U.S. patent application number 16/794071 was filed with the patent office on 2020-08-20 for methods and apparatuses for providing a holographic waveguide display using integrated gratings.
This patent application is currently assigned to DigiLens Inc.. The applicant listed for this patent is DigiLens Inc.. Invention is credited to Alastair John Grant, Sihui He, Edward Lao, Milan Momcilo Popovich, Roger Conley Smith, Jonathan David Waldern.
Application Number | 20200264378 16/794071 |
Document ID | 20200264378 / US20200264378 |
Family ID | 1000004670606 |
Filed Date | 2020-08-20 |
Patent Application | download [pdf] |
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United States Patent
Application |
20200264378 |
Kind Code |
A1 |
Grant; Alastair John ; et
al. |
August 20, 2020 |
Methods and Apparatuses for Providing a Holographic Waveguide
Display Using Integrated Gratings
Abstract
Systems and methods for providing holographic waveguide display
using integrated gratings in accordance with various embodiments of
the invention are illustrated. One embodiment includes a waveguide
display including a source of light, and a first waveguide
including a grating structure including first and second gratings,
and an input coupler configured to couple a first field-of-view
portion of light, and couple a second field-of-view portion of
light, wherein the first grating is configured to provide beam
expansion in a first direction for the first field-of-view portion
of light, and provide beam expansion in the first direction and
beam extraction towards a viewer for the second field-of-view
portion of light, the second grating is configured to provide beam
expansion in a second direction for the second field-of-view
portion of light, and provide beam expansion in the second
direction and beam extraction towards a viewer for the first
field-of-view portion of light.
Inventors: |
Grant; Alastair John; (San
Jose, CA) ; Waldern; Jonathan David; (Los Altos
Hills, CA) ; Popovich; Milan Momcilo; (Leicester,
GB) ; He; Sihui; (Sunnyvale, CA) ; Lao;
Edward; (Sunnyvale, CA) ; Smith; Roger Conley;
(Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens Inc.
Sunnyvale
CA
|
Family ID: |
1000004670606 |
Appl. No.: |
16/794071 |
Filed: |
February 18, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62806665 |
Feb 15, 2019 |
|
|
|
62813373 |
Mar 4, 2019 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 2027/0125 20130101;
G02B 27/0103 20130101; G02B 6/34 20130101; G02B 6/0016
20130101 |
International
Class: |
G02B 6/34 20060101
G02B006/34; F21V 8/00 20060101 F21V008/00; G02B 27/01 20060101
G02B027/01 |
Claims
1. A waveguide display, comprising: a source of light; and a first
waveguide comprising: a grating structure comprising first and
second gratings; and an input coupler configured to: couple a first
field-of-view portion of light from said source of light into said
first waveguide and towards said first grating; and couple a second
field-of-view portion of light from said source of light into said
first waveguide and towards said second grating; wherein: said
first grating is configured to: provide beam expansion in a first
direction for said first field-of-view portion of light; and
provide beam expansion in said first direction and beam extraction
towards a viewer for said second field-of-view portion of light;
said second grating is configured to: provide beam expansion in a
second direction for said second field-of-view portion of light;
and provide beam expansion in said second direction and beam
extraction towards a viewer for said first field-of-view portion of
light; said input coupler, said first grating, and said second
grating each comprises a grating vector; and said grating vectors
of said input coupler, said first grating, and said second grating
provide a resultant vector with substantially zero magnitude.
2. The waveguide display of claim 1, wherein: said first grating
comprises first and second grating prescriptions; and said second
grating comprises third and fourth grating prescriptions; wherein:
said first grating prescription is configured to provide said beam
expansion in said first direction for said first field-of-view
portion of light; said second grating prescription is configured to
provide beam expansion in said first direction and beam extraction
towards a viewer for said second field-of-view portion of light;
said third grating prescription is configured to provide said beam
expansion in said second direction for said second field-of-view
portion of light; and said fourth grating prescription is
configured to provide said beam expansion in said second direction
and beam extraction towards a viewer for said first field-of-view
portion of light.
3. The waveguide display of claim 2, wherein said first and second
grating prescriptions are at least partially multiplexed; and said
third and fourth grating prescriptions are at least partially
multiplexed.
4. The waveguide display of claim 3, wherein said first grating at
least partially overlaps said second grating.
5. The waveguide display of claim 4, wherein: said first waveguide
comprises first and second grating layers; said first grating is
disposed within said first grating layer; and said second grating
is disposed within said second grating layer.
6. The waveguide display of claim 5, wherein said first waveguide
further comprises a transparent layer disposed between and adjacent
said first and second grating layers.
7. The waveguide display of claim 6, further comprising a second
waveguide; wherein said first waveguide is configured to couple in
a first spectral band of light; and said second waveguide is
configured to couple in a second spectral band of light.
8. The waveguide display of claim 1, wherein said input coupler
comprises an input configuration selected from the group consisting
of: an input prism; an input grating; first and second input
gratings; and an input grating comprising two multiplexed grating
prescriptions.
9. The waveguide display of claim 1, wherein said grating vector of
said input coupler has a different magnitude than said grating
vector of said first grating.
10. The waveguide display of claim 1, wherein said source of light
provides at least two different wavelengths of light.
11. A method of displaying an image, the method comprising:
providing a waveguide display comprising a first waveguide
supporting an input coupler and a grating structure comprising
first and second gratings, wherein said input coupler, said first
grating, and said second grating each comprises a grating vector,
wherein said grating vectors of said input coupler, said first
grating, and said second grating provide a resultant vector with
substantially zero magnitude; coupling a first field of view
portion into said waveguide via said input coupler; coupling a
second field-of-view portion into said waveguide via said input
coupler; expanding said first field-of-view portion light in a
first direction using said first grating; expanding said first
field-of-view portion light in a second direction and extracting it
from said waveguide using said second grating; expanding said
second field-of-view portion light in said second direction using
said second grating; and expanding said second field of view
portion light in said first direction and extracting it from said
waveguide using said first grating.
12. The method of claim 11, wherein: said first grating comprises
first and second grating prescriptions; and said second grating
comprises third and fourth grating prescriptions; wherein: said
first field-of-view portion light is expanded in said first
direction using said first grating prescription; said second
field-of-view portion light is expanded in said first direction and
extracted from said waveguide using said second grating
prescription; said second field-of-view portion light is expanded
in said second direction using said third grating prescription; and
said first field-of-view portion light is expanded in said second
direction and extracted from said waveguide using said fourth
grating prescription.
13. The method of claim 12, wherein said first and second grating
prescriptions are at least partially multiplexed; and said third
and fourth grating prescriptions are at least partially
multiplexed.
14. The method of claim 13, wherein said first grating at least
partially overlaps said second grating.
15. The method of claim 14, wherein: said first waveguide comprises
first and second grating layers; said first grating is disposed
within said first grating layer; and said second grating is
disposed within said second grating layer.
16. The method of claim 15, wherein said first waveguide further
comprises a transparent layer disposed between and adjacent said
first and second grating layers.
17. The method of claim 16, wherein said waveguide display further
comprises a second waveguide; wherein said first waveguide is
configured to couple in a first spectral band of light; and said
second waveguide is configured to couple in a second spectral band
of light.
18. The method of claim 11, wherein said input coupler comprises an
input configuration selected from the group consisting of: an input
prism; an input grating; first and second input gratings; and an
input grating comprising two multiplexed grating prescriptions.
19. The method of claim 11, wherein said grating vector of said
input coupler has a different magnitude than said grating vector of
said first grating.
20. The method of claim 11, wherein said source of light provides
at least two different wavelengths of light.
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. 62/806,665 entitled "Methods and Apparatuses for
Providing a Color Holographic Waveguide Display Using Overlapping
Bragg Gratings," filed Feb. 15, 2019 and U.S. Provisional Patent
Application No. 62/813,373 entitled "Improvements to Methods and
Apparatuses for Providing a Color Holographic Waveguide Display
Using Overlapping Bragg Gratings," filed Mar. 4, 2019. The
disclosure of U.S. Provisional Patent Application Nos. 62/806,665
and 62/813,373 are hereby incorporated by reference in their
entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention generally relates to waveguide devices
and, more specifically, to holographic waveguide displays.
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 subclass includes
optical waveguides, which are structures that can guide
electromagnetic waves, typically those in the visible spectrum.
Waveguide structures can be designed to control the propagation
path of waves using 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. The resulting grating, which is commonly referred to as a
switchable Bragg grating (SBG), has all the properties normally
associated with volume or Bragg gratings but with much higher
refractive index modulation ranges combined with the ability to
electrically tune the grating over a continuous range of
diffraction efficiency (the proportion of incident light diffracted
into a desired direction). The latter can extend from
non-diffracting (cleared) to diffracting with close to 100%
efficiency.
[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 head-up displays (HUDs) and
helmet-mounted displays or head-mounted displays (HMDs) for road
transport, aviation, and military applications, and sensors for
biometric and laser radar (LIDAR) applications.
SUMMARY OF THE INVENTION
[0006] Systems and methods for providing holographic waveguide
display using integrated gratings in accordance with various
embodiments of the invention are illustrated. One embodiment
includes a waveguide display including a source of light, and a
first waveguide including a grating structure including first and
second gratings, and an input coupler configured to couple a first
field-of-view portion of light from the source of light into the
first waveguide and towards the first grating, and couple a second
field-of-view portion of light from the source of light into the
first waveguide and towards the second grating, wherein the first
grating is configured to provide beam expansion in a first
direction for the first field-of-view portion of light, and provide
beam expansion in the first direction and beam extraction towards a
viewer for the second field-of-view portion of light, the second
grating is configured to provide beam expansion in a second
direction for the second field-of-view portion of light, and
provide beam expansion in the second direction and beam extraction
towards a viewer for the first field-of-view portion of light, the
input coupler, the first grating, and the second grating each
includes a grating vector, and the grating vectors of the input
coupler, the first grating, and the second grating provide a
resultant vector with substantially zero magnitude.
[0007] In another embodiment, the first grating includes first and
second grating prescriptions, and the second grating includes third
and fourth grating prescriptions, wherein the first grating
prescription is configured to provide the beam expansion in the
first direction for the first field-of-view portion of light, the
second grating prescription is configured to provide beam expansion
in the first direction and beam extraction towards a viewer for the
second field-of-view portion of light, the third grating
prescription is configured to provide the beam expansion in the
second direction for the second field-of-view portion of light, and
the fourth grating prescription is configured to provide the beam
expansion in the second direction and beam extraction towards a
viewer for the first field-of-view portion of light.
