U.S. patent application number 11/120956 was filed with the patent office on 2005-09-22 for aberration correcting optical unit, optical pickup apparatus, and information recording/reproducing apparatus.
This patent application is currently assigned to PIONEER CORPORATION. Invention is credited to Iwasaki, Masayuki, Ogasawara, Masakazu.
Application Number | 20050207290 11/120956 |
Document ID | / |
Family ID | 26581221 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050207290 |
Kind Code |
A1 |
Iwasaki, Masayuki ; et
al. |
September 22, 2005 |
Aberration correcting optical unit, optical pickup apparatus, and
information recording/reproducing apparatus
Abstract
An aberration correcting optical unit includes an optical
element for causing a phase change to light passing therethrough by
the application of voltage and electrode layers for applying
voltages to the optical element. The optical element is sandwiched
between the electrode layers. At least one of the electrode layers
includes a plurality of electrodes which are electrically isolated
from one another. The plurality of electrodes are disposed such
that an electric field generated in a portion of the optical
element corresponding to a portion between the plurality of
electrodes is larger than a predetermined intensity when a
predetermined voltage is applied to the optical element.
Inventors: |
Iwasaki, Masayuki;
(Tsurugashima-shi, JP) ; Ogasawara, Masakazu;
(Tsurugashima-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
PIONEER CORPORATION
|
Family ID: |
26581221 |
Appl. No.: |
11/120956 |
Filed: |
May 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11120956 |
May 4, 2005 |
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09739452 |
Dec 19, 2000 |
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6909686 |
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Current U.S.
Class: |
369/44.23 ;
369/112.02; G9B/7.102; G9B/7.117 |
Current CPC
Class: |
G02F 1/134309 20130101;
G02F 2203/18 20130101; G11B 7/13925 20130101; G11B 7/1369 20130101;
G02B 27/0068 20130101 |
Class at
Publication: |
369/044.23 ;
369/112.02 |
International
Class: |
G11B 007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 1999 |
JP |
11-361217 |
Mar 7, 2000 |
JP |
2000-61451 |
Claims
1-16. (canceled)
17. An aberration correcting optical unit disposed in an optical
path of an optical system for irradiating a recording medium with a
light beam emitted from a light source and for guiding a reflected
light beam reflected by said recording medium, for correcting
aberration occurring in said reflected light beam, comprising: a
liquid crystal element for causing a phase change to light passing
therethrough by the application of voltage; first and second
electrode layers opposing to each other, said first and second
electrode layers sandwiching said liquid crystal element; said
first electrode layer includes a plurality of divided electrodes
electrically isolated from one another by gaps in the same plane,
and an insulating layer disposed between said liquid crystal
element and said first electrode layer, said insulating layer
having a thickness larger than the gaps of said first electrode
layer.
18. An aberration correcting optical unit according to claim 1,
wherein said insulating layer has a thickness larger than a
thickness of said liquid crystal element.
19. An aberration correcting optical unit according to claim 1,
further comprising a liquid crystal alignment layer provided
between said liquid crystal element and said insulating layer.
20. An aberration correcting optical unit according to claim 1,
further comprising: a second insulating layer disposed between said
liquid crystal element and said second electrode layer, wherein
said second electrode layer includes a plurality of divided
electrodes electrically isolated from one another by gaps in the
same plane, and said second insulating layer having a thickness
larger than the gaps of said second electrode layer.
21. An aberration correcting optical unit according to claim 4,
further comprising a liquid crystal alignment layer provided
between said liquid crystal element and said second insulating
layer.
22. An optical pickup apparatus including the aberration correcting
optical unit according to claim 1, comprising: a light source for
emitting a light beam; an optical system for irradiating a
recording medium with the light beam emitted from said light source
and for guiding a reflected light beam reflected by said recording
medium; and a photodetector for detecting said reflected light
beam.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an aberration correcting
optical unit for correcting aberration when information is recorded
on or reproduced from an information recording medium such as an
optical disc as well as an optical pickup apparatus having the
aberration correcting optical unit and an information recording
and/or reproducing apparatus (hereinafter, referred to as "an
information recording/reproducing apparatus").
[0003] 2. Description of the Related Art
[0004] Optical discs such as a CD (Compact Disc) and a DVD (Digital
Video Disc or Digital Versatile Disc) are known as information
recording media for optical information recording or reproduction.
In addition, a variety of different optical discs are now under
development, such as an optical disc specialized for reproduction,
a write-once optical disc capable of additionally recording
information thereon, a rewritable optical disc capable of erasing
information therefrom and re-recording information thereon, and so
on.
[0005] Research and development is being pursued relating to high
recording density optical discs and an optical pickup apparatus for
the high-density discs. In addition, research and development are
now under progress for a compatible pickup apparatus and
information recording/reproducing apparatus which are applicable to
different types of optical discs.
[0006] It is contemplated that the numerical aperture (NA) of an
objective lens provided in an optical pickup apparatus is increased
to irradiate an optical disc with a light beam of a smaller
irradiation diameter for supporting the higher density trend of the
optical disc. It is also contemplated that a short wavelength light
beam is used to support the higher density trend.
[0007] An increased numerical aperture of an objective lens and the
use of a short wavelength light beam, however, result in larger
aberration of the light beam by the optical disc, causing
difficulties in improving the accuracy of information recording and
information reproduction.
[0008] For example, an incident angle range of the light beam to
the optical disc becomes larger as the numerical aperture NA of an
objective lens is increased, thereby resulting in a larger
distribution width of the birefringence amount on the optical disc
pupil plane, which is an amount depending on the incident angle.
This causes a problem of a larger influence of spherical aberration
resulting from the birefringence. Also, when a light beam of a
short wavelength is used with an increased numerical aperture NA of
an objective lens, influence of coma aberration cannot be
negligible if the optical disc is inclined during recording or
reproducing information so as to incline an incident angle (tilt
angle) of the light beam with respect to the normal direction of
the optical disc.
[0009] Further, the influence of aberration such as the
above-described spherical aberration and coma aberration differs
depending on the type of a particular optical disc since different
types of optical discs such as CD and DVD have different structures
and recording densities, thereby making it difficult to develop
compatible optical pickup apparatus and information
recording/reproducing apparatus.
[0010] Conventionally, an optical pickup apparatus having a liquid
crystal unit for correcting aberration has been proposed for
reducing the influence of the aberration as mentioned above
(Laid-open Japanese Patent Application Kokai No. H10-20263).
[0011] This liquid crystal unit has a structure in which a liquid
crystal element C is sandwiched between mutually opposing
transparent electrodes A, B, as schematically illustrated in FIG.
