U.S. patent application number 15/098162 was filed with the patent office on 2017-10-19 for compact liquid crystal beam steering devices including multiple polarization gratings.
The applicant listed for this patent is Boulder Nonlinear Systems, Inc.. Invention is credited to Douglas J. McKnight, Steven A. Serati.
Application Number | 20170299941 15/098162 |
Document ID | / |
Family ID | 60040006 |
Filed Date | 2017-10-19 |
United States Patent
Application |
20170299941 |
Kind Code |
A1 |
Serati; Steven A. ; et
al. |
October 19, 2017 |
COMPACT LIQUID CRYSTAL BEAM STEERING DEVICES INCLUDING MULTIPLE
POLARIZATION GRATINGS
Abstract
Systems, methods, and apparatus are disclosed for attenuating an
incident polarized light beam using a plurality of LCPGs and one or
more switchable liquid crystal layers. When four LCPGs are used, a
spacing between first and second LCPGs can be equal to a spacing
between third and fourth LCPGs. Pi and FCL cells can also be used
in place of more traditional LC switches. Switching of the LC
switch can be imparted via an AC bias.
Inventors: |
Serati; Steven A.;
(Westminster, CO) ; McKnight; Douglas J.;
(Boulder, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Boulder Nonlinear Systems, Inc. |
Lafayette |
CO |
US |
|
|
Family ID: |
60040006 |
Appl. No.: |
15/098162 |
Filed: |
April 13, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/4277 20130101;
G02B 27/4261 20130101; G02F 1/292 20130101 |
International
Class: |
G02F 1/29 20060101
G02F001/29; G02B 5/30 20060101 G02B005/30; G02B 5/30 20060101
G02B005/30 |
Claims
1. A liquid crystal beam steering device, comprising: a first
polarization grating configured to direct incident light into first
and second beams having different directions of propagation than
that of the incident light, the first and second beams having
substantially orthogonal circular polarizations with respect to
each other; a liquid crystal layer configured to receive the first
and second beams from the first polarization grating, the liquid
crystal layer being switchable between first and second states for
introducing a first and second retardance, respectively, to the
first and second beams; a second polarization grating spaced apart
from the first polarization grating by a distance D and configured
to receive the first and second beams from the liquid crystal layer
and to alter the respective directions of propagation of the first
and second beams according to the first or second retardance
introduced to the first and second beams; a third polarization
grating configured to receive the first and second beams from the
second polarization grating and to further alter the respective
directions of propagation thereof; an intermediate region
configured to transmit the first and second beams from the third
polarization grating therethrough; a fourth polarization grating
configured to receive the first and second beams from the
intermediate region and to additionally alter the respective
directions of propagation thereof to provide output light; and an
aperture configured to transmit a first portion of both the first
and second beams from the fourth polarization grating when the
liquid crystal layer is in the first state, and to transmit a
second portion of both the first and second beams from the fourth
polarization grating therethrough when the liquid crystal layer is
in the second state, the first portion being greater than the
second portion, wherein at least one of the first and second
polarization gratings are arranged on a substrate, the substrate
arranged between the first and second polarization gratings, and
wherein the distance D between the first and second polarization
gratings is substantially equal to a distance D' between the third
and fourth polarization gratings.
2. The liquid crystal beam steering device of claim 1, wherein the
output light from the fourth polarization grating propagates in a
direction substantially: parallel to that of the incident light
when the liquid crystal layer is in the first state; and oblique to
that of the incident light when the liquid crystal layer is in the
second state.
3. The liquid crystal beam steering device of claim 1, wherein the
first, second, third, and fourth polarization gratings exhibit
substantially similar diffractive properties.
4. The liquid crystal beam steering device of claim 1, wherein in
the first state the liquid crystal layer introduces a retardance of
n.lamda., and wherein in the second state the liquid crystal layer
introduces a retardance of m + .lamda. 2 , ##EQU00007## where n and
m are selected from the set including integers and 0.
5. The liquid crystal beam steering device of claim 4, further
comprising an AC bias device configured to selectively apply an AC
bias to the liquid crystal layer in order to switch between the
first and second state.
6. The liquid crystal beam steering device of claim 4, wherein a
thickness of the liquid crystal layer is such that in the first
state the liquid crystal layer introduces a retardance of n.lamda.,
and wherein in the second state the liquid crystal layer introduces
a retardance of m + .lamda. 2 , ##EQU00008## where n and m are
selected from the set including integers and 0.
7. The liquid crystal beam steering device of claim 1, further
comprising one or more trim retarders arranged to one or both sides
of the liquid crystal layer and between the first and second
polarization gratings, the trim retarders shaped and arranged so as
to, in combination with the liquid crystal layer, impart no
retardance to the first and second beams when an AC bias device
imparts a finite AC bias to the liquid crystal layer, thereby
placing the liquid crystal layer in the first state.
8. The liquid crystal beam steering device of claim 1, wherein the
third polarization grating is configured to output off-axis beams
when the liquid crystal is in the second state.
9. A liquid crystal beam steering device, comprising: a first
polarization grating configured to direct incident light into first
and second beams having different directions of propagation than
that of the incident light, the first and second beams having
substantially orthogonal circular polarizations with respect to
each other; a liquid crystal layer configured to receive the first
and second beams from the first polarization grating, the liquid
crystal layer being switchable between first and second states for
introducing a first and second retardance, respectively, to light
traveling therethrough; a second polarization grating spaced apart
from the first polarization grating by a distance D1 and configured
to receive the first and second beams from the liquid crystal layer
to alter the respective directions of propagation of the first and
second beams in response to each of the first and second states of
the liquid crystal layer; a third polarization grating configured
to receive the first and second beams from the second polarization
grating to further alter the respective directions of propagation
thereof; an intermediate region having a thickness D2 and
configured to transmit the first and second beams from the third
polarization grating therethrough; a fourth polarization grating
spaced apart from the third polarization grating by a distance D2
and configured to receive the first and second beams from the third
polarization grating to additionally alter the respective
directions of propagation thereof to provide output light that
propagates in a direction substantially parallel to that of the
first and second beams output from the second polarization grating;
and an aperture configured to block both first and second beams
when the liquid crystal layer is in the first state, and to
transmit both first and second beams therethrough when the liquid
crystal layer is in the second state, wherein at least one of the
first and second polarization gratings are arranged on a substrate,
the substrate arranged between the first and second polarization
gratings, and wherein the distance D1 between the first and second
polarization gratings is substantially equal to distance D2 between
the third and fourth polarization gratings.
10. The liquid crystal beam steering device of claim 9, wherein the
intermediate region comprises a refractive component.
11. The liquid crystal beam steering device of claim 9, wherein D1
is equal to D2.
12. The liquid crystal beam steering device of claim 9, wherein D1
is not equal to D2.
13. A liquid crystal beam steering device, comprising: a first
polarization grating configured to direct incident light into first
and second beams having different directions of propagation than
that of the incident light, the first and second beams having
substantially orthogonal circular polarizations with respect to
each other; a liquid crystal layer configured to receive the first
and second beams from the first polarization grating, the liquid
crystal layer being switchable between first and second states for
introducing a first and second retardance, respectively, to light
traveling therethrough; a second polarization grating spaced apart
from the first polarization grating and configured to receive the
first and second beams from the liquid crystal layer to alter the
respective directions of propagation of the first and second beams
in response to each of the first and second states of the liquid
crystal layer; a third polarization grating configured to receive
the first and second beams from the second polarization grating to
further alter the respective directions of propagation thereof; an
intermediate region configured to transmit the first and second
beams from the third polarization grating therethrough while
modifying the respective directions of propagation thereof; a
fourth polarization grating configured to receive the first and
second beams from the intermediate region to additionally alter the
respective directions of propagation thereof to provide output
light that propagates in a direction substantially parallel to that
of the first and second beams output from the second polarization
grating; and an aperture configured to block both first and second
beams when the liquid crystal layer is in the first state, and to
transmit both first and second beams therethrough when the liquid
crystal layer is in the second state, wherein at least one of the
first and second polarization gratings are arranged on a substrate,
the substrate arranged between the first and second polarization
gratings, and wherein a first distance between the first and second
polarization gratings is substantially equal to a second distance
between the third and fourth polarization gratings.
14. The liquid crystal beam steering device of claim 13, wherein
the first and second polarization gratings are spaced apart by a
distance D, and wherein the intermediate region has a thickness
configured to separate the third and fourth polarization gratings
by the distance D.