[0008] In a further embodiment, the first and second grating
prescriptions are at least partially multiplexed, and the third and
fourth grating prescriptions are at least partially
multiplexed.
[0009] In still another embodiment, the first grating at least
partially overlaps the second grating.
[0010] In a still further embodiment, the first waveguide includes
first and second grating layers, the first grating is disposed
within the first grating layer, and the second grating is disposed
within the second grating layer.
[0011] In yet another embodiment, the first waveguide further
includes a transparent layer disposed between and adjacent the
first and second grating layers.
[0012] In a yet further embodiment, the waveguide display further
includes a second waveguide, wherein the first waveguide is
configured to couple in a first spectral band of light, and the
second waveguide is configured to couple in a second spectral band
of light.
[0013] In another additional embodiment, wherein the input coupler
includes an input configuration selected from at least one of an
input prism, an input grating, first and second input gratings, and
an input grating including two multiplexed grating
prescriptions.
[0014] In a further additional embodiment, the grating vector of
the input coupler has a different magnitude than the grating vector
of the first grating.
[0015] In another embodiment again, the source of light provides at
least two different wavelengths of light.
[0016] In a further embodiment again, a method of displaying an
image, the method including providing a waveguide display including
a first waveguide supporting an input coupler and a grating
structure including first and second gratings, wherein the input
coupler, the first grating, and the second grating each includes a
grating vector, wherein the grating vectors of the input coupler,
the first grating, and the second grating provide a resultant
vector with substantially zero magnitude, coupling a first field of
view portion into the waveguide via the input coupler, coupling a
second field-of-view portion into the waveguide via the input
coupler, expanding the first field-of-view portion light in a first
direction using the first grating, expanding the first
field-of-view portion light in a second direction and extracting it
from the waveguide using the second grating, expanding the second
field-of-view portion light in the second direction using the
second grating, and expanding the second field of view portion
light in the first direction and extracting it from the waveguide
using the first grating.
[0017] In still yet another embodiment, the first grating includes
first and second grating prescriptions, and the second grating
includes third and fourth grating prescriptions, wherein the first
field-of-view portion light is expanded in the first direction
using the first grating prescription, the second field-of-view
portion light is expanded in the first direction and extracted from
the waveguide using the second grating prescription, the second
field-of-view portion light is expanded in the second direction
using the third grating prescription, and the first field-of-view
portion light is expanded in the second direction and extracted
from the waveguide using the fourth grating prescription.
[0018] In a still yet further embodiment, the first and second
grating prescriptions are at least partially multiplexed, and the
third and fourth grating prescriptions are at least partially
multiplexed.
[0019] In still another additional embodiment, the first grating at
least partially overlaps the second grating.
[0020] In a still further additional embodiment, the first
waveguide includes first and second grating layers, the first
grating is disposed within the first grating layer, and the second
grating is disposed within the second grating layer.
[0021] In still another embodiment again, the first waveguide
further includes a transparent layer disposed between and adjacent
the first and second grating layers.
[0022] In a still further embodiment again, the waveguide display
further includes a second waveguide, wherein the first waveguide is
configured to couple in a first spectral band of light, and the
second waveguide is configured to couple in a second spectral band
of light.
[0023] In yet another additional embodiment, the input coupler
includes an input configuration selected from at least one of an
input prism, an input grating, first and second input gratings, and
an input grating comprising two multiplexed grating
prescriptions.
[0024] In a yet further additional embodiment, the grating vector
of the input coupler has a different magnitude than the grating
vector of the first grating.
[0025] In yet another embodiment again, the source of light
provides at least two different wavelengths of light.
[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.
[0028] FIG. 1 conceptually illustrates a waveguide display in
accordance with an embodiment of the invention.
[0029] FIG. 2 conceptually illustrates a color waveguide display
having two blue-green diffracting waveguides and two green-red
diffracting waveguides in accordance with an embodiment of the
invention.
[0030] FIGS. 3A-3C conceptually illustrate integrated gratings in
accordance with various embodiments of the invention.
[0031] FIGS. 4A-4C schematically illustrate ray propagation through
a grating structure having an input grating and two integrated
gratings in accordance with an embodiment of the invention.
[0032] FIGS. 5A-5E conceptually illustrate various grating vector
configurations in accordance with various embodiments of the
invention.
[0033] FIG. 6 conceptually illustrates a schematic plan view of a
grating architecture having an input grating and integrated
gratings in accordance with an embodiment of the invention.
[0034] FIG. 7 shows a flow diagram conceptually illustrating a
method of displaying an image in accordance with an embodiment of
the invention.
[0035] FIG. 8 shows a flow diagram conceptually illustrating a
method of displaying an image utilizing integrated gratings
containing multiple gratings in accordance with an embodiment of
the invention.
[0036] FIG. 9 conceptually illustrates a profile view of two
overlapping waveguide portions implementing integrated gratings in
accordance with an embodiment of the invention.
[0037] FIG. 10 conceptually illustrates a schematic plan view of a
grating architecture having two sets of integrated gratings in
accordance with an embodiment of the invention.
[0038] FIG. 11 conceptually illustrates a plot of diffraction
efficiency versus angle for a waveguide for diffractions occurring
at different field-of-view angles in accordance with an embodiment
of the invention.
[0039] FIG. 12 shows the viewing geometry provided by a waveguide
in accordance with an embodiment of the invention.
[0040] FIG. 13 conceptually illustrates the field-of-view geometry
for a binocular display with binocular overlap between the left and
right eye images provided by a waveguide in accordance with an
embodiment of the invention.
DETAILED DESCRIPTION
[0041] 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
electromagnetic radiation along rectilinear trajectories. The term
light and illumination may be used in relation to the visible and
infrared bands of the electromagnetic spectrum. Parts of the
following description will be presented using terminology commonly
employed by those skilled in the art of optical design. As used
herein, the term grating may encompass a grating comprised of a set
of gratings in some embodiments. For illustrative purposes, it is
to be understood that the drawings are not drawn to scale unless
stated otherwise.
[0042] Waveguide displays in accordance with various embodiments of
the invention can be implemented using many different techniques.
Waveguide technology can enable low cost, efficient, and versatile
diffractive optical solutions for many different applications. One
commonly used waveguide architecture includes an input grating for
coupling light from an image source into a TIR path in the
waveguide, a fold grating for providing beam expansion in a first
direction, and an output grating for providing a second beam
expansion in a direction orthogonal to the first direction and
extracting the pupil-expanded beam from the waveguide for viewing
from an exit pupil or eyebox. While effective at two-dimensional
beam expansion and extraction, this arrangement typically demands a
large grating area. When used with birefringent gratings, this
architecture can also suffer from haze that arises from millions of
grating interactions in the fold. A further issue is image
nonuniformity due to longer light paths incurring more beam
interactions with the substrates of the waveguide. As such, many
embodiments of the invention are directed towards wide angle, low
cost, efficient, and compact waveguide displays.
[0043] In many embodiments, the waveguide display includes at least
one input grating and at least two integrated gratings, each
capable of performing the functions of traditional fold and output
gratings. In further embodiments, a single multiplexed input
grating is implemented to provide input light with two bifurcated
paths. In other embodiments, two input gratings are implemented to
provide bifurcated optical paths. In addition to the different
configurations of the input grating(s), the integrated gratings can
also be configured in various ways. In some embodiments, the
integrated gratings contain crossed grating vectors and can be
configured to provide beam expansion in two directions and beam
extraction for light coming from the input grating(s). In several
embodiments, the integrated gratings are configured as overlapping
gratings with crossed grating vectors. The integrated nature of the
grating architecture can allow for a compact waveguide display that
is suitable for various applications, including but not limited to
AR, VR, HUD, and LIDAR applications. As can readily be appreciated,
the specific architecture and implementation of the waveguide
display can depend on the specific requirements of a given
application. For example, in some embodiments, the waveguide
display is implemented with integrated gratings to provide a
binocular field-of-view of at least 50.degree. diagonal. In further
embodiments, the waveguide display is implemented with integrated
gratings to provide a binocular field-of-view of at least
.about.100.degree. diagonal. Waveguide displays, grating
architecture, HPDLC materials, and manufacturing processes in
accordance with various embodiments of the invention are discussed
below in further detail.
Optical Waveguide and Grating Structures
[0044] Optical structures recorded in waveguides can include many
different types of optical elements, such as but not limited to
diffraction gratings. Gratings can be implemented to perform
various optical functions, including but not limited to coupling
light, directing light, and preventing the transmission of light.
In many embodiments, the gratings are surface relief gratings that
reside on the outer surface of the waveguide. In other embodiments,
the grating implemented is a Bragg grating (also referred to as a
volume grating), which are structures having a periodic refractive
index modulation. Bragg gratings can be fabricated using a variety
of different methods. One process includes interferential exposure
of holographic photopolymer materials to form periodic structures.
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, a property that can be
used to make lossy waveguide gratings for extracting light over a
large pupil.
[0045] One class of Bragg gratings used in holographic waveguide
devices is the Switchable Bragg Grating (SBG). SBGs can be
fabricated by first placing a thin film of a mixture of
photopolymerizable monomers and liquid crystal material between
substrates. The substrates can be made of various types of
materials, such glass and plastics. In many cases, the substrates
are in a parallel configuration. In other embodiments, the
substrates form a wedge shape. One or both substrates can support
electrodes, typically transparent tin oxide films, for applying an
electric field across the film. The grating structure in an SBG can
be recorded in the liquid material (often referred to as the syrup)
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 materials
in the mixture, and exposure temperature can determine the
resulting grating morphology and performance. As can readily be
appreciated, a wide variety of materials and mixtures can be used
depending on the specific requirements of a given application. In
many embodiments, HPDLC material is used. 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.
[0046] The resulting volume phase grating can exhibit very high
diffraction efficiency, which can be controlled by the magnitude of
the electric field applied across the film. When an electric field
is applied to the grating via transparent electrodes, the natural
orientation of the LC droplets can change, causing the refractive
index modulation of the fringes to lower and the hologram
diffraction efficiency to drop to very low levels. Typically, the
electrodes are configured such that the applied electric field will
be perpendicular to the substrates. In a number of embodiments, the
electrodes are fabricated from indium tin oxide (ITO). In the OFF
state with no electric field applied, the extraordinary axis of the
liquid crystals generally aligns normal to the fringes. The grating
thus exhibits high refractive index modulation and high diffraction
efficiency for P-polarized light. When an electric field is applied
to the HPDLC, the grating switches to the ON state wherein the
extraordinary axes of the liquid crystal molecules align parallel
to the applied field and hence perpendicular to the substrate. In
the ON state, the grating exhibits lower refractive index
modulation and lower diffraction efficiency for both S- and
P-polarized light. Thus, the grating region no longer diffracts
light. Each grating region can be divided into a multiplicity of
grating elements such as for example a pixel matrix according to
the function of the HPDLC device. Typically, the electrode on one
substrate surface is uniform and continuous, while electrodes on
the opposing substrate surface are patterned in accordance to the
multiplicity of selectively switchable grating elements.
