Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1998-09-29
2001-06-12
Dinh, Tan (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
active
06246641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording-reproducing method comprising the steps of (i) forming a record mark on a magneto-optical medium having a multi-layer structure; (ii) irradiating the magneto-optical medium with a light beam; (iii) displacing a domain wall of the record mark in a displacement layer without changing a record data in a memory layer by utilizing a temperature gradient in a temperature distribution caused by the irradiation of the light beam; and (iv) detecting a change in the polarization direction of reflected light of the light beam to reproduce the record mark. The present invention also relates to a magneto-optical recording reproducing apparatus used in such a method.
2. Related Background Art
As a rewritable high-density recording medium, a magneto-optical medium is known in which magnetic domains are formed in a magnetic thin layer thereof by thermal energy of a semiconductor layer to record information, and this information is read out by utilizing a magneto-optical effect. In recent years, there has been a strong demand for further enhancing of the recording density of this magneto-optical medium to provide a recording medium having a greater capacity.
The linear recording density of an optical disk, such as the magneto-optical medium, greatly depends on the laser wavelength and the numerical aperture of an objective lens of a reproducing optical system. More specifically, since the laser wavelength &lgr; and the numerical aperture NA of the objective lens of the reproducing optical system determine the diameter of a beam waist, the detectable range of the spatial frequency upon reproduction of a record mark is limited to about 2NA/&lgr;. Therefore, for actually achieving higher recording density with a conventional optical disk, it is necessary to shorten the laser wavelength &lgr; or enlarge the NA of the objective lens in the reproducing optical system. However, the improvements in the laser wavelength &lgr; and the numerical aperture NA of the objective lens are inherently limited.
Therefore, techniques have been developed where the composition and reading method of a recording medium are devised to improve the recording density.
For example, Japanese Patent Application Laid-Open No. 6-290496 proposes a signal-reproducing method and apparatus therefor, by which signals are recorded in a memory layer of a multi-layer film, having (i) a displacement layer and the memory layer magnetically connected to each other, and (ii) record marks of less than the optical diffraction limit of an optical system which are reproduced by displacing domain walls of record marks in the displacement layer without changing record data in the memory layer. This is done by utilizing a temperature gradient caused by irradiation of a light beam, magnetizing a part of the displacement layer in such a manner that the light beam-spotted region thereof becomes the same magnetization as the corresponding record mark, and detecting a change in the polarization direction of reflected light of the light beam. According to this method, the reproduction signals become rectangular as illustrated in
FIG. 2E
, so that record marks of frequency of less than the optical diffraction limit can be reproduced without decreasing the amplitude of reproduction signals depending on optical resolving power, whereby the magneto-optical medium and reproducing method can be greatly improved in recording density and transfer speed.
FIG. 1
illustrates an example of the construction of the conventional magneto-optical recording-reproducing apparatus. In
FIG. 1
, reference numeral
1
indicates a magneto-optical disk comprising a substrate
2
composed of a glass or plastic material, a magneto-optical layer
3
formed on the substrate, and a protective layer
4
formed on the magneto-optical layer
3
. The magneto-optical layer
3
has a multi-layer structure comprising a memory layer and a displacement layer and is capable of reproducing record marks of less than the optical diffraction limit of an optical system by displacing a domain wall of a record mark in the displacement layer without changing record data in the memory layer. This is done by utilizing a temperature gradient caused by irradiation of a light beam, enlarging magnetization within the light beam-spotted region of the displacement layer, and detecting a change in the polarization direction of reflected light of the light beam. The magneto-optical disk
1
is set to a spindle motor by a magnet chucking or the like, and is so constructed that it is rotatable on an axis of rotation.
Reference numerals
5
to
13
indicate individual parts that make up an optical head for irradiating the magneto-optical disk
1
with a laser beam and receiving information from reflected light. Specifically, the optical head comprises a condenser lens
6
, an actuator
5
for driving the condenser lens
5
, a semiconductor laser
7
, a collimator lens
8
, a beam splitter
9
, a &lgr;/2 plate
10
, a polarized light beam splitter
11
, photosensors
13
and condenser lenses
12
for the respective photosensors
13
. Reference numeral
14
indicates a differential amplification circuit for differentially amplifying signals condensed and detected in the respective polarization directions. The laser beam emitted from the semiconductor laser
7
is projected onto the magneto-optical disk
1
through the collimator lens
8
, the beam splitter
9
and the condenser lens
6
. At this time, the condenser lens
6
is controlled so as to move in a focusing direction and a tracking direction while under the control of the actuator
5
in response to the detected signals from the photosensor
13
to successively focus the laser beam on the magneto-optical layer
3
. This process is constructed so that it tracks along a guiding groove formed in the magneto-optical disk
1
.
The laser beam reflected on the magneto-optical disk
1
is deflected by the beam splitter
9
to an optical path toward the polarized light beam splitter
11
and then travels through the &lgr;/2 plate
10
and the polarized light beam splitter
11
. Light beams split by the polarized light beam splitter
11
are condensed by the condenser lenses
12
on the respective photosensor
13
in accordance with the magnetization polarity of the magneto-optical layer
3
. The outputs from the respective photosensors
13
are differentially amplified by the differential amplification circuit
14
in order to output magneto-optical reproduction signals.
A controller
16
receives information on the rotational speed of the magneto-optical disk
1
, recording radius, recording sectors, and so forth, and outputs recording power, recording signals and the like to control an LD (laser diode) driver
15
, a magnetic head driver
18
and the like. The LD driver
15
drives the semiconductor laser
7
and controls the recording power and reproduction power as desired.
Reference numeral
17
indicates a magnetic head for applying a modulation magnetic field to the laser irradiation site on the magneto-optical disk
1
upon the recording operation. The magnetic head is arranged in opposition to the condenser lens
6
with interposition of the magneto-optical disk
1
. Upon recording, the semiconductor laser
7
applies recording laser power by irradiation of DC (direct current) light, under the control of the LD driver
15
, and synchronously the magnetic head
17
produces magnetic fields of different polarities, under the control of the magnetic head driver
18
, in accordance with the recording signals. The magnetic head
17
moves with the optical head in a radius direction of the magneto-optical disk
1
, and applies a magnetic field successively upon recording onto the laser irradiation site of the magneto-optical layer
3
, thereby recording information.
Guiding groove portions formed in the magneto-optical layer
3
, between which respective land portions in a recording region have been formed, are preliminarily annealed at a high
Canon Kabushiki Kaisha
Dinh Tan
Fitzpatrick ,Cella, Harper & Scinto
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