Dynamic information storage or retrieval – Condition indicating – monitoring – or testing – Including radiation storage or retrieval
Reexamination Certificate
2003-02-24
2004-05-11
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Condition indicating, monitoring, or testing
Including radiation storage or retrieval
Reexamination Certificate
active
06735157
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical-pickup device and a tilt-detecting method thereof.
2. Description of the Related Art
Recently, enlargement of the storage capacity of an optical recording medium, typically, an optical disc, is strongly demanded. In order to increase the storage capacity without enlargement of the size of the optical recording medium itself, it is necessary to reduce the diameter of the light spot used in information recording/reproducing. Because the diameter of the light spot is inversely proportional to the numerical aperture NA of the objective lens, the storage capacity is proportional to the square of the NA. Therefore, increasing the NA has been attempted. For example, an optical disc (DVD: Digital Versatile Disk) for performing recording/reproducing in a condition where NA=0.6 has been developed although a conventional optical disc (CD-RW: Compact Disk ReWritable) is such that recording/reproducing is performed in a condition in which NA=0.5.
In an optical-pickup device, rays from a light source are focused, so as to form a light spot, on a recording plane of an optical disc via a transparent substrate. The shape of the light spot is distorted due to coma occurring due to a tilt of the disc substrate. The coma is proportional to the cube of NA. Because setting is made such that NA=0.6 (larger) for the DVD while setting is made such that NA=0.5 for the CD-RW, the thickness of the substrate of the DVD is ½ of that of the CD so as to reduce the influence of coma. However, because various margins (the defocus, the control error and so forth) are reduced due to increase in the storage capacity, the allowable amount of the tilt of the disc substrate is small. In order to obtain the best RF signal by causing the objective lens of the optical pickup to face the optical disc in a condition where the axis of the objective lens is perpendicular to the disc substrate although the disc substrate tilts, it is necessary to perform tilt-servo control. In order to perform the tilt-servo control, a tilt sensor which detects the tilt of the disc substrate is needed.
An example of such a tilt sensor in the related art will now be described with reference to FIG.
1
. This example of the tilt sensor consists of a combination of an LED
101
provided to face the recording plane of an optical disc
100
, a two-piece photodetector
102
and a subtracter
103
. The LED
101
emits an LED light to the recording plane of the optical disc
100
, the light reflected by the recording plane of the optical disc
100
is received by the photodetector
102
, the difference between the light-reception signals from the respective two areas of the two-piece photodetector is obtained through the subtracter
103
, and the thus-obtained difference signal is outputted as a tilt-error signal indicating the tilt of the optical disc
100
.
However, in this example, because the tilt sensor is provided completely separately from the optical-pickup device, an extra setting space is needed, thereby, miniaturization of the entire optical-disc apparatus is obstructed, and, also, the cost of the apparatus increases. Further, it is necessary to set the LED
101
in a manner in which the optical axis of the LED
101
is accurately parallel to the optical axis of the laser light of the optical-pickup device. Therefore, the adjustment work at the time of assembly is troublesome. Furthermore, because the tilt sensor shown in
FIG. 1
is located distantly from the objective lens, the accuracy in detection of the relative tilt between the optical disc
100
and the objective lens performed through the tilt sensor is not high. Furthermore, generally speaking, the tilt of the objective lens varies due to focusing and tracking. In such a case, it is not possible to detect the tilt of the objective lens accurately.
For example, Japanese Laid-Open Patent Application No. 9-147395 discloses an optical-pickup device in which these problems are solved. This optical-pickup device will now be described with reference to
FIGS. 2 and 3
. This optical-pickup device includes a laser diode
110
which is a light source. This device further includes a polarization beam splitter
113
which transmits the light emitted by the laser diode
110
as it is, and, thus, causes the light to travel to an optical disc
111
. The polarization beam splitter
113
also reflects light reflected by the optical disc
111
through the polarization plane thereof, and, thus, causes the light to travel to a four-piece photodetector
112
. The device further includes a collimator lens
114
which collimates the light, emitted by the laser diode
110
. The device further includes a quarter-wave plate
115
which rotates the vibration plane of the light, which the plate
115
transmits, by a ¼ wavelength. The device further includes a hologram element
116
which disperses the parallel light, provided from the quarter-wave plate
115
, into the 0-th-order light (non-diffracted light) and the first-order light. The device further includes an objective lens
117
, which is formed so as to be integral with the hologram element
116
, and which has a plurality of focuses so as to adapt to variation of the thickness of the substrate of the optical disc. A multiple-focus lens
118
is formed as a result of combining the objective lens
117
and the hologram element
116
. The device further includes a multiple lens
119
, acting as astigmatism means, provided in front of the photodetector
112
. The multiple-focus lens
118
disperses incident light into a plurality of beams of light. A beam L
0
(or L
1
) of the plurality of beams of light, which beam is focused on the recording plane of the optical disc
111
, is reflected thereby, and the thus-obtained light is received by the photodetector
112
through the multiple lens
119
. The thus-received light is used as a signal for reproducing information recorded on the optical disc
111
.
In addition to this basic arrangement, two light-receiving elements
121
and
122
are provided on both sides of the photodetector
112
, in a direction corresponding to the radial direction of the optical disc
111
, as shown in FIG.
3
. The difference between the output signals of these light-receiving elements
121
and
122
is obtained through a subtracter
123
. The thus-obtained difference is used as a tilt-error signal.
However, in the case of the tilt-detecting method shown in
FIGS. 2 and 3
, because the laser light emitted by the laser diode
110
is modulated at the time of a recording operation, it is not possible to obtain a stable tilt-error signal. Further, the extra light-receiving elements
121
and
122
are needed. Therefore, the arrangement is complicated, and, also, cost reduction and miniaturization of the device are obstructed.
Further, when an optical-axis deviation exists between the ideal optical axis and the optical axis of the objective lens at the time of tracking, the offset due to the optical-axis deviation is included in the tilt signal. However, it is not possible to determine whether this offset develops due to the optical-axis deviation or the tilt. Therefore, in a case where an optical system is such that the optical-axis deviation is large, a countermeasure such as to provide a member for detecting the optical-axis deviation is needed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical-pickup device and a tilt-detecting method in which, basically, it is not necessary to provide an extra member for detecting the tilt, and, also, the tilt can be always detected even during a recording/reproducing operation.
Another object of the present invention is to provide an optical-pickup device in which the tilt can be accurately detected without being affected by the offset due to the optical-axis deviation.
An optical-pickup device according to the present invention, comprises:
a plurality of light sources;
an objective lens for converging light from th
Akiyama Hiroshi
Hirai Hideaki
Koide Hiroshi
Dickstein , Shapiro, Morin & Oshinsky, LLP
Ricoh & Company, Ltd.
Tran Thang V.
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