U.S. patent application number 15/938677 was filed with the patent office on 2018-10-04 for light field generator devices with opposed saw modulators.
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, Michael G. Moebius, Joseph J. Register.
Application Number | 20180284562 15/938677 |
Document ID | / |
Family ID | 62002722 |
Filed Date | 2018-10-04 |
United States Patent
Application |
20180284562 |
Kind Code |
A1 |
Register; Joseph J. ; et
al. |
October 4, 2018 |
LIGHT FIELD GENERATOR DEVICES WITH OPPOSED SAW MODULATORS
Abstract
An electro-holographic light field generator device comprises
surface acoustic wave (SAW) optical modulators arranged in
different directions. Specifically, some embodiments have SAW
modulators arranged in pairs, nose-to-nose with each other, and
have output couplers that provide face-fire light emission. These
SAW modulators also possibly include SAW sense transducers and/or
viscoelastic surface material to reduce crosstalk.
Inventors: |
Register; Joseph J.;
(Cambridge, MA) ; Callahan; Dennis M.; (Wellesley,
MA) ; Moebius; Michael G.; (Somerville, MA) ;
Byrnes; Steven J.; (Watertown, MA) ; Favalora; Gregg
E.; (Bedford, MA) ; Frank; Ian W.; (Arlington,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Charles Stark Draper Laboratory, Inc. |
Cambridge |
MA |
US |
|
|
Family ID: |
62002722 |
Appl. No.: |
15/938677 |
Filed: |
March 28, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62477511 |
Mar 28, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 2203/023 20130101;
G10K 11/178 20130101; G02B 2006/0098 20130101; G03H 2225/11
20130101; G03H 2225/34 20130101; G02F 1/0353 20130101; G02F 2201/18
20130101; G02F 2203/28 20130101; G02F 2203/58 20130101; G03H
2225/36 20130101; G02F 2203/07 20130101; G02F 2203/24 20130101;
G03H 1/2294 20130101; G02B 6/0045 20130101; G02B 2006/12104
20130101; G03H 2223/17 20130101; G02F 2201/05 20130101; G02B 30/26
20200101; G02B 2006/12107 20130101; G10K 11/36 20130101; G02B
6/12007 20130101; G02B 2006/12085 20130101; G02B 6/005 20130101;
G02B 6/0078 20130101; G10K 11/17875 20180101; G02F 2201/302
20130101; G10K 11/002 20130101; G02F 2201/34 20130101; G02F 2203/22
20130101; G03H 2001/2292 20130101; G02B 6/105 20130101; G03H 1/02
20130101; G02F 1/125 20130101; G03H 2001/0224 20130101; G03H
2225/55 20130101; G02F 1/335 20130101; G10K 11/17873 20180101; G03H
2225/21 20130101; G02B 6/12004 20130101; G03H 2223/16 20130101 |
International
Class: |
G02F 1/335 20060101
G02F001/335; G02B 27/22 20060101 G02B027/22 |
Claims
1. A light field generator device, comprising: a substrate; an
array of waveguides extending across the substrate; and an array of
surface acoustic wave (SAW) modulators formed in the substrate, the
SAW modulators including SAW transducers for generating SAWs in the
substrate to diffract light from the waveguides; wherein the SAW
modulators are directed in different directions.
2. The light field generator device as claimed in claim 1, wherein
the SAW modulators that are directed in different directions share
a common waveguide.
3. The light field generator device as claimed in claim 1, wherein
the SAW modulators that are directed in different directions use
adjacent waveguides.
4. The light field generator device as claimed in claim 1, further
comprising output couplers for redirecting the light diffracted
from the waveguides.
5. The light field generator device as claimed in claim 4, wherein
the output couplers comprise mirror structures formed on the
optical substrate.
6. The light field generator device as claimed in claim 4, wherein
the output couplers comprise surface gratings on a distal side of
the substrate.
7. The light field generator device as claimed in claim 4, wherein
the output couplers comprise volume gratings fabricated within the
substrate.
8. The light field generator device as claimed in claim 4, wherein
the output couplers each comprise a plurality of chirped grating
structures, which are mapped to different wedges of output angles
in an output light field.
9. The light field generator device as claimed in claim 1, wherein
the SAW modulators are arranged together in pairs, wherein each
optical modulator in a pair contributes to half of an output light
field.
10. The light field generator device as claimed in claim 1, wherein
the device has folded light paths with out-of-plane coupling
elements.
11. The light field generator device as claimed in claim 1, wherein
each waveguide is configured to handle a single wavelength of input
light selected from the group of infrared, red, green, blue and
ultraviolet light.
12. The light field generator device as claimed in claim 1, wherein
each waveguide is configured to handle multiple wavelengths of
input light selected from the group of infrared, red, green, blue
and ultraviolet light.
13. A light field generator device, comprising: a substrate; an
array of waveguides extending across the substrate; and an array of
surface acoustic wave (SAW) modulators formed in the substrate, the
SAW modulators including SAW transducers for generating SAWs in the
substrate to diffract light from the waveguides; regions of
viscoelastic surface material on the substrate to reduce
crosstalk.
14. A 3D display system comprising one or more light field
generator devices, wherein each of the devices comprises: a
substrate; an array of waveguides extending across the substrate;
and an array of surface acoustic wave (SAW) modulators formed in
the substrate, the SAW modulators including SAW transducers for
generating SAWs in the substrate to diffract light from the
waveguides; wherein the SAW modulators are directed in different
directions.