15. The liquid crystal beam steering device of claim 13, wherein
the incident light is characterized by a wavelength .lamda., the
liquid crystal layer exhibits a first refractive index n1(.lamda.)
at the wavelength .lamda., the intermediate region exhibits a
second refractive index n2(.lamda.) at the wavelength .lamda., and
D1 and D2 are related by the equation
D1*.lamda.*n1(.lamda.)=D2*.lamda.*n2(.lamda.).
16. The liquid crystal beam steering device of claim 15, and
wherein the first, second, third, and fourth polarization gratings
exhibit substantially similar diffractive properties.
17. The liquid crystal beam steering device of claim 15, wherein
the first and second polarization gratings exhibit similar, first
diffractive properties, and wherein the third and fourth
polarization gratings exhibit similar, second diffractive
properties.
18. The liquid crystal beam steering device of claim 13, wherein
the incident light is incident on the first polarization grating at
a first angle with respect to an optical axis, wherein the first
and second beams exit the fourth polarization grating at a second
angle with respect to the optical axis when the liquid crystal
layer is in the second state, wherein the second angle is not equal
to the first angle.
Description
FIELD OF THE DISCLOSURE
[0001] The present disclosure relates to liquid crystal beam
steering devices and, more particularly, to switchable liquid
crystal-based beam steering devices including multiple liquid
crystal polarization gratings and related methods.
BACKGROUND OF THE DISCLOSURE
[0002] Recent advances in liquid crystal polarization grating
("LCPG") technology have enabled the use of passive LCPGs, singly
and in combination, to manipulate light, particularly in display
applications (See, for example, U.S. Pat. No. 8,537,310 to Escuti,
et al., which is incorporated herein in its entirety by reference).
In general, passive LCPGs possess a permanent, continuously varying
periodic polarization pattern to diffract incident light according
to its polarization.
[0003] More recently, LCPGs have been combined with switchable
liquid crystal ("LC") devices to provide low Size, Weight, and
Power ("SWaP") beam steering devices (See, for example, U.S. Pat.
No. 8,982,313 to Escuti, et al., and Boulder Nonlinear Systems
white paper, "Core Technologies," September 2014,
http://bnonlinear.com/wp-content/uploads/2014/09/Core-Technologies-White--
Paper.pdf, accessed 30 Sep. 2015, which are incorporated herein in
their entirety by reference). As an example, by incorporating fast
electro-optic half-wave polarization retarders as a switch to
control the handedness of polarization of the incident light,
switchable beam steering devices with faster speed and lower SWaP
compared to existing mechanical solutions, such as rotating Risley
prisms, can be achieved.
[0004] As described, for example, in U.S. Pat. No. 8,537,310, U.S.
Pat. No. 8,982,313 and "Core Technologies" whitepaper, passive
LCPGs generally consist of a nematic LC film that is surface
aligned and UV-cured to present a permanent, continuously varying
periodic polarization pattern. The structure of such LCPGs provides
an in-plane, uniaxial birefringence n that varies with position
(i.e., n(x)=[sin(.pi.x/.LAMBDA.), cos(.pi.x/.LAMBDA.), 0], where
.LAMBDA. is the period of the grating). Such transmissive gratings
efficiently (e.g., with greater than 99% efficiency) diffract
circularly polarized light to either the first positive or negative
order, based on the polarization handedness of the incident
light.
[0005] As used herein, "zero-order" light propagates in a direction
substantially parallel to that of the incident light, i.e., at a
substantially similar angle of incidence when the light is incident
on an optical system along an optical axis of the optical system,
and is also referred to herein as "on-axis" light. For example, if
the incident light is normally incident on the LCPG in a direction
parallel to the optical axis, "zero-order" or "on-axis" light would
also propagate substantially normally with respect to the first
polarization grating. In contrast, "non-zero-order light," such as
"first-order" light and/or "second-order light," propagates in a
direction that is not parallel to the incident light nor the
optical axis of the optical system. In particular, the second-order
light propagates at greater angles than the first-order light
relative to the angle of incidence. As such, first- and
second-order light are collectively referred to herein as
"off-axis" light.
[0006] LCPGs may be transparent, thin film, beam splitters that
periodically alter the local polarization state and propagation
direction of light traveling therethrough. Notably, during
diffraction, the LCPG causes the polarization handedness of the
incident light to flip to its orthogonal counterpart. Such
characteristics are in contrast to conventional polarizers, which
operate by permitting light of a first polarization state to travel
therethrough, but absorbing light of an orthogonal, second
polarization state.
[0007] A combination of two LCPGs may be aligned in parallel or in
antiparallel configurations. Specifically, a "parallel" LCPG
arrangement means the respective birefringence patterns of the two
LCPGs have substantially similar orientations. In contrast, an
"antiparallel" polarization grating arrangement means one LCPG has
a birefringence pattern that is inverted or rotated by about 180
degrees relative to that of the other LCPG.
[0008] Non-mechanical beam steering can be achieved with an
alternating stack of linear LCPGs and electro-optic half-wave
retardance switches, some embodiments of which are described in the
aforementioned U.S. Pat. No. 8,982,313. Non-mechanical beam
steering devices (also known as beam scanners) provide numerous
benefits over traditional gimbaled mechanical scanners due to their
vastly reduced SWaP requirements and their ability to perform
random access scanning. To achieve non-mechanical beam scanning
with LCPGs, a nematic or ferroelectric liquid crystal modulator
having an electronically controllable retardance is typically used
as the retardance switch, as mentioned above. In this case, the
retardance of the liquid crystal modulator is changed by applying a
voltage to either produce a half-wave of retardance or nearly zero
retardance through the cell. Since a half-wave retarder changes the
handedness of circularly polarized light while a cell with no
retardance does not affect the light's polarization, the incident
light can be steered to a selected angle by controlling the
handedness of circularly polarized light as it propagates through
the LCPG stack. LCPGs have to date been demonstrated with apertures
up to 50 mm.
[0009] It would be desirable to have alternative LCPG devices with
further SWaP and performance advantages.
SUMMARY OF THE DISCLOSURE
[0010] One aspect of the disclosure is a liquid crystal beam
steering device having a first polarization grating, a liquid
crystal layer, a second polarization grating, a third polarization
grating, an intermediate region, a fourth polarization grating, and
an aperture. The first polarization grating can be configured to
direct incident light into first and second beams having different
directions of propagation than that of the incident light. The
first and second beams can have substantially orthogonal circular
polarizations with respect to each other. The liquid crystal layer
can be configured to receive the first and second beams from the
first polarization grating. The liquid crystal layer can be
switchable between first and second states for introducing a first
and second retardance, respectively, to the first and second beams.
The second polarization grating can be spaced apart from the first
polarization grating by a distance D and can be configured to
receive the first and second beams from the liquid crystal layer.
The second polarization grating can also be configured to alter the
respective directions of propagation of the first and second beams
according to the first or second retardance introduced to the first
and second beams. The third polarization grating can be configured
to receive the first and second beams from the second polarization
grating and to further alter the respective directions of
propagation thereof. The intermediate region can be configured to
transmit the first and second beams from the third polarization
grating therethrough. The fourth polarization grating configured to
receive the first and second beams from the intermediate region and
to additionally alter the respective directions of propagation
thereof to provide output light. The aperture can be configured to
transmit a first portion of both the first and second beams from
the fourth polarization grating when the liquid crystal layer is in
the first state, and to transmit a second portion of both the first
and second beams from the fourth polarization grating therethrough
when the liquid crystal layer is in the second state. The first
portion can be greater than the second portion. The intermediate
region can have a thickness less than the distance D and can be
configured to separate the third and fourth polarization gratings
by the distance D.
[0011] Another aspect of the present disclose is a liquid crystal
beam steering device having a first polarization grating, a liquid
crystal layer, a second polarization rating, a third polarization
grating, an intermediate region, a fourth polarization grating, and
an aperture. The first polarization grating can be configured to
direct incident light into first and second beams having different
directions of propagation than that of the incident light. The
first and second beams can have substantially orthogonal circular
polarizations with respect to each other. The liquid crystal layer
can be configured to receive the first and second beams from the
first polarization grating. The liquid crystal layer can be
switchable between first and second states for introducing a first
and second retardance, respectively, to light traveling
therethrough. The second polarization grating can be spaced apart
from the first polarization grating by a distance D1 and can be
configured to receive the first and second beams from the liquid
crystal layer to alter the respective directions of propagation of
the first and second beams. Such altering of the directions of the
first and second beams can be in response to each of the first and
second states of the liquid crystal layer. The third polarization
grating can be configured to receive the first and second beams
from the second polarization grating to further alter the
respective directions of propagation thereof. The intermediate
region can have a thickness D2 and can be configured to transmit
the first and second beams from the third polarization grating
therethrough. The fourth polarization grating can be spaced apart
from the third polarization grating by a distance D2 and can be
configured to receive the first and second beams from the third
polarization grating to additionally alter the respective
directions of propagation thereof to provide output light that
propagates in a direction substantially parallel to that of the
first and second beams output from the second polarization grating.
