System and method for high throughput recording of holographic gratings in waveguide cells

Abstract

The present invention relates to systems and methods for high throughput recording of holographic gratings in waveguide cells, wherein a number of different methods and systems can be used to record holographic volume gratings in waveguide cells. One embodiment includes a holographic recording system comprising at least one laser source configured to emit a recording beam, 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, and 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 a second set of one or more stations.

Description

[0001] This application is a divisional application of the invention patent application with the application date of August 29, 2018, application number 201880085894.5, and invention name “System and method for high-throughput recording of holographic gratings in waveguide cells”. Technical Field

[0002] The present invention generally relates to processes and apparatus for recording gratings, and more specifically to processes and apparatus for recording holographic volume gratings in waveguide cells. Background Art

[0003] A waveguide can be referred to as a structure that has the ability to confine and guide waves (i.e., limit the region of space in which waves can propagate). A subclass includes optical waveguides, which are structures capable of guiding electromagnetic waves, typically those in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using many different mechanisms. For example, a planar waveguide can be designed to diffract incident light using a diffraction grating and couple the incident light into the waveguide structure so that the incident coupled light can continue to propagate within the planar structure via total internal reflection ("TIR").

[0004] Fabrication of a waveguide may include the use of a material system that allows for recording of holographic optical elements within the waveguide. One class of such materials includes polymer dispersed liquid crystal ("PDLC") mixtures, which are mixtures comprising photopolymerizable monomers and liquid crystals. Another subclass of such mixtures includes holographic polymer dispersed liquid crystal ("HPDLC") mixtures. Holographic optical elements, such as volume phase gratings, may be recorded in such liquid mixtures by illuminating the material with two mutually coherent laser beams. During the recording process, the monomers polymerize and the mixture undergoes photopolymerization-induced phase separation, thereby creating regions densely populated with liquid crystal droplets interspersed with transparent polymer regions. Alternating regions rich in liquid crystal and regions lacking liquid crystal form the fringe planes of the grating.

[0005] Waveguide optics such as those described above may be considered for a range of display and sensor applications. In many applications, waveguides containing one or more grating layers encoding multiple optical functions may be implemented using a variety of waveguide architectures and material systems, enabling new innovations in near-eye displays for augmented reality ("AR") and virtual reality ("VR"), compact head-up displays ("HUD") for aviation and road transportation, and sensors for biometric and laser radar ("LIDAR") applications. Summary of the invention

[0006] One embodiment includes 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 accommodate a first set of waveguide cells, a second set of one or more stations configured to accommodate 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 comprises a plurality of mirrors, 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 by directing the first set of one or more recording beams using the plurality of mirrors.

[0008] In another 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, and when the movable platform is in the first position, the first laser source is configured to emit a first recording beam toward a first set of one or more stations, and the second laser source is configured to emit a second recording beam toward a second set of one or more stations.

[0010] In yet another embodiment, the holographic recording system further includes a beam splitter, wherein 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.

[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 the first and second recording beams toward the first station.

[0012] 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.

[0013] In another additional embodiment, the holographic recording system further includes a beam splitter mounted on the movable platform, 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 the initial beam toward the beam splitter.

[0014] In another additional embodiment, the holographic recording system further comprises a pair of beam splitters mounted on the movable platform and a fixed beam splitter, wherein 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 comprises a first, second, third and fourth recording beams, and when the movable platform is in a first position, at least one laser source is configured to emit the first and second recording beams toward the first station and emit the third and fourth recording beams toward the second station, wherein the first, second, third and fourth recording beams are formed using the pair of beam splitters and the fixed beam splitter.

[0015] In another embodiment, the holographic recording system further includes a beam splitter, wherein 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] In 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 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 another embodiment, the holographic recording system further includes a beam splitter mounted on the movable platform, 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 the initial beam toward the beam splitter.

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

[0020] In yet another additional embodiment, each station within the first and second sets of stations comprises a filter for filtering out ambient light.

[0021] Another embodiment includes a method comprising emitting a first set of one or more recording beams using at least one laser source, directing the emitted first set of one or more recording beams toward a first set of one or more waveguide cells contained in a first set of one or more stations using at least one optical component mounted on a movable platform, recording a first set of one or more volume gratings in the first set of one or more waveguide cells, repositioning the movable platform, emitting a second set of one or more recording beams using at least one laser source, emitting a second set of one or more recording beams toward a second set of one or more waveguide cells contained in a second set of one or more stations using at least one optical component mounted on the movable platform, and recording a second set of one or more volume gratings in the second set of one or more waveguide cells.

[0022] In yet another embodiment, 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 another additional embodiment, the first and second recording beams are formed by transmitting an initial beam toward a beam splitter.

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

[0026] In yet another embodiment, 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 toward the first waveguide cell, and the emitted second recording beam is directed toward the second waveguide cell.

[0027] In another additional embodiment, the first and second recording beams are formed by transmitting an initial beam toward a beam splitter mounted on a movable platform.

[0028] In another additional embodiment, at least one optical component includes a first mounted beam splitter and a second mounted beam splitter, a first set of one or more waveguide cells includes a first waveguide cell and a second waveguide cell, at least one laser source is used to emit a first set of one or more recording beams by emitting an initial recording beam toward a fixed beam splitter to form a first recording beam and a second recording beam, directing the first recording beam toward the first mounted beam splitter to form a first recording sub-beam and a second recording sub-beam, and directing the second recording beam toward the second mounted beam splitter to form a third recording sub-beam and a fourth recording sub-beam, and the first set of one or more recording beams emitted is directed toward the first set of one or more waveguide cells by directing the first and third recording sub-beams toward the first waveguide cell and directing the second and fourth recording sub-beams toward the second waveguide cell.

[0029] In yet another additional embodiment, a single beam interferometry process is used to record a first set of one or more volume gratings.

[0030] Another additional embodiment includes a holographic recording system comprising: a laser source; first, second, third, and fourth stations, wherein each station includes an exposure stack and a waveguide cell stage, wherein the waveguide cell stage is configured to accommodate 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 micro-movement; a pair of fixed beam splitters; a movable platform mounted on a track, wherein the movable platform is configured to move along the track between a first position and a second position; three beam splitters mounted on the movable platform, wherein when the movable platform is in the first position, the laser source is configured to form a first set of three recording beams by emitting a first initial recording beam toward the pair of fixed beam splitters and moving the first set of three recording beams toward the three mounted beam splitters. The laser source is configured to simultaneously emit a first set of six recording sub-beams by directing the laser source to form a first set of six recording sub-beams, directing three recording sub-beams of the first set of six recording sub-beams toward a first station, and directing the other three recording sub-beams of the first set of six recording sub-beams toward a second station, and when the movable platform is located at a second position, the laser source is configured to simultaneously emit a second set of six recording sub-beams by emitting a second initial recording beam toward a pair of fixed beam splitters to form a second set of three recording beams and directing the second set of three recording beams toward three mounted beam splitters to form a second set of six recording sub-beams, directing three recording sub-beams of the second set of six recording sub-beams toward a third station, and directing the other three recording sub-beams of the second set of six recording sub-beams toward a fourth station.

