U.S. patent application number 16/203071 was filed with the patent office on 2019-07-11 for systems and methods for manufacturing waveguide cells.
This patent application is currently assigned to DigiLens, Inc.. The applicant listed for this patent is DigiLens, Inc.. Invention is credited to Shibu Abraham, Alastair John Grant, Sihui He, Ratson Morad, Milan Momcilo Popovich, Jonathan David Waldern.
Application Number | 20190212588 16/203071 |
Document ID | / |
Family ID | 67140737 |
Filed Date | 2019-07-11 |
![](/patent/app/20190212588/US20190212588A1-20190711-D00000.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00001.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00002.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00003.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00004.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00005.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00006.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00007.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00008.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00009.png)
![](/patent/app/20190212588/US20190212588A1-20190711-D00010.png)
View All Diagrams
United States Patent
Application |
20190212588 |
Kind Code |
A1 |
Waldern; Jonathan David ; et
al. |
July 11, 2019 |
Systems and Methods for Manufacturing Waveguide Cells
Abstract
Systems for the manufacturing of waveguide cells in accordance
with various embodiments can be configured and implemented in many
different ways. In many embodiments, various deposition mechanisms
are used to deposit layer(s) of optical recording material onto a
transparent substrate. A second transparent substrate can be
provided, and the three layers can be laminated to form a waveguide
cell. Suitable optical recording material can vary widely depending
on the given application. In some embodiments, the optical
recording material deposited has a similar composition throughout
the layer. In a number of embodiments, the optical recording
material spatially varies in composition, allowing for the
formation of optical elements with varying characteristics.
Regardless of the composition of the optical recording material,
any method of placing or depositing the optical recording material
onto a substrate can be utilized.
Inventors: |
Waldern; Jonathan David;
(Los Altos Hills, CA) ; Morad; Ratson; (Palo Alto,
CA) ; Grant; Alastair John; (San Jose, CA) ;
He; Sihui; (Sunnyvale, CA) ; Abraham; Shibu;
(Sunnyvale, CA) ; Popovich; Milan Momcilo;
(Leicester, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens, Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens, Inc.
Sunnyvale
CA
|
Family ID: |
67140737 |
Appl. No.: |
16/203071 |
Filed: |
November 28, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62663864 |
Apr 27, 2018 |
|
|
|
62614813 |
Jan 8, 2018 |
|
|
|
62614831 |
Jan 8, 2018 |
|
|
|
62614932 |
Jan 8, 2018 |
|
|
|
62667891 |
May 7, 2018 |
|
|
|
62703329 |
Jul 25, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B82Y 20/00 20130101;
G02B 5/1828 20130101; G02B 27/0172 20130101; G02B 6/13 20130101;
G02B 5/1871 20130101; G02B 2006/12116 20130101; B29D 11/00769
20130101; G03H 1/00 20130101; G02B 2006/12166 20130101; G03H 1/0248
20130101; G02B 5/1857 20130101; G02B 5/3016 20130101; G03H 1/02
20130101; G03H 2260/34 20130101; G02B 2006/12107 20130101; G02B
27/0081 20130101; G02B 6/122 20130101; B29D 11/00682 20130101; G02B
2207/101 20130101; G02F 1/1326 20130101; G02B 27/0944 20130101 |
International
Class: |
G02F 1/13 20060101
G02F001/13; G02B 6/13 20060101 G02B006/13; G02B 27/09 20060101
G02B027/09; G02B 6/122 20060101 G02B006/122 |
Claims
1. A method for manufacturing waveguide cells, the method
comprising; providing a first substrate; determining a predefined
grating characteristic; and depositing a layer of optical recording
material onto the first substrate using at least one deposition
head, wherein the optical recording material deposited over the
grating region is formulated to achieve the predefined grating
characteristic.
2. The method of claim 1, further comprising: providing a second
substrate; placing the second substrate onto the deposited layer of
optical recording material; and laminating the first substrate, the
layer of optical recording material, and the second substrate.
3. The method of claim 1, wherein depositing the layer of optical
recording material comprises: providing a first mixture of optical
recording material; providing a second mixture of optical recording
material; and depositing the first and second mixtures of optical
recording material onto the first substrate in a predetermined
pattern using the at least one deposition head.
4. The method of claim 3, wherein: the first mixture of optical
recording material comprises a first bead; and the second mixture
of optical recording material comprises a second bead that is a
different size from the first bead.
5. The method of claim 3, wherein the first mixture of optical
recording material has a different percentage by weight of liquid
crystals than the second mixture of optical recording material.
6. The method of claim 3, further comprising defining a grating
region and a nongrating region on the first substrate, wherein: the
first mixture of optical recording material comprises a liquid
crystal and a monomer; the second mixture of optical recording
material comprises a monomer; and depositing the first and second
mixtures of optical recording material onto the first substrate in
the predetermined pattern comprises: depositing the first mixture
of optical recording material over the grating region; and
depositing the second mixture of optical recording material over
the nongrating region.
7. The method of claim 3, wherein the first mixture of optical
recording material is a polymer dispersed liquid crystal mixture
comprising: a monomer; a liquid crystal; a photoinitiator dye; and
a coinitiator.
8. The method of claim 7, wherein the polymer dispersed liquid
crystal mixture comprises an additive selected from the group
consisting of: a photoinitiator, nano particles, low-functionality
monomers, additives for reducing switching voltage, additives for
reducing switching time, additives for increasing refractive index
modulation, and additives for reducing haze.
9. The method of claim 1, wherein the at least one deposition head
comprises at least one inkjet print head.
10. The method of claim 8, wherein depositing the layer of optical
recording material comprises: providing a first mixture of optical
recording material; providing a second mixture of optical recording
material; printing a first dot of the first mixture of optical
recording material using the at least one inkjet print head; and
printing a second dot of the second mixture of optical recording
material adjacent to the first dot using the at least one inkjet
print head.
11. The method of claim 9, wherein: the at least one inkjet print
head comprises a first inkjet print head and a second inkjet print
head; and depositing the layer of optical recording material
comprises: providing a first mixture of optical recording material;
providing a second mixture of optical recording material; printing
the first mixture of optical recording material onto the first
substrate using the first inkjet print head; and printing the
second mixture of optical recording material onto the first
substrate using the second inkjet print head.
12. The method of claim 1, wherein the predefined grating
characteristic comprises a characteristic selected from the group
consisting of: refractive index modulation, refractive index,
birefringence, liquid crystal director alignment, and grating layer
thickness.
13. The method of claim 1, wherein the predefined grating
characteristic comprises a spatial variation of a characteristic
selected from the group consisting of: refractive index modulation,
refractive index, birefringence, liquid crystal director alignment,
and grating layer thickness.
14. The method of claim 1, wherein the predefined grating
characteristic results in a grating after exposure, the grating
having a spatially varying diffraction efficiency.
15. A system for fabricating a grating, the system comprising: at
least one deposition head connected to at least one reservoir
containing at least one mixture of optical recording material; a
first substrate having at least one predefined region for
supporting gratings; a positioning element capable of positioning
the at least one deposition head across the first substrate,
wherein: the at least one deposition head is configured to deposit
the at least one mixture of optical recording material onto the
first substrate using the positioning element; and the deposited
material provides a predefined grating characteristic within the at
least one predefined grating region after holographic exposure.
16. The system of claim 15, wherein the at least one deposition
head is connected to a first reservoir containing a first mixture
of optical recording material and a second reservoir containing a
second mixture of optical recording material.
17. The system of claim 16, wherein the first mixture of optical
recording material comprises a liquid crystal and a monomer; and
the second mixture of optical recording material comprises a
monomer; wherein the at least one deposition head is configured to
deposit the first mixture of optical recording material onto the at
least one predefined grating region.
18. The system of claim 15, wherein the at least one deposition
head comprises at least one inkjet print head.
19. The system of claim 15, wherein the predefined grating
characteristic comprises a characteristic selected from the group
consisting of: refractive index modulation, refractive index,
birefringence, liquid crystal director alignment, and grating layer
thickness.
20. The system of claim 15, wherein the predefined grating
characteristic results in a grating after exposure, the grating
having a spatially varying diffraction efficiency.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The current application claims the benefit of and priority
under 35 U.S.C. .sctn. 119(e) to U.S. Provisional Patent
Application No. 62/663,864 entitled "Method and Apparatus for
Fabricating Holographic Gratings," filed Apr. 27, 2018, U.S.
Provisional Patent Application No. 62/614,813 entitled "Low Haze
Liquid Crystal Materials," filed Jan. 8, 2018, U.S. Provisional
Patent Application No. 62/614,831 entitled "Liquid Crystal
Materials and Formulations," filed Jan. 8, 2018, U.S. Provisional
Patent Application No. 62/614,932 entitled "Methods for Fabricating
Optical Waveguides," filed Jan. 8, 2018, U.S. Provisional Patent
Application No. 62/667,891 entitled "Method and Apparatus for
Copying a Diversity of Hologram Prescriptions from a Common
Master," filed May 7, 2018, and U.S. Provisional Patent Application
No. 62/703,329 entitled "Systems and Methods for Fabricating a
Multilayer Optical Structure," filed Jul. 25, 2018. The disclosures
of U.S. Provisional Patent Application Nos. 62/663,864, 62/614,813,
62/614,831, 62/614,932, 62/667,891, and 62/703,329 are hereby
incorporated by reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention generally relates to processes and
apparatuses for manufacturing waveguide cells and, more
specifically, manufacturing waveguide cells utilizing deposition
and printing techniques.
BACKGROUND
[0003] Waveguides can be referred to as structures with the
capability of confining and guiding waves (i.e., restricting the
spatial region in which waves can propagate). One class of
waveguides includes optical waveguides, which are structures that
can guide electromagnetic waves, typically those in the visible
spectrum. Waveguide structures can be designed to control the
propagation path of waves using a number of different mechanisms.
For example, planar waveguides can be designed to utilize
diffraction gratings to diffract and couple incident light into the
waveguide structure such that the in-coupled light can proceed to
travel within the planar structure via total internal reflection
("TIR").
[0004] Fabrication of waveguides can include the use of material
systems that allow for the recording of holographic optical
elements within the waveguides. One class of such material includes
polymer dispersed liquid crystal ("PDLC") mixtures, which are
mixtures containing photopolymerizable monomers and liquid
crystals. A further subclass of such mixtures includes holographic
polymer dispersed liquid crystal ("HPDLC") mixtures. Holographic
optical elements, such as volume phase gratings, can be recorded in
such a liquid mixture by illuminating the material with two
mutually coherent laser beams. During the recording process, the
monomers polymerize and the mixture undergoes a
photopolymerization-induced phase separation, creating regions
densely populated by liquid crystal micro-droplets, interspersed
with regions of clear polymer. The alternating liquid crystal-rich
and liquid crystal-depleted regions form the fringe planes of the
grating.
[0005] Waveguide optics, such as those described above, can be
considered for a range of display and sensor applications. In many
applications, waveguides containing one or more grating layers
encoding multiple optical functions can be realized using various
waveguide architectures and material systems, enabling new
innovations in near-eye displays for Augmented Reality ("AR") and
Virtual Reality ("VR"), compact Heads Up Displays ("HUDs") for
aviation and road transport, and sensors for biometric and laser
radar ("LIDAR") applications.
SUMMARY OF THE INVENTION
[0006] One embodiment includes a method for manufacturing waveguide
cells, the method including providing a first substrate,
determining a predefined grating characteristic, and depositing a
layer of optical recording material onto the first substrate using
at least one deposition head, wherein the optical recording
material deposited over the grating region is formulated to achieve
the predefined grating characteristic.
[0007] In another embodiment, the method further includes providing
a second substrate, placing the second substrate onto the deposited
layer of optical recording material, and laminating the first
substrate, the layer of optical recording material, and the second
substrate.
[0008] In a further embodiment, depositing the layer of optical
recording material includes providing a first mixture of optical
recording material, providing a second mixture of optical recording
material, and depositing the first and second mixtures of optical
recording material onto the first substrate in a predetermined
pattern using the at least one deposition head.
[0009] In still another embodiment, the first mixture of optical
recording material includes a first bead and the second mixture of
optical recording material includes a second bead that is a
different size from the first bead.
[0010] In a still further embodiment, the first mixture of optical
recording material has a different percentage by weight of liquid
crystals than the second mixture of optical recording material.
