U.S. patent application number 11/979131 was filed with the patent office on 2008-04-24 for optical pickup apparatus and optical disc apparatus using same.
Invention is credited to Toshimasa Kamisada, Tomoto Kawamura, Yasuo Kitada, Yoshiro Konishi, Kazuyoshi Yamazaki.
Application Number | 20080094976 11/979131 |
Document ID | / |
Family ID | 39317049 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080094976 |
Kind Code |
A1 |
Kamisada; Toshimasa ; et
al. |
April 24, 2008 |
Optical pickup apparatus and optical disc apparatus using same
Abstract
A method of detecting a servo signal of an optical disc having a
plurality of recording layers includes focusing and irradiating a
laser light onto the optical disc, dividing a laser light reflected
from the optical disc into a plurality of reflected laser lights by
a division element, wherein each of said reflected laser lights
having a different outgoing direction, and irradiating the
reflected laser lights onto a plurality of light receiving parts of
a light detector. A focus error signal is detected under a knife
edge method and a tracking error signal is detected.
Inventors: |
Kamisada; Toshimasa;
(Yokohama, JP) ; Kitada; Yasuo; (Odawara, JP)
; Yamazaki; Kazuyoshi; (Kawasaki, JP) ; Kawamura;
Tomoto; (Tokyo, JP) ; Konishi; Yoshiro;
(Yokohama, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
39317049 |
Appl. No.: |
11/979131 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11680705 |
Mar 1, 2007 |
|
|
|
11979131 |
|
|
|
|
Current U.S.
Class: |
369/53.15 ;
G9B/7.135 |
Current CPC
Class: |
G11B 7/094 20130101;
G11B 7/1353 20130101; G11B 7/0901 20130101; G11B 2007/0013
20130101; G11B 7/0943 20130101; G11B 7/1381 20130101; G11B 7/131
20130101; G11B 7/0906 20130101; G11B 7/133 20130101 |
Class at
Publication: |
369/53.15 |
International
Class: |
G11B 5/58 20060101
G11B005/58 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2006 |
JP |
2006-283245 |
Oct 18, 2006 |
JP |
2006-283248 |
Claims
1. A method of detecting a servo signal of an optical disc having a
plurality of recording layers, comprising the steps of: focusing
and irradiating a laser light onto said optical disc; dividing a
laser light reflected from said optical disc into a plurality of
reflected laser lights by a division element, each of said
reflected laser lights having a different outgoing direction, and
then irradiating said reflected laser lights onto a plurality of
light receiving parts of a light detector; detecting a focus error
signal under a knife edge method using reflected laser lights
passed through regions not including a center of a light flux among
said reflected laser lights passed through said division element
and irradiated onto a plurality of light receiving parts for
detecting a focus error signal of said light detector; and
detecting a tracking error signal using reflected laser lights
passed through regions not including a center of said light flux
and irradiated onto a plurality of light receiving parts for
detecting a tracking error signal of said light detector.
2. The method of detecting a servo signal according to claim 1,
wherein said light receiving parts and regions of said division
element are arranged so that when a laser light is focused into an
desired recording layer among said plurality of recording layers,
laser lights reflected from other recording layers are not
irradiated onto said plurality of said light receiving parts for
detecting said focus error signal and said tracking error signal.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 11/680,705, filed Mar. 1, 2007, the contents of which are
incorporated herein by reference. This application relates to U.S.
application Ser. Nos. ______, ______, and ______, filed Oct. 30,
2007, which are divisional applications of U.S. Ser. No.
11/680,705, filed Mar. 1, 2007.
INCORPORATION BY REFERENCE
[0002] The present application claims priorities from Japanese
applications JP2006-283248 filed on Oct. 18, 2006, JP2006-283245
filed on Oct. 18, 2006, the contents of which are hereby
incorporated by reference into this application.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to an optical disc apparatus
for recording information on or reproducing information from an
optical disc, and to an optical pickup apparatus used for the same.
More particularly, the present invention relates to an optical disc
apparatus for recording information on or reproducing information
from an optical disc that has a plurality of laminated information
recording layers, and to an optical pickup apparatus used for the
same.
[0004] A technology of a multilayer optical disc having laminated
information recording layers has been studied as a method of
increasing the storage capacity of the optical disc. In the
standard of a DVD (Digital Versatile Disc), BD (Blue-ray Disc) and
HD-DVD (High Density Digital Versatile Disc), a two-layer optical
disc is commercialized in which two information recording layers
are laminated at an interval of about 20 to 55 .mu.m. In addition,
a three- or more-layer optical disc has also been studied as a
technology of achieving a larger capacity.
[0005] When recording information on or reproducing information
from the multilayer optical disc, it is necessary to eliminate as
much as possible the offset of a servo signal such as a focus error
signal or a tracking error signal caused by a stray light from
other layer.
[0006] A method of eliminating the effect due to the stray light
from other layer is described, for example, in an article of
"Journal of Institute of Electronics Information and Communication
Engineers" CPM2005-149 (2005-10), which describes about the
placement of a tracking photodetector in a region free of a stray
light from other layer.
[0007] However, the above article (CPM2005-149(2005-10)) does not
describe about the effect that the stray light has on the focus
error signal.
[0008] Furthermore, in the above article (CPM2005-149(2005-10)), it
is required to dispose a light receiving part for the tracking
error signal outside the stray light from other layer that occurs
around the light receiving part for the focus error signal, thus
the size of a light detector being increased.
[0009] A background art of the optical pickup apparatus is proposed
in a Japanese Laid-open Patent Application JP-A-9-223321. In the
Laid-open Patent Application JP-A-9-223321, PROBLEM TO BE SOLVED
reads as follows: To provide an optical information reproducing
apparatus that can be simplified by reducing the number of optical
parts, and to provide a method of adjusting the optical information
reproducing apparatus that enables the adjustment of a tracking
error signal in accordance with the characteristic of an optical
disc. SOLUTION reads as follows: The optical information
reproducing apparatus comprises: an optical pickup having an
objective lens for irradiating the optical disc with light; a first
dividing means for dividing a light spot of a light emitted from
the optical disc substantially perpendicularly to the direction
equivalent to a track to form a light spot on an end region and a
light spot on a middle region relative to the center of the light
spot; a second dividing means for further dividing the light spots
on the end region and middle region in substantially parallel to
the direction equivalent to the track of the optical disc; a light
receiving element having a plurality of light receiving cells for
receiving the light divided by the first and second dividing means;
a light spot displacement signal detecting means for computing the
outputs of the light receiving cells that receive the light on the
middle region divided by the second dividing means to detect the
relative displacement of the light spots on the light receiving
element; a tracking error generating means for computing the
outputs of the light receiving cells that receive the light on the
middle region divided by the second dividing means to detect a
relative displacement between the track and objective lens; an
offset correction means for correcting the offset of the tracking
error signal by the computing the output signal of the light spot
displacement signal detecting means and the output signal of the
tracking error generating means; an objective lens driving device
for driving the objective lens in the direction across the track of
the optical disc; a tracking control means for drive-controlling
the objective lens driving device; and a switching means for
switching the input of the tracking control means to the output of
the light spot displacement signal detecting means during an
access, and for switching the input of the tracking control means
to the output of the tracking error generating means via the offset
correction means during reproducing of the information of the
optical disc.
SUMMARY OF THE INVENTION
[0010] In the optical pickup apparatus, generally, in order to
correctly irradiate a spot on a given record track in the optical
disc, an objective lens is displaced in the focusing direction
through the detection of a focus error signal, thus the objective
lens being adjusted in the focus direction. Furthermore, the
objective lens is displaced in the radial direction of a disc shape
recording medium through the detection of a tracking error signal,
thus the tracking adjustment is performed. These signals allow the
objective lens to be position-controlled.
[0011] While a push pull method is known as a tracking error signal
detection method of the above error signal detections, it has a
problem that a direct current fluctuation (referred to as a DC
offset hereinafter) is prone to occur. Therefore, a differential
push pull method is widely used that is capable of reducing the DC
offset.
[0012] The differential push pull method divides a light flux into
a main light flux and a sub light flux through a diffraction
grating and reduces the DC offset using a spot of the main light
flux and a spot of the sub light flux in the radial direction.
[0013] However, since the differential push pull method forms a
plurality of spots on the optical disc, light use efficiency of the
main light flux decreases. The main light flux not only generates a
focus error signal and a tracking error signal, but also has a
function of forming a record mark on the recording optical disc.