1. A voltage applied between the transparent electrodes A, B is
adjusted to change the alignment state of the liquid crystal
element C, such that when light incident on one of the transparent
electrode A (or B) passes through the liquid crystal element C, a
change in birefringence is given to the light in accordance with
the alignment state to emit the light to the other transparent
electrode B (or A).
[0012] Further, at least one of the transparent electrodes A, B is
divided into a plurality of transparent electrodes, for example,
a1, a2, a3 and b1, b2, b3. Also, the transparent electrodes a1, a2,
a3 are electrically isolated from one another, while the
transparent electrodes b1, b2, b3 are also electrically isolated
from one another.
[0013] Therefore, the liquid crystal element C can be adjusted in a
plurality of different alignment states when different voltages are
applied between transparent electrodes in an opposing relationship,
for example, between the transparent electrodes a1, b1; between the
transparent electrodes a2, b2; and between the transparent
electrodes a3, b3, so that changes in birefringence in accordance
with the respective alignment states can be simultaneously given to
light incident thereon.
[0014] Then, the liquid crystal unit is positioned on an optical
path between a light source for emitting laser light and an
objective lens. The liquid crystal unit gives changes in
birefringence in accordance with the plurality of alignment states
to the laser light, causing the laser light to transmit
therethrough to the objective lens. The objective lens converges
the transmitted laser light to generate a light beam which is
irradiated to an optical disc. Also, when reflected light produced
by irradiating the optical disc with the light beam impinges on the
liquid crystal unit through the objective lens, the reflected light
is given the changes in birefringence in accordance with the
plurality of alignment states, causing the reflected light to
transmit, and the transmitted reflected light is detected by a
photodetector. Therefore, the plurality of alignment states of the
liquid crystal unit are adjusted as appropriate to reduce the
influence of aberration such as spherical aberration and coma
aberration.
[0015] However, gaps (SP) are provided between the respective
transparent electrodes in the conventional liquid crystal unit to
electrically isolate the plurality of transparent electrodes a1,
a2, a3 and b1, b2, b3, as illustrated in FIG. 1. More specifically,
the gaps SP are provided along the respective boundaries of the
transparent electrodes a1, a2, a3, and the gaps SP are provided
along the respective boundaries of the transparent electrodes b1,
b2, b3.
[0016] Therefore, no voltage is applied to the gaps SP, so that the
foregoing structure suffers from an inability of controlling the
alignment states in the liquid crystal element C corresponding to
the gaps SP. As a result, the aberration correction can be made for
a light beam or reflected light passing through the transparent
electrodes a1, a2, a3 and b1, b2, b3, whereas no aberration
correction can be made for a light beam or reflected light passing
through the gaps SP, so that a highly accurate aberration
correction cannot be carried out for the light beam or the
reflected light.
[0017] Also, when the transparent electrodes are divided by a
larger number with the intention of making a finer correction for
the influence of aberration, a large number of transparent
electrodes are formed with required electrical insulating features
therebetween within a limited effective optical path range in which
the laser light or reflected light passes, resulting in an
increased number of the gaps SP and a larger area occupied thereby.
As a result, a fine aberration correction becomes difficult.
[0018] Further, different voltages applied to mutually adjoining
transparent electrodes result in abrupt discontinuous alignment
states produced in the liquid crystal element C corresponding to
the gaps SP intervening between the transparent electrodes.
OBJECT AND SUMMARY OF THE INVENTION
[0019] The present invention has been made to overcome the problems
of the prior art as mentioned above. It is, therefore, an object of
the present invention to provide an aberration correcting optical
unit capable of accurately correcting the influence of aberration
due to an information recording medium, as well as an optical
pickup apparatus including the aberration correcting optical unit,
and an information recording/reproducing apparatus including the
optical pickup apparatus.
[0020] It is another object of the present invention to provide an
aberration correcting optical unit capable of accurately correcting
the influence of aberration associated with the higher density
trend of the information recording media, as well as an optical
pickup apparatus including the aberration correcting optical unit,
and an information recording/reproducing apparatus including the
pickup apparatus.
[0021] According to the present invention, there is provided an
aberration correcting optical unit disposed in an optical path of
an optical system for irradiating a recording medium with a light
beam emitted from a light source and for guiding a reflected light
beam reflected by the recording medium, for correcting aberration
occurring in the optical path, which comprises an optical element
for causing a phase change to light passing therethrough by the
application of voltage; and electrode layers for applying voltages
to the optical element, the electrode layers sandwiching the
optical element, wherein at least one of the electrode layers
includes a plurality of electrodes electrically isolated from one
another, and the plurality of electrodes are disposed such that an
electric field generated in a portion of the optical element
corresponding to a portion between the plurality of electrodes is
larger than a predetermined intensity when a predetermined voltage
is applied to the optical element.
[0022] According to the present invention, there is provided an
aberration correcting optical unit disposed in an optical path
between a light source and an optical element for irradiating an
information recording medium with a light beam emitted from the
light source, in alignment with an optical axis, for correcting
aberration of light caused by the information recording medium,
which comprises a liquid crystal element exhibiting a predetermined
alignment state by applying a predetermined voltage; and mutually
opposing electrodes for applying voltages to the liquid crystal
element, wherein at least one of the mutually opposing electrodes
is formed with a plurality of electrodes of a multi-layer structure
arranged in the direction of the optical axis.
[0023] According to another aspect of the present invention, the
plurality of electrodes formed in a multi-layer structure is formed
such that the plurality of electrodes are arranged in the direction
of the optical axis without overlapping one another.
[0024] According to another aspect of the present invention, the
plurality of electrodes formed in a multi-layer structure is formed
such that the plurality of electrodes partially overlap one
another, and are arranged in the direction of the optical axis.
[0025] According to another aspect of the present invention, the
electrodes are applied with voltages which produce an
electro-optical effect opposite to aberration characteristics
caused by an information recording medium.
[0026] According to the aberration correcting optical unit of the
present invention having the foregoing structure, an
electro-optical effect of characteristics opposite to aberration
characteristics caused by the information recording medium can be
produced in the liquid crystal element by adjusting the voltages
applied across mutually opposing voltages, and aberration of light
transmitting the liquid crystal element can be corrected by this
electro-optical effect. Further, when at least one of the mutually
opposing electrodes is formed of a plurality of electrodes in a
multi-layer structure, the plurality of electrodes can be oriented
toward the liquid crystal element without gaps. For this reason, as
the plurality of electrodes are applied with voltages, the
electro-optical effect can be produced in the liquid crystal
element without gaps, thereby making it possible to correct the
aberration without omission. It is also possible to finely correct
the aberration.
