U.S. patent application number 12/738674 was filed with the patent office on 2010-11-25 for spatial light modulator using electrowetting cells.
This patent application is currently assigned to SeeReal Technologies S.A.. Invention is credited to Gerald Futterer, Stephan Reichelt.
Application Number | 20100296148 12/738674 |
Document ID | / |
Family ID | 38814129 |
Filed Date | 2010-11-25 |
United States Patent
Application |
20100296148 |
Kind Code |
A1 |
Reichelt; Stephan ; et
al. |
November 25, 2010 |
Spatial Light Modulator Using Electrowetting Cells
Abstract
A spatial light modulator comprising pixels, where for each
pixel, a light field amplitude transmitted by the pixel is
modulated by an electrowetting cell and/or a light field phase
transmitted by the pixel is modulated by an electrowetting
cell.
Inventors: |
Reichelt; Stephan; (Dresden,
DE) ; Futterer; Gerald; (Dresden, DE) |
Correspondence
Address: |
Saul Ewing LLP (Philadelphia);Attn: Patent Docket Clerk
Penn National Insurance Plaza, 2 North Second St., 7th Floor
Harrisburg
PA
17101
US
|
Assignee: |
SeeReal Technologies S.A.
Munsbach
LU
|
Family ID: |
38814129 |
Appl. No.: |
12/738674 |
Filed: |
October 17, 2008 |
PCT Filed: |
October 17, 2008 |
PCT NO: |
PCT/EP08/64052 |
371 Date: |
April 19, 2010 |
Current U.S.
Class: |
359/228 ;
359/290 |
Current CPC
Class: |
G03H 1/02 20130101; G03H
2225/55 20130101; G03H 2240/43 20130101; G02B 26/005 20130101; G03H
2001/0224 20130101; G03H 2225/33 20130101; G03H 1/22 20130101; G03H
1/2294 20130101; G03H 2225/32 20130101; G03H 2225/31 20130101; G03H
2001/2297 20130101; G03H 2225/24 20130101 |
Class at
Publication: |
359/228 ;
359/290 |
International
Class: |
G02B 26/02 20060101
G02B026/02; G02B 26/06 20060101 G02B026/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2007 |
GB |
0720484.5 |
Jun 27, 2008 |
GB |
0811795.4 |
Jun 27, 2008 |
GB |
0811797.0 |
Jun 27, 2008 |
GB |
0811802.8 |
Jun 27, 2008 |
GB |
0811805.1 |
Claims
1. A spatial light modulator comprising pixels, where for each
pixel, a light field amplitude transmitted by the pixel is
modulated by an electrowetting cell, wherein the electrowetting
cell is illuminated with converging light comprising a focus, the
convergent light is generated by a focusing element, wherein an
electrowetting cell comprises a cover substrate with a top face,
the top face being coated with an optically non-transparent layer,
which exhibits a centrally disposed optically transmitting
aperture, the aperture effecting a spatial filtering and wherein
the electrowetting cell is disposed near the focus of the light
such that the position of the aperture of the electrowetting cell
coincides with the position of the focus of the light.
2. The spatial light modulator of claim 1, especially for
modulating the light field amplitude, in which each electrowetting
cell comprises a first substantially transparent substrate coated
with a substantially transparent electrode and a hydrophobic
isolation layer, a pixel-separating side wall, at least two
immiscible liquids, one of the liquids being opaque or absorbing
and one of the liquids being electrically conductive or polar
liquid, and a second, substantially transparent substrate and where
the amount of light passing through the electrowetting cell is
controlled by a voltage applied to the electrically conductive or
polar liquid.
3. The spatial light modulator of claim 1, especially for
modulating the light field amplitude, in which each electrowetting
cell comprises a first substantially transparent substrate coated
with a substantially transparent electrode and hydrophobic
isolation layers, a pixel-separating side wall, a first opaque or
absorbing liquid and a second electrically conductive or polar
liquid where these two liquids are immiscible, and a second,
substantially transparent substrate and where the amount of light
passing through the electrowetting cell is controlled by a voltage
applied to the electrically conductive or polar liquid.
4. The spatial light modulator of claim 2 or 3, in which a contact
angle of the electrically conductive or polar liquid and the first
substantially transparent substrate is continuously variable by
applying different voltages thus realising a continuously variable
absorption in the cell.
5. The spatial light modulator of claims 2 or 3, in which the top
face of the second substrate is coated with an optically
non-transparent, layer, which exhibits an essentially centrally
disposed optically transmitting opening.
6. The spatial light modulator of claims 2 or 3, in which the
electrowetting cell is in the ON state if a DC or AC voltage is
applied between an electrode and a counter electrode, whereby the
electrically conductive or polar liquid is attracted to the
hydrophobic insulator layer caused by electrostatic forces, thereby
displacing the opaque or absorbing liquid, which is positioned
around a central spot on the first substantially transparent
substrate, and the cell is in its OFF state if no voltage is
applied.
7. The spatial light modulator of claims 2 or 3, in which the
opaque or absorbing liquid is disposed at fringes of the
electrowetting cell and is held in this position by suitable means
such that if no voltage is applied, the opaque or absorbing liquid
spreads across the base area; a small separation ring is positioned
in the centre of the cell, which ensures that there is permanent
contact to the electrically conductive or polar liquid and that the
opaque or absorbing liquid spreads homogeneously in all directions
when the cell is switched on.
8. A spatial light modulator comprising pixels, where for each
pixel, a light field phase transmitted by the pixel is modulated by
an electrowetting cell, in which each electrowetting cell comprises
at least three non-mixable liquid layers with at least two variably
adjustable optical interfaces, wherein at least two liquids exhibit
different optical properties, wherein the phase of an incident
light field is modulated independently in each individual pixel of
the spatial light modulator, wherein the at least two variably
adjustable optical interfaces are adjusted in a targeted manner
such that a relative phase lag is created among the light waves
which are controlled by individual pixels due to different optical
path lengths within individual electrowetting cells of the spatial
light modulator.
9. The spatial light modulator of claim 8, in which the liquid
layer in the middle of the three liquid layers forms an inclined,
essentially plane plate which is operated in a higher order for
phase modulation or in which the liquid layer in the middle of the
three liquid layers forms an inclined, essentially plane plate
which is operated in a higher order for phase modulation and in
which the liquid layer in the middle of the three liquid layers
forms an inclined, essentially plane plate, and a second
electrowetting cell is placed after the first cell to compensate
for lateral offset of light beams transmitting the first
electrowetting cell.
10-11. (canceled)
12. The spatial light modulator of claim 9, in which the liquid
layer in the middle of the three liquid layers forms an inclined,
essentially plane plate, and a fixed prism is placed on the beam
entrance side or on a beam exit side of the electrowetting cell to
compensate for lateral offset of light beams transmitting the
electrowetting cell.
13. The spatial light modulator of claim 9, in which the liquid
layer in the middle of the three liquid layers forms an inclined
plane plate, and an aperture is disposed in a central position on a
beam exit side of the electrowetting cell to prevent lateral offset
of light beams transmitting the electrowetting cell.
14. A spatial light modulator comprising pixels, where for each
pixel, the light field is modulated on a complex number basis using
two electrowetting cells in series for each pixel, one of the two
electrowetting cells being an electrowetting cell according to
claim 1 and/or one of the electrowetting cell being an
electrowetting cell according to claim 8, the two electrowetting
cells permitting independent modulation of amplitude and phase of
the complex number.
15. The spatial light modulator of claim 14, in which the two cells
are located in sufficient proximity that cross-talk between pixels
is zero or is kept to acceptable levels.
16. The spatial light modulator of claim 1, 8 or 14, in which
multiple pixels are arranged in the form of a line array or
matrix.
17. The spatial light modulator of claim 1, 8 or 14, in which the
light field amplitude transmitted by each pixel is modulated with a
switching time less than or equal to 5 ms and/or greater than or
equal to 100 microseconds.
18. The spatial light modulator of claim 17, in which the spatial
light modulator is operable at conventional switching frequencies,
preferably in the frequency range from 15 Hz to several KHz, or in
which the spatial light modulator is operable to maintain a
predetermined state for a predetermined period of time.
19. The spatial light modulator of claim 1, 8 or 14, in which the
electrowetting cell is positioned near a focus of a focusing
element or in which the electrowetting cell is positioned near a
focus of a focusing element and in which the size of the
electrowetting cell is smaller or much smaller than the size of the
focusing element.
20. (canceled)
21. The spatial light modulator of claim 1, 8 or 14, in which the
light transmitted through the electrowetting cell is transmitted
with a spherical or cylindrical outgoing wavefront, due to at least
one light beam forming means being assigned to the electrowetting
cell.
22-23. (canceled)
24. The spatial light modulator of claim 1, 8 or 14, in which the
spatial light modulator is used to form a secondary light source or
in which the spatial light modulator is used to form a light source
array with variable amplitude or in which the spatial light
modulator is used to form a light source array with variable
phase.
25-26. (canceled)
27. The spatial light modulator of claim 1, 8 or 14, in which the
spatial light modulator is used in transmission or in a reflective
geometry.
28. (canceled)
29. The spatial light modulator of claim 1, 8 or 14, in which the
spatial light modulator is used in a 3D display or in a holographic
display or in a stereoscopic display or in a two dimensional
amplitude modulating display or in which the spatial light
modulator is used in a holographic display and in which one or two
virtual observer windows for the eyes of one or more observers are
used.
30-32. (canceled)
33. Device or display device including the spatial light modulator
of claim 1, 8 or 14, in which the device is a phase and/or an
amplitude modulating device or in which the device is a complex
light wave modulating device.
34-36. (canceled)
37. The display device of claim 33, in which the display device is
a 2D phase modulating display device or a stereoscopic display
device or in which the display device is a holographic display
device and in which the holographic display device preferably uses
virtual observer windows for the eyes of the observer or
observers.
38. (canceled)
39. Method of using a display device of claim 33, the display
including a light source and an optical system to illuminate the
spatial light modulator; comprising the step of: for each pixel,
modulating the light field amplitude transmitted by each pixel
using an electrowetting cell and/or modulating the light field
phase transmitted by each pixel using an electrowetting cell.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The field of the invention is spatial light modulators, and
devices which contain such spatial light modulators, especially
holographic display devices.
[0003] 2. Technical Background
[0004] Spatial light modulators (SLMs) are known from the prior
art. There are various types of SLMs, based on various physical
principles. SLMs are optical devices that modulate an incident
light field in a spatial pattern in order to reflect or to transmit
an image or to generate a holographic reconstruction corresponding
to an electrical or optical input. An SLM typically comprises a
one- or two-dimensional array of addressable elements (pixels)
which are capable of transmitting or reflecting incident light
fields. Well-established examples are liquid crystal (LC) based
modulators, in which a voltage-induced birefringence is used to
modulate either the amplitude or phase of an incident light field.
Spatial light modulators are used in almost all areas of optical
technologies and optical information processing which take
advantage of variable or adaptive optical components. The
applications of spatial light modulators range from display and
projection systems, to microscopy, beam and wave front shaping,
optical metrology, maskless lithography, ultra-fast laser pulse
modulation to aberration correction in terrestrial telescopes.
[0005] Various types of SLMs are known from the prior art. These
include electrically addressable SLMs (EASLMs), optically
addressable SLMs (OASLMs) and magneto-optical SLMs (MOSLMs), for
example.
