U.S. patent application number 16/360491 was filed with the patent office on 2019-07-18 for method and apparatus for generating input images for holographic waveguide displays.
This patent application is currently assigned to DigiLens Inc.. The applicant listed for this patent is DigiLens Inc., Rockwell Collins, Inc.. Invention is credited to Alastair John Grant, Milan Momcilo Popovich, Jonathan David Waldern.
Application Number | 20190219822 16/360491 |
Document ID | / |
Family ID | 53794357 |
Filed Date | 2019-07-18 |
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United States Patent
Application |
20190219822 |
Kind Code |
A1 |
Popovich; Milan Momcilo ; et
al. |
July 18, 2019 |
Method and Apparatus for Generating Input Images for Holographic
Waveguide Displays
Abstract
An image generation device comprises: a spatial light modulator;
a source of light; a beam deflector; an illumination waveguide and
an image transport waveguide, each waveguide containing at least
one switchable grating; and a coupler for directing scanned light
into a first set of TIR paths in said illumination waveguide. A
switchable grating in the illumination waveguide diffracts light
onto said SLM, a switchable grating in said image transport
waveguide diffracting image-modulated from the SLM into a waveguide
path.
Inventors: |
Popovich; Milan Momcilo;
(Leicester, GB) ; Waldern; Jonathan David; (Los
Altos Hills, CA) ; Grant; Alastair John; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens Inc.
Rockwell Collins, Inc. |
Sunnyvale
Cedar Rapids |
CA
IA |
US
US |
|
|
Assignee: |
DigiLens Inc.
Sunnyvale
CA
Rockwell Collins, Inc.
Cedar Rapids
IA
|
Family ID: |
53794357 |
Appl. No.: |
16/360491 |
Filed: |
March 21, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15512500 |
Mar 17, 2017 |
10241330 |
|
|
PCT/GB2015/000203 |
Jun 29, 2015 |
|
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16360491 |
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62071277 |
Sep 19, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/425 20130101;
G02B 2027/0174 20130101; G02B 27/4205 20130101; G02B 27/017
20130101; G02F 1/29 20130101; G02B 6/0035 20130101; G02B 2027/0118
20130101; G02B 27/0103 20130101; G02B 27/01 20130101; G02B
2027/0165 20130101 |
International
Class: |
G02B 27/01 20060101
G02B027/01; G02F 1/29 20060101 G02F001/29; F21V 8/00 20060101
F21V008/00; G02B 27/42 20060101 G02B027/42 |
Claims
1. An image generation device comprising: an image source
configured to project collimated image-modulated light of a first
wavelength over a field of view comprising a plurality of portions
of the field of view; and a waveguide supporting a plurality of
switchable gratings each switchable between a non-diffracting state
and a diffracting state and configured to couple said
image-modulated light into said waveguide, wherein said plurality
of switchable gratings are configured to switch into their
diffracting states in synchronism with the projection of one or
more selected portions of said field of view by said image source,
wherein said plurality of switchable gratings in their diffracting
states provide an aperture for coupling light into said waveguide,
and wherein said switchable gratings are configured to provide a
different aperture for each said field of view portion.
2. The apparatus of claim 1, wherein said switchable gratings
exhibit high diffraction efficiency with no voltage applied and low
diffraction efficiency with a voltage applied.
3. The apparatus of claim 1, wherein said switchable gratings
exhibit low diffraction efficiency with no voltage applied and high
diffraction efficiency with a voltage applied.
4. The apparatus of claim 1, wherein said aperture is replicated by
total internal reflection within said waveguide.
5. The apparatus of claim 1, wherein said switchable gratings are
disposed in more than one layer.
6. The apparatus of claim 1, wherein said field of view portions
are projected in time sequence.
7. The apparatus of claim 1, wherein said image source provides
second wavelength light and said waveguide supports a second
plurality of switchable gratings for diffracting said second
wavelength.
8. The apparatus of claim 7, wherein one or more of said first
plurality of gratings are switched into a diffracting state when
said first wavelength image modulated light is projected and
wherein one or more of said second plurality of gratings are
switched into a diffracting state when said second wavelength image
modulated light is projected.