[0047] Typically, the SBG elements are switched clear in 30 .mu.s
with a longer relaxation time to switch ON. The diffraction
efficiency of the device can be adjusted, by means of the applied
voltage, over a continuous range. In many cases, the device
exhibits near 100% efficiency with no voltage applied and
essentially zero efficiency with a sufficiently high voltage
applied. In certain types of HPDLC devices, magnetic fields can be
used to control the LC orientation. In some HPDLC applications,
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 substrates used to form the HPDLC cell provide a
total internal reflection (TIR) 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.
[0048] In some embodiments, LC can be extracted or evacuated from
the SBG to provide a surface relief grating (SRG) that has
properties very similar to a Bragg grating due to the depth of the
SRG structure (which is much greater than that practically
achievable using surface etching and other conventional processes
commonly used to fabricate SRGs). The LC can be extracted using a
variety of different methods, including but not limited to flushing
with isopropyl alcohol and solvents. In many embodiments, one of
the transparent substrates of the SBG is removed, and the LC is
extracted. In further embodiments, the removed substrate is
replaced. The SRG can be at least partially backfilled with a
material of higher or lower refractive index. Such gratings offer
scope for tailoring the efficiency, angular/spectral response,
polarization, and other properties to suit various waveguide
applications.
[0049] Waveguides in accordance with various embodiments of the
invention can include various grating configurations designed for
specific purposes and functions. In many embodiments, the waveguide
is designed to implement a grating configuration capable of
preserving eyebox size while reducing lens size by effectively
expanding the exit pupil of a collimating optical system. The exit
pupil can be defined as a virtual aperture where only the light
rays which pass though this virtual aperture can enter the eyes of
a user. In some embodiments, the waveguide includes an input
grating optically coupled to a light source, a fold grating for
providing a first direction beam expansion, and an output grating
for providing beam expansion in a second direction, which is
typically orthogonal to the first direction, and beam extraction
towards the eyebox. As can readily be appreciated, the grating
configuration implemented waveguide architectures can depend on the
specific requirements of a given application. In some embodiments,
the grating configuration includes multiple fold gratings. In
several embodiments, the grating configuration includes an input
grating and a second grating for performing beam expansion and beam
extraction simultaneously. The second grating can include gratings
of different prescriptions, for propagating different portions of
the field-of-view, arranged in separate overlapping grating layers
or multiplexed in a single grating layer. Furthermore, various
types of gratings and waveguide architectures can also be
utilized.
[0050] In several embodiments, the gratings within each layer are
designed to have different spectral and/or angular responses. For
example, in many embodiments, different gratings across different
grating layers are overlapped, or multiplexed, to provide an
increase in spectral bandwidth. In some embodiments, a full color
waveguide is implemented using three grating layers, each designed
to operate in a different spectral band (red, green, and blue). In
other embodiments, a full color waveguide is implemented using two
grating layers, a red-green grating layer and a green-blue grating
layer. As can readily be appreciated, such techniques can be
implemented similarly for increasing angular bandwidth operation of
the waveguide. In addition to the multiplexing of gratings across
different grating layers, multiple gratings can be multiplexed
within a single grating layer--i.e., multiple gratings can be
superimposed within the same volume. In several embodiments, the
waveguide includes at least one grating layer having two or more
grating prescriptions multiplexed in the same volume. In further
embodiments, the waveguide includes two grating layers, each layer
having two grating prescriptions multiplexed in the same volume.
Multiplexing two or more grating prescriptions within the same
volume can be achieved using various fabrication techniques. In a
number of embodiments, a multiplexed master grating is utilized
with an exposure configuration to form a multiplexed grating. In
many embodiments, a multiplexed grating is fabricated by
sequentially exposing an optical recording material layer with two
or more configurations of exposure light, where each configuration
is designed to form a grating prescription. In some embodiments, a
multiplexed grating is fabricated by exposing an optical recording
material layer by alternating between or among two or more
configurations of exposure light, where each configuration is
designed to form a grating prescription. As can readily be
appreciated, various techniques, including those well known in the
art, can be used as appropriate to fabricate multiplexed
gratings.
[0051] In many embodiments, the waveguide can incorporate at least
one of: angle multiplexed gratings, color multiplexed gratings,
fold gratings, dual interaction gratings, rolled K-vector gratings,
crossed fold gratings, tessellated gratings, chirped gratings,
gratings with spatially varying refractive index modulation,
gratings having spatially varying grating thickness, gratings
having spatially varying average refractive index, gratings with
spatially varying refractive index modulation tensors, and gratings
having spatially varying average refractive index tensors. In some
embodiments, the waveguide can incorporate at least one of: a half
wave plate, a quarter wave plate, an anti-reflection coating, a
beam splitting layer, an alignment layer, a photochromic back layer
for glare reduction, and louvre films for glare reduction. In
several embodiments, the waveguide can support gratings providing
separate optical paths for different polarizations. In various
embodiments, the waveguide can support gratings providing separate
optical paths for different spectral bandwidths. In a number of
embodiments, the gratings can be HPDLC gratings, switching gratings
recorded in HPDLC (such switchable Bragg Gratings), Bragg gratings
recorded in holographic photopolymer, or surface relief gratings.
In many embodiments, the waveguide operates in a monochrome band.
In some embodiments, the waveguide operates in the green band. In
several embodiments, waveguide layers operating in different
spectral bands such as red, green, and blue (RGB) can be stacked to
provide a three-layer waveguiding structure. In further
embodiments, the layers are stacked with air gaps between the
waveguide layers. In various embodiments, the waveguide layers
operate in broader bands such as blue-green and green-red to
provide two-waveguide layer solutions. In other embodiments, the
gratings are color multiplexed to reduce the number of grating
layers. Various types of gratings can be implemented. In some
embodiments, at least one grating in each layer is a switchable
grating.
[0052] Waveguides incorporating optical structures such as those
discussed above can be implemented in a variety of different
applications, including but not limited to waveguide displays. In
various embodiments, the waveguide display is implemented with an
eyebox of greater than 10 mm with an eye relief greater than 25 mm.
In some embodiments, the waveguide display includes a waveguide
with a thickness between 2.0-5.0 mm. In many embodiments, the
waveguide display can provide an image field-of-view of at least
50.degree. diagonal. In further embodiments, the waveguide display
can provide an image field-of-view of at least 70.degree. diagonal.
The waveguide display can employ many different types of picture
generation units (PGUs). In several embodiments, the PGU can be a
reflective or transmissive spatial light modulator such as a liquid
crystal on Silicon (LCoS) panel or a micro electromechanical system
(MEMS) panel. In a number of embodiments, the PGU can be an
emissive device such as an organic light emitting diode (OLED)
panel. In some embodiments, an OLED display can have a luminance
greater than 4000 nits and a resolution of 4 k.times.4 k pixels. In
several embodiments, the waveguide can have an optical efficiency
greater than 10% such that a greater than 400 nit image luminance
can be provided using an OLED display of luminance 4000 nits.
Waveguides implementing P-diffracting gratings (i.e., gratings with
high efficiency for P-polarized light) typically have a waveguide
efficiency of 5%-6.2%. Since P-diffracting or S-diffracting
gratings can waste half of the light from an unpolarized source
such as an OLED panel, many embodiments are directed towards
waveguides capable of providing both S-diffracting and
P-diffracting gratings to allow for an increase in the efficiency
of the waveguide by up to a factor of two. In some embodiments, the
S-diffracting and P-diffracting gratings are implemented in
separate overlapping grating layers. Alternatively, a single
grating can, under certain conditions, provide high efficiency for
both p-polarized and s-polarized light. In several embodiments, the
waveguide includes Bragg-like gratings produced by extracting LC
from HPDLC gratings, such as those described above, to enable high
S and P diffraction efficiency over certain wavelength and angle
ranges for suitably chosen values of grating thickness (typically,
in the range 2-5 .mu.m).
Optical Recording Material Systems
[0053] HPDLC mixtures 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.|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 having: 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.
[0054] The patent and scientific literature contains many examples
of material systems and processes that can be used to fabricate
SBGs, including investigations into formulating such material
systems 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 SBG devices. Examples of recipes can also be found
in papers dating back to the early 1990s. Many of these materials
use acrylate monomers, including: [0055] 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 comprises 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. [0056] 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. [0057] 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. [0058] 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. [0059] 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. [0060] 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.
[0061] 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.
Modulation of Material Composition
[0062] High luminance and excellent color fidelity are important
factors in AR waveguide displays. In each case, high uniformity
across the FOV can be desired. 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. In applicable
cases, the biggest challenge is usually 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.
[0063] 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/deposited. The resolution of the deposition
mechanism can depend on the technical limitations of the system
utilized. In many embodiments, the spatial pattern can be
implemented to 30 micrometers resolution with full
repeatability.
[0064] 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 but not limited to refractive index modulation
and grating thickness, to be adjusted dynamically during the
deposition process without the need for a different master. 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.
[0065] 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. Various embodiments of the invention can be configured
to deposit different materials, or different material compositions,
in different areas on the substrate. 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
non-grating 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. The
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 non-grating areas of a
waveguide cell. In some embodiments, HPDLC material is deposited
onto the grating regions while only monomer is deposited onto the
non-grating regions. In several embodiments, the deposition
mechanism can be configured to deposit mixtures with different
component compositions.
[0066] 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. In many embodiments, the spraying mechanism is
configured for printing gratings in which at least one the material
composition, birefringence, and/or thickness can be controlled
using a deposition apparatus having at least two selectable spray
heads. In some embodiments, the manufacturing system provides an
apparatus for depositing grating recording material optimized for
the control of laser banding. In several embodiments, the
manufacturing system provides an apparatus for depositing grating
recording material optimized for the control of polarization
non-uniformity. In several embodiments, the manufacturing system
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. Inkjet print heads can also be
implemented to print different materials in different regions of
the substrate.
