U.S. patent application number 16/242966 was filed with the patent office on 2019-07-11 for methods for fabricating optical waveguides.
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, Milan Momcilo Popovich, Jonathan David Waldern.
Application Number | 20190212699 16/242966 |
Document ID | / |
Family ID | 67140759 |
Filed Date | 2019-07-11 |
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United States Patent
Application |
20190212699 |
Kind Code |
A1 |
Waldern; Jonathan David ; et
al. |
July 11, 2019 |
Methods for Fabricating Optical Waveguides
Abstract
Mastering systems and methods of fabricating waveguides and
waveguide devices using such mastering systems are described.
Mastering systems for fabricating holographic waveguides can
include using a master to control the application of energy (e.g. a
laser, light, or magnetic beam) onto a liquid crystal substrate to
fabricate a holographic waveguide into the liquid crystal
substrate. Mastering systems for fabricating holographic waveguides
in accordance with embodiments of the invention can include a
variety of features. These features include, but are not limited
to: chirp for single input beam copy (near i.e. hybrid contact
copy), dual chirped gratings (for input and output), zero order
grating for transmittance control, alignment reference gratings,
3:1 construction, position adjustment tooling to enable rapid
alignment, optimization of lens and window thickness for multiple
RKVs simultaneously, and avoidance of other orders and crossover of
the diffraction beam.
Inventors: |
Waldern; Jonathan David;
(Los Altos Hills, CA) ; Popovich; Milan Momcilo;
(Leicester, GB) ; Grant; Alastair John; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens, Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens, Inc.
Sunnyvale
CA
|
Family ID: |
67140759 |
Appl. No.: |
16/242966 |
Filed: |
January 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62614932 |
Jan 8, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 2006/1219 20130101;
G02B 6/00 20130101; G03H 2001/0439 20130101; G03H 2001/205
20130101; G03H 2223/16 20130101; G02B 27/0081 20130101; G03H 1/0248
20130101; G02B 27/0093 20130101; G03H 2260/00 20130101; G03H 1/202
20130101; G03H 1/20 20130101; G02B 27/0172 20130101; G03H 2001/2226
20130101; G02B 2006/12107 20130101; G03H 1/0408 20130101 |
International
Class: |
G03H 1/20 20060101
G03H001/20; G03H 1/04 20060101 G03H001/04; G03H 1/02 20060101
G03H001/02 |
Claims
1. A method for recording holograms, the method comprising:
providing a waveguide cell comprising a layer of polymer dispersed
liquid crystal mixture sandwiched between two substrates; providing
a master grating; emitting at least one recording beam toward the
master grating, wherein upon interaction with the master grating, a
portion of the at least one recording beam is diffracted towards
the waveguide cell; and recording at least one volume grating
within the waveguide cell using interferential exposure formed from
at least the diffracted portion of the at least one recording
beam.
2. The method of claim 1, wherein the master grating comprises an
amplitude grating.
3. The method of claim 2, wherein the master grating comprises a
chirped grating.
4. The method of claim 3, wherein the recorded volume grating
contains a rolled K-vector.
5. The method of claim 1, wherein the recorded volume grating
contains a multiplexed grating.
6. The method of claim 1, wherein the master grating comprises
three separate gratings.
7. The method of claim 6, wherein the three separate gratings are
designed to record an input grating, a fold grating, and an output
grating.
8. The method of claim 6, wherein the at least one volume grating
comprises three volume gratings.
9. The method of claim 8, wherein the at least one recording beam
comprises three recording beams.
10. The method of claim 1, wherein the interferential exposure is
formed from the zero order beam and diffracted portion of only one
recording beam.
11. A system for recording holographic gratings, the system
comprising: a waveguide cell comprising a layer of polymer
dispersed liquid crystal mixture sandwiched between two substrates;
a master grating; a light source configured to emit at least one
recording beam toward the master grating, wherein upon interaction
with the master grating, a portion of the at least one recording
beam is diffracted towards the waveguide cell and at least one
volume grating is recorded within the waveguide cell through
interferential exposure formed from at least the diffracted portion
of the at least one recording beam.
12. The system of claim 11, wherein the master grating comprises an
amplitude grating.
13. The system of claim 12, wherein the master grating comprises a
chirped grating.
14. The system of claim 13, wherein the recorded volume grating
contains a rolled K-vector.
15. The system of claim 11, wherein the recorded volume grating
contains a multiplexed grating.
16. The system of claim 11, wherein the master grating comprises
three separate gratings.
17. The system of claim 16, wherein the three separate gratings are
designed to record an input grating, a fold grating, and an output
grating.
18. The system of claim 16, wherein the at least one volume grating
comprises three volume gratings.
19. The system of claim 18, wherein the at least one recording beam
comprises three recording beams.
20. The system of claim 11, wherein the interferential exposure is
formed from the zero order beam and diffracted portion of only one
recording beam.
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. 62/614,932 entitled "Methods for Fabricating
Optical Waveguides," filed Jan. 8, 2018. The disclosure of U.S.
Provisional Patent Application No. 62/614,932 is hereby
incorporated by reference in its entirety for all purposes.
FIELD OF THE INVENTION
[0002] The present invention generally relates to methods for
manufacture of waveguides and more specifically for 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.
[0005] Waveguide optics, such as those described above, can be
considered for a range of display and sensor applications. In many
applications, waveguides containing one or more grating layers
encoding multiple optical functions can be realized using various
waveguide architectures and material systems, enabling new
innovations in near-eye displays for augmented reality ("AR") and
virtual reality ("VR"), compact heads-up displays ("HUDs") for
aviation and road transport, and sensors for biometric and laser
radar ("LIDAR") applications.
SUMMARY OF THE INVENTION
[0006] One embodiment includes a method for recording holograms,
the method including providing a waveguide cell comprising a layer
of polymer dispersed liquid crystal mixture sandwiched between two
substrates, providing a master grating, emitting at least one
recording beam toward the master grating, wherein upon interaction
with the master grating, a portion of the at least one recording
beam is diffracted towards the waveguide cell, and recording at
least one volume grating within the waveguide cell using
interferential exposure formed from at least the diffracted portion
of the at least one recording beam.
[0007] A further embodiment again includes a system for recording
holographic gratings, the system including a waveguide cell
including a layer of polymer dispersed liquid crystal mixture
sandwiched between two substrates, a master grating, a light source
configured to emit at least one recording beam toward the master
grating, wherein upon interaction with the master grating, a
portion of the at least one recording beam is diffracted towards
the waveguide cell and at least one volume grating is recorded
within the waveguide cell through interferential exposure formed
from at least the diffracted portion of the at least one recording
beam.
[0008] In another embodiment, the master grating includes an
amplitude grating.
[0009] In a further embodiment, the master grating includes a
chirped grating.
[0010] In still another embodiment, the recorded volume grating
contains a rolled K-vector.
[0011] In a still further embodiment, the recorded volume grating
contains a multiplexed grating.
[0012] In yet another embodiment, the master grating includes three
separate gratings.
[0013] In a yet further embodiment, the three separate gratings are
designed to record an input grating, a fold grating, and an output
grating.
[0014] In another additional embodiment, the at least one volume
grating includes three volume gratings.
[0015] In a further additional embodiment, the at least one
recording beam includes three recording beams.
[0016] In another embodiment again, the interferential exposure is
formed from the zero order beam and diffracted portion of only one
recording beam.
[0017] 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
[0018] The description will be more fully understood with reference
to the following figures and data graphs, which are presented as
exemplary embodiments of the invention and should not be construed
as a complete recitation of the scope of the invention. It will
apparent to those skilled in the art that the present invention may
be practiced with some or all of the present invention as disclosed
in the following description.
[0019] FIGS. 1A and 1B conceptually illustrate two volume Bragg
grating configurations in accordance with various embodiments of
the invention.
[0020] FIG. 2 conceptually illustrates a surface relief grating in
accordance with an embodiment of the invention.
[0021] FIGS. 3A and 3B conceptually illustrate HPDLC SBG devices
and the switching property of SBGs in accordance with various
embodiments of the invention.
[0022] FIGS. 4A-4D conceptually illustrate two-beam recording
processes in accordance with various embodiments of the
invention.
[0023] FIG. 5 conceptually illustrates a single-beam recording
process utilizing an amplitude grating in accordance with an
embodiment of the invention.
[0024] FIGS. 6A and 6B conceptually illustrate two implementations
of rolled K-vector gratings in accordance with various embodiments
of the invention.
[0025] FIG. 7 conceptually illustrates a multiplexed K-vector
grating in accordance with an embodiment of the invention.
[0026] FIG. 8 conceptually illustrates a waveguide utilizing
coupling gratings to diffract light into and out of the waveguide
in accordance with an embodiment of the invention.
[0027] FIGS. 9 and 10 conceptually illustrate waveguides utilizing
an output grating for exit pupil expansion in one dimension in
accordance with an embodiment of the invention.
[0028] FIG. 11 conceptually illustrates a waveguide system
utilizing two planar waveguides to provide exit pupil expansion in
two dimensions in accordance with an embodiment of the
invention.
[0029] FIG. 12 conceptually illustrates a waveguide utilizing a
three-grating structure to provide two dimensional exit pupil
expansion in accordance with an embodiment of the invention.
[0030] FIG. 13 conceptually illustrates a profile view of an RGB
stack of waveguides in accordance with an embodiment of the
invention.
[0031] FIG. 14 conceptually illustrates a dual axis expansion
waveguide display with two grating layers in accordance with an
embodiment of the invention.
[0032] FIG. 15 conceptually illustrates a plan view of a single
grating layer in accordance with an embodiment of the
invention.
