer of transparent material and the corresponding polyimide coating may be referred to herein collectively as a “substrate,” “substrate layer,” or “transparent layer”). The rubbing directions of substrates  202 ,  206 ,  208 ,  212 ,  224 ,  228 ,  230 , and  234  of  FIG.  8   , substrates  262 ,  266 ,  268 ,  272 ,  284 ,  288 ,  290 , and  294  of  FIG.  9   , and substrates  106 ,  114 ,  116 ,  124 ,  128 ,  136 ,  138 , and  146  of  FIG.  4    may be defined by the direction of rubbing performed on the corresponding polyimide coating of that substrate. There may also be thin film layers such as patterned transparent electrodes and inorganic or organic insulation layers formed over the substrate and underneath the polyimide layer to provide driving signals (omitted from  FIGS.  3 - 9    for the sake of clarity). The example of  FIGS.  3 - 8    where the liquid crystal molecules within each liquid crystal lens have opposite pretilt angles is merely illustrative. If desired, the pretilt angles within one or more of the liquid crystal lenses may have aligned pretilt angles in the same direction. 
     Electrode layers  108 ,  112 ,  118 ,  122 ,  130 ,  134 ,  140 , and  144  of  FIG.  4    and corresponding electrode layers in  FIGS.  8  and  9    (not shown in  FIGS.  8  and  9    for the sake of clarity) may each include a common (planar) electrode or elongated electrodes (e.g., multiple elongated conductors extending parallel to each other within a given plane and separated by gaps, where each elongated electrode is provided with a desired voltage by control circuitry  12 ). Any given liquid crystal layer in  FIGS.  4 ,  8 , and  9    may have a common electrode formed on one side (e.g., the upper or lower side) and elongated electrodes formed on the other side, may have a common electrode formed on both sides, or may have elongated electrodes formed on both sides. Any of the electrode layers of  FIGS.  4 ,  8 , and  9    may include two layers of electrodes separated by an insulator if desired (e.g., two layers of patterned electrodes such as two common electrodes, one common electrode and one layer of elongated electrodes, or two layers of elongated electrodes (e.g., extending parallel or orthogonal to each other) separated by an insulator layer may be formed on one side of the liquid crystal layer). 
     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: 20190724
Publication Date: 20231121
Grant Date: 20231121
Priority Date: 20180810
Inventors: YAN, Jin
YANG, YOUNG CHEOL
STAMENOV, Igor
DELAPP, SCOTT M.
AIETA, Francesco
STEELE, BRADLEY C.
DORJGOTOV, ENKHAMGALAN
OH, SE BAEK
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0035", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/137", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13306", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13471", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133784", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/1313", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/1337", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/1343", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133528", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0174", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/294", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/137", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0035", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133784", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13471", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13306", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 69405917