[0006] SLMs may comprise an array of pixels. The term "pixel"
derives from "picture element" and hence is a term associated with
digital imaging. In the context of SLMs, a "pixel" is the hardware
element which controls the display of a picture element of an image
which may be seen by a viewer. The image seen by a viewer may be a
holographic representation of a three-dimensional scene.
[0007] Prior art SLMs have various drawbacks. Most of the
liquid-crystal-based spatial light modulators which are
commercially available today exhibit refresh rates in a range of
60-120 Hz, which correspond to response times greater than 8
milliseconds. Such switching speeds are sufficient for many
applications. However, there are many applications which require a
much faster switching, i.e. higher frame rates. This includes in
particular applications which involve time multiplexing methods.
Possible applications of time multiplexing are displays that
present different information to different observers. Such displays
redirect the light to different observers and simultaneously change
the information content of the display designated for each
observer. As long as the refresh rate per observer is more than
about 60 Hz, i.e. the response time is below 17 ms, the observer
does not perceive any flickering of the image displayed. Examples
of possible applications are automotive displays, where the driver
wishes to see the navigation system whereas another passenger
wishes to see a movie. Another example is 3D autostereoscopic
displays where every observer wishes to see the 3D scene from their
own perspective.
[0008] An object of the implementations disclosed in this document
is to modulate the amplitude, or the phase, or the amplitude and
phase of a light field spatially, where the temporal modulation of
the desired values is fast compared with LC-based SLMs. The
amplitude is typically adjustable in the entire codomain (from 0 to
1, inclusive), whereas the phase is typically adjustable in the
entire codomain (from 0 to 2.pi., inclusive) and the target refresh
rate lies within the range of between some hundred Hertz and some
kHz, i.e. a response time of 5 milliseconds or less, but typically
greater than or equal to 100 microseconds. A further object of the
implementations is to cover the entire amplitude and/or phase range
by a relative change of the amplitude and/or phase values between
the individual pixels of a plane one- or two dimensional array.
[0009] It will be appreciated by those skilled in the art that the
SLMs conforming to this invention may be used in any known
application in which SLMs are employed. While the applications of
the spatial light modulators described here are not limited to
holographic displays, holographic displays are the preferred
application of the spatial light modulators described here. It will
be appreciated by those skilled in the art that the SLMs described
herein may be used in any known form of holographic display.
However, the preferred approach of the applicant to generating
computer-generated video holograms will be described below.
[0010] Computer-generated video holograms (CGHs) are encoded in one
or more spatial light modulators (SLMs); the SLMs may include
electrically or optically controllable cells. The cells modulate
the amplitude and/or phase of light by encoding hologram values
corresponding to a video-hologram. The CGH may be calculated e.g.
by coherent ray tracing, by simulating the interference between
light reflected by the scene and a reference wave, or by Fourier or
Fresnel transforms; CGH calculation methods are described for
example in US2006/055994 and in US2006/139710, which are
incorporated by reference. An ideal SLM would be capable of
representing arbitrary complex-valued numbers, i.e. of separately
controlling the amplitude and the phase of an incoming light wave.
However, a typical SLM controls only one property, either amplitude
or phase, with the undesirable side effect of also affecting the
other property. There are different ways to spatially modulate the
light in amplitude or phase, e.g. electrically addressed liquid
crystal SLM, optically addressed liquid crystal SLM,
magneto-optical SLM, micro mirror devices or acousto-optic
modulators. The modulation of the light may be spatially continuous
or composed of individually addressable cells, one-dimensionally or
two-dimensionally arranged, binary, multi-level or continuous.
[0011] In the present document, the term "encoding" denotes the way
in which regions of a spatial light modulator are supplied with
control values to encode a hologram so that a 3D-scene can be
reconstructed from the SLM.
[0012] In contrast to purely auto-stereoscopic displays, with video
holograms an observer sees an optical reconstruction of a light
wave front of a three-dimensional scene. The 3D-scene is
reconstructed in a space that stretches between the eyes of an
observer and the spatial light modulator (SLM), or possibly even
behind the SLM. The SLM can also be encoded with video holograms
such that the observer sees objects of a reconstructed
three-dimensional scene in front of the SLM and other objects on or
behind the SLM.
[0013] The cells of the spatial light modulator may be transmissive
cells which are passed through by light, the rays of which are
capable of generating interference at least at a defined position
and over a spatial coherence length of a few millimetres. This
allows holographic reconstruction with an adequate resolution in at
least one dimension. This kind of light will be referred to as
`sufficiently coherent light`. However, cells which operate in a
reflective geometry are also possible.
[0014] In order to ensure sufficient temporal coherence, the
spectrum of the light emitted by the light source must be limited
to an adequately narrow wavelength range, i.e. it must be
near-monochromatic. The spectral bandwidth of high-brightness LEDs
is sufficiently narrow to ensure temporal coherence for holographic
reconstruction. The diffraction angle at the SLM is proportional to
the wavelength, which means that only a monochromatic source will
lead to a sharp reconstruction of object points. A broadened
spectrum will lead to broadened object points and smeared object
reconstructions. The spectrum of a laser source can be regarded as
monochromatic. The spectral line width of a LED is sufficiently
narrow to facilitate good reconstructions.
[0015] Spatial coherence relates to the lateral extent of the light
source. Conventional light sources, like LEDs or Cold Cathode
Fluorescent Lamps (CCFLs), can also meet these requirements if they
radiate light through an adequately narrow aperture. Light from a
laser source can be regarded as emanating from a point source
within diffraction limits and, depending on the modal purity, leads
to a sharp reconstruction of the object, i.e. each object point is
reconstructed as a point within diffraction limits.
[0016] Light from a spatially incoherent source is laterally
extended and causes a smearing of the reconstructed object. The
amount of smearing is given by the broadened size of an object
point reconstructed at a given position. In order to use a
spatially incoherent source for hologram reconstruction, a
trade-off has to be found between brightness and limiting the
lateral extent of the source with an aperture. The smaller the
light source, the better is its spatial coherence.
[0017] A line light source can be considered to be a point light
source if seen from a right angle to its longitudinal extension.
Light waves can thus propagate coherently in that direction, but
incoherently in all other directions.
[0018] In general, a hologram reconstructs a scene holographically
by coherent superposition of waves in the horizontal and the
vertical directions. Such a video hologram is called a
full-parallax hologram. The reconstructed object can be viewed with
motion parallax in the horizontal and the vertical directions, like
a real object. However, a large viewing angle requires high
resolution in both the horizontal and the vertical direction of the
SLM.
[0019] Often, the requirements on the SLM are lessened by
restriction to a horizontal-parallax-only (HPO) hologram. The
holographic reconstruction takes place only in the horizontal
direction, whereas there is no holographic reconstruction in the
vertical direction. This results in a reconstructed object with
horizontal motion parallax. The perspective view does not change
upon vertical motion. A HPO hologram requires less resolution of
the SLM in the vertical direction than a full-parallax hologram. A
vertical-parallax-only (VPO) hologram is also possible but
uncommon. The holographic reconstruction occurs only in the
vertical direction and results in a reconstructed object with
vertical motion parallax. There is no motion parallax in the
horizontal direction. The different perspective views for the left
eye and right eye have to be created separately.
[0020] In some of the implementations described herein,
electrowetting cells are used. An early use of the term
"electrowetting" was in 1981; "electrowetting" was used in G. Beni
and S. Hackwood, Appl. Phys. Lett. 38, 4, pp. 207-209 (1981). The
electrowetting effect was originally defined as "the change in
solid electrolyte contact angle due to an applied potential
difference between the solid and the electrolyte". Since then a
number of devices based on electrowetting have been devised. The
phenomenon of electrowetting can be understood in terms of the
forces that result from the applied electric field. The fringing
field at the corners of the electrolyte droplet tend to pull the
droplet down onto the electrode, lowering the macroscopic contact
angle, and increasing the droplet contact area. Alternatively
electrowetting can be viewed from a thermodynamic perspective.
Since the surface tension of an interface is defined as the Gibbs
free energy required to create a certain area of that surface, it
contains both chemical and electrical components. The chemical
component is just the natural surface tension of the
solid/electrolyte interface with no electric field. The electrical
component is the energy stored in the capacitor formed between the
conductor and the electrolyte. In the present document the term
`electrowetting cell` describes in particular a single optical
element for changing the amplitude and/or phase of a wave field.
The electrowetting cell includes a chamber having cell walls filled
with at least two different non-miscible fluids or liquids,
especially a conductive polar fluid or liquid, like water, and an
insulating non-conductive fluid or liquid, like oil. It is noted
and understood that a fluid can be a liquid or a gas. In general, a
fluid is a subset of the phases of matter and include liquid,
(saturated) gas, plasma and, to some extent, plastic solid. It is
noted that the term "electrowetting" within the context of this
document is also to be understood as
"electrowetting-on-dielectrics" (EWOD).
[0021] 3. Discussion of Related Art
[0022] WO 2004/044659 (US2006/0055994) filed by the applicant
describes a device for reconstructing three-dimensional scenes by
way of diffraction of sufficiently coherent light; the device
includes a point light source or line light source, a lens for
focusing the light and a spatial light modulator. In contrast to
conventional holographic displays, the SLM in transmission mode
reconstructs a 3D-scene in at least one `virtual observer window`
(see Appendix I and II for a discussion of this term and the
related technology). Each virtual observer window is situated near
the observer's eyes and is restricted in size so that the virtual
observer windows are situated in a single diffraction order, so
that each eye sees the complete reconstruction of the
three-dimensional scene in a frustum-shaped reconstruction space,
which stretches between the SLM surface and the virtual observer
window. To allow a holographic reconstruction free of disturbance,
the virtual observer window size must not exceed the periodicity
interval of one diffraction order of the reconstruction. However,
it must be at least large enough to enable a viewer to see the
entire reconstruction of the 3D-scene through the window(s). The
other eye can see through the same virtual observer window, or is
assigned a second virtual observer window, which is accordingly
created by a second light source. Here, a visibility region i.e.
the range of positions from which an observer can see a correct
reconstruction, which would be rather large, is limited to the
locally positioned virtual observer windows. This virtual observer
window solution uses the larger area and high resolution of a
conventional SLM surface to generate a reconstruction which is
viewed from a smaller area which is the size of the virtual
observer windows. This leads to the effect that the diffraction
angles, which are small due to geometrical reasons, and the
resolution of current generation SLMs, are sufficient to achieve a
high-quality real-time holographic reconstruction using reasonable,
consumer level computing equipment. A mobile phone which generates
a three dimensional image is disclosed in US2004/0223049. However,
the three dimensional image disclosed in US2004/0223049 is
generated using autostereoscopy. One problem with
autostereoscopically generated three dimensional images is that
typically the viewer perceives the image to be inside the display,
whereas the viewer's eyes tend to focus on the surface of the
display. This disparity between where the viewer's eyes focus and
the perceived position of the three dimensional image leads to
viewer discomfort after some time in many cases. This problem does
not occur, or is significantly reduced, in the case of three
dimensional images generated by holography.
SUMMARY OF THE INVENTION
[0023] According to the invention, a spatial light modulator
comprising pixels, where for each pixel, a light field amplitude
transmitted by the pixel is modulated by an electrowetting cell
and/or a light field phase transmitted by the pixel is modulated by
an electrowetting cell.
