U.S. patent application number 17/341155 was filed with the patent office on 2022-02-24 for method for holographic mastering and replication.
This patent application is currently assigned to DigiLens Inc.. The applicant listed for this patent is DigiLens Inc.. Invention is credited to Alastair John Grant, Milan Momcilo Popovich, Jonathan David Waldern.
Application Number | 20220057749 17/341155 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-24 |
United States Patent
Application |
20220057749 |
Kind Code |
A1 |
Popovich; Milan Momcilo ; et
al. |
February 24, 2022 |
Method for Holographic Mastering and Replication
Abstract
A method for producing holograms with a multiplicity of
holographic prescriptions from a single master is provided. A
multiplicity of holographic substrates each containing a first
hologram is stacked on a second holographic recording medium
substrate. The first hologram is designed to diffract light from a
first direction into a second direction. When expose to
illumination from the first direction zero order and diffracted
light from each first hologram interfere in the second holographic
recording medium substrate forming a second hologram. The second
hologram is then copied into a third holographic recording medium
substrate to provide the final copy hologram.
Inventors: |
Popovich; Milan Momcilo;
(Leicester, GB) ; Waldern; Jonathan David; (Los
Altos Hills, CA) ; Grant; Alastair John; (San Jose,
CA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens Inc.
Sunnyvale
CA
|
Appl. No.: |
17/341155 |
Filed: |
June 7, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16517461 |
Jul 19, 2019 |
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17341155 |
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15502596 |
Feb 8, 2017 |
10359736 |
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PCT/GB2015/000228 |
Aug 5, 2015 |
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16517461 |
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61999867 |
Aug 8, 2014 |
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International
Class: |
G03H 1/20 20060101
G03H001/20; G03H 1/02 20060101 G03H001/02 |
Claims
1. A method for producing a pluarlity of holograms with a plurality
of prescriptions from common master hologram, the method
comprising: providing a master hologram configured to provide
diffracted and zero-order beams angles for each of a plurality of
incident beam directions; providing an intermediate holographic
medium; providing a contact copy holographic medium; providing a
light source to provide a plurality of incidence angles on said
master hologram; illuminating said master hologram with a light of
a first polarization in a first direction; diffracting said light
into a zero order beam in said first direction and a diffracted
beam in said second direction using said master hologram;
interfering said zero order beam and said diffracted beam in said
intermediate holographic medium to form an intermediate master
hologram; placing said intermediate master hologram in contact with
said contact copy holographic medium; further illuminating an
external surface of said intermediate master hologram with a second
light in said first direction, said intermediate master hologram
diffracting said second light into a second zero order beam in said
first direction and a second diffracted beam in said second
direction, said second diffracted and second first order beams
interfering in said contact copy holographic medium to form a
contact copy hologram.
2. The method of claim 1, wherein said steps of illuminating,
diffracting, interfering, placing and further illuminating are
repeated for a multiplicity of values of said first and second
directions.
3. The method of claim 1, wherein the first and second directions
falls within the diffraction efficiency angular bandwidth of said
master hologram.
4. The method of claim 1, wherein the master hologram is provided
by the steps of: configuring a laser holographic recording
apparatus to form a first recording beam in said first direction
and a second recording beam in said second direction; providing N
substrates each containing a master holographic medium; and
interfering said first and second beams within each said master
holographic medium substrate to form a first hologram in each said
substrate.
5. The method of claim 1, where said master hologram includes at
least one selected from the group consisting of: a liquid crystal
and polymer material system, a holographic photopolymer, and a
surface relief grating.
6. The method of claim 1, wherein said master hologram has a
modulation exceeding that required to achieve a target beam ratio
of said diffracted and zero order beams.
7. The method of claim 1, wherein said master hologram has a
modulation exceeding by up to 5% that required to achieve a target
beam ratio of said diffracted and zero order beams.
8. The method of claim 1, wherein said master hologram has a
modulation exceeding by up to 10% that required to achieve a target
beam ratio of said diffracted and zero order beams.
9. The method of claim 1, further comprising providing one of
either a half wave plate and a linear polarizer and disposing it
between said intermediate holographic medium and said master
hologram.
10. The method of claim 1, wherein said contact copy hologram is a
copy of said intermediate master hologram and said intermediate
master hologram is a copy of said master hologram.
11. The method of claim 1, wherein said zero order and diffracted
beams in at least one of the diffracting or further illuminating
steps have power substantially in the ratio of 1:1.
12. The method of claim 1, wherein said intermediate master
hologram and said contact copy holographic medium are separated by
an air gap.
13. The method of claim 1, wherein at least one index matching
optical layer is provided.
14. The method of claim 1, wherein said contact copy holographic
medium forms part of a mechanically translatable continuous
lamina.
15. The method of claim 1, further comprising providing a voltage
generator for applied a voltage across at least one of said
intermediate holographic medium and said contact copy holographic
medium characterized in that said voltage varies the refractive
index modulation of at least one of said intermediate master
hologram and said contact copy hologram during the further
illuminating.
16. The method of claim 1, further comprising providing a voltage
generator for applied a voltage across said master hologram
characterized in that said voltage varies the refractive index
modulation of said master hologram during the illuminating,
diffracting and interfering steps.
17. The method of claim 1, wherein the at least one medium selected
from the group of said intermediate holographic medium and said
contact copy holographic medium is one of a material selected from
the group of a photo thermal refractive or photopolymer, a forward
mode HPDLC mixture or a reverse mode HPDLC mixture.
