Methods of fabricating optical waveguides

Abstract

To provide mastering systems and methods for fabricating waveguides, and waveguide devices using such mastering systems.SOLUTION: A mastering system for fabricating holographic waveguides may employ a step of using a master to control application of energy (e.g., laser, light, or magnetic beam) onto a liquid crystal substrate to fabricate a holographic waveguide into the liquid crystal substrate. Mastering systems for fabricating holographic waveguides according to embodiments of the present invention may include a variety of features.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates generally to methods for the manufacture of waveguides, and more particularly to methods for waveguide displays. [Background technology]

[0002] A waveguide can be referred to as a structure with the ability to confine and guide waves (i.e., restrict the spatial region through which waves can propagate). One subclass includes optical waveguides, which are structures that can guide electromagnetic waves, typically those in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using several different mechanisms. For example, planar waveguides can be designed utilizing diffraction gratings to diffract and couple incident light into the waveguide structure, so that the internally coupled light can continue traveling within the planar structure via total internal reflection ("TIR").

[0003] Waveguide fabrication can include the use of material systems that enable the recording of holographic optical elements within the waveguide. One class of such materials includes polymer-dispersed liquid crystal ("PDLC") mixtures, which are mixtures containing a photopolymerizable monomer and a liquid crystal. A further subclass of such mixtures includes holographic polymer-dispersed liquid crystal ("HPDLC") mixtures. Holographic optical elements, such as volume phase gratings, can be recorded in such liquid mixtures by irradiating the material with two mutually coherent laser beams. During the recording process, the monomer polymerizes, and the mixture undergoes photopolymerization-induced phase separation, creating regions densely populated with liquid crystal microdroplets interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating.

[0004] Waveguide optics such as those described above can be considered for a variety of display and sensor applications. In many applications, waveguides containing one or more grating layers encoding multiple optical functions can be realized using various waveguide architectures and material systems, enabling new innovations in eyepiece displays for augmented reality ("AR") and virtual reality ("VR"), compact head-up displays ("HUDs") for aviation and road traffic, and sensors for biometric and laser radar ("LIDAR") applications. Summary of the Invention [Means for solving the problem]

[0005] One embodiment includes a method for recording a hologram, the method including the steps of providing a waveguide cell comprising a layer of a polymer dispersed liquid crystal mixture sandwiched between two substrates; providing a master grating; emitting at least one recording beam toward the master grating, wherein, upon interaction with the master grating, a portion of the at least one recording beam is diffracted toward the waveguide cell; and recording at least one volume grating in the waveguide cell using an interference exposure formed from at least the diffracted portion of the at least one recording beam.

[0006] Again, a further embodiment includes a system for recording holographic gratings, the system including a waveguide cell including a layer of a polymer dispersed liquid crystal mixture sandwiched between two substrates, a master grating, and a light source configured to emit at least one recording beam toward the master grating, wherein upon interaction with the master grating, a portion of the at least one recording beam is diffracted toward the waveguide cell, and at least one volume grating is recorded in the waveguide cell through an interference exposure formed from at least the diffracted portion of the at least one recording beam.

[0007] In another embodiment, the master grating comprises an amplitude grating.

[0008] In a further embodiment, the master grating comprises a chirped grating.

[0009] In yet another embodiment, the recorded volume grid contains a rolled K-vector.

[0010] In still a further embodiment, the recorded volume grid contains multiplexed grids.

[0011] In yet another embodiment, the master grating includes three separate gratings.

[0012] In yet a further embodiment, three separate lattices are designed to record the input lattice, the folded lattice, and the output lattice.

[0013] In another additional embodiment, the at least one volume grid includes three volume grids.

[0014] In yet an additional embodiment, the at least one recording beam includes three recording beams.

[0015] Again, in another embodiment, the interference exposure is formed from the diffracted portion of the zero order beam and only one recording beam.

[0016] Additional embodiments and features are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of this specification or may be learned by practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and drawings that form a part of this disclosure.

[0017] The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention. It will be apparent to those skilled in the art that the present invention may be practiced using some or all of the invention as disclosed in the following description. [Brief explanation of the drawings]

[0018] [Figure 1A] 1A and 1B conceptually illustrate two volume Bragg grating configurations according to various embodiments of the present invention. [Figure 1B] 1A and 1B conceptually illustrate two volume Bragg grating configurations according to various embodiments of the present invention. [Figure 2] FIG. 2 conceptually illustrates a surface relief grating according to one embodiment of the present invention. [Figure 3A] 3A and 3B conceptually illustrate the switching properties of an HPDLC SBG device and SBG according to various embodiments of the present invention. [Figure 3B] 3A and 3B conceptually illustrate the switching properties of an HPDLC SBG device and SBG according to various embodiments of the present invention. [Figure 4A] 4A-4D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 4B] 4A-4D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 4C] 4A-4D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 4D] 4A-4D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 5] FIG. 5 conceptually illustrates a single beam recording process utilizing an amplitude grating, according to one embodiment of the present invention. [Figure 6A] 6A-6B conceptually illustrate two implementations of a roll-axis rotating K-vector grating, according to various embodiments of the present invention. [Figure 6B] 6A-6B conceptually illustrate two implementations of a roll-axis rotating K-vector grating, according to various embodiments of the present invention. [Figure 7]FIG. 7 conceptually illustrates a multiplexed K-vector lattice in accordance with one embodiment of the present invention. [Figure 8] FIG. 8 conceptually illustrates a waveguide that utilizes a coupling grating to diffract light into and out of the waveguide, according to one embodiment of the present invention. [Figure 9] 9 and 10 conceptually illustrate a waveguide utilizing an output grating for exit pupil expansion in one dimension, according to one embodiment of the present invention. [Figure 10] 9 and 10 conceptually illustrate a waveguide utilizing an output grating for exit pupil expansion in one dimension, according to one embodiment of the present invention. [Figure 11] FIG. 11 conceptually illustrates a waveguide system utilizing two planar waveguides to provide exit pupil expansion in two dimensions, according to one embodiment of the present invention. [Figure 12] FIG. 12 conceptually illustrates a waveguide utilizing a three-grating structure to provide two-dimensional exit pupil expansion, according to one embodiment of the present invention. [Figure 13] FIG. 13 conceptually illustrates a contour diagram of an RGB stack in a waveguide, according to one embodiment of the present invention. [Figure 14] FIG. 14 conceptually illustrates a two-axis expanded waveguide display with two grating layers, according to one embodiment of the present invention. [Figure 15] FIG. 15 conceptually illustrates a plan view of a single grating layer, according to one embodiment of the present invention. [Figure 16] FIG. 16 conceptually illustrates a plan view of a two-grating layer configuration, according to one embodiment of the present invention. [Figure 17] FIG. 17 conceptually illustrates a two-axis expanded waveguide display according to one embodiment of the present invention. [Figure 18] FIG. 18 conceptually illustrates an eye tracker display according to one embodiment of the present invention. [Figure 19] FIG. 19 conceptually illustrates a dual magnification waveguide display with dynamic focusing elements and an eye tracker, according to one embodiment of the present invention. [Figure 20A] 20A and 20B conceptually illustrate a waveguide display coupled to an input image node by an opto-mechanical interface, according to one embodiment of the present invention. [Figure 20B] 20A and 20B conceptually illustrate a waveguide display coupled to an input image node by an opto-mechanical interface, according to one embodiment of the present invention. [Figure 21] 21-24 conceptually illustrate various input image node configurations according to various embodiments of the present invention. [Figure 22] 21-24 conceptually illustrate various input image node configurations according to various embodiments of the present invention. [Figure 23] 21-24 conceptually illustrate various input image node configurations according to various embodiments of the present invention. [Figure 24] 21-24 conceptually illustrate various input image node configurations according to various embodiments of the present invention. [Figure 25] FIG. 25 conceptually illustrates a system diagram showing components associated with a waveguide display, according to one embodiment of the present invention. [Figure 26] FIG. 26 conceptually illustrates an exposure process utilizing a chirped amplitude grating, according to one embodiment of the present invention. [Figure 27A] 27A and 27B conceptually illustrate an exposure process for simultaneously forming three gratings, according to one embodiment of the present invention. [Figure 27B] 27A and 27B conceptually illustrate an exposure process for simultaneously forming three gratings, according to one embodiment of the present invention. [Figure 28] FIG. 28 conceptually illustrates a processing setup for simultaneously exposing red, blue, and green gratings, according to one embodiment of the present invention. [Figure 29A] 29A-29C conceptually illustrate various approaches for generating RKV lattices, according to various embodiments of the present invention. [Figure 29B]29A-29C conceptually illustrate various approaches for generating RKV lattices, according to various embodiments of the present invention. [Figure 29C] 29A-29C conceptually illustrate various approaches for generating RKV lattices, according to various embodiments of the present invention. [Figure 30A] 30A and 30B conceptually illustrate various applications of chirped gratings, according to various embodiments of the present invention. [Figure 30B] 30A and 30B conceptually illustrate various applications of chirped gratings, according to various embodiments of the present invention. [Figure 31] FIG. 31 conceptually illustrates a mastering system utilizing a zero-order grating in conjunction with a chirped grating, in accordance with one embodiment of the present invention. [Figure 32] FIG. 32 conceptually illustrates a mastering system utilizing a reference grating, in accordance with one embodiment of the present invention. [Figure 33] FIG. 33 conceptually illustrates a mastering system configured to avoid other order beams from being created and / or interfering with the energy beam being focused onto the liquid crystal substrate, in accordance with one embodiment of the present invention. [Figure 34] FIG. 34 conceptually illustrates the effect of the positioning of the master grating on the resulting diffraction. DETAILED DESCRIPTION OF THE INVENTION

[0019] For purposes of describing embodiments, some well-known features of optical technology known to those skilled in the art of optical design and visual displays have been omitted or simplified so as not to obscure the underlying principles of the present invention. Unless otherwise noted, the term "on-axis" in reference to a ray or beam direction refers to propagation parallel to an axis normal to the surface of an optical component described in connection with the present invention. In the following description, the terms light, ray, beam, and direction are used synonymously and interrelatedly and may refer to the direction of propagation of electromagnetic radiation along a linear trajectory. The terms light and illumination may be used in connection with the visible and infrared bands of the electromagnetic spectrum. Portions of the following description will be presented using terminology commonly adopted by those skilled in the art of optical design. As used herein, the term grating may, in some embodiments, encompass a grating consisting of a set of gratings. It should be understood that for illustrative purposes, the drawings are not drawn to scale unless otherwise noted.