[0031] Additional embodiments and features are partially described in the following description, and will become apparent to those skilled in the art in part when reading the specification, or may be learned by practicing the invention. A further understanding of the nature and advantages of the invention may be achieved by reference to the remainder of the specification and drawings which constitute a part of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] This description will be more fully understood with reference to the following drawings, which are presented as exemplary embodiments of the present invention and should not be construed as a complete exposition of the scope of the present invention. It will be apparent to those skilled in the art that the present invention may be practiced with some or all of the invention disclosed in the following description.

[0033] Figure 1A A cross-sectional view of a waveguide unit cell according to an embodiment of the present invention is conceptually illustrated.

[0034] Figure 1B A waveguide unit cell having a tapered cross-section according to an embodiment of the present invention is conceptually illustrated.

[0035] Figure 1C A top view of a waveguide unit cell according to an embodiment of the present invention is conceptually illustrated.

[0036] Figures 2A-2D A two-beam recording process according to various embodiments of the present invention is conceptually illustrated.

[0037] Figure 3 A single beam recording process using an amplitude grating according to an embodiment of the present invention is conceptually illustrated.

[0038] Figure 4 A diagram conceptually illustrates a recording system using a single laser source according to an embodiment of the present invention.

[0039] Figure 5 Conceptually illustrated is an isometric view of a station configured to accommodate exposure stacks in accordance with an embodiment of the present invention.

[0040] Figure 6 A waveguide unit cell with marked exposure areas is conceptually illustrated in accordance with an embodiment of the present invention.

[0041] Figure 7 An exposure stack according to an embodiment of the present invention is conceptually illustrated.

[0042] Figure 8 A stage assembly for housing a waveguide unit cell according to an embodiment of the present invention is conceptually illustrated.

[0043] Fig.9A and 9BA rotation stage of a stage assembly according to an embodiment of the present invention is conceptually illustrated.

[0044] Fig. 10A and 10B A fixed stage assembly is conceptually illustrated in accordance with an embodiment of the present invention.

[0045] Fig.11A A top view of a recording system utilizing a single laser source and a movable platform is conceptually illustrated in accordance with an embodiment of the present invention.

[0046] Fig. 11B An isometric view of a recording system utilizing a single laser source and a movable platform is conceptually illustrated in accordance with an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to describe the embodiments, some well-known features of optical technology known to those skilled in the art of optical design and visual display have been omitted or simplified so as not to obscure the basic principles of the present invention. Unless otherwise stated, the term "coaxial" with respect to the direction of a light ray or beam refers to propagation parallel to an axis perpendicular to the surface of the optical component described with respect to the present invention. In the following description, the terms light, ray, beam and direction can be used interchangeably and are associated with each other to indicate the direction of propagation of light energy along a straight line trajectory. The parts described below will be presented using terms commonly used by those skilled in the art of optical design. For the purpose of illustration, it should be understood that, for the sake of clarity, the drawings are not drawn to scale unless otherwise stated. In addition, for the sake of clarity, each element in each drawing may be inappropriately proportional to each other element in the drawing.

[0048] Turning now to the accompanying drawings, systems and methods for recording holographic gratings in waveguide cell units are illustrated. According to various embodiments of the present invention, a system for recording an optical element, such as but not limited to a volume grating, in an optical recording medium may be implemented in many different ways. In many embodiments, the recording system is configured to record the volume grating in the optical recording medium of the waveguide cell unit. In other embodiments, the volume grating is recorded by exposing the recording medium to an interference pattern formed using at least one laser source. In some embodiments, the recording system is configured to record multiple volume gratings simultaneously. Multiple volume gratings may be recorded in one waveguide cell unit or across multiple waveguide cells. In several embodiments, multiple volume gratings are recorded in a stack of waveguide cells.

[0049] Depending on the specific application, different types of exposure sources can be used and can be configured accordingly. In addition, the number of exposure sources utilized can also vary. In some embodiments, multiple exposure sources are used to record multiple volume gratings simultaneously. In multiple embodiments, the recording system is configured to use a single laser source in combination with a beam splitter and a reflector to simultaneously record multiple volume gratings. The recording system can also be configured to record a collection of volume gratings using a movable platform. In such an embodiment, (one or more) exposure sources are configured to direct a recording beam toward a first set of waveguide cells to record a first set of volume gratings. The system can then be configured to reposition (one or more) components within the system using a movable platform, which can allow the recording beam from (one or more) exposure sources to be directed toward a second set of waveguide cells in order to record a second set of volume gratings. In several 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 on the recording plane. These configurations and additional systems and methods for recording optical elements in waveguide cells are discussed in further detail below.

[0050] Waveguide unit cell

[0051] A waveguide cell may be defined as a device containing an uncured and / or unexposed optical recording material, in which optical elements such as, but not limited to, gratings may be recorded. In many embodiments, optical elements may be recorded in a waveguide cell by exposing the optical recording material to certain wavelengths of electromagnetic radiation. Typically, a waveguide cell is constructed such that the optical recording material is sandwiched between two substrates, resulting in a three-layer waveguide cell. Depending on the application, the waveguide cell may be constructed in a variety of configurations. In some embodiments, the waveguide cell comprises more than three layers. In multiple embodiments, the waveguide cell comprises different types of layers that may be used for various purposes. For example, a waveguide cell may include a protective cover layer, a polarization control layer, and an alignment layer.

[0052] Substrates of various materials and shapes can be used in the construction of the waveguide unit cell. In many embodiments, the substrate is a plate made of a transparent material, such as but not limited to glass and plastic. Depending on the application, substrates of different shapes can be used, such as but not limited to rectangular and curved shapes. The thickness of the substrate can also vary depending on the application. The shape of the substrate can often determine the overall shape of the waveguide. In many embodiments, the waveguide unit cell includes two substrates of the same shape. In other embodiments, the substrates have different shapes. As can be easily appreciated, the shape, dimensions, and material of the substrate can vary and can depend on the specific requirements of a given application.

[0053] 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 to help prevent the two substrates from collapsing into each other. In some embodiments, a waveguide unit cell is composed of an optical recording layer sandwiched between two planar substrates. Depending on the type of optical recording material used, thickness control is difficult to achieve due to the viscosity of some optical recording materials and the lack of a defined perimeter of the optical recording layer. In a number of embodiments, the beads are relatively incompressible solids, which can allow the construction of waveguide units with consistent thickness. The size of the beads can determine the local minimum thickness of the area around a single bead. As such, the dimensions of the beads can be selected to help obtain the desired thickness of the optical recording layer. The beads can be made of any of a variety of materials, including but not limited to glass and plastic. In several embodiments, the material of the beads is selected so that its refractive index does not substantially affect the propagation of light in the waveguide unit cell.

