U.S. patent application number 09/879040 was filed with the patent office on 2001-10-18 for optical head, method of manufacturing the same and diffraction element suitable therefor.
This patent application is currently assigned to Asahi Glass Company, Ltd.. Invention is credited to Gunjima, Tomoki, Hotaka, Hiroki, Sato, Hiromasa, Tanabe, Yuzuru.
Application Number | 20010030732 09/879040 |
Document ID | / |
Family ID | 26344382 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030732 |
Kind Code |
A1 |
Tanabe, Yuzuru ; et
al. |
October 18, 2001 |
Optical head, method of manufacturing the same and diffraction
element suitable therefor
Abstract
An optically anisotropic diffraction grating for an optical
read/write head, comprising optically isotropic polymer liquid
crystal held between two transparent base plates. Gratings of
different refractive indexes are formed by periodically changing
the orientation of the polymer liquid crystal of the diffraction
grating. This enables a high light-utilization efficiency and a
high reliability to be obtained.
Inventors: |
Tanabe, Yuzuru;
(Yokohama-shi, JP) ; Gunjima, Tomoki;
(Yokohama-shi, JP) ; Sato, Hiromasa;
(Yokohama-shi, JP) ; Hotaka, Hiroki;
(Yokohama-shi, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Asahi Glass Company, Ltd.
1-2, Marunouchi 2-chome
Tokyo
JP
100
|
Family ID: |
26344382 |
Appl. No.: |
09/879040 |
Filed: |
June 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09879040 |
Jun 13, 2001 |
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09101776 |
Jul 22, 1998 |
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09101776 |
Jul 22, 1998 |
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PCT/JP97/00136 |
Jan 22, 1997 |
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Current U.S.
Class: |
349/201 ;
G9B/7.113; G9B/7.138 |
Current CPC
Class: |
G11B 7/1353 20130101;
G02F 1/133757 20210101; G02F 1/133365 20130101; G11B 7/22 20130101;
G02F 1/292 20130101; G02B 5/1833 20130101 |
Class at
Publication: |
349/201 |
International
Class: |
G02F 001/13 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 1996 |
JP |
8-9621 |
Jun 27, 1996 |
JP |
8-167803 |
Claims
1. An optical head which irradiates a ray from a light source to an
optically recording medium through a diffraction element so as to
read information and/or write information, characterized in that
the diffraction element includes an optically anisotropic
diffraction grating which comprises an optically anisotropic
polymer liquid crystal sandwiched between two transparent
substrates, and that the polymer liquid crystal has an alignment
direction thereof periodically changed.
2. The optical head according to claim 1, wherein both of the two
transparent substrates have electrodes mounted thereon, and at
least one of the electrodes comprises periodically formed electrode
pieces.
3. The optical head according to claim 2, wherein both of the
electrodes comprise periodically formed electrode pieces, and the
two electrodes are unsymmetrically arranged between the two
transparent substrates.
4. The optical head according to claim 2, wherein the periodically
formed electrode pieces of the at least one of the electrodes have
a width of 30-45% of a periodical pitch thereof.
5. The optical head according to claim 2, wherein both of the
electrodes comprise periodically formed electrode pieces, and the
periodically formed electrode pieces of both electrodes have a
width of 30-45% of a periodical pitch thereof.
6. The optical head according to claim 1, wherein at least one of
the transparent substrates has a side thereof contacting the
polymer liquid crystal formed with an alignment layer, and at least
one of the alignment layers includes alignment layer pieces which
periodically have different alignment forces.
7. The optical head according to claim 6, wherein the alignment
layers of both of the two transparent substrates respectively
include alignment layer pieces which periodically have different
alignment forces, and the alignment layer pieces which periodically
have different alignment forces are unsymmetrically arranged
between the two transparent substrates.
8. The optical head according to claim 1, wherein an alignment
layer is formed on an electrode, and the alignment layer is one
which is subjected to rubbing so as to extend perpendicularly to a
longitudinal direction of the electrode.
9. A method for manufacturing an optical head which irradiates a
ray from a light source to an optically recording medium through a
diffraction element so as to read information and/or write
information, characterized in that the method comprises providing
electrodes on two transparent substrates; forming at least one of
the electrodes so as to comprise periodically formed electrode
pieces; providing an alignment layer on a side of at least one of
the two transparent substrates which contacts a liquid crystal
material; sandwiching the liquid crystal material between the two
transparent substrates, the liquid crystal material being
polymerizable; applying a periodical electric field to the
electrodes to align the liquid crystal material; and polymerizing
the liquid crystal material in such an aligned state, thereby
fabricating a diffraction element which includes an optically
anisotropic diffraction grating.
10. The method for manufacturing an optical head according to claim
9, wherein the periodically formed electrode pieces include a
certain number of split electrodes included in one period, the
respective split electrodes in the one period have different
electric fields applied thereto, the liquid crystal material in the
one period has an aligned state thereof made bilaterally
unsymmetrical, and then the liquid crystal material is
polymerized.
11. The method for manufacturing an optical head according to claim
9, further comprising a step A for sandwiching the liquid crystal
material in an unpolymerized state between the two transparent
substrates and periodically polymerizing the liquid crystal
material by periodical masking exposure, and a step B for
polymerizing the entire liquid crystal material by non-masking
exposure after the first step, a voltage being applied in either
one of the step A and the step B.
