Systems and methods for high-throughput recording of holographic gratings in waveguide cells

Abstract

The holographic volume grating in the waveguide cell can be recorded using many different methods and systems in accordance with various embodiments of the invention.SOLUTION: One embodiment includes a holographic recording system including at least one laser source configured to emit recording beams, and a movable platform configured to move between a first position and a second position, wherein when the movable platform is in the first position, the at least one laser source is configured to emit a first set of one or more recording beams toward a first set of one or more stations; When the moveable platform is in the second position, the at least one laser source is configured to emit a second set of one or more recording beams toward a second set of one or more stations.SELECTED DRAWING: Figure 11B

Description

[Technical Field]

[0001] The present invention relates generally to a process and apparatus for recording gratings, and more particularly to a process and apparatus for recording holographic volume gratings in waveguide cells. [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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 7,018,563 [Patent Document 2] U.S. Patent No. 5,942,157 [Patent Document 3] U.S. Patent No. 5,751,452 Summary of the Invention [Means for solving the problem]

[0006] One embodiment includes a holographic recording system including at least one laser source configured to emit a recording beam; a first set of one or more stations configured to house a first set of waveguide cells; a second set of one or more stations configured to house a second set of waveguide cells; and a movable platform configured to move between a first position and a second position, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first set of one or more recording beams toward the first set of one or more stations, and when the movable platform is in the second position, the at least one laser source is configured to emit the second set of one or more recording beams toward the second set of one or more stations.

[0007] In another embodiment, the holographic recording system further includes a plurality of mirrors, and when the movable platform is in the first position, the at least one laser source is configured to emit the first set of one or more recording beams toward the first set of one or more stations by using the plurality of mirrors to direct the first set of one or more recording beams.

[0008] In a further embodiment, the first set of one or more recording beams includes a first recording beam and a second recording beam.

[0009] In yet another embodiment, the at least one laser source includes a first laser source and a second laser source, wherein when the movable platform is in the first position, the first laser source is configured to emit a first recording beam toward the first set of one or more stations and the second laser source is configured to emit a second recording beam toward the first set of one or more stations.

[0010] In still a further embodiment, the holographic recording system further includes a beam splitter, and the at least one laser source is configured to emit the first and second recording beams by emitting the initial beam towards the beam splitter.

[0011] In yet another embodiment, the first set of one or more stations includes a first station, and when the movable platform is in the first position, the at least one laser source is configured to emit first and second recording beams toward the first station.

[0012] In still further embodiments, the first set of one or more stations includes a first station and a second station, and when the movable platform is in the first position, the at least one laser source is configured to emit a first recording beam toward the first station and a second recording beam toward the second station.

[0013] In another additional embodiment, the holographic recording system further includes a beam splitter mounted on the movable platform, and when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams by emitting an initial beam toward the beam splitter.

[0014] In yet an additional embodiment, the holographic recording system further includes a beam splitter pair mounted on the movable platform and a stationary beam splitter, the first set of one or more stations includes a first station and a second station, the first set of one or more recording beams includes first, second, third, and fourth recording beams, and when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams toward the first station and the third and fourth recording beams toward the second station, and the first, second, third, and fourth recording beams are formed using the beam splitter pair and the stationary beam splitter.

[0015] Again, in another embodiment, the holographic recording system further includes a beam splitter, and the at least one laser source is configured to emit the first and second recording beams by emitting the initial beam toward the beam splitter.

[0016] Again, in a further embodiment, the first set of one or more stations includes a first station, and when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams toward the first station.

[0017] In yet another embodiment, the first set of one or more stations includes a first station and a second station, and when the movable platform is in the first position, the at least one laser source is configured to emit a first recording beam toward the first station and a second recording beam toward the second station.

[0018] In yet a further embodiment, the holographic recording system further includes a beam splitter mounted on the movable platform, and when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams by emitting an initial beam toward the beam splitter.

[0019] In yet another additional embodiment, the holographic recording system further includes a beam splitter pair mounted on the movable platform and a stationary beam splitter, the first set of one or more stations includes a first station and a second station, the first set of one or more recording beams includes first, second, third, and fourth recording beams, and when the movable platform is in a first position, the at least one laser source is configured to emit the first and second recording beams toward the first station and the third and fourth recording beams toward the second station, and the first, second, third, and fourth recording beams are formed using the beam splitter pair and the stationary beam splitter.

[0020] In still further additional embodiments, the stations in the first and second sets of stations each include an optical filter for filtering out ambient light.

[0021] Again, yet another embodiment includes a method including the steps of: emitting, using at least one laser source, a first set of one or more recording beams; directing, using at least one optical component mounted on a movable platform, the emitted first set of one or more recording beams toward a first set of one or more waveguide cells stored in a first set of one or more stations; recording the first set of one or more volume gratings in the first set of one or more waveguide cells; repositioning the movable platform; emitting, using at least one laser source, a second set of one or more recording beams; directing, using at least one optical component mounted on the movable platform, the second set of the emitted one or more recording beams toward a second set of one or more waveguide cells stored in a second set of one or more stations; and recording the second set of one or more volume gratings in the second set of one or more waveguide cells.

[0022] Again, in still further embodiments, the first set of one or more recording beams includes a first recording beam and a second recording beam.

[0023] In yet another additional embodiment, the at least one laser source includes a first laser source and a second laser source, and the first recording beam is emitted by the first laser source and the second recording beam is emitted by the second laser source.

[0024] In yet a further additional embodiment, the first and second recording beams are formed by projecting the initial beam towards a beam splitter.

[0025] Again, in yet another embodiment, the first set of one or more waveguide cells includes a first waveguide cell, and the emitted first and second recording beams are directed towards the first waveguide cell.

[0026] Again, in still further embodiments, the first set of one or more waveguide cells includes a first waveguide cell and a second waveguide cell, and the emitted first recording beam is directed towards the first waveguide cell and the emitted second recording beam is directed towards the second waveguide cell.

[0027] Again, in another additional embodiment, the first and second recording beams are formed by projecting the initial beam towards a beam splitter mounted on a movable platform.

[0028] Again, in a further additional embodiment, the at least one optical component includes a first mounted beam splitter and a second mounted beam splitter, the first set of one or more waveguide cells includes a first waveguide cell and a second waveguide cell, and the first set of one or more recording beams includes using at least one laser source to emit an initial recording beam toward the stationary beam splitter to form the first recording beam and the second recording beam, and direct the first recording beam toward the first mounted beam splitter. a first set of emitted one or more recording sub-beams directed toward a first set of one or more waveguide cells by directing the first and third recording sub-beams toward a first waveguide cell and directing the second and fourth recording sub-beams toward a second waveguide cell;

[0029] In yet another additional embodiment, the first set of one or more volume gratings is recorded using a single beam interference process.

