Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-06-09
2004-06-22
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013240, C369S013220, C369S275400
Reexamination Certificate
active
06754141
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a recording medium as well as a method for reading data from and writing data on the medium. A magnetic or magneto-optical recording medium has a large storing capacity, a high degree of reliability and a good portability as a removable recording medium. Therefore, the application field of the magneto-optical recording medium is rapidly widened for a data recording medium of a computer or a recording medium of image information. As the wide application, the request of the market for higher density and larger capacity has been increasing every year.
2. Description of the Prior Art
There are two methods for increasing a recording density (i.e., increasing a recording capacity) of a disk recording medium in which data are written linearly along tracks. The first method increases a bit density on a track, while the second method increases a track density.
The first method has a limitation caused by a diameter of the light beam spot for a magneto-optical recording medium. In order to read a bit written in a period smaller than a beam spot diameter, a smaller diameter of the light beam spot is required. However, the beam spot diameter cannot be smaller than the value determined by a wavelength &lgr; of a light source and a numerical aperture NA of an object lens.
Recently, a technique of reading a bit recorded in a period smaller than the beam spot diameter is proposed in Japanese unexamined patent publication No. 1-143041 or No. 3-93058. This technique utilizes a temperature distribution within the beam spot on the medium and making the recording medium in a multilayer structure for obtaining the effect as if the beam spot is focused more precisely.
FIG. 1
shows schematically the structure of the conventional recording medium and a method for recording and reproducing data using the medium disclosed in the Japanese unexamined patent publication No. 1-143041. This recording medium has three layers, i.e., a reproduction layer
101
, an intermediate layer
102
and a recording layer
103
. The bit
105
recorded in the recording layer
103
by a light modulation method is transferred from a low temperature area
104
a
in a beam spot
104
to the reproduction layer
101
by using an exchange connection via the intermediate layer
102
so as to reproduce the bit
105
. In a high temperature area
102
a
of the intermediate layer
102
whose temperature exceeds the Curie temperature, the exchange connection is interrupted and the magnetization of the reproduction layer
101
is directed to the magnetic field Hr for reading applied externally. Namely, a bit
105
a
of the recording layer
103
is hidden (is masked magnetically). Thus, among bits located in the beam spot
104
, only a bit located in the low temperature area
104
a
can be read. Accordingly, the reproducing resolution can be improved as if the beam spot is focused more precisely.
FIG. 2
shows schematically the structure of the recording medium and a method for reading data from and writing data on the medium disclosed in the Japanese unexamined patent publication No. 3-93058. This recording medium has four layers, i.e., a reproduction layer
201
, a control layer
202
, an intermediate layer
203
and a recording layer
204
. In this recording medium, the magnetization of the reproduction layer
201
forms the state where bits are hidden in the low temperature area
205
a
and the high temperature area
205
b
within the beam spot
205
. Only in a medium temperature area
205
c
, the bit is transferred from the recording layer
204
to the reproduction layer
201
to be read by the exchange connection. The magnetized state of the reproduction layer
201
in the high temperature area
205
b
is directed to the magnetic field (reproduction magnetic field) Hr applied externally by the same principle as the recording medium explained above.
In the low temperature area
205
a
, the magnetized state of the reproduction layer
201
keeps the magnetization direction formed by an initialization magnetic field HO (usually, a magnetic field of a few kilo Oe). This is caused by the fact that the composed coercive force of the reproduction layer
201
and the control layer (also referred to as an auxiliary reproduction layer)
202
in the low temperature area
205
a
is larger than the exchange connection force from the recording layer
204
via intermediate layer
203
. The medium temperature area
205
c
, i.e., a transfer area is formed when the above-mentioned inequality is inverted as the temperature rises.
The above-mentioned method for reading data from the magneto-optical recording medium having three or four layer structure is called a magnetic super resolution (MSR) method.
FIGS. 3A and 3B
show the relationship among the applied magnetic field, the laser light emission, the recorded mark and the reproduced data when using the conventional recording medium shown in
FIG. 1
or FIG.
2
.
FIG. 3A
shows the case of magnetic field modulation method in which the laser emits constant light, while
FIG. 3B
shows the case of magnetic field modulation method in which the laser emits pulsed light.
FIGS. 3A and 3B
are diagrams explaining the magnetic field modulation recording method. There is another method of light modulation recording method.
FIGS. 3A and 3B
show an example (the magnetic field modulation) of the recording methods, and another method (light modulation) can be used.
FIG. 4
is a block diagram showing the structure of the conventional recording and reproducing apparatus using the magneto-optical recording medium. A magnetic coil drive circuit
304
drives a magnetic coil
303
so as to generate the magnetic field for reading. The read signal obtained by the optical head
302
is amplified by an amplifier
305
, is adjusted about its gain by an automatic gain control (AGC) amplifier
306
, is equalized about its waveform by an equalizer
307
, and the high frequency noise of the signal is removed by a low pass filter (LPF)
308
. The signal is further digitized into a binary signal by a binary circuit
309
and becomes separate data after passing through a data discriminator
311
and a phase-locked loop (PLL) circuit
312
. The separate data are given to a demodulator
313
, which demodulates the data.
FIG. 5
shows a dependency of the repeatedly recorded mark on the magnetic field for reading. Namely,
FIG. 5
shows a relationship between the applied magnetic field for reading that is necessary when reading data from the recording medium and a ratio of carrier and noise (CNR). It is understood from
FIG. 5
that the CNR becomes larger than a predetermined level (CNRO) when the applied magnetic field for reading is within a predetermined range (the optimal reading magnetic field). If the applied magnetic field for reading is too stronger or too weaker than the optimal reading magnetic field, the signal will be deteriorated.
FIG. 6
shows data reproduction waveform and the binary data signal when the applied magnetic field for reading is a negative magnetic field. The reproduced waveform obtained by applying the negative magnetic field has a milder gradient in the leading edge than in the trailing edge, so a jitter of the binary signal for the same noise power becomes larger in the leading edge than in the trailing edge. Therefore, the quality of the reproduced data depends on the jitter in the leading edge.
FIG. 7
shows data reproduction waveform and the binary data signal when the applied magnetic field for reading is a positive magnetic field. In this case, on the contrary to the case of the negative magnetic field, the reproduced waveform has a milder gradient in the trailing edge than in the leading edge, so a jitter in the trailing edge determines the quality of the reproduced data.
As explained above, when the magnetic super resolution method used for reading a small bit recorded on a three layer or four layer structure of recording medium, the read signal becomes asymmetric so that the gradient of the leading edge or
Itakura Akihiro
Taguchi Masakazu
Uchida Akiyoshi
Fujitsu Limited
Greer Burns & Crain Ltd.
Neyzari Ali
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