When performing recording on the recording optical disc, its
writing speed becomes faster the larger the light amount of the
main light flux on the disc is. Therefore, it is disadvantageous to
use the diffraction grating for an outward optical system from a
viewpoint of writing speed.
[0014] Therefore, in the above JP-A-9-223321, one spot is formed on
the disc, and its reflective light is divided into a plurality of
regions, thus inspecting a stable tracking error signal free of the
DC offset even if the objective lens is displaced in the tracking
direction. This structure has an advantage that the writing speed
can be increased without reducing the light use efficiency.
(referred to as a one-beam method hereinafter)
[0015] However, when the detector is divided into regions as in the
above JP-A-9-223321, a problem occurs in a recording type optical
disc, such as, for example, BD-RE or BD-R. In the recording type
optical disc, there exist a region where recording is not performed
(referred to as an unrecorded region hereinafter) and a region
where recording is already performed (referred to as a recorded
region hereinafter). When the region is divided as described in the
above JP-A-9-223321, it is impossible to reduce the offset of the
tracking error signal occurring at the boundary between the
unrecorded region and recorded region on the disc, posing a
problem.
[0016] It is an object of the present invention to provide an
optical pickup apparatus capable of obtaining an stable servo
signal and an optical disc apparatus equipped with the same.
[0017] In order to solve the above problems, the optical pickup
apparatus according to the present invention comprises: a light
source; an objective lens for focusing a light flux emitted from
the light source on the optical disc; a dividing element for
dividing the light flux reflected from the optical disc into a
plurality of light fluxes; a condenser lens for condensing the
light flux reflected from the optical disc; and a light detector
for receiving the light flux condensed by the condenser lens with a
plurality of light receiving parts to convert it into an electrical
signal. The dividing element has a first divided region disposed
almost on the center; a second divided region comprised of four
regions which are divided by a first dividing line and disposed
along the direction of the first dividing line to sandwich the
first divided region; and a third divided region comprised of four
regions which are divided by a second dividing line perpendicular
to the first dividing line and disposed along the direction of the
second dividing line to sandwich the first divided region. Each of
the first to third divided regions is structured such that when a
target information recording layer of the optical disc is brought
into focus, a reflective light flux from the target information
recording layer is focused on the light receiving parts of the
light detector, and a reflective light flux from a recording
reproducing layer other than the target information recording layer
is not irradiated onto the light receiving parts of the light
detector.
[0018] The present invention enables the one beam tracking method
to obtain a stable focus error signal and tracking error
signal.
[0019] Furthermore, the present invention improves the offset of
the tracking error signal occurring at the boundary between the
unrecorded region and recorded region, which is a problem of the
above one-beam method. More specifically, the present invention
provides an optical pickup apparatus, an optical information
reproducing apparatus or an optical information recording and
reproducing apparatus that uses a novel tracing error detecting
means capable of detecting a stable tracking error signal even if
there exists a boundary between the unrecorded region and recorded
region on the optical disc.
[0020] It is an object of the present invention to provide the
optical pickup apparatus and optical information recording and
reproducing apparatus that are capable of detecting a stable
tracking error signal.
[0021] The above objects are implemented by the structure described
in the claim as an example.
[0022] Other objects, features and advantages of the invention will
become apparent from the following description of the embodiments
of the invention taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagram showing an optical pickup apparatus as
an embodiment of the present invention;
[0024] FIG. 2 is a diagram showing the shape of a polarizing
diffraction grating in an embodiment of the present invention;
[0025] FIG. 3 is a diagram showing a light detector and an light
pattern in an embodiment of the present invention;
[0026] FIGS. 4A to 4F are diagrams showing how light patterns in an
embodiment of the present invention change;
[0027] FIGS. 5A to 5E are diagrams showing how light patterns in an
embodiment of the present invention change;
[0028] FIGS. 6A to 6E are diagrams showing the shapes of light
patterns on a two-layer disc in an embodiment of the present
invention;
[0029] FIG. 7 is a diagram showing an embodiment 2 of the
polarizing diffraction grating in the present invention;
[0030] FIG. 8 is a diagram showing the shape of light patterns of
the embodiment 2 of the polarizing diffraction grating in the
present invention;
[0031] FIG. 9 is a diagram showing an embodiment 3 of the
polarizing diffraction grating in the present invention;
[0032] FIG. 10 is a diagram showing the shape of light patterns of
the embodiment 3 of the polarizing diffraction grating in the
present invention;
[0033] FIG. 11 is a schematic diagram of the optical disc apparatus
equipped with the optical pickup apparatus according to the present
invention;
[0034] FIG. 12 is a diagram explaining the disposition of an
optical pickup apparatus and an optical disc apparatus in an
embodiment 4;
[0035] FIG. 13 is a diagram explaining an optical pickup apparatus
using a one beam method in the embodiment 4;
[0036] FIG. 14 is a diagram explaining a light receiving part or a
polarizing diffraction grating of the embodiment 4 in the present
invention;
[0037] FIG. 15 is a diagram explaining the DC offset when the
objective lens of FIG. 12 is displaced on the inner and outer
periphery;
[0038] FIGS. 16A to 16C are diagrams schematically explaining the
offset occurring at the boundary between an recorded region and a
recorded region in the one beam method of the embodiment 4;
[0039] FIGS. 17A and 17B are diagrams comparing the characteristic
of the unrecorded region and recorded region in the embodiment 4
with JP-A-9-223321;
[0040] FIGS. 18A and 18B are diagrams explaining the effect of the
present invention by means of the light receiving part or a
difference in the method of dividing the surface of the diffraction
grating in the embodiment 4;
[0041] FIGS. 19A and 19B are diagrams showing a light receiving
part or a diffraction grating surface other than that shown in FIG.
14 in the embodiment 4;
[0042] FIG. 20 is a diagram explaining an optical pickup apparatus
using the one beam method in the embodiment 4;
[0043] FIG. 21 is a diagram explaining a light receiving part or
diffraction grating surface in an embodiment 5;
[0044] FIGS. 22A and 22B are diagrams comparing the characteristic
of the unrecorded region and recorded region in the embodiment 5
with JP-A-9-223321;
[0045] FIGS. 23A and 23B are diagrams showing a light receiving
part or a diffraction grating surface other than that shown in
FIGS. 19A and 19B in the embodiment 5;
[0046] FIG. 24 is a diagram explaining an optical pickup apparatus
using the one beam method in an embodiment 6;
[0047] FIGS. 25A and 25B are diagrams explaining a light receiving
part in the embodiment 6;
[0048] FIG. 26 is a diagram explaining an optical pickup apparatus
using the one beam method other than FIG. 24 in the embodiment
6;
[0049] FIG. 27 is a diagram explaining an optical pickup apparatus
using the one beam method in an embodiment 7;
[0050] FIG. 28 is a diagram explaining a diffraction grating
surface in the embodiment 7;
[0051] FIG. 29 is a diagram explaining light receiving parts in the
embodiment 7;
[0052] FIG. 30 is a diagram explaining an optical reproducing
apparatus in an embodiment 8; and
[0053] FIG. 31 is a diagram explaining an optical recording and
reproducing apparatus in an embodiment 9.
DESCRIPTION OF THE EMBODIMENTS
[0054] First, embodiments of the optical pickup apparatus according
to the present invention will be described. The optical pickup
apparatus according to the present invention is structured such
that for example a reflected light from a multilayer disc is
divided into a plurality of reflected light fluxes having different
outgoing directions and the divided light fluxes are focused on
different poisons on a light detector. Furthermore, the optical
pickup apparatus according to the present invention is structured
such that a photo focus error signal is detected using a reflected
light flux passing through a region that does not include the light
flux center out of the reflected light fluxes passing through a
dividing element according to a knife edge method, and a tracking
error signal is detected using a reflected light flux passing
through a region that does not include the light flux center.
Moreover, when a target layer is focused, each region of the
dividing element and the light receiving parts are disposed to
prevent a stray light from other layer from entering the light
receiving parts for a servo signal of the light detector.
[0055] An embodiment of the optical pickup apparatus and optical
disc apparatus equipped with the same according to the present
invention will be described with reference to FIG. 1 to FIGS. 5A to
5E.
Embodiment 1
[0056] FIG. 1 is a diagram showing the structure of the optical
pickup apparatus according to the present invention.
