Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-03-12
2001-01-30
Young, W. R. (Department: 2753)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230
Reexamination Certificate
active
06181667
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical recording medium drive employing the same.
2. Description of the Prior Art
An optical pickup apparatus employed for an optical recording medium drive such as an optical disk drive is adapted to record or read information in or from an optical recording medium such as an optical disk or detect a servo signal with a laser beam.
FIG. 20
schematically illustrates a conventional optical pickup apparatus disclosed in Japanese Patent Laying-Open Gazette No. 3-76035 (1991). This optical pickup apparatus performs tracking servo control by the three-beam method.
Referring to
FIG. 20
, symbols X, Y and Z denote the radial direction of an optical disk
1
, the track direction of the optical disk
1
, and a direction perpendicular to the disk plane of the optical disk
1
respectively.
A semiconductor laser device
102
emits a laser beam B in the direction Z. The beam B emitted from the semiconductor laser device
102
enters a diffraction grating
103
.
FIG. 21
is a plan view of the diffraction grating
103
. The diffraction grating
103
has a grating surface
103
a
formed by unevenness of regular pitches. The grating surface
103
a
divides the incident laser beam B into three beams, i.e., a 0th order diffracted beam (main beam), a +1st order diffracted beam (subbeam) and a −1st order diffracted beam (subbeam), and transmits the same through a transmission-type holographic optical element
104
.
Referring to
FIG. 20
, an objective lens
105
condenses the three beams transmitted through the transmission-type holographic optical element
104
on the optical disk
1
.
FIG. 22
is a model diagram showing the condensed states on the recording plane of the optical disk
1
. As shown in
FIG. 22
, the 0th order diffracted beam is condensed on a track surface TR of the recording plane as a main spot MO, and the ±1st order diffracted beams are condensed on both sides of the main spot MO as subspots S
1
and S
2
respectively.
The transmission-type holographic optical element
104
diffracts three returned beams (reflected beams) from the main spot MO and the subspots S
1
and S
2
in a plane substantially including the directions X and Z, so that a photodetector
106
detects these returned beams.
FIG. 23
is a typical plan view showing an exemplary photodetector
106
. This photodetector
106
includes a photodetection part
106
a
provided on the central portion for performing focus servo control with the astigmatism method and photodetection parts
106
b
and
106
c
provided on both sides of the photodetection part
106
a
for performing tracking servo control with the three-beam method. The returned beam corresponding to the main spot MO enters the central portion of the photodetection part
106
a
while the returned beams corresponding to the subspots S
1
and S
2
enter the photodetection parts
106
b
and
106
c
respectively.
The aforementioned optical pickup apparatus performs tracking control in the following manner: As shown in
FIG. 22
, the track surface TR recording information is different in light reflectance from a non-track surface. When the photodetection parts
106
b
and
106
c
detect the returned beams from the subspots S
1
and S
2
, the returned beams from the two subspots S
1
and S
2
entering the two photodetection parts
106
b
and
106
c
are equal in light intensity to each other if the main spot MO excellently tracks the track surface TR to be reproduced. If the main spot MO deviates to either side of the track surface TR, on the other hand, the photodetection part
106
a
or
106
b
relatively largely detects the light intensity of the returned beam from one of the subspots S
1
and S
2
. With output signals E and F from the photodetection parts
106
b
and
106
c
, therefore, the following tracking error signal TE is obtained:
TE=E−F
The optical pickup apparatus performs excellent tracking control when the tracking error signal TE is zero, and detects deterioration of the tracking state as the value of the tracking error signal TE increases.
When detecting the tracking error signal TE, the optical pickup apparatus moves the objective lens
105
in the radial direction (the direction X), for correcting the condensed positions of the main spot MO and the subspots S
1
and S
2
on the track surface TR of the optical disk
1
.
FIG. 24A
is a typical sectional view showing the condensed states of diffracted beams B
1
and B
2
diffracted by the diffraction grating
103
, and
FIG. 24B
shows typical plan views of the objective lens
105
. As shown in
FIG. 24A
, the diffracted beam B
1
diffracted by the diffraction grating
103
in the +1st order direction passes through the objective lens
105
, to be condensed as the subspot S
1
. The diffracted beam B
2
diffracted in the −1st order direction passes through the objective lens
105
, to be condensed as the subspot S
2
.
Referring to
FIG. 24B
, the grating surface
103
a
of the diffraction grating
103
is formed to be larger than the laser beam B, as shown in FIG.
20
. Therefore, the laser beam B incident on the grating surface
103
a
is diffracted over a region wider than an aperture
105
a
of the objective lens
105
, to result in regions B
1
a
and B
2
a
not entering the aperture
105
a of the objective lens
105
.
When the optical pickup apparatus performs a tracking operation in this state and moves the objective lens
105
in the direction X (the radial direction of the optical disk
1
), the incident states of the diffracted beams B
1
and B
2
on the objective lens
105
change from those on the left to those on the right in FIG.
24
B. The ratios of the diffracted beams B
1
and B
2
entering the aperture
105
a
of the objective lens
105
reduce following movement of the objective lens
105
. Therefore, the light quantities of the subspots S
1
and S
2
reduce on the recording plane
1
a
of the optical disk
1
, to result in reduction of the light quantities of the returned beams from the subspots S
1
and S
2
entering the photodetection parts
106
b
and
106
c
. When the objective lens
105
is moved during the tracking operation, therefore, the output of the tracking error signal TE disadvantageously reduces.
FIG. 25
is a model diagram for illustrating the diffracted state of the beam B diffracted by the diffraction grating
105
. Referring to
FIG. 25
, a light source
200
forms an emissive end of the semiconductor laser device
102
, so that the laser beam B emitted from this light source
200
is condensed on the recording plane
1
a
of the optical disk
1
as the two subspots S
1
and S
2
. The transmission-type holographic optical element
104
is omitted in FIG.
25
.
The grating surface
103
a
diffracts the laser beam B emitted from the light source
200
at least in the +1st order direction and the −1st order direction. In the laser beam B, the +1st order diffracted partial beam of a partial beam BE
1
passes through the objective lens
105
, to be condensed as the subspot S
1
. The +1st order diffracted partial beam of a partial beam BE
2
passes through a part beyond the objective lens
105
, not to be condensed on the subspot S
1
.
On the other hand, the −1st order diffracted partial beam of a partial beam BE
3
passes through the objective lens
105
, to be condensed on the subspot S
2
. Further, the −1st order diffracted partial beam of a partial beam BE
4
passes through a part beyond the objective lens
105
, not to be condensed on the subspot S
2
.
When an optical axis LP passing through the peak of the light intensity distribution of the laser beam B aligns with a central axis ZO passing through the center of the objective lens
105
, the light quantities of the partial beams BE
1
and BE
3
condensed on the subspots S
1
and S
2
respectively are equal to each other. Therefore, the correct tracking state can be detected by detecting the difference between the
Goto Takenori
Ibaraki Akira
Ide Daisuke
Inoue Yasuaki
Mori Kazushi
Armstrong, Westerman Hattori, McLeland & Naughton
Sanyo Electric Co,. Ltd.
Young W. R.
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