12. The method for manufacturing an optical head according to claim
9, wherein the alignment layer has been subjected to an alignment
treatment so that liquid crystal molecules are aligned in a
direction perpendicular to a longitudinal direction of the
electrodes.
13. A diffraction element which sandwiches an optically anisotropic
polymer liquid crystal between two transparent substrates,
characterized in that the liquid crystal sandwiched between the two
transparent substrates is a polymer liquid crystal, and that the
polymer liquid crystal has an alignment direction thereof
periodically changed.
14. The diffraction element according to claim 13, wherein both of
the two transparent substrates have electrodes mounted thereon, and
at least one of the electrodes comprises periodically formed
electrode pieces.
15. The diffraction element according to claim 14, wherein both of
the electrodes comprise periodically formed electrode pieces, and
the two electrodes are unsymmetrically arranged between the two
transparent substrates.
16. The diffraction element according to claim 14, wherein the
periodically formed electrode pieces of the at least one of the
electrodes have a width of 30-45% of a periodical pitch
thereof.
17. The diffraction element according to claim 14, wherein both of
the electrodes comprise periodically formed electrode pieces, and
the periodically formed electrode pieces of both electrodes have a
width of 30-45% of a periodical pitch thereof.
18. The diffraction element according to claim 13, wherein at least
one of the transparent substrates has a side thereof contacting the
polymer liquid crystal formed with an alignment layer, and at least
one of the alignment layers includes alignment layer pieces which
periodically have different alignment forces.
19. The diffraction element according to claim 18, wherein the
alignment layers of both of the two transparent substrates
respectively include alignment layer pieces which periodically have
different alignment forces, and the alignment layer pieces which
periodically have different alignment forces are unsymmetrically
arranged between the two transparent substrates.
20. The diffraction element according to claim 13, wherein an
alignment layer is formed on an electrode, and the alignment layer
is one which is subjected to rubbing so as to extend
perpendicularly to a longitudinal direction of the electrode.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical head for reading
and/or writing information on an optically recording medium such as
a CD (compact disc), a CD-ROM, another optical disc, and a
magneto-optical disc, and a diffraction element suitable for such
an optical head.
BACKGROUND ART
[0002] As an optical head which writes optical information or reads
optical information on an optical disc, a magneto-optical disc and
so on, there have been known one wherein a prism type beam splitter
is used as an optical part for directing (beam-splitting) a signal
ray reflected from a recording surface of a disc to a detector, and
one wherein a diffracting grating or a hologram element is used as
such an optical part.
[0003] In the past, the diffraction grating or the hologram element
for the optical head has had a rectangular grating with a
rectangular section (an isotropic diffracting grating in a relief
form) formed on a glass substrate or a plastic substrate by dry
etching or injecting molding, thereby reflecting a ray to obtain a
beam splitting function.
[0004] In order to increase the utilization efficiency of a ray in
comparison with an isotropic diffraction grating having about 10%
of light ray utilization efficiency, the utilization of
polarization can be thought out. When polarized light is utilized,
there has been a method wherein a prism type beam splitter is
combined with a quarter-wave plate to improve efficiency in a
forward path (a direction from a light source toward an optical
recording medium) and in a backward path (a direction from the
optical recording medium toward the light source and a detector) so
as to raise reciprocation efficiency.
[0005] However, the prism type polarization splitter is expensive,
and another system has been groped for. As a system, there has been
known a method wherein a flat plate of birefringent crystal such as
LiNbO.sub.3 is used and the flat plate has an anisotropic
diffraction grating formed thereon to obtain polarization
selectivity. However, such a birefringent crystal per se is
expensive, and application of the birefringent crystal to
commercial utilization is difficult. When the grating is formed by
proton exchanging, the proton in the proton exchange liquid is
usually liable to diffuse in a LiNbO.sub.3 substrate, creating a
problem in that it is difficult to provide the grating with a fine
pitch.
[0006] The isotopic diffraction grating has about 10% of
reciprocation efficiency at the maximum because the utilization
efficiency in the forward path is about 50% and the utilization
efficiency in the backward path is about 20% as stated earlier.
[0007] The present invention has been made to solve these problems,
and has an object to provide an optical head having high
utilization efficiency of light ray and high reliability.
DISCLOSURE OF INVENTION
[0008] The present invention provides an optical head which
irradiates a ray from a light source to an optically recording
medium through a diffraction element so as to read information
and/or write information, characterized in that the diffraction
element includes an optically anisotropic diffraction grating which
comprises an optically anisotropic polymer liquid crystal
sandwiched between two transparent substrates, and that the polymer
liquid crystal has an alignment direction thereof periodically
changed, and a diffraction element suitable for such an optical
head.
[0009] In a preferred mode of the present invention, both of the
two transparent substrates have electrodes mounted thereon, and at
least one of the electrodes comprises periodically formed electrode
pieces. By such arrangement, portions with the periodically formed
split electrodes and portions without the split electrodes can have
different alignment states in a liquid crystal material on
application of an electric field, facilitating formation of an
optically anisotropic diffraction grating by the electric
field.