[0057] A laser light 2 emitted from a semiconductor laser 1 is
reflected by a polarizing beam splitter 3 and converted into a
parallel light flux by a collimate lens 4. The parallel light flux
passes through a polarizing diffraction grating 5 and a one-quarter
wave plate 6, and is focused by a objective lens 7 on an optical
disc 8. The optical disc 8 is provided with a recording and
reproducing layer (information recording layer) comprising two
layers of a first layer 9 and a second layer 10, with each layer
being formed with a track (not shown) in the direction of arrow
11.
[0058] When any of the two recording and reproducing layers of the
optical disc is in focus, the laser light is reflected by the
optical disc 8 and passes through the objective lens 7 and
one-quarter wave plate 6. Then, the laser flux is divided by the
polarizing diffraction grating 5 to enter a plurality of regions,
with each light flux advancing in different directions. Then, the
light flux passes through the collimate lens 4 and polarizing beam
splitter 3, and is focused on a light detector 12.
[0059] A plurality of light receiving parts 13 are formed on the
light detector 12, and the light flux divided by the polarizing
diffraction grating 5 is irradiated onto each of the light
receiving parts 13. Electrical signals are outputted from the light
detector 12 in response to the amount of light irradiated onto the
light receiving parts 13. The outputs are computed to generate a
focus error signal and a tracking error signal.
[0060] In the description hereinafter, when the optical pickup
apparatus is disposed to face the optical disc for the purpose of
recording or reproducing, the direction perpendicular to the
surface of the optical disc 8 is defined as a Z axis, the track
direction as a Y axis, and the direction perpendicular to the track
as an X axis. The Z axis is substantially parallel with the optical
axis of the light flux emitted from the objective lens 7.
[0061] FIG. 2 shows the shape of the polarizing diffraction grating
5 of FIG. 1. The polarizing diffraction grating 5 is divided into a
plurality of regions. In FIG. 2, solid lines show boundary lines,
while an alternate long and two short dashes line schematically
shows the outside shape of the light flux of a laser light, and
shadow areas schematically show a push pull pattern occurring due
to the track of the optical disc.
[0062] The polarizing diffraction grating 5 is formed with a
dividing line 15 in the Y-axis direction that passes through the
light flux center 14, and with a dividing line 16 in the X-axis
direction. The polarizing diffraction grating 5 is also formed with
a divided region (first divided region) that comprises four regions
(regions C1 to C4) which are point-symmetrical to each other with
respect to the light flux center 14 and includes the light flux
center 14; a divided region (second divided region) that comprises
four regions (regions A1 to A4) which are point-symmetrical to each
other with respect to the light flux center 14, does not include
the light flux center 14, and includes part of the dividing line 16
in the X-axis direction; and a divided region (third divided
region) that comprises four regions (regions B1 to B4) which are
point-symmetrical to each other with respect to the light flux
center 14, does not include the light flux center 14, and includes
part of the dividing line 15 in the Y-axis direction.
[0063] When the optical pickup apparatus is disposed to face the
surface of the optical disc during recording or reproducing, the
dividing line 15 is substantially perpendicular to the track
direction of the optical disc, and the dividing line 16 is
substantially parallel to the track direction of the optical
disc.
[0064] The regions A1 to A4 are divided by the dividing line 16 in
the X-axis direction passing through the light flux center 14, two
dividing lines 17 in the Y-axis that do not pass through the light
flux center 14, four dividing lines 18 in the X-axis direction that
do not pass through the light flux center 14, and four dividing
lines 19 around the light flux center 14 that form angles of 30
degrees with respect to the Y-axis direction. The interval u in the
Y-axis direction of the four dividing lines 18 running in the
X-axis direction is set to include a push pull pattern in the range
of about 55% to 70% of the light flux diameter in the
embodiment.
[0065] The regions A1 to A4 are disposed to sandwich the regions C1
to C4. The regions A1 to A4 are formed such that the regions A1 and
A2 are line-symmetrical to the regions A4 and A3, respectively,
with respect to the dividing line 15.
[0066] Regions B1 to B4 are also provided to sandwich the regions
C1 to C4. The region B1 is provided to be line-symmetrical to the
region B2 with respect to the dividing line 16, while the region B4
is provided to be line-symmetrical to the region B3 with respect to
the dividing line 16.
[0067] The interval w between the two dividing lines 17 running in
the Y-axis direction is set to be as small as possible under the
condition that the region A includes the push pull pattern and the
stray light does not enter the light receiving parts 13 depending
on the shape of the light receiving parts 13 of the light detector
12. In the embodiment, it is set within the range of about of 25%
to 30% of the light flux diameter. The dividing lines 19 that form
angles of 30 degrees with respect to the Y-axis direction are
provided to prevent the entry of the stray light into the light
receiving parts 13.
[0068] The interval v between two dividing lines 20 running in the
X-axis direction on the boundary between the region B and region C
is set to be as small as possible under the condition that the
stray light does not enter the light receiving parts 13 in response
to the shape of the light receiving parts 13 of the light detector
12.
[0069] The shape of diffraction grating formed on the region C1 is
the same as that formed on the region C3, and that formed on the
region C2 is the same as that formed on region C4. However, the
shapes of diffraction gratings formed on other regions are
different with each other. In each diffraction grating, the light
flux is divided into plus(+)/minus(-) first-order diffracted light
before being irradiated onto the light detector 12.
[0070] FIG. 3 shows the shape of the light receiving parts 13 of
the light detector 12, and the shapes of light patterns irradiated
onto the light detector. In FIG. 3, the light pattern of only the
reflected light from the recording and reproducing layer is shown,
and the light pattern of the stray light from other layer is not
shown.
[0071] When the recording and reproducing layer is in focus, the
laser light reflected from the recording and reproducing layer is
focused at a point 21 on the light detector 12, and is irradiated
onto the light receiving part 13 comprising 18 light receiving
parts of A to Z formed on the light detector 12.
[0072] Light receiving parts M, N, O and P detect a focus error
signal using a double knife edge method. If the light flux of a
plus/minus first-order diffracted light which is diffracted on the
region A1 and irradiated onto the light detector 12 is represented
as a1+ and a1-, then a light flux a1- is irradiated onto the
boundary between the light receiving parts M and N, a light flux
a2- is irradiated onto the boundary between the light receiving
parts P and O, a light flux a3- is irradiated onto the boundary
between the light receiving parts P and N, a light flux a4- is
irradiated onto the boundary between the light receiving parts M
and O. When the outputs from light receiving parts A to J are
represented by a to j, respectively, and the outputs of light
receiving parts M to T are represented by m to t, respectively, an
focus error signal (FES) is obtained by the following computing
equation.
(FES)=(m+p)-(n+o)
[0073] Light receiving parts E, F, G and H and light receiving
parts Q, R, S and T are disposed outside light receiving parts A,
B, C and D and light receiving parts M, N, O and P, respectively.
The light receiving parts A, B, C and D are irradiated with light
fluxes a1+, a2+, a3+ and a4+. The light receiving parts E, F, G and
H are irradiated with light fluxes b1+, b2+, b3+ and b4+. The light
receiving parts Q, R, S and T are irradiated with light fluxes b1-,
b2-, b3- and b4-. They are used for detecting tracking error
signals. The outputs of the light receiving parts Q, R, S and T
shall be q, r, s and t, respectively.
[0074] The tracking error signal (TES) according to the push pull
method is obtained by the following computing equation.
(TES)=((a+e+b+f)-(c+g+d+h)-K((q+r)-(s+t))
where K is a constant, and the value of K is determined such that
an offset does not occur to (TES) when the objective lens 7 moves
in the X-axis direction due to a tracking operation.
[0075] The tracking error signal (DPD) according to a DPD method is
obtained by detecting the phase difference between (a+e, c+g) and
(b+f, d+h).
[0076] Light receiving parts I and J are disposed outside the light
receiving parts E, F, G and H, and the light receiving part I is
irradiated with light fluxes c1+ and c3+, while the light receiving
part J is irradiated with light fluxes c2+ and c4+. These are
combined with other signals and are used for the detection of
reproduced signals (RF), which are obtained using the following
computing equation.
(RF)=a+b+c+d+e+f+g+h+i+j
[0077] Light fluxes c1-, c2-, c3- and c4- are irradiated onto the
place where the light receiving parts do not exit, and so these
signals are not used for the signal detection.
