Akonia Holographics Patent | Hologram Recording Systems And Optical Recording Cells

Patent: Hologram Recording Systems And Optical Recording Cells

Publication Number: 20190278224

Publication Date: 20190912

Applicants: Akonia Holographics

Abstract

A system and method making one or more holographic optical elements is disclosed. The method may include at least partially submerging a recording medium in an index matching fluid residing in a fluid reservoir. A first surface of the fluid reservoir may include a surface of a first optical coupling element. The method may include positioning the recording medium with respect to the surface of the first optical coupling element. The method may also include applying a first recording beam through the first optical coupling element, the index matching fluid, and a first portion of the recording medium to form a hologram in the first portion of the recording medium.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims priority from co-pending U.S. Application Nos. 62/423,590, filed 17 Nov. 2016, and titled “HOLOGRAM RECORDING SYSTEMS AND METHODS OF USE,” and 62/423,761, filed 17 Nov. 2016, and titled “OPTICAL RECORDING CELLS, METHODS OF USE, AND METHODS OF MANUFACTURE.” The above applications are incorporated herein by reference for all purposes, in their entireties.

FIELD OF TECHNOLOGY

[0002] The present disclosure relates generally to optical reflective devices, and more specifically to manufacturing holographic optical elements.

BACKGROUND

[0003] Holograms may be implemented within optical media. Challenges associated with proper recording of the holograms within optical media and alignment of waveguide surfaces may introduce unwanted reflective characteristics of an optical reflective device. Accordingly, improved systems and methods to promote efficient hologram recording and manufacture holographic optical elements are desired.

SUMMARY

[0004] The described features generally relate to one or more improved methods, systems, or devices for recording optical signals in a recording medium. The optical signals are typically recorded as holograms. The recording medium may reside within or otherwise be supported by a substrate structure. The substrate structure may comprise one or more substrates oriented parallel to each other. A combination of the recording medium and substrate structure may be referred to as an optical recording cell. Fabrication of the optical recording cell may include deposition of a liquid medium mixture on or in the substrate structure, whereupon polymerization of matrix precursors within the medium mixture results in formation of a matrix polymer, which characterized transition of the medium mixture to become a recording medium. In contrast to the liquid medium mixture, the recoding medium is typically a rubbery solid at room temperature that may lend structural support to the optical recording cell. One or more assembly mechanisms may support and orient the substrates in order to sustain the substrates in a parallel orientation, at least until the solid recording medium forms from the media mixture. The assembly mechanisms may furthermore promote dispersion of the media mixture within the substrate structure. In addition, the described features relate to performing a hologram recording process within an environment that is substantially index-matched to the recording medium, to thereby produce a holographic optical element. Recording a hologram in the recording medium may be referred to as programming the recording medium or programming the holographic optical element. One or more methods may be implemented to enhance lateral and longitudinal mobility of an optical recording cell for performing pluralized hologram recording for a set of recording media. The recording media may be treated with spatially and/or temporally incoherent light following hologram recording, and singulated into respective holographic optical elements.

[0005] A method of making is described. The method may include at least partially submerging a recording medium in an index matching fluid residing in a fluid reservoir, wherein a first surface of the fluid reservoir comprises a surface of a first optical coupling element or a surface coupled to the first optical coupling element, positioning the recording medium with respect to the surface of the first optical coupling element, and applying a first recording beam through the first optical coupling element, the index matching fluid, and a first portion of the recording medium to form a hologram in the first portion of the recording medium.

[0006] An apparatus is described. The apparatus may be configured to at least partially submerging a recording medium in an index matching fluid residing in a fluid reservoir, wherein a first surface of the fluid reservoir comprises a surface of a first optical coupling element or a surface coupled to the first optical coupling element, position the recording medium with respect to the surface of the first optical coupling element, and apply a first recording beam through the first optical coupling element, the index matching fluid, and a first portion of the recording medium to form a hologram in the first portion of the recording medium.

[0007] Some examples of the method and apparatus described above may further include processes or features for applying a second recording beam through the first optical coupling element, the index matching fluid, and the first portion of the recording medium to form the hologram in the first portion of the recording medium. Some examples of the method and apparatus described above may further include processes or features for applying a second recording beam through the second optical coupling element, the index matching fluid, and the first portion of the recording medium to form the hologram in the first portion of the recording medium.

[0008] In some examples of the method and apparatus described above, a plane of the first surface of the fluid reservoir may be parallel to the a plane of the second surface of the fluid reservoir. In some examples of the method and apparatus described above, the hologram in the first portion of the recording medium may be formed based at least in part on interference between the first recording beam and the second recording beam.

[0009] Some examples of the method and apparatus described above may further include processes or features for applying a force to at least one of the first surface of the fluid reservoir or another portion of the fluid reservoir such that the first surface of the fluid reservoir moves closer to the recording medium. In some examples of the method and apparatus described above, the index matching fluid may have an index of refraction that may be within 0. In some examples of the method and apparatus described above, the index matching fluid may have an index of refraction that may be within 0.10 of an index of refraction of the first optical coupling element. In some examples of the method and apparatus described above, the index matching fluid may have an index of refraction that may be within 0.025 of an index of refraction of the first optical coupling element. In some examples of the method and apparatus described above, the index matching fluid may have an index of refraction that may be within 0.010 of an index of refraction of the first optical coupling element.