[0054] In some embodiments, the waveguide unit cell is constructed so that the two substrates are parallel or substantially parallel. In such embodiments, beads of relatively similar size can be dispersed throughout the optical recording material to help obtain a uniform thickness throughout the layer. In other embodiments, the waveguide unit cell has a tapered cross-section. A tapered waveguide unit cell 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 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.

[0055] The waveguide unit cell according to various embodiments of the present invention can be combined with a variety of photosensitive materials. In many embodiments, the waveguide unit cell is combined with a holographic polymer dispersed liquid crystal mixture as an optical recording medium. The HPDLC mixture according to various embodiments of the present invention generally includes a liquid crystal ("LC"), a monomer, a photoinitiator dye, and a co-initiator. The mixture (often referred to as a slurry) often also contains a surfactant. For the purpose of describing the present invention, a surfactant is defined as any chemical agent that reduces the surface tension of the total liquid mixture. The use of surfactants in HPDLC mixtures is known and can be traced back to the earliest research on HPDLC. For example, a paper by RL Sutherland et al., SPIE Vol. 2689, pp. 158-169, 1996 describes an HPDLC mixture comprising monomers, photoinitiators, co-initiators, chain extenders, and LC to which a surfactant can be added, the disclosure of which is incorporated herein by reference. Natarajan et al. also mentioned surfactants in Journal of Nonlinear Optical Physics and Materials Vol. 5 No. 1, pp. 89-98, 1996, the disclosure of which is incorporated herein by reference. In addition, U.S. Patent No. 7,018,563 to Sutherland et al. discusses a holographic polymer dispersed liquid crystal material for forming a polymer dispersed liquid crystal optical element having at least one acrylic monomer, at least one type of liquid crystal material, a photoinitiator dye, a co-initiator, and a surfactant. The disclosure of U.S. Patent No. 7,018,563 is incorporated herein by reference in its entirety.

[0056] The patent and scientific literature contains many examples of material systems and processes that can be used to make waveguides incorporating volume gratings, including studies of 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 combinations of monomers and liquid crystal materials suitable for making waveguides incorporating volume gratings. Examples of formulations can be found in papers from the early 1990s. Many of these materials use acrylate monomers, including:

[0057] RL 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 formulation includes a crosslinking multifunctional acrylate monomer; a chain extender N-vinyl pyrrolidone, LC E7, a photoinitiator Rose Bengal, and a co-initiator N-phenylglycine. In some variations, a surfactant, octanoic acid, is added.

[0058] · Fontecchio et al., SID 00 Digest, pp. 774-776, 2000, describes UV-curable HPDLCs for reflective display applications, including multifunctional acrylate monomers, LCs, photoinitiators, co-initiators, and chain terminators, the disclosure of which is incorporated herein by reference.

[0059] • YH Cho et al., Polymer International, 48, pp. 1085-1090, 1999, discloses HPDLC formulations including acrylates, the disclosure of which is incorporated herein by reference.

[0060] • Karasawa et al., Japanese Journal of Applied Physics, Vol. 36, pp. 6388-6392, 1997, describe acrylates of various functional orders, the disclosure of which is incorporated herein by reference.

[0061] • Multifunctional acrylate monomers are also described by TJ Bunning et al., Polymer Science: Part B: Polymer Physics, Vol. 35, pp. 2825-2833, 1997, the disclosure of which is incorporated herein by reference.

[0062] G. Slannacchione et al., Europhysics Letters, Vol. 36(6), pp. 425-430, 1996, describe PDLC mixtures comprising a pentaacrylate monomer, LC, a chain extender, a co-initiator and a photoinitiator, the disclosure of which is incorporated herein by reference.

[0063] Acrylates have the advantages of fast kinetics, good mixing with other materials, and good compatibility with film forming processes. Since acrylates are cross-linked, they tend to be mechanically flexible. For example, urethane acrylates with functionalities of 2 (di) and 3 (tri) have been widely used in HPDLC technology. Higher functional materials, such as pentagonal and hexagonal functional rods, have also been used. Although discussed above in conjunction with the appropriate use of HPDLC mixtures with specific components as optical recording materials in waveguide cells, the specific formulation of the optical recording material can vary widely and can depend on the specific requirements of a given application. These considerations can include diffraction efficiency ("DE"), haze, sunlight resistance, transparency, and switching requirements.

[0064] A waveguide unit cell can be constructed using a variety of different methods. In many embodiments, the waveguide unit cell is constructed by coating a first substrate with an optical recording material that can act as an optical recording medium. In multiple embodiments, the optical recording material is deposited onto the substrate using spin coating or spray coating. A second substrate layer can be combined to form the waveguide unit cell so that the optical recording material is sandwiched between the two substrates. In several embodiments, the second substrate can be a thin protective film coated on the exposed layer. In various embodiments, a substrate is used to make the unit cell, which is then filled with the optical recording material. A variety of different methods can be used to complete the filling process, such as, but not limited to, vacuum filling methods. In other embodiments, alignment layers and / or polarization layers can be added.

[0065] exist Figure 1A 1 is a cross-sectional view of a waveguide unit cell 100 according to an embodiment of the present invention. As shown, the waveguide unit cell 100 includes a layer of optical recording material 102, which can be used as a recording medium for an optical element (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 mixtures described in the above sections. 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. Figure 1B A cross-sectional view of a tapered waveguide unit cell 108 utilizing beads 110, 112, and 114 in accordance with an embodiment of the present invention is conceptually illustrated. As shown, the beads 110, 112, and 114 are of different sizes and are dispersed throughout an optical recording material 116 sandwiched between two glass plates 118, 120. During construction of the waveguide unit 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 the beads throughout the optical recording material in order of increasing size, a tapered waveguide unit cell can be constructed when a substrate is placed in contact with the beads. As discussed above, the thickness and shape of the substrate used in the waveguide unit cell can vary. In many embodiments, the substrate is rectangular in shape. In some embodiments, the shape of the waveguide unit cell is a combination of curvilinear components. Figure 1C A top view of a waveguide unit cell 122 having a curved shape is conceptually illustrated according to an embodiment of the present invention.

[0066] Although Figures 1A-1C A specific waveguide unit cell configuration and arrangement is illustrated, but the waveguide unit cells may be constructed in many different configurations and may use a variety of different materials depending on the specific requirements of a given application. For example, the substrate may be made of a transparent plastic polymer instead of glass. Furthermore, the shape and size of the waveguide unit cells may vary greatly and may be determined by a variety of factors, such as, but not limited to, the application of the waveguide, ergonomic considerations, and economic factors.