[0033] FIG. 16 conceptually illustrates a plan view of a two
grating layer configuration in accordance with an embodiment of the
invention.
[0034] FIG. 17 conceptually illustrates a dual axis expansion
waveguide display in accordance with an embodiment of the
invention.
[0035] FIG. 18 conceptually illustrates an eye tracker display in
accordance with an embodiment of the invention.
[0036] FIG. 19 conceptually illustrates a dual expansion waveguide
display with a dynamic focusing element and an eye tracker in
accordance with an embodiment of the invention.
[0037] FIGS. 20A and 20B conceptually illustrate a waveguide
display coupled to an input image node by an opto-mechanical
interface in accordance with an embodiment of the invention.
[0038] FIGS. 21-24 conceptually illustrate various input image node
configurations in accordance with various embodiments of the
invention.
[0039] FIG. 25 conceptually illustrates a system diagram showing
components for waveguide displays in accordance with an embodiment
of the invention.
[0040] FIG. 26 conceptually illustrates an exposure process
utilizing a chirped amplitude grating in accordance with an
embodiment of the invention.
[0041] FIGS. 27A and 27B conceptually illustrate an exposure
process for forming three gratings simultaneously in accordance
with an embodiment of the invention.
[0042] FIG. 28 conceptually illustrates a fabrication setup for
exposing red, blue, and green gratings simultaneously in accordance
with an embodiment of the invention.
[0043] FIGS. 29A-29C conceptual illustrate a variety of approaches
for generating RKV gratings in accordance with various embodiments
of the invention.
[0044] FIGS. 30A and 30B conceptually illustrate various
applications of chirped gratings in accordance with various
embodiments of the invention.
[0045] FIG. 31 conceptually illustrates a mastering system
utilizing a zero order grating along with a chirped grating in
accordance with an embodiment of the invention.
[0046] FIG. 32 conceptually illustrates a mastering system
utilizing reference gratings in accordance with an embodiment of
the invention.
[0047] FIG. 33 conceptually illustrates a mastering system
configured to avoid other order beams from being created and/or
interfering with the energy beam being focused on the liquid
crystal substrate in accordance with an embodiment of the
invention.
[0048] FIG. 34 conceptually illustrates the effect of positioning
of the master grating on the resulting diffraction.
DETAILED DESCRIPTION
[0049] 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.
[0050] Turning now to the drawings, mastering systems and methods
of fabricating waveguides and waveguide devices using such
mastering systems are described. Mastering systems for fabricating
holographic waveguides can include using a master to control the
application of energy (e.g. a laser, light, or magnetic beam) onto
a liquid crystal substrate to fabricate a holographic waveguide
into the liquid crystal substrate. After fabrication, the finished
holographic waveguide can be incorporated into a variety of display
systems. These mastering systems can employ one or more energy
beams. In many embodiments, the mastering system uses a single
energy beam, which can simplify the alignment of the various
components within the mastering system and can reduce wave front
error found in two-beam systems caused by the different paths of
the two beams. As such, single energy beam processes can be
compatible with high volume manufacturing processes where thermal
and vibrational considerations can introduce a variety of
complications in aligning multiple energy beams. Mastering systems
for fabricating holographic waveguides in accordance with
embodiments of the invention can include a variety of features.
These features include, but are not limited to: chirp for single
input beam copy (near i.e. hybrid contact copy), dual chirped
gratings (for input and output), zero order grating for
transmittance control, alignment reference gratings, 3:1
construction, position adjustment tooling to enable rapid
alignment, optimization of lens and window thickness for multiple
rolled K-vector gratings simultaneously, and avoidance of other
orders and crossover of the diffraction beam. Waveguide structures,
mastering systems, and exposure processes are described in the
sections below in further detail.
Waveguide Structures
[0051] Waveguide structures in accordance with various embodiments
can be implemented in many different ways. In many embodiments, the
waveguide structures are designed to be optical waveguides, which
are structures that can confine and guide electromagnetic waves in
the visible spectrum, or light. These optical waveguides can be
implemented for use in a number of different applications, such as
but not limited to helmet mounted displays, head mounted displays
("HMDs"), and HUDs. The term HUD is typically utilized to describe
a class of devices that incorporates a transparent display that
presents data without requiring users to change their usual visual
field. Optical waveguides can integrate various optical functions
into a desired form factor depending on the given application.
[0052] Optical waveguides in accordance with various embodiments
can be designed to manipulate light waves in a controlled manner
using various methods and waveguide optics. For example, optical
waveguides can be implemented using materials with higher
refractive indices than the surrounding environment to restrict the
area in which light can propagate. Light coupled into optical
waveguides made of such materials at certain angles can be confined
within the waveguide via total internal reflection. In a planar
waveguide, the angles at which total internal reflection occurs can
be given by Snell's law, which can determine whether the light is
refracted or entirely reflected at the surface boundary.
[0053] In many embodiments, waveguides incorporating Bragg gratings
are implemented for HUD applications. HUDs can be incorporated in
any of a variety of applications including (but not limited to)
near-eye applications. HUDs that utilize planar waveguides
incorporating Bragg gratings in accordance with various embodiments
of the invention can achieve significantly larger fields of view
and have lower volumetric requirements than HUDs implemented using
conventional optical components. In some embodiments, the HUDs
include at least one waveguide incorporating a number of gratings.
In further embodiments, the waveguide incorporates at least three
Bragg gratings that can be implemented to provide various optical
functions, such as but not limited to dual-axis beam expansion. For
example, in a number of embodiments, the waveguide incorporates an
input grating, a fold grating, and an output grating. HUDs
utilizing waveguides can be implemented using varying numbers of
waveguide. In many embodiments, a HUD is implemented using a single
waveguide. In other embodiments, the HUD is implemented using a
stack of waveguides. Multiple waveguides can be stacked and
implemented to provide different optical functions, such as but not
limited to implementing color displays. In several embodiments, the
HUDs incorporate three separate waveguides, one waveguide for each
of a Red, Green, and Blue color channel.
[0054] Waveguides utilizing Bragg gratings in accordance with
various embodiments of the invention can be designed to have
different types of fringes. Use of multiple waveguides having the
same surface pitch sizes but different grating slanted angles can
increase the overall couple-in angular bandwidth of the waveguide.
In a number of embodiments, one or more of the gratings within the
waveguide incorporate a rolling K-vector and/or a slant angle that
varies across the grating to modify the diffraction efficiency of
the grating. The K-vector can be defined as a vector orthogonal to
the plane of the associated grating fringe, which can determine the
optical efficiency for a given range of input and diffracted
angles. By incorporating a grating with rolled K-vectors ("RKVs"),
the gratings can be designed to vary diffraction efficiency in a
manner that achieves desirable characteristics across the eyebox of
the HUD display. Configurations of grating fringes (such as RKVs)
and other aspects relating to the structures and implementations of
waveguides for use in HUDs are discussed below in further
detail.
Diffraction Gratings
[0055] Optical waveguides can incorporate different optical
elements to manipulate the propagation of light waves. As can
readily be appreciated, the type of grating selected can depend on
the specific requirements of a given application. Optical
structures recorded in waveguides can include many different types
of optical elements, such as but not limited to diffraction
gratings. In many embodiments, the grating implemented is a Bragg
grating (also referred to as a volume grating). 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 is can be used to make lossy
waveguide gratings for extracting light over a large pupil. By
strategically placing volume Bragg gratings within a waveguide, the
propagation of light within the waveguide can be affected in a
controlled manner to achieve various effects. The diffraction of
light incident on the grating can be determined by the
characteristic of the light and the grating. As can readily be
appreciated, volume Bragg gratings can be constructed to have
different characteristics depending on the specific requirements of
the given application. In a number of embodiments, the volume Bragg
grating is designed to be a transmission grating. In other
embodiments, the volume Bragg grating is designed to be a
reflection grating. In transmission gratings, incident light
meeting the Bragg condition is diffracted such that the diffracted
light exits the grating on the side which the incident light did
not enter. For reflection gratings, the diffracted light exits on
the same side of the grating as where the incident light
entered.
[0056] FIGS. 1A and 1B conceptually illustrate two volume Bragg
grating configurations in accordance with various embodiments of
the invention. Depending on the side out of which a light ray exits
after diffraction, the grating can be classified as either a
reflection grating 100 or a transmission grating 150. The
conditions for refraction/reflection, or Bragg condition, can
depend several factors, such as but not limited to the refractive
indices of the medium, the grating period, the wavelength of the
incident light, and the angle of incidence. FIG. 1A shows a
reflection grating 100 recorded in a transparent material. As
shown, light rays 101, 102 are of different wavelengths and are
incident at the same angle on the reflection grating 100, which has
fringes 103 that are parallel to the grating surface. Light ray 101
does not meet the Bragg condition and is transmitted through the
grating. On the other hand, light ray 102 does meet the Bragg
condition and is reflected back through the same surface on which
it entered. Another type of grating is a transmission grating,
which is conceptually illustrated in FIG. 1B. In the illustrative
embodiment, the transmission grating 150 has fringes 151 that are
perpendicular to the grating surface. As shown, light rays 152, 153
with different wavelengths are incident on the transmission grating
150 at the same angle. Light ray 152 meets the Bragg condition and
is refracted, exiting on the opposite side of the grating on which
the light ray 152 entered. Light ray 153 does not meet the Bragg
condition and is transmitted through with its original path of
propagation. Depending on the efficiency of the grating, light can
be partially reflected or refracted. Although FIGS. 1A and 1B
illustrate specific volume grating structures, any type of grating
structure can be recorded in a waveguide cell in accordance with
various embodiments of the invention. For example, volume gratings
can be implemented with fringes that are tilted and/slanted
relative to the grating surface, which can affect the angles of
diffraction/reflection. Although the discussions above denote the
grating structures as either transmission or reflection, both types
of gratings behave in the same manner according to the standard
grating equation.