[0011] In yet another embodiment, the method further includes
defining a grating region and a nongrating region on the first
substrate, wherein the first mixture of optical recording material
includes a liquid crystal and a monomer, the second mixture of
optical recording material includes a monomer, and depositing the
first and second mixtures of optical recording material onto the
first substrate in the predetermined pattern includes depositing
the first mixture of optical recording material over the grating
region and depositing the second mixture of optical recording
material over the nongrating region.
[0012] In a yet further embodiment, the first mixture of optical
recording material is a polymer dispersed liquid crystal mixture
that includes a monomer, a liquid crystal, a photoinitiator dye,
and a coinitiator.
[0013] In another additional embodiment, the polymer dispersed
liquid crystal mixture includes an additive selected from the group
that includes a photoinitiator, nano particles, low-functionality
monomers, additives for reducing switching voltage, additives for
reducing switching time, additives for increasing refractive index
modulation, and additives for reducing haze.
[0014] In a further additional embodiment, the at least one
deposition head includes at least one inkjet print head.
[0015] In another embodiment again, depositing the layer of optical
recording material includes providing a first mixture of optical
recording material, providing a second mixture of optical recording
material, printing a first dot of the first mixture of optical
recording material using the at least one inkjet print head, and
printing a second dot of the second mixture of optical recording
material adjacent to the first dot using the at least one inkjet
print head.
[0016] In a further embodiment again, the at least one inkjet print
head includes a first inkjet print head and a second inkjet print
head and depositing the layer of optical recording material
includes providing a first mixture of optical recording material,
providing a second mixture of optical recording material, printing
the first mixture of optical recording material onto the first
substrate using the first inkjet print head, and printing the
second mixture of optical recording material onto the first
substrate using the second inkjet print head.
[0017] In still yet another embodiment, the predefined grating
characteristic includes a characteristic selected from the group
that includes refractive index modulation, refractive index,
birefringence, liquid crystal director alignment, and grating layer
thickness.
[0018] In a still yet further embodiment, the predefined grating
characteristic includes a spatial variation of a characteristic
selected from the group that includes refractive index modulation,
refractive index, birefringence, liquid crystal director alignment,
and grating layer thickness.
[0019] In still another additional embodiment, the predefined
grating characteristic results in a grating after exposure, wherein
the grating has a spatially varying diffraction efficiency.
[0020] A still further additional embodiment includes a system for
fabricating a grating, the system including at least one deposition
head connected to at least one reservoir containing at least one
mixture of optical recording material, a first substrate having at
least one predefined region for supporting gratings, a positioning
element capable of positioning the at least one deposition head
across the first substrate, wherein the at least one deposition
head is configured to deposit the at least one mixture of optical
recording material onto the first substrate using the positioning
element and the deposited material provides a predefined grating
characteristic within the at least one predefined grating region
after holographic exposure.
[0021] In still another embodiment again, the at least one
deposition head is connected to a first reservoir containing a
first mixture of optical recording material and a second reservoir
containing a second mixture of optical recording material.
[0022] In a still further embodiment again, the first mixture of
optical recording material includes a liquid crystal and a monomer
and the second mixture of optical recording material includes a
monomer, wherein the at least one deposition head is configured to
deposit the first mixture of optical recording material onto the at
least one predefined grating region.
[0023] In yet another additional embodiment, the at least one
deposition head includes at least one inkjet print head.
[0024] In a yet further additional embodiment, the predefined
grating characteristic includes a characteristic selected from the
group that includes refractive index modulation, refractive index,
birefringence, liquid crystal director alignment, and grating layer
thickness.
[0025] In yet another embodiment again, the predefined grating
characteristic results in a grating after exposure, wherein the
grating has a spatially varying diffraction efficiency.
[0026] Additional embodiments and features are set forth in part in
the description that follows, and in part will become apparent to
those skilled in the art upon examination of the specification or
may be learned by the practice of the invention. A further
understanding of the nature and advantages of the present invention
may be realized by reference to the remaining portions of the
specification and the drawings, which forms a part of this
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The description will be more fully understood with reference
to the following figures and data graphs, which are presented as
exemplary embodiments of the invention and should not be construed
as a complete recitation of the scope of the invention. It will
apparent to those skilled in the art that the present invention may
be practiced with some or all of the present invention as disclosed
in the following description.
[0028] FIG. 1A conceptually illustrates a profile view of a
waveguide cell in accordance with an embodiment of the
invention.
[0029] FIG. 1B conceptually illustrates a waveguide cell with a
wedge-shaped profile in accordance with an embodiment of the
invention.
[0030] FIG. 1C conceptually illustrates a top view of a waveguide
cell in accordance with an embodiment of the invention.
[0031] FIG. 2A conceptually illustrates a workcell cluster system
in accordance with an embodiment of the invention.
[0032] FIG. 2B conceptually illustrates a workcell cluster system
with two deposition workcells in accordance with an embodiment of
the invention.
[0033] FIG. 3A conceptually illustrates an isometric view of a
deposition workcell in accordance with an embodiment of the
invention.
[0034] FIG. 3B conceptually illustrates a top view of a deposition
workcell in accordance with an embodiment of the invention.
[0035] FIGS. 4A and 4B conceptually illustrate schematically the
use of reverse ray tracing to compute a compensated index
modulation pattern for coating in accordance with various
embodiments of the invention.
[0036] FIGS. 5A and 5B conceptually illustrate the fundamental
structural differences between SBGs and SRGs.
[0037] FIG. 6 conceptually illustrates a waveguide cell with marked
areas for gratings in accordance with an embodiment of the
invention.
[0038] FIGS. 7A and 7B conceptually illustrate operation of a
deposition mechanism utilizing a spray module in accordance with an
embodiment of the invention.
[0039] FIGS. 8A and 8B conceptually illustrate two operational
states of a spray module in accordance with an embodiment of the
invention.
[0040] FIG. 9 is a flow chart conceptually illustrating a method of
fabricating a holographic grating using a selective coating process
in accordance with an embodiment of the invention.
[0041] FIG. 10 conceptually illustrates a deposition head for
providing predefined grating characteristics within grating regions
in accordance with an embodiment of the invention.
[0042] FIG. 11 conceptually illustrates operation of a deposition
head for depositing material having regions with predefined grating
characteristics in accordance with an embodiment of the
invention.
[0043] FIG. 12 conceptually illustrates a deposition mechanism for
depositing two grating layers in accordance with an embodiment of
the invention.
[0044] FIG. 13 conceptually illustrates a system for depositing a
grating layer of material and for holographically exposing the
layer in accordance with an embodiment of the invention.
[0045] FIG. 14 is a flow chart conceptually illustrating a method
of depositing a film of material with regions having predefined
grating characteristics in accordance with an embodiment of the
invention.
[0046] FIG. 15 conceptually illustrates an inkjet printing
modulation scheme in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION
[0047] For the purposes of describing embodiments, some well-known
features of optical technology known to those skilled in the art of
optical design and visual displays have been omitted or simplified
in order to not obscure the basic principles of the invention.
Unless otherwise stated, the term "on-axis" in relation to a ray or
a beam direction refers to propagation parallel to an axis normal
to the surfaces of the optical components described in relation to
the invention. In the following description, the terms light, ray,
beam and direction may be used interchangeably and in association
with each other to indicate the direction of propagation of light
energy along rectilinear trajectories. Parts of the following
description will be presented using terminology commonly employed
by those skilled in the art of optical design. For illustrative
purposes, it is to be understood that the drawings are not drawn to
scale unless stated otherwise.
[0048] Turning now to the drawings, systems and methods for
manufacturing waveguide cells are illustrated. A waveguide cell can
be defined as a device containing uncured and/or unexposed optical
recording material in which optical elements, such as but not
limited to gratings, can be recorded through exposure to certain
wavelengths of electromagnetic radiation. Many techniques exist for
the manufacturing and construction of waveguide cells. In many
embodiments, a waveguide cell is constructed by placing a thin film
of optical recording material between two transparent substrates.
In further embodiments, a workcell cluster manufacturing system is
implemented to construct such waveguide cells. A workcell can be
defined as a set of machines assigned to a particular manufacturing
task. A cluster can be defined as a group of machines that performs
a similar function cooperatively. In some embodiments, the workcell
cluster includes a preparation workcell for preparing substrates
for deposition, a deposition workcell for depositing an optical
recording material onto a substrate, and a lamination workcell for
laminating various layers together to form a waveguide cell.
[0049] Workcells and workcell clusters in accordance with various
embodiments can be configured and implemented in many different
ways. For instance, preparation workcells can be configured to
prepare substrates for material deposition through various
processes, including but not limited to cleaning procedures and
protocols. In many embodiments, the preparation of substrates
includes glass cleaning procedures for ridding the surfaces of the
substrates of contaminants and particles. In some embodiments,
procedures for increasing the surface adhesion properties of the
substrates are implemented to further prepare the substrates for
material deposition.
[0050] Deposition workcells can be configured to deposit one or
more layers of optical recording material onto a transparent
substrate using a variety of different deposition and printing
mechanisms. In many embodiments, additive manufacturing techniques,
such as but not limited to inkjet printing, are used to deposit the
layer(s) of optical recording material. In several embodiments,
spraying techniques are utilized to deposit the layer(s) of optical
recording material. Suitable optical recording material can vary
widely depending on the given application. In some embodiments, the
optical recording material deposited has a similar composition
throughout the layer. In a number of embodiments, the optical
recording material spatially varies in composition, allowing for
the formation of optical elements with varying characteristics.
Regardless of the composition of the optical recording material,
any method of placing or depositing the optical recording material
onto a substrate can be utilized.
[0051] Lamination workcells can be configured to laminate various
layers to form a waveguide cell. In a number of embodiments, the
lamination workcell is configured to laminate and form a
three-layer composite of optical recording material and transparent
substrates. As can readily be appreciated, the number of layers and
types of materials used to construct the waveguide cells can vary
and depend on the given application. For example, in some
embodiments, waveguide cells can be constructed to include
protective cover layers, polarization control layers, and/or
alignment layers. In some embodiments, the system is configured for
the production of curved waveguides and waveguide cells. Specific
materials, systems, and methods for constructing waveguide cells
are discussed below in further detail.
[0052] Waveguide Cells
[0053] Waveguide cells can be configured and constructed in many
different ways in accordance with various embodiments of the
invention. As discussed above, in many waveguide configurations,
the waveguide cell includes a thin film of optical recording
material sandwiched between two substrates. Such waveguide cells
can be manufactured using various processes. In many embodiments,
waveguide cells can be constructed by coating a first substrate
with an optical recording material capable of acting as an optical
recording medium. Various optical recording materials can be used.
In some embodiments, the optical recording material is a
holographic polymer dispersed liquid crystal mixture (e.g., a
matrix of liquid crystal droplets). As can readily be appreciated,
the choice of optical recording material and types of mixtures
utilized can depend on the given application. The optical recording
material can be deposited using a variety of deposition techniques.
In a number of embodiments, the optical recording material can be
deposited onto the first substrate through inkjetting, spin
coating, and/or spraying processes. The deposition processes can be
configured to deposit one or more type of optical recording
material. In some embodiments, the deposition process is configured
to deposit optical recording material that spatially varies in
composition across a substrate. After deposition of the optical
recording material, a second substrate can be placed such that the
optical recording material is sandwiched between the two substrates
to form a waveguide cell. In several embodiments, the second
substrate can be a thin protective film coated onto the exposed
layer. In such embodiments, various techniques, including but not
limited to spraying processes, can be used to coat the exposed
layer with the desired film of material. In a number of
embodiments, the waveguide cell can include various additional
layers, such as but not limited to polarization control layers
and/or alignment layers. Other processes for manufacturing
waveguide cells can include filling empty waveguide cells
(constructed of two substrates) with an optical recording material
using processes such as but not limited to gravity filling and
vacuum filling methods.
[0054] Substrates used in the construction of waveguide cells are
often made of transparent materials. In some embodiments, the
substrate is an optical plastic. In other embodiments, the
substrate may be fabricated from glass. An exemplary glass
substrate is standard Corning Willow glass substrate (index 1.51)
which is available in thicknesses down to 50 micrometers. The
thicknesses of the substrates can vary from application to
application. In many embodiments, 1 mm thick glass slides are used
as the substrates. In addition to different thicknesses, substrates
of different shapes, such as but not limited to rectangular and
curvilinear shapes, can also be used depending on the application.