9. The apparatus of claim 1, wherein said waveguide is a component
of a waveguide display.
10. The apparatus of claim 1, wherein said waveguide further
comprises at least one set of gratings selected from the group of:
gratings providing beam expansion in at least one dimension,
gratings providing extraction of said light from said waveguide,
gratings providing field of view tiling, gratings with spatially
varying refractive index modulation, and gratings with spatially
varying thickness.
11. The apparatus of claim 1, wherein said switchable gratings each
have a unique angular diffraction characteristic.
12. The apparatus of claim 1, further comprising at least one of a
collimating lens and a despeckler.
13. The apparatus of claim 1, wherein said waveguide is selected
from the group of a curved waveguide and plastic waveguide.
14. The apparatus of claim 1, wherein said image source employs a
light source comprising one of a laser or a light emitting
diode.
15. The apparatus of claim 1, wherein said switchable gratings are
recorded in a liquid crystal and polymer holographic recording
material.
16. The apparatus of claim 1, wherein said switchable gratings
include at least one grating selected from the group of: a uniform
modulation HPDLC grating, a non-switching HPDLC grating a reverse
mode HPDLC grating and a surface relief grating.
17. The apparatus of claim 1, wherein said image source comprises:
a spatial light modulator; a source emitting first wavelength
light; a beam deflector for forming said light into a scanned beam;
an illumination waveguide containing at least one switchable
grating disposed in at least one layer; an image transport
waveguide containing at least one switchable grating disposed in at
least one layer; and a coupler for directing said scanned beam into
a first set of total internal reflection paths in said illumination
waveguide; wherein said at least one switchable grating in said
illumination waveguide is configured to diffract light out of said
first set of total internal reflection paths onto said spatial
light modulator; d) wherein said at least one switchable grating in
said image transport waveguide is configured to diffract the
image-modulated light from said spatial light modulator into a
second set of total internal reflection paths in said image
transport waveguide.
18. The method of claim 17, wherein the extent of said at least one
grating along said image transport waveguide defines a coupling
aperture, and wherein said coupling aperture defines a numerical
aperture for each pixel of said spatial light modulator.
19. A method of viewing an image comprising the steps of: providing
an image source for projecting collimated image-modulated light of
a first wavelength over a field of view comprising a plurality of
portions of the field of view, and a waveguide supporting a
plurality of switchable gratings each switchable between a
non-diffracting state and a diffracting state and configured to
couple said image-modulated light into said waveguide; providing an
output grating for extracting light from said waveguide for
viewing; projecting a first field of view portion; switching a set
of the plurality of switchable gratings into their diffracting
states to form an aperture for coupling said first field of view
portion into said waveguide; coupling said first field of view
portion into a total internal reflection path in said waveguide;
and extracting said first field of view portion light from said
waveguide via said output grating.
20. The method of claim 19, further comprising the steps of
projecting a second field of view portion and repeating the steps
of projecting, switching, coupling and extracting.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/512,500, entitled "Method and Apparatus for
Generating Input Images for Holographic Waveguide Displays," filed
Jun. 29, 2015, which is a U.S. National Phase of PCT Application
No. PCT/GB2015/000203, filed Jun. 29, 2015, which claims the
priority of U.S. Provisional Patent Application No. 62/071,277,
entitled "Method and Apparatus for Generating Input Images for
Holographic Waveguide Displays," filed Sep. 19, 2014, the
disclosures of which are incorporated herein by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an apparatus for generating an
image, and more particularly to an apparatus for generating an
input image for a holographic waveguide display.
[0003] The invention addresses the problem of providing uniform
output illuminination in holographic waveguide displays. U.S.
patent application Ser. No. 13/844,456 entitled TRANSPARENT
WAVEGUIDE DISPLAY, PCT Application No.: GB2012/000677 entitled
WEARABLE DATA DISPLAY, U.S. patent application Ser. No. 13/317,468
entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, 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 all of which are incorporated herein
by reference in their entireties.