The aperture can be configured to block both first and second beams
when the liquid crystal layer is in the first state. The aperture
can also be configured to transmit both first and second beams
therethrough when the liquid crystal layer is in the second state.
The incident light can be characterized by a wavelength .lamda..
The liquid crystal layer can exhibit a first refractive index
n1(.lamda.) at the wavelength .lamda.. The intermediate region can
exhibit a second refractive index n2(.lamda.) at the wavelength
.lamda.. The distances D1 and D2 can be related by the equation
D1*.lamda.*n1(.lamda.)=D2*.lamda.*n2(.lamda.).
[0012] Yet a further aspect of the disclosure can be described as a
liquid crystal beam steering device having a first polarization
grating, a liquid crystal layer, a second polarization grating, a
third polarization rating, an intermediate region, a fourth
polarization grating, and an aperture. The first polarization
grating can be configured to direct incident light into first and
second beams having different directions of propagation than that
of the incident light. The first and second beams can have
substantially orthogonal circular polarizations with respect to
each other. The liquid crystal layer can be configured to receive
the first and second beams from the first polarization grating. The
liquid crystal layer can be switchable between first and second
states for introducing a first and second retardance, respectively,
to light traveling therethrough. The second polarization grating
can be spaced apart from the first polarization grating and
configured to receive the first and second beams from the liquid
crystal layer to alter the respective directions of propagation of
the first and second beams in response to each of the first and
second states of the liquid crystal layer. The third polarization
grating can be configured to receive the first and second beams
from the second polarization grating to further alter the
respective directions of propagation thereof. The intermediate
region can be configured to transmit the first and second beams
from the third polarization grating therethrough while modifying
the respective directions of propagation thereof. The fourth
polarization grating can be configured to receive the first and
second beams from the intermediate region to additionally alter the
respective directions of propagation thereof to provide output
light that propagates in a direction substantially parallel to that
of the first and second beams output from the second polarization
grating. The aperture can be configured to block both first and
second beams when the liquid crystal layer is in the first state,
and to transmit both first and second beams therethrough when the
liquid crystal layer is in the second state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates a LCPG beam steering device including an
LC polarization switch, in accordance with an embodiment of the
present disclosure.
[0014] FIGS. 2 and 3 collectively illustrate the beam paths of
light transmitted through the LCPG beam steering device of FIG. 1.
FIG. 2 illustrates the beam path when the LC polarization switch is
in a first state, and FIG. 3 illustrates the beam path when the LC
polarization switch is in a second state.
[0015] FIG. 4 illustrates a LCPG beam steering device including an
LC polarization switch, in accordance with another embodiment of
the present disclosure.
[0016] FIG. 5 illustrates a LCPG beam steering device including an
LC polarization switch, in accordance with yet another embodiment
of the present disclosure,
[0017] FIG. 6 illustrates a LCPG beam steering device including an
LC polarization switch, in accordance with yet another embodiment
of the present disclosure.
[0018] FIG. 7 illustrates a LCPG beam steering device including an
LC polarization switch and trim retarders, in accordance with
another embodiment of the present disclosure.
[0019] FIG. 8 illustrates a retardance versus voltage plot for an
LC switch system that can be implemented in any of the
herein-described embodiments.
[0020] FIG. 9 shows another retardance versus voltage plot for a
switchable phase modification system having different parameters
than the system underlying FIG. 8.
[0021] FIG. 10 shows another retardance versus voltage plot, but
where the voltages for the first and second states are both
positive and thus the LC switch can be said to be on for both
states.
[0022] FIG. 11 shows another retardance versus voltage plot showing
the results of using a higher retardance LC switch than that of
FIG. 10.
[0023] FIG. 12 shows a retardance versus voltage plot for four
different states of the LC switch.
[0024] FIG. 13 illustrates a plot showing retardance as a function
of a voltage applied to a LC switch in an LCPG system; and
[0025] FIG. 14 illustrates an LCPG system having an AC bias and
optional feedback for controlling the AC bias.
[0026] FIG. 15 illustrates a method of operating an LCPG system
according to one embodiment of this disclosure.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE
[0027] The present disclosure is described more fully hereinafter
with reference to the accompanying drawings, in which embodiments
of the disclosure are shown. This disclosure 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 disclosure to
those skilled in the art. In the drawings, the size and relative
sizes of layers and regions may be exaggerated for clarity. Like
numbers refer to like elements throughout the specification.
[0028] It will be understood that, although the terms first,
second, third etc. may be used herein to describe various elements,
components, regions, layers and/or sections, these elements,
components, regions, layers and/or sections should not be limited
by these terms. These terms are only used to distinguish one
element, component, region, layer or section from another region,
layer or section. Thus, a first element, component, region, layer
or section discussed below could be termed a second element,
component, region, layer or section without departing from the
teachings of the present disclosure.
[0029] Spatially relative terms, such as "beneath," "below,"
"lower," "under," "above," "upper," and the like, may be used
herein for ease of description to describe one element or feature's
relationship to another element(s) or feature(s) as illustrated in
the figures. It will be understood that the spatially relative
terms are intended to encompass different orientations of the
device in use or operation in addition to the orientation depicted
in the figures. For example, if the device in the figures is turned
over, elements described as "below" or "beneath" or "under" other
elements or features would then be oriented "above" the other
elements or features. Thus, the exemplary terms "below" and "under"
can encompass both an orientation of above and below. The device
may be otherwise oriented (rotated 90 degrees or at other
orientations) and the spatially relative descriptors used herein
interpreted accordingly. In addition, it will also be understood
that when a layer is referred to as being "between" two layers, it
can be the only layer between the two layers, or one or more
intervening layers may also be present.
[0030] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the disclosure. As used herein, the singular forms "a," "an," and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" 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. As
used herein, the term "and/or" includes any and all combinations of
one or more of the associated listed items, and may be abbreviated
as "/".
[0031] It will be understood that when an element or layer is
referred to as being "on," "connected to," "coupled to," or
"adjacent to" another element or layer, it can be directly on,
connected, coupled, or adjacent to the other element or layer, or
intervening elements or layers may be present. In contrast, when an
element is referred to as being "directly on," "directly connected
to," "directly coupled to," or "immediately adjacent to" another
element or layer, there are no intervening elements or layers
present. Likewise, when light is received or provided "from" one
element, it can be received or provided directly from that element
or from an intervening element. On the other hand, when light is
received or provided "directly from" one element, there are no
intervening elements present.
[0032] Embodiments of the disclosure are described herein with
reference to cross-section illustrations that are schematic
illustrations of idealized embodiments (and intermediate
structures) of the disclosure. As such, variations from the shapes
of the illustrations as a result, for example, of manufacturing
techniques and/or tolerances, are to be expected. Thus, embodiments
of the disclosure should not be construed as limited to the
particular shapes of regions illustrated herein but are to include
deviations in shapes that result, for example, from manufacturing.
Accordingly, the regions illustrated in the figures are schematic
in nature and their shapes are not intended to illustrate the
actual shape of a region of a device and are not intended to limit
the scope of the disclosure.
[0033] 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
disclosure 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/or the present
specification and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0034] It will be understood by those having skill in the art that,
as used herein, a "transmissive" or "transparent" substrate may
allow at least some of the incident light to pass therethrough.
Accordingly, the transparent substrate may be, for example, formed
of glass, sapphire, or other materials.
[0035] Embodiments of the present disclosure are described herein
with reference to the accompanying figures. Referring first to FIG.
1, a LC beam steering device 100 is described. LC beam steering
device 100 includes a first LCPG 102, which is supported on a first
substrate 104. A second LCPG 112, supported on a second substrate
114, is spaced apart from first LCPG 102 by a distance D1. First
and second LCPGs 102 and 112 may be formed, for example, using
methods described in U.S. Pat. No. 7,196,758 to Crawford et al.,
which is incorporated herein in its entirety by reference.
