Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1998-08-27
2001-06-19
Psitos, Aristotelis M. (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
C369S275400
Reexamination Certificate
active
06249489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information recording-reproducing method comprising recording and reproducing information on and from a recording medium, and particularly to an information reproducing method utilizing domain wall displacement. The present invention also relates to a magneto-optical medium used in this method.
2. Related Background Art
In recent years, great expectation has been entertained of magneto-optical recording-reproducing apparatus using a magneto-optical disk as a recording medium in that they are portable, have a great memory capacity and are erasable and rewritable.
FIG. 1
illustrates an optical head for such a magneto-optical recording-reproducing apparatus. In
FIG. 1
, reference numeral
15
designates a semiconductor laser as a light source. A divergent flux of rays emitted from the semiconductor laser
15
is made parallel by a collimator lens
16
and then rectified to a parallel flux of rays of a circular form in section by a beam-shaping prism. In this case, linearly polarized light components perpendicular to each other are regarded as P polarized light and S polarized light, and this parallel flux of rays is regarded as linearly polarized light of P polarized light (here, linearly polarized light in a direction parallel to the drawing).
The light flux of P polarized light is incident on a polarized light beam splitter
18
. The polarized light beam splitter
18
is characterized by, for example, a transmittance of 60% and a reflectance of 40% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. The light flux of the P polarized light passed through the polarized light beam splitter
18
is focused by an objective lens
19
and is projected as a light spot on a magnetic layer of a magneto-optical disk
20
. An external magnetic field is applied from a magnetic head
21
to this light-spot projected portion, so that a magnetic domain (mark) is recorded on the magnetic layer by the irradiation of the light spot and the application of the external magnetic field.
Reflected light from the magneto-optical disk
20
is returned to the polarized light beam splitter
18
through the objective lens
19
. A part of the reflected light is separated here and afforded to a reproducing optical system. In the reproducing optical system, the light flux separated is further separated by a polarized light beam splitter
22
separately provided. The polarized light beam splitter
22
is characterized by, for example, a transmittance of 20% and a reflectance of 80% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. One light flux separated by the polarized light beam splitter
22
is guided to a half prism
29
through a condenser lens
28
. The light flux is divided into two portions here, and one is guided to a photosensor
30
, and the other to a photosensor
32
through a knife edge
31
. Error signals for auto-tracking and auto-focusing of a light spot are generated by these controlling optical systems.
The other light flux separated by the polarized light beam splitter
22
is guided to a half-wave plate
23
for turning the polarizing direction of the light flux by 45 degrees, a condenser lens
24
for focusing the light flux, a polarized light beam splitter
25
, and photosensors
26
and
27
for separately detecting light flux portions separated by the polarized light beam splitter
25
to reproduce information. The polarized light beam splitter
25
is characterized by a transmittance of 100% and a reflectance of 0% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. Signals detected by the photosensors
26
and
27
are differentially detected by a differential amplifier (not illustrated), thereby generating a reproduction signal.
In a magneto-optical medium, information is recorded by a difference in the direction of vertical magnetization. When the magneto-optical medium, in which information has been recorded by the difference in the direction of magnetization, is irradiated with linearly polarized light, the polarizing direction of light reflected therefrom is turned either clockwise or counterclockwise according to the difference in the direction of magnetization. For example, when a polarizing direction of linearly polarized light incident on the magneto-optical medium, and directions of reflected light for downward magnetization and reflected light for upward magnetization are regarded as directions of a coordinate axis P, R+ turned by +&thgr;k and R− turned by −&thgr;k, respectively, as illustrated in
FIG. 2
, and an analyzer is arranged in such a direction as illustrated in
FIG. 2
, light passed through the analyzer becomes A for R+ or B for −R. When this light is detected by a photosensor, information can be obtained as a difference in intensity of light. In the example illustrated in
FIG. 1
, the polarized light beam splitter
25
plays a part of the analyzer and serves as an analyzer in a direction turned by +45 degrees from the axis P for one light flux separated or in a direction turned by −45 degrees from the axis P for the other light flux. Namely, the signal components obtained by the photosensors
26
and
27
become antiphase. Therefore, the individual signals are differentially detected, whereby a reproduction signal can be obtained with reduced noise. In
FIG. 2
, axis S means an axis for S polarized light direction. Points S+ and S− on axis S mean S coordinates of points R+ and R− when letting the coordinates of points R+ and R− be (P+, S+) and (P+, S−) in P-S coordination system, respectively.
Recently, there has been a strong demand for enhancing the recording density of this magneto-optical medium. In general, the recording density of an optical disk such as a magneto-optical medium depends on the laser wavelength and the NA (numerical aperture) of an objective lens of a reproducing optical system. More specifically, since the laser wavelength &lgr; and the NA of the objective lens of the reproducing optical system decide the diameter of a light spot, the range of a reproducible magnetic domain is limited to about &lgr;/2NA. Therefore, for actually achieving higher recording density with a conventional optical disk, it has been necessary to shorten the laser wavelength or enlarge the NA of the objective lens in the reproducing optical system. However, the improvements in the laser wavelength and the NA of the objective lens are limited naturally. Therefore, techniques that the construction and reading method of a recording medium are devises to improve the recording density have been developed.
For example, in Japanese Patent Application Laid-Open No. 6-290496, there is proposed a domain wall displacement-reproduction system in which a reproduction signal is obtained after a light spot is scanned on a track on a magneto-optical medium composed of a laminate of plural magnetic layers, thereby transferring a magnetic domain (mark) recorded as vertical magnetization on a first magnetic layer to a third magnetic layer arranged with interposition of a second magnetic layer for adjusting exchange-coupling force, and a domain wall of the magnetic domain transferred to the third magnetic layer is displaced, thereby making the magnetic domain larger than the magnetic domain (mark) recorded on the first magnetic layer.
FIGS. 3A
to
3
D illustrate the principle of the domain wall displacement-reproduction method.
FIG. 3A
is a cross-sectional view of magnetic layers of a magneto-optical medium, and
FIG. 3B
is a plan view viewed from the side on which a light spot is incident. In
FIG. 3A
, an arrow A indicates a moving direction of the medium. Reference numeral
33
designates a magneto-optical disk as the magneto-optical medium. Reference numeral
34
indicates a first magnetic layer which is a memory layer for
Fujii Eiichi
Miyaoka Yasuyuki
Shiratori Tsutomu
Yamamoto Masakuni
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Psitos Aristotelis M.
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