[0057] Waveguide structures in accordance with various embodiments
of the invention can implement gratings in a number of different
ways. In addition to volume gratings, gratings can be implemented
as surface relief gratings. As the name suggests, surface relief
gratings can be implemented by physically forming grooves or
periodic patterns on the surface of the substrate. The periodicity
and angles formed by the grooves can determine the efficiency and
other characteristics of the grating. Any of a number of methods
can be used to form these grooves, such as but not limited to
etching and photolithography.
[0058] FIG. 2 conceptually illustrates a surface relief grating in
accordance with an embodiment of the invention. As shown, the
surface relief grating 200 contains periodic slanted grooves 201.
When light is incident on the grooves 201, diffraction can occur
under certain conditions. The slant and periodicity of the grooves
201 can be designed to achieve targeted diffraction behavior of
incident light.
[0059] Although FIGS. 1A-1B and 2 show specific grating structures,
it is readily appreciable that grating structures can be configured
in a number of different ways depending on the specific
requirements of a given application. Examples of such
configurations are discussed in the sections below in further
detail.
Switchable Bragg Gratings
[0060] One class of 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 glass plates or
substrates. In many cases, the glass plates are in a parallel
configuration. One or both glass plates 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.
[0061] 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.
[0062] Typically, the SBG elements are switched clear in 30 .mu.s
with a longer relaxation time to switch ON. Note that 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 glass plates 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.
[0063] FIGS. 3A and 3B conceptually illustrate HPDLC SBG devices
300, 350 and the switching property of SBGs in accordance with
various embodiments of the invention. In FIG. 3A, the SBG 300 is in
an OFF state. As shown, the LC molecules 301 are aligned
substantially normal to the fringe planes. As such, the SBG 300
exhibits high diffraction efficiency, and incident light can easily
be diffracted. FIG. 3B illustrates the SBG 350 in an ON position.
An applied voltage 351 can orient the optical axis of the LC
molecules 352 within the droplets 353 to produce an effective
refractive index that matches the polymer's refractive index,
essentially creating a transparent cell where incident light is not
diffracted. In the illustrative embodiment, an AC voltage source is
shown. As can readily be appreciated, various voltage sources can
be utilized depending on the specific requirements of a given
application.
[0064] In waveguide cell designs, in addition to the components
described above, adhesives and spacers can be disposed between the
substrates to affix the layers of the elements together and to
maintain the cell gap, or thickness dimension. In these devices,
spacers can take many forms, such as but not limited to materials,
sizes, and geometries. Materials can include, for example, plastics
(e.g., divinylbenzene), silica, and conductive spacers. They can
take any suitable geometry, such as but not limited to rods and
spheres. The spacers can take any suitable size. In many cases, the
sizes of the spacers range from 1 to 30 .mu.m. While the use of
these adhesive materials and spacers can be necessary in LC cells
using conventional materials and methods of manufacture, they can
contribute to the haziness of the cells degrading the optical
properties and performance of the waveguide and device.
HPDLC Material Systems
[0065] HPDLC mixtures in accordance with various embodiments of the
invention generally include LC, monomers, photoinitiator dyes, and
coinitiators. The mixture (often referred to as syrup) frequently
also includes a surfactant. For the purposes of describing the
invention, a surfactant is defined as any chemical agent that
lowers the surface tension of the total liquid mixture. The use of
surfactants in PDLC mixtures is known and dates back to the
earliest investigations of PDLCs. For example, a paper by R. L
Sutherland et al., SPIE Vol. 2689, 158-169, 1996, the disclosure of
which is incorporated herein by reference, describes a PDLC mixture
including a monomer, photoinitiator, coinitiator, chain extender,
and LCs to which a surfactant can be added. Surfactants are also
mentioned in a paper by Natarajan et al, Journal of Nonlinear
Optical Physics and Materials, Vol. 5 No. I 89-98, 1996, the
disclosure of which is incorporated herein by reference.
Furthermore, U.S. Pat. No. 7,018,563 by Sutherland; et al.,
discusses polymer-dispersed liquid crystal material for forming a
polymer-dispersed liquid crystal optical element comprising: at
least one acrylic acid monomer; at least one type of liquid crystal
material; a photoinitiator dye; a coinitiator; and a surfactant.
The disclosure of U.S. Pat. No. 7,018,563 is hereby incorporated by
reference in its entirety.
[0066] 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: [0067] 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. [0068] 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. [0069] 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. [0070] 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. [0071] 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. [0072] 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.
[0073] 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.
[0074] One of the known attributes of transmission SBGs is that the
LC molecules tend to align with an average direction normal to the
grating fringe planes (i.e., parallel to the grating or K-vector).
The effect of the LC molecule alignment is that transmission SBGs
efficiently diffract P polarized light (i.e., light with a
polarization vector in the plane of incidence), but have nearly
zero diffraction efficiency for S polarized light (i.e., light with
the polarization vector normal to the plane of incidence).
Recording Mechanisms for Volume Gratings
[0075] Volume gratings can be recorded in a waveguide cell using
many different methods in accordance with various embodiments of
the invention. The recording of optical elements in optical
recording materials can be achieved using any number and type of
electromagnetic radiation sources. Depending on the application,
the exposure source(s) and/or recording system can be configured to
record optical elements using varying levels of exposure power and
duration. As discussed above with regards to SBGs, techniques for
recording volume gratings can include the exposure of an optical
recording material using two mutually coherent laser beams, where
the superimposition of the two beams create a periodic intensity
distribution along the interference pattern. The optical recording
material can form grating structures exhibiting a refractive index
modulation pattern matching the periodic intensity distribution. In
HPDLC mixtures, the light intensity distribution results in
diffusion and polymerization of monomers into the high intensity
regions and simultaneous diffusion of liquid crystal into the dark
regions. This phase separation creates alternating liquid
crystal-rich and liquid crystal-depleted regions that form the
fringe planes of the grating. The grating structures can be formed
with slanted or non-slanted fringes depending on how the recording
beams are configured. FIGS. 4A-4D conceptually illustrate two-beam
recording processes in accordance with various embodiments of the
invention. As shown, two methods can be used to create two
different types of Bragg gratings--i.e., a transmission grating 400
and a reflection grating 401. Depending on how the two recording
beams 402, 403 are positioned, the interference pattern 404 can
record either a transmission or a reflection grating in an optical
recording material 405. Differences between the two types of
gratings can be seen in the orientation of the fringes (i.e., the
fringes of a reflection volume grating are typically substantially
parallel to the surface of the substrate, and the fringes of a
transmission grating are typically substantially perpendicular to
the surface of the substrate). During playback, a beam 406 incident
on the transmission grating 400 can result in a diffracted beam 407
that is transmitted. On the other hand, a beam 408 that is incident
on the reflection grating 401 can result in a beam 409 that is
reflected.
[0076] Another method for recording volume gratings in an optical
recording material includes the use of a single beam to form an
interference pattern onto the optical recording material. This can
be achieved through the use of a master grating. In many
embodiments, the master grating is a volume grating. In some
embodiments, the master grating is an amplitude grating. Upon
interaction with the master grating, the single beam can diffract.
The first order diffraction and the zero order beam can overlap to
create an interference pattern, which can then expose the optical
recording material to form the desired volume grating. A
single-beam recording process utilizing an amplitude grating in
accordance with an embodiment of the invention is conceptually
illustrated in FIG. 5. As shown, a beam 500 from a single laser
source (not shown) is directed through an amplitude grating 501.
Upon interaction with the grating 501, the beam 500 can diffract
as, for example, in the case of the rays interacting with the black
shaded region of the amplitude grating, or the beam 500 can
propagated through the amplitude grating without substantial
deviation as a zero-order beam as, for example, in the case of the
rays interacting with the cross-hatched region of the amplitude
grating. The first order diffraction beams 502 and the zero order
beams 503 can overlap to create an interference pattern that
exposes the optical recording layer 504 of a waveguide cell. In the
illustrative embodiment, a spacer block 505 is positioned between
the grating 501 and the optical recording layer 504 in order to
alter the distance between the two components.
[0077] Although specific methods of recording volume gratings are
discussed and shown in FIGS. 4A-4D and 5, recording systems in
accordance with various embodiments of the invention can be
configured to implement any of a number of methods for recording
volume gratings.
Rolled K-Vector Gratings and Multiplexed K-Vector Gratings
[0078] In addressing the limited range of wavelengths and angles
over which diffraction occurs in volume Bragg gratings, several
methods can be utilized to increase the diffraction bandwidth of
the gratings. In many embodiments, gratings can employ fringes that
vary with respect to their K-vectors. In a number of embodiments,
the change across the rolled K-vectors is typically such that the
direction of the change in K-vectors is out of plane with the
waveguide or grating element. Varying fringes, or rolled K-vectors,
can be implemented in a number of different ways. In some
embodiments, fringes of gratings are designed to vary in a
progressive manner across the grating. In other embodiments,
different discrete sets of gratings with different fringes are
place serially. Gratings with rolled K-vectors can be designed and
configured in a variety of ways. In many embodiments, the rolled
K-vectors are designed such that the peak diffraction efficiency of
each grating segment is optimized for its corresponding output
angle at that position. In some embodiments, the peak diffraction
efficiency of each grating at different positions is at an offset
with its corresponding output angle at that position. It has been
shown that by introducing this offset, eyebox homogeneity can be
improved. In several embodiments, offsets can improve total image
brightness by a factor of two compared to just matching the peak
diffraction efficiencies at different positions.
