Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
1998-08-31
2002-02-05
Psitos, Aristotelis M. (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013520
Reexamination Certificate
active
06345016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for reproducing signals recorded by magnetic orientation states in a magnetic material of a magnetic recording medium and an apparatus therefor.
2. Related Background Art
Magnetic recording mediums on which information is recorded by magnetic orientation states in a magnetic material such as magnetic recording mediums and magneto-optical mediums are attracting attention as rewritable high-density recording mediums. In recent years, still higher recording density of the magnetic recording medium is demanded for larger capacity of the recording medium.
In a magneto-optical recording system employing a magneto-optical medium and a recording-reproducing apparatus therefor, information is recorded by forming magnetic domains on a magnetic thin film by thermal energy of a semiconductor laser, and the recorded information is read out by utilizing magneto-optical effect. Generally, the linear recording density of an optical recording medium depends greatly on the laser wavelength of the reproducing optical system, and the numerical aperture NA of the objective lens. Specifically, the laser wavelength &lgr; and the numerical aperture NA of the objective lens of the reproducing optical system decide the diameter of the beam waist, whereby the detectable range of the spatial frequency of the recording pits is limited to about 2NA/&lgr;.
For achieving higher recording density with a conventional optical disk, the laser wavelength should be shorter, or the NA of the objective lens should be larger in the reproducing optical system. However, the laser wavelength &lgr; cannot readily be shortened owing to the efficiency limit and heat generation of the laser element, and the increase of the numerical aperture NA of the objective lens results in smaller focal depth, requiring higher mechanical accuracy, disadvantageously.
To solve the above problems, super-resolution techniques are being developed to improve recording density by changing the constitution of the recording medium and changing the reproducing method without changing the laser wavelength and the numerical aperture of the objective lens.
Japanese Patent Application Laid-Open No. 3-93058 discloses a signal-reproducing method. In this method, a multilayered film having a displacement layer and a memory layer coupled magnetically is provided, signals are recorded on the memory layer, the magnetization orientation in the displacement layer is made uniform, a laser light beam is projected thereto for heating, and the signals recorded on the memory layer is transferred to the heated region of the displacement layer for reading the recorded signals.
In this method, the size of the region heated by the laser up to the signal transfer temperature for signal detection can be made smaller than the laser spot diameter, whereby interference between the signs can be decreased to enable reproduction of signals of a cycle less than optical diffraction limit.
In any of the known super-resolution systems, the reproducing light is partly intercepted with a mask to limit the pit-reading aperture to a smaller region to improve the resolution limit. Thereby, the light intercepted by the mask is not utilized, and the amplitude of the reproduced signal is decreased, disadvantageously. In other words, the portion of the light intercepted by the mask does not contribute to the signal reproduction. Therefore, the smaller the aperture for higher resolution, the less is the effective light, and the lower is the signal level.
To solve the above problems, the inventors of the present invention disclosed a method for reproducing high-density recorded signal in Japanese Patent Application Laid-Open No. 6-290496 in which a special magnetic recording medium is employed, a magnetic domain wall existing at the border of the recorded mark is displaced by temperature gradient to the higher temperature side, and the domain wall displacement is detected to reproduce the high-density recorded signal.
In this method, however, since the temperature gradient is formed by heating the recording medium with the reproducing light beam itself, the peak of the temperature distribution is formed inside the reproducing light spot, and the displacement of the domain wall from the front side of the displacement of the region of the domain wall displacement and that from the rear side thereof are both read by the reproduction spot, not giving satisfactory signal reproduction. Therefore a separate means for controlling the temperature distribution is required in addition to the reproducing light beam, which complicates the reproduction apparatus.
FIG. 1
shows a constitution of a conventional system. In
FIG. 1
, magneto-optical disk
101
is constituted of substrate
102
, magneto-optical medium
103
formed thereon, and protection layer
104
formed further thereon. Substrate
102
is formed from glass or a plastic material. Magneto-optical medium
103
is comprised of a multiple layer comprising at least a memory layer and a displacement layer, and is capable of reproducing record marks of less than optical diffraction limit of the optical system by displacing a domain wall by utilizing temperature gradient caused by light beam irradiation without changing recorded data in the memory layer, magnetizing uniformly and almost entirely the reproducing light beam-spotted region on the displacement layer, and detecting the change of polarization direction of the reflected light beam. Magneto-optical disk
101
is set to a spindle motor by a magnet chucking or a like means to be rotatable on a rotation axis.
Parts
105
to
117
constitute an optical head for projecting a laser beam to magneto-optical disk
101
and for receiving information from reflected light. The parts comprise condenser lens
106
as an objective lens, actuator
105
for driving condenser lens
106
, semiconductor laser
107
of a wavelength of 680 nm for record reproduction, semiconductor laser
108
of wavelength of 1.3 &mgr;m for heating, collimator lenses
109
,
110
, dichroic mirror
111
for completely transmitting light of 680 nm and completely reflecting light of 1.3 &mgr;m, beam splitter
112
, dichroic mirror
113
for intercepting light of 1.3 &mgr;m and completely transmitting light of 680 nm to prevent leakage of light of 1.3 &mgr;m into the signal detecting system, &lgr;/2 plate
114
, polarized light beam splitter
115
, photosensors
117
, condenser lenses
116
for photosensor, differential amplification circuit
118
for differentially amplifying the condensed and detected signals for respective polarization direction, LD driver
119
, and controller
120
for recording power control.
The laser beams of 680 nm and 1.3 &mgr;m emitted respectively from semiconductor lasers
107
,
108
for recording-reproducing and heating are introduced through collimator lenses
109
,
110
, dichroic mirror
111
, beam splitter
112
, and condenser lens
106
to magneto-optical disk
101
. Condenser lens
106
moves in the focusing direction and the tracking direction under control by actuator
105
to focus the laser beams successively on magneto-optical medium
103
by tracking along a guiding groove formed on magneto-optical disk
101
. The light flux of 1.3 &mgr;m is made smaller than the aperture diameter of condenser lens
106
to make the NA smaller than that of the light of 680 nm which is condensed through the entire area of the aperture.
The heating spot, which is formed with a larger wavelength and a smaller NA, has a larger diameter of heating beam than the recording-reproducing spot of recording-reproducing beam as shown in
FIGS. 3A and 3B
. Thereby, a desired temperature gradient is produced in the recording-reproducing spot region on the moving medium face as shown in FIG.
3
D. The laser beam reflected by magneto-optical disk
101
is deflected by beam splitter
112
to the optical path toward polarized light beam splitter
115
, and travels through dichroic mirror
113
, &lgr;/2 plate
114
, and polarized l
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Psitos Aristotelis M.
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