[0067] Volume Bragg Grating

[0068] According to various embodiments of the present invention, many different types of gratings capable of exhibiting different optical properties may be recorded in an optical recording material. In many waveguide applications, diffraction gratings are implemented for various purposes and functions. As can be readily appreciated, the type of grating selected may depend on the specific requirements of a given application. One type of grating that may be recorded in a waveguide unit cell is a volume Bragg grating. A volume Bragg grating is a transparent medium that can diffract light of a specific wavelength incident at a specific angle due to periodic variations in the medium's refractive index. The diffraction of light incident on the grating may be determined by the characteristics of the light and the grating. A volume Bragg grating may have high efficiency with little to no light being diffracted into higher orders. The relative amount of diffracted and zero-order light may be varied by controlling the refractive index modulation of the grating. Using a volume Bragg grating within a waveguide, the propagation of light within the waveguide may be affected in a controlled manner to achieve a variety of effects.

[0069] Depending on the specific application, the volume Bragg grating can be constructed to have desired characteristics. In multiple embodiments, the volume Bragg grating is designed as a transmission grating. In other embodiments, the volume Bragg grating is designed as a reflection grating. In a transmission grating, incident light that satisfies the Bragg condition is diffracted so that the diffracted light leaves the grating on the side where the incident light did not enter. For a reflection grating, the diffracted light exits the same side of the grating as the incident light entered. The volume grating can also be designed to have fringes that are tilted and / or slanted relative to the grating surface, which can affect the angle of diffraction / reflection. Although the above discussion represents the grating structure as either transmission or reflection, according to the standard grating equation, the two types of gratings behave the same.

[0070] 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 in an HPDLC mixture. SBGs can be made by first placing a thin film of a mixture of a photopolymerizable monomer and a liquid crystal material between glass plates or substrates. In many cases, the glass plates are in a parallel configuration. The techniques for making and filling the glass unit cells are well known in the liquid crystal display industry. One or two glass plates can support electrodes, typically transparent tin oxide films, for applying an electric field across the film. The SBG can be implemented as a waveguide device in which the HPDLC mixture forms a waveguide core or evanescent coupling layer near the waveguide. The glass plates used to form the HPDLC unit cell can provide a total internal reflection lightguide structure. When the switchable grating diffracts light at an angle exceeding the TIR condition, light is coupled out of the SBG. The grating structure in the SBG can be recorded in a film of HPDLC material by photopolymerization-induced phase separation using interferometric exposure with spatially periodic intensity modulation. Controlling factors such as, but not limited to, the radiation intensity, the volume fraction of the components of the HPDLC material, and the exposure temperature can determine the resulting grating morphology and performance. During the recording process, the monomers polymerize and the mixture undergoes phase separation. The LC molecules aggregate to form discrete or coalesced droplets that are periodically distributed in the polymer network over the optical wavelength range. Alternating liquid crystal-rich and liquid crystal-poor regions form the fringe planes of the grating, which can produce Bragg diffraction with strong optical polarization caused by the orientation order of the LC molecules in the droplets.

[0071] Volume phase gratings can exhibit very high diffraction efficiency, which can be controlled by the strength of the electric field applied to the film. In the case where an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets may change, resulting in a reduced refractive index modulation of the fringes and a reduction in 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 many embodiments, the electrodes are made of indium tin oxide ("ITO"). In the OFF state, where no electric field is applied, the extraordinary axis of the liquid crystal is typically aligned perpendicular to the fringes. Therefore, the grating exhibits high refractive index modulation and high diffraction efficiency for P-polarized light. In the case of an electric field applied to the HPDLC, the grating switches to the ON state, where the extraordinary axes of the liquid crystal molecules are aligned parallel to the applied electric field and therefore aligned perpendicular to the substrate. In the ON state, the grating exhibits lower refractive index modulation and lower diffraction efficiency for both S-polarized light and P-polarized light. Therefore, the grating area no longer diffracts light. Depending on the function of the HPDLC device, each grating area can be divided into multiple grating elements, such as, for example, a pixel matrix. Typically, the electrodes on one substrate surface are uniform and continuous, while the electrodes on the opposing substrate surface are patterned according to a plurality of selectively switchable grating elements.

[0072] Typically, an SBG element is zeroed within 30 μs and switched on with a longer relaxation time. It is noted that the diffraction efficiency of the device can be adjusted over a continuous range with the aid of the applied voltage. In many cases, the device exhibits close to 100% efficiency without applied voltage and essentially zero efficiency when a sufficiently high voltage is applied. SBGs can also be fabricated and implemented with reverse mode operation. In such cases, when the applied voltage is zero, the grating is in its non-diffracting (clear) state, and when a voltage is applied across the electrodes, the grating switches to its diffraction state. In some types of HPDLC devices, a magnetic field can be used to control the LC orientation. In some HPDLC applications, the phase separation of the LC material from the polymer can be to the point where no discernible droplet structure is produced. SBGs can also be used as passive gratings. In this mode, its main advantage is the unique high refractive index modulation. SBGs can also be used to provide transmission or reflection gratings for free-space applications.

[0073] Volume Bragg gratings can be implemented in waveguides for a variety of different purposes, such as, 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 into a large eye box. Therefore, by effectively expanding the exit pupil of the collimating optical system, volume Bragg gratings can be used to preserve the eye box size while reducing the lens size. The exit pupil can be defined as a virtual aperture, and only light that passes through this virtual aperture can enter the user's eye.

[0074] In many embodiments, a volume Bragg grating is implemented as an input grating for coupling light into a waveguide by diffracting light at an angle within the TIR condition of the waveguide. Similarly, a volume Bragg grating may also be implemented as an output grating for coupling light out of a waveguide by diffracting light at an angle exceeding the TIR condition. A volume Bragg grating may also be implemented as a folded grating. In some embodiments, the Bragg fringes of the folded grating are oriented in a diagonal direction relative to the Bragg fringes of other gratings. Depending on the orientation of the folded grating, light may be directed in a specific direction after interacting with the folded grating. In many embodiments, the longitudinal edges of the folded grating are tilted to the alignment axis of the input coupler so that the folded grating is set on a diagonal relative to the propagation direction of the display light. The folded grating may be angled so that light from the input coupler is redirected to the output grating. In some embodiments, the folded grating is set at a forty-five degree angle relative to the direction in which the light is released from the input grating. This feature may enable a display image propagating along the folded grating to be transferred to the output grating. For example, in several embodiments, the folded grating causes the image to be rotated 90 degrees into the output grating. In various embodiments, each folded grating may have a partial diffraction structure. In multiple embodiments, each folded grating may have a full diffraction structure.

[0075] Different grating configurations and techniques can be combined 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 cause light to exit the waveguide from 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, such as but not limited to gratings with spatially varying K-vectors and multiplexed gratings. In many applications, waveguides are implemented using dual grating structures capable of dual-axis pupil expansion.