[0079] Rolled K-vector gratings can be used to maximize the peak
diffraction efficiency of in-couple light in accordance with an
embodiment of the invention. The use of rolled k-vectors enables
high efficiency input coupling into a grating, and also allows the
beam spread angle to be optimized to minimize the thickness of the
waveguide; this may need balancing the waveguide thickness, the
angular bandwidth of the grating, and the spread of field angles at
any given point on the grating. The low angular response of
gratings as the K-vector is rolled (and surface pitch maintained)
can prevent output coupling, allowing the waveguide thickness to be
minimized. In a number of embodiments, the design aim is to ensure
maximum input coupling at a point and to minimize the angular
diversity such that the grating thickness can be minimized without
reciprocally out-coupling at different point.
[0080] FIGS. 6A and 6B conceptually illustrate two implementations
of rolled K-vector gratings in accordance with various embodiments
of the invention. Referring first to FIG. 6A, in some embodiments a
rolled K-vector grating can be implemented as a waveguide portion
containing discrete grating elements 600 having different
K-vectors. Referring next to FIG. 6B, in several embodiments a
rolled K-vector grating can be implemented as a waveguide portion
containing grating elements 601 within which the K-vectors
undergoes a smooth monotonic variation in direction. As
illustrated, the change in the direction of the K-vectors is out of
plane with the waveguide.
[0081] In many embodiments, different sets of discrete fringes are
superimposed into the same grating, creating a multiplexed grating
with essentially multiple gratings inside the same volume that work
independently and without interfering with each other. For example,
if two volume gratings are recorded in the same device for two
different Bragg wavelengths at the same incidence angle, the device
can diffract the two selected wavelengths into different output
directions with limited crosstalk. Multiplexing can be used to
produce improved angular profiles by combining two gratings of
similar prescription to extend the diffraction efficiency angular
bandwidth and give better luminance uniformity and color balance
across the exit pupil and field of view. Multiplexing can also be
used to encode two distinct diffraction prescriptions which can be
design to project light into distinct field of regions or diffract
light of two different wavelengths into a given field of view
region. Steps can be taken to ensure that there is no competition
between gratings during recording leading to unequal diffraction
efficiencies and crosstalk between gratings in playback.
Multiplexing can also offer the significant benefit of reducing the
number of layers in the waveguide structure. In some embodiments,
at least one of the input, fold, or output gratings can combine two
or more angular diffraction prescriptions to expand the angular
bandwidth. Similarly, in several embodiments, at least one of the
input, fold, or output gratings can combine two or more spectral
diffraction prescriptions to expand the spectral bandwidth. For
example, a color multiplexed grating may be used to diffract two or
more of the primary colors.
[0082] FIG. 7 conceptually illustrates a multiplexed K-vector
grating in accordance with an embodiment of the invention. As
illustrated, the multiplexed grating 700 contains two sets of
fringes 701, 702. The first set 701 is depicted by solid diagonal
lines and has K-vector K.sub.1 and period .LAMBDA..sub.1. The
second multiplexed grating 702 is illustrated by dot-dash lines and
has K-vector K.sub.2 and period .LAMBDA..sub.2. In the illustrated
embodiment, the two grating periods are the same, but the K-vectors
differ in direction. In operation, both of the multiplexed gratings
701, 702 are active and can provide broader incidence and
diffraction bandwidths. The angular bandwidth of incidence
.theta..sub.i for the multiplexed gratings covers the angular range
including the overlapping .theta..sub.i1 and .theta..sub.i2. The
angular bandwidth of diffraction .theta..sub.d for the multiplexed
gratings 701, 702 covers the angular range including the
overlapping .theta..sub.d1 and .theta..sub.d2. In some embodiments,
more than two gratings are multiplexed.
[0083] Although specific grating structures with varying fringes
are discussed above, any of a number of fringe configurations can
be utilized in accordance with specific requirements of a given
application. For example, any number of gratings can be multiplexed
as allowed by manufacturing constraints. Rolled K-vector gratings
can be designed to have K-vectors rolled in any discrete unit.
Waveguides Implementing Pupil Expansion
[0084] Gratings can be implemented in waveguides in a variety of
different ways. In some embodiments, the gratings reside on the
outer surface of the waveguide. In other embodiments, volume
gratings are implemented inside the waveguide. Gratings can also be
implemented to perform different optical functions, such as but not
limited to coupling light, directing light, and preventing the
transmission of light. FIG. 8 conceptually illustrates a waveguide
utilizing coupling gratings to diffract light into and out of the
waveguide in accordance with an embodiment of the invention. As
shown, the waveguide 800 includes a first surface 801, a second
surface 802, an input grating element 803, and an output grating
element 804. Collimated light 805 from a projection lens enters the
waveguide through the first surface 801 at an orthogonal angle. The
light travels through the waveguide 800 at its original angle and,
before reaching the second surface 802 at the other side of the
waveguide 800, interacts with an input grating element 803. The
input grating element 803 can be designed to diffract the light 805
at an oblique angle such that the refracted light 806 is incident
on the second surface 802 at an angle at which total internal
reflection can occur. As such, the light 805 is coupled into the
waveguide and is confined within the first and second surfaces 801,
802 of the waveguide 800. In the illustrative embodiment, the light
travels within the waveguide 800 until it interacts with an output
grating 804, which refracts and couples the light out of the
waveguide 800 and into a user's eye 807.
[0085] In many embodiments, diffraction gratings can be used to
preserve eye box 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 a user's
eyes. FIGS. 9 and 10 conceptually illustrate waveguides utilizing
an output grating for exit pupil expansion in one dimension in
accordance with an embodiment of the invention. The waveguide 900
in FIG. 9 includes a first surface 901, a second surface 902, an
input grating element 903, and an output grating element 904. As
shown, light 905 is coupled into the waveguide 900 by the input
grating 902 and can travel through the waveguide 900 via total
internal reflection. In the illustrative embodiment, the output
grating 904 is extended and designed to refract a portion of the
waveguided light. The light can be refracted such that the
refracted light 906 is incident on the second surface 902 at an
angle at which total internal reflection does not occur, allowing
the light 906 to couple out of the waveguide 900. This lossy
extraction permits exit pupil expansion as the remaining light can
continue to travel within the waveguide 900 and, once the light is
again incident on the output grating 904, the scenario described
above can occur again. Utilizing this technique, a continuous
expanded exit pupil can also be achieved with the correct design,
as shown in FIG. 10.
[0086] Expanding upon the ideas in FIGS. 9 and 10, an optical
waveguide can be designed to expand the exit pupil in two
dimensions. In many embodiments, two waveguides can be stacked
together to create a system where light coupled into the waveguide
stack can achieve exit pupil expansion in two dimensions. FIG. 11
conceptually illustrates a waveguide system utilizing two planar
waveguides to provide exit pupil expansion in two dimensions in
accordance with an embodiment of the invention. As shown, the
system 1100 includes a first waveguide 1101 and a second waveguide
1102. The first waveguide 1101 can include a first input coupling
grating 1103 and a first output coupling grating 1104, and the
second waveguide 1102 can include a second input coupling grating
1105 and a second output coupling grating 1106. The first input
coupling grating 1103 can be designed to couple collimated light
1107 from an image source 1108 into the first waveguide 1101.
Similar to the systems as described in FIGS. 9 and 10, the confined
light can travel through the first waveguide 1101 via total
internal reflection until the light reaches the first output
coupling grating 1104. In the illustrative embodiment, the first
output coupling grating 1104 is designed to provide lossy exit
pupil expansion in a first dimension and to couple the light out of
the first waveguide 1101. The second input coupling grating 1105
can be designed to receive light outputted from the first waveguide
1101, which is expanded in the first dimension, and refract the
received light such that the received light travels through the
second waveguide 1102 via total internal reflection. In many
embodiments, the first output coupling grating 1104 and the second
input coupling grating 1106 are extended in a similar manner. The
light traveling through the second waveguide 1102 can then interact
with the second output coupling grating 1106. In the illustrative
embodiment, the second output coupling grating 1106 is designed to
provide lossy exit pupil expansion in a second dimension that is
different from the first dimension and to couple the light out of
the second waveguide 1102. As a result, the exit pupil is expanded
in two dimensions, allowing for a smaller lens size with respect to
the eye box size 1109.
[0087] In many embodiments, the optical waveguide utilizes a fold
grating, which can provide exit pupil expansion in one dimension
while directing the light within the waveguide. In further
embodiments, the fold grating directs the light towards an output
grating, which can provide exit pupil expansion in a second
dimension that is different from the first direction and also
couples the light out of the waveguide. By using the fold grating,
the waveguide display can require fewer layers than other systems
and methods of displaying information. In addition, by using fold
grating, light can travel by total internal refection within the
waveguide in a single rectangular prism defined by the waveguide
outer surfaces while achieving dual pupil expansion. As a result, a
two-dimension exit pupil expansion can be achieved using a single
waveguide. FIG. 12 conceptually illustrates a waveguide utilizing a
three-grating structure to provide two dimensional exit pupil
expansion in accordance with an embodiment of the invention. As
shown, the waveguide 1200 includes an input grating 1201, a fold
grating 1202, and an output grating 1203. Arrows 1204-1206 on the
gratings 1201-1203 show the k-vector associated with each grating.