[0078] FIGS. 4A to 4F show how the light pattern of each light flux
that is irradiated onto the light detector 12 changes during
defocusing, and the waveform of the focus error signal (FES). FIGS.
4A to 4C, 4E and 4F correspond to (a) to (e) of FIG. 4D.
[0079] The light pattern of a plus first-order diffracted light 22
is shown by a lattice pattern and the light pattern of a minus
first-order diffracted light 23 is shown by oblique lines. When the
focused focal point is positioned at (c), the light pattern is
focused on the boundary of the light receiving parts M to P and at
this time the focus error signal becomes 0. As the defocusing
increases, the light pattern becomes larger. At (b) or (d), the
focus error signal reaches a maximum or minimum value. Moreover, at
(a) and (e) where the light pattern becomes even larger, the light
receiving parts 13 cease to be irradiated with light, with the
focus error signal becoming 0.
[0080] As the defocusing increases, the light pattern becomes
larger around the focus point (c), and at this time, the light
pattern on the regions C1 to C4 also becomes larger. However, the
light pattern near the light flux center of the regions C1 to C4 is
not included in other regions than the regions of the light
receiving parts I and J, thus the light pattern deviating from the
light receiving part 13. In the light receiving parts M to P for
detecting the focus error signal, as the light pattern from the
region B1 to region B4 and from region C1 to region C4 become
larger, the light pattern deviates from the light receiving parts M
and P.
[0081] A detailed description will be given to how the light
pattern changes at the light receiving parts M to P for detecting
the focus error signal with reference to FIGS. 5A to SE. FIGS. 5A
to 5E show the change of the light pattern of a light flux a2-
which is condensed on the boundary between light receiving surfaces
O and P during focusing. (b), (c) and (d) correspond to the state
shown in FIGS. 4A to 4F, and (a') shows the state in the middle
between (a) and (b), and (e') shows the state in the middle between
(d) and (e) in FIG. 4D. At (c), a position corresponding to the
light flux center 14 is focused and the light pattern 25 expands
around the light focus point (light flux center) (c) as the
defocusing increases. At this time, since virtual light patterns 26
and 27 corresponding to light fluxes b2- and c2- which are
irradiated onto other light receiving parts expand, the light
pattern 25 extends off the light receiving parts M to P, and the
entire light pattern 25 lies outside the light receiving parts M to
P at (a') and (e').
[0082] In FIGS. 4A to 4F, the interval Wp between the light
patterns of the regions A1 to A4 in the X-axis direction during
defocusing is determined in response to the width w between the
dividing lines 17 of the polarizing diffraction grating. Therefore,
the relation between the shapes of the light receiving parts 13 and
the width w of between the dividing lines 17 is determined such
that the light patterns are not irradiated onto the light receiving
parts 13 during the defocusing.
[0083] Since the light receiving parts 13 are disposed on the
interval Wp, light receiving parts 13 can be disposed nearer to
each other compared with when the light receiving parts 13 are
disposed outside the light pattern during defocusing, thus making
it possible to reduce the size of the light detector.
[0084] Furthermore, if a light pattern of other region is
irradiated onto the light receiving parts M to P when the light
patterns expand with the increasing focusing, a distortion occurs
to the focus error signal waveform, causing an error during focus
withdrawal. A shaded area 24 of the region A, which is formed by
the dividing line 19 which forms an angle of 30 degrees with
respect to the Y-axis direction of the polarizing diffraction
grading 5, is provided to prevent the light patterns that are
irradiated onto the light receiving parts A to D from entering the
light receiving parts M to P when they expand with defocusing. The
shaded area becomes unnecessary depending on the deposition of the
light receiving parts.
[0085] FIGS. 6A to 6E show the shapes of light patterns on the
light detector 12 when the is comprises of two layers, and the
waveform of a focus error signal (FES). A light pattern 28
reflected from the first layer 9 is shown by a lattice pattern,
while the light pattern reflected from the second layer 10 is shown
with oblique lines. FIG. 6B corresponds to (a) of FIG. 6A. FIGS. 6C
and 6D correspond to (b) of FIG. 6A. FIG. 6E corresponds to (c) of
FIG. 6A.
[0086] The focus error signal waveform (FE) of the two-layer disc
is obtained by combining a focus error signal waveform (FE1)
generated at the first layer 9 and a focus error signal waveform
(FE2) generated at the second layer 10. FIG. 6B shows the shapes of
light patterns 28 and 29 when the first layer 9 is in focus,
wherein the light pattern 28 is focused on the light detector 12,
and the light pattern 29 (stray light) is irradiated onto the
outside of the light receiving parts 13 at that time.
[0087] As the focus shifts from the first layer 9 to the second
layer 10, the size of the light pattern 28 increases, while the
size of the light pattern 29 diminishes. At the midpoint (b)
between the first layer 9 and second layer 10, the size of the
light pattern 28 and that of light pattern 29 are substantially the
same as Figs. C and D show, and most of them are not irradiated
onto the light receiving parts M to P. At (c) where the second
layer 10 is in focus, the light pattern 29 is focused onto the
light detector 12, and the light pattern 28 (stray light) is
irradiated onto the outside of the light receiving parts 13.
[0088] While the focus error signal is obtained by computing the
outputs of the light receiving parts M to P, the stray light is not
irradiated on the light receiving parts M to P when the second
layer 10 is in focus. Therefore, an offset due to the stray light
does not occur there. In addition, the offset due to the stray
light does not occur even if the intensity distribution of a laser
light 2 varies and the light pattern is displaced from the light
receiving parts 13, thus making it possible to obtain a stable
focus error signal.
[0089] Furthermore, if the waveform (FE1) generated at the first
layer 9 overlaps a large part of the waveform (FE2) generated at
the second layer 10, a distortion occurs to the focus error signal
waveform (FE) of the second disc, and sometimes a focus withdrawal
error can occur. However, in the embodiment, since the light
pattern is hardly irradiated onto the light receiving parts M to P
at a location near the midpoint between the first layer 9 and the
second layer 10, the outputs of the (FE1) and (FE2) are small and
thereby distortion experienced by the focus error signal waveform
(FE) is also small.
[0090] By the same token, while a tracking error signal is obtained
by computing the outputs of the light receiving parts A to H and Q
to T, the stray light is not irradiated onto the light receiving
parts A to H and Q to T when the layer is in focus, thus making it
possible to obtain a stable focus error signal in which an offset
due to the stray light does not occur.
[0091] Since the light receiving parts I and J include the light
flux center, a light near the light flux center remains at the
light receiving parts and becomes a stray light even if the light
patterns expand. However, this portion is not used for detecting
the focus error signal and a tracking error signal, and is used
only for detecting a reproducing signal. Therefore, the existence
of the stray light causes no problem in practical use.
[0092] Since there is no influence from the stray light as
described above, it is possible to change the balance of light
amount of plus/minus first-order diffracted light which is
diffracted at the polarizing diffraction grating 5. It is possible
to improve the SN of a reproduced signal by increasing the light
amount of the plus first-order diffracted light 22 such that the
light amount of the light receiving parts for detecting the
reproduced signal increases. At this time, while the minus
first-order diffracted light 23 decreases, the offset due to the
stray light does not increase because of the reduction in the light
amount. Therefore, only electrical restriction has to be
considered.
[0093] In the present embodiment, the polarizing diffraction
grating 5 and one-quarter wave plate 6 may be fixed in one piece
with the objective lens 7 such that they operate together with the
objective lens 7. Alternatively, they may separately be fixed so
that they do not operate together with the objective lens 7. In the
case where the polarizing diffraction grating 5 and one-quarter
wave plate 6 are fixed separately from the objective lens 7, when
the objective lens 7 moves in the X direction due to the tracking
operation, the outside shape of the light flux, which is shown by
an alternate long and two short dashes line in FIG. 2, also moves
in the X direction, and the dividing line 15 lies off the light
flux center 14. However, the position and size of the light pattern
interval Wp that appears on the light detector 12 in response to
the width w between the dividing lines 17 do not change. Therefore,
the stray light is not irradiated onto the light receiving parts 13
either in this case. Since the outside shape of the light flux
moves in the X direction, the value of K in the computing equation
of the tracking error signal (TES) according to the push pull
method differs from that when they are fixed in one piece with the
objective lens 7.
