U.S. patent application number 10/796071 was filed with the patent office on 2004-09-09 for system and method for replicating volume holograms.
Invention is credited to Brandelik, Donna M., Pogue, Robert T., Sappington, John, Shepherd (nee Wendel), Christina K., Siwecki, Stephen A., Sutherland, Richard L..
Application Number | 20040175627 10/796071 |
Document ID | / |
Family ID | 24307532 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040175627 |
Kind Code |
A1 |
Sutherland, Richard L. ; et
al. |
September 9, 2004 |
System and method for replicating volume holograms
Abstract
The present invention offers increased efficiency and quality in
the duplication of a master hologram utilizing an improved method
of contact printing. This improved method of contact printing
employs a polymer-dispersed liquid crystal (PDLC) recording medium
as the duplication blank and/or the master hologram material. The
optical qualities of the PDLC material described herein provide an
improved method of duplication using single beam contact printing
regardless of the material comprising the master hologram. Thus,
master holograms originally recorded using highly complex optical
geometries (e.g., computer generated holograms) are capable of
duplication without the need for multiple beam power/intensity
balancing and long recording times. The improved hologram contact
printing method described herein works with virtually any type of
master hologram, including both reflection and transmission
holograms.
Inventors: |
Sutherland, Richard L.;
(Dayton, OH) ; Sappington, John; (West Alexandria,
OH) ; Brandelik, Donna M.; (New Carlisle, OH)
; Siwecki, Stephen A.; (Dayton, OH) ; Shepherd
(nee Wendel), Christina K.; (Dayton, OH) ; Pogue,
Robert T.; (Springboro, OH) |
Correspondence
Address: |
KILPATRICK STOCKTON LLP
607 14TH STREET, N.W.
SUITE 900
WASHINGTON
DC
20005
US
|
Family ID: |
24307532 |
Appl. No.: |
10/796071 |
Filed: |
March 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10796071 |
Mar 10, 2004 |
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09577166 |
May 24, 2000 |
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6730442 |
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Current U.S.
Class: |
430/1 |
Current CPC
Class: |
G03H 2250/33 20130101;
G03H 1/28 20130101; G03H 1/181 20130101; G03H 2001/2231 20130101;
G03H 2001/0413 20130101; G03H 2001/266 20130101; G03H 1/182
20130101; G03H 2240/52 20130101; G03H 2001/264 20130101; G03H
2001/2234 20130101; G03H 2001/2615 20130101; G03H 1/0248 20130101;
G03H 2001/0415 20130101; G03H 2001/2271 20130101; G03H 1/202
20130101; G02F 1/13342 20130101 |
Class at
Publication: |
430/001 |
International
Class: |
G03H 001/04; G03H
001/20 |
Claims
1. A system for duplicating a hologram comprising: a radiation
source for emitting a coherent beam of radiation; a hologram; and a
recording substrate comprised of a polymer-dispersed liquid crystal
material for recording a replica of the hologram therein, wherein
the hologram and the recording substrate are in optical contact
with one another and are placed in a path of the coherent beam of
radiation.
2. The system according to claim 1, wherein the polymer-dispersed
liquid crystal material is comprised of: (a) a polymerizable
monomer comprising at least one acrylate; (b) at least one type of
liquid crystal material; (c) a chain-extending monomer; (d) a
coinitiator; and (e) a photoinitiator.
3. The system according to claim 2, wherein the polymerizable
monomer comprises a mixture of di-, tri-, and penta-acrylates
4. The system according to claim 2, wherein the polymerizable
monomer is at least one acrylate selected from the group consisting
of triethyleneglycol diacrylate, trimethylolpropane triacrylate,
pentaerythritol triacrylate, pentaerythritol tetracrylate, and
dipentaerythritol penta-acrylate. comprises a mixture of tri- and
penta-acrylates.
6. The system according to claim 2, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate.
7. The system according to claim 1, wherein the polymer-dispersed
liquid crystal material further comprises a surfactant.
8. The system according to claim 7, wherein the surfactant is
octanoic acid.
9. The system according to claim 2, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate, the at least one
liquid crystal material comprises a mixture of cyanobiphenyls, the
chain-extending monomer is N-vinyl pyrrolidone, the coinitiator is
N-phenylglycine, and the photoinitiator is rose bengal.
10. The system according to claim 1, wherein the radiation source
is a laser.
11. The system according to claim 1, wherein a diffraction
efficiency of the hologram is continuously variable.
12. A method for duplicating a hologram comprising: directing a
coherent incident radiation beam at a first optical component;
transmitting the coherent incident radiation beam through the first
optical component forming a transmitted beam, to a second optical
component having a hologram recorded therein; and diffracting the
transmitted beam via the hologram forming a diffracted radiation
beam, wherein the coherent incident radiation beam and the
diffracted beam interfere within the first optical component to
form a replica of the hologram therein.
13. The method for duplicating a hologram according to claim 12,
wherein the first optical component is comprised of a
polymer-dispersed liquid crystal material. crystal material is
comprised of: (a) a polymerizable monomer comprising at least one
acrylate; (b) at least one type of liquid crystal material; (c) a
chain-extending monomer; (d) a coinitiator; and (e) a
photoinitiator.
15. The method according to claim 14, wherein the polymerizable
monomer comprises a mixture of di-, tri-, tetra-, and
penta-acrylates.
16. The method according to claim 14, wherein the polymerizable
monomer is at least one acrylate selected from the group consisting
of triethyleneglycol diacrylate, trimethylolpropane triacrylate,
plentaerytritol triacrylate, pentaerythritol tetracrylate, and
dipentaerythritol penta-acrylate.
17. The method according to claim 14, wherein the polymerizable
monomer comprises a mixture of tri- and pentaacrylates.
18. The method according to claim 14, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate.
19. The method according to claim 14, wherein the polymer-dispersed
liquid crystal material further comprises a surfactant.
20. The method according to claim 19, wherein the surfactant is
octanoic acid.
21. The method according to claim 14, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate, the at least one
liquid crystal material pyrrolidone, the coinitiator is
N-phenylglycine, and the photoinitiator is rose bengal.
22. A method for duplicating a hologram comprising: directing a
coherent radiation beam at a first optical component having a
hologram recorded therein; diffracting a first portion of the
coherent radiation beam via the hologram forming a diffracted
radiation beam; transmitting a second portion of the coherent
radiation beam through the first optical component forming a
transmitted beam; and interfering the diffracted radiation beam
with the transmitted radiation beam within a second optical
component to form a replica of the hologram therein.
23. The method for duplicating a hologram according to claim 22,
wherein the second optical component is comprised of a
polymer-dispersed liquid crystal material.
24. The method according to claim 23, wherein the polymer-dispersed
liquid crystal material is comprised of: (a) a polymerizable
monomer comprising at least one acrylate; (b) at least one type of
liquid crystal material; (c) a chain-extending monomer; (d) a
coinitiator; and (e) a photoinitiator.
25. The method according to claim 24, wherein the polymerizable
monomer comprises a mixture of di-, tri-, tetra-, and
penta-acrylates.
26. The method according to claim 24, wherein the polymerizable
monomer is at least one acrylate selected from the group consisting
of triethyleneglycoldiacrylate, trimethylolpropane triacrylate,
pentaerythritol triacrylate, pentaerythritol tetracrylate, and
dipentaerythritol pentaacrylate.
27. The method according to claim 24, wherein the polymerizable
monomer comprises a mixture of tri- and penta-acrylates.
28. The method according to claim 24, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate.
29. The method according to claim 24, wherein the polymer-dispersed
liquid crystal material further comprises a surfactant.
30. The method according to claim 29, wherein the surfactant is
octanoic acid.
31. The method according to claim 24, wherein the polymerizable
monomer comprises dipentaerythritol pentaacrylate, the at least one
liquid crystal material comprises a mixture of cyanobiphenyls, the
chain-extending monomer is N-vinyl pyrrolidone, the coinitiator is
N-phenylglycine, and the photoinitiator is rose bengal.
32. A method for contact recording at least one hologram
comprising: arranging at least a first master hologram and at least
a first holographic blank in optical contact to form a master/blank
assembly; exposing the master/blank assembly to a pre-recording
beam; and exposing the master/blank assembly to a recording beam,
wherein the master/blank assembly remains optically contacted
throughout each exposure.
33. The method according to claim 32, further comprising exposing
the master/blank assembly to a post-recording beam.
34. The method according to claim 32, wherein a diffraction
efficiency of the first master hologram is continuously
variable.
35. The method according to claim 34, wherein the continuously
variable diffraction efficiency of the first master hologram
includes at least the following two states, ON and OFF.
36. The method according to claim 32, wherein the first master
hologram is formed of a polymer-dispersed liquid crystal
material.
37. The method according to claim 35, wherein the continuously
variable first master hologram is switched OFF during exposure of
the master/blank assembly to the pre-recording beam and the first
master hologram is switched ON during exposure of the master/blank
assembly to the recording beam, thereby forming a first replica of
the first master hologram in the first holographic blank.