[0010] In another preferred mode of the present invention, both of
the electrodes comprise periodically formed electrode pieces, and
the two electrodes are unsymmetrical arranged between the two
transparent substrates. It means that both electrodes are
unsymmetrical with respect to a central plane located at the center
between the two transparent substrates and parallel to the two
transparent substrates in such a state that a polymer liquid
crystal cell has been formed. By such arrangement, the split
electrodes of both of the two electrodes confront one another,
having different locations and/or different sizes, and aligned
portions of the polymer liquid crystal formed by the split
electrodes can be unsymmetrical as a pair of upper and lower split
electrodes are viewed. The optically anisotropic diffraction
grating wherein either one of .+-.1st order diffraction rays has a
higher diffraction efficiency can be easily formed by the electric
field.
[0011] In these modes, it is preferable that the periodically
formed electrode pieces of the at least one of the electrodes have
a width of 30-45% of a periodical pitch thereof.
[0012] Both of the electrodes may comprise periodically formed
electrode pieces, and the periodically formed electrode pieces of
both electrodes may have a width of 30-45% of a periodical pitch
thereof. In this case, the upper and lower electrode pieces may
have the same width.
[0013] In another preferred embodiment of the present invention, at
least one of the transparent substrates has a side thereof
contacting the liquid crystal formed with an alignment layer, and
at least one of the alignment layers includes alignment layer
pieces which periodically have different alignment forces which
periodically have different alignment forces. By such arrangement,
the split alignment layers which are formed by the alignment layer
pieces having periodically different alignment forces can form a
certain distribution in the alignment state of the polymer liquid
crystal. When one period includes several split alignment layers,
the polymer liquid crystal can have an unsymmetrical alignment
state in the periodical direction. In this manner, the optically
anisotropic diffraction grating wherein either one of .+-.1st order
diffraction rays partially has a high diffraction efficiency can be
formed easily.
[0014] In another preferred mode of the present invention, the
alignment layers of both of the two transparent substrates
respectively include alignment layer pieces which periodically have
different alignment forces, and the alignment layer pieces which
periodically have different alignment forces are unsymmetrical
arranged between the two transparent substrates. It is mean that
the alignment layer pieces are unsymmetrical with respect to a
central plane locating at the center between the two transparent
substrates and parallel to the two transparent substrates in such a
state that the polymer liquid crystal cell has been formed. By such
arrangement, the respective split alignment layers on both
substrates which are formed by the alignment layer pieces
periodically having different alignment forces can make aligned
portions of the polymer liquid crystal by the split alignment
layers unsymmetrical as viewed a pair of upper and lower split
alignment layers are viewed. In this manner, the optically
anisotropic diffracting grating wherein either one of .+-.1st order
diffraction rays has a higher diffraction efficiency can be formed
easily by the alignment layers.
[0015] In another preferred mode of the present invention, an
alignment layer is formed on an electrode, the alignment layer is
one which is subjected to rubbing so as to extend perpendicularly
to a longitudinal direction of the electrode.
[0016] In addition, the present invention provides a method for
manufacturing an optical head which irradiates a ray from a light
source to an optically recording medium through a diffraction
element so as to read information and/or write information,
characterized in that the method comprises providing electrodes on
two transparent substrates; forming at least one of the electrodes
so as to comprise periodically formed electrode pieces; providing
an alignment layer on a side of at least one of the two transparent
substrates which contacts a liquid crystal material; sandwiching
the liquid crystal material between the two transparent substrates,
the liquid crystal material being polymerizable; applying a
periodical electric field to the electrodes to align the liquid
crystal material; and polymerizing the liquid crystal material in
such an aligned state, thereby fabricating a diffraction element
which includes an optically anisotropic diffraction grating.
[0017] In a preferred mode of the present invention, the
periodically formed electrode pieces include a certain number of
split electrodes included in one period, the respective split
electrodes in the one period have different electric fields applied
thereto, the liquid crystal material in the one period has an
alignment state made unsymmetrical, and then the liquid crystal
material is polymerized. By such a method, an unsymmetrical
diffraction grating which is equivalent to a brazed (sawtooth)
configuration of diffraction grating and so on can be easily formed
in one period by the electric fields in the periodical direction of
the electric fields. In this manner, the optically anisotropic
diffraction grating wherein either one of .+-.1st order diffraction
rays has a higher diffraction efficiency can be easily formed by
the electric fields.
[0018] In another preferred mode of the present invention, the
method further comprises a step A for sandwiching the liquid
crystal material in an unpolymerized state between the two
transparent substrates and periodically polymerizing the liquid
crystal material by periodical masking exposure, and a step B for
polymerizing the entire liquid crystal material by non-masking
exposure after the first step, a voltage being applied in either
one of the step A and the step B. By such a method, the following
processes are carried out.
[0019] The transparent substrates with the electrode and the
alignment layer thereon are subjected to a rubbing treatment,
providing the transparent substrates with a horizontal alignment
force. The two transparent substrates are overlapped so as to have
the rubbing direction conformed each other, and crystallizable
monomers having photopolymerization and having positive dielectric
anisotropy are sandwiched between the transparent substrates. When
reed screen shape of masks are put on the transparent substrates
and an ultraviolet beam is irradiated for exposure while a voltage
is applied across both of the transparent substrates (the step A),
a periodical structure with portions of polymer liquid crystal
polymerized in a vertical alignment state and portions of
crystallizable monomer unpolymerized is formed. When the masks are
removed and non-masking exposure is carried out without application
of a voltage (Step B), the portions of crystallizable monomer
unpolymerized change into portions of polymer liquid crystal in a
horizontal alignment state, providing an optically anisotropic
diffraction grating which has a periodical alignment state as a
whole.