[0004] The invention addresses a particular category of holographic
waveguides based on Switchable Bragg Gratings (SBGs). SBGs are
fabricated by first placing a thin film of a mixture of
photopolymerizable monomers and liquid crystal material between
parallel glass plates. One or both glass plates support electrodes,
typically transparent indium tin oxide films, for applying an
electric field across the film. A volume phase grating is then
recorded by illuminating the liquid material (often referred to as
the syrup) with two mutually coherent laser beams, which interfere
to form a slanted fringe grating structure. During the recording
process, the monomers polymerize and the mixture undergoes a phase
separation, creating regions densely populated by liquid crystal
micro-droplets, interspersed with regions of clear polymer. The
alternating liquid crystal-rich and liquid crystal-depleted regions
form the fringe planes of the grating. The resulting volume phase
grating can exhibit very high diffraction efficiency, which may 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
is changed causing the refractive index modulation of the fringes
to reduce and the hologram diffraction efficiency to drop to very
low levels. Note that the diffraction efficiency of the device can
be adjusted, by means of the applied voltage, over a continuous
range. 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
may be used to control the LC orientation. In certain types of
HPDLC phase separation of the LC material from the polymer may be
accomplished to such a degree that no discernible droplet structure
results. SBGs may be used to provide transmission or reflection
gratings for free space applications. In waveguide applications the
parallel glass plates used to form the HPDLC cell provide a total
internal reflection (TIR) light guiding structure. Light is
"coupled" out of the SBG when the switchable grating diffracts the
light at an angle beyond the TIR condition. Typically, the HPDLC
used in SBGs comprise liquid crystal (LC), monomers, photoinitiator
dyes, and coinitiators. The mixture frequently includes a
surfactant. The patent and scientific literature contains many
examples of material systems and processes that may be used to
fabricate SBGs. Two fundamental patents are: U.S. Pat. No.
5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et
al. Both filings describe monomer and liquid crystal material
combinations suitable for fabricating SBG devices. One of the known
attributes of transmission SBGs is that the LC molecules tend to
align normal to the grating fringe planes. The effect of the LC
molecule alignment is that transmission SBGs efficiently diffract P
polarized light (ie light with the polarization vector in the plane
of incidence) but have lower diffraction efficiency for S polarized
light (ie light with the polarization vector normal to the plane of
incidence.
[0005] Waveguides offer many features that are attractive in HMDs
and HUDs. They are thin and transparent. Wide fields of views can
be obtained by recording multiple holographs and tiling the field
of view regions formed by each hologram. A key feature of these
waveguides is that they provide pupil expansion in two orthogonal
directions. The pupil expansion in a given direction is achieved by
diffracting equal amounts of light out of output grating toward the
eye box at each beam grating interaction. Uniformity of output is
achieved by designing the output grating to have diffraction
efficiency varying from a low value near the input end of the
waveguide to a high value at the furthest extremity of the output
grating. The inventors refer to grating having such properties as
lossy gratings. The diffraction efficiency profile along the
waveguide may be controlled by varying one or both of the grating
refractive index modulation and the grating thickness. According to
the theory of Bragg gratings higher index modulations give higher
peak efficiency and narrow diffraction efficiency angular
bandwidths. Reducing the thickness of the grating leads to a
decrease in the diffraction efficiency and a broadening of the
diffraction efficiency angular bandwidth. The input image data is
provided by a microdisplay external to the waveguide. The
microdisplay which is usually a reflective array must be
illuminated via a beam splitter. The reflected image light is
collimated such that each pixel of the image provides a parallel
beam in a unique direction. Finally, the image light must be
coupled efficiently into the waveguide so that the image content
can by transferred to the waveguide components used for orthogonal
pupil expansion. The image light from the microdisplay is normally
coupled into the waveguide via an input grating. Alternatively a
prism may be used.