[0020] Turning now to the drawings, mastering systems and methods for fabricating waveguides and waveguide devices using such mastering systems are described. A mastering system for fabricating holographic waveguides can include using a master to control the application of energy (e.g., laser, light, or magnetic beam) onto a liquid crystal substrate to fabricate a holographic waveguide therein. After fabrication, the final holographic waveguide can be incorporated into various display systems. These mastering systems can employ one or more energy beams. In many embodiments, the mastering system uses a single energy beam, which can simplify the alignment of various components within the mastering system and reduce wavefront errors found in dual-beam systems caused by the different paths of the two beams. Thus, a single-energy beam process can be compatible with high-volume manufacturing processes where thermal and vibration considerations can introduce various challenges when aligning multiple energy beams. A mastering system for fabricating holographic waveguides according to embodiments of the present invention can include various features. These features include, but are not limited to, chirp for single input beam copy (nearby, i.e., hybrid contact copy), dual chirp gratings (for input and output), zero order gratings for transmittance control, alignment reference gratings, 3:1 structures, alignment tooling to enable rapid alignment, simultaneous optimization of lens and window thickness for multiple roll axis rotating K-vector gratings, and avoidance of other orders and crossovers of diffracted beams. The waveguide structure, mastering system, and exposure process are described in further detail in the following sections. Waveguide Structure

[0021] Waveguide structures according to various embodiments can be implemented in many different ways. In many embodiments, the waveguide structures are designed to be optical waveguides, which are structures that can confine and guide electromagnetic waves or light in the visible spectrum. These optical waveguides can be implemented for use in several different applications, such as, but not limited to, helmet-mounted displays, head-mounted displays ("HMDs"), and HUDs. The term HUD is typically utilized to describe a class of devices that incorporate a transparent display that presents data without requiring the user to alter their normal field of view. Optical waveguides can integrate various optical functions into a desired form factor, depending on a given application.

[0022] Optical waveguides according to various embodiments can be designed to manipulate light waves in a controlled manner using various methods and waveguide optics. For example, optical waveguides can be implemented using materials with a higher refractive index than the surrounding environment to limit the area through which light can propagate. Light coupled into an optical waveguide made from such a material at an angle can be confined within the waveguide via total internal reflection. In a planar waveguide, the angle at which total internal reflection occurs can be given by Snell's law, which can determine whether light is refracted or completely reflected at a surface boundary.

[0023] In many embodiments, a waveguide incorporating a Bragg grating is implemented for HUD applications. The HUD can be implemented in any of a variety of applications, including (but not limited to) eyepiece applications. HUDs utilizing planar waveguides incorporating Bragg gratings according to various embodiments of the present invention can achieve a significantly wider field of view and have lower volume requirements than HUDs implemented using conventional optical components. In some embodiments, the HUD includes at least one waveguide incorporating several gratings. In further embodiments, the waveguide incorporates at least three Bragg gratings, which can be implemented to provide various optical functions, such as, but not limited to, two-axis beam expansion. For example, in some embodiments, the waveguide incorporates an input grating, a folded grating, and an output grating. HUDs utilizing waveguides can be implemented using various numbers of waveguides. In many embodiments, the HUD is implemented using a single waveguide. In other embodiments, the HUD is implemented using a stack of waveguides. Multiple waveguides can be stacked and implemented to provide different optical functions, such as, but not limited to, implementing a color display. In some embodiments, the HUD incorporates three separate waveguides: one for the red, green, and blue channels.

[0024] Waveguides utilizing Bragg gratings according to various embodiments of the present invention can be designed to have different types of fringes. The use of multiple waveguides with the same surface pitch size but different grating tilt angles can increase the overall coupling internal angle bandwidth of the waveguide. In some embodiments, one or more of the gratings within the waveguide incorporate a roll-axis rotation K vector and / or tilt angle that varies across the grating to modify the grating's diffraction efficiency. The K vector can be defined as a vector orthogonal to the plane of the associated grating fringe, which can determine the optical efficiency for a given range of input and diffraction angles. By incorporating a grating with a roll-axis rotation K vector ("RKV"), the grating can be designed to vary the diffraction efficiency in a manner that achieves desired characteristics across the eyebox of a HUD display. Grating fringe configurations (such as RKVs) and other aspects related to the construction and implementation of waveguides for use in HUDs are discussed in further detail below. Diffraction Gratings

[0025] Optical waveguides can incorporate different optical elements to manipulate the propagation of light waves. As can be easily understood, the type of grating selected can depend on the specific requirements of a given application. The optical structure recorded in the waveguide can include many different types of optical elements, such as, but not limited to, diffraction gratings. In many embodiments, the implemented grating is a Bragg grating (also referred to as a volume grating). Bragg gratings can have high efficiency, with little light being diffracted into higher orders. The relative amount of light in the diffracted zeroth order can be varied by controlling the grating's refractive index modulation, a property that can be used to create lossy waveguide gratings for extracting light over a large pupil. By strategically placing a volume Bragg grating within a waveguide, the propagation of light within the waveguide can be influenced in a controlled manner to achieve various effects. The diffraction of light incident on the grating can be determined by the properties of the light and the grating. As can be easily understood, volume Bragg gratings can be constructed to have different properties depending on the specific requirements of a given application. In some embodiments, the volume Bragg grating is designed to be a transmission grating. In other embodiments, the volume Bragg grating is designed to be a reflection grating. In a transmission grating, incident light that satisfies the Bragg condition is diffracted such that diffracted light exits the grating on the side where the incident light did not enter. For a reflection grating, diffracted light exits the grating on the same side where the incident light entered.

[0026] 1A and 1B conceptually illustrate two volume Bragg grating configurations according to various embodiments of the present invention. Depending on the side from which light rays exit after diffraction, gratings can be classified as either reflection gratings 100 or transmission gratings 150. The refraction / reflection condition, or Bragg condition, can depend on several factors, including, but not limited to, the refractive index of the medium, the grating period, the wavelength of the incident light, and the angle of incidence. FIG. 1A shows a reflection grating 100 recorded in a transparent material. As shown, light rays 101 and 102 are of different wavelengths and are incident on the reflection grating 100 at the same angle, with fringes 103 parallel to the grating surface. Light ray 101 does not satisfy the Bragg condition and is transmitted through the grating. Meanwhile, light ray 102 satisfies the Bragg condition and is reflected back through the same surface where it entered. Another type of grating is a transmission grating, which is conceptually illustrated in FIG. 1B. In the illustrative embodiment, transmission grating 150 has fringes 151 that are perpendicular to the grating surface. As shown, light rays 152 and 153 with different wavelengths are incident on transmission grating 150 at the same angle. Light ray 152 satisfies the Bragg condition and is refracted, exiting on the opposite side of the grating from which light ray 152 entered. Light ray 153 does not satisfy the Bragg condition and is transmitted through its original propagation path. Depending on the efficiency of the grating, the light can be partially reflected or refracted. While FIGS. 1A and 1B illustrate a specific volume grating structure, any type of grating structure can be recorded within a waveguide cell in accordance with various embodiments of the present invention. For example, volume gratings can be implemented with fringes that are sloped and / or inclined relative to the grating surface, which can affect the angles of diffraction / reflection. While the above discussion represents grating structures as either transmission or reflection, both types of gratings behave identically according to standard grating equations.

[0027] Waveguide structures according to various embodiments of the present invention can implement gratings in several different ways. In addition to volume gratings, gratings can be implemented as surface relief gratings. As the name suggests, surface relief gratings can be implemented by physically forming grooves or periodic patterns on the surface of a substrate. The periodicity and angle formed by the grooves can determine the efficiency and other properties of the grating. Any of several methods can be used to form these grooves, including, but not limited to, etching and photolithography.

[0028] 2 conceptually illustrates a surface relief grating according to one embodiment of the present invention. As shown, surface relief grating 200 contains periodic tilted grooves 201. When light is incident on grooves 201, diffraction can occur under certain conditions. The tilt and periodicity of grooves 201 can be designed to achieve targeted diffraction behavior of the incident light.

[0029] 1A-1B and 2 show specific grating structures, it is readily apparent that the grating structure may be configured in a number of different ways depending on the specific requirements of a given application. Examples of such configurations are discussed in more detail in the following sections. Switchable Bragg Gratings

[0030] One class of grating used in holographic waveguide devices is the switchable Bragg grating ("SBG"). SBGs can be fabricated by first placing a thin film of a mixture of photopolymerizable monomer and liquid crystal material between glass plates or substrates. In many cases, the glass plates are in a parallel configuration. One or both glass plates can support electrodes, typically transparent tin oxide films, to apply an electric field across the film. The grating structure within the SBG can be recorded in a liquid material (often referred to as a syrup) through photopolymerization-induced phase separation using interference exposure with spatially periodic intensity modulation. Factors such as, but not limited to, control of the irradiation intensity, the volume fraction of the material components within the mixture, and the exposure temperature can determine the resulting grating morphology and performance. As can be readily appreciated, a wide variety of materials and mixtures can be used depending on the specific requirements of a given application. In many embodiments, HPDLC materials are used. During the recording process, the monomer polymerizes and the mixture undergoes phase separation. The LC molecules aggregate to form discrete or coalesced droplets that are periodically dispersed within the polymer network on the scale of optical wavelengths. The alternating liquid crystal-rich and liquid crystal-depleted regions form fringe planes of a grating that can generate Bragg diffraction with strong optical polarization resulting from the orientational order of the LC molecules within the droplets.

