Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
1999-11-03
2003-05-20
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S059100
Reexamination Certificate
active
06567348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a recording method, a recording and reproducing method, and a reproducing method for optical information for recording and reproducing the information on an optical recording medium by using a laser beam. The present invention also relates to an apparatus for carrying out the foregoing methods.
2. Description of the Related Art
The apparatus for recording and reproducing optical information is used such that an information mark is recorded on an information track of an optical recording medium by using a laser beam, and information is reproduced by detecting an optical change depending on the presence or absence of the information mark. Such an apparatus includes a focusing lens which is used to collect the laser beam onto the optical recording medium. The minimum diameter of a light spot formed on the optical recording medium is substantially represented by &lgr;/NA on the basis of the wavelength &lgr; of the laser beam and the numerical aperture NA of the focusing lens. In order to improve the recording density of the optical recording medium, it is necessary to decrease the arrangement interval (mark pitch) of information marks in the scanning direction of the light spot and the track interval (track pitch) for recording the information marks.
However, if the mark pitch and the track pitch are smaller than the light spot diameter, the following problem arises. That is, when one information mark is irradiated with the light spot, then a part of another information mark disposed therearound is simultaneously irradiated with the light spot, and a signal of the information mark to be reproduced is contaminated with any signal of the information mark disposed therearound. The contamination behaves as a noise component to cause interference, and the accuracy of reproduction is lowered. As described above, the contamination of the signal of the surrounding information mark greatly obstructs the realization of high density in the case of the system provided with the focusing lens and the laser having a specified wavelength.
The magnetic amplifying magneto-optical system (hereinafter referred to as “MAMMOS”) is known as a technique which makes it possible to solve the problem as described above (Japanese Patent Application Laid-Open No. 8-2350, WO98/02877, and WO98/02878). When MAMMOS is used, a minute magnetic domain, which is transferred from a recording layer to a reproducing layer, can be magnified by using a reproducing magnetic field to have a spot size of a reproducing light beam. Therefore, it is possible to remarkably increase the reproduced signal intensity. The magnified magnetic domain can be instantaneously erased after the reproduction. Therefore, the crosstalk is avoided. That is, in the case of MAMMOS, it is possible to greatly improve the reproduced signal intensity and S/N even when the mark pitch and the track pitch are decreased.
Japanese Patent Application Laid-Open No. 10-92036 describes a system which resides in a recording and reproducing method based on a principle different from that of MAMMOS. In this system, no magnetic field is applied during reproduction. The temperature gradient, which is generated by radiating a light beam for the magnetic domain included in a magnetic recording medium, is used to move the magnetic wall of a recording mark in a reproducing layer without changing the recording data in a recording layer so that the recording mark, which has a size of not more than a diffraction limit of the light beam, is reproduced. This reproducing method involves the following problem. That is, the movement timing is discrepant between the process of magnetic wall movement in the direction to magnify the recording mark and the process of magnetic wall movement in the direction to reduce the recording mark. As a result, the period of time corresponding to the recording mark is always detected as a period of time which is shorter by a certain period of time. In this method, in order to deal with this problem, the recording mark is formed (recorded) to have a length which is longer than ordinary one.
Japanese Patent Application Laid-Open Nos. 63-281229 and 4-265522 disclose a technique in which the edge position of the recording mark is corrected. The problem, i.e., the edge position of the mark is deviated from the position at which the recording is essentially performed, is solved by correcting the recording laser beam pulse in conformity with the mark length. However, in the case of the technique described in these patent documents, when a mark of nT is recorded, the mark length formed on the recording medium is still nT. There is no disclosure of the method of the present invention in which a recording mark shorter than nT is recorded for a recording signal corresponding n clocks.
As described above, MAMMOS is an extremely effective method to remarkably improve the recording density. However, according to an experiment performed by the present inventors, it has been revealed that when information is intended to be reproduced by using MAMMOS, the optimum values of the light power and the magnetic field intensity to be applied during reproduction of information differ depending on the mark length of a recorded mark. For this reason, it has been difficult in some cases to reliably reproduce pieces of data recorded as mark arrays including several types of marks having different lengths respectively. This problem will be explained below.
When the recording and the reproduction based on MAMMOS were evaluated, a laser having a wavelength &lgr;=680 nm and a focusing lens having an NA=0.55 were used, in which the NRZ modulation was applied with a channel bit length T=0.4 &mgr;m. The channel bit length herein means the length on the recording medium corresponding to the unit bit of the modulated data array. In the following description, the recording mark length is varied to use three types of 1T, 2T, and 3T. If the mark length is longer than the spot diameter (=&lgr;/NA=1.24 &mgr;m), the difference disappears concerning the characteristic when the reproduction is performed on the basis of MAMMOS. Therefore, explanation will be omitted for mark lengths of not less than 4T. Those having been known as the recording system for the magneto-optical recording medium include, for example, the optical modulation system, the magnetic field modulation system, and the optical magnetic field modulation system. In the experiment described below, the optical magnetic field modulation system was selected, in which the minute mark can be accurately formed.
As shown in
FIG. 7A
, the recording light power
203
was modulated at a constant cycle during the recording in accordance with the optical magnetic field modulation system, while the magnetic field of +Hw synchronized with the cycle was applied for a period of nT to record a mark having a length of nT (n is a natural number). For example, when it is intended to successively record a cycle of 1T mark and 1T gap, the magnetic field is an applied magnetic field
300
during recording at 1T cycle as shown in the drawing. When it is intended to successively record a cycle of 2T mark and 2T gap, the magnetic field to be applied during recording is an applied magnetic field
301
during recording at 2T cycle as shown in the drawing. When it is intended to successively record a cycle of 3T mark and 3T gap, the magnetic field to be applied during recording is an applied magnetic field
302
during recording at 3T cycle as shown in the drawing. The modulation of the light power and the waveform used when the magnetic field is applied are generated on the basis of a clock signal
113
. The clock signal
113
is generated by a conventional synchronized signal-generating circuit (PLL) for outputting a signal synchronized with clock pits previously formed on the recording medium.
For example, 1 cycle of the clock signal
113
generated in this experiment corresponds to 0.1 &mgr;m. Therefore, the following pr
Tani Manabu
Wakabayashi Kouichirou
Yoshihiro Masafumi
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