U.S. patent application number 13/337240 was filed with the patent office on 2012-04-26 for compound quarter-wave retarder for optical disc pickup heads.
This patent application is currently assigned to REALD INC.. Invention is credited to Gary D. Sharp.
Application Number | 20120099413 13/337240 |
Document ID | / |
Family ID | 36588553 |
Filed Date | 2012-04-26 |
United States Patent
Application |
20120099413 |
Kind Code |
A1 |
Sharp; Gary D. |
April 26, 2012 |
Compound quarter-wave retarder for optical disc pickup heads
Abstract
Chemically-bonded laminated polymer achromatic polarization
devices, such as circular polarizers, are disclosed for use in
optical disc (e.g., CD/DVD) pickup heads. Chemically-bonded
laminated polymer achromatic polarization devices have the benefit
of providing stable retardation and optic axis over an extended
wavelength range, thereby ensuring orthogonal polarization in
double-pass for two or more laser wavelengths. Moreover, the
chemically-bonded laminated polymer achromatic polarization devices
can be symmetric in construction, such that there is no specific
input and output side. This alleviates the need to produce
geometries that prohibit inversion of the part when installed in
the system. Manufacturing processes that produce chemically-bonded
laminated polymer achromatic polarization devices, with high light
efficiency, durability and robust performance in a variety of
environmental conditions are disclosed.
Inventors: |
Sharp; Gary D.; (Boulder,
CO) |
Assignee: |
REALD INC.
Beverly Hills
CA
|
Family ID: |
36588553 |
Appl. No.: |
13/337240 |
Filed: |
December 26, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11303904 |
Dec 16, 2005 |
8085644 |
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13337240 |
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60593172 |
Dec 16, 2004 |
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Current U.S.
Class: |
369/112.16 ;
G9B/7.112 |
Current CPC
Class: |
G11B 7/22 20130101; G11B
2007/0006 20130101; G02B 5/3083 20130101; G11B 7/1275 20130101;
G11B 7/1365 20130101 |
Class at
Publication: |
369/112.16 ;
G9B/7.112 |
International
Class: |
G11B 7/135 20120101
G11B007/135 |
Claims
1. An achromatic polarization device for an optical disc pickup
head, the achromatic polarization device comprising: a monolithic
retarder stack comprising: a first polymer retarder layer and a
second polymer retarder layer chemically bonded to the first
retarder polymer layer; wherein the monolithic retarder stack is
operable to transform, on a forward pass, linearly polarized light
to circularly polarized light, and on a reverse pass, circularly
polarized light to linearly polarized light, wherein the light on
the reverse pass is substantially orthogonal to the light on the
forward pass.
2. An achromatic polarization device according to claim 1, wherein
the monolithic retarder stack further comprises a dummy layer.
3. An achromatic polarization device according to claim 2, wherein
the dummy layer is bonded to the first polymer retarder layer.
4. An achromatic polarization device according to claim 2, wherein
the dummy layer has zero in-plane retardation.
5. An achromatic polarization device according to claim 2, wherein
the dummy layer has substantially little effect on the state of
polarization of light passing therethrough.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/303,904 entitled "Compound quarter-wave
retarder for optical disc pickup heads," filed on Dec. 16, 2005,
the entirety of which is incorporated herein by reference.
Additionally, this Application claims priority to Provisional
Patent Application No. 60/593,172, filed Dec. 16, 2004.
Incorporation by reference of the entire disclosure of that
provisional application is considered as being part of the
disclosure of the accompanying application and is herein
incorporated by reference in its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] The present disclosure relates to polarization control
elements for use in optical disc pickup heads. More specifically,
the present disclosure provides multilayer laminates exhibiting
wavelength stable retardation for two or more wavelengths.
[0004] 2. Description of the Related Art
[0005] Optical pickup heads are used both to read and to record
information on an optical disc. Generally, this information
includes file, audio, and video information. Different laser
wavelengths are used, depending on the format. For 650 megabyte
(Mb) CD, a 785 nm wavelength is used, for a 4.78 gigabyte (Gb)
DVD-ROM, a 650 nm wavelength is used, and for a DVD-R, a 650 nm
wavelength read wavelength is used with a 630 nm write wavelength.
The proposed next generation DVD media, for example Blu-ray and
HD-DVD, use a laser operating at a wavelength of 405 nm. There are
several types of optical disc drives in mass production: CD-R (or
compact disc recordable), CD-RW (or compact disc rewritable),
DVD.+-.R and DVD.+-.RW, which are respectively recordable and
rewritable optical discs, as well as next-generation 405 nm
drives.