Oftentimes, the shapes of the substrates can determine the overall
shape of the waveguide. In a number of embodiments, the waveguide
cell contains two substrates that are of the same shape. In other
embodiments, the substrates are of different shapes. As can readily
be appreciated, the shapes, dimensions, and materials of the
substrates can vary and depend on the specific requirements of a
given application.
[0055] In many embodiments, beads or other particles are dispersed
throughout the optical recording material to help control the
thickness of the layer of optical recording material and to help
prevent the two substrates from collapsing onto one another. In
some embodiments, the waveguide cell is constructed with an optical
recording material layer sandwiched between two planar substrates.
Depending on the type of optical recording material used, thickness
control can be difficult to achieve due to the viscosity of some
optical recording materials and the lack of a bounding edge for the
optical recording material layer. In a number of embodiments, the
beads are relatively incompressible solids, which can allow for the
construction of waveguide cells with consistent thicknesses. The
size of a bead can determine a localized minimum thickness for the
area around the individual bead. As such, the dimensions of the
beads can be selected to help attain the desired optical recording
material layer thickness. The beads can be made of any of a variety
of materials, including but not limited to glass and plastics. In
several embodiments, the material of the beads is selected such
that its refractive index does not substantially affect the
propagation of light within the waveguide cell.
[0056] In some embodiments, the waveguide cell is constructed such
that the two substrates are parallel or substantially parallel. In
such embodiments, relatively similar sized beads can be dispersed
throughout the optical recording material to help attain a uniform
thickness throughout the layer. In other embodiments, the waveguide
cell has a tapered profile. A tapered waveguide cell can be
constructed by dispersing beads of different sizes across the
optical recording material. As discussed above, the size of a bead
can determine the local minimum thickness of the optical recording
material layer. By dispersing the beads in a pattern of increasing
size across the material layer, a tapered layer of optical
recording material can be formed when the material is sandwiched
between two substrates.
[0057] Once constructed, waveguide cells can be used in conjunction
with a variety of processes for recording optical elements within
the optical recording material. For example, the process disclosed
may incorporated embodiments and teachings from the materials and
processes, such as but not limited to those described in U.S.
patent application Ser. No. 16/116,834 entitled "Systems and
Methods for High-Throughput Recording of Holographic Gratings in
Waveguide Cells," filed Aug. 29, 2018 and U.S. patent application
Ser. No. 16/007,932 entitled "Holographic Material Systems and
Waveguides Incorporating Low Functionality Monomers," filed Jun.
13, 2018 The disclosures of U.S. patent application Ser. Nos.
16/116,834 and 16/007,932 are hereby incorporated in their
entireties for all purpose.
[0058] A profile view of a waveguide cell 100 in accordance with an
embodiment of the invention is conceptually illustrated in FIG. 1A.
As shown, the waveguide cell 100 includes a layer of optical
recording material 102 that can be used as a recording medium for
optical elements, such as but not limited to gratings. 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 sections above. In the illustrative
embodiment, the optical recording material 102 is sandwich between
two parallel glass plates 104, 106. The substrates can be arranged
in both parallel and non-parallel configurations. FIG. 1B
conceptually illustrates a profile view of a tapered waveguide cell
108 utilizing beads 110, 112, and 114 in accordance with an
embodiment of the invention. As shown, beads 110, 112, and 114 vary
in size and are dispersed throughout an optical recording material
116 sandwiched by two glass plates 118, 120. During construction of
the waveguide cell, the local thickness of an area of the optical
recording material layer is limited by the sizes of the beads in
that particular area. By dispersing the beads in an increasing
order of sizes across the optical recording material, a tapered
waveguide cell can be constructed when the substrates are placed in
contact with the beads. As discussed above, substrates utilized in
waveguide cells can vary in thicknesses and shapes. In many
embodiments, the substrate is rectangular in shape. In some
embodiments, the shape of the waveguide cell is a combination of
curvilinear components. FIG. 1C conceptually illustrates a top view
of a waveguide cell 122 having a curvilinear shape in accordance
with an embodiment of the invention.
[0059] Although FIGS. 1A-1C illustrate specific waveguide cell
constructions and arrangements, waveguide cells can be constructed
in many different configurations and can use a variety of different
materials depending on the specific requirements of a given
application. For example, substrates can be made of transparent
plastic polymers instead of glass. Additionally, the shapes and
sizes of the waveguide cells can vary greatly and can be determined
by various factors, such as but not limited to the application of
the waveguide, ergonomic considerations, and economical factors. In
many embodiments, the substrates are curved, allowing for the
production of waveguides with curved cross sections.
Grating Structures
[0060] Waveguide cells in accordance with various embodiments of
the invention can incorporate a variety of light-sensitive
materials. In many embodiments, the waveguide cell incorporates a
holographic polymer dispersed liquid crystal mixture that functions
as an optical recording medium in which optical elements can be
recorded. Optical elements can include many different types of
gratings capable of exhibiting different optical properties. One
type of grating that can be recorded in waveguide cells is a volume
Bragg grating, which can be characterized as a transparent medium
with a periodic variation in its refractive index. This variation
can allow for the diffraction of incident light of certain
wavelengths at certain angles. Volume Bragg gratings can have high
efficiency with little light being diffracted into higher orders.
The relative amount of light in the diffracted and zero order can
be varied by controlling the refractive index modulation of the
grating.
[0061] One class of gratings used in holographic waveguide devices
is the Switchable Bragg Grating ("SBG"). An SBG is a diffractive
device that can be formed by recording a volume phase grating in an
HPDLC mixture (although other materials can be used). SBGs can be
fabricated by first placing a thin film of a mixture of
photopolymerizable monomers and liquid crystal material between
glass plates or substrates, which forms a waveguide cell. One or
both glass plates can support electrodes, typically transparent tin
oxide films, for applying an electric field across the film. SBGs
can be implemented as waveguide devices in which the HPDLC forms
either the waveguide core or an evanescently coupled layer in
proximity to the waveguide. The glass plates used to form the HPDLC
cell can provide a total internal reflection light guiding
structure. Light is coupled out of the SBG when the switchable
grating diffracts the light at an angle beyond the TIR
condition.
[0062] The grating structure in an SBG can be recorded in the film
of HPDLC material through photopolymerization-induced phase
separation using interferential exposure with a spatially periodic
intensity modulation. Factors such as but not limited to control of
the irradiation intensity, component volume fractions of the HPDLC
material, and exposure temperature can determine the resulting
grating morphology and performance. During the recording process,
the monomers polymerize and the mixture undergoes a phase
separation. The LC molecules aggregate to form discrete or
coalesced droplets that are periodically distributed in polymer
networks on the scale of optical wavelengths. The alternating
liquid crystal-rich and liquid crystal-depleted regions form the
fringe planes of the grating, which can produce Bragg diffraction
with a strong optical polarization resulting from the orientation
ordering of the LC molecules in the droplets. The resulting volume
phase grating can exhibit very high diffraction efficiency, which
may be controlled by the magnitude of the electric field applied
across the HPDLC layer. When an electric field is applied to the
hologram via transparent electrodes, the natural orientation of the
LC droplets is changed, causing the refractive index modulation of
the fringes to reduce and the hologram diffraction efficiency to
drop to very low levels. The diffraction efficiency of the device
can be adjusted, by means of the applied voltage, over a continuous
range from near 100% efficiency with no voltage applied to
essentially zero efficiency with a sufficiently high voltage
applied. In certain types of HPDLC devices, phase separation of the
LC material from the polymer can be accomplished to such a degree
that no discernible droplet structure results. An SBG can also be
used as a passive grating. In this mode, its chief benefit is a
uniquely high refractive index modulation. SBGs can be used to
provide transmission or reflection gratings for free space
applications. SBGs can be implemented as waveguide devices in which
the HPDLC forms either the waveguide core or an evanescently
coupled layer in proximity to the waveguide. The glass plates used
to form the HPDLC cell provide a total internal reflection light
guiding structure. Light can be coupled out of the SBG when the
switchable grating diffracts the light at an angle beyond the TIR
condition.
[0063] In many embodiments, SBGs are recorded in a uniform
modulation material, such as POLICRYPS or POLIPHEM having a matrix
of solid liquid crystals dispersed in a liquid polymer. Exemplary
uniform modulation liquid crystal-polymer material systems are
disclosed in United State Patent Application Publication No.:
US2007/0019152 by Caputo et al and PCT Application No.:
PCT/EP2005/006950 by Stumpe et al. both of which are incorporated
herein by reference in their entireties. Uniform modulation
gratings are characterized by high refractive index modulation (and
hence high diffraction efficiency) and low scatter. In some
embodiments, at least one of the gratings is recorded a reverse
mode HPDLC material. Reverse mode HPDLC differs from conventional
HPDLC in that the grating is passive when no electric field is
applied and becomes diffractive in the presence of an electric
field. The reverse mode HPDLC may be based on any of the recipes
and processes disclosed in PCT Application No.: PCT/GB2012/000680,
entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID
CRYSTAL MATERIALS AND DEVICES. Optical recording material systems
are discussed below in further detail.
Optical Recording Material Systems
[0064] HPDLC mixtures in accordance with various embodiments of the
invention generally include LC, monomers, photoinitiator dyes, and
coinitiators. The mixture (often referred to as syrup) frequently
also includes a surfactant. For the purposes of describing the
invention, a surfactant is defined as any chemical agent that
lowers the surface tension of the total liquid mixture. The use of
surfactants in PDLC mixtures is known and dates back to the
earliest investigations of PDLCs. For example, a paper by R. L
Sutherland et al., SPIE Vol. 2689, 158-169, 1996, the disclosure of
which is incorporated herein by reference, describes a PDLC mixture
including a monomer, photoinitiator, coinitiator, chain extender,
and LCs to which a surfactant can be added. Surfactants are also
mentioned in a paper by Natarajan et al, Journal of Nonlinear
Optical Physics and Materials, Vol. 5 No. I 89-98, 1996, the
disclosure of which is incorporated herein by reference.
Furthermore, U.S. Pat. No. 7,018,563 by Sutherland; et al.,
discusses polymer-dispersed liquid crystal material for forming a
polymer-dispersed liquid crystal optical element including: at
least one acrylic acid monomer; at least one type of liquid crystal
material; a photoinitiator dye; a coinitiator; and a surfactant.
The disclosure of U.S. Pat. No. 7,018,563 is hereby incorporated by
reference in its entirety.
[0065] The patent and scientific literature contains many examples
of material systems and processes that can be used to fabricate
waveguides incorporating volume gratings, including investigations
into formulating such material systems for achieving high
diffraction efficiency, fast response time, low drive voltage, and
so forth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No.
5,751,452 by Tanaka et al. both describe monomer and liquid crystal
material combinations suitable for fabricating waveguides
incorporating volume gratings. Examples of recipes can also be
found in papers dating back to the early 1990s, many of which
disclose the use of acrylate monomers, including: [0066] R. L.
Sutherland et al., Chem. Mater. 5, 1533 (1993), the disclosure of
which is incorporated herein by reference, describes the use of
acrylate polymers and surfactants. Specifically, the recipe
includes a crosslinking multifunctional acrylate monomer; a chain
extender N-vinyl pyrrolidinone, LC E7, photo-initiator rose Bengal,
and coinitiator N-phenyl glycine. Surfactant octanoic acid was
added in certain variants. [0067] Fontecchio et al., SID 00 Digest
774-776, 2000, the disclosure of which is incorporated herein by
reference, describes a UV curable HPDLC for reflective display
applications including a multi-functional acrylate monomer, LC, a
photoinitiator, a coinitiators, and a chain terminator. [0068] Y.
H. Cho, et al., Polymer International, 48, 1085-1090, 1999, the
disclosure of which is incorporated herein by reference, discloses
HPDLC recipes including acrylates. [0069] Karasawa et al., Japanese
Journal of Applied Physics, Vol. 36, 6388-6392, 1997, the
disclosure of which is incorporated herein by reference, describes
acrylates of various functional orders. [0070] T. J. Bunning et
al., Polymer Science: Part B: Polymer Physics, Vol. 35, 2825-2833,
1997, the disclosure of which is incorporated herein by reference,
also describes multifunctional acrylate monomers. [0071] G. S.