[0020] In this case, when the voltage is applied in Step B, a
periodical structure with the polymer liquid crystal portions
polymerized in a horizontal alignment state and portions of the
crystallizable monomers unpolymerized is provided in Step A, and
the portions of the crystallizable monomers which have not been
polymerized in Step B change into a polymer liquid crystal in a
vertical alignment state.
[0021] In addition, the following mode is preferable in the present
invention.
[0022] It is preferable that the direction of the alignment
treatment which is applied to the two transparent substrates of the
diffraction element extends perpendicularly to the periodical
grating of an electrode. By such arrangement, the ellipsoidal axis
of the refractive index of transient regions which are produced
around the boundary of the electrode on application of the electric
field can be directed to be parallel or vertical to an incident
polarization direction to facilitate maintenance of the
polarization linearity of a light ray transmitting through the
element. The anisotropy in refractive index with respect to the
polarization axis of an incident ray in the transient region can be
make larger in comparison with the alignment in parallel to the
periodical grating of the electrode.
[0023] It is more preferable that the angle (crossing angle) of the
alignment direction between the two transparent substrates is set
to 180.degree.. The present invention is implemented in such a
state that the alignment direction has an angle of about 0.degree.
or about 180.degree., and the angle of 180.degree. has superior
response to periodical on and off of the electric field in
comparison with the angle of 0.degree., allowing a steep grating
shape to be fabricated. In addition, it is preferable that the gap
between the two transparent substrates is set to not greater than 6
.mu.m in terms of a decrease in a driving voltage and a reduction
in estrangement from the rectangular grating shape caused by a
leakage electric field from the boundary of the electrode.
[0024] When the electrode pieces with different width are used, the
upper and lower electrode pieces can be arranged so as to be
unsymmetrical, increasing the diffraction efficiency of one side
portion. In particular, the width of narrower electrode pieces can
be set to 30-45% of the periodical pitch of the electrode pieces to
exceed the theoretical threshold level of a symmetrical grating of
40%. In addition, the provision of the gap of not greater than 6
.mu.m between the two transparent substrates as stated earlier can
restrain the leakage electric field from the electrode boundary to
reflect the unsymmetry of the electrode pieces to the alignment
portions more significantly. In order to make the alignment
portions unsymmetrical to increase the rate of the efficiency of
one side portion to that of the other side portion to not less than
1.5 times, it is preferable that the gap is set to not greater than
3 .mu.m.
[0025] Even when only one side portion is provided with a solid
electrode or both side portions are provided with upper and lower
symmetrical periodical electrode pieces, it is preferable that the
width of at least one of the electrode pieces is 30-45% of the
periodical pitch thereof. When a voltage is applied across the
upper and lower electrode pieces, the electric field expands in a
barrel shape so as to reach slightly outside of the electrode piece
width. From this standpoint, the diffraction efficiency can be
improved when the electrode piece width is set to not greater than
45% of the periodical pitch of the electrode pieces than when the
electrode piece width is set to 50% of the periodical pitch of the
electrode pieces. If the electrode piece width is set to less than
30% of the periodical pitch of the electrode pieces, the bulge of
the electric field in the barrel shape is relatively enlarged. As a
result, the liquid crystal is aligned along the electric field to
make the refractive index distribution of the grating unclear
easily, facilitating a decrease in the diffraction efficiency on
the contrary.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a side view in section of the diffraction element
for an optical head according to an embodiment of the present
invention.
[0027] FIG. 2 is a side view partly in section of the diffraction
element for an optical head according to an embodiment of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0028] In the present invention, at least one of two transparent
substrates has a side thereof contacting a polymer liquid crystal
formed with periodical electrodes (transparent electrodes). That is
to say, one of the transparent substrates has periodical electrodes
formed thereon and the other transparent substrate has a solid
electrode formed thereon, or both of the transparent substrates
have periodical electrodes formed thereon. An unpolymerized liquid
crystal material (crystallizable monomers) is sandwiched between
such two transparent substrates, and the liquid crystal material is
polymerized under application of an electric field across the
electrodes, providing an optically anisotropic diffraction grating
of a polymer liquid crystal having a periodical alignment
structure.
[0029] When a liquid crystal to be used has a positive dielectric
anisotropy, portions with the electric field applied thereto in the
unpolymerized liquid crystal material have liquid crystal molecules
aligned so as to be parallel with the electric field and vertical
to the transparent substrate. Portions without the electric field
applied thereto are aligned so as to be parallel with the
transparent substrates and parallel with the rubbing direction of
alignment layers.
[0030] When the alignment layers have ability for vertical
alignment and a liquid crystal material to be used has a negative
dielectric anisotropy, portions without the electric field applied
thereto are aligned so as to be vertical to the transparent
substrates and portions with the electric field applied thereto are
aligned so as to be parallel with the transparent substrates.