18. The method of claim 1, wherein said intermediate holographic
medium and contact copy holographic medium are supported by one of
either a glass or a plastic substrate.
19. The method of claim 1, wherein said intermediate holograhic
medium and contact copy holographic medium are supported by
substrates.
20. The method of claim 1, wherein said contact copy hologram forms
part of a waveguide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a continuation of U.S. patent
application Ser. No. 16/517,461 filed on Jul. 19, 2019, which is a
continuation of U.S. patent application Ser. No. 15/502,596 filed
on Feb. 8, 2017 and issued on Jul. 23, 2019 as U.S. Pat. No.
10,359,736, which is a U.S. National Phase of PCT Application No.
PCT/GB2015/000228 filed on Aug. 5, 2015, which claims the benefit
of U.S. Provisional Patent Application No. 61/999,867 filed Aug. 8,
2014, the disclosures of which are herein incorporated by reference
in their entireties.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to holography and more
particularly to an improved method for mastering and replicating
holograms.
[0003] Replication of holograms is normally carried out by
preparing a master hologram of the desired prescription which is
then copied into another holographic recording material using a
contact process. The master is usually made using a classical
two-beam holographic recording system comprising an object beam and
a reference beam. However, the master could itself be a copy of
another master. In the case of a transmission hologram the copying
process is based on interfering the diffracted and zero order beams
produced by master to form a grating within the copy hologram
material. Subject to processing variations such as shrinkage the
holographic pattern or grating formed in the copy should be
identical to the one in the master. This procedure may be used in
mass production roll-to-roll processes. The principles of
holographic replication and industrial processes for the mass
production of holograms are well documented in the literature.
[0004] The optical design benefits of diffractive optical elements
(DOEs) are well known, including unique and efficient form factors
and the ability to encode complex optical functions such as optical
power and diffusion into thin layers. Bragg gratings (also commonly
termed volume phase grating or holograms), which offer the highest
diffraction efficiencies, have been widely used in devices such as
Head Up Displays. An important class of Bragg grating devices is
known as a Switchable Bragg Grating (SBG). An SBG is a diffractive
device formed by recording a volume phase grating, or hologram, in
a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG
devices are fabricated by first placing a thin film of a mixture of
photopolymerizable monomers and liquid crystal material between
parallel glass plates or substrates. Techniques for making and
filling glass cells are well known in the liquid crystal display
industry. One or both glass substrates support electrodes,
typically transparent indium tin oxide films, for applying an
electric field across the PDLC layer. A volume phase grating is
then recorded by illuminating the liquid material with two mutually
coherent laser beams, which interfere to form the desired grating
structure. During the recording process, the monomers polymerize
and the HPDLC mixture undergoes a phase separation, creating
regions densely populated by liquid crystal micro-droplets,
interspersed with regions of clear polymer. The alternating liquid
crystal-rich and liquid crystal-depleted regions form the fringe
planes of the grating. The resulting volume phase grating can
exhibit very high diffraction efficiency, which may be controlled
by the magnitude of the electric field applied across the PDLC
layer. When an electric field is applied to the hologram via
transparent electrodes, the natural orientation of the LC droplets
is changed causing the refractive index modulation of the fringes
to reduce and the hologram diffraction efficiency to drop to very
low levels. Note that the diffraction efficiency of the device can
be adjusted, by means of the applied voltage, over a continuous
range from near 100% efficiency with no voltage applied to
essentially zero efficiency with a sufficiently high voltage
applied.
[0005] SBGs may be used to provide transmission or reflection
gratings for free space applications. SBGs may be implemented as
waveguide devices in which the HPDLC forms either the waveguide
core or an evanescently coupled layer in proximity to the
waveguide. In one particular configuration to be referred to here
as Substrate Guided Optics (SGO) the parallel glass plates used to
form the HPDLC cell provide a total internal reflection (TIR) light
guiding structure. Light is "coupled" out of the SBG when the
switchable grating diffracts the light at an angle beyond the TIR
condition. SGOs are currently of interest in a range of display and
sensor applications. Although much of the earlier work on HPDLC has
been directed at reflection holograms transmission devices are
proving to be much more versatile as optical system building blocks
and tend to be much easier to fabricate.
[0006] Typically, the HPDLC used in SBGs comprise liquid crystal
(LC), monomers, photoinitiator dyes, and coinitiators. The mixture
frequently includes a surfactant. The patent and scientific
literature contains many examples of material systems and processes
that may be used to fabricate SBGs. Two fundamental patents are:
U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452
by Tanaka et al. both filings describe monomer and liquid crystal
material combinations suitable for fabricating SBG devices.
[0007] One of the known attributes of transmission SBGs is that the
LC molecules tend to align normal to the grating fringe planes. The
effect of the LC molecule alignment is that transmission SBGs
efficiently diffract P polarized light (ie light with the
polarization vector in the plane of incidence) but have nearly zero
diffraction efficiency for S polarized light (ie light with the
polarization vector normal to the plane of incidence. Transmission
SBGs may not be used at near-grazing incidence as the diffraction
efficiency of any grating for P polarization falls to zero when the
included angle between the incident and reflected light is small. A
glass light guide in air will propagate light by total internal
reflection if the internal incidence angle is greater than about 42
degrees. Thus waveguide transmission SBGs may be used if the
internal incidence angles are in the range of 42 to about 70
degrees, in which case the light extracted from the light guide by
the gratings will be predominantly p-polarized.