In many embodiments, the fold grating 1202 can be designed to
provide exit pupil expansion in one dimension and to redirect the
direction of light propagating via total internal reflection from
the input grating 1201. In the illustrative embodiment, the fringes
of the fold gratings 1202 are at a 45 degree offset from either of
the other two gratings 1201, 1203. Light incident on the fold
grating is redirected 1207, 1208 to propagate towards the output
grating 1203, which provides exit pupil expansion in a second
dimension and couples the light out of the waveguide 1200.
[0088] Although the discussions above relating to FIGS. 8-12
describe specific waveguide structures, it is readily appreciated
that any number of waveguide structure configurations can be
utilized in accordance with specific requirements of a given
application. For example, gratings providing exit pupil expansion
can be designed with a gradient efficiency such that the portion of
light refracted changes depending on the area of incident.
Waveguide Layer Stacks
[0089] Waveguides in accordance with various embodiments of the
invention can be stacked together to implement certain optical
functions. For example, in many embodiments, the device can include
a stack of RGB diffracting layers, each layer comprising input and
output gratings. In each layer the SBGs are recorded to provide
peak diffraction efficiency vs. wavelength characteristics (along
the waveguide) shifted by small increments from the peak
wavelength. In some embodiments, RGB SBG layers are used and can be
switched sequentially and synchronously with RGB LEDs image
sources. FIG. 13 conceptually illustrates a profile view of an RGB
stack of waveguides 1300 in accordance with an embodiment of the
invention. In the illustrative embodiment, wavelength selective
absorptive layers 1301-1303 are used to selectively absorb unwanted
light in each waveguide layer 1304-1306. Dashed lines represent
weak coupling due to either off-polarization or off Bragg. The
stack of waveguides further includes various filters and waveplates
1307-1311. Polarization orientations are depicted with respect to
the input grating.
[0090] Although FIG. 13 illustrates a specific structure of a
waveguide stack, any of a number of stacking configuration can be
used in accordance with specific requirements of a given
application. For example, in many embodiments, only two layers, red
and blue/green, are used to implement an RGB stack. Such a system
can be achieved using several methods. In some embodiments,
multiplexed gratings containing different sets of gratings, each
correlating with an RGB color, are used to implement multiple color
waveguides in one waveguide layer.
Waveguide Displays
[0091] Waveguide displays in accordance with various embodiments of
the invention can be implemented and constricted in many different
ways. For example, waveguide displays can contain a varying number
of waveguide layers and different exit pupil expansion scheme. FIG.
14 conceptually illustrates a dual axis expansion waveguide display
with two grating layers in accordance with an embodiment of the
invention. As shown, the waveguide display 1400 includes a light
source 1401, a microdisplay panel 1402, and an input image node
("IIN") 1403 optically coupled to a waveguide 1404 having two
grating layers. In some embodiments, the waveguide is formed by
sandwiched the grating layers between glass or plastic substrates
to form a stack within which total internal reflection occurs at
the outer substrate and air interfaces. In several embodiments, the
stack can further comprise additional layers such as beam splitting
coatings and environmental protection layers. In the illustrative
embodiment, each grating layer contains an input grating 1405A,
1405B, a fold grating exit pupil expander 1406A, 1406B, and an
output grating 1407A, 1407B where characters A and B refer to the
first and second waveguide layers. The input grating, fold grating,
and the output grating can be holographic gratings, such as a
switchable or non-switchable SBG. As used herein, the term grating
may encompass a grating can include a set of gratings, such as
multiplexed gratings or sets of discrete rolled K-vector gratings.
In the illustrative embodiment, the IIN 1403 integrates the
microdisplay panel 1402, the light source 1401, and optical
components needed to illuminate the display panel, separate the
reflected light, and collimate it into the required FOV. In the
embodiment of FIG. 14 and in the embodiments to be described below,
at least one of the input, fold, and output gratings can be
electrically switchable. In many embodiments, all three grating
types are passive (i.e., non-switching). In a number of
embodiments, the IIN can project the image displayed on the
microdisplay panel such that each display pixel is converted into a
unique angular direction within the substrate waveguide. The
collimation optics contained in the IIN can include lens and
mirrors. In further embodiments, the lens and mirrors are
diffractive lenses and mirrors.
[0092] In the illustrative embodiment, the light path from the
source to the waveguide via the IIN is indicated by rays 1408-1411.
The input grating 1405A, 1405B of each grating layer can couple a
portion of the light into a TIR path in the waveguide 1404, such
path being represented by the rays 1412, 1413. The output gratings
1407A, 1407B can diffract light out of the waveguide into angular
ranges of collimated light 1414, 1415 respectively for viewing by
the eye 1416. The angular ranges, which correspond to the field of
view of the display, can be defined by the IIN optics. In some
embodiments, the waveguide gratings can encode optical power for
adjusting the collimation of the output. In several embodiments,
the output image is at infinity. In other embodiments, the output
image may be formed at distances of several meters from the eye
box. Typically, the eye is positioned within the exit pupil or eye
box of the display.
[0093] Different IIN implementations and embodiments can be
utilized as discussed and taught in U.S. patent application Ser.
No. 13/869,866, entitled Holographic Wide Angle Display, and U.S.
patent application Ser. No. 13/844,456, entitled Transparent
Waveguide Display, the disclosures of which are hereby incorporated
by reference in their entireties. In some embodiments, the IIN
contains a beamsplitter for directing light onto a microdisplay and
transmitting the reflected light towards the waveguide. In many
embodiments, the beamsplitter is a grating recorded in HPDLC and
uses the intrinsic polarization selectivity of such gratings to
separate the light illuminating the display and the image modulated
light reflected off the display. In several embodiments, the beam
splitter is a polarizing beam splitter cube. In a number of
embodiments, the IIN incorporates a despeckler. Despecklers are
discussed in U.S. Pat. No. 8,565,560, entitled Laser Illumination
Device, the disclosure of which is hereby incorporated by reference
in its entirety.
[0094] The light source can be a laser or LED and can include one
or more lenses for modifying the illumination beam angular
characteristics. The image source can be a micro-display or laser
based display. LED can provide better uniformity than laser. If
laser illumination is used, there is a risk of illumination banding
occurring at the waveguide output. In many embodiments, laser
illumination banding in waveguides can be overcome using the
techniques and teachings disclosed in U.S. patent application Ser.
No. 15/512,500, entitled Method and Apparatus for Generating Input
Images for Holographic Waveguide Displays, the disclosure of which
is hereby incorporated by reference in its entirety. In some
embodiments, the light from the light source is polarized. In
several embodiments, the image source is a liquid crystal display
(LCD) micro display or liquid crystal on silicon (LCoS) micro
display.
[0095] In some embodiments, similar to the one shown in FIG. 14,
each grating layer addresses half the total field of view.
Typically, the fold gratings are clocked (i.e., tilted in the
waveguide plane) at 45 degrees to ensure adequate angular bandwidth
for the folded light. In other embodiments, other clock angles can
be used to satisfy spatial constraints on the positioning of the
gratings that can arise in the ergonomic design of the display. In
some embodiments, at least one of the input and output gratings
have rolled k-vectors. Rolling the K-vectors can allow the angular
bandwidth of the grating to be expanded without the need to
increase the waveguide thickness.
[0096] In many embodiments, the fold grating's angular bandwidth
can be enhanced by designing the grating prescription to provide
dual interaction of the guided light with the grating. Exemplary
embodiments of dual interaction fold gratings are disclosed in U.S.
patent application Ser. No. 14/620,969, entitled Waveguide Grating
Device, the disclosure of which is hereby incorporated in its
entirety.
[0097] FIG. 15 conceptually illustrates a plan view 1500 of a
single grating layer similar to the ones used in FIG. 14 in
accordance with an embodiment of the invention. The grating layer
1501, which is optically coupled to the IIN 1502, includes input
grating 1503, a first beamsplitter 1504, a fold grating 1505, a
second beamsplitter 1506, and an output grating 1507. The
beamsplitters can be partially transmitting coatings which
homogenize the waveguided light by providing multiple reflection
paths within the waveguide. Each beamsplitter can include more than
one coating layer with each coating layer being applied to a
transparent substrate. Typical beam paths from the IIN up to the
eye 1508 are indicated by the rays 1509-1513.
[0098] FIG. 16 conceptually illustrates a plan view 1600 of a two
grating layer configuration in accordance with an embodiment of the
invention. As shown, the grating layers 1601A, 1601B, which are
optically coupled to the IIN 1602, includes input gratings 1603A,
1603B, first beamsplitters 1604A, 1604B, fold gratings 1605A,
1605B, second beamsplitters 1606A, 1606B and output gratings 1607A,
1607B, where the characters A, B refer to the first and second
grating layers, respectively. In the illustrated embodiment, the
gratings and beams splitters of the two layers substantially
overlap.
[0099] In many embodiments, the grating layer can be broken up into
separate layers. For example, in some embodiments, a first layer
includes the fold grating while a second layer includes the output
grating. In further embodiments, a third layer can include the
input grating. In such embodiments, the number of layers can then
be laminated together into a single waveguide substrate. In several
embodiments, the grating layer includes a number of pieces,
including the input coupler, the fold grating, and the output
grating (or portions thereof) that are laminated together to form a
single substrate waveguide. The pieces can be separated by optical
glue or other transparent material of refractive index matching or
substantially similar that of the pieces.
[0100] In many embodiments, the grating layer can be formed via a
cell making process by creating cells of the desired grating
thickness and vacuum filling each cell with SBG material for each
of the input coupler, the fold grating, and the output grating. In
some embodiments, the cell can be formed by positioning multiple
plates of glass with gaps between the plates of glass that define
the desired grating thickness for the input coupler, the fold
grating, and the output grating. In several embodiments, one cell
can be made with multiple apertures such that the separate
apertures are filled with different pockets of SBG material. Any
intervening spaces can then be separated by a separating material
(e.g., glue, oil, etc.) to define separate areas. In a number of
embodiments, the SBG material can be spin-coated onto a substrate
and then covered by a second substrate after curing of the
material.
