Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-02-25
2002-05-28
Neyzari, Ali (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013170, C369S126000, C369S044240
Reexamination Certificate
active
06396775
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magneto-optical head, magneto-optical device, and magneto-optical recording/reproducing method for use in an external memory device of a computer, etc., and for recording and reproducing of audio and video signals.
BACKGROUND OF THE INVENTION
Actual application of magneto-optical disks, which are one type of magneto- optical recording medium, has been realized as an external memory device of a computer.
The recording density of the magneto-optical disk is limited by the size of a light beam spot on the magneto-optical disk. That is, as the diameter and interval of recording marks become smaller than the size of the light beam spot, the light beam spot would contain plural bits, making it impossible to separately reproduce each recording bit.
The reproducing resolution of a signal is essentially determined by the wavelength &lgr; of a light source of a reproducing optical system and by the numerical aperture NA of an objective lens, and the spatial frequency 2NA/&lgr; sets a limit of reproduction. Thus, to increase recording density, one can take a measure of reducing the light spot diameter of the reproducing device by making the wavelength &lgr; of the light source shorter and/or by using a high NA lens.
As such, in recent years, to increase recording density of the magneto-optical disk, research has been made to actually reduce the spot diameter of the reproducing device by reducing the wavelength of the laser light used for recording and reproducing, and/or by using a high NA lens. For example, to reduce the wavelength of a laser light, there is research on a semiconductor blue laser, or research in which the wavelength of a laser light is reduced from around 800 nm to 400 nm with the use of a second harmonic wave generating element (SHG). These research has not reached the stage which can be brought into actual application due to their stability, performance, and cost. However, once realized, it is certain that they will bring higher density recording of information than that offered by the current optical disk systems.
However, the wavelength of a semiconductor laser as currently available in actual application is only around 650 nm.
Further, when a high NA lens is used, the depth of focus is reduced, and high precision is required in the distance between the lens and disk, and it becomes difficult to manufacture the optical disk with precision. For this reason, the NA of the lens cannot be increased substantially, and the lens NA which can be brought into actual application is only around 0.6. Thus, there is a limit in reducing the wavelength of the light source or increasing the NA of the lens, and as it currently stands, it is difficult to effectively increase recording density by these measures.
In view of these drawbacks, for example, Journal of The Magnetics Society of Japan, Vol. 19, Supplement, No. S1 (1995), pp. 421-424 (Document 1) recites a magnetically induced super resolution (“MSR” hereinafter) technique, which is the method for increasing recording density by improving the reproducing resolution with the use of a magneto-optical recording medium which is composed of magnetostatically coupled two magnetic layers and by utilizing a temperature distribution of the light beam spot.
Also, for example, Applied Physics Letter No. 69 (27), Dec. 30, 1996, pp. 4257-4259 (Document 2) recites that information is reproduced while applying an alternating magnetic field to an MSR medium employing magneto-static coupling so that a recording mark is enhanced when transferring the recording mark of the recording layer to the reproducing layer, thus increasing the amplitude of the reproduced signal.
As another method of increasing recording density without reducing the laser wavelength, the numerical aperture (NA) of the optical system is increased. For example, Applied Physics Letter, No. 68(2), Jan. 8, 1996, pp. 141-143 (Document 3) discloses a technique in which an effective NA is increased with the use of a solid immersion lens (SIL) so as to reduce the beam spot.
The following will describe Document 1 in more detail referring to FIG.
13
and FIG.
14
.
FIG. 13
shows a representative arrangement of a conventional MSR magneto-optical recording medium. On a transparent substrate
61
, there are deposited a transparent dielectric layer
62
, reproducing layer
63
, transparent dielectric layer
64
, recording layer
65
, and transparent dielectric layer
66
.
The recording layer
65
records magneto-optical information in the form of a change in length of recording marks. However, FIG.
13
and
FIG. 14
illustrate an example of only the shortest recording mark. This is because reproduction of the shortest recording mark would automatically allow reproduction of signals of longer recording marks. Thus, it is assumed that the lengths of recording marks are the same, and a region in which the shortest recording mark is to be formed is schematically divided into domains. In the following, each domain will be referred to as a magnetic domain.
Each of magnetic domains A through I records a signal as shown in FIG.
13
and FIG.
14
. Even though the reproducing layer
63
.is not divided into a plurality of magnetic domains unlike the recording layer
65
, for convenience of explanation, (refer to FIGS.
4
(
a
) and
4
(
b
) and FIG.
14
), domains of reproducing layer
63
corresponding in position to the magnetic domains A through I of the recording layer
65
are indicated by domains A′ through I′.
The following considers the case where laser light is projected on the center of magnetic domain E of the recording layer
65
, spreading over the region larger than the magnetic domain E. The temperature distribution of the recording layer
65
would then take the shape in which a high temperature portion (e.g., 150° C.) is found at the site of the recording mark E and the temperature decreases as it moves further from the magnetic domain E. Further, the size of saturation magnetization also takes the distribution which reflects the temperature distribution of the recording layer
65
and it becomes maximum in the magnetic domain E.
Meanwhile, the reproducing layer
63
has in-plane magnetization parallel to the plane of the film (perpendicular to the plane of the paper) at room temperature, and it has a “mask region” from which no signal is reproduced.
By being heated to a high temperature by projection of a laser beam, the magnetization of the reproducing layer
63
becomes smaller and the reproducing layer
63
comes to have perpendicular magnetization, forming an “aperture region” to which the magnetization of the recording layer
65
is transferred by a magneto-static force.
In reproduction, a temperature distribution is generated in the laser spot, and a signal is reproduced only from the aperture region formed at the high temperature portion of the temperature distribution. Namely, by the magnetic flux generated from the magnetic domain E, adjoining portion E′ of the reproducing layer
63
is subjected to a force (magneto-static force) which is in accordance with the saturation magnetization of the magnetic domain E, making saturation magnetization of magnetic domains E and E′ in line. In this manner, in the transfer of a recording mark of the recording layer
65
to the reproducing layer
63
, transfer of a signal to the reproducing layer
63
occurs only at the magnetic domain E, and the recording marks of the other magnetic domains (A through D and F through I) are not transferred and remain as a mask region, thus limiting the signal reproducing region and effectively reducing the reproduced spot.
Thus, even when the recording mark is smaller than the beam spot diameter, information can be read out without interfering with recording marks of adjacent magnetic domains, thus increasing the reproducing resolution of signals and realizing high density recording. Further, because the adjacent tracks at room temperature make up a mask region, a signal leak (cross talk) from adjacent tracks hardly
Neyzari Ali
Sharp Kabushiki Kaisha
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