[0006] Given that there are so many optical disc formats in
existence, the market demands interoperability of several formats
in a single disc drive. Accommodation of these several formats,
however, often requires numerous sets of optical components of
different types configured together. For example, U.S. Pat. No.
6,240,053 to Akiyama shows a conventional pickup head design in
which an optical isolator arrangement is used. Light from each
laser passes through a polarizing beam splitter and is incident as
a linear-polarized incident light on a quarter-wave plate (QWP),
which provides circular polarization. Return light from the optical
disc, makes a second pass of the QWP, and is thus converted to the
orthogonal linear state of polarization relative to the state of
polarization of the incident light. This light exits a separate
port of the polarizing beam splitter, the light being directed
toward a photodetector. As noted by Akiyama, when light of two
wavelengths shares the QWP, it is necessary for the QWP component
to generate circular polarization at both wavelengths.
[0007] Multi-layer retarder stacks that produce a two-or-more
wavelength stable responses are well known in the art. In 1948,
Destriau and Prouteau combined a quarter-wave and half-wave
retarder, with a 60.degree. angle between their optic axes, to
produce a circular polarization from linear polarization. In 1955,
Pancharatnam combined two half-wave retarders with a quarter-wave
retarder to produce an achromatic circular polarizer with a broader
spectral range. In the Pancharatnam design, input linear polarized
light passed through half-wave retarders at angles of 6.9.degree.
and 34.5.degree., respectively, followed by a quarter-wave retarder
at an angle of -79.7.degree.. Mindful that circular polarizers do
not behave precisely as quarter-wave plates (i.e., no optic axis),
Pancharatnam also generated the design for a three-layer
quarter-wave retarder. McIntyre and Harris (1968) disclosed designs
for achromatic visible waveplates using a network synthesis
technique. Koester (1958) showed that multiple half-wave retarders
could be combined to produce broad-band linear polarization
rotators. General properties of two-pass retarder networks were
also discussed by Ammann in 1966.
[0008] Such multilayer retarder stacks have conventionally been
manufactured using multiple components with different thermal,
optical and structural properties bonded together with an optical
adhesive. An example is provided with reference to FIG. 9. As
described below, such multilayer retarder stacks are complex to
manufacture and are susceptible to deviations in performance caused
by temperature variations.
[0009] Complexity of known optical pickup head designs leads to
increased manufacturing costs and reduced reliability because there
is a greater probability of failure as system complexity increases.
It is thus desirable to produce optical pickup heads with less
expense, and without significantly increasing component count, that
can accommodate two or more wavelengths, providing increased
compatibility among the various optical disc formats. In
configurations using a common path for each laser (or lasers
emitting multiple wavelengths), functional requirements of
components are thus expanded to cope with certain chromatic
effects.
BRIEF SUMMARY
[0010] Disclosed are multilayer chemically-bonded polymer laminates
that assign specific polarization states to two or more laser
wavelengths in an optical pickup head. These polymer laminates
include retarder films that are oriented at specific angles, so
that the net polarization transformation is specific to the
incident wavelength. Under certain circumstances, this polarization
state is uniform for all wavelengths (e.g., circular). In other
cases, the polarization can be unchanged at one wavelength, but is
partially transformed (e.g. circular or 45.degree. linear) at
another wavelength.
[0011] The multilayer polymer laminates are assembled using
chemical bonding techniques, such that the finished structure is of
high optical quality, is mechanically stable, and is highly
durable, while low in cost. These polymer multilayer laminates can
provide wavelength-specific polarization control laminates that are
symmetric in construction. Such structures may be constructed to
have no specified input and output side, thereby simplifying the
optical head assembly process. The multilayer polymer laminates
further comprise robust polarization performance in a variety of
environmental conditions. Such laminates can comprise layers of a
single material, and as such, issues related to differential
thermal expansion can be mitigated or eliminated compared to
polymer retarder materials bonded between glass using optical
adhesives, which would suffer from mismatch in thermal expansion.
The latter construction can cause stresses that affect the optic
axis and retardation stability.
[0012] Further disclosed are laminate structures that are
mechanically stable in a free-standing mode. Optically, polymer
stacks assembled with chemical bonding can be polished and directly
coated with an antireflection coating to yield parts that are
virtually free of internal and external reflections. At sufficient
thickness, such laminates may be rigid and flat enough that they
can be used in a free-standing mode.
