U.S. patent application number 14/617746 was filed with the patent office on 2016-08-11 for optical components.
The applicant listed for this patent is Pasi Kostamo, Pasi Saarikko. Invention is credited to Pasi Kostamo, Pasi Saarikko.
Application Number | 20160231477 14/617746 |
Document ID | / |
Family ID | 55447117 |
Filed Date | 2016-08-11 |
United States Patent
Application |
20160231477 |
Kind Code |
A1 |
Saarikko; Pasi ; et
al. |
August 11, 2016 |
Optical Components
Abstract
A substantially transparent optical component, which comprises
polymer, is moulded. The optical component has substantially
matching grating imprints on respective portions of its surface,
which imprints have a substantially zero relative orientation
angle. Substantially transparent molten polymer is forced between
two surfaces of a moulding component. The molten polymer is forced
into contact with surface modulations which form two substantially
matching gratings. An alignment portion is located so that light
which has interacted with both gratings is observable when the
substantially transparent polymer is between the surfaces. Whilst
the polymer is still liquid, the moulding component is reconfigured
from a current configuration to a new configuration in which the
fringe spacing of a fringe pattern formed by the two gratings is
substantially maximal, thus aligning the gratings to have a
substantially zero relative orientation angle. The new
configuration is maintained whilst the polymer sets.
Inventors: |
Saarikko; Pasi; (Espoo,
FI) ; Kostamo; Pasi; (Espoo, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Saarikko; Pasi
Kostamo; Pasi |
Espoo
Espoo |
|
FI
FI |
|
|
Family ID: |
55447117 |
Appl. No.: |
14/617746 |
Filed: |
February 9, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/1866 20130101;
G02B 27/0172 20130101; G02B 27/0103 20130101; B29D 11/00663
20130101; G02B 5/1857 20130101; G02B 5/1852 20130101; G02B
2027/0109 20130101 |
International
Class: |
G02B 5/18 20060101
G02B005/18; G02B 27/01 20060101 G02B027/01 |
Claims
1. A moulding process for making a substantially transparent
optical component which comprises polymer, the optical component
having substantially matching grating imprints on respective
portions of its surface, wherein the grating imprints have a
substantially zero relative orientation angle, the process
comprising: forcing substantially transparent molten polymer
between two surfaces of a moulding component, the surfaces having
surface modulations which form two substantially matching gratings,
the molten polymer forced into contact with the surface modulations
so as to imprint the gratings in the polymer, wherein the moulding
component is configurable to change the relative orientation angle
of the gratings; wherein at least an alignment portion of the
moulding component is substantially transparent, the alignment
portion located so that light which has interacted with both
gratings is observable from the alignment portion when the
substantially transparent polymer is between the surfaces, whereby
an observable fringe pattern is formed as the relative orientation
angle of the gratings is changed towards zero, the fringe pattern
exhibiting a fringe spacing which increases as the relative
orientation angle decreases, the process further comprising: whilst
the polymer is still liquid, reconfiguring the moulding component
from a current configuration to a new configuration in which the
fringe spacing of the fringe pattern is substantially maximal, thus
aligning the gratings to have a substantially zero relative
orientation angle, wherein the new configuration is maintained
whilst the polymer sets.
2. A moulding process according to claim 1 comprising capturing
images of the fringe pattern as the cavity is reconfigured and
performing an automatic image recognition procedure to detect the
fringe pattern in the images, wherein the step of reconfiguring is
based on the results of the image recognition procedure.
3. A moulding process according to claim 1 comprising sensing light
of only a small portion of the fringe pattern as the cavity is
reconfigured, wherein the step of reconfiguring is based on the
rate at which the intensity of that light changes.
4. A moulding process according to claim 1 wherein each of the
gratings lies substantially parallel to a plane, and the gratings
do not overlap or only partially overlap with one another when
viewed along a direction normal to the plane.
5. A moulding process according to claim 1 comprising illuminating
the gratings with an expanded laser beam, the fringe pattern formed
by light of the laser beam which has interacted with both
gratings.
6. A moulding process according to claim 5 comprising: receiving
the light of the laser beam at a detector, part of the received
light having been reflected from one of the gratings and another
part of the light having been reflected from the other of the
gratings, whereby the part and the other part interfere at the
detector to form the fringe pattern on a detection surface of the
detector; and using an output of the detector to control the
reconfiguring step.
7. A moulding process according to claim 1 wherein the opposing
portions of the cavity's surface are substantially parallel, so
that the opposing portions of the moulded optical component's
surface are substantially parallel.
8. A moulding process according to claim 1, wherein a first and a
second further grating are formed on other opposing portions of the
cavity's surface, the first further grating having a first
orientation angle .PHI.1 relative to the one of the gratings and
the second further grating having a second orientation angle .PHI.2
relative to the other of the gratings, so that the first and second
further gratings are imprinted in the polymer having a relative
orientation angle that is substantially |.PHI.2-.PHI.1| in the new
configuration.
9. A moulding process according to claim 1 wherein at least one of
the surfaces of the moulding component is curved so that the
polymer sets in a curved configuration.
10. A moulding process according to claim 1 wherein the moulding
component is arranged to provide a moulding cavity, the surfaces
being of the moulding cavity, and wherein the polymer is forced
into the moulding cavity to force the polymer into contact with the
surface modulations, the moulding component reconfigured to the new
configuration whilst the polymer in the cavity is still liquid.
11. A moulding process according to claim 1 wherein the polymer is
arranged in layers on the surface of a substantially transparent
substrate, whereby the gratings are imprinted in the layers, the
moulding component reconfigured to the new configuration whilst the
layers are still liquid, wherein the optical component comprises
the substrate and the layers once set.
12. A product obtained by the process of claim 1.
13. A moulding apparatus for moulding a substantially transparent
optical component which comprises polymer, the optical component
having substantially matching grating imprints on opposing portions
of its surface, wherein the grating imprints have a substantially
zero relative orientation angle, the apparatus comprising: a
moulding component having two surfaces, the surfaces having surface
modulations which form two substantially matching gratings, wherein
the moulding component is configurable to change the relative
orientation angle of the gratings; a drive mechanism coupled to the
moulding component controllable to configure the moulding
component; wherein at least an alignment portion of the moulding
component is substantially transparent, the alignment portion
located so that light which has interacted with both gratings is
observable from the alignment portion when the substantially
transparent polymer is between the surfaces, whereby an observable
fringe pattern is formed as the relative orientation angle of the
gratings is changed towards zero, the fringe pattern exhibiting a
fringe spacing which increases as the relative orientation angle
decreases, the apparatus further comprising: a light sensor
configured to receive at least some of the light which has
interacted with both gratings; and a controller configured, whilst
the polymer is still liquid, to control the drive mechanism based
on sensed data received from the image sensor to reconfigure the
moulding component from a current configuration to a new
configuration in which the fringe spacing of the fringe pattern is
substantially maximal, thus aligning the gratings to have a
substantially zero relative orientation angle, wherein the new
configuration is maintained whilst the polymer sets.