[0094] The polarizing diffraction grating 5 is not limited to the
shape shown in the above embodiment. Other embodiments of the
polarizing diffraction grating will be described below.
Embodiment 2
[0095] FIGS. 7 and 8 show the shapes of divided regions of the
polarizing diffraction grating 5 in an embodiment 2 and the shapes
of light patterns irradiated onto the light detector 12 at that
time. The light pattern 28 of the reflected light from the first
layer 9 is focused on the light detector 12, and the light pattern
29 (stray light) of the reflected light from the second layer 10 at
that time is shown by oblique lines. A difference from the
embodiment 1 is that the width Wc of the regions C1 to C4 in the
X-axis direction is narrower. The regions B1 to B4 are larger by
just that much. The shapes of the regions A1 to A4 are the same as
those of the embodiment 1. Since the regions B1 to B4 are larger,
the stray light is more likely to enter the light receiving parts E
to F and Q to T when the light detector 12 is displaced. However,
an effect is expected that increases the output of the tracking
error signal.
Embodiment 3
[0096] FIGS. 9 and 10 show the shapes of divided regions of the
polarizing diffraction grating 5 in an embodiment 3 and the shapes
of light patterns irradiated onto the light detector 12 at that
time. The light pattern 28 of the reflected light from the first
layer 9 is focused on the light detector 12, and the light pattern
29 (stray light) of the reflected light from the second layer 10 at
that time is shown by oblique lines.
[0097] A difference from the embodiment 1 is that there are not the
four dividing lines 18 in the X-axis direction that do not pass
through the light flux center 14, and the four dividing lines 19
that form an angle of 30 degrees with respect to the Y-axis
direction extend longer around the light flux center 14. Therefore,
the areas of the region B1 to B4 are reduced, and thereby the
outputs of the light receiving parts Q to T are reduced.
Accordingly, it is necessary to increase the value of K in the
following computing equation for the tracking error signal
according to the push pull method.
(TES)=((a+e+b+f)-(c+g+d+h))-K((q+r)-(s+t))
However, since the areas of regions A1 to A4 increase, an effect is
expected that increases the output of the error focus signal.
[0098] In above embodiment, the polarizing diffraction grating, as
a light flux dividing element, is disposed between the collimate
lens and one-quarter wave plate. However, an ordinary diffraction
grating may be disposed between a polarizing beam splitter and the
light detector.
[0099] Application can be expected for the optical disc apparatus
that records and reproduces information on and from an optical
disc.
[0100] When a target layer of the optical disc is in focus, the
stray light from other layer deviates from the light receiving
parts for a servo signal of the light detector. Therefore, it is
possible to receive only reflected light from the target layer to
obtain the servo signal, thus making it possible to obtain a stable
focus error signal and a tracking error signal free of the offset
due to the stray light.
[0101] Next, the optical disc apparatus equipped with the optical
pickup apparatus according to the present invention will be
described.
[0102] FIG. 11 shows a schematic diagram of a specific example of
the optical disc apparatus equipped with the optical pickup
apparatus shown in FIG. 1. A semiconductor laser 1, a polarizing
beam splitter 3, a polarizing diffraction grating 5, a one-quarter
wave plate 6 and a light detector 12 corresponding to those shown
in FIG. 1, and a mirror 30 for changing the direction of the laser
light, which is not shown in FIG. 1, are bonded and fixed to a case
31. A collimate lens 4 is fixed to the case 31 such that it can be
moved along the optical axis by a motion mechanism 32. The
collimate lens 4 can move to a position where the spherical
aberration of a laser light 2, which is focused on a light disc 8,
becomes minimal in each of the cases where recording and
reproducing are preformed on a first layer 9 and on a second layer
10 of the optical disc 8.
[0103] An objective lens 7 is attached to a holder 34 in which a
coil 33 is incorporated, and is combined with a magnet, which is
not shown, to form an actuator. The objective lens 7 can follow the
side-runout and decentering of the optical disc 8.
[0104] The case 31 can be moved in the radial direction of the
optical disc 8 by a motor 35 and a lead screw 36. The optical disc
8 is fixed to a spindle motor 37.
[0105] The operation of each component is controlled by a system
control circuit 47. When recording or reproducing is performed, the
spindle motor 37 is first driven by the operation of a spindle
motor driving circuit 46, and then the optical disc 8 is
rotated.
[0106] Next, the semiconductor laser 1 is radiated by the operation
of a laser driving circuit 41.
[0107] Focusing control is performed such that a servo signal
generating circuit 43 generates a focus error signal from the
output of the light detector 12, an actuator circuit 45 drives the
actuator based on the focus error signal, and the objective lens 7
focuses the laser light on the recording and reproducing layer.
[0108] When locating the focus point of the laser light 2 on the
first layer 9, the focus error signal is detected after the
collimate lens 5 is moved to a position corresponding to the first
layer 9. Waveforms shown in FIGS. 6A to 6E are obtained to the
focus error signal. Therefore, focusing control is performed such
that the focus point is located on the first layer.
[0109] Next, an access control circuit 44 is operated to rotate the
motor 35, and the case 31 is moved to a desired position on the
inner periphery or outer periphery of the optical disc through the
lead screw 36.
[0110] Then, tracking control is performed in which the actuator
circuit 45 drives the actuator based on the tracking error signal
generated by the servo signal generating circuit 43 from the output
of the light detector 12 to follow the focus point of the laser
light 2 on the track of the optical disc 8.
[0111] Then, data on the track of the optical disc 8 is reproduced
from the output of the light detector 12 by an information signal
generating circuit 42.
[0112] When information is recorded on the optical disc 8, a laser
driving circuit 41 is operated by the system control circuit 47 in
response to the information to be recorded, and a record mark is
formed on the track by modulating the output of the semiconductor
laser 1.
[0113] When moving the recording and reproducing layer from the
first layer 9 to the second layer 10, the focus point of the laser
light 2 is moved towards the second layer by stopping the focusing
control and operating the actuator driving circuit 45 at the same
time after the tracking control 45 is stopped by the system control
circuit 47. Then, the focusing control is performed such that the
actuator is driven at the timing that the focus point position of
the second layer of the focus error signal is detected and the
focus point of the laser light is located on the second layer.
Then, the tracking control is performed in which after the
collimate lens 4 is moved to a position corresponding to the second
layer 10, the actuator is driven based on the tracking error signal
to follow the focus point of the laser light 2 on the track. The
reproducing operation and recording operation are performed on the
second layer 10 in the same way as on the first layer 9.
[0114] While the polarizing diffraction grating 5 and one-quarter
wave plate 6 are fixed to the case 31 in the above embodiment, they
may be fixed to the holder 34, to which the objective lens 7 is
fixed, such that they move together with the objective lens 7.
[0115] While the optical pickup apparatus and optical disc
apparatus equipped with the same according to the present invention
have been described in detail by way of embodiments thereof in the
above, the present invention is not limited to the above
embodiments. The present invention can include various variations
and improvements without departing from the spirit of the present
invention.
[0116] For example, while recording or reproducing on or from the
optical disc in which two layers of recording and reproducing layer
(information recording layer) are laminated in the above
embodiments, the present invention is also adaptable to recording
or reproducing on or from an optical disc in which three layers or
more of recording and reproducing layer are laminated.
[0117] Furthermore, the disposition pattern of light receiving
parts of the light detector is not limited to the above examples.
The light receiving parts may be disposed in any way unless a
reflected light flux from other recording and reproducing layer
than the target recording and reproducing layer is not irradiated
onto the light receiving parts of the light detector when the
target information recording layer of the optical disc is in
focus.
[0118] In addition, while the first divided region comprises four
regions of C1 to C4 in the above embodiments, the present invention
is not limited to the same. The first divided region may comprise
only one region, two regions, or four or more regions.
[0119] Next, embodiments of the optical pickup apparatus according
to the present invention will be described.
Embodiment 4
[0120] FIG. 12 is a schematic diagram showing an exemplary optical
pickup apparatus according to the present invention.
[0121] The optical pickup apparatus 101 is structured such that it
can be driven by a drive mechanism 107 in the radial direction of
the optical disc 100 as is shown in FIG. 12. An actuator 105 on the
optical disc 100 is equipped with an objective lens 102. The
optical disc 100 is irradiated with light by the objective lens
102. The light emitted from the objective lens 102 forms a spot on
the disc and is reflected from the disc. A focus error signal and a
tracking error signal are generated by detecting the reflected
light.