[0027] According to the present invention, there is provided an
aberration correcting optical unit disposed in an optical path
between a light source and an optical element for irradiating an
information recording medium with a light beam emitted from the
light source, in alignment with an optical axis, for correcting
aberration of light caused by the information recording medium,
which comprises a liquid crystal element exhibiting a predetermined
alignment state by applying a predetermined voltage; and mutually
opposing electrodes for applying voltages to the liquid crystal
element, wherein at least one of the mutually opposing electrodes
is formed with a plurality of electrodes of a multi-layer structure
arranged in the direction of the optical axis.
[0028] According to the configuration as described, information can
be accurately reproduced based on the reflected light which has
been corrected for the influence of aberration.
[0029] An information reproducing apparatus according to the
present invention comprises the above-mentioned pickup apparatus,
and reproduces information by emitting information recording light,
and detecting reflected light from an information recording medium.
According to this configuration, information can be accurately
reproduced based on the reflected light which has been corrected
for the influence of aberration.
[0030] An information recording apparatus according to the present
invention comprises the above-mentioned pickup apparatus, and
records information on an information recording medium by emitting
information recording light. According to the configuration,
information can be accurately reproduced based on reflected light
reflected from the information recording medium and corrected for
the influence of aberration.
[0031] According to the present invention, there is provided an
aberration correcting optical unit disposed in an optical path of
an optical system for irradiating a recording medium with a light
beam emitted from a light source and for guiding a reflected light
beam reflected by the recording medium, for correcting aberration
occurring in the optical path, which comprises a liquid crystal
element for causing a phase change to light passing therethrough by
the application of voltage; mutually opposing electrode layers for
applying the liquid crystal element with voltages; and an
insulating layer disposed between the liquid crystal element and at
least one of the electrode layers, wherein the at least one of the
electrode layer includes a plurality of divided electrodes
electrically isolated from one another by gaps in the same plane,
and the insulating layer is of a thickness such that an electric
field generated in a portion of the optical element corresponding
to the gap between the plurality of divided electrodes is larger
than a predetermined intensity when a predetermined voltage is
applied to the optical element.
[0032] According to the present invention, there is provided an
optical pickup apparatus including the aberration correcting
optical unit mentioned above, which comprises a light source for
emitting a light beam; an optical system for irradiating a
recording medium with the light beam emitted from the light source
and for guiding a reflected light beam reflected by the recording
medium; and a photodetector for detecting the reflected light
beam.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a diagram schematically illustrating a
conventional liquid crystal unit;
[0034] FIG. 2 is a diagram illustrating the configuration of an
optical pickup apparatus provided in an information
recording/reproducing apparatus according to an embodiment of the
present invention;
[0035] FIG. 3 is a diagram illustrating the operational principle
of an aberration correcting optical unit provided in the optical
pickup apparatus;
[0036] FIG. 4 is a top plan view illustrating the shape of the
aberration correcting optical unit;
[0037] FIG. 5 is a cross-sectional view illustrating the vertical
cross-sectional structure of the aberration correcting optical
unit;
[0038] FIG. 6 is a diagram illustrating the principle of the
aberration correction by means of the aberration correcting optical
unit;
[0039] FIG. 7 is a top plan view illustrating the shape of another
aberration correcting optical unit;
[0040] FIG. 8 is a cross-sectional view illustrating the vertical
cross-sectional structure of the aberration correcting optical unit
in FIG. 7;
[0041] FIG. 9 is a graph showing the characteristic of coma
aberration;
[0042] FIG. 10 is a characteristic graph showing the result of
reducing the coma aberration by means of the aberration correcting
optical unit;
[0043] FIG. 11 is a graph showing the characteristic when the coma
aberration is not sufficiently reduced;
[0044] FIG. 12 is a cross-sectional view illustrating another
structure of the aberration correcting optical unit;
[0045] FIG. 13 is a cross-sectional view illustrating a further
another structure of the aberration correcting optical unit;
[0046] FIG. 14 is a cross-sectional view illustrating a further
another structure of the aberration correcting optical unit;
[0047] FIG. 15 is a perspective view schematically illustrating the
structure of an aberration correcting liquid crystal unit and a
change in alignment of crystal molecules;
[0048] FIG. 16 is a top plan view of an aberration correcting
optical unit for correcting spherical aberration;
[0049] FIG. 17 is a top plan view of an aberration correcting
optical unit for correcting coma aberration;
[0050] FIG. 18 is a cross-sectional view illustrating the structure
of the aberration correcting optical unit illustrated in FIG. 16,
taken along a line X-X;
[0051] FIG. 19 is a cross-sectional view for explaining electric
fields over a liquid crystal element in aberration correcting
regions and gap regions when each transparent electrode is applied
with a voltage;
[0052] FIG. 20 shows graphs of the electric field intensities on
interfaces between respective insulating layers and liquid crystal
element, with the thickness (THi) of the insulating layers used as
a parameter, when each electrode is applied with a voltage;
[0053] FIG. 21 is a graph showing an example of wave front
aberration (spherical aberration) produced in a light beam by an
optical disc;
[0054] FIG. 22 is a graph showing a phase difference (nm) produced
in a light beam passing through the liquid crystal element when
predetermined voltages are applied to the liquid crystal element,
when the thickness of an insulating layer is smaller than the
thickness of the liquid crystal element;
[0055] FIG. 23 is a graph showing a phase difference (nm) produced
in a light beam passing through the liquid crystal element when
predetermined voltages are applied to the liquid crystal element
when the thickness of the insulating layer is equal to or larger
than the thickness of the liquid crystal element; and
[0056] FIG. 24 is a graph showing a phase difference (nm) produced
in a light beam passing through the liquid crystal element when the
thickness of the insulating layer is smaller than the thickness of
the liquid crystal element in another case.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0057] Embodiments of the present invention will hereinafter be
described with reference to the accompanying drawings. FIG. 2 is a
diagram illustrating the configuration of an optical pickup
apparatus provided in an information recording/reproducing
apparatus.
[0058] In FIG. 2, the optical pickup apparatus PU includes a light
source 1 for emitting laser light H1, a polarizing beam splitter 3,
an aberration correcting optical unit 4, an objective lens 5, a
converging lens 6 and a photodetector 7. The components 1-7 are
arranged along an optical axis OA. A control circuit 8 is provided
in the optical pickup apparatus PU or in an information
recording/reproducing apparatus for controlling the aberration
correcting optical unit 4.