14. A moulding apparatus according to claim 13 wherein the light
sensor comprises a camera which captures images of the fringe
pattern as the cavity is reconfigured, and wherein the controller
comprises an image recognition module which performs an automatic
image recognition procedure to detect the fringe pattern in the
images, wherein the controller reconfigures the cavity based on the
results of the image recognition procedure.
15. A moulding apparatus according to claim 13 wherein the light
sensor senses light of only a small portion of the fringe pattern
as the cavity is reconfigured, and the controller is reconfigured
based on the rate at which the intensity of that light changes.
16. An optical component for use in an optical system, wherein the
optical component is substantially transparent and has two opposing
outer surfaces, wherein at least a respective portion of each of
the opposing surfaces is formed of polymer in which a respective
grating is imprinted, wherein the gratings substantially match one
another and have a relative orientation angle that is zero to
within one thousandth of a degree.
17. An optical component according to claim 16, wherein the
relative orientation angle is zero to within one half of one
thousandth of a degree.
18. An optical component according to claim 16, wherein the
opposing surface portions are substantially parallel.
19. An optical component according to claim 16, when used as a
waveguide in a display system to transport light of an image to a
user's eye.
20. An optical component according to claim 16, when used in a
wearable display system that is wearable by the user.
Description
BACKGROUND
[0001] Optical components can be used in optical systems to alter
the state of visible light in a predictable and desired manner, for
example in display systems to make a desired image visible to a
user. Optical components can interact with light by way of
reflection, refractions, diffraction etc. Diffraction occurs when a
propagating wave interacts with a structure, such as an obstacle or
slit. Diffraction can be described as the interference of waves and
is most pronounced when that structure is comparable in size to the
wavelength of the wave. Optical diffraction of visible light is due
to the wave nature of light and can be described as the
interference of light waves. Visible light has wavelengths between
approximately 390 and 700 nanometres (nm) and diffraction of
visible light is most pronounced when propagating light encounters
structures similar scale e.g. of order 100 or 1000 nm in scale.
[0002] One example of a diffractive structure is a periodic
structure. Periodic structures can cause diffraction of light which
is typically most pronounced when the periodic structure has a
spatial period of similar size to the wavelength of the light.
Types of periodic structures include, for instance, surface
modulations on a surface of an optical component, refractive index
modulations, holograms etc. When propagating light encounters the
periodic structure, diffraction causes the light to be split into
multiple beams in different directions. These directions depend on
the wavelength of the light thus diffractions gratings cause
dispersion of polychromatic (e.g. white) light, whereby the
polychromatic light is split into different coloured beams
travelling in different directions.
[0003] When the period structure is on a surface of an optical
component, it is referred to a surface grating. When the periodic
structure is due to modulation of the surface itself, it is
referred to as a surface relief grating (SRG). An example of a SRG
is uniform straight grooves in a surface of an optical component
that are separated by uniform straight groove spacing regions.
Groove spacing regions are referred to herein as "lines", "grating
lines" and "filling regions". The nature of the diffraction by a
SRG depends both on the wavelength of light incident on the grating
and various optical characteristics of the SRG, such as line
spacing, groove depth and groove slant angle. SRGs have many useful
applications. One example is an SRG light guide application. A
light guide (also referred to herein as a "waveguide") is an
optical component used to transport light by way of internal
reflection e.g. total internal reflection (TIR) within the light
guide. A light guide may be used, for instance, in a light
guide-based display system for transporting light of a desired
image from a light engine to a human eye to make the image visible
to the eye. Incoupling and outcoupling SRGs on surface(s) of the
light guide can be used for inputting light to and outputting light
from the waveguide respectively.
[0004] Surface gratings can be fabricated by way of a suitable
microfabrication process to create appropriate surface modulations
on a substrate. Microfabrication refers to the fabrication of
desired structures of micrometre scales and smaller (such as
surface gratings). Microfabrication may involve etching of and/or
deposition on a substrate (and possibly etching of and/or
deposition on a film deposited on the substrate) to create the
desired microstructure on the substrate (or film on the substrate).
As used herein, the term "patterning a substrate" or similar
encompasses all such etching of/deposition on a substrate or
substrate film. Whilst a substrate patterned with a surface grating
may be suitable for use as an optical component in an optical
system itself, a patterned substrate can also be used as a
production masters for manufacturing such optical components. For
example, a fused silica substrate (or similar), once patterned with
a surface grating, can then be used as part of a moulding component
for moulding optical components from polymer e.g. the moulding
component may be arranged to provide a moulding cavity with the
surface grating on the surface of the cavity. When liquid polymer
is forced into the moulding cavity, it is forced into contact with
the surface grating so as to imprint the surface grating in the
polymer, which then sets to form a solid polymer optical component
with the surface grating imprinted on its surface. Thus, large
numbers of polymer optical components can be mass-manufactured
using the same patterned substrate in an inexpensive, quick and
straightforward manner.
SUMMARY
[0005] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed
subject matter. Nor is the claimed subject matter limited to
implementations that solve any or all of the disadvantages noted in
the Background section.
[0006] A first aspect is directed to a moulding process for making
a substantially transparent optical component which comprises
polymer. The optical component has substantially matching grating
imprints on respective portions of its surface. The grating
imprints have a substantially zero relative orientation angle. The
process comprises the following steps. Substantially transparent
molten polymer is forced between two surfaces of a moulding
component. The surfaces have surface modulations which form two
substantially matching gratings. The molten polymer is forced into
contact with the surface modulations so as to imprint the gratings
in the polymer. The moulding component is configurable to change
the relative orientation angle of the gratings. At least an
alignment portion of the moulding component is substantially
transparent. The alignment portion is located so that light which
has interacted with both gratings is observable from the alignment
portion when the substantially transparent polymer is between the
surfaces, whereby an observable fringe pattern is formed as the
relative orientation angle of the gratings is changed towards zero.
