U.S. patent application number 09/085229 was filed with the patent office on 2002-02-14 for an astigmatism generating device to remove comma aberration and spherical aberration.
Invention is credited to KUBOTA, YOSHIHISA, SUGIURA, SATOSHI, TACHIBANA, AKIHIRO.
Application Number | 20020018434 09/085229 |
Document ID | / |
Family ID | 15230011 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018434 |
Kind Code |
A1 |
SUGIURA, SATOSHI ; et
al. |
February 14, 2002 |
An Astigmatism Generating Device To Remove Comma Aberration And
Spherical Aberration
Abstract
An optical pickup device includes a photodetector divided into
four light-receiving elements by two dividing lines, an incident
optical system for irradiating a light beam onto an optical
recording medium, and a receiving optical system for guiding a
return light from the optical recording medium to the
photodetector, wherein the receiving optical system includes
astigmatism generating means for generating astigmatism having an
astigmatism axis at an angle of 45 degree with respect to the two
dividing lines.
Inventors: |
SUGIURA, SATOSHI; (SAITAMA,
JP) ; TACHIBANA, AKIHIRO; (SAITAMA, JP) ;
KUBOTA, YOSHIHISA; (SAITAMA, JP) |
Correspondence
Address: |
MORGAN, LEWIS & BOCKIUS
1800 M ST NW
WASHINGTON
DC
20036
|
Family ID: |
15230011 |
Appl. No.: |
09/085229 |
Filed: |
May 27, 1998 |
Current U.S.
Class: |
369/112.15 ;
G9B/7.071; G9B/7.113; G9B/7.117; G9B/7.124 |
Current CPC
Class: |
G11B 2007/0006 20130101;
G11B 7/0909 20130101; G11B 7/1365 20130101; G11B 7/1353 20130101;
G11B 7/1381 20130101 |
Class at
Publication: |
369/112.15 |
International
Class: |
G11B 007/135 |
Foreign Application Data
Date |
Code |
Application Number |
May 28, 1997 |
JP |
9-138779 |
Claims
what is claimed is:
1. An optical pickup device comprising: a photodetector divided
into four light-receiving elements by two dividing lines; an
incident optical system for irradiating a light beam onto an
optical recording medium; and a receiving optical system for
guiding a return light from the optical recording medium to the
photodetector, wherein the receiving optical system includes
astigmatism generating means for generating astigmatism having an
astigmatism axis at an angle of 45 degree with respect to the two
dividing lines.
2. The optical pickup device according to claim 1, wherein the
astigmatism generating means is a hologram optical element formed
of a transparent parallel flat plate having a diffraction relief
formed thereon for deflecting the return light toward the
photodetector and generating the astigmatism.
3. The optical pickup device according to claim 2, wherein the
hologram optical element removes coma aberration and spherical
aberration from the return light and generates the astigmatism.
4. The optical pickup device according to claim 2, wherein the
hologram optical element focuses the return light onto the four
light-receiving elements of the photodetector.
5. The optical pickup device according to claim 1, further
including a 1/4 wavelength plate, wherein the hologram optical
element is a polarization hologram optical element including a flat
plate formed of a transparent uniaxial crystal.
6. The optical pickup device according to claim 5, wherein the
polarization hologram optical element includes a flat plate formed
of an isotropic material having a refractive index equal to one of
an ordinary index and an extraordinary index of the transparent
uniaxial crystal, the flat plate of the polarization hologram
optical element being joined to the flat plate of the uniaxial
crystal.
7. The optical pickup device according to claim 1, further
including a 114 wavelength plate, wherein the hologram optical
element is a polarization hologram optical element of a transparent
flat plate having a diffraction relief formed thereon for
generating astigmatism and having a uniaxial crystal in concave
portions of the diffraction relief.
8. An optical pickup device comprising: a light source for
irradiating an incident light beam onto an optical recording
medium; a photodetector having four light-receiving elements for
receiving a return light beam from the optical recording medium;
and a light deflecting device positioned in a forward light path
between the light source and the optical recording medium and in a
return light path between the optical recording medium and the
photodetector, wherein the light deflecting device passes the
incident light beam and deflects the return light towards the
photodetector.