[0010] In some examples of the method and apparatus described above, the index matching fluid has an index of refraction that is within 0.025 of an index of refraction of the first optical coupling element when subject to a wavelength of the first recording beam and has an index of refraction that is greater than 0.10 of the index of refraction of the first optical coupling element when subject to a wavelength of light different from the wavelength of the first recording beam. In some examples of the method and apparatus described above, the first recording beam may have a wavelength of approximately 405 nm. Some examples of the method and apparatus described above may further include processes or features for moving at least one of the recording medium, the first optical coupling element, or a position of the first recording beam with respect to at least one of the recording medium or the first optical coupling element. Some examples of the method and apparatus described above may further include processes or features for applying the first recording beam through the first optical coupling element, the index matching fluid, and a second portion of the recording medium different from the first portion to form a hologram in the second portion of the recording medium.

[0011] Some examples of the method and apparatus described above may further include processes or features for applying a second recording beam through the first optical coupling element, the index matching fluid, and a second portion of the recording medium different from the first portion to form a hologram in the second portion of the recording medium.

[0012] Some examples of the method and apparatus described above may further include processes or features for applying a second recording beam through the second optical coupling element, the index matching fluid, and a second portion of the recording medium different from the first portion to form a hologram in the second portion of the recording medium.

[0013] Another method of making is described. The method may include securing a first substrate substantially parallel to a second substrate, wherein the first substrate is spaced apart from the second substrate, adding a media mixture to a space between the first substrate and the second substrate, solidifying the media mixture to form a recording medium, and applying a first recording beam through a first portion of the recording medium to form a hologram in the first portion of the recording medium.

[0014] Another apparatus is described. The apparatus may be configured to secure a first substrate substantially parallel to a second substrate, wherein the first substrate is spaced apart from the second substrate, add a media mixture to a space between the first substrate and the second substrate, solidify the media mixture to form a recording medium, and apply a first recording beam through a first portion of the recording medium to form a hologram in the first portion of the recording medium.

[0015] Some examples of the method and apparatus described above may further include processes or features for adjusting, after adding a media mixture, a position of at least one of the first substrate or the second substrate. Some examples of the method and apparatus described above may further include processes or features for dispensing adhesive material on a surface of at least one of the first substrate or the second substrate, wherein the adhesive material may be dispensed proximal to a perimeter edge on the surface of the at least one of the first substrate or the second substrate for at least partially confining the media mixture between the first substrate and the second substrate.

[0016] In some examples of the method and apparatus described above, securing a first substrate substantially parallel to a second substrate comprises: applying a suction force to a surface of at least one of the first substrate or the second substrate. In some examples of the method and apparatus described above, at least one of a plurality of micrometers may be used to adjust the position of at least one of the first substrate or the second substrate. In some examples of the method and apparatus described above, an interferometry system may be used to adjust the position of at least one of the first substrate or the second substrate. In some examples of the method and apparatus described above, a spacer layer may be disposed between the first substrate and the second substrate. In some examples of the method and apparatus described above, the spacer layer includes two or more openings a space between the first substrate and the second substrate.

[0017] Some examples of the method and apparatus described above may further include processes or features for aligning the first substrate substantially parallel to the second substrate,* the aligning based at least in part on positioning one or more calibrated spacers between the first optical flat and second optical flat*

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A further understanding of the nature and advantages of implementations of the present disclosure may be realized by reference to the following drawings. In the appended figures, similar components or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

[0019] FIGS. 1A and 1B illustrate diagrams of systems that can be used for manufacturing holographic optical elements in accordance with various aspects of the disclosure.

[0020] FIGS. 2A through 2C illustrate diagrams of systems that can be used for manufacturing holographic optical elements in accordance with various aspects of the disclosure.

[0021] FIGS. 3A and 3B illustrate a diagram of a mechanical system that can be used for manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0022] FIG. 4 illustrates a diagram of a mechanical system that can be used for manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0023] FIGS. 5A and 5B illustrate examples of perspective views of a pre-sealed optical structure that supports manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0024] FIG. 6 illustrates properties of an optical structure that supports manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0025] FIGS. 7A and 7B illustrate properties of an optical structure that supports manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0026] FIGS. 8A and 8B illustrate properties of an optical structure that supports manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0027] FIG. 9 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0028] FIG. 10 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0029] FIG. 11 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0030] FIG. 12 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0031] FIG. 13 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0032] FIG. 14 illustrates an example of an optical system that can be used for edge coupling in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0033] FIG. 15 illustrates an example of an optical system that can be used for manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0034] FIGS. 16A and 16B illustrate examples of an optical system that can be used for manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0035] FIG. 17 illustrates an example of an optical system that can be used for high volume hologram recording in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0036] FIG. 18 illustrates an example of an optical system that can be used for fast hologram recording in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0037] FIG. 19 illustrates an example of an optical system that can be used for fast hologram recording in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0038] FIG. 20 illustrates an example of an optical system that can be used for fast hologram recording in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0039] FIG. 21 illustrates an example of an optical system that can be used for fast hologram recording in association with manufacturing a holographic optical element in accordance with various aspects of the present disclosure.

[0040] FIG. 22A is a diagram illustrating reflective properties of a holographic optical element in real space in accordance with various aspects of the disclosure.

[0041] FIG. 22B illustrates a holographic optical element in k-space in accordance with various aspects of the disclosure.

[0042] FIG. 23 is a diagram of an optical component illustrating a plurality of hologram recordings in accordance with various aspects of the disclosure.