[0067] 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 printing of dots in a two-material application is
essentially a binary system, averaging the printed dots across an
area can allow for discretization of a sliding scale of ratios of
the two materials to be printed. For example, 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. As such, there can be a range of different
concentration combinations, ranging from 100% of the first material
to 100% of the second material. As can readily be appreciated, 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, the concept of modulating material composition
using inkjet print heads can be expanded to use more than two
different material reservoirs and can vary in precision levels,
which largely depends on the types of print heads used.
[0068] 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 the formation of a waveguide with gratings that have spatially
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 the material is 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. 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.
[0069] The ability to vary the diffraction efficiency across a
waveguide can be used for various purposes. A waveguide is
typically designed to guide light internally by reflecting the
light many times between the two planar surfaces of the waveguide.
These multiple reflections can allow for the light path to interact
with a grating multiple times. In many embodiments, a layer of
material can be printed with varying composition of materials such
that the gratings formed have spatially 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 efficiency 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.
[0070] 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-layered 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 three-layered stack to implement a full
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.
[0071] 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 spacers. In many
embodiments, spacers are dispersed throughout the optical recording
material for structural support during the construction of the
waveguide cell. In some embodiments, different sizes of spacers are
dispersed throughout the optical recording material. The spacers
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 spacers, create a wedge-shaped layer of optical
recording material. spacers of varying sizes can be dispersed
similar to the modulation process described above. Additionally,
modulating spacer sizes can be combined with modulation of material
compositions. In several embodiments, reservoirs of HPDLC materials
each suspended with spacers of different sizes are used to print a
layer of HPDLC material with spacers of varying sizes strategically
dispersed to form a wedge-shaped waveguide cell. In a number of
embodiments, spacer size modulation is combined with material
composition modulation by providing a number of reservoirs equal to
the product of the number of different sizes of spacers 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 spacer sizes. In such an
embodiment, four reservoirs can be prepared: a liquid crystal-free
mixture suspension with spacers of a first size, a liquid
crystal-free mixture-suspension with spacers of a second size, a
liquid crystal-rich mixture-suspension with spacers of a first
size, and a liquid crystal-rich mixture-suspension with spacers of
a second size. Further discussion regarding material modulation can
be found in U.S. application Ser. No. 16/203,071 filed Nov. 18,
2018 entitled "SYSTEMS AND METHODS FOR MANUFACTURING WAVEGUIDE
CELLS." The disclosure of U.S. application Ser. No. 16/203,491 is
hereby incorporated by reference in its entirety for all
purposes.
Multi-Layered Waveguide Fabrication
[0072] Waveguide manufacturing in accordance with various
embodiments of the invention can be implemented for the fabrication
of multi-layered waveguides. Multi-layered waveguides refer to a
class of waveguides that utilizes two or more layers having
gratings or other optical structures. Although the discussions
below may pertain to gratings, any type of holographic optical
structure can be implemented and substituted as appropriate.
Multi-layered waveguides can be implemented for various purposes,
including but not limited to improving spectral and/or angular
bandwidths. Traditionally, multi-layered waveguides are formed by
stacking and aligning waveguides having a single grating layer. In
such cases, each grating layer is typically bounded by a pair of
transparent substrates. To maintain the desired total internal
reflection characteristics, the waveguides are usually stacked
using spacers to form air gaps between the individual
waveguides.
[0073] In contrast to traditional stacked waveguides, many
embodiments of the invention are directed to the manufacturing of
multi-layered waveguides having alternating substrate layers and
grating layers. Such waveguides can be fabricated with an iterative
process capable of sequentially forming grating layers for a single
waveguide. In several embodiments, the multi-layered waveguide is
fabricated with two grating layers. In a number of embodiments, the
multi-layered waveguide is fabricated with three grating layers.
Any number of grating layers can be formed, limited by the tools
utilized and/or waveguide design. Compared to traditional
multi-layered waveguides, this allows for a reduction in thickness,
materials, and costs as fewer substrates are needed. Furthermore,
the manufacturing process for such waveguides allow for a higher
yield in production due to simplified alignment and substrate
matching requirements.
[0074] Manufacturing processes for multi-layered waveguides having
alternating transparent substrate layers and grating layers in
accordance with various embodiments of the invention can be
implemented using a variety of techniques. In many embodiments, the
manufacturing process includes depositing a first layer of optical
recording material onto a first transparent substrate. Optical
recording material can include various materials and mixtures,
including but not limited to HPDLC mixtures and any of the material
formulations discussed in the sections above. Similarly, any of a
variety of deposition techniques, such as but not limited to
spraying, spin coating, inkjet printing, and any of the techniques
described in the sections above, can be utilized. Transparent
substrates of various shapes, thicknesses, and materials can be
utilized. Transparent substrates can include but are not limited to
glass substrates and plastic substrates. Depending on the
application, the transparent substrates can be coated with
different types of films for various purposes. Once the deposition
process is completed, a second transparent substrate can then be
placed onto the deposited first layer of optical recording
material. In some embodiments, the process includes a lamination
step to form the three-layer composite into a desired
height/thickness. An exposure process can be implemented to form a
set of gratings within the first layer of optical recording
material. Exposure processes, such as but not limited to
single-beam interferential exposure and any of the other exposure
processes described in the sections above, can be utilized. In
essence, a single-layered waveguide is now formed. The process can
then repeat to add on additional layers to the waveguide. In
several embodiments, a second layer of optical recording material
is deposited onto the second transparent substrate. A third
transparent substrate can be placed onto the second layer of
optical recording material. Similar to the previous steps, the
composite can be laminated to a desired height/thickness. A second
exposure process can then be performed to form a set of gratings
within the second layer of optical recording material. The result
is a waveguide having two grating layers. As can readily be
appreciated, the process can continue iteratively to add additional
layers. The additional optical recording layers can be added onto
either side of the current laminate. For instance, a third layer of
optical recording material can be deposited onto the outer surface
of either the first transparent substrate or the third transparent
substrate.
[0075] In many embodiments, the manufacturing process includes one
or more post processing steps. Post processing steps such as but
not limited to planarization, cleaning, application of protective
coats, thermal annealing, alignment of LC directors to achieve a
desired birefringence state, extraction of LC from recorded SBGs
and refilling with another material, etc. can be carried out at any
stage of the manufacturing process. Some processes such as but not
limited to waveguide dicing (where multiple elements are being
produced), edge finishing, AR coating deposition, final protective
coating application, etc. are typically carried out at the end of
the manufacturing process.
[0076] In many embodiments, spacers, such as but not limited to
beads and other particles, are dispersed throughout the optical
recording material to help control and maintain the thickness of
the layer of optical recording material. The spacers can also help
prevent the two substrates from collapsing onto one another. In
some embodiments, the waveguide cell is constructed with an optical
recording layer sandwiched between two planar substrates. Depending
on the type of optical recording material used, thickness control
can be difficult to achieve due to the viscosity of some optical
recording materials and the lack of a bounding perimeter for the
optical recording layer. In a number of embodiments, the spacers
are relatively incompressible solids, which can allow for the
construction of waveguide cells with consistent thicknesses. The
spacers can take any suitable geometry, including but not limited
to rods and spheres. The size of a spacer can determine a localized
minimum thickness for the area around the individual spacer. As
such, the dimensions of the spacers can be selected to help attain
the desired optical recording layer thickness. The spacers can take
any suitable size. In many cases, the sizes of the spacers range
from 1 to 30 .mu.m. The spacers can be made of any of a variety of
materials, including but not limited to plastics (e.g.,
divinylbenzene), silica, and conductive materials. In several
embodiments, the material of the spacers is selected such that its
refractive index does not substantially affect the propagation of
light within the waveguide cell.
[0077] In many embodiments, the first layer of optical recording
material is incorporated between the first and second transparent
substrates using vacuum filling methods. In a number of
embodiments, the layer of optical recording materials is separated
in different sections, which can be filled or deposited as
appropriate depending on the specific requirements of a given
application. In some embodiments, the manufacturing system is
configured to expose the optical recording material from below. In
such embodiments, the iterative multi-layered fabrication process
can include turning over the current device such that the exposure
light is incident on a newly deposited optical recording layer
before it is incident on any formed grating layers.
[0078] In many embodiments, the exposing process can include
temporarily "erasing" or making transparent the previously formed
grating layer such that they will not interfere with the recording
process of the newly deposited optical recording layer. Temporarily
"erased" gratings or other optical structures can behave similar to
transparent materials, allowing light to pass through without
affecting the ray paths. Methods for recording gratings into layers
of optical recording material using such techniques can include
fabricating a stack of optical structures in which a first optical
recording material layer deposited on a substrate is exposed to
form a first set of gratings, which can be temporarily erased so
that a second set of gratings can be recorded into a second optical
recording material layer using optical recording beams traversing
the first optical recording material layer. Although the recording
methods are discussed primarily with regards to waveguides with two
grating layers, the basic principle can be applied to waveguides
with more than two grating layers.
[0079] Multi-layered waveguide fabrication processes incorporating
steps of temporarily erasing a grating structure can be implemented
in various ways. Typically, the first layer is formed using
conventional methods. The recording material utilized can include
material systems capable of supporting optical structures that can
be erased in response to a stimulus. In embodiments in which the
optical structure is a holographic grating, the exposure process
can utilize a crossed-beam holographic recording apparatus. In a
number of embodiments, the optical recording process uses beams
provided by a master grating, which may be a Bragg hologram
recorded in a photopolymer or an amplitude grating. In some
embodiments, the exposure process utilizes a single recording beam
in conjunction with a master grating to form an interferential
exposure beam. In addition to the processes described, other
industrial processes and apparatuses currently used in the field to
fabricate holograms can be used.
[0080] Once a first set of gratings is recorded, additional
material layers can be added similar to the processes described
above. During the exposure process of any material layer after the
first material layer, an external stimulus can be applied to any
previously formed gratings to render them effectively transparent.
The effectively transparent grating layers can allow for light to
pass through to expose the new material layer. External
stimulus/stimuli can include optical, thermal, chemical,
mechanical, electrical, and/or magnetic stimuli. In many
embodiments, the external stimulus is applied at a strength below a
predefined threshold to produce optical noise below a predefined
level. The specific predefined threshold can depend on the type of
material used to form the gratings. In some embodiments, a
sacrificial alignment layer applied to the first material layer can
be used to temporarily erase the first set of gratings. In some
embodiments, the strength of the external stimulus applied to the
first set of gratings is controlled to reduced optical noise in the
optical device during normal operation. In several embodiments, the
optical recording material further includes an additive for
facilitating the process of erasing the gratings, which can include
any of the methods described above. In a number of embodiments, a
stimulus is applied for the restoration of an erased layer.