9. The optical pickup device according to claim 8, wherein the
light deflecting device is a hologram optical element having a
diffraction relief formed thereon.
10. The optical pickup device according to claim 9, wherein the
hologram optical element generates an astigmatism.
11. The optical pickup device according to claim 9, wherein the
hologram optical element is a transparent parallel flat plate.
12. The optical pickup device according to claim 10, wherein the
hologram optical element generates the astigmatism having an
astigmatism axis oriented at an angle of 45 degrees with respect to
dividing lines of the four light-receiving elements of the
photodetector.
13. The optical pickup device according to claim 8, further
including a 1/4 wavelength plate, wherein the deflecting device is
a polarization hologram optical element.
14. The optical pickup device according to claim 13, wherein the
polarization hologram optical element has a first transparent flat
portion formed of a uniaxial crystal.
15. The optical pickup device according to claim 14, wherein the
polarization hologram optical element has a second transparent flat
portion formed of the same uniaxial crystal as the transparent flat
portion.
16. The optical pickup device according to claim 15, wherein the
first transparent flat portion and the second transparent portion
are joined by a composition surface having a diffraction
pattern.
17. The optical pickup device according to claim 14, wherein the
polarization hologram optical element has a second transparent flat
portion formed of an isotropic material having a refractive index
equal to one of an ordinary index and an extraordinary index of the
uniaxial crystal.
18. The optical pickup device according to claim 17, wherein the
first transparent flat portion and the second transparent portion
are joined by a composition surface having a diffraction pattern.
Description
[0001] This application claims the benefit of Japanese Application
No. 9-138779, filed in Japan on May 28, 1997, which is hereby
incorporated by reference.
BACKGROUND OF THF INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical pickup device
for an optical recording and reproducing apparatus, such as an
optical disc player.
[0004] 2. Discussion of the Related Art
[0005] An optical disc player, which can reproduce recording
information from a recording media such as a laser disc (LD), a
compact disc (CD) or a digital video (or versatile) disc (DVD), is
well known. A so-called compatible disc player, which can reproduce
recording information from those different types of optical discs,
is also known.
[0006] In such a disc player, an optical pickup device has an
optical system for irradiating a light beam onto an optical disc
and reading a return light from the optical disc. Japanese patent
publication JP-B-2-8379 discloses an example of the optical pickup
device utilizing a diffraction grating formed on a transparent
parallel flat plate for beam deflection and guiding. As shown in
FIG. 11, the optical pickup device has a light beam from a light
source 1 focused on a pit train formed on an information recording
surface 5 of an optical disc by an objective lens 4. The return
light reflected from the information recording surface 5 again
passes through the objective lens 4, is deflected by a diffraction
grating formed on a parallel flat plate 25, and is guided and
focused onto four light-receiving elements of a photodetector
6.
[0007] In the optical pickup device described above, the four
light-receiving elements of the photodetector 6 are divided and
arranged parallel and perpendicular to the direction of the
information track of the disc. Accordingly, when astigmatism is
generated by the diffraction grating on the parallel flat plate 25,
the direction of an astigmatism axis coincides with a direction of
dividing lines of the four light-receiving elements, and a focus
error signal cannot be detected. When the four light-receiving
elements are inclined to a certain degree, a tracking error signal
may leak into the focus error signal. Namely, when the return light
is detected for controlling the position of the light beam relative
to an information track on the information recording surface, an
overlap of the detecting surface and the track direction, or a land
border, causes the leak of the detection signal.
SUMMARY OF THE INVENTION
[0008] Accordingly, the present invention is directed to an optical
pickup device that substantially obviates one or more of the
problems due to limitations and disadvantages of the related
art.
[0009] An object of the present invention is to use a hologram
optical element to improve the quality of the focus error
signal.
[0010] Additional features and advantages of the invention will be
set forth in the description which follows, and in part will be
apparent from the description, or may be learned by practice of the
invention. The objectives and other advantages of the invention
will be realized and attained by the structure particularly pointed
out in the written description and claims hereof as well as the
appended drawings.
