U.S. patent application number 15/883811 was filed with the patent office on 2018-08-02 for electro-holographic light field generators and displays.
The applicant listed for this patent is The Charles Stark Draper Laboratory, Inc.. Invention is credited to Steven J. Byrnes, Dennis M. Callahan, Gregg E. Favalora, Ian W. Frank, Juha-Pekka J. Laine, Michael G. Moebius, Joseph J. Register.
Application Number | 20180217414 15/883811 |
Document ID | / |
Family ID | 61198913 |
Filed Date | 2018-08-02 |
United States Patent
Application |
20180217414 |
Kind Code |
A1 |
Byrnes; Steven J. ; et
al. |
August 2, 2018 |
Electro-Holographic Light Field Generators and Displays
Abstract
An electro-holographic light field generator device is
disclosed. The light field generator device has an optical
substrate with a waveguide face and an exit face. One or more
surface acoustic wave (SAW) optical modulator devices are included
within each light field generator device. The SAW devices each
include a light input, a waveguide, and a SAW transducer, all
configured for guided mode confinement of input light within the
waveguide. A leaky mode deflection of a portion of the waveguided
light, or diffractive light, impinges upon the exit face. Multiple
output optics at the exit face are configured for developing from
each of the output optics a radiated exit light from the diffracted
light for at least one of the waveguides. An RF controller is
configured to control the SAW devices to develop the radiated exit
light as a three-dimensional output light field with horizontal
parallax and compatible with observer vertical motion.
Inventors: |
Byrnes; Steven J.;
(Watertown, MA) ; Favalora; Gregg E.; (Bedford,
MA) ; Register; Joseph J.; (Cambridge, MA) ;
Frank; Ian W.; (Arlington, MA) ; Callahan; Dennis
M.; (Natick, MA) ; Moebius; Michael G.;
(Allston, MA) ; Laine; Juha-Pekka J.; (Boston,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Charles Stark Draper Laboratory, Inc. |
Cambridge |
MA |
US |
|
|
Family ID: |
61198913 |
Appl. No.: |
15/883811 |
Filed: |
January 30, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62452281 |
Jan 30, 2017 |
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62453041 |
Feb 1, 2017 |
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62468455 |
Mar 8, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/0353 20130101;
G02F 2203/28 20130101; G02B 2006/0098 20130101; G03H 2225/55
20130101; G02B 6/105 20130101; G03H 2223/17 20130101; G02F 1/125
20130101; G03H 2225/36 20130101; G02F 2201/05 20130101; H01L 41/33
20130101; G02F 1/335 20130101; G02F 2203/58 20130101; H01L 41/1873
20130101; G02F 2201/307 20130101; G03H 2001/0224 20130101; G02F
1/0311 20130101; G03H 2225/21 20130101; H03H 9/02968 20130101; G02F
2203/07 20130101; G02F 2203/24 20130101; G02F 2202/20 20130101;
G02F 2203/023 20130101; G02F 2201/305 20130101; G03H 2001/2292
20130101; G03H 2223/18 20130101; G02F 2201/18 20130101; G02F
2201/302 20130101; G03H 1/02 20130101; G03H 2223/16 20130101; G03H
2223/24 20130101; G03H 2225/11 20130101; H01L 41/09 20130101; G03H
2223/23 20130101; G02F 1/11 20130101; G02F 2201/30 20130101; G02F
2001/311 20130101; G03H 1/2294 20130101; G02F 2201/34 20130101;
G03H 2225/32 20130101 |
International
Class: |
G02F 1/11 20060101
G02F001/11 |
Claims
1. An electro-holographic display, comprising: one or more stacks
of light field generator devices; and SAW modulators of the light
field generator devices that emit light from their end faces.
2. A display as claimed in claim 1, wherein the light field
generator devices are stacked such that end faces of the devices
form a plane.
3. A display as claimed in claim 1, wherein an edge cut angle of
end faces of the light field generator devices are obtuse.
4. A display as claimed in claim 1, wherein an edge cut angle of
end faces of the light field generator devices are acute.
5. A display as claimed in claim 1, further comprising optics on
the end faces.
6. A display as claimed in claim 5, wherein the optics are
transmissive.
7. A display as claimed in claim 5, wherein the optics are
diffractive optics.
8. A display as claimed in claim 5, wherein the optics are
gratings.
9. A display as claimed in claim 1, wherein the light field
generator devices are stacked such that distal faces of one light
field generator device are adjacent to a proximal faces of a next
light field generator device.
10. A display as claimed in claim 1, wherein the light field
generator devices are stacked in an alternating fashion such that
distal faces of one light field generator device are adjacent to
distal face of a next light field generator device.
11. A display as claimed in claim 1, wherein the light field
generator devices are stacked in an alternating fashion such that
proximal faces of one light field generator device are adjacent to
proximal face of a next light field generator device.
12. A method of providing an electro-holographic display,
comprising: stacking light field generator devices; and emitting
light from end faces of SAW modulators of the light field generator
devices.
13. A surface acoustic wave modulator, comprising: a SAW substrate;
a waveguide for transmitting light; and a transducer for generating
a surface acoustic wave to diffract light from the waveguide, the
transducer including fingers on either side of the waveguide.
14. An electro-holographic light field generator device,
comprising: a substrate with a series of waveguides; one or more
transducers for generating one or more surface acoustic waves to
diffract light from the waveguides; and a system for coupling light
into the waveguides from a light source.
15. A device as claimed in claim 1, wherein the system is a beam
switch.
16. A device as claimed in claim 1, wherein the system is a beam
splitter.
17. A method for fabricating a surface acoustic wave modulator,
comprising: bonding a SAW substrate to a support substrate; and
thinning the SAW substrate such that topside components are closer
a distal face of the substrate.
18. A method as claimed in claim 17, further comprising forming one
or more optics on the distal face.
19. A method as claimed in claim 18, wherein the optics are
sandwiched between the SAW substrate and the support substrate.
20. A method as claimed in claim 18, wherein the optics include an
array of diffractive optics.
21. A surface acoustic wave modulator, comprising: a SAW substrate;
a support substrate bonded to the SAW substrate.
22. A modulator as claimed in claim 21, further comprising one or
more optics on a distal face of the SAW substrate.
23. A modulator as claimed in claim 22, wherein the optics are
sandwiched between the SAW substrate and the support substrate.
24. A modulator as claimed in claim 22, wherein the optics include
an array of diffractive optics.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit under 35 USC 119(e) of
U.S. Provisional Application 62/452,281, filed Jan. 30, 2017, U.S.
Provisional Application 62/453,041, filed Feb. 1, 2017, and U.S.
Provisional Application No. 62/468,455, filed on Mar. 8, 2017,
which are incorporated herein by this reference in their
entirety.
[0002] This application is related to:
[0003] U.S. application Ser. No. ______ filed on an even date
herewith, entitled "SAW Modulators and Light Steering Methods,"
attorney docket number 0352.0046US1 (CSDL.2550.US.02), now U.S.
Patent Publication No.: ______; and International Application
number ______ filed on an even date herewith, entitled, "SAW
MODULATORS AND LIGHT STEERING METHODS," now International
Application Publication No.: ______; attorney docket number
0352.0046WO1 (CSDL.2550.WO.02).
[0004] All of the afore-mentioned applications are incorporated
herein by this reference in their entirety.
BACKGROUND OF THE INVENTION
[0005] Existing three dimensional (3D) display architectures
utilize a variety of techniques including scanning,
space-multiplexing, and steered illumination, among others. One
architecture, electro-holographic displays, relies principally on
diffractive phenomena, but it has not yet delivered on the promise
of high image quality and compactness. Examples of
electro-holographic displays are described in: Jason Geng,
Three-dimensional display technologies, Advances in Optics and
Photonics, 5, 456-535 (2013). (see pp. 508-516) and Yijie Pan et
al., A Review of Dynamic Holographic Three-Dimensional Display:
Algorithms, Devices, and Systems, IEEE Transactions on Industrial
Informatics, 12(4), 1599-1610 (August 2016).
[0006] A primary disadvantage of existing electro-holographic
displays, and their constituent modulators, is their low product of
display size and spatial frequency; this product is sometimes
called the space-bandwidth product. A large modulator, or a
modulator capable of being tiled into a large direct-view display,
is desirable because it obviates the need for intermediate scanners
or large output lenses. A high spatial fringe frequency is
desirable because it increases the field of view of the display:
diffraction angle increases with line pairs/mm. Pixel-based spatial
light modulators (SLMs) suffer from low space-bandwidth product
because they are typically impractically small (with areas on the
order of 1 cm.sup.2), and have pixels typically much larger than
the wavelength of light. Similarly, existing acousto-optical
modulators (AOMs) have small deflection angles and small active
areas. For example, the MIT Spatial Imaging Group Mark II
holographic video display employed 18 mirror-scanned TeO.sub.2 AOMs
to provide a 30.degree. view angle, an image volume of 150
millimeters (mm).times.75 mm.times.150 mm, and 144 vertical scan
lines, as described in St.-Hilaire et al., Advances in holographic
video, Proc. SPIE 1914, Practical Holography VII: Imaging and
Materials, vol. 188, pp. 188-96, (1993).
[0007] An alternative to the forgoing optical modulation modalities
is a surface acoustic wave (SAW) optical modulator, a device
category that provides controllable sub-holograms from which a
light field can be constructed. Briefly, in a SAW optical
modulator, a waveguide, patterned on an optical substrate, carries
a time-varying diffracting region that is formed by index changes
due to the substrate's piezoelectric effect under radio frequency
(RF) excitation (e.g., at 300 MHz), as described, for example, in
Onural et al., "New high-resolution display device for holographic
three-dimensional video: principles and simulations," Optical
Engineering, vol. 33(3), pp. 835-44 (1994); Matteo et al.,
Collinear Guided Wave to Leaky Wave Acoustooptic Interactions in
Proton-Exchanged LiNbO3 Waveguides, IEEE Trans. on Ultrasonics,
Ferroelectrics, and Frequency Control, 47(1), 16-28 (January 2000);
Smalley et al., Anisotropic leaky-mode modulator for holographic
video displays, Nature, 498, 313-317 (20 Jun. 2013); U.S. Pat. App.
Publ. US 2014/0300695, FULL-PARALLAX ACOUSTO-OPTIC/ELECTRO-OPTIC
HOLOGRAPHIC VIDEO DISPLAY.
[0008] One type of SAW modulator is the guided-to-leaky-mode device
fabricated using lithium niobate as described, for example, in
Hinkov et al., Collinear Acoustooptical TM-TE Mode Conversion in
Proton Exchanged Ti:LiNbO3 Waveguide Structures, J. Lightwave
Tech., vol. 6(6), pp. 900-08 (1988); Smalley et al., Anisotropic
leaky-mode modulator for holographic video displays, Nature, vol.
498, pp. 313-317 (2013), herein after "Smalley"; McLaughlin et al.,
Optimized guided-to-leaky-mode device for graphics processing unit
controlled frequency division of color, Appl. Opt., vol. 54(12),
pp. 3732-36 (2015); Qaderi et al., Leaky-mode waveguide modulators
with high deflection angle for use in holographic video displays,
Opt. Expr., vol. 24(18), pp. 20831-41 (2016), hereinafter "Qaderi";
and Savidis et al., Progress in fabrication of waveguide spatial
light modulators via femtosecond laser micromachining, Proc. of
SPIE Vol. 10115, 2017. The surface acoustic wave interacts with
input light and thereby causes at least some of the light to change
from a guided mode within the waveguide to a leaky mode that exits
the waveguide.