The fringe pattern exhibits a fringe spacing which increases as the
relative orientation angle decreases. Whilst the polymer is still
liquid, the moulding component is reconfigured from a current
configuration to a new configuration in which the fringe spacing of
the fringe pattern is substantially maximal, thus aligning the
gratings to have a substantially zero relative orientation angle.
The new configuration is maintained whilst the polymer sets.
[0007] A second aspect is directed to a moulding apparatus for
moulding a substantially transparent optical component which
comprises polymer. The optical component has substantially matching
grating imprints on opposing portions of its surface. The grating
imprints have a substantially zero relative orientation angle. The
apparatus comprises a moulding component, a drive mechanism, a
light sensor and a controller. The moulding component has two
surfaces, the surfaces having surface modulations which form two
substantially matching gratings. The moulding component is
configurable to change the relative orientation angle of the
gratings. The drive mechanism is coupled to the moulding component
and is controllable to configure the moulding component. At least
an alignment portion of the moulding component is substantially
transparent, the alignment portion located so that light which has
interacted with both gratings is observable from the alignment
portion when the substantially transparent polymer is between the
surfaces, whereby an observable fringe pattern is formed as the
relative orientation angle of the gratings is changed towards zero.
The fringe pattern exhibits a fringe spacing which increases as the
relative orientation angle decreases. The light sensor is
configured to receive at least some of the light which has
interacted with both gratings. The controller is configured, whilst
the polymer is still liquid, to control the drive mechanism based
on sensed data received from the image sensor to reconfigure the
moulding component from a current configuration to a new
configuration in which the fringe spacing of the fringe pattern is
substantially maximal, thus aligning the gratings to have a
substantially zero relative orientation angle. The new
configuration is maintained whilst the polymer sets.
[0008] Products obtained by any of the processes disclosed herein
are also provided. Such products include an optical component for
use in an optical system, which optical component is substantially
transparent, formed of polymer, and has substantially matching
gratings on opposing portions of its surface, the gratings having a
relative orientation angle that is zero to within one thousandth of
a degree.
BRIEF DESCRIPTION OF FIGURES
[0009] To aid understanding of the subject matter, reference will
now be made by way of example only to the following drawings in
which:
[0010] FIG. 1A is a schematic plan view of an optical
component;
[0011] FIG. 1B is a schematic illustration of an optical component,
shown interacting with incident light and viewed from the side;
[0012] FIG. 2A is a schematic illustration of a straight binary
grating, shown interacting with incident light and viewed from the
side;
[0013] FIG. 2B is a schematic illustration of a slanted binary
grating, shown interacting with incident light and viewed from the
side;
[0014] FIG. 2C is a schematic illustration of an overhanging
triangular grating, shown interacting with incident light and
viewed from the side;
[0015] FIG. 3 is a perspective view of an optical component;
[0016] FIGS. 4A, 4B and 4C are side, plan and perspective views of
parts of a moulding apparatus respectively;
[0017] FIG. 4D shows various views of a fringe observed at
different points in time during a moulding process of one
embodiment;
[0018] FIG. 5 is a block diagram of a moulding apparatus;
[0019] FIGS. 6A and 6B are side views of a moulding apparatus
during a moulding process of another embodiment.
DETAILED DESCRIPTION
[0020] FIGS. 1A and 1B show from the top and the side respectively
a substantially transparent optical component 2, such as a wave
guide, having an outer surface S. At least a portion of the surface
S exhibits surface modulations that form a surface grating 4, which
is a SRG. Such a portion is referred to as a "grating area". The
modulations comprise grating lines which are substantially parallel
and elongate (substantially longer than they are wide), and also
substantially straight in this example (though they need not be
straight in general).
[0021] FIG. 1B shows the optical component 2, and in particular the
grating 4, interacting with an incoming illuminating light beam I
that is inwardly incident on the SRG 4. The light I is white light
in this example, and thus has multiple colour components. The light
I interacts with the grating 4 which splits the light into several
beams directed inwardly into the optical component 2. Some of the
light I may also be reflected back from the surface S as a
reflected beam R0. A zero-order mode inward beam T0 and any
reflection R0 are created in accordance with the normal principles
of diffraction as well as other non-zero-order (.+-.n-order) modes
(which can be explained as wave interference). FIG. 1B shows
first-order inward beams T1, T-1; it will be appreciated that
higher-order beams may or may not also be created depending on the
configuration of the optical component 2. Because the nature of the
diffraction is dependent on wavelength, for higher-order modes,
different colour components (i.e. wavelength components) of the
incident light I are, when present, split into beams of different
colours at different angles of propagation relative to one another
as illustrated in FIG. 1B.
[0022] FIGS. 2A-2C are close-up schematic cross sectional views of
different exemplary SRGs 4a-4c (collectively referenced as 4
herein) that may be formed by modulations of the surface S of the
optical component 2 (which is viewed from the side in these
figures). Light beams are denoted as arrows whose thicknesses
denote approximate relative intensity (with higher intensity beams
shown as thicker arrows).
[0023] FIG. 2A shows an example of a straight binary SRG 4a. The
straight binary grating 4a is formed of a series of grooves 7a in
the surface S separated by protruding groove spacing regions 9a
which are also referred to herein as "filling regions", "grating
lines" or simply "lines". The grating 4a has a spatial period of d
(referred to as the "grating period"), which is the distance over
which the modulations' shape repeats. The grooves 7a have a depth h
and have substantially straight walls and substantially flat bases.
As such, the filling regions have a height h and a width that is
substantially uniform over the height h of the filling regions,
labelled "w" in FIG. 2A (with w being some fraction f of the
period: w=f*d).
[0024] For a straight binary grating, the walls are substantially
perpendicular to the surface S. For this reason, the grating 4a
causes symmetric diffraction of incident light I that is entering
perpendicularly to the surface, in that each +n-order mode beam
(e.g. T1) created by the grating 4a has substantially the same
intensity as the corresponding -n-order mode beam (e.g. T-1),
typically less than about one fifth (0.2) of the intensity of the
incident beam I.
[0025] FIG. 2B shows an example of a slanted binary grating 4b. The
slanted grating 4b is also formed of grooves, labelled 7b, in the
surface S having substantially straight walls and substantially
flat bases separated by lines 9b of width w. However, in contrast
to the straight grating 4a, the walls are slanted by an amount
relative to the normal, denoted by the angle .alpha. in FIG. 2B.