[0011] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described, in one aspect of the present invention there is provided
an optical pickup device including a photodetector divided into
four light-receiving elements by two dividing lines, an incident
optical system for irradiating a light beam onto an optical
recording medium, and a receiving optical system for guiding a
return light from the optical recording medium to the
photodetector, wherein the receiving optical system includes
astigmatism generating means for generating astigmatism having an
astigmatism axis at an angle of 45 degree with respect to the two
dividing lines.
[0012] In another aspect of the present invention there is provided
an optical pickup device including a light source for irradiating
an incident light beam onto an optical recording medium, a
photodetector having four light-receiving elements for receiving a
return light beam from the optical recording medium, and a light
deflecting device positioned in a forward light path between the
light source and the optical recording medium and in a return light
path between the optical recording medium and the photodetector,
wherein the light deflecting device passes the incident light beam
and deflects the return light towards the photodetector.
[0013] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DFSCRIPTION OF THIE ATTACHED DRAWINGS
[0014] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
[0015] In the drawings:
[0016] FIG. 1 is a schematic perspective view of an optical system
of an optical pickup device using a hologram optical element
according to a first embodiment of the present invention;
[0017] FIG. 2 is a plan view of a grating pattern of the hologram
optical element in the optical pickup device according to the
present invention;
[0018] FIG. 3 is a flowchart explaining a wavefront design process
of the hologram optical element in the optical pickup device
according to the present invention;
[0019] FIG. 4 is a schematic view explaining a wavefront design of
the hologram optical element in the optical pickup device according
to the present invention;
[0020] FIG. 5 is a schematic view explaining a wavefront design of
the hologram optical element in the optical pickup device according
to the present invention;
[0021] FIG. 6 is a schematic view explaining a wavefront design of
the hologram optical element in the optical pickup device according
to the present invention;
[0022] FIGS. 7A-7C are plan views of four light-receiving elements
of a photodetector according to the present invention;
[0023] FIG. 8 is an electrical diagram of the photodetector
according to the embodiment of the present invention;
[0024] FIG. 9 is a schematic perspective view of an optical system
of an optical pickup device using a hologram optical element
according to a second embodiment of the present invention;
[0025] FIG. 10 is a view explaining the performance of a
polarization hologram optical element in the optical pickup device
of the present invention; and
[0026] FIG. 11 is a schematic perspective view of an optical system
of an optical pickup device using a conventional diffraction
grating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings.
[0028] FIG. 1 is a schematic view of an optical pickup device in an
optical recording and reproducing apparatus according to the
present invention.
[0029] In an optical system of a forward light path in which a
light beam is incident onto an optical disc, the light beam from a
TE mode laser diode 1 (a light source) passes through a hologram
optical element 2 (a deflecting device). The light beam is then
focused onto a pit train formed on an information recording surface
5 of the optical disc by an objective lens 4. The pit train runs in
a y direction (a track direction). The laser diode 1 emits the
light beam (a Gaussian beam) having a plane of vibration in the
direction parallel to a junction interface and having an elliptic
cross-sectional intensity distribution with a major longitudinal
axis extending in a direction perpendicular to the junction
interface of the laser diode 1. Depending on a position of the
laser diode 1, an elliptical beam spot having the major
longitudinal axis in the direction either parallel or perpendicular
to a track direction is formed on the information recording surface
5. The laser diode 1 is mounted on a base plate along with four
light-receiving elements of a photodetector 6.
[0030] In a receiving optical system of the return light path
(which is the same as the optical system of the incident beam), the
reflected light from the information recording surface 5 again
passes through the objective lens 4 and the hologram optical
element 2 and is deflected from the forward light path. The light
deflected by the hologram optical element 2 is focused onto the
four light-receiving elements of the photodetector 6.
[0031] In the first embodiment, the hologram optical element 2 that
generates the astigmatism is employed as the deflecting device.
FIG. 2 is a plan view of a grating pattern of the hologram optical
element 2 as seen from the z direction. The hologram optical
element 2 is a parallel flat plate of a transparent material having
a diffraction relief formed on its main surface, generating the
astigmatism.
[0032] The hologram optical element 2 is designed based on the
interference of the light beam from the laser diode 1 and the
reflected light at the four light-receiving elements. It may be
designed by a computer method shown in FIG. 3 using any number of
well-known techniques. The wavefront for the grating pattern is
determined by a high refractive index method or a light following
method using a phase function method.
