Abstract:
Several unique configurations for interferometric recording of volumetric phase diffractive elements with relatively high angle diffraction for use in waveguides are disclosed. Separate layer EPE and OPE structures produced by various methods may be integrated in side-by-side or overlaid constructs, and multiple such EPE and OPE structures may be combined or multiplexed to exhibit EPE/OPE functionality in a single, spatially-coincident layer. Multiplexed structures reduce the total number of layers of materials within a stack of eyepiece optics, each of which may be responsible for displaying a given focal depth range of a volumetric image. Volumetric phase type diffractive elements are used to offer properties including spectral bandwidth selectivity that may enable registered multi-color diffracted fields, angular multiplexing capability to facilitate tiling and field-of-view expansion without crosstalk, and all-optical, relatively simple prototyping compared to other diffractive element forms, enabling rapid design iteration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. provisional patent application serial number 62/005,807 filed on May 30, 2014 entitled “METHODS AND SYSTEMS FOR VIRTUAL AND AUGMENTED REALTY”. This application is cross-related to U.S. Prov. Patent Application Ser. No. 61/909,174 filed on Nov. 27, 2013 under Atty. Dkt. No.: ML 30011.00 and entitled “VIRTUAL AND AUGMENTED REALITY SYSTEMS AND METHODS”, and U.S. Provisional Patent Application Ser. No. 61/845,907 filed on Jul. 12, 2013 under Atty. Dkt. No. 30007.00. The content of both provisional U.S. patent applications is hereby expressly incorporated by reference in its entirety. This application is also related to U.S. patent application serial number 14/690,401 filed on Apr. 18, 2015 and entitled “SYSTEMS AND METHODS FOR AUGMENTED AND VIRTUAL REALITY” and U.S. patent application Ser. No. 14/555,585 filed on Nov. 27, 2014 and entitled “VIRTUAL AND AUGMENTED REALITY SYSTEMS AND METHODS”. 
     
    
     BACKGROUND 
       [0002]    Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR”, scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR”, scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. 
         [0003]    When placing digital content (e.g., 3-D content such as a virtual chandelier object presented to augment a real-world view of a room, or 2-D content such as a planar/flat virtual oil painting object presented to augment a real-world view of a room), design choices may be made to control behavior of the objects. For example, the 2-D oil painting object may be head-centric, in which case the object moves around along with the user&#39;s head (e.g., as in a Google Glass approach); or the object may be world-centric, in which case it may be presented as though it is part of the real world coordinate system, so that the user may move his head or eyes without moving the position of the object relative to the real world. 
         [0004]    When placing virtual content into the augmented reality world presented with an augmented reality system, whether the object should be presented as world centric (i.e., the virtual object stays in position in the real world so that the user may move his body, head, eyes around it without changing its position relative to the real world objects surrounding it, such as a real world wall); body, or torso, centric, in which case a virtual element may be fixed relative to the user&#39;s torso, so that the user may move his head or eyes without moving the object, but such movement is slaved to torso movements; head centric, in which case the displayed object (and/or display itself) may be moved along with head movements, as described above in reference to Google Glass; or eye centric, as in a “foveated display” configuration wherein content is slewed around as a function of what the eye position is. 
         [0005]    Some conventional approaches uses optical waveguides having surface relief type diffractive elements (e.g., linear gratings) to redirect light beams from an image source to provide pupil expansion and to produce virtual content display to an observer&#39;s eye (in a monocular arrangement) or eyes (in a binocular arrangement). These waveguides having surface-relief type diffractive elements require complex designs of digital diffractive patterns. These complex designs are subsequently converted into high resolution binary mask information and then exposed onto a reticle or transferred to an electronic-beam writing device (e.g., lithographic writing equipment). These digital diffractive patterns are then authored or printed into a photoresist material and subsequently etched using various etching techniques. Such surface relief type diffractive elements are not only costly to manufacture, but the resulting structures are also fragile and vulnerable to inadvertent damages or contamination due to the existence of microscopic relief structures. 
         [0006]    Thus, there exists a need for methods and apparatus having enhanced diffractive elements for displaying virtual content for virtual or augmented reality. 
       SUMMARY 
       [0007]    Disclosed are a method and a system for virtual and augmented reality. Some embodiments are directed at an apparatus for virtual and augmented reality devices and applications. The apparatus may include an eyepiece including a diffractive optical element (DOE) having one or more layers, an in-coupling optic (ICO) element that receives light beams from, for example, a projector and transmits the light beams to a substrate in the DOE. Each layer may include OPE (orthogonal pupil expansion) diffractive elements and EPE (exit pupil expansion) diffractive elements. The OPE diffractive elements on a layer deflect some of the input light beams to the EPE diffractive elements which in turn deflect some of the deflected light beams toward the user&#39;s eye(s). It shall be noted that although the use of the term “gratings” does not imply or suggest that the diffractive structures in the “gratings” include only linear diffractive elements or structures. Rather, gratings (e.g., EPE gratings, OPE diffractive elements, etc.) may include linear diffractive structures, circular diffractive structures, radially symmetric diffractive structures, or any combinations thereof. The OPE diffractive elements and the EPE diffractive elements may include both the linear grating structures and the circular or radially symmetric diffractive elements to both deflect and focus light beams. 
         [0008]    The OPE diffractive elements and the EPE diffractive elements may be arranged in a co-planar or side-by-side manner on a layer in some embodiments. The OPE diffractive elements and the EPE diffractive elements may be arranged in a folded or overlaid manner on both sides of a layer in some embodiments. In some other embodiments, the OPE diffractive elements and the EPE diffractive elements may be arranged and recorded in a single, unitary, spatially-coincident layer to form a multiplexed layer having the functions of both the OPE diffractive elements and the functions of the EPE diffractive elements. Multiple such layers may be stacked on top of each other to form a multi-planar configuration where each layer may host its respective focal plane associated with its respective focal length. The multi-planar configuration may provide a larger focal range, and each layer in the multi-planar configuration may be dynamically switched on and off to present images that appear at different focal lengths to viewers. The OPE and EPE diffractive elements may be of the surface-relief type diffractive elements, the volumetric-phase type diffractive elements, or a combination thereof. 
         [0009]    Some embodiments are directed at a method for virtual and augmented reality. The method may transmit input light beams into a substrate of an eyepiece by using an in-coupling optic element, deflect the first portion of the input light beams toward second diffractive elements on a first layer of the eyepiece by using at least first diffractive elements on the first layer, and direct first exiting light beams toward a viewer&#39;s eye(s) by deflecting some of the first portion of the input light beams with the second diffractive elements on the first layer. 
         [0010]    Some first embodiments are directed at a method for generating stereoscopic images for virtual reality and/or augmented reality. Input light beams may be transmitted into a substrate of an eyepiece by using an in-coupling optic element; a first portion of the input light beams may be deflected toward second diffractive elements on a first layer of the eyepiece by using at least first diffractive elements on the first layer; and the first exiting light beams may further be directed toward a viewer by deflecting some of the first portion of the input light beams with the second diffractive elements on the first layer in these first embodiments. 
         [0011]    Some second embodiments are directed a process for implementing an apparatus for generating stereoscopic images for virtual reality and/or augmented reality. In these second embodiments, a first substrate may be identified (if already existing) or fabricated (if non-existent) for an eyepiece of the apparatus; first diffractive elements and second diffractive elements may be identified (if already existing) or fabricated (if non-existent) on one or more first films, wherein the first diffractive elements and second diffractive elements comprise linear diffractive elements and circular or radially symmetric diffractive elements; the one or more first films including the first diffractive elements and the second diffractive elements may be disposed on the first substrate; and an in-coupling optic element may also be integrated into the eyepiece to transmit input light beams from an input light source into the first substrate, wherein the first diffractive elements and the second diffractive elements are operatively coupled to the in-coupling optic element to deflect at least a portion of the input light beams. 
         [0012]    Some third embodiments are directed at a process for using or devising an apparatus for generating stereoscopic images for virtual reality and/or augmented reality. In these third embodiments, input light beams may be received from an in-coupling optical device; a first portion of the input light beams from the in-coupling optical device may be deflected into a first direction toward second diffractive elements with first diffractive elements in an eyepiece of the apparatus, wherein the first diffractive elements have a predetermined diffraction efficiency and a first orientation relative to a direction of propagation of the input light beams; and a second portion of the input light beams may be propagated through the second diffractive elements having a second orientation to produce stereoscopic images to an observer. 
         [0013]    Some fourth embodiments are directed at an apparatus for generating stereoscopic images for virtual reality and/or augmented reality. The apparatus comprises an eyepiece including a substrate; an in-coupling optic element to transmit input light beams into the substrate; and a first layer of the substrate comprising first diffractive elements and second diffractive elements that are operatively coupled to the in-coupling optic element and are disposed on one or more sides of the substrate, wherein the first diffractive elements and the second diffractive elements comprise linear diffractive elements and circular or radially symmetric diffractive elements. 
         [0014]    More details of various aspects of the methods and apparatuses for generating stereoscopic images for virtual reality and/or augmented reality are described below with reference to  FIGS. 1A-25D . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The drawings illustrate the design and utility of various embodiments of the present invention. It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. In order to better appreciate how to obtain the above-recited and other advantages and objects of various embodiments of the invention, a more detailed description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0016]      FIG. 1A  illustrates a simplified, schematic view of linear diffraction diffractive elements that deflects collimated light beam. 
           [0017]      FIG. 1B  illustrates a simplified, schematic view of radially symmetric diffractive elements that deflects collimated light beam. 
           [0018]      FIG. 1C  illustrates some embodiments described herein that include diffractive elements combining linear and radial structures. 
           [0019]      FIG. 1D  illustrates an example of the interaction between diffraction patterns or diffractive elements and the light beams carrying image information for an augmented or virtual reality display apparatus. 
           [0020]      FIG. 1E  illustrates another example of the interaction between diffraction patterns or diffractive elements and the light beams carrying image information for an augmented or virtual reality display apparatus. 
           [0021]      FIGS. 2A-B  illustrate some schematic representations of making and using volumetric phase diffractive elements in some embodiments. 
           [0022]      FIGS. 3A-B  illustrate some schematic representations of making and using volumetric phase diffractive elements for RGB (Red, Green, and Blue) in some embodiments. 
           [0023]      FIGS. 3C-D  illustrate some schematic representations of making and using volumetric phase diffractive elements for RGB (Red, Green, and Blue) in some embodiments. 
           [0024]      FIGS. 3E-F  illustrate some schematic representations of making and using steep-angle volumetric phase diffractive elements for RGB (Red, Green, and Blue) in some embodiments. 
           [0025]      FIGS. 4A-C  illustrate some schematic setups for recording volumetric phase diffractive elements or volumetric phase steep angle diffractive elements to fabricate EPEs, OPEs and/or combination EPE/OPEs in some embodiments. 
           [0026]      FIG. 5A  shows a schematic representation of one embodiment of an exit pupil expander recording stack of material and component layers and one of many possible recording geometries. 
           [0027]      FIG. 5B  shows a schematic representation of one embodiment of an exit pupil expander, orthogonal pupil expander, input coupling diffractive elements, or combination diffractive element recording stack of material and component layers and one of many possible recording geometries. 