[0013] Also described are multilayer retarder structures having
three or more layers that produce wavelength-controlled
polarization over extremely extended wavelength ranges, including
two or more, or all, of the wavelengths 405 nm, 630 nm, 650 nm and
785 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates a diagram of an optical disc pickup head
in accordance with the present disclosure;
[0015] FIG. 2a illustrates a diagram of a reflection-mode isolator
including an achromatic quarter-wave retarder stack in accordance
with the present disclosure;
[0016] FIG. 2b illustrates a diagram of an equivalent unfolded
arrangement of the retarder stack, as illustrated in FIG. 2a;
[0017] FIG. 3 is a graph showing the normalized relationship of the
transmission properties of (i) a single layer quarter-wave retarder
plate and (ii) a multilayer quarter-wave retarder as a function of
incident wavelength in accordance with the present disclosure;
[0018] FIG. 4a illustrates a diagram of a reflection-mode isolator
including an achromatic half-wave retarder stack in accordance with
the present disclosure;
[0019] FIG. 4b illustrates a diagram of an equivalent unfolded
arrangement of the retarder stack as illustrated in FIG. 4a;
[0020] FIG. 5 illustrates a diagram of a three-layer achromatic
quarter-wave retarder stack in accordance with the present
disclosure;
[0021] FIG. 6 illustrates a diagram of a five-layer achromatic
quarter-wave retarder stack in accordance with the present
disclosure;
[0022] FIG. 7 illustrates a diagram of an exemplary embodiment of a
retarder stack that converts a linear state of polarization,
wavelength selectively, to a rotated state in accordance with the
present disclosure;
[0023] FIG. 8 illustrates a diagram of an exemplary embodiment
including a pair of retarder stacks, that transform a linear state
of polarization, wavelength selectively, to a circular state in
accordance with the present disclosure;
[0024] FIG. 9 illustrates a diagram of a polarization optic
utilizing a two-layer liquid crystal polymer;
[0025] FIG. 10 illustrates a diagram of a multilayer polymer
free-standing polarization optic in accordance with the present
disclosure; and
[0026] FIG. 11 illustrates a logical flow diagram illustrating a
method for manufacturing the multilayer polymer free-standing
polarization optic in accordance with the present disclosure.
DETAILED DESCRIPTION
[0027] FIG. 1 shows an embodiment of an optical disc subsystem 100
having a pickup head 150 accommodating a plurality of wavelengths
(e.g., .lamda..sub.1, .lamda..sub.2, etc.) along a common path. The
pickup head 150 may include a light source 102, a first lens 106, a
polarizing beam splitter (PBS) 108, a polarization optic 110, a
reflective element 112, a second lens 114, a third lens 116, and a
photodetector 118 operably coupled to electronic circuitry 120,
arranged as shown, although not all of these elements are required
to be included in a pick-up head constructed in accordance with the
principles of this disclosure. In particular, the pick-up head may
or may not include a light source 102 or other components
described.
[0028] Light source 102 may generate multiple wavelengths of light,
e.g., 405 nm, 630 nm, 650 nm, and/or 785 nm. In generating these
multiple wavelengths, light source 102 may include a single laser
capable of emitting multiple wavelengths, or may include multiple
lasers combined using conventional means, or both. Additionally or
alternatively, light source 102 may employ light emitting diode
(LED) structures to produce the wavelengths of light, with LED
structures being a known alternative to lasers for such purposes.
Polarization optic 110 may, for example, be a circular polarizer.
Reflective element 112 may be a mirror, a total internal reflection
prism, or any other optical device providing a reflective
boundary.
[0029] In operation, light of two or more wavelengths from light
source 102 is combined by the first lens 106 to form a collimated
incident light beam along a common path. As mentioned above, this
incident light may originate from a single source or multiple
sources. Collimated light from the first lens 106 passes (on a
forward pass) through the PBS 108, where it emerges linearly
polarized. With the light continuing on the forward pass, a
polarization optic 110 circularly polarizes the incident light. The
light is then deflected to a second lens 114 by a reflective
element (e.g., a mirror or total internal reflection prism) 112,
where it is focused onto an optical disc (e.g., CD/DVD) 124.
[0030] Upon reflecting from the surface of the optical disc 124,
the return or reflected read-light retraces the path. The second
lens 114 re-collimates the light, and after being deflected by the
reflective element 112, the return beam makes a reverse pass of the
polarization optic 110. Upon the reverse pass, the reflected light
is preferably polarized substantially orthogonal to the
counter-propagating input beam of the forward pass, which may also
be referred to as the incident light. The read-light is then
reflected by the PBS 108, where it is focused by the third lens 116
onto the photodetector 108. The photodetector 118 converts the
read-light to an electrical current for processing by electronic
circuitry 120.