38. The method according to claim 37, wherein the first master
hologram is switched OFF during exposure of the master/blank
assembly to the post-recording beam.
39. The method according to claim 33, wherein the pre-recording
beam, the recording beam, and the post-recording beam are the same
beam.
40. The method according to claim 33, wherein of the pre-recording
beam, the recording beam, and the post recording beam at least one
is different from the others.
41. The method according to claim 37, wherein a diffraction
efficiency of the first replica is continuously variable.
42. The method according to claim 41, wherein the continuously
variable diffraction efficiency of the first replica includes at
least the following two states, ON and OFF.
43. The method according to claim 37, wherein the first replica is
formed of a polymer-dispersed liquid crystal material.
44. The method according to claim 42, wherein the master/blank
assembly further includes a second master hologram and a second
holographic blank in optical contact and the first master hologram
and the first replica are switched OFF during each of the
following, exposure of the second holographic blank to a
pre-recording beam, recording of the second master hologram in the
second holographic blank, and exposure of a resulting second
replica to a post-recording beam.
45. The method according to claim 44, wherein the first master
hologram and the second master hologram are the same master
hologram.
46. A method for contact recording at least one hologram
comprising: arranging at least a first master hologram and at least
first holographic blank in optical contact to form a master/blank
assembly; exposing the master/blank assembly to a recording beam;
and exposing the master/blank assembly to a post-recording beam,
wherein the master/blank assembly remains optically contacted
throughout each exposure.
47. A system for contact recording multiple holograms comprising: a
first, second, and third master hologram; a first, second, and
third holographic blank wherein the first, second, and third master
hologram and the first, second, and third holographic blanks are in
optical contact, forming a stack; and a first, second, and third
recording beam, wherein when the first recording beam is incident
upon the stack, the first master hologram is ON and the second and
third master holograms are OFF, forming a first replica hologram of
the first master hologram in the first holographic blank; when the
second recording beam is incident on the stack, the first and third
master holograms are OFF, the first replica hologram is OFF, and
the second master hologram is ON, forming a second replica hologram
of the second master hologram in the second holographic blank; when
the third recording beam is incident on the stack, the first and
second master holograms are OFF, the first and second replica
holograms are OFF, and the third master hologram is ON, forming a
third replica hologram of the third master hologram in the third
holographic blank.
48. A method for contact printing multiple master holograms
comprising: providing a stack comprised of first, second, and third
master holograms and first, second, and third holographic blanks
that are in optical contact; switching ON the first master
hologram; exposing the stack with a first recording beam, forming a
first replica hologram within the first holographic blank;
switching OFF the first master hologram and switching ON the second
master hologram; exposing the stack with a second recording beam,
forming a second replica hologram within the second holographic
blank; switching OFF the second master hologram and switching ON
the third master hologram; and exposing the stack with a third
recording beam, forming a third replica hologram within the third
holographic blank.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention pertains to the use of contact holography to
form multiple holograms and, more particularly, to the use of
contact holography and a master hologram to make replica gratings,
lenses, switches and other images wherein one or both of the master
hologram and corresponding replica comprise a polymer-dispersed
liquid crystal (PDLC) material.
[0003] 2. Description of the Related Art
[0004] Once a hologram has been recorded, whether using simple or
complex optical geometries, it is often desired to reproduce or
reconstruct the hologram so as to have multiple copies which are
substantially identical to the originally recorded hologram. There
are numerous methods for reconstructing holograms, however these
are cumbersome and involve retracing the steps used to create the
original or master hologram. Unfortunately, where complex
geometries are involved, this is neither an efficient nor a
practical method for performing mass reconstruction.
[0005] By way of example, U.S. Pat. No. 3,580,655 to Leith
("Leith") which is incorporated herein by reference, sets forth
multiple methods both for formation of the master hologram and for
reconstruction of the master hologram, either for viewing or for
permanent recording. While the subject matter of the current
invention is not centered on the formation of the master hologram,
the advantages of the current invention are readily apparent when
the complexity of this initial formation is recognized. For
example, FIG. 1 (FIG. 7 of Leith) illustrates one of the simplest
geometries for forming a master hologram. This simple configuration
illustrates the basic components for simple holographic
construction, including a coherent light source 10 emitting an
incident beam 12. From this incident beam 12, two separate beams
are formed. A prism 14 or similar light-splitting or directing
device intercepts part of the incident beam 12 and directs a
reference beam 16 to a detector plate 18. Simultaneously, a part of
the incident beam is diffused by a diffusion screen 20 and
diffracts off an object 22, forming an object beam 24, which also
passes onto the detector plate 18. The interaction between the
reference beam 16 and the object beam 24 produces an off-axis
hologram, in the form of multiple Fresnel patterns and interference
fringes.
[0006] Further in Leith, there is a method and system for using the
master hologram from FIG. 1 to produce replicas of the master
hologram. FIG. 2a represents the simplest system and method for
duplicating the master hologram. Referring to prior art FIG. 2a,
there is an incident beam 12 from a coherent light source 10 which
forms two separate beams, a reference beam 16 and an object beam
24. In this case, object beam 24 results from the interaction of
part of incident beam 12 with the master hologram 26. Due to the
grating effect of the master hologram 26, the object beam is
directed along the formation angle and a detector 18 is placed at
the intersection of the reference beam 16 and the object beam 24,
forming a replica of the interference pattern comprising master
hologram 26. In this case, the object beam forms a virtual image of
master hologram 26 which is recorded on the detector 18. The real
image is not used in the reproduction process.
[0007] As is clear to one skilled in the art, this method of master
hologram duplication, while viable, results in a number of
disadvantages. In any situation involving light traveling through
an optical train, there is the potential for misalignment of the
optical elements. Further there are inherent efficiency limits for
each optical element. These disadvantages can result in unwanted
diffraction, reflection, and in some cases aberration of the beams.
Additionally, while lasers have improved coherence parameters,
coherence length remains an issue. Even the simplest dual beam
recording and duplication systems described above require precise
alignment for optimal results. The conventional systems above also
require multiple optical elements even for the simplest holographic
formation geometries. Consequently, complex geometry hologram
formation is not available with the Leith system because of the
length and requisite multiple components of the optical train.
[0008] The prior art also contemplates a single beam master
hologram duplication system that greatly reduces the number of
necessary optical components. Referred to as contact printing, this
system for duplicating a master hologram resembles in many respects
the art of photography. The master hologram and a holographic
detection plate (e.g., emulsion plate) are placed in optical
contact with one another and exposed to light. Photographic
development of the holographic detection plate results in a replica
master hologram. For a fully successful reproduction, the optical
contact between the master hologram and the holographic detection
plate must be such that there is no loss of resolution within the
interference fringes. Establishing the requisite optical contact
has proved to be a significant limiting factor in attempts to use
contact printing for duplication of holograms. Consequently, the
prior art single-beam contact printing method, though it reduces
the number of optical elements necessary for duplication of a
master hologram, poses new optical hurdles to the art of hologram
replication.
[0009] Referring to FIG. 2b, a prior art single beam contact
printing system is illustrated in accordance with U.S. Pat. No.
5,547,786 to Brandstetter, et al. ("Brandstetter"), the
specification of which is incorporated herein by reference. The
system of Brandstetter includes a source of monochromatic,
collimated light of substantially fixed wavelength such as laser 10
which produces an output beam 12, referred to as the replication or
recording beam, and directs that beam through beam conditioning
means 80, which preferably comprises lenses 82 and 84, pinhole 86,
and filter 88. Lenses 82 and 84 and pinhole 86 are provided to
collimate beam 12 and to expand that beam to the desired size
filter 88 is provided to control or adjust the intensity or
amplitude of beam 12 across its profile as desired. Subsequent to
conditioning by means 80, the conditioned beam 12 is directed at a
desired angle onto master holographic optical element 26, passes
through, and directly enters a phase recording medium 18, such as a
photopolymer layer that has been applied onto the backside of the
master holographic optical element.
[0010] The method for forming the replica within the photopolymer
layer requires a polymerization step which is separate from the
recording step. Further, the resulting replica hologram is not
switchable. Further, the recording mediums currently available as
blanks for hologram duplication are limited in their ability to
provide optimal optical contact with the master hologram.
[0011] Accordingly, there remains a need for a system and method
for mass reproduction of holograms, having a single beam contact
printing method using an optically superior recording medium.
[0012] In conventional contact holography methods and systems,
situations exist wherein the use of a static, as opposed to a
switchable, master hologram is limiting. First, a static hologram
is limited to a single diffraction efficiency, which is always ON
(i.e., it cannot be turned OFF). Second, even though a
non-recording wavelength theoretically should pass through the
static hologram without causing recording in the blank, in practice
this is not the case. Instead, a non-recording, incoherent
wavelength passing through a static master may result in unwanted
scattering and cross-coupling of phase information which can
decrease diffraction efficiency, introduce cross-gratings, increase
haze, and generally decrease the signal-to-noise properties of the
replicated grating. These limitations of the static master hologram
result in difficulties with contact recording schemes that require
either in situ pre-recording or post-recording irradiation of the
blank.
