Dynamic information storage or retrieval – Condition indicating – monitoring – or testing – Including radiation storage or retrieval
Reexamination Certificate
2001-04-18
2004-03-16
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Condition indicating, monitoring, or testing
Including radiation storage or retrieval
C369S044320, C369S044260, C369S112120, C369S112150
Reexamination Certificate
active
06707773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an optical head apparatus and an optical information recording and playback apparatus which record on and play back from an optical recording medium, and particularly relates to an optical head apparatus and an optical information recording/playback apparatus which are capable of detecting a radial tilt of an optical recording medium.
2. Description of the Prior Art
A recording density in an optical information recording/playback apparatus is inversely proportional to a square of a diameter of a focused spot formed on an optical recording medium by an optical head apparatus. As the diameter of the focused spot becomes smaller, the recording density becomes higher. The diameter of the focused spot is inversely proportional to a numerical aperture of an objective lens in the optical head apparatus. Further, as the numerical aperture of the objective lens becomes higher, the diameter of the focused spot becomes smaller. Meanwhile, when the optical recording medium is tilted to a radial direction with respect to the objective lens, a shape of the focused spot is distorted due to a coma aberration caused on a substrate of the optical recording medium, and recording/playback characteristics are deteriorated. Since the coma aberration is proportional to a cube of the numerical aperture of the objective lens, as the numerical aperture of the objective lens becomes higher, a margin of a tilt of the optical recording medium in the radial direction with respect to the recording/playback characteristics (radial tilt) becomes smaller. Therefore, in the optical head apparatus and the optical information recording/playback apparatus in which the numerical aperture of the objective lens is made high in order to increase the recording density, it is necessary to detect and compensate the radial tilt of the optical recording medium so that the recording/playback characteristics are not deteriorated.
FIG. 27
shows a structure of a conventional optical head apparatus which is capable of detecting the radial tilt of the optical recording medium. This optical head apparatus is described in Japanese Patent Application Laid-Open No. 7-141673 (1995). A beam emitted from a semiconductor laser
257
is converted into a parallel beam by a collimating lens
258
, and about 50% of the beam is transmitted through a half mirror
259
and is focused on a disc
261
by an objective lens
260
. A beam reflected by the disc
261
is transmitted through the objective lens
260
in the opposite direction, and about 50% is reflected by a half mirror
259
, and is divided into a transmitted beam and diffracted beams by a holographic element
262
. The beams are transmitted through a lens
263
and are detected by a photo detector
264
.
FIG. 28
is a plan view of the holographic element
262
. The holographic element
262
has elliptical grating regions
265
and
266
which are positioned on the radial direction of the disc
261
. The directions of the gratings in both the regions
265
and
266
are approximately parallel with a tangential direction of the disc
261
, and the pattern of the gratings in both the regions
265
and
266
is off-axis concentric shape. Beams incident to the regions
265
and
266
are partially or fully diffracted as +1st order beams. Meanwhile, beams incident to the outside of the regions
265
and
266
are fully transmitted. Here, a dotted line in
FIG. 28
shows an effective diameter of the objective lens
260
.
FIG. 29
shows a pattern of detection portions of the photo detector
264
and an arrangement of focused spots on the photo detector
264
. A focused spot
271
corresponds to a beam transmitted from the outside of the regions
265
and
266
of the holographic element
262
, and it is received by detection portions
267
and
268
which are divided into two by a dividing line passing through an optical axis and parallel with the tangential direction of the disc
261
. A focused spot
272
corresponds to the +1st order beam diffracted by the inside of the region
265
of the holographic element
262
, and it is received by a single receiving area
269
. A focused spot
273
corresponds to the +1st order beam diffracted by the inside of the region
266
of the holographic element
262
, and it is received by a single receiving area
270
.
When outputs from the detection portions
267
to
270
are represented by V
267
to V
270
respectively, a tracking error signal is obtained by calculation of (V
267
+V
269
)−(V
268
+V
270
) according to the push-pull method. A radial tilt signal for detecting a radial tilt of the disc
261
is obtained by calculation of (V
267
+V
270
)−(V
268
+V
269
). Moreover, a playback signal is obtained by calculation of (V
267
+V
268
+V
269
+V
270
). A method of obtaining a focusing error signal is not described.
There is explained below the reason the radial tilt of the disc
261
can be detected by the above-mentioned calculation with reference to
FIGS. 30
to
32
.
FIGS. 30
to
32
show calculation examples of intensity distribution of the reflected beam from the disc
261
. In the drawings, a dark portion corresponds to a portion where the intensity is strong, and a beaming portion corresponds to a portion where the intensity is weak.
FIG. 30
shows the intensity distribution in the case where the disc
261
does not have the radial tilt. The intensity distribution is symmetrical with respect to a straight line which passes through the optical axis and is parallel to the tangential direction of the disc
261
. Further, the intensity is comparatively strong in regions
274
and
276
where the 0th order beam overlaps with the +1st order beam diffracted by the disc
261
. The intensity is also comparatively strong in regions
275
and
277
where the 0th order beam overlaps with the −1st order beam diffracted by the disc
261
. On the contrary, the intensity is comparatively weak in a region
278
where there is only the 0th order beam from the disc
261
.
FIG. 31
shows the intensity distribution in the case where the disc
261
has a positive radial tilt. As for regions
279
and
281
which are regions where the 0th order beam and the +1st order beam diffracted by the disc
261
are overlapped with each other, the intensity in the region
279
as a peripheral area is stronger than the intensity in the region
281
as a central section. As for regions
280
and
282
which are regions where the 0th order beam and the −1st order beam diffracted by he disc
261
are overlapped with each other, the intensity in the region
280
as a peripheral area is weaker than the intensity in the region
282
as a central section.
FIG. 32
shows the intensity distribution in the case where the disc
261
has a negative radial tilt. As for regions
283
and
285
which are regions where the 0th order beam and the +1st order beam diffracted by the disc
261
are overlapped with each other, the intensity in the region
283
as a peripheral area is weaker than the intensity in the region
285
as a central section. As for regions
284
and
286
which are regions where the 0th order beam and the −1st order beam diffracted by the disc
261
are overlapped with each other, the intensity in the region
284
as a peripheral area is stronger than the intensity in the region
286
as a central section.
In
FIGS. 30
to
32
, the peripheral area and the central area in the region where the 0th order beam and the +1st order beam diffracted by the disc
261
are overlapped with each other correspond to the detection portions
267
and
269
of the photo detector
264
shown in
FIG. 29
, and the peripheral area and the central area in the region where the 0th order beam and the −1st order beam diffracted by the disc
261
are overlapped with each other correspond to the detection portions
268
and
270
of the photo detector
264
shown in FIG.
29
.
When the radial ti
NEC Corporation
Tran Thang V.
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