[0031] For efficient transfer of read-light to the photodetector,
preferably, the optical isolator arrangement of the PBS 108 and
polarization optic 110 provides substantially orthogonal
polarization in a round trip at all relevant wavelengths (e.g., 405
nm, 630 nm, 650 nm, and/or 785 nm). For this to occur, the
polarization optic 110 preferably should provide substantially
uniform behavior at the two or more desired wavelengths used by the
light source 102.
[0032] Exemplary designs, constructions and properties of the
polarization optic 110 will be discussed in detail below with
reference to FIGS. 2a to 11. Due to the chromatic limitations of a
single layer quarter-wave retarder, a more elaborate solution may
be used to provide the desired achromatic polarization state. The
described optic 110 thereby provides a polarization mapping that
transforms an achromatic linear state of polarization to an
achromatic circular state of polarization for the two or more
desired wavelengths.
[0033] Such a solution for providing a polarization transforming an
achromatic linear state of polarization to an achromatic circular
state of polarization may be provided using some arrangement of
anisotropic inhomogeneity along the direction of propagation. As
used here, anisotropic inhomogeneity refers to the concept that
each layer has an independent anisotropy, such that as the light
beam passes through the layers of a multi-layer retarder stack, the
propagating light can encounter many different (and unrelated to
each other) optical axes. The optical axes of the layers can be
chosen to satisfy the design conditions. As in the present
disclosure, the inhomogeneity may be an engineered solution, such
as a stack of two or more laminated linear retarders. The
inhomogeneity may alternatively be a single liquid crystal polymer
layer, which self-assembles into a graded inhomogeneous structure
(such as a director twist with pitch large relative to the incident
wavelength). It may furthermore be a hybrid stacked liquid crystal
polymer. Regardless of the technique for providing the solution,
the solution provides a unitary transformation that entails a
lossless polarization mapping of a linear to a circular state of
polarization.
[0034] FIG. 2a illustrates a diagram of a reflection-mode isolator
including an achromatic quarter-wave retarder stack in accordance
with the present disclosure. In this exemplary embodiment, a
reflection-mode isolator arrangement 150 includes PBS 108 and
polarization optic 110. In operation, laser light with wavelength
.lamda..sub.i and amplitude E(.lamda..sub.i) passes through a PBS
108, then through a polarization optic 110 where it encounters a
double-pass of the structure. The polarization optic 110 provides
an achromatic quarter-wave retarder stack having N elements 152a
through 152N, each with retardation .GAMMA..sub.x and slow axis
orientation .alpha..sub.x, where x=1 corresponds to element 152a,
and x=N corresponds to element 152N. Subsequently, polarization
modified light is analyzed by the PBS 108, with reflected amplitude
E'(.lamda..sub.i).
[0035] FIG. 2b is the equivalent unfolded arrangement of the
retarder stack of polarization optic 110, which is convenient for
illustrative purposes. It shows that a structure containing an odd
number, 2N-1, of elements is effectively encountered in a round
trip of a forward and reverse pass. Second, it shows that the
effective stack includes a pair of retarders with a symmetric
arrangement; retarders on the output stack have the equivalent
angles to those of the input stack, but are presented in reverse
order. Moreover, it shows that the thickness of the central element
in the unfolded arrangement is effectively doubled.
[0036] The effect of the polarization optic 110 is conveniently
described using Jones Calculus, which propagates the state of
polarization through anisotropic materials. The unitary
transformation of a lossless linear retarder can be expressed in
the general form:
S + = ( a b - b * a * ) ##EQU00001##
where,
aa*+bb*=1
And the common-phase, which has no impact on the state of
polarization, has been omitted. It can furthermore be shown that
any stack of retarders 152a-152N (or LC polymers), with arbitrary
retardation and optic axis orientation, can be written in this
form, taking S.sup.+ to represent the forward pass of such a
polarization optic.
[0037] By suitable multiplication of Jones matrices, it can be
shown that the reverse pass matrix is always of the form:
S - = ( a - b * b a * ) ##EQU00002##
Thus, the double-pass matrix is given as the product of the forward
and reverse pass matrices:
M = S - S + = ( a - b * b a * ) ( a b - b * a * ) ##EQU00003##
which equals:
( a 2 + b * 2 ab - a * b * ab - a * b * a * 2 + b 2 )
##EQU00004##
This gives the important general result that the off-diagonal
components are identical in amplitude, which precludes polarization
rotation.