[0013] Accordingly, a need remains for a non-static master hologram
for use in a contact printing method and system.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention offers increased efficiency and
quality in the duplication of a master hologram utilizing an
improved method and system of contact printing. A first embodiment
of an improved method and system of contact printing employs a
polymer-dispersed liquid crystal (PDLC) recording medium as the
duplication blank. The optical qualities of the PDLC material
described herein provide an improved method of duplication using
single beam contact printing regardless of the material comprising
the master hologram. Thus, master holograms originally recorded
using highly complex optical geometries (e.g., computer generated
holograms) are capable of duplication without the need for multiple
beam power/intensity balancing and long recording times. The
improved hologram contact printing method and system described
herein works with virtually any type of master hologram, including
both reflection and transmission holograms.
[0015] A first embodiment of the present invention describes a
system for duplicating a hologram which includes a radiation source
for emitting a coherent beam of radiation, a hologram, and a
recording substrate comprised of a polymer-dispersed liquid crystal
material for recording a replica of the hologram therein. The
components of the system are arranged such that the hologram and
the recording substrate are in optical contact with one another and
they are placed in a path of the coherent beam of radiation.
[0016] A second embodiment of the present invention describes a
method for duplicating a hologram which includes the following
steps of (1) directing a coherent incident radiation beam at a
first optical component; (2) transmitting the coherent incident
radiation beam through the first optical component forming a
transmitted beam, to a second optical component having a hologram
recorded therein; and (3) diffracting the transmitted beam via the
hologram forming a diffracted radiation beam. The incident beam and
the diffracted beam interfere within the first optical component to
form a replica of the hologram therein.
[0017] A third embodiment of the present invention describes a
method for contact recording at least one hologram which includes
the following steps of: (1) directing a coherent radiation beam at
a first optical component having a hologram recorded therein and
(2) diffracting a first portion and transmitting a second portion
of the coherent radiation beam through the first optical component
to a second optical component. The transmitted beam and the
diffracted beam interfere within the second optical component to
form a replica of the hologram therein.
[0018] A fourth embodiment of the present invention describes a
method for contact recording at least one hologram which includes
the following steps of: (1) optically contacting at least one
master hologram to at least one holographic blank to form a
master/blank assembly; (2) exposing the master/blank assembly to a
pre-recording beam; (3) exposing the master/blank assembly to a
recording beam; and (4) exposing the master/blank assembly to a
post-recording beam, wherein the master/blank assembly remains
optically contacted throughout each exposure.
[0019] A fifth embodiment of the present invention describes a
method for contact recording at least one hologram which includes
the following steps: (1) optically contacting at least one master
hologram to at least one holographic blank to form a master/blank
assembly; (2) exposing the master/blank assembly to a recording
beam; and (3) exposing the master/blank assembly to a
post-recording beam, wherein the master/blank assembly remains
optically contacted throughout each exposure.
[0020] A sixth embodiment of the present invention describes a
system for contact recording at least one hologram which includes
at least one master hologram, at least one holographic blank, a
pre-recording beam, and a recording beam, wherein the at least one
master hologram and the at least one holographic blank are in
optical contact during exposure to the pre-recording beam and the
recording beam.
[0021] A seventh embodiment of the present invention describes a
system for contact recording at least one hologram which includes
at least one master hologram, at least one holographic blank, a
recording beam, and a post-recording beam, wherein the at least one
master hologram and the at least one holographic blank are in
optical contact during exposure to the recording beam and the
post-recording beam.
[0022] An eighth embodiment of the present invention describes a
system for contact recording at least one hologram which includes
at least one master hologram, at least one holographic blank, a
pre-recording beam, a recording beam, and a post-recording beam,
wherein the at least one master hologram and the at least one
holographic blank are in optical contact during exposure to the
pre-recording beam, the recording beam, and the post-recording
beam.
BRIEF DESCRIPTION OF THE FIGURES
[0023] In the drawings:
[0024] FIG. 1 is a schematic view of a conventional system for
forming a master transmission hologram;
[0025] FIG. 2a is a schematic view of a conventional system for
forming a replica of the master hologram of FIG. 1;
[0026] FIG. 2b is a schematic view of a conventional system for
forming a replica hologram via a contact printing method;
[0027] FIG. 3 is a schematic view of a system according to an
embodiment of the present invention for forming a replica hologram
in a PDLC blank from a master transmission hologram;
[0028] FIG. 4 is a schematic view of a system according to an
embodiment of the present invention for forming a replica hologram
in a PDLC blank from a master reflection hologram;
[0029] FIG. 5 is a schematic view of a PDLC blank according to an
embodiment of the present invention for forming a replica hologram
therein;
[0030] FIG. 6 is a schematic view of a recording system for forming
a transmission hologram according to the present invention;
[0031] FIGS. 7a and 7b are elevational views of a reflection
grating in accordance with the present invention having planes of
polymer channels and liquid crystal channels disposed parallel to
the front surface, in the absence of a field (FIG. 7a) and with an
electric field applied (FIG. 7b), wherein the liquid crystal
utilized in the formation of the grating has a positive dielectric
anisotropy;
[0032] FIG. 8a and 8b are elevational views of a reflection grating
in accordance with the invention having planes of polymer channels
and liquid crystal channels disposed parallel to the front surface
of the grating, in the absence of an electric field (FIG. 8a) and
with an electric field applied (FIG. 8b), wherein the liquid
crystal utilized in the formation of the grating has a negative
dielectric anisotropy;
[0033] FIG. 9a, 9b, 9c, 9d, 9e, and 9f are pre-recording, exposure,
and post-recording views of a contact printing system incorporating
a switchable H-PDLC master according to an embodiment of the
present invention;
[0034] FIG. 10a and 10b are reflection and transmission exposure
views wherein a switchable H-PDLC master is partially switched
according to an embodiment of the present invention;
[0035] FIG(s). 11(a)-(b) are multiple beam, multiple master,
multiple blank exposure views utilizing a reflective and
transmissive switchable H-PDLC master, according to embodiments of
the present invention;
[0036] FIG(s). 12(a)-(b) are multiple beam, multiple master,
multiple blank exposure views utilizing reflective and transmissive
switchable H-PDLC masters respectively, to form an RGB stacked
replica, according to embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] Referring to FIGS. 3 and 4, the improved contact printing
method and systems described herein comprise the following basic
components: a coherent light source 31 emitting an incident beam
33, a master hologram 35 emitting diffracted beam 34 and
transmitted beam 36, a polymer-dispersed liquid crystal ("PDLC")
blank 37 for recording a replica of the master hologram, and,
optionally, an absorption filter 39. As discussed further, the
order and placement of these elements with respect to one another,
vary depending on whether the master hologram is transmissive or
reflective.
[0038] The PDLC materials described herein may be used in both the
PDLC blank 37 and the master hologram 35. However, neither the
master hologram 35 nor the blank 37 are limited to this PDLC
material. The master hologram 35 may be formed on a conventional
photographic plate or similar emulsion-type recording medium, or by
computer generation, for example. Similarly, the blank 37 may be
formed of an appropriate photosensitive material as determined by
one skilled in the art.
[0039] In accordance with an embodiment of the present invention
there is provided a blank recording or master hologram medium
comprised of a PDLC material comprising a monomer, a liquid crystal
material, a cross-linking monomer, a coinitiator and a
photoinitiator dye. These PDLC materials exhibit clear and orderly
separation of the liquid crystal and cured polymer, whereby the
PDLC material advantageously provides high quality holographic
gratings. The PDLC materials of the present invention are also
advantageously formed in a single step. The present invention also
utilizes a unique photopolymerizable prepolymer material that
permits in situ control of significant characteristics of the
resulting gratings, such as domain size, shape, density, ordering,
and the like. Furthermore, the methods and materials of the present
invention can be used to prepare PDLC materials that function as
switchable transmission or reflection holograms or holographic
gratings.
[0040] Polymer-dispersed liquid crystal materials, methods, and
devices contemplated for use in the practice of the present
invention are also described in R. L. Sutherland et al., "Bragg
Gratings in an Acrylate Polymer Consisting of Periodic
Polymer-Dispersed Liquid-Crystal Planes," Chemistry of Materials,
No. 5, pp. 1533-1538 (1993) ("Chemistry of Materials"); R. L.
Sutherland, et al., "Electrically Switchable Volume Gratings in
Polymer-Dispersed Liquid Crystals," Applied Physics Letters, Vol.