15. A method of operation of a light field generator device,
comprising: providing an array of surface acoustic wave (SAW)
modulators formed in the substrate, the SAW modulators including
SAW transducers for generating SAWs in the substrate to diffract
light from waveguides; and directing the SAW modulators in
different directions.
16. The method as claimed in claim 15, wherein the SAW modulators
that are directed in different directions share a common
waveguide.
17. The method as claimed in claim 15, wherein the SAW modulators
that are directed in different directions use adjacent
waveguides.
18. The method as claimed in claim 15, wherein the SAW modulators
are arranged together in pairs, wherein each optical modulator in a
pair contributes to half of an output light field.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit under 35 USC 119(e) of
U.S. Provisional Application No. 62/477,511, filed on Mar. 28,
2017, which is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] A number of proposed autostereoscopic (naked-eye) 3D
displays or, more broadly, light field generator architectures
utilize a variety of scanning, diffraction, space-multiplexing,
steered illumination, and other techniques. One category,
electro-holographic displays, relies principally on diffractive
phenomena to shape and steer light. 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). Electro-holographic light field generators
hold the promise of projecting imagery with the ultimate in
realism: curved optical wavefronts, which can genuinely replicate
the real world. Such displays can theoretically provide nearly
perfect characteristics of visual depth information, color
rendering, optical resolution, and smooth transitions as viewers
changes their location. So far, displays built on this technology
have not achieved this theoretical level of performance.
[0003] One specific device category that provides controllable
sub-holograms from which a light field can be constructed uses what
is known as a surface acoustic wave (SAW) modulator. A SAW is
generated in a piezoelectric substrate under radio frequency (RF)
excitation. This creates a time-varying diffracting region that
interact with input light in the waveguide. This causes at least
some of the light to change from a guided mode within the waveguide
to a leaky mode that exits the waveguide. This is described more
fully, for example, in:
[0004] 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);
[0005] 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);
[0006] Smalley et al., Anisotropic leaky-mode modulator for
holographic video displays, Nature, 498, 313-317 (20 Jun.
2013);
[0007] U.S. Pat. App. Publ. US 2014/0300695; Full Parallax
Acousto-Optic/Electro-Optic Holographic Video Display;
[0008] Gneiting et al., Optimizations for Robust, High-Efficiency,
Waveguide-Based Holographic Video, Industrial Informatics (INDIN),
2016 IEEE 14th International Conference on, (19-21 Jul. 2016);
[0009] 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);
[0010] 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);
[0011] 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); and
[0012] Savidis et al., Progress in fabrication of waveguide spatial
light modulators via femtosecond laser micromachining, Proc. of
SPIE Vol. 10115, (2017).
[0013] FIG. 1 shows an exemplary prior art SAW optical modulator
100. It can be used to deflect light of the same or different
colors/wavelengths 101a, 101b, 101c from guided modes by different
angles simultaneously, or serially, in time.
[0014] The modulator 100 comprises a substrate 120 in which or on
which an optical waveguide 102 has been formed. The input light
101a, 101b, and/or 101c at one or more wavelengths (.lamda..sub.1,
.lamda..sub.2, .lamda..sub.3) enters waveguide 102. An in-coupling
device 106 is used 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 between an optical fiber or light in
free-space 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.
[0015] In such a SAW modulator 100, the waveguide 102, e.g., slab
waveguide, is typically created in a lithium niobate substrate 120
by proton-exchange. Transducers (e.g., interdigital transducers
(IDTs)) 110 are written on an aluminum side of the substrate 120.
The transducers 110 induce surface acoustic waves (SAWs) 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) drive signal 130.
[0016] The light interacts with the surface acoustic wave 140. The
result of this interaction between the SAW 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 leaky-mode or diffracted light 162 and
enters substrate 120 at angle .phi., measured from grazing 77. At
some point this diffracted light 162 exits the substrate 120 at an
exit face, which is possibly through the substrate's distal face
168 or end face 170 (as shown) as exit light 150 at an exit angle
of .theta.. The range of possible exit angles .theta. comprises the
angular extent, or exit angle fan, of the exit light 150.
[0017] Practical electronic constraints and materials properties
often limit the resulting angular deflection of SAW devices. Qaderi
(2016) reports that a total output angle of approximately
20.degree. can be achieved, significantly lower than the field of
view of contemporary 2D displays that approach 180.degree..
Existing electro-holographic 3D displays using SAW devices have
attempted to increase the exit angle fan of the diffracted output
light 150 in various ways such as by 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), by using edge-emitting
modulators having "right-angle" edges, by doubling the 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 are adequate to achieve an angle fan as high as 90.degree. in
any sort of flat form-factor.
SUMMARY OF THE INVENTION
[0018] Embodiments of the present invention are directed to
electro-holographic light field generator devices. In these
devices, the optical modulators are oriented with respect to the
directional reference line so that a substantially equal number of
optical modulators might he directed along the directional
reference line and directed against the directional reference line,
and the optical modulators are configured to develop the output fan
beams in coordination as a multiplexed output light field. For
example, the controlled output angle may specifically be
perpendicular to the optical substrate.