The grooves 7b have a depth h as measured along the normal. Due to
the asymmetry introduced by the non-zero slant, .+-.n-order mode
inward beams travelling away from the slant direction have greater
intensity that their .-+.n-order mode counterparts (e.g. in the
example of FIG. 2B, the T1 beam is directed away from the direction
of slant and has usually greater intensity than the T-1 beam,
though this depends on e.g. the grating period d); by increasing
the slant by a sufficient amount, those -Tn counterparts can be
substantially eliminated (i.e. to have substantially zero
intensity). The intensity of the T0 beam is typically also reduced
very much by a slanted binary grating such that, in the example of
FIG. 2B, the first-order beam T1 typically has an intensity of at
most about four fifths (0.8) the intensity of the incident beam
I.
[0026] The binary gratings 4a and 4b can be viewed as spatial
waveforms embedded in the surface S that have a substantially
square wave shape (with period d). In the case of the grating 4b,
the shape is a skewed square wave shape skewed by .alpha..
[0027] FIG. 2C shows an example of an overhanging triangular
grating 4c which is a special case of an overhanging trapezoidal
grating. The triangular 4c is formed of grooves 7c in the surface S
that are triangular in shape (and which thus have discernible tips)
and which have a depth h as measured along the normal. Filling
regions 9c take the form of triangular, tooth-like protrusions
(teeth), having medians that make an angle .alpha. with the normal
(a being the slant angle of the grating 4c). The teeth have tips
that are separated by d (which is the grating period of the grating
4c), a width that is w at the base of the teeth and which narrows
to substantially zero at the tips of the teeth. For the grating of
FIG. 4c, w.apprxeq.d, but generally can be w<d. The grating is
overhanging in that the tips of the teeth extend over the tips of
the grooves. It is possible to construct overhanging triangular
grating gratings that substantially eliminate both the
transmission-mode T0 beam and the .-+.n-mode beams, leaving only
.+-.n-order mode beams (e.g. only T1). The grooves have walls which
are at an angle .alpha. to the median (wall angle). The grating 4c
can be viewed as a spatial waveform embedded in S that has a
substantially triangular wave shape, which is skewed by a.
[0028] The grooves and spacing regions that form the gratings 4a-4c
constitute surface modulations.
[0029] Other type of grating are also possible, for example other
types of trapezoidal grating patterns (which may not narrow in
width all the way to zero), sinusoidal grating patterns etc. and
have a modulation width that can be readily defined in a suitable
manner. Such other patterns also exhibit depth h, linewidth w,
slant angle .alpha. and wall angles .gamma. which can be defined in
a similar manner to FIG. 2A-C.
[0030] A grating 4 has a grating vector (generally denoted as d),
whose size (magnitude) is the grating period d, and which is in a
direction perpendicular to the grating lines which form that
grating--see FIG. 1A.
[0031] In light guide-based display applications (e.g. where SRGs
are used for coupling of light into and out of a light guide of the
display system, and/or for providing beam expansion of beams
coupled into the waveguide), d is typically between about 250 and
500 nm, and h between about 30 and 400 nm. The slant angle .alpha.
is typically between about -45 and 45 degrees and is measured in
the direction of the grating vector.
[0032] FIG. 3A shows a perspective view of an optical component 2
having two separate gratings 4F and 4B on respective portions of
the component's surface, which are opposing, substantially parallel
and substantially flat. Viewed as in FIG. 3A, these are front and
rear portions of the surface. Each of the gratings 4B, 4F is formed
of substantially parallel, elongate grating lines and grooved,
which are also substantially straight in this example. The gratings
4B, 4F have respective grating periods d.sub.F, d.sub.B, which may
or may not be the same. The gratings 4B, 4F can be of the type
described above (and may, but need not be, of the same type).
[0033] The gratings 4F and 4B have respective grating vectors
d.sub.F, d.sub.B (front and back grating vectors) which run
parallel to their respective grating lines. A plane 3 is shown,
which has a normal {circumflex over (n)}(3) (unit vector
perpendicular to the plane 3) shown as a dotted arrow. In the
example of FIG. 3A, because the front and rear surface portions are
substantially parallel, they have substantially the same normals as
the plane (.apprxeq.{circumflex over (n)}(3)) so that the front and
rear surface portions and the plane 3 are all substantially
parallel (more generally, for non-parallel surface portions, the
plane 3 could be defined to have a normal {circumflex over (n)}(3)
in the approximate direction of the vector sum of the normals to
the front and rear surface portions as this represents a direction
of the mean of those normals, which normal {circumflex over (n)}(3)
is considered to substantially match those normals when so
defined).
[0034] Vectors 15F, 15B (shown as dashed arrows) lie in the plane
3, which are geometric projections of the front and back grating
vectors d.sub.F, d.sub.B onto the plane 3. The projections 15F, 15B
have an angular separation .DELTA..phi., which is an angle in the
plane 3 (azimuth), and which is the angular separation of d.sub.F,
d.sub.B when viewed along the normal {circumflex over (n)}(3). The
angular separation .DELTA..phi. is a measure of the relative
orientation of the gratings 4F, 4B and is referred to herein as the
relative orientation angle of the gratings 4F, 4B. When
.DELTA..phi.=0, the grating lines of the gratings 4F, 4B are
aligned, at least when viewed along the normal 3', and the gratings
4F, 4B are said to be aligned. In the example of FIG. 3, because
the front and rear surface portions are substantially parallel,
when .DELTA..phi.=0 the gratings 4F, 4B are aligned when viewed
from any viewpoint (more generally, this is true when the gratings
4F, 4B are arranged on opposing surface portions such that their
respective grating lines are parallel when .DELTA..phi.=0).
[0035] As will be apparent, the value of .DELTA..phi. affects the
optical characteristics of the optical component 2. In
waveguide-based display applications, in which the optical
component 2 forms part of a waveguide-based display system,
misalignment of the gratings (that is deviation from zero in
.DELTA..phi.) can--depending on the function of the gratings--cause
unwanted distortion of the image.
[0036] A moulding process for moulding optical components of the
type shown in FIG. 3 from polymer will now be described with
reference to FIGS. 4A-4D, which show various views of a moulding
apparatus 1 during the process. The polymer is substantially
transparent, which makes the process suitable for (among other
things) moulding waveguides for waveguide-based display systems
(see above).