[0101] In many embodiments, the input coupler, the fold grating,
and the output grating can be created by interfering two waves of
light at an angle within the substrate to create a holographic wave
front, thereby creating light and dark fringes that are set in the
waveguide substrate at a desired angle. Additional, such optical
elements can also be fabricated using any of the various methods
described in the above sections.
[0102] In one embodiment, the input coupler, the fold grating, and
the output grating embodied as SBGs can be Bragg gratings recorded
in a holographic polymer dispersed liquid crystal (HPDLC) (e.g., a
matrix of liquid crystal droplets), although SBGs may also be
recorded in other materials. In one embodiment, SBGs are recorded
in a uniform modulation material, such as POLICRYPS or POLIPHEM
having a matrix of solid liquid crystals dispersed in a liquid
polymer. The SBGs can be switching or non-switching in nature. In
its non-switching form a SBG has the advantage over conventional
holographic photopolymer materials of being capable of providing
high refractive index modulation due to its liquid crystal
component. Exemplary uniform modulation liquid crystal-polymer
material systems are disclosed in United State Patent Application
Publication No.: US2007/0019152 by Caputo et al and PCT Application
No.: PCT/EP2005/006950 by Stumpe et al. both of which are
incorporated herein by reference in their entireties. Uniform
modulation gratings are characterized by high refractive index
modulation (and hence high diffraction efficiency) and low
scatter.
[0103] In many embodiments, the input coupler, the fold grating,
and the output grating is made of a reverse mode HPDLC material.
Reverse mode HPDLC differs from conventional HPDLC in that the
grating is passive when no electric field is applied and becomes
diffractive in the presence of an electric field. The reverse mode
HPDLC may be based on any of the recipes and processes disclosed in
PCT Application No. PCT/GB2012/000680, entitled Improvements to
Holographic Polymer Dispersed Liquid Crystal Materials and Devices,
the disclosure of which is hereby incorporated in its entirety. The
grating can be recorded in any of the above material systems but
used in a passive (non-switching) mode. The fabrication process is
identical to that used for switched but with the electrode coating
stage being omitted. LC polymer material systems are highly
desirable in view of their high index modulation. In some
embodiments, the gratings are recorded in HPDLC but are not
switched.
[0104] In many embodiments, the input grating can be replaced by
another type of input coupler, such as but not limited to a prism
and a reflective surface. In some embodiments, the input coupler
can be a holographic grating, such as an SBG grating or a passive
grating, which can be a passive SBG grating. The input coupler can
be configured to receive collimated light from a display source and
to cause the light to travel within the waveguide via total
internal reflection between the first surface and the second
surface to the fold grating. The input coupler can be orientated
directly towards or at an angle relative to the fold grating. For
example, in several embodiments, the input coupler can be set at a
slight incline in relation to the fold grating. In a number of
embodiments, the fold grating can be oriented in a diagonal
direction. The fold grating can be configured to provide pupil
expansion in a first direction and to direct the light to the
output grating via total internal reflection inside the
waveguide.
[0105] In many embodiments, a longitudinal edge of each fold
grating is oblique to the axis of alignment of the input coupler
such that each fold grating is set on a diagonal with respect to
the direction of propagation of the display light. The fold grating
can be angled such that light from the input coupler is redirected
to the output grating. In some embodiments, the fold grating is set
at a forty-five-degree angle relative to the direction that the
display image is released from the input coupler. This feature can
cause the display image propagating down the fold grating to be
turned into the output grating. For example, in several
embodiments, the fold grating can cause the image to be turned 90
degrees into the output grating. In this manner, a single waveguide
can provide dual axis pupil expansion in both the horizontal and
vertical directions. In a number of embodiments, each of the fold
grating can have a partially diffractive structure. In some
embodiments, each of the fold gratings can have a fully diffractive
structure.
[0106] The output grating can be configured to provide pupil
expansion in a second direction different than the first direction
and to cause the light to exit the waveguide from the first surface
or the second surface. The output grating can receive the display
image from the fold grating via total internal reflection and can
provide pupil expansion in a second direction. In many embodiments,
the output grating includes multiple layers of substrate, thereby
comprising multiple layers of output gratings. Accordingly, there
is no requirement for gratings to be in one plane within the
waveguide, and gratings may be stacked on top of each other (e.g.,
cells of gratings stacked on top of each other).
[0107] In many embodiments, a quarter wave plate on the substrate
waveguide rotates polarization of a light ray to maintain efficient
coupling with the SBGs. The quarter wave plate can be coupled to or
adhered to the surface of substrate waveguide. For example, in some
embodiments, the quarter wave plate is a coating that is applied to
substrate waveguide. The quarter wave plate can provide light wave
polarization management. Such polarization management can help
light rays retain alignment with the intended viewing axis by
compensating for skew waves in the waveguide. The quarter wave
plate is optional and can increase the efficiency of the optical
design in implementations. In several embodiments, the waveguide
does not include the quarter wave plate. The quarter wave plate may
be provided as multi-layer coating.
[0108] In many embodiments, the waveguide display can be operated
in monochrome. In some embodiments, the waveguide display can be
operated in color. Operating in color can be achieved using a stack
of monochrome waveguides of similar design to the one in FIG. 14.
The design can use red, green, and blue waveguide layers as shown
or, alternatively, red and blue/green layers. FIG. 17 conceptually
illustrates a dual axis expansion waveguide display 1700 that
includes a light source 1701, a microdisplay panel 1702, and an IIN
1703 optically coupled to red, green, and blue waveguides 1704R,
1704G, 1704B, with each waveguide including two grating layers in
accordance with an embodiment of the invention. In the illustrative
embodiment, the three waveguides are separated by air gaps. In some
embodiments, the waveguides are separated by a low index material
such as a nanoporous film. As shown, the red grating layer labelled
by R includes an input grating 1705R, 1706R, a fold grating exit
pupil expander 1707R, 1708R, and an output grating 1709R, 1710R.
The grating elements of the blue and green waveguides are labeled
using the same numerals with B, G designating blue and green. In
some embodiments, the input, fold, and output gratings are all
passive, that is non-switching. In several embodiments, at least
one of the gratings is switching. In a number of embodiments, the
input gratings in each layer are switchable to avoid color
crosstalk between the waveguide layers. In many embodiments, color
crosstalk can be avoided by disposing dichroic filters 1711, 1712
between the input grating regions of the red and blue and the blue
and green waveguides. In a variety of embodiments, a color
waveguide can be implemented using just one grating layer in each
monochromatic waveguide
[0109] FIG. 18 conceptually illustrates an eye tracker display in
accordance with an embodiment of the invention. Waveguide device
based eye trackers are discussed in PCT Application No.
PCT/GB2014/000197, entitled Holographic Waveguide Eye Tracker, PCT
Application No. PCT/GB2015/000274, entitled Holographic Waveguide
Optical Tracker, and PCT Application No. PCT/GB2013/000210,
entitled Apparatus for Eye Tracking, the disclosures of which are
hereby incorporated in their entireties. Turning again to FIG. 18,
the eye tracked display 1800 includes a dual axis expansion
waveguide display based on any of the embodiments described above.
The waveguide display can include a waveguide 1801 containing at
least one grating layer incorporating an input fold and output
grating, the IIN 1802, an eye tracker including waveguide 1803,
infrared detector 1804, and infrared source 1805. The eye tracker
and display waveguides can be separated by an air gap or by a low
refractive material. As explained in the above references, the eye
tracker can comprise separate illumination and detector waveguides.
In the illustrative embodiment, the optical path from the infrared
source to the eye is indicated by the rays 1806-1808, and the
backscattered signal from the eye is indicated by the rays 1809,
1810. The optical path from the input image node through the
display waveguide to the eye box is indicated by the rays
1811-1813.
[0110] In many embodiments, a dual expansion waveguide display can
further include a dynamic focusing element. FIG. 19 conceptually
illustrates a dual expansion waveguide display 1900 with a dynamic
focusing element 1901 disposed in proximity to a principal surface
of the waveguide display and an eye tracker in accordance with an
embodiment of the invention. In some embodiments, the dynamic
focusing element is an LC device. In several embodiments, the LC
device combines an LC layer and a diffractive optical element. In a
number of embodiments, the diffractive optical element is an
electrically controllable LC-based device. In various embodiments,
the dynamic focusing element is disposed between the waveguide
display and the eye tracker. In a variety of embodiments, the
dynamic focusing element can be disposed in proximity to the
surface of the display waveguide furthest from the eye.
[0111] The dynamic focus device can provide a multiplicity of image
surfaces 1902. In light field display applications, at least four
image surfaces can be used. The dynamic focusing element can be
based on dynamic focusing elements described in U.S. patent
application Ser. No. 15/553,120 entitled, Electrically Focus
Tunable Lens, the disclosure of which is hereby incorporated in its
entirety. In some embodiments, a dual expansion waveguide display
having a dynamic focusing element and an eye tracker can provide a
light field display, such as those based on the teachings disclosed
in U.S. patent application Ser. No. 15/543,013, entitled
Holographic Waveguide Light Field Displays, the disclosure of which
is hereby incorporated by reference in its entirety.