[0036] As shown in FIG. 1, first and second substrates 104 and 114,
respectively, are configured to contain therebetween a liquid
crystal ("LC") switch 120. LC switch 120 includes liquid crystal
molecules that are configured to be switched between first and
second states, in response to voltages applied thereacross. The
surfaces of first and second substrates 104 and 114 that contain
the LC switch are treated with alignment layers and transparent
conductive layers (not shown) so as to align the LC switch in a
desired configuration as well as to allow the application of a
voltage across the LC switch. For example, the alignment layer may
be a commercial polyimide coating, such as Nissan Chemical
Industries SE-7492, and the transparent conductive layers may be
formed, for example, of standard coatings such as Indium Tin Oxide
("ITO") or Indium Molybdenum Oxide ("IMO"). In some embodiments, an
LC switch can also be referred to as a liquid crystal layer.
[0037] LC beam steering device 100 further includes a third LCPG
132, which is supported on a third substrate 134. Third LCPG 132 is
separated from second LCPG 112 by a distance d. In some
embodiments, the distanced may be the thickness of an optical
adhesive or index-matching layer (not shown) used to bond together
second and third LCPGs 112 and 132, respectively. Alternatively,
second LCPG 112 and third LCPG 132 may be placed in direct contact
with each other. In such embodiments, the distance d is much
smaller than the distance D1 shown in FIG. 1.
[0038] Still further, LC beam steering device 100 includes a fourth
LCPG 142, which is supported on a fourth substrate 134 and spaced
apart from third LCPG 132 and its supporting third substrate 134 by
a distance D2. A space 150 (or intermediate region) defined between
third substrate 134 and fourth substrate 144 may be filled, for
example, with a material such as an index-matching fluid, optical
adhesive, or air.
[0039] It should be emphasized that various components in the
figures described herein are not drawn to scale. For example, in
FIG. 1 and subsequent figures, the various substrates may have
thicknesses ranging from 50 to 2000 microns, or thicker, depending
on the material used and desired characteristics. Also, LC switch
120 and space 150 may have a thickness, for example, in the range
of 1 to 10 microns, depending on the material characteristics of
the material used therein. In certain cases, the LC switch
thickness may be less than one micron, or more than 10 microns,
depending on the optoelectronic characteristics of the LC
material.
[0040] LC beam steering device 100 additionally includes an
aperture 160. For example, the aperture may be an explicit aperture
in a piece of opaque material, as shown in FIG. 1. Alternatively,
the aperture may be an implicit aperture created by the finite
extent of another component, not shown, such as a lens or optical
fiber, or by the acceptance aperture of a subsequent optical
system. The components of LC beam steering device 100 in this
exemplary embodiment are aligned with respect to an optical axis
170.
[0041] First, second, third, and fourth LCPGs 102, 112, 132 and
142, respectively, may provide diffraction properties such as at
least one diffracted orders (such as +1 or -1 order), substantially
orthogonal circular polarizations of the non-zero orders, and/or
highly polarization-sensitive non-zero-orders, which may be
linearly proportional to the Stokes parameter of the LCPGs. For
example, the LCPGs may be polymerized LC films including
anisotropic periodic molecular structures with birefringence
patterns configured to diffract light incident thereon with a
diffraction efficiency of 50% or greater. Each one of first,
second, third, and fourth LCPGs 102, 112, 132 and 142,
respectively, may include multiple layers having periodic local
anisotropy patterns that are offset relative to one another to
define a phase modification therebetween and/or rotated by a twist
angle over respective thicknesses thereof. Additionally, one or
more of such multiple layers may be an actively switchable liquid
crystal layer such that the LCPG acts as a switchable liquid
crystal polarization grating.
[0042] First, second, third, and fourth LCPGs 102, 112, 132 and
142, respectively, may be identical in type, thickness,
periodicity, and/or molecular orientation, or one or more of the
LCPGs may have a characteristic distinct from the other LCPGs in LC
beam steering device 100. Furthermore, first, second, third, and
fourth LCPGs 102, 112, 132 and 142, respectively, may be arranged
in parallel or antiparallel orientation with respect to each
other.
[0043] The operation of an exemplary embodiment of LC beam steering
device 100 is illustrated in FIGS. 2 and 3. In FIG. 2, LC switch
120' is in a first state, in which a first retardance is introduced
to light traveling through the LC switch (retardance can also be
referred to as a phase modification between polarizations of light
passing through the LC switch 120). In FIG. 3, LC switch 120'' is
in a second state, in which a second retardance is introduced to
light traveling through the LC switch. In the exemplary embodiment
illustrated in FIGS. 2 and 3, first and second LCPGs 102 and 112,
respectively, are arranged in a parallel orientation (as indicated
by arrows 201 and 203), and third and fourth LCPGs 132 and 142,
respectively, are also arranged in a parallel orientation (as
indicated by arrows 205 and 207). In the illustrated example, the
orientation of the pair formed by first and second LCPGs 102 and
112, respectively, is antiparallel to the orientation of the pair
formed by third and fourth LCPGs 132 and 142, respectively. It is
assumed throughout the description of the exemplary embodiment in
FIGS. 2 and 3 that the diffractive properties, such as the grating
pitch and thickness, of first, second, third, and fourth LCPGs 102,
112, 132, and 142, respectively, are essentially identical.
However, in some cases, the diffractive properties of the first,
second, third, and fourth LCPGs 102, 112, 132, and 142 can be
substantially similar. While the diffractive properties can be the
same or similar, the orientations of one or more of the LCPGs can
be antiparallel.
[0044] As shown in FIG. 2, a light beam 200' is incident on first
LCPG 102 along optical axis 170. Light beam 200' is characterized
by a first polarization state. For instance, light beam 200' may be
characterized by right-hand circular polarization. However, in
other embodiments, the incident light beam 120 can be unpolarized,
elliptically polarized, or have some other polarization state. When
any but circularly-polarized light is incident on the first LCPG
102, two beams will emerge from the first LCPG 102.
[0045] Upon transmission therethrough, first LCPG 102 diffracts
light beam 200' by an angle A1' with respect to optical axis 170,
and the polarization state of light beam 200' is flipped to
left-hand circular polarization. LC switch 120' is in a first
state, which introduces a first retardance to light beam 200' upon
transmission therethrough. For example, the first retardance may be
a full-wave retardance; in this case, light beam 200' remains
left-hand circularly polarized when incident on second LCPG 112.
However, any multiple of a full wave, or .lamda. can be imparted by
the LC switch 120' in this first state. Said another way, in the
first state, the LC switch 120', along with any trim retarders, can
impart a retardance of n.lamda., where n can be selected from the
set of integers as well as 0. Assuming the first state imparts a
multiple of a full-wave retardance to the beam 200', and since LCPG
112 is oriented in parallel to first LCPG 102, second LCPG 112 then
diffracts light beam 200' to propagate substantially parallel to
optical axis 170, while flipping the handedness of the polarization
state such that light beam 200' emerging from second LCPG 112 is
again right-circularly polarized.
[0046] Light beam 200' is then incident on third LCPG 132. As third
LCPG 132 is oriented in an antiparallel manner with respect to
first and second LCPGs 102 and 112, light beam 200' is diffracted
at an angle -A1'. The polarization state of light beam 200' is once
again flipped such that light beam 200' emerging from third LCPG
132 is again left-circularly polarized.
[0047] Light beam 200' is subsequently transmitted through space
150 with its left-circular polarization intact until it is incident
on fourth LCPG 142. Fourth LCPG 142, being parallel in orientation
to third LCPG 132, diffracts light beam 200' back in alignment with
optical axis 170 and with right-hand circular polarization such
that light beam 200' is subsequently transmitted through aperture
160.
[0048] Turning now to FIG. 3, a light beam 200'' with right-hand
circular polarization is incident on first LCPG 102. First LCPG 102
diffracts light beam 200'' again at an angle A1' and flips the
polarization to a left-hand circular polarization. This time, LC
switch 120'' is in a second state such that light transmitted
therethrough experiences a second retardance. The second retardance
may be, for instance, a half-wave retardance; in this case, light
beam 200'' experiences a half-wave retardance during transmission
through LC switch 120'' such that light beam 200'' is characterized
by a right-hand circular polarization and consequently diffracted
by second LCPG 112 at an angle A2'', which is larger than angle
A1'. The handedness of the polarization of light beam 200'' again
is flipped upon diffraction by second LCPG 112 such that the light
beam emerging from second LCPG 112 is left-hand circularly
polarized. In other embodiments, the LC switch 120'' in this second
state, along with any trim retarders, can impart any m+.lamda./2
retardance to the light beam 200'', where m can be selected from
the set of integers as well as 0.