[0122] FIG. 13 shows an optical system for the above optical pickup
apparatus. While BD will be described here, HD-DVD and other
recording methods are also adaptable.
[0123] A light flux with a wavelength of about 405 nm is emitted
from a semiconductor laser 50 as a divergent light. The light flux
emitted from the laser 50 is converted by a collimate lens 51 into
a substantially parallel light. The light flux that passes through
the collimate lens 51 is reflected by a beam splitter 52. Part of
the light flux passes through the beam splitter 52 to enter a front
monitor 53. Generally, when information is recorded on a recording
type optical disc such as RD-RE or BD-R, a given amount of light is
irradiated onto the recording surface of the optical disc with.
Therefore, it is necessary to highly precisely control the light
amount of the semiconductor laser. For the purpose, the front
monitor 53 detects a change in the light amount of the
semiconductor laser 50 when information is recorded on the
recording type optical disc, and feeds back the result to the a
drive circuit (not shown) of the semiconductor laser 50. This
enables monitoring the light amount on the optical disc.
[0124] The light flux reflected from the beam splitter 52 enters a
beam expander 54. The beam expander 54 has a function to change the
diverging or converging state of the light flux. Therefore, the
beam expander 54 is used for compensating the spherical aberration
due to an error in thickness of a cover layer of the optical disc
100. The light flux emitted from the beam expander 54 is reflected
by a start-up mirror 55 and passes through a one-quarter wave plate
56, and thereafter the light flux is focused on the optical disc
100 by the objective lens 102 mounted on the actuator 105.
[0125] The light flux reflected by the optical disc 100 passes
through the objective lens 102, one-quarter wave plate 56, start-up
mirror 55, beam expander 54 and beam splitter 52. The light flux
passing through the beam splitter 52 is separated into a light flux
passing through a beam splitter 57 and a light flux reflected by
the beam splitter 57.
[0126] A focus error signal is detected from the light flux
reflected by the beam splitter 57 according to a knife edge method.
It should be noted that the knife edge method is used here as a
focus detecting method, but not limited to the knife edge method.
Since the knife edge method is publicly known, its description is
omitted here. After passing through the beam splitter 57, the light
flux enters a light detector 108. The light detector 108 detects a
signal on the disc and a tracking error signal.
[0127] FIG. 14 shows a pattern of a light receiving part 108
according to the present invention. The light receiving part 108
comprises four regions of a region I (region 1), a region J (region
2), a region G (region 3) and a region H (region 4). The region I
(region 1) and region G (region 3) are line-symmetrical to the
region J (region 2) and region H (region 4) with respect to the
center line of the light receiving part. In addition, the region I
(region 1) and region J (region 2) are characterized in that their
widths in the central axis 500 direction become narrower with the
distance away in the direction substantially perpendicular to the
central axis 500 from the central axis (or center line) 500.
[0128] Here, the principle of detecting a tracking error signal of
the one beam method will be described with reference to FIG. 14. On
the detector surface, there appears a region where a 0th-order
diffracted light and a plus/minus first-order diffracted light
interfere with each other. The interfering state of the regions
differs depending on the spot positions on the track. Therefore,
this can be used to dispose a spot on a desired track position.
Actually, a push pull signal is generated by computing the
difference between a signal obtained at an interference region Z1
of the 0th-order diffracted light and plus first-order diffracted
light, and a signal obtained at an interference region Z2 of the
0th-order diffracted light and minus first-order diffracted light.
The signal in the regions other than the interference hardly
depends upon the positions of the spot on the track. The one beam
method uses this characteristic.
[0129] The foregoing will be described in detail hereinafter. The
light flux is displaced on the light receiving part in the arrow
direction of FIG. 14 with the displacement of the objective lens,
and intensity distribution is also displaced in the same direction
at the same time. A DC offset occurs to the signal of (I-J) due to
the two effects. The DC offset also occurs to the signal of (G-H).
FIG. 15 shows the amount of offset of the (I-J) signal and (G-H)
signal relative to the amount of displacement of the objective
lens. It is known from FIG. 15 that the amount of DC offset
occurring to the (I-J) signal and (G-H) signal relative to the
displacement of the objective lens is nearly linear. Therefore, it
is known that a tracking error signal in which DC offset is
suppressed can be detected by performing the following
computation.
(Tracking error signal)=(I-J)-k(G-H) (equation 1)
where k is a coefficient for correcting the DC offset of the (I-J)
signal and DC offset of the (G-H) signal. In this manner, the one
beam method enables the detection of the tracking error signal in
which offset is suppressed.
[0130] Next, description will be made to the offset of a tracking
error signal occurring at the boundary between an unrecorded region
and a recorded region on the disc. FIGS. 16A to 16C schematically
show the waveform of a tracking error signal according to the one
beam method at the boundary between the unrecorded region and
recorded region. FIG. 16A shows when the offset can not be
suppressed. FIG. 16B shows when the offset occurs on the reverse
side due to overcorrection. FIG. 16C shows a tracking error signal
in which the offset is suppressed.
[0131] In the waveforms of the tracking error signals shown in
FIGS. 16A and 16B, the tracking error signals are more likely not
to cross the original point position due to variations or the like.
If the tracking error signal does not cross the original point
position, it is a problem in terms of servo control. (Tracking
control is performed by performing the servo control at the
original point position). Therefore, it is evident that the
waveform shown in FIG. 16C is desirable.
[0132] Here, the bottom ratio and top ratio are considered as an
indicator for the offset in the tracking error signal. The bottom
ratio is assumed to be (a-c)/(c+d) as shown in FIGS. 16A to 16C.
This indicates that, the difference is first obtained between a
tracking error signal bottom value in the recording region and a
tracking error signal bottom value at the boundary between the
unrecorded region and recorded region, and then the difference is
divided by the tracking error signal amplitude in the recorded
region. Specifically, it indicates how much bottom lies in the
lower side when compared with the tracking error signal amplitude
in the recording region. In contrast, the top ratio is assumed to
be (b-d)/(c+d). This indicates that the difference is first
obtained between a tracking error signal top value in the recording
region and a tracking error signal top value at the boundary
between the unrecorded region and recorded region, and then the
difference is divided by the tracking error signal amplitude in the
recorded region. Specifically, it indicates how much top lies in
the upper side when compared with the tracking error signal
amplitude in the recording region.
[0133] If the two indicators are positive values, the tracking
error signal amplitude gradually changes even if a spot shifts from
the unrecorded region to the recorded region, and thereby the servo
control stabilizes as is known from FIG. 16C. However, if one
indicator is a positive value and the other one is a negative
value, the offset can not suppressed, posing a problem. The DC
offset also occurs to the tracking error signal when the objective
lens is displaced in the tracking direction. Accordingly, the
offset due to the boundary between the unrecorded region and
recorded region and to the displacement of the objective lens must
be suppressed simultaneously.
[0134] The evaluation of the offset of the tracking error signal
that occurs at the boundary between the unrecorded region and
recorded region when the objective lens is displaced will be
performed based on the above indicators in the following sections.
Here, the calculation conditions when performing simulation are as
follows. [0135] wavelength: 405 nm [0136] objective lens NA: 0.85
[0137] track pitch: 0.32 .mu.m [0138] objective lens focal length:
1.41 mm
[0139] FIG. 17A shows the ratio of top when the objective lens is
displaced in the tracking direction while the light receiving parts
of the present invention and JP-A-9-223321 are used. FIG. 17B shows
the ratio of bottom in the same situation. The conditions of the
light receiving parts of the present invention are as follows.
t1=d2/d1=0.19, where t1 is the ratio of the interval between the
region I (region1) and region J (region 2) relative to the diameter
of the light flux incident to the detector. t2=d3/d1=0.5, where t2
is the ratio of the maximum width of the region I (region 1) and
region J (region 2) in the center axis direction relative to the
diameter of the light flux incident to the detector. The slope
angle .theta. of the outside shape of the region I (region 1) and
region J (region 2) with respect to the direction perpendicular to
the center axis is assumed to be 10 degrees.
[0140] FIGS. 17A and 17B show that the top ratio assumes negative
values at most of the objective lens displacement amount in the
case of JP-A-9-223321. In addition, FIGS. 17A and 17B show that the
bottom ratio assumes positive values. This indicates that the
offset is occurring.