[0008] Normally SBGs diffract when no voltage is applied and are
switching into their optically passive state when a voltage is
application other times. However SBGs can be designed to operate in
reverse mode such that they diffract when a voltage is applied and
remain optically passive at all other times. Methods for
fabricating reverse mode SBGs are disclosed in a U.S. Provisional
Patent Application No. 61/573,066 with filing date 24 Aug. 2011 by
the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER
DISPERSED LIQUID CRYSTAL MATERIALS AND which is incorporated by
reference herein in its entirety. The same reference also discloses
how SBGs may be fabricated using flexible plastic substrates to
provide the benefits of improved ruggedness, reduce weight and
safety in near eye applications.
[0009] The present invention is motivated by the requirement to
record SBGs of differing optical prescriptions for use in image
transmitting waveguides currently being designed for Head Up
Displays (HUDs) and Head Mounted Displays (HMDs). The holograms may
configured as stacks U.S. Pat. No. 8,233,204 entitled OPTICAL
DISPLAYS U.S. patent application Ser. No. 13/844,456 entitled WIDE
FIELD OF VIEW COLOR DISPLAY; or tessellated in single layers as
disclosed in U.S. patent application Ser. No. 13/869,866 entitled
APERTURE SAMPLING FOR DUAL AXIS SAMPLING. In such applications the
holograms are used to tile a field of view (FOV) space and/or
increase the size of the exit pupil. For large FOV full colour
displays the number of holographic prescriptions can be high as the
FOV of a holographic element is limited by diffraction efficiency
angular bandwidth. Since the cost of fabricating masters using
conventional holographic interferometry or ruling processes is
currently very high this can make the manufacture of large FOV
displays very expensive. Exemplary holographic masters and replicas
thereof) are provided by companies such as Holographix Inc. (MA).
Typically, masters are surface relief components fabricated using
holographic, binary grating etching or mechanical ruling processes.
Desirably, a mastering and replication process for large FOV
holographic waveguides should provide a range of optical
prescriptions spanning the required FOV space using a minimal
number of master components. Ideally this should be accomplished
with just one master. Applications such as HMDs and HUDs typically
demand tight control of the diffraction efficiency and geometrical
optical characteristics of the replicated holograms. In particular
there is a need for precise control of the intensities of the
diffracted and zero order beams. Currently available holographic
mastering process suffer from the problem that the relative
intensities of the diffracted and zero orders cannot be controlled
to better than .+-.5%. As disclosed in a co-pending patent
application PCT/GB2013/000273 the inventors have discovered that a
perfect copy can be made if the master hologram is "over-modulated"
by a small amount. Over-modulation in this context means that the
refractive index modulation of the hologram is a little above that
required to achieve the desired beam ratio. The next step is to
separately attenuate the master beams to bring them to the desired
ratio. Typically we require 50/50 or 1:1. However, the inventors
have found that making a perfect master with the appropriate level
of over-modulation, which is typically 5-10%, is very difficult in
practice. To the best of the inventors' knowledge the required
levels of index modulation control have not been achieved using
conventional holographic recording processes using currently
available holographic recording materials such as photopolymers and
Photo Thermo Refractive (PTR) materials. Desirably a holographic
mastering process should include methods for controlling the
hologram modulation.
[0010] There is requirement for an efficient and cost-effective
method for replicating holograms with a multiplicity of holographic
prescriptions from a single master.
SUMMARY OF THE INVENTION
[0011] There is provided a efficient and cost-effective method for
replicating holograms with a multiplicity of holographic
prescriptions from a single master.
[0012] The objects of the invention are achieved in a first
embodiment in which there is provided a method for mastering and
replicating holograms, the method comprising: [0013] a) providing N
substrates each containing a first hologram for diffracting
incident light from a first direction into diffracted light in a
second direction; providing a second holographic recording medium;
and providing a third holographic recording medium; [0014] b)
stacking in sequence the first holograms 1-N onto the second
holographic recording medium; [0015] c) illuminating external
surface of the first hologram N with light of a first polarization
in a first direction; [0016] d) the first holograms 1-N diffracting
the light into zero order light in the first direction and
diffracted light in the second direction; [0017] e) the first
direction light and the second direction light interfering in the
second holographic recording medium to form a second hologram;
[0018] f) placing the second hologram in contact with the third
holographic recording medium; [0019] g) illuminating external
surface of the second hologram with light in the first direction;
[0020] h) the second hologram diffracting the light into zero order
light in the first direction and diffracted light in the second
direction; [0021] i) the diffracted and first order light
interfering in the third holographic recording medium to form a
third hologram.
[0022] In one embodiment of the invention steps c) to i) are
repeated for a multiplicity of values of the first and second
directions. The first and second directions are limited by the
diffraction efficiency angular bandwidth of said first
hologram.
[0023] In one embodiment of the invention the first holograms 1,N
are provided by the steps of: configuring a laser holographic
recording apparatus to form a first recording beam in the first
direction and a second recording beams in the second direction;
providing N substrates each containing a first holographic medium;
and the first and second beams interfering within each the first
holographic medium substrate to form the first hologram in each the
substrate.
[0024] In one embodiment the first holograms 1,N are surface relief
structures. In one embodiment the first holograms 1,N are binary
structures.