[0033] In step S1, of FIG. 3 and schematically shown in FIG. 4, a
parallel flat plate 70 (refractive index n) having a thickness tl
is positioned in a light path of a light diverging from a point A
(wavelength .lambda.1), which corresponds to the laser diode.
Initial values of paraneters, i.e., the coordinates of the point A,
.lambda.1, t1 and n, are properly determined. With regard to the
light after passing through the parallel flat plate 70, the
wavefront at the coordinates of point B including spherical
aberration is calculated. The result of the calculation is stored.
The spherical aberration in the light caused by the parallel flat
plate 70 is removed by a correction in step S2 of FIG. 3. The
amount of the astigmatism generated by the parallel flat plate 70
is adjusted by changing the thickness tl.
[0034] In step S2, as shown in FIG. 5, the stored wavefront at the
point B is converged, The return light passes through the parallel
flat plate 70 having the thickness of tl (not shown) and is focused
onto the point A. Two parallel flat plates 71 each having a
thickness t2 and a refractive index n are positioned in the light
path of the return light (instead of the parallel flat plate 70 of
FIG. 4). The parallel flat plates 71 are positioned apart from each
other and inclined at an angle of .theta. and -.theta. with respect
to a plane perpendicular to a light axis in order to be a mirror
image with respect to the plane. The wavefront at a point C after
passing through the parallel flat plates 71 is calculated.
Parameters such as the coordinates of the point C, t2, .theta. and
-.theta. are properly selected.
[0035] As the light is converged from a point B, the wavefront
passing through the two parallel flat plates 71 has astigmatism and
spherical aberration, but no coma aberration. The spherical
aberration can be adjusted by changing the thickness t2 of the
parallel flat plates 71 and therefore the spherical aberration
caused in step SI can be removed. Thus, the wavefront at the point
C after passing through the parallel flat plates 71 and having a
certain amount of astigmatism, but no coma or spherical aberration,
is calculated and stored.
[0036] In step S6, as shown in FIG. 6, the stored wavefront at a
point C is again diverged. The wavefront at a point H that is
inclined at a certain angle (angle a) from the light axis is then
calculated. Here, the point to which the light is converged from
the stored wavefront at the point C corresponds to the position of
the four light-receiving elements. At the point H (the position of
the hologram optical element 2), the grating pattern of the
hologram optical element 2 can be designed based on the
interference of the calculated wavefront and the wavefront of the
light diverging from a point O(the position of the laser diode 1).
Parameters such as the coordinates of the point H and the point O
and the angle .alpha. are properly selected. The obtained
interference pattern at the point H is then stored and employed as
the grating pattern of the hologram optical element 2.
[0037] By forming the grating pattern obtained as described above
on a transparent substrate, the hologram optical element 2 shown in
FIG. 2 generates astigmatism without causing coma or spherical
aberration and functions as a lens for changing the focal length of
the light beam.
[0038] The hologram optical element 2 has a diffraction relief that
is so designed that the diffracted light forms a light spot near
the center of the four light-receiving elements of the
photodetector 6. When the light beam is in focus on the information
recording surface of the disc, a circular light spot is formed on
the four light-receiving elements as shown in FIG. 7A. When the
light beam is out of focus, an elliptic light spot is formed on the
four light-receiving elements in the direction of diagonal line of
the elements as shown in FIGS. 7B and 7C. Namely, the hologram
optical element 2 generates astigmatism.
[0039] The photodetector 6 has the four light-receiving elements
divided by two lines L1 and L2 crossing each other at right angles.
The light spot irradiated onto each of four light-receiving
elements is converted into electric signals (through photoelectric
conversion) and provided to a focus error detecting circuit 12, as
shown in FIG. 8. The focus error detecting circuit 12 produces a
focus error signal (FES) based on a signal from the photodetector 6
and provides the FES to an actuator drive circuit (not shown). The
actuator drive circuit supplies a focusing drive signal to an
actuator (not shown). The actuator moves the objective lens in the
direction of the optical axis in accordance with the focusing drive
signal.