[0009] A feature of SAW modulators is their inherently diffractive,
rather than pixelated, nature, and their potential for high
frequency bandwidth, which provides the benefit of higher
space-bandwidth product (and, thus, practical combinations of
diffractive fan angle and modulator area).
SUMMARY OF THE INVENTION
[0010] Improved SAW modulators are disclosed in various embodiments
herein. And these modulators are then included within
electro-holographic light field generator devices. In one example,
each electro-holographic light field generator devices is formed
from an array of SAW modulators. Further, systems can be added to
distribute light to multiple modulators.
[0011] Further, the invention extends to light field generator
devices and how the devices are combined, e.g., "stacked" in
different ways to form an electro-holographic display. The
electro-holographic display, in turn, might be a component of a
display system.
[0012] In addition, manufacturing techniques are also covered, as
well as transducer designs.
[0013] In general, according to one aspect, the invention features
an electro-holographic display. It comprises one or more stacks of
light field generator devices and SAW modulators of the light field
generator devices that emit light from their end faces.
[0014] In some embodiments, the light field generator devices are
stacked such that end faces of the devices form or fall in or near
common plane.
[0015] An edge cut angle of end faces of the light field generator
devices can be obtuse or acute.
[0016] Optics might further be placed on the end faces. The optics
could be transmissive and/or diffractive optics and/or
refractive.
[0017] In some cases, the light field generator devices are stacked
such that distal faces of one light field generator device are
adjacent to a proximal faces of a next light field generator
device.
[0018] In other cases, the light field generator devices are
stacked in an alternating fashion such that distal faces of one
light field generator device are adjacent to distal face of a next
light field generator device, and vice versa.
[0019] In general, according to another aspect, the invention
features a method of providing an electro-holographic display. The
method comprises stacking light field generator devices and
emitting light from end faces of SAW modulators of the light field
generator devices.
[0020] In general, according to another aspect, the invention
features a surface acoustic wave modulator. It comprises a SAW
substrate, a waveguide for transmitting light, and a transducer for
generating a surface acoustic wave to diffract light from the
waveguide. The transducer including fingers on either side of the
waveguide.
[0021] In general, according to another aspect, the invention
features an electro-holographic light field generator device. It
comprises a substrate with a series of waveguides, one or more
transducers for generating one or more surface acoustic waves to
diffract light from the waveguides, and a system for coupling light
into the waveguides from a light source.
[0022] In example, this system might be a beam switch or a beam
splitter.
[0023] In general, according to another aspect, the invention
features a method for fabricating a surface acoustic wave
modulator. The method comprises bonding a SAW substrate to a
support substrate and thinning the SAW substrate such that topside
components are closer a distal face of the substrate.
[0024] The method could include forming one or more optics on the
distal face and possibly sandwiching the optics between the SAW
substrate and the support substrate.
[0025] In examples, the optics include an array of diffractive
optics.
[0026] In general, according to another aspect, the invention
features a surface acoustic wave modulator. The modulator comprises
a SAW substrate and a support substrate bonded to the SAW
substrate.
[0027] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the particular method and device
embodying the invention are shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] In the accompanying drawings, reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
[0029] FIG. 1A is a schematic side view of a prior art SAW optical
modulator ("SAW device");
[0030] FIG. 1B is a schematic partial side view of a SAW device as
in FIG. 1A, illustrating dimensional and angular relationships
between beams traversing a substrate of the SAW device;
[0031] FIG. 1C is a schematic side view of a SAW device as in FIG.
1A, depicting the effects of Fresnel reflections at interfaces of
the SAW device;
[0032] FIG. 2A is a schematic side view of a proposed SAW device
having an angled end face as an exit face;
[0033] FIG. 2B is a schematic side view of a portion of a proposed
SAW device showing extremal deflection angles of diffracted light
within the SAW devices and corresponding extremal rays of exit
light for the extremal deflection angles, where the extremal rays
of exit light in FIG. 2B are emitted at a distal face of the SAW
device;
[0034] FIG. 2C is a schematic side view of a portion of a proposed
SAW device as in FIG. 2B, with the addition of a transmissive optic
such as a grating placed at the distal face;
[0035] FIG. 3 is a schematic side view of a portion of another
proposed SAW device, showing rays of diffracted light transmitting
through the substrate of the SAW device and towards a diffraction
grating as the transmissive optic, attached to a distal face of the
substrate;
[0036] FIG. 4 is a schematic plot of the decreasing spatial
frequency/decreasing deflection of the grating as a function of
position;
[0037] FIGS. 5A and 5B show schematic side views of a portion of
other proposed SAW devices that include powered refractive
elements, such as a concave optic, to increase the deflection
angle, and convex optics;
[0038] FIG. 6 is a ray trace that shows the rays of radiation of
diffracted light and exit light for the SAW device of FIG. 5, and
shows how the deflection angle is increased with the use of the
grating or the refractive optic;
[0039] FIG. 7A shows a ray trace of a beam of diffracted light
traversing a prior art SAW modulator device;
[0040] FIGS. 7B-7N show ray traces of beams of diffracted light
traversing SAW modulators with different end face geometries
according to the present invention;
[0041] FIG. 8A shows a proximal face of a proposed light field
generator device, where a partial array of SAW modulators within
the light field generator device is also shown;
[0042] FIG. 8B shows a distal face of the light field generator
device in FIG. 8A, which shows detail for a two dimensional array
of output optics of the SAW modulators within the light field
generator device;
[0043] FIG. 8C is a schematic cross-section of a light field
generator device as in FIGS. 8A and 8B, showing a SAW modulator
constructed with diverging diffractive lens output optics;
[0044] FIG. 9 is a schematic cross-section of a light field
generator device as in FIGS. 8A and 8B, showing a SAW modulator
constructed with converging diffractive lens output optics;
[0045] FIG. 10 is a schematic cross-section of a light field
generator device as in FIGS. 8A and 8B, showing a SAW modulator
constructed with diffractive lens output optics;
[0046] FIG. 11A is a schematic cross-section of a light field
generator device as in FIGS. 8A and 8B, where a SAW modulator
having a single output optic at an end face of the SAW modulator
functions as an exit face of the light field generator device;
[0047] FIG. 11B is a schematic cross-section of a light field
generator with two output optics at the end face;
[0048] FIG. 12 illustrates a process for creating a SAW device that
minimizes separation between waveguide and the distal/exit
face;
[0049] FIGS. 13A and 13B are diagrams that illustrate time
multiplexing of light and/or RF signal inputs to a light field
generator device including one or more SAW devices and optical
splitter of light to the SAW devices from one source;
[0050] FIG. 14 is a schematic section taken through planes of a
prior art holographic display system;
[0051] FIG. 15 shows an electro-holographic 3D display formed from
a stack of light field generator devices, where the light field
generator devices include SAW devices having angled end faces;
[0052] FIGS. 16A and 16B show electro-holographic 3D having
different stacking arrangements of light field generator devices
than in FIG. 15;
[0053] FIG. 17 shows a 3D display system that includes an
electro-holographic 3D display and other components for powering
and controlling the electro-holographic 3D display, where the
display is formed from a dual-column stack of light field generator
devices;
[0054] FIG. 18A is an enlarged top view of the prior art SAW device
in FIG. 1A, where the enlarged top view shows a layout of
transducer fingers of an interdigital transducer (IDT) within the
SAW device;
[0055] FIGS. 18B-18G are enlarged views of proposed SAW devices
showing different layouts of transducer fingers, where: FIGS. 18B
and 18C are cross-sectional top views of a SAW device showing an
optical waveguide channel with transducers located on either side
of, straddling, the waveguide; FIG. 18D is a cross-sectional side
view of the SAW devices in FIGS. 18B and 18C; FIG. 18E is a top
cross-sectional view of a SAW device showing several multimode
optical waveguide channels patterned within the SAW device; FIG.
18F is a cross-sectional top view of multiple multi-mode waveguide
paths within a SAW device that are fed by a common source of
illumination; and FIG. 18G is a cross-sectional side view of SAW
devices in either of the embodiments of FIGS. 18E and 18F.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] The invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which illustrative
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art.
[0057] As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items.
Further, the singular forms and the articles "a", "an" and "the"
are intended to include the plural forms as well, unless expressly
stated otherwise. It will be further understood that the terms:
includes, comprises, including and/or comprising, when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
Further, it will be understood that when an element, including
component or subsystem, is referred to and/or shown as being
connected or coupled to another element, it can be directly
connected or coupled to the other element or intervening elements
may be present.
[0058] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly
so defined herein.
[0059] It is to be understood that many materials and design
choices are available to the engineer in implementing the teachings
described herein, and all of them are subsumed within the scope of
the present invention. Thus, while lithium niobate as a substrate
material is discussed, for heuristic convenience, as a suitable
material, a person of ordinary skill in the art will appreciate
that various materials are available to the engineer, and that
lithium niobate is merely exemplary, as are various crystal
orientations, such as x-cut and y-cut, and waveguide architectures,
such as planar, ridge, rib, embedded, immersed, and bulged. Methods
described herein may be advantageously performed using waveguides
in y-cut, x-propagating lithium niobate, due to its high efficiency
of electrical to mechanical transduction. Doping, such as resulting
in MgO-doped lithium niobate, may be employed advantageously, in
some cases, to reduce photorefractive damage.
[0060] SAW Optical Modulator Architecture
[0061] FIG. 1A shows an exemplary prior art SAW device or modulator
100. It can be used to deflect light of the same or different
colors/wavelengths 101a, 101b, 101c from a guided mode by different
angles simultaneously. Due to fabrication, material, and power
constraints, the angular range is ordinarily extremely limited,
however, largely by the mode overlap between the guided mode and
the SAW. As described herein, using either diffractive or
refractive optics, there are methods for increasing this angular
distribution, in ways that are essentially angle magnifiers.
[0062] The device 100 comprises a substrate 120 in which or on
which an acousto-optic waveguide 102 has been formed. The input
light 101a, 101b, and 101c at one or more wavelengths
(.lamda..sub.1, .lamda..sub.2, .lamda..sub.3) enters waveguide 102.
Typically, an in-coupling device 106 is provided to couple the
input light 101 carried in an optical fiber, for example, into the
waveguide 102. Examples of in-coupling devices 106 include
in-coupling prisms, gratings, or simply butt-coupling techniques
between the optical fiber and the waveguide 102. The input light
101 is launched into a guided mode upon entry into the waveguide
102. Commonly, the TE (transverse electric) mode is guided.
[0063] In such a SAW device 100, the slab waveguide 102 is
typically created in a lithium niobate substrate 120 by
proton-exchange. Interdigital transducers 110 are written on an
aluminum side of the substrate 120. The transducers 110 induce
surface acoustic waves 140 in the substrate 120 that propagate
along the waveguide 102. Such transducers 110 are often driven
electrically, e.g. using a 300-500 MHz radio frequency (RF) input
signal 130.