[0025] In one embodiment step a) further comprises providing a half
wave plate (HWP) and step c) further comprises disposing the HWP
between the holographic recording medium substrate and the first
hologram stack. In a further embodiment step a) further comprises
providing a linear polarizer and in step c) further comprises
disposing the linear polarizer between the HWP and the first
hologram stack.
[0026] In one embodiment the first holographic recording medium is
a HPDLC for recording a SBG, the second holographic recording
medium is a holographic photopolymer and the third holographic
recording medium is a holographic photopolymer.
[0027] In one embodiment of the invention the third hologram is a
copy of the second hologram and the second hologram is a copy of
the first hologram.
[0028] In one embodiment of the invention the third holographic
recording medium comprises HPDLC material components for forming
one of a forward mode SBG or a reverse mode SBG.
[0029] In one embodiment of the invention the zero order light and
diffracted light in at least one step d) and step i) have power
substantially in the ratio of 1:1.
[0030] In one embodiment of the invention the third holographic
recording medium has a substrate fabricated from optical
plastic.
[0031] In one embodiment of the invention the second hologram and
the third holographic recording medium are separated by an air gap.
In one embodiment the second hologram and the third holographic
recording medium are in contact.
[0032] In one embodiment of the invention the third holographic
recording medium forms part of a mechanically translatable
continuous lamina.
[0033] In one embodiment of the invention there is further provided
a voltage generator for applied a voltage across at least one of
the second hologram and the third holographic recording medium. The
applied voltage varies the refractive index modulation of at least
one of the second hologram and the third hologram during steps g)
to i).
[0034] In one embodiment of the invention the second holographic
recording medium is one of a photo thermal refractive or
holographic photopolymer, a forward mode HPDLC mixture or a reverse
mode HPDLC mixture. In one embodiment of the invention the third
holographic recording medium is one of a photo thermal refractive
or photopolymer, a forward mode HPDLC mixture or a reverse mode
HPDLC mixture.
[0035] In one embodiment of the invention the diffracting thickness
of the first hologram is less than or equal to 1 micron. In one
embodiment of the invention the diffracting thickness of the first
hologram is less than or equal to 2 micron.
[0036] In one embodiment of the invention there is provided a
method of mastering and replicating holograms, the method
comprising: [0037] a) providing a laser apparatus for forming a
first recording beam in a first direction and a second recording
beams in a second direction; N substrates each containing a first
HPDLC mixture; a holographic photopolymer ; a copy holographic
substrate containing a second HPDLC mixture; a HWP; and a linear
polarizer; [0038] b) the first and second beams interfering within
each the first HPDLC mixture to form a first hologram in each
substrate; [0039] c) stacking in sequence the linear polarizer, HWP
and first holograms 1-N onto the second holographic photopolymer;
[0040] d) illuminating external surface of the hologram N with
light of a first polarization in the first direction; [0041] e) the
first holograms 1-N diffracting the light into zero order light in
the first direction and diffracted light in the second direction;
[0042] f) the HWF rotating the incident light polarization through
ninety degrees into a second polarization; [0043] g) the linear
polarizer removing residual first polarization light; [0044] h) the
first direction light and the second direction light interfering in
the holographic photopolymer to form a second hologram; [0045] i)
placing the second hologram in contact with the copy holographic
substrate; [0046] j) illuminating external surface of the second
hologram with light of the second polarization in the first
direction; [0047] k) the second hologram diffracting the light into
zero order light in the first direction and diffracted light in the
second direction; [0048] l) the diffracted and first order light
interfering in the copy holographic substrate to form a third
hologram.
[0049] In one embodiment of the invention steps d) to 1) are
repeated for a multiplicity of values of the first and second
directions, wherein the first and second directions are limited by
the diffraction efficiency angular bandwidth of the first
hologram.
[0050] In one embodiment of the invention the first polarization is
P-polarization and the second polarization is S-polarization.
[0051] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, wherein like index
numerals indicate like parts. For purposes of clarity, details
relating to technical material that is known in the technical
fields related to the invention have not been described in
detail.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
[0053] FIG. 2A is a cross section view of a stack of first
holograms in one embodiment.
[0054] FIG. 2B is a schematic cross section view of a stack
comprising first holograms, a linear polarizer, a half wave plate
and second holographic recording medium in one embodiment.
[0055] FIG. 2C is a schematic cross section view of a second
hologram in contact with a third holographic recording medium in
one embodiment of the invention.
[0056] FIG. 3A is a schematic cross section view of a stack of
first holograms in one embodiment of the invention.
[0057] FIG. 3B is a schematic cross section view of a stack
comprising a stack of first holograms, a linear polarizer, a half
wave plate and second holographic recording medium in one
embodiment of the invention.
[0058] FIG. 3C is a schematic cross section view of a second
hologram in contact with a third holographic recording medium in
one embodiment of the invention.
[0059] FIG. 4A is a schematic cross section view of a first
hologram in one embodiment of the invention.
[0060] FIG. 4B is a schematic cross section view of a stack of
first holograms showing ray paths in one embodiment of the
invention.
[0061] FIG. 4C is a schematic cross section view of a stack
comprising a stack of first holograms, a linear polarizer, a half
wave plate and second holographic recording medium showing ray
paths for a first illumination direction in one embodiment of the
invention.
[0062] FIG. 4D is a schematic cross section view of a stack
comprising a stack of first holograms, a linear polarizer, a half
wave plate and second holographic recording medium showing ray
paths for a second illumination direction in one embodiment of the
invention.