           [0028]      FIG. 6  shows an illustrative configuration of one embodiment of the ICO, EPE, and OPE components in a single wafer substrate, and their functions when illuminated with an image projection system. 
           [0029]      FIG. 7  illustrates a schematic arrangement of a co-planar OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. 
           [0030]      FIG. 8  illustrates a schematic arrangement of an overlaid or folded OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. 
           [0031]      FIG. 9  illustrates another schematic arrangement of an overlaid or folded OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. 
           [0032]      FIGS. 10A-B  illustrate another schematic arrangement of an overlaid or folded OPE and EPE arrangement in some embodiments. 
           [0033]      FIG. 11  illustrates another schematic arrangement of an overlaid or folded OPE and EPE and a beam multiplying layer arrangement in some embodiments. 
           [0034]      FIGS. 12A-C  illustrate some schematic representations of the interactions between diffractive elements and light carrying image information for an observer in some embodiments. 
           [0035]      FIG. 12D  illustrates a schematic representation of a multi-planar configuration for a virtual reality and/or augmented reality apparatus in some embodiments. 
           [0036]      FIGS. 13A-B  illustrate schematic representations of a switchable layer in some embodiments. 
           [0037]      FIG. 14  illustrates a schematic representation of a multiplexed expander element in some embodiments. 
           [0038]      FIG. 15A  illustrates a portion of a schematic representation of a multiplexed expander element in some embodiments. 
           [0039]      FIG. 15B  illustrates another pictorial representation of a multiplexed expander assembly in some other embodiments. 
           [0040]      FIG. 16  shows an illustration of a user using a virtual reality or augmented reality device described herein to view an image. 
           [0041]      FIG. 17  illustrates a portion of  FIG. 16  for illustration purposes. 
           [0042]      FIG. 18  illustrates another perspective of a portion of  FIG. 16  for illustration purposes. 
           [0043]      FIG. 19  illustrates another perspective of a portion of  FIG. 16  for illustration purposes. 
           [0044]      FIG. 20  illustrates a close-up view of  FIG. 19  to provide a view of various elements of the diffractive optical element. 
           [0045]      FIG. 21  illustrates a side view of an illustration of a user using a virtual reality or augmented reality device to view an image. 
           [0046]      FIG. 22  illustrates a close-up view of the diffractive optical element (DOE) in some embodiments. 
           [0047]      FIG. 23A  illustrates a high level flow diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in some embodiments. 
           [0048]      FIGS. 23B-C  jointly illustrate a more detailed flow diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in some embodiments. 
           [0049]      FIG. 24A  illustrates a high level block diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. 
           [0050]      FIG. 24B  illustrates a more detailed block diagram for the process of generating stereoscopic images for virtual reality and/or augmented reality illustrated in  FIG. 24A  in one or more embodiments. 
           [0051]      FIG. 24C  illustrates a more detailed block diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. 
           [0052]      FIG. 25A  illustrates a high level block diagram for generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. 
           [0053]      FIGS. 25B-D  jointly illustrate some additional, optional acts  2500 B that may be individually performed or jointly performed in one or more groups for the process of generating stereoscopic images for virtual reality and/or augmented reality illustrated in  FIG. 25A . 
       
    
    
     DETAILED DESCRIPTION 
       [0054]    Various embodiments of the invention are directed to methods and systems for generating virtual content display virtual or augmented reality in a single embodiment or in some embodiments. Other objects, features, and advantages of the invention are described in the detailed description, figures, and claims. 
         [0055]    Some embodiments are directed to an apparatus for generating virtual content display. The apparatus includes diffractive elements to propagate light beams carrying image information from an image source to an observer&#39;s eye (monocular) or eyes (binocular). More specifically, the apparatus includes a first waveguide having OPE diffractive elements to deflect the light beams carrying image information from the image source to the second waveguide having EPE diffractive elements. The EPE diffractive elements in the second waveguide further redirect the light beams from the first waveguide to an observer&#39;s eye or eyes. 
         [0056]    A simplified mode of interactions between the EPE and OPE diffractive elements and the light beams for an augmented or virtual reality display apparatus may be explained with the following example with reference to  FIGS. 1D-E . In this example, light carrying the image information enters a waveguide ( 118 ), and the OPE diffractive elements in the waveguide ( 118 ) may deflect the incoming light toward the DOE or EPE diffractive elements ( 120 ) in the planar waveguide ( 116 ). A diffraction pattern, a “diffractive optical element” (or “DOE”), or EPE diffractive elements ( 120 ) are embedded within a planar waveguide ( 116 ) such that as a collimated light is totally internally reflected along the planar waveguide ( 116 ), the collimated light intersects the EPE diffractive elements ( 120 ) at a multiplicity of locations. In some embodiments described herein, the EPE diffractive elements ( 120 ) have a relatively low diffraction efficiency so that only a portion of the light is deflected away toward the eye ( 158 ) with each intersection of the EPE diffractive elements ( 120 ) while the rest of the light continues to move through the planar waveguide ( 116 ) via total internal reflection (TIR). 
         [0057]    The light beams carrying the image information is thus divided into a number of related light beams that exit the waveguide ( 116 ) at a multiplicity of locations and the result is a fairly uniform pattern of exit emission toward the eye ( 158 ) for this particular collimated beam bouncing around within the planar waveguide ( 116 ), as shown in  FIG. 1D . The exit beams toward the eye ( 158 ) are shown in  FIG. 1D  as substantially parallel, because, in this example, the EPE diffractive elements ( 120 ) has only a linear diffraction pattern. Referring to  FIG. 1E , with changes in the radially symmetric diffraction pattern component of the embedded EPE diffractive elements ( 220 ), the exit beam pattern may be rendered more divergent from the perspective of the eye ( 158 ) and require the eye to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a viewing distance closer to the eye than optical infinity. 
         [0058]    One of the advantages of the apparatus described herein is that a virtual content display apparatus described herein may include volumetric type diffractive elements that may be manufactured in a more robust and cost effective manner, without requiring the use of lithographic and etching processes. The volumetric type diffractive elements may be fabricated (e.g., by imprinting) for one or more waveguides for the apparatus in some embodiments and thus completely eliminates various problems associated with the fabrication, integration, and use of surface relief type diffractive elements in conventional approaches. These diffractive elements may be further arranged in different arrangements for a virtual content display apparatus to serve their intended purposes as described below in greater details. 
         [0059]    Various embodiments will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and the examples below are not meant to limit the scope of the present invention. Where certain elements of the present invention may be partially or fully implemented using known components (or methods or processes), only those portions of such known components (or methods or processes) that are necessary for an understanding of the present invention will be described, and the detailed descriptions of other portions of such known components (or methods or processes) will be omitted so as not to obscure the invention. Further, various embodiments encompass present and future known equivalents to the components referred to herein by way of illustration. 
         [0060]    Disclosed are method and systems for virtual and augmented reality. In optical instruments such as a human wearable stereoscopic glasses for the application of virtual reality or augmented reality, the user&#39;s eye may be aligned with and be of a similar size to the instrument&#39;s exit pupil in order to properly couple the instrument to the eye(s) of the user. The location of the exit pupil may thus determine the eye relief, which defines the distance from the last surface of an eyepiece of the instrument at which the user&#39;s eye may obtain full viewing angle to an observer&#39;s eye(s), and thereby the field of view, of the eyepiece. 
         [0061]    The eye relief is typically devised to be of certain distance (e.g., 20 mm) for use&#39;s comfort. If the eye relief is too large, the exiting light from the eyepiece may be lost and fail to reach the pupil. On the other hand, the view defined by the exiting light from the eyepiece or a waveguide coupled with the diffractive optical element (DOE) may be vignette if the eye relief is too small such that the exit pupil is smaller than the size of the pupil. Various embodiments described herein use volumetric phase diffractive elements with high angle diffraction to produce exit pupil expansion (EPE) structures or expanders and orthogonal pupil expansion (EPE) structures or expanders for a virtual reality or augmented reality system. 
         [0062]    As presented in this disclosure, the production of OPE and/or EPE surface-relief structures implements design of the complex digital diffractive pattern that will perform the desired pupil expansion and out-coupling functions. The design may then be converted to high resolution binary mask information, exposed onto a reticle or transferred to a special electron-beam writing device, authored into a photoresist material, and etched using chemical techniques. The resulting structure is somewhat fragile, because it is a microscopic physical relief, vulnerable to damage and contamination that will disrupt the diffractive function. 
         [0063]    In comparison, volume phase holograms may be authored by either piece-wise or monolithic (wide area-simultaneous) exposure of photosensitive materials (for example, photopolymers, silver halides, polymer-dispersed liquid crystal mixtures, etc.) with laser light, in a holographic (two-beam or more) approach. The special fringe orientation and spacing required or desired for these structures may be achieved through recording the holograms on thick dielectric substrates, such as glass or transparent or translucent plastic, which enable formation of fringes through index-matched coupling of the laser light in steep angle geometries. Some embodiments include the superimposed OPE/EPE combination of volumetric phase and surface relief structures. 
         [0064]    One of the benefits of a combined element may be to utilize unique properties of both types of structures which, when combined, produce a superior function, as compared to an all-digital (e.g., all surface relief) or all-volumetric-phase approach. The recording of volumetric phase holograms is inexpensive, rapid, and more flexible than the digital design/computation/authoring approach in surface-relief structures because the volumetric phase authoring optical system may be easily reconfigured, modified, and customized using a variety of off-the-shelf components and implementation techniques. Highly sensitive, easy-to-use, dry-process photopolymer materials may also provide another advantage in using the volumetric phase techniques in producing the EPE/OPE structures. 
         [0065]    Volumetric phase approaches possess the inherent ability to modulate diffraction efficiency without introducing unwanted or undesired artifacts. In the case of EPE and OPE functions, both the EPE and the OPE structures rely on successive redirection and out-coupling of collimated wavefronts propagating through large area waveguides via total internal reflection in some embodiments. With each interaction with the diffractive elements, some light is redirected, or coupled out of the structure entirely (as designed), resulting in a reduction in the amount of light left for successive interactions. This may result in some undesirable reduction in image field brightness distribution across the eyepiece as the light propagates from the projection injection point. To mitigate this problem, the diffraction efficiency of the eyepiece components may be ramped in some embodiments, such that the initial interaction between the light rays and the structures use less of the available light than later interactions. 
         [0066]    Moreover, re-distribution of grating diffraction efficiency uniformity is straightforward in volumetric-phase recording methods, achieved by modulating the recording beam intensities, and/or the ratio of intensities between the two interfering beams. In contrast, surface-relief structures, being binary in nature, may not as readily be modified to achieve the same effect, particularly without introducing ghosting images, additional diffracted orders, and other unwanted or undesired artifacts. Volumetric phase-type structures may also be desired or required for polymer-dispersed liquid crystal (PDLC) components, including switchable diffractive elements that may enable time-multiplexed distribution of projected images to multiple focal-plane imaging elements. Some embodiments combine volumetric-phase approaches with PDLC and apply the combination to the OPE/EPE and the in-coupling optics (ICO). 