[0019] The output couplers may specifically comprise micro-minor
structures on the optical substrate, surface grating structures
fabricated on a back face of the optical substrate, or volume
grating structures fabricated within the optical substrate.
[0020] In some specific embodiments, the optical modulators might
be arranged together in pairs, where each optical modulator in a
pair contributes to part of the output light field.
[0021] There may be a region of patterned viscoelastic surface
material for each pair of optical modulators or other arrangement.
This material will lower SAW crosstalk.
[0022] In general, according to one aspect, the invention features
a light field generator device. It comprises a substrate and an
array of waveguides extending across the substrate. An array of
surface acoustic wave (SAW) modulators are also formed in the
substrate. They include SAW transducers for generating SAWs in the
substrate to diffract light from the waveguides. According to the
invention, the SAW modulators are directed in different
directions.
[0023] In disclosed embodiments, the SAW modulators are arranged in
pairs, and in some cases "nose-to-nose," in face-fire
configurations.
[0024] In embodiments, the SAW modulators that are directed in
different directions share a common waveguide. But in other
embodiments, the SAW that are directed in different directions use
adjacent waveguides.
[0025] Different output couplers are used for redirecting the light
diffracted from the waveguides. Examples include mirror structures
formed on the optical substrate, surface gratings on a distal side
of the substrate, volume gratings fabricated within the substrate.
Chirped grating structures, which are mapped to different wedges of
output angles in an output light field, are a further
possibility.
[0026] Preferably, the SAW modulators are arranged together in
pairs, wherein each optical modulator in a pair contributes to half
of an output light field.
[0027] Further examples include the device with folded light paths
with out-of-plane coupling elements.
[0028] 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
[0029] 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:
[0030] FIG. 1 shows a side view of a prior art SAW modulator;
[0031] FIG. 2 is an elevated perspective view of an
electro-holographic light field generator device according to an
embodiment of the present invention;
[0032] FIG. 3 is a front plan view of the light field generator
device, looking down on z-axis;
[0033] FIG. 4A is a side cross-sectional view of a SAW optical
modulator in the light field generator device;
[0034] FIG. 4B is a side cross-sectional view of a cooperating pair
of SAW optical modulators in the light field generator device;
[0035] FIG. 4C is a side cross-sectional view of a cooperating pair
of SAW optical modulators in the center of the array of the light
field generator device;
[0036] FIG. 5 is a schematic diagram showing the lines of
modulators and their shared waveguides of a light field generator
device, where each line of modulators is shown in side
cross-section to illustrate their time-multiplex operation,
according to the present invention;
[0037] FIG. 6 is a side cross-sectional view of a SAW optical
modulator in the light field generator device using a surface
grating output coupler;
[0038] FIG. 7 is a side cross-sectional view of a SAW optical
modulator in the light field generator device using a volume
grating output coupler;
[0039] FIG. 8 is a schematic diagram showing the lines of
modulators and their shared waveguides of a light field generator
device, where each line of modulators is shown in side
cross-section to illustrate their operation in which the light
field generator device has a folded light path to scan a single
line according to an embodiment of the present invention;
[0040] FIG. 9 is a side cross-sectional view of a SAW optical
modulator in the light field generator device using series of
chirped volume grating output couplers;
[0041] FIG. 10A is a side cross-sectional view of a SAW optical
modulator in the light field generator device having a sensing
transducer for detecting SAWs in the substrate;
[0042] FIG. 10B is a side cross-sectional view of a SAW optical
modulator in the light field generator device in which the
transducers can function within a SAW generation mode or a SAW
sensing mode by the incorporation of a mode switch;
[0043] FIG. 11 is a side cross-sectional view of a cooperating pair
of SAW optical modulators with feedback SAW dampening;
[0044] FIG. 12 is a side cross-sectional view of a cooperating pair
of SAW optical modulators with feedforward SAW dampening;
[0045] FIG. 13 is a side cross-sectional view of a pair of SAW
optical modulators in the light field generator device employing
patterned viscoelastic surface material to reduce crosstalk;
and
[0046] FIG. 14 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.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] 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.
[0048] 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.
[0049] 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 he
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.
[0050] Different embodiments of the present invention described
herein below can address several drawbacks that are present in
existing light field generators. These drawbacks are of particular
importance to their use as 3D display systems and other systems
that involve light field generation. One such drawback concerns the
fact that existing SAW devices often have an insufficiently broad
exit fan of the output light field, and that in most cases, the
diffracted output light field is centered inconveniently away from
the normal to the device's exit face, such as the distal face.
Embodiments of the present invention can be used to increase the
field of view (FOV) of a diffractive 3D display system using
modulators in a "face-fire" configuration by combining modulators
oriented in different directions to effectively increase the exit
fan. This improves displays in at least two ways: centering the
output angle of the output light field about convenient direction
(e.g. the device normal), and broadening the angular field of view
of the output light field so that it becomes more useful for a
variety of applications.
[0051] FIGS. 2 and 3 show an elevated perspective view and front
view respectively of an electro-holographic light field generator
device 300 in the specific form of a 3D display module according to
an embodiment of the present invention. A 3D display system, for
example, would employ one or more such devices 300 in possibly a
regular grid in order to generate a light field of sufficient
extent for a desktop or wall-mounted 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.
