U.S. patent application number 12/183250 was filed with the patent office on 2009-02-05 for optical disk apparatus.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Jun-ichi Asada, Yusuke Kanda, Kazuo Momoo, Seiji Nishiwaki, Kenji Otani.
Application Number | 20090034401 12/183250 |
Document ID | / |
Family ID | 35136276 |
Filed Date | 2009-02-05 |
United States Patent
Application |
20090034401 |
Kind Code |
A1 |
Nishiwaki; Seiji ; et
al. |
February 5, 2009 |
OPTICAL DISK APPARATUS
Abstract
It is intended to provide an optical disk apparatus which
detects a light amount greater than zero even when used in
conjunction with an optical disk substrate having a large
birefringence, so that it is possible to properly read a signal
without errors and properly perform optical disk controls. The
optical disk apparatus includes: a light source for emitting light;
an objective lens for converging the light onto a signal surface of
an optical disk; a polarized beam diffraction element for
diffracting the light reflected from the optical disk; a
photodetector for detecting the light diffracted from the polarized
beam diffraction element; and a wavelength plate disposed between
the optical disk and the polarized beam diffraction element. The
wavelength plate has a two-dimensional array of a plurality of
birefringent regions including first and second regions, the first
and second regions differing in birefringent phase difference
and/or optic axes from each other, and the plurality of
birefringent regions including the first and second regions cause
the light to have different polarization states.
Inventors: |
Nishiwaki; Seiji; (Kobe-shi,
JP) ; Momoo; Kazuo; (Hirakata-shi, JP) ;
Asada; Jun-ichi; (Kobe-shi, JP) ; Otani; Kenji;
(Ikoma-shi, JP) ; Kanda; Yusuke; (Settsu-shi,
JP) |
Correspondence
Address: |
MARK D. SARALINO (PAN);RENNER, OTTO, BOISSELLE & SKLAR, LLP
1621 EUCLID AVENUE, 19TH FLOOR
CLEVELAND
OH
44115
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
35136276 |
Appl. No.: |
12/183250 |
Filed: |
July 31, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11112998 |
Apr 22, 2005 |
7463569 |
|
|
12183250 |
|
|
|
|
Current U.S.
Class: |
369/112.23 ;
427/162; 427/595; G9B/7.112 |
Current CPC
Class: |
G11B 7/1353 20130101;
G11B 7/1381 20130101; G11B 7/1365 20130101 |
Class at
Publication: |
369/112.23 ;
427/162; 427/595; G9B/7.112 |
International
Class: |
G11B 7/135 20060101
G11B007/135; B05D 5/06 20060101 B05D005/06; C08F 2/48 20060101
C08F002/48 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2004 |
JP |
2004-127855 |
Apr 28, 2004 |
JP |
2004-133108 |
Claims
1-7. (canceled)
8. An optical disk apparatus comprising: a light source for
emitting light of a wavelength .lamda.1 and light of a wavelength
.lamda.2 (where .lamda.1.noteq..lamda.2); an objective lens for
converging the light onto a signal surface of an optical disk; a
polarized beam diffraction element for diffracting the light
reflected from the optical disk; a photodetector for detecting the
light diffracted from the polarized beam diffraction element; and a
wavelength plate disposed between the optical disk and the
polarized beam diffraction element, wherein, an imaginary line L on
the beam splitter is defined, the line L being perpendicular to a
radial direction of the optical disk, and intersecting an optical
axis of the objective lens; the beam splitter at least has a
regional, a region a2, a region a3, a region A1, a region A2, and a
region A3, such that the regional, the region a2, and the region a3
are on a same side of the line L on the beam splitter, and the
region A1, the region A2, and the region A3 are substantially
symmetrical regions to the regional, the region a2, and the region
a3, respectively, with respect to the line L; the photodetector at
least has two regions b and B; light of the wavelength .lamda.1
entering the region a3, the regional, and the region A2 of the beam
splitter produces 1.sup.st order diffracted light which is
projected onto the region b of the photodetector, and light of the
wavelength .lamda.1 entering the region A3, the region A1, and the
region a2 of the beam splitter produces 1.sup.st order diffracted
light which is projected onto the region B of the photodetector;
light of the wavelength .lamda.2 entering the region a3 of the beam
splitter produces 1.sup.st order diffracted light which is
projected onto the region B of the photodetector, and light of the
wavelength .lamda.2 entering the region A3 of the beam splitter
produces 1.sup.st order diffracted light which is projected onto
the region b of the photodetector; and based on a difference
between a detection signal from the region b and a detection signal
from the region B, the optical disk apparatus generates a tracking
error signal for the optical disk or a correction signal for
correcting the tracking error signal.
9. The optical disk apparatus according to claim 8, wherein, the
photodetector further has at least two regions b' and B'; light
from a first light source or a second light source entering the
region a3, the regional, and the region a2 of the beam splitter
produces -1.sup.st order diffracted light which is projected onto
the region b' of the photodetector, and light from the first light
source or the second light source entering the region A3, the
region A1, and the region A2 of the beam splitter produces
-1.sup.st order diffracted light which is projected onto the region
B' of the photodetector; and the optical disk apparatus generates a
difference signal based on a difference between a detection signal
from the region b' and a detection signal from the region B', and
generates a tracking error signal for the optical disk by adding to
the difference signal a value obtained by multiplying the
correction signal by an arbitrary coefficient.
10. An optical disk apparatus comprising: a light source for
emitting light of a wavelength .lamda.1 and light of a wavelength
.lamda.2 (where .lamda.1.noteq..DELTA.2); an objective lens for
converging the light onto a signal surface of an optical disk; a
polarized beam diffraction element for diffracting the light
reflected from the optical disk; a photodetector for detecting the
light diffracted from the polarized beam diffraction element; and a
wavelength plate disposed between the optical disk and the
polarized beam diffraction element, wherein, an imaginary line L on
the beam splitter is defined, the line L being perpendicular to a
radial direction of the optical disk, and intersecting an optical
axis of the objective lens; the beam splitter at least has eight
regions a1, a2, a3, A1, A2, A3, and A4 such that the regional a1,
the region a2, the region a3, and the region a4 are on a same side
of the line L on the beam splitter, and the region A1, the region
A2, the region A3, and the region A4 are substantially symmetrical
regions to the region a1, the region a2, the region a3, and the
region a4, respectively, with respect to the line L; the
photodetector at least has six regions b, B, b', B', b', and B'';
light of the wavelength .lamda.1 entering the region A2 and the
regional of the beam splitter produces -1.sup.st order diffracted
light which is projected onto the region b of the photodetector,
and light of the wavelength .lamda.1 entering the region a2 and the
region A1 of the beam splitter produces -1.sup.st order diffracted
light which is projected onto the region B of the photodetector,
the optical disk apparatus generating a tracking error signal for
the optical disk based on a difference between a detection signal
from the region b and a detection signal from the region B; light
of the wavelength .lamda.2 entering the region a3 and the region a4
of the beam splitter produces -1.sup.st order diffracted light
which is projected onto the region b' of the photodetector, and
light of the wavelength .lamda.2 entering the region A3 and the
region A4 of the beam splitter produces -1.sup.st order diffracted
light which is projected onto the region B' of the photodetector,
the optical disk apparatus generating a difference signal based on
a difference between a detection signal from the region b' and a
detection signal from the region B'; and light of the wavelength
.lamda.2 entering the region a3 further produces 1.sup.st order
diffracted light which is projected onto the region b'' of the
photodetector, and light of the wavelength .lamda.2 entering the
region A3 further produces 1.sup.st order diffracted light which is
projected onto the region B'' of the photodetector, the optical
disk apparatus generating a correction signal based on a difference
between a detection signal from the region b'' and a detection
signal from the region B''; the optical disk apparatus generates a
tracking error signal for the optical disk by adding to the
difference signal a value obtained by multiplying the correction
signal by an arbitrary coefficient.
11. An optical element comprising a two-dimensional array of a
plurality of birefringent regions including first and second
regions, the first and second regions differing in birefringent
phase difference and/or optic axes from each other, wherein the
plurality of birefringent regions including the first and second
regions cause the light to have different polarization states.
12. The optical element according to claim 11, wherein optic axes
of the first and second regions are parallel to each other, and the
first and second regions have different retardations from each
other.
13. The optical element according to claim 11, wherein optic axes
of the first and second regions are oriented in different
directions from each other.
14. The optical element according to claim 11, wherein a plurality
of said first regions and a plurality of said second regions
alternate within a plane perpendicular to an optical axis.
15. The optical element according to claim 14, wherein each of the
first and second regions has a shape selected from the group
consisting of: a strip shape, a checker shape, and an annular
shape.
16. The optical element according to claim 11, further comprising a
polarization filter.
17. The optical element according to claim 16, wherein the
polarization filter is a polarization hologram.
18. The optical element according to claim 13, wherein, the optic
axis of the first region is at
45.degree.+.delta..+-..alpha.(-10.degree.<.delta.<10.degree.,
0.degree.<.alpha..ltoreq.15.degree.) with respect to a
polarization direction of incident light; and the optic axis of the
second region is at 45.degree.+.delta.-.alpha. with respect to a
polarization direction of incident light.
19. The optical element according to claim 18, wherein, with
respect to light of at least one wavelength among light of a
plurality of wavelengths traveling back and forth through the
optical element, an average retardation .DELTA. of the plurality of
birefringent regions is set equal to (2m+1).pi./2 (where m is an
integer).
20. The optical element according to claim 19 which is a broadband
wavelength plate having a same retardation .DELTA. for light of
different wavelengths.
21. The optical element according to claim 13, wherein optic axes
of some of the plurality of birefringent regions are at 45.degree.
with respect to a polarization direction of incident light.
22. An optical pickup comprising: a light source for emitting two
or more kinds of laser light of different wavelengths; a lens for
converging the light emitted from the light source onto an optical
information medium; and a photodetector for receiving light
reflected from the optical information medium, wherein the optical
pickup further comprises the optical element according to claim 11,
the optical element being disposed in a region common to an optical
path from the light source to the optical information medium and an
optical path from the optical information medium to the
photodetector.
23. The optical pickup according to claim 21, wherein the light
source and the photodetector are integrally formed.
24. A method for producing an optical element having a
two-dimensional array of a plurality of birefringent regions
including first and second regions, the first and second regions
differing in birefringent phase difference and/or optic axes from
each other, the plurality of birefringent regions including the
first and second regions causing the light to have different
polarization states, the method comprising the steps of: (a)
forming on a substrate an alignment film including a plurality of
regions having different alignment directions from one another; and
(b) forming a liquid crystal layer on the alignment film and
controlling the alignment direction of each region of the liquid
crystal layer.
25. The method according to claim 24, wherein the step (a)
comprises the substeps of: (a1) depositing a photo-alignable film
on the substrate; (a2) subjecting a portion of the photo-alignable
film to an exposure with ultraviolet light to form a first aligning
region having a first alignment direction; and (a3) subjecting
another portion of the alignment film to an exposure with
ultraviolet light to form a second aligning region having a second
alignment direction, the second alignment direction being different
from the first alignment direction.
26. The method according to claim 24, wherein, the step (b)
comprises the substeps of: (b1) forming on the alignment film a
liquid crystal layer containing a UV-curing material, and
controlling the alignment directions within the liquid crystal
layer in accordance with first and second alignment directions; and
(b2) curing the liquid crystal layer by irradiating the liquid
crystal layer with ultraviolet light.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/112,998 filed on Apr. 22, 2005, which is based on
Japanese Patent Application Nos. 2004-127855 filed Apr. 23, 2004
and 2005-133108 filed Apr. 28, 2004, which is hereby incorporated
herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical disk apparatus
which is capable of writing data to an optical disk and/or reading
data from an optical disk. The present invention also relates to an
optical element which is suitable for use in such an optical disk
apparatus, and a method for producing the same.
[0004] 2. Description of the Related Art
[0005] An optical disk apparatus comprises a motor for rotating an
optical disk, an optical pickup which irradiates the optical disk
with a light beam, a signal processing section for processing
recording or reproduced data, and like elements. Among others, the
optical pickup, which is a most vital component to enhanced storage
density, comprises a light source for generating a light beam,
lenses for converging the light beam onto the recording surface of
the optical disk, and a photodetector for detecting light which has
been reflected from the optical disk (reproduction light or signal
light) and converting the detected light into an electrical
signal.
[0006] A known optical disk apparatus is disclosed in, for example,
Japanese laid-open patent publication No. 2000-132848.
[0007] Hereinafter, referring to FIGS. 19A and 19B, the structure
of the conventional optical pickup disclosed in Japanese Laid-Open
Patent Publication No. 2000-132848 will be described.
[0008] FIG. 19A shows an optical pickup structure in a conventional
optical disk apparatus. FIG. 19B shows the neighborhood of a light
source 1 thereof.
[0009] As shown in FIG. 19A, this optical pickup comprises a
photodetection substrate 9 on which the light source 1 (e.g., a
semiconductor laser) is mounted, as well as an optical system. The
optical system includes a collimating lens 4, a polarization
hologram substrate 2, a 1/4 wavelength plate 3, and an objective
lens 5, which are provided along an optical axis 7. The 1/4
wavelength plate 3, which is formed on the same substrate as a
hologram surface 2a of the polarization hologram substrate 2, moves
integrally with the objective lens 6.
[0010] The surface of the photodetection substrate 9 includes a
region (detection surface 9a) in which a plurality of
photosensitive portions such as photodiodes are formed, and a
region in which the light source 1 is mounted. As shown in FIG.
19B, a reflection mirror 10 is formed on the surface of the
photodetector substrate 9, the reflection mirror 10 reflecting
light emitted from the light source 1 in a direction which is
substantially perpendicular to the surface of the photodetection
substrate 9.
[0011] Laser light which has been emitted from the light source 1
is reflected from the reflection mirror 10 on the photodetection
substrate 9, and thereafter collimated into parallel light by the
collimating lens 4. The parallel light is transmitted through the
polarization hologram substrate 2 in the form of P-polarized light.
The polarization hologram substrate 2 is characterized so that it
does not diffract P-polarized light, but diffracts S-polarized
light. In the case where the incident light is S-polarized light,
the polarization hologram substrate 2 has a diffraction efficiency
of about 0% for the 0.sup.th order light, and about 41% for the
.+-.1.sup.st order light, for example.
[0012] The light transmitted through the polarization hologram
substrate 2 is converted by a 1/4 wavelength plate 3' from linearly
polarized light (P-polarized light) into circularly polarized
light. The circularly polarized light is converged by the objective
lens 5 onto a signal surface 6a of the optical disk substrate 6.
The 1/4 wavelength plate 3', which is constructed on the same
substrate as the hologram surface 2a, moves integrally with the
objective lens 6.