[0063] FIG. 4E is a schematic cross section view of a second
hologram in contact with a third holographic recording medium
showing ray paths in one embodiment of the invention.
[0064] FIG. 5 is a schematic illustration illustrating the use of
an electric field to control the refractive index modulations of
the first holograms during the recording of the second hologram in
one embodiment of the invention
[0065] FIG. 6 is a schematic illustration illustrating the use of
an electric field to control the refractive index modulations of
the second hologram during the recording of the third hologram in
one embodiment of the invention
[0066] FIG. 7 is a schematic illustration of an optical arrangement
for recording on of the first holograms in one embodiment of the
invention.
[0067] FIG. 8 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
[0068] FIG. 9 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
[0069] FIG. 10 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
[0070] FIG. 11 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
[0071] FIG. 12 is a flowchart illustrating a process for mastering
and replicating holograms in one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0072] The invention will now be further described by way of
example only with reference to the accompanying drawings. It will
apparent to those skilled in the art that the present invention may
be practiced with some or all of the present invention as disclosed
in the following description. For the purposes of explaining the
invention well-known features of optical technology known to those
skilled in the art of optical design and visual displays have been
omitted or simplified in order not to obscure the basic principles
of the invention. Unless otherwise stated the term "on-axis" in
relation to a ray or a beam direction refers to propagation
parallel to an axis normal to the surfaces of the optical
components described in relation to the invention. In the following
description the terms light, ray, beam and direction may be used
interchangeably and in association with each other to indicate the
direction of propagation of light energy along rectilinear
trajectories. Parts of the following description will be presented
using terminology commonly employed by those skilled in the art of
optical design. The term "grating" may be used to describe a
hologram. It should also be noted that in the following description
of the invention repeated usage of the phrase "in one embodiment"
does not necessarily refer to the same embodiment.
[0073] The present invention provides a method for producing
holograms with a multiplicity of holographic prescriptions from a
single master. The master which will be described as a first
hologram is characterised by a wide angular bandwidth. Desirably,
the first hologram also has large index modulation. This allows a
wide range of input and diffracted beam angles to be generated by
the first hologram. For each set of input and diffracted beam angle
an intermediate master (second) hologram is recorded. The resulting
set of intermediate master (second) holograms may then be used to
contact copy the hologram into the desired copy medium to provide a
copy (third) hologram. A wide bandwidth hologram will have a small
thickness which results have relatively low diffraction efficiency.
In the proposed method the problem of low diffraction efficiency is
overcome by stacking a multiplicity of holographic substrates each
containing the first hologram. This stack is then overlaid on
stacked on a second holographic recording medium substrate. The
first hologram is designed to diffract light from a first direction
into a second direction. When exposed to illumination from the
first direction zero order and diffracted light from each (first)
hologram in the stack interfere in the second holographic recording
medium substrate forming a second hologram. The second hologram is
then copied into a third holographic recording medium substrate to
provide the final copy hologram. The invention may be used to
master and replicate any type of hologram in any type of
holographic recording material. The invention may be used to master
and replicated passive or switchable holograms. The holograms may
be single elements or switchable arrays as described in
PCT/GB2013/000273. Voltages may be applied across the second
hologram to control the index modulation and hence fine tune beam
ratios during the final contact copying stage. Voltages may also be
applied across the first holograms during the recording of the
second hologram.
[0074] In one embodiment of the invention there is provided a
method for mastering and replicating holograms, the method
comprising: [0075] a) providing N substrates each containing a
first hologram for diffracting incident light from a first
direction into diffracted light in a second direction; providing a
second holographic recording medium; and providing a third
holographic recording medium; [0076] b) stacking in sequence the
first holograms 1-N onto the second holographic recording medium;
[0077] c) illuminating external surface of the first hologram N
with light of a first polarization in a first direction; [0078] d)
the first holograms 1-N diffracting the light into zero order light
in the first direction and diffracted light in the second
direction; [0079] e) the first direction light and the second
direction light interfering in the second holographic recording
medium to form a second hologram; [0080] f) placing the second
hologram in contact with the third holographic recording medium;
[0081] g) illuminating external surface of the second hologram with
light in the first direction; [0082] h) the second hologram
diffracting the light into zero order light in the first direction
and diffracted light in the second direction; [0083] i) the
diffracted and first order light interfering in the third
holographic recording medium to form a third hologram.
[0084] In one embodiment of the invention steps c) to i) are
repeated for a multiplicity of values of the first and second
directions. The first and second directions are limited by the
diffraction efficiency angular bandwidth of said first
hologram.
[0085] A method of replicating a hologram in one embodiment of the
invention in accordance with the basic principles of the invention
is shown in the flow diagram in FIG. 1. Referring to the flow
diagram, we see that the method comprises the following steps:
[0086] At step 2001 provide N hologram substrates each containing a
first hologram having construction angles in first and second
directions; a second holographic recording medium; and a third
holographic recording medium. [0087] At step 2002 stack hologram
substrates 1-N onto the second holographic recording medium [0088]
At step 2003 illuminate hologram substrates (1-N) with light in the
first direction [0089] At step 2004 hologram substrates (1-N)
provide 0-order light in the first direction and diffracted light
in the second direction.
[0090] At step 2005 the first and second direction light interferes
to form a second hologram in the second holographic recording
medium. [0091] At step 2006 place the second hologram in contact
with the third holographic recording medium. [0092] At step 2007
illuminate the second hologram with light in the first direction.