[0141] In contrast, in the case of the present invention, both the
top ratio and bottom ratio are positive in most of the objective
lens displacement amount, indicating that the offset at the
boundary between the recorded region and unrecorded region is
suppressed.
[0142] Next, effects will be described that are provided by
inclining the dividing lines of the region I and J. FIGS. 18A and
18B show the result of the simulation performed when the dividing
lines are inclined. FIG. 18A shows the top ratio, while FIG. 18B
shows the bottom ratio. The conditions of the light receiving parts
are t1=0.19, t2=0.5 and .theta.=10 degree, and t1=0.19, t2=0.5, and
.theta.=0 degree.
[0143] The top ratio assumes positive values in most regions where
the objective lens is displaced, and the bottom ratio is
significantly improved at the regions where the objective lens
displacement is negative. In this manner, inclined dividing lines
would be able to suppress the offset at the boundary between the
unrecorded region and recorded region. Especially, a larger
improvement effect will be provided in suppressing the DC offset
and the offset at the boundary between the unrecorded region and
recorded region when 0 degree<.theta.<15 degree, and
0<t1<0.35, and 0<t2<0.70, where t1 and t2 are figures
relative to the diameter of the light flux entering the light
receiving parts of the light detector 10.
[0144] When simply thinking, as shown in FIG. 14, the tracking
error signal amplitude seems to extremely decrease if the widths of
the regions I and J in the direction of the center axis 500 become
smaller as the regions I and J move away from the center axis 500
in the substantially perpendicular direction, because when the
objective lens is displaced, the area to be detected decreases in
the interference regions (interference region Z1 or interference
region Z2) on the light receiving parts region (region I or region
J) in the direction of the objective lens displacement. However,
actually, when the objective lens is displaced, the intensity
distribution of the light flux is displaced at the same time in the
objective lens displacement direction by two times the objective
lens displacement amount. Therefore, while the area decreases, the
intensity increases on the light receiving part region (region I or
region J) in the objective lens displacement direction. Moreover,
while the intensity decreases, the area increases on the light
receiving part region (region J or region I) opposite to the
objective lens displacement direction. Therefore, the tracking
error signal amplitude is less prone to decrease, and the DC offset
is more likely to be corrected. Moreover, the offset at the
boundary between the unrecorded region and recorded region greatly
occurs at a location near the interference region (interference
region Z1 or interference region Z2). Accordingly, it is effective
in suppressing the offset at the boundary between the unrecorded
region and recorded region to enter the light of the regions other
than the interference region onto the light receiving parts on the
DC offset detection side when the objective lens is displaced.
[0145] While FIG. 14 shows the dividing lines inside the detector
by straight lines that are substantially parallel with the track,
and straight lines that extend from there to form other angles, it
does not matter at all whether the dividing lines inside the
detector are arc lines as shown in FIG. 19A, or straight lines as
shown in FIG. 19B. Furthermore, while the pattern of the light
receiving parts is shown here, it is needless to say that a similar
effect is provided by disposing a diffraction grating having the
same pattern as that of the light receiving parts like the optical
system of FIG. 20, and by changing the diffraction directions and
angles in each region to detect signals with a plurality of light
receiving parts.
Embodiment 5
[0146] FIG. 21 shows a pattern of the light receiving part which
relates to an embodiment 5 and differs from that of the embodiment
4. A difference from the embodiment 4 lies in that the pattern of
the embodiment 5 is provided with a center region Y (region 5). The
ratio of the length in the center axis direction of the center
region relative to the diameter of the light flux entering the
light receiving part of the detector 108 is t3. The ratio of the
length in the direction perpendicular to the center axis of the
center region relative to the diameter of the light flux entering
the light receiving part of the detector 108 is t4. The light
receiving part is capable of generating a tracking error signal by
performing the following computation.
(tracking error signal)=(C-D)-k{(A-B)+(E-F)} (equation 2)
[0147] FIG. 22A and FIG. 22B show the top ratio and bottom ratio,
respectively, when the light receiving parts of the present
invention and JP-A-9-223321 are used, and the objective lens is
displaced in the tracking direction. The light receiving parts of
the present invention are calculated under the condition that
t1=0.19, t2=0.54, t3=0.19, t4=0.19 and .theta.=10 degree.
[0148] As FIGS. 22A and 22B show, the tracking error signal of
JP-A-9-223321 changes a lot with a change in the characteristic at
the boundary between the unrecorded region and recorded region and
the displacement of the objective lens. In contrast, the tracking
error signal of the present invention does not change with the
displacement of the objective lens, thus making it unnecessary to
control particularly the displacement of the lens. Both the top
ratio and bottom ratio are positive in most of the objective lens
displacement amount, indicating that the offset at the boundary
between the recorded region and unrecorded region is
suppressed.
[0149] The use of the detector pattern such as that of the present
invention enables stable tracking control even if the objective
lens is displaced. The DC offset as well as the offset at the
boundary between the unrecorded region and recorded region are
particularly effectively suppressed under the condition that 0
degrees<.theta.<15 degrees, 0<t1<0.35, 0<t2<0.70,
0<t3<0.35 and 0<t4<0.35, where t1, t2, t3 and t4 are
ratios relative to the diameter of light flux entering the light
receiving parts of the detector 10.
[0150] As described in the embodiment 4, the offset of the boundary
between the unrecorded region and recorded region occurs greatly at
locations near the interference region (interference region Z1 or
interference region Z2), and the center part of the detecting
surface occurs little offset. Furthermore, since the region is not
detected for a tracking error signal, the coefficient k can be set
to an appropriate value. As a result, it is possible to improve the
effect of suppressing the offset at the boundary between the
unrecorded region and recorded region.
[0151] While the dividing lines inside the detector are shown in
straight lines that are substantially parallel with the track and
straight lines that extend from there to form angles in FIG. 21,
the dividing lines inside the light receiving part could be arc
lines as shown in FIG. 23A, or straight lines as shown in FIG.
23B.
[0152] While patterns of the light receiving parts are shown here,
it is needless to say that similar effects are provided by
disposing a diffraction grating 61 having the same pattern as that
of the light receiving parts shown in FIG. 4 to detect signals with
a plurality of light receiving parts on the detector.
Embodiment 6
[0153] FIG. 24 shows an optical system of an optical pickup
apparatus relating to an embodiment 6 of the present invention. In
the embodiment 6, like numerals are used for like and corresponding
parts of the embodiment 4 of the present invention shown in FIG.
13. Although BD will be described here, it could be HD-DVD or other
recording type methods.
[0154] A light flux with a wavelength of about 405 nm is emitted
from a semiconductor laser 50 as a divergent light. The light flux
emitted from the laser 50 is reflected by a beam splitter 52. Part
of the light flux passes through the beam splitter 52 to enter a
front monitor 53. The light flux reflected by the beam splitter 52
is converted by a collimate lens 51 into a substantially parallel
light flux. The light flux passing through the collimate lens 51
enters a beam expander 54. The light flux emitted from the beam
expander 54 is reflected by a start-up mirror 55, passes through a
one-quarter wave plate 56 and is condensed on an optical disc 100
by an objective lens 102 mounted on an actuator 105.
[0155] The light flux reflected by the optical disc 100 passes
through the objective lens 2, one-quarter wave plate 56, start-up
mirror 55, beam expander 54, collimate lens 51 and beam splitter
52.
[0156] The light flux passing through the beam splitter 52 is
divided by a diffraction grating 63 into a light flux for
generating a focus error signal (0th-order diffracted light) and a
light flux for generating a tracking error signal (plus first-order
diffracted light or minus first-order diffracted light). While a
description is made here using the diffracting grating of FIG. 14
relating to the embodiment 4, the diffraction grating of FIG. 19A
or FIG. 19B relating to the embodiment 4, or that of FIG. 21, FIG.
23A or FIG. 23B relating to the embodiment 5 can also be used. The
light flux divided by the diffraction grating 63 enters a detection
lens. The light flux divided by the diffraction grating 63 enters a
detection lens. When passing through the detection lens, the light
flux is given a predetermined astigmatism, which is used for the
detection of the focus error signal. The light flux for generating
the tracking error signal is given an astigmatism and spherical
aberration when diffracting the diffraction grating 63. Therefore,
the light flux passing through the detection lens 59 is condensed
on the light receiving parts.