[0024] In a preferred embodiment of the spatial light modulator
especially for modulating the light field amplitude, each
electrowetting cell comprises a first substantially transparent
substrate coated with a substantially transparent electrode and a
hydrophobic isolation layer, a pixel-separating side wall, at least
two immiscible liquids, one of the liquids being opaque or
absorbing and one of the liquids being electrically conductive or
polar liquid, and a second, substantially transparent substrate and
where the amount of light passing through the electrowetting cell
is controlled by a voltage applied to the electrically conductive
or polar liquid. Even though it is mentioned that the
electrowetting cell comprises at least two immiscible liquids, in
general instead of a liquid of the electrowetting cell, immiscible
fluids could be used.
[0025] Preferably each electrowetting cell comprises a first
substantially transparent substrate coated with a substantially
transparent electrode and hydrophobic isolation layers, a
pixel-separating side wall, a first opaque or absorbing liquid and
a second electrically conductive or polar liquid where these two
liquids are immiscible, and a second, substantially transparent
substrate and where the amount of light passing through the
electrowetting cell is controlled by a voltage applied to the
electrically conductive or polar liquid.
[0026] A contact angle of the electrically conductive or polar
liquid and the first substantially transparent substrate could be
continuously variable by applying different voltages thus realising
a continuously variable absorption in the cell.
[0027] The top face of the second substrate could be coated with an
optically non-transparent, layer, which exhibits an essentially
centrally disposed optically transmitting opening.
[0028] Preferably, the electrowetting cell is in the ON state if a
DC or AC voltage is applied between an electrode and a counter
electrode, whereby the electrically conductive or polar liquid is
attracted to the hydrophobic insulator layer caused by
electrostatic forces, thereby displacing the opaque or absorbing
liquid, which is positioned around a central spot on the first
substantially transparent substrate, and the cell is in its OFF
state if no voltage is applied.
[0029] In one embodiment, the opaque or absorbing liquid could be
disposed at fringes of the electrowetting cell and is held in this
position by suitable means such that if no voltage is applied, the
opaque or absorbing liquid spreads across the base area; a small
separation ring is positioned in the centre of the cell, which
ensures that there is permanent contact to the electrically
conductive or polar liquid and that the opaque or absorbing liquid
spreads homogeneously in all directions when the cell is switched
on.
[0030] In a preferred embodiment of the spatial light modulator
especially for modulating the light field phase, each
electrowetting cell comprises at least three non-mixable liquid
layers with at least two variably adjustable optical interfaces,
where at least two liquids exhibit different optical properties.
The liquid layer in the middle of the three liquid layers could
form an inclined, essentially plane plate which is operated in a
higher order for phase modulation.
[0031] The liquid layer in the middle of the three liquid layers
could form an inclined, essentially plane plate, and a second
electrowetting cell is placed after the first cell to compensate
for lateral offset of light beams transmitting the first
electrowetting cell if necessary.
[0032] The liquid layer in the middle of the three liquid layers
could form an inclined, essentially plane plate, and a fixed prism
is placed on a beam exit side of the electrowetting cell to
compensate for lateral offset of light beams transmitting the
electrowetting cell if necessary.
[0033] The liquid layer in the middle of the three liquid layers
could form an inclined, essentially plane plate, and a fixed prism
is placed on the beam entrance side of the electrowetting cell to
compensate for lateral offset of light beams transmitting the
electrowetting cell if necessary.
[0034] The liquid layer in the middle of the three liquid layers
could form an inclined plane plate, and an aperture is disposed in
a central position on a beam exit side of the electrowetting cell
to prevent lateral offset of light beams transmitting the
electrowetting cell if necessary.
[0035] In another embodiment of the invention, for each pixel, the
light field is modulated on a complex number basis using two
electrowetting cells in series for each pixel, the two
electrowetting cells permitting independent modulation of amplitude
and phase of the complex number. The two cells could be located in
sufficient proximity that cross-talk between pixels is zero or is
kept to acceptable levels.
[0036] Multiple pixels could be arranged in the form of a line
array or matrix.
[0037] The light field amplitude transmitted by each pixel could be
modulated with a switching time less than or equal to 5 ms and/or
greater than or equal to 100 microseconds. The spatial light
modulator could be operable at conventional switching frequencies,
preferably in the frequency range from 15 Hz to several KHz.
Alternatively or additionally, the spatial light modulator could be
operable to maintain a predetermined state for a predetermined
period of time.
[0038] The electrowetting cell could be positioned near a focus of
a focusing element. The size of the electrowetting cell could be
smaller or much smaller than the size of the focusing element.
[0039] The light transmitted through the electrowetting cell could
be transmitted with a spherical or cylindrical outgoing wavefront,
due to at least one light beam forming means being assigned to the
electrowetting cell.
[0040] The modulated light could be visible light and/or near infra
red light and/or near ultraviolet light. The spatial light
modulator could be used in military applications, especially in
laser radar systems. The spatial light modulator could be used to
form a secondary light source. The spatial light modulator could be
used to form a light source array with variable amplitude.
Alternatively, the spatial light modulator could be used to form a
light source array with variable phase. The spatial light modulator
could be used in transmission or in a reflective geometry.
[0041] The spatial light modulator could be used in a 3D display.
The spatial light modulator could be used in a holographic display
or in a stereoscopic display or in an auto stereoscopic display.
One or two virtual observer windows for the eyes of one or more
observers could be used.
[0042] The spatial light modulator could be used in a two
dimensional amplitude modulating display.
[0043] According to an aspect of the invention, a device includes
the spatial light modulator of any of the Claims 1 to 31, in which
the device is a phase and/or an amplitude modulating device or in
which the device is a complex light wave modulating device.
[0044] According to another aspect of the invention, a display
device includes the spatial light modulator of any of Claims 1 to
31. The display device could have up to several million pixels. The
display device could contain a diffuser foil.
[0045] The display device could be a 2D phase modulating display
device or a stereoscopic display device. Alternatively, the display
device could be a holographic display device. The holographic
display device preferably uses virtual observer windows for the
eyes of the observer or observers.
[0046] According to still another aspect of the invention, a method
uses a display device of any of Claims 33 to 37, the display
including a light source and an optical system to illuminate the
spatial light modulator; the method comprising the step of:
[0047] for each pixel, modulating the light field amplitude phase
transmitted by each pixel using an electrowetting cell and/or
modulating the light field phase transmitted by each pixel using an
electrowetting cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a schematic cross-sectional view of one
electrowetting cell with a central absorbing oil droplet, where
sub-figure (a) shows the cell in its ON state, sub-figure (b) shows
the cell in its OFF state, and sub-figure (c) shows the cell in a
partially light-attenuating state.
[0049] FIG. 2: FIGS. 2A and 2B are cross-sectional views of one
display pixel, comprising the electrowetting cell positioned near
the focus of a first focusing element and a second, confocally
positioned focusing element.
[0050] FIG. 3: FIGS. 3A and 3B are cross-sectional views of one
display pixel, comprising the electrowetting cell positioned near
the focus of a focusing element which is followed by a diffuser
foil.
[0051] FIG. 4: FIGS. 4A and 4B show the combination of a spherical
focusing element with an electrowetting cell comprising a circular
pinhole aperture and an alternative combination of a cylindrical
focusing element with an electrowetting cell comprising a slit
aperture, respectively.
[0052] FIG. 5: FIGS. 5A and 5B show a schematic cross-sectional
view of one electrowetting cell with a ring-shaped absorbing oil
droplet, with the cell in its ON state and the cell in its OFF
state, respectively.
[0053] FIG. 6 shows how a controllable refractive index n.sub.LC of
a liquid crystal inside a surface relief grating leads to a
controllable intensity of a fixed focal point. The part of the
diffracted light (dashed line) depends on the diffraction
efficiency .eta. of the diffractive structure. The diffraction
efficiency is changed by the change of the modulation
.DELTA.n(.orgate.)=n.sub.LC(.orgate.)-n.sub.substrate.
[0054] FIG. 7 shows different arrangements of a combination of a
surface relief grating and a liquid crystal to realize an amplitude
modulation.
[0055] FIG. 8 shows how a circular focus can be used to obtain an
amplitude modulating device with a high contrast. By changing the
focal length of a lens the intensity value being transmitted can be
chosen. A circular spot is realized by a combination of an axicon
and a lens. Also a circular phase function in front of a lens can
be used to obtain a circular focus. An enlarged circular focus will
be stopped by the aperture stop AS in a way that no light will pass
the central clear area of the aperture stop AS. Thus a high
contrast can be obtained.
[0056] FIG. 9 shows a setup of a lens and an axicon which is placed
behind the lens.
[0057] FIG. 10 shows a side view of a combination of a lens and an
axicon which forms a circular focus.
[0058] FIG. 11 shows a circular focus behind a combination of a
lens and an axicon.
[0059] FIG. 12 shows a side view and beam path through a
combination of a lens and an axicon which forms a circular focus.
The diameter of the circular focus can be changed continuously by
changing the focal length of the lens.
[0060] FIG. 13 shows generating a circular focus by combining a
lens with an axicon in a single element.
[0061] FIG. 14 shows a realized circular focus in the image plane
of the setup shown in FIG. 13.
[0062] FIG. 15 shows a realized focus in the image plane of the
setup shown in FIG. 13 in the case of an increased focal length f.
Compared to FIG. 14, the focal length is enlarged by thirty percent
in this example.
[0063] FIG. 16 shows an optical phase modulating element, using an
electrowetting cell.
[0064] FIG. 17 shows an optical phase modulating element, using an
electrowetting cell.
[0065] FIG. 18 shows an optical phase modulating element, using an
electrowetting cell.
[0066] FIG. 19 shows an optical phase modulating element, using an
electrowetting cell, with a prism for altering the beam propagation
direction.
[0067] FIG. 20 shows an optical phase modulating element, using an
electrowetting cell, with an aperture on the side from which the
beam exits.
[0068] FIGS. 21A, B and C show the lateral beam offset or lateral
shift of the beam, the optical path length difference and the phase
lag, respectively, as a function of the inclination angle .gamma.,
calculated for the example shown in FIG. 16.
[0069] FIG. 22 shows an optical arrangement of a preferred
embodiment of the present invention, wherein amplitude and phase
modulation is sequentially applied.
[0070] FIG. 23 shows a part of an optical arrangement of a
preferred embodiment of the present invention, wherein a light
source array can be provided, the single light sources of the light
source array comprise variable/adjustable phase values.
[0071] FIG. 24 shows a part of another optical arrangement of a
preferred embodiment of the present invention, wherein a light
source array can be provided, the single light sources of the light
source array comprise variable/adjustable phase and amplitude
values.
DETAILED DESCRIPTION
[0072] Various Implementations will now be described.
[0073] A. Spatial Light Modulator for Modulating Light Field
Amplitude and Display Device Using Electrowetting Cells
[0074] This implementation relates to a spatial light modulator,
and in particular to a spatial light modulator comprising pixels,
where for each pixel, a light field amplitude transmitted by the
pixel is modulated by an electrowetting cell. The spatial light
modulator may be used to generate a desired video hologram.
[0075] This implementation relates to a spatial light modulator,
and in particular to a spatial light modulator suitable for
displaying dynamic computer-generated holograms, where the
amplitude of a light field is spatially modulated. It also relates
to an active matrix display device incorporating a spatial light
modulator according to the implementation, more particularly to an
electrowetting display device. It further relates to a switchable
light source and light source array with an individually adjustable
intensity incorporating a light modulator of the present
implementation.
[0076] It is an object of the present implementation to provide a
fast or very fast amplitude modulation of a light field using the
electrowetting principle, and a corresponding display device.
However, the spatial light modulator may also be operable at more
conventional switching frequencies.