[0038] Moreover, using,
a=|a|e.sup.-i.alpha.
and
b=|b|e.sup.-i.beta.
gives
(ab-a*b*)=-i2|a.parallel.b|sin(.alpha.+.beta.)
showing that the off-diagonal amplitude is in general
imaginary.
[0039] If a double-pass arrangement in accordance with the present
disclosure is used to convert linear polarized incident light to
the orthogonal state of polarization for the reflected light, the
diagonal components must vanish, forcing the constraint,
|a|=|b|=1/ {square root over (2)}
(.alpha.+.beta.)=.pi.(1/2+m)
where m is an integer. Using the above, the double pass matrix
reduces to the form,
S = ( 0 - - 0 ) ##EQU00005##
which represents a pure polarization reflection about an axis at
.pi./4.
[0040] Physically, the above matrix represents the transformation
of an ideal half-wave retarder, with fast axis oriented along
.pi./4. This proves an important point about the double-pass of any
unitary structure: If the double-pass converts a linear
polarization state to its orthogonal linear polarization state,
then it behaves as a half-wave retarder at .pi./4. This means that
the double-pass structure has an optic axis at 45.degree. and has
zero polarization rotation for all wavelengths satisfying this
conversion.
[0041] Inserting the above constraint into the forward-pass matrix
gives the result:
S + = 1 2 ( - .alpha. - .alpha. - - .alpha. .alpha. ) = 1 2 ( 1 - -
1 ) ( - .alpha. 0 0 .alpha. ) ##EQU00006##
where we have used the specific case (m=0). The latter
decomposition represents a linear retarder with arbitrary
retardation 2.alpha., oriented parallel to the input polarization,
followed by a pure quarter-wave retarder with orientation .pi./4.
The above matrix illustrates a second point: A circular state
exists after the forward pass of the retarder stack to achieve full
polarization conversion in double-pass.
[0042] Accordingly, from a design standpoint, an effective
double-pass converter can be provided by constraining the single
pass to produce a very precise circular state. Note that the above
is not the matrix of an ideal quarter-wave retarder, which would
further require linear eigen-polarizations, but is more
appropriately termed a circular polarizer. A pure quarter-wave
retarder is thus a further constrained subset of the circular
polarizer family, where .alpha.=0.
[0043] There are often multiple solutions for retarder stack-based
circular polarizers that provide identical or substantially similar
polarization performance. For example, each design may have the
same set and order of retardances but with different sets of
retarder orientations. Also, each design may have a unique
"compound retardation" .GAMMA., which is hidden in practice by the
fixed uniform linear input. This compound retardation often becomes
problematic when the input polarization of the light into the
optical system does not conform to the intended or designed-for
polarization input.
[0044] The reverse-pass stack can be written as a quarter-wave
retarder with orientation .pi./4, followed by a linear retarder
with retardation 2.alpha. and orientation also parallel to the
input. In double-pass, the net half-wave central retardation
reflects the state of polarization, such that external retardation
is substantially nullified. Since a stack possessing the above
symmetry converts at least two wavelengths to the orthogonal linear
state, it can be regarded as a half-wave retarder for those at
least two wavelengths. In addition to having stable retardation,
the structure can furthermore be considered to have wavelength
stable eigenpolarizations for those at least two wavelengths.
Accordingly, the round-trip matrix can be diagonalized via a .pi./4
rotation. A compound element with stable behavior over an extended
band is called an achromatic half-wave retarder. Based on the
above, an achromatic half-wave retarder can be designed by pairing
an achromatic circular polarizer with the reverse order stack.
[0045] Based on the above discussion, a multi-layer polarization
optic may be provided using either of two approaches. The first
approach is performed by providing a stack that produces a precise
circular state at all relevant wavelengths. The second approach is
to provide an achromatic half-wave retarder with reverse-order
symmetry, and divide the stack in half.
[0046] FIG. 3 is a graph showing the normalized relationship of the
transmission properties of (i) a single layer quarter-wave retarder
plate and (ii) a multilayer quarter-wave retarder as a function of
incident wavelength in accordance with the present disclosure.
[0047] A single layer quarter-wave retarder plate (with the
exception of some dispersion controlled copolymers), may provide
appropriate retardation at a single wavelength. Graph 180 shows the
double-pass crossed-polarizer transmission of a single-layer
retarder, which gives unity transmission only at the half-wave
wavelength 184.
