Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-04-10
2004-10-12
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S053190
Reexamination Certificate
active
06804180
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head assembly for recording and reproducing data on and from an optical disk medium and to an optical information recording/reproducing device. In particular, the present invention relates to an optical head assembly and an optical information recording/reproducing device with which tangential tilt of optical recording media can be detected.
2. Description of the Prior Art
Recording density obtained in optical information recording/reproducing devices is in inverse proportion to the square of the diameter of focused light spots formed on an optical recording medium with an optical head assembly. Thus, the smaller the diameter of the focused light spot is, the higher the recording density is. The diameter of the focused light spot is in inverse proportion to the numerical aperture of the objective lens in the optical head assembly. Thus, the higher the numerical aperture of the objective lens is, the smaller the diameter of the focused light spot is.
Tilting of the optical recording medium in a tangential direction relative to the objective lens deforms the focused light spots due to coma aberration for which the substrate of the optical recording medium is responsible, badly affecting recording/reproducing properties. The coma aberration is in proportion to the cubic of the numerical aperture of the objective lens. Thus, the higher the numerical aperture of the objective lens is, the smaller a margin for the tangential tilt of the optical recording medium with respect to the recording/reproducing properties is. Accordingly, it is necessary to detect and correct the tangential tilt of optical recording media to avoid degradation of recording/reproducing properties in optical head assemblies and optical information recording/reproducing device that employ an objective lens with a higher numerical aperture for a higher recording density.
FIG. 17
shows a configuration of a conventional optical head assembly with which the tangential tilt of optical recording media can be detected. This optical head assembly is disclosed in Japanese Patent Laid-Open No. 9-161293. The light beam emitted from a semiconductor laser
105
is converted into parallel rays with a collimator lens
106
. The parallel rays are then incident on a diffraction grating
107
where they are divided into the zero order diffracted beam, the plus first order diffracted beam and the minus first order diffracted beam.
These beams are directed to a half mirror
108
where about 50% of the light is passed through it and are focused on a disk D by an objective lens
109
. The three beams reflected from the disk D are transmitted through the objective lens
109
in the opposite direction, about 50% of which are reflected from the half mirror
108
. The reflected light is passed through a cylindrical lens
111
and a lens
112
, and is then received by a photodetector
113
. The photodetector
113
is disposed on the midway of the line focuses of the cylindrical lens
111
and the lens
112
.
FIG. 18
is a plan view of the diffraction grating
107
. The diffraction grating
107
serves to impart, to the plus and minus first order diffracted beams, a coma aberration in the tangential direction of the disk D. The direction of grating in the diffraction grating
107
is generally in parallel with the radial direction of the disk D. A grating pattern is such that the upper lines (upper half in the figure) are curved upward while the lower lines (lower half in the figure) are curved downward.
FIG. 19
shows location of the focused light spots on the disk D. Focused light spots L
115
, L
116
, and L
117
correspond to the zero order diffracted beam, the plus first order diffracted beam and the minus first order diffracted beam, respectively, from the diffraction grating
107
. These spots are located on the same track D
1
where pits are formed. The focused light spots L
116
and L
117
have side lobes on upper and lower sides thereof, respectively, in the tangential direction of the disk D.
FIG. 20
shows a pattern of light receiving elements of the photodetector
113
and location of the focused light spots on the photodetector
113
. A light spot L
124
corresponds to the zero order diffracted beam from the diffraction grating
107
and is received by light receiving elements
118
to
121
which are four divisions defined by the dividing line parallel to the tangential direction of the disk D traversing across the optical axis and the other dividing line parallel to the radial direction. A light spot L
125
corresponds to the plus first order diffracted beam from the diffraction grating
107
and is received by a single light receiving element
122
. A light spot L
126
corresponds to the minus first order diffracted beam from the diffraction grating
107
and is received by a single light receiving element
123
. The row of the focused light spots L
115
to L
117
on the disk D is oriented in the tangential direction while the row of the light spots L
124
to L
126
on the photodetector
113
is oriented in the radial direction due to an effect of the cylindrical lens
111
and the lens
112
(the up-and-down direction corresponds to the radial direction and the side-to-side direction corresponds to the tangential direction in FIG.
20
).
The outputs from the light receiving elements
118
to
123
are herein represented by V
118
to V
123
, respectively. A focus error signal may be obtained, using the astigmatic method, from the following arithmetic operation:
(V118+V121)−(V119+V120).
A tracking error signal may be obtained, using the push-pull method, from the following arithmetic operation:
(V118+V120)−(V119+V121).
A playback signal produced by the focused light spot L
115
may be obtained from the following arithmetic operation:
V118+V119+V120+V121.
The tangential tilt of the disk D may be detected by either one of the following two approaches. The first approach is to obtain a tangential tilt signal by subtracting V
123
from V
122
. The second approach is to obtain a tangential tilt signal according to a difference in bit error rates of a playback signal produced by the focused light spot L
116
from the output V
122
and a playback signal produced by the focused light spot L
117
from the output V
123
.
When the first approach is used to detect the tangential tilt of an optical recording medium in the conventional optical head assembly, there is a defect of not being capable of detecting the tangential tilt at a high sensitivity because the change in the outputs V
122
and V
123
for the tangential tilt is significantly small.
On the other hand, when the second approach is used to detect the tangential tilt of the optical recording medium in the conventional optical head assembly, it is necessary to measure the bit error rates in the playback signals. The tangential tilt can be detected only with optical recording media for playback only, where such signals are recorded previously. No tangential tilt can be detected with recordable and rewritable optical recording media where no such signal is recorded previously.
BRIEF SUMMARY OF THE INVENTION
Object of the Invention
An object of the present invention is to overcome the above-mentioned problem in the conventional optical head assembly with which the tangential tilt of the optical recording media can be detected. Another object of the present invention is to provide an optical head assembly and an optical information recording/reproducing device which allows detection of the tangential tilt at a high sensitivity and with which the tangential tilt can be detected even on the recordable and rewritable optical recording media where no signal is recorded previously.
SUMMARY OF THE INVENTION
An optical head assembly according to the present invention comprises: a light source; an objective lens which focuses transmitted light from the light source onto an optical recording medium; and a photodetector adapted to receive refle
NEC Corporation
Tran Thang V.
Vuong Bach
Whitham Curtis & Christofferson, P.C.
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