64, No. 9, pp. 1074-1076 (1984); and T. J. Bunning, et al., "The
Morphology and Performance of Holographic Transmission Gratings
Recorded in Polymer-Dispersed Liquid Crystals," Polymer, Vol. 36,
No. 14, pp. 2699-2708 (1995), all of which are fully incorporated
by reference into this specification.
[0041] The PDLC material employed in the practice of the present
invention creates a switchable hologram in a single step. A new
feature of a preferred PDLC material is that illumination by an
inhomogeneous, coherent light pattern initiates a patterned,
anisotropic diffusion (or counter-diffusion) of polymerizable
monomer and second phase material, particularly liquid crystal.
Thus, alternating well-defined channels of second phase-rich
material, separated by well-defined channels of nearly pure
polymer, are produced in a single-step process.
[0042] A resulting preferred PDLC material has an anisotropic
spatial distribution of phase-separated liquid crystal droplets
within the photochemically cured polymer matrix. Conventional PDLC
materials made by a single-step process can achieve at best only
regions of larger liquid crystal droplets and smaller liquid
crystal droplets in a polymer matrix. The large bubble sizes are
highly scattering, producing a hazy appearance and multiple order
diffractions, in contrast to the well-defined first order
diffraction and zero order diffraction resulting from the small
liquid crystal droplets of the preferred PDLC material in
well-defined channels of liquid crystal-rich material. Reasonably
well-defined alternately liquid crystal-rich channels and nearly
pure polymer channels in a PDLC material are possible by multi-step
processes, but such processes do not achieve the precise control of
morphology over liquid crystal droplet size and distribution of
size and width of the polymer and liquid crystal-rich channels made
possible by a preferred PDLC material.
[0043] The features of the PDLC material are influenced by the
components used in the preparation of the homogeneous starting
mixture and, to a lesser extent, by the intensity of the incident
light pattern. In a preferred embodiment, the prepolymer material
comprises a mixture of a photopolymerizable monomer, a second phase
material, a photoinitiator dye, a coinitiator, a chain extender (or
cross-linker), and, optionally, a surfactant.
[0044] In a preferred embodiment, the two major components of the
prepolymer mixture are the polymerizable monomer and the second
phase material, which are preferably completely miscible. Highly
functionalized monomers are preferred because they form densely
cross-linked networks which shrink to some extent and tend to
squeeze out the second phase material. As a result, the second
phase material is moved anisotropically out of the polymer region
and, thereby, separated into well-defined, polymer-poor, second
phase-rich regions or domains. Highly functionalized monomers are
also preferred because the extensive cross-linking associated with
such monomers yields fast kinetics, allowing the hologram to form
relatively quickly, whereby the second phase material will exist in
domains of less than approximately 0.1 .mu.m.
[0045] Highly functionalized monomers, however, are relatively
viscous. As a result, these monomers do not tend to mix well with
other materials, and they are difficult to spread into thin films.
Accordingly, it is preferable to utilize a mixture of
pentaacrylates in combination with di-, tri-, and/or
tetra-acrylates in order to optimize both the functionality and
viscosity of the prepolymer material. Suitable acrylates, such as
triethyleneglycol diacrylate, trimethylolpropane triacrylate,
pentaerythritol triacrylate, pentaerythritol tetraacrylate,
pentaerythritol pentaacrylate, and the like can be used in the
present invention. In a preferred embodiment, it has been found
that an approximately 1:4 mixture of tri- to penta-acrylate
facilitates homogeneous mixing, while providing a favorable mixture
for forming 1-100 .mu.m thin films on the optical plates.
[0046] The second phase material of choice for use in the practice
of the present invention is a liquid crystal. This also allows an
electro-optical response in the resulting hologram. The
concentration of liquid crystal employed should be large enough to
allow a significant phase separation to occur in the cured sample,
but not so large as to make the sample opaque or very hazy. Below
about 20% by weight very little phase separation occurs and
diffraction efficiencies are low. Above about 35% by weight, the
sample becomes highly scattering, reducing both diffraction
efficiency and transmission. Samples fabricated with approximately
25% by weight of liquid crystal typically yield good diffraction
efficiency and optical clarity. In prepolymer mixtures utilizing a
surfactant, the concentration of liquid crystal may be increased to
35% by weight without loss in optical performance by adjusting the
quantity of surfactant. Suitable liquid crystals contemplated for
use in the practice of the present invention include the mixture of
cyanobiphenyls marketed as E7 by Merck,
4'-n-pentyl-4-cyanobiphenyl, 4'-n-heptyl-4-cyanobiphenyl,
4'-octaoxy-4-cyanobiphenyl,
4'-pentyl-4-cyanoterphenyl,4-methoxybenzylide- ne-4'-butylaniline,
and the like. Other second phase components are also possible.
[0047] By way of example, a polymer-dispersed liquid crystal
material employed in the practice of the present invention is
formed from a prepolymer material that is a homogeneous mixture of
a polymerizable monomer comprising dipentaerythritol
hydroxypentaacrylate (available, for example, from Polysciences,
Inc., Warrington, Pa.), approximately 10-40% by weight of the
liquid crystal E7 (which is a mixture of cyanobiphenyls marketed as
E7 by Merck and also available from BDH Chemicals, Ltd., London,
England), the chain-extending monomer N-vinylpryrrolidone("NVP")
(available from the Aldrich Chemical Company, Milwaukee, Wis.),
coinitiator N-phenylgylycine ("NPG") (also available from the
Aldrich Chemical Company, Milwaukee, Wis.), and the photoinitiator
dye rose bengal ester;
(2,4,5,7-tetraiodo-3',4',5',6'-tetrachlorofluroescein-6-ace- tate
ester) marketed as RBAX by Spectragraph, Ltd., Maumee, Ohio). Rose
bengal is also available as rose bengal sodium salt (which must be
esterfied for solubility) from the Aldrich Chemical Company. This
system has a very fast curing speed which results in the formation
of small liquid crystal micro-droplets.
[0048] A preferred example of a PDLC duplication blank 38 is
illustrated in FIG. 5. This cross-sectional view of a duplication
blank 38 is formed of a layer 50 of the PDLC material sandwiched
between a pair of indium-tin-oxide ("ITO") coated glass slides 52a
and 52b and spacers 54.
[0049] In an exemplary embodiment, the master hologram 35, as shown
in FIG. 3 and FIG. 4, is formed from PDLC material. In this
embodiment, the interior of master hologram 35 reveals a Bragg
transmission grating 56 (FIG. 6) formed when PDLC material layer 50
is exposed to an interference pattern from two intersecting beams
of coherent laser light. In FIG. 6, there is shown an exemplary
system for recording a master transmission hologram using PDLC
materials of the present invention. A coherent light source 62
(e.g., Ar ion laser) is incident upon a spatial filter 64 and a
collimating lens 66 prior to being divided via a dual slit aperture
68 and impinging upon a prism 70 causing the dual beams to
interfere within the layer of PDLC material 50. Further within this
set-up, similar to FIG. 5, the PDLC material is sandwiched between
layers of ITO glass slides 52a and 52b, separated by spacers 54.
Also, in order to insure optical homogeneity, neutral density
filters 57 are placed before slide 52a and after slide 52b,
separated by an index matching fluid 53. Finally, in order to allow
control of the liquid crystal orientation within the PDLC material,
both during and after formation of the hologram, so as to ascertain
desired Bragg grating angles, electrodes 55 are provided in
electrical contact with the ITO glass slides 52a and 52b.
Similarly, one skilled in the art will appreciate the variations
and additions of reflective material necessary to form a Bragg
reflection grating as opposed to a Bragg transmission grating.
[0050] The PDLC material is a mixture of liquid crystal and
prepolymer material homogenized to form a viscous solution by
suitable means (e.g., ultrasonification) and spread between, for
example, ITO coated glass slides with spacers of nominally 1-100
.mu.m thickness and, preferably, 4-20 .mu.m thickness. The ITO is
electrically conductive and serves as an optically transparent
electrode. Preparation, mixing and transfer of the prepolymer
material onto the glass slides are preferably performed in
conditions outside of the absorbance spectrum of the selected
photoinitiator dye.
[0051] The sensitivity of the prepolymer materials to light is
dependent on the photoinitiator dye and its concentration. Higher
dye concentration leads to higher sensitivity. In most cases,
however, the solubility of the photoinitiator dye limits the
concentration of the dye and, thus, the sensitivity of the
prepolymer material. Nevertheless, it has been found that for most
general applications photoinitiator dye concentrations in the range
of 0.2-0.4% by weight are sufficient to achieve desirable
sensitivities and allow for a complete bleaching of the dye in the
recording process, resulting in colorless final samples.
Photoinitiator dyes that are useful in generating PDLC materials in
accordance with the present invention are rose bengal ester
(2,4,5,7-tetraiodo-3',4',5',6'-te- trachlorofluroescein-6-acetate
ester); rose bengal sodium salt; eosin; eosin sodium salt;
4,5-diiodosuccinyl fluorescein; camphorquinone; methylene blue; and
the like. These dyes allow a sensitivity to recording wavelengths
across the visible spectrum from nominally 400 nm to 700 nm.