[0030] Yet still further additional embodiments include a laser source; first, second, third, and fourth stations, each station including an exposure stack and a waveguide cell stage configured to store a waveguide cell, position the waveguide cell so that a surface of the waveguide cell is parallel to a surface of the exposure stack, and maintain the position of the waveguide cell while accounting for micromotion; a pair of stationary beam splitters; a movable platform mounted on a track, the movable platform configured to move along the track between a first position and a second position; and three beam splitters mounted on the movable platform, wherein when the movable platform is in the first position, the laser source emits a first initial recording beam toward the pair of stationary beam splitters to form a first set of three recording beams, and directs the first set of three recording beams toward the three mounted beam splitters to form a first set of six recording sub-beams. and when the movable platform is in a second position, the laser source is configured to simultaneously emit a first set of six recording sub-beams by forming a second initial recording beam toward the pair of stationary beam splitters, form a second set of three recording beams, and direct the second set of three recording beams toward the three mounted beam splitters, to simultaneously emit the second set of six recording sub-beams by forming a second set of six recording sub-beams, directing three recording sub-beams in the second set of six recording sub-beams toward the third station and directing the other three recording sub-beams in the second set of six recording sub-beams toward the fourth station.

[0031] 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.

[0032] The description will be more fully understood with reference to the following figures, 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]

[0033] [Figure 1] Figure 1A conceptually illustrates a profile view of a waveguide cell according to an embodiment of the present invention, Figure 1B conceptually illustrates a waveguide cell with a tapered profile according to an embodiment of the present invention, and Figure 1C conceptually illustrates a top view of a waveguide cell according to an embodiment of the present invention. [Figure 2AB] 2A-2B conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 2C-D] 2C-2D conceptually illustrate a two-beam recording process according to various embodiments of the present invention. [Figure 3] 1 conceptually illustrates a single beam recording process utilizing an amplitude grating, according to an embodiment of the present invention. [Figure 4] 1 conceptually illustrates a schematic diagram of a recording system utilizing a single laser source, according to an embodiment of the present invention. [Figure 5] 1 conceptually illustrates an isometric view of a station configured to store an exposure stack, according to an embodiment of the present invention. [Figure 6] 1 conceptually illustrates a waveguide cell with a marked exposed area, according to one embodiment of the present invention. [Figure 7]1 conceptually illustrates an exposure stack, according to an embodiment of the present invention. [Figure 8] 1 conceptually illustrates a stage assembly for storing a waveguide cell, according to one embodiment of the present invention. [Figure 9A] 1 conceptually illustrates a rotary stage of a stage assembly, according to one embodiment of the present invention. [Figure 9B] 1 conceptually illustrates a rotary stage of a stage assembly, according to one embodiment of the present invention. [Figure 10A] 1 conceptually illustrates an affixed stage assembly according to one embodiment of the present invention. [Figure 10B] 1 conceptually illustrates an affixed stage assembly according to one embodiment of the present invention. [Figure 11A] 1 conceptually illustrates a top view of a recording system utilizing a single laser source and a movable platform, according to an embodiment of the present invention. [Figure 11B] 1 conceptually illustrates an isometric view of a recording system utilizing a single laser source and a movable platform, in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] 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 light ray or beam direction refers to propagation parallel to an axis normal to the surface of the 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 light energy along a straight trajectory. Portions of the following description will be presented using terminology commonly adopted by those skilled in the art of optical design. For illustrative purposes, it should be understood that, for clarity, the drawings are not drawn to scale unless otherwise noted. Additionally, each element of each drawing may not be in proper proportion to each of the other elements in the drawing for clarity.

[0035] Turning now to the drawings, systems and methods for recording holographic gratings in waveguide cells are illustrated. Systems for recording optical elements, such as, but not limited to, volume gratings, in optical recording media can be implemented in many different ways according to various embodiments of the present invention. In many embodiments, the recording system is configured to record a volume grating in the optical recording media of a waveguide cell. In further embodiments, the volume grating is recorded by exposing the recording media to an interference pattern formed using at least one laser source. In some embodiments, the recording system is configured to simultaneously record multiple volume gratings. Multiple volume gratings can be recorded in a single waveguide cell or across multiple waveguide cells. In some embodiments, multiple volume gratings are recorded in a stack of waveguide cells.

[0036] Different types of exposure sources can be utilized and configured depending on the specific application. Additionally, the number of exposure sources utilized can also vary. In some embodiments, multiple exposure sources are used to simultaneously record multiple volume gratings. In some embodiments, the recording system is configured to utilize a single laser source in conjunction with a beam splitter and mirrors to simultaneously record multiple volume gratings. The recording system can be further configured to record a set of volume gratings using a movable platform. In such embodiments, the exposure source is configured to direct a recording beam to a first set of waveguide cells to record the first set of volume gratings. The system can then be configured to reposition components within the system using a movable platform, which can enable the recording beam from the exposure source to be directed toward a second set of waveguide cells to record a second set of volume gratings. In some embodiments, the exposure delivered to any given waveguide cell can be configured to have one or more exposure energies, exposure durations, and / or exposure on / off schedules that vary spatially across the recording surface. These configurations and additional systems and methods for recording optical elements in waveguide cells are discussed in further detail below.

[0037] Waveguide Cell A waveguide cell can be defined as a device containing uncured and / or unexposed optical recording material onto which optical elements, such as, but not limited to, gratings, can be recorded. In many embodiments, optical elements can be recorded within a waveguide cell by exposing the optical recording material to electromagnetic radiation of a certain wavelength. Typically, waveguide cells are constructed such that the optical recording material is sandwiched between two substrates, creating a three-layer waveguide cell. Depending on the application, waveguide cells can be constructed in various configurations. In some embodiments, a waveguide cell contains more than three layers. In some embodiments, a waveguide cell contains different types of layers that can serve various purposes. For example, a waveguide cell can include a protective cover layer, a polarization control layer, and a matching layer.

[0038] Substrates of various materials and shapes can be used in the construction of waveguide cells. In many embodiments, the substrate is a plate made of a transparent material, such as, but not limited to, glass and plastic. Substrates of different shapes, such as, but not limited to, rectangular and curved shapes, can be used depending on the application. The thickness of the substrate can also vary depending on the application. In many cases, the shape of the substrate can determine the overall shape of the waveguide. In some embodiments, the waveguide cell contains two substrates that are the same shape. In other embodiments, the substrates are different shapes. As can be readily understood, the shape, dimensions, and materials of the substrates can vary and depend on the specific requirements of a given application.

