Method and system for optically recording and playing back information on a recording medium having magnetization film thereon

Disclosed is a method and system for recording and playing back information utilizing a recording medium having a vertical magnetization film on and from which the information is recorded and read out by an optical means, characterized by changing the direction of the magnetic field applied on the recording medium during the application of a recording light beam so as to maintain the recording track of portions of the recording medium on which the previously recorded information is erased by the application of the recording light beam during editing operations.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and a system for recording and 
playing back information, and more particularly to a method and a system 
which utilizes a recording medium having a vertical magnetization film on 
and from which the information is recorded, and which has a readout 
mechanism utilizing optical means. 
2. Description of the Prior Art 
In recent years, recording systems have been proposed wherein information 
is recorded and played back by utilizing the magneto-optical 
characteristics of materials such as an amorphous (noncrystalline) alloy 
used as the recording medium. The amorphous alloy thin films such as 
Gadlinium Iron (GdFe) and Gadlinium Cobalt (GdCo) are examples of the 
"vertical magnetization films" in which the direction of magnetization is 
perpendicular to the surface of the film. 
Recording of information on the amorphous thin film takes place in a manner 
such that the orientation or the direction of magnetization of the desired 
portions of the amorphous alloy film, which is initially magnetized in a 
preselected direction, is turned over by heating those portions above the 
Curie temperature or a compensation temperature while applying a weak 
external magnetic field. 
More specifically, the amorphous alloy thin film which was previously 
downwardly (corresponding to the binary "0" signal) magnetized is 
subjected to a weak upwardly biased magnetic field, while only those 
portions of the thin film on which the "1" signal is to be recorded are 
applied with a laser beam so as to raise the temperature of those portions 
above the Curie temperature or a compensation temperature, thereby causing 
a change in the direction of magnetization. 
On the other hand, the read out of information recorded on the thin film 
takes place by utilizing the magnetic Kerr effect, which is an interaction 
between the magnetization of substance and a light beam. When a linearly 
polarized laser beam is reflected by the magnetized thin film, the plane 
of polarization is rotated in a direction which is determined by the 
direction of the magnetization. Therefore, the information recorded on the 
amorphous thin film is read out by detecting the direction of rotation of 
the plane of polarization of the reflected laser beam by means of an 
analyzer. 
Since this recording medium has the advantage that new information can be 
rewritten at will, referred to as "reversible recording", so called 
"editing" of the recorded information becomes possible. In editing, the 
reading out of the previously recorded information and the erasure of the 
unnecessary portion of the recording track and rewriting of new 
information, are usually performed in parallel. 
However, the prior art systems of this type have suffered from the problem 
that certain portions of the recording track are completely erased during 
editing, which may cause difficulty in tracking the recording beam spot 
during the recording of new information. That is, there may be a shift in 
registry between the originally recorded and the newly-recorded 
information. 
In such a case, the discrepancy, or lack of registry, between the newly 
recorded track and the pre-recorded track results in tracking problems 
during playback. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide a method and a 
system for recording and playing back information in which the 
above-mentioned problem is eliminated. 
According to the present invention, the method for recording and playing 
back information comprises the step of recording the information on a 
vertical magnetization film in a manner such that a spot of the recording 
light beam is applied, in accordance with the information to be recorded 
on a vertical magnetization film having a first uniform magnetization 
orientation, under the application of a weak magnetic field having a 
second and reverse magnetization orientation. Then, if it is desired 
subsequently to erase a portion of the recorded track, the recording light 
beam is applied on the desired portion of the recording track to be 
erased, under the application of a weak magnetic field having the same 
orientation as was used during the original recording step. This is done 
in a manner such that the desired portion of the recording track is erased 
subsequent to the reproduction (play back) of the recorded information, so 
that tracking and focusing are made possible. 
The result of erasing by using a magnetic field of the same polarity as 
originally used for the recording step is that the integrity of the 
recording track is maintained. Because the entire disc originally was 
biased on a first magnetic direction and the recorded information oriented 
in a second and opposite direction, erasure in the second direction 
"stands out" on the disc having the vast majority of its bias in the first 
direction. Hence, the continuity of the recording track is maintained, 
thus facilitating the tracking step during rerecording and the application 
of new information in alignment with such track. In this regard, it should 
be noted that automatic alignment and focusing are contemplated in the 
inventive system, and attention is directed to copending U.S. application, 
entitled "Information Recording System" and filed by the same Applicant, 
Ser. No. 384,241, now copending, filed on June 2, 1984. 