[0052] An optical substrate 120 of the device 300 has a proximal
face 160 (shown facing in FIGS. 2 and 3) and is characterized by a
directional reference line 306 along the proximal face 160. In the
specific embodiment shown, the proximal face 160 is planar, but
that is not necessarily the case in other specific embodiments. For
example, in some embodiments the modulator face may be convex or
concave, or concave spherical curvature or convex spherical
curvature.
[0053] The optical substrate 120 may be made, for example, of a
suitable piezoelectric material such as lithium niobate
(LiNbO.sub.3), quartz (SiO.sub.2), or lithium tantalate
(LiTaO.sub.3) following known processes, e.g., Smalley et al, 2013.
Many other materials and design choices are available including
other piezoelectric materials and different 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.
[0054] The optical substrate 120 may range in x- or y-dimensions of
1 centimeter (cm) (for near-eye display applications) to over 20 cm
on each of the x and y dimensions (for larger displays at larger
viewing distances). Typically, the thickness (z-dimension) of the
optical substrate 120 ranges from 0.5 mm to 3 mm.
[0055] At opposing ends of the optical substrate 120 are left and
right light in-coupling devices 106-L, 106-R that are utilized to
produce (counter-propagating) light vectors within each of a
plurality of waveguides 102 that are distributed across the
proximal face 160 of the optical substrate 120 in an orientation
defined with relation to the reference line 306. Corresponding SAW
transducers 110 generate SAWs in the substrate that interact with
the waveguide light.
[0056] The illustrated electro-holographic light field generator
300 comprises a two dimensional array of SAW modulators 100.
Specifically, in the illustrated embodiment, there are twenty-two
shared waveguides 102 extending parallel to each other along the
y-axis of the device 300. On the other hand, there are eighteen
shared SAW transducers 110 along the x-axis direction of the device
300 for each of the waveguides 102.
[0057] FIG. 4A is a side cross-sectional view of one of the
modulators along the far left side of the electro-holographic light
field generator 300 such as modulator 100-E, and FIG. 4B shows how
multiple optical modulators are arranged in cooperating pairs
100-E, 100-F, along the edges of the generator 300. FIG. 4C shows
another exemplary cooperating pair of modulators 100-G, 100-H from
the center of the device 300.
[0058] The optical modulators 100 share waveguides 102 defined
within the optical substrate 120. A light source 210 provides input
light 101 through the in-coupling device 106-L (e.g., an optical
grating or prism) to the waveguide 102 for TE-like (transverse
electric) guided mode confinement of waveguide light within the
waveguide 102.
[0059] There is at least one surface acoustic wave (SAW) transducer
110 (e.g., an interdigital transducer (IDT)) for each modulator
100. The transducers, in response to the RF input,
piezoelectrically generate SAWs 140 in the modulator's substrate
120 that propagate collinearly, in one example, with the waveguide
102 and are timed with the pulses of waveguide light to interact to
convert a portion of the light to transverse magnetic (TM)
polarization.
[0060] Birefringence of the waveguide 102 and the optical substrate
120 (and/or the wave-vector change from the interaction) causes the
deflection of light in the waveguide 102 to create a leaky mode fan
of diffracted light 162. In typical operation, the light is timed
with the SAW generation so that the waveguide light is diffracted
out of the waveguide within a defined waveguide output coupling
region 102-O of the waveguide 102 in the optical substrate 120.
[0061] The SAW transducers 110 can occupy various specific
locations and specific orientations with respect to the waveguide
102; for example. The illustrated embodiment has the SAW
transducers 110 near the in-coupling device 106-L (see FIG.
4B).
[0062] When integrated into the electro-holographic light field
generator 300, there could be multiple SAW transducers 110 for each
light source 210/in-coupling device 106/waveguide 102 and upstream
of an output coupler. Then each of the SAW transducers 110 that are
distributed along the waveguide 102 is responsible for a different
specific bandwidth around a given center frequency (e.g.: 100-200
MHz, 200-300 MHz, and 300-400 MHz), in one implementation.
[0063] The SAW transducers 110 typically are interdigital
transducer (IDT) features fabricated from patterned metal films
such conductive material including metals (e.g., aluminum,
titanium, or gold), conductive polymers, or conductive oxides such
as indium tin oxide (ITO). Patterning the SAW transducers 110 may
be performed though photolithography (etching or lift-off), laser
ablation of metal film, or direct-writing techniques such as
described in Datta et al, Direct-laser metal writing of surface
acoustic wave transducers far integrated-optic spatial light
modulators in lithium niobate, Proc. SPIE Adv. Fab. Tech. for
Micro/Nano Optics and Photonics X, 10115 (2017). Each SAW
transducer 110 is fabricated with separate drive signal
connections, allowing for each to be individually addressed, in one
implementation. In another implementation, each a line of SAW
transducers of the electro-holographic light field generator 300 of
FIGS. 2 and 3, or other group of transducers, is fed with a common
RF signal.
[0064] As shown in FIGS. 4B and 4C, in specific embodiments, pairs
of optical modulators 100-E/100-F and 100-G/100-H may be arranged
along a shared waveguide 102, where one of the optical modulators
100-E, 100-G in each pair is directed along the directional
reference line 306, and one of the optical modulators 100-F, 100-H
in each pair is directed against the directional reference line
306. In such an embodiment, the modulators 100 across the extent of
the device 300 shown in FIGS. 2 and 3 are paired in this way.