[0064] The light interacts with the surface acoustic wave 140. The
result of this interaction between the surface acoustic wave 140
and the light in the waveguide 102 is that a portion of the guided
light is polarization-rotated out of the guided mode and into a
leaky mode having the transverse magnetic (TM) polarization. The
light then exits the waveguide 102 as polarized leaky-mode or
diffracted light 162 and enters substrate 120. At some point this
diffracted light 162 exits the substrate 120, either through the
substrate's side as in FIG. 1A or its bottom as in FIG. 1B, as exit
light 150 at an exit angle fan .alpha.. The range of possible exit
angles comprises the angular extent, or exit angle fan, of the exit
light 150.
[0065] Due to fabrication, material, and power constraints, the
angular range of exit angles .alpha. is ordinarily very limited.
Qaderi (2016), for example, reports that a total exit angle .alpha.
of approximately 20.degree. can be achieved, which is significantly
lower than the field of view of contemporary 2-D displays that
approach 180.degree..
[0066] FIG. 1B shows the relationships between different beams of
diffracted light 162 in the SAW device 100 of FIG. 1A to illustrate
its limited angular range. Once diffracted from the waveguide 102,
the diffracted light 102 propagates through thickness h of
substrate 120. Diffracted light 162-1 propagates through the
substrate 120 at deflection angle .PHI..sub.1. The deflection angle
.PHI..sub.1 is measured relative to grazing, such as proximal face
160 of substrate 120. The diffracted light 162 exits distal face
168 of substrate 120 at a distance
h tan .phi. 1 ##EQU00001##
from the point 165 normal to where the light left the waveguide
102.
[0067] Depending on the crystal phase and the values of the overlap
integral for the waveguide mode, using a higher or lower RF drive
frequency of the RF input signals 130, a higher deflection angle
.phi. may be achieved. This is illustrated by diffracted light
162-2, at deflection angle .phi..sub.2 to proximal face 160 of
substrate 120. Thus, as might be expected, the finite thickness h
of substrate 120 gives rise to a spatial variation of where the
beams of diffracted light 162 impinge upon distal face 168 of
substrate 120. The distance from the point normal 165 to the point
of incidence 199 of diffracted light beam 162-1 on distal face 168
shall be referred to herein as the "finite substrate propagation
displacement" 182. This range of possible diffraction angles .PHI.
and the corresponding exit angle .alpha., i.e., the exit fan, of
the exit light 150 is generally limited due to fabrication,
material, and power constraints.
[0068] Additionally, the high refractive index contrast between the
substrate material forming the SAW modulator and the ambient medium
105 (typically air) leads to significant Fresnel reflections. These
Fresnel reflections reduce the wall plug efficiency of modulator
100, and also lead to stray light in a display in which each pixel
might include a separate SAW modulator 100, thereby reducing the
visual contrast of the imagery. Additionally, there is a
possibility of total internal reflection, which prevents the
modulator from working at all in certain configurations.
[0069] FIG. 1C illustrates the effect of Fresnel reflections at
successive interfaces between substrate 120 of SAW device 100 and
ambient medium 105. These Fresnel reflections are indicated in FIG.
1C as Fresnel reflected components 164 and give rise to stray exit
light 174 and other problems.
[0070] Here, radiation 101 in the form of input light is introduced
at in-coupling device 106. Within waveguide 102, the input light
101 is diffracted by the surface acoustic wave 140 as diffracted
light 162. The diffracted light 162 impinges upon distal face 168
as an exit face, at an angle .theta..sub.1. The diffracted beam 162
is transmitted through the distal face 168 as exit light 150, which
exits the substrate 120 through the distal face 168 at exit angle
.alpha.. The exit light 150 is the primary intended signal to be
transmitted out the SAW device 100.
[0071] However, the index discontinuity at the distal face 168 will
also create a Fresnel reflected component 164. The Fresnel
reflected component 164 retraverses the substrate 120, with a
portion emitted as stray exit light 174 at proximal face 160, in
one example. Several successive reflections are depicted at angles
.theta..sub.2 and .theta..sub.3 at proximal face 160 and end face
170, respectfully, in examples. As a result, stray exit light 174
exits the substrate 120 at unwanted locations and/or output
faces.
[0072] Existing electro-holographic displays using SAW devices have
attempted to increase the angular subtense/exit angle .alpha. of
the exit light 150 (field of view) in various ways. In examples,
these ways include: experimentally optimizing various modulator
parameters to increase the useful bandwidth of the RF driver such
as waveguide depth and IDT design (in published systems, the output
angle is a function of IDT drive frequency); using edge-emitting
modulators having "right-angle" edges; doubling the exit angle fan
via waveguides on both sides of the wafer, and/or by
demagnification (i.e. using a large lens to demagnify an area of
numerous modulators to provide a smaller visible display area
having larger field of view). But it does not appear that any of
these methods are adequate to achieve an exit angle fan such as
30.degree. or more to as high as 90.degree. or more in any sort of
flat form-factor. Other conventional approaches are based on
building a 3D display using a diffractive lens array including the
diffractive-patch version of integral photography such as in J. H.
Kulick, et al., Partial pixels: a three-dimensional diffractive
display architecture, JOSA A, 12(1), 73-83 (1995), and D. Fattal et
al., A multi-directional backlight for a wide-angle, glasses-free
three-dimensional display, Nature 495, p 348-351 (21 Mar. 2013);
both of which are incorporated herein by reference in their
entireties.
[0073] Embodiments of the present invention are disclosed herein
below that provide improved SAW devices 200. These improved SAW
devices increase the angular extent of the exit angle .alpha. of
the exit light 150 as compared to existing SAW devices 100.
[0074] FIG. 2A shows a proposed SAW device 200 utilizing different
approaches for increasing the exit angle .alpha. of the exit light
150 as compared to prior art SAW devices 100. These approaches
include providing an angled end face 170 and/or placing a
transmissive optic 180 such as a grating on an exit face of the SAW
device 200 to add divergence to the exiting rays of exit light
150.
[0075] In more detail, input light 101 enters the substrate 120 via
the in-coupling device 106 at the input end 340 and travels through
the substrate 120 in the waveguide 102 as a wave of guided light
301. When the surface acoustic waves 140 produced by the
interdigital transducer 110 interact with the guided light 301, the
light 301 is diffracted and coupled into a leaky mode, which is no
longer guided by the waveguide 102. The light of the leaky mode
leaves the waveguide 102 and ultimately exits the substrate 120 as
exit light 150.
[0076] The illustrated SAW device 200 shows a number of innovations
that can be used separately or in conjunction with each other in
order to improve the performance of the SAW device 200 such as by
increasing the divergence of the exiting rays of exit light 150.
One of those innovations is a non-orthogonal end face 170.
Specifically, in one example, an end face 170 is fabricated or
machined at a non-right angle relative to the proximal face 160
and/or the distal face 168 of the substrate 120. A second
innovation is to include a transmissive optic 180, such as a
diffractive optic or grating or refractive optic, within or upon an
exit face, which is the distal face 168 in the illustrated device
200. The transmissive optic 180 might be patterned within/upon the
exit faces during fabrication of the SAW device 200, in one
example.
[0077] In more detail, the end face 170 of the substrate 120 is
planar and angled by an edge cut angle .beta. relative to the
proximal face 160. The edge cut angle .beta. is measured from the
plane of the proximal face 160 to the end face 170. The edge cut
angle .beta. is typically chosen such that a deflection of at least
half of the available cone of the deflection angle .phi., or
.phi./2, is normal or near normal to the distal face 168 of the
substrate 120 after the diffracted light 162 reflects off of the
end face 170. Diffracted light 162 that reflects off of an exit
face, such as end face 170, is referred to as reflected diffracted
light 162'. The reflected diffracted light 162' is then directed
toward the transmissive optic 180, such as a grating. In certain
embodiments of the invention, a reflective substance such as a
metal coating or dielectric coating may be placed on the end face
170 to increase reflectivity and thus the intensity of the
reflected diffracted light 162' traveling toward the optic or
grating 180. As the light 162' interacts with the grating 180, the
light is dispersed, increasing the overall exit angle .alpha. of
the exit light 150.
[0078] In one embodiment, the transmissive optic 180 is a
subwavelength grating that is deposited or patterned on or
otherwise fabricated on the exit/distal face 168 of the substrate
120 of the SAW device 200. As is well known, a diffraction grating
uses perturbations of the refractive index of different materials
or dopants, such as: etched grid lines, deposited grid lines,
etched holes, deposited cylinders, or other techniques, to alter
the k-vector of the light. The standard grating equation states
that the spatial frequency of the grating will alter the momentum,
such that the following relation holds:
k.sub.initial-k.sub.final=k.sub.grating
[0079] Insofar as the foregoing standard grating equation is a
vector equation, it shows that the grating interaction changes the
direction of the light.
[0080] As discussed above, there is a finite distance, referred to
herein as the finite substrate propagation displacement 182
(introduced in connection with FIG. 1B), between where the light is
deflected by the surface acoustic wave 140 from the waveguide 102
and where it impinges on the proposed angle-enhancing grating 180.
This distance 182 leads to a spatial spread in the exit angle
.alpha. of the exit light 150 for different deflection angles
.phi..
[0081] In FIGS. 2B and 2C, on the other hand, when the rays
traverse the grating (or other transmissive optic 180), extremal
deflection angles .phi..sub.1 and .phi..sub.2 of the diffracted
light 162-1 and 162-2 may be mapped to extremal emergent rays of
exit light 150-1 and 150-2. In both FIGS. 2B and 2C, .phi..sub.1
and .phi..sub.2 denote the minimum and maximum deflection angles,
respectively.
[0082] FIG. 2B is a proposed SAW device 200 that does not include a
transmissive optic 180. In the absence of a transmissive optic 180,
the diffracted light 162-1 and 162-2 impinges upon distal face 168
at points A and B, respectfully. The diffracted light 162-1 and
162-2 are emitted from the distal face 168 as extremal rays of exit
light 150-1 and 150-2. Here, the extremal rays of exit light
150-1/150-2 are simply given by Snell's law, as the rays traverse
the interface between the refractive index of the substrate 120
(say, n.about.2.3 for LiNbO3) to that of the ambient medium
105.
[0083] FIG. 2C is a proposed SAW device 200 with the addition of
the transmissive optic 180, such as a chirped grating. The grating
180 is located at the distal face 168. When diffracted light 162-1
and 162-2 impinge upon points C and D of the grating 180,
respectfully, extremal rays of exit light 150-1 and 150-2 are
emitted. Here, the extremal rays of exit light 150-1 and 150-2
extend at angles, indicated by theta (.theta.), from nearly
90.degree. in one direction to 90.degree. in the other direction
with respect to the normal 111 to the substrate 120. Exemplary
angle .theta..sub.2 for extremal ray of exit light 150-2 is
shown.
[0084] In more detail, the remapping of the diffracted light 162 is
accomplished by rearranging the grating equation to obtain angle
.theta..sub.i between each emergent ray of exit light 150 (in the
first diffraction order) and the normal 111:
.theta. i = arcsin ( .lamda. d - cos .phi. i n ) ##EQU00002##
where n is the refractive index of the substrate 120, and use is
made of the fact that the angle of incidence of a ray deflected
toward the transmissive optic 180 is the complement of its angle
with respect to the proximal face 160. As a well-known consequence
of this equation, any possible emergent ray angle can be created
with a suitable choice of spatial frequency of the grating 180 for
a specified input angle.