[0006] A major design challenge is coupling the image content into
the waveguide efficiently and in such a way the waveguide image is
free from chromatic dispersion and brightness non uniformity. To
overcome chromatic dispersion and to achieve the best possible
collimation it is desirable to use lasers. However, lasers and
other narrow band sources such as LEDs suffer from the problem of
pupil banding artifacts which manifest themselves as output
illumination non uniformity. Banding artifacts are formed when the
collimated pupil is replicated (expanded) in a TIR waveguide. In
very basic terms the light beams diffracted out of the waveguide
each time the beam interacts with the grating have gaps or
overlaps. This leads to an illumination ripple. The degree of
ripple is a function of field angle, waveguide thickness, and
aperture thickness.
[0007] The inventors have found by experiment and simulation that
the effect of banding can be smoothed by dispersion with broadband
sources such as LEDs. The effects are therefore most noticed in
narrowband (e.g. laser) illumination sources.
[0008] There is a requirement for an image generator for
illuminating a microdisplay, collimating the reflected image light
from the microdisplay and efficiently coupling it into a thin
holographic waveguide with high efficiency, with low chromatic
dispersion and with high illumination uniformity.
[0009] There is a further requirement for a waveguide display
comprising an image generator for illuminating a microdisplay,
collimating the reflected image light from the microdisplay and
efficiently coupling it into a thin holographic waveguide with high
efficiency, with low chromatic dispersion and with high
illumination uniformity, and holographic waveguides for providing
pupil expansion in orthogonal directions.
SUMMARY OF THE INVENTION
[0010] It is a first object of the invention to provide an image
generator for illuminating a microdisplay, collimating the
reflected image light from the microdisplay and efficiently
coupling it into a thin holographic waveguide with high efficiency,
with low chromatic dispersion and with high illumination
uniformity.
[0011] It is a second object of the invention to provide a
waveguide display comprising an image generator for illuminating a
microdisplay, collimating the reflected image light from the
microdisplay and efficiently coupling it into a thin holographic
waveguide with high efficiency, with low chromatic dispersion and
with high illumination uniformity, and holographic waveguides for
providing pupil expansion in orthogonal directions.
[0012] The objects of the invention are met in a first embodiment
in which there is provided an image generation device comprising: a
spatial light modulator (SLM); a source emitting first wavelength
light; a beam deflector for forming the light into a scanned beam;
an illumination waveguide containing at least one switchable
grating disposed in at least one layer; an image transport
waveguide containing at least one switchable grating disposed in at
least one layer; and a coupler for directing the scanned beam into
a first set of TIR paths in the illumination waveguide. The at
least one switchable grating in the illumination waveguide
diffracts light out of the first set of TIR paths onto the SLM. The
at least one switchable grating in the image transport waveguide
diffracts image-modulated from the SLM into a second set of TIR
paths in the image transport waveguide.
[0013] In one embodiment the extent of the at least one grating
along the image transport waveguide defines a coupling aperture,
wherein the coupling aperture defines a numerical aperture for each
pixel of the SLM.
[0014] In one embodiment at least one grating in the illumination
waveguide and at least one grating in the image transport waveguide
are switched into their diffracting states simultaneously.
[0015] In one embodiment the switchable grating in the illumination
waveguide comprises an array of elements each having a unique
angular diffraction characteristic.
[0016] In one embodiment the switchable grating in the image
transport waveguide comprises an array of elements each having a
unique angular diffraction characteristic.
[0017] In one embodiment regions of the SLM are illuminated
cyclically the regions being updated with new image information in
phase with the illumination, wherein the at least one grating in
the illumination waveguide and the at least one grating in the
image transport waveguide are switched synchronously with the SLM
image formation updates.
[0018] In one embodiment a collimating lens is disposed between the
illumination waveguide and the image transport waveguide.
[0019] In one embodiment the apparatus further comprises a source
emitting second wavelength light wherein the illumination grating
waveguide and the image transport waveguide each contain at least
one grating for diffracting the first wavelength light and at least
one grating for diffraction second wavelength light, wherein the
SLM displays first wavelength image information when the first
wavelength diffracting gratings are in their diffracting states,
wherein the SLM displays second wavelength image information when
the second wavelength diffracting gratings are in their diffracting
states.