Iannacchione et al., Europhysics Letters Vol. 36 (6). 425-430,
1996, the disclosure of which is incorporated herein by reference,
describes a PDLC mixture including a penta-acrylate monomer, LC,
chain extender, coinitiators, and photoinitiator.
[0072] Acrylates offer the benefits of fast kinetics, good mixing
with other materials, and compatibility with film forming
processes. Since acrylates are cross-linked, they tend to be
mechanically robust and flexible. For example, urethane acrylates
of functionality 2 (di) and 3 (tri) have been used extensively for
HPDLC technology. Higher functionality materials such as penta and
hex functional stems have also been used.
[0073] Although HPDLC mixtures with specific components are
discussed above in relation with their suitable uses as the optical
recording material in a waveguide cell, specific formulations of
optical recording materials can vary widely and can depend on the
specific requirements of a given application. Such considerations
can include diffraction efficiency ("DE"), haze, solar immunity,
transparency, and switching requirements.
Embodiments of S & P Polarized RMLCM Materials
[0074] The S and P polarization response of a grating containing LC
can depend on the average LC director orientations relative to the
grating K-vector. Typically, the directors are substantially
parallel to the K-vector, giving a strong P-response and a weaker
S-response. If the LC directors are not aligned, the grating can
have a strong S-response. Many embodiments of the invention include
reactive monomer liquid crystal mixture ("RMLCM") material systems
configured to incorporate a mixture of LCs and monomers (and other
components including: photoinitiator dye, coinitiators,
surfactant), which under holographic exposure undergo phase
separation to provide a grating in which at least one of the LCs
and at least one of the monomers form a first HPDLC morphology that
provides a P polarization response and at least one of the LCs and
at least one of the monomers form a second HPDLC morphology that
provides a S polarization response. In various such embodiments,
the material systems include an RMLCM, which includes
photopolymerizable monomers composed of suitable functional groups
(e.g., acrylates, mercapto-, and other esters, among others), a
cross-linking agent, a photo-initiator, a surfactant and a liquid
crystal.
[0075] Turning to the components of the material formulation, any
encapsulating polymer formed from any single photo-reactive monomer
material or mixture of photo-reactive monomer materials having
refractive indices from about 1.5 to 1.9 that crosslink and phase
separate when combined can be utilized. Exemplary monomer
functional groups usable in material formulations according to
embodiments include, but are not limited to, acrylates, thiol-ene,
thiol-ester, fluoromonomers, mercaptos, siloxane-based materials,
and other esters, etc. Polymer cross-linking can be achieved
through different reaction types, including but not limited to
optically-induced photo-polymerization, thermally-induced
polymerization, and chemically-induced polymerization.
[0076] These photopolymerizable materials can be combined in a
biphase blend with a second liquid crystal material. Any suitable
liquid crystal material having ordinary and extraordinary
refractive indices matched to the polymer refractive index can be
used as a dopant to balance the refractive index of the final RMLCM
material. The liquid crystal material can be manufactured, refined,
or naturally occurring. The liquid crystal material includes all
known phases of liquid crystallinity, including the nematic and
smectic phases, the cholesteric phase, the lyotropic discotic
phase. The liquid crystal can exhibit ferroelectric or
antiferroelectric properties and/or behavior.
[0077] Any suitable photoinitiator, co-initiator, chain extender
and surfactant (such as for example octanoic acid) suitable for use
with the monomer and LC materials can be used in the RMLCM material
formulation. It will be understood that the photo-initiator can
operate in any desired spectral band including the in the UV and/or
in the visible band.
[0078] In various embodiments, the LCs can interact to form an LC
mixture in which molecules of two or more different LCs interact to
form a non-axial structure which interacts with both S and P
polarizations. The waveguide can also contain an LC alignment
material for optimizing the LC alignment for optimum S and P
performance. In many embodiments, the ratio of the diffraction
efficiencies of the P- and S-polarized light in the PDLC morphology
is maintained at a relative ratio of from 1.1:1 to 2:1, and in some
embodiments at around 1.5:1. In other embodiments, the measured
diffraction efficiency of P-polarized light is from greater than
20% to less than 60%, and the diffraction efficiency for
S-polarized light is from greater than 10% to less than 50%, and in
some embodiments the diffraction efficiency of the PDLC morphology
for P-polarization is around 30% and the diffraction efficiency of
the PDLC morphology for S-polarization is around 20%. This can be
compared with conventional PDLC morphologies where the diffraction
efficiency for P-polarization is around 60% and for S-polarization
is around 1 (i.e., the conventional P-polarization materials have
very low or negligible S-components).
Mixtures Incorporating Nanoparticles
[0079] In many embodiments, the reactive monomer liquid crystal
mixture can further include chemically active nanoparticles
disposed within the LC domains. In some such embodiments, the
nanoparticles are carbon nanotube ("CNT") or nanoclay nanoparticle
materials within the LC domains. Embodiments are also directed to
methods for controlling the nanoclay particle size, shape, and
uniformity. Methods for blending and dispersing the nanoclay
particles can determine the resulting electrical and optical
properties of the device. The use of nanoclays in HPDLC is
discussed in PCT Application No.: PCT/GB2012/000680, entitled
IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL
MATERIALS AND DEVICES.
[0080] The nanoclay nanoparticles can be formed from any naturally
occurring or manufactured composition, as long as they can be
dispersed in the liquid crystal material. The specific nanoclay
material to be selected depends upon the specific application of
the film and/or device. The concentration and method of dispersion
also depends on the specific application of the film and/or device.
In many embodiments, the liquid crystal material is selected to
match the liquid crystal ordinary index of refraction with the
nanoclay material. The resulting composite material can have a
forced alignment of the liquid crystal molecules due to the
nanoclay particle dispersion, and the optical quality of the film
and/or device can be unaffected. The composite mixture, which
includes the liquid crystal and nanoclay particles, can be mixed to
an isotropic state by ultrasonication. The mixture can then be
combined with an optically crosslinkable monomer, such as acrylated
or urethane resin that has been photoinitiated, and sandwiched
between substrates to form a cell (or alternatively applied to a
substrate using a coating process).
[0081] In various embodiments, nanoparticles are composed of
nanoclay nanoparticles, preferably spheres or platelets, with
particle size on the order of 2-10 nanometers in the shortest
dimension and on the order of 10 nanometers in the longest
dimension. Desirably, the liquid crystal material is selected to
match the liquid crystal ordinary index of refraction with the
nanoclay material. Alternatively, the nanoparticles can be composed
of material having ferroelectric properties, causing the particles
to induce a ferroelectric alignment effect on the liquid crystal
molecules, thereby enhancing the electro-optic switching properties
of the device. In another embodiment of the invention, the
nanoparticles are composed of material having ferromagnetic
properties, causing the particles to induce a ferromagnetic
alignment effect on the liquid crystal molecules, thereby enhancing
the electro-optic switching properties of the device. In another
embodiment of the invention, the nanoparticles have an induced
electric or magnetic field, causing the particles to induce an
alignment effect on the liquid crystal molecules, thereby enhancing
the electro-optic switching properties of the device. Exemplary
nanoparticles used in other contexts including thermoplastics,
polymer binders, etc. are disclosed in U.S. Pat. Nos. 7,068,898;
7,046,439; 6,323,989; 5,847,787; and U.S. Patent Pub. Nos.
2003/0175004; 2004/0156008; 2004/0225025; 2005/0218377; and
2006/0142455, the disclosures of which are incorporated herein by
reference.
[0082] The nanoclay can be used with its naturally occurring
surface properties, or the surface can be chemically treated for
specific binding, electrical, magnetic, or optical properties.
Preferably, the nanoclay particles will be intercalated, so that
they disperse uniformly in the liquid crystalline material. The
generic term "nanoclay" as used in the discussion of the present
invention can refer to naturally occurring montmorillonite
nanoclay, intercalated montmorillonite nanoclay, surface modified
montmorillonite nanoclay, and surface treated montmorillonite
nanoclay. The nanoparticles can be useable as commercially
purchased, or they may need to be reduced in size or altered in
morphology. The processes that can be used include chemical
particle size reduction, particle growth, grinding of wet or dry
particles, milling of large particles or stock, vibrational milling
of large particles or stock, ball milling of particles or stock,
centrifugal ball milling of particles or stock, and vibrational
ball milling of particles or stock. All of these techniques can be
performed either dry or with a liquid suspension. The liquid
suspension can be a buffer, a solvent, an inert liquid, or a liquid
crystal material. One exemplary ball milling process provided by
Spex LLC (Metuchen, N.J.) is known as the Spex 8000 High Energy
Ball Mill. Another exemplary process, provided by Retsch (France),
uses a planetary ball mill to reduce micrometer size particles to
nanoscale particles.
[0083] The nanoparticles can be dispersed in the liquid crystal
material prior to polymer dispersion. Dry or solvent suspended
nanoparticles can be ultrasonically mixed with the liquid crystal
material or monomers prior to polymer dispersion to achieve an
isotropic dispersion. Wet particles may need to be prepared for
dispersion in liquid crystal, depending on the specific materials
used. If the particles are in a solvent or liquid buffer, the
solution can be dried, and the dry particles dispersed in the
liquid crystal as described above. Drying methods include
evaporation in air, vacuum evaporation, purging with inert gas like
nitrogen and heating the solution. If the particles are dispersed
in a solvent or liquid buffer with a vapor pressure lower than the
liquid crystal material, the solution can be mixed directly with
the liquid crystal, and the solvent can be evaporated using one of
the above methods leaving behind the liquid crystal/nanoparticle
dispersion. In one embodiment of the invention, the optical film
includes a liquid crystal material and a nanoclay nanoparticle,
where a nanoparticle is a particle of material with size less than
one micrometer in at least one dimension. The film can be
isotropically distributed.
[0084] Although nanoclay materials are discussed, in many
embodiments CNT is used as an alternative to nanoclay as a means
for reducing voltage. The properties of CNT in relation to PDLC
devices are reviewed by E. H. Kim et. al. in Polym. Int. 2010; 59:
1289-1295, the disclosure of which is incorporated herein by
reference in its entirety. PDLC films have been fabricated with
varying amounts of multi-walled carbon nanotubes ("MWCNTs") to
optimize the electro-optical performance of the PDLC films. The
MWCNTs were well dispersed in the prepolymer mixture up to 0.5 wt
%, implying that polyurethane acrylate ("PUA") oligomer chains wrap
the MWCNTs along their length, resulting in high diffraction
efficiency and good phase separation. The hardness and elastic
modulus of the polymer matrix were enhanced with increasing amounts
of MWCNTs because of the reinforcement effect of the MWCNTs with
intrinsically good mechanical properties. The increased elasticity
of the PUA matrix and the immiscibility between the matrix and the
liquid crystals gradually increased the diffraction efficiency of
the PDLC films. However, the diffraction efficiency of PDLC films
with more than 0.05 wt % MWCNTs was reduced, caused by poor phase
separation between the matrix and LCs because of the high viscosity
of the reactive mixture. PDLC films showing a low driving voltage
(75%) could be obtained with 0.05 wt % MWCNTs at 40 wt % LCs.
[0085] In embodiments where the PDLC materials incorporate such
nanoparticles, reductions of switching voltage and improvements to
the electro-optic properties of a polymer dispersed liquid crystal
film and/or polymer dispersed liquid crystal device can be obtained
by including nanoparticles in the liquid crystal domains. The
inclusion of nanoparticles serves to align the liquid crystal
molecules and to alter the birefringent properties of the film
through index of refraction averaging. In addition, the inclusion
of the nanoparticles improves the switching response of the liquid
crystal domains.
Monomer Functionality
[0086] RMLCM material systems in accordance with various
embodiments can be formulated in a variety of ways. In many
embodiments, the material system is an RMLCM that includes at least
one LC, at least one multi-functional monomer, a photo-initiator, a
dye, and at least one mono-functional monomer. Along with several
factors, such as but not limited to recording beam
power/wavelength, grating periodicity, and grating thickness, the
specific mixture of components and their percent composition can
determine the diffraction efficiency of the resulting HPDLC
gratings. Inhomogeneous polymerization due to the spatially
periodic irradiation intensity of the exposure can be the driving
force to segregate monomers and LCs and to order the orientation of
LC molecules, which can influence the diffraction efficiencies of
the HPDLC gratings. Oftentimes, the diffusion coefficient of
monomers depends on their molecular weight and reactivity. It has
been shown that a variety of monomer molecular weights or
functional numbers can yield a complex distribution of polymer and
LC phases. In many cases, molecular functionality can be critical
in achieving efficient phase separation and the formation of
gratings with high diffraction efficiency. As such, many
embodiments of the invention include material systems formulated
with specific mixes of monomers that are chosen, at least in part,
for their functionality so as to influence the diffraction
efficiency and index modulation of the resulting grating structure.