[0085] A variation on the technique that has been described is to
spatially vary the grating period (the reciprocal of the spatial
frequency) along the extent of the grating 180. This spatial
variation can be mapped to the incoming deflection angle .PHI. due
to finite substrate propagation displacement. For example, the
grating frequencies may be chosen so that light hitting the grating
at the surface normal 111 is transmitted without additional
deflection, whereas light striking the grating 180 at increasingly
oblique angles will experience greater and greater deflection. This
deflection increases to the point where the maximum angle .phi.
deflected by the surface acoustic wave 140 (for example
+10.degree.) will experience an additional deflection due to the
exit-surface grating 180, thereby experiencing a total deflection
from the normal 111 at or approaching 90.degree.. The variation in
angle .theta. provided by the grating 180 for extremal (e.g.
minimum and maximum) deflection angles .phi..sub.1 and .phi..sub.2
has two benefits: it may advantageously allow for the modest
initial deflection angles to be amplified, and it may
advantageously suppress Fresnel reflections, the undesirability of
which has been discussed above with reference to FIG. 1C.
[0086] A mirror or other reflective feature can be used to center
the "deflection cone" of the surface acoustic wave 140, such that
the central value is normal to the grating 180. This may provide
symmetric deflection.
[0087] A refinement to this technique is to tailor the grating 180
to work equally well at multiple wavelengths, such as red, green
and blue (R, G, B) when used for visual display applications.
Although R, G, B are referenced here, all other ways of
representing color may also be used within the scope of the present
invention. The grating may be constructed through optimization to
work over the entire color spectrum in one embodiment. This is
accomplished through numerical optimization, with the starting
point given by the grating equation. The grating layout is then
optimized either through genetic algorithms or other well-known
numerical methods to produce the highest efficiency and best
angular distribution. The actual spatial pattern will depend on the
choice of substrate material used for the SAW modulator 200, as
well as the maximum and minimum deflection of the surface acoustic
wave 140, and the material choice for the grating 180.
[0088] A grating in accordance with embodiments of the present
invention may be fabricated in any of the following ways, provided
as examples and without limitation: [0089] Etching directly
into/onto the modulator wafer [0090] Depositing metal dots or lines
[0091] Depositing dielectric dots or lines
[0092] Descriptions of exemplary genetic algorithms and numerical
grating optimization techniques may be found in: [0093] Zhou et
al., "Genetic local search algorithm for optimization design of
diffractive optical elements," Appl. Opt., vol. 38(20), pp. 4281-90
(1999); [0094] Lin et al., "Optimization of random diffraction
gratings in thin-film solar cells using genetic algorithms," Solar
Energy Materials and Solar Cells, vol. 92(12), pp. 1689-96 (2008);
[0095] Qing et al., "Crowding clustering genetic algorithm for
multimodal function optimization," Appl. Soft Computing, vol. 8(1),
pp. 88-95 (2008); [0096] Taillaert et al., "Compact efficient
broadband grating coupler for silicon-on-insulator waveguides,"
Opt. Lett., vol. 29(23), pp. 2749-51 (2004); [0097] Shokooh-Saremi
et al., "Particle swarm optimization and its application to the
design of diffraction grating filters," Opt. Lett., vol. 32(8), pp.
894-96 (2007); and [0098] Byrnes et al., "Designing large,
high-efficiency, high-numerical-aperture, transmissive meta-lenses
for visible light." Opt. Exp. 24 (5), pp. 5110-5124 (2016).
[0099] FIG. 3 depicts an example of reflected diffracted light 162'
incident upon a transmissive optic 180 such as a grating of a
proposed SAW device 200. As shown, the deflection is symmetrical
about the point x.sub.2, which is normal of the exit distal face
168, and the spatial spread of the deflection angle .PHI. is
approximately equal to x.
[0100] As in FIGS. 2A and 2B, .phi..sub.1 and .phi..sub.2 are the
minimum and maximum deflection angles possible for the diffracted
light 162. The diffracted light 162 reflects off end face 170 as
reflected diffracted light 162'. Here, the edge cut angle .beta. of
the end face 170 is typically chosen such that a deflection of at
least half of the available cone of the deflection angle .phi., or
.phi./2, is normal or near normal to the distal face 168.
Spatial Spread -> ( = ) .times. So Deflection = .PHI. 2 = )
normal = 0 = ) - .PHI. 2 = .PHI. = ) .PHI. 2 ##EQU00003##
[0101] FIG. 4 is a plot of the spatial frequency of the grating 180
shown in FIG. 3 as function of position. As shown, the amount of
deflection is symmetric with respect to the distance defined by x
(i.e. the size of the grating) in one example. This symmetry is not
required in all embodiments of this invention, but is merely
illustrative, however.
[0102] FIG. 4 is a plot of the spatial frequency for three
embodiments of the grating 180 shown in FIG. 3 as function of
position. As shown, the amount of deflection is different at
different locations within the grating.
[0103] Due to finite substrate propagation displacement, different
initial deflection angles .phi. will experience the varying spatial
frequencies as depicted in FIG. 4. Thus, more deflection of the
diffracted light 162 will occur when the light reaches the grating
180 proximate to points x1, while less deflection will occur at the
proximate to the point x3 when the grating is constructed with a
spatial frequency according to curve A. The result is an increase
in the angular fan of the exit angle .alpha. of the exit light 150
beyond that created by initial interaction of the guided light 301
with the surface acoustic wave 140.
[0104] On the other hand, when the grating is constructed with a
spatial frequency according to curve C, more deflection of the
diffracted light 162 will occur when the light reaches the grating
180 proximate to points x3, while less deflection will occur at the
proximate to the point x1.
[0105] On the other hand, when the grating is constructed with a
spatial frequency according to curve B, more deflection of the
diffracted light 162 will occur when the light reaches the grating
180 proximate to points x1 and x3, while less deflection will occur
at the proximate to the point x2, at the center of the grating.
[0106] A second family of techniques to increase the angular extent
of the exit angle .alpha. of the fan of exit light 150 is to use
geometrical optical techniques, i.e. adding or "hollowing-out"
diverging optical features on, or near, the SAW modulator 200.
These features may be used to increase or decrease the optical
power to optimize the fan of the output light.
[0107] It should be understood by one of ordinary skill in the art
that the various described techniques of creating a refractive lens
at an output/exit face, and adding optical power to the system may
be done using hybrid diffractive-refractive optics. Such optical
systems are generally known to those of ordinary skill in the art
and can be found in Stone et al., "Hybrid diffractive-refractive
lenses and achromats," Appl. Opt., vol. 27(14), pp. 2960-71
(1988).
[0108] FIG. 5A is an illustrative example for yet another
transmissive optic 180. Here, the optic 180 is formed by removing
material from the lithium niobate substrate 120 at the distal face
168 as an exit face to create a concave optical surface 180. The
curved optical surface 180 provides diverging optical power,
thereby broadening the angular extent of the exit angle .alpha. of
the fan of exit light 150. In other examples, optical elements that
provide the optical power might be placed upon an exit face,
patterned flat on a surface such as an exit face, and included as
part of a later optical train.
[0109] In more detail, the guided light 301 is diffracted from the
waveguide 102 by the surface acoustic wave 140 and propagates
through the substrate 120 as diffracted light 162. In the
illustrated embodiment, this diffracted light 162 is reflected by
the end face 170 as reflected diffracted light 162'. As shown, the
distal face 168 is made (e.g. patterned) to have a concave optical
surface 180 to provide optical power to the wave of reflected
diffracted light 162', thus expanding the divergent nature of the
light wave. The light wave 162' can then interact with the optical
surface 180 to diverge, further creating an exit angle .alpha. of
greater than 90 degrees.
[0110] Due to imperfections in the substrate 120 and differences
between the indices of refraction of substrate 120 and ambient
medium 105, some of the diffracted light 162 might exit the SAW
device 200 as unwanted stray exit light 174 from end face 170. In
one example, an opaque layer may be applied to the end face 170 to
minimize or eliminate the stray exit light 174.
[0111] FIG. 5B is another example of the transmissive optic 180.
Here, the optic 180 is formed by adding material to the lithium
niobate substrate 120 at the distal face 168 as an exit face to
create a convex optical surface 180.
[0112] FIG. 6 shows a ray trace illustrating the propagation of
light in the embodiment of the SAW device in FIG. 5. The diffracted
light 162 propagates toward the end face 170. This reflects the
light 162 as reflected diffracted light 162'. This light 162' then
interacts with the transmissive optic 180, which in the illustrated
embodiment is a concave optical surface. Other embodiments of the
present invention in which the subtense of the exit angle .alpha.
of the fan of exit light 150 for a SAW modulator 200 might be
enhanced are now described with reference to FIGS. 6B and
7A-7N.
[0113] The following table describes each of the ray traces in FIG.
7A-7N. Each row/entry in the table describes a separate ray trace
FIG. 7A-7N. Fields within each row includes typical values for the
internal deflection angle .phi. and edge cut angle .beta., and
resulting output angular subtense .alpha.. A "comments" field is
also included. More detail for each of the ray trace FIGS. 7A-7N
accompanies the descriptions of these figures, provided herein
below.
TABLE-US-00001 Approx. Output Edge Exit angle Internal Cut fan or
Angle Angle Subtense FIG. (.phi..degree.) (.beta..degree.)
(.alpha..degree.) Comments 7A 10 90 25 Base case, showing one small
active area as possible subset of grating. Diffractive structure on
top face (e.g., SAW or other grating). Right-angle edge face. 7B 10
90 Varies Base case, illustrating diffractive fan as modulated by
three regions along, e.g., a SAW. 7C 10 40 40 Exits face opposite
modulator channel. Grazing beam exits approximately normal to exit
face. 7D 10 50 35 Exits face opposite modulator channel. Edge cut
to position median diffracted ray normal to output. 7E 10 52 33
Exits face opposite modulator channel. Edge cut to position
maximally diffracted ray to exit normal. 7F 10 60 ~60 Exits face
opposite modulator channel. Edge cut so that grazing ray just
misses total internal reflection (TIR) at exit face. 7G 10 63 N/A
Edge cut so that grazing ray just avoids TIR, and exits the edge
face. 7H 10 80 ~35 Edge cut so that the maximally-diffracted ray
just misses TIR and exits nearly grazing to the output edge. 7I 10
100 30 Exits edge. Edge cut to make central diffracted ray exit
normal to edge face. 7J 10 120 (example) Exits edge. Edge cut such
that grazing input ray just missed TIR condition. 7K 20 <90 ~90
Exits edge. Diffraction period and edge angle chosen such (89) that
the output subtends approximately 90 degrees. 7L 20 105 50 Exits
edge. Median ray exits normal to exit face. 7M 20 140 (example)
Single-reflection TIR. Fan exits same face as diffracting structure
(SAW). 7N 20 160 (example) Double-reflection TIR. Fan exits edge
face.
[0114] It is to be understood that, within the scope of certain
embodiments of the present invention, that the various faces may
serve as the exit face, either with an intervening reflection at a
face or without. It is also to be understood that the length of the
SAW modulator 200 and/or the proximal face 160 and/or distal face
168 may be varied within the scope of the present invention.