[0031] Making use of the difference between the alignment layers in
terms of alignment ability, a photolithography technique and a
rubbing technique can be combined to provide a periodical pattern
with vertical alignment regions and horizontal alignment regions
periodically included therein. An alternating electric field can be
applied to improve an electric field distribution. In this case,
the alignment layers can be omitted.
[0032] By the method just stated above, the entire liquid crystal
material can be polymerized by heat, a ultraviolet ray and so on
with a certain distribution applied to the alignment in the liquid
crystal material, being solidified with the alignment distribution
fixed.
[0033] The polymer liquid crystal is a polymer formed from
crystallizable monomers, and means a polymer having an anisotropy
in refractive index of not less than 0.02.The polymer liquid
crystal per se does not necessarily exhibit a liquid crystal
property.
[0034] It is preferable that the polymer liquid crystal is produced
by polymerizing crystallizable monomers with light or heat. In
particular, crystallizable monomers which can be polymerized with
an ultraviolet ray or a visible ray are preferable since a polymer
liquid crystal can be produced on sight (directly on the
substrates) by a photolithography process.
[0035] A crystallizable monomer means a monomer which exhibits
liquid crystal property at a room temperature or at temperature on
photopolymerization. The liquid crystal property means to exhibit a
known liquid crystal phase such as a nematic phase, a smectic phase
and a cholesteric phase though a liquid crystal phase with a short
helical pitch as in the cholesteric phase is not appropriate since
it is unsuitable for the present invention.
[0036] The crystallizable monomer is preferably selected from
esters of e.g. acrylic acid or methacrylic acid. The alcohol
residue constituting the ester preferably contains at least one,
especially two or three, phenyl groups. Further, the alcohol
residue constituting the ester may contain one cyclohexyl group.
The crystallizable monomer can be provided by two components or
more in order to expand the temperature range where the
crystallizable monomer can exist as a liquid crystal.
[0037] An optical head according to the present invention optically
functions as follows. It is assumed that a diffraction element is
prepared so that the rubbing direction of an alignment layer is
aligned in a direction perpendicular to the grating of an electrode
(in the right and left directions in FIG. 1). In the following
explanation, a P wave means a ray which has been polarized in a
direction parallel to the paper surface of FIG. 1. An S wave (a ray
polarized in a direction vertical to the paper surface of FIG. 1)
which has entered from a light source side of the diffraction
element (a lower side in FIG. 1) enters an optically anisotropic
diffraction grating from the lower side. At the time, no change is
effected in the S wave since the optically anisotropic diffraction
grating has the vertical aligned portions and the horizontally
aligned portions of the polymer liquid crystal optically provided
in a uniform manner with respect to the S wave. The S wave enters a
quarter-wave plate as it is, is changed into a circularly polarized
ray, transmits an aspherical lens (objective lens), and reaches a
recording surface of an optically recording medium with about 100%
of amount of light.
[0038] The ray which has reflected from the optically recording
medium and returned through the aspherical lens transmits the
quarter-wave plate again, and changes into a P wave with the
polarization direction shifted to 90.degree.. When the P wave
enters the optically an isotropic diffraction grating, the
diffracting grating exhibits its function with respect to the P
wave since the vertically aligned portions and the horizontally
aligned portions of the polymer liquid crystal have different
refractive indices. At the time, about 40% of diffraction
efficiency as a 1st order diffraction ray and about 40% of
diffraction efficiency as a -1st order diffraction ray are
obtained. When an optical detector is arranged on one side, 40% of
utilization efficiency of light, and when an optical detector is
arranged on each of sides, 80% of utilization efficiency of light
is totally obtained.
[0039] Two periodical electrodes which are formed on the respective
two transparent substrates can be unsymmetrical in terms of
location and/or size to provide an unsymmetrical structure to
portions of the polymer liquid crystal which correspond to the
electrode portions and have been aligned in a certain direction by
the electric field. As a result, the optical anisotropic
diffraction grating can be obtained in such a way that either one
of the .+-.1st order diffraction rays has a higher diffraction
efficiency.
EXAMPLES
Example 1
[0040] As shown in FIG. 1, a glass substrate 1 which has 3 mm in
thickness and 120 mm.times.120 mm in square has a solid transparent
electrode 2 of ITO formed on a surface thereof. A similar glass
plate 4 was prepared, and transparent electrodes 2 of ITO were
periodically formed on a surface of the glass plate by a
photolithography technique and a dry etching technique. The
periodical transparent electrodes were formed so that the ratio of
the width of each of the electrodes to that of a portion without an
electrode was about 1:1.
[0041] After that, a polyimide film 3 having a thickness of about
100 nm was formed on the surfaces of the two glass substrates 1 and
4 with the transparent electrodes 2 formed thereon by spin coating.
The polyimide film 3 was subjected a rubbing treatment for a
horizontal alignment. At that time, the rubbing direction on the
substrates with the electrodes formed thereon was vertical to the
grating of the transparent electrodes 2 (in a periodical
direction). The two glass substrates were arranged so that the
transparent electrodes 2 confronted one another, the angle of the
alignment direction between the upper and lower substrates was
180.degree. and the gap between the two glass substrates was 3
.mu.m.