[0039] In many embodiments, beads or other particles are dispersed throughout the optical recording material to help control the thickness of the optical recording material layer and prevent the two substrates from collapsing relative to one another. In some embodiments, a waveguide cell is constructed with an optical recording layer sandwiched between two planar substrates. Depending on the type of optical recording material used, thickness control can be difficult to achieve due to the viscosity of some optical recording materials and the lack of a perimeter boundary for the optical recording layer. In some embodiments, the beads are relatively incompressible solids, which can enable the construction of waveguide cells with consistent thicknesses. The size of the beads can determine the local minimum thickness relative to the area surrounding each individual bead. Therefore, the dimensions of the beads can be selected to help achieve the desired optical recording layer thickness. The beads can be made from any of a variety of materials, including, but not limited to, glass and plastic. In some embodiments, the bead material is selected so that its refractive index does not substantially affect the propagation of light within the waveguide cell.

[0040] In some embodiments, the waveguide cells are constructed such that the two substrates are parallel or nearly parallel. In such embodiments, beads of relatively similar size can be dispersed throughout the optical recording material to help achieve a uniform thickness throughout the layer. In other embodiments, the waveguide cells have a tapered profile. Tapered waveguide cells can be constructed by dispersing beads of different sizes across the optical recording material. As discussed above, the size of the beads can determine the local minimum thickness of the optical recording material layer. By dispersing the beads in a pattern of increasing size across the material layer, a tapered layer of optical recording material can be formed when the material is sandwiched between two substrates.

[0041] Waveguide cells according to various embodiments of the present invention can incorporate various photosensitive materials. In many embodiments, the waveguide cells incorporate a holographic polymer-dispersed liquid crystal mixture as the optical recording medium. HPDLC mixtures according to various embodiments of the present invention generally include a liquid crystal (LC), a monomer, a photoinitiator dye, and a 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 HPDLC mixtures is known and dates back to the earliest investigations of HPDLC. For example, a paper by R.L. Sutherland et al., SPIE Vol. 2689, pp. 158-169, 1996 (the disclosure of which is incorporated herein by reference), describes an HPDLC mixture including a monomer, a photoinitiator, a coinitiator, a chain extender, and an LC to which a surfactant may be added. Surfactants are also mentioned in the article by Natarajan et al., Journal of Nonlinear Optical Physics and Materials, Vol. 5, No. 1, pp. 89-98, 1996, the disclosure of which is incorporated herein by reference. Furthermore, U.S. Patent No. 7,018,563 by Sutherland et al. discusses a holographic polymer-dispersed liquid crystal material for forming a polymer-dispersed liquid crystal optical element, the material comprising 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.

[0042] The patent and scientific literature contains many examples of material systems and processes that can be used to fabricate waveguides incorporating volume gratings, 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 waveguides incorporating volume gratings. 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, LCE7, the photoinitiator Rose Bengal, and the coinitiator 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) describe a UV-curable HPDLC for reflective display applications that includes a multifunctional acrylate monomer, a LC, a photoinitiator, a co-initiator, and a chain terminator. YH Cho et al., Polymer International, 48, 1085-1090, 1999, the disclosure of which is incorporated herein by reference, discloses HPDLC recipes 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 describes multifunctional acrylate monomers. GSI Annacchione et al., Europhysics Letters, Vol. 36(6), 425-430, 1996, the disclosure of which is incorporated herein by reference, describes PDLC mixtures containing pentaacrylate monomers, LC, chain extenders, coinitiators, and photoinitiators.

[0043] Acrylates offer the benefits of fast dynamics, 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 in HPDLC technology. Higher functionality materials, such as penta- and hexa-functional stems, have also been used. While HPDLC mixtures with specific components are discussed above in connection with their suitable use as optical recording materials in waveguide cells, the specific formulation of the optical recording material can vary widely and depend on the specific requirements of a given application. Such considerations include diffraction efficiency ("DE"), haze, solar resistance, transparency, and switching requirements.

[0044] Waveguide cells can be constructed using a variety of different methods. In many embodiments, the waveguide cell is constructed by coating a first substrate with an optical recording material capable of acting as an optical recording medium. In some embodiments, the optical recording material is deposited onto the substrate using spin coating or spraying. A second substrate layer can be incorporated to form the waveguide cell such that the optical recording material is sandwiched between two substrates. In some embodiments, the second substrate can be a thin protective film coated onto the exposure layer. In various embodiments, a substrate is used to create the cell, which is then filled with the optical recording material. The filling process can be accomplished using a variety of different methods, such as, but not limited to, a vacuum filling method. In further embodiments, matching layers and / or polarizing layers can be added.

[0045] A profile view of a waveguide cell 100 according to one embodiment of the present invention is conceptually illustrated in FIG. 1A. As shown, the waveguide cell 100 includes a layer of optical recording material 102, which can be used as a recording medium for optical elements such as, but not limited to, a grating. The optical recording material 102 can be any of a variety of compounds, mixtures, or solutions, such as, but not limited to, the HPDLC mixture described in the previous section. In an illustrative embodiment, the optical recording material 102 is sandwiched between two parallel glass plates 104, 106. In other embodiments, the substrates are arranged in a non-parallel configuration. FIG. 1B conceptually illustrates a profile view of a tapered waveguide cell 108 utilizing beads 110, 112, and 114 according to one embodiment of the present invention. As shown, the beads 110, 112, and 114 vary in size and are dispersed throughout the optical recording material 116 sandwiched between two glass plates 118, 120. During construction of the waveguide cell, the local thickness of an area of ​​the optical recording layer is limited by the size of the beads in that particular area. By dispersing beads in increasing order of size across the optical recording material, a tapered waveguide cell can be constructed when a substrate is placed in contact with the beads. As discussed above, the substrate utilized in the waveguide cell can vary in thickness and shape. In many embodiments, the substrate is rectangular in shape. In some embodiments, the shape of the waveguide cell is a combination of curvilinear components. FIG. 1C conceptually illustrates a top view of a waveguide cell 122 having a curved shape, according to one embodiment of the present invention.

[0046] 1A-1C illustrate specific waveguide cell structures and arrangements, waveguide cells can be constructed in many different configurations and can use a variety of different materials depending on the specific requirements of a given application. For example, the substrate can be made from a transparent plastic polymer instead of glass. In addition, the shape and size of the waveguide cells can vary greatly and can be determined by various factors, such as, but not limited to, the application of the waveguide, ergonomic considerations, and economic factors.