[0037] FIG. 4A shows the apparatus 1 from the side. The apparatus
comprises blocks 5F, 5B (front, back), 5U, 5D (upper,
lower--visible in FIG. 4A only) and 5L (left--visible in FIG. 4B
only), which are formed of a rigid material. The reference numeral
5 is used to refer to the blocks collectively. The blocks are
arranged in contact so as to form a cavity 11 (moulding cavity),
with regions of their surfaces (inner surface regions) forming the
surface of the cavity 11. The rigid blocks 5 constitute a moulding
component.
[0038] Portions of the front and back blocks' inner surface regions
are modulated to form respective gratings 4'F, 4'B (front and rear
cavity gratings) on each of those inner surface portions, which
have structures corresponding to the gratings 4F, 4B of the optical
component 2 shown in FIG. 3 respectively--in this case, each cavity
grating 4'F, 4'B is formed by surface modulations in the form of
substantially parallel, elongate and substantially straight grating
lines/grooves, and have periods d.sub.F, d.sub.B respectively.
These inner surface portions constitute opposing portions of the
surface of the cavity 11, which are also substantially parallel to
one another.
[0039] The cavity gratings 4'F, 4'B can be patterned on the front
and rear blocks 5F, 5B, for instance, by way of a suitable
microfabrication process, or they may themselves be moulded from a
suitably patterned substrate.
[0040] An injection component 10 forces polymer 8 into the moulding
cavity 11 (from the right as viewed in FIG. 4B) when the polymer 8
is in a molten (and thus liquid) state. In this manner, the liquid
polymer 8 is forced into contact with the front and rear cavity
gratings 4'F, 4'B--that is, into contact with the grooved and lines
that form those gratings, which has the effect of imprinting the
structure of the cavity gratings 4'F, 4'B in the polymer 8. The
blocks 5 are sufficiently rigid to resist distortion from the force
of the liquid polymer, so the gratings are imprinted undistorted.
This is ultimately the mechanism by which the gratings 4F, 4B are
formed on the optical component 2, itself formed by the polymer 8
upon setting, and for this reason the gratings 4F, 4B of the final
optical component 2 are referred to hereinbelow as front and rear
"imprint gratings" or equivalently "grating imprints" 4F, 4B. The
front and rear surface portions of the final optical component 2 on
which the imprint gratings 4F, 4B are formed correspond to the
front and rear surface portions of the moulding cavity. The overall
size and shape of the final component 2 matches that of the cavity
11 when the polymer was allowed to set therein.
[0041] In FIGS. 4A-4D, the z-direction is that of the normal
{circumflex over (n)}(3) as defined in relation to the final
optical component (which is perpendicular to the cavity surface
portions on which the cavity gratings 4'F, 4'B are formed in this
example), the xy-plane corresponds to the plane 3 of FIG. 3 (which
lies parallel to those cavity surface portions in this example),
and the cavity gratings 4'F, 4'B have a relative orientation angle
.DELTA..phi.' that is defined in an equivalent manner to that of
the imprint gratings 4F, 4B (i.e. as their angular separation
measured in the xy-plane).
[0042] The arrangement of the rigid blocks 5 is not fixed: at least
one of the front and back blocks 5B, 4F (the back block 5B in this
example) is susceptible to xy-rotation whilst still maintaining the
integrity of the moulding cavity 11 so that it can be rotated
whilst continuing to hold the liquid polymer in the cavity 11.
Controlled xy-rotation of the back block 5B is effected by
controlling a suitable drive mechanism coupled to the back block
5B. Using commercially available drive mechanisms, it is possible
to controller xy-rotation of the back block 5B to effect controlled
rotation of the back block 5B by miniscule amounts (fractions of a
thousandth of a degree, or less) in a regulated manner.
[0043] By adjusting the xy-orientation angle of the front and back
blocks 5B, 5F relative to one another so as to adjust the relative
orientation angle .DELTA..phi.' of the cavity gratings 4'F, 4'B, it
is possible to precisely align the cavity gratings 4'F, 4'B (that
is, to have a substantially zero .DELTA..phi.') before the polymer
8 sets. By maintaining a substantially zero .DELTA..phi.' whilst
the polymer sets, the imprint gratings 4F, 4B on the optical
component 2--as formed when the polymer 8 finished setting--are as
aligned with equal precision as (i.e. with substantially zero
.DELTA..phi.=.DELTA..phi.'). The mechanism by which this precise
alignment is achieved will now be described with reference to FIGS.
4C and 4D.
[0044] FIG. 4C shows a perspective view of components of the
moulding apparatus 1. A light sensor 6 (also shown in FIGS. 4A-4B)
is positioned forward of the moulding cavity 5 to receive light
propagating towards the sensor along a line of sight (LOS--shown as
a dotted line) that has passed through a portion 7 of the moulding
component 5 (alignment portion), which is a portion of the front
block 4'F in this example. The LOS is oriented so as to intersect
both of the cavity gratings 4F, 4B. At least the alignment portion
7 of the moulding component is substantially transparent along the
LOS, so that light which has interacted with both gratings can
propagate out of the moulding component along the LOS.
[0045] The disclosure recognizes that, when the cavity gratings
4'F, 4'B are in near alignment, an observable fringe pattern is
formed that is observable along the LOS. A "fringe pattern" means a
pattern created when light interacts with two substantially
matching gratings (in this cast, the patterns of the cavity
gratings 4'F, 4'B, which are perceived to overlap when viewed along
the LOS) to create a pattern with fringes, the fringe spacing of
which depends on the relative orientation angle of the gratings.
The fringe pattern is formed of a series of alternating light and
dark fringes, whose spacing increases as the relative orientation
angle of the cavity gratings 4'F, 4'B is changed towards zero, at
which the fringe spacing become maximal (theoretically infinite
were the patterns to be exactly aligned with a relative orientation
angle of exactly zero). "Near alignment" means that .DELTA..phi.'
is within a range near zero that the fringe spacing is detectable
(i.e. not so close to zero that the fringe spacing is too large to
be detectable, but not so far from zero that the fringe spacing is
too small to be detectable).
[0046] In practice, the Fringe pattern is best observed using
diffracted light from the gratings. The diffracted light will
generally propagate along almost the same path as the incident
light but in the opposite direction. The path along which
incident/diffracted light propagates is labelled I/D in FIG. 4A (R
denotes the path followed by light reflected from the back grating
4'B) The LOS is substantially parallel to I/D, thus light visible
along the LOS will include light which has interacted with both of
the cavity gratings 4'F, 4'B, including when the polymer 8 is in
the cavity 11 (as the polymer 8 is also substantially transparent
and thus permits the passage of such light to the sensor 6). Thus,
the sensor 6 is able to receive light from inside the moulding
cavity which has interacted with both cavity gratings 4'F, 4'B. In
the example of FIG. 4A, this light will have been reflected from
the back grating 4'B (the reflected light being of a reflective
diffraction mode) before passing through the front grating 4'F.