[0112] Although specific waveguide structures are discussed above,
any of a number of waveguide structures can be implemented
depending on the specific requirements of a given application. For
example, in many waveguide configurations, the input, fold, and
output gratings are formed in a single layer sandwiched by
transparent substrates. Such a configuration is shown in FIG. 14,
where two layers are stacked as such. In some embodiments, the
waveguide includes just one grating layer. In several embodiments,
switching transparent electrodes are applied to opposing surfaces
of the substrate layers sandwiching the switching grating. In a
number of embodiments, the cell substrates can be fabricated from
glass. One glass substrate that can be used is standard Corning
Willow glass substrate (index 1.51) which is available in
thicknesses down to 50 micrometers. In other embodiments, the cell
substrates can be optical plastics.
[0113] In many embodiments, the waveguide display is coupled to the
IIN by an opto-mechanical interface that allows the waveguide to be
easily retracted from the IIN assembly. The basic principle is
conceptually illustrated in FIG. 20A. FIG. 20A shows a dual axis
expansion waveguide display 2000 including a waveguide 2001
containing an input grating 2002, a fold grating 2003, an output
grating 2004, and an IIN 2005. The apparatus further includes an
optical link 2006 connected to the waveguide, a first optical
interface 2007 terminating the optical link, and a second optical
interface 2008 forming the exit optical port of the IIN. The first
and second optical interfaces can be decoupled as indicated by gap
2009 shown in FIG. 20B. In some embodiments, the optical link is a
waveguide. In several embodiments, the optical link is curved. In a
number of embodiments, the optical link is a GRIN image relay
device. In a variety of embodiments, the optical connection is
established using a mechanical mechanism. In some embodiments, the
optical connection is established using a magnetic mechanism. The
advantage of decoupling the waveguide from the IIN in helmet
mounted display applications is that the near eye portion of the
display can be removed when not in used. In some embodiments where
the waveguide includes passive gratings, the near eye optics can be
disposable.
[0114] FIG. 21 conceptually illustrates an IIN 2100 having a
microdisplay panel 2101, a spatially-varying NA component 2102, and
microdisplay optics 2103 in accordance with an embodiment of the
invention. As shown, the microdisplay optics 2103 accepts light
2104 from an illumination source (not illustrated) and deflects the
light onto the microdisplay in the direction indicated by ray 2105.
The light reflected from the microdisplay is indicated by the
divergent ray pairs 2106-2108 with numerical aperture ("NA") angles
varying along the X axis. In the illustrative embodiment, the
spatially-varying NA component is disposed between the microdisplay
optics and the microdisplay. In other embodiments, the
spatially-varying NA component is disposed adjacent the output
surface of the microdisplay optics. FIG. 22 conceptually
illustrates such an embodiment, shown by spatially-varying NA
component 2200.
[0115] In many embodiments, the microdisplay is a reflective
device. In some embodiments, the microdisplay is a transmission
device, typically a transmission LCoS device. FIG. 23 conceptually
illustrates an IIN 2300 including a backlight 2301, a microdisplay
2302, and a variable NA component 2303 in accordance with an
embodiment of the invention. Light from the backlight indicated by
the rays 2304-2306, which typically has a uniform NA across the
backlight, illuminates the back surface of the microdisplay and,
after propagation through the variable NA component, is converted
into output image modulated light indicated by the divergent ray
pairs 2307-2309 with NA angles varying along the X axis.
[0116] In many embodiments, the principles of the invention may be
applied to an emissive display. Examples of emissive displays for
use with the invention include ones based on LED arrays and light
emitting polymers arrays. FIG. 24 conceptually illustrates an IIN
2400 having an emissive microdisplay 2401 and a spatially-varying
NA component 2402 in accordance with an embodiment of the
invention. Light from the microdisplay indicated by rays 2403-2405,
which typically has a uniform NA across the emitting surface of the
display, illuminates the spatially-varying NA component and is
converted into output image modulated light indicated by divergent
ray pairs 2406-2408 with NA angles varying along the X axis.
[0117] In many embodiments, the microdisplay optics includes a
polarizing beam splitter cube. In some embodiments, the
microdisplay optics includes an inclined plate to which a beam
splitter coating has been applied. In a number of embodiments, the
microdisplay optics includes a waveguide device comprising a SBG,
which acts as a polarization selective beam splitter. Details
relating to such embodiments are discussed U.S. patent application
Ser. No. 13/869,866, entitled Holographic Wide Angle Display, and
U.S. patent application Ser. No. 13/844,456, entitled Transparent
Waveguide Display, the disclosures of which are hereby incorporated
in their entireties. In several embodiments, the microdisplay
optics contains at least one of a refractive component and curved
reflecting surfaces or a diffractive optical element for
controlling the numerical aperture of the illumination light. In
some embodiments, the microdisplay optics contains spectral filters
for controlling the wavelength characteristics of the illumination
light. In a number of embodiments, the microdisplay optics contains
apertures, masks, filter, and coatings for controlling stray light.
In many embodiments, the microdisplay optics incorporate birdbath
optics.
[0118] Although FIGS. 14-24 describe specific waveguide displays
and structures, any waveguide display system and configuration can
be used as appropriate to the specific requirements of a given
application. At its core, waveguides are simply used to manipulate
the direction of light. This property can be used generally in a
variety of different systems. An example of a general system that
can utilize waveguides is shown in FIG. 25. FIG. 25 conceptually
illustrates a system diagram showing components for waveguide
displays in accordance with various embodiments of the invention.
As shown, the system 2500 utilizes a light source 2501 that can
output light into a waveguide 2502. Light sources used can be a
variety of different systems. In some embodiments, the light source
2501 is from a projector. In many embodiments, the light source
2501 further includes a microdisplay panel and optical components
needed to illuminate the display panel. In further embodiments, the
light source 2501 includes collimators and other optical components
for manipulating the light into a desired form before entry into
the waveguide. In other embodiments, the light source 2501 is
natural light. Once the light source 2501 outputs light into the
waveguide 2502, the waveguide 2502 can then manipulate and redirect
light in a desired manner out and into a receiver 2503. Waveguides
can be any general waveguides known within the art and/or one of
the waveguides as described above. A receiver can be any of a
number of components capable of receiving light from the waveguide.
In many embodiments, the receiver 2503 is a user's eye(s). In some
embodiments, the receiver 2503 is another waveguide. In several
embodiments, the receiver 2503 is a display capable of displaying
the light from the waveguide 2502. In further embodiments, the
display is simply glass that can reflect the light onto another
receiver. The system 2500 can optionally include a switching device
2504 and electrical components for use in conjunction with SBGs. In
many embodiments, the switching device 2504 can optionally receive
data from the light source in order to introduce a voltage to turn
the SBGs in an ON position at the appropriate times.
Waveguide Exposure Processes and Mastering Systems
[0119] In addition to the exposure schemes described in FIGS. 4A-4D
for the recording of both transmission and reflection waveguides,
many mastering techniques can be used to perform such recordings
and to form various waveguide structures and gratings. In various
embodiments, the mastering system includes the use of an amplitude
grating ("AG"). Such gratings can be used to form various types of
gratings with different configurations. In many embodiments, an
amplitude master grating is used to form an RKV grating within a
waveguide. In further embodiments, the amplitude grating contains a
linear variation in the grating period, called a chirp. Chirped
gratings can be utilized in many different ways. FIG. 26
conceptually illustrates an exposure process utilizing a chirped
amplitude grating in accordance with an embodiment of the
invention. As shown, the process includes the use of an input beam
2600 interacting with a focusing element 2601 to provide 1D focus.
A zero order input beam 2602 can be directed towards a chirped AG
2603, providing a diffraction profile with a linear variation. The
two beams can then combine to form a desired interference pattern
to expose a waveguide substrate 2604. As shown, separation distance
from the exposure plane and the origin of the diffracted beam(s)
can be crucial. In the illustrative embodiment, a transparent
spacer block 2605 is used to control the separation distance.
[0120] In many embodiments, a single beam exposure system can be
used in conjunction with the amplitude grating to form gratings
within a waveguide. FIG. 5 conceptually illustrates one such
embodiment. Use of a single beam in a near to contact copy mode can
be considered a hybrid between a direct contact copy and a separate
two-beam contact copy--i.e., a hybrid contact copy. This approach
can be useful where it is not possible to make a direct contact
copy, such as where the separation distance from the master plane
to the exposure plane is not negligible. In such circumstances, the
separation distance can be critical. For example, in the exposure
process for an RKV grating, the separation distance can be
important and should be accounted for in order to preserve the
surface projected fringe period across the full RKV grating
(without which full waveguide path reciprocity cannot be
maintained). In several embodiments, a single plane wavefront input
beam can be configured to interact with a cylindrical lens to
provide 1D focus. In further embodiments, at least a portion of the
light can generate a diffractive beam through interaction with a
chirped master, and another portion can pass through (with
attenuation) as zero order, preserving the original 1D focus
function of the cylindrical lens.
[0121] In many embodiments, a master can be designed to incorporate
more than one amplitude grating. By incorporating multiple
amplitude gratings in a single master, alignment errors can be
reduced compared to systems utilizing a single master for each
grating. In some embodiments, the mastering system includes a
master with three amplitude gratings. In several embodiments, the
master can be developed to incorporate the RKV functionality in the
simultaneous exposure of three patterns written in one plate. The
input and/or output master gratings can be chirped gratings, with
additional gratings as needed in zero order regions where there is
no overlap with the chip.
[0122] In many embodiments, mastering multiple grating elements
within a waveguide structure can involve the use of multiple
exposures. In such embodiments, a multi-step process can be used
wherein different regions corresponding to different grating
elements of the contact copy element are exposed. In many such
embodiments, the process can include sequentially exposing the
contact copy. For example, the process can include first exposing
an output grating region (e.g., using a large area O/P only master
or part of a multi-grating master) and then multiple exposures to
form the fold grating region.