[0040] As shown in FIG. 8, the focus error detecting circuit 12 is
connected to the photodetector 6. The photodetector 6 is divided
into four light-receiving elements DET1 to DET4 by the two dividing
lines L1 and L2 crossing each other at right angles. The four
elements DET1 to DET4 correspond to first through fourth quadrants
that are mutually independent. The photodetector 6 is positioned so
that one division line (L1 or L2) is parallel to the track
direction and the other division line is parallel to the radial
direction of the optical disc. The photoelectric conversion outputs
from the elements DET1 and DET3, which are located symmetrically
with respect to the center O of the light-receiving surface, are
added by an adder 22. Similarly, the photoelectric conversion
outputs from the elements DET2 and DET4 are added by an adder 21.
The output signals from the adder 21 and 22 are fed into a
differential amplifier 23. The differential amplifier 23 calculates
a difference between the output signals and outputs the
differential signal as the focus error signal FES.
[0041] As explained above, the focus error detection circuit 12
produces the focus error components by adding the output of the
four light-receiving elements of the photodetector 6 using the
adders 21 and 22 and calculating the differential signal of the
outputs thereof using the differential amplifier 23. When the light
beam is in focus, the intensity distribution is symmetrical with
respect to the center O, i.e., symmetrical with respect to the
track direction and the radial direction, and the circular light
spot shown in FIG. 7A is formed on the photodetector 6. In this
case, values obtained by adding the photoelectric conversion
outputs of the elements existing on the diagonal lines are equal to
each other and the focus error is 0. When the light beam is out of
focus, the elliptic light spot having an axis in the diagonal
direction is formed on the photodetector 6, as shown in FIGS. 7B
and 7C. Accordingly, values obtained by adding the photoelectric
conversion outputs of the diagonal pairs of the light-receiving
elements are different from each other.
[0042] In the optical system described above, when the light beam
traverses the pit train (track) due to an erroneous tracking
operation, the amount of the return light at a portion
corresponding to a shadow of the track fluctuates on the four
light-receiving elements of the photodetector 6. In the astigmatism
focusing method, the focus error signal is obtained from the
difference between outputs of the diagonal light-receiving
elements. Thus, if the shadow of the track is projected onto the
diagonal pairs of light-receiving elements, the tracking error
signal influences the focus error signal. Therefore, it is
necessary for the extending direction of the shadow of the pit
train (track) to coincide with the direction of the dividing line
(L1 or L2) of the four light-receiving elements. For the above
reason, the four light-receiving elements in FIG. 1 must have the
dividing lines L1 and L2 coincide with the directions of the x axis
and the y axis, respectively. This arrangement also coincides with
the direction of the division when the tracking servo is controlled
by phase difference (or time difference) methods. Accordingly, the
directions of the dividing lines of the four light-receiving
elements are determined depending on the direction of the pit train
(track) on the information recording surface. Thus, when the light
path of the incident light and that of the return light are
separated using the deflecting device, and the astigmatism method
is employed for the focus servo control, the direction of the
astigmatism axis is restricted in accordance with the direction of
the light-receiving elements.
[0043] In the astigmatism method using the four light-receiving
elements, as shown in FIG. 1, the direction of the astigmatism
generated for the focus servo control is limited to the direction
at an angle of 45 degrees in the x-y plane shown in FIG. 1.
[0044] In the embodiment described above, the TE mode laser diode
is used. However, a TM mode laser diode may similarly be used. With
the TM mode laser diode, the light intensity distribution on the
information recording surface extends in the radial direction even
if the plane of vibration of the incident beam is in the y
direction, and the direction of astigmatism is similarly limited to
a direction of 45 degrees in the x-y plane.
[0045] A second embodiment of the present invention will now be
explained. In the second embodiment, an optical pickup irradiates
the light beam on the optical recording medium via a 1/4 wavelength
plate and an objective lens. The optical pickup employs a
transparent flat polarization hologram optical element formed of a
uniaxial crystal having a diffraction relief for generating
astigmatism. With this, the structure of the compatible disc player
for the DVD and DVD-RAM can be simplified, miniaturized, and its
cost can be reduced.