[0093] At step 2008 the second hologram provides 0-order light in
the first direction and diffracted light in the second direction.
[0094] At step 2009 the first and second direction light interferes
to form the third hologram.
[0095] Note that in terms of defining the holographic prescription
a hologram having construction angles in the first and second
directions is equivalent to the same hologram diffracting incident
light from a first direction into diffracted light in a second
direction.
[0096] FIG. 2A shows the stack of first holograms labelled by
11-12. FIG. 2B shows the stack of first holograms overlaying of the
second holographic recording medium 40. FIG. 2C shows the second
hologram overlaying the third holographic recording medium.
[0097] In one embodiment of the invention the first holograms 1,N
are provided by the steps of firstly, configuring a laser
holographic recording apparatus to form a first recording beam in
the first direction and a second recording beams in the second
direction; secondly, providing N substrates each containing a first
holographic medium; and, thirdly, the first and second beams
interfering within each the first holographic medium substrate to
form the first hologram in each substrate. The present invention
does not assume that any particular holographic recording process
or HPDLC material is used to fabricate the first holograms. Any of
the processes and material systems currently used to fabricate SBGs
may be used such as for example the ones disclosed in U.S. Pat. No.
5, 942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka.
The master may be recorded using currently available industrial
processes such as the ones provided by companies such as
Holographix LLC (MA). Ideally, the master would be recorded using
remote computer controlled equipment, which by removing human
presence eliminates vibrations and thermal variations that may
adversely affect the quality of the recording process. Ideally, the
master recording laboratory should be protected from vibrations
from external disturbances. Desirably, the master hologram
recording equipment will provide active fringe stabilization.
[0098] In the preferred embodiments the first hologram and third
(copy) holograms are SBGs. In one embodiment the SBGs are reverse
mode such the hologram diffracts when a voltage is applied and
remains optically passive at all other times. A reverse mode SBG
will provide lower power consumption. A reverse mode HPDLC and
methods for fabricating reverse mode SBG devices is disclosed in
U.S. Provisional Patent Application No. 61/573,066 with filing date
24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO
HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND which is
incorporated by reference herein in its entirety. Ultimately, the
inventors aim to make replica SBGs with plastic substrates and
flexible transparent conductive coatings (to replace ITO). Plastic
SBG technology suitable for the present invention is also disclosed
in U.S. Provisional Patent Application No. 61/573,066. A reverse
mode SBG is more ideally suited to mastering as it avoids the
degradation of SBG material that occurs with UV recording.
Advantageously, the SBGs will used thin flexible glass substrates
such as the ones developed by Corning and Schott driven by the
touch panel and smart phone industries.
[0099] In one embodiment of the invention the first holograms 1,N
are surface relief structures such as binary structures. Such
holograms would typically require index matching layers between the
hologram layers
[0100] In one embodiment of the invention step a) further comprises
providing a half wave plate (HWP) and step c) further comprises
disposing the HWP between the holographic recording medium
substrate and the first hologram stack. In a further embodiment
step a) further comprises providing a linear polarizer and in step
c) further comprises disposing the linear polarizer between the HWP
and the first hologram stack.
[0101] In one embodiment of the invention the first holographic
recording medium is a HPDLC for recording a SBG, the second
holographic recording medium is a holographic photopolymer and the
third holographic recording medium is a holographic photopolymer.
In one embodiment of the invention the third hologram is copy of
the second hologram and the second hologram is a copy of the first
hologram. In one embodiment of the invention the third holographic
recording medium comprises HPDLC material components for forming
one of a forward mode SBG or a reverse mode SBG. In one embodiment
of the invention the zero order light and diffracted light in at
least one step d) and step i) have power substantially in the ratio
of 1:1. In one embodiment of the invention the third holographic
recording medium has a substrate fabricated from optical plastic.
In one embodiment of the invention the second hologram and the
third holographic recording medium are separated by an air gap. In
one embodiment of the invention the second hologram and the third
holographic recording medium are in contact. In one embodiment of
the invention the third holographic recording medium forms part of
a mechanically translatable continuous lamina.
[0102] In one embodiment of the invention there is further provided
a voltage generator for applied a voltage across at least one of
the second hologram and the third holographic recording medium
according to the principles disclosed in PCT/GB2013/000273 entitled
ELECTRICALLY CONTROLLABLE MASTER HOLOGRAM FOR CONTACT COPYING. The
voltage varies the refractive index modulation of at least one of
the second hologram and the third during steps g) to i). FIG. 5
illustrates the application of a voltage across the first holograms
using the voltage generator 15 via the connectors 16. FIG. 6
illustrates the application of a voltage across the second hologram
using the voltage generator 15 via the connectors 17.
[0103] In one embodiment of the invention the second holographic
recording medium is one of a photo thermal refractive or
holographic photopolymer, a forward mode HPDLC mixture or a reverse
mode HPDLC mixture. In one embodiment of the invention the third
holographic recording medium is one of a photo thermal refractive
or photopolymer, a forward mode HPDLC mixture or a reverse mode
HPDLC mixture. In one embodiment of the invention the diffracting
thickness of the first hologram is less than or equal to 1 micron.
In one embodiment of the invention the diffracting thickness of the
first hologram is less than or equal to 2 micron.
[0104] In one embodiment of the invention illustrated in FIGS.