[0047] Volume Bragg Grating Many different types of gratings, capable of exhibiting different optical properties, can be recorded in optical recording materials according to various embodiments of the present invention. In many waveguide applications, diffraction gratings are implemented for a variety of purposes and functions. As can be readily understood, the type of grating selected can depend on the specific requirements of a given application. One type of grating that can be recorded in a waveguide cell is a volume Bragg grating. A volume Bragg grating is a transparent medium that can diffract light of a certain wavelength incident at a certain angle due to periodic variations in the refractive index of the medium. The diffraction of light incident on the grating can be determined by the properties of the light and the grating. Volume Bragg gratings can have high efficiency, with little light being diffracted into higher orders. The relative amount of light in the zeroth order diffracted can be varied by controlling the refractive index modulation of the grating. Utilizing a volume Bragg grating in a waveguide allows the propagation of light within the waveguide to be influenced in a controlled manner to achieve various effects.

[0048] Volume Bragg gratings can be constructed to have desired properties depending on the specific 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 the diffracted light exits the grating on the side on which the incident light did not enter. For a reflection grating, the diffracted light exits the grating on the same side on which the incident light entered. Volume gratings can also be designed with fringes that are sloped and / or inclined relative to the grating surface, which can affect the angle of diffraction / reflection. While the above discussion represents grating structures as either transmission or reflection, both types of gratings behave in the same manner according to the standard grating equation.

[0049] One class of Bragg grating elements includes switchable Bragg gratings ("SBGs"). SBGs are diffractive devices that can be formed by recording a volume phase grating within an HPDLC mixture. 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. Techniques for fabricating and filling glass cells are well known in the liquid crystal display industry. One or both glass plates can support electrodes, typically transparent tin oxide films, to apply an electric field across the film. SBGs can be implemented as waveguide devices, in which the HPDLC mixture forms either the waveguide core or an evanescent coupling layer adjacent to the waveguide. The glass plates used to form the HPDLC cell can provide a total internal reflection light-guiding structure. When the switchable grating diffracts light at angles beyond the TIR condition, the light is coupled out of the SBG. The grating structure within the SBG can be recorded in a film of HPDLC material 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 HPDLC material components, and the exposure temperature can determine the resulting grating morphology and performance. 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 the fringe planes of the grating, which can generate Bragg diffraction with strong optical polarization resulting from the orientational order of the LC molecules within the droplets.

[0050] Volume phase gratings 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.

[0051] 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. SBGs can also be fabricated and implemented with reversed-mode operation. In such cases, the grating is in its non-diffracting (cleared) state when zero voltage is applied and switches to its diffracting state when a voltage is applied across the electrodes. 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 inherently high refractive index modulation. SBGs can also be used to provide transmission or reflection gratings for free-space applications.

[0052] Volume Bragg gratings can be implemented within waveguides for a variety of different purposes, including, but not limited to, redirecting light and preventing light transmission. Volume Bragg gratings can also be used to provide beam expansion. For example, in many waveguide applications, volume Bragg gratings are used to provide beam expansion in two orthogonal directions. In display applications, this translates to a large eyebox. Therefore, volume Bragg 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, where only light rays that pass through this virtual aperture can enter the user's eye.

[0053] In many embodiments, a volume Bragg grating is implemented as an input grating for coupling light into a waveguide by diffracting the light at angles within the TIR condition of the waveguide. Similarly, a volume Bragg grating can also be implemented as an output grating for coupling light out of a waveguide by diffracting the light at angles beyond the TIR condition. A volume Bragg grating can also be implemented as a folded grating. In some embodiments, the Bragg fringes of the folded grating are oriented diagonally relative to the Bragg fringes of the other gratings. Depending on the orientation of the folded grating, light can be directed toward a specific direction depending on its interaction with the folded grating. In many embodiments, the longitudinal edges of the folded grating are oblique to the axis of alignment of the input coupler so that the folded grating is oriented diagonally relative to the propagation direction of the display light. The folded grating can be angled so that light from the input coupler is redirected to the output grating. In some embodiments, the folded grating is oriented at a 45-degree angle relative to the direction in which light is emitted from the input grating. This feature can direct a display image propagating through the folding grating to an output grating. For example, in some embodiments, the folding grating directs the image by 90 degrees to the output grating. In various embodiments, each folding grating can have a partial diffractive structure. In some embodiments, each folding grating can have a full diffractive structure.

[0054] Different grating configurations and technologies can be incorporated into a single waveguide. A folded grating can be configured to provide pupil expansion in one direction and direct light to an output grating via TIR inside the waveguide. The output grating can be configured to provide pupil expansion in a second direction different from the first direction and direct light from the waveguide out of the waveguide. In this way, a single waveguide can provide pupil expansion in both horizontal and vertical directions. As can be readily appreciated, volume Bragg gratings can be implemented in many different configurations, including, but not limited to, gratings with spatially varying K-vectors and multiplexed gratings. In many applications, the waveguide is implemented using a two-grating structure capable of two-axis pupil expansion.

[0055] Due to 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, a recording system is configured to record a volume grating with fringes having a spatially varying K vector. The K vector (also referred to in the literature as the grating 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. Each K vector is associated with a fringe tilt angle (as defined in Kogelnik theory). In some embodiments, the plane within which the K vector varies is out of the plane of the waveguide or grating element. A varying fringe tilt angle 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 sets of discrete gratings are placed in series. A grating with a roll-axis rotation K vector can be implemented 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 for its corresponding output angle at that location. In other embodiments, the peak diffraction efficiency of each grating at a different location is offset from its corresponding output angle at that location, thereby expanding the effective angular bandwidth of the grating. By introducing this offset, eyebox uniformity can be improved. In some embodiments, the offset can improve overall image brightness by a factor of two compared to simply matching the peak diffraction efficiencies at different locations.

[0056] In many embodiments, different sets of fringes are superimposed or overlapped to create multiplexed gratings, with multiple gratings inside the same volume that can function independently and without interfering with each other. For example, if two volume gratings are recorded in the same device for two different Bragg wavelengths at the same angle of incidence, the device can diffract the two selected wavelengths in different output directions with limited crosstalk. Multiplexing can be used to combine two gratings of similar prescriptions to produce improved angular profiles, expand 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 are no conflicts between the gratings during recording, which would lead to unequal diffraction efficiencies and crosstalk between the gratings upon playback. In some embodiments, at least one of the input, folded, or output gratings can combine two or more angular diffraction 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 formulations to expand the spectral bandwidth, e.g., a color-multiplexed grating can be used to diffract two or more of the primary colors.

[0057] Although specific lattice structures are discussed above, those skilled in the art will understand that recording systems according to various embodiments of the present invention may be configured to record any type of volumetric lattice, including but not limited to those described above.