[0076] Due to the limited range of wavelengths and angles over which diffraction occurs in a volume Bragg grating, several methods can be used to increase the diffraction bandwidth of the grating. In many embodiments, a recording system is configured to record a volume grating having fringes with a spatially varying K vector. A K vector (also referred to in the literature as a grating vector) can be defined as a vector orthogonal to the plane of the associated grating fringes, which can determine the optical efficiency for a given range of input angles and diffraction angles. Each K vector is associated with an edge tilt angle (as defined by Kogelnik theory). In many embodiments, the plane in which the K vector varies is not in the plane with the waveguide or grating element. A varying fringe tilt angle or a rolling K vector can be implemented in a variety of different ways. In some embodiments, the fringes of the grating are designed to vary in a progressive manner across the grating. In other embodiments, different sets of discrete gratings are placed in series. Gratings with a rolling K vector can be implemented in a variety of ways. In many embodiments, the rolling K vector is designed so that the peak diffraction efficiency of each grating segment is optimized for its corresponding output angle at that position. In other embodiments, the peak diffraction efficiency of each grating at different positions is offset from its corresponding output angle at that position, thereby expanding the effective angular bandwidth of the grating. By introducing this offset, the uniformity of the eye movement range can be improved. In some embodiments, the offset can increase the total image brightness by a factor of two compared to simply matching the peak diffraction efficiency at different positions.

[0077] In many embodiments, different sets of fringes are superimposed or overlapped, creating a multiplexed grating with multiple gratings within the same volume that can operate 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, then the device can diffract the two selected wavelengths to different output directions with limited crosstalk. By combining two gratings of similar prescriptions, multiplexing can be used to produce improved angular profiles to expand the diffraction efficiency angular bandwidth and provide better brightness uniformity and color balance across the exit pupil and field of view. Multiplexing can also be used to encode two different diffraction prescriptions, which can be designed to project light into different fields of view of an area, or to diffract two different wavelengths of light into a given field of view of an area. Steps can be taken to ensure that there is no competition between gratings during recording that results in unequal diffraction efficiencies and crosstalk between gratings during playback. Multiplexing can also provide the significant benefit of reducing the number of layers in the waveguide structure. In some embodiments, at least one of the input, fold, or output gratings can combine two or more angular diffraction prescriptions to extend the angular bandwidth. Similarly, in several embodiments, at least one of the input, fold, or output gratings can combine two or more spectral diffraction prescriptions to extend the spectral bandwidth. For example, a color multiplexed grating can be used to diffract two or more primary colors.

[0078] Although specific grating structures are discussed above, those of ordinary skill in the art will recognize that recording systems according to various embodiments of the present invention may be configured to record any type of volume grating, including but not limited to those described above.

[0079] Recording volume grating

[0080] According to various embodiments of the present invention, volume gratings may be recorded in waveguide cells using many different methods. Any number and type of electromagnetic radiation sources may be used to achieve the recording of optical elements in optical recording materials. Depending on the application, (one or more) exposure sources and / or recording systems may be configured to record optical elements using varying exposure powers and durations. As discussed above with respect to SBG, techniques for recording volume gratings may include exposing an optical recording material using two mutually coherent laser beams, wherein the superposition of the two beams forms a periodic intensity distribution along an interference pattern. The optical recording material may form a grating structure exhibiting a refractive index modulation pattern that matches the periodic intensity distribution. In an HPDLC mixture, the light intensity distribution causes the monomers to diffuse and polymerize into high intensity regions, while the liquid crystal diffuses into dark regions. This phase separation produces alternating regions rich in liquid crystal and regions depleted of liquid crystal, which form the fringe planes of the grating. Depending on how the recording beams are configured, the grating structure may be formed with tilted or non-tilted stripes. Figures 2A-2D A dual-beam recording process according to various embodiments of the present invention is conceptually illustrated. As shown, two methods can be used to create two different types of Bragg gratings - namely, a transmission grating 200 and a reflection grating 201. Depending on how the two recording beams 202, 203 are positioned, the interference pattern 204 can record either a transmission or reflection grating in the optical recording material 205. The difference between the two types of gratings can be seen in the orientation of the fringes (i.e., the fringes of the reflection volume grating are typically substantially parallel to the surface of the substrate, while the fringes of the transmission grating are typically substantially perpendicular to the surface of the substrate). During playback, a beam 206 incident on the transmission grating 200 can result in a transmitted diffracted beam 207. On the other hand, a beam 208 incident on the reflection grating 201 can result in a reflected beam 209.

[0081] Another method for recording a volume grating in an optical recording material includes using a single light beam to form an interference pattern on the optical recording material. This can be achieved by using a master grating. In many embodiments, the master grating is a volume grating. In some embodiments, the master grating is an amplitude grating. After interacting with the master grating, the single light beam is diffracted. The first order diffraction and zero order beams can overlap to produce an interference pattern, and then the optical recording material can be exposed to form the desired volume grating. Figure 3, a single beam recording process utilizing an amplitude grating according to an embodiment of the present invention is conceptually illustrated in FIG. As shown, a beam 300 from a single laser source (not shown) is directed through an amplitude grating 301. After interacting with the grating 302, the beam 300 may be diffracted, such as when the light interacts with a black shaded region of the amplitude grating, or the beam 300 may propagate through the amplitude grating without substantial deviation as a zero-order beam, such as when the light interacts with a cross-hatched region of the amplitude grating. The first-order diffracted beam 304 and the zero-order beam 306 may overlap to produce an interference pattern that exposes an optical recording layer 308 of a waveguide unit cell. In the illustrative embodiment, a spacer block 310 is located between the grating 302 and the optical recording layer 308 to modify the distance between the two components.

[0082] Although in Figures 2A-2D Specific methods for recording volume gratings are discussed and illustrated in and 3, but the recording system according to various embodiments of the present invention can be configured to implement any of a variety of methods for recording volume gratings, such as but not limited to general lithography techniques.

[0083] Recording system configuration

[0084] A recording system for recording a volume grating in a waveguide cell can be configured in many different ways. In multiple embodiments, the recording system includes at least one exposure source and multiple stations configured to accommodate an exposure stack containing the waveguide cell. The exposure source can be derived from any suitable electromagnetic radiation source, which can depend on the type of photosensitive material used. In some embodiments, the electromagnetic radiation source is a laser source. In several embodiments, the station and the exposure stack are configured so 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 varies spatially across the recording plane. During operation, (one or more) laser sources can output light of appropriate wavelengths so as to expose the waveguide cell housed in the station to form a volume grating within the waveguide cell. Various methods can be used to record the volume grating, such as described in the above sections. For example, in many embodiments, a single beam recording method is used along with a main grating. In other embodiments, a two-beam recording method is used.

[0085] Depending on the application and the waveguide cell, one or more volume gratings may be recorded in a single waveguide cell. In many embodiments, at least three volume gratings may be recorded simultaneously in a single waveguide cell using one or more laser sources. In some embodiments, one or more laser sources may be used to simultaneously expose at least two waveguide cells residing in one or more stations. In further embodiments, (one or more) laser sources may be used to simultaneously record at least three volume gratings in each of a plurality of waveguide cells. As can be readily appreciated, the number of waveguide cells exposed and the number of volume gratings simultaneously recorded per waveguide cell may vary widely and may depend on the specific requirements of a given application. In addition, the number and type of exposure sources used may vary depending on several factors, such as, but not limited to, space and power requirements. For example, in embodiments where a large number of gratings are to be recorded simultaneously, a high power laser source or multiple laser sources may be used to provide sufficient exposure power. In embodiments where a single laser source is utilized to record multiple gratings and / or expose multiple waveguide cells, a beam splitter may be used to create sub-beams that may allow for simultaneous exposure in different areas. The recording system may also include mirrors and other optical elements to manipulate and direct light from (one or more) laser sources into (one or more) desired workstations. In some embodiments, the initial beam is expanded to cover the appropriate exposure area.