[0042] A liquid crystal material (crystallizable monomers) in
liquid form which included a
4'-.omega.-acryloyloxyalkylxoy-4-cyanobiphenyl and a
4-.omega.-acryloyloxyalkylxoybenzoic acid 4'-n-alkyloxyphenylester
as main components was injected into the gap to be sandwiched
between the two glass substrates. At that time, the crystallizable
monomers had 1% of benzoin isopropyl ether added therein as a
photopolymerizable initiator to obtain a crystallizable monomer
composition having an ultraviolet curable property.
[0043] After that, a voltage of 5V was applied across the
electrodes, and the crystallizable monomer composition was
vertically aligned at portions corresponding to the split
electrodes which were prepared by periodically forming the
electrodes (vertically aligned portions 7). Portions which did not
correspond to the split electrodes became horizontally aligned
portions 8. The split electrodes had a pitch (period) of 4 .mu.m,
and each split electrode had a width of 2 .mu.m. After that, an
ultraviolet ray having a wavelength of 360 nm was irradiated to the
entire crystallizable monomer composition to polymerize and cure
the entire crystallizable monomer composition with the alignment
state held, fixing the entire structure.
[0044] The glass substrate 4 had a quarter-wave plate 5 laminated
and bonded to an upper surface thereof (a surface opposite to the
liquid crystal) by a transparent adhesive agent, and a glass
substrate 6 having good flatness was laminated and bonded to an
upper surface of the quarter-wave plate 5 by a transparent adhesive
agent. Although the glass substrate 6 is provided to improve the
wave front aberration of a light incident and outgoing surface of
the entire diffraction element, the quarter-wave plate 5 can be
omitted if it has good flatness. In this manner, the diffraction
element was fabricated so as to have an optically anisotropic
diffraction grating.
[0045] The diffraction element had a refractive index of 1.52
(ordinary ray refractive index) at portions with electric field
applied thereto (the vertically aligned portions 7) and a
refractive index of 1.53 (ordinary ray refractive index) at
portions without the electric field applied thereto (the
horizontally aligned portions 8) with respect to an S wave (a ray
polarized in a direction vertical to the paper surface in FIG. 1).
With respect to a P wave (a ray polarized in a direction
perpendicular to the paper surface in FIG. 1), a refractive index
of 1.54 (ordinary ray refractive index) at the potions with the
electric field applied thereto and a refractive index of 1.66
(extraordinary ray refractive index) at the portions without the
electric field applied thereto were obtained, and a refractive
index difference of about 0.12 was obtained.
[0046] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as a optical detector were used to fabricate an optical head. The
light transmission with respect to an incident ray (S wave) having
a light wavelength 780 m was about 80%, and the diffraction
efficiency to a P wave which a reflected ray (circularly polarized
ray) from the optical disc was transformed into by the quarter-wave
plate was obtained at a value of about 25% with respect to both of
.+-.1st order diffraction rays.
Example 2
[0047] A diffraction element was fabricated as in the Example 1
except that the structure of the transparent electrodes 2 was
modified as described below.
[0048] As shown in FIG. 2, the transparent electrodes 2 on the side
of the glass substrate 1 were periodically formed, and each split
electrode had a width of 4 .mu.m and a pitch of 8 .mu.m. The
transparent electrodes 2 on the side of the glass substrate 4 were
also periodically formed, and each split electrode had a width of 2
.mu.m and a pitch of 8 .mu.m. In this case as well, each of the
substrates had an alignment direction extended in a direction
perpendicular to the grating (a periodic direction) and the angle
of the alignment direction between the upper and lower substrates
was 180.degree..
[0049] In this case, a refractive index of 1.52 (ordinary ray
refractive index) at portions with the electric field applied
thereto (vertically aligned portions 7) and a refractive index of
1.52 (ordinary ray refractive index) at portions without the
electric field applied thereto (horizontally aligned portions 8)
were obtained with respect to an S wave. With respect to a P wave,
a refractive index of 1.53 (ordinary ray refractive index) at the
portions with the electric field applied thereto and a refractive
index of 1.65 n(extraordinary ray refractive index) at the portions
without the electric field applied thereto were obtained, and a
refractive index difference of about 0.12 was obtained
[0050] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as an optical detector were used to fabricate an optical head. The
light transmittance with respect to an incident ray (S wave) having
a light wavelength of 780 m was about 78%, and the diffraction
efficiency to a P wave which a reflected ray (circularly polarized
ray) from the optical disc was transformed into by the quarter-wave
plate was obtained at a value of about 28% for a +1st order
diffraction ray and about 19% for a -1st order diffraction ray.
Example 3
[0051] A diffraction element was fabricated as in the Example 1
except that no transparent electrodes 2 were formed and alignment
layers were provided as explained below.