[0031] The resulting volume phase grating can exhibit very high diffraction efficiency, which can be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating through transparent electrodes, the natural orientation of the LC droplets changes, reducing the refractive index modulation of the fringes and reducing the hologram diffraction efficiency to very low levels. Typically, the electrodes are configured so that the applied electric field is perpendicular to the substrate. In some embodiments, the electrodes are fabricated from indium tin oxide (ITO). In the off state, when no electric field is applied, the extraordinary axis of the liquid crystal is generally aligned normal to the fringes. The grating therefore exhibits high refractive index modulation and high diffraction efficiency for P-polarized light. When an electric field is applied to the HPDLC, the grating switches to the on state, and the extraordinary axis of the liquid crystal molecules aligns parallel to the applied electric field and therefore perpendicular to the substrate. In the on state, the grating exhibits lower refractive index modulation and lower diffraction efficiency for both S- and P-polarized light. Thus, the grating regions no longer diffract light. Each grating region can be divided into multiple grating elements, e.g., a pixel matrix, as in the function of an HPDLC device. Typically, the electrodes on one substrate surface are uniform and continuous, while the electrodes on the opposing substrate surface are patterned according to multiple selectively switchable grating elements.

[0032] Typically, the SBG element switches clear in 30 μs, with a longer relaxation time for switching on. Note that the diffraction efficiency of this device can be tuned over a continuous range using the applied voltage. In many cases, the device exhibits near 100% efficiency when no voltage is applied and essentially zero efficiency when a sufficiently high voltage is applied. In some types of HPDLC devices, a magnetic field can be used to control the LC orientation. In some HPDLC applications, phase separation of the LC material from the polymer can be achieved to the point where no discernible droplet structure results. SBGs can also be used as passive gratings. In this mode, their primary benefit is the uniquely high refractive index modulation. SBGs can be used to provide transmission or reflection gratings for free-space applications. SBGs can be implemented as waveguide devices, with the HPDLC forming either the waveguide core or the evanescent coupling layer adjacent to the waveguide. The glass plates used to form the HPDLC cell provide a total internal reflection ("TIR") light-guiding structure. If the switchable grating diffracts the light at an angle that exceeds the TIR condition, the light can be coupled out of the SBG.

[0033] 3A and 3B conceptually illustrate HPDLC SBG devices 300, 350 and the switching properties of SBGs according to various embodiments of the present invention. In FIG. 3A, the SBG 300 is in the off state. As shown, the LC molecules 301 are aligned nearly normal to the fringe plane. Therefore, the SBG 300 exhibits high diffraction efficiency, and incident light can be easily diffracted. FIG. 3B illustrates the SBG 350 in the on position. An applied voltage 351 can align the optic axes of the LC molecules 352 within the droplets 353, producing an effective refractive index that matches the refractive index of the polymer, essentially creating a transparent cell in which incident light is not diffracted. In the illustrative embodiment, an AC voltage source is shown. As can be readily understood, various voltage sources can be utilized depending on the specific requirements of a given application.

[0034] In waveguide cell designs, in addition to the components described above, adhesives and spacers can be placed between the substrates to affix the layers of elements together and maintain the cell gap or thickness dimension. In these devices, the spacers can take many forms, including but not limited to, materials, sizes, and geometries. Materials can include, for example, plastics (e.g., divinylbenzene), silica, and conductive spacers. They can take any suitable geometric shape, including but not limited to, rods and spheres. The spacers can take any suitable size. In many cases, the size of the spacers ranges from 1 to 30 μm. While the use of these adhesive materials and spacers may be necessary in LC cells using conventional materials and manufacturing methods, they can contribute to cell haze, which degrades the optical properties and performance of the waveguide and device. HPDLC Material Systems

[0035] HPDLC mixtures according to various embodiments of the present invention generally include LC, monomer, photoinitiator dye, and coinitiator. The mixture (often referred to as a syrup) also frequently includes a surfactant. For purposes of describing the present invention, a surfactant is defined as any chemical that reduces the surface tension of the overall liquid mixture. The use of surfactants in PDLC mixtures is known and dates back to the earliest investigations of PDLC. For example, an article by R.L. Sutherland et al., SPIE Vol. 2689, pp. 158-169, 1996 (the disclosure of which is incorporated herein by reference), describes a PDLC mixture containing a monomer, photoinitiator, coinitiator, chain extender, and LC to which a surfactant may be added. Surfactants are also mentioned in an article by Natarajan et al., Journal of Nonlinear Optical Physics and Materials, Vol. 5, No. 1, 89-98, 1996 (the disclosure of which is incorporated herein by reference). Additionally, U.S. Patent No. 7,018,563 to Sutherland et al. discusses a polymer dispersed liquid crystal material for forming a polymer dispersed liquid crystal optical element, the material including at least one acrylic acid monomer, at least one type of liquid crystal material, a photoinitiator dye, a coinitiator, and a surfactant. The disclosure of U.S. Patent No. 7,018,563 is incorporated herein by reference in its entirety.

[0036] The patent and scientific literature contains many examples of material systems and processes that can be used to fabricate SBGs, including investigations into formulating such material systems to achieve high diffraction efficiency, fast response times, low drive voltages, etc. U.S. Patent No. 5,942,157 to Sutherland and U.S. Patent No. 5,751,452 to Tanaka et al. both describe suitable monomer and liquid crystal material combinations for fabricating SBG devices. Examples of recipes can also be found in papers dating back to the early 1990s. Many of these materials use acrylate monomers, including: R.L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the disclosure of which is incorporated herein by reference, describes the use of acrylate polymers and surfactants. Specifically, the recipe includes a crosslinking multifunctional acrylate monomer, the chain extender N-vinylpyrrolidinone, LC E7, the photoinitiator Rose Bengal, and the co-initiator N-phenylglycine. The surfactant octanoic acid was added in some variations. Fontecchio et al., SID 00 Digest 774-776, 2000 (the disclosure of which is incorporated herein by reference) describes a UV-curable HPDLC for reflective display applications, containing a multifunctional acrylate monomer, LC, photoinitiator, coinitiator, and chain terminator. YH Cho, et al., Polymer International, 48, 1085-1090, 1999 (the disclosure of which is incorporated herein by reference) discloses an HPDLC recipe containing acrylates. Karasawa et al., Japanese Journal of Applied Physics, Vol. 36, 6388-6392, 1997 (the disclosure of which is incorporated herein by reference) describes acrylates in various functional states.TJ Bunning et al., Polymer Science: Part B: Polymer Physics, Vol. 35, 2825-2833, 1997 (the disclosure of which is incorporated herein by reference) also describe multifunctional acrylate monomers. GS Iannacchione et al., Europhysics Letters Vol. 36 (6). 425-430, 1996 (the disclosure of which is incorporated herein by reference) describe PDLC mixtures containing pentaacrylate monomers, LCs, chain extenders, coinitiators, and photoinitiators.

[0037] Acrylates offer the benefits of fast kinetics, good mixing with other materials, and compatibility with film-forming processes. Because acrylates are crosslinked, they tend to be mechanically robust and flexible. For example, urethane acrylates with functionalities of 2 (di) and 3 (tri) have been widely used for HPDLC technology. Higher functionality materials, such as penta- and hexa-functional stems, have also been used.

[0038] One of the known attributes of transmission SBGs is that the LC molecules tend to align with their average direction normal to the plane of the grating fringes (i.e., parallel to the grating or K vector). The effect of LC molecular alignment is that transmission SBGs efficiently diffract P-polarized light (i.e., light with a polarization vector in the plane of incidence), but have near-zero diffraction efficiency for S-polarized light (i.e., light with a polarization vector normal to the plane of incidence). Volume Grating Recording Mechanism

[0039] Volume gratings can be recorded in waveguide cells using many different methods according to various embodiments of the present invention. Recording of optical elements in optical recording materials can be achieved using any number and type of electromagnetic radiation source. Depending on the application, the exposure source and / or recording system can be configured to record the optical elements using various levels of exposure power and duration. As discussed above with respect to SBG, techniques for recording volume gratings can include exposing the optical recording material using two mutually coherent laser beams, where the overlap of the two beams creates a periodic intensity distribution along the interfering beam. The optical recording material can form a grating structure exhibiting a refractive index modulation pattern that matches the periodic intensity distribution. In HPDLC mixtures, the light intensity distribution results in the diffusion and polymerization of monomers into the high-intensity regions and the simultaneous diffusion of liquid crystals into the dark regions. This phase separation creates alternating liquid crystal-rich and liquid crystal-depleted regions, which form the fringe planes of the grating. The grating structure can be formed with tilted or non-tilted fringes, depending on how the recording beam is configured. 4A-4D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. As shown, two methods can be used to create two different types of Bragg gratings: a transmission grating 400 and a reflection grating 401. Depending on how the two recording beams 402, 403 are positioned, an interference pattern 404 can record either a transmission grating or a reflection grating in the optical recording material 405. The difference between the two types of gratings can be seen in the orientation of the fringes (i.e., the fringes of a reflection volume grating are typically approximately parallel to the surface of the substrate, while the fringes of a transmission grating are typically approximately perpendicular to the surface of the substrate). During playback, a beam 406 incident on the transmission grating 400 can result in a transmitted diffracted beam 407. Meanwhile, a beam 408 incident on the reflection grating 401 can result in a reflected beam 409.

[0040] Another method for recording a volume grating in an optical recording material involves using a single beam to form an interference pattern on the optical recording material. This can be achieved through the use of a master grating. In many embodiments, the master grating is a volume grating. In some embodiments, the master grating is an amplitude grating. Upon interaction with the master grating, the single beam can diffract. The first-order diffracted and zero-order beams can overlap, creating an interference pattern that can then expose the optical recording material to form the desired volume grating. A single-beam recording process utilizing an amplitude grating according to one embodiment of the present invention is conceptually illustrated in FIG. 5. As shown, a beam 500 from a single laser source (not shown) is directed through an amplitude grating 501. Upon interaction with the grating 501, the beam 500 can be diffracted, as in the case of a ray interacting with the black-shaded areas of the amplitude grating, or the beam 500 can propagate through the amplitude grating without substantial deviation as a zero-order beam, as in the case of a ray interacting with the cross-hatched areas of the amplitude grating. The first order diffracted beam 502 and the zero order beam 503 can overlap to create an interference pattern that exposes the waveguide cell's optical recording layer 504. In an illustrative embodiment, a spacer block 505 is positioned between the grating 501 and the optical recording layer 504 to modify the distance between the two components.