[0046] FIG. 9 is a schematic diagram of the optical pickup using
the polarization hologram optical element in the optical recording
and reproducing apparatus. In the light path of the incident beam,
a light beam from a laser diode 1 is focused onto a pit train on an
information recording surface 5 of an optical disc by an objective
lens 4 via a polarization hologram optical element 2 (a deflecting
device) and a 1/4 wavelength plate 3. The incident light beam has a
plane of vibration in the x direction, which is parallel to a
junction interface and an emitting surface (cleavage plane) of the
laser diode 1. Namely, the laser diode 1 is arranged so that the
plane of vibration exists in the x direction. The incident light
beam has an elliptic Gaussian distribution with a minor axis
oriented in the x direction and a major longitudinal axis oriented
in a y direction. As shown in FIGS. 9 and 10, the incident light
beam passes through the polarization hologram optical element 2 and
is converted from linear polarization into circular polarization by
the 1/4 wavelength plate 3. The light beam is then focused onto the
information recording surface 5 by the objective lens 4.
[0047] In a receiving optical system of a return light path, the
light beam having the circular polarization has been reflected and
diffracted by the information recording surface 5 and again passes
through the objective lens 4. The light beam of the circular
polarization is then converted by the 114 wavelength plate 3 into a
linear polarization beam having a phase difference of 90 degrees
with respect to the incident light beam. The plane of vibration is
also rotated to the y direction. The light beam having the linear
polarization is diffracted by the polarization hologram optical
element 2 and is thus separated from the incident light path. As
mentioned above, the reflected light from the information recording
surface 5 is deflected by the polarization hologram optical element
2 via the objective lens 4 and the 1/4 wavelength plate 3 by which
the plane of vibration is inclined at an angle of 90 degrees from
the incident light beam. The deflected light beam is then focused
onto the four light-receiving elements of the photodetector 6. As
shown in FIGS. 9 and 10, the incident light beam passes through the
polarization hologram optical element 2, which produces astigmatism
when the plane of vibration is oriented in the x direction. When
the plane of vibration is oriented in the y direction, the incident
light beam is diffracted by the polarization hologram optical
element 2.
[0048] As shown in FIG. 10, the polarization hologram optical
element 2 in the second embodiment includes a first transparent
portion 11 formed of a transparent uniaxial crystal and a second
transparent portion 12 having a refractive index substantially
identical to an ordinary index n.sub.o or an extraordinary index n,
of the uniaxial crystal. The first and the second transparent
portions II and 12 are joined via a composition surface 13 having a
diffraction relief formed thereon. The polarization hologram
optical element 2 is a flat plate having parallel surfaces on both
sides. The diffraction grating pattern for generating the
astigmatism is formed on the composition surface 13 by the method
described in the description of the first embodiment.
[0049] The angle of the optical crystal axis with respect to the
optical axis of the incident light, and the ordinary index n.sub.o
of the uniaxial crystal for the first transparent portion 11 are
properly selected. The material of the second transparent portion
12 is also selected to have the same refractive index as the
ordinary index n.sub.u, of the uniaxial crystal. Thus, the
polarization hologram optical element 2 performs different
functions depending on the polarization of the incident light
beam.
[0050] As shown in FIG. 10, for example, the first transparent
portion 11 of the polarization hologram optical element 2 is formed
of a negative (i e., n.sub.o>n.sub.c) uniaxial optical crystal.
The second transparent portion 12 is formed of the same material as
the first transparent portion 11, having the crystal axis in the
direction of the optical axis and joined to the first transparent
portion 11 via the composition surface 13. When the optical axis of
the first transparent portion 11 is not parallel to the optical
axis of the incident light, but is perpendicular to the plane of
the paper, for example, the light beam having the plane of
vibration parallel to the plane of the paper is an ordinary ray.
The refractive index of the first transparent portion 11 is
n.sub.o. Since the refractive index of the second transparent
portion 12 is also n.sub.o, the polarization hologram optical
element 2 functions as a transparent parallel flat plate having the
refractive index n, as a whole.
[0051] The return light has its plane of vibration perpendicular to
the plane of the paper because the light passes through the 114
wavelength plate twice. In the second transparent portion 12, the
return light is the ordinary ray, and therefore the refractive
index is n.sub.o, same as the incident light. In the first
transparent portion 11, however, the light behaves as the
extraordinary ray, and the refractive index is n.sub.e.