DETAILED DESCRIPTION

[0043] Holographic optical elements may be fabricated by deposition of a liquid medium mixture on or in the substrate structure, whereupon polymerization of matrix precursors within the medium mixture results in formation of a matrix polymer, which characterized transition of the medium mixture to become a recording medium. One or more assembly mechanisms may support and orient the substrates in order to sustain the substrates in a parallel orientation, at least until the solid recording medium forms from the media mixture. The assembly mechanisms may furthermore promote dispersion of the media mixture within the substrate structure. In addition, the described features relate to performing a hologram recording process within an environment that is substantially index-matched to the recording medium, to thereby produce a holographic optical element. The additional features may include at least edge sealed structural materials to protect media from environmental degradation, and partially reflective surfaces and/or coatings employed at portions of the substrates as a means to promote pupil homogenization. In some embodiments, the edge seal may reduce escape of a volatile component from the recording medium or holographic optical element caused therefrom. Pupil homogenization may refer to the replication of incident light at the recording cell, without invariance or interference between reflected light beams (e.g., modes). Mechanisms for orienting the parallel substrates of the recording cell, including the spacing of parallel substrates, as well as edge coupling (e.g., cutting and polishing one or more substrate edges at an angular offset) may also aid sustaining parallelism.

[0044] A mechanical assembly (e.g., a jig assembly) may be used to fabricate the optical recording cell via the implementation of optical flats mounted to oriented bearings. An adjustment apparatus of the mechanical assembly and/or one or more spacers may allow for variant adjustment of orientation and spacing between optical flats. Vacuum channels may be cut into the optical flats for statically sustaining placed substrates, and a mounted mechanism may dispense the medium mixture onto at least one of the substrates of the optical recording cell. The adjustment apparatus, which may comprise a micrometer, may be configured to make precise adjustments.

[0045] In some embodiments, a medium mixture may be dispersed within substrates prior to sealing of the substrates and fabrication of the optical recording cell. For example, substrates may be cleaned and placed on optical flats of the mechanical assembly. At least one of the substrates may contain an edge seal for optical cell composition. A mount of the mechanical assembly may dispense the medium mixture onto at least one of the substrates. Micrometers or other adjustment apparatus may precisely adjust top optical flat toward bottom optical flat, registering a configured spacing between the substrates correlated to the desired media thickness and allowing the medium mixture to spread to fill the desired region. The substrates may then be registered mechanically and/or visually for orientation and the promotion of edge alignment and parallelism between the substrates of the constructed optical cell. In other examples, the substrates may be cleaned and placed on the optical flats of the mechanical assembly. A mount of the mechanical assembly may dispense the medium mixture onto at least one of the substrates. The top optical flat may be adjusted toward the bottom optical flat to a precisely configured distance between the optical flats. The configured distance may be indicated or determined by calibrated spacers, which may reside between the optical flats. The distance associated with the spacers may correlate to the desired media thickens and allow the medium mixture to spread to fill the desired region bounded by the substrates. Similarly, the substrates may then be registered mechanically and/or visually for orientation and the promotion of edge alignment and parallelism between the substrates of the constructed optical cell.

[0046] Alternatively, in other embodiments, substrates may be oriented and sealed to construct an optical recording cell prior to dispersion of media components. Adhesive and/or structural materials may be dispensed onto the substrates in a path that follows the interior perimeter of the surface edges. The volume of the adhesive may be sufficient to join both substrates, and may include a gap or edge (i.e., aperture) to dispose the medium mixture into the optical cell. The pre-sealed optical recording cell sustain structural integrity sufficient to sustain a substantially parallel orientation of the substrates while allowing the recording cell to be filled with the medium mixture from the gap or edge.

[0047] The medium mixture may include a matrix precursor configured to polymerize to form a matrix polymer, along with a photoimageable system. In some embodiments, the matrix polymer can be referred to as a support matrix. The medium mixture is a usually a liquid at 20.degree. C. After casting, matrix precursors typically polymerize approximately to completion to form the matrix polymer. The resulting composition, now referred to as a recording medium, is typically no longer a liquid at 20.degree. C. The recording medium is usually a solid or elastomer at 20.degree. C. and includes a photoimageable system as described herein, along with the matrix polymer. Typically, but not necessarily, medium mixture embodiments include matrix precursors such as a polyol and an isocyanate, polymerization of which results in a matrix polymer comprising a polyurethane.

[0048] Recording medium embodiments can include a matrix polymer formed by polymerization of one or more matrix precursors, and a photoimageable system configured to form a photopolymer upon light induced polymerization. The photoimageable system may comprise a photoactive monomer and an initiator, and the matrix polymer typically comprises a cross-linked support matrix. In some embodiments, the photoimageable system further comprises a terminator. The matrix precursor and the photoimageable system (or the polymers generated therefrom) are typically compatible with each other, and thus avoid phase separation before or after polymerization of either of the matrix precursor or the photoimageable system. The matrix precursor and photoimageable systems furthermore polymerize by reactions sufficiently independent from each other that the photoimageable system remains photosensitive after formation of the matrix polymer but prior to exposure to photoinitiating light. Polymerization of the matrix precursors typically, but not necessarily, commences at room temperature (i.e., at approximately 20.degree. C.) upon mixing of all medium mixture components. In some cases, dispersion of the polymerized medium mixture may include distributive deviations throughout the optical recording cell (e.g., resulting in thickness and/or parallelism variation). The resulting deviation can be determined by determining an optical path length (OPL) variance across the recording cell. As a result, the OPL variance may be compensated for prior to introduction of recording beams at the recording medium.

[0049] One or more coupling elements may be oriented with reference to the optical recording cell, including the polymerized recording medium inset between the substrates of the optical recording cell. The one or more coupling elements may promote the introduction of recording beams at the recording medium at one or more angular ranges exceeding the total internal reflection (TIR) angular range of the optical recording cell. Additionally, the optical means of the recording beams may be translated and/or rotated with respect to the recording medium to achieve hologram recording characteristics which exceed static implementation.

[0050] In some cases, the optical recording cell may be sandwiched between the one or more coupling elements with small amounts of fluid at the glass-to-glass interface. The fluid may be index-matched to the refractive index of each of the coupling elements, subject to a wavelength or range of wavelengths of the recording beams for hologram recording. The optical recording cell may be oriented as to substantially place the recording medium within a common surface region of each of the coupling elements.