[0052] Horizontal alignment layers (polyimide film) and vertical
alignment layers were alternately formed on each of the glass
substrates 1 and 4 by a photolithography technique and a mask
rubbing technique. The vertical alignment layers were formed by
coating a fluorine type surfactant
(C.sub.8F.sub.17SO.sub.2NH(CH.sub.2).sub.3N.sup.+
(CH.sub.3).sub.3.I.sup.- -). The glass substrates 1 and 4 were
layered and bonded together with a gap of 5 .mu.m so that a
horizontal alignment layer confronted a corresponding horizontal
alignment layer and a vertical alignment layer confronted a
corresponding vertical alignment layer (the alignment layers were
symmetrical between the glass substrates 1 and 4). Each horizontal
alignment layer was a width of 4 .mu.m, and each vertical alignment
layer had a width of 4 .mu.m to provide the alignment layers having
a pitch of 8 .mu.m, which had an alignment force periodically
changed which had an alignment force periodically changed. In this
case, horizontally aligned portions had an alignment direction
extended in the periodical direction with the horizontally aligned
portions alternately formed. The angle of the alignment direction
between the upper and lower substrates was 180.
[0053] In this case, a refractive index of 1.53 (ordinary ray
refractive index) at the vertically aligned portions and a
refractive index of 1.53 ordinary ray refractive index) at the
horizontally aligned portions were obtained with respect to an S
wave. With respect to a P wave, a refractive index of 1.54
(ordinary ray refractive index) at the vertically aligned portions
and a refractive index of 1.65 (extraordinary ray reflective index)
at the horizontally aligned portions were obtained, and a
refractive index difference was obtained at a value of about
0.11.
[0054] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as an optical detector were used to fabricate an optical head. The
light transmittance to an incident ray (S wave) having a light
wavelength of 780 m was about 70%, and the diffraction efficiency
to a P wave which a reflected ray (circularly polarized ray) from
the optical disc was transformed into by the quarter-wave plate was
obtained at a value of about 20% for a .+-.1st order diffraction
ray and about 20% for a -1st order diffraction ray.
Example 4
[0055] A diffraction element was fabricated as in the Example 1
except that the structure of the transparent electrodes 2 were
modified as explained below.
[0056] The transparent electrodes 2 were periodically formed on the
glass substrates 1 and 4. One period was determined as 9 .mu.m, and
sets of a non-electrode portion having a width of about 3 .mu.m and
two split electrodes having a width of about 3 .mu.m in one period
were provided from an end in the order as stated earlier. The glass
substrates 1 and 4 were layered and bonded together with a gap of 5
.mu.m so that a non-electrode portion confronted a corresponding
non-electrode portion and a split electrode confronted a
corresponding split electrode (the non-electrode portions and the
split electrodes were symmetrical between the glass substrates 1
and 4). A central split electrode D in one period had a voltage of
2V applied thereto, and another split electrode E had a voltage of
5V applied thereto.
[0057] Thus, the crystallizable monomer composition at the
non-electrode portion was changed into a horizontally aligned
portion, a portion corresponding to the split electrode E was
changed into a vertically aligned portion, and a portion
corresponding to the split electrode E was changed into an
alignment state which was almost intermediate between a horizontal
alignment and a vertical alignment. As a result, the alignment
state in one period was unsymmetrical. After that, an ultraviolet
ray having a wavelength of 360 nm was irradiated to the entire
crystallizable monomer composition to polymerize and cure the
entire crystallizable monomer composition with the alignment state
held, fixing the entire structure.
[0058] In this case, with respect to an S wave, a refractive index
of 1.52 (ordinary ray refractive index) at the vertically aligned
portions, a refractive index of 1.52 (ordinary ray refractive
index) at the intermediately aligned portions, and a refractive
index of 1.52 (ordinary ray refractive index) at the horizontally
aligned portions were obtained. With respect to a P wave, a
refractive index of 1.66 (ordinary ray refractive index) at the
vertically aligned portions, a refractive index 1.60 at the
intermediately aligned portions, and a refractive index of 1.54
(extraordinary ray refractive index) at the horizontally aligned
portions were obtained, and the refractive indices were stepwise
changed.
[0059] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as an optical detector were used to fabricate an optical head. The
light transmittance to an incident ray (S wave) having a light
wavelength of 780 m was about 80%, and the diffraction efficiency
to a P wave which a reflected ray (circularly polarized ray) from
the optical disc was transformed into by the quarter-wave plate was
obtained at a value about 29% for a +1st order diffraction ray and
at a value of about 20% for a -1st order diffraction ray.
Example 5
[0060] As shown in FIG. 1, glass substrates 1 and 4 which are 1.1
mm in thickness and 120 mm.times.120 mm in square have transparent
electrodes 2 of ITO having a thickness 1000 .ANG. formed on a
surface thereof. The ITO electrodes on the glass substrates 1 and 4
were subjected to a photolithography technique and a wet etching
technique to be modified into periodically formed ITO transparent
electrodes 2. At that time, the periodically formed transparent
electrodes were formed so that the ratio of the width of an
electrode portion to a width a non-electrode portion was about
0.8:1.2.
[0061] After that, a polyimide film 3 having a thickness about 100
nm was formed the surfaces of the two glass substrates 1 and 4 with
the transparent electrodes 2 formed thereon by a spin coating
technique. The polyimide film 3 was subjected to a rubbing
treatment to extend in a direction perpendicular to the ITO
electrode gratings for horizontal alignment. The two glass
substrates were arranged so that the transparent electrodes 2
confronted one another, the angle of the alignment direction
between the upper and lower substrates was 180.degree., and the gap
between the two glass substrates was 5 .mu.m.