         [0067]    The PDLC material includes micro-droplets that have a diffraction pattern in a host medium, and the refraction index of the host medium or the micro-droplets may be switched to an index that does not match that of the substrate. Switchable diffractive elements may also be made of materials including lithium niobate. Volumetric phase structures may be more angularly selective than surface relief structures, and thus may not as readily diffract light from external, possibly ambient sources. This may constitute another advantage for using at least some of the described embodiments in eyewear applications, where the diffractive elements may be exposed to sunlight or other light sources in addition to the intended image projection source. In addition or in the alternative, some embodiments utilize a single-layer multiplexed OPE/EPE structure whose function may be difficult or entirely impossible to produce using alternative approaches such as surface-relief type diffractive structures or elements. One of the reasons for such difficulty or impossibility may be due to the fact that surface-relief type diffractive elements are more dispersive than volumetric phase type diffractive elements, and thus may introduce crosstalk and multiple diffraction orders that could be wasteful of projection light and visually distracting. Another reason for such difficulty or impossibility is that the complexity of the required pattern or the etch depth and orientation required to produce the necessary pattern in binary form is difficult to attain. 
         [0068]    Various embodiments entail specific volume phase holographic recording techniques and geometries for producing OPEs, EPEs, combinations of these two in separate layers, and combination of these two functions in a single layer that comprise a waveguide distribution-based eyepiece for augmented reality display. Although Bayer Bayfol holographic photopolymer may be used as the primary recording medium for the orthogonal pupil expansion and exit pupil expansion structures, various embodiments are not limited to this specific material for achieving the intended purposes or performing intended functions. Rather, various objectives, purposes, and functions are independent from any proprietary elements or characteristics of the Bayer Bayfol material. For instance, the PDLC material that was used in constructing some switchable EPEs behaved very similarly to the Bayer material in terms of photosensitivity, processing, clarity, etc. Additionally, DuPont OmniDex photopolymer materials may also be used with similar effect. 
         [0069]      FIG. 1A  illustrates a simplified, schematic view of linear diffractive elements that deflect collimated light beam. As it can be seen from  FIG. 1A , linear diffractive elements  102 A including a linearly arranged periodic structures diffract the collimated incident light beam  104 A into the exiting light beam  106 A travelling in a different direction than the incident light direction.  FIG. 1B  illustrates a simplified, schematic view of a radially symmetric diffractive elements that deflect collimated light beam. More specifically, the collimated, incident light beam  104 B passes through a zone plate or circular diffractive elements  102 B including a radially symmetric structures and become diffracted towards a “focal” point due to the radially symmetric structures of the circular diffractive elements  102 B. 
         [0070]    In these embodiments, the zone plate or circular diffractive elements  102 B effectively focuses the collimated, incident light beam  104 B to form the focused exiting light beam  106 B.  FIG. 1C  illustrates some embodiments described herein that includes diffractive elements  102 C combining linear and radial structures. The diffractive elements  102 C both deflect and focus the incident light beam  104 C to form the exiting light beam  106 C. In some embodiments, circular or radially symmetric diffractive elements may be configured or devised to cause the exiting light beams to diverge. 
         [0071]    Some embodiments use volumetric phase holograms that are authored or written by using, for example, piece-wise or monolithic (e.g., wide area-simultaneous) exposure of photosensitive materials that may include photopolymers, silver halides, polymer-dispersed liquid crystal mixtures, etc. with laser light in a holographic (two-beam or more) approach.  FIGS. 2A-B  illustrate some schematic representation of making and using volumetric phase type diffractive elements in some embodiments. More specifically,  FIG. 2A  illustrates that two laser beams or other light sources  202 B and  204 B (the “recording beams”) intersect within a photopolymer film  206 B and produce a volumetric interference pattern. The interference pattern may be permanently recorded as a phase pattern in the photopolymer  206 B. 
         [0072]      FIG. 2B  illustrates some broad-band (e.g., white light) light is directed toward the diffractive elements from the direction (the opposite direction of the first recording beam in  FIG. 2A ) of one of the recording beams, some of the broad-band light may be refracted and deflected to travel in the same direction (the opposite direction of the second recording beam in  FIG. 2A ) as the second light beam  204 C. Because of the refractive index of the photopolymer film  206 C, only a relatively narrow band of color may be diffracted. Therefore, the exiting light beam appears approximately the same color as the recording beam that is used to record the diffractive elements. The line plot corresponding to  FIG. 2A  illustrates the wavelength (about 600 nanometers in this example) of the recording spectrum of the recording beams. The line plots corresponding to  FIG. 2B  illustrate the output spectrum of the exiting light beam  204 C (also about 600 nanometers in this example) as well as the illumination spectrum of the broad-band light source  208 C. 
         [0073]      FIGS. 3A-B  illustrate some schematic representation of making and using volumetric phase type diffractive elements for the three primary-color model—RGB (Red, Green, and Blue) color model—in some embodiments. More specifically,  FIG. 3A  illustrates the use of three recording light beams (e.g., a red laser beam, a blue laser beam, and a green laser beam) for recording the volumetric phase interference pattern in photopolymer films. Each of the three recording beams  302 A and  304 A records a separate superimposed diffractive elements  308 A within the photopolymer film  306 A in an identical or substantially similar manner as that described for monochromatic recording light beam in  FIGS. 2A-B . 
         [0074]      FIG. 3B  illustrates an example of a use case when broad-band light  308 B (e.g., white light) is directed toward a fabricated RGB diffractive elements  306 A. Due to the wavelength selective nature of the RGB diffractive elements  306 A, each color of the broad-band light is diffracted by its own diffractive elements of the RGB diffractive elements  306 A. Consequently, only a narrow color band of each color may be diffracted when the broad-band light passes through the RGB diffractive elements  306 A. Therefore, the exiting light beam for an incident light beam component (e.g., red, blue, or green) appears approximately the same color as the incident recording light beam component that is used to record the diffractive elements. 
         [0075]    As a result, the exiting light beam  304 B appears approximately full color as a result. The line plot corresponding to  FIG. 3A  illustrates the wavelengths of the recording spectrum of the recording beams having three peaks that respectively represent the red, green, and blue light components of the recording light beam. The line plots corresponding to  FIG. 3B  illustrate the output spectrum of the exiting light beam  304 B as well as the illumination spectrum of the broad-band light source  308 B. 
         [0076]      FIGS. 3C-D  illustrate some schematic representation of making and using volumetric phase type diffractive elements for RGB (Red, Green, and Blue) in some embodiments. More specifically,  FIG. 3C  illustrates the use of three recording light beams (e.g., a red laser beam, a blue laser beam, and a green laser beam) for recording the volumetric phase interference pattern in photopolymer films. Each of the three recording beams  302 C records a separate superimposed diffractive elements  308 A within the photopolymer film  306 C in an identical or substantially similar manner as that described for monochromatic recording light beam in  FIGS. 2A-B  and  3 A. 
         [0077]      FIG. 3D  illustrates a use case when narrow-band or laser-source RGB illumination light  308 D is directed toward a fabricated RGB diffractive elements  306 C. When the RGB laser light beam is directed toward the RGB diffractive elements, each color is diffracted or reflected by its respective diffractive elements. Each laser color of the RGB laser illumination light  308 D is reflected or diffracted when the RGB light passes through its own diffractive elements in the RGB diffractive elements  306 C due to the wavelength selective nature of the RGB diffractive elements  306 D. Therefore, the exiting light beam for an incident light beam component (e.g., red, blue, or green) appears approximately the same color as the corresponding light component of the incident RGB light beam that is used to record the diffractive elements. As a result, the exiting light beam  304 D also appears approximately full color. 
         [0078]    The line plot corresponding to  FIG. 3C  illustrates the wavelengths of the recording spectrum of the recording beams having three peaks that respectively represent the red, green, and blue light components of the recording light beam. The line plots corresponding to  FIG. 3D  illustrate the output spectrum of the exiting light beam  304 D as well as the illumination spectrum of the broad-band light source  308 D. The deviation between the recording RGB recording beams ( 302 C and  304 C) and the reconstruction (e.g.,  308 D) may cause angular displacement of the diffracted light beam, and significant amount of deviation of wavelength may result in decreased diffraction efficiency due to Bragg condition mismatch. 
         [0079]      FIGS. 3E-F  illustrate some schematic representation of making and using steep-angle volumetric phase type diffractive elements for RGB (Red, Green, and Blue) in some embodiments. More specifically,  FIG. 3E  illustrates the use of two recording beams  302 E and  304 E to record the volumetric phase interference pattern in photopolymer films or polymer-dispersed liquid crystal materials. The two recording beams  302 E and  304 E interfere to produce diffractive elements  308 E within the photopolymer film  306 E in an identical or substantially similar manner as that described for monochromatic recording light beam in  FIGS. 2A-B . 
         [0080]    In  FIG. 3E , the second recording beam  304 E is directed at a relative steep angle to the photopolymer film  306 E. In some embodiments, a waveguide made of relative high refractive index host medium  310 E (e.g., glass, transparent or translucent plastic, etc.) coupled with a diffractive optical element (DOE) may be used to control or improve the steep angle incident recording light beam  304 E.  FIG. 3F  illustrates broad-band light (e.g., white illumination light) directed toward the diffractive elements from the direction (the same direction of the second recording beam  304 E in  FIG. 3E ) of one of the recording beams, some of the broad-band light may be diffracted and deflected in the same direction as the first recording light beam  302 E due to the steep angle of the second recording light beam  304 E in the fabrication process of the volumetric phase interference pattern. Because of the refractive index of and the interference pattern structures in the photopolymer film  306 E, only light beams  308 F of a relatively narrow band of color may be diffracted. Therefore, the exiting light beam  304 F appears approximately the same color as the recording light beam ( 302 E and  304 E) that is used to record the diffractive elements. The line plot corresponding to  FIG. 3F  illustrates the output spectrum of the exiting light beam  304 F. 
         [0081]    In some embodiments, the volumetric phase steep angle diffractive elements for the EPEs and OPEs may be made by using, for example Nd: YAG (neodymium-doped yttrium aluminum garnet or Nd:Y 3 Al 5 O 12 ) or the Nd:YLF (Neodymium-doped yttrium lithium fluoride or Nd:LiYF 4 ) as the lasing medium for solid-state lasers for recording the interference patterns in photopolymer films including Bayer Bayol® HX self-developing photopolymer film. The recording dosage may range from a few millijoules per square centimeter (mJ/cm 2 ) to tens of millijoules per square centimeter with varying recording times. 
         [0082]    For example, the volumetric phase interference patterns may be fabricated with 10 mJ/cm2 for a period of 10 seconds or shorter to fabricate the EPEs or OPEs in some embodiments. The laser beam distribution may be offset from the center to produce an intensity ramp on the diffractive element recoding plane to produce a variation in the diffraction efficiency in some embodiments. The variation in diffraction efficiency may result in a more uniform distribution of diffracted beams from the TIR-illuminated construct (total internal reflection-illuminated construct). Some illustrative setups for recording volumetric phase type diffractive elements or volumetric phase steep angle diffractive elements by using one or more lens-pinhole spatial filters (LPSF), collimators (COLL), and various other optic elements to fabricate EPEs and/or OPEs are shown in  FIGS. 4A-C . 