[0041] Although specific methods for recording volumetric gratings are discussed and illustrated in Figures 4A-4D and 5, recording systems according to various embodiments of the present invention can be configured to implement any of several methods for recording volumetric gratings: Roll-axis rotated K-vector gratings and multiplexed K-vector gratings.

[0042] When addressing the limited range of wavelengths and angles over which diffraction occurs in a volume Bragg grating, several methods can be utilized to increase the diffraction bandwidth of the grating. In many embodiments, the gratings can employ fringes that vary with respect to their K vector. In some embodiments, the variation across the roll-axis rotation K vector is typically such that the direction of the K vector variation is out of the plane of the waveguide or grating element. The varying fringes or roll-axis rotation K vector can be implemented in several different ways. In some embodiments, the fringes of the grating are designed to vary in a gradual manner across the grating. In other embodiments, different discrete sets of gratings with different fringes are placed in series. Gratings with roll-axis rotation K vectors can be designed and configured in various ways. In many embodiments, the roll-axis rotation K vector is designed so that the peak diffraction efficiency of each grating section is optimized with respect to its corresponding output angle at that location. In some embodiments, the peak diffraction efficiency of each grating at different locations is offset from its corresponding output angle at that location. It has been shown that introducing this offset can improve eyebox uniformity. In some embodiments, the offset can improve the overall image brightness by a factor of two compared to just matching the peak diffraction efficiencies at different locations.

[0043] Roll-axis rotated K-vector gratings can be used to maximize the peak diffraction efficiency of in-coupled light in accordance with certain embodiments of the present invention. The use of roll-axis rotated K-vectors allows for highly efficient input coupling into the grating and also allows the beam divergence angle to be optimized to minimize the waveguide thickness, which may require balancing the waveguide thickness, the angular bandwidth of the grating, and the field-of-view angular spread at any given point on the grating. The low-angle response of the grating as the K-vector is roll-rotated (and the surface pitch is maintained) can prevent output coupling and allow the waveguide thickness to be minimized. In some embodiments, the design goal is to ensure maximum input coupling at a point and minimize angular diversity so that the grating thickness can be minimized without mutual outcoupling at different points.

[0044] 6A and 6B conceptually illustrate two implementations of a roll-axis rotated K-vector grating, according to various embodiments of the present invention. Referring first to FIG. 6A, in some embodiments, a roll-axis rotated K-vector grating can be implemented as a waveguide section containing discrete grating elements 600 having different K-vectors. Referring next to FIG. 6B, in some embodiments, a roll-axis rotated K-vector grating can be implemented as a waveguide section containing grating elements 601 in which the K-vector undergoes a smooth monotonic variation of direction. As shown, the change in direction of the K-vector is out of the plane of the waveguide.

[0045] In many embodiments, different sets of discrete fringes are superimposed within the same grating, creating a multiplexed grating, essentially multiple gratings within the same volume, functioning independently and without mutual interference. For example, if two volume gratings are recorded within the same device for two different Bragg wavelengths at the same angle of incidence, the device can diffract the two selected wavelengths into different output directions with limited crosstalk. Multiplexing can be used to combine two gratings of similar prescriptions to produce improved angular profiles, extend the diffraction efficiency angular bandwidth, and provide better luminance uniformity and color balance across the exit pupil and field of view. Multiplexing can also be used to encode two distinctly different diffraction prescriptions, which can be designed to project light into distinct fields of area or diffract light of two different wavelengths into a given field of view. Measures can be taken to ensure there is no conflict between gratings during recording, which would lead to unequal diffraction efficiencies and crosstalk between gratings upon playback. Multiplexing can also provide the significant benefit of reducing the number of layers in a waveguide structure. In some embodiments, at least one of the input, folding, or output gratings can combine two or more angular diffractive prescriptions to expand the angular bandwidth. Similarly, in some embodiments, at least one of the input, folding, or output gratings can combine two or more spectral diffractive prescriptions to expand the spectral bandwidth. For example, a color-multiplexed grating may be used to diffract two or more of the primary colors.

[0046] FIG. 7 conceptually illustrates a multiplexed K-vector grating according to one embodiment of the present invention. As shown, multiplexed grating 700 contains two sets of fringes 701, 702. The first set 701 is depicted by a solid diagonal line and has a K-vector K1 and a period Λ1. The second multiplexed grating 702 is depicted by a dashed line and has a K-vector K2 and a period Λ2. In the illustrated embodiment, the two grating periods are identical, but the K-vectors are in different directions. In operation, both multiplexed gratings 701, 702 are active, which can provide wider incidence and diffraction bandwidths. The angular bandwidth of incidence θ for the multiplexed gratings i is the overlapping θ i1 and θ i2 The angular bandwidth of diffraction for the multiplexed gratings 701, 702, θ d is the overlapping θ d1 and θ d2 In some embodiments, more than two gratings are multiplexed.

[0047] Although specific grating structures with varying fringes are discussed above, any of several fringe configurations can be utilized according to the specific requirements of a given application. For example, any number of gratings can be multiplexed as permitted by manufacturing constraints. Roll-axis rotated K-vector gratings can be designed to have the K-vector roll-rotated in any discrete unit. Waveguides that implement pupil expansion

[0048] Gratings can be implemented within waveguides in a variety of different ways. In some embodiments, the grating is on the exterior surface of the waveguide. In other embodiments, a volume grating is implemented inside the waveguide. Gratings can also be implemented to perform different optical functions, such as, but not limited to, coupling light, directing light, and preventing light transmission. FIG. 8 conceptually illustrates a waveguide utilizing a coupling grating to diffract light into and out of the waveguide, in accordance with an embodiment of the present invention. As shown, waveguide 800 includes a first surface 801, a second surface 802, an input grating element 803, and an output grating element 804. Collimated light 805 from a projection lens enters the waveguide through first surface 801 at an orthogonal angle. The light travels through waveguide 800 at its original angle and interacts with input grating element 803 before reaching second surface 802 on the other side of waveguide 800. The input grating element 803 can be designed to diffract the light 805 at an oblique angle, such that the refracted light 806 is incident on the second surface 802 at an angle where total internal reflection can occur. Thus, the light 805 is coupled into the waveguide and is confined within the first and second surfaces 801, 802 of the waveguide 800. In an illustrative embodiment, the light travels within the waveguide 800 until it interacts with the output grating 804, which refracts the light out of the waveguide 800 and into the user's eye 807.

[0049] In many embodiments, diffraction gratings can be used to preserve eyebox size while reducing lens size by effectively expanding the exit pupil of a collimating optical system. The exit pupil can be defined as a virtual aperture, and only light rays that pass through this virtual aperture can enter the user's eye. Figures 9 and 10 conceptually illustrate a waveguide utilizing an output grating for exit pupil expansion in one dimension, in accordance with an embodiment of the present invention. The waveguide 900 of Figure 9 includes a first surface 901, a second surface 902, an input grating element 903, and an output grating element 904. As shown, light 905 can be coupled into the waveguide 900 by the input grating 902 and travel through the waveguide 900 via total internal reflection. In an illustrative embodiment, the output grating 904 is expanded and designed to refract a portion of the guided light. The light can be refracted such that the refracted light 906 strikes the second surface 902 at an angle where total internal reflection does not occur, allowing the light 906 to couple out of the waveguide 900. This lossy extraction allows for exit pupil expansion, as the remaining light can continue traveling within the waveguide 900, and once the light re-enters the output grating 904, the scenario described above can occur again. Utilizing this technique, a continuous expanded exit pupil can also be achieved with the right design, as shown in FIG.

[0050] Extending the concepts of FIGS. 9 and 10 , optical waveguides can be designed to expand the exit pupil in two dimensions. In many embodiments, two waveguides can be stacked together to create a system in which light coupled into the waveguide stack can achieve exit pupil expansion in two dimensions. FIG. 11 conceptually illustrates a waveguide system utilizing two planar waveguides to provide exit pupil expansion in two dimensions, according to an embodiment of the present invention. As shown, system 1100 includes a first waveguide 1101 and a second waveguide 1102. First waveguide 1101 can include a first input coupling grating 1103 and a first output coupling grating 1104, and second waveguide 1102 can include a second input coupling grating 1105 and a second output coupling grating 1106. The first input coupling grating 1103 can be designed to couple collimated light 1107 from the image source 1108 into the first waveguide 1101. Similar to the systems described in FIGS. 9 and 10 , the trapped light can travel through the first waveguide 1101 via total internal reflection until it reaches the first output coupling grating 1104. In an illustrative embodiment, the first output coupling grating 1104 is designed to provide lossy exit pupil expansion in the first dimension and couple light out of the first waveguide 1101. The second input coupling grating 1105 can be designed to receive light output from the first waveguide 1101, which is expanded in the first dimension, and refract the received light so that it travels through the second waveguide 1102 via total internal reflection. In many embodiments, the first output coupling grating 1104 and the second input coupling grating 1106 are expanded in a similar manner. Light traveling through the second waveguide 1102 can then interact with the second output coupling grating 1106. In an illustrative embodiment, the second output coupling grating 1106 is designed to provide a lossy exit pupil expansion in a second dimension different from the first dimension to couple light out of the second waveguide 1102. As a result, the exit pupil is expanded in two dimensions, allowing for a smaller lens size relative to the eyebox size 1109.