[0059] The aberration correcting optical unit 4 has an
electro-optic element which exhibits an electro-optic effect that
varies depending on an electric field applied thereto. More
specifically, the aberration correcting optical unit 4 has a liquid
crystal optical element. Birefringence characteristics of the
liquid crystal optical element changes in response to a control
voltage Vi applied thereto by the control circuit 8.
[0060] In particular, the aberration correcting optical unit 4 has
a structure, as schematically illustrated in FIG. 3, in which a
liquid crystal element 14 is encapsulated between two transparent
insulating substrates 10, 11 such as glass substrates. Formed
between the opposing surfaces of the glass substrates 10, 11 are
electrodes 12, 13; insulating films 23, 24; and liquid crystal
alignment films 21, 22.
[0061] The alignment state of the liquid crystal element 14 changes
when a control voltage Vi is applied between the electrodes 12, 13,
in response to an electric field Ei produced by the control voltage
Vi. As a result, light passing through the liquid crystal element
14 receives birefringence of the liquid crystal element 14 to
change in a polarizing state (phase). The polarizing state (phase)
can be controlled by the control voltage Vi applied to the liquid
crystal element 14.
[0062] The aberration correcting optical unit 4 also has
bidirectional light transmissivity, so that any side of the
insulating substrates 10, 11 may be oriented to the objective lens
5.
[0063] The aberration correcting optical unit 4 is partitioned into
a plurality of aberration correcting regions AR1-ARi which have
been determined in correspondence to the distribution of aberration
caused by the optical disc 9, as illustrated in a top plan view of
FIG. 4. The aberration correcting regions AR1-ARi are implemented
by transparent electrode (ITO: Indium Tin Oxide) layers formed in
the electrodes 12, 13. It should be noted that while FIG. 4
illustrates a typical example of the aberration correcting regions
AR1, AR2-ARi for correcting spherical aberration caused by the
optical disc 9, the aberration correcting optical unit 4 is
actually partitioned into a variety of shapes in correspondence to
the distribution of particular aberration caused by the optical
disc 9. For example, for correcting coma aberration caused by the
optical disc 9 which is inclined during recording or reproduction
of information, the aberration correcting optical unit 4 is
provided with aberration correcting regions BR1-BR9 of shapes as
illustrated in FIG. 7. Also, the number of sections of the
aberration correcting regions is determined in correspondence to
the distribution of aberration caused by the optical disc 9.
[0064] When the aberration correcting optical unit 4 is provided
with the concentric aberration correcting regions AR1-ARi as
illustrated in FIG. 4, the electrode 12 includes transparent
electrode layers A1-Ai embedded in a transparent insulating layer
15 in an electrically isolated relationship with each other; and
transparent electrode layers B1-Bj embedded in the insulating layer
15 opposed to a plurality of gaps W1 existing between the
respective transparent electrode layers A1-Ai, as illustrated in a
cross-sectional view of FIG. 5 (a diagram illustrating the
cross-sectional structure taken along a line X-X in FIG. 4). Also,
the group of the transparent electrodes A1-Ai and the group of the
transparent electrode layers B1-Bj are formed in a two-stage
structure within the insulating layer 15 along the optical axis OA.
For convenience of description, the insulating films 23, 24 and the
liquid crystal alignment films 21, 22 are omitted in the
cross-sectional view of FIG. 5 (the same is applied likewise to
FIGS. 6, 8, 12, 13, 14).
[0065] The transparent electrode layer A1 is formed in a shape that
conforms to the aberration correcting region AR1 (circular in FIG.
4); the transparent electrode A2 is formed in a shape that conforms
to to the aberration correcting region AR2 (annular in FIG. 4); and
the remaining transparent electrode layers A3-Ai are likewise
formed in shapes which conform to the corresponding aberration
correcting regions AR3-ARi.
[0066] The transparent electrode layer B1 in turn is formed in an
extremely narrow annular shape that conforms to the shape of the
gap W1 for electrically isolating the transparent electrode layers
A1, A2. The transparent electrode layer B2 is likewise formed in an
extremely narrow annular shape that conforms to the shape of the
gap W1 for electrically isolating the transparent electrode layers
A2, A3, and the remaining transparent electrode layers B3-Bj are
formed in a similar manner.
[0067] In other words, the aberration correcting regions AR1-ARi
illustrated in FIG. 4 are implemented by forming the transparent
electrodes A1-Ai in an electrically isolated structure, while the
respective gaps BK1-BKj between the aberration correcting regions
AR1-ARi are implemented by forming the transparent electrodes
B1-Bj.
[0068] It should be noted that while these transparent electrodes
B1-Bj may be arranged opposed to all of the existing gaps W1, they
may be formed opposed to a number of gaps W1 which may be required
in accordance with the characteristic of a particular aberration to
be corrected.
[0069] On the other hand, the electrode 13 has a similar two-stage
structure comprised of a group of transparent electrode layers
C1-Ci embedded in a transparent insulating layer 16 in an
electrically isolated relationship with each other, and a group of
transparent electrode layers D1-Dj embedded in the insulating layer
16 opposed to a plurality of gaps W2 existing between the
respective transparent electrodes C1-Ci. The transparent electrode
layers C1-Ci are in an opposed relationship with the transparent
electrode layers A1-Ai of the electrode 12, while the transparent
electrode layers D1-Dj are in an opposing relationship with the
transparent electrode layers B1-Bj of the electrode 12.
[0070] It should be noted that while the transparent electrode
layers D1-Dj may also be arranged opposed to all of the existing
gaps W2, they may be formed opposed to a number of gaps required in
accordance with the characteristic of particular aberration to be
corrected.
[0071] Then, as schematically illustrated in FIG. 6, when
appropriate, different voltages V1-Vk are applied respectively
across the respective transparent electrode layers (A1, C1)-(Ai,
Ci) of the aberration correcting regions AR1-ARi in a mutually
opposed relationship as well as the respective transparent
electrode layers (B1,D1)-(Bj, Di) of the gaps BK1-BKj by the
control voltage Vi from the control circuit 8, the applied voltages
V1-Vk cause a plurality of alignment states to occur within the
liquid crystal element 14. The voltages V1-Vk are determined to be
such voltages that provide the alignment states of the liquid
crystal element 14 in the respective aberration correcting regions
AR1-ARi and the gaps BK1-BKj with the characteristics opposite to
the characteristic of aberration caused by the optical disc 9.
[0072] In this way, even with the gaps BK1-BKj interposed between
the respective aberration correcting regions AR1-ARi, the
transparent electrode layers B1-Bj, D1-Dj are provided for the gaps
BK1-Bkj, so that the alignment states can be adjusted for
correcting aberration over the entire region of the liquid crystal
element 14, and the alignment states can be finely adjusted.