[0020] In one embodiment the image transfer waveguide provides an
optical path to a waveguide display device.
[0021] In one embodiment further comprises a lossy grating
operative to provide a spatial variation of intensity across the
beam.
[0022] In one embodiment the coupler comprises a prism or
grating.
[0023] In one embodiment the SLM, the illumination waveguide and
the image transport waveguide are configured in a stack.
[0024] In one embodiment the apparatus further comprises
despeckler.
[0025] In one embodiment the switchable grating layers are recorded
in one of a HPDLC reverse mode HPDLC, uniform modulation.
[0026] In one embodiment the source is a laser or LED.
[0027] In one embodiment the SLM is a liquid crystal device.
[0028] In one embodiment of the invention there is provided a
method of generating a wave guided image comprising the steps of:
[0029] a) providing a SLM, a illumination waveguide containing at
least one switchable grating layer, an image transport waveguide
containing at least one switchable grating layer, a laser module, a
micro mirror and a coupler; [0030] b) the micro-mirror sweeping
light from the laser module, through a defined angular range the
light being injected into the illumination waveguide; [0031] c)
switching a SBG layer in the illumination waveguide to diffract
light onto the SLM; [0032] d) switching a SBG layer in the image
transport waveguide to diffract light reflected from the SLM into a
waveguide path in the image transport waveguide.
[0033] In one embodiment of the invention there is provide a method
of generating a wave guided image wherein at least one grating in
the illumination waveguide and at least one grating in the image
transport waveguide are switched into their diffracting states
simultaneously.
[0034] In one embodiment there is provided a method of generating a
wave guided image comprising the steps of: [0035] a) providing a
SLM, a illumination waveguide containing at least one switchable
grating layer, an image transport waveguide containing at least one
switchable grating layer, a laser module, a micro mirror and a
coupler; [0036] b) the micro-mirror sweeping light from the laser
module through a defined angular range the light being injected
into the illumination waveguide; [0037] c) sequentially update
regions of the SLM with image information; [0038] d) switching a
SBG layer in the illumination waveguide to diffract light onto a
region of the SLM. [0039] e) switching a SBG layer in the image
transport waveguide to diffract light reflected from the SLM into a
waveguide path in the image transport waveguide.
[0040] In one embodiment there is provide a method of generating a
wave guided image wherein simultaneously at least one grating in
the illumination waveguide is switched into a first diffracting
state and least one grating in the image transport waveguide is
switched into a second diffracting state and a region of the SLM is
updated with image information.
[0041] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, wherein like index
numerals indicate like parts. For purposes of clarity, details
relating to technical material that is known in the technical
fields related to the invention have not been described in
detail.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a schematic cross section view of an image
generator in one embodiment of the invention.
[0043] FIG. 2 is a schematic cross section view of the image
generator of FIG. 1 illustrating its operation in a pupil switching
mode in one embodiment of the invention.
[0044] FIG. 3 is a schematic cross section view of an image
generator in one embodiment of the invention.
[0045] FIG. 4 is a schematic cross section view of the image
generator of FIG. 3 illustrating its operation in an aperture
switching mode in one embodiment of the invention.
[0046] FIG. 5 is a schematic cross section view of an image
generator incorporating a waveguide despeckler in one embodiment of
the invention.
[0047] FIG. 6 is a flow chart illustrated a method of generating an
image using a pupil switching mode.
[0048] FIG. 7 is a flow chart illustrated a method of generating an
image using an aperture switching mode.
DETAILED DESCRIPTION OF THE INVENTION
[0049] The invention will now be further described by way of
example only with reference to the accompanying drawings. 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. For the purposes of explaining the
invention 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 not to obscure the basic principles
of the invention. Unless otherwise stated the term "on-axis" in
relation to a ray or a beam direction refers to propagation
parallel to an axis normal to the surfaces of the optical
components described in relation to the invention. In the following
description the terms light, ray, beam and direction may be used
interchangeably and in association with each other to indicate the
direction of propagation of light energy along rectilinear
trajectories. Parts of the following description will be presented
using terminology commonly employed by those skilled in the art of
optical design. It should also be noted that in the following
description of the invention repeated usage of the phrase "in one
embodiment" does not necessarily refer to the same embodiment.