Suitable near-infrared dyes, such as cationic cyanine dyes with
trialkylborate anions having absorption from 600-900 nm, as well as
merocyanine dyes derived from spiropyran, should also find utility
in connection with the present invention.
[0052] The coinitiator employed in the practice of the present
invention controls the rate of curing in the free radical
polymerization reaction of the prepolymer material. Optimum phase
separation and, thus, optimum diffraction efficiency in the
resulting PDLC material are a function of curing rate. It has been
found that favorable results can be achieved utilizing the
coinitiator in the range of 2-3% by weight. Suitable coinitiators
include N-phenylglycine; triethylene amine; triethanolamine;
N,N-dimethyl-2,6-diisopropyl aniline; and the like.
[0053] Other suitable dyes and dye-coinitiator combinations that
should be suitable for use in the present invention, particularly
for visible light, include eosin and triethanolamine;
camphorquinone and N-phenyglycine; fluorescein and triethanolamine;
methylene blue and triethanolamine or N-phenylglycine; erythrosin B
and triethanolamine; indolinocarbocyanine and triphenyl borate;
iodobenzospiropyran and triethylamine; and the like.
[0054] The chain extender (or cross-linker) employed in the
practice of the present invention helps to increase the solubility
of the components in the prepolymer material, as well as increase
the speed of polymerization. The chain extender is preferably a
smaller vinyl monomer as compared with the pentaacrylate, whereby
it can react with the acrylate positions in the pentaacrylate
monomer, which are not easily accessible to neighboring
pentaacrylate monomers as a result of steric hindrance. Thus,
reaction of the chain extender monomer with the polymer increases
the propagation length of the growing polymer and results in high
molecular weights. It has been found that a chain extender in the
range of 10-18% by weight maximizes the performance in terms of
diffraction efficiency. In a preferred embodiment, it is expected
that suitable chain extenders can be selected from the following:
N-vinyl pyrrolidone; N-vinyl pyridine; acrylonitrile; N-vinyl
carbazole; and the like.
[0055] It has been found that the addition of a surfactant
material, for example, octanoic acid, in the prepolymer material
lowers the switching voltage and improves the diffraction
efficiency. In particular, the switching voltage for PDLC materials
containing a surfactant are significantly lower than those of a
PDLC material made without the surfactant. While not wishing to be
bound by any particular theory, it is believed that these results
may be attributed to the weakening of the anchoring forces between
the polymer and the phase-separated liquid crystal droplets.
Scanning electron microscopy ("SEM") studies have shown that
droplet sizes in PDLC materials including surfactants are reduced
to the range of 30-50 nm and the distribution is more homogeneous.
Random scattering in such materials is reduced due to the dominance
of smaller droplets, thereby increasing the diffraction efficiency.
Thus, it is believed that the shape of the droplets becomes more
spherical in the presence of surfactant, thereby contributing to
the decrease in switching voltage.
[0056] For more general applications, it has been found that
samples with as low as 5% by weight of surfactant exhibit a
significant reduction in switching voltage. It has also been found
that, when optimizing for low switching voltages, the concentration
of surfactant may vary up to about 10% by weight (dependent mostly
on liquid crystal concentration) after which there is a large
decrease in diffraction efficiency, as well as an increase in
switching voltage (possibly due to a reduction in total phase
separation of liquid crystal). Suitable surfactants include
octanoic acid; heptanoic acid; hexanoic acid; dodecanoic acid;
decanoic acid; and the like.
[0057] In samples utilizing octanoic acid as the surfactant, it has
been observed that the conductivity of the sample is high,
presumably owing to the presence of the free carboxyl (COOH) group
in the octanoic acid. As a result, the sample increases in
temperature when a high frequency (.about.2 KHz) electrical field
is applied for prolonged periods of time.
[0058] Thus, it is desirable to reduce the high conductivity
introduced by the surfactant, without sacrificing the high
diffraction efficiency and the low switching voltages. It has been
found that suitable electrically switchable gratings can be formed
from a polymerizable monomer, vinyl neononanoate ("VN") C.sub.8
H.sub.17 CO.sub.2 CH.dbd.CH.sub.2, commercially available from the
Aldrich Chemical Co. in Milwaukee, Wis. Favorable results have also
been obtained where the chain extender NVP and the surfactant
octanoic acid are replaced by 6.5% by weight VN. VN also acts as a
chain extender due to the presence of the reactive acrylate monomer
group. In these variations, high optical quality samples were
obtained with about 70% diffraction efficiency, and the resulting
gratings could be electrically switched by an applied field of 6
V/.mu.m.
[0059] PDLC materials in accordance with the present invention may
also be formed using a liquid crystalline bifunctional acrylate as
the monomer ("liquid crystal monomer"). The liquid crystal monomers
have an advantage over conventional acrylate monomers due to their
high compatibility with the low molecular weight nematic liquid
crystal materials, thereby facilitating formation of high
concentrations of low molecular weight liquid crystal and yielding
a sample with high optical quality. The presence of higher
concentrations of low molecular weight liquid crystals in the PDLC
material greatly lowers the switching voltages (e.g., to .about.2
V/.mu.m). Another advantage of using liquid crystal monomers is
that it is possible to apply low AC or DC fields while recording
holograms to pre-align the host liquid crystal monomers and the low
molecular weight liquid crystal so that a desired orientation and
configuration of the nematic directors can be obtained in the
liquid crystal droplets. The chemical formulae of several suitable
liquid crystal monomers are as follows:
CH.sub.2.dbd.CH--COO--(CH.sub.2).sub.6O--C.sub.6
H.sub.5--C.sub.6H.sub.5--- COO--CH.dbd.CH.sub.2 I.
CH.sub.2
CH--(CH.sub.2).sub.8--COO--C.sub.6H.sub.5--COO--(CH.sub.2).sub.8--
-CH.dbd.CH.sub.2 II.
H(CF.sub.2).sub.10
CH.sub.2O--CH.sub.2--C(.dbd.CH.sub.2)--COO--(CH.sub.2CH- .sub.2O)
.sub.3CH.sub.2CH.sub.2O--COO--CH.sub.2--C(.dbd.CH.sub.2)--CH.sub.-
2O(CF.sub.2).sub.10 H III.
[0060] Semifluorinated polymers are known to show weaker anchoring
properties and also significantly reduced switching fields. Thus,
it is believed that semifluorinated acrylate monomers, which are
bifunctional and liquid crystalline will find suitable application
in the present invention.
[0061] In a preferred embodiment, the prepolymer material utilized
to make a reflection grating comprises a monomer, a liquid crystal,
a cross-linking monomer, a coinitiator, and a photoinitiator dye.
In a preferred embodiment, the reflection grating is formed from
prepolymer material comprising by total weight the monomer
dipentaerythritol hydroxypentaacrylate ("DPHPA"), 34% by total
weight of a liquid crystal comprising a mixture of cyano biphenyls
(known commercially as "E7"), 10% by total weight of a
cross-linking monomer comprising NVP, 2.5% by weight of the
coinitiator N-phenylglcine ("NPG"), and 10.sup.-5 to 10.sup.-6 gram
moles of a photoinitiator dye comprising rose bengal ester.
Further, as with transmission gratings, the addition of surfactants
is expected to facilitate the same advantageous properties
discussed above in connection with transmission gratings. It is
also expected that similar ranges and variation of prepolymer
starting materials will find ready application in the formation of
suitable reflection gratings.
[0062] It has been determined by low voltage, high resolution
scanning electron microscopy ("LVHRSEM") that the resulting
material comprises a fine grating with a periodicity of 165 nm with
the grating vector perpendicular to the plane of the surface. Thus,
as shown schematically in FIG. 7a, grating 130 includes periodic
planes of polymer channels 130a and PDLC channels 130b which run
parallel to the front surface 134. The grating spacing associated
with these periodic planes remains relatively constant throughout
the full thickness of the sample from the air/film to the
film/substrate interface.
[0063] Although interference is used to prepare both transmission
and reflection gratings, the morphology of the reflection grating
differs significantly. In particular, it has been determined that,
unlike transmission gratings with similar liquid crystal
concentrations, very little coalescence of individual droplets was
evident. Furthermore, the droplets that were present in the
material were significantly smaller, having diameters between 50
and 100 nm. Furthermore, unlike transmission gratings where the
liquid crystal-rich regions typically comprise less than 40% of the
grating, the liquid crystal-rich component of a reflection grating
is significantly larger. Due to the much smaller periodicity
associated with reflection gratings, i.e., a narrower grating
spacing (.about.0.2 microns), it is believed that the time
difference between completion of curing in high intensity regions
versus low intensity regions is much smaller. Thus, gelation occurs
more quickly and droplet growth is minimized. It is also believed
that the fast polymerization, as evidenced by small droplet
diameters, traps a significant percentage of the liquid crystal in
the matrix during gelation and precludes any substantial growth of
large droplets or diffusion of small droplets into larger
domains.