Additionally, either end face 170 and/or distal face 168 may be
cleaved and/or polished or otherwise angled, within the scope of
the present invention. For example, the wafer containing the SAW
devices 200 may be lapped at an angle. All techniques for
fabricating a SAW modulator 200 with faces at non orthogonal angles
are within the scope of the present invention. Also, the
interaction region (the places where the SAW and the light interact
and kick out the leaky mode light) has a finite extent, e.g. 4 mm,
8 mm. A consequence of this, the values the exit angle fan or
subtense are generally approximations.
[0115] Ray traces are shown, by way of example only, for a variety
of geometrical configurations of SAW modulator 200 in FIGS. 7A-7N,
and are by no means intended to limit the application of the
principles taught herein. FIGS. 7A-7N assume the absence of an
anti-reflective (AR) coating. However, the effect of such an AR
coating can be readily calculated, given the indices of refraction
of substrate 120 and ambient medium 105 The AR coating would
typically be applied, insofar as an AR coating is beneficial, as
elsewhere discussed herein.
[0116] FIG. 7A is a base ray trace example, in accordance with the
prior art. End face 170 is at a 90.degree. edge cut angle .beta.
with respect to the proximal face 160. A typical value for the
deflection angle .phi. is 10.degree.. Here, the diffracted light
162 exits near orthogonal to end face 170 as exit light 150.
[0117] FIGS. 7B-7N depict exemplary ray traces in proposed SAW
devices 200 constructed according to embodiments of the present
invention.
[0118] FIG. 7B illustrates a fan of exit light 150 that is
diffracted at regions 190, 192, 194. There is a finite interaction
region along which the waveguided light interacts with the SAW to
become leaky-mode light. This illustrates an interaction region
from 190 to 194, with an intermediate point 192 shown. If a
single-frequency RF signal drives the IDT, there is essentially one
"ray" emitted at each of 190, 192, and 194, and at intermediate
points. This "fills" the output face in this illustration. As in
FIG. 7A, the end face 170 is at a 90.degree. edge cut angle .beta.
with respect to the proximal face 160. A typical value for the
deflection angle .phi. is 10.degree.. Here, the exit angle fan
.alpha. is defined as approximately the maximum subtense of the
steered light, in air and varies due to the modulation.
[0119] FIG. 7C-7H are ray traces for SAW devices 200 having an
acute edge cut angle .beta.. In each of FIG. 7C-7H, a typical value
for the deflection angle .phi. is 10.degree..
[0120] FIG. 7C shows a ray trace in which the diffracted light 162
reflects off of the end face 170 as reflected diffracted light
162'. This end face 170 has an edge cut angle .beta. of about
40.degree.. Then, the reflected diffracted light 162' propagates
through the substrate 120 towards the distal face 168, which
functions as the exit face. The reflected diffracted light 162'
exits the SAW device 200 as exit light 150, approximately normal to
the distal face 168. Here, an exemplary value for the exit angle
fan .alpha. is as high as 40.degree..
[0121] FIG. 7D shows a similar configuration as in FIG. 7C. The
diffracted light 162 reflects off the end face 170 as reflected
diffracted light 162'. Here, however, the end face 170 is at a
greater edge cut angle .beta. to the proximal face 160, such as
about 50.degree.. This has the effect of changing the general
direction of the exit light 150 with respect to the distal face
168. Specifically, the end face 170 is edge cut to position median
diffracted rays of the reflected diffracted light 162' to be normal
to the distal face 168 as the exit face. The reflected diffracted
light 162' is transmitted out the distal face 168 as exit light
150. Here, an exemplary value for the exit angle .alpha. is as high
as 35.degree..
[0122] FIG. 7E shows a similar configuration as in FIG. 7D. Here,
however, the exit face 170 is at a greater edge cut angle .beta. to
the proximal face 160, such as about 52.degree.. This has the
effect of further changing the general direction of the exit light
150. Specifically, the end face 170 is edge cut to position
maximally diffracted rays of the reflected diffracted light 162' to
be normal to the distal face 170 as the exit face. Here, an
exemplary value for the exit angle fan .alpha. is as high as
33.degree..
[0123] FIG. 7F shows a similar configuration as in FIG. 7E. Here,
however, the end face 170 is at a still greater edge cut angle
.beta. to the proximal face 160, such as about 60.degree.. This has
the effect of further changing the general direction of the exit
light 150. Specifically, the end face 170 is edge cut so that
grazing rays of the reflected diffracted light 162' just miss being
totally internally reflected (TIR) at the end face 170, but instead
are emitted near the Brewster angle, thus minimizing undesirable
Fresnel reflections. The reflected diffracted light 162' exits the
SAW device 200 at distal face 168 as exit light 150, approximately
normal to the distal face 168. Here, an exemplary value for the
exit angle .alpha. is approximately 60.degree.. Also shown is a
Fresnel reflected component 164 arising at the distal face 168.
[0124] The Fresnel reflected component 164 can be addressed. In one
example, this component is preferably minimized by adding an AR
coating to the distal face 168 to minimize or possibly eliminate
the Fresnel reflected component 164.
[0125] FIG. 7G shows a similar configuration as in FIG. 7F. Here,
however, the end face 170 is at a greater edge cut angle .beta. to
the proximal face 160, such as about 63.degree.. This has multiple
effects. First, not all of the diffracted light 162 is transmitted
out the end face 170 as exit light 150. Rather, some of the
diffracted light 162 is reflected internally as stray internally
reflected light 172. Second, when the stray internally reflected
light 172 impinges on the distal face 168, the stray internally
reflected light 172 is totally internally reflected.
[0126] The stray internally reflected light 172 can also be
corrected. In one example, an antireflective (AR) coating might be
applied to the exit face (here, end face 168) to minimize or
possibly eliminate the stray internally reflected light 172.
Further, an absorber material should be added to the distal face
168 to absorb light reflected light and prevent further stray light
reflections.
[0127] FIG. 7H shows a similar configuration as in FIG. 7G. Here,
however, the exit face 170 is at a greater edge cut angle .beta. to
the proximal face 160, approaching 80.degree.. As in FIG. 7G, some
of the diffracted beam 162 is reflected internally as stray
internally reflected light 172 and is totally internally reflected
at the distal face 168. Here, an exemplary value for the exit angle
fan .alpha. is approximately 35.degree..
[0128] FIGS. 7I-7N are described below and show various
combinations of internal reflections and exit angles .alpha. for
additional edge cut SAW devices 200. In this group of figures, with
the exception of FIG. 7K, the edge cut angle .beta. is obtuse.
[0129] In more detail, in FIG. 7I, the end face 170 is edge cut to
make centrally diffracted light 162 exit normal to the end face 170
as exit light 150. Some of the diffracted light 162 might also be
reflected internally as stray internally reflected light 172. The
edge cut angle .beta. is about 100.degree. and provides an exit
angle .alpha. of approximately 30.degree.. A typical value for the
deflection angle .phi. is 10.degree..
[0130] In FIG. 7J, the end face 170 is edge cut such that grazing
input rays just miss the TIR condition at the end face 170. The
diffracted light 162 exits the end face 170 as exit light 150. The
edge cut angle .beta. is about 120.degree. and the exit angle
.alpha. varies, as shown.
[0131] In FIG. 7K, the diffraction period and edge cut angle .beta.
are chosen such that the exit angle .alpha. is approximately 90
degrees. The diffracted light 162 exits the end face 170 as exit
light 150. Here, the edge cut angle .beta. is typically less than
90.degree. (a value of 89.degree. was selected here). A typical
value for the deflection angle .phi. is 20.degree..
[0132] According to FIG. 7L, median rays of the diffracted light
162 exits normal to the end face 170 as exit light 150. Here, the
edge cut angle .beta. is 105.degree. and the output angular
subtense .alpha. is typically 50.degree.. A typical value for the
deflection angle .phi. is 20.degree..
[0133] In FIG. 7M, a single-reflection-TIR case is shown. The
diffracted light 162 exits as exit light 150 from the same exit
face (here, proximal face 160) as the SAW propagates along. Here,
the edge cut angle .beta. is 140.degree.. Unwanted stray exit light
174 is also transmitted out of end face 170. A typical value for
the deflection angle .phi. is 20.degree..
[0134] FIG. 7N shows a double-reflection TIR case. The diffracted
light 162 exits the edge face 170 as exit light 150. Here, the edge
cut angle .beta. is 160.degree.. Unwanted stray exit light 174 is
also transmitted out of proximal face 160. A typical value for the
deflection angle .phi. is 20.degree..
[0135] It is to be understood that antireflective (AR) coatings are
preferred at the exit faces of the SAW modulator 200, given the
Fresnel reflections due to the typically large discontinuity of
index of refraction between substrate 120 and the ambient medium
105. The design of antireflective coatings for the spectral and
angular ranges involved here is well within the capabilities of a
person of ordinary skill in the art, as is calculating the effect
of coating the face on the emerging fan. All antireflective coating
techniques are within the scope of the present invention.
[0136] When assembled into a display operating in a
horizontal-parallax-only mode (i.e. the diffracted fans steer
horizontally), it is often also desirable to spread the beams of
diffracted light 162/reflected diffracted light 162' vertically.
This is typically done with a vertical diffuser. Such methods may
be applied, within the scope of the present invention, to all of
the embodiments discussed above. Moreover, all of the foregoing
teachings may also be applied for vertical displacement. Thus, if
the beams 162/162' exit the narrow edge of exit face 170, the
narrow edge may be augmented with a transmissive optic 180, such as
a lens or a grating, as taught above, to further condition the exit
light 150.
[0137] Electro-Holographic Light Field Generator Device
Architecture
[0138] FIG. 8A shows a proximal face 160, FIG. 8B shows a
distal/exit face 168, and FIG. 8C shows a side cross-sectional view
(all not to scale) of an electro-holographic light field generator
device 300 according to an embodiment of the present invention.
[0139] In general, the electro-holographic light field generator
device 300 comprises an array of SAW devices or modulators 200.
These SAW devices 200 are fabricated in a common substrate 120. As
best shown in FIG. 8A, the longitudinal axes of each of these SAW
devices 200 extend parallel to each other across the light field
generator device 300 in the x-axis direction. Note: the axes x, y,
z are used here to orient the geometry of the light field generator
system and its components for clarity. This coordinate system has
no relation to the x,y,z crystallographic axes of lithium niobate
or any other material.
[0140] In more detail, and as described hereinabove, the substrate
120 may be made, for example, of lithium niobate following known
processes such as that disclosed in Smalley. Many other materials
and design choices are available including other piezoelectric
materials and crystallographic orientations, and waveguide
architectures such as planar, ridge, rib, embedded, immersed, and
bulged. Doping such as MgO-doped lithium niobate may be useful, in
some cases.
[0141] The array of surface acoustic wave (SAW) optical modulators
200 is arranged in the y-axis direction across the width of the
common substrate 120. Each SAW optical modulator 200 includes an
in-coupling device 106 (e.g., a laser in-coupling grating or
prism), a waveguide 102 and a SAW transducer 110 (e.g., an IDT, for
example).
[0142] As described before, the waveguides 102 provide confinement
of the input light in a TE (transverse electric) guided mode 301,
in one example. The SAW transducers 110 are driven by an RF input
signal 130 that creates a corresponding surface acoustic wave 140
that propagates collinearly with the light 301 in the waveguide 102
and which interacts with the light to convert part of the light to
the transverse magnetic (TM) polarization, leaky mode.