[0048] Graph 182 shows that a multilayer quarter-wave retarder
stack can expand the half-wave retardance range to encompass a
bandwidth .alpha..lamda. 186. If the unfolded stack is comprised of
uniform thickness waveplates, a spectrum similar to graph 182 is
typical. That is, a substantially flat response (centered on the
half-wave wavelength 184) can exist between minima corresponding to
a full-wave retardance. At the full-wavelengths 188, it is possible
to have no polarization manipulation, making this an exercise in
narrowing the notches centered on the full-wave wavelengths 188.
The broadest coverage occurs when using zero-order half-wave
retarders (m=0), where there is no actual minimum at wavelengths
longer than the half-wave wavelength 184.
[0049] As discussed above, a circular polarizer may be designed by
providing an achromatic half-wave retarder with reverse-order
symmetry and dividing the stack in half. For such a circular
polarizer design, a large solution set can exist including N
retarders with arbitrary orientation and retardation. However, it
is generally most convenient to minimize the number of unique
retardance values (e.g., all films are laminated using the same
base retardance). Moreover, since solutions using half-wave
retarders are symmetric about the half-wave wavelength, the
broadest half-wave bandwidth (.DELTA..lamda. 186) occurs when the
zero-order (m=0) half-wave wavelength is roughly centered in that
band.
[0050] FIG. 4a illustrates a diagram of a reflection-mode isolator
including an achromatic half-wave retarder stack in accordance with
the present disclosure. The reflection-mode isolator 210 includes
PBS 108 and an achromatic half-wave retarder 220. Retarder 220
includes retarder elements 222a to 222N. FIG. 4b illustrates a
diagram of an equivalent unfolded arrangement of the retarder stack
as illustrated in FIG. 4a.
[0051] Achromatic half-wave retarder 220 is a circular polarizer
having an odd-number of zero-order half-wave layers in
reverse-order configuration, and thus has a broad spectral coverage
with a minimal number of layers. While compound retardation can
exist along the input direction, it is nullified in double-pass.
Therefore, the orientation tolerance is no greater or less than
that of a zero-order quarter-wave retarder. There is, however, a
specific input side in this architecture. In order to ensure that
light enters the half-wave side, it is frequently necessary to cut
unsymmetrical shapes, such as trapezoids. This is an inconvenience
and reduces the packing density of parts on the mother sheet.
[0052] Given that any design is likely to encompass the half-wave
wavelength, it is beneficial to consider any constraints imposed at
that wavelength. It is straightforward to show that the Jones
matrix for a series of half-wave retarders with arbitrary
orientations may be expressed as a pure rotator, whereas an odd
number of half-wave retarders may be expressed as a polarization
reflector (or half-wave plate). Thus, a solution based on half-wave
retarder films will use an odd number of layers. As FIG. 4a shows,
this is a convenience, since only an odd-number solution is
possible. Note that the uniform retardance requirement forces the
retarder 222N directly adjacent the optical disc 124 to be half the
thickness of the other elements (nominally quarter-wave). As the
number of half-wave layers 222a to 222(N-1) is added, the potential
for broader bandwidth achromatic half-wave retarders exists, using
a suitable set of angles. For example, angles for circular
polarizers include the (15.degree./75.degree.) design of Destraiu
and Proteau, and the (6.9.degree./34.5.degree./-79.7.degree.)
design of Pancharatnam.
[0053] A different solution set can result when a retarder stack is
required to behave as a pure retarder in single-pass. Achromatic
quarter-wave retarders are symmetric, at least in behavior,
permitting part flipping with no functional change. Achromatic
multilayer waveplate designs exist that, like the above circular
polarizer designs, also use only two retardance values in their
construction. While there are many such solutions, one solution set
comprises stacks possessing the reverse order symmetry shown in the
unfolded double-pass arrangement of FIG. 2b. In this arrangement,
the external retarders, have a unique retardance value, sandwiching
an odd number of half-wave retarders. Such designs are again
symmetric about the half-wave wavelength. However, the additional
requirement for a compound optic axis (.alpha.=0), tends to
diminish the spectral coverage of uniform retardation
(.DELTA..lamda.). For instance, an optimized three-layer AQW has
the spectral coverage of a two-layer ACP. However, the cost
associated with the additional lamination to achieve comparable
performance is relatively low. In addition, as discussed with
reference to FIG. 10, a free-standing monolithic element will use
additional layers regardless, because mechanical stability and
established thickness standards force a certain number of
laminations. A typical film thickness of a retarder is
approximately 65 microns, so several layers are needed to meet the
thickness standards. Often, these are "dummy" layers, which
increase thickness, with no impact on the state of
polarization.