[0049] Since third LCPG 132 is in an antiparallel orientation with
respect to first and second LCPGs 102 and 112, respectively, light
beam 200'' is further diffracted to an angle A3'', which is larger
than angle A2'', upon transmission through third LCPG 132. Light
beam 200'' then propagates through space 150 with right-hand
circular polarization, then is further diffracted by fourth LCPG
142 into an angle A4'', which is still larger than angle A3'', with
left-hand circular polarization. Finally, light beam 200'' emerging
from fourth LCPG 142 can be blocked by aperture 160. If, for
instance, the aperture is instead an implicit aperture, as
previously discussed, light beam 200'' does not enter the optical
component or system located further along optical axis 170 from
fourth LCPG 142.
[0050] For the exemplary embodiment illustrated in FIGS. 1, 2, and
3, in other words, light beam 200 is transmitted through aperture
160 when LC switch 120 is in a first state, and light beam 200 is
blocked by aperture 160 when LC switch is in a second state. In
other embodiments, the system 100 can be arranged such that
switching the LC switch 120 results in a partial transmission or
blocking of the light beam 200. For instance, when the LC switch
120 is in a first state, the light beam 200 may be at least
partially transmitted through the aperture 160, while the light
beam 200 may be at least partially blocked by the aperture 160 when
the LC switch 120 is in a second state. As a further example, the
LC switch 120 in a first state, either in combination with one or
more trim retarders, may impart other than a multiple of a half
wave of retardance to the light beam 200'. For instance, in a first
state (or an "off" state) the LC switch 120' plus one or more trim
retarders or other retarding mechanisms can apply a retardance or
phase modification of
.lamda. 10 ##EQU00001##
waves to the light beam 200' between LCPG 102 and LCPG 112. The
result would be a slight change in the polarization of the light
beam 200 that causes an output light beam 200' to split into two
components of different powers, one following the path shown in
FIG. 2, and one following the path shown in FIG. 3. In this way,
the first state of the LC switch 120' can result in some
attenuation of the light beam 200', although this attenuation is
not equal to nor comparable to the attenuation seen when the LC
switch 120' is in a second or "on" state.
[0051] Similarly, a second state of the LC switch, either alone or
in combination with one or more trim retarders, may impart other
than a multiple of a quarter wave retardance to the light beam
200''. In this way, the light beam 200'' changes polarization
between LCPG 102 and LCPG 112 (sees a phase modification between
polarizations or a retardance), but does not undergo a full
90.degree. or quarter-wave change in polarization. Rather, the
retardance can be close to a 90.degree. or quarter-wave retardance,
but not equal thereto. The result, is that the second state of the
LC switch 120'' results in less than full attenuation at the
aperture 160. In other words, if the system 100'' is spaced and
sized appropriately, the second state of the LC switch 120'' can
result in some portion of the light beam 200'' passing through the
aperture 160.
[0052] In another embodiment, a first state of the LC switch 120'
results in an entirety of the light beam 200' passing through the
aperture 160, while a second state of the LC switch 120'' results
in some, but not all of the light beam 200'', passing through the
aperture 160. A third state of the LC switch 120, between the first
and second states of the LC switch 120, results in some portion of
the light beam 200 passing through the aperture 160, where this
portion is larger than that transmitted given the first state of
the LC switch 120', yet smaller than that transmitted given the
second state of the LC switch 120''. Third, fourth, fifth, etc.
states of the LC switch 120 can also be implemented in order to
increase the selectivity of transmission amounts through the
aperture 160.
[0053] These examples show that the LC switch 120 may have
intermediate states that result in less than a maximum contrast
between first and second states, or on and off states.
Alternatively, there may be more than two states, and thereby
variable attenuation can be achieved.
[0054] In one embodiment, such variable attenuation can be
accomplished through an AC bias applied to the LC switch 120, where
the AC bias is configured to apply a variety of AC biases to switch
the LC switch 120 between various states between and including the
first and second states. Different biases result in a different
level of alignment within the LC switch 120 and hence different
amounts of retardance can be imparted to the light beam 120. In
other words, the attenuation of the light beam 120 can be a
function of the AC bias applied to the LC switch 120. In an
embodiment, the AC bias applied in either the first or second state
is 0V. In another embodiment, the AC bias for both the first and
second state can be greater than 0V.
[0055] Returning briefly to FIG. 1, it should be noted that the
distances D1 and D2 are strategically determined to achieve the
appropriate performance by LC beam steering device 100. In one
example, assuming the material composition and thicknesses of
first, second, third, and fourth substrates 104, 114, 134, and 144
are essentially identical (i.e., essentially identical indices of
refraction), then distances D1 and D2 may be set to be equal if the
refractive indices of the materials comprising LC switch 120 and
space 150 are similar. If the thicknesses of LC switch 120 and
space 150 are small compared to the thickness of the substrates, as
is likely in a practical implementation, the precision required of
the refractive index match between LC switch 120 and any material
contained within space 150 may be reduced. Such a setting may be
achieved, for example, by using the same spacer arrangement (not
shown) in setting the thicknesses of both LC switch 120 and space
150. Suitable spacer arrangements may include, for example, the use
of spacer beads or spacer rods suspended in optical adhesive.
Further, the above-mentioned relationship of D1 and D2 assumes that
bonding layers such as glues, have a negligible effect on beam
steering. Where such layers do have a noticeable effect on beam
steering, their influence can be factored into the relationship
between D1 and D2.
[0056] In a further refined calculation, the angles of deviation of
the gratings and the propagation through multiple layers are
"balanced" so that the light beam, in the transmitting state, is
correctly returned to the optical axis. This calculation is
performed by tracing a ray through the stack and evaluating the
result of refraction at layer boundaries using Snell's law. Such a
holistic, optical system view of the LC beam steering device allows
the inclusion of manufacturability considerations into the device
design, thereby greatly increasing the configuration flexibility of
the entire system. We have recognized that factoring
manufacturability and LC and LCPG material issues into the LC beam
steering device design is essential to the implementation of a
practical and consistently manufacturable devices with superior
SWaP characteristics.
[0057] As an example, setting D1=D2, as shown in FIGS. 1-3, may be
useful such that, light beam 200 may be inserted into LC beam
steering device 100 along optical axis 170 and subsequently exit
through aperture 160 again along optical axis 170 when LC switch
120 is in a first state, as shown in FIG. 2. This choice of setting
D1=D2 assumes that the refractive index of substrates 104, 114,
134, and 144 as well as the refractive indices of LC switch 120 and
any material contained within space 150 are substantially the same
at the wavelength of interest or, if a layer has a significantly
different refractive index, its thickness is sufficiently small so
as to not displace the beam too far (e.g., bonding layers such as
glues or optical fillers). Additionally, manufacturing tolerances
can cause small changes in D1 and D2 that do not cause untenable
beam displacement. The allowed tolerances can depend on beam
diameter and other factors, but, for instance, the inventors found
that given 700 .mu.m thick substrates 104, 114, 134, 144 having
manufacturing tolerances of +/-50 .mu.m each, the influence on beam
displacement of these small divergences from D1=D2 were acceptable.
Given smaller beam diameters, smaller tolerances may be preferred.
Thus, one could say that D1=D2 within the bounds of manufacturing
tolerances for a given application (e.g., given a certain beam
diameter among others).
[0058] Additionally, setting D1=D2, in the system 100, enables a
single type LCPG to be used for all four LCPGs 102, 112, 132, 142.
In other words, from a manufacturing standpoint, setting D1=D2
enables a single type of LCPG having singular parameters to be used
for all four of the LCPGs 102, 112, 132, 142 in the system 100. For
instance, the same manufacturing setup can be used for all four
LCPGs 102, 112, 132, 142 (e.g., two or more of the LCPGs 102, 112,
132, 142 can be formed on the same substrate). Alternatively, a
large LCPG can be made, and many small gratings can be formed
therefrom by cutting the large LCPG with a dicing saw or
scribe-and-break system. The use of four identical, or
nearly-identical, LCPGs provides cost and manufacturing advantages
over three-grating systems, where each LCPG would need to be
different.