[0065] For example, the optical modulators 100 in each pair may
share a single common waveguide 102. Or pairs of optical modulators
100 may be arranged with waveguides 102 that are adjacent and
parallel to each other, again with one of the optical modulators
100 directed along the directional reference line 306, and one of
the optical modulators 100 directed against the directional
reference line 306.
[0066] Optical modulators pairs may be packed relatively tightly
with shared waveguides or separate waveguides, with a separation
between the waveguides 102 of between 10 micrometers (.mu.m)-1000
.mu.m, for example, 50 .mu.m. The waveguide length may be 1-10
centimeters (e.g., 5 cm) or even longer if multiple SAW transducers
110 and/or multiple light in-coupling devices 106 are used to
mitigate acoustic and optical attenuation respectively. In this
context, a greater waveguide length reduces system complexity and,
if tiled into a larger display, it minimizes tile-borders
("grout"). Since the SAW 140 moves at the speed of sound, the light
sources 402 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.
[0067] Each waveguide 102 of the electro-holographic light field
generator 300 of FIGS. 2 and 3 may be configured for a single
specific wavelength of input light, 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 wavelengths of infrared, red, green, blue or
ultraviolet light. 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.
[0068] For each SAW optical modulator 100, there are one or
multiple output couplers. In the examples of FIGS. 4A and 4B, the
output couplers are micro-mirrors 410 formed, such as deposited, on
angled endfaces 170 of each of the SAW modulator 100. In the
illustrated example, an edge-cut angle .beta. of the endfaces 170
relative to the proximal face 160 is approximately 45 degrees.
[0069] The output couplers are distributed along the waveguide 102
and adjacent to the output coupling region 102-O and configured for
reflecting the leaky mode fan of exit light 150 as an output fan
beam that is directed out of the plane from the optical substrate
120 at an output angle towards a display viewer. In the illustrated
embodiments, the output fan beam is directed in a range of angles
around perpendicular to the plane of proximal face 160 of the
substrate 120. The micro-mirrors 410 are configured at an angle
(typically less than 45 degrees) such that, for a pair of optical
modulators 100-E, 100-F, each side handles half of the total field
of view (FOV) of the output fan beam 150. In one example, the
mirrors 410 are at about 40 degrees, i.e., less than 45 degrees, so
that rays 162 from modulator 100-E have some "margin" of being
reflected into the region labelled "B" and rays 162 from modulator
100-F are reflected into the region labeled "A", to provide some
overlap between the output fans of each of the two modulators in a
pair.
[0070] An RF controller 405 includes at least one hardware
implemented processor device which provides control instructions to
translate a desired 3D image into an appropriate RF waveform to the
SAW transducer 110 of each optical modulator pair for the device
300 to develop the output fan beams 150 as a multiplexed
three-dimensional output light field. For example, the output fan
beams 150 may specifically be perpendicular to the optical
substrate 120. 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).
[0071] Once the RF controller 405 determines how much light needs
to be put into a given view of a given pixel, it then determines
what RF waveforms need to be applied to the SAW transducers 110 to
produce that outcome. For example, the RF controller 405 may do a
computational back-propagation of that light via the micro-mirror
output couplers 410 and back into the corresponding waveguide 102.
The computational interference between that back-propagated light
and the waveguide light finally determines a specific SAW waveform
to be used. In specific embodiments, the back-propagation can be
pre-computed into a lookup table.
[0072] FIG. 5 schematically shows an array of SAW modulators of an
electro-holographic light field generator 300 and one possible mode
of operation. The illustrated example as four rows of modulators
and five columns of modulators (denoted 100-[row], [column]). Each
line is shown in cross-section to illustrate the operation.
[0073] In more detail, in the illustrated light field generator
300, there are four shared waveguides 102-1, 102-2, 103-3 and
102-4. It should be noted, however, that in other embodiments, each
of these waveguides 102-1, 102-2, 103-3 and 102-4 might actually be
a pair of closely spaced waveguides that carry light in opposite
directions with respect to each other.
[0074] Light is coupled into the each of these waveguides 102 via
left and right in-coupling devices. For example, for shared
waveguide 102-1, there is a left in coupling device 106-1-L and a
right in-coupling device 106-1-R.
[0075] Then, for each shared waveguide 102, there are a series of
SAW modulators 100. For example, for the top shared waveguide
102-1, there are SAW modulators 100-1,1 to 100-1,5.
[0076] Each of these SAW modulators has an associated SAW
transducer 110, and one or two output couplers. In the illustrated
example, the output couplers are micro-mirrors 410. However, in
other embodiments, different output couplers could be used such as
gratings, volume gratings, discrete optics, or the like.
[0077] For example, the end SAW modulators, such as 100-1,1 have a
single output coupling mirror 410-1,1, in the illustrated
embodiment. On the other hand, SAW modulators that are not on the
end have two associated output couplers. For example, SAW modulator
100-1,4 has a left output coupler 410-1,4L and a right output
coupler 410-1,4R. Thus, depending on the direction of light
propagation in the shared waveguides 102, most of the SAW
modulators can output light at either of their two output
couplers.