[0013] The light (signal light) which has been reflected from the
signal surface 6a of the optical disk substrate 6 propagates in the
opposite direction of the forward path. This light (signal light)
travels through the objective lens 5 and enters the 1/4 wavelength
plate 3'. The light transmitted through the 1/4 wavelength plate 3'
is converted from circularly polarized light into linearly
polarized light (S-polarized light). The S-polarized light enters
the hologram surface 2a of the polarization hologram substrate 2 so
as to be diffracted. Through this diffraction, 1.sup.st order
diffracted light 8 and -1.sup.st order diffracted light 8' are
formed with respect to the optical axis 7 as an axis of symmetry.
The diffracted light 8 and 8' is each converged on the detection
surface 9a on the detector 9 via the collimating lens 4. The
detection surface 9a is located substantially at the focal plane of
the collimating lens 4 (i.e., an imaginary emission point on the
light source 1).
[0014] Generally-used optical disk systems are designed on the
premise that the optical disk substrate 6 does not have any
birefringence. In reality, however, there are some low-quality
optical disk substrates 6 which do suffer from a large
birefringence, thus inviting the following problems.
[0015] Assuming that the laser light which is emitted from the
light source 1 has a wavelength of .lamda., the birefringence of
the optical disk substrate 6 may cause a birefringent phase
difference (retardation: phase delay) exceeding .lamda./2, over the
course of the back and forth trips of light. When converted into an
angle, .lamda./2 equals 180.degree.. Hereinafter, any birefringent
phase difference will be expressed in terms of angle.
[0016] Assuming that the birefringent phase difference ascribable
to the optical disk substrate 6 is 180.degree. over the course of
the back and forth trips of light, and when taken together with the
birefringent phase difference (180.degree.) of the 1/4 wavelength
plate 3' over the course of the back and forth trips of light,
there is a birefringent phase difference of 360.degree.. As a
result, the signal light entering the polarization hologram
substrate 2 is P-polarized, instead of being S-polarized. Since the
polarization hologram substrate 2 is characterized so as not to
diffract P-polarized light, the light in the return path, which is
P-polarized, is not diffracted. This means that the light amounts
of the diffracted light 8 and 8' shown in FIG. 19 are zero.
Therefore, the photodetector 9 cannot receive the signal light
reflected from the signal surface 6a. Thus, not only is it
impossible to read the signal, but it is also impossible to perform
focusing and tracking controls, etc.
SUMMARY OF THE INVENTION
[0017] In order to overcome the problems described above, preferred
embodiments of the present invention provide an optical disk
apparatus which detects a light amount greater than zero even when
used in conjunction with an optical disk substrate having a large
birefringence, so that it is possible to properly read a signal
without errors and properly perform optical disk controls.
[0018] An optical disk apparatus according to the present invention
comprises: a light source for emitting light; an objective lens for
converging the light onto a signal surface of an optical disk; a
polarized beam diffraction element for diffracting the light
reflected from the optical disk; a photodetector for detecting the
light diffracted from the polarized beam diffraction element; and a
wavelength plate disposed between the optical disk and the
polarized beam diffraction element, wherein, the wavelength plate
has a two-dimensional array of a plurality of birefringent regions
including first and second regions, the first and second regions
differing in birefringent phase difference and/or optic axes from
each other, and the plurality of birefringent regions including the
first and second regions cause the light to have different
polarization states.
[0019] In a preferred embodiment, optic axes of the first and
second regions of the wavelength plate are oriented in different
directions from each other.
[0020] In a preferred embodiment, the first region has a
birefringent phase difference of .lamda./4+.alpha. and the second
region has a birefringent phase difference of .lamda./4-.alpha.,
where .lamda. is a wavelength of the light emitted from the light
source.
[0021] In a preferred embodiment, the first region has a
birefringent phase difference of .lamda./4+.alpha. and the second
region has a birefringent phase difference of -3.lamda./4-.alpha.,
where .lamda. is a wavelength of the light emitted from the light
source.
[0022] In a preferred embodiment, .alpha. is in a range of
-.lamda./8<.alpha.<.lamda./8.
[0023] In a preferred embodiment, a plurality of said first regions
and a plurality of said second regions alternate on the wavelength
plate, each first region and each second region having a strip
shape.
[0024] In a preferred embodiment, the light source is capable of
emitting first laser light of a wavelength .lamda.1 and second
laser light of a wavelength .lamda.2 (where
.lamda.2>.lamda.1).
[0025] An optical disk apparatus according to the present invention
comprises: a light source for emitting light of a wavelength
.lamda.1 and light of a wavelength .lamda.2 (where
.lamda.1.noteq..lamda.2); an objective lens for converging the
light onto a signal surface of an optical disk; a polarized beam
diffraction element for diffracting the light reflected from the
optical disk; a photodetector for detecting the light diffracted
from the polarized beam diffraction element; and a wavelength plate
disposed between the optical disk and the polarized beam
diffraction element, wherein, an imaginary line L on the beam
splitter is defined, the line L being perpendicular to a radial
direction of the optical disk, and intersecting an optical axis of
the objective lens; the beam splitter at least has a regional, a
region a2, a region a3, a region A1, a region A2, and a region A3,
such that the regional, the region a2, and the region a3 are on a
same side of the line L on the beam splitter, and the region A1,
the region A2, and the region A3 are substantially symmetrical
regions to the regional, the region a2, and the region a3,
respectively, with respect to the line L; the photodetector at
least has two regions b and B; light of the wavelength .lamda.1
entering the region a3, the regional, and the region A2 of the beam
splitter produces 1.sup.st order diffracted light which is
projected onto the region b of the photodetector, and light of the
wavelength .lamda.1 entering the region A3, the region A1, and the
region a2 of the beam splitter produces 1.sup.st order diffracted
light which is projected onto the region B of the photodetector;
light of the wavelength .lamda.2 entering the region a3 of the beam
splitter produces 1.sup.st order diffracted light which is
projected onto the region B of the photodetector, and light of the
wavelength .lamda.2 entering the region A3 of the beam splitter
produces 1.sup.st order diffracted light which is projected onto
the region b of the photodetector; and based on a difference
between a detection signal from the region b and a detection signal
from the region B, the optical disk apparatus generates a tracking
error signal for the optical disk or a correction signal for
correcting the tracking error signal.
[0026] In a preferred embodiment, the photodetector further has at
least two regions b' and B'; light from a first light source or a
second light source entering the region a3, the regional, and the
region a2 of the beam splitter produces -1.sup.st order diffracted
light which is projected onto the region b' of the photodetector,
and light from the first light source or the second light source
entering the region A3, the region A1, and the region A2 of the
beam splitter produces -1.sup.st order diffracted light which is
projected onto the region B' of the photodetector; and the optical
disk apparatus generates a difference signal based on a difference
between a detection signal from the region b' and a detection
signal from the region B', and generates a tracking error signal
for the optical disk by adding to the difference signal a value
obtained by multiplying the correction signal by an arbitrary
coefficient.
[0027] Alternatively, an optical disk apparatus according to the
present invention comprises: a light source for emitting light of a
wavelength B1 and light of a wavelength .lamda.2 (where
.lamda.1.noteq..lamda.2); an objective lens for converging the
light onto a signal surface of an optical disk; a polarized beam
diffraction element for diffracting the light reflected from the
optical disk; a photodetector for detecting the light diffracted
from the polarized beam diffraction element; and a wavelength plate
disposed between the optical disk and the polarized beam
diffraction element, wherein, an imaginary line L on the beam
splitter is defined, the line L being perpendicular to a radial
direction of the optical disk, and intersecting an optical axis of
the objective lens; the beam splitter at least has eight regions
a1, a2, a3, A1, A2, A3, and A4 such that the regional, the region
a2, the region a3, and the region a4 are on a same side of the line
L on the beam splitter, and the region A1, the region A2, the
region A3, and the region A4 are substantially symmetrical regions
to the regional, the region a2, the region a3, and the region a4,
respectively, with respect to the line L; the photodetector at
least has six regions b, B, b', B', b'', and B''; light of the
wavelength .lamda.1 entering the region A2 and the regional of the
beam splitter produces -1.sup.st order diffracted light which is
projected onto the region b of the photodetector, and light of the
wavelength .lamda.1 entering the region a2 and the region A1 of the
beam splitter produces -1.sup.st order diffracted light which is
projected onto the region B of the photodetector, the optical disk
apparatus generating a tracking error signal for the optical disk
based on a difference between a detection signal from the region b
and a detection signal from the region B; light of the wavelength
.lamda.2 entering the region a3 and the region a4 of the beam
splitter produces -1.sub.st order diffracted light which is
projected onto the region b' of the photodetector, and light of the
wavelength .lamda.2 entering the region A3 and the region A4 of the
beam splitter produces -1.sup.st order diffracted light which is
projected onto the region B' of the photodetector, the optical disk
apparatus generating a difference signal based on a difference
between a detection signal from the region b' and a detection
signal from the region B'; and light of the wavelength .lamda.2
entering the region a3 further produces 1.sup.st order diffracted
light which is projected onto the region b'' of the photodetector,
and light of the wavelength .lamda.2 entering the region A3 further
produces 1.sup.st order diffracted light which is projected onto
the region B'' of the photodetector, the optical disk apparatus
generating a correction signal based on a difference between a
detection signal from the region b'' and a detection signal from
the region B''; the optical disk apparatus generates a tracking
error signal for the optical disk by adding to the difference
signal a value obtained by multiplying the correction signal by an
arbitrary coefficient.
[0028] An optical element according to the present invention
comprises a two-dimensional array of a plurality of birefringent
regions including first and second regions, the first and second
regions differing in birefringent phase difference and/or optic
axes from each other, wherein the plurality of birefringent regions
including the first and second regions cause the light to have
different polarization states.
[0029] In a preferred embodiment, optic axes of the first and
second regions are parallel to each other, and the first and second
regions have different retardations from each other.
[0030] In a preferred embodiment, optic axes of the first and
second regions are oriented in different directions from each
other.
[0031] In a preferred embodiment, a plurality of said first regions
and a plurality of said second regions alternate within a plane
perpendicular to an optical axis.
[0032] In a preferred embodiment, each of the first and second
regions has a shape selected from the group consisting of: a strip
shape, a checker shape, and an annular shape.
[0033] In a preferred embodiment, the optical element further
comprises a polarization filter.
[0034] In a preferred embodiment, the polarization filter is a
polarization hologram.
[0035] In a preferred embodiment, the optic axis of the first
region is at
45.degree.+.delta..+-..alpha.(-10.degree.<.delta.<10.degree.,
0.degree.<.alpha..ltoreq.15.degree.) with respect to a
polarization direction of incident light; and the optic axis of the
second region is at 45.degree.+.delta.-.alpha. with respect to a
polarization direction of incident light.
[0036] In a preferred embodiment, with respect to light of at least
one wavelength among light of a plurality of wavelengths traveling
back and forth through the optical element, an average retardation
.DELTA. of the plurality of birefringent regions is set equal to
(2m+1).pi./2 (where m is an integer).
[0037] In a preferred embodiment, the optical element is a
broadband wavelength plate having a same retardation .DELTA. for
light of different wavelengths.
[0038] In a preferred embodiment, optic axes of some of the
plurality of birefringent regions are at 45.degree. with respect to
a polarization direction of incident light.
[0039] An optical pickup according to the present invention
comprises: a light source for emitting two or more kinds of laser
light of different wavelengths; a lens for converging the light
emitted from the light source onto an optical information medium;
and a photodetector for receiving light reflected from the optical
information medium, wherein the optical pickup further comprises
the optical element according to the present invention, the optical
element being disposed in a region common to an optical path from
the light source to the optical information medium and an optical
path from the optical information medium to the photodetector.
[0040] In a preferred embodiment, the light source and the
photodetector are integrally formed.
[0041] A method according to the present invention for producing an
optical element having a two-dimensional array of a plurality of
birefringent regions including first and second regions, the first
and second regions differing in birefringent phase difference
and/or optic axes from each other, the plurality of birefringent
regions including the first and second regions causing the light to
have different polarization states, comprises the steps of: (a)
forming on a substrate an alignment film including a plurality of
regions having different alignment directions from one another; and
(b) forming a liquid crystal layer on the alignment film and
controlling the alignment direction of each region of the liquid
crystal layer.
[0042] In a preferred embodiment, the step (a) comprises the
substeps of: (a1) depositing a photo-alignable film on the
substrate; (a2) subjecting a portion of the photo-alignable film to
an exposure with ultraviolet light to form a first aligning region
having a first alignment direction; and (a3) subjecting another
portion of the alignment film to an exposure with ultraviolet light
to form a second aligning region having a second alignment
direction, the second alignment direction being different from the
first alignment direction.
[0043] In a preferred embodiment, the step (b) comprises the
substeps of: (b1) forming on the alignment film a liquid crystal
layer containing a UV-curing material, and controlling the
alignment directions within the liquid crystal layer in accordance
with first and second alignment directions; and (b2) curing the
liquid crystal layer by irradiating the liquid crystal layer with
ultraviolet light.
[0044] According to the present invention, regardless of the exact
birefringence of the optical disk substrate, the birefringent phase
difference of returned light has a distribution (fluctuations).
Therefore, a more than zero detected light amount is obtained, so
that signal reading errors or control failure can be prevented. In
the case of adopting a structure where two light sources are
comprised, it is possible to provide countermeasures against
birefringence in accordance with each light source, so that control
signals and a reproduction signal for various kinds of optical
disks can be detected with the same photodetector.
[0045] Furthermore, there is provided an optical disk apparatus
which enables tracking control free of off-tracking through
calculations of detection signals even in the following cases: (1)
where the objective lens or the polarized beam diffraction element
has an eccentricity along a radial direction of the optical disk;
(2) where the optical disk substrate is tilted; and (3) where a
light spot rests on a border between a recorded region and an
unrecorded region on the optical disk and is susceptible to the
influence from an adjoining track.
[0046] Other features, elements, processes, steps, characteristics
and advantages of the present invention will become more apparent
from the following detailed description of preferred embodiments of
the present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1A is an essential structural diagram showing an
optical disk apparatus according to one embodiment of the present
invention. FIG. 1B is a side view of a light source section
thereof.
[0048] FIG. 2A is a structural diagram showing a detection surface
according to the above embodiment.
[0049] FIG. 2B is a structural diagram showing a hologram surface
according to the above embodiment.
[0050] FIGS. 3A and 3B are a plan view and a cross-sectional view,
respectively, showing a distributed-type wavelength plate according
to the above embodiment.
[0051] FIG. 4A is a diagram illustrating a
birefringence-counteracting principle applicable to the forward
path, as realized by a distributed-type wavelength plate according
to the above embodiment. FIG. 4B shows one
birefringence-counteracting principle applicable to the return
path. FIG. 4C shows another birefringence-counteracting principle
applicable to the return path.