[0143] Birefringence of the waveguide 102 and the optical substrate
120 (and/or the wave-vector change from the interaction) causes the
TM leaky mode portion of the light propagating in the waveguide 102
to leak out of the waveguide 102 into the optical substrate 120 as
diffracted light 162 towards the exit face, which is the distal
face 168, in this embodiment.
[0144] In different embodiments, the IDTs 110 can occupy a variety
of specific locations and specific orientations with respect to the
waveguide 102. For example, in the illustrated embodiment, the
transducers 110 are located near the end face 170 so that the
surface acoustic waves 140 will propagate in a direction opposite
the propagation of the light in the waveguides 102. In other
embodiments, however, the transducers 110 are located near the
in-coupling devices 106 so that the surface acoustic waves 140 will
co-propagate in the direction of the light in the waveguides
102.
[0145] Also, there could be multiple SAW transducers 110 for each
in-coupling device 106/waveguide 102, with each SAW transducer 110
responsible for a different specific bandwidth around a given
center frequency (e.g.: 100-150 MHz, 150-250 MHz, and 250-400
MHz).
[0146] In a specific embodiment, the array of SAW optical
modulators 200 may be packed relatively tightly with a waveguide
separation 206 of between 10 .mu.m-400 .mu.m, for example, 50
.mu.m. The waveguide length 207 may be 1-10 centimeters (e.g., 5
cm) or even longer if multiple SAW transducers 110 and/or multiple
laser inputs 106 are used to mitigate acoustic and optical
attenuation respectively. In this context, a greater waveguide
length 207 reduces system complexity and, if tiled into a larger
display, it minimizes tile-borders ("grout"). Since the surface
acoustic waves 140 move at the speed of sound, the light inputs 101
may be strobed at a repetition rate equal to or lower than the
inverse acoustic transit time, at a pulse width sufficiently narrow
(for example, in the range of nanoseconds to microseconds) to cause
acceptably low blurring.
[0147] Each waveguide 102 may be configured for a single specific
wavelength of input light 101, which in this context should be
understood to include at least one of visible light, infrared light
and ultraviolet light, or for multiple different light wavelengths.
For example, for 3D display applications, each waveguide 102 may
carry one or more of red, green, or blue light 101. In other
specific light field generation applications, other wavelength
combinations may be useful including more or fewer than three
colors and/or non-visible wavelengths.
[0148] FIG. 8B shows the distal face 168 of a light field generator
device 300, which is the exit face in the illustrated embodiment.
According to the embodiment, the optical substrate 160 includes a
two dimensional array 310 of output optics 303 for shaping output
exit light 150. In illustrated example, the exit light 150 is
collimated into a beam, focused at infinity.
[0149] The output optics 303 are diffractive lenses arranged into
lens strips 302 (e.g., one strip for each waveguide). Each of the
strips 302 is aligned under a respective waveguide 102. Each
individual output optic 303 is the length of a display pixel (100
.mu.m-2 mm, typically about 1 mm in the x-axis direction). Thus,
with a waveguide pitch 206 of 50 .mu.m each diffractive lens output
optic 303 would be 1 mm.times.50 .mu.m, and each diffractive lens
strip 302 may be about 5 cm.times.50 .mu.m.
[0150] The diffractive lens strips 302 may be chirped gratings with
components that redirect light in both directions. That is,
rectangular sections of a diffractive lens, which may or may not
have different focal lengths in the horizontal and vertical
directions, where "horizontal" means parallel to the length of the
respective waveguide 102 (x-axis direction) and "vertical" means
across the width of the respective waveguide 102 (y-axis
direction).
[0151] Diffractive lens output optics 303 may be fabricated, for
example and without limitation, by etching directly into/onto the
substrate 120, depositing metal dots or lines, or depositing
dielectric dots or lines or pillars. Descriptions of exemplary
generic algorithms and numerical grating optimization techniques
may be found in: [0152] Zhou et al., Genetic local search algorithm
for optimization design of diffractive optical elements, Appl.
Opt., vol. 38(20), pp. 4281-90 (1999); [0153] Lin et al.,
Optimization of random diffraction gratings in thin-film solar
cells using genetic algorithms, Solar Energy Materials and Solar
Cells, vol. 92(12), pp. 1689-96 (2008); [0154] Qing et al.,
Crowding clustering genetic algorithm for multimodal function
optimization, Appl. Soft Computing, vol. 8(1), pp. 88-95 (2008);
[0155] Taillaert et al., Compact efficient broadband grating
coupler for silicon-on-insulator waveguides, Opt. Lett., vol.
29(23), pp. 2749-51 (2004); [0156] Shokooh-Saremi et al., Particle
swarm optimization and its application to the design of diffraction
grating filters, Opt. Lett., vol. 32(8), pp. 894-96 (2007); and
[0157] Byrnes et al., Designing large, high-efficiency,
high-numerical-aperture, transmissive meta-lenses for visible
light, Opt. Exp. 24 (5), pp. 5110-5124 (2016).
[0158] The surface acoustic wave (SAW) optical modulators 200 on
the proximal face 160 and the two dimensional array 310 of output
optics 303 on the distal face 168 need to be carefully aligned in
the width (y-axis) direction so that each waveguide 102 sends light
into its corresponding diffractive lens strip 302. Their alignment
in the longitudinal direction (x-axis) is less critical because it
can be corrected for in the operating software during a calibration
step in which a wafer thickness profile (including thickness
non-uniformity) also can be measured and corrected.
[0159] FIG. 8C is a schematic cross-section of a light field
generator device 300, showing a SAW modulator 200 constructed with
diverging diffractive lens output optics 303.
[0160] An RF controller 405 includes at least one hardware
implemented processor device that is controlled by software
instructions to translate a desired 3D image into an appropriate RF
waveform to control the respective SAW optical modulators 200 of
the electro-holographic light field generator device 300. The
objective is to develop the exit light 150 produced from all of the
modulators 200 of the light field generator device 300 as a
three-dimensional output light field. Software for driving
electro-holographic displays of a variety of forms is described,
for example, in Mark Lucente, Computational holographic bandwidth
compression, IBM Systems Journal, 35(3 & 4), 349-365 (1996);
Quinn et al., Interactive holographic stereograms with
accommodation cues, Practical Holography XXIV: Materials and
Applications, ed. Hans I. Bjelkhagen and Raymond K. Kostuk, SPIE
(2010); and Jolly et al., Computation of Fresnel holograms and
diffraction-specific coherent panoramagrams for full-color
holographic displays based on leaky-mode modulators, Proc. SPIE
Practical Holography XXIX, 9386, 93860A (Mar. 10, 2016).
[0161] The resulting three-dimensional output light field is
similar to integral photography 3D displays in which there are
known algorithms for deciding how much light to put into which
views of which pixels. Such integral photography algorithms are
usable in this context, or they can be modified for even better
performance, for example, by adding in aspects of algorithms for
electro-holographic display such as how to choose and adjust
wavefront curvature (See, e.g., Smithwick et al., Interactive
holographic stereograms with accommodation cues, Practical
Holography XXIV: Materials and Applications, ed. Hans I. Bjelkhagen
and Raymond K. Kostuk, SPIE (2010)). Another algorithm improvement
is that the number of views that can be calculated and projected
can be determined and updated in software to optimize the display
quality rather than being fixed in hardware. In addition, the view
direction can be continuously adjustable in one dimension.
[0162] Once the RF controller 405 determines how much light 101
needs to be put into a given view of a given pixel, the RF
controller 405 then determines what waveforms of the RF drive
signals 130 need to be applied to the transducers 110 of the
respective SAW optical modulators 200 to produce that outcome. This
involves determining the appropriate output optics 303 to use--each
output optic 303 is able to send light of a particular wavelength
into any of typically 10-100 different non-overlapping views.
[0163] Once the appropriate output optics 303 are determined, the
RF controller 405 executes a computational back-propagation of that
light through the output optics 303 and back into the corresponding
waveguide 102. The computational interference between that
back-propagated light and the waveguided light finally determines a
specific waveform of the surface acoustic wave 140 to be used. In
specific embodiments, the back-propagation can be pre-computed into
a lookup table. For example, to create an approximately-collimated
beam of exit light 150 (focused at infinity), a given specific
chirped RF waveform creates a corresponding surface acoustic wave
140 in the waveguide 102, which creates out-coupled diffracted
light 162 as shown in FIG. 8C. This diffracted light 162
approximately converges towards a certain point on the horizontal
focal plane 404 of the diffractive lens output optic 303 (either in
front of or behind the output optic 303 depending on whether the
lens of the output optic 303 is diverging (as shown in FIG. 8C) or
converging (as shown in FIG. 9 discussed below).
[0164] In the specific embodiment shown in FIG. 8C, each
diffractive lens output optic 303 has the same length as an output
pixel and spreads the radiated exit light 150 into a desirable
field-of-view in the horizontal direction, for example, 90.degree..
This information, together with the RF bandwidth and substrate
index of refraction of the substrate 120 all are associated with a
chirped spatial frequency profile of the diffractive lens output
optic 303 and the diffractive lens strips 302 (which again acts as
a slice of a diffractive lens in this specific embodiment).
[0165] It should be understood that in this context there is not
some specific unique output optic 303 geometry that is optimal for
a desired field-of-view. Rather there are multiple possible
nominally acceptable designs with different characteristics such as
different focal lengths, different lens centers, and/or specific
optical aberrations. For example, some designs may be sharpest in
the center of the field of view, while others are sharper towards
the sides of the field-of-view but blurrier in the center.
Selection of a specific profile for the output optics 303 is a
matter of design choice.
[0166] Thus the RF controller 405 is configured to develop a hybrid
three-dimensional output light field formed from the exit light 150
transmitted by SAW devices 200 of the light field generator devices
300. The output light field is holographic horizontally and
spatially-multiplexed horizontally, vertically, or both to improve
the angular range or resolution horizontally, vertically, or both,
and also to incorporate the three colors. In the vertical
direction, each diffractive lens output optic 303 can collimate (or
approximately collimate as desired) the leaky mode diffracted
portion of the waveguide light 162 coming out of the respective
waveguide 102 and send it into a specific uniform vertical
direction. With 50 .mu.m-spaced (see reference 206) waveguides 102
and 1 mm.times.1 mm pixel size, then .about.7 different vertical
views may be produced, 7.times.3.times.50 .mu.m.apprxeq.1 mm (where
the 3 is to support red, green, and blue). The horizontal views can
be continuously adjusted though there is a limit caused by the
extent to which the radiation output 150 can be horizontally
collimated; i.e. the blurriness of the horizontal view direction.
The number of not-substantially-overlapping horizontal views may be
in the 10-100 range, and depends on the RF bandwidth driving the
SAW optical modulators 200.
[0167] A specific embodiment may utilize horizontal spatial
multiplexing by putting two or more output optics 303 horizontally
within each display pixel 304, in order to trade-off between the
number of vertical views and horizontal views. The diffractive lens
output optics 303 are optimized for maximum efficiency, so they
should have anti-reflective properties.
[0168] Within each display pixel 304, the waveguides 102 can be
arranged in a blocked order (e.g. RRRRGGGGBBBB) or in an
interleaved configuration (e.g. RGBRGBRGBRGB). In blocked order,
the diffractive lens output optics 303 may be configured such that
vertically-neighboring output optics 303 merge into each other and
are combined together into larger diffractive lens structures 304
(e.g., larger rectangular sections cut out of a single diffractive
lens). This can help mitigate the issue when some light from a
waveguide 102 travels vertically (in the y-axis direction in the
Figures) and hits an unintended output optic 303. Interleaved order
could also help with this same issue in a different way, if the
diffractive lens output optics 303 respond in a narrow-band way and
reject the light from an unintended neighboring waveguide 102.