[0051] In other cases, the coupling elements may be oriented and structured such that a reservoir structure may be disposed between the two coupling elements, either directly or subject to one or more glass surface planes to which the coupling elements are adhered. The surface planes may be substantially parallel, and each of the coupling elements may be mounted at the surface center of the planes. In some cases, the reservoir structure may include a sealing edge or chamber for sustaining material properties of the reservoir. The reservoir structure may be filled with a fluid index-matched to the refractive index of each of the coupling elements, subject to a wavelength or range of wavelengths of the recording beams for hologram recording. Prior to the hologram recording process, the recording medium may be at least partially submerged within the residing index-matched fluid of the disposed reservoir.

[0052] Methods for increasing throughput and efficiency of hologram programming at a recording media may include motorized stages implemented at the optical recording cell and/or motorized stages implemented at the substrates of the corresponding coupling elements, surface planes, and/or reservoir regions. The motorized stages may promote lateral and longitudinal translation of optical elements to obtain desired recording beam and recording medium portions. In some cases, employed motorized stages may allow for simultaneously recording multiple holograms for a plurality of optical recording cells, as part of a hologram array.

[0053] After hologram recording is complete, a the polymerized recording medium of an optical recording cell may be treated with spatially and/or temporally incoherent light. Spatial incoherence may correspond to a lack of correlation between distinct points, in the extent of one or more mode waveforms. Temporal incoherence may correspond to a lack of correlation at a single reference point during disparate temporal instances. The spatially and/or temporally incoherent light may substantially eliminate photosensitivity of the photopolymer precursors contained within the recording medium. The holography programmed (i.e., inclusion of hologram recordings) optical recording cell, including the treated recording medium, may be referred to as a holographic optical element.

[0054] One or more holographic optical element type components or devices may be employed in a light coupling device (e.g., an input coupler, an output coupler, and/or a cross coupler). Utilizing holographic optical element technology in the one or more light coupling devices may improve viewing capability and optical clarity of an associated image projection. A holographic optical element type device may exhibit achromatic characteristics. A holographic optical element type device (e.g., an output coupler embodiment) may be Bragg-mismatched to one reflection of TIR mode input light that is reflected between substrates and to input light passing straight through the holographic optical element type device (e.g., external light incident on a substrate). The hologram recordings may be configured to reflect light, of a wavelength, about a reflective axis offset from surface normal of the structure, at a plurality of particular incident angles. Aspects of the disclosure are further illustrated by and described with reference to apparatus diagrams, system diagrams, and flowcharts that relate to manufacturing holographic optical elements.

[0055] The aforementioned description provides examples, and is not intended to limit the scope, applicability or configuration of implementations of the principles described herein. Rather, the ensuing description will provide those skilled in the art with an enabling description for implementing implementations of the principles described herein. Various changes may be made in the function and arrangement of elements.

[0056] Thus, various implementations may omit, substitute, or add various procedures or components as appropriate. For instance, it should be appreciated that the methods may be performed in an order different than that described, and that various steps may be added, omitted or combined. Also, aspects and elements described with respect to certain implementations may be combined in various other implementations. It should also be appreciated that the following systems, methods, devices, and software may individually or collectively be components of a larger system, wherein other procedures may take precedence over or otherwise modify their application.

[0057] FIG. 1A illustrates a system 100-a for manufacturing a holographic optical element in accordance with various aspects of the disclosure. System 100-a may include a sample stage carrier 105, a sample carrier rail 110, a first recording beam 115-a, a signal mirror 120, a second recording beam 125-a, a reference mirror 130, a reference mirror carrier rail 135, a reference mirror carrier 140, a recording medium 145-a, a hologram 150, a first coupling element 155-a, and a second coupling element 160-a.

[0058] System 100-a may include global coordinates (xG, yG, zG) and holographic optical element coordinates (x, y, z). The origin may be defined to be in the center of the recording medium 145-a. In some cases, the recording medium 145-a may comprise a generally rectangular shape where z corresponds to the thickness of the recording medium 145-a, x corresponds to the length of the in-plane side of the recording medium 145-a, and y corresponds to the length of the in-plane side of the recording medium 145-a. The global angle for recording, .theta.G, may be defined as the angle of the first recording beam 115-a with respect to the xG-axis inside recording medium 145-a. Holographic optical element coordinates (x, y, z) may be converted to global coordinates by the following equation:

[ x G y G z G ] = [ sin .phi. G 0 cos .phi. G 0 - 1 0 cos .phi. G 0 - sin .phi. G ] [ x y z ] ( 8 ) ##EQU00001##

[0059] In an implementation, the system 100-a may dispose rotating mirrors such as the signal mirror 120 and the reference mirror 130 to create the correct angles for the first recording beam 115-a and the second recording beam 125-a. The angle of the signal mirror 120 may be changed to produce a desired angle (.theta.G1) of first recording beam 115-a with width .about.dEB. The sample stage carrier 105 and the reference mirror carrier 140 may be positioned so as to illuminate the correct location with the recording beams for each exposure. The sample stage carrier 105 of the system 100-a may be positioned on the sample carrier rail 110 to facilitate the illumination of the recording medium 145-a with the first recording beam 115-a in the desired location. The reference mirror carrier 140 may be positioned on the reference mirror carrier rail 135 to facilitate the illumination of the recording medium 145-a with the second recording beam 125-a in the desired location. The recording medium 145-a may be referred to as a recording medium prior to or during hologram recording, and may include a photopolymer. In some embodiments, the recording medium may comprise photorefractive crystals, dichromatic gelatin, photo-thermo-refractive glass, and/or film containing dispersed silver halide particles. In some cases, the medium mixture may include mostly unreacted support matrix precursors including at least polyol and isocyanate.