[0052] FIG. 5A is an essential structural diagram showing an
optical disk apparatus according to another embodiment of the
present invention. FIG. 5B is a side view showing a light source
section thereof.
[0053] FIG. 6 is a structural diagram showing a hologram surface of
a polarization hologram substrate according to the above
embodiment.
[0054] FIG. 7A is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of first laser light emitted from a first
emission point.
[0055] FIG. 7B is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of second laser light emitted from a second
emission point.
[0056] FIG. 8 is a structural diagram showing a hologram surface of
a polarization hologram substrate to be used in an optical disk
apparatus according to another embodiment of the present
invention.
[0057] FIG. 9A is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of first laser light emitted from a first
emission point.
[0058] FIG. 9B is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of second laser light emitted from a second
emission point.
[0059] FIG. 10 is a structural diagram showing a hologram surface
of a polarization hologram substrate 2 to be used in an optical
disk apparatus according to another embodiment of the present
invention.
[0060] FIG. 11A is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of first laser light emitted from a first
emission point.
[0061] FIG. 11B is a structural diagram showing a photodetection
surface according to the above embodiment, and also an explanatory
diagram showing a light distribution thereon, illustrating light
spots of returned light of second laser light emitted from a second
emission point.
[0062] FIG. 12 is an essential structural diagram showing an
optical pickup according to an embodiment of the present
invention.
[0063] FIG. 13A is a plan view showing a wavelength plate according
to the above embodiment. FIG. 13B is a partial side view showing an
optical pickup including the wavelength plate. FIG. 13C is a
diagram showing changes in the polarization state obtained with the
use of the wavelength plate.
[0064] FIG. 14A is a plan view showing a wavelength plate according
to another embodiment of the present invention. FIG. 14B is a plan
view showing a wavelength plate according to still another
embodiment of the present invention. FIG. 14C is a plan view
showing a wavelength plate according to still another embodiment of
the present invention.
[0065] FIG. 15 is an essential structural diagram showing an
optical pickup according to another embodiment of the present
invention.
[0066] FIG. 16A is a diagram showing a conventional optical
element, as well as behavior of light having a wavelength
.lamda..sub.1 being led therethrough. FIG. 16B is a diagram showing
the conventional optical element, as well as behavior of light
having a wavelength .lamda..sub.2 being led therethrough.
[0067] FIG. 17A includes a plan view and a side view of an optical
element according to another embodiment of the present invention.
FIG. 17B includes a plan view and a side view of an optical element
according to still another embodiment of the present invention.
[0068] FIGS. 18A, 18B, 18C, and 18D are diagrams showing one
embodiment of a method for producing a distribution wavelength
plate according to the present invention.
[0069] FIG. 19A is an essential structural diagram showing a
conventional optical disk apparatus. FIG. 19B is a side view
showing a light source section thereof.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0070] Hereinafter, preferred embodiments of the present invention
will be described with reference to the accompanying drawings.
Embodiment 1
[0071] With reference to FIGS. 1A and 1B to FIGS. 4A-4C, an optical
disk apparatus according to a first embodiment of the present
invention will be described.
[0072] First, FIG. 1A will be referred to FIG. 1A shows essential
components of an optical pickup for an optical disk apparatus
according to the present embodiment. FIG. 1B is a side view showing
a light source section 1 and a neighborhood thereof.
[0073] As shown in FIG. 1A, the optical pickup according to the
present embodiment comprises a photodetection substrate 9 on which
the light source 1 (e.g., a semiconductor laser) is mounted, as
well as an optical system. The optical system includes a
collimating lens 4, a polarization hologram substrate 2, a
distributed-type wavelength plate 3, and an objective lens 5, which
are provided along an optical axis 7. The distributed-type
wavelength plate 3, which is formed on the same substrate as a
hologram surface 2a of the polarization hologram substrate 2, moves
integrally with the objective lens 6. A most characteristic feature
of the present embodiment is the distribution wavelength plate 3.
As used herein, a "distribution wavelength plate" is defined as a
wavelength plate having birefringent regions of different
characteristics arranged along the plane of a principal face
thereof.
[0074] The surface of the photodetection substrate 9 includes a
region (detection surface 9a) in which a plurality of
photosensitive portions such as photodiodes are formed, and a
region in which the light source 1 is mounted. As shown in FIG. 1B,
a reflection mirror 10 is formed on the surface of the
photodetector substrate 9, the reflection mirror 10 reflecting
light emitted from the light source 1 in a direction which is
substantially perpendicular to the surface of the photodetection
substrate 9.
[0075] Laser light which has been emitted from the light source 1
is reflected from the reflection mirror 10 on the photodetection
substrate 9, and thereafter collimated into parallel light by the
collimating lens 4. The parallel light is transmitted through the
polarization hologram substrate 2 in the form of P-polarized light.
The polarization hologram substrate 2 is characterized so that it
does not diffract P-polarized light, but diffracts S-polarized
light. In the case where the incident light is S-polarized light,
the polarization hologram substrate 2 has a diffraction efficiency
of about 0% for the 0.sup.th order light, and about 41% for the
.+-.1.sup.st order light, for example.
[0076] The light which has been transmitted through the
polarization hologram substrate 2 is converted by the
distributed-type wavelength plate 3 into light comprising a spatial
mixture of two types of polarization states (which hereinafter may
be referred to as "mixed-polarized light"). The detailed structure
and functions of the distributed-type wavelength plate 3 will be
described later. The mixed-polarized light is converged by the
objective lens 5 onto a signal surface 6a of the optical disk
substrate 6.
[0077] The light (signal light) which has been reflected from the
signal surface 6a of the optical disk substrate 6 propagates in the
opposite direction of the forward path. This light (signal light)
travels through the objective lens 5 and enters the
distributed-type wavelength plate 3. The light transmitted through
the distributed-type wavelength plate 3 enters the hologram surface
2a of the polarization hologram substrate 2 so as to be diffracted.
Through this diffraction, 1.sup.st order diffracted light 8 and
-1.sup.st order diffracted light 8' are formed with respect to the
optical axis 7 as an axis of symmetry. The diffracted light 8 and
8' is each converged on the detection surface 9a on the detector 9
via the collimating lens 4. The detection surface 9a is located
substantially at the focal plane of the collimating lens 4 (i.e.,
an imaginary emission point on the light source 1).
[0078] FIG. 2A shows the structure of the photodetection surface 9a
of the photodetector 9. FIG. 2B shows the structure of the hologram
surface 2a of the polarization hologram substrate 2. Both FIGS. 2A
and 2B are plan views showing the photodetection surface 9a and the
hologram surface 2a, respectively, as viewed from the side of the
optical disk 6.
[0079] Referring to FIG. 2B, the structure of the hologram surface
2a will be described. The hologram surface 2a is divided into four
portions (quadrants) by two lines (X and Y axes) which
perpendicularly intersect each other at an intersection 20 between
the hologram surface 2a and the optical axis 7. The Y axis
corresponds to a radial direction on the signal surface 6a of the
optical disk substrate 6. Each quadrant is divided along the Y axis
into strip regions 21B and 21F; 22B and 22F; 23B and 23F; or 24B
and 24F. Each strip region extends along the X axis.
[0080] Next, by referring to FIG. 2A, the structure of the
detection surface 9a will be described. When an intersection
between the detection surface 9a and the optical axis 7 is defined
as an "intersection 90", x and y axes of a coordinate system whose
origin is at the intersection 90 are parallel to, respectively, the
X axis and the Y axis shown in FIG. 2B. The light source 1 is
mounted at a point on the x axis, and emits laser light from an
emission point la thereof.
[0081] As shown in FIG. 2A, in a region corresponding to the "+"
side of the y axis on the detection surface 9a, strip-like focus
detection cells F1a, F2a, F1b, F2b, F1c, F2c, F1d, F2d, F1e, and
F2e are formed, each of which extends along the y axis. In a region
corresponding to the "-" side of the y axis, trapezoidal tracking
detection cells 7T1, 7T2, 7T3, and 7T4 are formed. These detection
cells are placed in a symmetrical arrangement with respect to the y
axis. Note that light which is emitted from the emission point 1a
of the light source 1 travels along a direction parallel to the x
axis, within a plane which contains the x axis and is perpendicular
to the plane of FIG. 2A, so as to be reflected by the reflection
mirror 10 in the optical axis direction (i.e., a direction which
extends through the point 90 and perpendicularly to the plane of
FIG. 2A).
[0082] FIG. 2B shows (with a circular broken line 80) the outer
shape of a beam cross section of light entering the hologram
surface 2a. Out of the light entering the hologram surface 2a,
1.sup.st order diffracted light which has been diffracted at the
strip regions 21B and 21F located in the first quadrant of the
hologram surface 2a is respectively converged at light spots 81BS
and 81FS, each of which lies astride the border between the
detection cells F2a and F1b. The -1.sup.st order diffracted light
is converged at light spots 81BS' and 81FS', which fit within the
detection cell 7T1.
[0083] The 1.sup.st order diffracted light which has been
diffracted at the regions 22B and 22F located within the second
quadrant is respectively converged at light spots 82BS and 82FS,
each of which lies astride the border between the detection cells
F1b and F2b. The -1.sup.st order diffracted light is converged at
light spots 82BS' and 82FS', which fit within the detection cell
7T2.
[0084] The 1.sup.st order diffracted light which has been
diffracted at the regions 23B and 23F located within the third
quadrant is respectively converged at light spots 83BS and 83FS,
each of which lies astride the border between the detection cells
F1d and F2d. The -1.sup.st order diffracted light is converged at
light spots 83BS' and 83FS', which fit within the detection cell
7T3.
[0085] The 1.sup.st order diffracted light which has been
diffracted at the regions 24B and 24F located within the fourth
quadrant is respectively converged at light spots 84BS and 84FS,
each of which lies astride the border between the detection cells
F2d and F1e. The -1.sup.st order diffracted light is converged at
light spots 84BS' and 84FS', which fit within the detection cell
7T4.
[0086] Some of the detection cells are electrically interconnected
so that the following six types of signals F1, F2, T1, T2, T3, and
T4 are output from the photodetector 9:
F1=signal obtained from the detection cell F1a
+signal obtained from the detection cell F1b
+signal obtained from the detection cell F1c
+signal obtained from the detection cell F1d
+signal obtained from the detection cell F1e
F2=signal obtained from the detection cell F2a
+signal obtained from the detection cell F2b
+signal obtained from the detection cell F2c
+signal obtained from the detection cell F2d
+signal obtained from the detection cell F2e
T1=signal obtained from the detection cell 7T1
T2=signal obtained from the detection cell 7T2
T3=signal obtained from the detection cell 7T3
T4=signal obtained from the detection cell 7T4
[0087] It is assumed that the y axis and the Y axis shown in FIGS.
2A and 2B, respectively, are parallel to a radial direction on the
signal surface 6a of the optical disk substrate 6. In this case, a
focus error signal FE from the signal surface 6a, a tracking error
signal TE, and a reproduction signal RF are to be detected based on
eq. 1 to eq. 3 below:
FR=F1-F2 (eq. 1)
TE=T1+T2-T3-T4 (eq. 2)
RF=F1+F2+T1+T2+T3+T4 (eq. 3)
[0088] Next, with reference to FIGS. 3A and 3B, the structure of
the distributed-type wavelength plate 3 will be described. FIGS. 3A
and 3B are a plan view and a cross-sectional view, respectively,
showing the distributed-type wavelength plate 3. Note that FIG. 3A
is a plan view as viewed from the side of the optical disk
substrate 6. Herein, two lines perpendicularly intersecting each
other at an intersection 30 between the surface of the
distributed-type wavelength plate 3 and the optical axis 7 are
defined as X and Y axes. The X and Y axes here correspond to the X
and Y axes on the hologram surface 2a. The Y axis here is parallel
to the radial direction 6R (hereinafter referred to as the "disk
radial direction") on the signal surface 6a of the optical disk
substrate 6.
[0089] The distributed-type wavelength plate 3 is divided into a
plurality of strip regions 3A and 3B, each of which has a
longitudinal axis along the disk radial direction 6R. Each strip
region 3A is responsible for a birefringent phase difference of
90+.alpha..degree., whereas each strip region 3B is responsible for
a birefringent phase difference of 90-.alpha..degree.. The fast
axis is oriented in a 45.degree. direction with respect to the
optical disk radial direction 6R. The strip regions 3A and the
strip regions 3B alternate with each other.
[0090] As shown in FIG. 3B, the distributed-type wavelength plate 3
has a birefringent layer 3c (having a thickness of c) formed on the
polarization hologram substrate 2, as well as birefringent layers
3a and transparent layers 3b which are arranged on the birefringent
layer 3c. The transparent layers 3a form the strip regions 3A,
whereas the transparent layers 3b form the strip regions 3B. Each
birefringent layer 3a and each transparent layer 3b have
thicknesses of a and b, respectively. Although FIG. 3B illustrates
a case where b<a, it might also be possible that b=a or b>a.
Each transparent layer 3b can be regarded as a phase correcting
layer functioning to ensure phase alignment between the light
transmitted through the transparent layers 3b and the light
transmitted through the transparent layers 3a.
[0091] The distributed-type wavelength plate 3 as above can be
produced in the following manner, for example.
[0092] First, the birefringent layer 3c is deposited so as to have
a uniform thickness on the polarization hologram substrate 2. After
depositing the birefringent layer 3a on the birefringent layer 3,
the birefringent layer 3a is patterned by photolithography and
etching techniques. Through this patterning, those portions of the
birefringent layer 3a in which the strip regions 3B shown in FIG.
3A are to be formed are removed, thus leaving a plurality of
openings. Next, each opening is filled with a transparent layer 3b,
whereby the structure as shown in FIG. 3B is obtained.
[0093] In the present embodiment, the birefringent layer 3c is
responsible for a birefringent phase difference of
90-.alpha..degree., whereas each birefringent layer 3a is
responsible for a birefringent phase difference of
2.alpha..degree.. Both fast axes are tilted at 45.degree. with
respect to the optical disk radial direction 6R. The
distributed-type wavelength plate 3 may further comprise another
transparent layer covering the birefringent layers 3a and the
transparent layers 3b; and the further transparent layer may be a
transparent substrate. Moreover, the birefringent layers 3a may be
located below the birefringent layer 3c instead.
[0094] The region marked by the circular broken line 10 in FIG. 3A
schematically shows a cross section of a light beam entering the
distributed-type wavelength plate 3. In response to P-polarized
light entering the distributed-type wavelength plate 3, light
(mixed-polarized light) comprising a spatial mixture of two types
of polarization states (both being elliptical polarization close to
circular polarization) goes out from the distributed-type
wavelength plate 3.