[0059] Another advantage of the four-grating system 100 is enhanced
contrast ratio or dynamic range as compared to three-grating
systems. Komanduri et al. (A High Throughput Liquid Crystal Light
Shutter for Unpolarized Light Using Polymer Polarization Gratings;
Acquisition, Tracking, and Laser Systems Technologies XXV, Proc. of
SPIE Vol. 8052, 2011) discuss a three-grating system for use in
displays and describes contrast ratios as high as 230:1. The herein
disclosed four-grating system is able to achieve much higher
contrast ratios including those greater than 1000:1.
[0060] Alternatively, D1 and D2 may be set to be purposefully
unequal such that light beam 200 emerges at an off-axis angle or
off-set from optical axis 170. Such embodiments may be useful in
certain system configurations that require off-axis inputs and/or
outputs.
[0061] FIG. 4 shows another variation in which first and second
LCPGs 102 and 112, respectively, may be a matched pair sharing
substantially similar periodic birefringence patterns, while third
and fourth LCPGs 132 and 142, respectively, are another matched
pair sharing substantially similar periodic birefringence patterns
with respect to each other, although different from the periodic
birefringence patterns of first and second LCPGs 102 and 112,
respectively. For a light beam 401 incident on LC beam steering
device 400 along optical axis 170, the corresponding D1' and D2'
values are related by the beam propagation angles B1 and B2 so
that:
D1'*sin(B1)+D2'*sin(B2)=0 Eq. (1)
[0062] One of skill in the art will recognize that optical
tolerances of up to 10% are common, and therefore, tolerances of up
to around 10% in D1' and D2' are acceptable without departing from
Eq. (1). For instance, a thickness of the liquid crystal switch
(e.g., 3 .mu.m) 420 is unlikely to have a noticeable effect on Eq.
(1) in many use cases. For instance, where the substrates 104, 114,
134, 144 are around 700 .mu.m in thickness, nominal changes in
thickness (e.g., +/-30 .mu.m), for instance, that of LC switch 420,
the space 150, and the thicknesses of the LCPGs 102, 112, 132, 142,
are unlikely to have a noticeable effect on Eq. (1). Tolerances of
D1' and D2' may depend on incident beam diameter: wider beams may
suggest greater tolerance, while narrower beams may suggest lesser
tolerance. In other words, various manufacturing tolerances on D1'
and D2' are envisioned, and those of skill in the art will be able
to apply Eq. (1) given acceptable tolerances for a given
application. The angles shown in this figure illustrate the
situation in which the refractive indices of LC switch 420 and any
material contained within space 150 are similar to that of first,
second, third, and fourth substrates 104, 114, 134, and 144,
respectively.
[0063] FIG. 5 illustrates another exemplary embodiment, in which
thickness D52 of third substrate 534, thickness D53 of space 550,
and thickness D54 of fourth substrate 544 are set to be
significantly different from each other and distance D1.
Diffractive angles are traced through media of different refractive
indices. In this example, a light beam 501 is first deflected at
first LCPG 102 up to an angle C1, at which it propagates for
distance D1. Upon encountering second LCPG 112, light beam 501 is
then redirected to a direction substantially parallel to the
optical axis. Light beam 501 then propagates for distance d until
it encounters a third LCPG 532, at which point light beam 501 is
then directed down to an angle C2 for propagation distance D52
through a third substrate 534. In this example, the material in a
space 550 is assumed to have a lower refractive index than that of
substrate 534, so light beam 501 deviates further down to an angle
C3 as it propagates for distance D53 through space 550. Light beam
501 is then incident on a fourth substrate 544 which causes a
refraction to an angle C4. It may be noted that, if third substrate
534 and fourth substrate 544 are formed of different materials,
then angles C2 and C4 would not be equal. Finally, a fourth LCPG
542, causes a deflection of the beam back parallel to optical axis
170. Snell's law may be used to determine the angle of propagation
after the transition into a new medium such as from substrate 534
to space 550.
[0064] In the situation which is shown in FIG. 5, which has the
exit aperture positioned on-axis with respect to optical axis 170,
the deflection angles and distances should be chosen so that:
D1*sin(C1)+D52*sin(C2)+D53*sin(C3)+D54*sin(C4)=0 Eq. (2)
In this way, a variety of material and thickness configurations may
be accommodated to achieve an effective and practical device
design. Again, manufacturing tolerances appropriate for the use
case are envisioned relative to the distances specified in Eq.
(2).
[0065] In yet another variation the functions of the third and
fourth polarization gratings, above, may be combined into a single
LCPG, as illustrated in FIG. 6. In this exemplary embodiment, a
first LCPG 602 deflects a light beam 601 to an angle of A61. The
beam then passes through a LC switch 620, which may "flip" the
polarization state to the orthogonal handedness, depending on the
state of the liquid crystal material contained therein. A second
LCPG 612 may be configured such that light beam 601 may be
deflected back towards optical axis 170. Finally, light beam 601 is
brought back into alignment with the optical axis by a third LCPG
642. In this configuration, the diffractive properties of first,
second, and third LCPGs 602, 612, and 642, respectively, are chosen
so that the resulting deflection angles are related by the
following equation:
D61*sin(A61)+D62*sin(A62)=0 Eq. (3)
Note that the magnitude of the deflection angle effected by second
LCPG 612 is equal to the sum of the magnitudes of A61 and A62, and
the deflection effected by third LCPG 642 is equal in magnitude to
A62. Again, manufacturing tolerances appropriate for the use case
are envisioned relative to the distances specified in Eq. (3).
[0066] In yet another variation, the LC switch may be switched
between two states that are separated by approximately a half wave
of retardance, with the values of retardance chosen for reasons of
LC switching speed, convenience of cell assembly, drive voltage
range, or a combination of these factors. For example, the LC cell
could be an untwisted electrically-controlled birefringence ("ECB")
cell configured to switch between a high-voltage state, with a
retardance of less than a quarter-wave, and a low-voltage state,
with a retardance approximately one half-wave greater. One or more
retarders, external to the LC cell, may be added to "trim" the
effective retardance of the ECB cell such that, in one of the LC
cell's states, the light's polarization is substantially unaltered
and, in the other state, the light's polarization is changed to the
orthogonal circular polarization state.
[0067] An example of an ECB cell implementation of an LC beam
steering device is shown in FIG. 7. FIG. 7 shows a LC beam steering
device 700 including first, second, third, and fourth LCPGs 102,
112, 132, and 142, respectively, supported on first, second, third,
and fourth substrates 104, 114, 134, and 144, respectively. Rather
than having an LC switch supported between first and second
substrates 104 and 114, respectively, a combination of elements are
supported between first and second substrates 104 and 114. As shown
in FIG. 7, a first trim retarder 715 and an optional second trim
retarder 725 is/are placed on either side of an ECB cell 735. ECB
cell arrangement includes fifth and sixth substrates 754 and 764,
respectively, supporting a liquid crystal layer 740
therebetween.
[0068] In an exemplary embodiment, first trim retarder 715 or the
combination of first and second trim retarders 715 and 725,
respectively, may be chosen to compensate for any residual
retardance of ECB cell 735 so the combination of ECB cell 735, in
one state, and trim retarder(s) leaves the incident light's
polarization substantially unaffected. For instance, ECB cell 735
may be configured and driven to a high or first voltage for one if
its switched states. If, in this high or first voltage state, the
residual retardance of ECB cell 735 is 80 nm, as an example, then
first trim retarder 715 may be selected to exhibit a retardance of
80 nm at the wavelength of interest and second trim retarder 725
may be eliminated from LC beam steering device 700. First trim
retarder 715 may be oriented, for example, with its in-plane
slow-axis at 90 degrees to an in-plane slow-axis of ECB cell 735.
This choice of orientation results in the residual retardance of
ECB cell 735 and the retardance of first trim retarder 715
cancelling each other such that the polarization state of the light
transmitted through the combination of first trim retarder 715 and
ECB cell 735, in the high or first voltage state, is essentially
unaltered.
[0069] First and second trim retarders 715 and 725 may be formed,
for instance, by combining a plurality of retarders in order to
obtain the required retardance value to cancel out the residual
retardance of the particular ECB cell selected to be used within
the system. Continuing the previous example, if it proves
inconvenient to purchase or make 80 nm retarder material, it may be
more convenient to acquire retarders of other values and combine
them appropriately. For instance, a retarder of 350 nm could be
crossed with (i.e., oriented at 90 degrees to) a retarder of 270 nm
to yield a composite retarder of 80 nm. Similarly, a retarder of 30
nm could be additively combined with a retarder of 50 nm to achieve
a composite retarder of 80 nm, by combining them with their slow
axes in parallel. If the trim retarder is made from two or more
separate parts, these component parts may be placed on either or
both sides of ECB cell 735 as first and second trim retarders 715
and 725, respectively.