[0060] With the rotation of the signal mirror 120 and the reference mirror 130 set, the mirrors may be arranged to direct the first recording beam 115-a and the second recording beam 125-a such that the recording beams intersect and interfere with each other to form an interference pattern that is recorded as a hologram 150 in the recording medium 145-a. The system 100-a may form multiple hologram recordings, each configured to reflect light of a particular wavelength about the skew axis 125-a at a plurality of incidence angles. Each hologram may be formed by an exposure of the recording medium 145-a to coherent light having a particular wavelength. The plurality of incidence angles corresponding to each hologram may be offset from one another by a minimum range of angles.

[0061] In some implementations, the recording beams may have widths that differ from each other, or they may be the same. The recording beams may each have the same intensity as each other, or intensity can differ among the beams. The intensity of the beams may be non-uniform. The recording medium 145-a may be secured in place between the first coupling element 155-a (e.g., a first prism) and the second coupling element 160-a (e.g., a second prism) using a fluid 175-a at the glass-to-glass interfaces of the recording medium 145-a with first coupling element 155-a and the second coupling element 160-a. The fluid 175-a may be index-matched to one or more of the coupling elements and the recording medium substrates. A skew axis 125-a resides at a skew angle relative to the surface normal 170-a. As depicted in FIG. 1A, skew angle may be -30.25 degrees relative to the surface normal 170-a. The angle between the first and second recording beams may reside in a range from 0 to 180 degrees. The recorded skew angle relative to surface normal 170-a then becomes .PHI.’=(.theta..sub.R1+.theta..sub.R2-180.degree.)/2+.PHI..sub.G for in-plane system 100-a. For the nominal case where .theta..sub.G2=180.degree.-.theta..sub.G1, .PHI.’=.PHI..sub.G. In FIG. 1A, .PHI..sub.G shows the nominal skew angle relative to surface normal. Additionally, in FIG. 1A, the exact depiction of angles of .theta..sub.G1 and .theta..sub.G2 are not shown. The angles of .theta.’.sub.G1 and .theta.’.sub.G2 are illustrated and correspond to the angles of .theta..sub.G1 and .theta..sub.G2. The angles of .theta..sub.G1 and .theta..sub.G2 are in relation to the first recording beam 115-a and the second recording beam 125-a beam, respectively, within the first coupling element 155-a and the second coupling element 160-a. The angles of .theta.’.sub.G1 and .theta.’.sub.G2 will be different from angles of .theta..sub.G1 and .theta..sub.G2 because of an index of refraction mismatch at the boundary between air and the coupling elements when the recording beams enter the coupling elements (e.g., the effects of Snell’s Law or the law of refraction).

[0062] The first recording beam 115-a and the second recording beam 125-a may be nominally symmetrical about the skew axis 125-a such that the sum of first recording beam internal angle relative to the skew axis and the second recording beam internal angle relative to the skew axis equates to 180 degrees. Each of the first and second recording beams may be collimated plane wave beams originating from a laser light source.

[0063] Refraction at air/coupling element boundaries, for example where the first recording beam 115-a intersects an air/coupling element boundary of the first coupling element 155-a and where the second recording beam 125-a intersects an air/coupling element boundary of the second coupling element 160-a, is shown figuratively rather than strictly quantitatively. Refraction at the coupling element/recording medium boundary may also occur. In implementations, the recording medium and coupling elements each have an index of refraction of approximately 1.5471 at the recording beam wavelength of 405 nm.

[0064] A skew angle for a hologram (including a mean skew angle for a collection of holograms) can be substantially identical to a reflective axis angle, meaning the skew angle or mean skew angle is within 1.0 degree of the reflective axis angle. Given the benefit of the present disclosure, persons skilled in the art will recognize that the skew angle and reflective axis angle can be theoretically identical. However, due to limits in system precision and accuracy, shrinkage of recording medium that occurs during recording holograms, and other sources of error, the skew angle or mean skew angle as measured or estimated based on recording beam angles may not perfectly match the reflective axis angle as measured by incidence angles and reflection angles of light reflected by a holographic optical element. Nevertheless, a skew angle determined based on recording beam angles can be within 1.0 degree of the reflective axis angle determined based on angles of incident light and its reflection, even where medium shrinkage and system imperfections contribute to errors in estimating skew angle and reflective axis angle. It is understood that these medium shrinkage and system imperfections can be made arbitrarily small in the manufacture of holographic optical elements. In this regard, these medium shrinkage and system imperfections may be considered analogous to flatness of an ordinary or conventional mirror. In some examples, a fundamental limit associated with the manufacture of holographic optical elements using volume holograms may be based on thickness of the recording medium.

[0065] A skew axis/reflective axis is generally called a skew axis when referring to making a holographic optical element (for example when describing recording a hologram in a recording medium), and as a reflective axis when referring to light reflective properties of a holographic optical element. A skew angle for a hologram (including a mean skew angle for a collection of holograms) can be substantially identical to a reflective axis angle, meaning the skew angle or mean skew angle is within 1.0 degree of the reflective axis angle. Persons skilled in the art given the benefit of the present disclosure will recognize that the skew angle and reflective axis angle can be theoretically identical. However, due to limits in system precision and accuracy, shrinkage of recording medium that occurs during recording holograms, and other sources of error, the skew angle or mean skew angle as measured or estimated based on recording beam angles may not perfectly match the reflective axis angle as measured by incidence angles and reflection angles of light reflected by a holographic optical element. Nevertheless, a skew angle determined based on recording beam angles can be within 1.0 degree of the reflective axis angle determined based on angles of incident light and its reflection, even where medium shrinkage and system imperfections contribute to errors in estimating skew angle and reflective axis angle. Given the benefit of the present disclosure, persons skilled in the art will recognize that the skew angle for a given hologram is the same as the grating vector angle for that hologram.