[0095] As shown in FIG. 1, the mixed-polarized light transmitted
through the distributed-type wavelength plate 3 is converged by the
objective lens 5 onto the signal surface 6a of the optical disk
substrate 6. Although the diameter of the focused beam spot formed
on the signal surface 6a might be slightly increased from the
conventional value, the increase would be small. For example,
assuming that .alpha.=20.degree., NA=0.5, and .lamda.=790 nm, the
increase in the spot diameter will be about 1/1000 .mu.m, which
would correspond to a 2% to 3% deterioration of Strehl (which
refers to normalized peak intensity).
[0096] Next, referring to FIGS. 4A to 4C, the functions of the
distributed-type wavelength plate 3 will be described.
[0097] For simplicity, FIGS. 4A to 4C illustrate an example in
which the distributed-type wavelength plate 3 is divided into two
strip regions 3A and 3B. Specifically, the distributed-type
wavelength plate 3 is equally divided into two regions 3A and 3B by
a line L which extends along the optical disk radial direction 6R.
The region 3A is responsible for a birefringent phase difference of
90+.alpha..degree., whereas the region 3B is responsible for a
birefringent phase difference of 90-.alpha.+. The fast axes of the
regions 3A and 3B are both oriented in a 45.degree. direction with
respect to the optical disk radial direction 6R.
[0098] FIG. 4A shows a relationship between incident light 10 and
the distributed-type wavelength plate 3 in the forward path.
[0099] Out of incident light 10 which is transmitted through the
distributed-type wavelength plate 3, light 10A transmitted through
the right region of the line L has a birefringent phase difference
of 90+.alpha..degree.. On the other hand, light 10B transmitted
through the left region of the line L has a birefringent phase
difference of 90-.alpha..degree..
[0100] FIG. 4B shows a relationship between incident light 80 and
the distributed-type wavelength plate 3 in the return path.
[0101] Since the incident light 80 is reflected light from the
signal surface 6a of the optical disk substrate 6, its light
distribution is inverted. In other words, out of the incident light
80 entering the distributed-type wavelength plate 3, light 80A
transmitted through the right region of the line L has a
birefringent phase difference of 90-.alpha..degree.. On the other
hand, light 80B transmitted through the left region of the line L
has a birefringent phase difference of 90+.alpha..degree.. Note
that it is herein assumed that the optical disk substrate 6 does
not cause any change in the birefringent phase difference.
[0102] FIG. 4C shows a relationship between the incident light 80
and the distributed-type wavelength plate 3 in the return path, in
the case where a signal pit pattern exists on the signal surface 6a
of the optical disk substrate 6. It is assumed here that pits
having a sufficiently broad width along the optical disk radial
direction 6R are arranged along a disk rotation direction 6T at an
equal pitch.
[0103] Due to such a pit pattern, the reflected light from the
signal surface 6a is diffracted along the disk rotation direction
6T, so that 1.sup.st order diffracted light 81A and -1.sup.st order
diffracted light 81B are produced. The birefringent phase
differences of these rays of diffracted light correspond to those
of the incident light 80A and 80B in FIG. 4B being swapped from
left to right and vice versa. In other words, the 1.sup.st order
diffracted light 81A has a birefringent phase difference of
90-.alpha..degree., whereas the -1.sup.st order diffracted light
81B has a birefringent phase difference of 90+.alpha..degree. at
entry into the distributed-type wavelength plate 3.
[0104] Therefore, after the 1.sup.st order diffracted light 81A and
the -1.sup.st order diffracted light 81B have been transmitted
through the distributed-type wavelength plate 3, the diffracted
light 81A has a birefringent phase difference of
180-2.alpha..degree., whereas the diffracted light 80B has a
birefringent phase difference of 180+2.alpha..degree.. Again, it is
assumed herein that the optical disk substrate 6 does not cause any
change in the birefringent phase difference.
[0105] Next, a case will be considered where the optical disk
substrate 6 introduces a birefringent phase difference during
transmission of light therethrough.
[0106] If the birefringent phase difference ascribable to the
optical disk substrate 6 is -180.degree. over the course of the
back and forth trips of light, the diffracted light 81A will have a
birefringent phase difference of -2.alpha..degree., whereas the
light 80B will have a birefringent phase difference of
+2.alpha..degree.. Regardless of the exact birefringence of the
optical disk substrate 6, the birefringent phase differences of the
diffracted light 81A and the diffracted light 80B will not be
simultaneously zero. Therefore, the returned light (signal light)
entering the polarization hologram substrate 2 will always have
some polarized component to be diffracted at the hologram surface
2a.
[0107] Since pits, embossed portions, signal marks, and the like
exist on the signal surface 6a of the optical disk substrate 6, the
reflected light from the signal surface 6a will undergo a more
complicated diffraction. However, regardless of the exact
birefringence of the optical disk substrate 6, the birefringent
phase difference of the returned light (signal light) will always
have a spatial distribution. Such a distribution can be obtained as
long as the distributed-type wavelength plate 3 includes a
two-dimensional array of plural birefringent regions including
first and second regions which impart different birefringent phase
differences to the same incident linearly polarized light. When
light is transmitted through such plural birefringent regions,
different phase differences occur depending on the incident
position of light. The number and shapes of birefringent regions to
be formed in the distributed-type wavelength plate 3 can be
arbitrary.
[0108] With respect to the distributed-type wavelength plate 3
having the strip regions 3A and 3B shown in FIG. 3A, a detected
light amount S.sub.0 in the case where the birefringent phase
difference ascribable to the optical disk substrate 6 is 0.degree.
over the course of the back and forth trips of light, and a
detected light amount S.sub.180 in the case where the birefringent
phase difference ascribable to the optical disk substrate 6 is
180.degree. over the course of the back and forth trips of light
were determined, and the ratio of detected light amounts
S.sub.180/S.sub.0 was calculated.
[0109] With respect to a random disk signal from a CD-ROM, the
ratio of detected light amounts was 15%, in the case where
.alpha.=20.degree., NA=0.5, .lamda.=790 nm. In the case where
.alpha.=36.degree., the ratio of detected light amounts was 60%. In
either case, the calculation showed no substantial deterioration in
the optical jitter.
[0110] Thus, according to the present embodiment, a more than zero
detected light amount is obtained even with respect to an optical
disk substrate 6 having a large birefringence, so that signal
reading errors or control failure as in the conventional example
will not occur.
[0111] Although the present embodiment illustrates an example where
the distributed-type wavelength plate 3 is divided into strip-like
regions, any other manner of division may be adopted as long as two
kinds of birefringent phase differences are produced. Similar
effects can be obtained also in the case where the division is made
so as to produce two or more kinds of birefringent phase
differences. The same is also true to each of the following
embodiments.
Embodiment 2
[0112] Next, with reference to FIGS. 5A and 5B to FIGS. 7A and 7B,
an optical disk apparatus according to a second embodiment of the
present invention will be described.
[0113] In the present embodiment, there are two emission points in
the light source 1. Moreover, the pattern of the polarization
hologram surface 2a, the detection pattern on the photodetection
surface 9a, and the light distribution thereupon are different from
those in Embodiment 1. Otherwise, the optical disk apparatus of the
present embodiment is identical in construction to the optical disk
apparatus of Embodiment 1. Therefore, any descriptions which would
be similar to those in Embodiment 1 will be omitted. Those
components which have identical counterparts in the optical disk
apparatus of Embodiment 1 are denoted by the same reference
numerals as those used therein.
[0114] The light source 1 may include two different types of
semiconductor laser chips, or include a single semiconductor laser
chip which is capable of emitting laser light of different
wavelengths. Thus, the light source 1 can output laser light of an
appropriate wavelength in accordance with the type of optical disk
which is mounted on the optical disk apparatus.
[0115] As shown in FIG. 5A, laser light (wavelength .lamda.1) which
has been emitted from a first emission point 1a of the light source
1 mounted on the photodetection substrate 9 is reflected from the
reflection mirror 10 on the photodetection substrate 9, and
thereafter collimated into parallel light by the collimating lens
4. The parallel light is transmitted through the polarization
hologram substrate 2 in the form of P-polarized light. The
polarization hologram substrate 2 is characterized so that it does
not diffract P-polarized light, but diffracts S-polarized light. In
the case where the incident light is S-polarized light, the
polarization hologram substrate 2 has a diffraction efficiency of
about 0% for the 0.sup.th order light, and about 41% for the
.+-.1.sup.st order light, for example. For convenience, FIG. 5A
simultaneously illustrates the first optical disk substrate 6 and
the second optical disk substrate 6'. In practice, however, either
the first optical disk substrate 6 or the second optical disk
substrate 6' is to be loaded to the optical disk apparatus
separately. Laser light of the wavelength .lamda.1 is emitted from
the first emission point 1a in the case where the first optical
disk substrate 6 is loaded.
[0116] The light which has been transmitted through the
polarization hologram substrate 2 is converted by the
distributed-type wavelength plate 3 into light comprising a spatial
mixture of two types of polarization states (mixed-polarized
light). The detailed structure and functions of the
distributed-type wavelength plate 3 will be described later. The
mixed-polarized light is converged by the objective lens 5 onto a
signal surface 6a of the first optical disk substrate 6.
[0117] The light (signal light) which has been reflected from the
signal surface 6a of the first optical disk substrate 6 propagates
in the opposite direction of the forward path. This light (signal
light) travels through the objective lens 5 and enters the
distributed-type wavelength plate 3. The light transmitted through
the distributed-type wavelength plate 3 enters the hologram surface
2a of the polarization hologram substrate 2 so as to be diffracted.
Through this diffraction, 1.sup.st order diffracted light 8 and
-1.sup.st order diffracted light 8' are formed with respect to the
optical axis 7 as an axis of symmetry. The diffracted light 8 and
8' is each converged on the detection surface 9a on the detector 9
via the collimating lens 4. The detection surface 9a is located
substantially at the focal plane of the collimating lens 4 (i.e.,
an imaginary emission point on the light source 1).
[0118] The light source 1 in the present embodiment is also capable
of emitting light of a different wavelength from that of the first
laser light. In the present embodiment, second laser light
(wavelength .lamda.2, where .lamda.2>.lamda.1) is emitted from
the second emission point 1a' on the light source 1 in the case
where data is to be recorded on or read from the second optical
disk substrate 6'. The second laser light which has been emitted
from the second emission point 1a' is reflected from the reflection
mirror 10, and thereafter collimated into parallel light by the
collimating lens 4. The parallel light is transmitted through the
polarization hologram substrate 2 in the form of P-polarized light.
The polarization hologram substrate 2 is characterized so that it
does not diffract P-polarized light, but diffracts S-polarized
light.
[0119] The light which has been transmitted through the
polarization hologram substrate 2 is converted by the
distributed-type wavelength plate 3 into light comprising a spatial
mixture of two types of polarization states (mixed-polarized
light). The detailed structure and functions of the
distributed-type wavelength plate 3 will be described later. The
mixed-polarized light is converged by the objective lens 5 onto a
signal surface 6a' of the second optical disk substrate 6'.
[0120] The light (signal light) which has been reflected from the
signal surface 6a' of the second optical disk substrate 6'
propagates in the opposite direction of the forward path. This
light (signal light) travels through the objective lens 5 and
enters the distributed-type wavelength plate 3. The light
transmitted through the distributed-type wavelength plate 3 enters
the hologram surface 2a of the polarization hologram substrate 2 so
as to be diffracted. Through this diffraction, 1.sup.st order
diffracted light 8 and -1.sup.st order diffracted light 8' are
formed with respect to the optical axis 7 as an axis of symmetry.
Since the second laser light has the wavelength of .lamda.2, which
is greater than the wavelength .lamda.1 of the first laser light,
the diffraction efficiency for the .+-.1.sup.st order light is
about 10% lower than that for the wavelength .lamda.1. The
diffracted light 8 and 8' is each converged on the detection
surface 9a on the detector 9 via the collimating lens 4.
[0121] FIG. 6 shows the structure of the hologram surface 2a of the
polarization hologram substrate 2 in the present embodiment. FIGS.
7A and 7B show the structure of the photodetection surface 9a in
the present embodiment. Specifically, FIG. 7A illustrates light
spots of returned light of first laser light emitted from the first
emission point 1a, whereas FIG. 7B illustrates light spots of
returned light of second laser light emitted from the second
emission point 1a'.
[0122] As shown in FIG. 6, the structure of the hologram surface 2a
in the present embodiment is similar to that of the hologram
surface 2a shown in FIG. 2B. FIGS. 7A and 7B show x and y axes,
which perpendicularly intersect each other at an intersection 90
(or 90') between the detection surface 9a and the optical axis 7
(or 7'). The x and y axes are parallel to the X and Y axes shown in
FIG. 6, respectively.
[0123] As shown in FIGS. 7A and 7B, in a region corresponding to
the "-" side of the y axis on the detection surface 9a, strip-like
focus detection cells F1a, F2a, F1b, F2b, F1c, F2c, F1d, and F2d
are formed, each of which extends along the y axis. In a region
corresponding to the "+" side of the y axis, rectangular tracking
detection cells 7T1, 7T2, 7T3, and 7T4 are formed. These detection
cells are placed in a symmetrical arrangement with respect to the y
axis.
[0124] Light which is emitted from the first emission point 1a of
the light source 1 travels along a direction parallel to the x
axis, within a plane which contains the x axis and is perpendicular
to the plane of FIGS. 7A and 7B, so as to be reflected by the
reflection mirror 10 in the optical axis direction (i.e., a
direction which extends through the point 90 and perpendicularly to
the plane of FIGS. 7A and 7B). On the other hand, light which is
emitted from the second emission point 1a' of the light source 1
travels along a direction parallel to the x axis, within a plane
which contains the x axis and is perpendicular to the plane of FIG.
6, so as to be reflected by the reflection mirror 10 in the optical
axis direction (i.e., a direction which extends through the point
90' and perpendicularly to the plane of FIG. 6).
[0125] Out of the light 80 entering the hologram surface 2a,
1.sup.st order diffracted light which has been diffracted at the
strip regions 21B and 21F located in the first quadrant is
respectively converged at light spots 81BS and 81FS, each of which
lies astride the border between the detection cells F2c and F1d. On
the other hand, -1.sup.st order diffracted light is converged at
light spots 81BS' and 81FS', which fit within the detection cell
7T1.
[0126] Moreover, 1.sup.st order diffracted light which has been
diffracted at the strip regions 22B and 22F located in the second
quadrant is respectively converged at light spots 82BS and 82FS,
each of which lies astride the border between the detection cells
F1c and F2d. On the other hand, -1.sup.st order diffracted light is
converged at light spots 82BS' and 82FS', which fit within the
detection cell 7T2.