[0086] Figure 4 A diagram of a recording system 400 utilizing a single laser source 402 is illustrated in accordance with an embodiment of the present invention. As shown, a starting beam 404 is directed toward a beam splitter 406 for creating three sub-beams 408. The sub-beams 408 are directed toward a station 410 using a mirror 412. In the illustrative embodiment, the three sub-beams 408 are used to record three volume gratings in a single waveguide unit cell housed in the station 410. As can be readily appreciated, these conceptual elements may be implemented using any suitable optical frame, removable adapter, exposure plate, etc., as may be required to allow for the fixing of optical elements to implement such a recording system. Furthermore, while Figure 4 A specific recording system configuration is illustrated, but any configuration may 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 several embodiments, multiple laser sources are utilized. Furthermore, the propagation paths of the beams may be manipulated in any of a number of different ways. For example, while Figure 4 A recording system designed to direct a recording beam across a planar surface is shown, but the recording system can also be configured to direct the beam to propagate in 3D space. In such a configuration, a compact design and other process efficiency improvements can be achieved.

[0087] Station Configuration

[0088] Due to the characteristic size of the volume grating, the recording process can require high precision in positioning and leveling. As such, replacing an exposed waveguide cell with a new waveguide cell and recording a volume grating in the replaced waveguide cell can be time and / or resource intensive. In many embodiments, a station that accommodates an exposure stack is implemented to allow multiple waveguide cells to be quickly exposed. In some embodiments, the station is configured to allow the exchange of waveguide cells, thereby allowing an exposed waveguide cell to be replaced with an unexposed waveguide cell. In these embodiments, a waveguide cell can be removed and replaced with another waveguide cell with little to no interference with the rest of the system. This can be achieved in a variety of ways. In several embodiments, each station includes a base or recess that can accommodate a waveguide cell, thereby allowing the waveguide cells to be swapped. In several embodiments, additional components can be incorporated to align the waveguide cells. For example, a mounting edge designed to hold the edge (one or more) of a waveguide cell can be implemented in the station to facilitate alignment of the waveguide cells. In various embodiments, the base or recess can be removed from the station. In several embodiments, the station is configured to allow the entire exposure stack to be removed and swapped. In various embodiments, the stage assembly is implemented to accommodate a waveguide unit cell. The stage assembly can be configured to position the waveguide unit cell at a desired location relative to the exposure stack. In such embodiments, interchange of the waveguide unit cells can be easily achieved while maintaining consistency in the positioning of the waveguide unit cells.

[0089] Figure 5 A station 500 for housing an exposure stack is conceptually illustrated in accordance with an embodiment of the present invention. In the illustrative embodiment, the station 500 is configured to house a single exposure stack using a mounting recess 502. The station 500 includes a laser tube 504 and a mirror 506 for directing incoming light toward the exposure stack. As can be readily appreciated, the orientation of the exposure stack relative to the axis on which the light beam travels can determine whether the station housing the exposure stack includes additional mirrors to change the axis on which the recording light beam travels. Although Figure 5 A specific station configuration is illustrated, but the station may be configured in a variety of different ways according to various embodiments of the invention. For example, in some embodiments, the station is configured to accommodate multiple exposure stacks. Given the light-sensitive nature of the waveguide unit cells, a cover such as, but not limited to, a filter may be used to prevent ambient light from entering the station. In several embodiments, the cover includes at least one cutout for allowing light to pass through to expose the desired region(s) of the waveguide unit cells.

[0090] The exposure stack may include various components designed to manipulate light transmitted from (one or more) laser sources into an exposure area of ​​a waveguide cell. The exposure area is a designated area on the waveguide cell to which the light is to be exposed. As can be readily appreciated, the size and shape of the exposure area can vary and can depend in large part on the volume grating to be written. For example, in some applications, different types of volume gratings requiring different exposure levels are recorded in 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. Figure 6 A waveguide unit cell 600 with marked exposure regions for three gratings is conceptually illustrated in accordance with an embodiment of the present invention. The waveguide unit cell 600 has a curved shape and is designed to implement an input grating, a fold grating, and an output grating. In the illustrative embodiment, exposure regions for an input grating 602, a fold grating 604, and an output grating 606 are shown. Although Figure 6 A specific waveguide unit cell is illustrated having specific exposure regions, but according to various embodiments of the present invention, the waveguide unit cell may have any number of exposure regions of any shape and size.

[0091] The exposure stack can be constructed with different combinations of components. In many embodiments, the exposure stack includes a master grating and a waveguide unit cell. In some embodiments, the master grating is an amplitude grating. In other embodiments, the master grating is a chrome master consisting of a transparent layer and a chrome layer that defines the grating structure. During recording, various optical components (such as but not limited to mirrors and beam splitters) can be used to direct light from one or more laser sources toward the exposure stack. In a single-beam recording system, a single beam is directed toward the master grating in the exposure stack. After interacting with the master grating, the beam diffracts, and the first-order diffraction and the zero-order beam can form an interference pattern that exposes the waveguide unit cell to form a volume grating.

[0092] The recording system can be configured to position the master grating in a variety of different ways. In multiple embodiments, the master grating is located within the exposure stack so that the surface of the master grating is parallel to the surface of the waveguide unit cell. In some embodiments, the master grating is positioned parallel to the surface of the optical recording layer of the waveguide unit cell. The offset between the master grating and the surface of the waveguide unit 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 several embodiments, the master grating is in direct contact with the waveguide unit cell. In other embodiments, different layers of material within the exposure stack separate the optical recording layer from the master grating. As can be easily recognized, the position of the master grating and the waveguide unit cell can vary and can depend on the specific requirements of a given application. For example, in various embodiments, the optical recording material is a FIPDLC mixture encapsulated between two glass substrates. Therefore, in such embodiments, during the recording process, there is at least a glass layer between the master grating and the optical recording layer. In several embodiments, the exposure stack includes a protective layer, such as but not limited to a glass plate, which can 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 continuity in the refractive index.

[0093] 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 typically travel and reflect a second time 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 generally 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 a pseudo grating. As such, in some embodiments, an additional material layer is added and positioned between the master grating and the optical recording layer to help prevent exposure of the reflected light. In many embodiments, the additional layer is a glass layer. With the additional layer, the distance between the master grating and the waveguide unit 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 the second reflection, thereby changing the position at which the reflected light will be incident on the optical recording layer. The system can be designed so that this position is in an insignificant area of ​​the optical recording layer. In several embodiments, this position is not on the optical recording layer at all.