[0051] In many embodiments, optical waveguides utilize folded gratings, which can provide exit pupil expansion in one dimension while directing light within the waveguide. In further embodiments, the folded grating directs light toward an output grating, which can provide exit pupil expansion in a second dimension different from the first direction, and also couples light out of the waveguide. By using folded gratings, waveguide displays can require fewer layers than other systems and methods for displaying information. Additionally, by using folded gratings, light can travel by total internal reflection within the waveguide within a single right-angle prism defined by the waveguide exterior surface, achieving dual pupil expansion. As a result, two-dimensional exit pupil expansion can be achieved using a single waveguide. FIG. 12 conceptually illustrates a waveguide utilizing a three-grating structure to provide two-dimensional exit pupil expansion, according to one embodiment of the present invention. As shown, waveguide 1200 includes input grating 1201, folded grating 1202, and output grating 1203. Arrows 1204-1206 on gratings 1201-1203 indicate the k-vector associated with each grating. In many embodiments, folded grating 1202 can be designed to provide exit pupil expansion in one dimension and redirect the direction of light propagating from input grating 1201 via total internal reflection. In an illustrative embodiment, the fringes of folded grating 1202 are offset 45 degrees from either of the other two gratings 1201, 1203. Light entering the folded grating is redirected 1207, 1208 and propagates toward output grating 1203, which provides exit pupil expansion in a second dimension and couples light out of waveguide 1200.

[0052] While the above discussion relating to Figures 8-12 describes specific waveguide structures, it will be readily understood that any number of waveguide structure configurations may be utilized according to the specific requirements of a given application. For example, a grating that provides exit pupil expansion can be designed with a gradient efficiency such that the fraction of light that is refracted varies with incident area. Waveguide Layer Stack

[0053] Waveguides according to various embodiments of the present invention can be stacked together to implement certain optical functions. For example, in many embodiments, the device can include a stack of RGB diffractive layers, each layer comprising an input and output grating. In each layer, an SBG is recorded to provide a peak diffraction efficiency versus wavelength characteristic (along the waveguide) that is shifted by a small increment from the peak wavelength. In some embodiments, RGB SBG layers are used and can be switched continuously and synchronously with an RGB LED image source. Figure 13 conceptually illustrates a profile view of an RGB stack of a waveguide 1300 according to one embodiment of the present invention. In the illustrative embodiment, wavelength-selective absorption layers 1301-1303 are used to selectively absorb unwanted light in each waveguide layer 1304-1306. Dashed lines represent weak coupling due to polarization off or Bragg off. The waveguide stack further includes various filters and waveplates 1307-1311. The polarization orientation is depicted relative to the input grating.

[0054] While Figure 13 illustrates a specific structure of a waveguide stack, any of several stack configurations can be used according to the specific requirements of a given application. For example, in many embodiments, only two layers, red and blue / green, are used to implement an RGB stack. Such a system can be achieved using several methods. In some embodiments, multiplexed gratings, each containing a different set of gratings correlated with an RGB color, are used to implement multiple color waveguides within one waveguide layer. Waveguide Displays

[0055] Waveguide displays according to various embodiments of the present invention can be implemented and constructed in many different ways. For example, waveguide displays can contain various numbers of waveguide layers and different exit pupil expansion schemes. FIG. 14 conceptually illustrates a two-axis expanded waveguide display with two grating layers according to an embodiment of the present invention. As shown, the waveguide display 1400 includes a light source 1401, a microdisplay panel 1402, and an input image node (“IIN”) 1403 optically coupled to a waveguide 1404 having two grating layers. In some embodiments, the waveguide is formed by sandwiching a grating layer between glass or plastic substrates, forming a stack in which total internal reflection occurs at the outer substrate and air interfaces. In some embodiments, the stack can further comprise additional layers, such as beam-splitting coatings and environmental protection layers. In the illustrative embodiment, each grating layer contains input gratings 1405A, 1405B, folded grating exit pupil expanders 1406A, 1406B, and output gratings 1407A, 1407B, with the letters A and B referring to the first and second waveguide layers. The input, folded, and output gratings may be holographic gratings, such as switchable or non-switchable SBGs. As used herein, the term grating may encompass gratings, which may include multiplexed gratings or sets of gratings, such as sets of discrete roll-axis rotating K-vector gratings. In the illustrative embodiment, the IIN 1403 integrates the microdisplay panel 1402, the light source 1401, and the optical components required to illuminate the display panel, separate the reflected light, and collimate it into the required FOV. In the embodiment of FIG. 14 and the embodiments to be described below, at least one of the input, folded, and output gratings may be electrically switchable. In many embodiments, all three grating types are passive (i.e., non-switching). In some embodiments, the IIN can project an image to be displayed on a microdisplay panel such that each display pixel is translated to a unique angular orientation within the substrate waveguide. Collimation optics contained within the IIN can include lenses and mirrors.In a further embodiment, the lenses and mirrors are diffractive lenses and mirrors.

[0056] In an illustrative embodiment, the light path from the source to the waveguide through the IIN is shown by rays 1408-1411. Input gratings 1405A, 1405B in each grating layer can couple a portion of the light into a TIR path within the waveguide 1404; such paths are represented by rays 1412, 1413. Output gratings 1407A, 1407B can diffract the light from the waveguide into an angular range of collimated light 1414, 1415, respectively, for viewing by the eye 1416. The angular range corresponding to the field of view of the display can be defined by the IIN optics. In some embodiments, the waveguide grating can encode the optical output to adjust the collimation of the output. In some embodiments, the output image is at infinity. In other embodiments, the output image may be formed several meters away from the eyebox. Typically, the eye is positioned within the exit pupil or eyebox of the display.

[0057] Different IIN implementations and embodiments can be utilized as discussed and taught in U.S. patent application Ser. No. 13 / 869,866, entitled "Holographic Wide Angle Display," and U.S. patent application Ser. No. 13 / 844,456, entitled "Transparent Waveguide Display," the disclosures of which are incorporated herein by reference in their entireties. In some embodiments, the IIN contains a beam splitter for directing light onto the microdisplay and transmitting reflected light toward the waveguide. In many embodiments, the beam splitter is a grating recorded in HPDLC, and the inherent polarization selectivity of such gratings is used to separate the light illuminating the display and the image-modulated light reflected from the display. In some embodiments, the beam splitter is a polarizing beam splitter cube. In some embodiments, the IIN incorporates a despeckler. Despecklers are discussed in U.S. Patent No. 8,565,560, entitled "Laser Illumination Device," the disclosure of which is incorporated herein by reference in its entirety.

[0058] The light source may be a laser or an LED and may include one or more lenses to modify the illumination beam angular characteristics. The image source may be a microdisplay or a laser-based display. LEDs may provide better uniformity than lasers. When laser illumination is used, there is a risk of illumination banding at the waveguide output. In many embodiments, laser illumination banding in a waveguide can be overcome using techniques and teachings disclosed in U.S. Patent Application No. 15 / 512,500, entitled "Method and Apparatus for Generating Input Images for Holographic Waveguide Displays," the disclosure of which is incorporated herein by reference in its entirety. In some embodiments, the light from the light source is polarized. In some embodiments, the image source is a liquid crystal display (LCD) microdisplay or a liquid crystal on silicon (LCoS) microdisplay.

[0059] In some embodiments, each grating layer addresses half of the total field of view, similar to that shown in FIG. 14. Typically, the folded gratings are offset 45 degrees clockwise (i.e., sloped in the waveguide plane) to ensure adequate angular bandwidth for the folded light. In other embodiments, other clock angles can be used to meet spatial constraints on grating positioning that may arise in the ergonomic design of the display. In some embodiments, at least one of the input and output gratings has a roll-axis rotated K vector. Roll-axis rotating the K vector can allow the angular bandwidth of the grating to be expanded without the need to increase the waveguide thickness.

[0060] In many embodiments, the angular bandwidth of the folded grating can be enhanced by designing the grating formulation to provide dual interaction of the guided light with the grating. An exemplary embodiment of a dual interaction folded grating is disclosed in U.S. Patent Application No. 14 / 620,969, entitled "Waveguide Grating Device," the disclosure of which is incorporated herein in its entirety.

[0061] FIG. 15 conceptually illustrates a plan view 1500 of a single grating layer similar to that used in FIG. 14 , in accordance with one embodiment of the present invention. The grating layer 1501, optically coupled to the IIN 1502, includes an input grating 1503, a first beam splitter 1504, a folded grating 1505, a second beam splitter 1506, and an output grating 1507. The beam splitters may be partially transmissive coatings that homogenize the guided light by providing multiple reflection paths within the waveguide. Each beam splitter may include more than one coating layer, with each coating layer applied to a transparent substrate. A typical beam path from the IIN to the eye 1508 is shown by rays 1509-1513.

[0062] 16 conceptually illustrates a plan view 1600 of a two-grating layer configuration according to one embodiment of the present invention. As shown, grating layers 1601A, 1601B optically coupled to an IIN 1602 include input gratings 1603A, 1603B, first beam splitters 1604A, 1604B, folded gratings 1605A, 1605B, second beam splitters 1606A, 1606B, and output gratings 1607A, 1607B, where the letters A and B refer to the first and second grating layers, respectively. In the illustrated embodiment, the gratings and beam splitters of the two layers substantially overlap.

[0063] In many embodiments, the grating layer can be divided into separate layers. For example, in some embodiments, a first layer includes a folded grating, while a second layer includes an output grating. In further embodiments, a third layer can include an input grating. In such embodiments, several layers can then be stacked together into a single waveguide substrate. In some embodiments, the grating layer includes several components, including an input coupler, a folded grating, and an output grating (or portions thereof), that are stacked together to form a single substrate waveguide. The components can be separated by optical glue or other transparent material with a refractive index that matches or is substantially similar to that of the components.

[0064] In many embodiments, the grating layer can be formed through a cell fabrication process by creating cells of the desired grating thickness and vacuum-filling each cell with SBG material for the input coupler, folded grating, and output grating. In some embodiments, the cells can be formed by positioning multiple glass plates with gaps between them that define the desired grating thicknesses for the input coupler, folded grating, and output grating. In some embodiments, a single cell can be fabricated with multiple openings such that separate openings are filled with different pockets of SBG material. Any intervening spaces can then be separated by a separating material (e.g., glue, oil, etc.) to define separate areas. In some embodiments, the SBG material can be spin-coated onto a substrate and then covered by a second substrate after the material hardens.