         [0083]      FIGS. 4A-C  illustrate some schematic setups for recording volumetric phase type diffractive elements or volumetric phase steep angle diffractive elements to fabricate EPEs, OPEs and/or combined EPE/OPEs in some embodiments. More specifically,  FIG. 4A  shows an illustrative recording system design that uses the neodymium-doped yttrium aluminum garnet (Nd: YAG) lasing medium for solid-state laser to record volumetric-phase type diffractive elements for EPEs, OPEs, and/or combination EPEs and OPEs. The solid-state Nd: YAG lasers  400 A emit light at, for example, 532 nm, and the laser light travels through a series of optic elements including the variable beam splitter  412 A, beam-splitters, beam combiners, or transparent blocks  406 A, various mirrors  404 A, spatial filters  414 A, collimators  408 A, and lens and eventually perform the recording function to fabricate the desired or required diffractive elements on a film material positioned on the DOE (diffractive optic element) plane  402 A. 
         [0084]    In these embodiments illustrated in  FIG. 4A , a prism  418 A is used to couple the laser light into one side of the substrate carrying the film. It shall be noted that although the distance from the focal point  416 A of the optic element  410 A to the DOE recording plane  402 A in this illustrated embodiment is 1-meter, this distance may be varied to accommodate different design configurations for different recording systems and thus shall not be considered or interpreted as limiting the scope of other embodiments or the scope of the claims, unless otherwise specifically recited or claimed. 
         [0085]      FIG. 4B  shows another illustrative recording system design in some embodiments. In addition to the Nd:YAG laser  454 B generating green-colored laser light beams  408 B, the illustrative recording system in  FIG. 4B  uses two additional solid-state laser  452 B (Neodymium-doped yttrium lithium fluoride or Nd:YLF) generating blue-colored laser light beams  410 B and  456 B (Krypton Ion laser) generating red-colored laser light beams  406 B to record volumetric-phase type diffractive elements for EPEs, OPEs, and/or combination EPEs and OPEs. The red, green, and blue colored light beams are combined with a series of optic elements (e.g., beam-splitter, beam-combiner, or transparent block  412 B, wavelength-selective beam combining mirrors  414 B, variable beam-splitters  416 B) to form RGB (red, green, and blue) light beams  404 B that are further transmitted through a plurality of optic elements (e.g., spatial filters  418 B, collimators  420 B, focusing lens  422 B, and prism  424 B) to fabricate the desired or required diffractive elements on a film located on the DOE (diffractive optical element) recording plane  402 B. 
         [0086]    Similar to the recording system illustrated in  FIG. 4A , the recording system illustrated in  FIG. 4B  includes the prism  424 B to couple light beams into the film on the DOE recording plane  402 B. Also similar to the recording system illustrated in  FIG. 4A , although the distance from the focal point  426 B of the optic element  422 B to the DOE recording plane  402 B in this illustrated embodiment is 1-meter, this distance may be varied to accommodate different design configurations for different recording systems and thus shall not be considered or interpreted as limiting the scope of other embodiments or the scope of the claims, unless otherwise specifically recited or claimed. In one embodiment, the internal angle may be 73-degree from the normal direction of the prism  418 A or  424 B although different angles may also be used for different but similar configurations. 
         [0087]      FIG. 4C  shows another illustrative recording system design in some embodiments. For the ease of illustration and explanation , the illustrative record system in  FIG. 4C  includes for example, the Nd:YAG laser  402 C (or other lasing medium or media for different or additional light beams) to generate light beams for recording diffractive elements on a film located on the DOE recording plane  420 C. The laser light beams are transmitted through a plurality of optic elements including, for example, beam-splitter, beam-combiner, or transparent block  404 C, wavelength-selective beam combining mirrors  408 C, variable beam-splitters  406 C, spatial filters  410 C, collimators  412 C, beam-splitter  404 C, and periscope  408 C and are eventually coupled into the film or substrate located on a glass block  418 C to record the diffractive elements on the film or substrate. 
         [0088]      FIG. 4C  also shows the top view  450 C and the side view  460 C of a part of the recording system. In this illustrative recording system in  FIG. 4C , the light beams used for recording diffractive elements are coupled into the substrate or film by using a glass block  418 C, rather than a prism as shown in  FIGS. 4A-B . The use of a glass block (e.g.,  418 C) allows access from four sides of the glass block for the light beams rather than two sides from the prism as shown in  FIGS. 4A-B . In one embodiment, the internal angle may be 30-degree from the normal direction of the glass block  418 C although different angles may also be used for different but similar configurations. In addition or in the alternative, the distance between the spatial filter  410 C and the DOE recording plane  420 C is 0.5 meter, although it shall be noted that this distance may be varied to accommodate different design configurations for different recording systems and thus shall not be considered or interpreted as limiting the scope of other embodiments or the scope of the claims, unless otherwise specifically recited or claimed. 
         [0089]      FIG. 5A  shows a schematic representation of the recording configuration for one embodiment of EPE diffractive elements. Expanded laser beams  510  and reference laser  504  intersect at a steep angle (shown as 73° here, but arbitrarily adjustable) within the recording material  514  through index-matching coupling prism  502  and index-matching coupling fluid  512 , a substrate  514 , a photopolymer layer  516 , and a dielectric layer  518 , all of which have nominally high (˜1.51) or similar index of refraction. Use of index-matching elements enables coupling of light into the recording material that would otherwise be highly-reflected from the surface of the material and not coupled in contribute to diffractive element recording. 
         [0090]      FIG. 5B  shows a schematic representation of an alternative recording configuration for various embodiments of EPE, OPE or ICO diffractive elements. Expanded laser beams  502 B and  505 B intersect at a steep angle (shown as 60° here, but arbitrarily adjustable) within the recording material  507 B through index-matching block  509 B and index-matching coupling fluid  501  B and substrate  508 B, all of nominally high and matched indices of refraction (˜1.46), but lower than the index of refraction of recording material  507 B. Anti-reflection coated and or also absorbing layer  504 B, nominally glass or plastic, is coupled to the recording stack with index-matching fluid layer  503 B. Layer  504 B and its associated anti-reflection coatings prevent total-internal reflection (TIR) of beam  502 B, to mitigate recording of secondary diffractive elements from that reflected light. 
         [0091]    The illustrative EPE diffractive element recording stack in  FIG. 5A  is disposed on one side of a rectangular side  508  of the triangular prism. It shall be noted that in  FIG. 5A , the EPE diffractive element recording stack appears to be disposed on a rectangular side  508  for the ease of illustration and explanation purposes. The EPE may be disposed in a variety of different manners as will be described in subsequent paragraphs with reference to  FIGS. 7-15 . The EPE diffractive element recording stack comprises a film  512  of xylenes (n˜1.495) or mineral oil (n˜1.46), a film  514  of mic. slide (n˜1.51) stacked on the xylenes or mineral oil film, a film  516  of Bayer Bayfol HX photopolymer film (n˜1.504) stacked on the mic. slide film, and a film  518  of polycarbonate (n˜1.58). In  FIG. 5B , an EPE or OPE diffractive element recording stack comprises a film  508 B of Cargille 1.46 index matching oil (n˜1.46), a film  508 B of quartz or fused silica microscope slide stacked on the index matching oil film, a film  507 B of Bayer Bayfol HX photopolymer film (n˜1.504) stack on the microscope slide film, and a film  506 B of polyamide (n 1.52). Further, a film of Cargille 1.52 index matching oil (n˜1.52) is stacked on to film  506 B, and a film of anti-reflection-coated and/or absorbing gray glass  504 B is stacked onto the index-matching oil. 
         [0092]    In contrast, when the reference beam  504  in  FIG. 5A  is directed toward a rectangular side  506  of the triangular prism  502 , the refractive index of the triangular prism causes the beam to deflect toward the EPE diffractive element recording stack which may be configured as shown to deflect the reference beam  504  such that the normal beam  510  interferes with it, and produces diffractive elements which are recorded in  516 . When the reference beam  502 B in  FIG. 5B  is directed toward a rectangular side of the block  509 B, the refractive index of the block causes the beam to deflect toward the EPE/OPE diffractive element recording stack which may be configured as shown to deflect the reference beam  502 B such that the beam  505 B interferes with it and produces diffractive elements which are recorded in  507 B. 
         [0093]    In some embodiments, the diffractive optical element (DOE) may be sandwiched in, coupled with, or otherwise integrated with a waveguide and may have relative low diffraction efficiency so only a smaller portion of the light, rather than the light in its entirety, is deflected toward the eyes while the rest propagates through the planar waveguide via, for example, total internal reflection (TIR). It shall be noted that the light propagates within a waveguide, and diffraction occurs when the light encounters the diffractive optical element (DOE) coupled with the DOE due to the interference of light waves in some embodiments. Therefore, one of ordinary skill in the art will certain appreciate the fact that the diffractive optical element constitutes the “obstacle” or “slit” to cause diffraction, and that the waveguide is the structure or medium that guides the light waves. 
         [0094]      FIG. 6  shows an illustrative configuration of an apparatus for virtual and/or augmented reality applications in some embodiments. More specifically,  FIG. 6  illustrates a co-planar OPE/EPE configuration for a virtual or augmented reality device. In these embodiments illustrated in  FIG. 6 , the OPE  112  and EPE  110  are arranged in a substantially co-planar manner on a, for example, glass or transparent or translucent plastic substrate  114  which also serves as a waveguide to guide the light waves propagating therewithin. During operation of the illustrative apparatus, the input light beam  604  may be transmitted from a source  602  which may include one of a fiber scanning system, a fiber scanner, a pico-projector, a bundle of projectors, micro-array displays, LCoS or Liquid Crystal on Silicon, or DLP or Digital Light Processing, or any other sources that may be used to provide input light beams. 
         [0095]    The input light beams from the source  602  is transmitted to scanning optics and/or an in-coupling optics (ICO)  606  and directed toward to the OPE diffractive elements  112  that are disposed or integrated on the substrate  114 . The OPE diffractive elements  112  cause the light beams to continue to propagate along the array of OPE diffractive elements  112  within a waveguide  114  as shown by the arrowheads  116 . Every time when the light beams hit the slanted OPE diffractive elements  112 , a portion of the light beams is thus deflected by the OPE diffractive elements  112  toward the EPE diffractive elements  110  as shown by the arrowheads  118 . When the portion of the light beams that are deflected to the EPE diffractive elements  110  hits the EPE diffractive elements, the EPE diffractive elements  110  deflect the incoming light beams into exiting light beams  108  toward the user&#39;s eye(s)  106 . 
         [0096]      FIG. 7  illustrates a schematic arrangement of a co-planar OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. The OPE and EPE diffractive elements may be arranged in a substantially co-planar manner on a substrate  702  such as a glass or transparent or translucent plastic substrate. In some of these embodiments, the OPE diffractive elements  704  and/or the EPE diffractive elements  706  may comprise the surface-relief type diffractive elements that may be produced optically with, for example, laser beam interference or be produced digitally with, for example, computer-designed structures and microscopic fringe-writing techniques. 