[0054] FIG. 5 illustrates a diagram of a three-layer achromatic
quarter-wave retarder stack in accordance with the present
disclosure. This exemplary achromatic quarter-wave retarder stack
250 includes three retarder plates: 252, 254, and 256, having
retardance values of 115.degree., 180.degree., and 115.degree.; and
optic axis values of 76.degree., 5.7.degree., and 76.degree.
respectively. This three-layer achromatic quarter-wave retarder
stack 250 performs a linear-to-circular transformation when the
optic axis of the input light is at 45 degrees relative to the
illustrated reference orientation 260.
[0055] FIG. 6 illustrates a diagram of a five-layer achromatic
quarter-wave retarder stack in accordance with the present
disclosure. The exemplary achromatic quarter-wave plate retarder
stack 280 includes five retarder plates: 282, 284, 286, 288 and
290, having retardance values of 75.degree., 180.degree.,
180.degree., 180.degree., and 75.degree.; and optic axis values of
-9.1.degree., 40.9.degree., -68.1.degree., 40.9.degree., and
-9.1.degree. respectively. This five-layer achromatic quarter-wave
retarder stack performs a linear-to-circular transformation when
the optic axis of the input light is at 45 degrees relative to the
illustrated reference orientation 295.
[0056] Under certain circumstances, it is beneficial to assign
distinct polarization states to each laser wavelength. For
instance, the needs of write light and read light are different,
requiring distinct polarization states. Thus, FIG. 7 illustrates a
diagram of an exemplary embodiment of a retarder stack that
converts a linear state of polarization, wavelength selectively, to
a rotated state in accordance with the present disclosure.
[0057] A use of the disclosed technology is to provide uniform
polarization over a range of wavelengths (e.g. circular or
45-degree linear), while assigning a different polarization state
to another range of wavelengths. Such stack designs can be designed
directly, or can be assembled from separate structures with
distinct polarization functionality.
[0058] Retarder stacks can be designed which provide, for example,
a 45-degree rotation to linear polarization. FIG. 7 shows one such
example, where polarization optic 310, with a base retardation of
three waves, is used to manipulate 405 nm and 650 nm light, while
leaving 785 nm light unchanged. In order to achieve this, retarder
layers 312/314/316/318/320 have optic axis orientations of
.alpha.=20.0.degree./-5.1.degree./-70.6.degree./-8.6.degree./14.5.degree.
respectively. It should be noted that other solutions may exist to
provide a similar result, and that these values are provided by way
of example.
[0059] FIG. 8 illustrates a diagram of an exemplary embodiment
including a pair of retarder stacks, that transform a linear state
of polarization, wavelength selectively, to a circular state in
accordance with the present disclosure. Here, an achromatic
quarter-wave retarder stack 330, which possesses linear
eigenpolarizations) (.alpha.=0.degree.), can be combined with the
polarization optic 310 of FIG. 7 to yield a circular state of
polarization in predetermined spectral bands (e.g., 405 nm and 650
nm), with no change in the polarization state in another spectral
band (e.g., 780 nm).
[0060] FIG. 9 illustrates a diagram of a polarization optic
utilizing a two-layer liquid crystal polymer retarder. Polarization
optic 450 comprises liquid crystal polymer retarder layers 452 and
454, bonded together by optical adhesive 453. Glass plates 458 are
bonded to the delicate liquid crystal polymer retarder layers 452
and 454 with optical adhesive 455 to provide structural support.
The external faces of the glass plates are coated with an
antireflection layer 460.
[0061] When dissimilar materials are used in polarization optic
assemblies, as shown in FIG. 9, there is some risk that the
manufacturing processes and thermal variations will produce
undesirable stress birefringence. Thermal variations may include
uniform and nonuniform effects, the latter tending to induce more
stress birefringence. Polymer films 452 and 454 typically expand at
a rate roughly ten times that of glass 458, while optical adhesives
453 and 455 usually have yet a higher rate of expansion. The strain
on the actual retarder film(s) from this can induce a spatial
variation in the optic axis (e.g. optic axis rotation in the
corners), and retardance (e.g., retardation shift at the center of
each edge). Moreover, strain associated with differential thermal
expansion can produce changes in the spatial uniformity of the
optical properties of the component. This can result in temperature
dependence in the transmitted wavefront distortion.