The step of recording the new information is done in a manner such that a 
recording beam corresponding to the new information is applied on the 
recording track which resulted from the erasure, under the application of 
a magnetic field having a direction reverse from that used in the erasure. 
The foregoing and other objects and advantages of the invention will become 
more clearly understood from the following description taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is first made to FIG. 1, wherein an embodiment of the recording 
and playback system according to the present invention is illustrated. 
In FIG. 1, there is shown a disc 1 comprising a base 11 of a dielectric 
material such as glass, and an amorphous thin film 12 formed thereon. The 
amorphous thin film 12 is made of Gadlinium Iron (GdFe) or Gadlinium 
Cobalt (GdCo), for example. 
An output light beam of a laser source 2 is fed to an A/O (Acoustic 
Optical) modulator 3 where the light beam is modulated by an output signal 
of a driver 4 which receives a signal corresponding to the informaiton to 
be recorded. The modulation signal, i.e., the output signal of the driver 
4, takes the form of a digital pulse signal produced by modulating the 
frequency of a carrier signal having a predetermined frequency by the 
signal to be recorded, and by limiting the amplitude of the signal by 
means of a limiter circuit. 
The output beam of the A/O modulator 3 takes the form of a pulse train of 
light corresponding to the pulse signal from the driver 4, and passes 
throught a lens 5 and a dichroic mirror 6. The beam thus is formed into a 
recording beam spot, and is focused on the recording surface 12 of the 
disc 1 by means of a focus lens 7. 
The system is also provided with a readout laser source 8, the output beam 
of which has a frequency different from that of the recording laser beam 
from source 2. The readout laser beam is also introduced to the focus lens 
7, through a diffraction grating 9, a lens 13, a half prism 14, and a 
mirror 15, and forms a readout beam spot which is focused on the recording 
surface 12 of the disc 1. 
The reflection of the readout laser beam travels the same path as does the 
incidental beam, until it is received by the half prism 14 at which time 
the reflection beam is directed to a half prism 151. Half prism 151 splits 
the reflection beam received form half prism 14 into a first beam directed 
to a first light detector 18 through a cylindrical lens 16 and a first 
analyzer 17, and a second beam directed to a second light detector 20 
through a second analyzer 19. 
If the first and second analyzers 17 and 19 are arranged appropriately, a 
playback RF signal having a good S/N ratio can be produced by means of a 
differentiation amplifier (not shown) which receives the output signals of 
the first and second light detectors 18 and 20, wherein noises inherent to 
the laser beam can be sufficiently cancelled. 
In addition, the output signal of the first light detector 18 is utilized 
to control a so called "focus servo" system in which the distance between 
the focus lens 7 and the recording surface 12 of the disc is controlled to 
maintain the beam from the focus lens 7 in focus on the recording surface 
12. 
A coil 21 for generating a bias magnetic field is disposed directly beneath 
the portion of the disc 1 at which the recording laser beam is focused, 
and is fed with a DC voltage from a DC voltage source 22 via a polarity 
switching device 23. 
In FIG. 2, there is illustrated the relationship between the recording 
track and a plurality of beam spots used in the system. 
For purposes of explanation, let it be assumed that the recording track (or 
the recording portion of the disc) moves in the direction indicated by the 
arrow shown in the figure. A recording beam spot 25 shown at the right 
hand portion of the figure has its center at the central portion of the 
recording track 24. A playback beam spot 26 is also indicated with its 
center at the central portion of the recording track 24. 
A pair of spots 27 and 28, positioned at both sides of the playback beam 
spot 26, are utilized for the tracking purpose and a first order light 
beam from the diffraction grating 17 is utilized for these tracking beam 
spots 27 and 28. On the other hand, a zero order output light beam from 
the diffraction grating 17 is utilized for the playback beam spot 26. 