Blocked order may also make the optical layout (laser in-coupling
configuration) simpler.
[0169] Depending on the specific application, the in-coupling
device 106 can deliver the input light 101 into the waveguides 102
from either direction, in some cases simultaneously, but more often
with interleaved strobes so that each optical direction obtains its
own RF waveform. This could double the number of views per RF
bandwidth. For example, we can have one laser direction send light
into the left half of the field-of-view, and the other into the
right half--one light input 106 would diffract off the +1 grating
order and the other (co-located with SAW transducers 110) off the
-1 grating order of the same diffractive lens output optic 303.
[0170] The waveguides 102 can carry red, green, and blue light
either simultaneously or in interleaved strobes. The diffractive
lens output optics 303 can be optimized for broadband and/or to
impart phase independently for the three colors (e.g. Aieta et al.,
Multiwavelength achromatic metasurfaces by dispersive phase
compensation, Science 347, 1342 (2015), which is incorporated
herein by reference in its entirety).
[0171] FIG. 9 shows a side cross-sectional view of an alternate
embodiment for one of the SAW modulators 200 of an
electro-holographic light field generator 300 in which the
diffractive lens output optics 303 are converging, rather than
diverging as in FIG. 8C. In this embodiment, the leaky mode
diffracted light 162 from the waveguides 102 is modulated by the
surface acoustic waves 140 to converge at a focal plane 404 within
the substrate 120 so that it is diverging as it enters an output
optic 303. The output optic 303 then converges the radiated exit
light 150 into a collimated beam (focused at infinity).
[0172] FIG. 10 shows a side cross-sectional view of an alternate
mode of operation for an electro-holographic light field generator
300. In this mode, the RF controller 405 operates the SAW optical
modulators 200 so that the leaky mode diffracted light 162 passing
through the output optics 303 converges towards a virtual focus 601
above the focal plane 404 of a lens output optic 303 by controlling
the RF drive waveform. As a result, the radiated exit light 150
from each output optic 303 has a selected output focal point 603
beyond the focal plane 404 before the location of the observer. The
surface acoustic wave 140 can be controlled to locate the output
focal point 603 at any desired specific distance (within reasonable
limits).
[0173] In some embodiments, the output optics 303 may be reflective
rather than transmissive, e.g. as a curved mirror or as a
reflective diffractive lens. This could entail some modification of
the grating period profile and/or an anti-reflective coating on the
surface of the wafer through which the light enters the air. This
surface may be the waveguide surface, or may be an edge.
[0174] The diffractive lens periodicity profile may not
specifically be a section of a conventional diffractive lens, but
may be modified--for example, including some positive or negative
spherical aberration--in order to optimize the distribution,
blurriness, wavefront curvature (focus), and other properties of
the views. For the same reason, the RF encoding may be more
complicated than described in the embodiments described above, and
more specifically may be modified from the back-propagation
algorithm results, for example by apodization of the RF waveform in
the time or frequency domain. The diffractive lens output optics
may have a helical property such that horizontal position
determines vertical deflection, instead of (or in addition to)
horizontal deflection. In such a case, the display may usefully be
oriented with vertical rather than horizontal diffractive
lenses.
[0175] FIG. 11A shows a side cross-sectional view of another
electro-holographic light field generator 300. A SAW device 200
within the light field generator 200 is shown, in which the end
face 170 functions as the exit face. Thus, the exit face need not
necessarily be the distal face 168. A single output optic 303 is at
the distal face 168. In this example, the RF controller 405
operates the SAW optical modulators 200 so that the leaky mode
portion of the diffracted light 162 converges towards a focal point
603 beyond the side exit surface/end face 170, which is deflected
by the output optic 303 as a side radiated exit light 150. The
output optic is a refractive lens or diffractive lens/grating in
different examples.
[0176] By using a variety of different focal depths per pixel, a
display constructed from the light field generators 200 can be
improved in several metrics including image quality and depth,
vergence-accommodation conflict, and astigmatism. In the horizontal
(x-axis) direction, the operating software can accomplish that by
using an RF encoding to the SAW optical modulators 200 that sends
light towards a point in front of or behind the diffractive lens
focal plane. In the vertical (y-axis) direction, the diffractive
lens output optics 303 can have different focal lengths in
different areas of the substrate 120. The feature of
continuously-adjustable view direction (as opposed to discrete
views) can be helpful for display quality, particularly alleviating
some aliasing issues common with other 3D displays (e.g. Zwicker et
al., Antialiasing for Automultiscopic 3D Displays, Eurographics
Symposium on Rendering, 2006). This is somewhat related to ability
to manipulate wavefront curvature as described above, which
similarly helps improve display quality.
[0177] The approaches described above enable a higher display
quality due to the small effective (sub) pixel size, lack of
horizontal aliasing, and ability to continuously adjust the
horizontal wavefront curvature. The conventional
acousto-optic-modulator-based holographic 3D display (e.g. "MIT
Mark 1", 2, or 3) is horizontal-parallax-only (HPO) made with an
acousto-optic or SAW modulator in a descanning configuration (e.g.
with a spinning polygon). But such descanning is very challenging
at best in a thin display with no-moving-parts. Descanning can be
avoided using strobe lights (e.g. Jolly et al., Near-to-eye
electroholography via guided-wave acousto-optics for augmented
reality, Proc. of SPIE Vol. 10127, 2017), but the thin form factor
also prevents overall demagnification and hence leads to a very
limited angle exit fan.
[0178] There are some known ways to increase the angle fan in an
electro-holographic display without pixelating the holo-line as
described above, but no one has previously suggested breaking the
holo-line into pixels and using lenses to increase the angle spread
of each pixel. Of course, this is not a conventional holographic
technique, but is more like a hybrid with integral photography.
Cutting up the hologram as described above does reduce the image
quality compared to a full proper hologram, but degradation is
likely to be acceptable in many practical applications and
certainly can be better than non-holographic alternatives.
[0179] The combined configuration of the lens focal plane 404,
waveguide 102, and RF encoding such as that shown in FIG. 8C is
particularly beneficial over known conventional approaches to
encoding the discrete data (which views of which pixels should be
turned on in each video display frame) into an RF waveform such as
encoding the data-points in different time-slots or in different
frequency-slots in the waveform. Rather, each data point
contributes a specific chirped waveform spread out in both time and
frequency, which greatly increases flexibility as to the location
and orientation of the diffractive lens array. That allows the use
of a flat pattern on the back side of a wafer-shaped optical
substrate, which can be very convenient in practice.
[0180] In some embodiments, it may be useful to have a second layer
of diffractive lenses or other optical components, for example, to
deal with diffraction effects. In addition or alternatively, the
transmissive diffractive lenses can be replaced or supplemented by
refractive or reflective optics; e.g., either cut out of the
optical substrate 102 or attached to the optical substrate. Some
embodiments may also omit the vertical parallax response, instead
making a horizontal-parallax-only (HPO) display. Such a display may
be simpler to implement with fewer waveguides, which can be larger
and spaced farther apart, which in turn may make the system easier
to build for various reasons, including reducing the number of RF
and optical connections, switches, and drivers.
[0181] FIG. 11B is another implementation of the
electro-holographic light field generator 300 in FIG. 11B. Here,
two output optics 303 provided on end face 170.
[0182] In some embodiments, it may be useful to maximize thinning
of the optical substrate 120 to reduce the distance between the
waveguide layer and diffractive lens output layer. In some
instances, this gap may need to be so thin that these two
components cannot be on two opposite sides of a single
self-supporting wafer.
[0183] FIG. 12 illustrates one possible process of creating a
substrate 120 that minimizes component separation.
[0184] In step 1, a piezoelectric material wafer 410 and a support
substrate (e.g. glass) are selected. In one example, the
piezoelectric material is lithium niobate.
[0185] In step 2, the piezoelectric material is planarized and then
patterned with output optics such as a two dimensional array 310 of
output optics 303 and described previously.
[0186] The piezoelectric material wafer 410 is then bonded to the
separate support substrate or wafer 412 using a wafer bonding
process in step 3. This encapsulates the output optic array 310
between the two wafers or substrates.
[0187] Then, the piezoelectric material wafer 410 is mechanically
thinned down in step 4 to produce final thickness for the device
substrate 120. This can be performed using CMP, for example. CMP is
chemical mechanical polishing/planarization, which thins and
smooths the surface of the wafer using a combination of chemical
and mechanical forces.
[0188] According to step 5, topside components 414, such as
waveguides 102, incoupling devices 106, and IDTs 110, are patterned
within/upon the surface of the now-thinned piezoelectric material
410 which will function as substrate 120. If a second layer of
diffractive lenses or micro-lenses is needed, it can be provided on
the back of the piezoelectric material 410 as layer 414, or the
back of the substrate could be made reflective, enabling two passes
through the output optics. As with other surfaces, the back of the
substrate could also have an anti-reflective coating.
[0189] FIG. 13A illustrates time-multiplexing of input light 101
and/or RF input signals 130 applied to a light field generator
device 300. The light field generator device 300 includes an array
of SAW devices 200.
[0190] One challenge of SAW-based electro-holographic displays is
the large number of illumination in-coupling ports 106 and RF drive
lines 130. The present embodiment addresses the issue of coupling
light into each of the waveguides 102 by relying on
time-multiplexing.
[0191] As before, the proximal face 160 includes an array of
waveguides 102. These multiple waveguides 102 are fed with an
optical signal input light source 902 using beam displacer 905. The
input light source 902 provides one or more wavelengths of input
light 101, such as red, green and blue. In one example, the input
light source 902 is a laser or system of lasers.
[0192] The beam displacer 905 operates via any of several
techniques for steering or displacing the laser beam of input light
101 into the waveguides 102-1 to 102-N (e.g., liquid crystal
steering (as in S R Davis et al., Analog, non-mechanical
beam-steerer with 90 degree field of regard, Proc. of SPIE Vol.
6971, 69710G, (2008)), SAW-based steering (as in C S Tsai et al.,
Guided-Wave Two Dimensional Acousto-Optic Scanner Using
Proton-Exchanged Lithium Niobate Waveguide, Fiber and Integrated
Optics 17, 157 (1998)), wavelength tuning accompanied by a pair of
diffraction gratings, and so on. In other variations, a surface
acoustic wave beam may be displaced instead of the laser beam of
input light 101, or the beam of input light 101 may be steered
instead of (or in addition to) displacing it, and so on.
[0193] At the other end, a series of IDT's 110 generate the SAWs
the propagate along waveguides 102. RF controller 405 generates RF
input signals 130-1 . . . 130-N for each of the IDTs 110-1 . . .
110-N.
[0194] In specific embodiments, all of the waveguides 102 may be
strobed simultaneously or in a sequence. Additionally, the
waveguides 102 can be included in different groupings. In this way,
the waveguides 102 within each group might be strobed
simultaneously, and the different groups strobed sequentially, in
another example.