[0078] Also illustrated is how the electro-holographic light field
generator 300 employs time-multiplexing of multiple light inputs,
multiple SAW transducers, and multiple output couplers. This can be
used to piecewise scan a single line of a 3D display system
according to an embodiment of the present invention.
[0079] In more detail, the specific light output couplers is chosen
through coordinating the timing of the light inputs and the
energization of the SAW transducers so that inactive SAW
transducers can be in the OFF state (not necessarily required, but
reasonable for power management reasons), and the light from the
selected waveguide interacts with the SAW generated by the one or
more ON SAW transducer(s). The SAW from the ON transducer then
deflects the waveguide light into the leaky mode fan and to an
output coupler.
[0080] The duty cycle and phase (time-synchronization) of the input
light in this arrangement is controlled to be low duty cycle and
synchronized with the velocity of the corresponding SAW so that the
waveguide is illuminated at least for the window of time during
which the SAW will diffract the waveguide light to the appropriate
intended output coupler. (Again, the SAW transducer could be active
beyond that time window, but for power management reasons probably
would not).
[0081] Alternatively, a different embodiment might run multiple
SAWs transducers at once and adjust for the variation in intensity
as the light is deflected by each subsequent SAW.
[0082] For example, in the illustrated example, the light source
210-1-L is "on" to generate light that is to be coupled into the
shared waveguide 102-1 via the left input-coupler 106-1-L. At the
same time, a RF drive signal is delivered to the SAW transducer 110
of SAW modulator 100-1,1 so that the light in the waveguide 102-1
is deflected to exit the proximal face 160 of the device 300 via
output coupler 410-1,1L.
[0083] In a similar way, input optical source 210-4-R is also
activated to generate light that is coupled into waveguide 102-4 by
input coupler 106-4-R. At the same time, an RF drive signal is
delivered to the SAW transducer 110 of the SAW modulator 100-4,5 so
that light is deflected by the output coupler 410-4,5-R to also
exit the proximal face 160 of the device 300.
[0084] In a further example, input optical source 210-3-L is also
activated to generate light that is coupled into waveguide 102-3 by
input coupler 106-3-L. At the same time, an RF drive signal is
delivered to the SAW transducer 110 of the SAW modulator 100-3,4 so
that light is deflected by the output coupler 410-3,4-L to also
exit the proximal face 160 of the device 300.
[0085] FIG. 6 shows a side cross-sectional view of an
electro-holographic light field generator according to an
alternative embodiment of the present invention which uses a
surface gratings output coupler 410-G fabricated on the distal face
168 of the optical substrate 120 instead of micro-mirror structures
described earlier. In general, in the previously discussed light
field generator 300 of FIGS. 2 and 3, the surface grating 410-G
could be substituted for micro-mirrors 412.
[0086] The surface gratings output coupler 410-G can be fabricated
via standard photolithography or laser writing processes such as in
Taillaert et al., An out-of-plane grating coupler for efficient
butt-coupling between compact planar waveguides and single-mode
fibers, IEEE Journal of Quantum Electronics 38.7 (2002): 949-955,
which is incorporated herein by reference in its entirety. Further
descriptions of exemplary generic algorithms and numerical grating
optimization techniques may be found in:
[0087] Zhou et al., Genetic local search algorithm for optimization
design of diffractive optical elements, Appl. Opt., vol. 38(20),
pp. 4281-90 (1999);
[0088] 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);
[0089] Qing et al., Crowding clustering genetic algorithm for
multimodal function optimization, Appl. Soft Computing, vol. 8(1),
pp. 88-95 (2008);
[0090] Taillaert et al., Compact efficient broadband grating
coupler for silicon-on-insulator waveguides, Opt. Lett., vol.
29(23), pp. 2749-51 (2004);
[0091] 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
[0092] Byrnes et al., Designing large, high-efficiency,
high-numerical-aperture, transmissive meta-lenses for visible
light, Opt. Exp. 24 (5), pp. 5110-5124 (2016).
[0093] FIG. 7 shows a side cross-sectional view of one side of a
SAW modulator of an electro-holographic light field generator 300
according to another embodiment of the present invention. Here
volume gratings output coupler 410-V formed in the optical
substrate 120 is used as the output couplers. Volume gratings 410-V
can be configured for multi-wavelength composite response and have
higher diffraction efficiency than thinner surface gratings. These
volume gratings 410-V can be fabricated via known interference
photolithography (holographic recording) or laser writing by
processes such as described in Guo et al., Design of a multiplexing
grating for color holographic waveguide, Optical Engineering 54.12
(2015): 125105-125105, (see pp. 1223); and in Gattass and Mazur,
Femtosecond laser micromachining in transparent materials, Nature
photonics 2.4 (2008): 219-225.
[0094] FIG. 8 schematically shows an array of SAW modulators of an
electro-holographic light field generator 300 that employ a folded
light path to scan each single line of the light field generator
300 according to an embodiment of the present invention.
[0095] Optical out-of-plane coupling elements 420 (e.g., a
micro-fabricated prism or mirror) enables single-edge illumination
of the electro-holographic light field generator device 300.
Arranging the light inputs and the associated in-coupling devices
106 along a single edge of the optical substrate 120 may simplify
the display connections and allow for a single spatial light
modulator (SLM) to be used to address all of the light inputs.