Other considerations in formulating such a mixture can include but
are not limited to the properties of the recording beam and the
thickness of the gratings. For the purposes of describing this
invention, the functionality of a monomer refers to the number of
reactive sites on each monomer unit.
[0087] The effects of varying monomer functionality in HPDLC
material systems have been studied to some degree in the scientific
literature. Such studies have generally examined the effects of the
effective, or average, functionality of a mixture with regards to
grating formation and performance. For example, in a paper by Pogue
et al., Polymer 41 (2000) 733-741, the disclosure of which is
incorporated herein by reference, investigations were conducted in
floodlit PDLCs and holographic PDLC gratings to show that a
decrease in effective monomer functionality general leads to
decreased LC phase separation.
[0088] Many embodiments in accordance with the invention include
investigations into mixtures with specific blends of monomers of
low functionality that can result in the formation of gratings
having high diffraction efficiency and efficient phase separation.
While the scientific literature typically emphasizes the use of
high functionality monomers, various embodiments in accordance with
the invention are focused on the use of monomers of low
functionality in certain applications. In some embodiments, the
monomers within the mixture are either mono-functional monomers or
bi-functional monomers. In a number of embodiments, tri-functional
monomers are also included. In such mixtures, the tri-functional
monomers are typically included at a low concentration, such as
lower than 5 wt %.
[0089] Mixtures including low functional monomers can behave
differently depending on a variety of factors, such as but not
limited to the wavelength sensitivity of the material system,
thickness of the HPDLC to be formed, and exposure temperature. In
the scientific literature, investigations into PDLC material
systems typically include UV sensitive material systems since
material reaction efficiency in general is typically poor with
visible light systems. However, formulations in accordance with
various embodiments of the invention have been able to reach high
diffraction efficiency (>80%) with low haze using low
functionality monomers that are sensitive (polymerizes) to visible
light. In further embodiments, the material systems include
monomers that are sensitive to green light, such as light with
wavelengths ranging from 495-570 nm. In addition to different light
systems, performance of the HPDLC mixtures can depend on the
thickness of the waveguide cell in which gratings are formed. For
example, for a given material system, different thicknesses of
deposited films can form waveguides with different amounts of haze.
Although grating thicknesses have been explored in the patent and
scientific literature, such investigations are focused on
relatively thick gratings. In a number of embodiments, the material
system is formulated for use in waveguides with thin form factors.
In further embodiments, the material system is formulated for use
in manufacturing waveguides having HPDLC layers with thicknesses of
less than 10 .mu.m. and gratings with more than 80% diffraction
efficiency. In further embodiments, the material system is
formulated for use in a waveguide having a 2-3 .mu.m thick HPDLC
layer and gratings with 80-90% diffraction efficiency. The material
system can also be formulated for manufacturing such waveguides
with low haze. In several embodiments, the material system can form
HPDLC layers having less than 1% haze. Waveguide haze is the
integrated effect of light interacting with material and surface
inhomogeneities over many beam bounces. The impact on the ANSI
contrast, the ratio of averaged white to black measurements taken
from a checkerboard pattern, can be dramatic owing to the scatter
contribution to the black level. Haze is mostly due to wide-angle
scatter by LC droplets and other small particles or scattering
centers resulting from incomplete phase separation of the
LC/monomer mixture during grating recording. Haze can also arise,
at least partly, from narrow angle scatter generated by large-scale
nonuniformities, leading to a loss of see-through quality and
reduced image sharpness. Some waveguide applications such as
aircraft HUDs, which use 1-D beam expansion in thick waveguides,
produce as few as 7 bounces, allowing up to 80:1 contrast. However,
in thin waveguides of the type use in near eye displays the number
of bounces may increase by a factor of 10 making the need for haze
control more acute.
[0090] RMLCM recipes can be optimized for specific thicknesses of
HPDLC layers. In many embodiments, the RMLCM recipe is optimized
for a .about.3 .mu.m thick uniform modulation gratings designed to
have a refractive index modulation of .about.0.16. As can readily
be appreciated, the specific thickness of the waveguide parts to be
fabricated can vary and can depend on the specific requirements of
a given application. In a number of embodiments, the waveguide
parts can be fabricated with 90% transmission and 0.3% haze. In
other embodiments, the waveguide parts can be fabricated with
.about.0.1% haze (with .about.0.01% haze recorded in unexposed
samples of the same material). In some embodiments, the RMLCM can
be formulated for fabricating waveguide parts containing haze of
less than 0.05%.
[0091] Transmission haze can be defined as the percentage of light
that deviates from desired beam direction by more the 2.5 degrees
on average (according to the ASTM D1003 standard). The clarity of a
waveguide can be characterized by the amount of narrow angle
scattered light (at an angle less than 2.5.degree. from the normal
to the waveguide surface). Transmission can be defined as the
amount of light transmitted through the waveguide without being
scattered. To assess general material haze, the scatter can be
measured around a vector normal to a waveguide TIR surface. To
assess holographic haze, the scatter can be measured around
principal diffraction directions (passing through the center of the
eye box). The procedures for measurement of haze, clarity and
transmission are defined in the ASTM D1003 International test
standards, in which "Procedure A" uses a haze meter and "Procedure
B" uses a spectrophotometer. An exemplary instrument for measuring
haze is the BYK-Gardner HAZE Guard II equipment.
[0092] In many embodiments, the RMLCM mixture includes a liquid
crystal mixture, a complex mixture of acrylates and acrylate
esters, Dynasylan.RTM. MEMO, and photoinitiators. In further
embodiments, the RMLCM includes EHA and DFHA. Depending on the
specific mix of components and their percent composition, the
resulting grating can have vastly different characteristics. In
some embodiments, the proportion of LC by weight is greater than
30%. In further embodiments, the proportion of LC is greater than
35 wt %. In some embodiments, the mixture includes liquid crystal
with high birefringence. In further embodiments, the high
birefringence liquid crystal accounts for more than 20 wt % of the
mixture. In a number of embodiments, dye and photo-initiators
account for less than 5 wt % of the mixture.
[0093] Nematic LC materials can provide a range of birefringence
(which can translate to refractive index modulation). Low to medium
birefringence typically covers the range of 0.09-0.12. However,
gratings can be designed using much lower birefringence values,
including gratings in which the birefringence varies along the
grating. Such gratings can be used to extract light from waveguides
with low efficiency at one end of the grating and high efficiency
at the other end of the grating to provide spatially uniform output
illumination. High birefringence (nematic LC) is typically the
range of 0.2-0.5. Even higher values are possible. Nematic liquid
crystals, compounds, and mixtures with positive dielectric
anisotropies (i.e., LCs for which the dielectric constant is
greater in the long molecular axis than that in the other
directions) are review in a paper by R. Dabrowski et al., "High
Birefringence Liquid Crystals"; Crystals; 2013; 3; 443-482, the
disclosure of which is incorporated herein by reference.
[0094] The functionality of the monomers in the mixtures can
greatly influence the diffraction efficiency of the resulting
grating. In many embodiments, the mixture includes at least one
mono-functional monomer and at least one multifunctional monomer in
varying concentrations. In several embodiments, the concentration
of mono-functional monomer within the mixture ranges from 1-50 wt
%. The monofunctional monomer can include aliphatic/aromatic groups
and an adhesion promoter. In some embodiments, the proportion of
multi-functional monomers present in the mixture is in the range of
2-30 wt %. Multi-functional monomers in accordance with various
embodiments of the invention typically include monomers of low
functionality. In a number of embodiments, the mixture includes a
bi-functional monomer at a low concentration. In further
embodiments, the mixture includes bi-functional monomers at less
than 15 wt %. Depending on the type and concentration of
bi-functional monomer in the mixture, adequate phase separation and
grating formation can occur. In the illustrative embodiment, the
mono-functional monomer, bi-functional monomer and LC have relative
weight ratios of 30%, 14%, and 40%, which resulted in a formulation
that allowed for the recording of gratings with a diffraction
efficiency higher than 90% and an index modulation of around
0.12.
[0095] As can readily be appreciated, percent composition of each
component within an RMLCM can vary widely. Formulations of such
material systems can be designed to achieve certain characteristics
in the resulting gratings. In many cases, the RMLCM is formulated
to have as high a diffraction efficiency as possible.
Workcell Cluster for Manufacturing Waveguide Cells
[0096] Waveguide cell manufacturing systems in accordance with
various embodiments of the invention can be implemented as workcell
clusters. By compartmentalizing different manufacturing steps into
workcells, modular systems can be implemented. In many embodiments,
a workcell cluster includes a preparation workcell for preparing
substrates for material deposition, a deposition workcell for
depositing an optical recording material onto a substrate, and a
lamination workcell for laminating various layers together to
construct a waveguide cell. Workcells can be configured in various
ways to implement different manufacturing processes for waveguide
cells. In some embodiments, the workcells are linked and configured
such that the output of one workcell is transferred to another
workcell, forming a manufacturing assembly line. The transferring
mechanism can be implemented in a variety of ways, such as but not
limited to the use of mechanical arms, suction, and/or a conveyor
system. In several embodiments, the products are manually
transferred. FIG. 2A conceptually illustrates a workcell cluster
system 200 in accordance with an embodiment of the invention. In
the illustrative embodiment, the system 200 includes a preparation
workcell 202, a deposition workcell 204, and a lamination workcell
206. As shown, arrows 208 indicate a sequential workflow
relationship among the workcells.
[0097] One advantage in a modular system is the ability to vary the
number of workcells dedicated to a particular task to improve
throughput by optimizing workcell use and reducing workcell
downtime. For example, a waveguide cell manufactured with different
optical recording materials may result in different deposition
times. In such embodiments, the number of deposition workcells can
vary accordingly to balance out the task completion time of each
workcell such as to minimize the overall downtime of the workcells.
FIG. 2B conceptually illustrates a workcell cluster system 210 with
two deposition workcells 212, 214 in accordance with an embodiment
of the invention. In the illustrative embodiment, the system 210
includes a preparation workcell 216, two deposition workcells 212,
214, and a lamination workcell 218. Dotted arrows 220 indicate that
output from the preparation workcell 216 can be received by either
deposition workcell 212, 214. Such a system can be ideally
implemented when the completion time for a single deposition
process is approximately twice as long as the completion time for
other processes.
[0098] Although FIGS. 2A and 2B conceptually illustrate specific
workcell cluster system configurations, workcell clusters in
accordance with various embodiments of the invention can be
configured in numerous ways depending on the specific requirements
of the given application. For example, workcell clusters can be
configured to have different workflow paths, types of workcells,
and/or numbers of workcells.
[0099] Due to the sensitive nature of some materials and processes
associated with waveguide cell fabrication, workcells can be
configured to provide protection from environmental light and
contaminants. In many embodiments, optical filters cover the
workcell in order to reduce and/or prevent unwanted light from
interacting with the optical recording material, which is typically
a photosensitive material. Depending on the specific type of
optical recording material, the deposition workcell can be lined
with an appropriate optical filter that prevent light of certain
wavelengths from entering the workcell and exposing the optical
recording material. In addition to the reduction/prevention of
light contamination, workcells can also be configured to reduce
particulate contamination. In several embodiments, the workcell is
configured to operate in an environment with minimal air
contamination. A low-particulate environment can be achieved in
many different ways, including but not limited to the use of air
filters. In a number of embodiments, air filters employing laminar
airflow principles are implemented. Contamination
reduction/prevention systems such as those described above can be
implemented separately or in combination. Although specific systems
are described, workcells in accordance with various embodiments of
the invention can be constructed in various ways as to alter the
working environment in a desired manner. For example, in several
embodiments, the workcell is configured to operate in a vacuum.
Specific workcells and their implementations and constructions are
described in the sections below in further detail.