[0081] The clearing and restoration of a recorded layer described
in the process above can be achieved using many different methods.
In many embodiments, the first layer is cleared by applying a
stimulus continuously during the recording of the second layer. In
other embodiments, the stimulus is initially applied, and the
grating in the cleared layer can naturally revert to its recorded
state over a timescale that allows for the recording of the second
grating. In other embodiments, the layer stays cleared after
application of an external stimulus and reverts in response to
another external stimulus. In several embodiments, the restoration
of the first optical structure to its recorded state can be carried
out using an alignment layer or an external stimulus. An external
stimulus used for such restoration can be any of a variety of
different stimuli, including but not limited to the
stimulus/stimuli used to clear the optical structure. Depending on
the composition material of the optical structure and layer to be
cleared, the clearing process can vary. Further discussion
regarding the multi-layered waveguide fabrication utilizing
external stimuli can be found in U.S. application Ser. No.
16/522,491 filed Jul. 25, 2019 entitled "Systems and Methods for
Fabricating a Multilayer Optical Structure." The disclosure of U.S.
application Ser. No. 16/522,491 is hereby incorporated by reference
in its entirety for all purposes.
Waveguides Incorporating Integrated Gratings
[0082] Waveguides in accordance with various embodiments of the
invention can include different grating configurations. In many
embodiments, the waveguide includes at least one input coupler and
at least two integrated gratings. In some embodiments, at least two
integrated gratings can be implemented to work in combination to
provide beam expansion and beam extraction for light coupled into
the waveguide by the input coupler. Multiple integrated gratings
can be implemented by overlapping integrated gratings across
different grating layers or by multiplexing the integrated
gratings. In a number of embodiments, the integrated gratings are
partially overlapped or multiplexed. Multiplexed gratings can
include the superimposition of at least two gratings having
different grating prescriptions within the same volume. Gratings
having different grating prescriptions can have different grating
vectors and/or grating slant with respect to the waveguide's
surface. The magnitude of the grating vector of a grating can be
defined as the inverse of the grating period while its direction
can be defined as the direction orthogonal to the fringes of the
grating.
[0083] In several embodiments, an integrated can be implemented to
perform both beam expansion and beam extraction. An integrated
grating can be implemented with one or more grating prescriptions.
In a number of embodiments, the integrated grating is implemented
with at least two grating prescriptions. In further embodiments,
the integrated grating is implemented with at least three grating
prescriptions. In many embodiments, two grating prescriptions
within the integrated grating have similar clock angles. In some
embodiments, the two grating prescriptions have different slant
angles. An integrated grating in accordance with various
embodiments of the invention can be implemented using a variety of
types of gratings, such as but not limited to SRGs, SBGs,
holographic gratings, and other types of gratings including those
described in the sections above. In a number of embodiments, the
integrated grating includes two surface relief gratings. In other
embodiments, the integrated grating includes two holographic
gratings.
[0084] The integrated grating can include at least two grating
prescriptions that are at least partially overlapped or
multiplexed. In further embodiments, the integrate grating includes
at least two grating prescriptions that are fully overlapped or
multiplexed. In a number of embodiments, the integrated grating
includes multiplexed or overlapping gratings that have different
sizes and/or shapes--i.e., one grating may be larger than the
other, resulting in only partial multiplexing of the larger
grating. As can readily be appreciated, various multiplexed and
overlapping configurations may be implemented as appropriate
depending on the specific requirements of a given application.
Although the discussions below may describe configurations as
implementing multiplexed or overlapping gratings, such gratings can
be substituted for one another as appropriate depending on the
application. In several embodiments, the integrated gratings are
implemented by a combination of both multiplexed and overlapping
gratings. For example, two or more sets of multiplexed gratings can
be overlapped across two or more grating layers.
[0085] Integrated gratings in accordance with various embodiments
of the invention can be utilized for various purposes including but
not limited to implementing full color waveguides and addressing
some key problems in conventional waveguide architectures. Other
advantages include reduced material and waveguide refractive index
requirements and reduced waveguide dimensions resulting from the
overlapping and/or multiplexing nature of the integrated gratings.
Such configurations can allow for large field-of-view waveguides,
which would ordinarily incur unacceptable increases in waveguide
form factor and refractive index requirements. In many embodiments,
a waveguide is implemented with at least one substrate having a low
refractive index. In some embodiments, the waveguide is implemented
with a substrate having a refractive index of lower than 1.8. In
further embodiments, the waveguide is implemented with a substrate
having a refractive index of not more than .about.1.5.
[0086] Integrated gratings that can provide beam expansion and beam
extraction--i.e., the functions of conventional fold and output
gratings--can result in a much smaller grating area, enabling a
small form factor and lower fabrication cost. By integrating the
functions of beam expansion and extraction, instead of performing
them serially as in traditional waveguides, beam expansion and
extraction can be accomplished with .about.50% of the grating
interactions normally required, cutting down haze in the same
proportion in the case of birefringent gratings. A further
advantage is that, as a result of the greatly shortened light
paths, the number of beam bounces at glass/air interface(s) is
reduced, rendering the output image less sensitive to substrate
nonuniformities. This can enable higher quality images and the
potential to use less expensive, lower specification
substrates.
[0087] In many embodiments, the grating vectors of the input
coupler and integrated gratings are arranged to provide a
substantially zero resultant vector. The grating vectors of the
input coupler and integrated gratings can be arranged to form a
triangular configuration. In several embodiments, the grating
vectors can be arranged in an equilateral triangular configuration.
In some embodiments, the grating vectors can be arranged in an
isosceles triangular configuration where at least two grating
vectors have equal magnitudes. In further embodiments, the grating
vectors are arranged in an isosceles right triangular
configuration. In a number of embodiments, the grating vectors are
arranged in a scalene triangular configuration. Another waveguide
architecture includes integrated diffractive elements with grating
vectors aligned in the same direction for providing horizontal
expansion for one set of angles and extraction for a separate set
of angles. In several embodiments, one or more of the integrated
gratings are asymmetrical in their general shape. In some
embodiments, one or more of the integrated gratings has at least
one axis of symmetry in their general shape. In a number of
embodiments, the gratings are designed to sandwich an
electro-active material, enabling switching between clear and
diffracting states for certain types of gratings such as but not
limited to HPDLC gratings. The gratings can be a surface relief or
a holographic type.
[0088] In many embodiments, a waveguide supporting at least one
input coupler and first and second integrated gratings is
implemented. The grating structures can be implemented in single-
or multi-layered waveguide designs. In single-layered designs, the
integrated gratings can be multiplexed. In embodiments where each
integrated grating contains at least two multiplexed gratings, the
multiplexed integrated gratings can contain at least four
multiplexed gratings. As described above, any individual
multiplexed grating can be partially or completely multiplexed with
the other gratings. In some embodiments, a multi-layered waveguide
is implemented with overlapping integrated gratings. In further
embodiments, the integrated gratings are partially overlapped. Each
of the integrated gratings can be a separate grating or multiplexed
gratings.
[0089] In many embodiments, the waveguide architecture is designed
to couple the input light into two bifurcated paths using an input
coupler. Such configurations can be implemented in various ways. In
some embodiments, a multiplexed input grating is implemented to
couple input light into two bifurcated paths. In other embodiments,
two input gratings are implemented to separately couple input light
into two bifurcated paths. The two input gratings can be
implemented in the same layer or separately in two layers. In a
number of embodiments, two overlapping or partially overlapping
input gratings are implemented to couple input light into two
bifurcated paths. In many embodiments, the input coupler includes a
prism. In further embodiments, the input coupler includes a prism
and any of the input grating configuration described above.
[0090] In addition to various input coupler architectures, the
first and second integrated gratings can be implemented in a
variety of configurations. Integrated gratings in accordance with
various embodiments of the invention can be incorporated into
waveguides to perform the dual function of two-dimensional beam
expansion and beam extraction. In several embodiments, the first
and second integrated gratings are crossed gratings. As described
above, some waveguide architectures include designs in which input
light is coupled into two bifurcated paths. In such designs, the
two bifurcated paths are each directed towards a different
integrated grating. As can readily be appreciated, such
configurations can be designed to bifurcate the input light based
on various light characteristics, including but not limited to
angular and spectral bandwidths. In some embodiments, light can be
bifurcated based on polarization states--e.g., input unpolarized
light can be bifurcated into S and P polarization paths. In many
embodiments, each of the integrated gratings performs either beam
expansion in a first direction or beam expansion in a second
direction different from the first direction according to the
field-of-view portion being propagated through the waveguide. The
first and second directions can be orthogonal to one another. In
other embodiments, the first and second directions are not
orthogonal to one another. Each integrated grating can provide
expansion of the light in a first dimension while directing the
light towards the other integrated grating, which provides
expansion of the light in a second dimension and extraction. For
example, many grating architectures in accordance with various
embodiments of the invention include an input configuration for
bifurcating input light into first and second portions of light. A
first integrated grating can be configured to provide beam
expansion in a first direction for the first and second portions of
light and to provide beam extraction for the second portion of
light. Conversely, the second integrated grating can be configured
to provide beam expansion in a second direction for the first and
second portions of light and to provide beam extraction for the
first portion of light.
[0091] In a number of embodiments, the first integrated grating
includes multiplexed first and second grating prescriptions, and
the second integrated grating includes multiplexed third and fourth
grating prescriptions. In such embodiments, the first grating
prescription can be configured to provide beam expansion in a first
direction for the first portion of light and to redirect the
expanded light towards the fourth grating prescription. The second
grating prescription can be configured to provide beam expansion in
the first direction for the second portion of light and to extract
the light out of the waveguide. The third grating prescription can
be configured to provide beam expansion in a second direction for
the second portion of light and to redirect the expanded light
towards the second grating prescription. The fourth grating
prescription can be configured to provide beam expansion in the
second direction for the first portion of light and to extract the
light out of the waveguide. As can readily be appreciated, the
integrated gratings can be implemented with overlapping grating
prescriptions instead of multiplexed grating prescriptions. In many
embodiments, the first and second grating prescriptions have the
same clock angle but different grating slants. In some embodiments,
the third and fourth grating prescriptions have the same clock
angle, which is different from the clock angles of the first and
second grating prescriptions. In a number of embodiments, the
first, second, third, and fourth grating prescriptions all have
different clock angles. In several embodiments, the first, second,
third, and fourth grating prescriptions all have different grating
periods. In a number of embodiments, the first and third grating
prescriptions have the same grating period, and the second and
fourth grating prescriptions have the same grating period.