[0064] Analysis of the reflection notch in the absorbance spectrum
supports the conclusion that a periodic refractive index modulation
is disposed through the thickness of the film. For example, given a
PDLC materials formed with a 488 nm line of an argon ion laser, a
resulting reflection notch may have a reflection wavelength at
approximately 472 nm for normal incidence and a relatively narrow
bandwidth. This small difference between the writing wavelength and
the reflection wavelength (approximately 3%) indicates that
shrinkage of the film is not a significant problem under these
exemplary conditions. Further, one skilled in the art recognizes
that multiple write/reflect conditions are easily obtainable
according to desired specifications and manipulations and
adjustments to the recording geometry may also account for
shrinkage, thus minimizing any resulting negative effects.
Moreover, it has been found that the performance of such gratings
is stable over periods of many months.
[0065] In addition to the materials utilized in a preferred
embodiment described above, it is believed that suitable PDLC
materials could be prepared utilizing monomers such as
triethyleneglycol diacrylate, trimethylolpropane triacrylate,
pentaerythritol triacrylate, pentaerythritol tetraacrylate,
pentaerythritbl pentaacrylate, and the like. Similarly, other
coinitiators such as triethylamine, triethanolamine,
N,N-dimethyl-2,6-diisopropylaniline, and the like could be used
instead of N-phenylglycine. Where it is desirable to use the 458
nm, 476 nm, 488 nm or 514 nm lines of an argon ion laser, the
photoinitiator dyes rose bengal sodium salt, eosin, eosin sodium
salt, fluorescein sodium salt and the like will give favorable
results. Where the 633 nm line is utilized, methylene blue will
find ready application. Finally, it is believed that other liquid
crystals, such as 4'-pentyl-4-cyanobiphenyl or
4'-heptyl-4-cyanobiphenyl, can be utilized in accordance with the
invention.
[0066] Referring again to FIG. 7a, there is shown an elevational
view of a reflection grating 130 in accordance with the invention
having periodic planes of polymer channels 130a and PDLC channels
130b disposed parallel to the front surface 134 of the grating 130.
The symmetry axis 136 of the liquid crystal domains is formed in a
direction perpendicular to the periodic channels 130a and 130b of
the grating 130 and perpendicular to the front surface 134 of the
grating 130. Thus, when an electric field E is applied, as shown in
FIG. 7b, the symmetry axis 136 is already in a low energy state in
alignment with the field E and will not reorient. Thus, reflection
gratings formed in accordance with the procedure described above
will not normally be switchable.
[0067] In general, a reflection grating tends to reflect a narrow
wavelength band, such that the grating can be used as a reflection
filter. In a preferred embodiment, however, the reflection grating
is formed so that it will be switchable. In accordance with the
present invention, switchable reflection gratings can be made
utilizing negative dielectric anisotropy liquid crystals (or liquid
crystals with a low cross-over frequency), an applied magnetic
field, an applied shear stress field, or slanted gratings.
[0068] It is known that liquid crystals having a negative
dielectric anisotropy (.DELTA..epsilon.) will rotate in a direction
perpendicular to an applied field. As shown in FIG. 8a, the
symmetry axis 136 of the liquid crystal domains formed with a
liquid crystal having a negative .DELTA..epsilon. will also be
disposed in a direction perpendicular to the periodic channels 130a
and 130b of the grating 130 and to the front surface 134 of the
grating. However, when an electric field E is applied across such
gratings, as shown in FIG. 8b, the symmetry axis of the negative
.DELTA..epsilon. liquid crystal will distort and reorient in a
direction perpendicular to the field E, which is perpendicular to
the film and the periodic planes of the grating. As a result, the
reflection grating can be switched between a state where it is
reflective and a state where it is transmissive. The following
negative .DELTA..epsilon. liquid crystal and others are expected to
find ready application in the methods and devices of the present
invention: 1
[0069] Liquid crystals can be found in nature (or synthesized) with
either positive or negative .DELTA..epsilon.. Thus, in more
detailed aspects of the invention, it is possible to use a liquid
crystal which has a positive .DELTA..epsilon. at low frequencies,
but becomes negative at high frequencies. The frequency (of the
applied voltage) at which .DELTA..epsilon. changes sign is called
the cross-over frequency. The cross-over frequency will vary with
liquid crystal composition, and typical values range from 1-10 kHz.
Thus, by operating at the proper frequency, the reflection grating
may be switched. In accordance with the invention, it is expected
that low cross-over frequency materials can be prepared from a
combination of positive and negative dielectric anisotropy liquid
crystal. A suitable positive dielectric liquid crystal for use in
such a combination contains four ring esters as shown below: 2
[0070] A strongly negative dielectric liquid crystal suitable for
use in such a combination is made up of pyridazines as shown below:
3
[0071] Both liquid crystal materials are available from LaRoche
& Co., Switzerland. By varying the proportion of the positive
and negative liquid crystals in the combination, crossover
frequencies from 1.4-2.3 kHz are obtained at room temperature.
Another combination suitable for use in the present embodiment is a
combination of the following:
p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy) benzoate and
4-(p-pentylbenzoyloxy) benzoate and
p-heptylphenyl-2-chloro-4-(p-octylben- zoyloxy) benzoate. These
materials are available from Kodak.RTM. Company.
[0072] Now that the unique and advantageous optical and electrical
qualities of the PDLC material used herein have been set forth, the
technique for reproducing holograms within this type of PDLC
material will be described.
[0073] Referring once again to FIG. 3 a system 30 for reproducing a
switchable transmission hologram from a master hologram 35 is
shown. For recording a transmission hologram, the master hologram
35 is mounted in the path of an incident beam 33 at the Bragg
angle. A PDLC blank 37 is optically contacted to the master
hologram 35 at the same angle to form a master/blank assembly.
Finally, an absorption filter 30 (e.g., neutral density) may
optionally be placed behind the PDLC blank 37 in order to absorb
extraneous radiation and avoid spurious reflections which may lead
to the formation of unwanted interferograms within the PDLC blank
37. The master/blank assembly of master hologram 35, PDLC blank 37,
and, optionally, absorption filter 30 is then illuminated using a
single coherent incident beam 33 from radiation source 31.
[0074] Utilizing the hologram duplication system of FIG. 3, the
radiation source 31 emits a coherent radiation beam 33 (e.g.,
laser) that is directed towards a first surface of the master
hologram 35. Within master hologram 35, part of incident beam 33 is
diffracted by the holographic grating 56, forming diffracted beam
34, and part of the incident beam 33 is transmitted undiffracted,
forming transmitted beam 36. Both diffracted beam 34 and
transmitted beam 36 pass through the second surface of the master
hologram 35 and into the optically contacted first surface of the
PDLC blank 37. Within the PDLC blank 37, the transmitted beam 36
and the diffracted beam 34 interfere, forming a replica of
holographic grating 56 therein.
[0075] Given a master hologram diffraction efficiency of 50%,
wherein 50% of the incident beam power is diffracted as diffracted
beam 34 and 50% of the incident beam power is transmitted as
transmitted beam 36, a 1:1 beam ratio will result. While 50% is
often an ideal diffraction efficiency in the master for optimal
hologram reproduction, varying diffraction efficiencies will still
result in a replica hologram.
[0076] Referring to FIG. 4, a system 40 for reproducing reflection
holograms according to an embodiment of the present invention is
shown. While similar to the transmission reproduction system 30 of
FIG. 3, the reflection reproduction system 40 requires that PDLC
blank 37 be located before the master hologram 35 within the
optical path, followed by an absorption filter 39. As in FIG. 3,
the PDLC blank 37, master hologram 35 and filter 39 configuration
is oriented at the Bragg angle with respect to the coherent
incident beam 33 from radiation source 31.
[0077] Utilizing the hologram duplication system of FIG. 4, the
radiation source 31 emits a coherent radiation beam 33 (e.g.,
laser) that is directed towards a first surface of the PDLC blank
37. The incident beam 33 is transmitted through the PDLC blank 37
as transmitted beam 36, passing through the second surface thereof
and enters the first surface of the master hologram 35 which is
optically contacted to the second surface of the PDLC blank 37.
Within the master hologram 35, the incident beam 33 is diffracted
by the holographic grating 56 located therein, creating diffracted
beam 34. Transmitted beam 36 and diffracted beam 34 interfere
within the PDLC blank 37 forming a replica of holographic grating
56 therein.
[0078] In the case of a reflection hologram, given a master
hologram with a diffraction efficiency of 100%, wherein 100% of
incident beam 33 is diffracted as beam 34 and the amount of
incident beam 33 that is transmitted 36 is negligible, a 1:1 beam
ratio will result. However, a master hologram with a lower
diffraction efficiency will still result in a replica hologram.