[0058] Volume lattice recording 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 with two mutually coherent laser beams, where the overlap of the two beams creates a periodic intensity distribution along an interference pattern. 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. Depending on how the recording beam is configured, the grating structure can be formed with tilted or non-tilted fringes. 2A-2D 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 200 and a reflection grating 202. Depending on how the two recording beams 204, 206 are positioned, an interference pattern 208 can record either a transmission grating or a reflection grating in the optical recording material 210. 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 212 incident on the transmission grating 200 can result in a transmitted diffracted beam 214. Meanwhile, a beam 216 incident on the reflection grating 202 can result in a reflected beam 218.

[0059] 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. 3. As shown, a beam 300 from a single laser source (not shown) is directed through an amplitude grating 302. Upon interaction with the grating 302, the beam 300 can be diffracted, as in the case of a ray interacting with the black-shaded areas of the amplitude grating, or the beam 300 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 304 and the zero order beam 306 can overlap to create an interference pattern that exposes the waveguide cell's optical recording layer 308. In an illustrative embodiment, a spacer block 310 is positioned between the grating 302 and the optical recording layer 308 to modify the distance between the two components.

[0060] Although specific methods of recording volumetric gratings are discussed and illustrated in Figures 2A-2D and 3, recording systems according to various embodiments of the present invention can be configured to implement any of several methods for recording volumetric gratings, such as, but not limited to, general photolithography techniques.

[0061] Recording system configuration A recording system for recording a volume grating in a waveguide cell can be configured in many different ways. In some embodiments, the recording system includes at least one exposure source and a plurality of stations configured to store an exposure stack containing the waveguide cells. The exposure source can be from any suitable electromagnetic radiation source, which may depend on the type of photosensitive material used. In some embodiments, the electromagnetic radiation source is a laser source. In some embodiments, the stations and exposure stacks are configured such that the exposure delivered to any given waveguide cell has one or more of an exposure energy, exposure duration, and / or exposure on / off schedule that vary spatially across the recording surface. During operation, the laser source can output light within an appropriate wavelength to expose the waveguide cells stored in the stations and form a volume grating in the waveguide cells. Various methods for recording a volume grating, such as those described in the above sections, can be used. For example, in many embodiments, a single-beam recording method is used in conjunction with a master grating. In other embodiments, a two-beam recording method is used.

[0062] Depending on the application and the waveguide cell, one or more volume gratings can be recorded in a single waveguide cell. In many embodiments, one or more laser sources can be used to simultaneously record at least three volume gratings in a single waveguide cell. In some embodiments, one or more laser sources can be used to simultaneously expose at least two waveguide cells present in one or more stations. In further embodiments, a laser source can be used to simultaneously record at least three volume gratings in each of multiple waveguide cells. As can be readily appreciated, the number of waveguide cells exposed and the number of volume gratings simultaneously recorded per waveguide cell can vary greatly and depend on the specific requirements of a given application. Furthermore, the number and type of exposure sources used can vary depending on several factors, such as, but not limited to, space and power requirements. For example, in embodiments where multiple gratings are to be recorded simultaneously, a high-power laser source or multiple laser sources can be used to provide sufficient exposure power. In embodiments utilizing a single laser source to record multiple gratings and / or expose multiple waveguide cells, a beam splitter can be used to create sub-beams that can enable simultaneous exposure in different areas. The recording system may also include mirrors and other optical elements to steer and direct the light from the laser source into the desired station. In some embodiments, the initial beam is expanded to cover the appropriate exposure area.

[0063] FIG. 4 shows a schematic diagram of a recording system 400 utilizing a single laser source 402, in accordance with an embodiment of the present invention. As shown, an outgoing beam 404 is directed toward a beam splitter 406, which is used to create three sub-beams 408. The sub-beams 408 are directed toward a station 410 using a mirror 412. In an illustrative embodiment, the three sub-beams 408 are used to record three volume gratings within a single waveguide cell housed within the station 410. As can be readily appreciated, these conceptual elements can be implemented using any suitable optical frame, movable adapter, exposure plate, etc., required to enable fixturing of the optical elements for implementing such a recording system. Furthermore, while FIG. 4 illustrates a specific recording system configuration, any configuration can be implemented in accordance with various embodiments of the present invention. For example, in some recording systems, more than three sub-beams are created and directed across multiple stations. In some embodiments, multiple laser sources are utilized. Furthermore, the propagation path of the beam can be manipulated in any of several different ways. For example, while Figure 4 shows a recording system designed to direct the recording beam across a plane, the recording system can also be configured to direct the beam to propagate through 3D space. In such a configuration, a compact design can be achieved along with other process efficiency improvements.

[0064] Station Configuration Due to the feature size of the volume grating, the recording process may require high precision in positioning and leveling. Therefore, replacing an exposed waveguide cell with a new waveguide cell and recording a volume grating into the replaced waveguide cell may be time- and / or resource-intensive. In many embodiments, a station for storing exposure stacks is implemented to enable exposure of multiple waveguide cells in a rapid manner. In some embodiments, the station is configured to allow waveguide cells to be swapped, enabling replacement of an exposed waveguide cell with an unexposed waveguide cell. In these implementations, a waveguide cell can be removed and replaced with another waveguide cell with little or no disturbance to the rest of the system. This can be implemented in various ways. In some embodiments, the stations each contain a pedestal or recess that can store a waveguide cell, allowing the waveguide cells to be swapped. In some embodiments, additional components can be incorporated to align the waveguide cells. For example, a mounting edge designed to hold the edge of the waveguide cell can be implemented in the station to facilitate alignment of the waveguide cell. In various embodiments, the pedestal or recess can be removed from the station. In some embodiments, the station is configured to allow the entire exposure stack to be removed and swapped. In some embodiments, a stage assembly is implemented to store the waveguide cell. The stage assembly can be configured to position the waveguide cell at a desired location relative to the exposure stack. In such embodiments, swapping of the waveguide cell can be easily implemented while maintaining consistency in the positioning of the waveguide cell.

[0065] FIG. 5 conceptually illustrates a station 500 for storing exposure stacks, according to one embodiment of the present invention. In the illustrative embodiment, station 500 is configured to store a single exposure stack using a mounting recess 502. Station 500 includes a laser pipe 504 and a mirror 506 to direct incident light toward the exposure stack. As can be readily appreciated, the orientation of the exposure stack relative to the axis along which the beam travels can dictate whether the station storing the exposure stack includes additional mirrors to change the axis along which the recording beam propagates. While FIG. 5 illustrates a specific station configuration, the station can be configured in a variety of different ways according to various embodiments of the present invention. For example, in some embodiments, the station is configured to store multiple exposure stacks. Given the photosensitive nature of the waveguide cell, a cover, such as, but not limited to, an optical filter, can be used to prevent ambient light from entering the station. In some embodiments, the cover includes at least one cutout to allow light to pass through to expose a desired area of ​​the waveguide cell.