Accordingly, for the return light, the polarization hologram
optical element 2 functions as a diffraction grating in which the
diffraction relief is formed as a borderline.
[0052] When the light beam (the arrow pointing right) having the
plane of vibration parallel to the plane of the paper is incident
on the optical disc via the polarization hologram optical element 2
(the parallel flat plate), the return light (the arrow pointing
left) reflected from the optical disc passes through the
polarization hologram optical element 2 and is detected by the
photodetector 6. In the polarization hologram optical element 2,
the first and second transparent portions 11 and 12 are joined so
that their optical crystal axes cross at right angles. Thus, the
difference in the refractive index between the first and second
transparent portions 11 and 12 can be maximized. Furthermore, the
deflecting angle of the return light can be adjusted by changing
the angle of the composition surface 13 with respect to the optical
axis, which is perpendicular to the optical axis in the second
embodiment.
[0053] In the second embodiment, the refractive index of the second
transparent portion 12 is substantially identical to the ordinary
index n, of the first transparent portion 11. However, even if the
refractive index of the second transparent portion 12 is
substantially identical to the extraordinary index n.sub.e of the
first transparent portion 11, the polarization hologram optical
element 2 functions as a parallel flat plate and a hologram for the
incident light and the return light, respectively.
[0054] In the second embodiment, the first and the second
transparent portions 11 and 12 may be formed of different uniaxial
crystal materials selected from among a variety of materials.
Furthermore, in addition to the negative uniaxial crystal, a
positive uniaxial material may also be used.
[0055] Both of the first and the second transparent portions 11 and
12 are not necessarily formed of anisotropic materials. Namely, it
is also permissible to form the first transparent portion 11 of a
uniaxial crystal and the second transparent portion of an isotropic
material, and vice versa
[0056] The second transparent portion 12 may be formed of an
isotropic material such as the optical glass having a refractive
index n.sub.g equal to the ordinary index no of the first
transparent portion 11 of the negative uniaxial crystal. The
polarization hologram optical element 2 with such a structure
functions as a transparent parallel flat plate having the
refractive index n.sub.o (because n.sub.o=n.sub.g) for the incident
light beam. For the return light, on the other hand, the
polarization hologram optical element 2 functions as a diffraction
grating because n.sub.en.sub.g.
[0057] The polarization hologram optical element 2 may be formed as
a single flat plate of a transparent uniaxial crystal without
combining two portions. The polarization hologram optical element
may also be formed as a single flat plate having a diffraction
relief for generating astigmatism on at least one main surface and
having a uniaxial crystal filled in a concave portion of the
diffraction relief.
[0058] In the embodiments described above, the return light is
separated from the incident light path by the hologram optical
element 2. However, other deflecting devices, such as a
polarization beam splitter, may also be used instead of the
hologram optical element 2. With such a structure, the return light
from the optical disc is reflected by the polarization beam
splitter at a right angle in the x direction and advances toward
the photodetector 6 that is positioned so that the four
light-receiving elements are located in the y-z plane in FIG. 1.
The direction of the dividing lines of the light-receiving elements
and the direction of the astigmatism axis are at an angle of 45
degrees with respect to the y axis or the z axis. Between the
polarization beam splitter and the photodetector 6, an astigmatism
generating device such as a convex cylindrical lens is inserted for
generating astigmatism having an axis at an angle of 45
degrees.
[0059] In the embodiments described above, the optical pickup of a
finite conjugate type system is described. However, the optical
pickup of an infinite conjugate type system may also be used in
which the objective lens, the hologram optical element 2, and the
1/4 wavelength plate can be controlled as an integral member and a
collimating lens is inserted in front of the light source.
[0060] According to the present invention, because a deflecting
device that generates astigmatism is employed, the efficiency of
the light being utilized is improved and the number of optical
parts can be reduced. The efficiency of the light being utilized
can be further improved by using a polarization element as the
deflecting device.
[0061] It will be apparent to those skilled in the art that various
modifications and variations can be made in the optical pickup
device of the present invention without departing from the spirit
or scope of the invention. Thus, it is intended that the present
invention cover the modifications and variations of this invention
provided they come within the scope of the appended claims and
their equivalents.
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