[0066] In a variation of the system 100-a, a variable wavelength laser may be used to vary the wavelength of the first and second recording beams. Incidence angles of the first and second recording beams may be, but are not necessarily, held constant while the wavelength of the first and second recording beams is changed. Wavelengths may be comprised of visible red light wavelength, visible blue light wavelength, visible green light wavelength, ultraviolet (UV) wavelength, and/or infrared (IR) wavelength. Each hologram of the system 100-a may reflect an incidence angle at a wavelength that is different than another hologram recording. The system 100-a may have reflective properties that allow it to reflect light at a substantially different wavelength, and in particular a considerably longer wavelength, than the recording beam wavelength.

[0067] FIG. 1B illustrates a system 100-b for manufacturing a holographic optical element in accordance with various aspects of the disclosure. System 100-b may include a first recording beam 115-b, a second recording beam 125-b, a recording medium 145-b, a first coupling element 155-b, a second coupling element 160-b, and skew axis 125-b. System 100-b may be an expanded view in reference to embodiments discussed in reference to FIG. 1A.

[0068] In some cases, one or more holographic optical elements may be fabricated for a light coupling device. A holographic optical element disposed in a horizontal waveguide of a light coupling device may be referred to as a cross coupler. Alternatively, a holographic optical element disposed in a vertical waveguide of a light coupling device may be referred to as an output coupler. In some cases, each reflective axis of the disposed holographic optical elements may be either parallel or angularly offset to the surfaces of the one or more waveguides. For example, a cross coupler having a crossed holographic optical element cross coupler configuration may be fabricated by re-orienting the recording medium 145-b within the first coupling element 155-b (e.g., a first prism) and the second coupling element 160-b (e.g., a second prism). A fluid 175-a may be utilized at the glass-to-glass interfaces of the recording medium 145-b with the first coupling element 155-b and/or the second coupling element 160-b. The fluid 175-a may be index-matched to both the coupling elements and the recording medium. In some recording implementations, the second coupling element 160-b may be omitted and replaced with a component for securing or stabilizing the recording medium 145-b. The component for securing or stabilizing the recording medium 145-b that may also include light absorbing characteristics. For example, the first recording beam 115-b and the second recording beam 125-b may both enter the first coupling element 155-b when configuring a cross coupler.

[0069] In some cases, a second holographic optical element orientation may be recorded on the re-oriented recording medium 145-b. The second holographic optical element may be oriented in an at least partially overlapping, or non-overlapping manner with the first holographic optical element. Thus, a cross holographic optical element configuration is formed in a given volume of the recording medium 145-b (i.e., the recording medium after reorienting and curing processes). The re-orienting process may be repeated to record all desired skew axes of the light coupling device. In some cases, the second skew holographic optical element may be oriented in a non-overlapping manner with the first holographic optical element.

[0070] FIG. 2A illustrates a system 200-a for manufacturing a holographic optical element in accordance with various aspects of the disclosure. System 200-a may include a sample stage carrier 205, a sample carrier rail 210, a first recording beam 215, a signal mirror 220, a second recording beam 225, a reference mirror 230, a reference mirror carrier rail 235, a reference mirror carrier 240, a recording medium 245, a hologram 250, a first coupling element 255-a, and a second coupling element 260-a. Reservoir 275-a may be disposed between the first coupling element 255-a (e.g., first prism) and the second coupling element 260-a (e.g., second prism). In some cases, reservoir 275-a may be a generally rigid structure with respect to the coupling elements 255-a and 260-a, as shown. In other cases, reservoir 275-a may span a volume extending beyond the coupling elements 255-a and 260-a to which the coupling elements 255-a and 260-a are oriented and adhered at substrates of the reservoir 275. In some cases, reservoir 275-a may include a sealing edge or chamber for sustaining material properties. The sealing edge or chamber may exhibit pliability subject to a force exhibited by coupling element 255-a and/or coupling element 260-a. For example, one or more actuators may be used to exert the force to coupling element 255-a and/or coupling element 260-a. Reservoir 275-a may be filled with a fluid 280-a, the fluid may be index-matched to the refractive index of each of the recording medium substrates 255-a and 260-a at a range of wavelengths. Recording medium 245-a may be at least partially submerged within the index-matched fluid 280-a of reservoir 275. In some cases recording medium 245-a may be substantially parallel to the proximal substrates of at least one of coupling elements 255-a and 260-a. In other cases, recording medium 245-a may be translated according to a lateral and/or longitudinal offset from center orientation of the reservoir 275-a, according to the inset orientation axis and/or angularly offset from the proximate substrates of at least one of coupling elements 255-a and 260-a.

[0071] In an implementation, the system 200-a may dispose rotating mirrors such as the signal mirror 220 and the reference mirror 230 to create the correct angles for the first recording beam 215-a and the second recording beam 225. The angle of the signal mirror 220 may be changed to produce a desired angle (.theta.G1) of first recording beam 215-a with width .about.dEB. The sample stage carrier 205 and the reference mirror carrier 240 may be positioned so as to illuminate the correct location with the recording beams for each exposure. The sample stage carrier 205 of the system 200-a may be positioned on the sample carrier rail 210 to facilitate the illumination of the first recording beam 215-a through coupling element 255-a, the index-matched fluid 280-a resident at reservoir 275-a, and incident at the recording medium 245, in the desired location. The reference mirror carrier 240 may be positioned on the reference mirror carrier rail 235 to facilitate the illumination of the second recording beam 225-a through coupling element 255-a, the index-matched fluid 280-a resident at reservoir 275-a, and incident at the recording medium 245, in the desired location. The recording medium 245-a and may include a polymerized photopolymer that includes substantially unreacted photopolymer precursors. In some embodiments, the recording medium may comprise photorefractive crystals, dichromatic gelatin, photo-thermo-refractive glass, and/or film containing dispersed silver halide particles. In some cases, the medium mixture may include mostly unreacted support matrix precursors including at least polyol and isocyanate.