[0050] In one embodiment the apparatus comprises a microdisplay 1,
illumination waveguide layer 2, an image transport waveguide layer
which is divided into image transport waveguide aperture 3A and an
image transport waveguide layer 3B, The illumination waveguide
layer contains a three layer SBG 4 with the separate layers
indicated by 4A,4B,4C. The image transport grating aperture
contains a SBG layer 5 with the separate layers indicated by
5A,5B,5C. The apparatus further comprises laser module 6 micro
mirror 7, prism 8, a SBG module comprising lossy SBG layer 9
sandwiched by the substrates 10,11 and a coupling prism 12 abutting
the illumination waveguide layer 2 for coupling scanned light into
the illumination waveguide layer, and a collimation lens 13.
[0051] Note that although three SBG layers are used in the
illumination waveguide layer, the invention may be applied with
more or fewer SBG layers according to the angular range
requirement. The number layers will be roughly equal to the angular
range to be covered divided by the diffraction efficiency angular
bandwidth of the SBGs. The micro mirror will have a rotation range
equal to cover the required angular range to cover the angular
acceptance range of the microdisplay. The collimation lens 13 has a
relative aperture of typically F/2. However other relative aperture
may be used as dictated by the system requirements. The image
transport waveguide is typically less than 1 mm in thickness. The
coupling prism 12 is not an essential feature of the invention and
could be replaced by a grating. The purpose of the lossy SBG 9 is
to expand the input laser beam across the height of the
microdisplay pixel column. Typically this distance would be a few
millimeters.
[0052] The invention provides a method and apparatus for
time-sequential aperture switching using SBGs to create a
multiplicity of time distributed spatially discrete pupil apertures
in a waveguide optic that are temporally integrated to construct a
spatially homogenized expanded pupil. The temporal integration of
the multiple pupils serves to reduce the contrast of banding
artifacts that arise from spatially discrete pupil replication
across the waveguide. Perfect homogenization will normally only be
achieved for a singe field angle. Typically, this will be at the
centre of the field of view. In other regions of the field of view
bright banding or dark banding may occur depending on the field
angle range. Pupil artifacts manifest themselves in two different
ways according to whether the display is beyond the limits of eye
accommodation or within the eye accommodation range. Beyond the
limits of eye accommodation: that is when the display is close to
the eye, as in the case of Head Mounted Displays (HMDs) and smart
eyewear (eye relief around 30 mm or less), uniformity variations
are seen as a function of field angle. The contrast of these
artifacts can vary as a function of pupil diameter (influenced
display brightness and/or see-thru brightness). Within the eye
accommodation distance pupil artifacts can be directly imaged when
the eye focuses on the pupil and not at infinity; this can occurs
in Head Up Displays (HUDs) and Head Down Displays (HDDs) where the
display is several inches from the eye. This is distracting to
display users, impairing the functionality of the display; a user
focusing on a pupil artifact will then not be focusing on the far
field image projected by the display.
