Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-12-15
2003-07-15
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
Reexamination Certificate
active
06594206
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical head and an optical information recording/reproducing device, and more particularly to an optical head and an optical information recording/reproducing device which store informations to an optical storage medium and reproduce the informations stored in the optical storage medium, and also which are capable of detecting a radial tilt of the optical storage medium.
A recording density of the optical information recording/reproducing device is inversely proportional to the square of a diameter of a beam spot on the optical storage medium. As the diameter of the beam spot is small, then the recording density is high. The diameter of the beam spot is inversely proportional to the numerical aperture of an objective lens of the optical head. Namely, as the numerical aperture of the objective lens is high, the diameter of the beam spot is small and the recording density is high.
As the optical storage medium is tilted in a radial direction with reference to the objective lens, a distortion in shape or a deformation in shape of the beam spot is caused due to a frame aberration, whereby characteristics of the recording and reproducing operations are made deteriorated. Since the frame aberration is proportional to the third power of the numerical aperture of the objective lens, the increase in the numerical aperture of the objective lens makes narrow the acceptable margin in the radial tilt of the optical storage medium to keep the recording and reproducing characteristics. If the numerical aperture of the objective lens is increased in order to increase the recording density, then it is necessary to detect and correct the radial tilt of the optical recording medium to keep the recording and reproducing characteristics.
FIG. 1
is a schematic view illustrative of a first conventional optical head which is capable of detecting the radial tilt of the optical recording medium. This first conventional optical head is disclosed in Japanese laid-open patent publication No. 9-161293. The first conventional optical head has a semiconductor laser
121
, a collimator lens
122
, a diffraction optical device
123
, a half mirror
124
, an objective lens
125
, an optical disk
126
, a cylindrical lens
127
, an additional lens
128
and a photo-detector
129
. A laser beam is emitted from the semiconductor laser
121
, and transmitted through the collimator lens
122
where the laser beam is collimated. The collimated laser beam is then transmitted through the diffraction optical device
123
, where the collimated laser beam is divided into three parts, for example, 0-order light, +1-order diffracted light, and −1-order diffracted light. The divided three lights reach the half mirror
124
, wherein about 50% of the divided three lights pass through the half mirror
124
and then are transmitted through the objective lens
125
, wherein the lights are condensed onto the optical disk
126
. The lights are then reflected from the optical disk
126
and further transmitted through the objective lens
125
to reach the half mirror
124
, wherein about 50% of the reflected three lights are reflected by the half mirror
124
. The further reflected three lights are then transmitted through the cylinder lens
127
and the additional lens
128
to reach the photo-detector
129
. The photo-detector
129
is positioned at an intermediate point between focal points of the cylinder lens
127
and the additional lens
128
.
FIG. 2
is a plane view illustrative of the diffraction optical device of the first conventional optical head shown in FIG.
1
. The diffraction optical device
123
to provide both the +1-order diffracted light and the −1-order diffracted light with the frame aberration in the radial direction of the disk
126
. A lattice direction of the diffraction optical device
123
is almost parallel to the radial direction of the disk
126
. A lattice pattern of the diffraction optical device
123
is such that a left half region has a downwardly-arched pattern and a right half region has a upwardly-arched pattern, wherein the left and right regions are bounded by a center line crossing an optical axis of the diffraction optical device
123
and in a tangential direction perpendicular to the radial direction.
FIG. 3
is a plane view illustrative of an arrangement of beam spots of an alignment of tracks of the optical disk of the first conventional optical head shown in FIG.
1
. Each of the tracks has a single alignment of pits. First, second and third beam spots
131
,
132
, and
133
correspond to the 0-order diffracted light, the +1-order diffracted light and the −1-order diffracted light, respectively. The first, second and third beam spots
131
,
132
, and
133
are aligned on a single track
130
. The second beam spot
132
has a right side lobe in a right side with reference to the radial direction. The third beam spot
133
has a left side lobe in a left side with reference to the radial direction.
FIG. 4
is a plane view illustrative of an arrangement of the beam spots and an alignment of patterns of photo-receiving parts of the photo-detector in the first optical head shown in FIG.
1
. First, second and third beam spots
140
,
141
and
142
correspond to the 0-order diffracted light, the +1-order diffracted light and the −1-order diffracted light, respectively. The first beam spot
140
is received by divided photo-receiving areas
134
,
135
,
136
and
137
which are bounded by both a first dividing line crossing the optical axis and being parallel to the tangential line of the disk
126
, and a second dividing line crossing the optical axis and being parallel to the radial direction. The second beam spot
141
is received by a single photo-receiving area
138
. The third beam spot
142
is received by a single photo-receiving area
139
. The alignment of the first, second and third beam spots
131
,
132
, and
133
are parallel to the tangential direction, whilst the alignment of the first, second and third beam spots
140
,
141
and
142
are parallel to the radial direction perpendicular to the tangential direction due to the functions of the cylinder lens
127
and the additional lens
128
.
Outputs from the photo-receiving areas
134
through
139
are represented by V
134
to V
139
. Focus error signals are obtained by an astigmatism method, wherein an operation (V
134
+V
137
)−(V
135
+V
136
) is made. Track error signals are obtained by a push-pull method, wherein an operation (V
134
+V
136
)−(V
135
+V
137
) is made. A reproducing signal by the beam spot
131
is obtained by an operation (V
134
+V
135
+V
136
+V
137
).
The radial tilt of the disk
126
may be detectable by either one of the following two methods. First method is to obtain the radial tilt signal from the operation (V
138
−V
139
). Second method is to obtain the radial tilt signal from a difference in bit error rate of between a first reproducing signal by the beam spot
132
obtained from the V
138
and a second reproducing signal by the beam spot
133
obtained from the V
139
.
If the radial tilt is detected by the above first method, then a variation of V
138
and V
139
with reference to the radial tilt is extremely small, for which reason it is difficult to realize a highly sensitive detection of the radial tilt. If the radial tilt is detected by the above second method, then it is necessary to measure the bit error rate in the reproducing signals, for which reason it is possible to detect the radial tilt for the reproducing only disk having already stored the signals, whilst it is impossible to detect the radial tilt for the write-enable disk.
In the above circumstances, it had been required to develop a novel optical head and a novel optical information recording/reproducing device free from the above problem.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a novel optical head capable of detecti
NEC Corporation
Tran Thang V.
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