[0073] Also, as the voltages V1-Vk across the respective
transparent electrode layers (B1, D1)-(Bj, Di) are adjusted as
appropriate, their alignment states can be changed continuous with
the alignment states produced by the voltages applied across the
transparent electrode layers (A1, C1)-(Ai, Ci), without causing any
abrupt change.
[0074] Likewise, with an aberration correcting optical unit 4
having the aberration correcting regions BR1-BR9 for correcting
coma aberration as illustrated in FIG. 7, transparent electrode
layers (reference numerals of which are omitted) corresponding to
the aberration correcting regions BR1-BR9, and transparent
electrode layers corresponding to gaps between the aberration
correcting regions BR1-BR9 are formed on the electrodes 12, 13,
respectively, in a two-stage structure, as illustrated in a
cross-sectional view of FIG. 8 (which is a diagram illustrating the
cross-sectional structure taken along a line X-X in FIG. 7). It
should be noted that FIG. 8 shows that the transparent electrode
layers corresponding to the gaps between the respective aberration
correcting regions BR1-BR9 are not formed opposed to all the gaps
but are provided only for those gaps in which the influence of coma
aberration should be particularly reduced, as indicated by
reference numerals F1-F4, G1-G4.
[0075] The operation of the optical pickup apparatus PU having the
aberration correcting optical unit 4 constructed as described will
be explained with reference to FIGS. 2 and 7-11. The operation of
the apparatus will be explained as a representative example in
which the optical pickup apparatus PU is provided with the
aberration correcting optical unit 4 for correcting coma aberration
illustrated in FIGS. 7 and 8.
[0076] When an optical disc 9 is loaded at a so-called clamp
position in an information recording/reproducing apparatus, and the
user instructs the information recording/reproducing apparatus to
start reproduction of information, a system controller (not shown)
provided in the information recording/reproducing apparatus
commands the control circuit 8 to output a control voltage Vi for
correcting coma aberration. Consequently, appropriate voltages are
applied respectively across the transparent electrode layers in the
opposed relationship corresponding to the respective aberration
correcting regions BR1-BR9 of the aberration correcting optical
unit 4 illustrated in FIGS. 7 and 8, and across the transparent
electrode layers F1-F4, G1-G4 in the opposed relationship
corresponding to the gaps, causing a plurality of alignment states
to change in accordance with associated electric fields produced by
the applied voltages in the liquid crystal element 14.
[0077] The system controller drives a spindle motor (not shown)
provided in the information recording/reproducing apparatus for
rotation. The system controller also drives a carriage (not shown)
for moving the optical pickup apparatus PU in a radial direction of
the optical disc 9. The optical disc 9 is rotated at a
predetermined line velocity.
[0078] Further, the light source 1 emits linearly polarized laser
light H1 having a constant power as the system controller supplies
a driving signal to the light source 1. The laser beam H1 is
transformed into collimated beam by a collimator lens 2. The
collimated beam, then, transmits the polarizing beam splitter 3,
and is incident on the aberration correcting optical unit 4.
[0079] The laser beam incident on the aberration correcting optical
unit 4 is subjected to birefringence in accordance with the
alignment state of the liquid crystal element 14 when it transmits
the aberration correcting optical unit 4. The laser beam subjected
to the birefringence is converged by the objective lens 5, and a
resulting light beam having a smaller irradiation diameter is
irradiated on the optical disc 9.
[0080] Further, reflected light produced by the light beam
reflected off a pupil plane of the optical disc 9 is incident on
the objective lens 5. The reflected light transmitting the
objective lens 5 is again subjected to the birefringence by the
aberration correcting optical unit 4 and transmits the same, and
then is reflected by the polarizing beam splitter 3 toward the
converging lens 6. Then, the reflected light is converged by the
converging lens 6 to be received by the photodetector 7. The
photodetector 7 transduces the received reflected light to output
an optoelectrically-transduced signal having information recorded
on the optical disc 9. The transduced signal is supplied to a
reproduced signal processing circuit (not shown) provided in the
information recording/reproducing apparatus. Finally, the
reproduced signal processing circuit performs decoding and so on
based on the transduced signal to produce a reproduced signal such
as an audio signal and/or a video signal.
[0081] Coma aberration occurs on the pupil plane of the optical
disc 9 when the optical disc 9 is inclined and an incident angle of
the light beam is inclined (tilt angle) with respect to the normal
direction of the optical disc 9.
[0082] FIG. 9 is a graph which shows the influence of the coma
aberration occurring on the pupil plane of the optical disc 9. The
curve represents a normalized wave front aberration amount, where
the horizontal axis represents an effective optical path range of
the objective lens 5 (i.e., lens diameter).
[0083] When the coma aberration as shown in FIG. 9 occurs, the
aberration correcting optical unit 4 causes birefringence to the
laser beam incident thereon from the polarizing beam splitter 3 so
as to reduce the influence of the coma aberration as described
above, so that the optical disc 9 is irradiated beforehand with a
light beam which can reduce the influence of the coma aberration
through the objective lens 5. Further, when reflected light
returning from the optical disc 9, influenced by the coma
aberration, is again incident on the aberration correcting optical
unit 4 through the objective lens 5, the light beam is given the
birefringence to reduce the influence of the coma aberration on the
reflected light, and passes through the aberration correcting
optical unit 4 to the polarizing beam splitter 3. Consequently, the
reflected light with suppressed influence of the coma aberration is
incident on the photodetector 7 through the converging lens 6,
thereby enabling highly accurate information reproduction.
[0084] Further, the gaps between the respective aberration
correcting regions BR1-BR9 of the aberration correcting optical
unit 4 are provided with the transparent electrode layers F1-F4,
G1-G4 in an opposing relationship as illustrated in FIGS. 7 and 8,
so that the coma aberration can be reduced over the entire
aberration correcting optical unit 4, and the coma aberration can
be accurately reduced.
[0085] FIG. 10 is a graph showing the influence of coma aberration
reduced by the aberration correcting optical unit 4, which is
represented as a normalized wave front aberration amount, wherein
the horizontal axis represents an effective optical path range of
the objective lens 5 (i.e., lens diameter). As is apparent from
FIG. 10, a significant improvement is recognized as compared with
the coma aberration before the correction shown in FIG. 9.