[0066] An exposure stack can include various components designed to manipulate light incident from a laser source into the exposure area of ​​a waveguide cell. The exposure area is a designated area on the waveguide cell that the light is intended to expose. As can be easily understood, the size and shape of the exposure area can vary and can depend greatly on the volume grating to be written. For example, in some applications, different types of volume gratings requiring different levels of exposure are recorded within the same waveguide cell. In many embodiments, the recording system is configured to expose each individual exposure area with different levels of power and / or duration of light, which can be specifically tailored to the type of volume grating to be recorded. FIG. 6 conceptually illustrates a waveguide cell 600 with marked exposure areas for three gratings, in accordance with one embodiment of the present invention. The waveguide cell 600 has a curved shape and is designed to implement an input grating, a folded grating, and an output grating. In the illustrative embodiment, exposure areas for an input grating 602, a folded grating 604, and an output grating 606 are shown. Although FIG. 6 illustrates a specific waveguide cell with a specific exposure area, a waveguide cell can have any number of exposure areas of any shape and size according to various embodiments of the present invention.

[0067] The exposure stack can be constructed using different combinations of components. In many embodiments, the exposure stack includes a master grating and a waveguide cell. In some embodiments, the master grating is an amplitude grating. In further embodiments, the master grating is a chrome master made of a transparent layer and a chrome layer that define a grating structure. During the recording process, light from one or more laser sources can be directed toward the exposure stack using various optical components, such as, but not limited to, mirrors and beam splitters. In a single-beam recording system, a single light beam is directed toward the master grating in the exposure stack. Upon interaction with the master grating, the light beam can be diffracted, and the first-order diffraction and zero-order beams can form an interference pattern that exposes the waveguide cell, forming a volume grating.

[0068] The recording system can be configured to position the master grating in a variety of different ways. In some embodiments, the master grating is positioned within the exposure stack so that the surface of the master grating is parallel to the surface of the waveguide cell. In some embodiments, the master grating is positioned so that it is parallel to the surface of the optical recording layer of the waveguide cell. The offset between the master grating and the surface of the waveguide cell / optical recording layer can vary depending on several considerations, such as, but not limited to, the dimensions of the grating to be formed. In some embodiments, the master grating is in direct contact with the waveguide cell. In other embodiments, a different layer of material in the exposure stack separates the optical recording layer from the master grating. As can be easily understood, the positions of the master grating and the waveguide cell can vary and depend on the specific requirements of a given application. For example, in various embodiments, the optical recording material is an HPDLC composite encapsulated between two glass substrates. As a result, in such embodiments, at least a glass layer is present between the master grating and the optical recording layer during the recording process. In some embodiments, the exposure stack includes a protective layer, such as, but not limited to, a glass plate, that may be placed adjacent to the master grating to help prevent mechanical damage to the grating. In various embodiments, optical oil may be used between the various layers to help provide a continuum of refractive index.

[0069] In some cases, the light exposing the optical recording material may be partially reflected at the surface of the optical recording material. The reflected light may travel and reflect a second time, typically at the surface of the master grating. After the second reflection, the light may then travel back and expose the optical recording material. This secondary exposure is typically undesirable because it can result in degradation of the desired grating (e.g., reduction in grating refractive index modulation contrast) and / or the formation of spurious gratings. Therefore, in some embodiments, an additional layer of material is added and positioned between the master grating and the optical recording layer to help prevent exposure from the reflected light. In many embodiments, the additional layer is a layer of glass. With the additional layer, the distance between the master grating and the waveguide cell can be controlled. By increasing this distance, light reflected at a certain angle at the surface of the optical recording layer can travel further before being reflected a second time, changing the location where the reflected light will enter the optical recording layer. The system can be designed so that this location is in an insignificant area of ​​the optical recording layer. In some embodiments, the location is not on the optical recording layer at all.

[0070] A profile view of an exposure stack 700 according to one embodiment of the present invention is conceptually shown in FIG. 7. In an exemplary embodiment, the exposure stack 700 is configured for a single-beam recording process. As shown, the exposure stack 700 can include a master grating 702, a protective glass layer 704, a spacer plate 706, and a waveguide cell having an optical recording layer 708 between two glass plates 710. The spacer plate 706 can be used to increase the distance between the master grating 702 and the optical recording layer 708 while keeping the surfaces of the two components parallel. The additional distance can help reduce / prevent unwanted exposure from reflected light from the initial exposure. In an exemplary embodiment, the master grating 702 is an amplitude grating implemented using a chrome master having a glass layer 712 and a chrome layer 714 that define a grating structure (not shown). During the recording process, light from a laser source can be directed toward the master grating. Upon interaction with the grating surface, the light can diffract. The first diffraction order and zeroth order beams can be combined to form an interference pattern, which exposes the optical recording layer 708 (similar to the process shown in FIG. 3 ). While FIG. 7 illustrates a specific exposure stack configuration, many configurations can be implemented in accordance with various embodiments of the present invention. For example, in embodiments utilizing a stage assembly, the waveguide cells can be held and separately positioned by the stage assembly. In many embodiments, the materials of the additional layers are chosen to have similar or matching refractive indices to prevent any unwanted refraction of the exposure beam. In further embodiments, optical oil is added between the various layers to further improve refractive index matching. In some embodiments, the holographic optical recording material is coated on a substrate, which is then delivered to the stations and exposure stack described above in a roll-to-roll holographic fabrication process.

[0071] Stage for storing the waveguide cell In many embodiments, the exposure system utilizes a waveguide cell stage within the station to position the waveguide cell in a desired manner. In some embodiments, the stage assembly is designed to provide the functionality and adjustability needed to position the waveguide cell so that one face of the waveguide cell is held relative to the horizontal plane of the exposure stack. In some embodiments, the face of the waveguide cell is held relative to the master of the exposure stack. The stage assembly can be configured to position the waveguide cell in a desired orientation relative to features on the master. The stage assembly can also be designed to store specific types of waveguide cells. In some embodiments, the stage assembly includes a holder subassembly for storing the waveguide cell. In various embodiments, the holder subassembly is customized to hold a waveguide cell of a specific shape. In some embodiments, the holder subassembly is designed to allow the waveguide cell to be swapped and repositioned in a consistent manner.