[0094] exist Figure 7A cross-sectional view of an exposure stack 700 according to an embodiment of the present invention is conceptually illustrated in FIG. In an illustrative embodiment, the exposure stack 700 is configured for a single beam recording process. As shown, the exposure stack 700 may include a master grating 702, a protective glass layer 704, a spacer 706, and a waveguide unit cell having an optical recording layer 708 between two glass plates 710. The spacer 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 unnecessary exposure caused by reflected light from the initial exposure. In an illustrative 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 interacting with the grating surface, the light can diffract. The first order diffraction and zero order beams can combine to form an interference pattern that exposes the optical recording layer 708 (similar to Figure 3 ). Although Figure 7 A specific exposure stack configuration is illustrated, but many configurations may be implemented in accordance with various embodiments of the invention. For example, in embodiments utilizing a stage assembly, the waveguide unit cells are held by the stage assembly and may be separately positionable. In many embodiments, the materials of the additional layers are selected to have similar or matching refractive indices to prevent any unwanted refraction of the exposure beam. In other embodiments, optical oil is added between the layers to further improve the matching of the refractive indices. In some embodiments, the holographic optical recording material is coated onto a substrate and then delivered to the above-described stations and exposure stacks in a roll-to-roll holographic manufacturing process.

[0095] Stage that holds the waveguide unit cell

[0096] 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 required to position the waveguide cell in such a way that one face of the waveguide cell is held against a horizontal surface of the exposure stack. In several embodiments, the face of the waveguide cell is held against a master of the exposure stack. The stage assembly can be configured to position the waveguide cell to a desired orientation relative to the features of the master. The stage assembly can also be designed to accommodate a specific type of waveguide cell. In multiple embodiments, the stage assembly includes a holder subassembly for accommodating the waveguide cell. In various embodiments, the holder subassembly is customized to hold waveguide cells of specific properties. In multiple embodiments, the holder subassembly is designed to allow the waveguide cells to be interchanged and repositioned in a consistent manner.

[0097] Figure 8A stage assembly for accommodating a waveguide unit cell 812 according to an embodiment of the present invention is conceptually illustrated. As shown, the stage assembly 800 includes a base component 802, an XY linear translation stage 804, and a rotating stage 806. The rotating stage 806 may include a fork subassembly 808 and a holder subassembly 810 that accommodates a waveguide unit cell 812. In multiple embodiments, one end of the base component 802 is designed to be fixed to a station of an exposure system. Finally, the XY linear translation stage 804 can be implemented to allow the waveguide unit cell 810 to be positioned across a plane. Combined with the functionality of the rotating stage 806, the stage assembly 800 can be configured to allow the waveguide unit cell to be positioned in three-dimensional space.

[0098] Fig.9A and 9B A rotating stage of a stage assembly according to an embodiment of the present invention is conceptually illustrated. Fig.9A A perspective view of a rotating stage 900 is shown, which includes a fork subassembly 902 and a holder subassembly 904. As shown, the rotating stage 900 can be configured to allow the waveguide unit cell 906 to be positioned across several rotational axes. In an illustrative embodiment, the fork subassembly 902 and the holder subassembly 904 are configured to allow the mounted waveguide unit cell 906 to rotate around two axes 908, 910 that are perpendicular to each other. The rotating stage 900 can be configured so that the holder subassembly 904 pivots around precision hardware 912 located in the fork subassembly, while the fork subassembly 902 pivots around ball bearing guides 914. The range of rotation of each axis can depend on the specific requirements of a given application. The combination of these two subassemblies 902, 904 can allow the waveguide unit 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 position changes of the system due to environmental disturbances (such as but not limited to thermal changes). In many embodiments, the rotating stage 900 can be configured to allow in-plane rotation of the waveguide unit cell 906. In the illustrative embodiment, the rotating stage 900 is configured to allow the waveguide unit cell 906 to rotate about an axis 916 passing through a dimensional center 918 of the waveguide unit cell 906. This movement can allow "pitch matching" to occur between the waveguide unit cell 906 and features of a master utilized during an exposure process.

[0099] Fig. 9B A side view of the rotating stage 900 is shown. As shown, the rotating stage 900 can also be configured to provide an upward preload of the fork subassembly 902, which can be used to maintain a constant indeterminate alignment between the top surface of the waveguide unit cell 906 and the bottom surface of the exposure stack. The contact force between the waveguide unit cell 906 and the exposure stack can be determined by the spring constant k of the spring used to load the fork subassembly 902 upward. This range 922 of upward rotation can be mechanically limited by the design of the system.

[0100] The stage assembly can be implemented in a variety of ways depending on the specific requirements of a given application. In various embodiments, the stage assembly is configured to be mounted to a station while maintaining the ability to reposition the waveguide unit cells. Fig. 10A and 10B A fixed stage assembly according to an embodiment of the present invention is conceptually illustrated. As shown, the stage assembly 1000 is fixed to a fixed component 1002 of the station via a base component 1004. In the illustrated embodiment, the stage assembly 1000 includes a fork subassembly 1006 and a holder subassembly 1008, which are capable of positioning a waveguide unit cell 1010 relative to a cutout 1012 on which an exposure stack can be accommodated. The stage assembly can be designed to maintain 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.

[0101] Example of Cyclic Exposure

[0102] In many embodiments, the recording system includes a movable platform. In other embodiments, the movable platform can reposition optical components, such as but not limited to mirrors, to redirect a beam or (one or more) sub-beams to a different set of stations. As discussed above, the station can be configured to swap out waveguide cells. This form of "hot swap" and the implementation of multiple workstations and movable platforms allow the recording system to operate continuously to record volume gratings in an unlimited number of waveguide cells. In many embodiments, the interchange of waveguide cells is done manually. In other embodiments, an automated system interchanges waveguide cells. As can be easily appreciated, the exact configuration of the movable platform and its repositioned (one or more) components 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 housed in a first set of stations. The movable platform can then reposition the mirror, which can change the propagation path of the laser source, and then record a second set of volume gratings housed in a second set of stations. During the second exposure set, the first set of waveguide cells can be exchanged for a new set of unexposed waveguide cells to be housed in the first set of stations. The movable platform can then reposition the mirror to direct the (one or more) recording beams to expose a new set of waveguide cells in the first set of stations. In this configuration, recording of volume gratings 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 exposure cycles with different numbers of stages, which can depend on the number of workstations and the physical constraints of the system.