[0070] Alternatively, it may be convenient to use a trim retarder
arrangement that modifies the combined retardance of the assembly
to a value that does change the polarization state of the incident
light when ECB cell 735 is in the high or first voltage state. For
instance, if ECB cell 735 exhibits a residual retardance of 80 nm
in the high or first voltage state and the wavelength of interest
is 500 nm, then it may be convenient to trim the cell from 80 nm to
250 nm by using a first and/or second retarder 715 and 725,
respectively, with an effective trim retardance of 170 nm. The
value of the low or second voltage may then be chosen such that ECB
cell 735 exhibits a retardance of approximately 330 nm in the low
or second voltage state, so that the retardance of the combined ECB
cell and trim retarder arrangement would be approximately 500 nm
(i.e., one wave at the wavelength of interest). In this case, the
high voltage state (or first state) of ECB cell 735 would flip the
polarization state of the incident light, and the low voltage state
(or second state) would leave the polarization state of the
incident light substantially unchanged, thus resulting in different
beam propagation paths through LC beam steering device 700.
[0071] There are a variety of ways to arrange the combination of
ECB cell and trim retarder(s) to provide the required switching
function. The appropriate switching function may be achieved with a
configuration that provides a combined retardance approximately
equal to an even number of half-waves in one state of the ECB cell,
and an odd number of half-waves in the other state of the ECB cell.
The choice of components will depend on the relative importance of
engineering factors, such as switching speed, available voltage,
temperature range requirements, manufacturing cost and availability
of retarders at the desired retardance values.
[0072] In another variation, ECB cell 735 in FIG. 7, may be
replaced with a pi cell (also known as an Optically Compensated
Bend cell) (See, for example, P. J. Bos and K. R. Koehler-Beran,
Mol. Cryst. Liq. Cryst. 113, 329 (1984)). This alternative may be a
good choice of LC configuration for applications requiring
sub-millisecond switching speed, and is an example of an LC cell
configuration that works well with a trim retarder. Although a pi
cell may be constructed and driven between a half-wave and a full
wave of retardance, one can obtain faster performance by driving
between two states of lower retardance. Consequently, by combining
a pi cell with one or more trim retarders, faster switching speeds
may be obtained.
[0073] In yet a further variation, ECB cell 735 in FIG. 7, may be
replaced with one or more ferroelectric liquid crystals (FLCs).
This alternative may be a good choice of LC configuration for
applications requiring greater speed than those using a pi cell.
Each FLC is typically arranged to comprise a quarter-wave of
retardance. The orientation of the FLC can be in-plane, oriented at
an angle that depends on the state of an applied voltage to the
FLC. The FLC material is selected to switch between two states
separated by approximately 45 degrees. Thus, when two FLCs are used
in combination and two states of a voltage are applied to the pair
of FLCs, the combined FLCs can either add, to apply a half-wave of
retardance if they are parallel, or subtract, to apply no
retardance if they are oriented 90 degrees to each other.
[0074] While FLCs have been around for some time and were seen as
having great potential for use in displays, they tend to produce
patchy images when used in displays due to difficulties in
achieving uniform alignment and their less-than-desired response to
analogue switching inputs. Thus, FLCs are considered to have
inherent disadvantages that make them unlikely contenders for
switching applications. Yet, the inventors recognized that a much
greater attenuation tolerance could be afforded in certain
applications, such as where a single beam is being directed through
an aperture. Unexpectedly, FLCs have application here despite their
inherent disadvantages for switching applications.
[0075] Turning back now to switching of the LC switch, a better
understanding may be possible via reference to the following
equations and FIGS. 8-10. Equation 4 represents a retardance
imparted by any combination of one or more LC switches or LC
layers, along with any one or more trim retarders in a first state
(where the LC switch is on), where n is selected from the set of
integers as well as 0 (i.e., the retardance of an even number of
half wave retarders). Equation 5 represents a phase modification
imparted by any combination of one or more LC switches or LC
layers, along with any one or more trim retarders in a second state
(where the LC switch is off), where m is selected from the set of
integers as well as 0 (i.e., the retardance of an odd number of
half wave retarders). For instance, an LC switch could impart a
2.lamda. retardance in the first state, where n=2 or a .lamda.
retardance in the first state, where n=1.
n .lamda. Eq . ( 4 ) m + .lamda. 2 Eq . ( 5 ) ##EQU00002##
[0076] FIGS. 8-10 show some different scenarios where equations 4
and 5 are used to explain or design a system of one or more LC
switches, or one or more LC switches in combination with one or
more trim retarders. In each figure a cumulative retardance of the
one or more LC switches and optional one or more trim retarders is
shown on the y-axis, while an AC voltage applied to the LC switch
at a first state, V.sub.1, and at a second state, V.sub.2, are
shown on the x-axis.
[0077] FIG. 8 illustrates a retardance versus voltage plot for an
LC switch system that can be implemented in any of the
above-described embodiments. The spline curve represents a
retardance imparted by one or more LC switches, or one or more LC
switches in combination with one or more trim retarders, for
different AC voltages applied to the one or more LC switches. For
simplicity, the one or more LC switches, or one or more LC switches
in combination with one or more trim retarders will be referred to
as a "switchable retardance system." One will recognize that in
practice the voltage applied in a multi-LC-switch configuration may
be more complicated than that shown since different voltages may be
applied to each LC switch. However, for purposes of these
illustrations, one can assume that the same voltage is applied to
the one or more LC switches.
[0078] In a first state, a voltage V.sub.1 is applied to the
switchable retardance system, and a relative phase modification of
0 is imparted to light beams passing through the switchable
retardance system. In a second state, no voltage is applied to the
switchable retardance system, and a half wave, or
.lamda. 2 , ##EQU00003##
of retardance is imparted to any light beams passing through the
switchable retardance system. In the second state, the
retardance,
.lamda. 2 , ##EQU00004##
is the inherent or default retardance of the LC switch. A thickness
and type of the one or more components in the switchable retardance
system can dictate the shape of the curve and the spline's
intersections with the x and y axes (although in some cases the
curve does not intersect the x-axis due to residual
retardance).
[0079] Typical LC switches are unable to impart a retardance of 0
due to residual retardance. Even at very large voltages, the
retardance curve for most LC switches does not intersect the
x-axis. Instead, an infinite voltage is required to apply zero
retardance, and such a voltage is not practical. FIG. 13 shows the
voltage versus retardance plot for such an LC switch.
[0080] One way to achieve a retardance of 0, as shown for the first
state in FIG. 8, is to form a switchable retardance system
comprising an LC switch and a trim retarder, where the trim
retarder shifts the retardance curve of the LC switch seen in FIG.
13. The LC switch thickness can then be increased such that the
spline curve's intersection of the y-axis still occurs at
.lamda. 2 , ##EQU00005##
as seen in FIG. 8. While this is one switchable retardance system
that can achieve the plot in FIG. 8, this example shows that a
variety of other switchable retardance systems can also achieve the
plot in FIG. 8.
[0081] Further, FIG. 8 shows that switching between two applied AC
voltages enables a switchable retardance system to impart either a
half wave of retardance or no retardance, as shown in FIGS. 2-3.
One can also see that by applying different voltages between
V.sub.1 and V.sub.2 other retardances between 0 and a half-wave of
retardance can be imparted. Thus, states other than the first and
second state are also possible and therefore a variable tuning of
the beam steering and attenuation is possible (see FIGS.
10-12).
[0082] FIG. 9 shows another retardance versus voltage plot for a
switchable retardance system having different parameters than the
system underlying FIG. 8. One can see that in the second state, the
retardance is equal to three half waves,
3 2 .lamda. , ##EQU00006##
which is effectively a half-wave retardance. In the first state,
there is no retardance. As can be seen, the systems underlying
FIGS. 8 and 9 impart the same effective retardance in the first and
second states.
[0083] FIG. 10 shows another retardance versus voltage plot, but
where the voltages for the first and second states are both
positive and thus the LC switch can be said to be on for both
states. Moreover, the two states do not correspond to a maximum and
minimum retardance that can be imparted, as was the case in FIGS. 8
and 9. In this case, the second state applies a voltage greater
than in the first state, and the retardance imparted by the second
state is a half wave while the retardance imparted by the first
state is a full wave. In some cases, LC switching is improved by
using higher voltages, and thus configuring the switchable
retardance system such that the first and second states both
involve finite voltages, or greater than 0V, may enhance switching
(e.g., make for faster LC switching). There may be other reasons
for desiring to use non-zero bias voltages, so FIG. 10 demonstrates
that the switchable retardance system can be configured to achieve
this goal while still enabling a half-wave switching of
polarization in a second state (V.sub.2), and no change to the
polarization in a first state (V.sub.1).