[0072] FIG. 8 conceptually illustrates a stage assembly for storing a waveguide cell, according to one embodiment of the present invention. As shown, stage assembly 800 includes a base component 802, an XY linear translation stage 804, and a rotation stage 806. Rotation stage 806 can include a fork subassembly 808 and a holder subassembly 810 that stores a waveguide cell 812. In some embodiments, one end of base component 802 is designed to be affixed to a station of an exposure system. On the end, XY linear translation stage 804 can be implemented to enable positioning of waveguide cell 812 across a plane. In combination with the functionality of rotation stage 806, stage assembly 800 can be configured to enable positioning of the waveguide cell in three-dimensional space.

[0073] 9A and 9B conceptually illustrate a rotational stage of a stage assembly according to an embodiment of the present invention. FIG. 9A shows a perspective view of a rotational stage 900 including a fork subassembly 902 and a holder subassembly 904. As shown, the rotational stage 900 can be configured to allow positioning of a waveguide cell 906 across several rotational axes. In an illustrative embodiment, the fork subassembly 902 and the holder subassembly 904 are configured to allow the mounted waveguide cell 906 to be rotated about two mutually perpendicular axes 908, 910. The rotational stage 900 can be configured such that the holder subassembly 904 pivots about precision hardware 912 located within the fork subassembly, while the fork subassembly 902 pivots about ball bearing guides 914. The range of rotation for each axis can depend on the specific requirements of a given application. The combination of these two subassemblies 902, 904 can allow the waveguide cell 906 to remain parallel to the bottom surface of the exposure stack regardless of the nominal position of the exposure stack and / or any positional changes of the system resulting from environmental disturbances such as, but not limited to, thermal changes. In many embodiments, the rotational stage 900 can be configured to allow in-plane rotation of the waveguide cell 906. In an illustrative embodiment, the rotational stage 900 is configured to allow rotation of the waveguide cell 906 about an axis 916 that passes through a dimensional center 918 of the waveguide cell 906. This movement can allow a "pitch match" to occur between the waveguide cell 906 and features of a master utilized in the exposure process.

[0074] 9B shows a side view of the rotation stage 900. As shown, the rotation stage 900 can also be configured to provide an upward preload on the fork subassembly 902, which can be used to maintain a constant, indefinite alignment between the top surface of the waveguide cell 906 and the bottom surface of the exposure stack. The force of contact between the waveguide cell 906 and the exposure stack can be dictated by the spring constant k of the spring used to load the fork subassembly 902 upward. The range 922 of this upward rotation can be mechanically limited by the design of the system.

[0075] The stage assembly can be implemented in a variety of ways, depending on the specific requirements of a given application. In some embodiments, the stage assembly is configured to be mounted to a station while maintaining the ability to reposition the waveguide cell. FIGS. 10A and 10B conceptually illustrate an affixed stage assembly according to an embodiment of the present invention. As shown, the stage assembly 1000 is affixed to a stationary component 1002 of the station through a base component 1004. In the illustrated embodiment, the stage assembly 1000 includes a fork subassembly 1006 and a holder subassembly 1008 capable of positioning the waveguide cell 1010 relative to a cutout 1012 onto which the exposure stack can be stored. The stage assembly can be designed to maintain its position while accounting for micro-movements of the exposure system due to various environmental factors, such as, but not limited to, thermal changes and mechanical disturbances.

[0076] Cyclic Exposure Embodiment In many embodiments, the recording system includes a movable platform. In further embodiments, the movable platform can reposition optical components, such as, but not limited to, mirrors, to redirect the beam or sub-beams to a different set of stations. As discussed above, the stations can be configured to swap out waveguide cells. This form of “hot swapping,” along with the implementation of multiple stations and movable platforms, allows continuous operation of the recording system to record volume gratings in an infinite number of waveguide cells. In many embodiments, the swapping of waveguide cells is performed manually. In other embodiments, an automated system swaps the waveguide cells. As can be readily understood, the exact configuration of the movable platform and the components it repositions can depend on the specific requirements of a given application. For example, in some embodiments, a single laser source is used to expose a first set of waveguide cells stored in a first set of stations. The movable platform can then reposition a mirror, which changes the propagation path of the laser source and can then record a second set of volume gratings stored in a second set of stations. During the second set of exposures, the first set of waveguide cells can be replaced with a new set of unexposed waveguide cells to be stored in the first set of stations. The movable platform can then reposition the mirror and direct the recording beam to expose the new set of waveguide cells in the first set of stations. In this configuration, recording of the volume grating in multiple waveguide cells can be performed in a cyclic manner. Although a two-stage cyclic exposure configuration is discussed, the recording system can be configured to perform different cycles of exposures with different numbers of stages, which can depend on the number of stations and the physical constraints of the system.

[0077] 11A and 11B conceptually illustrate top and isometric views, respectively, of a recording system 1100 utilizing a single laser source 1102 and a movable platform 1104, in accordance with an embodiment of the present invention. During operation, a beam 1106 originating from the laser source 1102 passes through a beam splitter 1108, which can form three sub-beams 1110. The beam splitter can be implemented in several different ways. In many embodiments, a partially reflective mirror is used as the beam splitter. A beam expanding component 1112 can be used to manipulate the size of the sub-beam 1110 and collimate it. In an illustrative embodiment, the system is designed to direct the sub-beams 1110 toward a beam splitter 1114 and mirror 1116 mounted on the movable platform 1104. A beam splitter 1114 mounted on the movable platform 1104 can further split the three sub-beams 1110 into six sub-beams, which are directed by the beam splitter 1114 and mirror 1116 to simultaneously record three volume gratings on each of the two waveguide cells. As shown, the movable platform 1104 can be positioned so that the mounted beam splitter 1114 and mirror 1116 can redirect six sub-beams at a time into two different sets of stations. Within station 1118, a mirror is also implemented to redirect the incident beam toward the exposure stack. Once the recording process is completed for two waveguide cells, the movable platform 1104 can travel along a track 1120 to reposition the beam splitter 1114 and mirror 1116 and direct the sub-beams into two other stations. This process can continue in a cyclical manner by replacing exposed waveguide cells with unexposed waveguide cells during cycles in which recording is occurring at different stations. In some embodiments, station 1118 includes a sheet cover to help reduce / prevent ambient light from affecting the exposure stack.The cover may include a cutout designed to allow the incident exposure beam to pass through.

[0078] 11A and 11B conceptually illustrate an exemplary recording system, any of several different configurations can be implemented in accordance with various embodiments of the present invention. For example, any number of stations can be implemented depending on the specific requirements of a given application. In some embodiments, nine stations are implemented. In some embodiments, the stations and exposure stacks shown in the figures can contain ports for the input and extraction of diagnostic laser beams and reflected light for use in monitoring the holographic exposure process. In some embodiments, the stations are positioned in 3D space, and a movable platform is configured to move in 3D space and redirect the recording beam accordingly.