2A-2C, 3A-3C and 4A-4E there is provided a method of mastering and
replicating holograms, the method comprising: [0105] a) providing a
laser apparatus for forming a first recording beam in a first
direction 1000 and a second recording beam in a second direction
1001; a stack 10 of substrates 11-14 each containing a first HPDLC
mixture; a holography photopolymer 40 on a substrate 50; a copy
holographic substrate 60 containing a second HPDLC mixture 70; a
HWP 20; and a linear polarizer 30 in FIGS. 3A-3C and 4A-4E; [0106]
b) the first and second beams interfering within each the first
HPDLC mixture to form a first hologram in each substrate; [0107] c)
stacking in sequence the linear polarizer, HWP and first holograms
(1-N) onto the second holographic photopolymer; [0108] d)
illuminating external surface of the holograms (1-N) with light of
a first polarization in the first direction, as indicated by rays
1015-1018 in FIG. 4C; [0109] e) the first holograms (1-N)
diffracting the light into zero order light in the first direction
1000 and diffracted rays 1011-1014 in FIG. 4C in the second
direction; [0110] f) the HWP rotating the incident light
polarization through ninety degrees into a second polarization;
[0111] g) the linear polarizer removing residual first polarization
light; [0112] h) the first direction light and the second direction
light interfering in the holographic photopolymer to form a second
hologram; [0113] i) placing the second hologram in contact with the
copy holographic substrate; [0114] j) illuminating external surface
of the second hologram with light of the second polarization in the
first direction 1050; [0115] k) the second hologram diffracting the
light into zero order light 1051 in the first direction and
diffracted light in the second direction 1052; [0116] l) the
diffracted and first order interfering in the copy holographic
substrate to form a third hologram.
[0117] In one embodiment of the invention steps d) to 1) are
repeated for a multiplicity of values of the first and second
directions, wherein the first and second directions are limited by
the diffraction efficiency angular bandwidth of the first hologram.
For example FIG. 4D shows the step of FIG. 4C in which the first
direction is at an opposing angle to the first direction of FIG. 4C
as indicated by the rays 1021,1022. This results in a different
diffraction direction as indicated by the rays 1030-1033 and
1040-1043. The first direction of FIG. 4D is also used in FIG.
4E.
[0118] In the embodiment of FIGS. 2-3 the first polarization is P
and the second polarization is S. The first hologram is ideally a
thin wide angular bandwidth SBG. Thinner grating will provide a
broader angular bandwidth. Typically the grating thickness may
range from 0.5-1.0 microns depending on the angular bandwidth
required. The SBGs are recorded using S-polarized light. The second
hologram is recorded into a high quality holographic photopolymer
material such as the ones supplied by Bayer Inc. In this case the
second hologram is illuminated with P-polarized light as SBGs will
only diffract P. To achieve the best contrast in the second
hologram the exposure must use S-polarized light. This is done by
means of the half wave plate (for rotating P to S) and linear
polarizer (for removing residual P-polarised light). Fine tuning of
the beam ration is provided by optimizing the angle of the HWP
relative to the linear polarizer. The aim is to have the ratio of
diffracted to zero order light in the second hologram as close to
1:1 as possible to provide the optimal beam ratio for the recording
of the third hologram. At this point it will be desirable to
increase the source brightness to compensate for light losses
through the polarizing components. As discussed above beam
intensity may be fined tune by applying voltages to the first
holograms. The third hologram is created using a contact copy
process in which the second hologram is illuminated by S-polarized
light
[0119] FIG. 7 is a schematic illustration of an optical arrangement
using a dove prism for recording on of the first holograms in one
embodiment of the invention. The dove prism 90 with acute angle W
(typically 45 degree) has its base in contact with the first
hologram substrate 10. Construction beams in the directions 1072,
1073 at angles U1, V1 to the surface normals 1070, 1071 interfere
in the substrate 10 to form the first hologram.
[0120] A method of replicating a hologram according to a preferred
embodiment of the invention in accordance with the basic principles
of the invention is shown in the flow diagram in FIG. 8. Referring
to the flow diagram, we see that the method comprises the following
steps: [0121] At step 2011 provide: N master SBGs having
construction angles in the first and second directions; a
holographic photopolymer recording medium; a HPDLC recording
medium; a half wave plate; and a linear polarizer. [0122] At step
2012 stack the master SBGs (1-N), linear polarizer and half wave
plate onto the second holographic recording medium. [0123] At step
2013 illuminate the master SBGs (1-N) with P-polarized light in the
first direction.
[0124] At step 2014 master SBGs (1-N) provide 0-order light in the
first direction and diffracted light in the second direction.
[0125] At step 2015 the first and second direction light interferes
to form the second hologram in the holographic photopolymer
recording medium. [0126] At step 2016 place the second hologram in
contact with the HPDLC recording medium.
[0127] At step 2017 illuminate the second hologram with S-polarized
light in the first direction. [0128] At step 2018 the second
hologram provides 0-order light in the first direction and
diffracted light in the second direction. [0129] At step 2019 the
first and second direction light interferes to form a copy SBG.
[0130] In one embodiment of the invention the first hologram is a
surface relief hologram such as binary grating. A method of
replicating a hologram in one embodiment of the invention in
accordance with the basic principles of the invention is shown in
the flow diagram in FIG. 9. Referring to the flow diagram, we see
that the method comprises the following steps: [0131] At step 2021
provide: N substrates each containing a surface relief hologram
having input and diffraction angles in first and second directions;
a second holographic recording medium; and a third holographic
recording medium. [0132] At step 2022 stack substrates (1-N) onto
the second holographic recording medium. [0133] At step 2023
illuminate substrates (1-N) with light in the first direction.