[0065] In many embodiments, the input coupler, folded grating, and output grating are created by interfering two light waves at an angle within the substrate to create a holographic wavefront, which can create bright and dark fringes set within the waveguide substrate at desired angles. Additionally, such optical elements can also be fabricated using any of the various methods described in the sections above.

[0066] In one embodiment, the input coupler, folded grating, and output grating embodied as an SBG may be Bragg gratings recorded in holographic polymer dispersed liquid crystal (HPDLC) (e.g., a matrix of liquid crystal droplets), although the SBG may also be recorded in other materials. In one embodiment, the SBG is recorded in a uniform modulation material such as POLICRYPS or POLIPHEM, which has a matrix of solid liquid crystal dispersed in a liquid polymer. The SBG may be inherently switching or non-switching. In its non-switching form, the SBG has the advantage over conventional holographic photopolymer materials of being able to provide high refractive index modulation due to its liquid crystal component. Exemplary uniformly modulating liquid crystal polymer material systems are disclosed in U.S. Patent Application Publication No. US2007 / 0019152 by Caputo et al. and PCT Application No. PCT / EP2005 / 006950 by Stumpe et al., both of which are incorporated herein by reference in their entireties. Uniform modulated gratings are characterized by high refractive index modulation (and therefore high diffraction efficiency) and low scattering.

[0067] In many embodiments, the input coupler, folded grating, and output grating are fabricated from reverse-mode HPDLC material. Reverse-mode HPDLC differs from conventional HPDLC in that the grating is passive when no electric field is applied and becomes diffractive in the presence of an electric field. Reverse-mode HPDLC may be based on any of the recipes and processes disclosed in PCT Application No. PCT / GB2012 / 000680, entitled "Improvements to Holographic Polymer Dispersed Liquid Crystal Materials and Devices," the disclosure of which is incorporated herein in its entirety. Gratings can be recorded in any of the above material systems but used in a passive (non-switching) mode. The fabrication process is the same as that used for switching, but the electrode coating step is omitted. LC polymer material systems are highly desirable due to their high refractive index modulation. In some embodiments, gratings are recorded in HPDLC but are not switched.

[0068] In many embodiments, the input grating can be replaced by another type of input coupler, such as, but not limited to, a prism and a reflective surface. In some embodiments, the input coupler can be a passive SBG grating, a holographic grating, such as an SBG grating or a passive grating. The input coupler can be configured to receive collimated light from a display source and direct the light to the folded grating within the waveguide via total internal reflection between the first and second surfaces. The input coupler can be oriented directly toward the folded grating or at an angle relative to it. For example, in some embodiments, the input coupler can be set at a slight angle with respect to the folded grating. In some embodiments, the folded grating can be oriented diagonally. The folded grating can be configured to provide pupil expansion in a first direction and direct the light to the output grating via total internal reflection inside the waveguide.

[0069] In many embodiments, the vertical edges of each folded grating are oblique to the axis of alignment of the input coupler so that each folded grating is positioned diagonally relative to the propagation direction of the display light. The folded gratings can be angled so that light from the input coupler is redirected to the output grating. In some embodiments, the folded gratings are positioned at a 45-degree angle relative to the direction in which the display image is emitted from the input coupler. This feature can direct the display image propagating along the folded grating to the output grating. For example, in some embodiments, the folded grating can direct the image by 90 degrees to the output grating. In this way, a single waveguide can provide two-axis pupil expansion in both the horizontal and vertical directions. In some embodiments, each folded grating can have a partial diffractive structure. In some embodiments, each folded grating can have a full diffractive structure.

[0070] The output grating can be configured to provide pupil expansion in a second direction different from the first direction and to output light from the waveguide through the first surface or the second surface. The output grating can receive the display image from the folded grating via total internal reflection and can provide pupil expansion in the second direction. In many embodiments, the output grating includes multiple layers of substrate, thereby comprising multiple layers of output grating. Thus, there is no requirement that the grating lie in a single plane within the waveguide, and the gratings may be stacked on top of each other (e.g., the cells of the grating are stacked on top of each other).

[0071] In many embodiments, a quarter-wave plate on the substrate waveguide rotates the polarization of the light beam to maintain efficient coupling with the SBG. The quarter-wave plate can be bonded or glued to the surface of the substrate waveguide. For example, in some embodiments, the quarter-wave plate is a coating applied to the substrate waveguide. The quarter-wave plate can provide light wave polarization management. Such polarization management can help keep the light beam aligned with the intended viewing axis by compensating for skew waves within the waveguide. The quarter-wave plate is optional and can increase the efficiency of the optical design in implementation. In some embodiments, the waveguide does not include a quarter-wave plate. The quarter-wave plate may be provided as a multi-layer coating.

[0072] In many embodiments, a waveguide display can be operated in monochrome. In some embodiments, a waveguide display can be operated in color. Color operation can be achieved using a stack of monochrome waveguides with a design similar to that of FIG. 14. This design can use red, green, and blue waveguide layers as shown, or alternatively, red and blue / green layers. FIG. 17 conceptually illustrates a two-axis expanded waveguide display 1700 including a light source 1701, a microdisplay panel 1702, and an IIN 1703 optically coupled to red, green, and blue waveguides 1704R, 1704G, and 1704B, each of which includes two grating layers, in accordance with an embodiment of the present invention. In an illustrative embodiment, the three waveguides are separated by an air gap. In some embodiments, the waveguides are separated by a low-index material, such as a nanoporous film. As shown, the red grating layer, labeled R, includes input gratings 1705R, 1706R, folded grating exit pupil expanders 1707R, 1708R, and output gratings 1709R, 1710R. The grating elements of the blue and green waveguides are labeled using the same numerals, with B and G designating blue and green. In some embodiments, the input, folded, and output gratings are all passive, i.e., unswitched. In some embodiments, at least one of the gratings is switched. In some embodiments, the input gratings in each layer are switchable to avoid color crosstalk between the waveguide layers. In many embodiments, color crosstalk can be avoided by placing dichroic filters 1711, 1712 between the input grating regions of the red and blue and blue and green waveguides. In various embodiments, the color waveguides can be implemented using only one grating layer in each monochromatic waveguide.

[0073] 18 conceptually illustrates an eye tracker display according to one embodiment of the present invention. Waveguide device-based eye trackers are discussed in PCT Application No. PCT / GB2014 / 000197, entitled "Holographic Waveguide Eye Tracker," PCT Application No. PCT / GB2015 / 000274, entitled "Holographic Waveguide Optical Tracker," and PCT Application No. PCT / GB2013 / 000210, entitled "Apparatus for Eye Tracking," the disclosures of which are incorporated herein in their entireties. Turning again to FIG. 18 , the eye tracking display 1800 includes a two-axis magnified waveguide display based on any of the embodiments described above. The waveguide display can include a waveguide 1801 containing at least one grating layer incorporating input, folded, and output gratings, an IIN 1802, an eye tracker 1803 including the waveguide, an infrared detector 1804, and an infrared source 1805. The eye tracker and display waveguide can be separated by an air gap or a low-refractive index material. As discussed in the above references, the eye tracker can include separate illumination and detector waveguides. In an illustrative embodiment, the optical path from the infrared source to the eye is shown by rays 1806-1808, and the backscattered signal from the eye is shown by rays 1809 and 1810. The optical path from the input image node through the display waveguide to the eyebox is shown by rays 1811-1813.

[0074] In many embodiments, the dual-expansion waveguide display further includes a dynamic focusing element. FIG. 19 conceptually illustrates a dual-expansion waveguide display 1900 with a dynamic focusing element 1901 and an eye tracker positioned proximate a major surface of the waveguide display, in accordance with an embodiment of the present invention. In some embodiments, the dynamic focusing element is an LC device. In some embodiments, the LC device combines an LC layer and a diffractive optical element. In some embodiments, the diffractive optical element is an electrically controllable LC-based device. In various embodiments, the dynamic focusing element is positioned between the waveguide display and the eye tracker. In various embodiments, the dynamic focusing element can be positioned proximate a surface of the display waveguide farthest from the eye.

[0075] The dynamic focusing device can provide multiple image planes 1902. In light field display applications, at least four image planes can be used. The dynamic focusing element can be based on the dynamic focusing element described in U.S. patent application Ser. No. 15 / 553,120, entitled "Electrically Focus Tunable Lens," the disclosure of which is incorporated herein in its entirety. In some embodiments, a dual magnification waveguide display having a dynamic focusing element and an eye tracker can provide a light field display such as one based on the teachings disclosed in U.S. patent application Ser. No. 15 / 543,013, entitled "Holographic Waveguide Light Field Displays," the disclosure of which is incorporated herein by reference in its entirety.

[0076] Although specific waveguide structures are discussed above, any of several waveguide structures can be implemented depending on the specific requirements of a given application. For example, in many waveguide configurations, the input, folded, and output gratings are formed in a single layer sandwiched by transparent substrates. Such a configuration is shown in FIG. 14, where two layers are stacked in this manner. In some embodiments, the waveguide includes only one grating layer. In some embodiments, switching transparent electrodes are applied to opposing surfaces of the substrate layers that sandwich the switching grating. In some embodiments, the cell substrate can be fabricated from glass. One glass substrate that can be used is standard Corning Willow glass substrate (refractive index 1.51), which is available in thicknesses up to 50 micrometers. In other embodiments, the cell substrate can be an optical plastic.