[0047] When the relative orientation angle
.DELTA..phi.'.apprxeq.(5/1000).degree., the fringe pattern will
typically have a fringe spacing around 2 mm, which is readily
observable. As this angle .DELTA..phi.' is decreased, the fringe
spacing increases to the point at which it becomes substantially
maximal--this is the point at the fringe spacing is so large that
the pattern is no longer observable because the fringes are larger
than the cavity gratings, or at least larger than a portion of the
grating being if only that portion is being observed. At this point
of substantially maximal fringe spacing, .DELTA..phi.' is
substantially zero--in practice, when .DELTA..phi.' is no more than
about (0.5*1/1000).degree. to (1/1000).degree..
[0048] This is exploited present moulding process as follows.
Whilst the polymer 8 in the moulding cavity 11 is still liquid, the
front and back blocks 5B, 5F are brought into near alignment if
they are not already in near alignment, so that the fringe pattern
is observable along the LOS (current cavity configuration). Their
relative orientation angle .DELTA..phi.' is then fine-tuned until
the fringe spacing becomes substantially maximal, at which point
.DELTA..phi.' is substantially zero (new and final cavity
configuration). That new configuration (with the substantially zero
.DELTA..phi.') is maintained whilst the polymer 8 sets to form the
optical component 2, with the relative orientation angle
.DELTA..phi. of the imprint gratings 4F, 4B being substantially
zero (equal to .DELTA..phi.' as reached in the new and final
configuration) in the final component 2.
[0049] FIG. 4D shows views of the alignment portion 7 along the LOS
at various points in time during the moulding process. A fringe
pattern is visible at these points in time, which exhibits a
changing fringe spacing D. The left-most view represents a view at
a point in time when the gratings are in near alignment. Moving to
the right, views are shown at points in time as the relative
orientation angle .DELTA..phi.' is changed towards zero (with D
increasing accordingly) until reaching the point at which D is
substantially maximal as shown in the left-most view (which
represents an exemplary view in or near the new and final
configuration).
[0050] In practice, visibility of the fringe pattern can be
increased by suitable illumination of the apparatus. For instance,
to enhance the visibility of the fringe pattern, a laser (not
shown) may be used to provide a beam that is directed towards the
alignment portion 7. The beam is reflectively diffracted back of
the back grating 4'B and the diffracted beam then passes thought
the front grating 4'F towards the sensor 6. A beam expander (not
shown) may be used to expand the beam before reaching the alignment
portion 7, so as to increase the area over which the visibility is
enhanced. For example, the beam may be expanded to encompass the
cavity gratings 4'F, 4'B to provide the enhanced visibility of the
fringe patterns over the full extent of the cavity gratings 4'F,
4'B. Curved components can be made using a curved mould i.e. the
surfaces of the moulding component on which the gratings 4'F and
4'B care formed can be curved, whereby the curvature is imparted to
the polymer as well as the structure of the gratings 4'f, 4'B.
[0051] FIG. 5 is a block diagram of the moulding apparatus 1, which
comprises a controller 20 connected to control both the drive
mechanism 22 and the injection component 10, and to receive sensed
data from the sensor 6. The drive mechanism is coupled to at least
one of the back blocks 5F, 5B that form part of the moulding
component (back block 5B in this example) for fine-tuning the
relative orientation angle .DELTA..phi.' of the front and back
cavity gratings 4'B, 4'B. The controller 20 can adjust
.DELTA..phi.' automatically by controlling the drive mechanism 22,
once it has controlled the injection component 10 force the polymer
8 into the cavity 11.
[0052] The controller 20 receives the sensed data from the sensor
6, and adjusts the relative orientation angle .DELTA..phi.' of the
front and back cavity gratings 4'B, 4'F based on the sensed data
until .DELTA..phi.' is substantially zero by effecting the
procedure outlined above. The controller may be implemented by code
executed on a processor.
[0053] In a first embodiment, the sensor 6 comprises an image
sensing component in the form of a camera, which supplies images of
the alignment portion 7, taken along the LOS, to the controller 20
(such images capturing the views shown in FIG. 4D). The controller
comprises an image recognition module which performs an automatic
image recognition procedure on the received images to detect the
fringes of the fringe pattern when captured in the images. The
controller adjusts .DELTA..phi.' until the results of the image
recognition procedure indicate that the fringe spacing D is
maximal, and maintains that .DELTA..phi.' until the polymer has
set.
[0054] When illuminated with the laser beam, the fringe pattern is
formed by light of the laser beam which has interacted with both
gratings. The fringe pattern may not, and need not, be visible on
any surface of either mould as the fringe pattern obtained with
expanded laser beam can be recorded directly to a pixelated
detector (for example, an array of individual pixel detectors),
i.e. light reflected back from both alignment gratings interferes
and creates the fringe patters on a detection surface of detector.
The detector for example may part of the camera. In this manner,
the pattern is observed on the surface of a detector instead on the
surface of the moulds. The detector is used to detect the fringe
spacing as created on the detector, and the moulding process is
controlled based on the detected fringe spacing to align the
gratings to the maximal fringe spacing.
[0055] In a second embodiment, the sensor 6 comprises a photodiode,
which is shielded from surrounding light but for a small
pinhole--e.g. having a diameter .about.1 mm (order of
magnitude)--through which only a small portion of the fringe
pattern is observable. That is, such that the only light received
by the photodiode is from a small portion of the fringe pattern the
size of the pinhole, so that once the cavity gratings are in near
alignment, the fringes are larger than the pinhole. The controller
20 then changes .DELTA..phi.', e.g. at a uniform rate. As the
cavity gratings 4'F, 4'B are brought into alignment, the fringe
spacing increases, which effectively results in movement of the
fringes (this is evident in FIG. 4D). Thus the intensity of the
light received by the photodiode oscillates between high (when only
part of a light fringe is observable through the pinhole) and low
(when only part of a dark fringe is perceivable through the
pinhole) as .DELTA..phi.' is changed. As the fringe spacing
increases, the rate of this oscillation will decrease due to the
light and dark fringes becoming progressively larger so that the
rate of oscillation is minimal as .DELTA..phi.' becomes
substantially zero--in the second embodiment, the controller
adjusts .DELTA..phi.' until that minimum rate of oscillation is
achieved, and maintains that .DELTA..phi.' until the polymer has
set.