[0134] At step 2024 substrates (1-N) provide 0-order light in the
first direction and diffracted light in the second direction.
[0135] At step 2025 the first and second direction light interferes
to form the second hologram in the second holographic recording
medium. [0136] At step 2026 place the second hologram in contact
with the third holographic recording medium. [0137] At step 2027
illuminate the second hologram with light in the first direction.
[0138] At step 2028 the second hologram provides 0-order light in
the first direction and diffracted light in second direction.
[0139] At step 2029 the first and second direction light interferes
to form a copy SBG.
[0140] As illustrated in FIGS. 5-6 in one embodiment of the
invention there is further provided a voltage generator for applied
a voltage across at least one of the second hologram and the third
holographic recording medium. A method of replicating a hologram in
one embodiment of the invention in accordance with the basic
principles of the invention is shown in the flow diagram in FIG.
10. Referring to the flow diagram, we see that the method comprises
the following steps:
[0141] At step 2031 provide: N hologram substrates each containing
a first hologram having construction angles in first and second
directions; a second holographic recording medium; a third
holographic recording medium; and a voltage generator. [0142] At
step 2032 stack the hologram substrates (1-N) onto the second
holographic recording medium. [0143] At step 2033 illuminate the
hologram substrates (1-N) with light in the first direction. [0144]
At step 2034 the hologram substrates (1-N) provide 0-order light in
the first direction and diffracted light in the second direction.
[0145] At step 2035 the first and second direction light interferes
to form the second hologram in the second holographic recording
medium.
[0146] At step 2036 place the second hologram in contact with the
third holographic recording medium and connect the voltage
generator to the second hologram. [0147] At step 2037 illuminate
the second hologram with light in the first direction. Apply a
voltage across the second hologram. [0148] At step 2038 the second
hologram provides 0-order light in the first direction and
diffracted light in the second direction. [0149] At step 2039 the
first and second direction light interferes to form a third
hologram.
[0150] A further method of replicating a hologram in one embodiment
of the invention in accordance with the basic principles of the
invention is shown in the flow diagram in FIG. 11. Referring to the
flow diagram, we see that the method (based on the preferred
embodiment discussed above and illustrated in FIG. 8) comprises the
following steps: [0151] At step 2041 provide: N master SBGs having
construction angles in first and second directions; a holographic
photopolymer recording medium; a HPDLC recording medium; a half
wave plate; a linear polarizer; and a voltage generator. [0152] At
step 2042 stack the master SBGs (1-N), linear polarizer and half
wave plate onto the second holographic recording medium. [0153] At
step 2043 illuminate the master SBGs (1-N) with P-polarized light
in the first direction. [0154] At step 2044 the master SBGs (1-N)
provide 0-order light in the first direction and diffracted light
in the second direction. [0155] At step 2045 the first and second
direction light interferes to form the second hologram in the
holographic photopolymer recording medium. [0156] At step 2046
place the second hologram in contact with the HPDLC recording
medium and connect the voltage generator to the second hologram.
[0157] At step 2047 illuminate the second hologram with S-polarized
light in the first direction. Apply a voltage across the second
hologram. [0158] At step 2048 the second hologram provides 0-order
light in the first direction and diffracted light in the second
direction. [0159] At step 2049 the first and second direction light
interferes to form a copy SBG.
[0160] In one embodiment of the invention steps are repeated for a
predefined number of holographic prescriptions, that is, for a
multiplicity of vectors defining the first and second directions.
One first hologram (master) is used to produce all replicas at each
prescription. The first holograms (1-N) are illumination by each
first direction vector of a predefined set in turn. The first and
second directions are limited by the diffraction efficiency angular
bandwidth of said first hologram.
[0161] A method of replicating a hologram in one embodiment of the
invention (based on the embodiment of FIG. 1) in accordance with
the basic principles of the invention is shown in the flow diagram
in FIG. 12. Referring to the flow diagram, we see that the method
comprises the following steps: [0162] At step 2051 provide N
hologram substrates each containing a first hologram having
construction angles in first and second directions; a second
holographic recording medium; and a third holographic recording
medium. [0163] At step 2052 stack hologram substrates 1-N onto the
second holographic recording medium [0164] At step 2053 illuminate
hologram substrates (1-N) with light in the first direction [0165]
At step 2054 hologram substrates (1-N) provide 0-order light in the
first direction and diffracted light in the second direction.
[0166] At step 2055 the first and second direction light interferes
to form a second hologram in the second holographic recording
medium. [0167] At step 2056 place the second hologram in contact
with the third holographic recording medium. [0168] At step 2057
illuminate the second hologram with light in the first direction.
[0169] At step 2058 the second hologram provides 0-order light in
the first direction and diffracted light in the second direction.
[0170] At step 2059 the first and second direction light interferes
to form the third hologram. [0171] At step 2060 a new first
direction vector is selected from a pre-defined set. [0172] At step
2061 the process is repeated from step 2053 onwards until the last
vector in the pre-defined set has been selected.
[0173] It should be understood by those skilled in the art that
while the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. Various
modifications, combinations, sub-combinations and alterations may
occur depending on design requirements and other factors insofar as
they are within the scope of the appended claims or the equivalents
thereof.
* * * * *