[0077] Each electrowetting cell comprises at least a first
substantially transparent substrate coated with a substantially
transparent electrode and hydrophobic isolation layers, a
pixel-separating side wall, a first opaque or absorbing liquid and
a second electrically conductive or polar liquid where these two
liquids are immiscible, and a second, substantially transparent
substrate. The amount of light passing through the electrowetting
cell is controlled by a voltage applied to the electrically
conductive or polar liquid.
[0078] According to a first implementation, a spatial light
modulator is provided having a plurality of electrowetting cells.
In a preferred example, each cell comprises the following: [0079] A
first, substantially transparent substrate coated with a
substantially transparent electrode and hydrophobic isolation
layers; [0080] A pixel-separating side wall [0081] A first, opaque
or light absorbing liquid and a second, electrically conductive or
polar liquid where those two liquids are non-mixable; [0082] A
second, substantially transparent substrate optionally coated with
a substantially light absorbing layer with a centrally disposed
transparent aperture,
[0083] where the electrowetting cell is positioned near the focus
of a focusing element, and where the amount of light passing
through the electrowetting cell is controlled by a voltage applied
to the second liquid such that the contact angle of said first
opaque liquid is changed, and therefore the shape of the interface
between said immiscible liquids is modified, and as a consequence,
more or less light is absorbed by the opaque liquid. The
electrowetting cell is smaller or much smaller than the focusing
element. The applied voltage is applied directly such as from a
controllable source of electrical potential difference, such as in
an electrically addressable SLM. The contact angle of the
electrically conductive or polar liquid and the first substantially
transparent substrate is continuously variable by applying
different voltages thus realising a continuously variable
absorption in the cell.
[0084] A display device according to the present implementation
comprises a light source, a first focusing element, said
electrowetting cell, and a second focusing element. The minimum
pixel pitch of the display is defined by the size of the light
focusing element.
[0085] According to a second implementation, a switchable point
source or point source array having one or more electrowetting
cells is provided. A switchable point source or point source array
according to the present implementation comprises a light source, a
focusing element, and said electrowetting cell.
[0086] The terms `opaque`, `absorbing` and `transparent` denote
wavelength-dependent material properties, i.e. they are related to
the wavelength of the electromagnetic radiation whose amplitude is
to be modulated with the help of the modulator according to this
implementation. The modulator according to this implementation is
thus not limited to the spectral range of the visible light, but
includes the near infra red and near ultraviolet. For example,
military applications in the near infra red are possible, such as
in laser radar systems.
[0087] The implementation will now be described in detail with the
help of particular examples. The examples relate to the
electrowetting approach and can be combined with various focusing
elements to realize amplitude modulating spatial light modulators.
These spatial light modulators can be used in display devices,
especially in holographic display devices. It is also possible to
use these spatial light modulators to form a secondary light source
or a light source array with variable amplitude. Secondary light
sources may be used in back light units (BLU) of display
devices.
[0088] FIG. 1 illustrates a first example of an electrowetting
cell. It comprises a hermetically sealed hollow body which is
filled with two immiscible liquids. One of the liquids is optically
transparent, polar and electrically conductive; it will be referred
to below as a water-based liquid. It is known from the prior art
that this liquid can be water with added salts, or a polar or
conductive liquid, or any other liquid which is made conductive by
adding ionic components. A second liquid comprises an optically
opaque, light absorbing liquid, e.g. an oil-based liquid. This
second liquid is electrically insulating or non-polar; suitable
substances are for example oils, alkanes or alkane mixtures. It is
known that both liquids preferably have the same or similar
density, so as to prevent shape deviations due to gravitational
force or mechanical vibrations.
[0089] The shape of the light absorbing liquid can be changed in a
specific manner taking advantage of the electrowetting principle
such that the transmitted optical radiation is not attenuated, or
is partly or fully attenuated. The electrowetting cell according to
this implementation comprises a transparent substrate (e.g. glass
or plastic) on to which a thin electrode film (e.g. an indium-tin
oxide (ITO) layer, approx. 50-100 nm thick) is applied which is
optically transparent and electrically conductive. The ITO coating
can be applied for example with the help of sputtering processes.
Then, for example, an approx. 1 .mu.m thick, hydrophobic dielectric
insulator layer is applied on to the electrode film, such as by way
of clip coating and curing. This insulator film can for example be
made using an amorphous fluoro-polymer (e.g. Teflon) dissolved in a
fluoride solution. An additional centring means may be disposed in
the centre of the cell, for example a hydrophobic spot, which
enables the oil-based liquid droplet to be held in a preferred
position. The side walls of the cell can be made by shaping
silicon, e.g. with the help of commonly used etching processes,
such as reactive ion etching (RIE) or plasma etching (ICP).
Alternatively, optical structuring methods can be used, and the
side walls can be formed using photoresist. In the case silicon
walls are used, these directly form the counter electrode; if
photoresist walls are used, these may be additionally coated with a
conductive material or the electrical supply line ends directly in
the water-based liquid volume. A further thin cover substrate seals
the cell hermetically.
[0090] According to a preferred example of the electrowetting cell,
the top face of the cover substrate is coated with an optically
non-transparent, preferably absorbing layer, which exhibits a
centrally disposed optically transmitting opening (pinhole
aperture). This aperture effects a spatial filtering and represents
a secondary light source with adjustable intensity. In this
implementation, the light is transmitted through the electrowetting
cell with a spherical or cylindrical outgoing wavefront, where the
cell is positioned near a focal point or near the beam waist of a
ray bundle or of a Gaussian beam. The cell is in the ON state if a
DC or AC voltage is applied between electrode and counter
electrode, as shown in FIG. 1(a). In this state, the electrically
conductive or polar liquid or water-based liquid is attracted to
the hydrophobic insulator layer caused by electrostatic forces,
thereby displacing the opaque or absorbing or oil-based droplet,
which is positioned around the central spot. As a consequence, a
large portion of the light is transmitted through the cell. By
applying different voltages, the water contact angle can be varied
continuously, thus realising a continuously variable absorption in
the cell, resulting in an amplitude A of the light passing through
the electrowetting cell, as shown in FIG. 1(c). The cell is in its
OFF state if no voltage is applied, as shown in FIG. 1(b). Due to
the hydrophobic coating of the dielectric base substrate, the oil
droplet spreads across the entire base area or at least across
large parts of it. The light which is incident on the cell is fully
absorbed in the OFF state.
[0091] FIG. 5 illustrates a second example of an electrowetting
cell. In contrast to the first example in FIG. 1, the opaque or
absorbing liquid or oil-based liquid is disposed at the fringes of
the electrowetting cell and is held in this preferred position by
suitable means. If no voltage is applied, the oil-based liquid
spreads across the base area. A small separation ring can
preferably be positioned in the centre of the cell, which ensures
that there is permanent contact to the water-based solution and
that the oil spreads homogeneously in all directions when the cell
is switched on. Other examples of electrowetting cell
configurations will be obvious to those skilled in the art.
[0092] FIG. 2 shows the use of an electrowetting cell in
combination with two converging or focusing elements according to a
first example of a display device. The combination of these three
elements represents a pixel of a display. The minimum pixel pitch
is determined by the size of the focusing element. The
electrowetting (EW) cell may be small or very small compared with
the lateral dimension of a focusing element, which allows very fast
switching times to be achieved, because the liquid volume to be
moved is small, and the distance to be moved is small. A typical
switching time is in the range from 100 microseconds to 5
milliseconds, but it strongly depends on the cell size. The cell is
disposed near the focal point of the first focusing element or near
the intermediate image of a light source. The position is
preferably chosen such that the exit pupil of the electrowetting
cell coincides with the position of the intermediate focus. The
axial shift of the intermediate focus due to refraction at the
electrowetting cell is preferably taken into account. The exit
pupil of the electrowetting cell represents a secondary light
source, which is imaged through a second optical element into the
desired position. FIG. 2 shows a refractive microlens which
collimates the light again. Alternatively, the second optical
element could have a diverging effect or be disposed non-confocally
to the intermediate focus. The effect of the optical elements shown
can be refractive, but also reflective or diffractive. The two
sub-figures of FIG. 2 illustrate the conditions for a pixel in the
ON state and in the OFF state. Here, and in some other Figures,
only one pixel is shown, but it will be appreciated by those
skilled in the art that a real display device may have any number
of pixels, up to several million pixels or more. It is noted that
the single pixels shown in FIGS. 2 and 3 do not comprise a scale.
The length of a side of a pixel of FIG. 2 would be approximately
the same size as the diameter of the lens shown on the tight
side.
[0093] FIG. 3 shows the use of an electrowetting cell in
combination with a converging or focusing element and a diffuser
foil according to a second example of a display device. Such an
arrangement is preferable if no further imaging is required, and if
the diffuser foil is to be used as an illuminated display surface.
The two sub-figures of FIG. 3 illustrate the conditions for a pixel
in the ON state and one in the OFF state. Here only one pixel is
shown, but it will be appreciated by those skilled in the art that
a real display device may have any number of pixels, up to several
million pixels or more.
[0094] FIG. 4 illustrates various examples of the electrowetting
cell. The aperture of the electrowetting cell can have various
forms. A preferred example is a circular aperture, as already
described above, which allows a spherical secondary wave to be
formed when the light is emitted through the opening. Another
preferred example is a slit aperture, which allows a cylindrical
secondary wave to be formed when the light is emitted through the
opening. In the latter example, the electrowetting cell has a
rectangular base, where the absorbing liquid is for example
arranged in the form of a line.
[0095] According to a preferred arrangement, multiple pixels are
arranged in the form of a line array or matrix. The individual
pixels are discretely controllable. Because of their small size,
they are capable of switching fast or very fast. An arrangement in
the form of a matrix is preferred in the context of display
applications. Colour contents may be presented on the display by
switching on the primary colours red, green and blue one after
another using a time multiplexing method. The colour mixture may be
achieved by way of pulse width modulation, which is realised either
in the light source, on the way to the display pixel according to
this implementation, or directly in the electrowetting cell. The
latter is achieved by varying the hold time of the cell in the ON
or OFF state. However, individual cells for the display of primary
colours are also possible.
[0096] A further example according to this implementation relates
to a variable light source or to a variable light source array. The
light source has therein preferably the form of a point or line
light source. The term `variable` is used here to describe a
variable intensity of the respective source. An arrangement as
sketched exemplarily in FIG. 4 can be used to generate spherical or
cylindrical secondary waves with variable amplitude. Variable or
switchable light sources are of interest for example for
applications such as in holographic displays, amplitude modulation
displays, or in optical measuring equipment.
[0097] An advantage of electrowetting cells is that the moving
parts are liquid. The absence of moving solid parts reduces device
wear compared to devices in which moving solid parts are in
mechanical contact with other solid parts, where device wear
reduces device lifetime and consistency of performance over
time.
[0098] One skilled in the art will appreciate that amplitude
modulation may be implemented on a pixel by pixel basis, and that a
display may contain up to several million pixels, or more. The
amplitude spatial modulator described may be used in a 3D display,
such as a holographic display, especially in a holographic display
in which the viewer views the holographic reconstruction through
virtual observer windows. One or two virtual observer windows for
the eyes of each of one or more observers are used. The amplitude
spatial modulator described may also be used in a two dimensional
amplitude modulating display, or in other applications in which
amplitude modulating spatial light modulators are employed. The
amplitude spatial modulator described may be used in transmission,
or in a reflective geometry.