[0077] In many embodiments, the waveguide display is coupled to the IIN by an opto-mechanical interface, which allows the waveguide to be easily retracted from the IIN assembly. The basic principle is conceptually illustrated in FIG. 20A . FIG. 20A shows a two-axis expanded waveguide display 2000 including a waveguide 2001 containing an input grating 2002, a folded grating 2003, and an output grating 2004, and an IIN 2005. The device further includes an optical link 2006 connected to the waveguide, a first optical interface 2007 terminating the optical link, and a second optical interface 2008 forming the output optical port of the IIN. The first and second optical interfaces can be decoupled, as indicated by the gap 2009 shown in FIG. 20B . In some embodiments, the optical link is a waveguide. In some embodiments, the optical link is curved. In some embodiments, the optical link is a GRIN image relay device. In various embodiments, the optical connection is established using a mechanical mechanism. In some embodiments, the optical connection is established using a magnetic mechanism. An advantage of decoupling the waveguide from the IIN in helmet-mounted display applications is that the eyepiece portion of the display can be removed when not in use. In some embodiments where the waveguide includes a passive grating, the eyepiece optics can be disposable.

[0078] FIG. 21 conceptually illustrates an IIN 2100 having a microdisplay panel 2101, a spatially varying NA component 2102, and microdisplay optics 2103, in accordance with an embodiment of the present invention. As shown, microdisplay optics 2103 receives light 2104 from an illumination source (not shown) and deflects the light onto the microdisplay in a direction indicated by light ray 2105. Light reflected from the microdisplay is indicated by diverging ray pair 2106-2108, whose numerical aperture (“NA”) angle varies along the X-axis. In an illustrative embodiment, the spatially varying NA component is located between the microdisplay optics and the microdisplay. In other embodiments, the spatially varying NA component is located adjacent to the output face of the microdisplay optics. FIG. 22 conceptually illustrates such an embodiment, indicated by spatially varying NA component 2200.

[0079] In many embodiments, the microdisplay is a reflective device. In some embodiments, the microdisplay is a transmissive device, typically a transmissive LCoS device. Figure 23 conceptually illustrates an IIN 2300 including a backlight 2301, a microdisplay 2302, and a variable NA component 2303, in accordance with an embodiment of the present invention. Light from the backlight, represented by rays 2304-2306, which typically has a uniform NA across the backlight, illuminates the rear surface of the microdisplay and, after propagation through the variable NA component, is converted to output image-modulated light, represented by diverging ray pair 2307-2309, whose NA angle varies along the X-axis.

[0080] In many embodiments, the principles of the present invention may be applied to emissive displays. Examples of emissive displays for use with the present invention include those based on LED arrays and light-emitting polymer arrays. Figure 24 conceptually illustrates an IIN 2400 having an emissive microdisplay 2401 and a spatially varying NA component 2402, in accordance with one embodiment of the present invention. Light from the microdisplay, represented by rays 2403-2405, which typically has a uniform NA across the emitting surface of the display, illuminates the spatially varying NA component and is converted into output image-modulated light, represented by diverging ray pair 2406-2408, whose NA angle varies along the X-axis.

[0081] In many embodiments, the microdisplay optics includes a polarizing beam splitter cube. In some embodiments, the microdisplay optics includes a tilting plate having a beam splitter coating applied thereto. In some embodiments, the microdisplay optics includes a waveguide device with an SBG that acts as a polarization-selective beam splitter. Details regarding such embodiments are discussed in U.S. patent application Ser. No. 13 / 869,866 entitled "Holographic Wide Angle Display" and U.S. patent application Ser. No. 13 / 844,456 entitled "Transparent Waveguide Display," the disclosures of which are incorporated herein in their entireties. In some embodiments, the microdisplay optics includes at least one of a refractive component and a curved reflective surface or a diffractive optical element to control the numerical aperture of the illumination light. In some embodiments, the microdisplay optics includes a spectral filter to control the wavelength characteristics of the illumination light. In some embodiments, the microdisplay optics includes apertures, masks, filters, and coatings to control stray light. In many embodiments, the microdisplay optics incorporate birdbath optics.

[0082] While Figures 14-24 illustrate specific waveguide displays and structures, any waveguide display system and configuration can be used as appropriate for the specific requirements of a given application. At its core, the waveguide is simply used to manipulate the direction of light. This property can generally be used in a variety of different systems. An example of a general system that can utilize a waveguide is shown in Figure 25. Figure 25 conceptually illustrates a system diagram showing components related to a waveguide display, according to various embodiments of the present invention. As shown, system 2500 utilizes a light source 2501 that can output light into a waveguide 2502. The light source used can be a variety of different systems. In some embodiments, light source 2501 is from a projector. In many embodiments, light source 2501 further includes a microdisplay panel and the optical components needed to illuminate the display panel. In further embodiments, light source 2501 includes a collimator and other optical components to manipulate the light into a desired form before entering the waveguide. In other embodiments, light source 2501 is natural light. Once light source 2501 outputs light into waveguide 2502, waveguide 2502 can then steer and redirect the light in a desired manner into or out of receiver 2503. The waveguide can be any general waveguide known in the art and / or one of the waveguides described above. The receiver can be any of several components capable of receiving light from a waveguide. In many embodiments, receiver 2503 is the user's eye. In some embodiments, receiver 2503 is another waveguide. In some embodiments, receiver 2503 is a display capable of displaying light from waveguide 2502. In further embodiments, the display is simply glass that can reflect light onto another receiver. System 2500 can optionally include a switching device 2504 and electrical components for use with SBG. In many embodiments, the switching device 2504 can optionally receive data from the light source to introduce a voltage to direct the SBG to the ON position at the appropriate time.Waveguide exposure process and mastering system.

[0083] In addition to the exposure schemes illustrated in FIGS. 4A-4D for recording in both transmission and reflection waveguides, many mastering techniques can be used to perform such recording and form various waveguide structures and gratings. In various embodiments, the mastering system includes the use of amplitude gratings (“AGs”). Such gratings can be used to form various types of gratings with different configurations. In many embodiments, amplitude master gratings are used to form RKV gratings in waveguides. In further embodiments, amplitude gratings contain linear variations in the grating period, called chirp. Chirped gratings can be utilized in many different ways. FIG. 26 conceptually illustrates an exposure process utilizing a chirped amplitude grating, according to an embodiment of the present invention. As shown, the process includes the use of an input beam 2600 interacting with a focusing element 2601 to provide a 1D focus. A zero-order input beam 2602 is directed toward a chirped AG 2603, which can provide a diffraction profile with linear variations. The two beams are then combined to form the desired interference pattern, which can expose a waveguide substrate 2604. As shown, the separation distance from the exposure surface and the origin of the diffracted beam can be important. In the illustrative embodiment, transparent spacer blocks 2605 are used to control the separation distance.

[0084] In many embodiments, a single-beam exposure system can be used in conjunction with an amplitude grating to form a grating in a waveguide. Figure 5 conceptually illustrates one such embodiment. The use of a single beam in near-contact replication mode can be considered a hybrid between direct contact replication and separate two-beam contact replication, i.e., hybrid contact replication. This approach can be useful when direct contact replication is not possible, such as when the separation distance from the master surface to the exposure surface is not negligible. In such situations, the separation distance can be important. For example, in an exposure process involving an RKV grating, the separation distance can be important and should be considered to preserve the surface-projected fringe period across the complete RKV grating (without which the complete waveguide path correlation cannot be maintained). In some embodiments, a single plane wavefront input beam can be configured to interact with a cylindrical lens to provide a 1D focus. In further embodiments, at least a portion of the light can generate a diffracted beam through interaction with the chirped master, and another portion can pass through (with attenuation) as the zeroth order, preserving the original 1D focusing function of the cylindrical lens.

[0085] In many embodiments, a master can be designed to incorporate more than one amplitude grating. By incorporating multiple amplitude gratings within a single master, alignment errors can be reduced compared to systems that utilize a single master per grating. In some embodiments, a mastering system includes a master with three amplitude gratings. In some embodiments, a master can be developed to incorporate RKV functionality in the simultaneous exposure of three patterns written within one plate. The input and / or output master gratings can be chirped gratings, with additional gratings as needed in the zero-order region where there is no overlap with the chip.

[0086] In many embodiments, mastering multiple grating elements in a waveguide structure can involve the use of multiple exposures. In such embodiments, a multi-step process can be used, in which different areas of a contact replica element corresponding to different grating elements are exposed. In many such embodiments, the process can include sequentially exposing the contact replica. For example, the process can include first exposing the output grating area (e.g., using a large-area O / P-only master or a portion of a multi-grating master), and then exposing multiple times to form the folded grating area.

[0087] Masters incorporating more than one amplitude grating can also be utilized for simultaneous exposure of more than one grating. In such systems, a collimated or coherent incident light beam is generally focused onto the desired area of ​​contact printing via optics through the master AG and through a suitable transparent substrate material. Figures 27A and 27B conceptually illustrate an exposure process for simultaneously forming three gratings according to an embodiment of the present invention. As shown, in many such embodiments, optics are used to simultaneously direct collimated light onto the desired grating areas, resulting in the formation of multiple gratings (e.g., input, output, and fold) in a single exposure. In an illustrative embodiment, the process involves the use of a master 2700 containing the gratings, a glass plate 2701 for adjusting the separation distance, and a contact printing substrate 2702 as the material on which the gratings are recorded. The process utilizes a collimated beam 2703 directed at three mirrors 2704-2706, which redirect the beam 2703 toward the master grating 2700. Once beam 2703 is incident on master grating 2700, an exposure / curing process similar to that described in FIG. 5 can occur, simultaneously forming three different gratings on contact copy 2702. In many embodiments, a 3:1 mastering process can be utilized to fabricate a holographic waveguide having input, folded, and output gratings in a single exposure. In various such embodiments, the input, folded, and / or output gratings are RKV gratings. In some embodiments, the folded grating may be segmented into multiple sections. While any number of sections can be utilized according to the requirements of a specific application, in some embodiments of the present invention, the folded grating may be divided into five sections.

[0088] While Figures 27A and 27B show a specific number and arrangement of gratings to be formed, it should be understood that any number and arrangement of such gratings can be provided. Similarly, any number and arrangement of illumination beams can be provided. For example, if a three-color waveguide is desired, as shown in Figure 28, the beams can be optically arranged to allow beams for the red 2800, green 2801, and blue 2802 channels to be incident through the master and onto the contact copy area. While Figures 27A, 27B, and 28 show conceptual diagrams of mastering systems and arrangements, it should be understood that these conceptual elements can take the form of any suitable optical frame, movable adapter, exposure plate, etc., required to allow fixation of optical elements relative to the master and contact copy area.