[0056] In some optical components, it may be desirable to have
additional surface gratings that have a relative orientation angle,
which does not deviate from a non-zero amount .PHI. by more than an
amount which is substantially zero (i.e. which is
.PHI.+.DELTA..phi., where .DELTA..phi. is substantially zero). In
this case, the gratings 4'F, 4'D as shown in FIG. 4C can be used in
the same way as described above, with a first further grating
formed on a distinct portion of the front block's inner surface
that is oriented at an angle .PHI.1 relative to 4'F, and a second
further grating formed on a distinct portion of the rear block's
inner surface that is oriented at an angle D2 relative to 4'B. The
angles .PHI.1, .PHI.2 are such that .PHI.=|.PHI.2-.PHI.1|, which
can be achieved to a high level of accuracy using conventional
techniques e.g. conventional microfabrication techniques. When the
gratings 4'F, 4'B are aligned to have a substantially zero relative
orientation angle .DELTA..phi.' (relative to one another) using the
above techniques, the further gratings will have an orientation
angle relative to one another that is substantially .PHI. i.e. that
deviates from .PHI. by at most an amount of the order of
.DELTA..phi.' (which is, of course, substantially zero). The
further gratings will also be imprinted in the polymer as the
polymer is forced into contact with these gratings when liquid in a
similar manner to 4'F, 4'B, so that the further gratings as
imprinted in the polymer have substantially the desired relative
orientation angle .PHI..
[0057] FIGS. 6A and 6B exemplify an alternative moulding process.
In this process a transparent substrate, such as a glass or
suitable plastic plate 30. Thin layers of polymer on the substrate
are used to replicate the gratings from the mould i.e. the
substrate acts as a "back bone" of the optical component and the
gratings are formed on thin layers of polymer 8 on the
substrate.
[0058] FIG. 6A show an alternative moulding apparatus 1' in an
initial arrangement, in which the plate 30, having thin layers of
liquid polymer 8 deposited on portions of its outer surface, is
disposed between two blocks 5F, 5B. These blocks can be
substantially the same as in the apparatus of FIGS. 4A-C, with
equivalent gratings 4'F, 4'B. The blocks 5F, 5B are then forced
towards one another so that the gratings 4'F, 4'B are forced into
contact with the liquid polymer layers as shown in FIG. 6B. In this
manner, their structure is imprinted in the polymer layers. The
relative orientation angle of the modulations 4'F, 4'B is then
changed to substantially zero using the fringe pattern formed by
the gratings 4'F, 4'B, and remains thus whilst the polymer layers
set. The final optical component comprises the plate 30 and the set
polymer layers on the plate's surface.
[0059] Note that, in this case, the area in which the polymer is
imprinted does not necessarily have to be sealed, and the alignment
portion could alternatively be an uncovered gap between the
components 5F and 5B (e.g. the sensor 6 could be located below the
apparatus 1' to receive light reflected of both gratings 4'F, 4'B,
the alignment portion being the gap between 5F and 5B at the bottom
of the apparatus 1').
[0060] As will be apparent, the alternative apparatus 1' does not
need an injection component, but otherwise has a similar
configuration to that shown in FIG. 5.
[0061] Whilst in the above, the exemplary gratings 4F,4B
(equivalently 4'F, 4'B) match due to the fact that they are both
formed of substantially straight grating lines, in general gratings
which are considered to "substantially match" do not necessarily
have to be formed of straight grating lines, nor do they have to be
formed of identically shaped curved grating lines. In general, two
gratings "substantially match" provided some parts of their
respective structures are similar enough for it to be possible to
create an observable fringe pattern that exhibits a discernible
fringe spacing by overlaying those parts (even though other parts
of their structure may be markedly different).
[0062] Note that the alignment gratings need not overlap, provided
it is possible to receive light which has interacted with (e.g.
been reflected from) both at a location in space (e.g. at a
detector) so that a fringe pattern is formed at that location.
[0063] Whilst in the above, gratings are formed on opposing,
substantially parallel surfaces, in general the terminology
"opposing surfaces portions" (or similar) encompasses surface
portions which are not parallel. Note that the definition of the
relative orientation angle (azimuth) between two gratings as set
out above with reference to FIG. 3B can be applied to gratings on
non-parallel surface portions.
[0064] Whilst the above has been described with reference to
opposing gratings, the techniques can be applied to non-opposing
gratings, whereby the fringe pattern is formed for instance by a
beam which has been guided by reflection onto both gratings, and
which thus interacts with both.
[0065] The cavity gratings 4'F, 4'B (and thus the imprint gratings
4F, 4B) can be binary (slanted/non-slanted), sinusoidal,
trapezoidal (e.g. triangular) in shape (among others) and need not
have the same shape, slant a, width w, depth h etc. as one another
(though this is not excluded).
[0066] Whilst the above considers a substantially
software-implemented controller 20, the functionality of the
controller can be implemented using software, firmware, hardware
(e.g., fixed logic circuitry), or a combination of these
implementations. The terms "module," "functionality," "component"
and "logic" as used herein generally represent, where applicable,
software, firmware, hardware, or a combination thereof. In the case
of a software implementation, the module, functionality, or logic
represents program code that performs specified tasks when executed
on a processor (e.g. CPU or CPUs). The program code can be stored
in one or more computer readable memory devices. The features of
the techniques described below are platform-independent, meaning
that the techniques may be implemented on a variety of commercial
computing platforms having a variety of processors.
[0067] For example, the apparatus may also include an entity (e.g.
software) that causes hardware of a computer of the apparatus to
perform operations, e.g., processors functional blocks, and so on.
For example, the computer may include a computer-readable medium
that may be configured to maintain instructions that cause the
computer, and more particularly the operating system and associated
hardware of the computer to perform operations. Thus, the
instructions function to configure the operating system and
associated hardware to perform the operations and in this way
result in transformation of the operating system and associated
hardware to perform functions. The instructions may be provided by
the computer-readable medium to the computer through a variety of
different configurations.
[0068] One such configuration of a computer-readable medium is
signal bearing medium and thus is configured to transmit the
instructions (e.g. as a carrier wave) to the computing device, such
as via a network. The computer-readable medium may also be
configured as a computer-readable storage medium and thus is not a
signal bearing medium. Examples of a computer-readable storage
medium include a random-access memory (RAM), read-only memory
(ROM), an optical disc, flash memory, hard disk memory, and other
memory devices that may us magnetic, optical, and other techniques
to store instructions and other data.