[0062] A liquid crystal material (crystallizable monomers) in
liquid form which includes a
4'-.omega.-acryloyloxyalkyloxy-4-cyanobiphenyl and a
4-.omega.-acryloyloxyalkyloxybenzoic acid 4'-n-alkyloxyphenylester
as main components was injected into the gap to be sandwiched
between the two glass substrates. At that time, the crystallizable
monomers had benzoin isopropyl ester of 1% added thereinto as a
photopolymerization initiator, providing a crystallizable monomer
composition having an ultraviolet curable property.
[0063] After that, a rectangular wave of a.c. voltage was applied
across the electrodes at 5V and 100 Hz to vertically align the
crystallizable monomer composition at portions which corresponded
to the periodically formed split electrodes (vertically aligned
portions 7). Portions which did not correspond to the split
electrodes became horizontally aligned portions 8. The pitch
(period) of the split electrodes was 20 .mu.m, and the width of a
split electrode was 8 .mu.m. After that, an ultraviolet ray having
a wavelength of 360 nm was irradiated to the entire crystallizable
monomer composition to polymerize and cure the crystallizable
monomer composition with the alignment state held, fixing the
entire structure.
[0064] The glass substrate 4 had a quarter-wave plate 5 layered on
and bonded to an upper surface thereof (a surface opposite to the
liquid crystal) by a transparent adhesive agent, and the
quarter-wave plate 5 had a glass substrate 6 having good flatness
layered on and bonded to an upper surface thereof by a transparent
adhesive agent. Although the glass substrate 6 was provided to
improve the wavefront aberration on a light incident and outgoing
surface of the entire diffraction element, it can be omitted if the
quarter-wave plate 5 has good flatness. In this manner, the
diffraction element was fabricated so as to have an optically
anisotropic diffraction grating.
[0065] The diffraction element had a refractive index of 1.52
(ordinary ray refractive index) at the electric field applied
portions (the vertically aligned portions 7), and a refractive
index of 1.53 (ordinary ray refractive index) at the non-electric
field applied portions (the horizontally aligned portions 8) with
respect to an S wave (a ray polarized in a direction perpendicular
to the paper surface in FIG. 1). With regard to a P wave (a ray
polarized in parallel with the paper surface in FIG. 1), a
refractive index of 1.54 (ordinary ray refractive index) at the
electric field applied portions, and a refractive index of 1.66
(extraordinary ray refractive index) at the non-electric field
applied portions were obtained, and the refractive index difference
was obtained at a value of about 0.12.
[0066] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as an optical detector were used to fabricate an optical head. The
light transmittance to an incident ray (S wave) having a light
wavelength of 780 m was about 84%, and the diffraction efficiency
to a P wave which a reflected ray (circularly polarized ray) from
the optical disc was transformed into by the quarter-wave plate was
obtained at a value of about 32% for .+-.1st order diffraction
rays.
Example 6
[0067] A diffraction element was fabricated as in the Example 5
except that the structure of the transparent electrodes 2 was
modified as described below.
[0068] As shown in FIG. 2, the transparent electrodes 2 on the
glass substrate 1 were periodically formed, and each split
electrode had a width of 8 .mu.m and a pitch of 16 .mu.m. The
transparent electrodes 2 on the glass substrate 4 were also
periodically formed, and each split electrode was a width of 6
.mu.m and a pitch of 16 .mu.m. The gap between the two substrates
was 3 .mu.m. A rectangular wave of a.c. voltage was applied at 7V
and 100 Hz on irradiation of an ultraviolet ray.
[0069] In this case, with respect to an S wave, a refractive index
of 1.52 (ordinary ray refractive index) at the electric field
applied portions (the vertically aligned portions 7), and a
refractive index of 1.52 (ordinary ray refractive index) at the
non-electric field applied portions (the horizontally aligned
portions 8) were obtained. With respect to a P wave, a refractive
index of 1.53 (ordinary ray refractive index) at the electric field
applied portions, and a refractive index of 1.65 (extraordinary
refractive index) at the non-electric field applied portions were
obtained, and the refractive index difference was obtained at a
value of about 0.12.
[0070] A semiconductor laser (a light wavelength of 780 nm) as a
light source, the diffraction element, a quarter-wave plate, an
aspherical lens (objective lens), an optical disc, and a photodiode
as an optical detector were used to fabricate an optical head. The
light transmittance to an incident ray (S wave) having a light
wavelength of 780 m was about 89%, and the diffraction efficiency
to a P wave which a reflected ray (circularly polarized ray) from
the optical disc was transformed into by the quarter-wave plate was
obtained at a value of about 40% for a +1st order diffraction ray
and a value of about 26% for a -1st order diffraction ray.
Industrial Applicability
[0071] In accordance with the present invention, the electrodes can
be periodically formed or the alignment direction of the alignment
layers can be periodically changed to easily provide an optically
anisotropic diffraction grating having a small pitch (period).
Since split electrodes or split alignment layers can be
unsymmetrically arranged between two transparent substrates to
easily form an unsymmetrical optically anisotropic diffraction
grating, a diffraction element wherein either one of .+-.1st order
diffraction rays has a higher diffraction efficiency can be easily
fabricated. Since crystallizable monomers can be aligned by an
electric field using only electrodes, alignment layers can be
eliminated.
* * * * *