[0092] FIG. 1 conceptually illustrates a waveguide display in
accordance with an embodiment of the invention. As shown, the
apparatus 100 includes a waveguide 101 supporting an input grating
102 and a grating structure 103. Each grating can be characterized
by a grating vector defining the orientation of the grating fringes
in the plane of the waveguide. A grating can also be characterized
by a K-vector in 3D space, which in the case of a Bragg grating is
defined as the vector normal to the grating fringes. The waveguide
reflecting surfaces are parallel to the XY plane of the Cartesian
reference frame inset into the drawing. In some embodiments, the X
and Y axes can correspond to global horizontal and vertical axes in
the reference frame of a user of the display.
[0093] In the illustrative embodiment of FIG. 1, the input grating
102 includes a Bragg grating 104. In other embodiments, the input
grating 102 is a surface relief grating. The input grating 102 can
be implemented to bifurcate input light into two different
portions. In further embodiments, the input grating 102 includes
two multiplexed gratings having different grating prescriptions. In
other embodiments, the input grating 102 includes two overlaid
surface relief gratings. The grating structure 103 includes two
effective gratings 105,106 that have different grating vectors. The
gratings 105,106 can be integrated gratings implemented as surface
relief gratings or volume gratings. In many embodiments, the
gratings 105,106 are multiplexed in a single layer. In several
embodiments, the waveguide 101 provides two effective gratings at
all points across the grating structure 103 by overlaying more than
two separated gratings in the grating structure. For ease of
clarity, the gratings 105,106 that form the grating structure 103
will be referred to as first and second integrated gratings since
their role in the grating structure includes providing beam
expansion by changing the direction of the guided beam in the plane
of the waveguide and beam extraction. In various embodiments, the
integrated gratings 105,106 perform two-dimensional beam expansion
and extraction of light from the waveguide 101. The field-of-view
coupled into the waveguide can be partitioned into first and second
portions, which can be bifurcated as such by the input grating 102.
In many embodiments, the first and second portions correspond to
positive and negative angles, vertically or horizontally. In some
embodiments, the first and second portions may overlap in angle
space. In a number of embodiments, the first portion of the
field-of-view is expanded in a first direction by the first
integrated grating and, in a parallel operation, expanded in a
second direction and extracted by the second integrated grating.
When a ray interacts with a grating fringe, some of the light that
meets the Bragg condition is diffracted while non-diffracted light
proceeds along its TIR path up to the next fringe, continuing the
expansion and extraction process. Considering next the second
portion of the field-of-view, the role of the gratings is reversed
such that the second portion of the field is expanded in the second
direction by the second integrated grating and expanded in the
first direction and extracted by the first integrated grating.
[0094] In many embodiments, the integrated gratings 105,106 in the
grating structure 103 can be asymmetrically disposed. In some
embodiments, the integrated gratings 105,106 have grating vectors
of different magnitudes. In several embodiments, the input grating
102 can have a grating vector offset from the Y-axis. In a number
of embodiments, it is desirable that the vector combination of the
grating vectors of the input grating 102 and the integrated
gratings 105,106 in the grating structures 103 gives a resultant
vector of substantially zero magnitude. As described above, the
grating vectors can be arranged in an equilateral, isosceles, or
scalene triangular configuration. Depending on the application,
certain configurations may be more desirable.
[0095] In many embodiments, at least one grating parameter selected
from the group of grating vector direction, K-vector direction,
grating refractive index modulation, and grating spatial frequency
can vary spatially across at least one grating implemented in the
waveguide for the purposes of optimizing angular bandwidth,
waveguide efficiency, and output uniformity to increase the angular
response and/or efficiency. In some embodiments, at least one of
the gratings implemented in the waveguide can employ rolled
K-vectors--i.e., spatially varying K-vectors. In several
embodiments, the spatial frequencies of the grating(s) are matched
to overcome color dispersion.
[0096] The apparatus 100 of FIG. 1 further includes an input image
generator. In the illustrative embodiment, the input image
generator includes a laser scanning projector 107 that provides a
scanned beam 107A over a field-of-view that is coupled into total
internal reflection paths (TIR paths) (108A,108B, for example) in
the waveguide by the input grating 102 and is directed towards the
integrated gratings 105,106 to be expanded and extracted (as shown
by rays 109A,109B, for example). In some embodiments, the laser
projector 107 is configured to inject a scanned beam into the
waveguide. In several embodiments, the laser projector 107 can have
a scan pattern modified to compensate for optical distortions in
the waveguide. In a number of embodiments, the laser scanning
pattern and/or grating prescriptions in the input grating 102 and
grating structure 103 can be modified to overcome illumination
banding. In various embodiments, the laser scanning projector 107
can be replaced by an input image generator based on a microdisplay
illuminated by a laser or an LED. In many embodiments, the input
image can be provided by an emissive display. A laser projector can
offer the advantages of improved color gamut, higher brightness,
wider field-of-view, high resolution, and a very compact form
factor. In some embodiments, the apparatus 100 can further include
a despeckler. In further embodiments, the despeckler can be
implemented as a waveguide device. Although FIG. 1 shows a specific
waveguide application implementing integrated gratings, such
structures and grating architectures can be utilized for various
applications. In a number of embodiments, a waveguide having
integrated gratings can be implemented in a single grating layer
for a full color application. In many embodiments, more than one
grating layer implementing integrated gratings are implemented.
Such configurations can be implemented to provide wider angular or
spectral bandwidth operation. In some embodiments, a multi-layered
waveguide is implemented to provide a full color application. In
several embodiments, a multi-layered waveguide is implemented to
provide a wider field-of-view. In many embodiments, a full color
waveguide having at least a .about.50.degree. diagonal
field-of-view is implemented using integrated gratings. In some
embodiments, a full color waveguide having at least a
.about.100.degree. diagonal field-of-view is implemented using
integrated gratings.
[0097] FIG. 2 conceptually illustrates a color waveguide display
having two blue-green diffracting waveguides and two green-red
diffracting waveguides in accordance with an embodiment of the
invention. FIG. 2 schematically illustrates an apparatus 200 with
an architecture similar to that of FIG. 1 but includes the use of
four stacked waveguides 201A-201D, including two blue-green
diffracting waveguides and two green-red diffracting waveguides. As
shown, the apparatus 200 includes a laser scanning projector 202
that provides scanning beams 202A-202D. In the illustrative
embodiment, the waveguides providing each color band can be
configured to propagate different field-of-view portions. For
example, in some embodiments, each of the waveguides operating in a
given color band provides a field-of-view of 35.degree.
h.times.35.degree. v (50.degree. diagonal), yielding 70.degree.
h.times.35.degree. v (78.degree. diagonal) field-of-view for each
color band when the two fields of view are combined. In many
embodiments, the scanning beams can be generated using red, green,
and blue laser emitters with each light of two laser wavelengths
selected from red, green, and blue being injected into each
waveguide according to the color band intended to be propagated by
the waveguide. The laser beam intensities can be modulated for the
purposed of color balancing. The stacked waveguides can be arranged
in any order. In several embodiments, consideration of factors such
as but not limited to color crosstalk can influence the stack
order. In a number of embodiments, the integrated gratings of one
waveguide are partially or completely overlapped with the
integrated gratings of another waveguide. As described above, the
integrated gratings can be implemented in various configurations.
In some embodiments, the integrated gratings are implemented across
more than one grating layer. In several embodiments, each of the
integrated gratings includes two multiplexed grating
prescriptions.
[0098] In many embodiments, the optical geometrical requirements
for combining waveguide paths for more than one field-of-view or
color band can dictate an asymmetric arrangement of the gratings
used in the input grating(s) and the integrated gratings. In other
words, the grating vectors of the input grating and the integrated
gratings are not equilaterally disposed or symmetrically disposed
about the Y axis.
[0099] Although FIGS. 1 and 2 show specific configurations of
waveguide architectures, various structures can be implemented as
appropriate depending on the specific requirements of a given
application. In some embodiments, a six-layered waveguide is
implemented for full color applications. The six-layered waveguide
can be implemented with three pairs of layers configured for color
bands of red, green, and blue, respectively. In such embodiments,
waveguides within each pair can be configured for different
field-of-view portions.
[0100] In some embodiments, to perform beam expansion and
extraction, the waveguide is designed such that each point of
interaction of a ray with a grating structure occurs in a region of
overlapping effective gratings. In a non-fully overlapped grating
configuration, the grating structures will have regions in which
the first and second effective gratings only partially overlap such
that some rays interact with only one of the effective gratings. In
many embodiments, the grating structures are formed from two
multiplexed gratings. The first of the multiplexed grating 300,
which is shown in FIG. 3A, multiplexes a first effective grating
301 with one 302 having a different effective grating vector (or
clock angle). The second multiplexed grating 310, which is shown in
FIG. 3B, multiplexes a second effective grating 311 with one 312
having a different effective grating vector. FIGS. 3A-3B are
intended to illustrate the relative orientations of the multiplexed
gratings and do not represent the shapes of the gratings as
implemented. In some embodiments, the gratings 301,302 and 311,312
may differ in shape from each other. In the embodiments of FIGS.
3A-3B, the grating vector (clock angle) of the second multiplexed
grating is identical to the first grating vector of the first
multiplexed grating. Likewise, the grating vector of the first
multiplexed grating is identical to the second grating vector of
the second multiplexed grating. Turning now to FIG. 3C, it should
be apparent that when the gratings are overlapped 320, there are
two gratings of different clock angles at any point in the grating
structures (e.g., in the regions of partial overlap--labeled by
numerals 2-4 in FIG. 3C) of the effective gratings. In the regions
of full overlap (labelled by numeral 1 in FIG. 3C) of the effective
gratings, there will be four gratings overlapping any point in the
grating structures. However, in such regions, each pair of gratings
having the same clock angle results in only two overlapping
effective gratings. It should be appreciated from the above
description that, in many embodiments, the two pairs of multiplexed
gratings could be implemented as one multiplexed grating formed
from the four gratings 301,302 and 311, 312.