[0157] FIGS. 25A and 25B show a detector 64 and light fluxes to be
detected. The detector 64 is divided into focus detecting regions
40 to 43 and tracking error signal regions 44 to 47. Since the
focus error signals are publicly known, its description is omitted
here. The directions of the light fluxes that diffracted the
diffracting grating are different in each region, and a light flux
diffracting a region G of FIG. 14 enters a region 45 of FIG. 25B, a
light flux diffracting a region H enters a region 46, a light flux
diffracting a region I enters a region 44, and a light flux
diffracting a region J enters a region 47. This causes the tracking
error signals to be generated. Here, the RF signal can be detected
by obtaining the total of the focus error signals, the total of the
tracking error signals, or the total of the focus error signals and
tracking error signals. The use of the regions 40 to 43 for focus
error signals would also enable DPD (Differential Phase Detection)
based on the tracking error signal detection method which is
adopted for a DVD-ROM or the like.
[0158] With such an optical system structure as described above, it
becomes possible to obtain not only the tracking error signals but
also other signals. While the diffracting grating 63 is disposed on
the detector side here, instead a polarizing diffraction grating 65
can be disposed near the objective lens as shown in FIG. 26.
Embodiment 7
[0159] FIG. 27 shows an optical system of an optical pickup
apparatus relating to an embodiment 7 of the present invention. In
the embodiment 7, like numerals are used for like and corresponding
parts of the embodiment 4 shown in FIG. 13. Although BD will be
described here, it could be HD-DVD or other recording type methods
instead.
[0160] A P-polarization light flux with a wavelength of about 405
nm is emitted from a semiconductor laser 90 as a divergent light.
The light flux emitted from the laser 90 passes through the beam
splitter 91 and is reflected by a mirror 92. Part of the light flux
outside the pitch diameter enters a front monitor 93. The light
flux reflected by the mirror 92 enters an auxiliary lens 94 and
then a collimate lens 95. The collimate lens 95, which can be
driven in the light axis direction by a driving mechanism (not
shown), can change the diverging or converging state of the light
flux thereby to compensate the spherical aberration due to the
thickness error of a covering layer of an optical disc 100.
[0161] The P-polarization light flux passing through the collimate
lens 95 enters a polarizing diffraction grating 66 of the present
invention. The P-polarization light flux that entered the
polarizing diffraction grating 66 passes through the diffraction
grating 66, is reflected by a start-up mirror 96, passes through a
one-quarter wave plate 97, and thereafter becomes a circularly
polarized light. The light flux that became a circularly polarized
light is condensed on the optical disc 100 by the objective lens
102 which is equipped with an actuator 105.
[0162] The light flux reflected by the optical disc 100 passes
through the objective lens 102 and one-quarter wave plate 97. The
circularly polarized light is converted into an S-polarized light
by the one-quarter wave plate 97. The S-polarized light flux is
reflected by the start-up mirror 96 and enters the polarizing
diffraction grating 66. The S-polarized light entering the
polarizing diffraction grating 66 is divided by the polarizing
diffraction grating 66 into a plurality of light fluxes. The light
fluxes passing through the polarizing diffraction grating 66 are
reflected by the beam splitter after passing through the collimate
lens 95, auxiliary lens 94 and mirror lens 92, and then enters a
detector 67.
[0163] FIGS. 28 and 29 show patterns of the polarizing diffraction
grating which consider the tracking error signals as well as focus
error signals. FIGS. 27 and 29 show detector 67. In the polarizing
diffraction grating 66 is a diffracting grating in which only
plus/minus first-order light is diffracted, and the diffraction
direction and diffraction angle of each of the diffracted gratings
are different in each region. For the sake of simplicity, FIG. 29
shows the light fluxes that are diffracted from each region of the
polarizing diffraction grating shown in FIG. 28 by means of
characters of the regions. Additionally, a subscript "+" added to
the character indicates a plus first-order diffracted light, while
a subscript "-" added to the character indicates a minus
first-order diffracted light. For example, a plus first-order
diffracted light of a region L of the polarizing diffraction
grating 66 of FIG. 28 enters a region 74 of a detector 67 of FIG.
29, and a minus first-order diffracted light enters a region
83.
[0164] The focus error detection method is based on the knife edge
method. Detection is performed by the minus first-order diffracted
light diffracted in regions N, P, Q and O of the polarizing
diffraction grating 66. Since the knife edge method is publicly
known, its description is omitted here. The detection of the
tracking error signal can be obtained by performing the following
computation using the detection signals of regions 70 to 79 and
regions 81 to 84.
(Tracking error
signal)={(N.sub.++L.sub.+)+(P.sub.++R.sub.+)-(O.sub.++M.sub.+)+(Q.sub.++S-
.sub.+)}-k{(L.sub.-+R.sub.-)+(M.sub.-+S.sub.-)} (equation 3)
[0165] While the polarizing diffraction grating 66 is divided into
a plurality of regions for the purpose of detecting focuses or the
like, it is the same detection method as FIG. 21 of the embodiment
2 from the viewpoint of the tracking error signal. Moreover, the RF
signal detection is obtained by performing the following
computation using the detection signals of regions 70 to 79.
(RF
signal)=N.sub.++P.sub.++Q.sub.++O.sub.++L.sub.++R.sub.++S.sub.++M.su-
b.++T.sub.++U+ (equation 4)
[0166] DPD signal detection is also obtained by performing the
following computation using the detection signals of regions 70 to
79.
(DPD
signal)={(N.sub.++L.sub.+)+(Q.sub.++S.sub.+)}-{(P.sub.++R.sub.+)+(O-
.sub.++M.sub.+)} (equation 5)
Such an optical system structure enables obtaining not only the
tracking error signals but also other signals.
Embodiment 8
[0167] In an embodiment 8, an optical reproducing apparatus
equipped with an optical pickup apparatus 101 will be described.
FIG. 30 is a schematic structure of the optical reproducing
apparatus. The optical pickup apparatus 101 is provided with a
mechanism for allowing the optical pickup apparatus to move in the
radial direction of the optical disc and is position-controlled in
response to an access control signal from an access control circuit
172.
[0168] A predetermined laser driving current is supplied to a
semiconductor laser in the pickup apparatus 101 from a laser
lighting circuit 177, and a laser light of a predetermined light
amount is emitted from the semiconductor laser in response to
reproduction. It should be noted that the laser lighting circuit
177 can be installed in the optical pickup apparatus 101.
[0169] A signal outputted from a light detector in the optical
pickup apparatus 101 is transferred to a servo signal generating
circuit 174 and information signal generating circuit 175. A servo
signal such as a focus error signal, a tracking error signal or a
tilt control signal is generated at the servo signal generating
circuit 174 based on the signal from the light detector. An
objective lens is position-controlled by controlling an actuator in
the pickup apparatus 101 via the actuator circuit 173 based on the
servo signal.
[0170] At the information signal reproducing circuit 175,
information signals stored in the optical disc 100 are reproduced
based on the information from the light detector. Part of the
signals obtained at the servo signal generating circuit 174 and
information reproducing circuit 175 is transferred to a control
circuit 176. A spindle motor driving circuit 171, the access
control circuit 172, the servo signal generating circuit 174, the
laser lighting circuit 177, a spherical aberration correction
element driving circuit 179 and the like are connected to the
control circuit 176. The control circuit 176 controls the rotation,
access direction and access position of a spindle motor 180 that
rotates the optical disc 100, servo-controls the objective lens,
controls the amount of light emitted by the semiconductor laser in
the optical pickup apparatus 101, corrects the spherical aberration
due to a difference in the disc thickness, and performs others.
Embodiment 9
[0171] In an embodiment 9, an optical recording and reproducing
apparatus equipped with an optical pickup apparatus 101 will be
described. FIG. 31 is a schematic structure of the optical
recording and reproducing apparatus. A difference of the optical
recording and reproducing apparatus of the embodiment 9 from the
optical information reproducing apparatus shown in FIG. 30 lies in
that the apparatus of this embodiment is provided with an
information signal recording circuit 178 between the control
circuit 176 and laser lighting circuit 177, and is added with a
function for controlling the lighting of the laser light circuit
177 based on the record controlling signal from the information
signal recording circuit 178 to write desired information to the
optical disc 100.
[0172] It should be further understood by those skilled in the art
that although the foregoing description has been made on
embodiments of the invention, the invention is not limited thereto
and various changes and modifications may be made without departing
from the spirit of the invention and the scope of the appended
claims.
* * * * *