[0072] With the rotation of the signal mirror 220 and the reference mirror 230 set, the mirrors may be arranged to direct the first recording beam 215-a and the second recording beam 225-a such that the recording beams intersect and interfere with each other to form an interference pattern that is recorded as a hologram 250 in the recording medium 245. The system 200-a may form multiple hologram recordings, each configured to reflect light of a particular wavelength about the skew axis 225-a at a plurality of incidence angles. Each hologram may be formed using an exposure of the recording medium 245-a to coherent light having a particular wavelength.

[0073] In some implementations, the recording beams may have widths that differ from each other, or they may be the same. The recording beams may each have the same intensity as each other, or intensity can differ among the beams. The intensity of the beams may be non-uniform. The recording medium 245-a may inset within reservoir 275-a disposed between first light coupling device 255-a and second light coupling device 260-a. Recording medium 245-a may be at least partially submerged within a fluid 280-a contained within reservoir 275-a. The fluid may be index-matched to both the coupling elements 255-a and 260-a and the recording medium 245. A skew axis 225-a resides at a skew angle relative to the surface normal 270. The angle between the first and second recording beams may reside in a range from 0 to 180 degrees. The recorded skew angle relative to surface normal 270 then becomes .PHI.’=(.theta..sub.R1+.theta..sub.R2-180.degree.)/2+.theta..sub.G for in-plane system 200-a. For the nominal case where .theta..sub.G2=180.degree.-.theta..sub.G1, .PHI.’=.PHI..sub.G. In FIG. 2, .PHI..sub.G shows the nominal skew angle relative to surface normal. Additionally, in FIG. 2, the exact depiction of angles of .theta..sub.m and .theta..sub.G2 are not shown. The angles of .theta.’.sub.G1 and .theta.’.sub.G2 are illustrated and correspond to the angles of .theta..sub.G1 and .theta..sub.G2. The angles of .theta..sub.G1 and .theta..sub.G2 are in relation to the first recording beam 215-a and the second recording beam 225, respectively, within the first coupling element 255-a and the second coupling element 260-a. The angles of .theta.’.sub.G1 and .theta.’.sub.G2 will be different from angles of .theta..sub.G1 and .theta..sub.G2 because of an index of refraction mismatch at the boundary between air and the coupling elements when the recording beams enter the coupling elements (e.g., the effects of Snell’s Law or the law of refraction).

[0074] Refraction at air/coupling element boundaries, for example where the first recording beam 215-a intersects an air/coupling element boundary of the first coupling element 255-a and where the second recording beam 225-a intersects an air/coupling element boundary of the second coupling element 260-a, is shown figuratively rather than strictly quantitatively. Index-matched fluid 280-a may be substantially matched to at least one of the first coupling element 255-a and the second coupling element 260-a. The index-matched fluid 280-a may be classified according to an index of refraction of the fluid being within a variant threshold of the index of refraction of the respective coupling element. For example, in some embodiments, the index-matched fluid may have an index of refraction, at a specified wavelength or range of wavelengths, within 0.10 of the index of refraction of the corresponding coupling element (e.g., first coupling element 255-a, second coupling element 260-a), and classified as “matched” to the coupling element. In other embodiments, the index-matched fluid may have an index of refraction, at a specified wavelength or range of wavelengths, within 0.025 of the index of refraction of the corresponding coupling element (e.g., first coupling element 255-a, second coupling element 260-a), and classified as “closely matched” to the coupling element. Furthermore, in other cases, the index-matched fluid may have an index of refraction, at a specified wavelength or range of wavelengths, within 0.010 of the index of refraction of the corresponding coupling element (e.g., first coupling element 255-a, second coupling element 260-a), and classified as “very closely matched” to the coupling element. The classification parameters provided are not intended to be exclusionary, rather they are provided as examples of index-matched fluid characterization. Incident light passing from the coupling elements 255-a and/or 260-a to recording medium 245-a may be refracted. Additionally or alternatively, refraction may occur at the boundary between a substrate of the recording cell and contained recording medium 245-a of the recording cell. In implementations, the recording medium and coupling elements each have an index of refraction of approximately 1.5302, the substrates of the recording cells have an index of 1.5225, the index-matched fluid 280-a has an index between 1.5302 and 1.5225, at the recording beam wavelength of 405 nm. Optionally, the refractive index of the fluid may be 1.5263, which is between the refractive index of the recording medium substrate and the refractive index of the coupling element.

[0075] Mechanical mounts 285 may be integrated with an optical recording cell containing recording medium 245. The mechanical mounts may include a clamp or fastener mechanism to hold the recording medium 245, while sustaining stability and characteristic properties (e.g., parallelism) of the recording medium 245, and surrounding substrates contained within the optical recording cell. A motorized stage and/or robotic mechanism may be implemented with the mounts 285 to translate and/or rotate the optical recording cell within the reservoir 275-a, for subsequent hologram recording at recording medium 245. Translation of the optical recording cell may include a lateral or longitudinal offset of the optical recording cell within reservoir 275-a, to position recording medium 245-a at an orientation between coupling elements 255-a and 260-a for subsequent hologram recording.