[0079] In the representative embodiments described above, the
master hologram is not limited to a PDLC hologram. The master
hologram may be, but is not limited to, a computer generated
hologram, any of a variety of emulsion-type holograms, a
photopolymer hologram, a photochromic hologram, a polymer dispersed
liquid crystal hologram, silver halide photographic emulsion
hologram, dichromated gelatin hologram, photoresist hologram,
photothermoplastic hologram, photorefractive crystal hologram,
multiplexed hologram, white light hologram, rainbow hologram, thin
holograms, in-line hologram, off-axis hologram, fourier hologram,
fraunhofer hologram, diffractive optical element (DOE), holographic
optical element (HOE), evanescent-wave hologram, image hologram,
amplitude hologram, phase hologram, volume hologram, surface
hologram, transmission hologram, reflection hologram, or
silver-halide sensitized gelatin hologram. Further, in order to
fully take advantage of the switching characteristics inherent in
the liquid crystal component of the PDLC material, electrodes may
be attached to ITO glass slides (as described in FIG. 6) of either
one or the other or both the master hologram 35 and the PDLC blank
37. By controlling the orientation of the liquid crystal layer of
either the master hologram 35 or the PDLC blank 37 during the
duplication process, the replica holograms may be formed so as to
diffract either with the application of a specified voltage or in
the absence of the application of a specified voltage, in order to
meet design and system specifications.
[0080] While the aforementioned embodiments do not require a
switchable holographic-PDLC ("H-PDLC") master, there are situations
where it is advantageous to utilize a switchable H-PDLC master in
the hologram replication process. In the following embodiments, the
blanks are not restricted to the RDLC material described herein.
One skilled in the art recognizes the alternative materials which
may be used for the replica in the proceeding embodiments of the
present invention.
[0081] In an embodiment of the present invention wherein an H-PDLC
switchable master is advantageously utilized, the blank is exposed
prior to recording in order to improve uniformity of the hologram
and for aesthetic reasons. Pre-recording in this context means
prior to exposure of the H-PDLC master. One skilled in the art will
recognize when such pre-recording irradiation is necessary based on
the composition of the blank. For example, the materials discussed
in K. T. Weitzel et al., "Hologram recording in DuPont photopolymer
films by use of pulse exposure," Opt. Lett. 22, 1899 (1997) and V.
N. Mikhailov et al., "Pulse Hologram Recording in DuPont's
Photopolymer Films," Proc. SPIE 3011, 200 (1997) both of which are
incorporated by reference herein, benefit from pre-recording
irradiation.
[0082] Since the method of reproduction in the embodiments of the
present invention is contact reproduction, the master and the blank
are in close physical and optical proximity. If the master hologram
is not capable of being switched OFF, any exposure radiation
through the master hologram, will necessarily cause a pattern to be
created within the blank. But, in the case where the master
hologram is an H-PDLC master, the holographic nature of the master
may actually be turned OFF and ON under electrical control, such
that in the OFF state, the H-PDLC master resembles a piece of
transparent glass as shown in FIG. 9a. In FIG. 9a, the H-PDLC
master reflection hologram 35 is turned OFF, such that the liquid
crystals do not orient so as to form a holographic grating within
the H-PDLC master. Consequently, a pre-recording beam 5 passes
through the PDLC blank 37 and the H-PDLC master 35 without being
reflected back through the PDLC blank and forming an interference
pattern therein. In the case of a transmission H-PDLC master, as
shown in FIG. 9d the order of the PDLC blank 37 and the H-PDLC
master 35 is reversed, such that the pre-recording beam 5 first
passes through the master 35 which is OFF and then passes through
the blank 37, without forming any interference pattern with in the
blank 37. Pre-recording beam 5 may be the same beam as recording
incident beam 33 or it may be an auxiliary beam.
[0083] Similar to the pre-recording embodiment described above, one
skilled in the art recognizes that post-recording irradiation may
also be advantageous for certain contact replication scenarios.
Post-recording exposure is useful with certain recording
photopolymer materials (see e.g., Chemistry of Materials) for
eliminating unconverted monomers and bleaching unconsumed
photosensitive dye. For example, it may be necessary to further
expose the replicated hologram in order to permanently set the
recording therein. In FIG. 9b, a reflection H-PDLC master 35 is ON
when incoming beam 33 is incident thereon and is reflected as beam
34 so as to interfere within the blank 37 with incident beam 33,
forming an interference region therein. In a situation where
post-recording irradiation of the replication is necessary, the
master hologram 35 is turned OFF and a post-recording beam 6 is
incident upon what is now the replica of the master but, for
simplicity, what we will continue to refer to as the blank 37 but,
is not reflected back through by the master 35 because the grating
therein is not ON (see FIG. 9c).
[0084] As with the pre-recording situation, post-recording may also
take place when replicating a transmission hologram. Referring to
FIG. 9e replication of a transmission master 35 occurs when an
incident beam 33 passes through an ON master 35, wherein part of
the incident beam is deflected by the hologram pattern formed
therein resulting in diffracted beam 34. Beams 33 and 34 interfere
within the blank 37 to form a replica of the hologram pattern of
the master 35. After this replication step has occurred, it is
preferable in some situations to irradiate the now replica (used to
be blank) 37 with a post-recording beam 6 as shown in FIG. 9f.
Consequently, after the blank 37 has been exposed with the hologram
of the master 35, the master 35 is turned OFF and a post-recording
beam 6 is direct through the master 35 and into the replica 37,
without re-forming the hologram of the master 35 but merely
allowing the beam to pass unmodulated, through the master 35.
Post-recording beam 6 may be the same beam as recording incident
beam 33 or it may be an auxiliary beam.
[0085] Both the pre-recording and the post-recording embodiments
utilizing the H-PDLC master improve the efficiency of contact
printing. Without the benefit of the switchable master, there are
significant physical changes to the printing component which must
occur before and after the actual exposure. First, the blank would
have to be pre-exposed without being in optical contact with the
master, next for the recording step, the master would have to be
optically contacted thereto, and finally, for post-recording, the
master would have to be removed from the blank (now replica) prior
to irradiation of the blank with the post-recording beam. These
multiple configurations for pre-recording exposure, recording
exposure, and post-recording exposure add significant time to the
contact printing process and reduce efficiency. Using the H-PDLC
master, allows for a single configuration during the performance of
all three exposure steps.
[0086] In another embodiment of the present invention, using a
switchable H-PDLC master allows for substantial control of
diffraction efficiency. When contact printing planar transmission
gratings, the incident beam is split into two beams, a transmitted
beam and a diffracted beam. The optimum beam balance between the
transmitted and diffracted beams is often 1:1 since the beams are
overlapping to form the replica holographic transmission grating
within the blank. With a static master hologram, the diffraction
efficiency may not be selectively turned OFF and ON and it is not
variable. Consequently, given a static hologram which does not meet
the 1:1 requirement, there is a limitation in the quality of the
replica due to the non-optimal diffraction efficiency. In an
embodiment wherein the master is a switchable H-PDLC, the
diffraction efficiency is voltage controlled. Consequently, the
optimum efficiency can in effect be tuned in. This ability to
change or tune the diffraction efficiency in order to achieve the
optimal 1:1 power ratio is extremely useful where the hologram to
be replicated is complex. For example, where the master hologram is
a lens or some other complicated fringe pattern, the diffraction
efficiency may be manipulated in order to find the beam ratio, be
it 1:1 or otherwise, that produces the replica with highest
fidelity.
[0087] Referring to FIGS. 10a and 10b, incident recording beam 33
is split into transmitted beam 36 and diffracted beam 34 by only
partially switching the master H-PDLC hologram 35. The ability to
partially switch the master is analogized to an analog signal as
opposed to a digital signal. Instead of choosing from two states,
all ON or all OFF, a partially switchable H-PDLC master allows for
a range of diffraction efficiencies. For example, as the applied
voltage varies through a range, the diffraction efficiency of the
master also varies through a range. In FIG. 10a, a reflective
master is contact printed when the incident beam 33 and the
diffracted beam 34 interfere within the blank 37. Due to the use of
the switchable H-PDLC master, the power of the diffracted beam may
be varied with voltage as desired. Similarly, in FIG. 10b, the
transmitted beam 36 and the diffracted beam 34 interfere within the
blank 37 and both the transmitted beam power and the diffracted
beam power are capable of voltage control.
[0088] In another embodiment of the present invention, the
switchable H-PDLC master is utilized in order to create replica
holograms with tailored diffraction efficiency through decreased
fringe contrast. In many material systems, holographic exposure
with low fringe contrast will result in a replica with less than
optimum diffraction efficiency. By adjusting the diffraction
efficiency of the switchable master, the writing beams become
imbalanced (or the optimum balance for the system is altered) and
thus a decreased replica diffraction efficiency results. There are
situations where a power ratio other than 1:1 is desired during the
replication process (see e.g., D. J. Lougnot et al., "Photopolymers
for Holographic Recording: IV. New Self-processing Formulations
Based on .beta.-hydroxy ethyloxazolidone acrylate," Pure Appl. Opt.
