Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-04-14
2001-08-07
Huber, Paul W. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S118000
Reexamination Certificate
active
06272099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens for converging the light emitted from a light source onto the information recording surface of an optical disk, and also relates to an optical head and an optical disk device for optically recording/reproducing information onto/from an optical disk using the objective lens.
2. Description of the Related Art
An objective lens used for an optical head is designed in view of the predetermined base material thickness of an optical disk. Thus, when an optical disk having a different base material thickness from the predetermined thickness is installed, a spherical aberration is caused, so that the convergence performance is deteriorated and it becomes difficult to precisely record/reproduce information onto/from the optical disk. All of conventional optical disks, including a so-called “compact disk (CD)”, i.e., a read-only disk for music replay, a video disk and a magneto-optical disk for data storage, have a uniform base material thickness of about 1.2 mm. Therefore, it has heretofore been possible to record/reproduce information from optical disks of various types by using a single optical head.
On the other hand, a digital video disk (DVD), the specifications of which have recently been unified, uses an objective lens having an increased numerical aperture (NA) in order to realize a high density. If the numerical aperture is increased, then the optical resolution of an optical disk is improved. As a result, the width of a frequency band on which the recording/reproducing operations are enabled can be expanded. However, if an optical disk to be installed has a tilt, then a coma is adversely increased. In general, an optical disk is tilted to a certain degree against an objective lens, because the optical disk itself has a certain deflection and some inclination is almost always involved when the optical disk is installed into an optical disk device. Consequently, a kind of aberration called “coma” is generated in a converged light spot. The coma disadvantageously prevents the convergence performance from being improved even when the numerical aperture is increased.
Thus, the base material thickness of a DVD (it is noted that the “base material thickness” of a DVD corresponds to the thickness of one of a pair of bonded substrates, unlike the cases of conventional optical disks) is reduced to about 0.6 mm such that the coma is not increased even when the numerical aperture of an objective lens is increased. However, reducing the base material thickness of an optical disk means changing the predetermined base material thickness for an objective lens for recording/reproducing information onto/from the optical disk. As a result, the objective lens for the changed base material thickness can no longer be used for recording/reproducing information onto/from conventional optical disks (i.e., CDs, magneto-optical disks for data storage, etc.), i.e., the objective lens is no longer compatible with the conventional optical disks.
In order to solve such a problem, a device using two optical heads such as that shown in
FIG. 12
is proposed. The device shown in
FIG. 12
includes two optical heads
70
and
83
. The optical head
70
is used for recording/reproducing information onto/from an optical disk
10
having a base material thickness of about 0.6 mm, while the optical head
83
is used for recording/reproducing information onto/from an optical disk
11
having a base material thickness of about 1.2 mm. It is noted that, in
FIG. 12
, the left half portion of the optical disk
10
having the base material thickness of about 0.6 mm and the right half portion of the optical disk
11
having the base material thickness of about 1.2 mm are selectively illustrated.
In the optical head
70
, the radiated light having a wavelength of about 650 nm which has been emitted from a semiconductor laser device
71
is condensed by a condenser lens
72
to be transformed into a luminous flux
73
of substantially parallel light beams. The luminous flux
73
is p-polarized light, which is incident onto and transmitted through a polarization beam splitter
74
and transformed by a quarter-wave plate
75
into substantially circularly polarized light. The circularly polarized light is reflected by a reflective mirror
76
to be incident onto an objective lens
77
. The luminous flux
73
transmitted through the objective lens
77
is converged onto the information recording surface of the optical disk
10
having the base material thickness of about 0.6 mm, thereby forming a light spot
78
thereon.
The luminous flux reflected by the optical disk
10
passes through the objective lens
77
, the reflective mirror
76
and the quarter-wave plate
75
again to be incident onto the polarization beam splitter
74
. Since the reflected luminous flux is transformed by the quarter-wave plate
75
into s-polarized light, the s-polarized light is reflected by the polarization beam splitter
74
, passed through a converging lens
79
and a cylindrical lens
80
, and is received by a photodetector
81
. The photodetector
81
photoelectrically converts the received reflected luminous flux to form a reproduced signal, forms a focusing control signal in accordance with an astigmatism method, forms a tracking control signal in accordance with a phase difference method and a push-pull method, and then outputs these signals.
An objective lens driver
82
drives the objective lens
77
in the focusing direction and the tracking direction, thereby making the light spot
78
follow the tracks on the surface of the recording medium onto/from which the information is recorded/reproduced.
On the other hand, in the optical head
83
, the radiated light having a wavelength of about 780 nm which has been emitted from a semiconductor laser device
84
is condensed by a condenser lens
85
to be transformed into a luminous flux
86
of substantially parallel light beams. The luminous flux
86
is p-polarized light, which is incident onto and transmitted through a polarization beam splitter
87
and transformed by a quarter-wave plate
88
into substantially circularly polarized light. The circularly polarized light is reflected by a reflective mirror
89
to be incident onto an objective lens
90
. The luminous flux
86
transmitted through the objective lens
90
is converged onto the information recording surface of the optical disk
11
having the base material thickness of about 1.2 mm, thereby forming a light spot
91
thereon.
The luminous flux reflected by the optical disk
11
passes through the objective lens
90
, the reflective mirror
89
and the quarter-wave plate
88
again to be incident onto the polarization beam splitter
87
. Since the reflected luminous flux is transformed by the quarter-wave plate
88
into s-polarized light, the s-polarized light is reflected by the polarization beam splitter
87
, passed through a converging lens
92
and a cylindrical lens
93
, and received by a photodetector
94
. The photodetector
94
photoelectrically converts the received reflected luminous flux to form a reproduced signal, forms a focusing control signal in accordance with an astigmatism method, forms a tracking control signal in accordance with a phase difference method and a push-pull method, and then outputs these signals.
An objective lens driver
95
drives the objective lens
90
in the focusing direction and the tracking direction, thereby making the light spot
91
follow the tracks on the surface of the recording medium onto/from which the information is recorded/reproduced.
In the above-described arrangement, in the case of recording/reproducing information onto/from the optical disk
11
having a base material thickness of about 1.2 mm such as a CD, the optical head
83
is operated and controlled such that the light spot
91
is formed on the information recording surface of the optical disk
11
. On the other hand, in the case of recording/reproducing information onto/from the optical disk
10
having
Komma Yoshiaki
Mizuno Sadao
Huber Paul W.
Matsushita Electric - Industrial Co., Ltd.
Renner, Otto, Boiselle & Sklar
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