[0086] FIG. 11 in turn shows the influence of coma aberration when
only the aberration correcting regions BR1-BR9 are used to correct
the aberration without providing the transparent electrode layers
in the gaps between the respective aberration correcting regions
BR1-BR9 of the aberration correcting optical unit 4, i.e., when the
aberration is corrected without providing the transparent electrode
layers F1-F4, G1-G4 illustrated in FIG. 8, for the purpose of
comparison with the characteristic graph of FIG. 10.
[0087] It can be seen that the graph of FIG. 11 presents large
peaks P1-P4 in the aberration in portions where the transparent
electrode layers F1-F4, G1-G4 are not provided, whereas the peaks
are largely reduced as shown in FIG. 10.
[0088] As is also apparent from the result of the experiment
described above, it is confirmed that according to the aberration
correcting optical unit 4 of the embodiment, the influence of the
coma aberration can be largely reduced by the transparent electrode
layers F1-F4, G1-G4 provided between the gaps between the
respective aberration correcting regions BR1-BR9.
[0089] The operation of the optical pickup apparatus PU for
recording information will be described.
[0090] As the user instructs the information recording/reproducing
apparatus to start recording of information, the system controller
provided in the information recording/reproducing apparatus
commands a recording signal processing circuit (not shown) to
perform modulation, encoding and so on based on an input signal
such as an audio signal and a video signal supplied from the
outside, and provides the light source 1 with a recording signal
produced by such processing, causing the light source 1 to emit
laser light H1 modulated by the recording signal.
[0091] The laser beam H1 is transformed into collimated light beam
by the collimator lens 2, transmits the polarizing beam splitter 3,
and is incident on the aberration correcting optical unit 4. The
laser beam incident on the aberration correcting optical unit 4 is
subjected to birefringence in accordance with the alignment state
of the liquid crystal element 14 when it transmits the aberration
correcting optical unit 4. The laser beam subjected to the
birefringence is converged by the objective lens 5, and a resulting
light beam having a smaller irradiation diameter is irradiated to
the optical disc 9 for recording information with optical energy of
the light beam.
[0092] Further, reflected light produced by the light beam
reflected from a pupil plane of the optical disc 9 is incident on
the objective lens 5. The reflected light transmitting the
objective lens 5 is again subjected to the birefringence by the
aberration correcting optical unit 4 and transmits the same, and
then is reflected by the polarizing beam splitter 3 toward the
converging lens 6. Then, the reflected light is converged by the
converging lens 6, such that the resulting converged light beam is
received by the photodetector 7. The photodetector 7
optoelectrically transduces the received reflected light to output
the transduced signal, which is supplied to a servo circuit (not
shown) provided in the information recording/reproducing
apparatus.
[0093] The servo circuit detects a focus error, for example, by an
astigmatism method, and drives the objective lens 5 in a focus
servo scheme based on the result of the detection. Since the focus
servo is performed based on the optoelectrically transduced signal
with significantly reduced influence of coma aberration, highly
accurate focus servo can be accomplished.
[0094] When the optical pickup apparatus is provided with the
aberration correcting optical unit 4 for correcting spherical
aberration illustrated in FIGS. 4 and 5, the pickup apparatus can
have similar effects to those produced when it is provided with the
aberration correcting optical element 4 for correcting coma
aberration illustrated in FIGS. 7 and 8, so that the influence of
various types of aberration can be significantly reduced.
[0095] As described above, according to the optical pickup
apparatus PU and the information recording/reproducing apparatus of
the embodiment, it is possible to significantly reduce the
influence of aberration by the optical disc 9 and to finely control
the reduction of the aberration, since the optical pickup apparatus
PU is provided with the aberration correcting optical unit 4, and
the transparent electrode layers are disposed corresponding to the
gaps defined between the respective aberration correcting regions
of the aberration correcting optical unit 4, as illustrated in
FIGS. 5 and 8.
[0096] While the foregoing embodiment has been described for the
aberration correcting optical unit 4 which includes the transparent
electrode layers corresponding to the gaps defined between the
respective aberration correcting regions, the aberration correcting
optical unit of the present invention is not limited to such a
structure.
[0097] Alternatively, as illustrated in a cross-sectional view of
FIG. 12, transparent electrode layers may be provided corresponding
to gaps defined between respective aberration correcting regions on
one electrode 12, while a transparent electrode layer 17 is
provided over the entirety of the effective optical path range on
the other electrode 13, such that the transparent electrode layer
17 is used as a common electrode to apply appropriate voltages
respectively between the respective transparent electrode layers in
the electrode 13 and the transparent electrode layer 17.
[0098] Also alternatively, as illustrated in a cross-sectional view
of FIG. 13, a plurality of transparent electrode layers overlapping
one another may be formed along the optical axis OA in a
multi-stage structure in each of the electrodes 12, 13, such that
appropriate voltages may be applied respectively between the
transparent electrode layers in a mutually opposing relationship
having shapes corresponding to associated aberration correcting
regions. In other words, portions of the respective transparent
electrode layers opened to the liquid crystal element 14 are made
to conform to the shapes of the aberration correcting regions,
thereby allowing for a correction of aberration. Since such a
structure is free from gaps otherwise existing between the
aberration correcting regions, the transparent electrode layers
such as those illustrated in FIGS. 5 and 8 are not required
corresponding to the gaps. Further effects can be provided in
realizing simplification of the control circuit 8 resulting from a
reduction in the number of wirings for applying voltages to the
transparent electrode layers, and a reduction in the number of
types of voltages to be applied, and so on.
[0099] Also, in the structure of FIG. 13, a single transparent
electrode layer, similar to the transparent electrode layer 17
illustrated in FIG. 12, may be formed on one electrode 12 (or 13),
such that the single transparent electrode layer is used as a
common electrode.
[0100] Further alternatively, as illustrated in a cross-sectional
view of FIG. 14, transparent electrode layers of shapes
corresponding to aberration correcting regions may be formed in a
multi-stage structure along the optical axis OA, such that
appropriate voltages are respectively applied across transparent
electrode layers in a mutually opposing relationship.
[0101] According to the foregoing structure, it is possible to
finely adjust the alignment states which should be caused in the
liquid crystal element 14, since the shapes of the transparent
electrode layers can be formed to conform to the shapes of the
aberration correcting regions. Thus, realization of more accurate
aberration correcting optical unit can be attained. Also, the
transparent electrode layers such as those illustrated in FIGS. 5
and 8 are not required corresponding to the gaps, since no gaps
exist between transparent electrode layers. Further effects can be
provided in realizing simplification of the control circuit 8
resulting from a reduction in the number of wirings for applying
voltages to the transparent electrode layers, and a reduction in
the number of types of voltages to be applied, and so on.