[0084] FIG. 11 is a variation of FIG. 10 showing the results of a
thicker LC switch. In particular, the retardance induced by the
first state is 4.lamda., which effectively imparts no polarization
change to the light beam, and the retardance induced by the second
state is 3.5.lamda., which effectively flips the polarization of
the light beam (e.g., changing right-hand circular to left-hand
circular).
[0085] FIG. 12 shows a retardance versus voltage plot for four
different states of the LC switch. The first and second states are
identical to those shown and described relative to FIG. 8. However,
third and fourth states are also shown, where applied voltages are
between V.sub.1 and V.sub.2, and result in retardance between that
of the first and second states. As can be seen, this enables four
levels of attenuation or beam steering. The first state
(corresponding to V.sub.1) can enable full transmission through an
aperture, or no beam steering from an incident direction. The
second state (corresponding to V.sub.2) can enable full blocking or
attenuation through an aperture, and a maximum of beam steering for
these four states. The third and fourth states can result in
partial attenuation where an aperture at the output is used, and a
level of beam steering greater than in the first state, but less
than in the second state.
[0086] FIG. 14 illustrates an LCPG system having an AC bias and
optional feedback for controlling the AC bias. The LCPG system 1402
can comprise any number of LCPGs as described above, including
three and four-grating systems. An LC switch 1408 can be used to
alter the polarization of an incident light beam at some point
within the LCPG system 1402, for instance, between a first and
second LCPG (e.g., in a four-grating system). A first state of the
LC switch 1408 can cause the outgoing light beam to follow a path
coincident with that of the incident light beam and thereby pass
through an aperture 1410. A second state of the LC switch 1408 can
cause the outgoing light beam to follow a path oblique to that of
the incident light beam and thereby impact and be attenuated by the
aperture 1410. The LC switch 1408 can be controlled via application
of an AC bias having a first state and a second state, each
corresponding to a respective one of the first and second states of
the LC switch 1408. An AC bias device 1404 can control the AC bias
applied to the LC switch 1408 and an optional bias controller
circuit 1406 can control the AC bias device 1404. Optionally, the
bias controller circuit 1406 may be coupled to one or more sensors
that measure an intensity of the outgoing light beam at one or more
locations and receive feedback from these one or more sensors.
These one or more sensors could use a nominal amount of light to
make readings such that optical throughput is negligibly affected.
The bias controller circuit 1406 could then instruct the AC bias
1404 to adjust the LC switch 1408 to achieve a desired light
intensity at one or more of the sensors. For instance, where the
bias controller circuit 1406 receives feedback indicating an amount
of light that is on-axis, the AC bias device 1404 could be
instructed to alter the LC switch 1408 until the on-axis light
intensity increases beyond a threshold or is optimized (i.e., a
desired level of on-axis intensity is found). Although FIG. 14
shows two possible locations for feedback, certain embodiments may
only provide feedback from a single position (e.g., an on-axis
position). In some embodiments, measurements of light intensity can
be taken for both off-axis beams (e.g., at 1412 and 1414) since,
depending on a polarization of the incident beams, off-axis
intensity may not be the same at both off-axis measurement
positions 1412, 1414. Optionally, the LC switch 1408 can have more
than two states, each selectable via a different AC bias from the
AC bias device 1404.
[0087] FIG. 15 illustrates a method of attenuating a light beam
using an LCPG system. The method 1500 can include receiving an
incident light beam at a first liquid crystal polarization grating
(LCPG), the incident light beam optionally being circularly
polarized. The first LCPG can deflect a path of the light beam
based on a polarization of the incident beam (1502), thereby
forming a first deflected beam. Given a certain polarization, a
left circular component of the incident beam may be deflected
off-axis at a first angle while a right circular component of the
incident beam may be deflected off-axis at a second angle equal to
the first angle, but angled in an opposite direction relative to an
axis of the incident beam. If the incident light is not circularly
polarized, or at least not perfectly circular, then the incident
beam will be deflected into two first deflected beams. The
remainder of the description of FIG. 15 assumes circularly
polarized incident light, and thus only a single first deflected
beam. However, those of skill in the art will recognize that the
method 1500 is also applicable to non-circular incident light and
thus those situations where two deflected beams spread out from the
first LCPG.
[0088] The first deflected beam can pass through a first substrate
without deflection, the first substrate configured to support the
first LCPG. The first deflected beam can be received at a first
liquid crystal switch (LC switch). The liquid crystal switch can
impart a phase retardance to the first deflected beam (Block 1504)
based on a bias applied to the LC switch (Block 1506). The first
deflected beam can then pass through a second substrate without
deflection, wherein the second substrate can support a second LCPG.
The second LCPG can receive the first deflected beam, and the
second LCPG can deflect the first deflected beam based on a
polarization of the first deflected beam (Block 1508) thereby
forming a second deflected beam. A third LCPG can receive the
second deflected beam and deflect the second deflected beam based
on a polarization of the second deflected beam (Block 1510),
thereby forming a third deflected beam. The third deflected beam
can pass through a third substrate without deflection, wherein the
third substrate can be configured to support the third LCPG. The
third deflected beam can also pass through a space between the
third substrate and a fourth substrate, the fourth substrate
configured to support a fourth LCPG. The space can be configured to
cause a distance between the first and second LCPGs to equal a
space between the third and fourth LCPGs (Block 1512). The third
deflected beam can then pass through the fourth substrate without
deflection, and then be received by the fourth LCPG. The fourth
LCPG can deflect the third deflected beam based on a polarization
of the third deflected beam (Block 1514) thereby forming a fourth
deflected beam. If the LC switch is in a first state, then the
fourth deflected beam may pass primarily through an aperture
(Decision 1516 and Block 1518). If the LC switch is in a second
state, then the fourth deflected beam may be primarily attenuated
by the aperture (Decision 1516 and Block 1520).
[0089] The foregoing is illustrative of the present disclosure and
is not to be construed as limiting thereof. Although a few
exemplary embodiments of this disclosure have been described, those
skilled in the art will readily appreciate that many modifications
are possible in the exemplary embodiments without materially
departing from the novel teachings and advantages of this
disclosure. For example, the distance, d, in FIG. 7 could be
increased to allow a fold mirror or other optical components to be
inserted between the two sub-assemblies. Furthermore, it should be
understood that analog attenuation may be achieved by setting the
LC cell to an intermediate state. This setting directs some of the
light along the path shown in FIG. 2 and some of the light along
the path shown in FIG. 3, with amounts depending on the retardance
of the LC cell.
[0090] Accordingly, many different embodiments stem from the above
description and the drawings. It will be understood that it would
be unduly repetitious and obfuscating to literally describe and
illustrate every combination and subcombination of these
embodiments. As such, the present specification, including the
drawings, shall be construed to constitute a complete written
description of all combinations and subcombinations of the
embodiments described herein, and of the manner and process of
making and using them, and shall support claims to any such
combination or sub combination.
[0091] In the specification, there have been disclosed embodiments
of the disclosure and, although specific terms are employed, they
are used in a generic and descriptive sense only and not for
purposes of limitation. Although a few exemplary embodiments of
this disclosure have been described, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this disclosure. For instance, the
herein disclosed systems, methods, and apparatus could be employed
for beam-steering applications. For instance, using an LC switch
having a variety of states and polarized input light, enables an
output light beam with a selectable exit angle. The use of two or
more LC switches could increase the number of exit angles that can
be selected, or simplify the circuitry needed to enable such beam
steering. Further, in a beam steering application, the location of
the one or more LC switches can be varied (for instance residing
between LCPG 132 and LCPG 142 or LCPG 102 and LCPG 112).
Accordingly, all such modifications are intended to be included
within the scope of this disclosure as defined in the claims.
Therefore, it is to be understood that the foregoing is
illustrative of the present disclosure and is not to be construed
as limited to the specific embodiments disclosed, and that
modifications to the disclosed embodiments, as well as other
embodiments, are intended to be included within the scope of the
appended claims. The disclosure is defined by the following claims,
with equivalents of the claims to be included therein.
* * * * *
References