Preparation Workcell
[0100] Waveguide cells in accordance with various embodiments of
the invention are typically composed of a layer of optical
recording material sandwiched between two substrates. Manufacturing
techniques for constructing such waveguide cells in accordance with
various embodiments of the invention can include a deposition step
where a layer of optical recording material is deposited onto one
of the substrate. In many embodiments, a preparation workcell can
be implemented to perform a cleaning/preparation procedure on the
substrates to prepare them for the deposition step. Preparing
substrates, such as but not limited to glass plates, can include
ridding the surfaces of contaminants and increasing the surface
adhesion properties for better material deposition.
[0101] Preparation workcells can be configured to implement various
cleaning and preparation protocols. Mechanical arms and/or suction
apparatuses can be used to maneuver the substrates throughout the
workcell. In many embodiments, the preparation workcells are
configured to clean glass substrates using various solvents and
solutions, including but not limited to soap solutions, acid
washes, acetone, and various types of alcohols. In some
embodiments, several types of solvents and/or solutions are used in
conjunction. For example, in several embodiments, methanol or
isopropanol can be administered after acetone to rinse off excess
acetone. In a number of embodiments, deionized water is used to
rinse off excess solvents or solutions. The solvents can be
administered in several ways, including but not limited to the use
of nozzles and baths. After cleaning, the workcell can be
configured to dry the substrates using an inert gas, such as
nitrogen, and/or a heating element.
[0102] In many embodiments, the cleaning process includes a
sonication step. In several embodiments, the substrate is placed in
a chamber containing a solution and a transducer is used to produce
ultrasonic waves. The ultrasonic waves can agitate the solution and
remove contaminants adhered to the substrates. The treatment can
vary in duration depending on several factors and can be performed
with different types of substrates. Deionized water or cleaning
solutions/solvents can be used depending on the type of
contamination and the type of substrate.
[0103] In many embodiments, the preparation workcell is configured
to implement a plasma chamber to plasma treat the surfaces of the
substrates. In some embodiments, the substrates are made of glass.
Existing in the form of ions and electrons, plasma is essentially
an ionized gas that has been electrified with extra electrons in
both negative and positive states. Plasma can be used to treat the
surface of the substrate to remove contaminants and/or prepare the
surface for material deposition by increasing the surface energy to
improve adhesion properties. In a number of embodiments, the
workcell includes a vacuum pump, which can be used to create a
vacuum under which the plasma treatment can be performed.
[0104] As can readily be appreciated, preparation workcells in
accordance with various embodiments of the invention can be
configured to perform combinations of various steps to implement a
specific cleaning protocol according to the requirements of a given
application. Although specific preparation workcells for preparing
glass plates are discussed above, preparation workcells can be
implemented to preform various preparatory steps for a variety of
different substrates, including but not limited to plastics.
Deposition Workcell
[0105] Waveguide cell manufacturing systems can utilize various
techniques for placing optical recording materials in between two
substrates. Manufacturing systems in accordance with various
embodiments of the invention can utilize a deposition process where
a film of optical recording material is deposited onto a substrate,
and the composite is laminated along with a second substrate to
form a three-layer laminate. In many embodiments, the manufacturing
system is a workcell cluster that includes a deposition workcell
for depositing a film of optical recording material onto a
substrate. Such deposition workcells can be configured to receive
substrates from preparation workcells. In some embodiments, the
deposition workcell includes a stage for supporting the substrate
and at least one deposition mechanism for depositing material onto
the substrate. Any of a variety of deposition heads can be
implemented to perform as the deposition mechanism. In several
embodiments, spraying mechanisms such as but not limited to
spraying nozzles are implemented to deposit optical recording
material onto a substrate. In some embodiments, the optical
recording material is deposited using a printing mechanism.
Depending on the type of deposition mechanism/head implemented,
several different deposition capabilities can be achieved. In a
number of embodiments, the deposition head can allow for the
deposition of different materials and/or mixtures that vary in
component concentrations. As can readily be appreciated, the
specific deposition mechanism utilized can depend on the specific
requirements of a given application.
[0106] The components within the deposition workcell can be
configured to move in various ways in order to deposit the optical
recording material onto the substrate. In many embodiments, the
deposition head and/or the stage are configured to move across
certain axes in order to deposit one or multiple layers of optical
recording material. In some embodiments, the deposition head is
configured to move and deposit material across three dimensions,
such as in a three-dimensional Euclidean space, which allows for
the deposition of multiple layers onto the substrate. In a number
of embodiments, the deposition head is only configured to move in
two axes to deposit a single layer. In other embodiments, the stage
and, consequently, the substrate are configured to move in three
dimensions while the deposition head is stationary. As can readily
be appreciated, deposition applications can be implemented to
deposit material in various dimensions by configuring the degrees
of motion freedom of the print head(s) and/or stage. The stage and
deposition head can be configured such that their combination of
degrees of motion freedom allows for depositing material in
n-dimensional Euclidean space, where n is the desired dimension.
For example, in several embodiments, the deposition head is
configured to move back and forth to deposit material in one axis
while the stage moves in a different axis, allowing for the
deposition of material in a two-dimensional Euclidean plane. In a
number of embodiments, the stage is implemented using a conveyor
belt. The system can be designed such that the conveyor belt
receives the substrate from a different workcell, such as the
preparation workcell. Once received, the conveyor system can move
the substrate along as a deposition head deposits a layer of
material onto the substrate. At the end of the conveyor path, the
substrate can be delivered into another workcell.
[0107] In a number of embodiments, the deposition workcell includes
an inkjet print head configured to deposit optical recording
material onto the substrate. Conventionally, inkjet printing refers
to a printing method that deposits a matrix of ink dots to form a
desired image. In typical operation, an inkjet print head contains
a large amount of small individual nozzles that can each deposit a
dot of material. In additive manufacturing applications, inkjet
printing can be used to create complex patterns and structures with
high precision due to the size and number of nozzles in a typical
inkjet print head. Applying these principles to waveguide cell
manufacturing applications, inkjet printing can be used to print a
uniform or near-uniform, in terms of thickness and composition,
layer of optical recording material. Depending on the application
and inkjet print head, one or multiple layers of the optical
recording material can be printed onto the substrate. Various
optical recording materials, such as those described in the
sections above, can be used in conjunction with an inkjet print
head. In addition to the capability of printing in different
materials, the printing system can be configured for use with
various types of substrates. As can readily be appreciated, the
choice of material to be printed and the substrates used can depend
on the specific requirements of a given application. For instance,
choices in material systems can be selected based on printing
stability and accuracy. Other considerations can include but are
not limited to viscosity, surface tension, and density, which can
influence several factors such as but not limited to droplet
formability and the ability to form layers of uniform
thickness,
[0108] A deposition workcell 300 in accordance with an embodiment
of the invention is conceptually illustrate in FIGS. 3A and 3B.
FIG. 3A shows an isometric view of the deposition workcell 300
while FIG. 3B shows a top view of the same deposition workcell 300.
As shown, the deposition workcell 300 is constructed with a frame
that can hold optical glass filters to prevent particulate
contamination and environmental light from exposing optical
recording materials within the workcell 300. The workcell includes
chambers 302, 304 for receiving substrates and outputting waveguide
cells. In the illustrative embodiment, the stage is implemented as
a conveyor belt 306 that moves received substrates along one
direction. The deposition workcell 300 further includes an inkjet
printer 308 implemented as a deposition mechanism. The inkjet
printer 308 is configured to print across a direction different
from the movement of the conveyor belt 306, allowing for the
deposition of a layer of optical recording material across the
planar surface of the substrates. Additionally, the deposition
workcell 300 implements a roller laminator 310 for laminating the
printed layer and two substrates to construct a waveguide cell. The
workcell 300 is also implemented as a glovebox with gloves 312 that
allow for the manual manipulation of the devices within the
workcell 300 while maintaining a clean environment.
[0109] Although FIGS. 3A and 3B depict a specific deposition
workcell configuration, deposition workcells can be configured in
many ways in accordance with various embodiments of the invention.
For example, the laminator can be implemented in a separate
lamination workcell. In several embodiments, automatic system
configurations can be implemented. In many embodiments, multiple
inkjet print heads are used. In other embodiments, spraying nozzles
are used as the deposition mechanism.
Modulation of Material Composition
[0110] High luminance and excellent color fidelity are important
factors in AR waveguide displays. In each case, high uniformity
across the FOV can be essential. However, the fundamental optics of
waveguides can lead to non-uniformities due to gaps or overlaps of
beams bouncing down the waveguide. Further non-uniformities may
arise from imperfections in the gratings and non-planarity of the
waveguide substrates. In SBGs, there can exist a further issue of
polarization rotation by birefringent gratings. The biggest
challenge is the fold grating where there are millions of light
paths resulting from multiple intersections of the beam with the
grating fringes. Careful management of grating properties,
particularly the refractive index modulation, can be utilized to
overcome non-uniformity in accordance with various embodiments of
the invention.
[0111] Out of the multitude of possible beam interactions
(diffraction or zero order transmission), only a subset contributes
to the signal presented at the eye box. By reverse tracing from the
eyebox, fold regions contributing to a given field point can be
pinpointed. The precise correction to the modulation that is needed
to send more into the dark regions of the output illumination can
then be calculated. Having brought the output illumination
uniformity for one color back on target, the procedure can be
repeated for other colors. Once the index modulation pattern has
been established, the design can be exported to the deposition
mechanism, with each target index modulation translating to a
unique deposition setting for each spatial resolution cell on the
substrate to be coated. In many embodiments, the spatial pattern
can be implemented to 30 micrometers resolution with full
repeatability.
[0112] FIGS. 4A and 4B conceptually illustrate schematically the
use of reverse ray tracing to compute a compensated index
modulation pattern for coating in accordance with various
embodiments of the invention. The procedure can determine the
optimum usable area of the fold grating and the refractive index
modulation variation across the fold grating needed to provide
uniform illumination at the eye box. FIG. 4A shows a mathematical
model of a basic waveguide architecture that includes an input
grating 402, a fold grating that is divided up into a calculation
mesh 404, and an output grating 406. By tracing rays from points
across the eye box through the output grating and through the fold
grating, the fold grating cells which contribute to the eyebox
illumination for a given FOV direction can be identified. Reverse
beam paths from the output grating are indicated by the rays
408-414. By repeating the ray trace for different FOV angles the
maximum extent of the fold grating needed to fill the eye box can
be determined. This ensures that the area of HPDLC material to be
deposited/printed can be kept to a minimum, thereby reducing haze
in the finished waveguide part. The procedure can also identify
which cells need to have their index modulation increased (or
decreased) in order to maintain illumination uniformity across the
eyebox. For example, in the embodiment of FIG. 4A, most of the fold
grating region has a refractive index modulation of 0.03. However,
certain calculation cells encircled by 416 (such as cell 418, for
example) and encircled by 420 (such as cell 422, for example)
should have index modulations of 0.07, while the calculation cells
lying within the rectangular zone 424 should have index modulation
0.05. Typically, the map of index modulation values is exported as
an AutoCAD DXF (Drawing Interchange Format) file into the processor
controlling the deposition mechanism. FIG. 4B is a plan view 450 of
the final waveguide part 452 onto which is superimposed the index
modulation map of the printed grating layer (corresponding to the
model of FIG. 4A) as would be revealed by examining the printed
grating under cross polarizers. The grating regions include the
input 454, output 456, and fold 458 gratings. In the illustrative
embodiment, the fold grating contains the high index modulation
regions 460, 462, and 464 corresponding to the cells identified in
regions 416, 420, and 424 of FIG. 4A. The grating regions of FIG.
4B are surrounded by a clear polymer region 466. Although FIGS. 4A
and 4B illustrate a specific way of computing a compensated index
modulation pattern, any of a variety of techniques can be utilized
to compute such a pattern.