[0099] B. Amplitude Modulating Device for Imaging Means and
Holographic Displays
[0100] The aim is to realize fast amplitude modulating devices
which can be used in 2D or 3D displays. 3D displays include
holographic displays, especially holographic displays which use the
applicant's preferred approach to holography, as described for
example in US2006/055994, US2006/139711, and in US2006/139710,
which are incorporated by reference. A fast modulation of the
pixels gives the opportunity to implement techniques like
temporally multiplexed viewing windows or cross talk reduction by
sparse object reconstruction. Sparse object reconstruction means
that only a part of the grid of all object points is reconstructed
in a given frame. Thus, the amount of cross talk between
neighbouring object points can be reduced. For example, if only
each second object point in the x and the y direction is
reconstructed in one frame then four frames are needed to
reconstruct all object points. This is one reason why faster SLM
pixels are desirable. For example, if only the fourth object point
is reconstructed, in x and y direction respectively, then sixteen
frames of this sparse reconstruction will reconstruct all object
points.
[0101] The response time of the light modulating devices should be
fast and the number of realized intensity values of reconstructed
(or displayed) object points should be high enough to provide a
viewer with an acceptable quality image. However, the spatial light
modulator may also be operable at more conventional switching
frequencies. The SLM may have amplitude modulating pixels, phase
modulating pixels or complex value generating pixels.
[0102] One opportunity is to use a surface relief grating which
acts as a diffractive lens, where a liquid crystal is used to fill
the grooves of the surface grating structure. In an index matched
situation where n.sub.LC=n.sub.substrate, the device will act as a
plane plate i.e. a plane wave will propagate through this device
without any propagation direction change. In other words, the plane
wave sees no grating-like structure in this case.
[0103] If a voltage U is applied, the refractive index of the
liquid crystal n.sub.LC experienced by the input light wave is
changed. The electrodes can be made transparent, e.g. by using ITO.
In a refractive setup, a continuous change of the voltage will
cause a continuous shift of the focus point. In a diffractive
setup, a continuous change of the voltage will cause a continuous
change of the diffraction efficiency of the grating. Thus, the
binary surface relief grating can realize a fixed focal point with
different intensity values from zero to one hundred percent of the
initial intensity. The part which is not diffracted will pass
through the element as a plane wave. This non-diffracted wave is
shown in FIG. 6 by the dotted lines propagating straight ahead for
light exiting the device.
[0104] In FIG. 6, a controllable refractive index n.sub.LC of a
liquid crystal inside a surface relief grating leads to a
controllable intensity at a fixed focal point. The part of the
diffracted light (dashed lines for light exiting the device in FIG.
6) depends on the diffraction efficiency .eta. of the diffractive
structure. The diffraction efficiency is changed by the change of
the modulation given by .DELTA.n(U)=n.sub.LC(U)-n.sub.substrate. In
FIG. 6, gradient discontinuities are present at the boundaries
between the liquid crystal domains and the host material.
[0105] If an aperture stop (AS) is placed behind a variable lens,
then the intensity which propagates behind the aperture stop is
controlled by the voltage being applied. In the case of a
diffractive lens the aperture stop is placed in the fixed focal
plane with a distance of the focal length f to the lens, or light
modulation element. This is shown in FIG. 7A. To obtain a
collimated plane wave, a second lens is added to the arrangement of
FIG. 7A, leading to the setup shown in FIG. 7B. If the inner area
of the aperture stop is too large, then a significant part of the
non-diffracted light will still propagate behind the aperture stop.
Thus the contrast which can be realized might be too low in respect
to the specific application of the modulator. This problem can be
reduced by placing a spherical lens or ball lens inside the
aperture stop, as shown in FIG. 7C. As shown in FIG. 7C, the light
being focused by the variable lens will pass through the small
sphere without a change. A plane wave entering the sphere will
leave the sphere with a strong divergence, as shown by the strongly
diverging rays for light exiting the spherical lens in FIG. 7C.
That means that the part of the plane wave which enters the
transmitting area of the aperture stop will spread out. Thus the
proportion of this unwanted light is reduced significantly.
[0106] The benefit of using a device consisting of a surface relief
grating being filled with a liquid crystal is the opportunity to
realize a fast switching time of less than 5 ms, and in a preferred
case less than 2 ms, but still typically greater than 100
microseconds if liquid crystals are used. For a small numerical
aperture of NA<0.4 it can be assumed that the realized
functionality is independent of the polarisation of the light used.
It is also possible to use electro-optical materials. Such
materials are used for instance in Kerr cells or Pockels cells.
Low-voltage crystalline materials need at least 100 V and
high-voltage materials need several thousand volts in order to
switch, but the switching time may be less than 100
microseconds.
[0107] It is also possible to use a multi order Fresnel lens
instead of a binary surface relief structure. Thus the modulator
can be optimized to work for several wavelengths in the same
way.
[0108] It is also possible to fill a continuously shaped (i.e. no
abrupt edges are present, or equivalently, no gradient
discontinuities are present) surface relief pattern with a liquid
crystal. Thus, a continuous shift of the focal point can be
achieved by applying a voltage U. If the focal length f(U) is
chosen to be equivalent to the distance of the aperture stop (AS),
then approximately all the intensity is transmitted through the
modulator. Only a small part of the propagating light will pass the
aperture stop if the focal length is set to infinity. The part
which propagates behind a second lens which is used to recollimate
the light, analogously to FIG. 7B, can be reduced significantly.
This can be done by implementing a small spherical lens inside the
centre of the aperture stop. With such a spherical lens, the setup
is analogous to FIG. 7C.
[0109] One opportunity to achieve a variable focal length used for
an amplitude modulating element is to use an electro wetting cell.
In this case possible setups are equivalent to the setups shown in
FIG. 7. The diffractive lens is replaced by an electro wetting lens
which realizes a variable focal length f(U). Electrowetting cells
may be as discussed elsewhere in this document.
[0110] If a phase shift is realized by a variable focal length and
in addition to that an aperture stop is used which is made of a
light absorbing electro wetting fluid, then an element realizing
complex values of the propagating field is obtained. If one looks
at the part of the light which is on axis then the change of the
focal length of an electro wetting lens is equivalent to a change
of the optical path length. Thus the phase is changed if the
central thickness of a lens is changed. It is known that light
absorbing oil may be used to form optical valves in flat panel
displays using electro wetting. The same oil can be used for
different wavelengths.
[0111] There are different opportunities to enhance the contrast
which can be obtained by the modulating element. One opportunity is
to generate a circular focus. For instance this can be done by
combining a lens function with a circular aperture, a Fabry-Perot
interferometer or with an axicon. An axicon is a specialized type
of lens which has a conical surface. It is also sometimes called a
cone lens. An axicon transforms a collimated laser beam into Bessel
beam. If in addition to that a convex lens is used, then a ring is
formed. The focal distance f(U) can be chosen in a way that the
complete circular spot will pass the aperture stop. This is shown
in FIG. 8. By changing the focal length, the focal spot can be
enlarged in a way that no propagating light will pass the aperture
stop. Thus a continuous intensity range from zero to one hundred
percent of the initial value can be chosen by applying the
appropriate voltage.
[0112] For the case of a Fabry-Perot interferometer, if a
Fabry-Perot interferometer is illuminated with a converging
spherical wave then a set of circular rings can be seen at the exit
plane of the Fabry-Perot etalon. This behaviour of an etalon is
well known. A change of the focal length of the lens which is
placed in front of the Fabry-Perot etalon can be used to change the
diameter of a circular ring.
[0113] In FIG. 8, a circular focus can be used to obtain an
amplitude modulating device with a high contrast. By changing the
focal length of a lens the intensity value being transmitted can be
chosen.
[0114] A circular spot is realized by a combination of an axicon
and a lens. Also a circular phase function in front of a lens can
be used to obtain a circular focus. An enlarged circular focus will
be stopped by the aperture stop AS in a way that no light will pass
the central clear area of the aperture stop AS. Thus a high
contrast can be obtained. A small enough ring will be fully
transmitted whereas a large ring will be fully blocked. If inside
the inner area of the aperture stop, shown in FIG. 8, a diffuser is
placed, then a second lens will collimate the propagating light in
the required way without a change of the z position of this second
lens or without a change of the focal length of this second
lens.
[0115] FIG. 9 shows the generation of a circular focus by combining
a lens with an axicon. The diameter of the circular focus can be
changed continuously by changing the focal length of the lens.
[0116] As a modification of the setup discussed so far, the
diameter of the circular spot also can be changed by implementing a
variable axicon. A liquid crystal combined with a cone can act as a
variable axicon.
[0117] FIG. 10 shows a side view of a combination of a lens and an
axicon which forms a circular focus.
[0118] FIG. 11 shows a circular focus behind a combination of a
lens and an axicon. The image analysis is performed using Zemax
software supplied by ZEMAX Development Corporation, 3001 112th
Avenue NE, Suite 202, Bellevue, Wash. 98004-8017 USA.
[0119] A reduction of elements needed to obtain a ring focus can be
obtained by using one surface of the lens to create the axicon.
This is shown in FIG. 12, which was created using Zemax software.
FIG. 12 shows a side view and beam path through a combination of a
lens and an axicon which forms a circular focus. The diameter of
the circular focus can be changed continuously by changing the
focal length of the lens. The axicon also can be realized as a
diffractive circular structure. But as long as the numerical
aperture is low (e.g. NA<0.4), the functionality of the setups
discussed here is independent of the polarization state of the
light used.
[0120] FIG. 13 shows the generation of a circular focus by
combining a lens with an axicon in a single element. In FIG. 13 a
3D layout of the setup shown in FIG. 12 is shown. The light enters
the setup from the left-hand side and the plane where the aperture
stop is placed is the plane at the right-hand side of FIG. 13.
[0121] FIG. 14 shows a realized circular focus in the image plane
of the setup shown in FIG. 13, obtained using Zemax image analysis
software. FIG. 14 shows a two dimensional intensity distribution in
the plane where an aperture stop is placed. By changing the focal
length f(U) of the lens or the prism apex angle .kappa.(.orgate.)
of the cone forming the axicon, the diameter of the circular spot
can be changed. The propagating light can be stopped by using an
aperture stop AS if the diameter of the circular focus d(.orgate.)
is chosen to be large enough.
[0122] FIG. 15 shows a realized focus in the image plane of the
setup shown in FIG. 13 in the case of an increased focal length f,
obtained using Zemax image analysis software. Compared to FIG. 14,
the focal length is enlarged by thirty percent.
[0123] Another approach is to use cylindrical grooves of a
substrate filled with birefringent material. This can be done in a
way that one polarization sees an index matched situation and the
perpendicular polarization sees a cylindrical lens. At the focal
distance of this lens a slit is placed. Thus the amplitude can be
changed by changing the polarization state. If a slit is used to
form the aperture, then the transmission can be changed by changing
the polarization state of the light entering the configuration
described.
[0124] One skilled in the art will appreciate that amplitude
modulation may be implemented on a pixel by pixel basis, and that a
display may contain up to several million pixels, or more. The
amplitude spatial modulator described may be used in a holographic
display, especially in a holographic display in which the viewer
views the holographic reconstruction through virtual observer
windows. The amplitude spatial modulator described may also be used
in a two dimensional amplitude modulating display, or in other
applications in which amplitude modulating spatial light modulators
are employed.