[0089] As described above, the holographic waveguide implemented in connection with the mastering and processing embodiments can be a single component and / or a stack of waveguides, depending on the requirements of a particular application of an embodiment of the present invention. For example, a holographic waveguide can include three layers, one for each of the colors red, blue, and green. A conceptual diagram of a holographic waveguide having three layers is shown and discussed in connection with FIG. 13.

[0090] In a typical RKV grating, the grating vector rolls in the same plane as the plane of incidence of the structure beam. In a folded grating, the grating vector can roll perpendicular to the plane of incidence of the structure beam. Turning now to Figures 29A-29C, conceptual diagrams of various approaches for generating RKV gratings are shown, according to some embodiments of the present invention. Many embodiments include a stepped folded RKV, where the angle at each segment varies orthogonally to the K-vector direction, as shown in Figures 29B and 29C. The cross-term MUX limit is overcome by a single input angle of the scanned beam in any single approach. Some embodiments include a scanned folded RKV grating, where the scanned beam exposes the RKV at different angles in discrete steps across the aperture of the planar folded master grating to generate a stepped master. As discussed above, in some embodiments, only a single input beam angle is used to illuminate the folded master at any given time.

[0091] In addition to the above discussion, mastering systems according to many embodiments of the present invention can employ chirped gratings for various other purposes, as shown in FIG. 30A. These chirped gratings can help compensate for when the incident beam on the exposure surface is not collimated and for differences between the master and the holographic waveguide being fabricated. For example, holographic waveguides typically reside inside waveguide cells of finite thickness, while masters typically have a thin protective cover applied to prevent damage to the chrome. Chirped gratings can be utilized to compensate for these additional layers. Any of a variety of protective coatings, such as glass and SiO2 protective layers, can be utilized as appropriate for the requirements of a particular application of an embodiment of the present invention.

[0092] A conceptual diagram of a dual-chirped grating master according to one embodiment of the present invention is shown in FIG. 30B. The 3:1 dual-chirped grating offers various advantages over prior art fabrication techniques. The RKV can dramatically improve the efficiency and uniformity of the holographic waveguide. The RKV input can provide additional input coupling to the waveguide, and the RKV output can provide better pupil shaping, allowing for improved brightness. As discussed above, allowing simultaneous exposure of the RKV input and output gratings (and foldback) can reduce total fabrication / process time. In many embodiments, both gratings share the same spacer and / or optical density between the master and the holographic waveguide, and therefore the RKV profile and spacer window may be configured to balance.

[0093] Various mastering systems according to embodiments of the present invention utilize zero-order gratings. The zero-order grating can be used to control the transmittance of the zero-order beam, which will therefore be close to that of the chirped grating and allow for a continuous beam ratio. This prevents discontinuities in the exposure (and therefore the diffraction efficiency in the replicated grating portion). A conceptual diagram of a mastering system utilizing a zero-order grating in conjunction with a chirped grating according to an embodiment of the present invention is shown in Figure 31. In various embodiments, the zero-order grating has no diffraction orders, or the diffraction orders do not interfere with the system. The orientation of the master and / or energy beam can be used to control the direction of unwanted diffracted beams, but then the relative polarization of the grating and the zero-order beam must be considered. To eliminate diffraction, in various embodiments, the period of the grating can be smaller than the limit for obtaining evanescent diffracted waves. In various embodiments, using a master grating with a transmittance and / or period similar to that of the chirped grating at the interface allows seamless replication to be created within the liquid crystal substrate.

[0094] In many embodiments, the mastering system utilizes a reference grating to align the lens position and obtain the correct grating period. A conceptual diagram of such a system is provided in Figure 32, where the input chirp (K 11 ) and the 0th plane (K 12 ) grating, output chirp (K 32 ) and the 0th order planar lattice (K 32 ), and the reference lattice K for the folded lattice (K2) 13 (input) and K 33 (Output). Reference gratings can help improve the 3:1 structure by allowing simultaneous exposure of input, folded, and output gratings, reducing total fabrication time and providing high accuracy. The lattice matching accuracy can be accurate to 0.1 nm, given the master and compact design, making this attractive for higher volume manufacturing. In some embodiments, the energy beam is collimated. To ensure a collimated input beam for RKV grating generation is used, reverse ray tracing can be employed by tracing the ideal structural ray from the holographic waveguide to the lens; modifying the window thickness can be used to change the focus of the beam relative to the hologram plane; shifting the lens can achieve a collimated beam output from the master to the liquid crystal substrate.

[0095] In some embodiments, the mastering system avoids other beam orders from being created and / or interfering with the energy beam being focused onto the liquid crystal substrate, as shown in Figure 33. To avoid undesired beam orders hitting the grating area, window thickness can be adjusted, glass can be modified, RKV profile can be changed, lens and / or incident beam angle changes can be changed, and / or low index materials can be used to completely reflect the undesired orders off the low index material.

[0096] In some embodiments, the mastering system utilizes one or more diffraction means, so that the same location on the master must generate two different refracted beams, which is not possible unless they are different diffraction orders. To simplify processing, the master is positioned in a configuration where there is no crossover of diffracted beams. In various embodiments, the master is placed as close as possible to the liquid crystal substrate. In some embodiments, the master is placed far enough away that the crossover ends, but this can induce a large separation between the zero order and the diffracted beams. If the master is placed too far away, crossover of diffracted beams can occur (see, for example, Figure 34). If the master is placed too close, unwanted diffracted orders can impinge on the liquid crystal substrate. Therefore, the mastering system according to embodiments of the present invention limits the distance from the liquid crystal substrate to the master plate to a certain range. To increase this range, high-index glass can be used to reduce the ray angle so that the diffracted beams have a longer travel distance to cross over. In addition, shorter wavelength exposure can also reduce the ray angle, which is similarly helpful. In many embodiments where high index plates are used in the master stack, Fresnel reflections need to be managed, particularly with respect to the high index plates between the master and copy planes.

[0097] Although specific systems and methods are discussed above, many different embodiments can be implemented in accordance with the present invention. It should be understood, therefore, that the present invention may be practiced otherwise than as specifically described without departing from the scope and spirit of the present invention. Accordingly, the present embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the present invention should, therefore, be determined not by the illustrated embodiments, but by the appended claims and their equivalents. While specific embodiments have been described in detail in this disclosure, many modifications (e.g., variations in the size, dimensions, structure, shape, and proportions of various elements, parameter values, mounting arrangements, use of materials, color, orientation, etc.) are possible. For example, the positions of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be within the scope of the present disclosure. The order or sequence of any process or method steps may be varied or resequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the exemplary embodiments without departing from the scope of the present disclosure.

Claims

1. 1. A method for recording a hologram, said method comprising: providing a waveguide cell comprising a layer of a polymer dispersed liquid crystal mixture sandwiched between two substrates; providing a master grid; emitting at least one recording beam towards said master grating; scanning the at least one recording beam in a first direction at at least one different angle across an aperture of the master grating in discrete steps, wherein, in response to interaction with the master grating, a portion of the at least one recording beam is diffracted towards the waveguide cell; recording a first volume grating in the waveguide cell using an interference exposure formed from at least the diffracted portion of the at least one recording beam; scanning the at least one recording beam across an aperture of the master grating in discrete steps at at least one different angle in a second direction; recording a second volume grating in the waveguide cell, wherein the first volume grating and the second volume grating are superimposed; A method comprising:

2. The method of claim 1 , wherein the master grating comprises an amplitude grating.

3. The method of claim 2 , wherein the master grating comprises a chirped grating.

4. The method of claim 3 , wherein at least one of the recorded volume grids contains a roll axis rotation K vector.

5. The method of claim 1 , wherein at least one of the recorded volumetric gratings is combined with another grating in a multiplexed grating.

6. The method of claim 1 , wherein the master grating comprises three separate gratings.

7. The method of claim 6 , wherein the three separate lattices are designed to record an input lattice, a folded lattice, and an output lattice.

8. The method of claim 6 , wherein at least one of the volumetric grids comprises three volumetric grids.

9. The method of claim 8 , wherein the at least one recording beam comprises three recording beams.

10. 10. The method of claim 1, wherein the interference exposure is formed from a zero order beam and diffracted portions of only one recording beam.

11. 2. The method of claim 1, wherein scanning the at least one recording beam across the aperture of the master grating in discrete steps results in a volume grating vector that rotates in a plane different from a plane of incidence of the at least one recording beam.

12. 1. A system for recording a holographic grating, said system comprising: a waveguide cell comprising a layer of a polymer dispersed liquid crystal mixture sandwiched between two substrates; a master lattice; a light source configured to emit at least one recording beam towards the master grating, the at least one recording beam scanning across an aperture of the master grating in discrete steps and at least one different angle; Equipped with In response to interaction with the master grating, a portion of the at least one recording beam is diffracted toward the waveguide cell, and at least one volume grating is recorded in the waveguide cell through an interference exposure formed from at least the diffracted portion of the at least one recording beam.

13. The system of claim 12 , wherein the master grating comprises an amplitude grating.

14. The system of claim 13 , wherein the master grating comprises a chirped grating.

15. The system of claim 14 , wherein the recorded volume grid contains a roll axis rotation K vector.

16. The system of claim 12 , wherein the recorded volumetric grid comprises a multiplexed grid.

17. The system of claim 12 , wherein the master grid comprises three separate grids.

18. 20. The system of claim 17, wherein the three separate lattices are designed to record an input lattice, a folded lattice, and an output lattice.

19. The system of claim 17 , wherein the at least one volumetric grid comprises three volumetric grids.

20. 20. The system of claim 19, wherein the at least one recording beam comprises three recording beams.

21. The system of claim 12 , wherein the interference exposure is formed from a zero order beam and a diffracted portion of only one recording beam.

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