Micro-fabricated out-of-plane coupling elements 420 can be
fabricated though known processes such as described in Van Erps et
al., Discrete out-of-plane coupling components for printed circuit
board-level optical interconnections, IEEE Photonics Technology
Letters 19.21 (2007): 1753-1755, which is incorporated herein by
reference in its entirety.
[0096] In the illustrated example, optical source 210-1-L is
activated to couple light into the shared waveguide 102-1 via input
coupler 106-L-1 SAW modulator 100-1,1 is active to deflect that
light out of the device 300, through its proximal face.
[0097] On the other hand, as illustrated in the second line of the
device 300, the right side optical source 210-2-R is actually
located on the left side to couple light into the shared waveguide
102-2 via the input coupler 106-R-2. This input light is
transmitted along the backside portion of the shared waveguide
102-2-B. It is then coupled into the front side portion of the
shared waveguide 102-2 via the coupler 420-2. It is then
transmitted down the front side portion of the waveguide 102-2 to
the SAW modulator 100-2,2, where it is coupled out of the waveguide
102-2 and through the proximal face of the device 300 due to the
SAW generated by that modulator's transducer 110.
[0098] FIG. 9 shows a side cross-sectional view of one side of an
electro-holographic light field generator according to an
alternative embodiment of the present invention where output
couplers each may comprise multiple chirped grating output couplers
410 arranged with adjacent grating structures being mapped to
different wedges of output angles in the output light field
150.
[0099] This effectively spreads the output light field 150 across a
larger area to increase the FOV of the display. Each individual
chirped grating output coupler 410-1, 410-2, 410-3 can be selected
by the RF controller 405 by controlling the SAW/light pulse timing.
This approach trades off temporal bandwidth (display refresh rate)
for a higher FOV.
[0100] FIG. 10A shows a side cross-sectional view of an
electro-holographic light field generator according to another
embodiment of the present invention configured to address changes
in the waveguide propagation that occur due to thermal and other
effects. Such changes in waveguide propagation time will affect the
reflection angle of the output coupler grating 410-G and the
resulting output light field 150. This can be addressed by adding
one or more SAW sensing transducers 110-S for each optical
modulator pair that are arranged over the waveguide of the optical
substrate 120. These SAW sensing electrodes 110-S, which are IDTs
in one example, provide a feedback signal that can be used to more
precisely time subsequent light pulses in a closed-loop manner.
Specifically, a high-Z amplifier 1004 then amplifies the small
electrode feedback signal (microvolts to millivolts), and closed
loop controller 1005 and RF controller 405 then coordinate the
timing of the light input and the RF drive signal that creates the
SAW signal 140. Such a closed loop timing arrangement allows
elimination of baseline drift due to thermal drift or other
material properties that change over time.
[0101] More specific discussion of the use of such IDT sensing
electrodes is set forth in Bevies et al., Comparison between BAW
and SAW sensor principles, IEEE transactions on ultrasonics,
ferroelectrics, and frequency control 45.5 (1998): 1314-1330.
[0102] In general, the sensing transducers 110-S are placed with
known locations and orientations on the substrate 120. Typical
distances of the sensing transducers 110-S to one or more SAW
transducers 110 that induce surface acoustic waves would be on the
5-20 mm range, but could be as close as 0.1 mm, or as far as 100
mm.
[0103] In a more typical case, the SAW transducers 110 and sensing
transducers 110-S would be on either end of a waveguide 102.
[0104] The sensing transducers 110-S could either be just beyond
the waveguide (i.e., the waveguide is between, but not under them)
or the waveguide could extend beneath one or both of them.
[0105] In some embodiments, the sensing transducers 110-S are
chirped, i.e., there is a change in spatial frequency of their
fingers along the transducer's length. Then they could either be
pointed towards or away from each other, or in the same
direction.
[0106] In other embodiments, roles of the sensing transducers 110-S
and the SAW transducers 110 could be shared, such that at different
points in the operation of the device 300 some of the transducers
would function in a sensing mode whereas other transducers function
in a SAW generation mode, followed by swapping of roles.
[0107] FIG. 10B shows an embodiment that has a switch 480 between
the RF amplifier 405 and the high-Z amplifier 1004. Specifically,
the switch 480 allows the RF amplifier 405 to drive either one of
the transducers 110/110-S-A, 110/110-S-B of the modulator 100. As a
result, either one of the transducers 110/110-S-A, 110/110-S-B may
function in a SAW generation mode when driven by the RF amplifier
405. Then the other transducer will function in a SAW sensing mode
in which it is connected to the high-Z amplifier 1004 by the switch
480. Then, the switch 480 can swap the two roles of those
transducers so that the transducer that was in SAW sensing mode is
now in SAW generation mode, and the transducer that was in SAW
generation mode is now in SAW sensing mode.
[0108] Generally, the sensing transducers 110-S are be used to
modulate or calibrate the amplitude profile of the SAW modulators
110, as to better induce surface acoustic waves with the desired
amplitude, and also correct for variations due to thermal
environmental changes, or aging, or photorefractive damage.