[0101] FIGS. 4A-4C schematically illustrate ray propagation through
a grating structure 400 having an input grating 401 and two
integrated gratings 402,403 in accordance with an embodiment of the
invention. The ray propagation is illustrated using unfolded ray
paths to clarify the interaction between the rays and gratings. As
shown in the schematic diagram of FIG. 4A, light from a first
portion of the FOV shows a ray 404A coupled into a TIR path in the
waveguide by the input grating 401, a TIR ray 405A leading to the
first integrated grating 402, a TIR ray 406A diffracted by the
first integrated grating 403 (which also provides beam expansion in
a first direction), and a ray 407A diffracted out of the waveguide
by the second integrated grating 403 (which also provides beam
expansion in a second direction). Turning now to the propagation of
the second portion of the FOV, which is shown in FIG. 4B, the ray
path includes a ray 404B coupled into a TIR path in the waveguide
by the input grating 401, a TIR ray 405B leading to the second
integrated grating 403, a TIR ray 406B diffracted by the second
integrated grating 403 (which also provides beam expansion in the
second direction), and a TIR ray 407B diffracted out of the
waveguide by the first integrated grating 402 (which also provides
beam expansion in the first direction). FIG. 4C shows the combined
paths of FIGS. 4A-4B with the integrated gratings overlaid. FIG. 4C
also shows the partial overlapping nature of the integrated
gratings along the paths of the rays. As can readily be
appreciated, such configurations can be modified as appropriate
depending on the specific requirements of a given application.
Gratings of various shapes can be utilized. An integrated grating
can include two multiplexed gratings, one providing the function of
a traditional fold grating and another for extracting the light
similar to a traditional output grating. Each of the two
multiplexed gratings within a single integrated grating can be
configured to act on different portions of light bifurcated by the
input configuration. In a number of embodiments, the two
multiplexed gratings within a single integrated grating can have
different shapes--i.e., certain areas of one or both of the
gratings are not multiplexed. In some embodiments, more than two
gratings are multiplexed for a single integrated grating. In many
embodiments, the integrated gratings are multiplexed in a single
grating layer. In several embodiments, the integrated gratings are
fully multiplexed or overlapped. In other embodiments, only
portions of the integrated gratings are multiplexed overlapped.
[0102] As described above, grating architectures including those
implementing integrated gratings can be described and visualized
using grating vectors. In many embodiments, three grating vectors,
which can represent traditional input, fold, and output functions,
can be implemented with a substantially zero resultant vector. FIG.
5A conceptually illustrates a grating vector configuration with a
substantially zero resultant vector in accordance with an
embodiment of the invention. As shown, the configuration 500
includes three grating vectors 501-503 represented as k.sub.1,
k.sub.2, and k.sub.3, respectively. With three grating vectors,
configurations having a substantially zero resultant vector can
provide various triangular configurations, such as but not limited
to equilateral triangles, isosceles triangles, and scalene
triangles. In the case of architectures utilizing integrated
gratings, more than one triangular configuration can be visualized.
FIG. 5B conceptually illustrates one such embodiment. As shown, the
configuration 510 illustrates two triangular configurations. One
triangular configuration is formed by grating vectors k.sub.1,
k.sub.2, and k.sub.3 (511-513), and a second configuration is
formed by grating vectors k.sub.1, k.sub.4, and k.sub.5 (511, 514,
and 515). In the illustrative embodiment, grating vector k.sub.1
represents the function of the input coupler, grating vectors
k.sub.2 and k.sub.5 represent the functions of a first integrated
grating, and grating vectors k.sub.4 and k.sub.3 represent the
functions of a second integrated grating.
[0103] In many embodiments, the grating vector configuration
implemented can include various triangular configurations.
Typically, the magnitudes of the grating vectors can dictate the
resulting triangular configuration. In some embodiments, an
equilateral triangular configuration is implemented where all
grating vectors are of similar, or substantially similar,
magnitude. In cases where integrated gratings are implemented, the
configuration can include two triangular configurations. In a
number of embodiments, the grating vector configuration includes at
least one isosceles triangle where at least two of the grating
vectors have similar, or substantially similar, magnitudes. FIG. 5C
conceptually illustrates a grating vector configuration with two
isosceles triangles in accordance with an embodiment of the
invention. As shown, the configuration 520 forms two isosceles
triangles due to grating vectors k.sub.2-k.sub.5 having similar
magnitudes. In several embodiments, the grating configuration
includes at least one scalene triangle. FIG. 5D conceptually
illustrates a grating vector configuration with two scalene
triangles in accordance an embodiment of the invention. As shown,
the configuration 530 forms two scalene triangles. In the
illustrative embodiment, the two scalene triangles are
mirrored--i.e., grating vectors k.sub.2 and k.sub.4 are equal in
magnitude, and grating vectors k.sub.3 and k.sub.5 are equal in
magnitude. FIG. 5E conceptually illustrates a grating vector
configuration with two different scalene triangles in accordance
with an embodiment of the invention. As shown, the configuration
540 includes two different scalene triangles with grating vectors
k.sub.2-k.sub.5 having different magnitudes.
[0104] Although FIGS. 5A-5E illustrate specific grating vector
configurations, various other configurations can be implemented as
appropriate depending on the specific requirements of a given
application. For example, in some embodiments, the input coupler is
implemented to have two different grating vectors. Such
configurations utilize an input grating having two different
grating prescriptions, which can implemented using overlapping or
multiplexed grating prescriptions. In the embodiments illustrated
in FIGS. 5B-5E, the configurations shown can be due to the
implementation of integrated gratings. In many embodiments, grating
vectors k.sub.2 and k.sub.5 represent the functions of a first
integrated grating, and grating vectors k.sub.4 and k.sub.3
represent the functions of a second integrated grating. In several
embodiments, each grating vector k.sub.i represent a different
grating prescription. For example, many grating architectures in
accordance with various embodiments of the invention can implement
integrated gratings that each contain two different grating
prescriptions. In such cases, grating vectors k.sub.2 and k.sub.5
can respectively represent the two different grating prescriptions
of a first integrated grating, and grating vectors k.sub.4 and
k.sub.3 can respectively represent the two different grating
prescriptions of a second integrated grating.
[0105] FIG. 6 conceptually illustrates a schematic plan view of a
grating architecture 600 having an input grating and integrated
gratings in accordance with an embodiment of the invention. As
shown, the grating architecture 600 includes an input coupler 601.
The input coupler 601 can be a Bragg grating or a surface relief
grating. In many embodiments, the input coupler 601 includes at
least two gratings. In such embodiments, individual input gratings
can be configured to couple in different portions of input light,
which can be based on angular or spectral characteristics. In some
embodiments, the input couple 601 includes two overlapped gratings.
In other embodiments, the input coupler 601 includes two
multiplexed gratings. The grating architecture 600 further includes
first (bold lines) and second (dashed lines) integrated gratings.
In the illustrative embodiment, the first integrated grating
includes a first grating 602 having a first grating prescription
and a second grating 603 having a second grating prescription. As
shown, the second grating 603 is smaller than the first grating 602
and can be entirely multiplexed within the volume of the first
grating 602. In some embodiments, the first and second gratings
602,603 are overlapped across different grating layers. In several
embodiments, the first and second gratings 602,603 are adjacent or
nearly adjacent one another and are neither overlapped nor
multiplexed. In a number of embodiments, the first and second
gratings 602,603 have the same clock angles but different grating
prescriptions.
[0106] In many embodiments, the configuration of the first
integrated grating is applied similarly to the second integrated
grating but flipped about an axis. For example, the illustrative
embodiment in FIG. 6 shows the second integrated grating having
third 604 and fourth 605 gratings with shapes corresponding to the
first and second gratings 602,603, respectively. The third grating
604 has a third grating prescription, and the fourth grating 605
has a fourth grating prescription. Similar to the first integrated
grating, the third and fourth gratings 604,605 can have the same
clock angles but different grating prescriptions. In a number of
embodiments, the first and second gratings 602,603 are clocked at
an angle different from the third and fourth gratings 604,605.
Again, the overlapping and multiplexing nature of the third and
fourth gratings 604,605 can be implemented in a similar manner as
the first and second gratings 602,603.
[0107] In the illustrative embodiment of FIG. 6, the first and
third integrated gratings are partially overlapped with one another
such that the second and fourth gratings 603,605 are also partially
overlapped. In the illustrative embodiment, the second and fourth
gratings 603,605 are multiplexed within the first and third
gratings 602,604, and, as such, the waveguide architecture includes
an area 606 where four grating prescriptions are active. In
embodiments where the first and second integrated gratings are
implemented in a single layer, area 606 would contain four
multiplexed gratings. In other embodiments, the first and second
integrated gratings are implemented across different grating
layers.
[0108] During operation, input light incident on the input grating
601 can be bifurcated into two portions of light traveling in TIR
paths within the waveguide. One portion can be directed towards the
first grating 602 while the other portion can be directed towards
the third grating 604. The first grating 602 can be configured to
provide beam expansion in a first direction for incident light and
to redirect the incident light towards the fourth grating 605. The
fourth grating 605 can be configured to provide beam expansion in a
second direction for incident light and to extract the light out of
the waveguide. On the other hand, the third grating 604 can be
configured to provide beam expansion in the second direction for
incident light and to redirect the incident light towards the
second grating 603. The second grating 603 can be configured to
provide beam expansion in the first direction for incident light
and to extract the light out of the waveguide.
[0109] FIG. 7 shows a flow diagram conceptually illustrating a
method of displaying an image in accordance with an embodiment of
the invention. Referring to the flow diagram, the method 700
includes providing (701) a waveguide supporting an input grating, a
first integrated grating, and a second integrated grating. In many
embodiments, the first integrated grating partially overlaps the
second integrated grating. In some embodiments, the integrated
gratings are fully overlapped. The first and second integrated
gratings can include multiplexed pairs of different K-vector
gratings. A first field-of-view portion can be coupled (702) into
the waveguide via the input grating and directed towards the first
integrated grating. A second field-of-view portion can be coupled
(703) into the waveguide via the input grating and directed towards
the second integrated grating. The first field-of-view portion
light can be expanded (704) in a first direction using the first
integrated grating. The first field-of-view portion light can be
expanded in a second direction and extracted (705) from the
waveguide using the second integrated grating. The second
field-of-view portion light can be expanded in the second direction
(706) using the second integrated grating. The second field-of-view
portion light can be expanded in the first direction and extracted
(707) from the waveguide using the first integrated grating.