[0062] FIG. 10 illustrates a diagram of a multilayer polymer for
free-standing polarization optic in accordance with the present
disclosure. In an embodiment, a polarization optic 500 has a
monolithic structure including a stack of polymer retarder films
510a through 510n, which are laminated together using a chemical
bonding process (where a=1 and n is the number of polymer retarder
layers). Such laminates are comprised of a single material, and as
such, issues related to differential thermal expansion are less
relevant. In contrast, polymer retarder materials bonded between
glass using optical adhesives (as shown in FIG. 9) suffer from
mismatch in thermal expansion. This can cause stresses that affect
the optic axis and retardation stability. Other layers, such as
dummy layers, antireflection coatings, etc., may be provided as
external faces 520 for structural stability, durability, and
desired optical properties.
[0063] FIG. 11 illustrates a logical flow diagram illustrating a
method for manufacturing the multilayer polymer free-standing
polarization optic in accordance with the present disclosure. In
this process, polymer retarder films can be chemically bonded
together at step 602. Exemplary processes and materials are as
described in commonly-owned U.S. Pat. No. 6,638,583, which is
herein incorporated by reference. Because the individual retarder
films may not be optically flat, the resulting stack can show
unacceptable transmitted wavefront characteristics. This problem
can be overcome using some form of planarization. One planarization
technique involves chemical bonding of additional dummy layers
(often with zero in-plane retardation) on the external faces of the
polarization optic at step 604. A double-side polishing technique
may then be used to produce a planar polarization optic at step
606. Optionally, the finished element may be further planarized
using wet coatings that reflow, or planarize directly, at step 608.
Another optional step is to deposit other optically thin layers,
such as hard coats and primers, at step 610. At step 612, a
low-temperature antireflection coating may be directly applied to
the external faces to yield parts that are relatively free of
internal and external reflections. The result is a polarization
optic with no significant internal stress, and relative
insensitivity to the thermal issues that plague designs based on
stacks of dissimilar materials.
[0064] It will be appreciated by those of ordinary skill in the art
that the invention can be embodied in other specific forms without
departing from the spirit or essential character thereof. Any
disclosed embodiment may be combined with one or several of the
other embodiments shown and/or described. This is also possible for
one or more features of the embodiments. The steps herein described
and claimed do not need to be executed in the given order. The
steps can be carried out, at least to a certain extent, in any
other order.
[0065] As may be used herein, the terms "substantially" and
"approximately" provide an industry-accepted tolerance for its
corresponding term and/or relativity between items. Such an
industry-accepted tolerance ranges from less than one percent to
ten percent and corresponds to, but is not limited to, component
values, angles, et cetera. Such relativity between items ranges
between less than one percent to ten percent
[0066] Further, it will be appreciated by one of ordinary skill in
the art that various retardance and optic axis values for numerous
retarder designs may be combined to perform other desired
transformations and for various other wavelengths. As used herein,
the term achromatic plates, achromatic polarization device, or
achromatic polarization rotators refer to devices that are operable
to provide a consistent polarization rotation at two or more
wavelengths. It will also be appreciated that the compound retarder
stack disclosed herein may be combined with various other optical
components to perform similar results. The presently disclosed
embodiments are therefore considered in all respects to be
illustrative and not restrictive. The scope of the invention is
indicated by the appended claims rather than the foregoing
description, and all changes that come within the meaning and
ranges of equivalents thereof are intended to be embraced
therein.
[0067] Additionally, the section headings herein are provided for
consistency with the suggestions under 37 C.F.R. .sctn.1.77 or
otherwise to provide organizational cues. These headings shall not
limit or characterize the invention(s) set out in any claims that
may issue from this disclosure. Specifically and by way of example,
although the headings refer to a "Technical Field," the claims
should not be limited by the language chosen under this heading to
describe the so-called technical field. Further, a description of a
technology in the "Background of the Invention" is not to be
construed as an admission that technology is prior art to any
invention(s) in this disclosure. Neither is the "Brief Summary of
the Invention" to be considered as a characterization of the
invention(s) set forth in the claims found herein. Furthermore, any
reference in this disclosure to "invention" in the singular should
not be used to argue that there is only a single point of novelty
claimed in this disclosure. Multiple inventions may be set forth
according to the limitations of the multiple claims associated with
this disclosure, and the claims accordingly define the
invention(s), and their equivalents, that are protected thereby. In
all instances, the scope of the claims shall be considered on their
own merits in light of the specification, but should not be
constrained by the headings set forth herein
* * * * *