[0125] C. Spatial Light Modulator for Modulating Light Field Phase
Based on Electrowetting Cells and Display Device
[0126] This implementation relates to a spatial light modulator
which comprises an array of liquid-filled cells which can be
discretely controlled with the help of the electrowetting principle
such that they modulate the phase of an incident light field. The
phase is modulated independently in each individual pixel of the
liquid cell array. A cell (pixel) comprises at least three
non-mixable liquid layers with at least two variably adjustable
optical interfaces, where at least two liquids exhibit different
optical properties. In general, the two variably adjustable optical
interfaces may be parallel, or they may be non-parallel, such that
a prism shape results. Taking advantage of the electrowetting
principle, the contact angle of the liquids can be modified, thus
causing a variable refraction at the variable optical interfaces.
The variable interfaces are adjusted in a targeted manner such that
the wave emitted by the pixel (i.e. the parallel bundle of rays)
runs parallel to the waves emitted by the other pixels. Due to
different optical path lengths within individual cells of a pixel
array, a relative phase lag can be created among the waves which
are transmitted or controlled by individual pixels.
[0127] The present implementation relates to a spatial light
modulator for phase modulation of a light field, and to the
manufacture of such a spatial light modulator.
[0128] Various designs of spatial light modulators (SLM) are known
from the prior art under various names, and some of them are
discussed elsewhere in this document. The best known example is a
liquid crystal (LC) based modulator, where a voltage-induced
birefringence is used for either phase or amplitude modulation of a
light field. Spatial light modulators are used in a wide range of
applications which are based on optical technologies and where
variable or adaptive elements are required. The fields of
application of spatial light modulators range from display and
projection systems for the consumer goods sector to microscopy
(optical tweezers, phase filter, digital holographic microscopy,
in-vivo imaging), beam and wave front forming using dynamic
diffractive elements (laser material processing, measuring
equipment, focus control) optical measuring equipment (digital
holography, fringe projection, Shack-Hartmann sensor) and
applications in maskless lithography, ultra-fast laser pulse
modulation (dispersion compensation) or in terrestrial telescopes
(dynamic aberration correction).
[0129] Most of the liquid-crystal-based spatial light modulators
which are commercially available today exhibit switching speeds
which allow refresh rates of 60-120 Hz to be achieved i.e the
switching time is greater than 8 ms. These switching speeds are
sufficient for many applications. However, there are many
applications which require lower switching times or higher refresh
rates. This includes in particular applications which involve time
multiplexing methods.
[0130] An object of the present implementation is to spatially
modulate the phase of a light field, where the desired phase values
are altered quickly or very quickly in contrast to LC-based SLMs.
The phase .phi. should be adjustable in a range of
0.ltoreq..phi.<m2.pi., m>1 and m being a natural number and
the refresh rates which are aimed at lie in a range of between some
hundred Hertz and some kHz i.e. the response time should be less
than or equal to 5 ms, but typically greater than or equal to 100
microseconds. However, the spatial light modulator may also be
operable at more conventional switching frequencies. A further
object is to cover the entire range of phase values by a relative
modification of the phase values among the individual pixels of an
areal matrix.
[0131] The physical functional principle of the spatial light
modulator according to this implementation is based on the phase
lag as a result of variable optical path lengths within an
electrowetting cell. An electrowetting cell comprises at least
three transparent optical liquids through which the light is
transmitted one after another, seen in the direction of light
propagation. The optical path length within a cell is changed with
the help of variably adjustable interfaces between the immiscible
liquids.
[0132] The modulator according to this implementation is not
limited to the spectral range of the visible light, but includes
the near infra red and near ultraviolet. For example, military
applications in the near infra red are possible, such as in laser
radar systems.
[0133] Examples of this implementation are explained in detail
below and are illustrated by the accompanying drawings.
[0134] A first and preferred example (shown in FIG. 16) is based on
the functional principle of a variable, pivoting plate with
parallel sides. It is known from e.g. Malacara, D., Servin, M., and
Malacara, Z., Interferogram Analysis for Optical Testing, 2.sup.nd
Ed. (Taylor & Francis, New York, 2005) pages 360 to 363, that a
solid plate with parallel sides which is inclined out of its
vertical position causes both a phase lag and a parallel offset of
the transmitted wave. Here it is disclosed that this principle can
be made use of in an electrowetting-actuated liquid cell if it is
configured appropriately.
[0135] An electrowetting liquid cell may comprise three non-mixable
liquids disposed one after another: oil- and water-based solutions
can for example be used. The centrally disposed liquid exhibits
optical properties (in particular a refractive index n) which
differ from those of the two outer liquids. The two outer liquids
may have an identical refractive index. It is known from the
literature that plane interfaces can be achieved between two
liquids if particular voltage differences are applied between two
opposing electrodes, as shown for example by Smith, N. R.,
Abeysinghe, D. C., Haus, J. W., and Heikenfeld, J. Optics Express
14 (2006) 6557-6563. This principle is employed here. However, in a
preferred example, three liquids are used and controlled such that
the two interfaces between the three liquids are parallel. In the
initial state, the two interfaces are parallel to the outer, fixed
substrate interfaces (inclination angle .gamma.=0). By applying
defined voltage differences, the optical interfaces can be inclined
while maintaining their planarity. The inclination angle is denoted
by the letter .gamma.. Further, it is provided that the inclination
angles .gamma..sub.1; .gamma..sub.2 of the two variable interfaces
are identical, i.e. both interfaces preferably are parallel:
.gamma..sub.1=.gamma..sub.2 (see FIG. 16). This way, the optical
functionality of a pivoting coplanar plate is realised, which,
however, differs from its solid counterpart known from classic
optics in that the thickness of the liquid plate here changes (it
reduces) as the liquid plate is inclined. This is due to the liquid
volume constancy within a cell. The phase lag .DELTA..phi. can be
derived from the geometrical conditions and is given by
.DELTA. .phi. = ( 2 .pi. .lamda. ) { dn 2 cos [ arcsin ( n 1 n 2
sin .gamma. ) ] + v n 3 tan .gamma. } ( 1 ) ##EQU00001##
[0136] where n.sub.i is the refractive index of liquid number i
(i=1, 2, or 3), .gamma. is the inclination angle, d is the plate
thickness, .lamda. is the optical wavelength in the vacuum, and v
is the lateral offset, as shown in FIG. 16. The effective plate
thickness of the embedded liquid is defined as
d = h b sin ( .pi. 4 - .gamma. ) ( 2 ) ##EQU00002##
[0137] The lateral offset v is defined as
v = d sin .gamma. ( 1 - n 1 cos .gamma. n 2 2 - n 1 2 sin 2 .gamma.
) ( 3 ) ##EQU00003##
[0138] FIG. 16 is a cross-sectional view of a first example of an
electrowetting cell of the spatial light modulator of this
implementation. Three transparent optical liquids are disposed in
layers in a cell, which is hermetically sealed by side walls and by
transparent cover substrates. In the exemplary case, an
electrically insulating or non-polar liquid (e.g. oil-based
solution) is sandwiched between two polar, electrically conductive
liquids (e.g. water-based solutions). Four electrodes are disposed
on the side walls and can be addressed discretely. More electrodes
can be disposed on the other walls being arranged parallel to the
drawing plane (not shown), These electrodes can be controlled such
that a predetermined angle between an interface between two
adjacent liquids and the respective sidewall can be adjusted. The
predetermined angle preferably is set to about 90 degrees. The
optical liquids have the refractive indices n.sub.1; n.sub.2;
n.sub.3, where according to a preferred example n.sub.1=n.sub.3.
FIG. 16(a) shows the initial state of the electrowetting cell,
where the voltages U.sub.T1; U.sub.T2; U.sub.B1; U.sub.B1, which
are applied to the electrodes, and are chosen such that the water
contact angles .theta..sub.T1; .theta..sub.T2; .theta..sub.B1;
.theta..sub.B1, are all 90.degree. in the initial state. The side
walls are coated with a thin, for example approx. 1 .mu.m thick,
hydrophobic insulation layer. The thickness of the hydrophobic
insulation layer can range from about 50 nm to up to some The
initial thickness of the central liquid layer is denoted by
h.sub.b. FIG. 16(b) shows the cell in an actuated state. The
voltage pattern applied to the electrodes is here chosen such that
the central liquid layer is inclined by an angle .gamma.. The water
contact angles are .theta..sub.T1=.theta..sub.B2 and
.theta..sub.B1=.theta..sub.T2. This reduces the thickness d of the
central liquid layer, where the thickness is here measured on the
surface normal to the optical interface. The optical path length of
the light which passes through the electrowetting cell changes as a
result of the refraction at the optical interfaces. This leads to a
phase lag of .DELTA..phi. and to a parallel offset v.
[0139] For phase modulation, each electrowetting cell comprises at
least three transparent optical liquids which are disposed in
layers in a cell, which is hermetically sealed by side walls and by
transparent cover substrates, where an electrically insulating or
non-polar liquid is sandwiched between two polar, electrically
conductive liquids, the optical liquids having the refractive
indices n.sub.1; n.sub.2; n.sub.3, where four electrodes are
disposed on the side walls and can be addressed discretely, and
where the side walls are coated with a hydrophobic insulation
layer. The optical path length of the light which passes through
the electrowetting cell changes as a result of the refraction at
the optical interfaces. Alternatively, the cell may comprise a
layer of polar, electrically conductive liquid which is sandwiched
between two electrically insulating or non-polar liquids.
[0140] FIG. 17 is a cross-sectional view of a second example of an
electrowetting cell of the spatial light modulator according to
this implementation, which permits a controllable phase change for
negligible change in the beam propagation direction. The general
arrangement is similar to that of the first example shown in FIG.
16; however, the electrical addressing and thus the optical
functionality of the cell are different. The optical functionality
of a prism is achieved with the help of three liquids which have
the refractive indices n.sub.1; n.sub.2; n.sub.3. It is not
necessary that n.sub.1=n.sub.3; it is also possible to use
different liquids for liquids 1 and 3. Different liquids 1 and 3
may be able to correct to some extent for each other's dispersion
properties, and correct to some extent for the dispersion
properties of liquid 2, for example. The general idea is that the
same deflection angles .beta. can be realised with different
combinations of prism angles .gamma..sub.1; .gamma..sub.2. As a
result of the refraction at the optical interface, the light passes
through the electrowetting cell on different paths and is thus
given a different phase lag than an adjacent cell with differently
set prism angles .gamma..sub.1; .gamma..sub.2, while the deflection
angle .beta. remains the same. FIG. 17(a) shows the cell in its
initial state and FIG. 17(b) shows the cell in its actuated state.
Generally, any state can be referred to as the initial state: this
term is only used to denote a reference state to which the phase
values of other states are related.
[0141] FIG. 18 is a cross-sectional view of a preferred example of
an electrowetting cell with the objective of reducing the volume of
the liquid to be moved when switching occurs, and reducing the
distance the liquid has to move when switching occurs. A reduced
volume of liquid to be moved, and a reduced distance to be moved,
will reduce the switching time. This idea can be realised in both
types of cells above, i.e. those according to the first example
(FIG. 16) and those according to the second example (FIG. 17). Two
sub-cells are disposed one after another, thus representing a
phase-lagging pixel of a spatial light modulator. An optically
transparent separating substrate is sandwiched between the two
sub-cells in the centre of the cell. The separating substrate can
preferably be index-matched to the refractive index of the
surrounding liquid, so that there will be no loss of light caused
by reflections. In the example according to FIG. 16, two liquids
which differ in their optical properties are used, and in the
example according to FIG. 17, at least two different liquids are
used. The optical axes of the two sub-cells do not have to
coincide, and preferably they exhibit a constant lateral offset.
This constant lateral offset is preferred in order to reduce the
dynamic part of the lateral shift, which would occur in the
solution according to FIG. 16, in particular if the inclined plane
plate is operated in a higher order for phase modulation. An
example of a higher order for phase modulation is shown in FIG.