[0103] Fig.11A and 11BA top view and an isometric view of a recording system 1100 utilizing a single laser source 1102 and a movable platform 1104 according to an embodiment of the present invention are conceptually illustrated, respectively. During operation, a beam 1106 originating from the laser source 1102 can pass through a beam splitter 1108 to form three sub-beams 1110. The beam splitter can be implemented in several different ways. In many embodiments, a partial reflector is used as a beam splitter. A beam expansion component 1112 can be used to manipulate the size of the sub-beams 1110 and collimate them. In an illustrative embodiment, the system is designed to direct the sub-beams 1110 toward a beam splitter 1114 and a mirror 1116 mounted on the movable platform 1104. The beam splitter 1114 mounted on the movable platform 1104 can also split the three sub-beams 1110 into six sub-beams, which are directed by the beam splitter 1114 and the mirror 1116 to simultaneously record three volume gratings onto each of the two waveguide units. As shown, the movable platform 1104 can be positioned so that the mounted beam splitter 1114 and reflector 1116 can redirect six sub-beams at a time into a set of two different stations. Within station 1118, a reflector is also implemented to redirect the incident beam to the exposure stack. Once the recording process for two waveguide cells is completed, the movable platform 1104 can travel along the track 1120 to reposition the beam splitter 1114 and reflector 1116 to direct the sub-beams into two more stations. The process can be continued in a cyclic manner by replacing the exposed waveguide cells with unexposed waveguide cells during the period of recording in the 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 incoming exposure beam to pass through.

[0104] Although Fig.11A and 11B A specific recording system is conceptually illustrated, but any of a number of different configurations may be implemented in accordance with various embodiments of the invention. For example, any number of stations may 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 may include ports for inputting and extracting diagnostic laser beams and reflected light for monitoring the holographic exposure process. In several embodiments, the stations are positioned in 3D space and the movable platform is configured to move in 3D space to redirect the recording beam accordingly.

[0105] Although specific systems and methods for recording holographic gratings in waveguide units are discussed above, many different configurations may be implemented according to many different embodiments of the present invention. Therefore, it should be understood that the present invention may be practiced in a manner different from that specifically described without departing from the scope and spirit of the present invention. Therefore, the embodiments of the present invention should be considered in all aspects as illustrative and not restrictive. Therefore, the scope of the present invention should not be determined by the embodiments shown, but by the appended claims and their equivalents.

Claims

1. A holographic recording system, comprising: at least one laser source configured to emit at least one recording beam; a first set of one or more stations configured to accommodate a first set of waveguide cells, wherein the first set of one or more stations comprises a first station configured to support a first waveguide of the first set of waveguide cells and a second station configured to support a second waveguide of the first set of waveguide cells; a second set of one or more stations configured to accommodate a second set of waveguide cells, wherein the second set of one or more stations includes a third station and a fourth station, the third station being configured to support a third waveguide of the second set of waveguide cells, and the fourth station being configured to support a fourth waveguide of the second set of waveguide cells; as well as 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 recording beam is simultaneously directed to a first waveguide of a first set of waveguide cells supported by the first station and a second waveguide of the first set of waveguide cells supported by the second station; as well as When the movable platform is in the second position, the at least one recording beam is simultaneously directed to a third waveguide of the second set of waveguide cells supported by the third station and a fourth waveguide of the second set of waveguide cells supported by the fourth station.

2. The holographic recording system of claim 1, further comprising a plurality of mirrors mounted on the movable platform, wherein: When the movable platform is in the first position, the plurality of mirrors are configured to redirect the at least one recording beam toward the second station.

3. The holographic recording system of claim 2, wherein when the movable platform is in the second position, the plurality of mirrors are configured to redirect the at least one recording beam toward the fourth station. 4 . The holographic recording system of claim 1 , further comprising one or more beam splitters configured to split the at least one recording beam into a plurality of sub-beams.

5. The holographic recording system as described in claim 1 further includes a plurality of beam splitters mounted on the movable platform, wherein the plurality of beam splitters are configured to receive the at least one recording beam and to split the at least one recording beam into a plurality of sub-beams, wherein when the movable platform is in a first position, a first group of sub-beams are directed to a first station and a second group of sub-beams are directed to a second station.

6. The holographic recording system according to claim 5, wherein: When the movable platform is in the second position, the first group of sub-beams are directed into the third station and the second group of sub-beams are directed into the fourth station.

7. The holographic recording system of claim 1, further comprising one or more beam expansion components for manipulating a size of the at least one recording beam and collimating the at least one recording beam.

8. The holographic recording system of claim 1, wherein each station within the first set of stations and the second set of stations comprises a filter for filtering out ambient light.

9. The holographic recording system of claim 1, wherein each waveguide unit cell of the first set of waveguide unit cells and / or the second set of waveguide unit cells is configured in an exposure stack including a master grating.

10. The holographic recording system of claim 9, wherein the exposure stack further comprises at least one component selected from the group consisting of: Transparent layer; Protective layer; An optical layer for providing continuity of refractive index; an optical layer for controlling reflection from a surface of an optical recording material; and Partition.

11. The holographic recording system of claim 1 , wherein each waveguide unit cell in the first set of waveguide units and / or the second set of waveguide units comprises a layer of holographic optical recording material supported by a film and delivered to a station in a roll-to-roll holographic manufacturing process.

12. A method for recording a volume grating, the method comprising: placing the movable platform at a first set of one or more stations, wherein the first set of one or more stations is configured to accommodate a first set of waveguide cells, wherein the first set of one or more stations comprises a first station configured to support a first waveguide of the first set of waveguide cells and a second station configured to support a second waveguide of the first set of waveguide cells; emitting at least one recording beam using at least one laser source; simultaneously directing the at least one recording beam to a first waveguide of a first set of waveguide cells supported by a first station and to a second waveguide of a first set of waveguide cells supported by a second station; repositioning the movable platform to a second set of one or more stations, wherein the second set of one or more stations is configured to accommodate a second set of waveguide cells, wherein the second set of one or more stations includes a third station and a fourth station, the third station being configured to support a third waveguide of the second set of waveguide cells, and the fourth station being configured to support a fourth waveguide of the second set of waveguide cells; as well as The at least one recording beam is simultaneously directed to a third waveguide of a second set of waveguide cells supported by a third station and to a fourth waveguide of a second set of waveguide cells supported by a fourth station.

13. The method of claim 12, further comprising redirecting the at least one recording beam toward the second station using a plurality of mirrors mounted on the movable platform when the movable platform is in the first position.

14. The method of claim 13, further comprising redirecting the at least one recording beam toward the fourth station using a plurality of mirrors mounted on the movable platform when the movable platform is in the second position.

15. The method of claim 12, further comprising splitting the at least one recording beam into a plurality of sub-beams using one or more fixed beam splitters.

16. The method of claim 12, further comprising splitting the at least one recording beam into a plurality of sub-beams using a plurality of beam splitters mounted on a movable platform, wherein when the movable platform is in a first position, a first group of sub-beams are directed into a first station and a second group of sub-beams are directed into a second station.

17. The method of claim 16, wherein: When the movable platform is in the second position, the first group of sub-beams are directed into the third station and the second group of sub-beams are directed into the fourth station.

18. The method of claim 12, further comprising resizing or collimating the at least one recording beam using one or more beam expansion components.

19. The method of claim 12, wherein each station within the first set of stations and the second set of stations comprises a filter for filtering out ambient light.

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