[0102] As described above, according to the foregoing embodiment,
at least one of mutually opposing electrodes for applying a voltage
to the liquid crystal element of the aberration correcting optical
unit is formed by a plurality of electrodes in a multi-layer
structure, so that the plurality of electrodes can be oriented
toward the liquid crystal element without gaps. Thus, an
electro-optical effect is produced in the liquid crystal element
without any gap when voltages are applied to the plurality of
electrodes, so that the aberration can be corrected without any
omission. Also, the aberration can be finely corrected.
[0103] Further, it is possible to promote a higher NA of an
objective lens and a shorter wavelength for light emitted from a
light source, which is associated with the trend of increasing the
density of the information recording media, because of the ability
of appropriately correcting aberration by an information recording
medium, thereby providing an aberration correcting optical unit
which is effective for the trend of increasing the density.
[0104] Also, highly accurate information recording and information
reproduction can be accomplished since the optical pickup apparatus
as well as the information reproducing apparatus and the
information recording apparatus of the present invention comprise
the aforementioned aberration correcting optical element to make an
aberration correction.
[0105] The following description will be made on a liquid crystal
element and an aberration correcting optical unit which has
insulating layers between electrode layers according to another
embodiment of the present invention.
[0106] In the embodiment, an optical unit 4 has a structure in
which a liquid crystal element 114 is sandwiched between two
transparent insulating substrates 110, 111 such as glass
substrates, as schematically illustrated in FIG. 15. There are
formed electrodes 112, 113, insulating layers 123, 124, and liquid
crystal alignment layers 121, 122, respectively, on the mutually
opposed surfaces of the insulating substrates 110, 111.
[0107] Alignment of liquid crystal molecules in the liquid crystal
element 114 changes in response to an electric field Ei produced by
a control voltage Vi which is applied between the electrodes 112,
113. As a result, light passing through the liquid crystal element
114 is subject to birefringence of the liquid crystal element 114
to change the phase. The polarization state (phase) can be
controlled by the control voltage Vi applied to the liquid crystal
element 114.
[0108] The aberration correcting optical unit 4 also has
bidirectional light transmissivity, so that any side of the
insulating substrates 10, 11 may be oriented to the objective lens
5.
[0109] The aberration correcting optical unit 4 is partitioned into
a plurality of aberration correcting regions AR1-ARi which have
been determined in correspondence to the distribution of aberration
caused by the optical disc 9, as illustrated in a top plan view of
FIG. 16. The aberration correcting regions AR1-ARi are implemented
by transparent electrode (ITO: indium tin oxide) layers formed in
the electrodes 112, 113. It should be noted that while FIG. 16
illustrates a typical example of the aberration correcting regions
AR1-ARi for correcting spherical aberration caused by the optical
disc 9, the aberration correcting optical unit 4 is actually
partitioned into a variety of shapes in correspondence to the
distribution of aberration caused by the optical disc 9. For
example, the aberration correcting optical unit 4 is provided with
aberration correcting regions BR1-BRi of shapes as illustrated in
FIG. 17 for correcting coma aberration caused by the optical disc 9
which is inclined during recording or reproduction of information.
Also, the number of sections of these aberration correcting regions
is determined in correspondence to the distribution of aberration
by the optical disc 9.
[0110] In the following, description will be made on an example of
the aberration correcting optical unit 4 which is provided with
concentric aberration correcting regions AR1-ARi as illustrated in
FIG. 16. FIG. 18 is a cross-sectional view illustrating the
cross-sectional structure taken along a line X-X in FIG. 16. As
illustrated, the electrode 112 has a structure comprised of
transparent electrodes A1-Ai electrically isolated from one another
by a plurality of gaps P1-Pi existing between the respective
transparent electrodes A1-Ai.
[0111] The transparent electrode A1 is formed in a shape that
conforms to the aberration correcting region AR1 (circular in FIG.
16); the transparent electrode A2 is formed in a shape that
conforms to the aberration correcting region AR2 (annular in FIG.
16); and the remaining transparent electrodes A3-Ai are likewise
formed in shapes which conform to the corresponding aberration
correcting regions AR3-ARi. Also, the gaps P1-Pi isolating the
transparent electrodes A1-Ai are formed in an annular shape.
[0112] The electrode 113 has a similar structure comprised of a
plurality of transparent electrodes C1-Ci electrically isolated
from one another by a plurality of gaps Q1-Qi existing between the
respective transparent electrodes C1-Ci.
[0113] It should be noted that the electrode 113 need not be
isolated if the electrode 112 is formed as isolated electrodes. For
example, the electrode 113 may be formed as a single electrode
extending over the entire plane of the layer, or may be formed in a
shape required in accordance with the characteristic of particular
aberration to be corrected, or formed separately in a required
number.
[0114] It is disclosed that spherical aberration and coma
aberration can be corrected by a single liquid crystal unit in
Laid-open Japanese Patent Application Kokai No. H10-289465.
Specifically, in the present invention, an upper electrode may be
formed in a shape for correcting spherical aberration, while a
lower electrode may be formed in a shape for correcting coma
aberration, to correct the spherical aberration and the coma
aberration with a single liquid crystal unit. In this event, an
insulating layer between the upper electrode and a liquid crystal
element and an insulating layer between the lower electrode and the
liquid crystal element may have suitable thicknesses for allowing
the corrections of the associated aberration, respectively.
[0115] Referring to FIG. 19, the following description will be made
on the correction of aberration when the respective transparent
electrodes are applied with voltages. For convenience of
description, FIG. 19 illustrates a cross-section of the liquid
crystal element in the radial direction from the center of the
element. As schematically illustrated in FIG. 19, when different
predetermined voltages V1-Vk are applied by the control circuit 8
across the respective transparent electrodes (A1, C1)-(Ak, Ck) of
aberration correcting regions AR1-ARk which are in a mutually
opposed relationship, a plurality of alignment states occur in the
liquid crystal element 114 in accordance with electric fields
(E1-Ek) produced by the applied voltages V1-Vk. The applied
voltages V1-Vk are determined such that the alignment states of the
liquid crystal element 114 in the respective aberration correcting
regions AR1-ARk present the characteristics opposite to the
characteristic of aberration caused by the optical disc 9, i.e.,
such that the aberration is corrected.