2, 383 (1993) incorporated herein by reference). In a particular
embodiment benefiting from this method, a single switchable H-PDLC
master is able to produce multiple replicated static holograms with
identical prescriptions but with various diffraction efficiencies.
Without the partial switching capability of the H-PDLC master,
multiple static masters with varying photopolymer recipes must be
used to create this result. This method and system are cumbersome
and inefficient. Additionally, decreased diffraction efficiency can
also be achieved by decreasing the overall incident power. However,
the total incident power in that case would not be maintained so as
to preserve reaction rates within the blank. This embodiment of the
present invention preserves the total power incident on the blank
to preserve reaction rates.
[0089] Further embodiments of the present invention comprise a
stack of switchable H-PDLC masters. As with non-holographic lens
configurations forming an optical train (e.g., camera lenses), two
or more stacked holographic lenses, either reflection or
transmission, comprising an optical train also require precise
predetermined alignment of their optical centers or registration,
as it is referred to by those skilled in the art. An embodiment of
the present invention utilizes two or more switchable H-PDLC
masters to consecutively record two or more independent holographic
lenses. Referring to FIG. 11(a), when contact printing multiple
reflection masters in accordance with a preferred embodiment of the
present invention, the first replica lens 37a is recorded with the
first master 35a switched ON and the second master 35b switched OFF
(not shown). Next, the second replica lens 37b is recorded with the
second master 35b switched ON and the first master 35a switched
OFF. Similarly, referring to FIG. 11(b), when contact printing
multiple transmission masters in accordance with a preferred
embodiment of the present invention, the first replica lens 37a is
recorded with the first master 35a switched ON and the second
master 35b switched OFF (not shown). Next, the second replica lens
37b is recorded with the second master 35b switched ON and the
first master 35a switched OFF. By sequentially switching the
masters, interference and cross talk are eliminated during
replication. One skilled in the art will appreciate that the
masters must be designed, recorded and registered appropriately,
considering parameters such as the distance between corresponding
master/replica pairs, spectral properties, etc., in order to yield
the proper optical properties and registration in the replica
stack.
[0090] Another preferred embodiment of the present invention is a
system and method for mass producing a Red-Green-Blue (RGB)
holographic lens stack according to the principles illustrated with
regard to FIG(s) 11(a)-(b). Holographic lenses have unique
applications in the art of personal displays in that their optical
properties cannot be reproduced with conventional refractive
optics. In such personal displays a color image is produced by
rapidly sequencing independent red, green, and blue images. To
avoid color cross talk and interference, switchable H-PDLC lenses
are employed, allowing only the appropriate color hologram(s) to be
ON during the RGB sequence. Precise registration of the RGB H-PDLC
stack is required so that the three monochrome images overlap.
Current manufacturing procedures require each monochrome hologram
to be independently recorded in independent complex optical rigs
that require painstaking alignment. This alignment must be such
that each hologram is placed in a precise location in a substrate
blank of specific dimensions. Each monochrome hologram is recorded
using a laser of a similar wavelength. If successful, the three
monochrome holograms will register when they are physically stacked
and aligned. Registration can be compromised if any one recording
rigs become misaligned or the blank substrate dimensions is out of
specification. This manufacturing technique is subject to serious
downtime and costly maintenance. The following method and system of
an embodiment of the present invention eliminate the complex
alignment in the mass production step and thus reduce the risk of
production delays.
[0091] Initially, a master set of holograms is designed, recorded,
and registered into a master switchable holographic stack, taking
into account the required replica optical design, registration,
blank substrate dimensions, etc. A fixture is provided to hold the
master and replica stacks in optical contact. Next, stable,
optically clean and uniform laser beams of three different
wavelengths (e.g., red, green, blue) are provided to illuminate the
master/replica assembly. The angle of incidence of each of the RGB
beams onto the master/replica assembly is set according to the
master designs.
[0092] Following the sequence set forth in TABLE 1 and referencing
FIG(s). 12(a)-(b), the recording of the master switchable
holographic stack is described. In FIG. 12(a), a stack of
reflective RGB H-PDLC masters is contact printed. First, if
necessary based on the material comprising the blue blank 37a, a
pre-recording irradiation is performed while all masters and blanks
are OFF, utilizing the blue laser beam or an appropriate auxiliary
beam. Next, while recording the blue H-PDLC master 35a onto the
designated blue blank 37a, all masters and blanks excepting the
blue H-PDLC master 35a are switched OFF, leaving only the blue
H-PDLC master 35a ON and irradiated by the blue laser. Following
the recording step, if post-recording exposure is necessary, again,
all masters, remaining red and green blanks and the blue replica
are switched OFF while the blue laser or an auxiliary beam
irradiates the blue replica.
[0093] After the blue blank (now replica) 37a has been recorded,
the green blank 37b, is pre-exposed (if necessary) utilizing the
green laser or an auxiliary beam, while all masters, the red and
green blanks, and the blue replica are switched OFF. As is
specifically shown in FIG. 12(a), while recording the green H-PDLC
master 35b utilizing green incident beam 33b, the blue and red
masters 35a, 35c, the red and green blanks 37b, 37c, and the blue
replica 37a, remain switched OFF, while the green master 35b is
switched ON. Green incident beam 33b and diffracted beam 34b
interfere within the green blank 37b to form the replica. Following
the recording step, if post-recording exposure is necessary, all
masters, the red blank, and the blue and green replicas are
switched OFF while the green replica is irradiated with the green
laser or an auxiliary beam.
[0094] Finally, after the blue blank (now replica) 37a and green
blank (now replica) 37b have been recorded, the red blank 37c, is
pre-exposed (if necessary) utilizing the red laser or an auxiliary
beam, while all masters, the red blank, and the blue and green
replicas are switched OFF. While recording the red H-PDLC master
35c utilizing the red laser, the blue and green masters, the red
blank, and the blue and green replicas, remain switched OFF, while
the red master 35c is switched ON. Following the recording step, if
post-recording exposure is necessary, all masters and the blue,
green, and red replicas are switched OFF while the red replica is
irradiated with the red laser or an auxiliary beam.
[0095] When recording a stack of transmission RGB H-PDLC masters,
the sequence described above is applicable. The only difference
lies in the interacting beams used to form the interference pattern
within the appropriate blank during the recording step. For
example, referring to FIG. 12(b), while recording the green H-PDLC
master 35b utilizing green incident beam 33b, the blue and red
masters 35a, 35c, the red and green blanks 37b, 37c, and the blue
replica 37a, remain switched OFF, while the green master 35b is
switched ON. Unlike in the reflection recording step, the
diffracted beam 34b and the transmitted beam 36b interfere within
the green blank 37b to form the replica.
1TABLE 1 Sample Switching Sequence in an RGB Stack Produced by
Contact Holography Red Blank/ Green Blank/ Blue Blank/ Replica
Replica Replica Red Master Green Master Blue Master Pre-ex Blue N/A
N/A N/A Off Off Off Expose Blue N/A N/A N/A Off Off ON Post-ex Blue
N/A N/A N/A Off Off Off Pre-ex Green N/A N/A Off Off Off Off Expose
N/A N/A Off Off ON Off Green Post-ex N/A N/A Off Off Off Off Green
Pre-ex Red N/A Off Off Off Off Off Expose Red N/A Off Off ON Off
Off Post-ex Red N/A Off Off Off Off Off
[0096] In constructing the system described in the previous
embodiment for contact printing of multiple stacked holograms of
varying colors, considerations must be given to the photo-sensitive
materials used in the replica blanks to ensure that they do not
spectrally overlap. Further, one skilled in the art recognizes that
the aforementioned embodiments are merely representative of the
multiple useful configurations and methods which are possible when
a switchable PDLC master and/or blank are incorporated therein.
[0097] The replica PDLC holograms resulting from the systems and
methods described above, because of the switching properties of the
liquid crystal component of the PDLC material, may be switched
accordingly from a diffractive to a transmissive state.
Consequently, the duplication method described above allows for
increased efficiency and reproducibility of switchable holographic
elements. These switchable holographic elements find use in all
areas of technology utilizing optical switches including the
telecommunications and imaging technologies.
[0098] Given the aforementioned examples of hologram reproduction
using a single beam approach with a PDLC blank, one skilled in the
art is readily aware of the importance of this improved method when
considering the manufacturing process of, for example, holographic
lenses which use complex optical geometries. The set-up time is
considerably reduced and the overall reproducibility from one
hologram to another is significantly increased.
[0099] Further, one skilled in the art recognizes that the
embodiments illustrated herein are merely exemplary and that the
inventive concepts may be practiced in any number of alternate
embodiments while remaining within the scope of the present
invention.
* * * * *