[0127] Moreover, 1.sup.st order diffracted light which has been
diffracted at the strip regions 23B and 23F located in the third
quadrant is respectively converged at light spots 83BS and 83FS,
each of which lies astride the border between the detection cells
F1a and F2b. On the other hand, -1.sup.st order diffracted light is
converged at light spots 83BS' and 83FS', which fit within the
detection cell 7T3.
[0128] Moreover, 1.sup.st order diffracted light which has been
diffracted at the strip regions 24B and 24F located in the fourth
quadrant is respectively converged at light spots 84BS and 84FS,
each of which lies astride the border between the detection cells
F2a and F1b. On the other hand, -1.sup.st order diffracted light is
converged at light spots 84BS' and 84FS', which fit within the
detection cell 7T4.
[0129] Some of the detection cells are electrically interconnected
so that signals F1, F2, T1, T2, T3, and T4 are obtained from the
following equations.
F1=signal obtained from the detection cell F1a
+signal obtained from the detection cell F1b
+signal obtained from the detection cell F1c
+signal obtained from the detection cell F1d
=signal obtained from the detection cell F2a
+signal obtained from the detection cell F2b
+signal obtained from the detection cell F2c
+signal obtained from the detection cell F2d
T1=signal obtained from the detection cell 7T1
T2=signal obtained from the detection cell 7T2
T3=signal obtained from the detection cell 7T3
T4=signal obtained from the detection cell 7T4
[0130] In FIG. 7B, the second emission point 1a' of the light
source 1 is shifted in the y axis direction as compared to the
position of the first emission point 1a shown in FIG. 7B. Moreover,
the light emitted from the second emission point 1a' has the
wavelength .lamda.2, which is greater than the wavelength .lamda.1.
Therefore, the hologram has a greater diffraction angle for the
light emitted from the second emission point 1a', thus causing
changes in the positions of the light spots formed on the detection
surface. However, as shown in FIG. 7B, the detection cells 7T1,
7T2, 7T3, and 7T4 are capable of receiving such shifted light
spots. Furthermore, on the detection cells F1a, F1b, F1c, F1d, F2a,
F2b, F2c, and F2d, light spots will move along the division line
(the y axis direction). However, since these detection cells are
elongated along the y axis direction, and since the there is little
change in the distance between each light spot and the division
line, the light of the wavelength .lamda.2 also permits accurate
detection of a focus error signal (FE), as does light of the
wavelength .lamda.1.
[0131] In the present embodiment, with respect to light of the
wavelength .lamda.1, the distributed-type wavelength plate 3
imparts a birefringent phase difference of 90+.alpha..degree. in
the strip regions 3A (FIG. 3A), and imparts a birefringent phase
difference of 90-.alpha..degree. in the strip regions 3B (FIG. 3A).
With respect to light of the wavelength .lamda.2, the strip regions
3A imparts a birefringent phase difference of
(.lamda.1/.lamda.2).times.(90+.alpha..degree.) whereas the strip
regions 3B imparts a birefringent phase difference of
(.lamda.1/.lamda.2).times.(90-.alpha.).degree.. Therefore, with
respect to either light wavelength, a more than zero detected light
amount is obtained with respect to an optical disk substrate 6
having a large birefringence, so that signal reading errors or
control failure as in the conventional example will not occur.
[0132] An alternative structure may be one where, with respect to
the wavelength .lamda.1, the distributed-type wavelength plate 3
imparts a birefringent phase difference of 90+.alpha..degree. in
the strip regions 3A, and imparts a birefringent phase difference
of -270-.alpha..degree. in the strip regions 3B. Such a structure
may be realized in the case where, in FIG. 3B, the birefringent
layer 3c imparts a birefringent phase difference of
90+.alpha..degree., whereas the birefringent layers 3a impart a
birefringent phase difference of -360-2.alpha..degree., for
example. In this case, with respect to the wavelength .lamda.2, the
strip regions 3A will impart a birefringent phase difference of
(.lamda.1/.lamda.2).times.(90+.alpha.).degree., whereas the strip
regions 3B will impart a birefringent phase difference of
(.lamda.1/.lamda.2).times.(-270-.alpha.).degree.. Assuming
.alpha.=0.degree.; .lamda.1=660 nm; and .lamda.2=790 nm, for
example, this situation corresponds to the case where there is no
phase difference between the strip regions 3A and 3B with respect
to the wavelength .lamda.1, whereas there is a phase difference of
60.degree. between the strip regions 3A and 3B with respect to the
wavelength .lamda.2. In this case, a countermeasure to
birefringence is being provided only with respect to the wavelength
.lamda.2, whereas the same optical performance as in the
conventional example is provided with respect to the wavelength
.lamda.1. On the other hand, assuming that .alpha.=15.degree.;
.lamda.1=660 nm; and .lamda.2=790 nm, there is a phase difference
of 300 between the strip regions 3A and 3B with respect to the
wavelength .lamda.1, whereas there is a phase difference of
34.degree. between the strip regions 3A and 3B with respect to the
wavelength .lamda.2. In this case, a countermeasure to
birefringence is being provided for both wavelengths .lamda.1 and
.lamda.2, with a stronger countermeasure being provided for the
wavelength .lamda.2. By changing the value of .alpha., the phase
difference balance can be adjusted.
Embodiment 3
[0133] Next, with reference to FIGS. 8, 9A, and 7B, an optical disk
apparatus according to a third embodiment of the present invention
will be described. Except for the pattern of the polarization
hologram surface 2a, the detection pattern on the photodetection
surface 9a, and the light distribution thereupon, the optical disk
apparatus of the present embodiment is identical in construction to
the optical disk apparatus of Embodiment 2. Therefore, any
descriptions which would be similar to those in Embodiment 2 will
be omitted.
[0134] FIG. 8 shows the structure of the hologram surface 2a of the
polarization hologram substrate 2 in the present embodiment. FIGS.
9A and 9B show the photodetection surface 9a in the present
embodiment. Both FIG. 8 and FIGS. 9A and 9B are plan views showing
the hologram surface 2a and the photodetection surface 9a,
respectively, as viewed from the side of the optical disk 6.
Specifically, FIG. 9A illustrates light spots of returned light of
first laser light emitted from a first emission point 1a, whereas
FIG. 9B illustrates light spots of returned light of second laser
light emitted from a second emission point 1a'.
[0135] As shown in FIG. 8, the hologram surface 2a is divided into
four portions (quadrants) by two lines (X and Y axes) which
perpendicularly intersect each other at an intersection 20 between
the hologram surface 2a and the optical axis 7. The Y axis
corresponds to a radial direction. The first quadrant is divided
into two regions 21a and 21b; the second quadrant is divided into
two regions 22a and 22b; the third quadrant is divided into two
regions 23a and 23b; and the fourth quadrant is divided into two
regions 24a and 24b.
[0136] Although not explicitly shown in FIG. 8, each region is
further divided into strip-like regions with the suffix B and
strip-like regions with the suffix F (e.g., regions 21aB and
regions 21aF), each strip region extending along the X direction,
in a manner similar to FIG. 6 of Embodiment 2. Portions of the
regions 21a and 24a which lie within the aperture (denoted by a
circle 80) are some of the regions which do not contain any
.+-.1.sup.st order diffracted light from a disk groove on a CD-R/RW
or the like. Portions of the regions 22a and 23a which lie within
the aperture are some of the regions which do not contain any
.+-.1.sup.st order diffracted light from a disk groove on a
DVD-R/RW or the like. Consistently with earlier descriptions, the
suffix B represents +1.sup.st order diffracted light which is
converged after the detection surface, whereas the suffix F
represents light which is converged before the detection surface.
For simplicity, FIGS. 9A and 9B only show light spots corresponding
to the suffix B.
[0137] FIGS. 9A and 9B show x and y axes, which perpendicularly
intersect each other at an intersection 90 (or 90') between the
detection surface 9a and the optical axis 7 (or 7'). The x and y
axes are parallel to the X and Y axes shown in FIG. 8,
respectively. In a region corresponding to the "-" side of the y
axis, strip-like focus detection cells F1a, F2a, F1b, F2b, F1c,
F2c, F1d, and F2d are formed, each of which extends along the y
axis, and detection cells 7T5 and 7T6 for tracking correction are
also formed. In a region corresponding to the "+" side of the y
axis, rectangular tracking detection cells 7T1, 7T2, 7T3, and 7T4
are formed. These detection cells are placed in a symmetrical
arrangement with respect to the y axis. Light which is emitted from
the emission point 1a or 1a' of the light source 1 travels along a
direction parallel to the x axis, within a plane which contains the
x axis and is perpendicular to the plane of FIGS. 9A and 9B, so as
to be reflected by the reflection mirror 10 in the optical axis
direction (i.e., a direction which extends through the point 90 or
90' and perpendicularly to the plane of FIGS. 9A and 9B).
[0138] Out of the light (incident light 80) entering the hologram
surface 2a, +1.sup.st order diffracted light which has been
diffracted at the strip regions 21aB and 21aF in the region 21a and
the strip regions 21bB and 21bF in the region 21b located in the
first quadrant is respectively converged at light spots 81aBS and
81aFS and light spots 81bBS and 81bFS, each of which lies astride
the border between the detection cells F2c and F1d. On the other
hand, -1.sup.st order diffracted light is converged at light spots
81aBS' and 81aFS' and light spots 81bBS' and 81bFS', which fit
within the detection cell 7T1.
[0139] Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 22aB and 22aF in the region 22a
located in the second quadrant is respectively converged at light
spots 82aBS and 82aFS, each of which lies astride the border
between the detection cells F1a and F2b. On the other hand,
-1.sup.st order diffracted light is converged at light spots 82aBS'
and 82aFS', which fit within the detection cell 7T3. Furthermore,
+1.sup.st order diffracted light which has been diffracted at the
strip regions 22bB and 22bF in the region 22b located in the second
quadrant is respectively converged at light spots 82bBS and 82bFS,
each of which lies astride the border between the detection cells
F1c and F2d. On the other hand, -1.sup.st order diffracted light is
converged at light spots 82bBS' and 82bFS', which fit within the
detection cell 7T2.
[0140] Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 23aB and 23aF in the region 23a
located in the third quadrant is respectively converged at light
spots 83aBS and 83aFS, each of which lies astride the border
between the detection cells F1c and F2d. On the other hand,
-1.sup.st order diffracted light is converged at light spots 83aBS'
and 83aFS', which fit within the detection cell 7T2. Furthermore,
+1.sup.st order diffracted light which has been diffracted at the
strip regions 23bB and 23bF in the region 23b located in the third
quadrant is respectively converged at light spots 83bBS and 83bFS,
each of which lies astride the border between the detection cells
F1a and F2b. On the other hand, -1.sup.st order diffracted light is
converged at light spots 83bBS' and 83bFS', which fit within the
detection cell 7T3.
[0141] Moreover, 1.sup.st order diffracted light which has been
diffracted at the strip regions 24aB and 24aF in the region 24a and
the strip regions 24bB and 24bF in the region 24b located in the
fourth quadrant is respectively converged at light spots 84aBS and
84aFS and light spots 84bBS and 84bFS, each of which lies astride
the border between the detection cells F2a and F1b. On the other
hand, -1.sup.st order diffracted light is converged at light spots
84aBS' and 84aFS' and light spots 84bBS' and 84bFS', which fit
within the detection cell 7T4.
[0142] Some of the detection cells are electrically interconnected
so that eight signals F1, F2, T1, T2, T3, T4, T5, and T6 are
obtained as follows.
F1=signal obtained from the detection cell F1a
+signal obtained from the detection cell F1b
+signal obtained from the detection cell F1c
+signal obtained from the detection cell F1d
F2=signal obtained from the detection cell F2a
+signal obtained from the detection cell F2b
+signal obtained from the detection cell F2c
+signal obtained from the detection cell F2d
T1=signal obtained from the detection cell 7T1
T2=signal obtained from the detection cell 7T2
T3=signal obtained from the detection cell 7T3
T4=signal obtained from the detection cell 7T4
T5=signal obtained from the detection cell 7T5
T5=signal obtained from the detection cell 7T6
[0143] In FIG. 9B, the emission point of the light source 1 is at
the point 1a', where the light source emits light of the wavelength
.lamda.2, which is greater than .lamda.1. Therefore, the hologram
has a greater diffraction angle for the light emitted from the
emission point 1a', thus causing changes in the positions of the
light spots. The detection cells 7T1, 7T2, 7T3, and 7T4 receive
light spots similar to those shown in FIG. 9A. The light spots
81aBS and 81aFS and the light spots 84aBS and 84aFS fit within the
detection cells 7T5 and 7T6, respectively, whereas the light spots
81bBS and 81bFS and the light spots 84bBS and 84bFS fall outside
the detection cells. On the other hand, the positions of the light
spots 82aBS, 82aFS, 83bBS, 83bFS, 82bBS, 82bFS, 83aBS, and 83aFS
are changed, but after all, these light spots are received by
detection cells in a similar manner to FIG. 9A.
[0144] The y axis shown in FIGS. 9A and 9B is parallel to a radial
direction of the optical disk substrate 6. A focus error signal FE
for the optical disk signal surface 6a, a tracking error signal TE1
for the optical disk corresponding to the wavelength .lamda.1, a
tracking error signal TE2 for the optical disk corresponding to the
wavelength .lamda.2, and a reproduction signal RF from the optical
disk signal surface 6a are detected based on eq. 4, eq. 5, eq. 6,
and eq. 7 below.
FE=F1-F2 (eq. 4)
TE1=.alpha.(T1-T4)+.beta.(T2-T3) (eq. 5)
TE2=(T1-T4)+.gamma.(T5-T6) (eq. 6)
RF=T1+T2+T3+T4 (eq. 7)
[0145] For example, eq. 5 is used for an optical disk such as a
DVD-RAM or a DVD-R/RW. In the case of an optical disk such as a
DVD-RAM, a and .beta. are prescribed so that a=1, .beta.=0. In the
case of an optical disk such as a DVD-R/RW, a and .beta. are
prescribed so that a=0, .beta.=1.
[0146] On the other hand, eq. 6 is used for an optical disk such as
a CD-R/RW. The signal (T1-T4) corresponds to the usual TE signal
being detected through a semicircular aperture, and is identical in
characteristics to the usual TE signal. The signal (T2-T3) is a TE
signal detected with some of the regions (22a and 23a) in the
aperture being swapped. Since the swapped regions (22a and 23a)
will not contain any .+-.1.sup.st order diffracted light for a
DVD-R/RW disk, no deterioration in the TE sensitivity for a
DVD-R/RW disk will result. The swapping serves to cancel influences
such as: the influence of the eccentricity of the objective lens
along an optical disk radial direction; the influence of any tilt
of the optical disk substrate 6; and the influence exerted when a
light spot rests on a border between a recorded region and an
unrecorded region on the optical disk signal surface 6a.
