Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-07-03
2004-12-28
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044290
Reexamination Certificate
active
06836452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup apparatus enabling to read two or more kinds of recording mediums having different reading wavelengths from each other such as a compatible optical pickup apparatus for DVD/CD, and particularly to an optical pickup apparatus using a semiconductor laser element formed of a one-chip laser diode for emitting two laser beams having different wavelengths from each other.
2. Description of the Related Art
Conventionally, a DVD/CD compatible reproducing apparatus sharing an optical pickup of a DVD reproducing apparatus with a CD reproducing apparatus has actively been proposed, and there are forms such as a DVD/CD compatible reproducing apparatus using an optical pickup of one-wavelength/bifocal type and a DVD/CD compatible reproducing apparatus using an optical pickup of two-wavelength/bifocal type.
Comparing a structure of a CD with that of a DVD, a thickness of a protective layer of the DVD (0.6 mm) is about one-half that of a protective layer of the CD. In case of reproducing both of optical disks using an optical pickup of unifocal type, when a light beam is gathered to achieve optimization to an information recording surface of the DVD, the protective layer of the CD through which the light beam passes is thicker than that of the DVD. Therefore, aberration such as spherical aberration occurs in the light beam and thus the light beam cannot be gathered optimally to an information recording surface of the CD. Also, since the CD is different from the DVD in a size of information pits formed for recording, it is necessary to form a beam spot having an optimum size to each of information pits on the information recording surface of the CD or the DVD in order to exactly read each of information pits.
A size of the beam spot is proportional to a ratio of a wavelength of the light beam to a numerical aperture of an objective lens for gathering the light beam on the information recording surface. That is, when it is assumed that a wavelength of the light beam is constant, the beam spot becomes smaller as the numerical aperture becomes larger. Thus, in case of reproducing the CD and the DVD by the optical pickup of unifocal type, when it is constructed so that a wavelength of the light beam is constant and the numerical aperture is adapted for, for example, the information pits of the DVD, the beam spot is too small with respect to the information pits of the CD and distortion occurs in a reproduction signal on reproducing the CD and an exact reading becomes difficult. Hence, a DVD/CD compatible reproducing apparatus using an optical pickup of bifocal type enabling to focalize on different positions on the same straight line and applying two laser beams for forming a beam spot with a proper size in correspondence with a size of each of information pits has become the mainstream.
For example, an optical pickup apparatus shown in
FIG. 16
is a DVD/CD compatible reproducing apparatus mixing an optical path from a first light source
10
for CD and an optical path from a second light source
15
for DVD are mixed by a first beam splitter
13
acting as a prism to apply the mixed optical path to a bifocal lens comprising an objective lens and a diffraction element. A configuration and operation thereof will be described briefly.
In
FIG. 16
, according to a driving signal from a first driving circuit
11
, the first light source
10
generates a laser beam (shown by a broken line) having a wavelength (780 nm) most suitable for an information reading from a CD, and this laser beam is applied to the first beam splitter
13
through a grating
12
for generating three beams. The first beam splitter
13
reflects the laser beam from the first light source
10
to guide the reflected light to a second beam splitter
14
.
On the other hand, according to a driving signal from a second driving circuit
16
, the second light source
15
placed at a 90° angle with respect to the first light source
10
generates a laser beam (shown by a solid line) having a wavelength (650 nm) most suitable for an information reading from a DVD, and this laser beam is applied to the first beam splitter
13
through a grating
17
for generating three beams. The first beam splitter
13
transmits the laser beam from the second light source
15
to guide the laser beam to the second beam splitter
14
.
The second beam splitter
14
guides the laser beam supplied through the first beam splitter
13
, namely the laser beam from the first light source
10
or the second light source
15
to a bifocal lens
19
through a collimator lens
18
. The bifocal lens
19
applies a information reading light gathered the laser beam from the second beam splitter
14
at one point to an information recording surface of an optical disk
21
rotated and driven by a spindle motor
20
.
The laser beam from the first light source
10
(shown by a broken line) is gathered by the bifocal lens
19
to focus on an information recording surface C of the optical disk
21
. Also, the laser beam from the second light source
15
(shown by a solid line) is gathered by the bifocal lens
19
to focus on an information recording surface D of the optical disk
21
.
The reflected light, which is caused by applying the information reading light from the bifocal lens
19
to the optical disk
21
, passes through the bifocal lens
19
and the collimator lens
18
, is reflected by the second beam splitter
14
, passes through a cylindrical lens
22
which is an astigmatism generation element, and is applied to a photodetector
23
. The photodetector
23
generates an analog electrical signal having a level corresponding to the quantity of an applied light to supply the signal to an information data reproducing circuit
24
and a disk determination circuit
25
as a reading signal.
The information data reproducing circuit
24
generates a digital signal based on the obtained reading signal and further performs demodulation and error correction to the digital signal and reproduces an information data. The disk determination circuit
25
identifies a kind of the optical disk
21
based on a size of a beam spot formed at the time of applying a laser beam to the optical disk
21
to supply a disk identification signal to a controller
26
as disclosed in, for example, JP-A-10-255274 by the present applicant. According to the disk identification signal, the controller
26
drives and controls any one of the first driving circuit
11
and the second driving circuit
16
in order to selectively bring about a drive state.
The controller
26
drives only the first driving circuit
11
when the disk identification signal indicating the CD is obtained from the disk determination circuit
25
. Accordingly, a laser beam emitted from the first light source
10
is applied to the optical disk
21
through an optical system comprising the grating
12
, the first beam splitter
13
, the second beam splitter
14
, the collimator lens
18
and the bifocal lens
19
. Then, the reflected light (return light) reflected by the information recording surface C of the optical disk
21
passes through the bifocal lens
19
and the collimator lens
18
, is reflected by the second beam splitter
14
, passes through the cylindrical lens
22
, and is applied to the photodetector
23
.
The controller
26
drives only the second driving circuit
16
when the disk identification signal indicating the DVD is obtained from the disk determination circuit
25
. Accordingly, a laser beam emitted from the second light source
15
is applied to the optical disk
21
through an optical system comprising the grating
17
, the first beam splitter
13
, the second beam splitter
14
, the collimator lens
18
and the bifocal lens
19
. Then, the reflected light (return light) reflected by the information recording surface D of the optical disk
21
passes through the bifocal lens
19
and the collimator lens
18
, is reflected by the second beam splitter
14
, passes through the cylindrical lens
22
Pioneer Corporation
Tran Thang V.
Vuong Bach
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