         [0097]    Diffractive elements produced in this manner may be replicated through embossing or casting and usually exhibit dispersive behavior like a prism. In some other embodiments, the OPE diffractive elements  704  and/or the EPE diffractive elements  706  may comprise the volumetric-phase type diffractive elements that may be produced and replicated optically through, for example, contact copying. The volumetric-phase type diffractive elements may be produced in lamintable photopolymer films (e.g., Bayer Bafol HX) or in polymer-dispersed liquid crystal layers (PDLC layers) in some embodiments. The volumetric-phase type diffractive elements may be wavelength selective and behavior like a dichroic mirror. In some other embodiments, at least a first portion of the OPE diffractive elements or the EPE diffractive elements may be of the surface-relief type diffractive elements, and at least another portion of the OPE diffractive elements or the EPE diffractive elements may be of the volumetric-phase type diffractive elements. 
         [0098]    During operation, the in-coupling optics  712  receives input light beams from, for example, a fiber scanner or a pico-projector (not shown in  FIG. 7 ) and refracts the input light beams toward the OPE diffractive elements  704  as shown by the input light beams  710 . The OPE diffractive elements  704  may be configured in a slanted orientation to deflect some of the input light beams toward the EPE diffractive elements  706  as shown by the light beams  708 . In addition or in the alternative, the OPE diffractive elements  704  may be configured or devised to have relative low diffraction efficiency such that a desired portion of the input light beams  710  continues to propagate within the substrate  702  via, for example, total internal reflection (TIR), and that the remaining portion of the input light beam from the ICO  712  is deflected toward the EPE diffractive elements  706 . 
         [0099]    That is, every time the input light beam hits the OPE diffractive elements, a portion of it will be deflected toward the EPE diffractive elements  706  while the remaining portion will continue to transmit within the substrate, which also functions as a waveguide to guide the light waves propagating therewithin. The diffraction efficiency of the OPE diffractive elements  704  and/or that of the EPE diffractive elements  706  may be configured or devised based at least in part upon one or more criteria including the brightness or uniformity of the exiting light beams from the EPE diffractive elements  706 . The EPE diffractive elements  706  receives the light beams  708  deflected from the OPE diffractive elements  704  and further deflect the light beams  708  toward the user&#39;s eye. 
         [0100]      FIG. 8  illustrates a schematic arrangement of an overlaid or folded OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. In these embodiments, the OPE diffractive elements  804  and the EPE diffractive elements  806  may be disposed or mounted on both sides of a substrate  802  (e.g., a glass or transparent or translucent plastic substrate) that also functions as a waveguide to guide the light waves propagating therewithin. The OPE diffractive elements  804  and the EPE diffractive elements  806  may be separated fabricated as two film structures (e.g., on a photopolymer film or a polymer-dispersed liquid crystal layer) and then be integrated to the substrate  802  in some embodiments. 
         [0101]    In some other embodiments, both the OPE diffractive elements  804  and the EPE diffractive elements  806  may be fabricated on a single film or layer and subsequently folded to be integrated with the substrate  802 . During operation, the in-coupling optics  808  may receive input light beams from a source (e.g., a fiber scanner or a pico-projector) and refracts the input light beams into the side of the substrate  802 . The input light beams may continue to propagate within the substrate  802  via, for example, total internal reflection (TIR) as shown by  810 . When the input light beams hit the OPE diffractive elements  804 , a portion of the input light beams are deflected by the OPE diffractive elements  804  toward the EPE diffractive elements  806  as shown by  812  and the remaining portion of the input light beams may continue to propagate within the substrate as shown by  810 . 
         [0102]    The remaining portion of the input light beams  810  continues to propagate in the direction within the substrate  802  and hits the EPE diffractive elements  806  disposed on the other side of the substrate  802  as shown by  816 . A portion of this remaining portion of the input light beams  810  is thus deflected by the EPE diffractive elements  806  and becomes the existing light beams  814  to the user&#39;s eye(s) (not shown), and the remaining portion of the input light beams  810  further continues to propagate as light beams  818  within the substrate  802 . The same also applies to the deflected input light beams  812  along the horizontal direction (as shown by  FIG. 8 ). That is, the input light beams through the ICO  808  bounce within the substrate  802 . 
         [0103]    When a portion of the input light beams hit the OPE diffractive elements  804 , this portion of the input light beams is deflected to travel in the direction orthogonal (as shown by  812 ) to the incident direction (as shown by  810 ) and continues to bounce within the substrate  802  while the remaining portion continues to travel along the original direction within the substrate  802 . When the light beams hit the EPE diffractive elements  806 , the EPE diffractive elements  806  deflect the light beams toward the user&#39;s eye as shown by  814 . One of the advantage of this folded or overlaid OPE/EPE configuration is that the OPE and EPE do not occupy as much space as the co-planar configuration ( FIG. 7 ) does. Another advantage of this overlaid or folded OPE/EPE configuration is that the diffraction efficiency in the transmission of light due to the more confined propagation of light beams in this overlaid or folded configuration. In some embodiments, the EPE diffractive elements intercept the incident light beams and direct them toward the user&#39;s eye(s) by deflection (as shown by  814 ), reflection (as shown by the reflected light beams of  820 ), or by both deflection and reflection. 
         [0104]      FIG. 9  illustrates another schematic arrangement of an overlaid or folded OPE and EPE arrangement operatively coupled to an in-coupling optic device in some embodiments. More specifically,  FIG. 9  illustrates a substantially similar overlaid or folded OPE/EPE configuration as that in  FIG. 8 . Nonetheless, the overlap between the OPE diffractive elements  904  and the EPE diffractive elements  906  is different from that in  FIG. 8 . In some embodiments, the degree or extent of overlap or how the OPE and EPE diffractive elements overlap may be determined based at least in part upon one or more design criteria or requirements and/or the desired or required uniformity of the exiting light beams. 
         [0105]      FIGS. 10A-B  illustrate another schematic arrangement of an overlaid or folded OPE and EPE arrangement in some embodiments.  FIG. 10A  shows the OPE diffractive elements  1004 A and the EPE diffractive elements  1006 A disposed on both sides of a substrate (e.g., a glass or transparent or translucent plastic substrate)  1002 A.  FIG. 10B  also shows the OPE diffractive elements  1004 B and the EPE diffractive elements  1006 B disposed on both sides of a substrate (e.g., a glass or transparent or translucent plastic substrate)  1002 B. Nonetheless, the thickness of the substrate  1002 B is smaller than that of the substrate  1002 A. 
         [0106]    As a result of the thinner substrate  1002 B, the density of the output light beams  1010 B is higher than the density of the output light beams  1010 A because the light beams  1008 B travels for a shorter distance than the light beams  1010 A in  FIG. 10A  before the light beams  1008 B hit the OPE diffractive elements  1004 B or the EPE diffractive elements  1006 B in  FIG. 10B . As  FIGS. 10A-B  shows, thinner substrate thickness results in higher output light beam density. The thickness of the substrate may be determined based at least in part upon one or more factors in some embodiments. The one or more factors may include, for example, the desired our required output beam density, the attenuation factor, etc. In some embodiments, the thickness of the substrate may be within the range of 0.1-2 mm. 
         [0107]      FIG. 11  illustrates another schematic arrangement of an overlaid or folded OPE and EPE arrangement in some embodiments. More specifically, the overlaid or folded OPE and EPE arrangement illustrated in  FIG. 11  includes a beam-splitting surface  1104  embedded in the substrate  1102  or sandwiched between two separate substrates  1102 . As other overlaid or folded OPE/EPE configurations, the OPE diffractive elements  1106  and the EPE diffractive elements  1108  are disposed on both sides of the substrate  1102 . In these embodiments, the beam-splitting surface may be embedded, sandwiched, or otherwise integrated with the substrate(s)  1102  to increase the output light beam density. 
         [0108]    As  FIG. 11  shows, the beam splitter splits a light beam into two—the reflected light beam and the transmitted light beam—as the light beam passes through the beam splitter. The beam splitter may include a thin coating on a surface of a first substrate that is subsequently glued, bonded, or otherwise attached to a second substrate. Illustrative coating may include, for example, metallic coating (e.g., silver, aluminum, etc.), dichroic optical coating, adhesives (e.g., epoxy, polyester, urethane, etc.) In some embodiments, the ratio of reflection to transmission of the beam splitter may be adjusted or determined based at least in part upon the thickness of the coating. A beam-splitter may include a plurality of small perforations to control the ratio of reflection to transmission of the beam splitter in some of these embodiments. 
         [0109]      FIG. 12D  illustrates a schematic representation of a multi-planar configuration for a virtual reality and/or augmented reality apparatus in some embodiments. In these embodiments illustrated in  FIG. 12D , multiple eyepieces may be stacked on top of each other, and each eyepiece or layer of the multiple eyepieces hosts a distinct focal plane to produce images at its respective focal distance.  FIGS. 12A-C  illustrate some schematic representations of the interactions between diffractive elements in the multi-planar configuration illustrated in  FIG. 12D  and light carrying image information for an observer in some embodiments. More specifically, the multiple layers may include one layer that hosts the focal plane with the infinity focal length as shown in  FIG. 12A  to simulate the images as if the images are located at a substantially long distance from the user such that the light beams for forming the image are substantially parallel to each other. 
         [0110]      FIG. 12B  illustrates that the multi-planar configuration may also include a layer that hosts the focal plane with specific focal length (e.g., four meters) to produce images as if they are located four meters from the user. This may be achieved with using a combination of linear diffractive elements and radially symmetric diffractive elements as described in the preceding paragraphs with reference to  FIGS. 1A-C .  FIG. 12C  illustrates that the multi-planar configuration may also include a layer that hosts the focal plane with a relative close in focal length (e.g., 0.5-meter) to produce images as if they are located half a meter from the user. It shall be noted that these focal lengths are provided in these figures for the ease of illustration and explanation and are not intended to limit the scope of other embodiments or the scope of the claims, unless otherwise specifically recited or claimed. 
         [0111]    The multi-planar approach may also include layers having different or additional focal lengths.  FIG. 12D  illustrates a schematic representation of a six-layer multi-planar configuration for the eyepiece  1202 D where the overall thickness  1204 D of the six-layer eyepiece  1202 D may be no more than 4 millimeters in some embodiments. One or more of these six layers may comprise a switchable layer (e.g., a PDLC or polymer-dispersed liquid crystal layer) that may be switched on and off by using control signals to change the focal planes of the produced images. This illustrative multi-planar configuration may also operatively coupled to a rapidly switching in-coupling optics (ICO)  1206 D that may be further operatively coupled to a light source such as a fiber, a bundle of fibers, a multi-fiber projector, or a pico-projector, etc. 
         [0112]    During operation, the source transmits light beams to the ICO which refracts or deflects the light beams into the plane of the eyepiece. The control signal from a controller (not shown) may further switch on a designated layer such that the diffractive elements (e.g., OPE diffractive elements and EPE diffractive elements) on the layer perform their respective functions as described above with reference to  FIGS. 5-11  to produce the images at the designated focal plane as observed by the user&#39;s eye(s). Depending on where the images are intended to be observed by the user, the controller may further transmit further control signals to switch on one or more other layers and switch off the remaining layers to change the focal lengths as observed by the user&#39;s eye(s). The multi-planar configuration may provide a larger focal range by having one primary focal plane and one or more focal planes with positive margins in the focal lengths and one or more focal planes with negative margins in the focal lengths in some embodiments. 