[0147] On the other hand, the signal (T5-T6) which is obtained in
the case shown in FIG. 9B is a difference signal detected by
extracting only some of the regions (21a and 24a) within the
aperture, and is a difference signal in the regions which will not
contain any .+-.1.sup.st order diffracted light for a CD-R/RW disk.
Thus, the signal (T5-T6) has a zero TE sensitivity for a CD-R/RW
disk, and as compared to the usual TE signal (i.e., the signal
(T1-T4)), the signal (T5-T6) has a quite different dependence on
influences such as: the influence of the eccentricity of the
objective lens along an optical disk radial direction; the
influence of any tilt of the optical disk substrate 6; and the
influence exerted when a light spot rests on a border between a
recorded region and an unrecorded region on the optical disk signal
surface 6a. Therefore, through the calculation as expressed by eq.
6, which also involves the signal (T1-T4), such influences can be
canceled without degrading the TE sensitivity. Although the focus
error signal FE of the case shown in FIG. 9B is detected through a
semicircular aperture, there is little disk-groove related
influences since the aperture constitutes one of the semicircles as
divided along an optical disk radial direction. Thus, substantially
equivalent characteristics to those attained by full circle
detection, which is a conventional detection technique, can be
obtained.
[0148] In the present embodiment, a distributed-type wavelength
plate 3 which is similar to that of Embodiment 2 is used.
Therefore, the counteracting effects against any birefringence of
the optical disk substrate 6 are quite similar to those provided in
Embodiment 2. Furthermore, by allowing the calculation result of
eq. 5 or eq. 6 to be used as a tracking error signal, the present
embodiment enables tracking control free of off-tracking, even in
the case where the objective lens has an eccentricity along an
optical disk radial direction, where the optical disk substrate 6
is tilted, or where a light spot rests on a border between a
recorded region and an unrecorded region on the optical disk
recording surface 6a and is susceptible to the influence from an
adjoining track.
Embodiment 4
[0149] Next, with reference to FIGS. 10 and 11, an optical disk
apparatus according to a fourth embodiment of the present invention
will be described. Except for the pattern of the polarization
hologram surface 2a, the detection pattern on the photodetection
surface 9a, and the light distribution thereupon, the optical disk
apparatus of the present embodiment is identical in construction to
the optical disk apparatus of Embodiment 2. Therefore, any
descriptions which would be similar to those in Embodiment 2 will
be omitted.
[0150] FIG. 10 shows the structure of the hologram surface 2a of
the polarization hologram substrate 2 in the present embodiment.
FIGS. 11A and 11B show the photodetection surface in the present
embodiment. Both FIG. 10 and FIGS. 91A and 11B are plan views
showing the hologram surface and the photodetection surface,
respectively, as viewed from the side of the optical disk 6.
Specifically, FIG. 11A illustrates light spots of returned light of
first laser light emitted from a first emission point 1a, whereas
FIG. 11B illustrates light spots of returned light of second laser
light emitted from a second emission point 1a'.
[0151] As shown in FIG. 10, the hologram surface 2a is divided into
four portions (quadrants) by two lines (X and Y axes) which
perpendicularly intersect each other at an intersection 20 between
the hologram surface 2a and the optical axis 7. The Y axis
corresponds to a radial direction. The first quadrant is divided
into three regions 21a, 21b, and 21c; the second quadrant only has
one region 22b; the third quadrant only has one region 23b; and the
fourth quadrant is divided into three regions 24a, 24b, and 24c.
Although not explicitly shown in FIG. 10, each region is further
divided into strip-like regions with the suffix B and strip-like
regions with the suffix F (e.g., regions 21aB and regions 21aF),
each strip region extending along the X direction, in a manner
similar to FIG. 6 of Embodiment 2. Portions of the regions 21a and
24a which lie within the aperture (denoted by a circle 80) are some
of the regions which do not contain any .+-.1.sup.st order
diffracted light from a disk groove on a CD-R/RW or the like.
Portions of the regions 21b and 24b which lie within the aperture
are some of the regions which do not contain any .+-.1.sup.st order
diffracted light from a disk groove on a DVD-R/RW or the like.
Consistently with earlier descriptions, the suffix B represents
+1.sup.st order diffracted light which is converged after the
detection surface, whereas the suffix F represents light which is
converged before the detection surface. For simplicity, FIGS. 11A
and 11B only show light spots corresponding to the suffix B.
[0152] FIGS. 11A and 11B show x and y axes, which perpendicularly
intersect each other at an intersection 90 (or 90') between the
detection surface 9a and the optical axis 7 (or 7'). The x and y
axes are parallel to the X and Y axes shown in FIG. 10,
respectively. In a region corresponding to the "-" side of the y
axis, strip-like focus detection cells F1a, F2a, F1b, F2b, F1c,
F2c, F1d, and F2d are formed, each of which extends along the y
axis, and detection cells 7T5 and 7T6 for tracking correction are
also formed. In a region corresponding to the "+" side of the y
axis, rectangular tracking detection cells 7T1, 7T2, 7T3, and 7T4
are formed. These detection cells are placed in a symmetrical
arrangement with respect to the y axis. Light which is emitted from
the emission point 1a or 1a' of the light source 1 travels along a
direction parallel to the x axis, within a plane which contains the
x axis and is perpendicular to the plane of FIGS. 11A and 11B, so
as to be reflected by the reflection mirror 10 in the optical axis
direction (i.e., a direction which extends through the point 90 or
90' and perpendicularly to the plane of FIGS. 11A and 11B).
[0153] Out of the light (incident light 80) entering the hologram
surface 2a, +1.sup.st order diffracted light which has been
diffracted at the strip regions 21aB and 21aF in the region 21a and
the strip regions 21cB and 21cF in the region 21c located in the
first quadrant is respectively converged at light spots 81aBS and
81aFS and light spots 81cBS and 81cFS, which fit within the
detection cell 7T5. On the other hand, -1.sup.st order diffracted
light is converged at light spots 81aBS' and 81aFS' and light spots
81cBS' and 81cFS', which fit within the detection cell 7T1.
Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 21bB and 21bF in the region 21b
located in the first quadrant is respectively converged at light
spots 81bBS and 81bFS, which fit within the detection cell 7T6. On
the other hand, -1.sup.st order diffracted light which has been
diffracted at the strip regions 21bB and 21bF in the region 21b
located in the first quadrant is respectively converged at light
spots 81bBS' and 81bFS', which fit within the detection cell
7T1.
[0154] Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 22bB and 22bF in the region 22b
located in the second quadrant is respectively converged at light
spots 82bBS and 82bFS, each of which lies astride the border
between the detection cells F1c and F2d. On the other hand,
-1.sup.st order diffracted light is converged at light spots 82bBS'
and 82bFS', which fit within the detection cell 7T2.
[0155] Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 23bB and 23bF in the region 23b
located in the third quadrant is respectively converged at light
spots 83bBS and 83bFS, each of which lies astride the border
between the detection cells F1a and F2b. On the other hand,
-1.sup.st order diffracted light is converged at light spots 83bBS'
and 83bFS', which fit within the detection cell 7T3.
[0156] Moreover, +1.sup.st order diffracted light which has been
diffracted at the strip regions 24aB and 24aF in the region 24a and
the strip regions 24cB and 24cF in the region 24c located in the
fourth quadrant is respectively converged at light spots 84aBS and
84aFS and light spots 84cBS and 84cFS, which fit within the
detection cell 7T6. On the other hand, -1.sup.st order diffracted
light is converged at light spots 84aBS' and 84aFS' and light spots
84cBS' and 84cFS', which fit within the detection cell 7T4.
Furthermore, +1.sup.st order diffracted light which has been
diffracted at the strip regions 24bB and 24bF in the region 24b
located in the fourth quadrant is respectively converged at light
spots 84bBS and 84bFS, which fit within the detection cell 7T5. On
the other hand, -1.sup.st order diffracted light is converged at
light spots 84bBS' and 84bFS', which fit within the detection cell
7T4.
[0157] Some of the detection cells are electrically interconnected
so that eight signals F1, F2, T1, T2, T3, T4, T5, and T6 are
obtained as follows.
F1=signal obtained from the detection cell F1a
+signal obtained from the detection cell F1b
+signal obtained from the detection cell F1c
+signal obtained from the detection cell F1d
=signal obtained from the detection cell F2a
+signal obtained from the detection cell F2b
+signal obtained from the detection cell F2c
+signal obtained from the detection cell F2d
T1=signal obtained from the detection cell 7T1
T2=signal obtained from the detection cell 7T2
T3=signal obtained from the detection cell 7T3
T4=signal obtained from the detection cell 7T4
T5=signal obtained from the detection cell 7T5
T6=signal obtained from the detection cell 7T6
[0158] In FIG. 11B, the emission point of the light source 1 is at
the point 1a', where the light source emits light of the wavelength
.lamda.2, which is greater than .lamda.1. Therefore, the hologram
has a greater diffraction angle for the light emitted from the
emission point 1a', thus causing changes in the positions of the
light spots. The detection cells 7T1, 7T2, 7T3, and 7T4 receive
light spots similar to those shown in FIG. 11A. The light spots
81aBS and 81aFS and the light spots 84aBS and 84aFS fit within the
detection cells 7T6 and 7T5, respectively, whereas the light spots
81bBS, 81bFS, 81cBS, and 81cFS and the light spots 84bBS, 84bFS,
84cBS, and 84cFS fall outside the detection cells. On the other
hand, the positions of the light spots 82bBS, 82bFS, 83bBS, and
83bFS are changed, but after all, these light spots are received by
detection cells in a similar manner to FIG. 11A.
[0159] Assuming that the y axis shown in FIGS. 11A and 11B is
parallel to a radial direction of the optical disk substrate 6, a
focus error signal FE for the optical disk signal surface 6a and a
reproduction signal RF from the optical disk signal surface 6a are
detected based on eq. 4 and eq. 7 above. On the other hand, a
tracking error signal TE1 for the optical disk corresponding to the
wavelength .lamda.1 and a tracking error signal TE2 for the optical
disk corresponding to the wavelength .lamda.2 are detected based on
eq. 8 and eq. 9 below.
TE1=.alpha.(T1+T2-T3-T4)+.beta.(T5-T6) (eq. 8)
TE2=(T1+T2-T3-T4)+.gamma.(T6-T5) (eq. 9)
[0160] For example, eq. 8 is used for an optical disk such as a
DVD-RAM or a DVD-R/RW. In the case of an optical disk such as a
DVD-RAM, a and .beta. are prescribed so that a=1, .beta.=0. In the
case of an optical disk such as a DVD-R/RW, a and .beta. are
prescribed so that .alpha.=0, .beta.=1. On the other hand, eq. 9 is
used for an optical disk such as a CD-R/RW. The signal
(T1+T2-T3-T4) corresponds to the usual TE signal.
[0161] The signal (T5-T6) in the case shown in FIG. 11A is a TE
signal detected with some of the regions (21b and 24b) in the
aperture being swapped. Since the swapped regions (21b and 24b)
will not contain any .+-.1.sup.st order diffracted light for a
DVD-R/RW disk, no deterioration in the TE sensitivity for a
DVD-R/RW disk will result. The swapping serves to cancel influences
such as: the influence of the eccentricity of the objective lens
along an optical disk radial direction; the influence of any tilt
of the optical disk substrate 6; and the influence exerted when a
light spot rests on a border between a recorded region and an
unrecorded region on the optical disk signal surface 6a. On the
other hand, the signal (T6-T5) which is obtained in the case shown
in FIG. 11A is a difference signal detected by extracting only some
of the regions (21a and 24a) within the aperture, and is a
difference signal in the regions which will not contain any
.+-.1.sup.st order diffracted light for a CD-R/RW disk. Thus, the
signal (T6-T5) has a zero TE sensitivity for a CD-R/RW disk, and as
compared to the usual TE signal (i.e., the signal (T1+T2-T3-T4)),
the signal (T6-T5) has a quite different dependence on influences
such as: the influence of the eccentricity of the objective lens
along an optical disk radial direction; the influence of any tilt
of the optical disk substrate 6; and the influence exerted when a
light spot rests on a border between a recorded region and an
unrecorded region on the optical disk signal surface 6a. Therefore,
through the calculation as expressed by eq. 9, which also involves
the signal (T1+T2-T3-T4), such influences can be canceled without
degrading the TE sensitivity. Although the focus error signal FE of
the case shown in FIGS. 11A and 11B is detected through a
semicircular aperture, there is little disk-groove related
influences since the aperture constitutes one of the semicircles as
divided along an optical disk radial direction. Thus, substantially
equivalent characteristics to those attained by full circle
detection, which is a conventional detection technique, can be
obtained.
[0162] In the present embodiment, a distributed-type wavelength
plate 3 which is similar to that of Embodiment 2 is used.
Therefore, the counteracting effects against any birefringence of
the optical disk substrate 6 are quite similar to those provided in
Embodiment 2. Furthermore, by allowing the calculation result of
eq. 8 or eq. 9 to be used as a tracking error signal, the present
embodiment enables tracking control free of off-tracking, even in
the case where the objective lens has an eccentricity along an
optical disk radial direction, where the optical disk substrate 6
is tilted, or where a light spot rests on a border between a
recorded region and an unrecorded region on the optical disk
recording surface 6a and is susceptible to the influence from an
adjoining track.
Embodiment 5
[0163] Referring to FIGS. 12 and 13, an optical disk apparatus
according to a fifth embodiment of the present invention will be
described. Hereinafter, like components will be denoted by like
reference numerals.
[0164] FIG. 12 is an essential structural diagram showing an
optical pickup of the optical disk apparatus according to the
present embodiment. The optical pickup includes a light source 101,
which incorporates a laser chip capable of emitting light of
different wavelengths. The light source 101 emits light of a
relatively short wavelength for DVDs and light of a relatively long
wavelength for CDs.
[0165] Although FIG. 12 illustrates both an optical information
medium 107 and an optical information medium 108, in reality, an
arbitrarily selected one of the optical information mediums 107 and
108 is to be mounted. Depending on the type of optical information
medium mounted, light of an appropriate wavelength is to be emitted
from the light source 101. Light (signal light or reproduction
light) which is reflected from the optical information medium 107
or 108 enters a photodetector 110, which is used in common for both
DVDs and CDs.
[0166] The optical path of light from the light source 101 to the
optical information medium 107 or 108 and the optical path of light
(signal light) being reflected from the optical information medium
107 or 108 and traveling toward the photodetector 110 are separated
by a prism having a polarization beam splitter film 103 formed on
its surface. Assuming that the linearly polarized light emitted
from the light source 101 is P-polarized light, the polarization
beam splitter film 103 is designed so as to allow P-polarized light
to be transmitted therethrough. The P-polarized light having been
transmitted through the polarization beam splitter film 103 is
transmitted through the wavelength plate 105, and thereafter
reflected from the optical information medium 107 or 108, so as to
be transmitted back through the wavelength plate 105 in the
opposite direction. The returned light (signal light), at entry
into the polarization beam splitter film 103, is in such a
polarization state that it contains a large amount of S-polarized
light components having a polarization axis which is substantially
perpendicular to the polarization axis of p-polarized light.