[0079] While specific systems and methods for recording holographic gratings in waveguide cells are discussed above, many different configurations can be implemented in accordance with the many different embodiments of the present invention. It should therefore be understood 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 exemplified embodiments, but by the appended claims and their equivalents.

Claims

1. 1. A holographic recording system comprising: at least one laser source configured to emit a recording beam; a first set of one or more stations configured to store a first set of waveguide cells; a second set of one or more stations configured to store a second set of waveguide cells; a movable platform configured to move between a first position and a second position; when the movable platform is in the first position, the at least one laser source is configured to emit a first set of one or more recording beams toward the first set of one or more stations; A holographic recording system, wherein when the movable platform is in the second position, the at least one laser source is configured to emit a second set of one or more recording beams toward the second set of one or more stations.

2. further comprising a plurality of mirrors; 2. The holographic recording system of claim 1, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first set of one or more recording beams toward the first set of one or more stations by using the plurality of mirrors to direct the first set of one or more recording beams.

3. The holographic recording system of claim 1 , wherein the first set of one or more recording beams includes a first recording beam and a second recording beam.

4. the at least one laser source comprises a first laser source and a second laser source; 4. The holographic recording system of claim 3, wherein when the movable platform is in the first position, the first laser source is configured to emit the first recording beam toward the first set of one or more stations, and the second laser source is configured to emit the second recording beam toward the first set of one or more stations.

5. 4. The holographic recording system of claim 3, further comprising a beam splitter, wherein the at least one laser source is configured to emit the first and second recording beams by emitting an initial beam toward the beam splitter.

6. the first set of one or more stations comprises a first station; 4. The holographic recording system of claim 3, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams toward the first station.

7. the first set of one or more stations comprises a first station and a second station; 4. The holographic recording system of claim 3, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first recording beam toward the first station and the second recording beam toward the second station.

8. a beam splitter mounted on the movable platform; 8. The holographic recording system of claim 7, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first and second recording beams by emitting an initial beam toward the beam splitter.

9. a pair of beam splitters mounted on the movable platform and a stationary beam splitter; the first set of one or more stations comprises a first station and a second station; the first set of one or more recording beams includes a first recording beam, a second recording beam, a third recording beam, and a fourth recording beam; 2. The holographic recording system of claim 1, wherein when the movable platform is in the first position, the at least one laser source is configured to emit the first recording beam and the second recording beam toward the first station and the third recording beam and the fourth recording beam toward the second station, and the first recording beam, the second recording beam, the third recording beam, and the fourth recording beam are formed using the beam splitter pair and the stationary beam splitter.

10. The holographic recording system of claim 1 , wherein each station in the first set of stations and the second set of stations comprises an optical filter for filtering out ambient light.

11. 1. A method for recording a volume grid, comprising: emitting a first set of one or more recording beams using at least one laser source; directing, using at least one optical component mounted on a movable platform, a first set of the emitted one or more recording beams toward a first set of one or more waveguide cells stored in a first set of one or more stations; recording a first set of one or more volume gratings within the first set of one or more waveguide cells; repositioning the movable platform; emitting a second set of one or more recording beams using the at least one laser source; directing, using the at least one optical component mounted on the movable platform, a second set of the emitted one or more recording beams toward a second set of one or more waveguide cells stored in a second set of one or more stations; and recording a second set of one or more volume gratings within the second set of one or more waveguide cells.

12. The method of claim 11 , wherein the first set of one or more recording beams includes a first recording beam and a second recording beam.

13. the at least one laser source comprises a first laser source and a second laser source; 13. The method of claim 12, wherein the first recording beam is emitted by the first laser source and the second recording beam is emitted by the second laser source.

14. The method of claim 12 , wherein the first recording beam and the second recording beam are formed by projecting an initial beam toward a beam splitter.

15. the first set of one or more waveguide cells comprises a first waveguide cell; The method of claim 12 , wherein the emitted first and second recording beams are directed toward the first waveguide cell.

16. the first set of one or more waveguide cells comprises a first waveguide cell and a second waveguide cell; 13. The method of claim 12, wherein the emitted first recording beam is directed toward the first waveguide cell and the emitted second recording beam is directed toward the second waveguide cell.

17. 17. The method of claim 16, wherein the first recording beam and the second recording beam are formed by projecting an initial beam toward a beam splitter mounted on the movable platform.

18. the at least one optical component comprises a first mounted beam splitter and a second mounted beam splitter; the first set of one or more waveguide cells comprises a first waveguide cell and a second waveguide cell; the first set of one or more recording beams using at least one laser source to emit an initial recording beam toward a stationary beam splitter to form a first recording beam and a second recording beam; directing the first recording beam towards the first mounted beam splitter to form a first recording sub-beam and a second recording sub-beam; directing the second recording beam towards the second mounted beam splitter to form a third recording sub-beam and a fourth recording sub-beam; The emitted first set of one or more recording beams directing the first and third recording sub-beams toward the first waveguide cell; 12. The method of claim 11 , wherein the second and fourth recording sub-beams are directed toward a first set of one or more waveguide cells by: directing the second and fourth recording sub-beams toward the second waveguide cell.

19. The method of claim 11 , wherein the first set of one or more volume gratings is recorded using a single-beam interference process.

20. 1. A holographic recording system comprising: a laser source; a first station, a second station, a third station, and a fourth station, each station comprising an exposure stack and a waveguide cell stage, the waveguide cell stage comprising: storing the waveguide cell; positioning the waveguide cell so that a surface of the waveguide cell is parallel to a surface of the exposure stack; maintaining the position of said waveguide cell while accounting for micro-movements; a first station, a second station, a third station, and a fourth station configured to perform the following: a pair of stationary beam splitters; a movable platform mounted on a track, the movable platform configured to move along the track between a first position and a second position; three beam splitters mounted on the movable platform; When the movable platform is in the first position, the laser source: emitting a first initial recording beam toward the pair of stationary beam splitters to form a first set of three recording beams, and simultaneously emitting the first set of six recording sub-beams by directing the first set of three recording beams toward the three mounted beam splitters to form a first set of six recording sub-beams; directing three recording sub-beams in the first set of six recording sub-beams toward the first station; directing three other recording sub-beams in the first set of six recording sub-beams towards the second station; When the movable platform is in the second position, the laser source: simultaneously emitting the second set of six recording sub-beams by emitting a second initial recording beam toward the pair of stationary beam splitters to form a second set of three recording beams, and directing the second set of three recording beams toward the three mounted beam splitters to form a second set of six recording sub-beams; directing three recording sub-beams in the second set of six recording sub-beams toward the third station; directing three other recording sub-beams in the second set of six recording sub-beams toward the fourth station.

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