21C, where a phase shift between 0 and 2.pi. can be achieved for
values of .gamma. between about 25 degrees and 34 degrees. What
this means is that (referring to FIG. 21A), when the apex angle
.gamma. varies from 0.degree. to 25.degree., a lateral shift from 0
to 10 microns will occur. On the other hand, if the apex angle
varies from 25.degree. to 34.degree., the lateral shift is in
between 10 and 14 microns. In this case, it is preferred to choose
a constant lateral offset (due to laterally shifted assembly of
both cells, like a decentering) of say 12 microns. Then, the
variable part of the lateral shift is reduced to plus/minus 2
microns.
[0142] In FIG. 18, and indeed more generally, what is desired is
the ability to produce a phase shift between zero and 2.pi.
radians, and to achieve this by introducing a change to the tilt of
the interface between two liquid layers which is relatively small,
because the smaller the change in the tilt angle required to
achieve a given phase shift, the faster the phase modulator should
be. If the liquid interface, or a plane parallel plate, is tilted
to start with (i.e. if there is a shifted lateral offset), then the
additional tilt which has to be introduced to obtain the needed
phase shift of up to 2.pi. is smaller or much smaller than in the
case of a non tilted initial state. That is why a pre tilted liquid
cell is preferred. But if we have this large tilt in the initial
state, we also have a large lateral shift in the beam propagation
direction for a normally incident beam in the initial state of the
device, because plane waves propagating normal to the cell will be
propagating off-normal to the interfaces between the liquids and
hence will experience refraction. For instance this lateral offset
(i.e. the lateral deflection of a beam propagating normal to the
cell, when traversing the cell), defining the zero position or
initial state of the cell, may be 20 percent of the width of a
cell, it is calculated. To correct for this, we arrange the second
array of cells (of which the cell on the right hand side of FIG. 18
is an example) to compensate for the lateral offset of 20 percent
of the width of a single cell. There is still a lateral offset
v(.gamma.) in dependence on the introduced phase shift, and vice
versa, but the dynamic range of the variable lateral shift is
greatly reduced.
[0143] FIG. 19 shows an example where the fixed prism angle .beta.,
which occurs when realising the example shown in FIG. 17, is
compensated for with the help of a fixed prism on the beam exit
side of the device. Alternatively, the fixed prism may be on the
beam entrance side of the device, as would be appreciated by one
skilled in the art.
[0144] FIG. 20 shows an example which may be used to counteract the
parallel offset of the beam, which occurs when realising the
example shown in FIG. 17. In FIG. 20, light propagates from left to
right. An aperture is disposed in a central position on the exit
surface of the pixel, where said aperture is designed such that it
is always completely illuminated. A part of the parallel-offset
light wave is absorbed by the aperture. The exit surface of a pixel
thus remains localised at the position of the aperture and is
independent of the liquid interface inclination angle.
[0145] FIGS. 21A, B and C show the lateral beam offset or lateral
shift of the beam, the optical path length difference and the phase
lag, respectively, as a function of .gamma., calculated for the
example shown in FIG. 16. Parameters used are:
n.sub.1=n.sub.3=1.33; n.sub.2=1.6, h.sub.b=130 .mu.m. In order to
achieve a phase lag of 2.pi., several inclination regimes are
possible. For example, a 2.pi. phase lag is achieved by an
inclination within the range of
0.degree..ltoreq..gamma..ltoreq.25.degree. or within the range of
25.degree..ltoreq..gamma..ltoreq.34.degree.. Because of the more
favourable linearity and a smaller variable parallel offset, it is
preferred to use a higher order, i.e. to define the initial state
at a value of .gamma. which is different from zero, such as in the
range 25.degree..ltoreq..gamma..ltoreq.34.degree.. The initial
offset of the plane parallel shift can be compensated for by an
equivalent but opposite plane parallel shift of the second sub-cell
in respect to the first sub-cell, or by using an inclined
cylindrical cavity which is filled with the liquids.
[0146] One skilled in the art will appreciate that phase modulation
may be implemented on a pixel by pixel basis, and that a display
may contain up to several million pixels, or more. The phase
spatial modulator described may be used in a holographic display,
especially in a holographic display in which the viewer views the
holographic reconstruction through virtual observer windows. The
phase spatial modulator described may also be used in a two
dimensional phase modulating display, or in other applications in
which phase modulating spatial light modulators are employed.
[0147] D. Complex Spatial Light Modulator and Display Device Using
Electrowetting Cells and Display Device
[0148] The spatial light modulators of parts A and C above may be
combined to provide a complex spatial light modulator using
electrowetting cells, which may be used in a display device. For
complex modulation of a light wave, it is necessary to be able to
modulate the amplitude and phase of a light wave independently. By
using the spatial light modulators of parts A and C above in
series, which respectively modulate the amplitude and the phase of
a light wave, complex modulation of the light wave is enabled. The
spatial light modulators of parts A and C above must be placed in
sufficient proximity that cross-talk between pixels is zero or is
kept to acceptable levels i.e. display artifacts which result are
acceptably small for the viewer or viewers. The refresh rates which
are aimed at lie in a range of between some hundred Hertz and some
kHz i.e. the response time should be less than or equal to 5 ms,
but typically greater than or equal to 100 microseconds. However,
the spatial light modulator may also be operable at more
conventional switching frequencies.
[0149] The modulator according to this implementation is thus not
limited to the spectral range of the visible light, but includes
the near infra red and near ultraviolet. For example, military
applications in the near infra red are possible, such as in laser
radar systems.
[0150] FIG. 22 shows a schematic representation of an optical
arrangement of a preferred embodiment of the present invention,
wherein amplitude and phase modulation is sequentially applied.
From the left to the right--in the direction of the propagation of
the light--the following optical components are shown: a pinhole, a
macro lens and a first and a second sandwich. The pinhole denotes a
primary or secondary light source. The light source can be of a
point-like or of a line-like shape. The macro lens comprises a
spherical or a cylindrical shape, collimating the light emitted by
the light source. The diameter or the cross sectional size of the
macro lens can be in the range of several mm, for example 3 to 10
mm. The first sandwich comprises micro lenses on both surfaces
perpendicular to the optical axis. The diameter or the cross
sectional size of the micro lenses can be 20 to 100 .mu.m, for
example. The micro lenses comprise a spherical or a cylindrical
shape. A micro lens of the left surface of the first sandwich as
shown in FIG. 22 focuses the collimated light into an
electrowetting cell. Such an electrowetting cell is comparable to
an electrowetting cell as shown in FIGS. 1 to 3, i.e. it comprises
a pinhole or a slit. The first sandwich comprises a plurality of
such electrowetting cells being arranged next to each other in one
or more directions forming a line type or a matrix type arrangement
(below each other in the representation of FIG. 22, not shown). The
first sandwich is operable such that it realises a SLM modulating
the amplitude of the light directed towards the first sandwich. The
light passing through the electrowetting cell--depending on its
switching state being comparable to the ones shown in FIG. 1a to
FIG. 1c--is collimated by the micro lens of the right surface of
the first sandwich as shown in FIG. 22. The first sandwich can be
spaced apart from the second sandwich by spacers (not shown) or the
first sandwich can be in direct contact with the second sandwich.
The order of the two sandwiches can be inverted. The second
sandwich comprises a plurality of electrowetting cells being
arranged next to each other in one or more directions forming a
line type or a matrix type arrangement (below each other in the
representation of FIG. 22, not shown). Light coming from an
electrowetting cell of the first sandwich passes through an
electrowetting cell of the second sandwich. The electrowetting
cells of the second sandwich are of the type as shown in FIG. 18.
The electrowetting cells of the second sandwich are operable such
that they realises a SLM modulating the phase of the light directed
through the second sandwich. The optical arrangement as shown in
FIG. 22 can be seen as a cut-out of a display extending in vertical
direction of FIG. 22 and therefore comprising more pinholes/light
sources and more macro lenses, respectively. The light emitted by
the pixels of this display can be modulated by the first and second
sandwich in amplitude and/or in phase and thus can provide complex
values. Such a display can be used as a hologram bearing medium of
a holographic display into which a hologram is encoded in order to
visualize a holographic representation of a three-dimensional
scene. Such a holographic display is described for example in
Appendix I. If light being emitted by the pixels of the display
needs to be deflected, e.g. in order to realize eye tracking, an
additional optical layer or sandwich can be added on the right hand
side of the second sandwich (not shown in FIG. 22).
[0151] FIG. 23 shows a schematic representation of a part of an
optical arrangement of a preferred embodiment of the present
invention, wherein a light source array can be provided. The single
light sources of the light source array comprise
variable/adjustable phase values. Collimated light coming from the
left side passes through an electrowetting cell being adapted to
modulate or alter the phase of the light passing through the
electrowetting cell. The electrowetting cell is of the type as
shown e.g. in FIGS. 16 to 20. The transmitted and still collimated
light is focused by the spherical or cylindrical shaped lens into a
pinhole or into a slit, respectively. The pinhole or slit light can
be regarded as a single point or as a line light source--if light
is passing through the pinhole or slit--whose phase can be altered
depending on the control of the electrowetting cell. The optical
arrangement as shown in FIG. 23 can be seen as a cut-out of an
array of a plurality of light sources, electrowetting cells, lenses
and pinholes being arranged in vertical direction of FIG. 23. The
phase values of the light sources of this array can be modulated
independently from each other.
[0152] FIG. 24 shows a schematic representation of a part of
another optical arrangement of a preferred embodiment of the
present invention, wherein a light source array can be provided.
The single light sources of the light source array comprise
variable/adjustable phase and amplitude values. Collimated light
coming from the left side passes through an electrowetting cell
being adapted to modulate or alter the phase of the light passing
through the electrowetting cell. The electrowetting cell is of the
type as shown e.g. in FIGS. 16 to 20. The transmitted and still
collimated light is focused by the spherical or cylindrical shaped
lens into a second electrowetting cell comprising a pinhole or a
slit, respectively. The second electrowetting cell can be one as
shown in FIGS. 1 to 3. Therefore, the second electrowetting cell is
adapted to modulate the amplitude of the light passing it in
dependence of its switching state. The pinhole or slit of the
second electrowetting cell can be regarded as a single point or
line light source--if light is passing through the pinhole or
slit--whose phase and/or amplitude can be altered depending on the
control of the two electrowetting cells. The optical arrangement as
shown in FIG. 24 can be seen as a cut-out of an array of a
plurality of light sources, electrowetting cells and lenses being
arranged in vertical direction of FIG. 24. The phase and/or
amplitude values of the light sources of this array can be
modulated independently from each other.
[0153] One skilled in the art will appreciate that complex
modulation may be implemented on a pixel by pixel basis, and that a
display may contain up to several million pixels, or more. The
complex spatial modulator described may be used in a holographic
display, especially in a holographic display in which the viewer
views the holographic reconstruction through one or two virtual
observer windows. The complex spatial modulator described may also
be used in other applications, as would be obvious to one skilled
in the art.
[0154] Notes
[0155] While the implementations have been illustrated and
described in detail by the foregoing description in conjunction
with the accompanying drawings, such illustration and description
shall be considered illustrative and exemplary and not restrictive.
The implementations shall not be limited to the disclosed examples.
Other variations in the disclosed examples can be understood and
effected by those skilled in the art in practicing the
implementations, from a study of the drawings and the
disclosure.
[0156] In the Figures herein, the relative dimensions shown are not
necessarily to scale.