[0116] However, the electric fields (Eg1-Egk) produced in liquid
crystal element portions corresponding to the gaps P1-Pk, Q1-Qk
between the respective transparent electrodes vary depending on the
thickness of the insulating layers 123, 124. FIG. 20 shows the
intensity of the electric field on the interfaces of the insulating
layers and the liquid crystal element portions, when the respective
electrodes (here A1-A6) are applied with voltages, with the
thickness of the insulating layers (THi) used as a parameter. In
the graphs, it is assumed that the liquid crystal element has a
thickness of 5 micrometers (.mu.m), and the gaps P1-Pk, Q1-Qk have
a width of 5 .mu.m. The intensity of the electric field is shown
for three thicknesses (THi) of the insulating films, i.e., THi=1,
6, 15 .mu.m. Also, the respective electrodes are applied with
voltages in increment of 2 volts, i.e., A1=10 (V), A2=12 (V), . . .
, A6=20 (V).
[0117] As shown in FIG. 20, the electric field abruptly drops at
the liquid crystal element portions corresponding to the gaps with
a smaller thickness of the insulating films (THi=1 .mu.m).
Therefore, the alignment of the liquid crystal cannot be
sufficiently changed in these portions. In other words, it is
difficult to correct the aberration since no phase difference can
be produced in light passing therethrough. On the other hand, the
reduction in the electric field is smaller when the thickness of
the insulating films THi=6 .mu.m. The electric field exhibits a
substantially flat profile when the thickness of the insulating
films THi=15 .mu.m, so that the aberration can be sufficiently
corrected by the liquid crystal element even in the gaps.
[0118] FIG. 21 shows an example of wave front aberration (spherical
aberration) produced by the optical disc 9. The amount of phase
difference (in nanometer: nm) in a light incident plane of the
liquid crystal element 114 is plotted in the radial direction with
reference to the value at the center of the liquid crystal element
114. More specifically, a light beam incident on the liquid crystal
element 114 has the phase difference of the profile shown in FIG.
21 with respect to the radial direction of the liquid crystal
element 114. The aberration can be corrected by applying
predetermined voltages to the liquid crystal element 114 for
canceling out the phase difference to cause an in-plane phase
change in the incident light beam.
[0119] FIG. 22 shows a phase difference (nm) produced in a light
beam passing through the liquid crystal element 114 by a
predetermined voltage applied to each electrode (A1-Ak, C1-Ck),
when the insulating layer 123 sandwiched by the liquid crystal
element 114 and the electrode layer 112 is smaller in thickness
than the liquid crystal element 114 (for example, the thickness of
the insulating layer 123 is equal to or less than approximately 1
.mu.m, while the thickness of the liquid crystal element 114 is 5
.mu.m). The phase difference is plotted with reference to the
values in the gaps. In the aberration correcting region (AR1-ARk)
of the liquid crystal element 114 corresponding to each electrode,
a desired phase difference is obtained by the application of the
voltage. On the other hand, since the electric field intensity is
small in regions of the liquid crystal element 114 corresponding to
the gaps P1-Pk, Q1-Qk, a phase change hardly occurs. Thus, abrupt
discontinuity occurs in the phase difference, thereby significantly
impeding the correction for the aberration.
[0120] FIG. 23 shows a phase difference produced in a light beam
passing through the liquid crystal element 114 by the application
of predetermined voltages when the insulating layer 123 of the
liquid crystal element 114 has a thickness equal to or larger than
the thickness of the liquid crystal element 114. As shown in the
graph, the phase difference in a region of the liquid crystal
element 114 corresponding to each of the gaps P1-Pk, Q1-Qk shows
the value between the phase differences produced by two adjacent
electrodes, so that the aberration is sufficiently corrected. It
should be noted that the amount of phase difference is optimally
coincident with the phase difference curve shown in FIG. 21 in the
most favorable conditions.
[0121] FIG. 24 shows a phase difference produced in a light beam
passing through the liquid crystal element 114 by the application
of predetermined voltages, when the insulating layer 123 is smaller
in thickness than the liquid crystal element 114 (for example, the
thickness of the insulating layer 123 is approximately 3 .mu.m,
while the thickness of the liquid crystal element 114 is 5 .mu.m).
As shown in the graph, the phase differences in the gap regions are
larger than the case shown in FIG. 22. More specifically, the
aberration correcting unit of the present invention is improved to
ensure sufficient phase differences even in the gap regions and
enables the correction for aberration in these regions, which has
not been possible in the prior art. It should be noted that the
value of the phase difference required in each gap region depends
on a variety of conditions and parameters such as a particular
optical system, the type of disc, a control circuit, and so on for
use therewith. Therefore, the thickness of the insulating layers
may be determined to ensure desired phase differences in accordance
with the conditions. In other words, the thickness of the
insulating layers may be determined such that an electric field
applied to a portion of the liquid crystal element corresponding to
each gap region is equal to or larger than a predetermined
intensity.
[0122] It should be noted that at least one of the insulating
layers sandwiched between the liquid crystal element 114 and the
electrode layers may be formed sufficiently thick, for example,
thicker than the liquid crystal element, and both insulating layers
sandwiching the liquid crystal element need not be formed
thick.
[0123] As described above, sufficient phase changes can be provided
even in the gap regions, thereby making it possible to accurately
correct the influence of aberration occurring in an optical
path.
[0124] While the foregoing embodiments have been described from a
viewpoint of the correction for aberration of reflected light
caused by the optical disc, the aberration correcting optical unit
according to the present invention may be disposed at any position
as long as it is on the optical path from the light source to the
photodetector. Also, the aberration correcting optical unit
according to the present invention is not limited for spherical
aberration and coma aberration, but may be applied to a correction
for a variety of aberrations such as astigmatism. Further, the
present invention can be applied to an optical pickup apparatus
which has a plurality of light sources or a plurality of optical
paths. For example, the present invention can be applied to an
optical pickup apparatus which includes light sources such as a
two-wavelength laser or the like having different wavelengths for
recording and reproducing CD and DVD, respectively.
[0125] According to the present invention, as described in detail,
an electrode layer for applying voltages to the optical element
(liquid crystal) is configured such that an appropriate electric
field should be applied to a portion of the optical element
corresponding to a portion between the plurality of electrodes
(i.e., a gap) in order to correct aberration. In particular, the
electrode layer is configured to have a multi-layer structure or an
insulating layer of appropriate thickness is provided between the
optical element and the electrode layer.
[0126] As is apparent from the foregoing, the present invention can
provide high performance aberration correcting optical unit and an
optical pickup apparatus capable of accurately correcting
aberration occurring in an optical path.
[0127] The invention has been described with reference to the
preferred embodiments thereof. It should be understood by those
skilled in the art that a variety of alterations and modifications
may be made from the embodiments described above. It is therefore
contemplated that the appended claims encompass all such
alternations and modifications.
* * * * *