Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Utility Patent
1998-10-16
2001-01-02
Edun, Muhammad (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044290, C369S044350
Utility Patent
active
06169714
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magneto-optical information recording/reproducing apparatus, and particularly to a magneto-optical information recording/reproducing apparatus in which the recording/reproducing characteristic can be effectively prevented from being lowered by relative tilt between a magneto-optical information recording medium and a pickup, or the like.
Recently, the magneto-optical information recording/reproducing apparatus has been put into practical use as a large-capacity data file, and the magneto-optical information recording/reproducing apparatus is expected that its capacity is more increased.
A magneto-optical disk as a representative magneto-optical information recording medium has a magnetic film having perpendicular magnetic anisotropy. Information recording on the magneto-optical disk is performed by a process in which a laser beam is focussed onto the magneto-optical disk to locally heat the magnetic film, and, at the same time, a magnetic field is given thereto from the outside to thereby form a magnetic domain having a magnetizing direction modulated in accordance with information.
On the other hand, the readout of the recorded information from such a magneto-optical disk is performed by irradiation of a linear-polarization laser beam having power smaller than that at the recording time onto the magneto-optical disk. Because magnetization having a direction in accordance with information remains in the magnetic film of the magneto-optical disk, a plane of polarization of the laser beam reflected from the magneto-optical disk rotates. This phenomenon is called Kerr effect. The rotation angle of the plane of polarization is called Kerr rotation angle. Because the Kerr rotation angle changes in dependence on the direction and magnitude of magnetization, the recorded information can be readout by detection of the Kerr rotation angle.
To increase recording density in the afore-mentioned recording/reproducing system, it is necessary to reduce a recording mark to thereby narrow a track pitch. Generally, in an optical disk including a magneto-optical disk in which a recording signal is readout by a laser beam, the spot size of the laser beam to be focussed onto the optical disk, that is, the resolution at the time of the readout is determined on the basis of both the wavelength of a readout laser beam and the numerical aperture of an objective lens. This is a limitation to increase recording density.
A large number of techniques called magnetically induced super resolution for readout a smaller recording mark than the spot size of the readout laser beam have been proposed to solve the above problem in the magneto-optical information recording/reproducing apparatus. In these techniques, at least a recording magnetic film and a readout magnetic film are provided in the magneto-optical disk and the phenomenon that the temperature on the magneto-optical disk on the basis of the laser beam varies according to the light spot is utilized. That is, the resolution is increased such that only in a limited temperature range, the magnetization of the recording magnetic film is transferred to the readout magnetic film, while in temperature ranges other than the limited temperature range, a magnetic mask is formed so that the magnetization of the readout magnetic film is unidirectionally oriented independently of the direction and magnitude of magnetization of the recording magnetic film.
A conventional example using one of the magnetically induced super resolution techniques is described in JP-A-5-012731. A magneto-optical recording medium used in the conventional example and the principle of information readout will be described below with reference to FIG.
14
. In
FIG. 14
, the reference numeral
100
designates a readout laser beam;
200
, a magneto-optical information recording medium;
201
, a recording magnetic layer; and
202
, a readout magnetic layer.
The magneto-optical information recording medium
200
is constituted by a laminate of the recording magnetic layer
201
and the readout magnetic layer
202
. In the recording magnetic layer
201
, recorded magnetic domains based on recorded information are formed so that the size of each recorded magnetic domain is smaller than the beam spot size of the readout laser beam
100
. The readout magnetic layer
202
has such a characteristic that the layer
202
serves as an in-plane magnetization film at room temperature and as a perpendicular magnetization film at a temperature not lower than a predetermined value, for example, 80° C.
To readout recorded information from the magneto-optical information recording medium
200
, beam-like readout laser light
100
is irradiated from the readout magnetic layer
202
side. When information is readout at a relatively low readout velocity, the temperature on the magneto-optical information recording medium
200
becomes maximum in the vicinity (hereinafter referred to as “the highest temperature point”) of the center of the beam spot of the readout laser beam
100
so that the temperature decreases as the position approaches a fringe of the beam spot. Accordingly, the readout magnetic layer
202
exhibits a perpendicular magnetic characteristic only in a region at the center portion of the beam spot where the temperature is not lower than a predetermined value, for example, 80° C. In this region, a recorded magnetic domain formed in the recording magnetic layer
201
is transferred to the readout magnetic layer
202
by exchange coupling force, so that a Kerr effect occurs in the reflected readout laser beam
100
. The region in which a recorded magnetic domain is transferred (hereinafter referred to as transfer temperature region) is smaller than the size of the beam spot of the readout laser beam
100
, so that a high-density recorded information signal can be readout.
As described above, the magnetically induced super resolution techniques are very effective and excellent means for enhancing resolution and reducing crosstalk. The following problems, however, arise when the magneto-optical information recording medium has a tilt with respect to the optical axis of a bundle of readout laser light (readout laser beam), that is, when a surface of the magneto-optical information recording medium is not perpendicular to the optical axis of the readout laser beam.
When recorded information is to be readout from the magneto-optical disk as a magneto-optical information recording medium, tracking control is performed so that the spot of the readout laser beam accurately tracks a recorded information track formed in the magneto-optical disk. As a method for performing such tracking control, a method called push-pull method is generally applied. This method utilizes a magneto-optical disk having a configuration in which magnetic layers are laminated on a transparent substrate having a guide groove formed therein (in the case of a magneto-optical disk
200
configured as shown in
FIG. 14
, a readout magnetic layer
202
is laminated on a transparent substrate and a recording magnetic layer
201
is laminated on the readout magnetic layer
202
). That is, the tracking displacement is detected by use of a diffraction beam which is generated by the guide groove when a readout laser beam is irradiated onto the magneto-optical disk
200
.
This method will be described below with reference to
FIG. 15
which shows a radial section of the magneto-optical disk
200
. When a readout laser beam is irradiated onto the magneto-optical disk
200
by means not shown, a reflected laser beam
300
including a diffraction beam generated by the guide groove on the magneto-optical disk
200
is generated. The reflected laser beam
300
enters into a 2-split detector
402
constituted by two light-receiving elements
402
a
and
402
b
disposed symmetrically with respect to the center line of the track, through optical means such as an objective lens
401
, etc. Output signals of levels corresponding to the quantities of beams received in the light-receiving el
Kurebayashi Masaaki
Maeda Shigemi
Maeda Takeshi
Matsubayashi Nobuhide
Matsuura Michio
Antonelli Terry Stout & Kraus LLP
Edun Muhammad
Hitachi , Ltd.
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