         [0113]      FIGS. 13A-B  illustrate schematic representations of a switchable layer in some embodiments. In these embodiments, the apparatus may include the PDLC (polymer-dispersed liquid crystal) for ICO (in-coupling optics) and/or EPE switching. The apparatus includes the PDLC-filled area  1302 A and the ITO (Indium tin oxide) active area  1304 A that captures only one TIR (total internal reflection) bounce. The apparatus may also be operatively coupled to the ICO  1306 A.  FIG. 13A  illustrates the produced image when the voltage is off, and  FIG. 13B  illustrates the produced image when the voltage is on. In some of these embodiments, the PDLC-filled area or a portion thereof may be transmissive when no voltage or current is applied. 
         [0114]    The switchable layers in, for example, a diffractive optical element (DOE) including at least the substrate, the OPE diffractive elements, and the EPE diffractive elements may switch and thus adjust or shift focus at tens to hundreds of megahertz (MHz) so as to facilitate the focus state on a pixel-by-pixel basis in some embodiments. In some other embodiments, the DOE may switch at the kilohertz range to facilitate the focus on a line-by-line basis so the focus of each scan line may be adjusted. In some embodiments, a matrix of switchable DOE elements may be used for scanning, field of view expansion and/or the EPE. In addition or in the alternative, a DOE may be divided into multiple smaller sections, each of which may be uniquely controlled by its own ITO or other control lead material to be in an on state or an off state. 
         [0115]      FIG. 14  illustrates a schematic representation of a multiplexed expander element in some embodiments. The multiplexed expander element  1406  combines the OPE functionality by the diagonal OPE diffractive elements  1402  and the functionality of the EPE diffractive elements  1404  in a single element on a single layer. In some embodiments, a multiplexed expander may be formed by performing an exclusive OR between the OPE diffractive element surface  1402  and the EPE diffractive element surface  1404  with the computer-designed structures and microscopic fringe-writing techniques. One of the advantages of this approach is that the resulting multiplexed element may have fewer issues with scattering and diffractive elements cross terms. 
         [0116]    In some other embodiments, a multiplexed expander element may be formed by representing the OPE diffractive elements as a phase ramp and add the phase ramp to the lens functions in its continuous polynomial form and subsequently discretize a binary structure. One of the advantages of this second approach for fabricating multiplexed expander elements is that the high diffractive efficiency of the resulting multiplexed expander elements. In some other embodiments, a multiplexed expander element may be formed by pattern the combined patterns successively on the surface of the element, either before or after etching. 
         [0117]      FIG. 15A  illustrates a portion of a schematic representation of a multiplexed expander element in some embodiments. The multiplexed expander element  1502  includes the diagonal OPE diffractive elements and the out-coupling circular EPE diffractive elements in a single element on a single layer. When an incident light beam  1504  propagates within the layer (e.g., by total internal reflection or TIR) and hits the diagonal OPE diffractive elements, the diagonal OPE diffractive elements deflects a portion of the incident light beam  1504  to form the deflected light beam  1506 . A portion of the deflected light beam  1506  interacts with the out-coupling circular EPE diffractive elements and deflects a portion of the deflected light beam to the user&#39;s eye(s). 
         [0118]    The remaining portion of the incident light beam  1504  continues to propagate within the layer and interacts with the diagonal OPE diffractive elements in a substantially similar manner to continue to deflect a portion of the propagated light beams across the multiplexed element. It shall be noted that the combined diffraction or cross terms from both the diagonal OPE diffractive elements and the out-coupling EPE circular diffractive elements will be evanescent. The deflected light beam  1506  also propagates within the layer and interacts with both the diagonal OPE diffractive elements and the out-coupling circular EPE diffractive elements in a substantially similar manner. 
         [0119]      FIG. 15B  illustrates another pictorial representation of a multiplexed expander assembly in some other embodiments. In these embodiments illustrated in  FIG. 15B , the multiplexed expander assembly  1500 A includes three individual expander elements  1502 A,  1504 A, and  1506 A that are stacked on top of each other. The incident RGB (red, green, and blue) light  1508 A from the light source enters the multiplexed expander assembly  1500 A via an, for example, input coupling optic element (ICO) as described above. The multiplexed expander assembly  1500 A may include a first wavelength selective or wavelength specific filter (hereinafter color filter)  1510 A between the individual expander element  1502 A and  1504 A to allow light components of certain wavelength(s) to pass through while reflecting light components of other wavelength(s). For example, the first color filter may include a blue and green pass dichroic filter such that the blue and green light components in the incident light  1508 A pass through the first color filter  1510 A while the red light components are reflected and henceforth propagated with the individual expander element  1502 A by, for example, total internal reflection to interact with the OPE and/or the EPE diffractive elements. 
         [0120]    The multiplexed expander assembly  1500 A may include a second color filter  1512 A between the individual expander element  1504 A and  1506 A to allow light components of certain wavelength(s) to pass through while reflecting light components of other wavelength(s). For example, the second color filter may include a blue dichroic filter such that the blue light components in the incident light  1508 A pass through the second color filter  1512 A while the green light components are reflected and henceforth propagated with the individual expander element  1504 A by, for example, total internal reflection to interact with the OPE, EPE, and/or the focus adjustment diffractive elements (e.g., the circular or radially symmetric diffractive elements having optical powers) as shown in  FIG. 15B . 
         [0121]    The blue light components may also propagate within the individual expander element  1506 A by, for example, total internal reflection to interact with the OPE, EPE, and/or the focus adjustment diffractive elements (e.g., the circular or radially symmetric diffractive elements) as shown in  FIG. 15B . In some of the illustrated embodiments, the incident light  1508 A is transmitted into the multiplexed expander assembly  1500 A at an angle greater than the respective critical angles such that the respective light components may propagate within the respective individual expander element by total internal reflection. In some other embodiments, the multiplexed expander assembly  1500 A may further include a reflective coating to cause or enhance the efficiency of total internal reflection of the blue light components in the individual expander element  1506 A. 
         [0122]    The difference between the multiplexed expander assembly  1500 A and those illustrated in  FIGS. 14-15 , the multiplexed expander assembly  1500 A includes three individual expander elements, each of which includes its own OPE, EPE, and focus adjustment diffractive elements and is responsible for the corresponding light components of specific wavelength(s). The volumetric-phase diffractive elements used in  FIGS. 14-15  may be fabricated all at once with a single recording process or multiple recording processes on a single film or substrate as described above. Nonetheless, both the volumetric-phase diffractive elements as illustrated in  FIGS. 14-15  and multiplexing multiple individual expander elements illustrated in  FIG. 15B  provide multiplexed expander elements, each of which may include the OPE, EPE, and/or the focus adjustment diffractive elements for all three primary colors in the incident input light. 
         [0123]      FIG. 16  shows an illustration of a user  1602  using a virtual reality or augmented reality device  1604  described herein to view an image  1606 . Due to the multiple, switchable focal planes provided by the virtual reality or augmented reality device, the image  1606  appear to the user that the object in the image  1606  is located at the designated focal distance(s) from the user. When the object in the image is to move further away from the user, the virtual reality or augmented reality device may switch on a designated layer having certain circular diffractive element patterns that render the object on the focal plane with a longer focal distance hosted by the designated layer. 
         [0124]    When the object in the image is to move closer to the user, the virtual reality or augmented reality device may switch on another designated layer having certain circular diffractive element patterns that render the object on another focal plane with a shorter focal distance hosted by the designated layer. As a result of the use of different circular diffractive element patterns that change the focal points of the light beams forming the image, the object in the image may appear to the user that it is moving toward or away from the user. The virtual reality or augmented reality device  1604  may include the switchable, co-planar OPE diffractive elements and EPE diffractive elements, folded or overlaid OPE diffractive elements and EPE diffractive elements, multi-planar eyepieces, or a single-layer multiplexed OPE diffractive elements and EPE diffractive elements in different embodiments as previously described. The OPE diffractive elements and the EPE diffractive elements may include the surface relief type diffractive elements, the volumetric-phase type diffractive elements, or a combination thereof. 
         [0125]    Moreover, the OPE diffractive elements and/or the EPE diffractive elements may include linear diffractive elements that are summed with circular or radially symmetric diffractive elements to deflect and focus exiting light beams. The linear diffractive elements and the circular or radially symmetric diffractive elements may exist on a single film or on two separate films. For example, the DOE (diffractive optical element) diffractive elements (the OPE diffractive elements and/or the EPE diffractive elements) may include a first film having linear diffractive elements and attached to a second film having circular or radially symmetric diffractive elements. In some embodiments, the virtual reality or augmented reality device may employ time-varying diffractive element control to expand the field of view as observed by the user&#39;s eye(s) and/or to compensate for chromatic aberration. Both the linear and circular DOEs may be modulated or controlled over time (e.g., on a frame sequential basis) to, for example, produce tiled display configurations or expanded field of view for the light existing toward the eyes of a user. 
         [0126]      FIG. 17  illustrates a portion of  FIG. 16 . More specifically,  FIG. 17  shows the diffractive optical element  1702  including a substrate  1704  integrated with the OPE diffractive elements  1706  on the side of the substrate near the user and EPE diffractive elements  1708  on the other side of the substrate away from the user. The ICO  1710  transmits light beams into the substrate  1704 , and the OPE diffractive elements and EPE diffractive elements deflect the light beams as described above into the exiting light beams  1712  observed by the user&#39;s eye(s). 
         [0127]      FIG. 18  illustrates another perspective of a portion of  FIG. 16 . More specifically,  FIG. 18  shows the diffractive optical element  1802  including a substrate  1804  integrated with the OPE diffractive elements  1806  on the side of the substrate near the user and EPE diffractive elements  1808  on the other side of the substrate away from the user. The ICO  1810  transmits light beams into the substrate  1804 , and the OPE diffractive elements  1806  and EPE diffractive elements  1808  deflect the light beams as described above into the exiting light beams  1812  forming an image  1820  observed by the user&#39;s eye(s). The DOE  1802  includes both linear diffractive elements and circular or radially symmetric diffractive elements to not only deflect the light beams from the ICO  1810  but also produce exiting light beams  1818  to appear as if the exiting light beams were emanating from the object being observed at the focal distance defined by the focal plane of a specific layer that hosts the focal plane. 
         [0128]      FIG. 19  illustrates another perspective of a portion of  FIG. 16 . More specifically,  FIG. 19  shows the diffractive optical element  1902  including a substrate  1904  integrated with the OPE diffractive elements  1906  on the side of the substrate near the user and EPE diffractive elements  1908  on the other side of the substrate away from the user. The ICO  1910  transmits light beams into the substrate  1904 , and the OPE diffractive elements  1906  and EPE diffractive elements  1908  deflect the light beams as described above into the exiting light beams  1912  forming an image  1920  observed by the user&#39;s eye(s). The DOE  1902  includes both linear diffractive elements and circular or radially symmetric diffractive elements to not only deflect the light beams from the ICO  1910  but also produce exiting light beams  1918  to appear as if the exiting light beams were emanating from the object being observed at the focal distance defined by the focal plane of a specific layer that hosts the focal plane. 