As shown, these beam spots 26 to 28 are so arranged that beam spots 27 and 
28 are positioned at the peripheral portion of the recording track when 
the playback beam spot 26 is located at the center of the recording track 
24. Therefore, the tracking error of the playback beam spot 26 can be 
detected by means of the difference in intensities between the reflections 
of the tracking beams 27 and 28. The thus obtained tracking error signal 
is used to effect operation of the tracking servo system. 
Taking account the above described arrangement of the recording beam spot, 
playback beam spot, and tracking beam spots, the recording and playback 
method according to the present invention will now be described with 
reference to FIGS. 3A to 3D. 
Reference is first made to FIG. 3A, where there is illustrated the 
magnetization direction of the recording film at the initial state (before 
any recording) wherein the magnetization direction is unified in the 
upward direction. 
At the time of recording information on this recording film, the direction 
of the weak magnetic field produced by the coil 21 is controlled by means 
of the polarity switching device 23 so that the magnetic field is directed 
in the downward direction. In accordance with the signal applied to the 
driver 4, the recording laser beam is controlled to turn on and off, 
thereby changing the localized direction of magnetization of the recording 
film in a manner such that only the portions of the recording film on 
which the recording laser beam is applied becomes downwardly magnetized as 
shown in FIG. 3B. At the time of the recording operation, the disc is 
translated along its radial direction at a constant speed, so that the 
recording tracks are spaced at constant intervals. 
During the recording operation, the focus servo system is controlled in 
accordance with the output signal of the light detector 18, which receives 
the reflected beam of the playback beam spot 26, so that the recording 
beam spot 25 is accurately focused on the recording film 12. This focusing 
is described in detail in the aforementioned copending application. 
Then, when it is desired to erase some of the information recorded on the 
recording track, the source 8 of the playback laser beam is actuated to 
playback (monitor) the recorded information and simultaneously to operate 
the tracking servo system and the focus servo system. When the portion of 
the recording track bearing the unnecessary information reaches the 
position of the recording beam spot in accordance with the movement of the 
disc 1, the source 2 of the recording laser beam is actuated in accordance 
with an erasure command signal. 
Since the direction of the magnetic field of the coil 21 is the same as the 
magnetization direction used during the original recording operation, the 
magnetization direction of the erased portion is inverse to the 
magnetization direction of the original unrecorded disc, shown in FIGS. 3A 
and 3C. Therefore, position of the recording track can be readily 
detected, so that with the tracking operation enabled during the recording 
of new information, it is ensured that there is continuity between the 
recording tracks of the old and new information. 
It is necessary to change the direction of the magnetic field at the time 
of recording new information on the erased portion, and since the 
magnetization of the erased portion has a downward direction, the magnetic 
field is directed upwardly at the time of recording new information. 
FIG. 3D shows an example of the recording track on which the new 
information is recorded on the previously erased portion of the recording 
track. As noted above, the continuity of recording tracks of old and new 
information is maintained because the tracking servo and focus servo 
systems are operated during the recording of new information. 
In order to change the direction of the magnetic field produced by the coil 
21 during the rewriting of new information, it is sufficient to place a 
sign or a detectable pattern on the lead portion of the erased track, so 
that the sign or pattern is detected to operate the polarity switching 
device 23 at the time of rewriting. 
It will be appreciated from the foregoing that a precise tracking is 
enabled in accordance with the present invention. By timing the playback 
beam spots so that they precede the recording beam spots (timed by 
controlling the light emission from laser sources 8 and 2, respectively), 
monitoring becomes possible during the erasure of the information, which 
ensures the continuity of recording track. Furthermore, this arrangement 
enables the detection of the sign or pattern recorded at the lead portion 
of the erased track at the time of rewriting new information. 
Above, a preferred embodiment of the present invention has been described. 
It should be understood, however, that the foregoing description has been 
for illustrative purpose only, and is not intended to limit the scope of 
the invention. Rather, there are numerous equivalents to the preferred 
embodiments, and such are intended to be covered by the appended claims. 
As only one of many examples, the amorphous alloy used as the material of 
the thin film may be replaced by a polycrystalline material such as 
Manganese Bismuth (MnBi).