[0110] As described in the sections above, integrated gratings can
be implemented in a variety of different ways. In many embodiments,
an integrated grating is implemented with two gratings that have
the same clock angle but different grating prescriptions. In
further embodiments, the two gratings are multiplexed. FIG. 8 shows
a flow diagram conceptually illustrating a method of displaying an
image utilizing integrated gratings containing multiple gratings in
accordance with an embodiment of the invention. Referring to the
flow diagram, the method 800 includes providing (801) a waveguide
supporting an input grating, first and second gratings having a
first clock angle, and third and fourth gratings having a second
clock angle, where the first and third grating at least partially
overlaps. In many embodiments, the first integrated grating
partially overlaps the second integrated grating. In some
embodiments, the integrated gratings are fully overlapped. The
first and second integrated gratings can include multiplexed pairs
of different K-vector gratings. A first field-of-view portion can
be coupled (802) into the waveguide via the input grating and
directed towards the first grating. A second field-of-view portion
can be coupled (803) into the waveguide via the input grating and
directed towards the third grating. The first field-of-view portion
light can be expanded (804) in a first direction using the first
grating and redirected towards the fourth grating. The first
field-of-view portion light can be expanded in a second direction
and extracted (805) from the waveguide using the fourth grating.
The second field-of-view portion light can be expanded in the
second direction (806) using the third grating and redirected
towards the second grating. The second field-of-view portion light
can be expanded in the first direction and extracted (807) from the
waveguide using the second grating.
[0111] Although FIGS. 6-8 illustrate specific waveguide
configurations and methods of displaying an image, many different
methods can be implemented in accordance with various embodiments
of the invention. For example, in some embodiments, more than one
input grating is utilized. In other embodiments, the input
configuration includes a prism. Such methods and implemented
waveguides can also be configured to improve performance and/or
provide various different functions. In many embodiments, the
waveguide apparatus includes at least one grating with
spatially-varying pitch. In some embodiments, each grating has a
fixed K vector. In a number of embodiments, at least one of the
gratings is a rolled k-vector grating according to the embodiments
and teachings disclosed in the cited references. Rolling the
K-vectors can allow the angular bandwidth of the grating to be
expanded without the need to decrease the grating thickness or to
utilize multiple grating layers. In some embodiments a rolled
K-vector grating includes a waveguide portion containing discrete
grating elements having differently aligned K-vectors. In some
embodiments, a rolled K-vector grating comprises a waveguide
portion containing a single grating element within which the
K-vectors undergo a smooth monotonic variation in direction. In
some of the embodiments describe rolled K-vector gratings are used
to input light into the waveguide. In some embodiments, waveguides
having two integrated gratings can be implemented as single-layered
or multi-layered waveguides. In several embodiments, a
multi-layered waveguide is implemented with more than two
integrated gratings. As can readily be appreciated, the specific
architecture and configuration implemented can depend on a number
of different factors. In some embodiments, the position of the
input grating relative to the integrated gratings can be dictated
by various factors, including but not limited to projector relief
and the input pupil diameter and vergence. In many applications, it
is desirable for the distance between the input grating and the
integrated gratings to be minimized to provide a waveguide having a
small form factor. The field ray angle paths required to fill the
eyebox typically dominate the waveguide height. In many cases, the
height of waveguide grows non-linearly with projector relief. In
some embodiments, the pupil diameter does not have a significant
impact on the footprint of the waveguide. A converging or diverging
pupil can be used to reduce the local angle response at any
location on the input grating.
[0112] In some embodiments, the waveguide configuration implemented
can depend on the configuration of the input image
generator/projector. FIG. 9 conceptually illustrates a profile view
900 of two overlapping waveguide portions implementing integrated
gratings in accordance with an embodiment of the invention. In the
illustrative embodiment, the two-layered waveguide is designed for
a high field-of-view application implemented with a converging
projector pupil input beam, indicated by rays 901. As shown, the
apparatus includes a first waveguide 902 containing a first grating
layer 903 having a first set of two integrated gratings and a
second waveguide 904 containing a second grating layer 905 having a
second set of two integrated gratings that partially overlaps the
first set of two integrated gratings. The grating layers 903,905
having integrated gratings can operate according to the principles
discussed in the sections above. The output beam from the
waveguides is generally indicated by rays 906 intersecting the
eyebox 907. In the illustrated embodiment, the eyebox has
dimensions 10.5 mm..times.9.5 mm., an eye relief of 13.5 mm, and a
laser projector to waveguide separation of 12 mm. As can readily be
appreciated, such dimensions and specifications can be specifically
tailored depending on the requirements of a given application.
[0113] FIG. 10 conceptually illustrates a schematic plan view 1000
of a grating architecture having two sets of integrated gratings in
accordance with an embodiment of the invention. As shown, the
grating configuration includes first and second input gratings
1001,1002, forming the combined input grating area 1003 indicated
by the shaded area. In some embodiments, each of the input gratings
includes a set of multiplexed or overlapping gratings. The grating
configuration further includes a first set of grating structures
having first and second integrated gratings 1004,1005 and a second
set of grating structures having third and fourth integrated
gratings 1006,1007. In the illustrative embodiment, each set of
integrated gratings is shaped and disposed asymmetrically. Such
configurations can be implemented as appropriate depending on
several factors. In the embodiment of FIG. 10, the asymmetrical
grating architecture can be implemented for operation with a
converging projector pupil configuration, such as the one shown in
FIG. 9. Furthermore, different grating characteristics can be
implemented and tuned for different applications. FIG. 11
conceptually illustrates a plot 1100 of diffraction efficiency
versus angle for a waveguide for diffractions occurring at
different field-of-view angles in accordance with an embodiment of
the invention. As shown, the waveguide is tuned to have three
different peak diffraction efficiencies, with two different peaks
1101,1102 for the "fold" interaction and one 1103 for the "output."
In some embodiments, light undergoes a dual interaction within the
grating. Such gratings can be designed to have high diffraction
efficiencies for two different incident angles. Turning back to
FIG. 10, the first and second set of grating structures can be
implemented as partially overlapping structures, forming a combined
output grating area 1008 as indicated by the shaded area. The
eyebox 1009 is overlaid on the drawing and is indicated by the dark
shaded area. In the illustrative embodiment, the waveguide
apparatus is configured to provide a FOV of 120 degrees diagonal.
As shown in FIGS. 9-10, in some embodiments, displays providing a
FOV of 120 degrees diagonal can be configured with a projector to
waveguide distance of 12 mm and an eye relief of 13.5 mm., which is
compatible with many glasses inserts. In some embodiments, the
display provides an eyebox of 10.5 mm..times.9.5 mm., which can
provide easy wearability. FIG. 12 shows the viewing geometry of
such a waveguide. As can readily be appreciated, the grating
configuration illustrated by FIG. 10 can be implemented in a
variety of waveguide architectures. In some embodiments, both input
gratings and both sets of grating structures are implemented in a
single grating layer, with the overlapping portions multiplexed. In
several embodiments, the first input grating and the first set of
grating structures are implemented in a first grating layer while
the second input grating and the second set of grating structures
are implemented in a second grating layer. In a number of
embodiments, the first, second, third, and fourth integrated
gratings are implemented across four grating layers.
[0114] FIG. 13 conceptually illustrates the field-of-view geometry
for a binocular display with binocular overlap between the left and
right eye images provided by a waveguide in accordance with an
embodiment of the invention. Binocular displays utilizing various
grating architectures, such as the one described in FIGS. 9-10. can
be implemented. In the illustrated embodiment, the waveguide is a
color waveguide that includes a stack of four waveguides: two
blue-green layers and two green-red layers. Each of the waveguides
can provide a field-of-view of 35.degree. h.times.35.degree. v
(.about.50.degree. diagonal) for a single-color band, yielding
70.degree. h.times.35.degree. v (.about.78.degree. diagonal)
field-of-view for each color band. Each waveguide set for the left
and right eyes can be overlapped by 50.degree. horizontally to
achieve .about.100.degree. diagonal binocular field-of-view. As can
readily be appreciated, various binocular configurations can be
implemented as appropriated depending on the specific requirements
of a given application. In many embodiments, the waveguide is raked
at an angle of at least 5.degree., which can facilitate the
implementation of some binocular overlapped field-of-view
applications. In further embodiments, the waveguide is raked at an
angle of at least 10.degree.. In some embodiments, the
field-of-views for both the left and right eyes are completely
overlapped.
Other Waveguide Embodiments
[0115] In some embodiments, a prism may be used as an alternative
to the input grating. In many embodiments, this can require that an
external grating is provided for grating vector closure purposes.
In several embodiments, the external grating may be disposed on the
surface of the prism. In some embodiments, the external grating may
form part of a laser despeckler disposed in the optical train
between the laser projector and the input prims. The use of a prism
to couple light into a waveguide has the advantage of avoiding the
significant light loss and restricted angular bandwidth resulting
from the use of a rolled K-vector grating. A practical rolled
K-vector input grating typically cannot match the much large
angular bandwidth of the fold grating, which can be around 40
degrees or more.
[0116] Although the drawings may indicate a high degree of symmetry
in the grating geometry and layout of the gratings in the different
wavelength channels, the grating prescriptions and footprints can
be asymmetric. The shapes of the input, fold, or output gratings
can depend on the waveguide application and could be of any
polygonal geometry subject to factors such as the required beam
expansion, output beam geometry, beam uniformity, and ergonomic
factors.
[0117] In some embodiments, directed at displays using unpolarized
light sources, the input gratings can combine gratings orientated
such that each grating diffracts a particular polarization of the
incident unpolarized light into a waveguide path. Such embodiments
may incorporate some of the embodiments and teachings disclosed in
the PCT application PCT/GB2017/000040 "METHOD AND APPARATUS FOR
PROVIDING A POLARIZATION SELECTIVE HOLOGRAPHIC WAVEGUIDE DEVICE" by
Waldern et al., the disclosure of which is incorporated herein in
by reference in its entirety. The output gratings can be configured
in a similar fashion so that the light from the waveguide paths is
combined and coupled out of the waveguide as unpolarized light. For
example, in some embodiments, the input grating and output grating
each combine crossed gratings with peak diffraction efficiency for
orthogonal polarizations states. In a number of embodiments, the
polarization states are S-polarized and P-polarized. In several
embodiments, the polarization states are opposing senses of
circular polarization. The advantage of gratings recorded in liquid
crystal polymer systems, such as SBGs, in this regard is that owing
to their inherent birefringence, they exhibit strong polarization
selectivity. However, other grating technologies that can be
configured to provide unique polarization states can also be
used.
[0118] In some embodiments using gratings recorded in liquid
crystal polymer material systems, at least one polarization