[0069] Moreover, whilst the above alignment process is automated,
manual or partially manual processes are not excluded.
[0070] In embodiments of the various aspects set out above, images
of the fringe pattern may be captured as the cavity is reconfigured
and an automatic image recognition procedure may be performed to
detect the fringe pattern in the images, the step of reconfiguring
being based on the results of the image recognition procedure.
[0071] Light of only a small portion of the fringe pattern may be
sensed as the cavity is reconfigured, the step of reconfiguring
being based on the rate at which the intensity of that light
changes.
[0072] Each of the gratings may lie substantially parallel to a
plane, and the gratings may not overlap or may only partially
overlap with one another when viewed along a direction normal to
the plane.
[0073] The gratings may be illuminated with an expanded laser beam,
the fringe pattern being formed by light of the laser beam which
has interacted with both gratings. The light of the laser beam may
for instance be received at a detector, part of the received light
having been reflected from one of the gratings and another part of
the light having been reflected from the other of the gratings,
whereby the part and the other part interfere at the detector to
form the fringe pattern on a detection surface of the detector. An
output of the detector may be used to control the reconfiguring
step.
[0074] The opposing portions of the cavity's surface may be
substantially parallel, so that the opposing portions of the
moulded optical component's surface are substantially parallel.
[0075] A first and a second further grating may be formed on other
opposing portions of the cavity's surface, the first further
grating having a first orientation angle .PHI.1 relative to the one
of the gratings and the second further grating having a second
orientation angle .PHI.2 relative to the other of the gratings, so
that the first and second further gratings are imprinted in the
polymer having a relative orientation angle that is substantially
|.PHI.2-.PHI.1| in the new configuration.
[0076] At least one of the surfaces of the moulding component may
be curved so that the polymer sets in a curved configuration.
[0077] The moulding component may be arranged to provide a moulding
cavity, the surfaces being of the moulding cavity, and the polymer
may be forced into the moulding cavity to force the polymer into
contact with the surface modulations, the moulding component
reconfigured to the new configuration whilst the polymer in the
cavity is still liquid.
[0078] The polymer may be arranged in layers on the surface of a
substantially transparent substrate, whereby the gratings are
imprinted in the layers, the moulding component reconfigured to the
new configuration whilst the layers are still liquid, the optical
component comprising the substrate and the layers once set.
[0079] The light sensor may comprise a camera which captures images
of the fringe pattern as the cavity is reconfigured, the controller
may comprise an image recognition module which performs an
automatic image recognition procedure to detect the fringe pattern
in the images, and the controller may reconfigure the cavity based
on the results of the image recognition procedure.
[0080] The light sensor may sense light of only a small portion of
the fringe pattern as the cavity is reconfigured, and the
controller may be reconfigured based on the rate at which the
intensity of that light changes.
[0081] According to a third aspect an optical component for use in
an optical system is substantially transparent and has two opposing
outer surfaces. At least a respective portion of each of the
opposing surfaces is formed of polymer in which a respective
grating is imprinted. The gratings substantially match one another
and have a relative orientation angle that is zero to within one
thousandth of a degree.
[0082] The relative orientation angle may for instance be zero to
within one half of one thousandth of a degree.
[0083] The opposing surface portions may be substantially
parallel.
[0084] The optical component may be used as a waveguide in a
display system to transport light of an image to a user's eye, for
example a wearable display system that is wearable by the user.
[0085] The gratings may be binary, trapezoidal or sinusoidal in
shape.
[0086] Another aspect of the subject matter is directed to a
moulding apparatus for moulding a substantially transparent optical
component which comprises polymer, the optical component having
substantially matching grating imprints on opposing portions of its
surface, wherein the grating imprints have a substantially zero
relative orientation angle, the apparatus comprising: a moulding
component having two surfaces, the surfaces having surface
modulations which form two substantially matching gratings, wherein
the moulding component is configurable to change the relative
orientation angle of the gratings; a drive mechanism coupled to the
moulding component controllable to configure the moulding
component; wherein at least an alignment portion of the moulding
component is substantially transparent, the alignment portion
located so that light which has interacted with both gratings is
observable from the alignment portion when the substantially
transparent polymer is between the surfaces, whereby an observable
fringe pattern is formed as the relative orientation angle of the
gratings is changed towards zero, the fringe pattern exhibiting a
fringe spacing which increases as the relative orientation angle
decreases, the apparatus further comprising: a light sensor
configured to receive at least some of the light which has
interacted with both gratings; and a controller configured, whilst
the polymer is still liquid, to control the drive mechanism based
on sensed data received from the image sensor to reconfigure the
moulding component from a current configuration to a new
configuration in which the fringe spacing of the fringe pattern is
substantially maximal, thus aligning the gratings to have a
substantially zero relative orientation angle, wherein the new
configuration is maintained whilst the polymer sets
[0087] Yet another aspect is directed to a moulding process for
moulding a substantially transparent optical component from
polymer, the optical component having substantially matching
grating imprints on opposing portions of its surface, wherein the
grating imprints have a substantially zero relative orientation
angle, the process comprising: forcing substantially transparent
molten polymer into a moulding cavity provided by a moulding
component, the cavity's surface having surface modulations which
form two substantially matching gratings on opposing portions of
the cavity's surface, the molten polymer forced into contact with
the surface modulations so as to imprint the gratings in the
polymer, wherein the cavity is configurable to change the relative
orientation angle of the gratings; wherein at least an alignment
portion of the moulding component is substantially transparent
along a line of sight that intersects both gratings so that light
which has interacted with both gratings is observable along the
line of sight when the substantially transparent polymer is in the
cavity, whereby an observable fringe pattern is formed as the
relative orientation angle of the gratings is changed towards zero,
the fringe pattern exhibiting a fringe spacing which increases as
the relative orientation angle decreases, the process further
comprising: whilst the polymer in the cavity is still liquid,
reconfiguring the cavity from a current configuration to a new
configuration in which the fringe spacing of the fringe pattern is
substantially maximal, thus aligning the gratings to have a
substantially zero relative orientation angle, wherein the new
configuration is maintained whilst the polymer sets.
[0088] Although the subject matter has been described in language
specific to structural features and/or methodological acts, it is
to be understood that the subject matter defined in the appended
claims is not necessarily limited to the specific features or acts
described above. Rather, the specific features and acts described
above are disclosed as example forms of implementing the
claims.
* * * * *