[0076] FIG. 2B illustrates a system 200-b for manufacturing a holographic optical element in accordance with various aspects of the disclosure. System 200-b may include a first recording beam 215-b, a second recording beam 225-b, a recording medium 245-b contained within a reservoir 275-b, disposed between a first coupling element 255-b and a second coupling element 260-b, and a skew axis 225-b. The one or more substrates of an optical recording cell, including recording medium 245-b, may be at least partially submerged in an index matched fluid 280-b contained within reservoir 275-b. System 200-b may be an expanded view in reference to embodiments discussed in reference to FIG. 2A.

[0077] In some cases, reservoir 275-b may extend beyond the surface area of at least one of coupling elements 255-b (e.g., first prism) and 260-b (e.g., second prism). Coupling elements 255-b and 260-b may be oriented and attached to substrate planes of the reservoir 275-b for subsequent hologram recording at the at least partially submerged recording medium 245-b. Via emission of the recording beams 215-b and 225-b, one or more holograms may be programmed at recording medium 245-b and one or more holographic optical elements may be fabricated at the optical recording cell containing at least recording medium 245-b. In some cases, each reflective axis of the disposed holographic optical elements may be either parallel or angularly offset to the surfaces of reservoir 275-b and/or the proximal planar edges of coupling elements 255-b and 260-b.

[0078] In some cases, a second holographic optical element orientation may be recorded on the re-oriented recording medium 245-b. The second holographic optical element may be oriented in an at least partially overlapping, or non-overlapping manner with the first holographic optical element. Thus, a cross holographic optical element configuration is formed in a given volume of the recording medium 245-b (i.e., the recording medium after reorienting and curing processes). The re-orienting process may be repeated to record all desired skew axes of the light coupling device. In some cases, the second skew holographic optical element may be oriented in a non-overlapping manner with the first holographic optical element.

[0079] FIG. 2C illustrates additional aspects of system 200-c for manufacturing a holographic optical element using tiger coupling elements, in accordance with various aspects of the disclosure. System 200-c may include first recording beam 215-c, second recording beam 225-c, recording medium 245-c, first coupling element 255-c, and second coupling element 260-c. Recording medium 245-c may be contained within a reservoir 275-c disposed between first coupling element 255-c and second coupling element 260-c. The one or more substrates of an optical recording cell, including recording medium 245-c, may be at least partially submerged in an index matched fluid 280-c contained within reservoir 275-c. The first recording beam 215-c, second recording beam 225-c, recording medium 245-c may be similar (but are not necessarily required to be identical) to these same numbered elements described with respect to FIG. 2B. In some embodiments, manufacturing holographic optical elements may include the method and configurations as described in reference to FIG. 2A. System 200-c may likewise include global coordinates (x.sub.G, y.sub.G, z.sub.G) and holographic optical element coordinates (x, y, z). In some examples, first coupling element 255-c and second coupling element 260-c may be an example of tiger (total internal grazing-extension rotation) prisms (e.g., oblique-faced prisms) coupling elements or the like. In some cases, first coupling element 255-c may “overhang” second coupling element 260-c and recording medium 245-c. In other examples, second coupling element 260-c may “undercut” first coupling element 255-c and recording medium 245-c. First coupling element 255-c and second coupling element 260-c may each have a surface that is oblique to the base of the coupling element and form an angle of .PHI..sub.G with respect to the y.sub.G-axis. That is, first coupling element 255-c and second coupling element 260-c may allow recording medium 245-c surface normal to be angled by .PHI..sub.G out of the plane. First coupling element 255-c and second coupling element 260-c may allow the recording medium 245-c to be rotated -90.degree. about the x.sub.G-axis compared to FIG. 2A in order to “split the difference” between the first recording beam 215-c and second recording beam 225-c angles.

[0080] The bottom portion of FIG. 2C illustrates a collapsed plane plan view (i.e., the x and z planes shown in the same plane) of the recording medium 245-c to more clearly show aspects associated with or resulting from the oblique orientation of the recording medium 245-c within the tiger coupling element configuration. A perspective view of the first coupling element 255-c and second coupling element 260-c with perspective view coordinates is illustrated above the collapsed plane plan view of the recording medium 245-c. The first coupling element 255-c and second coupling element 260-c are spaced apart in the perspective view to show where the recording medium 245-c would be positioned in the tiger coupling element configuration. Reservoir 275-c may thus have a longitudinally angular shape.

[0081] As described herein, first coupling element 255-c and second coupling element 260-c may have coupling element faces that comprise the tiger coupling element configuration. For example, first coupling element 255-c may have a first coupling element face 285 that is oblique to the base of first coupling element 255-c and form an angle of .PHI..sub.G with respect to the y.sub.G-axis. First coupling element 255-c may also have a second coupling element face 290 where the first recording beam 215-c may enter the first coupling element 255-c. Second coupling element 260-c may have a third coupling element face 295 that is oblique to the base of the second coupling element 260-c and form an angle of .PHI..sub.G with respect to the y.sub.G-axis. Second coupling element 260-c may also have a fourth coupling element face 297 where the second recording beam 225-c may enter the second coupling element 260-c.

[0082] While the use of first coupling element 255-c and second 260-c (e.g., tiger coupling elements), may be used to write equivalent hologram recordings having grating vectors aligned with the x.sub.G-axis, first coupling element 255-c and second coupling element 260-c may able to access lower recording beam difference angles, alpha, than are accessible with in-plane coupling elements. That is, first coupling element 255-c and second coupling element 260-c may be used to record holograms having a lower frequency than can be written using in-plane coupling elements (using recording beams having the same wavelength). In some cases, a different set of first coupling element 255-c and second 260-c may be used to record holograms having a different vector angle, i.e., a different skew axis. First coupling element 255-c and second coupling element 260-c may also be index matched to recording medium 245-c and may affect the ability to perform hologram programming.

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