[0053] The invention provides two primary modes of operation based
on the switching of the micro mirror and the SBG elements in the
two waveguides. The first mode, which is illustrated in FIGS. 1-2,
is pupil switching that is aperture switching at the input to the
image transport waveguide aperture. The second mode, which is
illustrated in FIGS. 3-4, is field switching, that is switching at
the LCoS to control the portion of the LCoS that is illuminated at
any given time. We next consider the propagation of light through
the embodiment of FIG. 1. Collimated light 1000 from the laser is
swept by the microscanner into a range of ray directions such as
1001. The light is then expanded by the lossy SBG to provide the
light 1002 incident on the coupling prism which enters the TIR path
indicated by 1003. The function of the SBGs in the illumination
waveguide is to illuminate the desired microdisplay column, the
SBGs being switch in synchronism with the rotation of the micro
mirror. Each SBG will have a unique diffracting characteristic
design to diffract a set of angularly separated ray directions from
the microscanner into a range of output angularly separated ray
directions incident on the image surface of the microdisplay. The
incident and diffracted angles from a given SBG element are given
by the Bragg equation. In FIG. 1 an active SBG 4D element deflects
the TIR beam towards a column 1A of the microdisplay in the
direction 1004. From consideration of FIG. 1 it should be apparent
that by sweeping the micro mirror through a given angular range and
selectively switching SBG elements into their diffracting state
during the sweep time any given column of pixels will be
illuminated by a range of incidence angles. For a reflective
microdisplay as shown in FIG. 1 these rays are in turn reflected
into opposing angles towards the collimation lens. The extreme rays
reflected from the microdisplay column 1A are indicated by 1005 and
1007 which give rise to collimated beam represented by the extreme
rays 1008,1009 which enter the image transport waveguide aperture
to illuminate the SBG array 5. In FIG. 1 the SBG element 5B is in
its diffracting state and all others are in their non-diffracting
states. The block arrow 1012 indicates the direction of scrolling
of the SBG elements in synchronism with the scanning of the beam.
The ray 1008 incident on element 5B is diffracted in to the TIR
path indicated by 1010 within the image transport waveguide
aperture and proceeds into the image transport waveguide as the TIR
light 1011. In the envisaged applications of the invention the
light 1011 would be wave guided to separate waveguide grating
elements for pupil expansion in orthogonal directions. FIG. 2
illustrates the pupil switching process used in the embodiment of
FIG. 1 in more detail with the filed regions diffracted by the
elements of transport waveguide being indicated by the boundary
rays originating at the microdisplay column 1A indicated
1006A-1006F ad -th boundary collimated rays by 1013A-1013B.
[0054] We next consider the aperture switching mode referring to
FIG. 3. The apparatus is similar to that of FIG. 1 except that the
image transport waveguide aperture 20A now contains three SBG array
layers, 21,22,23. Each stack of elements such as 21A,22A,23A
receives light from three separate portions of the microdisplay.
Only one stack diffracts at any time. All elements of all other
SBGs are in their non diffracting states. This process is shown in
more detail in FIG. 4 which shows the microdisplay divided into the
three regions 1B,1C,1D. Ray paths from the centre column of each
region to the SBG stack elements 21A,22A,23A by the rays paths
1021-1026, which include the paths through the collimator lens 13
and the SBG stack, are illustrated. In one embodiment all three
SBGs in the stack are in their active state simultaneously. In one
embodiment the SBGs are switched simultaneously. In one embodiment
the SBGs are switched synchronously with the switching of the
microdisplay columns such that for example: when the microdisplay
columns 1B are displayed the element 21A is in its active state,
when the microdisplay columns 1C are displayed the element 22A is
in its active state, and so on. The captured light proceeds via TIR
into the image transport waveguide 20A.
[0055] It should be apparent from consideration of the drawings and
the above description that pupil switching and aperture switching
can be combined in a single image generation process.
[0056] This embodiment would use the same optical components as the
embodiments discussed above with an appropriate switching procedure
for the illumination waveguide and image transport waveguide SBGs.
The chief limitation on the switching scheme would be the switching
on and off times for a SBG element.
[0057] The above described pupil switching and aperture switching
schemes are characterised by five synchronized switching events
which are described below with reference to FIG. 2 and FIG. 4. In a
first event the micro-mirror sweeps a laser beam, through a defined
angular range the light being injected into the illumination
waveguide. The following two events (second and third) are
concerned with pupil switching. In the second event the diffracting
SBG grating layer in the illumination waveguide is switched
(between the three layers). This SBG switching covers the angular
bandwidth required to fill the full acceptance angle of the
microdisplay. The angular range covered by each SBG layer and
microscanner sweep corresponds with the image transport waveguide
aperture element selected. In the third event the SBG element under
the illuminated portion of the image transport waveguide aperture
is switched to a diffracting state. The following two events
(fourth and fifth) are concerned with field switching. In the
fourth event three defined portions of the microdisplay are time
sequentially illuminated by SBG elements of the illumination
waveguide layer. Note that only a small field angle is illuminated
at a given time. In the fifth event SBG elements in stack of SBG
elements in the image transfer waveguide aperture are switching
into their diffracting states in synchronism with the switching of
the SBG elements of the illumination waveguide layer.