[0195] Alternatively, if the strobe lights illuminate different
waveguides in sequence rather than simultaneously, a single large
SAW transducer 110 can produced the SAW's for multiple waveguides,
or many SAW transducers 110 may be coupled to a single RF input
signal 130 feed.
[0196] FIG. 13B illustrates a different embodiment. The beam
displacer or beam switch 905 is replaced with a beam splitter 906.
It divides the input light 101 from input light source 902 equally
into the waveguides 102-1 to 102-N. This allows one light source to
drive multiple SAW devices 200. In one embodiment, the splitter is
formed by waveguide formed in or on the SAW substrate 120.
[0197] It also may be helpful to account for speed-of-sound
frequency dispersion, for acoustic attenuation with distance, and
for optical attenuation with distance (due partly to SAW
scattering) when encoding the waveform of the RF light signals 130.
This may be done with higher accuracy by calibrating each unit
separately, controlling the unit's temperature and/or measuring the
unit's temperature to adjust the corrections.
[0198] In the specific application of a 3D display system 920,
there may be two additional optical polarizers--one optical
polarizer between the light input element and the SAW optical
modulator 200, and the other optical polarizer at the exit face. It
is noted that in this context, leaky mode SAW optical modulators
have the property of placing the exit light 150 in a polarization
state that is rotated with respect to the relatively high level of
background noise that is present from non-modulated light
reflecting within the modulator.
[0199] U.S. Pat. No. 6,927,886, to Plesniak et al., incorporated
herein by reference, hereinafter known as Plesniak, describes
software to manipulate the viewability of a holographic image. FIG.
14 is also FIG. 11A in Plesniak.
[0200] In FIG. 14, relationships between a modulator plane 1110, a
spatially distinct image plane 1120, and a viewzone 1130 are shown.
The image depth 1140 and the hologram depth 1150 are also shown.
Using embodiments of the proposed system, optical power can be
added by virtue of hardware embodiments not just for the expansion
(or contraction) of the output's angular subtense, but to
effectively "throw" the vertex of the output fan towards or behind
the modulator plane 1110 itself. This has benefit in fields such as
holographic display, in which the perceived resolution drops with
distance from the display. The image as viewed from the view zone
is formed in the image plane 1120. By adding or subtracting optical
power, a surface of best resolution can be created at surfaces
other than the modulator's exit face(s). Operation in this context
is described, for example, in Plesniak et al., "Reconfigurable
image projection holograms," Opt. Eng., vol. 45(11), (2006). In an
arrangement of this sort, diffraction gratings, or the additional
refractive or reflective optical element(s), can be "tuned" during
manufacture or computationally to best match the features of the
reconstructed holographic imagery, when the SAW device is used as a
component of an electronic display.
[0201] In accordance with embodiments of the present invention, an
optical engine of electro-holographic displays may be provided,
such as: a desktop 3-D computer display, a head-worn near-eye
virtual reality/augmented reality/mixed reality display, a virtual
sand table, or the walls of a room creating immersive imagery.
Applications of such displays include: battlefield visualization,
interventional medical imaging for procedure planning and guidance,
molecular visualization, and entertainment.
[0202] Electro-Holographic Display Architecture
[0203] FIG. 15 shows multiple electro-holographic light field
generator devices 300 stacked to form an electro-holographic 3D
display 900, according to one embodiment.
[0204] In more detail, edge-fire SAW optical modulators 200 are
employed in the electro-holographic light field generator devices
300. In an edge-fire SAW device 200, the exit light 150 is emitted
out the end face 170 of each optical modulator 200 of the stacked
light field generator devices 300. Further, these SAW optical
modulators 200 preferably utilize non-orthogonal end faces 170.
[0205] The light field generator devices 300 have longitudinal
axes, defined by the direction of the waveguide 102 of the SAW
modulator devices 200 that form the light field generator devices
300. These axes are parallel to line 914. Further, the light field
generator devices 300 are arranged in a stack 901 such that the
direction of their longitudinal axes 914 is at an angle 922 to the
direction 912 of observers 910. Typically, the angle 922 is between
20 and 70 degrees.
[0206] Moreover, the light field generator devices 300 are
arranged, one on top of the other, such that the distal faces 168
of the modulator devices 200 of one light field generator device
300 are adjacent to the proximal face 160 of the modulator devices
200 of the next light field generator device 300. Moreover, the end
faces 170 of the separate SAW devices 200, generator devices 300
all lie in approximately the same vertically extending plane.
[0207] FIG. 16A shows electro-holographic light field generator
devices 300 stacked to form an electro-holographic 3D display 900,
according to another embodiment.
[0208] This embodiment also employs edge-fire SAW optical
modulators 200 in the electro-holographic light field generator
devices 300. And, these optical modulators 200 preferably utilize
non-orthogonal end faces 170 in which the edge cut angles are
acute. In this example, however, the longitudinal axis 914 of each
of the modulator devices 200 is pointed at the observers 910, such
that angle 922 is less than 15 degrees, and possibly 0 degrees.
[0209] Moreover, the light field generator devices 300 are arranged
in a stack 901, in pairs 340. Two pairs 340-1 and 340-2 within the
stack 901 of light field generator devices 300 are shown.
[0210] In each pair 340, the proximal faces 160 of the modulator
devices 200/light field generator devices 300 are adjacent to each
other. The pairs 340 are then stacked such that the distal faces
168 of the modulator devices 200 of one pair 340-1 are adjacent to
the distal faces 168 of the modulator devices 200 of the next pair
340-1 in the stack 901.
[0211] FIG. 16B also shows electro-holographic light field
generator devices 300 stacked to form an electro-holographic 3D
display 900, according to another embodiment.
[0212] This embodiment also employs edge-fire SAW optical
modulators 200 in the electro-holographic light field generator
devices 300. And, these optical modulators 200 preferably utilize
non-orthogonal end faces 170, but here the edge cut angles are
obtuse.
[0213] FIG. 17 shows a 3D display system 920. The 3D display system
920 includes an electro-holographic 3D display 900 and also
includes additional components. The additional components include
an illumination source 902, an RF driver 907, and a processor 909.
Here, the electro-holographic light field generator devices 300 of
the electro-holographic 3D display 900 are arranged in a dual
column stack 901. The light field generator devices 300 are stacked
and arranged side by side to obtain a 3D display system 920 with a
wider display field in the lateral direction.
[0214] Each of the electro-holographic light field generator
devices 300 within the electro-holographic 3D display 900 receives
a beam of input light 101 generated by illumination source 902. The
illumination source 902 might be a laser such as a pulsed laser, to
cite one of many possible examples of illumination light sources
902. The laser might illuminate the generator devices 300 together
in a beam. Separate in-coupling prisms could be used to couple
light into each of the separate waveguides.
[0215] The light field generator devices 300 are driven by the RF
driver 907 (also referred to herein as a "controller"). The RF
driver 907 is governed by processor 909 on the basis of typically
digitized graphical data resident or derived in a format
appropriate for the electro-optical subsystem of the light field
display 900. The 3D display system 920 produces a modulated exit
beam 930, in accordance with any of the teachings provided herein
above, such that observers 910 in the far field perceive an object
950 to be projected in three dimensions.
[0216] It is to be understood that the teachings presented herein
may be applied to SAW device 200 configurations described herein
but may also be applied to any other SAW device 200 configurations,
whether currently known or developed in the future.
[0217] It is also important to note that such light field
generators 900, though described in the specific context of 3D
display systems, also can usefully be applied to other applications
such as optogenetics, 3D printing, cloaking, and near-eye displays
for augmented reality/virtual reality (AR/VR).
[0218] Embodiments of the invention may be implemented in part in
any conventional computer programming language such as VHDL,
SystemC, Verilog, ASM, Python, C, C++, MATLAB etc. Alternative
embodiments of the invention may be implemented as pre-programmed
hardware elements such as, without limitation, combinations of one
or more of a field-programmable gate array (FPGA), graphics
processing unit (GPU), central processing unit (CPU) and other
related components, or as a combination of hardware and software
components.
[0219] Embodiments can be implemented in part as a computer program
product for use with a computer system. Such implementation may
include a series of computer instructions fixed either on a
tangible medium, such as a computer readable medium (e.g., a
diskette, CD-ROM, ROM, or fixed disk) or transmittable to a
computer system, via a modem or other interface device, such as a
communications adapter connected to a network over a medium. The
medium may be either a tangible medium (e.g., optical or analog
communications lines) or a medium implemented with wireless
techniques (e.g., microwave, infrared or other transmission
techniques). The series of computer instructions embodies all or
part of the functionality previously described herein with respect
to the system. Those skilled in the art should appreciate that such
computer instructions can be written in a number of programming
languages for use with many computer architectures or operating
systems. Furthermore, such instructions may be stored in any memory
device, such as semiconductor, magnetic, optical or other memory
devices, and may be transmitted using any communications
technology, such as optical, infrared, microwave, or other
transmission technologies. It is expected that such a computer
program product may be distributed as a removable medium with
accompanying printed or electronic documentation (e.g., shrink
wrapped software), preloaded with a computer system (e.g., on
system ROM or fixed disk), or distributed from a server or
electronic bulletin board over the network (e.g., the Internet or
World Wide Web). Of course, some embodiments of the invention may
be implemented as a combination of both software (e.g., a computer
program product) and hardware. Still other embodiments of the
invention are implemented as entirely hardware, or entirely
software (e.g., a computer program product).
[0220] IDT Architecture
[0221] FIG. 18A shows detail for a prior art IDT 110.
[0222] In the `slab` configuration which waveguide 102 assumes in
FIG. 1A, the IDT 110 is comprised of transducer fingers 188. These
transducer fingers 188 are typically patterned at a higher layer
than the waveguide 102, rather than at the same layer as the
waveguide. As a result, sound waves (e.g. the surface acoustic
waves 140) propagate at a lower "altitude" than the IDTs.
Embodiments of the SAW devices 200 disclosed herein might utilize
an IDT 110 having transducer fingers 188 in accordance with FIG.
18A.
[0223] FIG. 18B through FIG. 18G show different layouts of
transducer fingers 188 that can be constructed for the various
embodiments of the SAW devices 200 proposed herein.
[0224] FIG. 18B, for example, shows transducer fingers 188 of a
waveguide that uses the geometry of a channel rather than that of
the `slab` configuration of the waveguide shown in FIG. 1A. The
transducer fingers 188 are disposed astride the channel waveguide
102. Longitudinal acoustic wave 189 is shown propagating in the
channel waveguide 102. The channel waveguide 102 may support
multiple modes of electromagnetic radiation. Here, the
configuration of the transducer fingers 188 may advantageously
provide enhanced electro-mechanical efficiency when compared with
the prior art slab configuration of FIG. 18A.
[0225] FIG. 18C shows a view similar to that of FIG. 18B,
additionally depicting an emergent fan of deflected exit light 150.
FIG. 18D shows a side view of the transducer fingers 188 in FIGS.
18B and 18C, and also shows diffracted light 162 inside substrate
120, which emerge as rays of exit light 150 outside the substrate
120.
[0226] In the embodiment shown in FIG. 18F, multiple waveguides 102
may be patterned into the material used for SAW device 200, such as
lithium niobate, for example. Light used to illuminate multiple
waveguides 102 as shown in FIG. 18F may also be derived from a
single input light source 902. FIG. 18G is a cross-sectional side
view of either of the embodiments of FIG. 18E or 18F.
[0227] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
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