[0109] Still another mode is noise-cancelling/SAW cancelling/active
damping mode. In physically small devices 300, SAW
back-reflections, occurring from chip edges and other features,
exist and are problematic. There can be crosstalk between adjacent
modulators. This SAW noise is sensed by the sensing transducers
110-S and then the sensing transducers 110-S or the SAW transducers
110 or dedicated noise damping transducers are driven with the
"opposite" (i.e., 180 degrees out of phase) signal in order to
quench the SAW noise. In general, this can be done in a feedback or
feedforward configuration. A hybrid feedback/feedforward system
could also be used. The SAW velocity can be well-measured, so the
SAW can be squelched or damped with the same IDT that senses it, or
with a different IDT nearby.
[0110] FIG. 11 shows an embodiment that uses a feedback noise
cancellation/SAW dampening system. Specifically, an exemplary pair
of SAW modulators 100-G and 100-H are shown from the
electro-holographic light field generator device 300, as previously
discussed. These SAW modulators 100-G, 100-H have their respective
SAW transducers 110 and output coupling regions 102-O of the
waveguide 102 at which the light is diffracted out of the waveguide
102 by the SAWs generated by the SAW transducers 110.
[0111] Also provided are SAW sensing and noise reduction/dampening
transducers 110-S-N. Specifically, each of the modulators 110-G in
110-H has respective sensing and noise reduction transducers
110-S-N. These are driven by feedback RF noise cancellation
circuits 182-G and 182-H. Specifically, these circuits 182-G, 182-H
detect the SAWs as generated by the respective transducers 110 and
then are driven by the feedback RF noise cancellation circuits
182-G, 182-H to suppress or dampen the noise or any other parasitic
SAW noise that they detect. Specifically, sense transducers/IDTs
will sense the acoustic (mechanical) waves (SAWs) in the form of
electric fields induced by the piezoelectric properties of the
substrate 120. Then the feedback circuits 182-G, 182-H will drive
those sense transducers/IDTs 110-S-N or other transducers to dampen
those SAWs.
[0112] FIG. 12 shows an embodiment that uses a feedforward noise
cancellation system. Specifically, in this embodiment, there are
separate sense transducers 110-S and noise suppression transducers
110-N downstream of the output coupling regions 102-O of the
waveguide 102, for each of the adjacent SAW modulators 100-G,
100-H.
[0113] In general, the sense acoustic (mechanical) waves in the
form of electric fields detect the SAWs typically generated by the
respective SAW modulators 110 via their sense transducers/IDTs
110-S. Specifically, the IDTs sense acoustic (mechanical) waves in
the form of electric fields induced by the piezoelectric properties
of the substrate 120. They then drive the noise cancellation/SAW
dampening transducers 110-N with the inverse signal corrected for
the phase delay between the sense transducers 110-S and the noise
cancellation transducers 110-N. This will help to cancel or damped
these SAW signals after they have propagated through the output
coupling region 102-O and thereafter would simply contribute to
noise and crosstalk.
[0114] FIG. 13 shows a side cross-sectional view an
electro-holographic light field generator according to an
alternative embodiment of the present invention. This embodiment
includes patterned viscoelastic surface materials, SAW absorber
material 1101 between the sites of the SAW transducers 110. These
material islands are configured to absorb the remainder of each SAW
140, which thereby prevents crosstalk in adjacent sites that would
otherwise result in blurring of the output images. The addition of
such SAW absorber material 1101 allows the RF controller 405 to
simultaneously operate the light input source 210 and SAW
transducer 110 at each end of each optical modulator pair so that
each light input 402 and corresponding SAW transducer 110
contributes to half the output field of view. Convenient materials
to use for the SAW absorber material 1101 are polymers or oxides
that can be patterned though standard microfabrication techniques
such as described, for example, in Hamidon and Yunusa, Sensing
Materials for Surface Acoustic Wave Chemical Sensors, Progresses in
Chemical Sensor (2016).
[0115] There are reports in the existing literature of the field of
increasing the angular subtense of a SAW's diffractive fan. But
these reported techniques do not include the idea presented above
of alternate firing of illumination on opposing sides or splitting
the angular reach into "halves." For example, FIG. 8 of (Quaderi
and Smalley, 2016) depicts two schemes for broadening the output
fan that stacks devices together "front to front" and "back to
back", and patterns waveguides and IDTs on both sides of an optical
modulator, but amplitude variations as a function of angle suggest
the need for alternative approaches.
[0116] Similarly, there are some research disclosures on linear
array SAW devices (e.g., Jolly et al., Near-to-eye
electroholography via guided-wave acousto-optics for augmented
reality, Proc. of SPIE Vol. Vol. 10127. 2017; incorporated herein
by reference in its entirety), but no reference has been made to
changing the propagation vector of light through a hardware
implementation so as to double the display field of view.
[0117] FIG. 14 shows a 3D display system 920. The 3D display system
920 includes an electro-holographic 3D display 900. The RF
controller 405 and a processor 909 are also shown. Here, the
electro-holographic light field generator devices 300 are arranged
in a dual column stack 901.
[0118] 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 210. The
illumination source 210 might be a laser such as a pulsed laser, to
cite one of many possible examples of illumination light sources
210. The laser might illuminate the generator devices 300 together
in a beam. Separate in-coupling prisms or gratings could be used to
couple light into each of the separate waveguides formed in the
devices.
[0119] The light field generator devices 300 are driven by the RF
controller 405. The RF driver 405 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.
[0120] It is also important to note that such light field
generators 300, 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).
[0121] 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.
[0122] Note that such light field generators, 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 AR/VR.
[0123] 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.
* * * * *