[0113] Compared with waveguides utilizing surface relief gratings
("SRGs"), SBG waveguides implementing manufacturing techniques in
accordance with various embodiments of the invention can allow for
the grating design parameters that impact efficiency and
uniformity, such as refractive index modulation and grating
thickness, to be adjusted dynamically during the deposition
process. As such, there is no need for a new master for the grating
recording process. With SRGs where modulation is controlled by etch
depth, such schemes would not be practical as each variation of the
grating would entail repeating the complex and expensive tooling
process. Additionally, achieving the required etch depth precision
and resist imaging complexity can be very difficult. FIGS. 5A and
5B conceptually illustrate the fundamental structural differences
between SBGs and SRGs. FIG. 5A shows a cross-sectional view 500 of
a portion of an SRG. In the illustrative embodiment, the grating
includes a substrate 502 supporting slanted surface relief elements
504 separated by air gaps 506. Typically, the surface relief
elements and substrate are formed from a common material. The
grating pitch is indicated by the symbol p and the grating depth by
symbol h. FIG. 5B shows a cross-sectional view 550 of an SBG. In
contrast to an SRG, the SBG includes alternating slanted Bragg
fringes formed from low index monomer-rich fringes such as 552 and
higher index LC-rich fringes such as 554. The index difference is
characterized by the refractive index modulation .delta.n, which
plays an equivalent role in determining grating diffraction
efficiency to the grating depth in a SRG. The variation of index
modulation is represented by the superimposed plot 556 of index
modulation versus distance z along the grating. In some
embodiments, the index modulation has a sinusoidal profile as shown
in FIG. 5B. In embodiments in which the SBG is formed in a uniform
modulation HPLDC, the index modulation profile can include
near-rectangular LC-rich and polymer-rich regions.
[0114] Deposition processes in accordance with various embodiments
of the invention can provide for the adjustment of grating design
parameters by controlling the type of material that is to be
deposited. Similar to multi-material additive manufacturing
techniques, various embodiments of the invention can be configured
to deposit different materials, or different material compositions,
in different areas on the substrate. In many embodiments, a layer
of optical recording material can be deposited with different
materials in different areas. For example, deposition processes can
be configured to deposit HPDLC material onto an area of a substrate
that is meant to be a grating region and to deposit monomer onto an
area of the substrate that is meant to be a nongrating region. In
several embodiments, the deposition process is configured to
deposit a layer of optical recording material that varies spatially
in component composition, allowing for the modulation of various
aspects of the deposited material. Modulation schemes and
deposition processes for different types of materials and mixtures
are discussed below in further detail.
[0115] The choice in material printed in a specific area can depend
on the optical element that will later be recorded in that area.
For example, in some embodiments, the deposition head is configured
to deposit a layer of optical recording material for a waveguide
cell intended to be recorded with three different gratings. The
layer can be deposited such that the materials printed in each of
the areas designated for the three gratings are all different from
one another. FIG. 6 conceptually illustrates a waveguide cell 600
with marked areas intended to be recorded with various gratings in
accordance with an embodiment of the invention. As shown, areas for
an input grating 602, a fold grating 604, and an output grating 606
are outlined. Such areas can each be composed of a different
material or different mixture composition depending on the given
application. In a number of embodiments, different materials can be
deposited to produce different diffraction efficiencies among the
recorded gratings. In the illustrative embodiment, the waveguide
cell is in a curvilinear shape, which, along with the positions,
sizes, and shapes of the gratings, is designed to be a waveguide
for near-eye applications.
[0116] Deposition of material with different compositions can be
implemented in several different ways. In many embodiments, more
than one deposition head can be utilized to deposit different
materials and mixtures. Each deposition head can be coupled to a
different material/mixture reservoir. Such implementations can be
used for a variety of applications. For example, different
materials can be deposited for grating and nongrating areas of a
waveguide cell. In some embodiments, HPDLC material is deposited
onto the grating regions while only monomer is deposited onto the
nongrating regions. In several embodiments, the deposition
mechanism can be configured to deposit mixtures with different
component compositions.
[0117] In some embodiments, spraying nozzles can be implemented to
deposit multiple types of materials onto a single substrate. In
waveguide applications, the spraying nozzles can be used to deposit
different materials for grating and non-grating areas of the
waveguide. FIGS. 7A and 7B conceptually illustrate operation of a
deposition mechanism utilizing a spray module in accordance with an
embodiment of the invention. As shown, the apparatus 700 includes a
coating module 702 that includes a first spray module 704 connected
via a pipe 706 to a first reservoir 708 containing a first mixture
of a first material and a second spray module 710 connected via a
pipe 712 to a second reservoir 714 containing a second mixture of a
second material. In the illustrative embodiment, the first material
includes at least a liquid crystal and a monomer while the second
material includes only a monomer. Such a configuration allows for
the deposition of a layer of optical recording material with
defined grating and non-grating areas. As can readily be
appreciated, any configurations of different mixtures can be
utilized as appropriate depending on the specific application.
[0118] In FIGS. 7A and 7B, the first and second spray modules
provide jets of liquid droplets over a controllable divergence
angle as represented by 716, 718. The apparatus further includes a
support for a transparent substrate 720 having predefined regions
for supporting gratings as illustrated by the shaded regions
722-726, regions of gratings that do not transmit light into the
eyebox as indicated by 728, 730, and regions surrounding the
gratings indicated by 732. In some embodiments, the regions 728,
730 are identified by a reverse ray trace of the waveguide from the
eyebox. During operation, the regions for supporting gratings
providing diffracted light that enters the eye box are coated with
the first mixture. The regions 728, 730 are coated with the second
mixture. The apparatus further includes a positioning apparatus 734
connected to the coating apparatus by a control link 736 for
traversing the coating apparatus across the substrate. The
apparatus further includes a switching mechanism for activating the
first spray module and deactivating the second spray module when
the coating apparatus is positioned over a substrate region for
supporting a grating and for deactivating the first spray module
and activating the second spray module when the coating apparatus
is positioned over a substrate region that does not support a
grating.
[0119] Two operational states of the apparatus are conceptually
illustrated in FIGS. 8A and 8B, which show a detail of the
substrate. As shown in FIG. 8A, when the coating apparatus is over
a nongrating-supporting region 800 (located in the upper region of
the strip bounded by the edges 802, 804), the second spray module
is activated, and the first spray module is deactivated so that a
layer of monomer 806 is sprayed onto the substrate. As shown in
FIG. 8B, when the coating apparatus is over a substantially
grating-supporting region 808 (located in the lower region of the
strip bounded by the edges 802, 804), the second spray module is
deactivated, and the first spray module is activated so that a
layer of liquid crystal and monomer mixture 810 is sprayed onto the
substrate.
[0120] Although FIGS. 7A-8B illustrate specific applications and
configurations of spraying mechanisms, spraying mechanisms and
deposition mechanisms in general can be configured and utilized for
a variety of applications. In many embodiments, the spraying
mechanism is configured for printing gratings in which at least one
of the material composition, birefringence, and thickness can be
controlled using a coating apparatus having at least two selectable
spray heads. In some embodiments, the deposition workcell provides
an apparatus for depositing grating recording material optimized
for the control of laser banding. In several embodiments, the
deposition workcell provides an apparatus for depositing grating
recording material optimized for the control of polarization
non-uniformity. In some embodiments, the deposition workcell
provides an apparatus for depositing grating recording material
optimized for the control of polarization non-uniformity in
association with an alignment control layer. In a number of
embodiments, the deposition workcell can be configured for the
deposition of additional layers such as beam splitting coatings and
environmental protection layers. Additionally, although FIGS. 7A-8B
discuss the capabilities of spraying nozzles, these capabilities
can be implemented in other deposition mechanisms. For example,
inkjet print heads can also be implemented to print different
materials in grating and nongrating regions of the substrate.
[0121] FIG. 9 is a flow chart conceptually illustrating a method of
fabricating a holographic grating using a selective coating process
in accordance with an embodiment of the invention. Referring to
FIG. 9, the method 900 includes providing (902) a transparent
substrate for coating. A grating supporting and
non-grating-supporting regions of the substrate can be defined
(904). Depending on the specific application, gratings of various
sizes and shapes can be defined. In some embodiments, a grating
region supports an input, a fold, or an output grating. In many
embodiments, the substrate has regions defined for gratings made of
a combination of the aforementioned types of gratings. A first
mixture for coating containing a liquid crystal and monomer and a
second mixture for coating containing a monomer can be provided
(906). A first spray head can be provided (908) for coating the
first mixture onto the substrate. A second spray head can be
provided (910) for coating the second mixture. The first and second
spray heads integrated together can be considered a coating
apparatus. The coating apparatus can be set (912) to its starting
position (k=1). The coating apparatus can be moved (914) to the
current position over the substrate. A decision can be made (916)
on whether the current coating apparatus is positioned over a
grating supporting region or a non-grating-supporting region. If
the coating apparatus is over a grating region, the first spray
head can be activated and the second spray head can be deactivated
(918). If the coating module is over a grating-supporting region,
the first spray head can be deactivated and the second spray head
can be activated (920). A decision can be made (922) regarding the
coating status. If all specified regions have been coated, the
process can be terminated (924). If the specified regions have not
all been coated, the next region (increment k) to be coated can be
selected (926) and the deposition steps can be repeated.
[0122] Although FIG. 9 illustrates a specific method for depositing
different materials over a substrate, the deposition mechanism can
be configured to produce a film of material having characteristics
that can vary spatially and across regions. FIG. 10 conceptually
illustrates a deposition head for providing predefined grating
characteristics within grating regions in accordance with an
embodiment of the invention. Referring to FIG. 10, the deposition
head 1000 includes a first spray module 1002 fed via pipe 1004 from
a reservoir 1006 containing a mixture of at least one of a liquid
crystal and a monomer, which is dispersed into the spray jet 1008
by the spray module 1002 for coating a transparent substrate. The
substrate has predefined regions for supporting gratings. There is
also provided an X-Y displacement controller 1010 for traversing
the spray module across the substrate and a means for controlling
the spray characteristics from the module over each grating region
to deposit a film that provides a predefined grating characteristic
within the grating region following holographic exposure. The
holographic exposure may be carried out using any current
holographic process, include any of the processes disclosed in the
reference documents. In the illustrative embodiment, the deposition
head 1000 further includes a mixture controller 1012 for
controlling one or more of the temperature, dilution and relative
concentrations of chemical components of the mixture. The
deposition head 1000 can also include a spray controller 1014 for
controlling one or more of the spray angle relative to the
substrate, the spray divergence angle, and the durations of the
spray on and off states. In several embodiments, the predefined
grating characteristic includes one or more of refractive index
modulation, refractive index, birefringence, liquid crystal
director alignment, and grating layer thickness. As can readily be
appreciated, deposition heads can be implemented and configured in
many different ways. In many embodiments, any combination and
subset of the X-Y displacement controller, mixture controller, and
spray controller can be utilized. In some embodiments, additional
controllers are utilized to configure the spraying mechanism and
the material deposited.
[0123] FIG. 11 conceptually illustrates operation of a deposition
head for depositing material having regions with predefined grating
characteristics in accordance with an embodiment of the invention.
As discussed above, the deposition head can be configured to
deposit material having a spatial variation across the grating
region of one or more of refractive index modulation, refractive
index, birefringence, liquid crystal director alignment and grating
layer thickness. As shown in FIG. 11, the spray module 1100 follows
a spraying path 1102 across the substrate 1104. The spray can be
dynamically controlled during transit along the path 1102 to vary
the predefined grating characteristics in areas of the predefined
grating regions such as 1106, 1108, for example. In some
embodiments, the deposition mechanism provides, after exposure, a
grating with a spatially varying diffraction efficiency. For
example, referring again to FIG. 11, the coating areas 1106, 1108
(after holographic exposure) exhibit diffraction efficiency (DE)
versus angle (U) characteristics represented by the curves 1110,
1112 respectively.
[0124] FIG. 12 conceptually illustrates a deposition mechanism for
depositing two grating layers in accordance with an embodiment of
the invention. As shown, the system 1200 is similar to that of FIG.
11 but further includes a second spray module 1202 providing a jet
1204 for coating the second grating layer 1206. In many
embodiments, the grating layers are coated using different mixture
compositions. In some embodiments, similar to the one of FIG. 7A,
the system includes a first spray module connected to a first
reservoir containing a first mixture that includes at least one of
a first liquid crystal and a first monomer and a second spray
module connected to a second reservoir containing a second mixture
that includes at least one of a second liquid crystal and a second
monomer.
[0125] FIG. 13 conceptually illustrates a system for depositing a
grating layer of material and for holographically exposing the
layer using recording beams with on and off states synchronized
with the coating module. As shown, the system 1300 includes a
coating apparatus similar to that of FIG. 12 following a spraying
path 1302 across the substrate 1304 providing predefined grating
regions 1306, 1308. While the coating process is taking place, a
holographic exposure apparatus 1310, which provides a recording
beam 1312, can expose coated predefined grating regions 1314. In
many embodiments, the holographic exposure apparatus is based on a
master grating which contact copies the required grating into the
predefined grating region.