         [0129]      FIG. 20  illustrates a close-up view of  FIG. 19  to provide a view of various elements of the diffractive optical element. More specifically,  FIG. 20  shows a portion of the DOE including the substrate  2004 , the OPE diffractive elements  2006  on one side of the substrate  2004  near the user, and the EPE diffractive elements  2008  on the other side of the substrate  2004 . The ICO  2010  is disposed relative to the substrate to refract and transmit input light beams into the substrate. The input light beams are propagated within the substrate  2004  via total internal reflection (TIR) and interact with the OPE diffractive elements  2006  and EPE diffractive elements  2008  to deflect the input light beams into the exiting light beams  2012  observed by the user&#39;s eye(s). 
         [0130]      FIG. 21  illustrates a side view of an illustration of a user using a virtual reality or augmented reality device to view an image. The diffractive optical element  2102  includes a substrate  2104  operatively coupled to the OPE diffractive elements  2106  disposed on the near side of the substrate  2004  and the EPE diffractive elements  2108  disposed on the far side of the substrate  2104 . The shapes  2112  represent the exiting light beams observable by the user&#39;s eye(s). The shapes  2130  represent the light beams bouncing between the OPE diffractive elements  2106  and the EPE diffractive elements  2108  along the vertical direction (as shown in  FIG. 21 ) within the substrate  2104 . The input light beams from, for example, the ICO element also bounce between the OPE diffractive elements  2106  and the EPE diffractive elements  2108  along the Z-direction (pointing into or out of the plane as shown in  FIG. 21 ) in a substantially similar manner. Each time the light beams hits the OPE diffractive elements  2106 , the OPE diffractive elements deflect a portion of the light beams toward the EPE diffractive elements  2108  which in turn deflects a portion of the deflected portion of the light beams toward the user&#39;s eye(s). 
         [0131]      FIG. 22  illustrates a close-up view of the diffractive optical element (DOE) in some embodiments. The DOE includes the combination OPE/EPE diffractive elements  2204  disposed on one side of the substrate  2202 . The input light beams  2214  are transmitted into the substrate via the in-coupling optics  2206  and propagate within the substrate  2202  via total internal reflection (TIR). The input light beams bounce within the substrate  2202  and interact with both the combination OPE/EPE diffractive elements  2204 . More specifically, the combination of OPE/EPE diffractive elements  2204  deflects a portion of the input light beams in orthogonal directions which are substantially parallel to the surfaces of substrate  2202 . 
         [0132]    It shall be noted that although the combination OPE/EPE diffractive elements  2204  may be designed or intended to deflect light beams in orthogonal directions that are perfectly parallel to the surface of the substrate  2202 , the tolerances, slacks, and/or allowances in the fabrication process(es) may nonetheless cause some deviations in the fabricated product. In addition or in the alternative, the tolerances, slacks, and/or allowances in the arrangement or relative positioning of various devices and components or the variations in the uniformity of various properties of the materials used may also cause the aforementioned orthogonal directions to deviate from being perfectly parallel to the surface of the substrate  2202 . Therefore, the aforementioned “orthogonal directions” are “substantially parallel” to the surface of the substrate  2202  to accommodate such variations in the fabrication process(es), the arrangement, the relative position, and/or various variations. 
         [0133]    The EPE diffractive elements deflect a portion of the deflected portion of the input light beams into the exiting light beams  2208  toward the user&#39;s eye(s). The shapes  2208  represent the exiting light beams observable by the user&#39;s eye(s). The shapes  2208  in  FIG. 22  represent infinitely-focused image information, however any other focal distance may be produced using this approach. In some embodiments where the EPE diffractive elements include circular or radially symmetric diffractive elements in addition to linear diffractive elements, each of these shapes may have a conical form with the apex at the focal point of the circular or radially symmetric diffractive elements. 
         [0134]    The zigzagged shapes  2210  represent a portion of the input light beams bouncing within the substrate and interacting with the combination OPE/EPE diffractive elements  2204 . Each time when the portion of the light beams hits the combination OPE/EPE diffractive elements  2204 , the OPE component diffractive elements deflect a portion of the light beams laterally through the substrate. Each time when the portion of deflected light beams hits the combination OPE/EPE diffractive elements  2204 , the EPE component diffractive elements deflect a portion of the light beams toward the user&#39;s eye(s) and thus form the light beams  2208  observable by the user&#39;s eye(s). 
         [0135]    The remainder of the portion of the light beams not deflected by the combination OPE/EPE diffractive elements  2204  continues to propagate within the substrate  2202  as shown by  2210 . Due to the refraction index and/or the diffraction efficiency, the remaining part of the deflected portion of the light beams not deflected by the combination OPE/EPE diffractive elements continues to propagate with the substrate as indicated by the zigzagged shapes  2212 . As a result, the DOE including the combination OPE/EPE diffractive elements effectively transform the input light beams into a matrix of exiting light beams forming the images perceived by the user&#39;s eye(s). 
         [0136]      FIG. 23A  illustrates a high level flow diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in some embodiments. Input light beams may be transmitted at  2302 A into a substrate of an eyepiece for virtual reality and/or augmented reality using at least an in-coupling optical element (e.g., reference numeral  606  of  FIG. 6 , reference numeral  712  of  FIG. 7 , reference numeral  808  of  FIG. 8 , etc.) The substrate may comprise a translucent or transparent dielectric material. 
         [0137]    A first portion of the input light beams may be deflected using the first diffractive elements toward the second diffractive elements at  2304 A. For example, first diffractive elements may be arranged at an acute or obtuse orientation to the direction of propagation of the first portion of the input light beams coming out of the in-coupling optical element to deflect the first portion of first portion of the input light beams toward the second diffractive elements. An example of deflecting the first portion light using the first diffractive elements toward the second diffractive elements is described above with reference to  FIG. 7 . In some of these embodiments, the first diffractive elements comprise exit pupil expansion (EPE) structures or diffractive elements or exit pupil expanders. 
         [0138]    At  2306 A, the first exiting light beams may be directed or redirected toward an observer by deflecting at least a portion of the first portion of the input light beams using the second diffractive elements. In some of these embodiments, the second diffractive elements comprise orthogonal pupil expansion (OPE) structures or diffractive elements or orthogonal pupil expanders. 
         [0139]      FIGS. 23B-C  jointly illustrate a more detailed flow diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in some embodiments. In some embodiments, the process may first transmit input light beams into a substrate of an eyepiece at  2302 . For example, the process may involve transmitting light beams from a projector through one or more fibers to an in-coupling optic element described above with reference to at least  FIG. 5 , and the in-coupling optic element further relays the input light beams to the substrate of an eyepiece via, for example, refraction. The process may further optionally switch on a first layer of one or more layers of a diffractive optical element (DOE) at  2304 . 
         [0140]    The first layer includes the first diffractive elements (e.g., OPE diffractive elements described above) and the second diffractive elements (e.g., EPE diffractive elements described above). The first diffractive elements and the second diffractive elements may be arranged in a co-planar or side-by-side manner or a folded or overlaid manner in some embodiments. In some other embodiments, the first diffractive elements and the second diffractive elements may be fabricated and co-exist in a multiplexed manner on a single layer of film as described in some of the preceding paragraphs. The DOE may include multiple such layers that are stacked on top of each other to form a multi-planar DOE as described earlier. 
         [0141]    The first diffractive elements and second diffractive elements may include the surface-relief type diffractive elements, the volumetric-phase type diffractive elements, or a combination thereof. The first diffractive elements or the second diffractive elements may include both linear diffractive elements and circular or radially symmetric diffractive elements to deflect as well as focus input light beams. With both the linear diffractive elements and the circular or radially symmetric diffractive elements, the first layer may therefore host a first focal plane associated with a first focal length such that an image of an object created by the light beams deflected from the first layer may appear to be at the focal length to a user&#39;s eye(s) as if the user is observing the object that were physically located at the location defined by the focal length in real world. 
         [0142]    In some embodiments, the DOE may include multiple layers, each hosting its own focal plane with a unique focal length. Each of these multiple layers may comprise a switchable layer that may be switched on and off by using control signals. At  2306 , the process may deflect a first portion of the input light beams toward the second diffractive elements by using the first diffractive elements on the first layer. For example, the process may use the OPE diffractive elements described earlier to deflect a portion of the input light beams toward the EPE diffractive elements. 
         [0143]    The process may then direct the first exiting light beams toward a user&#39;s eye via the eyepiece by deflecting some of the first portion of input light beams with the second diffractive elements at  2308 . For example, the process may use the EPE diffractive elements described earlier to deflect a portion of the input light beams deflected from the OPE diffractive elements toward the user&#39;s eye. At  2310 , the process may further transmit the remaining portion of the input light beams that is not deflected to the second diffractive elements within the substrate of the eyepiece. The amount of the remaining portion of the input light beams depends on the diffraction efficiency, the refraction indices, desired or required uniformity of the final output light beams, the diffractive elements involved, or any other pertinent factors. 
         [0144]    The process may further deflect some of the remaining portion of the input light beams toward the second diffractive elements by using the first diffractive elements of the first layer at  2312 . For example, some of the input light beams that continue to propagate within the substrate of the eyepiece due to the transmissive property of the first diffractive elements may hit different portion of the first diffractive elements and be deflected by this different portion of the first diffractive elements toward the second diffractive elements due to the reflective property of the first diffractive elements. At  2314 , the process may direct the second exiting light beams toward the user&#39;s eye(s) by deflecting some of the remaining portion of the input light beams with the second diffractive elements. For example, the process may use the EPE diffractive elements to deflect some of the incoming light beams from the OPE diffractive elements toward the user&#39;s eye(s) at  2314 . 
         [0145]    At  2316 , the remaining portion of the first portion of input light beams continues to propagate with the substrate of the eyepiece via, for example, total internal reflection (TIR) due to the transmissive property of the second diffractive elements. At  2318 , the remaining portion of the first portion of input light beams propagates within the substrate and thus interacts with both the first diffractive elements and the second diffractive elements. When some of the remaining portion hits the first diffractive elements, the first diffractive elements deflect the light beams toward the second diffractive elements which in turn deflect these light beams into the additional exiting light beams toward the viewer&#39;s eye(s). The process may then generate a first image for the viewer to perceive via the eyepiece with the first exiting light beams, the second exiting beams, and the additional exiting light beams at  2320 . 
         [0146]    In some embodiments where both the linear diffractive elements and the circular or radially symmetric diffractive elements are utilized, the first layer may therefore host a first focal plane associated with a first focal length such that the image of an object created by these exiting light beams deflected from the first layer may appear to be at the focal length to the viewer&#39;s eye(s) as if the viewer is observing the object that were physically located at the location defined by the focal length in real world. An image may include a static image such as a picture or may be a dynamic image such as a part of a motion picture. At  2322 , the process may further optionally switch a second layer that hosts a second focal plane with a second focal length. A second image for the view may be generated at  2324  by using at least the third diffractive elements and the fourth diffractive elements. 
         [0147]    The second layer may include its own third diffractive elements and fourth diffractive elements such as the OPE diffractive elements and the EPE diffractive elements described above. The process may then repeat the steps of  2302  through  2320  to generate a second image of an object for the viewer as described immediately above. The second image may appear to be at the second focal length to the viewer&#39;s eye(s) as if the viewer is observing the object that were physically located at the location defined by the second focal length in real world. In some of these embodiments illustrated in  FIG. 23 , these multiple layers of the diffractive optical element may be dynamically switchable at a rate ranging from one or higher kilohertz (KHz) to hundreds of megahertz (MHz) to facilitate the focus state on a line-by-line basis or on a pixel-by-pixel basis. These multiple layers may include PDLC layers and may be switched on and off by using control signals to change the focal planes of the produced images. This illustrative multi-layer approach may also operatively coupled to a rapidly switching in-coupling optics (ICO)  1206 D that may be further operatively coupled to a light source such as a fiber, a bundle of fibers, a multi-fiber projector, or a pico-projector, etc. 