[0058] In the embodiment of FIG. 5, which is identical to that of
FIG. 1, the illumination waveguide also includes a waveguide
despeckler 30. The despeckler is desirable a waveguide despeckler
based on principles disclosed in PCT Application No.:
PCT/GB2013/000500 entitled WAVEGUIDE FOR HOMOGENIZING ILLUMINATION,
and U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATION DEVICE both
of which are incorporated herein by reference in their
entireties.
[0059] In one embodiment illustrated in the flow diagram of FIG. 6
a method of generating a image using the pupil switching scheme of
the embodiment of FIG. 1 is provided. Referring to the flow
diagram, we see that the said method comprises the following
steps:
[0060] At step 2000 provide a SLM, a illumination waveguide layer
containing at least one switchable grating layer, an image
transport waveguide layer containing at least one switchable
grating layer, an image transport waveguide containing at least one
switchable grating layer, a laser module, a micro mirror and a
coupler;
[0061] At step 2001 the micro-mirror sweeps light from said laser
module, through a defined angular range, the light being injected
into the illumination waveguide
[0062] At step 2002 switch a grating layer in the illumination
waveguide to diffract light onto the SLM.
[0063] At step 2003 switch a grating layer in the image transport
waveguide to diffract light reflected from the SLM into a waveguide
path in the image transport waveguide.
[0064] In one embodiment illustrated in the flow diagram of FIG. 7
a method of generating a image using the pupil switching scheme of
the embodiment of FIG. 3 is provided. Referring to the flow
diagram, we see that the said method comprises the following
steps:
[0065] At step 2010 provide a SLM, a illumination waveguide layer
containing at least one switchable grating layer, an image
transport waveguide layer containing at least one switchable
grating layer, an image transport waveguide containing at least one
switchable grating layer, a laser module, a micro mirror and a
coupler;
[0066] At step 2011 the micro-mirror sweeps light from said laser
module, through a defined angular range the light being injected
into the illumination waveguide
[0067] At step 2012 sequentially update regions of the SLM with
image information.
[0068] At step 2013 switch a grating layer in the illumination
waveguide to diffract light onto a region of the SLM.
[0069] At step 2014 switch a grating layer in the image transport
waveguide to diffract light reflected from the SLM into a waveguide
path in the image transport waveguide.
[0070] It should be emphasized that the drawings are exemplary and
that the dimensions have been exaggerated. For example thicknesses
of the SBG layers have been greatly exaggerated. In any of the
above embodiments the waveguides may be curved or formed from a
mosaic of planar or curved facets.
[0071] An image generator based on any of the above-described
embodiments may be implemented using plastic substrates using the
materials and processes disclosed in PCT Application No.:
PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER
DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. Advantageously, the
SBGs are recorded in a reverse mode HPDLC material in which the
diffracting state of SBG occurs when an electric field is applied
across the electrodes. An eye tracker based on any of the
above-described embodiments may be implemented using reverse mode
materials and processes disclosed in the above PCT application.
[0072] While the invention may be applied with gratings of any type
including switching or non-switching gratings based on Bragg
(volume) holograms, or surface-relief gratings the preferred
grating technology is a SBG, which offers the advantages of fast
switching, high optical efficiency and transparency and high index
modulation.
[0073] The method of fabricating the SBG pixel elements and the ITO
electrodes used in any of the above-described embodiments of the
invention may be based on the process disclosed in the PCT
Application No. US2006/043938, entitled METHOD AND APPARATUS FOR
PROVIDING A TRANSPARENT DISPLAY.
[0074] It should be understood by those skilled in the art that
while the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. Various
modifications, combinations, sub-combinations and alterations may
occur depending on design requirements and other factors insofar as
they are within the scope of the appended claims or the equivalents
thereof.
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