[0126] FIG. 14 is a flow chart conceptually illustrating a method
of depositing a film of material with regions having predefined
grating characteristics in accordance with an embodiment of the
invention. As shown, the method 1400 includes providing (1402) a
transparent substrate for coating. A grating supporting and
non-grating-supporting regions of the substrate can be defined
(1404). A mixture containing a liquid crystal and monomer can be
provided (1406). In several embodiments, the material utilized
includes one or more of a photoinitiator, nano-particles,
low-functionality monomers, additives for reducing switching
voltage, additives for reducing switching time, additives for
increasing refractive index modulation and additives for reducing
haze. A spray module for coating the mixture onto the substrate can
be provided (1408). The spray module can be set (1410) to its
starting position (k=1). The spray module can be moved (1412) to
the current position over the substrate. A decision can be made
(1414) on whether the current coating apparatus is positioned over
a grating supporting region or a non-grating-supporting region. If
the coating apparatus is over a grating region, the spray module
can be activated (1416) to provide a spray characteristic for
achieving a predefined grating characteristic within the grating
region. The grating region can be coated (1418). A decision can be
made (1420) regarding the coating status. If all specified regions
have been coated, the process can be terminated (1422). If all
specified regions have not been coated, the next region to be
coated can be selected (1424) and the deposition steps can be
repeated with k incremented.
[0127] Although FIGS. 10-14 illustrate specific implementations and
methods of depositing material with regions having predefined
grating characteristics, any of a variety of configurations can be
implemented. For example, in many embodiments, multiple spray
modules or deposition heads are utilized. Various predefined
grating characteristics can be controlled and/or modulated
depending on the specific application. Modulation of material
composition utilizing more than one deposition head is discussed
below in further detail.
[0128] As discussed above, deposition processes can be configured
to deposit optical recording material that varies spatially in
component composition. Modulation of material composition can be
implemented in many different ways. In a number of embodiments, an
inkjet print head can be configured to modulate material
composition by utilizing the various inkjet nozzles within the
print head. By altering the composition on a "dot-by-dot" basis,
the layer of optical recording material can be deposited such that
it has a varying composition across the planar surface of the
layer. Such a system can be implemented using a variety of
apparatuses including but not limited to inkjet print heads.
Similar to how color systems use a palette of only a few colors to
produce a spectrum of millions of discrete color values, such as
the CMYK system in printers or the additive RGB system in display
applications, inkjet print heads in accordance with various
embodiments of the invention can be configured to print optical
recording materials with varying compositions using only a few
reservoirs of different materials. Different types of inkjet print
heads can have different precision levels and can print with
different resolutions. In many embodiments, a 300 DPI ("dots per
inch") inkjet print head is utilized. Depending on the precision
level, discretization of varying compositions of a given number of
materials can be determined across a given area. For example, given
two types of materials to be printed and an inkjet print head with
a precision level of 300 DPI, there are 90,001 possible discrete
values of composition ratios of the two types of materials across a
square inch for a given volume of printed material if each dot
location can contain either one of the two types of materials. In
some embodiments, each dot location can contain either one of the
two types of materials or both materials. In several embodiments,
more than one inkjet print head is configured to print a layer of
optical recording material with a spatially varying composition.
Although the printed dots for a two-material application are
essentially a binary system, in practical applications, averaging
the printed dots across an area can allow for discretization of a
sliding scale of ratios of the two materials to be printed.
[0129] FIG. 15 conceptually illustrates an inkjet printing
modulation scheme in accordance with an embodiment of the
invention. As shown, eighteen discrete unit-squares are each
capable of being printed with a varying ratios of two different
types of materials. In the illustrated embodiment, the inkjet print
head is capable of printing sixty-four dots within each of the
eighteen unit squares. Each dot can be printed with either one of
two types of material. A close up 1500 of unit square 1502 shows
that all sixty-four dot locations within the unit square is printed
with the first material. Similarly, a close up 1504 of unit square
1506 is printed completely with the second material. Unit square
1508 shows an intermediate composition where thirty out of the
sixty-four dot locations are printed with the first material while
the remaining dot locations are printed with the second material.
As such, unit square 1508, as a whole, contains an intermediate
level of concentrations from both materials. Utilizing this
modulation scheme, any pattern of varying material characteristics
can be achieved.
[0130] The amount of discrete levels of possible
concentrations/ratios across a unit square is given by how many dot
locations can be printed within the unit square. In the
illustrative embodiment, sixty-four discrete dots can be printed
within the unit square, which thus results in each unit square
having a possibility of sixty-five different concentration
combinations, ranging from 100% of the first material to 100% of
the second material. Although FIG. 15 discusses the areas in terms
of a unit square, the concepts are applicable to real units and can
be determined by the precision level of the inkjet print head.
Although specific examples of modulating the material composition
of the printed layer are discussed, it can readily be appreciated
that the concept of modulating material composition using inkjet
print head can be expanded to use more than two different material
reservoirs and can vary in precision levels, which largely depends
on the type of print head used.
[0131] Varying the composition of the material printed can be
advantageous for several reasons. For example, in many embodiments,
varying the composition of the material during deposition can allow
for a waveguide with gratings that have varying diffraction
efficiencies across different areas of the gratings. In embodiments
utilizing HPDLC mixtures, this can be achieved by modulating the
relative concentration of liquid crystals in the HPDLC mixture
during the printing process, which creates compositions that can
produce gratings with varying diffraction efficiencies when
exposed. In several embodiments, a first HPDLC mixture with a
certain concentration of liquid crystals and a second HPDLC mixture
that is liquid crystal-free are used as the printing palette in an
inkjet print head for modulating the diffraction efficiencies of
gratings that can be formed in the printed material. In such
embodiments, discretization can be determined based on the
precision of the inkjet print head. For example, if a 150 DPI
inkjet print head is utilized, each square inch can be printed with
22,501 discrete levels of liquid crystal concentration. A discrete
level can be given by the concentration/ratio of the materials
printed across a certain area. In this example, the discrete levels
range from no liquid crystal to the maximum concentration of liquid
crystals in the first PDLC mixture.
[0132] The ability to vary the diffraction efficiency across a
waveguide can be used for various purposes. Waveguides are
typically designed such that light can be reflected many times
between the two planar surfaces of a waveguide. These multiple
reflections can allow for a light path to interact with a grating
multiple times. In many embodiments, a waveguide cell can be
printed with varying compositions such that the gratings formed
from the optical recording material layer have varying diffraction
efficiencies to compensate for the loss of light during
interactions with the gratings to allow for a uniform output
intensity. For example, in some waveguide applications, an output
grating is configured to provide exit pupil expansion in one
direction while also coupling light out of the waveguide. The
output grating can be designed such that when light within the
waveguide interact with the grating, only a percentage of the light
is refracted out of the waveguide. The remaining portion continues
in the same light path, which remains within TIR and continues to
be reflected within the waveguide. Upon a second interaction with
the same output grating again, another portion of light is
refracted out of the waveguide. During each refraction, the amount
of light still traveling within the waveguide decreases by the
amount refracted out of the waveguide. As such, the portions
refracted at each interaction gradually decreases in terms of total
intensity. By varying the diffraction efficiencies of the grating
such that it increases with propagation distance, the decrease in
output intensity along each interaction can be compensated,
allowing for a uniform output intensity.
[0133] Varying the diffraction efficiency can also be used to
compensate for other attenuation of light within a waveguide. All
objects have a degree of reflection and absorption. Light trapped
in TIR within a waveguide are continually reflected between the two
surfaces of the waveguide. Depending on the material that makes up
the surfaces, portions of light can be absorbed by the material
during each interaction. In many cases, this attenuation is small,
but can be substantial across a large area where many reflections
occur. In many embodiments, a waveguide cell can be printed with
varying compositions such that the gratings formed from the optical
recording material layer have varying diffraction efficiencies to
compensate for the absorption of light from the substrates.
Depending on the substrates, certain wavelengths can be more prone
to absorption by the substrates. In a multi-layer waveguide design,
each layer can be designed to couple in a certain range of
wavelengths of light. Accordingly, the light coupled by these
individual layers can be absorbed in different amounts by the
substrates of the layers. For example, in a number of embodiments,
the waveguide is made of a 3-layer stack to implement a color
display, where each layer is designed for one of Red, Green, and
Blue. In such embodiments, gratings within each of the waveguide
layers can be formed to have varying diffraction efficiencies to
perform color balance optimization by compensating for color
imbalance due to loss of transmission of certain wavelengths of
light.
[0134] In addition to varying the liquid crystal concentration
within the material in order to vary the diffraction efficiency,
another technique includes varying the thickness of the waveguide
cell. This can be accomplished through the use of beads. In many
embodiments, beads are dispersed throughout the optical recording
material for structural support during the construction of the
waveguide cell. In some embodiments, different sizes of beads are
dispersed throughout the optical recording material. The beads can
be dispersed in ascending order of sizes across one direction of
the layer of optical recording material. When the waveguide cell is
constructed through lamination, the substrates sandwich the optical
recording material and, with structural support from the varying
sizes of beads, create a wedge shaped layer of optical recording
material. Beads of varying sizes can be dispersed similar to the
modulation process described above. Additionally, modulating bead
sizes can be combined with modulation of material compositions. In
several embodiments, reservoirs of HPDLC materials each suspended
with beads of different sizes are used to print a layer of HPDLC
material with beads of varying sizes strategically dispersed to
form a wedge shaped waveguide cell. In a number of embodiments,
bead size modulation is combined with material composition
modulation by providing an amount of reservoirs equal to the
product of the number of different sizes of beads and the number of
different materials used. For example, in one embodiment, the
inkjet print head is configured to print varying concentrations of
liquid crystal with two different bead sizes. In such an
embodiment, four reservoirs can be prepared: a liquid crystal-free
mixture-suspension with beads of a first size, a liquid
crystal-free mixture-suspension with beads of a second size, a
liquid crystal-rich mixture-suspension with beads of a first size,
and a liquid crystal-rich mixture-suspension with beads of a second
size.
Lamination Workcell
[0135] In many embodiments, the workcell cluster includes a
lamination workcell for laminating the waveguide cell. After the
deposition of optical recording material onto a substrate, a second
substrate can be placed onto the optical recording material,
creating a three-layer composite. Oftentimes, the second substrate
will be made of the same material and in the same dimensions as the
first substrate. In many embodiments, the deposition workcell is
configured to place the second substrate onto the optical recording
material. In other embodiments, the lamination workcell is
configured to place the second substrate onto the optical recording
material. The second substrate can be placed manually or through
the use of mechanical arms and/or suction mechanisms. Once the
second substrate is placed, the three-layer composite may be too
unstable to handle manually and, thus, in many embodiments, a
laminator is implemented to compact the composite.
[0136] The three-layer composite can be laminated in various ways.
In many embodiments, a press is implemented to provide downward
pressure onto the composite. In other embodiments, the lamination
workcell is configured to feed the composite through a roller
laminator. The compacted composite and adhesion properties of the
optical recording material can result in a waveguide cell with
enough stability to be handled manually. In some embodiments, the
layer of optical recording material includes beads. Consequently,
these relatively incompressible beads can define the height of the
layer of optical recording material within the compacted composite.
As discussed in the sections above, differently sized beads can be
placed throughout the optical recording material. Upon lamination,
the sizes of the beads can each determine the local thickness of
the waveguide cell. By varying the sizes of the beads, a wedge
shaped waveguide cell can be constructed. As can readily be
appreciated, the lamination of the substrates-optical recording
material layer composite can be achieved using lamination workcells
that can be configured and implemented in many different ways. In
several embodiments, the lamination workcell is a modular workcell
within the workcell cluster. In other embodiments, the lamination
workcell is simply a laminator implemented within the deposition
workcell, such as the one shown in FIGS. 3A and 3B.
[0137] Although specific systems and methods for manufacturing
waveguide cells are discussed above, many different configurations
can be implemented in accordance with many different embodiments of
the invention. It is therefore to be understood that the present
invention can be practiced in ways other than specifically
described, without departing from the scope and spirit of the
present invention. Thus, embodiments of the present invention
should be considered in all respects as illustrative and not
restrictive. Accordingly, the scope of the invention should be
determined not by the embodiments illustrated, but by the appended
claims and their equivalents.
* * * * *