         [0148]      FIG. 24A  illustrates a high level block diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. A first substrate for an eye piece may be identified (if already existing) or fabricated (if non-existent) for an eyepiece at  2402 . In some of these one or more embodiments, a first substrate may include a translucent or transparent dielectric material having a single layer or multiple layers. The first diffractive elements and the second diffractive elements may be identified (if already existing) or fabricated (if non-existent) on or in one or more first films at  2404 . A film comprises a sheet of material whose thickness is smaller than a predetermined percentage of the length or width of the material in some embodiments. 
         [0149]    In some of these embodiments, the first diffractive elements comprise exit pupil expansion (EPE) structures or diffractive elements or exit pupil expanders. In some of these embodiments, the second diffractive elements comprise exit orthogonal pupil expansion (OPE) structures or diffractive elements or orthogonal pupil expanders. The one or more films may then be disposed on the first substrate at  2406  in some embodiments. In some other embodiments, the one or more films accommodating the first diffractive elements and the second diffractive elements may be identified at  2406  on the first substrate. With the one or more first films accommodating the first and second diffractive elements and disposed on the first substrate, input light beams may be transmitted at  2408  from an input light source into the first substrate. In some of these embodiments, the input light source comprises an in-coupling optic element disposed in or on the eyepiece and coupled with the first diffractive elements or the second diffractive elements. 
         [0150]      FIG. 24B  illustrates a more detailed block diagram for the process of generating stereoscopic images for virtual reality and/or augmented reality illustrated in  FIG. 24A  in one or more embodiments. More specifically,  FIG. 24B  illustrates more details about the act of disposing the one or more first films on the first substrate. In some these embodiments, the first diffractive elements and the second diffractive elements may be identified or arranged at  2402 B in a co-planar arrangement on one side of the first substrate. An example of this co-planar arrangement is illustrated in  FIG. 7 . 
         [0151]    Alternatively, the first diffractive elements and the second diffractive elements may be identified or arranged at  2404 B in a folded or partially or completely overlaid arrangement on one side or two sides of the first substrate. Some examples of this folded or overlaid arrangement are illustrated in  8 - 9 ,  10 A-B, and  11 . In some embodiments where the first diffractive elements and second diffractive elements are already implemented, the arrangement of the first diffractive elements and second diffractive elements may be identified at  2402 B or  2404 B. With the arrangement of the first and second diffractive elements identified or devised on a unitary, inseparable layer disposed on one side of the first substrate, the first diffractive elements and the second diffractive elements may be multiplexed at  2406 B. 
         [0152]      FIG. 24C  illustrates a more detailed block diagram for a process of generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. In these embodiments, a first substrate for an eyepiece may be identified (if already existing) or fabricated (if not yet devised) at  2402 C. The first diffractive elements and the second diffractive elements may also be identified (if already existing) or fabricated (if not yet devised) on one or more first films at  2404 C. That is, the first and second diffractive elements may be devised in a single film or layer of material in some of these embodiments by using, for example, volumetric phase recording techniques, surface-relief type diffractive element techniques, or a combination of both the volumetric phase recording techniques and the surface-relief type diffractive element techniques. 
         [0153]    Alternatively, the first diffractive elements and the second diffractive elements may be devised on two or more separate layers or films that are optically coupled with each other. For example, the first diffractive elements may be devised on a first film, and the second diffractive elements may be devised on a second film in some of these embodiments. At  2406 C, the one or more first films accommodating the first and second diffractive elements may be disposed on the first substrate. Input light beams from an input light source including, for example, an in-coupling optic element or device may be transmitted into the first substrate at  2408 C. The input light source may be disposed in or on the eyepiece and may also be coupled with the first diffractive elements, the second diffractive elements, or a combination of both the first and second diffractive elements. A second substrate may similarly be identified or fabricated for the eyepiece at  2410 C as the first substrate is at  2402 C. 
         [0154]    The third diffractive elements and the fourth diffractive elements may also be identified (if already existing) or fabricated (if not yet devised) on one or more first films at  2412 C. That is, the third and fourth diffractive elements may be devised in a single film or layer of material in some of these embodiments by using, for example, volumetric phase recording techniques, surface-relief type diffractive element techniques, or a combination of both the volumetric phase recording techniques and the surface-relief type diffractive element techniques. 
         [0155]    Alternatively, the third diffractive elements and the fourth diffractive elements may be devised on two or more separate layers or films that are optically coupled with each other. For example, the third diffractive elements may be devised on a third film, and the fourth diffractive elements may be devised on a fourth film in some of these embodiments. In some of these embodiments, the third diffractive elements may comprise linear, circular, radially symmetric, or any combinations of linear, circuit, or radially symmetric diffractive elements. In addition or in the alternative, the fourth diffractive elements may include linear, circular, radially symmetric, or any combinations of linear, circuit, or radially symmetric diffractive elements while the third and fourth diffractive elements are different from each other. 
         [0156]    The one or more second films may be disposed or identified on the second substrate at  2414 C. The second substrate may further be disposed on the first substrate at  2416 C. In some embodiments, the first and second diffractive elements on the first substrate may be dynamically switchable between two states (e.g., on and off states) by using, for example, electrical currents or voltages. In addition or in the alternative, the third and fourth diffractive elements on the first substrate may be dynamically switchable between two states (e.g., on and off states) also by using, for example, electrical currents or voltages. Dynamically switchable diffractive elements may enable time-multiplexed distribution of projected images to multiple focal-plane imaging elements. The switch rate may range from one kilohertz (1 KHz) to hundreds of megahertz (MHz) to facilitate the focus state on a line-by-line basis or on a pixel-by-pixel basis. 
         [0157]      FIG. 25A  illustrates a high level block diagram for generating stereoscopic images for virtual reality and/or augmented reality in one or more embodiments. More specifically,  FIG. 25A  together with  FIGS. 25B-D  illustrate more details about propagating input light beams through diffractive elements to produce stereoscopic images for virtual reality and/or augmented reality. In these one or more embodiments, input light beams may be received at  2502 A from an input light source including, for example, an in-coupling optic element or device. 
         [0158]    In some embodiments, the first diffractive elements may be arranged at a first orientation that forms an acute or obtuse angle with respect to the incident direction of the input light beams. The first portion of the input light beams propagated from the input light source into the first diffractive elements may be deflected at  2504 A with the first diffractive elements toward the second diffractive elements in the eyepiece. In some embodiments, the first diffractive elements may include the exit pupil expansion (EPE) diffractive elements or expanders, and the second diffractive elements may include the orthogonal pupil expansion (OPE) diffractive elements or expanders. 
         [0159]    A second portion of the input light beams may be propagated through the second diffractive elements having a second orientation different from the first orientation to produce the stereoscopic images to an observer at  2506 A. In some embodiments, the ratio between the first portion and the second portion may be determined based in part or in whole upon the transmissive and reflective properties of the first or second diffractive elements. In some embodiments, the second portion may constitute the remaining portion of the input light beams exiting the input light source and may propagate through the second diffractive elements via total internal reflection (TIR). 
         [0160]      FIGS. 25B-D  jointly illustrate some additional, optional acts  2500 B that may be individually performed or jointly performed in one or more groups for the process of generating stereoscopic images for virtual reality and/or augmented reality illustrated in  FIG. 25A . It shall be noted that some of the acts illustrated in  FIGS. 25B-D  may be individually performed and thus are not connected to other acts with arrowheads in  FIGS. 25B-D . In these embodiments, input light beams may be received at  2502 B from an input light source including, for example, an in-coupling optic element or device as similarly described above with reference to  FIG. 25A . 
         [0161]    The first portion of the input light beams propagated from the input light source into the first diffractive elements may be deflected at  2504 B with the first diffractive elements toward the second diffractive elements in the eyepiece. A second portion of the input light beams may be propagated through the second diffractive elements having a second orientation different from the first orientation to produce the stereoscopic images to an observer at  2506 B. During any point in time between receiving the input light beams at  2502 B and finally producing the stereoscopic images at  2506 B, one or more of the additional, optional acts  2500 B may be performed. For example, artifacts in the stereoscopic images may be reduced by at least modulating the diffraction efficiency of the first diffractive elements or the second diffractive elements or a combination of the first and second diffractive elements at  2508 B in some embodiments. 
         [0162]    A host medium for the first diffractive elements and/or the second diffractive elements may be identified at  2510 B. In some embodiments, the host medium may include at least one of a dry-process photopolymer material, a single-layer silver halides, or single-layer polymer-dispersed liquid crystal mixture material. Propagation of the input light beams may be guided at  2512 B by at least successively redirecting the first light wave-fronts of at least the first portion of the input light beams with the first diffractive elements. 
         [0163]    Propagation of the input light beams may be further guided at  2512 B by out-coupling the redirected first light wave-fronts with at least the second portion of the input light beams that propagate through the second diffractive elements. The earlier part and later part of interactions (in terms of temporal or spatial order) between the input light beams and the first and/or the second diffractive elements may be controlled at  2514 B by at least ramping a diffraction efficiency of one or more components in the eyepiece with different diffraction efficiencies. In these embodiments, the diffraction efficiency of the eyepiece components may be ramped such that the initial interaction between the light rays and the structures use less of the available light than later interactions to reduce or eliminate the reduction in image field brightness distribution across the eyepiece as the light propagates. 
         [0164]    A grating diffraction efficiency may also be distributed at  2516 B for the first and/or the second diffractive elements by at least modulating the recording beam intensities or a ratio of the recording beam intensities in preparing the first and/or the second diffractive elements. Time-multiplexed distribution of projected images may be provided at  2518 B to multiple focal-plane image elements by using switchable diffractive elements for the first and/or the second diffractive elements. In some embodiments, polymer-dispersed liquid crystal (PDLC) components may be identified at  2520 B for the first and/or the second diffractive elements. In some embodiments involving the PDLC components, a host medium for the PDLC components may be identified at  2522 B, and structural elements in the host medium of the PDLC components may be identified at  2524 B. 
         [0165]    A refraction index of the host medium or the structural elements may then be determined at  25328  to be an index that mismatches the refraction index of the substrate that accommodates the first diffractive elements and the second diffractive elements. That is the refraction index of the host medium or the structural elements may be different from the refraction index of the substrate in these embodiments. In some embodiments, a single-layer structure may be identified at  2526 B, and the first diffractive elements and the second diffractive elements may be identified or devised at  2528 B in the single-layer structure. With the single-layer structure, crosstalk in diffraction of the propagation of the input light beams in at least a portion of the eyepiece may be reduced at  2530 B by at least multiplexing the first and the second diffractive elements in the single-layer structure. 
         [0166]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the above-described process flows are described with reference to a particular ordering of process actions. However, the ordering of many of the described process actions may be changed without affecting the scope or operation of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.