[0123] Masters incorporating more than one amplitude grating can
also be utilized for the simultaneous exposure of more than one
grating. In such systems, a collimated or coherent incident light
beam is generally brought into focus via optics through a master AG
and onto the desired regions of the contact copy through a suitable
transparent substrate material. FIGS. 27A and 27B conceptually
illustrate an exposure process for forming three gratings
simultaneously in accordance with an embodiment of the invention.
As shown, in many such embodiments, optics are used to directed
collimated light onto the desired grating regions simultaneously to
effect formation of multiple gratings (e.g., input, output and
fold) in one exposure. In the illustrative embodiment, the process
includes the use of a master 2700 containing grating, a glass plate
2701 for adjusting the separation distance, and a contact copy
substrate 2702 as material in which the gratings are recorded. The
process utilizes a collimated beam 2703 directed at three mirrors
2704-2706, which redirects the beam 2703 towards the master grating
2700. Once the beam 2703 is incident upon the master grating 2700,
an exposure/curing process similar to one described in FIG. 5 can
occur, forming three different gratings simultaneously on the
contact copy 2702. In many embodiments, a 3:1 mastering process can
be utilized to fabricate a holographic waveguide having input, fold
and output gratings in a single exposure. In a variety of such
embodiments, the input, fold, and/or output gratings are RKV
gratings. In a number of embodiments, the fold grating may be
segmented into multiple zones. Although any number of zones can be
utilized in accordance with the requirements of specific
applications, in some embodiments of the invention the fold grating
may be divided into 5 segments.
[0124] Although FIGS. 27A and 27B show a specific number and
arrangement of gratings to be formed, it will be understood that
any number and arrangement of such gratings can be provided.
Similarly, any number and arrangement of illumination beams can be
provided. For example, as shown in FIG. 28, where a three color
waveguide is required, beams can be optically arranged to allow for
beams for red 2800, green 2801, and blue 2802 channels to be
incident through the master onto the contact copy area. Although
FIGS. 27A, 27B, and 28 show conceptual drawings of the mastering
systems and arrangement, it will be understood that these
conceptual elements can take the form of any suitable optical
frames, movable adapters, exposure plates, etc. required to allow
for the fixation of optical elements relative to the master and
contact copy regions.
[0125] As previously described, the holographic waveguides
implemented in association with the mastering and fabrication
embodiments can be a single piece and/or as a stack of waveguides
in accordance with the requirements of specific applications of
embodiments of the invention. For example, a holographic waveguide
can include three layers, one for each of red, blue, and green. A
conceptual illustration of a holographic waveguide having three
layers is shown and discussed in relation to FIG. 13.
[0126] In the typical RKV grating, the grating vector rolls in the
same plane as the incident plane of the construction beams. In the
fold grating, the grating vector can roll perpendicular to the
incident plane of the construction beams. Turning now to FIGS.
29A-29C, conceptual illustrations of a variety of approaches for
generating RKV gratings in accordance with a number of embodiments
of the invention are shown. Many embodiments include a stepped fold
RKV, where the angle in each section changes orthogonally to the
K-vector direction, as shown in FIGS. 29B and 29C. Cross term MUX
limitations are overcome by the scanned beam, single input angle at
any single time approach. Several embodiments include scanned RKV
fold gratings, where a scanned beam exposes the RKV with a
different angle in discrete steps across the aperture of the plane
fold master grating to generate a stepped master. As previously
discussed, in a number of embodiments, only a single input beam
angle is used to illuminate the fold master at any given time.
[0127] In addition to the discussion above, mastering systems in
accordance with many embodiments of the invention can employ
chirped gratings for various other purposes, as shown in FIG. 30A.
These chirped gratings can aid in correcting when the incident beam
on the exposure plane is not collimated and for differences between
the master and the holographic waveguide being fabricated. For
example, the holographic waveguide is typically inside a waveguide
cell of finite thickness while the master typically has a thin
protective cover applied to prevent damage to the chrome. Chirped
gratings can be utilized to compensate for these additional layers.
Any of a variety of protective coatings, such as glass and
SiO.sub.2 protective layers, can be utilized as appropriate to the
requirements of specific applications of embodiments of the
invention.
[0128] A conceptual illustration of a dual chirped grating master
in accordance with an embodiment of the invention is shown in FIG.
30B. 3:1 dual chirped gratings provide a variety of advantages over
prior art manufacturing techniques. RKVs can dramatically improve
efficiency and uniformity of the holographic waveguides. RKV inputs
can provide more input coupling to the waveguide, and RKV outputs
can provide better pupil forming, allowing for improved brightness.
As discussed above, allowing exposure of RKV input and output
grating (and fold) at the same time can reduce the total
manufacturing/process time. In many embodiments, both gratings
share the same spacer and/or optical density between master and
holographic waveguide, so the RKV profiles and the spacer windows
may be configured to be balanced.
[0129] A variety of mastering systems in accordance with
embodiments of the invention utilize zero order gratings. Zero
order gratings can be used to control the transmittance of the zero
order beam, so that it would be close to the transmittance of
chirped grating to allow a continuous beam ratio. This prevents a
discontinuity on the exposure (and hence diffraction efficiency in
the copied grating part). A conceptual illustration of a mastering
system utilizing a zero order grating along with a chirped grating
in accordance with an embodiment of the invention in shown in FIG.
31. In a variety of embodiments, a zero order grating does not have
a diffraction order or the diffraction order does not interfere
with the system. The orientation of the master and/or energy beam
can be used to control the direction of the unwanted diffracting
beam, but then account needs to be taken of the relative
polarizations of the grating and the zero order beam. To eliminate
the diffraction, in various embodiments, the period of the grating
can be smaller than the limit to get evanescent diffraction wave.
In a variety of embodiments, using a master grating having a
similar transmittance and/or period as the chirp grating at the
boundary allows for a seamless copy to be created in the liquid
crystal substrate.
[0130] In many embodiments, mastering systems utilize a reference
grating to align the lens position to get accurate grating period.
A conceptual illustration of such a system is provided in FIG. 32,
showing reference gratings K.sub.13 (input) and K.sub.33 (output)
relative to input chirp (K.sub.11) and 0 order plane (K.sub.12)
gratings, output chirp (K.sub.32) and 0 order plane gratings
(K.sub.32), and fold grating (K.sub.2). The reference grating can
help improve the 3:1 construction by allowing exposure of input,
fold, and output gratings at the same time, which reduces the total
manufacture time, providing high accuracy: the accuracy of the
grating alignment is given by the master, which can be accurate to
0.1 nm, and a compact design, which make it attractive for larger
volume manufacture. In several embodiments, the energy beam is
collimated. To ensure that a collimated incident beam for the
generation or RKV grating is being used, a reverse ray tracing can
be employed by tracing the ideal construction rays from the
holographic waveguide to the lens, modifying the window thickness
is used to change the focus of the beam relative to the hologram
plane, and shifting the lens can achieve collimated beam output
from the master to the liquid crystal substrate.
[0131] In several embodiments, mastering systems avoid other order
beams from being created and/or interfering with the energy beam
being focused on the liquid crystal substrate, as shown in FIG. 33.
In order to avoid the undesired order beam hitting the grating
area, the window thickness can be adjusted, the glass can be
modified, the RKV profile can be changed, the lens and/or change
the incident beam angle can be changed, and/or a low refractive
index material can be used to make the undesired order totally
reflected by the low refractive index material.
[0132] In a number of embodiments, mastering systems utilize one or
more diffraction means, where the same position in the master needs
to generate two different refracted beams, which is not possible
unless they are different diffraction orders. To simplify the
fabrication, the master is disposed in a configuration where there
is no cross over of diffracted beams. In a variety of embodiments,
the master is placed as close as possible to the liquid crystal
substrate. In several embodiments, the master is placed far enough
where the cross over ends, but this can induce large separation
between the 0 order and the diffracted beam. When the master is
placed too far away, crossover of the diffracted beam can occur
(see, e.g., FIG. 34). When the master is placed too close,
undesired diffracted orders can hit the liquid crystal substrate.
Therefore, mastering systems in accordance with embodiments of the
invention limit the distance from the liquid crystal substrate to
master plate is to a range. To increase this range, high index
glass can be used to reduce the ray angle so that it takes longer
travel for the diffracted beam to cross over. Additionally, short
wavelength exposure can also reduce the ray angle, which helps as
well. In many embodiments where high index plates are used in the
master stack, Fresnel reflections need to be managed; particularly
for high index plates between the master and the copy plane.
Doctrine of Equivalents
[0133] Although specific systems and methods are discussed above,
many different embodiments can be implemented in accordance with
the invention. It is therefore to be understood that the present
invention can be practiced in ways other than specifically
described, without departing from the scope and spirit of the
present invention. Thus, embodiments of the present invention
should be considered in all respects as illustrative and not
restrictive. Accordingly, the scope of the invention should be
determined not by the embodiments illustrated, but by the appended
claims and their equivalents. Although specific embodiments have
been described in detail in this disclosure, many modifications are
possible (for example, variations in sizes, dimensions, structures,
shapes and proportions of the various elements, values of
parameters, mounting arrangements, use of materials, colors,
orientations, etc.). For example, the position of elements may be
reversed or otherwise varied and the nature or number of discrete
elements or positions may be altered or varied. Accordingly, all
such modifications are intended to be included within the scope of
the present disclosure. The order or sequence of any process or
method steps may be varied or re-sequenced according to alternative
embodiments. Other substitutions, modifications, changes, and
omissions may be made in the design, operating conditions and
arrangement of the exemplary embodiments without departing from the
scope of the present disclosure.
* * * * *