[0167] Since the polarization beam splitter film 103 reflects
S-polarized light, most of the signal light is reflected toward the
photodetector 110. This reflected light is diffracted by a hologram
109 so as to enter the photodetector 110.
[0168] FIG. 13A shows a planar structure of the wavelength plate
105, whereas FIG. 13B is a diagram illustrating how the light
traveling from the light source toward the optical information
medium 11 and the reflected light from the optical information
medium 11 are led through the wavelength plate 105. FIG. 13C is a
diagram illustrating exemplary polarizations conversion by the
wavelength plate 105.
[0169] As shown in FIG. 13A, the wavelength plate 105 is divided
into four regions. Regions (i.e., regions A or regions B) having
the same characteristics are formed at two symmetrical positions
with respect to the optical axis center. The two regions A have an
axis of optical anisotropy (optic axis) having an angle of
.theta..sub.1 with respect to the x axis direction. On the other
hand, the two regions B have an axis of optical anisotropy (optic
axis) having an angle of .theta..sub.2 with respect to the x axis
direction.
[0170] It is assumed that the linearly polarized light which enters
the wavelength plate 105 from the light source side has a direction
of polarization which coincides with the x axis. The angles
.theta..sub.1 and .theta..sub.2 are 45.degree.-.alpha. and
45.degree.+.alpha., respectively, with respect to the x axis
direction, where 0<.alpha..ltoreq.15.degree.. In accordance with
the region splitting scheme of the present embodiment, a portion of
the light from the light source 101 which travels through one of
the regions A of the wavelength plate 105 is converged by the lens
106, and thereafter reflected from the optical information medium
11; then, the reflected light travels through the other region A
which is at a symmetrical position with respect to the optical axis
center. On the other hand, a portion of the light which travels
through the one of the regions B is similarly reflected from the
optical information medium 11 to travel through the other region B
in the return path.
[0171] Assuming that the wavelength plate 105 has a birefringence
(difference in refractive index) of .DELTA.n, a thickness of d, and
a wavelength of .lamda., the wavelength plate 105 has a retardation
of 2.pi..DELTA.nd/.lamda.. If .alpha.=0, the regions A and the
regions B of the wavelength plate 105 will have the same optical
characteristics. In this case, if the retardation
(2.pi..DELTA.nd/.lamda.) of the wavelength plate 105 were set to a
value which is equal to .pi./2, the wavelength plate 105 would
serve the same function as that of a conventional 1/4 wavelength
plate. In other words, if linearly polarized light whose electric
field vector direction is parallel to the x axis direction entered
the wavelength plate 105, the linearly polarized light would be
converted into circularly polarized light for output. When the
light (circularly polarized light) reflected from the optical
information medium 107 or 108 travels back through the wavelength
plate in the opposite direction, the reflected light is converted
into linearly polarized light whose polarization direction
coincides with the y axis direction. In the present embodiment,
.alpha. is set to a value other than 0 to introduce a difference
between the action of the regions A and the action of the regions B
with respect to the same polarized light.
[0172] FIG. 13C shows polarization state conversion processes
realized by the wavelength plate 105. Since a is not 0, if linearly
polarized light I whose polarization direction coincides with the x
axis direction is led through the wavelength plate 105, the
linearly polarized light I is converted into elliptically polarized
light which is slightly more elongated than circularly polarized
light. Since the direction of the axis of optical anisotropy (optic
axis) of the region A is shifted from the direction of the axis of
optical anisotropy (optic axis) of the region B, the difference as
shown in FIG. 13C emerges between the elliptically polarized light
II obtained through the regions A and the elliptically polarized
light II obtained through the regions B.
[0173] In the case where the optical information medium 107 or 108
does not have birefringence, the light (signal light) which is
reflected from the optical information medium 107 or 108 is
elliptically polarized light III as shown in FIG. 13C. This
elliptically polarized light III is close to linearly polarized
light having a polarization axis which is perpendicular to the
polarization direction of the light in the forward path. If
.alpha.=0, the light (signal light) reflected from the optical
information medium 107 or 108 would be converted into linearly
polarized light.
[0174] On the other hand, in the case where the optical information
medium 107 or 108 has birefringence, polarized light III' as shown
in FIG. 13C may be obtained. For example, consider a case where the
light in the return path which has been led through the regions A
has substantially the same polarization state as that of the light
in the forward path which has exited the light source 101 and
entered one of the regions A. In this case, the light in the return
path will not be reflected from the polarization beam splitter 103
shown in FIG. 12, but instead return to the light source 101.
However, even in such a case, the light in the return path which
has been led through the regions B has a different polarization
state from that of the light in the return path which has been led
through the regions A. In other words, the light in the return path
which has been led through the region B is in an elliptically
polarized state containing S-polarized light components, which will
be reflected by the polarization beam splitter 103. As a result,
irrespective of the amount of birefringence of the optical
information medium 107 or 108, the signal light is prevented from
completely disappearing.
[0175] By using such a device, it becomes possible to realize an
optical system which provides a high playability for disks having
birefringence, even when adopting a so-called "polarization optical
system", which is an optical system having a high transmission
efficiency in the forward path and in the return path.
[0176] The reason why .alpha. is prescribed to be equal to or less
than 15.degree. in the present embodiment is that, if a were overly
increased, light comprising a mixture of extremely different
polarization states would be formed. Light comprising a mixture of
extremely different polarization states is difficult to be
converged by the lens 106 because merging two light components
having greatly different polarization states results in poor
coherence of light.
[0177] In the present embodiment, the directions of the two optic
axes are shifted from each other by a symmetrical angle .alpha.,
with respect to a direction which is at 45.degree. from the
polarization direction of incident light. In general, the
birefringence (if any) of an optical disk substrate is stronger in
one polarity than in the other. In this respect, an offset .delta.
may be introduced to the center (reference) direction between the
optic axes; that is, the optic axis of the regions A may be rotated
by 45.degree.+.delta.+.alpha. from the polarization direction of
incident light, whereas the optic axis of the regions B may be
rotated by 45.degree.+.delta.-.alpha. from the polarization
direction of incident light. In either type of regions, it is
preferable to satisfy -15.degree..ltoreq..delta..ltoreq.15.degree.
in order to obtain polarization states which are as orthogonal as
possible over the course of the back and forth trips of light.
[0178] Note that the number of optic axis directions assigned to
the respective regions of the distribution wavelength plate is not
limited to two, but may be three or more. The retardation does not
need to be 90.degree., but may be an integer multiple of
90.degree., or a value obtained by adding an offset to an integer
multiple of 90.degree.. For example, if the retardation of the
wavelength plate is prescribed to be a value which allows the
wavelength plate to function as a 1/4 wavelength plate with respect
to light for DVDs (wavelength: 650 nm), then there will be a
retardation which is about 650/800 of 1/4 wavelength with respect
to light for CDs (wavelength: 800 nm). However, by utilizing the
wavelength dependence of the reflectance, etc., of the material
used for the distribution wavelength plate, it becomes possible to
allow the distribution wavelength plate to function substantially
as a 1/4 wavelength plate with respect to either type of light.
[0179] For example, assuming that the wavelength plate has an
optical anisotropy of .DELTA.n.sub.1 with respect to light for DVDs
(wavelength .lamda..sub.1) and an optical anisotropy of
.DELTA.n.sub.2 with respect to light for CDs (wavelength
.lamda..sub.2), the aforementioned condition can be satisfied by
prescribing the optical parameters of the material of the
wavelength plate (which may be a liquid crystal layer in the
present embodiment) so that eq. 10 below holds true.
2.pi..DELTA.n.sub.1d/.lamda..sub.1=2.pi..DELTA.n.sub.2d/.lamda..sub.2=.p-
i./2 (eq. 10)
[0180] By prescribing such optical parameter values, the efficiency
in the return path can be maximized with respect to either
wavelength. Although the present embodiment illustrates an example
where the light source 101 emits light for DVDs and light for CDs,
the types of light to be emitted by the light source 101 are not
limited thereto. Alternatively, a light source which is capable of
emitting light of an even shorter wavelength, as used for Blu-ray
discs, for example, may be employed.
Embodiment 6
[0181] Referring to FIGS. 14A to 14C, a distribution wavelength
plate according to another embodiment of the present invention will
be described.
[0182] First, FIG. 14A will be referred to. A distribution
wavelength plate 131 shown in FIG. 14A includes a plurality of
alternating regions D.sub.3 and D.sub.4 each having a strip shape.
The regions D.sub.3 have a different optic axis direction from that
of the regions D.sub.4.
[0183] A distribution wavelength plate 132 shown in FIG. 14B
includes rows and columns (a checker pattern) of regions Ds and
D.sub.6. The regions D.sub.5 have a different optic axis direction
from that of the regions D.sub.6.
[0184] In the case where the distribution wavelength plate 105
shown in FIG. 13A is used, light which is transmitted through
either the regions A or the regions B will not be detected if the
optical disk substrate has about the same birefringence as that of
a 1/4 wavelength plate. In other words, information which is
contained in regions corresponding to a half of the cross section
of the light beam which is transmitted through the distribution
wavelength plate 105 is lost in this case. Since the regions whose
information is lost in this manner are located in diagonal
positions, the spatial frequency characteristics of pit images are
deteriorated. Stated otherwise, image reproducibility of the minute
pits present on the optical disk, as detected on the detector
surface, is deteriorated. As a result, although a sufficient signal
light amount can be obtained, the signal waveform may be distorted,
thus rendering the reproduction performance insufficient.
[0185] However, in the case where a distribution wavelength plate
as shown in FIG. 14A or 14B is employed, whose surface is divided
into a multitude of finer regions, the regions in which information
is lost are small and dispersed, whereby the reproduction
performance can be improved.
[0186] The region splitting scheme for the distribution wavelength
plate is not limited to those illustrated in FIG. 14A and FIG. 14B.
As long as a plurality of regions having different optic axis
directions are arranged in a two-dimensional array within the plane
of the wavelength plate, the shape and size of each region may be
arbitrary.
[0187] A wavelength plate 133 shown in FIG. 14C is divided into an
annular region D.sub.9 and an inner circular region. The circular
region is further divided into strip regions D.sub.7 and D.sub.8,
where the regions D.sub.7 have a different optic axis direction
from that of the regions D.sub.8. The optic axis directions of the
regions D.sub.7 and the regions D.sub.8 are set at, respectively,
45.degree.+.alpha. and 45.degree.+.alpha. with respect to the
polarization direction of incident light. The annular region
D.sub.9 is not divided, and has an optic axis direction which is at
45.degree. with respect to the polarization direction of incident
light.
[0188] The outer diameter (d.sub.2) of the annular region D.sub.9
corresponds to the aperture diameter of a lens having a high NA
value, which is used for optical disks of higher recording density,
e.g., DVDs. On the other hand, the inner diameter (d.sub.1) of the
annular region D.sub.9 corresponds to the aperture diameter of a
lens having a low NA value, which is used for optical disks of
lower recording density, e.g., CDs. By employing the
distributed-type wavelength plate 133 shown in FIG. 14C, good
reproduction characteristics are obtained in the case of using a
low-NA lens (i.e., in the case where a medium whose substrate has a
large birefringence, e.g., a CD, is used), whereas deterioration in
the spatial frequency characteristics is prevented in the case of
using a high-NA lens.
Embodiment 7
[0189] Referring to FIG. 15, an optical disk apparatus according to
another embodiment of the present invention will be described. FIG.
15 is an essential structural diagram showing an optical pickup of
the optical disk apparatus according to the present embodiment.
[0190] The optical pickup shown in FIG. 15 is capable of writing
data to a plurality of types of optical disks, and/or reading data
from a plurality of types of optical disks.
[0191] This device comprises a light source 141 capable of
producing a plurality of light beams of different wavelengths. The
light source 141 typically includes a plurality of semiconductor
laser chips, but may alternatively be composed of a single
semiconductor laser chip which is arranged to emit light beams of
different wavelengths.
[0192] The optical pickup comprises: objective lens 148 for
converging a light beam and producing a light spot on a signal
surface 139 or 149 of an optical disk; a polarization hologram 145
and a wavelength plate 146 disposed between the light source 101
and the objective lens 148; and a photodetector 143 for detecting
the intensity of the light beam reflected from the optical
disk.
[0193] The polarization hologram 145 is disposed in a portion
common to an optical path from the light source 101 to the
objective lens 148 and an optical path reflecting from the optical
disk signal surface 139 or 149 to the photodetector 143.
[0194] The photodetector 143 in the present embodiment is formed on
a semiconductor substrate such as a silicon chip. A laser chip
which emits two kinds of laser light, i.e., wavelength
.lamda..sub.1 and wavelength .lamda..sub.2, is mounted on the
substrate. The photodetector 143 is composed of a plurality of
photodiodes for converting light into electrical signals by
photoelectric effects. As for the laser light to be radiated by the
laser chip, the wavelength .lamda..sub.1 is about 650 nm, and the
wavelength .lamda..sub.2 is about 800 nm, for example. The laser
light of the wavelength .lamda..sub.1 may be used for DVDs, whereas
the laser light of the wavelength .lamda..sub.2 may be used for
CDs, for example.
[0195] The light of the wavelength .lamda..sub.1 which is emitted
from the laser chip is collimated by a collimating lens 144, and
thereafter transmitted through a polarization element 147. The
polarization element 147 is an optical element which integrates the
polarization hologram 145 and the wavelength plate 146. The
polarization element 147 is attached to a supporting member 137
together with the objective lens 148, and is driven by an actuator
138 integrally with the objective lens 148. In order to facilitate
the understanding of the function of the polarization element 147,
a case where the wavelength plate 146 is a conventional wavelength
plate which shows uniform retardation, rather than being a
distribution wavelength plate, will be described first.
[0196] The light (wavelength .lamda..sub.1) which has been
transmitted through the polarization element 147 is converged by
the objective lens 148 onto the optical disk signal surface 149,
and reflected therefrom. The reflected light again goes through the
objective lens 148, and is diffracted by the polarization element
147. The light which has been diffracted by the polarization
element 147 goes through the collimating lens 144 and enters the
photodetector 143. The photodetector 143 generates electrical
signals which are in accordance with changes in the light amount.
These electrical signals are a focusing control signal, a tracking
control signal, and an RF signal.