[0157] Various modifications and alterations of this invention will
become apparent to those skilled in the art without departing from
the scope of this invention, and it should be understood that this
invention is not to be unduly limited to the illustrative examples
and implementations set forth herein.
APPENDIX I
[0158] Technical Primer
[0159] The following section is meant as a primer to several key
techniques used in some of the systems that implement the present
invention.
[0160] In conventional holography, the observer can see a
holographic reconstruction of an object (which could be a changing
scene); his distance from the hologram is not however relevant. The
reconstruction is, in one typical optical arrangement, at or near
the image plane of the light source illuminating the hologram and
hence is at the Fourier plane of the hologram. Therefore, the
reconstruction has the same far-field light distribution of the
real world object that is reconstructed.
[0161] One early system (described in WO 2004/044659 and US
2006/0055994) defines a very different arrangement in which the
reconstructed object is not at or near the Fourier plane of the
hologram at all. Instead, a virtual observer window zone is at the
Fourier plane of the hologram; the observer positions his eyes at
this location and only then can a correct reconstruction be seen.
The hologram is encoded on a LCD (or other kind of spatial light
modulator) and illuminated so that the virtual observer window
becomes the Fourier transform of the hologram (hence it is a
Fourier transform that is imaged directly onto the eyes); the
reconstructed object is then the Fresnel transform of the hologram
since it is not in the focus plane of the lens. It is instead
defined by a near-field light distribution (modeled using spherical
wavefronts, as opposed to the planar wavefronts of a far field
distribution). This reconstruction can appear anywhere between the
virtual observer window (which is, as noted above, in the Fourier
plane of the hologram) and the LCD or even behind the LCD as a
virtual object.
[0162] There are several consequences to this approach. First, the
fundamental limitation facing designers of holographic video
systems is the pixel pitch of the LCD (or other kind of light
modulator). The goal is to enable large holographic reconstructions
using LCDs with pixel pitches that are commercially available at
reasonable cost. But in the past this has been impossible for the
following reason. The periodicity interval between adjacent
diffraction orders in the Fourier plane is given by .lamda.D/p,
where .lamda. is the wavelength of the illuminating light, D is the
distance from the hologram to the Fourier plane and p is the pixel
pitch of the LCD. But in conventional holographic displays, the
reconstructed object is in the Fourier plane. Hence, a
reconstructed object has to be kept smaller than the periodicity
interval; if it were larger, then its edges would blur into a
reconstruction from an adjacent diffraction order. This leads to
very small reconstructed objects--typically just a few cm across,
even with costly, specialised small pitch displays. But with the
present approach, the virtual observer window (which is, as noted
above, positioned to be in the Fourier plane of the hologram) need
only be as large as the eye pupil. As a consequence, even LCDs with
a moderate pitch size can be used. And because the reconstructed
object can entirely fill the frustum between the virtual observer
window and the hologram, it can be very large indeed, i.e. much
larger than the periodicity interval. Further, where an OASLM is
used, then there is no pixelation, and hence no periodicity, so
that the constraint of keeping the virtual observer window smaller
than a periodicity interval no longer applies.
[0163] There is another advantage as well, deployed in one variant.
When computing a hologram, one starts with one's knowledge of the
reconstructed object--e.g. you might have a 3D image file of a
racing car. That file will describe how the object should be seen
from a number of different viewing positions. In conventional
holography, the hologram needed to generate a reconstruction of the
racing car is derived directly from the 3D image file in a
computationally intensive process. But the virtual observer window
approach enables a different and more computationally efficient
technique. Starting with one plane of the reconstructed object, we
can compute the virtual observer window as this is the Fresnel
transform of the object. We then perform this for all object
planes, summing the results to produce a cumulative Fresnel
transform; this defines the wave field across the virtual observer
window. We then compute the hologram as the Fourier transform of
this virtual observer window. As the virtual observer window
contains all the information of the object, only the single-plane
virtual observer window has to be transformed to the hologram and
not the multi-plane object. This is particularly advantageous if
there is not a single transformation step from the virtual observer
window to the hologram but an iterative transformation like the
Iterative Fourier Transformation Algorithm. Each iteration step
comprises only a single Fourier transformation of the virtual
observer window instead of one for each object plane, resulting in
significantly reduced computation effort.
[0164] Another interesting consequence of the virtual observer
window approach is that all the information needed to reconstruct a
given object point is contained within a relatively small section
of the hologram; this contrasts with conventional holograms in
which information to reconstruct a given object point is
distributed across the entire hologram. Because we need encode
information into a substantially smaller section of the hologram,
that means that the amount of information we need to process and
encode is far lower than for a conventional hologram. That in turn
means that conventional computational devices (e.g. a conventional
digital signal processor (DSP) with cost and performance suitable
for a mass market device) can be used even for real time video
holography.
[0165] There are some less than desirable consequences however.
First, the viewing distance from the hologram is important--the
hologram is encoded and illuminated in such a way that only when
the eyes are positioned at the Fourier plane of the hologram is the
optimal reconstruction seen; whereas in normal holograms, the
viewing distance is not important. There are however various
techniques for reducing this Z sensitivity or designing around it,
and in practice the Z sensitivity of the holographic reconstruction
is usually not extreme.
[0166] Also, because the hologram is encoded and illuminated in
such a way that optimal holographic reconstructions can only be
seen from a precise and small viewing position (i.e. precisely
defined Z, as noted above, but also X and Y co-ordinates), eye
tracking may be needed. As with Z sensitivity, various techniques
for reducing the X, Y sensitivity or designing around it exist. For
example, as pixel pitch decreases (as it will with LCD
manufacturing advances), the virtual observer window size will
increase. Furthermore, more efficient encoding techniques (like
Kinoform encoding) facilitate the use of a larger part of the
periodicity interval as virtual observer window and hence the
increase of the virtual observer window.
[0167] The above description has assumed that we are dealing with
Fourier holograms. The virtual observer window is in the Fourier
plane of the hologram, i.e. in the image plane of the light source.
As an advantage, the undiffracted light is focused in the so-called
DC-spot. The technique can also be used for Fresnel holograms where
the virtual observer window is not in the image plane of the light
source. However, care must be taken that the undiffracted light is
not visible as a disturbing background. Another point to note is
that the term transform should be construed to include any
mathematical or computational technique that is equivalent to or
approximates to a transform that describes the propagation of
light. Transforms are merely approximations to physical processes
more accurately defined by Maxwellian wave propagation equations;
Fresnel and Fourier transforms are second order approximations, but
have the advantages that (a) because they are algebraic as opposed
to differential, they can be handled in a computationally efficient
manner and (ii) can be accurately implemented in optical
systems.
[0168] Further details are given in US patent application
2006-0138711, US 2006-0139710 and US 2006-0250671, the contents of
which are incorporated by reference.
APPENDIX II
Glossary of Terms Used in the Description
[0169] Computer Generated Hologram
[0170] A computer generated video hologram CGH is a hologram that
is calculated from a scene. The CGH may comprise complex-valued
numbers representing the amplitude and phase of light waves that
are needed to reconstruct the scene. The CGH may be calculated e.g.
by coherent ray tracing, by simulating the interference between the
scene and a reference wave, or by Fourier or Fresnel trans
form.
[0171] Encoding
[0172] Encoding is the procedure in which a spatial light modulator
(e.g. its constituent cells, or contiguous regions for a continuous
SLM like an OASLM) are supplied with control values of the video
hologram. In general, a hologram comprises of complex-valued
numbers representing amplitude and phase.
[0173] Encoded Area
[0174] The encoded area is typically a spatially limited area of
the video hologram where the hologram information of a single scene
point is encoded. The spatial limitation may either be realized by
an abrupt truncation or by a smooth transition achieved by Fourier
transform of a virtual observer window to the video hologram.
[0175] Fourier Transform
[0176] The Fourier transform is used to calculate the propagation
of light in the far field of the spatial light modulator. The wave
front is described by plane waves.
[0177] Fourier Plane
[0178] The Fourier plane contains the Fourier transform of the
light distribution at the spatial light modulator. Without any
focusing lens the Fourier plane is at infinity. The Fourier plane
is equal to the plane containing the image of the light source if a
focusing lens is in the light path close to the spatial light
modulator.
[0179] Fresnel Transform
[0180] The Fresnel transform is used to calculate the propagation
of light in the near field of the spatial light modulator. The wave
front is described by spherical waves. The phase factor of the
light wave comprises a term that depends quadratically on the
lateral coordinate.
[0181] Frustum
[0182] A virtual frustum is constructed between a virtual observer
window and the SLM and is extended behind the SLM. The scene is
reconstructed inside this frustum. The size of the reconstructed
scene is limited by this frustum and not by the periodicity
interval of the SLM.
[0183] Imaging Optics
[0184] Imaging optics are one or more optical components such as a
lens, a lenticular array, or a microlens array used to form an
image of a light source (or light sources). References herein to an
absence of imaging optics imply that no imaging optics are used to
form an image of the one or two SLMs as described herein at a plane
situated between the Fourier plane and the one or two SLMs, in
constructing the holographic reconstruction.
[0185] Light System
[0186] The light system may include either of a coherent light
source like a laser or a partially coherent light source like a
LED. The temporal and spatial coherence of the partially coherent
light source has to be sufficient to facilitate a good scene
reconstruction, i.e. the spectral line width and the lateral
extension of the emitting surface have to be sufficiently
small.
[0187] Virtual Observer Window (VOW)
[0188] The virtual observer window is a virtual window in the
observer plane through which the reconstructed 3D object can be
seen. The VOW is the Fourier transform of the hologram and is
positioned within one periodicity interval in order to avoid
multiple reconstructions of the object being visible. The size of
the VOW has to be at least the size of an eye pupil. The VOW may be
much smaller than the lateral range of observer movement if at
least one VOW is positioned at the observer's eyes with an observer
tracking system. This facilitates the use of a SLM with moderate
resolution and hence small periodicity interval. The VOW can be
imagined as a keyhole through which the reconstructed 3D object can
be seen, either one VOW for each eye or one VOW for both eyes
together.
[0189] Periodicity Interval
[0190] The CGH is sampled if it is displayed on a SLM composed of
individually addressable cells. This sampling leads to a periodic
repetition of the diffraction pattern. The periodicity interval is
.lamda.D/p, where .lamda. is the wavelength, D the distance from
the hologram to the Fourier plane, and p the pitch of the SLM
cells. OASLMs however have no sampling and hence there is no
periodic repetition of the diffraction pattern; the repetitions are
in effect suppressed.
[0191] Reconstruction
[0192] The illuminated spatial light modulator encoded with the
hologram reconstructs the original light distribution. This light
distribution was used to calculate the hologram. Ideally, the
observer would not be able to distinguish the reconstructed light
distribution from the original light distribution. In most
holographic displays the light distribution of the scene is
reconstructed. In our display, rather the light distribution in the
virtual observer window is reconstructed.
[0193] Scene
[0194] The scene that is to be reconstructed is a real or computer
generated three-dimensional light distribution. As a special case,
it may also be a two-dimensional light distribution. A scene can
constitute different fixed or moving objects arranged in a
space.
[0195] Spatial Light Modulator (SLM)
[0196] A SLM is used to modulate the wave front of the incoming
light. An ideal SLM would be capable of representing arbitrary
complex-valued numbers, i.e. of separately controlling the
amplitude and the phase of a light wave. However, a typical
conventional SLM controls only one property, either amplitude or
phase, with the undesirable side effect of also affecting the other
property.
* * * * *