[0197] On the other hand, the light of the wavelength .lamda..sub.2
which has exited the laser chip is also collimated by the
collimating lens 144, and is transmitted through the polarization
element 147. The light which has been transmitted through the
polarization element 147 is converged by the objective lens 148
onto the signal surface 139 of an optical disk having a different
substrate thickness from that of the optical disk having the signal
surface 149, and reflected from the signal surface 139. The
reflected light again goes through the objective lens 148, and is
diffracted by the polarization element 147. The diffracted light
goes through the collimating lens 144 and enters the photodetector
143. The photodetector 143 generates electrical signals which are
in accordance with changes in the light amount. These electrical
signals are a focusing control signal, a tracking control signal,
and an RF signal.
[0198] FIGS. 16A and 16B are diagrams schematically showing
polarization dependence of the diffraction which occurs when a
conventional polarization element is employed as the polarization
element 147 in FIG. 15. In the following description, the optical
path of light traveling from the light source to the disk will be
referred to as a "forward path" of the optical system, and the
optical path of light reflected from the disk and traveling toward
the photodetector will be referred to as a "return path" of the
optical system.
[0199] FIG. 16A schematically shows cases where light of the
wavelength .lamda..sub.1 travels through the polarization element
147 in the forward and return paths. Light of the wavelength
.lamda..sub.1 which enters the polarization element 147 from the
light source side (i.e., the lower side in the figure) is, for
example, linearly polarized light having a polarization direction
which is parallel to the plane of FIG. 16A. Such light is able to
be transmitted through the polarization hologram 145 having a
periodic structure 111. The periodic structure 111 of the
polarization hologram 145 has polarization dependence such that,
when linearly polarized light (wavelength .lamda..sub.1) whose
polarization direction is parallel to the plane of FIG. 16A is
transmitted through the polarization hologram 145, a phase
difference of 2N.pi. (where N is an integer other than 0) occurs in
the transmitted light, depending on the incident position on the
periodic structure 111. The polarization hologram 145 is quite
different from a generally-used conventional polarization hologram
in that N is not zero. Since the periodic phase difference
occurring in the light transmitted through the polarization
hologram 145 is equal to an integer multiple of 2.pi. (i.e., any
optical path difference occurring in the polarization hologram 145
is equal to an integer multiple of the wavelength .lamda..sub.1),
according to the diffraction principle of light, a condition
stipulating absence of diffraction through the periodic structure
111 (a perfect transmission condition) is satisfied with respect to
light of the wavelength .lamda..sub.1.
[0200] The light which has thus been transmitted through the
polarization hologram 145 then travels through the wavelength plate
146. The wavelength plate 146 functions as a 5/4 wavelength plate
with respect to light of the wavelength .lamda..sub.1 (650 nm).
Therefore, linearly polarized light of the wavelength .lamda..sub.1
is converted by the wavelength plate 146 into circularly polarized
light.
[0201] The light (circularly polarized light) which has been
reflected back by the optical disk (not shown) is converted into
linearly polarized light by the wavelength plate 146. The
polarization direction (which is perpendicular to the plane of FIG.
16A) of this linearly polarized light is perpendicular to the
polarization direction of the light which has entered the
polarization hologram 145 from the light source side. To such
linearly polarized light, the periodic structure 111 of the
polarization hologram 145 periodically imparts a phase difference
of (2M+1).pi. (where M is an integer) depending on the incident
position. Therefore, the linearly polarized light is completely
diffracted, according to the diffraction principle of light. In
theory, assuming that a phase difference of .phi. is caused by the
periodic structure of the hologram, a transmittance T for the
0.sup.st order light traveling through the hologram is expressed by
eq. 11 below.
T=cos.sup.2(.phi./2) (eq. 11)
[0202] If the phase difference .phi. is (2M+1).pi., it follows that
T=0, that is, the perfect diffraction condition is satisfied.
[0203] Next, with reference to FIG. 16B, the operation of the
conventional polarization element 147 with respect to the light of
the wavelength .lamda..sub.2 will be described. As shown in FIG.
16B, when the light of the wavelength .lamda..sub.2 (linearly
polarized light whose polarization direction is parallel to the
plane of FIG. 16B) entering the polarization hologram 145 from the
light source is incident to the polarization element 147, a phase
difference of about 2N.pi..lamda..sub.1/.lamda..sub.2 is caused by
the periodic structure 111 of the polarization hologram 145. Since
N is not 0, the phase difference caused is not zero. Moreover,
assuming that .lamda..sub.1=650 nm and .lamda..sub.2=800 nm, the
value of N must be set quite high in order to make
N.lamda..sub.1/.lamda..sub.2 an integer. Therefore, the
polarization hologram 145 deviates from the perfect transmission
condition, so that light of the wavelength .lamda..sub.2 is
partially diffracted.
[0204] Assuming that .lamda..sub.1=650 nm (light of the wavelength
for DVDS); .lamda..sub.2=800 nm (light of the wavelength for CDs);
and N=1, the transmission efficiency of the non-diffracted light
(0.sup.th order light) is expressed by eq. 12 below.
cos.sup.2((2.pi..lamda..sub.1/.lamda..sub.2)/2)=cos.sup.2((2.pi..times.6-
50/800)/2)=69% (eq. 12)
[0205] From eq. 12, it can be seen that about 31% of the incident
light is diffracted by the polarization hologram 145.
[0206] The light of the wavelength .lamda..sub.2 which has thus
been transmitted through the polarization hologram 145 next travels
through the wavelength plate 146. Since the wavelength plate 146 is
a 5/4 wavelength plate with respect to light of the wavelength
.lamda..sub.1 (650 nm), the wavelength plate 146 functions
substantially as a 1 wavelength plate with respect to light of the
wavelength .lamda..sub.2 (800 nm). Therefore, the linearly
polarized light of the wavelength .lamda..sub.2 passes through the
wavelength plate 146 without being subjected to polarization
conversion by the wavelength plate 146.
[0207] On the other hand, since the light of the wavelength
.lamda..sub.2 returned from the optical disk is not subjected to
polarization conversion by the wavelength plate 146 any more than
in the forward path, a phase difference of
2N.pi..lamda..sub.1/.lamda..sub.2 is similarly caused by the
periodic structure 111 of the polarization hologram 145. Therefore,
between light of the wavelength .lamda..sub.1 and light of the
wavelength .lamda..sub.2, it would be impossible to set diffracted
light for both light to be 0, unless the light having the
relatively greater wavelength equals an integer multiple (twice,
three times, . . . etc.) of the wavelength of the other light.
[0208] Assuming that .lamda..sub.1=650 nm (light for DVDs);
.lamda..sub.2=800 nm (light for CDs); and M=1, the diffraction
efficiencies of the 1.sup.st order diffracted light are expressed
by eq. 13 below.
(2/.pi.).sup.2.times.cos.sup.2((.pi..lamda..sub.1/.lamda..sub.2)/2)=cos.-
sup.2((.pi..times.650/800)/2)=8.4% (eq. 13)
[0209] Any light other than the .+-.1.sup.st order diffracted light
is mostly transmitted through the diffraction grating as 0.sup.th
order light.
[0210] The above diffraction efficiency for the 1.sup.st order
diffracted light would be true if the disk substrate did not have
any birefringence so that the substrate would exert no polarization
influence. Note that, when the substrate of the CD has the highest
birefringence, i.e., a birefringence substantially equivalent to
that of a 1/4 wavelength plate, the linearly polarized light is in
a direction perpendicular to that when entering. Since the
diffraction efficiency of the 1.sup.st order diffracted light in
this case satisfies the perfect diffraction condition, the light
amount of the signal light will be more increased than decreased.
In other words, the amount of returned light may vary depending on
various polarization states, but is non-zero even in the worst
cases.
[0211] Light of the wavelength .lamda..sub.1 is used for optical
disks (such as DVDs) whose substrate thickness is so thin that
substrate birefringence is not likely to occur during the
production process but which require a short wavelength and thus
hinder high-power implementation. The use of such a polarization
element makes it possible to attain a high efficiency with respect
to light of the wavelength .lamda..sub.1. On the other hand, light
of the wavelength .lamda..sub.2 is used for optical disks (such as
CDs) for which a sufficient light amount can be secured with a
high-power laser (which in itself is relatively easy to produce)
notwithstanding a low efficiency but which have such a large
substrate thickness that products having a large amount of optical
birefringence are likely to be formed during the production
process. With respect to light of the wavelength .lamda..sub.2, the
use of such a polarization element ensures that the signal level
does not become zero even if the polarization state of the returned
light from the disk has been changed due to the birefringence of
the disk substrate, whereby stable signal reproduction and control
can be performed.
[0212] Moreover, the use of such a polarization element also makes
it possible to realize, in a compact construction, an optical
pickup which supports optical storage media of different standards.
The reason is that, while independent beam splitters corresponding
to different wavelengths have conventionally been used (from the
aforementioned perspective) to guide light from a disk to the
photodetector, the use of the aforementioned polarization element
realizes the same function with the use of a single hologram. As a
result, the optical path from the laser light source to the optical
storage medium (forward path) and the optical path from the optical
storage medium to the photodetector (return path) can be entirely
unified, whereby the number of elements in the optical system can
be reduced, and the optical system can be accommodated in a small
space.
[0213] According to the present embodiment, in a device having the
above-described structure, a distributed-type wavelength plate 146
is used in the place of the wavelength plate 146 having a uniform
retardation as shown in FIGS. 16A and 16B.
[0214] With respect to the polarization direction of the light from
the laser light source 141, the polarization hologram 145 does not
diffract light of either wavelength in the forward path of the
optical system. Therefore, transmission efficiency losses are
prevented, and the incident light is converted by the distribution
wavelength plate 146 into substantially circularly polarized light,
which is then converged onto the optical disk signal surface 139 or
149. By being reflected from the optical disk signal surface 139 or
149 and going again through the distribution wavelength plate 146
in the return path, light of either wavelength becomes light whose
main axis of polarization is substantially in a direction
perpendicular to the polarization direction in the forward path. By
being led through the polarization hologram 145, light of different
wavelengths can both be diffracted by the polarization hologram 145
with a high efficiency, so as to be led to the photodetector 143.
In this case, the amount of signal light depends on the
polarization diffraction direction components of the hologram.
Therefore, when an optical disk having a large birefringence is
used, in the very worst case, the signal light amount may become
zero if the conventional, uniform wavelength plate is employed.
According to the present embodiment, however, the use of the
distribution wavelength plate 146 provides adequate countermeasures
against the birefringence of the optical disk, while maintaining a
high efficiency in the forward path.
[0215] Through a thin film formation/processing procedure such as
vapor deposition, sputtering, or etching, a thin film structure may
be formed on the distribution wavelength plate 146. For example, as
shown in FIG. 17A, a transmittance filter 152 providing different
aperture sizes for light of different wavelengths may be formed on
the distribution wavelength plate 155. Moreover, as shown in FIG.
17B, for disks having different substrate thicknesses, light of one
of the wavelengths may be transmitted in the form of plane waves,
whereas light of the other wavelength may be diffused, and the
spherical aberration which occurs due to the different substrate
thicknesses may be corrected by a phase filter 153 formed on the
distribution wavelength plate 156. By adopting such a structure,
the optical pickup can be further downsized.
Embodiment 8
[0216] Next, referring to FIGS. 18A to 18D, one embodiment of a
method for producing a distribution wavelength plate which can be
suitably used in each of the above embodiments will be
described.
[0217] First, as shown in FIG. 18A, transparent substrates 161a and
161b on which transparent electrode films 162a and 162b (made of
ITO), respectively, are formed are prepared. An alignment material
is applied onto the transparent conductive films 162a and 162b,
thus forming liquid crystal alignment films 163a and 163b,
respectively. As the alignment material, a photo-alignable material
is used which, when irradiated with linearly-polarized ultraviolet
rays and subjected to an exposure, acquires an alignment in the
polarization direction.
[0218] Next, as shown in FIG. 18B, when forming regions having an
optic axis in the direction defined by the angle .theta..sub.1, the
transparent substrate 161a (or 161b) is irradiated with ultraviolet
rays while covering the other regions with a mask 164a, the
ultraviolet rays being linearly polarized in the direction defined
by the angle .theta..sub.1. On the other hand, when forming regions
having an optic axis in the direction defined by the angle
.theta..sub.2, the transparent substrate 161a (or 161b) is
irradiated with ultraviolet rays while covering the other regions
with a mask 164b, the ultraviolet rays being linearly polarized in
the direction defined by the angle .theta..sub.2.
[0219] Next, as shown in FIG. 18C, the transparent substrate 161a
and the transparent substrate 161b are placed so as to oppose each
other, and after the peripheries are attached by means of an
adhesive, a liquid crystal material 167 containing a UV-curing
resin is injected into the interior, through an aperture 166. Once
the liquid crystal material 167 is injected, the longer axis of
each liquid crystal molecule will be aligned in accordance with the
alignment directions of the liquid crystal alignment films 163a and
163b.
[0220] In order to further enhance the alignment uniformity of the
liquid crystal layer 168, it is preferable to apply a voltage
between the transparent electrode films 162a and 162b, thus
creating an electric field across the liquid crystal layer 168. In
the case where no such electric field is to be applied, the
transparent electrode films 162a and 162b may be omitted.
[0221] Next, as shown in FIG. 18D, the liquid crystal layer 168
with irradiated with unpolarized ultraviolet rays, thus curing the
liquid crystal layer 168.
[0222] Alignment restriction for a liquid crystal layer is
typically performed by rubbing the surface of the alignment film in
a predetermined direction, with a cloth on which fine fibers, e.g.,
polyamide type synthetic fibers, are formed. However, the present
embodiment adopts an optical alignment technique in order to obtain
different alignment directions within the same plane. By using such
an optical alignment technique, it is possible to obtain a desired
alignment distribution. Note that at least one of the transparent
conductive films 162a and 162b may be patterned in accordance with
the division pattern of the regions. By thus patterning the
transparent conductive film 162a and/or 162b, it becomes possible
to apply different voltages to different regions, thus facilitating
a region-by-region control of the alignment state.
[0223] According to the present invention, it is possible to obtain
a necessary amount of detected light, irrespective of the
birefringence of an optical disk substrate. Therefore, it is
possible to support various types of optical disks.
[0224] Moreover, an optical pickup according to the present
invention can be used for a plurality of different types of optical
storage medium, and therefore is suitably used for recording-type
optical disk apparatuses (e.g., CDs, DVDs, or Blu-ray discs) which
need to be reduced in size and cost.
[0225] While the present invention has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed invention may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention that
fall within the true spirit and scope of the invention.
[0226] This application is based on Japanese Patent Applications
No. 2004-127855 filed Apr. 23, 2004, No. 2004-133108 filed Apr. 28,
2004, and No. 2005-121245 filed Apr. 19, 2005, the entire contents
of which are hereby incorporated by reference.
* * * * *